{"paper_id":"4a1dd9d0-775d-4915-b4e9-6cc0b2cba03b","body_text":"Operando ESR elucidation of charge accumulation and molecular orientation in ternary polymer solar cell materials using organic electrochemical transistor structures | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Operando ESR elucidation of charge accumulation and molecular orientation in ternary polymer solar cell materials using organic electrochemical transistor structures Jiaxi Wang, Dong Xue, Satoshi Inai, Itaru Osaka, Kazuhiro Marumoto This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5982163/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract In recent years, polymer solar cells have been investigated extensively because of their cost-effectiveness and flexibility. Notably, inverted type polymer solar cells using PTzBT((2,5-bis(3-(2-butyloctyl)thiophen-2-yl)thiazolo[5,4-d]thiazole)-alt-(2,5-bis(3-(2-hexyldecyl)thiophen-2-yl)thiazolo[5,4-d]thiazole)) have gained prominence because of their superior conversion efficiency and stability, particularly with the incorporation of non-fullerene acceptor ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene) into the active layer. Nevertheless, a comprehensive understanding of charge accumulation states and molecular orientation within PTzBT remains elusive. For this study, electron spin resonance (ESR) spectroscopy was used to clarify the issues above in conjunction with organic electrochemical transistor structures, which are recognized for their low-voltage operation and flexibility. Our operando ESR investigation revealed the accumulation of positive holes within the PTzBT molecules, simultaneously revealing anisotropy in the ESR spectra upon altering the external magnetic field direction. Intriguingly, an additional observation surfaced: angle variation of the g -factor exhibited discernible changes related to the gate voltage. This finding demonstrates that charges are injected into distinct orientations in PTzBT molecules depending on the amount of accumulated charge, thereby contributing to improvement of solar cell performance. Physical sciences/Materials science/Materials for energy and catalysis/Solar cells Physical sciences/Energy science and technology/Renewable energy Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The pressing need for the advancement of next-generation energy sources is underscored by the adverse effects of extensive fossil fuel consumption. Polymer solar cells have emerged as a focal point of research and development for next-generation solar technologies 1–3 , by virtue of their unique attributes such as flexibility, low cost, and semi-transparency. Their potential benefits pose challenges for traditional silicon solar cells. A notable example of polymer solar cells involves the utilization of poly(2,5-bis(3-(2-butyloctyl)thiophen-2-yl)thiazolo[5,4-d]thiazole)-alt-(2,5-bis(3-(2-hexyldecyl)thiophen-2-yl)thiazolo[5,4-d]thiazole) (PTzBT) 4–6 (Fig. 1 a) in conjunction with the n-type semiconductor [6,6]-phenyl C 61 (or C 71 )-butyric acid methyl ester (PC 61 BM or PC 71 BM) 7,8 (Fig. 1 b). Incorporation of a small amount of the narrow-bandgap non-fullerene acceptor, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) 9,10 (Fig. 1 c), has been reported to enhance the conversion efficiency considerably 4,9 . Moreover, the addition of ITIC contributes to PTzBT solar cell stability, mitigating the decline in power conversion efficiency (PCE) after prolonged (1000 h) storage in a nitrogen-filled glove box at 85°C in the dark. The decrease in PCE is mitigated: from 30% without the additive to a mere 10%. Consequently, PTzBT ternary solar cells can exhibit both high PCE and stability, positioning ITIC as a highly beneficial additive for augmenting photovoltaic conversion performance 4 . To elucidate the charge states and molecular orientation in solar cells from a microscopic perspective, electron spin resonance (ESR) method, which is both non-destructive and highly sensitive, has been applied for molecular-level observation of the spins of charges in solar cells. 11–20 From an earlier study using ESR measurements of PTzBT:ITIC:PC 61 BM solar cells, findings from the ESR anisotropy measurements indicated that ITIC addition can enhance the molecular orientation of PTzBT, particularly with hole accumulation in the active layer of the solar cells 21 . It is particularly interesting that the increase in the number of spins ( N spin ) in the solar cells with ITICs was smaller than in those without ITICs. This insight by microscopy provides a possible explanation for the higher performance and stability of solar cells with ITIC compared to those without ITIC. However, a noteworthy discrepancy was identified between the molecular orientation of PTzBT observed in ESR measurements and results found from an earlier study using X-rays 21,22 . The ESR measurements indicated that the charges accumulate mainly in molecules with molecule planes perpendicular to the substrate plane (edge-on orientation). By contrast, earlier X-ray research indicated the predominant orientation of PTzBT molecules as having a molecule plane parallel to the substrate plane, so-called face-on orientation. This contradiction highlights the need for additional investigation into the charge state of PTzBT and its molecular orientation. For this study, to clarify this contradiction and to study the charge accumulation state and molecular orientation of PTzBT, we use an organic electrochemical transistor (OECT) structure able to change the amount of charge accumulation in the active layer. The OECT design facilitated a short distance between charges and provided a large electrical capacity, thereby enhancing the efficiency of charge injection. Moreover, this construction allowed the device to operate at low voltages 23–25 . The operando ESR investigation revealed the accumulation of positive holes within the PTzBT molecules 5,21 . Simultaneously, alterations in the external magnetic field direction induced anisotropy in the ESR spectra. An intriguing observation was obtained: the angle variation of the g -factor exhibited noticeable changes in response to the gate voltage ( V G ). This finding demonstrates that charges are injected into distinct orientations within PTzBT molecules, depending on the accumulated charge amount. Therefore, comprehensive understanding of charge accumulation states and molecular orientation in PTzBT solar cells is imperative to support additional advancements with this promising solar technology. Experimental For this study, we fabricated devices with an organic electrochemical transistor (OECT) structure (Fig. 1 d) using an active layer of PTzBT:PC 61 BM:ITIC and PTzBT. As the insulating layer, we used an ion gel consisting of a mixture of an ionic liquid ([EMIM][TFSI]) and a polymer (PS-PMMA-PS) (Supplementary Fig. 1). The gate electrode was composed of Ni/Au (3 nm/57 nm), whereas the source and drain electrodes were both made of Au (each 60 nm). ESR measurements were conducted at room temperature (298 K) under applied gate voltage ( V G ). The V G dependence and the external magnetic field direction dependence on the device substrate surface were examined. Results and Discussion Operando ESR signals of PTzBT:ITIC:PC 61 BM OECT To evaluate the charge state of the PTzBT:ITIC:PC 61 BM OECT, we took ESR measurements under device operation, that is, operando ESR measurements. As shown in Fig. 2 a, when negative V G is applied to the OECT, no signal was detected until V G reached − 1.0 V, after which the ESR signal intensity increased, which indicates that holes were progressively accumulating in the semiconductor layer because of hole injection. To identify the signal source, we fabricated an OECT device using PTzBT as the active layer. The ESR measurements are shown in Supplementary Fig. 2a. The similar g -factor from both measurement results confirmed that the observed signal originated from the holes in the PTzBT. For a more quantitative analysis of the spin state of the observed ESR spectra, we calculated the number of spins ( N spin ) derived from the accumulated charge carriers. Specifically, N spin was calculated by integrating the observed ESR spectra twice and comparing the integrated intensity of a standard Mn 2+ marker sample. Figure 2 b presents the V G dependence of N spin for the PTzBT:ITIC:PC 61 BM OECT. As V G increases from − 1.0 V, where the signal is first observed, to − 2.0 V, N spin exhibits a monotonic increase similarly to the trend observed for the PTzBT OECT (Supplementary Fig. 2b). These findings further corroborate the occurrence of hole injection into the active layer. Molecular orientation from angular dependence of ESR signals Observing the anisotropy of the g -factor in ESR signals is a useful method for analyzing molecular orientation. We conducted detailed analysis of the molecular orientation influenced by charge accumulation by varying the angle ( θ ) between the direction of the external magnetic field ( H ) and the substrate plane and by measuring the θ dependence of the ESR signal's g -factor. We rotated the fabricated OECT devices in an increment of 30° during ESR measurements. For the fabricated PTzBT:PC 61 BM:ITIC OECT devices, ESR measurements were taken while applying V G from − 1.0 V to − 2.0 V. Figure 3 a presents the observed angular dependence of the ESR signal's g -factor. Here, θ is defined as the angle between the plane of the OECT substrate and the H direction (Fig. 3 b). Therefore, θ = 90° corresponds to H ⊥ substrate; θ = 0° corresponds to H // substrate. We defined the molecular plane perpendicular to the substrate plane as an edge-on orientation, which inhibits charge transport in solar cells. Also, we defined the molecular plane parallel to the substrate plane as face-on orientation, which helps with charge transport in solar cells (Fig. 3 b) 26–28 . As Fig. 3 a shows, the anisotropy of the g -factor changed with variations in V G . When V G = − 1.0 V, we observed a minimum g -factor (2.00236) when the substrate plane was parallel to the H direction ( θ = 0°). As θ increased, the g -factor also increased, reaching its maximum value (2.00266) at θ = 90°. As V G increased to − 1.8 V, the g -factors at various angles became almost identical, showing almost no anisotropy. By contrast, when V G = − 2.0 V, the g -factor exhibited completely opposite anisotropy compared to that found when V G = − 1.0 V. The g- factor (2.00272) has its maximum value at θ = 0° and minimum value (2.00249) at θ = 90°. Density functional theory (DFT) analysis for PTzBT For more accurate evaluation of the anisotropy of the PTzBT molecular orientation with accumulated holes, we conducted density functional theory (DFT) calculation using Gaussian 16W 18,29–31 . The g -factor shift is useful to determine the presence of charge accumulation at the ends of polymer chains. Reportedly, residual bromine exists at the ends of polymer chains in conducting polymers, even after chain-end treatment 18,29–31 . To elucidate this point, model molecules of PTzBT without residual bromine ends (monomer, dimer, and trimer denoted as TzBT-H, 2TzBT-H, and 3TzBT-H, respectively) and PTzBT with residual bromine ends (monomer, dimer, and trimer denoted as TzBT-Br, 2TzBT-Br, and 3TzBT-Br, respectively) were calculated using the 6-31G(d,p) basis set and the UB3LYP functional (Supplementary Fig. 3). Supplementary Table 1 presents the DFT results for all model molecules. The averaged g -factors ( g µν ) are calculable with a root mean square of principal values of g tensors g µ and g ν ( µ, ν = x, y, z ) as \\(\\:{g}_{\\mu\\:\\nu\\:}=\\sqrt{\\frac{{{g}_{\\mu\\:}}^{2}+{{g}_{\\nu\\:}}^{2}}{2}}\\) , ( μ, ν = x, y, z ).(1) Because the g -factors of the trimer model-molecule 3TzBT-Br are closest to the experimentally obtained value, we use the g -factors of 3TzBT-Br for comparison with the experimentally obtained value. According to the g -factors of 3TzBT-Br (Supplementary Table 1), g y of all the model molecules are larger than other calculated g -factors. At a gate voltage of V G = − 1.0 V, the observed large g -factor at θ = 90° ( g = 2.00266) corresponds to g y . The observed small g -factor at θ = 0° ( g = 2.00241) corresponds to g xz , which indicates that the orientation of molecules with charge accumulation is found to be “edge-on” (Fig. 3 b). Presumably, when charge accumulation begins, holes are stored primarily in molecules with a shallow highest occupied molecular orbital (HOMO) level in edge-on orientation area of PTzBT (Fig. 3 c) 32 . However, at V G = − 2.0 V, the observed small g -factor at θ = 90° ( g = 2.00249) can be described by the g z . The observed large g -factor at θ = 0° ( g = 2.00285) can be described by g xy . Consequently, the orientation of molecules with charge accumulation is found to be “face-on” (Fig. 3 d), which is consistent with the finding obtained from an earlier study of 2D-GIXD showing that the predominant orientation of PTzBT molecules is face-on orientation 4,22,32 . Consequently, as charge accumulation begins, holes are accumulated primarily in molecules with edge-on orientation for shallow HOMO levels 32 . As V G increases, the charge accumulation amount increases. The charge accumulation is observed mainly in molecules with face-on orientation, which are the majority in PTzBT. Changes in the charge accumulation location Our study also revealed that, with a constant angle of θ = 0°, the g -factor increases monotonically as V G increases (Fig. 4 a), which indicates that charge accumulation occurs on molecules with different orientations as V G increases. The g -factor increases sharply from − 1.8 V to − 2.0 V, which results from the charge accumulation observed changing from molecules with edge-on orientation to face-on orientation. We also analyze the ESR signal line width by measuring the full width of a peak at half maximum value (Δ H 1/2 ) and the width between a spectral peak and valley; that is the peak-to-peak ESR line width (Δ H pp ). As shown in Figs. 4 b and 4 c, we found that the line width reaches a maximum at − 1.8 V, probably because, at this voltage, the g -factor of the ESR signal shows almost no variation with changes in θ , indicating that the charge accumulation on face-on oriented molecules is roughly equal to that on edge-on oriented molecules. Because of the large difference in the two g -factors, the two signals cannot overlap completely. That incomplete overlap causes the line width of the ESR signal to reach the maximum at this point. Here, we evaluate the density of charges accumulated in edge-on oriented molecules which degrade the performance of solar cells. As described above, the numbers of charges accumulated in the edge-on and face-on oriented molecules are roughly equal at V G = − 1.8 V. From Fig. 2 b, when V G = − 1.8 V, N spin is obtained as 7.74 × 10 14 . The number of charges accumulated in edge-on oriented molecules is half of the N spin at V G = − 1.8 V, calculated as 3.87 × 10 14 . The volume of active semiconductor layer is calculated as 2.08 × 10 − 5 cm − 3 from the active area (1 mm × 23 mm = 23 mm 2 ) and the measured film thickness (90.3 nm). Therefore, the density of charges accumulated in edge-on oriented molecules per unit volume is estimated as 1.87 × 10 20 cm − 3 . Reducing the density of charges accumulated in edge-on oriented molecules is crucially important for improving solar cell performance, which can be conducted effectively using operando ESR technique presented in this work. For the region between − 1.0 V and − 1.2 V, the initial charge accumulation and the presence of more trapped charges cause the line width at − 1.0 V to be larger. Because the voltage increases to − 1.2 V, more charges are injected, resulting in fewer trapped charges and enhanced charge mobility, thereby leading to a decrease in line width at − 1.2 V, i.e., motional narrowing of ESR line width occurs Conclusions For this study, we investigated charge accumulation and molecular orientation in the active layer of PTzBT-based polymer solar cells using ESR spectroscopy combined with OECTs. Our results indicate that hole injection into PTzBT molecules engenders charge accumulation, with notable changes in molecular orientation depending on V G . The ESR measurements showed that, as V G increases from − 1.0 V to − 1.6 V, charge accumulation occurs primarily in edge-on oriented molecules. At V G = − 1.8 V, the g -factor exhibits no anisotropy, indicating an approximately equal distribution of charges between edge-on and face-on oriented molecules. When V G is increased further to − 2.0 V, charge accumulation observed shifts predominantly to face-on oriented molecules. Additionally, marked changes in the g -factor occur at V G = − 1.8 V, where the ESR linewidth also reaches its maximum. At this point, the accumulated charges are nearly equally distributed between edge-on and face-on oriented molecules. Because of the different g -factors of these two orientated molecules, their respective ESR signals are out of phase, resulting in a maximum in the linewidth. These findings obtianed from a microscopic perspective underscore the crucially important role of molecular orientation and charge accumulation in affecting the performance of PTzBT-based polymer solar cells. Because ESR can detect molecules with edge-on orientation, which are unfavorable for charge transport in solar cells, we can improve the film-formation methods continuously to reduce the number of edge-on oriented molecules and to enhance the efficiency of polymer solar cells based on operando ESR measurement. Methods Active layer fabrication A solution of PTzBT or PTzBT:ITIC:PC 61 BM (1:0.2:2 w/w) was prepared by dissolving them in chlorobenzene solvent at a concentration of approximately 5 g L - ¹ (based on PTzBT). The solution was stirred at 100°C for 30 min. Subsequently, the active layer was fabricated by spin-coating the solution onto a quartz substrate inside a nitrogen atmosphere glovebox (O 2 < 0.2 ppm, H 2 O < 0.5 ppm) at 600 rpm for 20 s. Electrode fabrication The Au source and drain (S-D) electrodes were deposited in three stages with different deposition rates: 0.1 Å s - ¹ up to 3 nm, 0.2 Å s - ¹ from 3 to 10 nm, and 0.3 Å s - ¹ for the remaining 50 nm, producing total thickness of 60 nm. The Ni/Au gate electrode was deposited onto cleaned PET substrates in three stages using different deposition rates: for Ni 0.1 Å s - ¹ up to 3 nm, and for Au 0.2 Å s - ¹ from 3 to 10 nm, and 0.3 Å s - ¹ for the remaining 50 nm, producing total thickness of 60 nm. Fabrication of ion-gel film on gate electrode The ion gel used for this study was prepared by first placing the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), the triblock copolymer poly(styrene-b-methylmethacrylate-b-styrene) (PS-PMMA-PS), and ethyl acetate in a screw cap vial at a mass ratio of 10:1:10. A magnetic stir bar was then added. The mixture was stirred at 300 rpm for 24 h at room temperature (298 K) using a programmable hot-stirrer. The resulting ion-gel solution was drop-cast onto a PET substrate/gate electrode, leaving the wiring area of the gate electrode exposed. Finally, the substrate was vacuum-annealed at 70°C for 24 h to complete the fabrication process. OECT structure fabrication The quartz substrate with the active layer film and S-D electrodes was fixed onto the PET substrate using double-sided tape. Thin copper wires were then connected to the S-D electrodes using silver paste. The PET substrate with the stack of gate electrode and ion-gel film was laminated onto the active layer film with the S-D electrodes, ensuring that the ion-gel film adhered to the surface of the active layer and the S-D electrodes. Additionally, a thin copper wire was connected to the gate electrode using silver paste. Finally, the fabricated organic electrochemical transistor (OECT) device was placed into an ESR sample tube and was sealed under a nitrogen atmosphere inside the glovebox. Declarations Data availability The data supporting these study findings are available from the corresponding authors upon reasonable reques and can also be found at the following online repository: https://doi.org/10.6084/m9.figshare.28244369 Acknowledgments This work was supported by the Japan Science and Technology Agency MIRAI (Grants No. JPMJMI20C5, JPMJMI22C1, and JPMJMI22E2), Japan, by the New Energy and Technology Development Organization, Green Innovation, Japan, by the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research (KAKENHI) (Grant No. 24K01325), Japan, by the University of Tsukuba, Organization for the Promotion of Strategic Research Initiatives, Japan, and by JST SPRING (Grant No. JPMJSP2124), Japan. Author contributions J.W. and K.M. planned the study. I.O. synthesized PTzBT and ITIC molecules. J.W., S.I., and K.M. fabricated the device. J.W., S.I., D.X., and K.M. measured and analyzed the data. J.W.and K.M. wrote the paper. All authors discussed the results and reviewed the manuscript. Competing interests The authors declare that they have no competing interest. Additional information Correspondence should be addressed to Kazuhiro Marumoto. References Günes, S., Neugebauer, H. & Sariciftci, N. S. Conjugated polymer-based organic solar cells. Chem. Rev. 107 , 1324-1338 (2007). Clarke, T. M. & Durrant, J. R. Charge photogeneration in organic solar cells. Chem Rev 110 , 6736-6767 (2010). Søndergaard, R., Hösel, M., Angmo, D., Larsen-Olsen, T. T. & Krebs, F. C. Roll-to-roll fabrication of polymer solar cells. Materials Today 15 , 36-49 (2012). Saito, M. et al. Significantly Sensitized Ternary Blend Polymer Solar Cells with a Very Small Content of the Narrow-Band Gap Third Component That Utilizes Optical Interference. 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Adv. Energy Mater . 6 , 1600171 (2016). Rachmat, V. A. A., Kubodera, T., Son, D., Cho, Y. & Marumoto, K. Molecular Oriented Charge Accumulation in High-efficiency Polymer Solar Cells as Revealed by Operando Spin Analysis. ACS Appl. Mater. Interfaces 11 , 31129-31138 (2019). Heimel, G., Salzmann, I., Duhm, S., Rabe, J. P. & Koch, N. Intrinsic surface dipoles control the energy levels of conjugated polymers. Adv. Funct. Mater. 19 , 3874-3879 (2009). Additional Declarations No competing interests reported. Supplementary Files JWangnpjFlexibleElectronicsSupplementaryInformation.pdf Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-5982163\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":true,\"archivedVersions\":[],\"articleType\":\"Article\",\"associatedPublications\":[],\"authors\":[{\"id\":420108946,\"identity\":\"d00a7df7-3034-4107-b432-130aac09c2c5\",\"order_by\":0,\"name\":\"Jiaxi Wang\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"University of Tsukuba\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Jiaxi\",\"middleName\":\"\",\"lastName\":\"Wang\",\"suffix\":\"\"},{\"id\":420108947,\"identity\":\"1d70a15b-116d-4cce-8de6-ac47db3f43f3\",\"order_by\":1,\"name\":\"Dong Xue\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"University of Tsukuba\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Dong\",\"middleName\":\"\",\"lastName\":\"Xue\",\"suffix\":\"\"},{\"id\":420108948,\"identity\":\"7a65d083-ad8f-46d3-a789-d13524f8e716\",\"order_by\":2,\"name\":\"Satoshi Inai\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"University of Tsukuba\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Satoshi\",\"middleName\":\"\",\"lastName\":\"Inai\",\"suffix\":\"\"},{\"id\":420108951,\"identity\":\"6bb4bf37-df1e-4ba3-b701-58c90cd95ea2\",\"order_by\":3,\"name\":\"Itaru Osaka\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Hiroshima University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Itaru\",\"middleName\":\"\",\"lastName\":\"Osaka\",\"suffix\":\"\"},{\"id\":420108952,\"identity\":\"eb6728c4-89a7-41b7-bc11-7024b549c936\",\"order_by\":4,\"name\":\"Kazuhiro Marumoto\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABB0lEQVRIiWNgGAWjYDADfmYoQ4KBB8JgbMCrwYBBshlEJ5CixeAAuhZcQLf97DOpGxV/5IyPcyd/YPxRlzhzRu7BDww1dgzMs7FbY3Ym3Uw654yBsdlh3m0SDAmHE2dL5CVLMBxLZmCccwC7lgNpbNK5bQaJ24BagA47kDhPIsdAgoHtAAPjjATsWs4/g2jZ3My7+QNDQh1Ii/EPhn94tNyA2rKBmXcD0GHMQIflmEkwtuHT8ozZOueMsbEEyC8JaYeNZ/a8S7NI7EvmwemX82mMt3Mq5OT4+89u/vDBpk52xvHcwzc+fLOTM8QRYqggAYnBYziDCB2oQF6CZC2jYBSMglEwPAEAN5tam151PXAAAAAASUVORK5CYII=\",\"orcid\":\"\",\"institution\":\"University of Tsukuba\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Kazuhiro\",\"middleName\":\"\",\"lastName\":\"Marumoto\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2025-02-07 14:53:19\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-5982163/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-5982163/v1\",\"draftVersion\":[],\"editorialEvents\":[],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":77654330,\"identity\":\"a72390b3-bdea-44f2-ab65-cb076018a2ab\",\"added_by\":\"auto\",\"created_at\":\"2025-03-04 03:05:02\",\"extension\":\"png\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":85167,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eChemical structures and device structure of PTzBT:ITIC:PC\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e61\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003eBM OECT. a, b, c \\u003c/strong\\u003eChemical structures of PTzBT, PC\\u003csub\\u003e61\\u003c/sub\\u003eBM, and ITIC.\\u003cstrong\\u003e d \\u003c/strong\\u003eSchematic of a PTzBT:ITIC:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM OECT used for ESR study.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage1.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5982163/v1/7864444372fe4d5ae57de277.png\"},{\"id\":77654325,\"identity\":\"8fcb91c2-3be0-4d99-afc2-068e99c5970f\",\"added_by\":\"auto\",\"created_at\":\"2025-03-04 03:05:02\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":68763,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eOperando ESR signals of PTzBT:PC\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e61\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003eBM:ITIC OECT. a \\u003c/strong\\u003e\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e dependence of ESR spectra of the PTzBT:ITIC:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM OECT (\\u003cem\\u003eH\\u003c/em\\u003e ⊥ plane, \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eD\\u003c/sub\\u003e = 0 V). \\u003cstrong\\u003eb\\u003c/strong\\u003e \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG \\u003c/sub\\u003edependence of\\u003cem\\u003e N\\u003c/em\\u003e\\u003csub\\u003espin\\u003c/sub\\u003e of the PTzBT:ITIC:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM OECT (\\u003cem\\u003eH\\u003c/em\\u003e ⊥ plane, \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eD\\u003c/sub\\u003e = 0 V).\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage2.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5982163/v1/0f26129b7315d03c26708226.png\"},{\"id\":77655147,\"identity\":\"1553d764-3af1-436d-869d-db256522c53f\",\"added_by\":\"auto\",\"created_at\":\"2025-03-04 03:13:02\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":181031,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eLine width of ESR signals\\u003c/strong\\u003e \\u003cstrong\\u003eof PTzBT:PC\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e61\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003eBM:ITIC OECT. a \\u003c/strong\\u003eVariation of ESR\\u003cem\\u003e g\\u003c/em\\u003e-factor against the direction of the magnetic field with different values of \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e. \\u003cstrong\\u003eb\\u003c/strong\\u003e Schematic of molecular orientation (face-on and edge-on) and the angle (\\u003cem\\u003eθ\\u003c/em\\u003e) between the external magnetic field direction and the substrate plane. \\u003cstrong\\u003ec, d\\u003c/strong\\u003e Charge accumulation at low \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e and high \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5982163/v1/994c18ace21b9a5b13c75a35.png\"},{\"id\":77654333,\"identity\":\"aeef028f-d3cf-4205-b5f8-672fed436734\",\"added_by\":\"auto\",\"created_at\":\"2025-03-04 03:05:02\",\"extension\":\"png\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":82900,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eVariation of \\u003c/strong\\u003e\\u003cem\\u003e\\u003cstrong\\u003eg\\u003c/strong\\u003e\\u003c/em\\u003e\\u003cstrong\\u003e-factor and line width of ESR signals of PTzBT:PC\\u003c/strong\\u003e\\u003csub\\u003e\\u003cstrong\\u003e61\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003eBM:ITIC OECT with\\u003c/strong\\u003e\\u003cem\\u003e\\u003cstrong\\u003e V\\u003c/strong\\u003e\\u003c/em\\u003e\\u003csub\\u003e\\u003cstrong\\u003eG.\\u003c/strong\\u003e\\u003c/sub\\u003e\\u003cstrong\\u003e a \\u003c/strong\\u003eVariation of \\u003cem\\u003eg\\u003c/em\\u003e-factor with \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e at\\u003cem\\u003e H\\u003c/em\\u003e //\\u003cem\\u003e \\u003c/em\\u003eplane (\\u003cem\\u003eθ \\u003c/em\\u003e= 0°). \\u003cstrong\\u003eb, c\\u003c/strong\\u003e Variation of line width (Δ\\u003cem\\u003eH\\u003c/em\\u003e\\u003csub\\u003e1/2\\u003c/sub\\u003e and Δ\\u003cem\\u003eH\\u003c/em\\u003e\\u003csub\\u003epp\\u003c/sub\\u003e) with \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e at\\u003cem\\u003e H\\u003c/em\\u003e //\\u003cem\\u003e \\u003c/em\\u003eplane (\\u003cem\\u003eθ \\u003c/em\\u003e= 0°).\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"floatimage4.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5982163/v1/50e14e07fdc287a6ddf61e04.png\"},{\"id\":77655681,\"identity\":\"818db041-1d12-4c2b-a995-5075546b35eb\",\"added_by\":\"auto\",\"created_at\":\"2025-03-04 03:21:03\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":1204834,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5982163/v1/177c96e3-1101-42b8-a8ae-74de4358423c.pdf\"},{\"id\":77654326,\"identity\":\"53001c51-5655-4ef9-bcd0-dfe797599ec3\",\"added_by\":\"auto\",\"created_at\":\"2025-03-04 03:05:02\",\"extension\":\"pdf\",\"order_by\":1,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":357386,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"JWangnpjFlexibleElectronicsSupplementaryInformation.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-5982163/v1/2318c8af6e5b6bf4f580708a.pdf\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"Operando ESR elucidation of charge accumulation and molecular orientation in ternary polymer solar cell materials using organic electrochemical transistor structures\",\"fulltext\":[{\"header\":\"Introduction\",\"content\":\"\\u003cp\\u003eThe pressing need for the advancement of next-generation energy sources is underscored by the adverse effects of extensive fossil fuel consumption. Polymer solar cells have emerged as a focal point of research and development for next-generation solar technologies \\u003csup\\u003e1\\u0026ndash;3\\u003c/sup\\u003e, by virtue of their unique attributes such as flexibility, low cost, and semi-transparency. Their potential benefits pose challenges for traditional silicon solar cells.\\u003c/p\\u003e \\u003cp\\u003eA notable example of polymer solar cells involves the utilization of poly(2,5-bis(3-(2-butyloctyl)thiophen-2-yl)thiazolo[5,4-d]thiazole)-alt-(2,5-bis(3-(2-hexyldecyl)thiophen-2-yl)thiazolo[5,4-d]thiazole) (PTzBT)\\u003csup\\u003e4\\u0026ndash;6\\u003c/sup\\u003e (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ea) in conjunction with the n-type semiconductor [6,6]-phenyl C\\u003csub\\u003e61\\u003c/sub\\u003e (or C\\u003csub\\u003e71\\u003c/sub\\u003e)-butyric acid methyl ester (PC\\u003csub\\u003e61\\u003c/sub\\u003eBM or PC\\u003csub\\u003e71\\u003c/sub\\u003eBM)\\u003csup\\u003e7,8\\u003c/sup\\u003e (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003eb). Incorporation of a small amount of the narrow-bandgap non-fullerene acceptor, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2\\u0026rsquo;,3\\u0026rsquo;-d\\u0026rsquo;]-s-indaceno[1,2-b:5,6-b\\u0026rsquo;]dithiophene (ITIC)\\u003csup\\u003e9,10\\u003c/sup\\u003e (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ec), has been reported to enhance the conversion efficiency considerably\\u003csup\\u003e4,9\\u003c/sup\\u003e. Moreover, the addition of ITIC contributes to PTzBT solar cell stability, mitigating the decline in power conversion efficiency (PCE) after prolonged (1000 h) storage in a nitrogen-filled glove box at 85\\u0026deg;C in the dark. The decrease in PCE is mitigated: from 30% without the additive to a mere 10%. Consequently, PTzBT ternary solar cells can exhibit both high PCE and stability, positioning ITIC as a highly beneficial additive for augmenting photovoltaic conversion performance\\u003csup\\u003e4\\u003c/sup\\u003e.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eTo elucidate the charge states and molecular orientation in solar cells from a microscopic perspective, electron spin resonance (ESR) method, which is both non-destructive and highly sensitive, has been applied for molecular-level observation of the spins of charges in solar cells.\\u003csup\\u003e11\\u0026ndash;20\\u003c/sup\\u003e From an earlier study using ESR measurements of PTzBT:ITIC:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM solar cells, findings from the ESR anisotropy measurements indicated that ITIC addition can enhance the molecular orientation of PTzBT, particularly with hole accumulation in the active layer of the solar cells\\u003csup\\u003e21\\u003c/sup\\u003e. It is particularly interesting that the increase in the number of spins (\\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003espin\\u003c/sub\\u003e) in the solar cells with ITICs was smaller than in those without ITICs. This insight by microscopy provides a possible explanation for the higher performance and stability of solar cells with ITIC compared to those without ITIC.\\u003c/p\\u003e \\u003cp\\u003eHowever, a noteworthy discrepancy was identified between the molecular orientation of PTzBT observed in ESR measurements and results found from an earlier study using X-rays\\u003csup\\u003e21,22\\u003c/sup\\u003e. The ESR measurements indicated that the charges accumulate mainly in molecules with molecule planes perpendicular to the substrate plane (edge-on orientation). By contrast, earlier X-ray research indicated the predominant orientation of PTzBT molecules as having a molecule plane parallel to the substrate plane, so-called face-on orientation. This contradiction highlights the need for additional investigation into the charge state of PTzBT and its molecular orientation.\\u003c/p\\u003e \\u003cp\\u003eFor this study, to clarify this contradiction and to study the charge accumulation state and molecular orientation of PTzBT, we use an organic electrochemical transistor (OECT) structure able to change the amount of charge accumulation in the active layer. The OECT design facilitated a short distance between charges and provided a large electrical capacity, thereby enhancing the efficiency of charge injection. Moreover, this construction allowed the device to operate at low voltages\\u003csup\\u003e23\\u0026ndash;25\\u003c/sup\\u003e. The operando ESR investigation revealed the accumulation of positive holes within the PTzBT molecules\\u003csup\\u003e5,21\\u003c/sup\\u003e. Simultaneously, alterations in the external magnetic field direction induced anisotropy in the ESR spectra. An intriguing observation was obtained: the angle variation of the \\u003cem\\u003eg\\u003c/em\\u003e-factor exhibited noticeable changes in response to the gate voltage (\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e). This finding demonstrates that charges are injected into distinct orientations within PTzBT molecules, depending on the accumulated charge amount. Therefore, comprehensive understanding of charge accumulation states and molecular orientation in PTzBT solar cells is imperative to support additional advancements with this promising solar technology.\\u003c/p\\u003e\"},{\"header\":\"Experimental\",\"content\":\"\\u003cp\\u003eFor this study, we fabricated devices with an organic electrochemical transistor (OECT) structure (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003ed) using an active layer of PTzBT:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM:ITIC and PTzBT. As the insulating layer, we used an ion gel consisting of a mixture of an ionic liquid ([EMIM][TFSI]) and a polymer (PS-PMMA-PS) (Supplementary Fig.\\u0026nbsp;1). The gate electrode was composed of Ni/Au (3 nm/57 nm), whereas the source and drain electrodes were both made of Au (each 60 nm). ESR measurements were conducted at room temperature (298 K) under applied gate voltage (\\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e). The \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e dependence and the external magnetic field direction dependence on the device substrate surface were examined.\\u003c/p\\u003e\"},{\"header\":\"Results and Discussion\",\"content\":\"\\u003cdiv id=\\\"Sec4\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eOperando ESR signals of PTzBT:ITIC:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM OECT\\u003c/h2\\u003e \\u003cp\\u003eTo evaluate the charge state of the PTzBT:ITIC:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM OECT, we took ESR measurements under device operation, that is, operando ESR measurements. As shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003ea, when negative \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e is applied to the OECT, no signal was detected until \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e reached \\u0026minus;\\u0026thinsp;1.0 V, after which the ESR signal intensity increased, which indicates that holes were progressively accumulating in the semiconductor layer because of hole injection. To identify the signal source, we fabricated an OECT device using PTzBT as the active layer. The ESR measurements are shown in Supplementary Fig.\\u0026nbsp;2a. The similar \\u003cem\\u003eg\\u003c/em\\u003e-factor from both measurement results confirmed that the observed signal originated from the holes in the PTzBT.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eFor a more quantitative analysis of the spin state of the observed ESR spectra, we calculated the number of spins (\\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003espin\\u003c/sub\\u003e) derived from the accumulated charge carriers. Specifically, \\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003espin\\u003c/sub\\u003e was calculated by integrating the observed ESR spectra twice and comparing the integrated intensity of a standard Mn\\u003csup\\u003e2+\\u003c/sup\\u003e marker sample. Figure\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eb presents the \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e dependence of \\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003espin\\u003c/sub\\u003e for the PTzBT:ITIC:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM OECT. As \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e increases from \\u0026minus;\\u0026thinsp;1.0 V, where the signal is first observed, to \\u0026minus;\\u0026thinsp;2.0 V, \\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003espin\\u003c/sub\\u003e exhibits a monotonic increase similarly to the trend observed for the PTzBT OECT (Supplementary Fig.\\u0026nbsp;2b). These findings further corroborate the occurrence of hole injection into the active layer.\\u003c/p\\u003e \\u003c/div\\u003e\\n\\u003ch3\\u003eMolecular orientation from angular dependence of ESR signals\\u003c/h3\\u003e\\n\\u003cp\\u003eObserving the anisotropy of the \\u003cem\\u003eg\\u003c/em\\u003e-factor in ESR signals is a useful method for analyzing molecular orientation. We conducted detailed analysis of the molecular orientation influenced by charge accumulation by varying the angle (\\u003cem\\u003eθ\\u003c/em\\u003e) between the direction of the external magnetic field (\\u003cem\\u003eH\\u003c/em\\u003e) and the substrate plane and by measuring the \\u003cem\\u003eθ\\u003c/em\\u003e dependence of the ESR signal's \\u003cem\\u003eg\\u003c/em\\u003e-factor.\\u003c/p\\u003e \\u003cp\\u003eWe rotated the fabricated OECT devices in an increment of 30\\u0026deg; during ESR measurements. For the fabricated PTzBT:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM:ITIC OECT devices, ESR measurements were taken while applying \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e from \\u0026minus;\\u0026thinsp;1.0 V to \\u0026minus;\\u0026thinsp;2.0 V. Figure\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ea presents the observed angular dependence of the ESR signal's \\u003cem\\u003eg\\u003c/em\\u003e-factor. Here, \\u003cem\\u003eθ\\u003c/em\\u003e is defined as the angle between the plane of the OECT substrate and the \\u003cem\\u003eH\\u003c/em\\u003e direction (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eb). Therefore, \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;90\\u0026deg; corresponds to \\u003cem\\u003eH\\u003c/em\\u003e \\u0026perp; substrate; \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;0\\u0026deg; corresponds to \\u003cem\\u003eH //\\u003c/em\\u003e substrate. We defined the molecular plane perpendicular to the substrate plane as an edge-on orientation, which inhibits charge transport in solar cells. Also, we defined the molecular plane parallel to the substrate plane as face-on orientation, which helps with charge transport in solar cells (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eb) \\u003csup\\u003e26\\u0026ndash;28\\u003c/sup\\u003e. As Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ea shows, the anisotropy of the \\u003cem\\u003eg\\u003c/em\\u003e-factor changed with variations in \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e. When \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;1.0 V, we observed a minimum \\u003cem\\u003eg\\u003c/em\\u003e-factor (2.00236) when the substrate plane was parallel to the \\u003cem\\u003eH\\u003c/em\\u003e direction (\\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;0\\u0026deg;). As \\u003cem\\u003eθ\\u003c/em\\u003e increased, the \\u003cem\\u003eg\\u003c/em\\u003e-factor also increased, reaching its maximum value (2.00266) at \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;90\\u0026deg;. As \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e increased to \\u0026minus;\\u0026thinsp;1.8 V, the \\u003cem\\u003eg\\u003c/em\\u003e-factors at various angles became almost identical, showing almost no anisotropy. By contrast, when \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;2.0 V, the \\u003cem\\u003eg\\u003c/em\\u003e-factor exhibited completely opposite anisotropy compared to that found when \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;1.0 V. The \\u003cem\\u003eg-\\u003c/em\\u003efactor (2.00272) has its maximum value at \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;0\\u0026deg; and minimum value (2.00249) at \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;90\\u0026deg;.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e\\n\\u003ch3\\u003eDensity functional theory (DFT) analysis for PTzBT\\u003c/h3\\u003e\\n\\u003cp\\u003eFor more accurate evaluation of the anisotropy of the PTzBT molecular orientation with accumulated holes, we conducted density functional theory (DFT) calculation using Gaussian 16W\\u003csup\\u003e18,29\\u0026ndash;31\\u003c/sup\\u003e. The \\u003cem\\u003eg\\u003c/em\\u003e-factor shift is useful to determine the presence of charge accumulation at the ends of polymer chains. Reportedly, residual bromine exists at the ends of polymer chains in conducting polymers, even after chain-end treatment \\u003csup\\u003e18,29\\u0026ndash;31\\u003c/sup\\u003e. To elucidate this point, model molecules of PTzBT without residual bromine ends (monomer, dimer, and trimer denoted as TzBT-H, 2TzBT-H, and 3TzBT-H, respectively) and PTzBT with residual bromine ends (monomer, dimer, and trimer denoted as TzBT-Br, 2TzBT-Br, and 3TzBT-Br, respectively) were calculated using the 6-31G(d,p) basis set and the UB3LYP functional (Supplementary Fig.\\u0026nbsp;3). Supplementary Table\\u0026nbsp;1 presents the DFT results for all model molecules. The averaged \\u003cem\\u003eg\\u003c/em\\u003e-factors (\\u003cem\\u003eg\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003e\\u0026micro;ν\\u003c/em\\u003e\\u003c/sub\\u003e) are calculable with a root mean square of principal values of \\u003cem\\u003eg\\u003c/em\\u003e tensors \\u003cem\\u003eg\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003e\\u0026micro;\\u003c/em\\u003e\\u003c/sub\\u003e and \\u003cem\\u003eg\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003eν\\u003c/em\\u003e\\u003c/sub\\u003e (\\u003cem\\u003e\\u0026micro;, ν\\u0026thinsp;=\\u0026thinsp;x, y, z\\u003c/em\\u003e) as\\u003c/p\\u003e \\u003cp\\u003e \\u003cspan class=\\\"InlineEquation\\\"\\u003e \\u003cspan class=\\\"mathinline\\\"\\u003e\\\\(\\\\:{g}_{\\\\mu\\\\:\\\\nu\\\\:}=\\\\sqrt{\\\\frac{{{g}_{\\\\mu\\\\:}}^{2}+{{g}_{\\\\nu\\\\:}}^{2}}{2}}\\\\)\\u003c/span\\u003e \\u003c/span\\u003e, (\\u003cem\\u003eμ, ν = x, y, z\\u003c/em\\u003e).(1)\\u003c/p\\u003e \\u003cp\\u003eBecause the \\u003cem\\u003eg\\u003c/em\\u003e-factors of the trimer model-molecule 3TzBT-Br are closest to the experimentally obtained value, we use the \\u003cem\\u003eg\\u003c/em\\u003e-factors of 3TzBT-Br for comparison with the experimentally obtained value. According to the \\u003cem\\u003eg\\u003c/em\\u003e-factors of 3TzBT-Br (Supplementary Table\\u0026nbsp;1), \\u003cem\\u003eg\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003ey\\u003c/em\\u003e\\u003c/sub\\u003e of all the model molecules are larger than other calculated \\u003cem\\u003eg\\u003c/em\\u003e-factors. At a gate voltage of \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;1.0 V, the observed large \\u003cem\\u003eg\\u003c/em\\u003e-factor at \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;90\\u0026deg; (\\u003cem\\u003eg\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;2.00266) corresponds to \\u003cem\\u003eg\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003ey\\u003c/em\\u003e\\u003c/sub\\u003e. The observed small \\u003cem\\u003eg\\u003c/em\\u003e-factor at \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;0\\u0026deg; (\\u003cem\\u003eg\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;2.00241) corresponds to \\u003cem\\u003eg\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003exz\\u003c/em\\u003e\\u003c/sub\\u003e, which indicates that the orientation of molecules with charge accumulation is found to be \\u0026ldquo;edge-on\\u0026rdquo; (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003eb). Presumably, when charge accumulation begins, holes are stored primarily in molecules with a shallow highest occupied molecular orbital (HOMO) level in edge-on orientation area of PTzBT (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ec)\\u003csup\\u003e32\\u003c/sup\\u003e.\\u003c/p\\u003e \\u003cp\\u003eHowever, at \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;2.0 V, the observed small \\u003cem\\u003eg\\u003c/em\\u003e-factor at \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;90\\u0026deg; (\\u003cem\\u003eg\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;2.00249) can be described by the \\u003cem\\u003eg\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003ez\\u003c/em\\u003e\\u003c/sub\\u003e. The observed large \\u003cem\\u003eg\\u003c/em\\u003e-factor at \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;0\\u0026deg; (\\u003cem\\u003eg\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;2.00285) can be described by \\u003cem\\u003eg\\u003c/em\\u003e\\u003csub\\u003exy\\u003c/sub\\u003e. Consequently, the orientation of molecules with charge accumulation is found to be \\u0026ldquo;face-on\\u0026rdquo; (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003ed), which is consistent with the finding obtained from an earlier study of 2D-GIXD showing that the predominant orientation of PTzBT molecules is face-on orientation\\u003csup\\u003e4,22,32\\u003c/sup\\u003e. Consequently, as charge accumulation begins, holes are accumulated primarily in molecules with edge-on orientation for shallow HOMO levels\\u003csup\\u003e32\\u003c/sup\\u003e. As \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e increases, the charge accumulation amount increases. The charge accumulation is observed mainly in molecules with face-on orientation, which are the majority in PTzBT.\\u003c/p\\u003e\\n\\u003ch3\\u003eChanges in the charge accumulation location\\u003c/h3\\u003e\\n\\u003cp\\u003eOur study also revealed that, with a constant angle of \\u003cem\\u003eθ\\u003c/em\\u003e\\u0026thinsp;=\\u0026thinsp;0\\u0026deg;, the \\u003cem\\u003eg\\u003c/em\\u003e-factor increases monotonically as \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e increases (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ea), which indicates that charge accumulation occurs on molecules with different orientations as \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e increases. The \\u003cem\\u003eg\\u003c/em\\u003e-factor increases sharply from \\u0026minus;\\u0026thinsp;1.8 V to \\u0026minus;\\u0026thinsp;2.0 V, which results from the charge accumulation observed changing from molecules with edge-on orientation to face-on orientation. We also analyze the ESR signal line width by measuring the full width of a peak at half maximum value (Δ\\u003cem\\u003eH\\u003c/em\\u003e\\u003csub\\u003e1/2\\u003c/sub\\u003e) and the width between a spectral peak and valley; that is the peak-to-peak ESR line width (Δ\\u003cem\\u003eH\\u003c/em\\u003e\\u003csub\\u003epp\\u003c/sub\\u003e). As shown in Figs.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003eb and \\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003ec, we found that the line width reaches a maximum at \\u0026minus;\\u0026thinsp;1.8 V, probably because, at this voltage, the \\u003cem\\u003eg\\u003c/em\\u003e-factor of the ESR signal shows almost no variation with changes in \\u003cem\\u003eθ\\u003c/em\\u003e, indicating that the charge accumulation on face-on oriented molecules is roughly equal to that on edge-on oriented molecules. Because of the large difference in the two \\u003cem\\u003eg\\u003c/em\\u003e-factors, the two signals cannot overlap completely. That incomplete overlap causes the line width of the ESR signal to reach the maximum at this point.\\u003c/p\\u003e \\u003cp\\u003e \\u003c/p\\u003e \\u003cp\\u003eHere, we evaluate the density of charges accumulated in edge-on oriented molecules which degrade the performance of solar cells. As described above, the numbers of charges accumulated in the edge-on and face-on oriented molecules are roughly equal at \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;1.8 V. From Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003eb, when \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;1.8 V, \\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003espin\\u003c/sub\\u003e is obtained as 7.74 \\u0026times; 10\\u003csup\\u003e14\\u003c/sup\\u003e. The number of charges accumulated in edge-on oriented molecules is half of the \\u003cem\\u003eN\\u003c/em\\u003e\\u003csub\\u003espin\\u003c/sub\\u003e at \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;1.8 V, calculated as 3.87 \\u0026times; 10\\u003csup\\u003e14\\u003c/sup\\u003e. The volume of active semiconductor layer is calculated as 2.08 \\u0026times; 10\\u003csup\\u003e\\u0026minus;\\u0026thinsp;5\\u003c/sup\\u003e cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;3\\u003c/sup\\u003e from the active area (1 mm \\u0026times; 23 mm\\u0026thinsp;=\\u0026thinsp;23 mm\\u003csup\\u003e2\\u003c/sup\\u003e) and the measured film thickness (90.3 nm). Therefore, the density of charges accumulated in edge-on oriented molecules per unit volume is estimated as 1.87 \\u0026times; 10\\u003csup\\u003e20\\u003c/sup\\u003e cm\\u003csup\\u003e\\u0026minus;\\u0026thinsp;3\\u003c/sup\\u003e. Reducing the density of charges accumulated in edge-on oriented molecules is crucially important for improving solar cell performance, which can be conducted effectively using operando ESR technique presented in this work.\\u003c/p\\u003e \\u003cp\\u003eFor the region between \\u0026minus;\\u0026thinsp;1.0 V and \\u0026minus;\\u0026thinsp;1.2 V, the initial charge accumulation and the presence of more trapped charges cause the line width at \\u0026minus;\\u0026thinsp;1.0 V to be larger. Because the voltage increases to \\u0026minus;\\u0026thinsp;1.2 V, more charges are injected, resulting in fewer trapped charges and enhanced charge mobility, thereby leading to a decrease in line width at \\u0026minus;\\u0026thinsp;1.2 V, i.e., motional narrowing of ESR line width occurs\\u003c/p\\u003e\"},{\"header\":\"Conclusions\",\"content\":\"\\u003cp\\u003eFor this study, we investigated charge accumulation and molecular orientation in the active layer of PTzBT-based polymer solar cells using ESR spectroscopy combined with OECTs. Our results indicate that hole injection into PTzBT molecules engenders charge accumulation, with notable changes in molecular orientation depending on \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e. The ESR measurements showed that, as \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e increases from \\u0026minus;\\u0026thinsp;1.0 V to \\u0026minus;\\u0026thinsp;1.6 V, charge accumulation occurs primarily in edge-on oriented molecules. At \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;1.8 V, the \\u003cem\\u003eg\\u003c/em\\u003e-factor exhibits no anisotropy, indicating an approximately equal distribution of charges between edge-on and face-on oriented molecules. When \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e is increased further to \\u0026minus;\\u0026thinsp;2.0 V, charge accumulation observed shifts predominantly to face-on oriented molecules. Additionally, marked changes in the \\u003cem\\u003eg\\u003c/em\\u003e-factor occur at \\u003cem\\u003eV\\u003c/em\\u003e\\u003csub\\u003eG\\u003c/sub\\u003e\\u0026thinsp;=\\u0026thinsp;\\u0026minus;\\u0026thinsp;1.8 V, where the ESR linewidth also reaches its maximum. At this point, the accumulated charges are nearly equally distributed between edge-on and face-on oriented molecules. Because of the different \\u003cem\\u003eg\\u003c/em\\u003e-factors of these two orientated molecules, their respective ESR signals are out of phase, resulting in a maximum in the linewidth. These findings obtianed from a microscopic perspective underscore the crucially important role of molecular orientation and charge accumulation in affecting the performance of PTzBT-based polymer solar cells. Because ESR can detect molecules with edge-on orientation, which are unfavorable for charge transport in solar cells, we can improve the film-formation methods continuously to reduce the number of edge-on oriented molecules and to enhance the efficiency of polymer solar cells based on operando ESR measurement.\\u003c/p\\u003e\"},{\"header\":\"Methods\",\"content\":\"\\u003cdiv id=\\\"Sec10\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eActive layer fabrication\\u003c/h2\\u003e \\u003cp\\u003eA solution of PTzBT or PTzBT:ITIC:PC\\u003csub\\u003e61\\u003c/sub\\u003eBM (1:0.2:2 w/w) was prepared by dissolving them in chlorobenzene solvent at a concentration of approximately 5 g L\\u003csup\\u003e-\\u003c/sup\\u003e\\u0026sup1; (based on PTzBT). The solution was stirred at 100\\u0026deg;C for 30 min. Subsequently, the active layer was fabricated by spin-coating the solution onto a quartz substrate inside a nitrogen atmosphere glovebox (O\\u003csub\\u003e2\\u003c/sub\\u003e\\u0026thinsp;\\u0026lt;\\u0026thinsp;0.2 ppm, H\\u003csub\\u003e2\\u003c/sub\\u003eO\\u0026thinsp;\\u0026lt;\\u0026thinsp;0.5 ppm) at 600 rpm for 20 s.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec11\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eElectrode fabrication\\u003c/h2\\u003e \\u003cp\\u003eThe Au source and drain (S-D) electrodes were deposited in three stages with different deposition rates: 0.1 \\u0026Aring; s\\u003csup\\u003e-\\u003c/sup\\u003e\\u0026sup1; up to 3 nm, 0.2 \\u0026Aring; s\\u003csup\\u003e-\\u003c/sup\\u003e\\u0026sup1; from 3 to 10 nm, and 0.3 \\u0026Aring; s\\u003csup\\u003e-\\u003c/sup\\u003e\\u0026sup1; for the remaining 50 nm, producing total thickness of 60 nm.\\u003c/p\\u003e \\u003cp\\u003eThe Ni/Au gate electrode was deposited onto cleaned PET substrates in three stages using different deposition rates: for Ni 0.1 \\u0026Aring; s\\u003csup\\u003e-\\u003c/sup\\u003e\\u0026sup1; up to 3 nm, and for Au 0.2 \\u0026Aring; s\\u003csup\\u003e-\\u003c/sup\\u003e\\u0026sup1; from 3 to 10 nm, and 0.3 \\u0026Aring; s\\u003csup\\u003e-\\u003c/sup\\u003e\\u0026sup1; for the remaining 50 nm, producing total thickness of 60 nm.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec12\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eFabrication of ion-gel film on gate electrode\\u003c/h2\\u003e \\u003cp\\u003eThe ion gel used for this study was prepared by first placing the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), the triblock copolymer poly(styrene-b-methylmethacrylate-b-styrene) (PS-PMMA-PS), and ethyl acetate in a screw cap vial at a mass ratio of 10:1:10. A magnetic stir bar was then added. The mixture was stirred at 300 rpm for 24 h at room temperature (298 K) using a programmable hot-stirrer. The resulting ion-gel solution was drop-cast onto a PET substrate/gate electrode, leaving the wiring area of the gate electrode exposed. Finally, the substrate was vacuum-annealed at 70\\u0026deg;C for 24 h to complete the fabrication process.\\u003c/p\\u003e \\u003c/div\\u003e \\u003cdiv id=\\\"Sec13\\\" class=\\\"Section2\\\"\\u003e \\u003ch2\\u003eOECT structure fabrication\\u003c/h2\\u003e \\u003cp\\u003eThe quartz substrate with the active layer film and S-D electrodes was fixed onto the PET substrate using double-sided tape. Thin copper wires were then connected to the S-D electrodes using silver paste. The PET substrate with the stack of gate electrode and ion-gel film was laminated onto the active layer film with the S-D electrodes, ensuring that the ion-gel film adhered to the surface of the active layer and the S-D electrodes. Additionally, a thin copper wire was connected to the gate electrode using silver paste. Finally, the fabricated organic electrochemical transistor (OECT) device was placed into an ESR sample tube and was sealed under a nitrogen atmosphere inside the glovebox.\\u003c/p\\u003e \\u003c/div\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003eData availability\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe data supporting these study findings are available from the corresponding authors upon reasonable reques and can also be found at the following online repository: https://doi.org/10.6084/m9.figshare.28244369\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAcknowledgments\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThis work was supported by the Japan Science and Technology Agency MIRAI (Grants No. JPMJMI20C5, JPMJMI22C1, and JPMJMI22E2), Japan, by the New Energy and Technology Development Organization, Green Innovation, Japan, by the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research (KAKENHI) (Grant No. 24K01325), Japan, by the University of Tsukuba, Organization for the Promotion of Strategic Research Initiatives, Japan, and by JST SPRING (Grant No. JPMJSP2124), Japan.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAuthor contributions\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eJ.W. and K.M. planned the study. I.O. synthesized PTzBT and ITIC molecules. J.W., S.I., and K.M. fabricated the device. J.W., S.I., D.X., and K.M. measured and analyzed the data. J.W.and K.M. wrote the paper. All authors discussed the results and reviewed the manuscript.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eCompeting interests\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe authors declare that they have no competing interest.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAdditional information\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eCorrespondence\\u0026nbsp;\\u003c/strong\\u003eshould be addressed to Kazuhiro Marumoto.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\n\\u003cli\\u003eG\\u0026uuml;nes, S., Neugebauer, H. \\u0026amp; Sariciftci, N. S. Conjugated polymer-based organic solar cells. \\u003cem\\u003eChem. 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Mater.\\u003c/em\\u003e \\u003cstrong\\u003e19\\u003c/strong\\u003e, 3874-3879 (2009).\\u003c/li\\u003e\\n\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":true,\"hideJournal\":true,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":false,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true},\"keywords\":\"\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-5982163/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-5982163/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eIn recent years, polymer solar cells have been investigated extensively because of their cost-effectiveness and flexibility. Notably, inverted type polymer solar cells using PTzBT((2,5-bis(3-(2-butyloctyl)thiophen-2-yl)thiazolo[5,4-d]thiazole)-alt-(2,5-bis(3-(2-hexyldecyl)thiophen-2-yl)thiazolo[5,4-d]thiazole)) have gained prominence because of their superior conversion efficiency and stability, particularly with the incorporation of non-fullerene acceptor ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene) into the active layer. Nevertheless, a comprehensive understanding of charge accumulation states and molecular orientation within PTzBT remains elusive. For this study, electron spin resonance (ESR) spectroscopy was used to clarify the issues above in conjunction with organic electrochemical transistor structures, which are recognized for their low-voltage operation and flexibility. Our operando ESR investigation revealed the accumulation of positive holes within the PTzBT molecules, simultaneously revealing anisotropy in the ESR spectra upon altering the external magnetic field direction. Intriguingly, an additional observation surfaced: angle variation of the \\u003cem\\u003eg\\u003c/em\\u003e-factor exhibited discernible changes related to the gate voltage. This finding demonstrates that charges are injected into distinct orientations in PTzBT molecules depending on the amount of accumulated charge, thereby contributing to improvement of solar cell performance.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Operando ESR elucidation of charge accumulation and molecular orientation in ternary polymer solar cell materials using organic electrochemical transistor structures\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2025-03-04 03:04:57\",\"doi\":\"10.21203/rs.3.rs-5982163/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true}}],\"origin\":\"\",\"ownerIdentity\":\"31fd3a1f-7d62-447b-9628-f323183531d7\",\"owner\":[],\"postedDate\":\"March 4th, 2025\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"posted\",\"subjectAreas\":[{\"id\":44783171,\"name\":\"Physical sciences/Materials science/Materials for energy and catalysis/Solar cells\"},{\"id\":44783172,\"name\":\"Physical sciences/Energy science and technology/Renewable energy\"}],\"tags\":[],\"updatedAt\":\"2025-03-04T03:04:57+00:00\",\"versionOfRecord\":[],\"versionCreatedAt\":\"2025-03-04 03:04:57\",\"video\":\"\",\"vorDoi\":\"\",\"vorDoiUrl\":\"\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-5982163\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-5982163\",\"identity\":\"rs-5982163\",\"version\":[\"v1\"]},\"buildId\":\"8U1c8b4HqxoKbykW_rLl7\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}