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Doping is one of the most important methods to control charge carrier concentration in semiconductors. Ideally, the introduction of dopants should not perturb the ordered microstructure of the semiconducting host. In some systems, such as modulation-doped inorganic semiconductors or molecular charge transfer crystals, this can be achieved by spatially separating the dopants from the charge transport pathways. However, in conducting polymers, dopants tend to be randomly distributed within the conjugated polymer, and as a result the transport properties are strongly affected by the resulting structural and electronic disorder. Here, we show that in the highly ordered lamellar microstructure of a regioregular thiophene-based conjugated polymer, a small-molecule p-type dopant can be incorporated by solid state diffusion into the layers of solubilizing side chains without disrupting the conjugated layers. In contrast to more disordered systems, this allows us to observe coherent, free-electron-like charge transport properties, including a nearly ideal Hall effect in a wide temperature range, a positive magnetoconductance due to weak localization and the Pauli paramagnetic spin susceptibility. Small molecules diffusing into a thin film of thiophene-based conjugated polymers act as p-type dopants without altering the ordered microstructure of the film. As a result, free-electron-like charge transport is also observed at high doping levels.

Spin-states and charge-trappings in blue organic light-emitting diodes (OLEDs) are important issues for developing high-device-performance application such as full-color displays and white illumination. However, they have not yet been completely clarified because of the lack of a study from a microscopic viewpoint. Here, we report operando electron spin resonance (ESR) spectroscopy to investigate the spin-states and charge-trappings in organic semiconductor materials used for blue OLEDs such as a blue light-emitting material 1-bis(2-naphthyl)anthracene (ADN) using metal–insulator–semiconductor (MIS) diodes, hole or electron only devices, and blue OLEDs from the microscopic viewpoint. We have clarified spin-states of electrically accumulated holes and electrons and their charge-trappings in the MIS diodes at the molecular level by directly observing their electrically-induced ESR signals; the spin-states are well reproduced by density functional theory. In contrast to a green light-emitting material, the ADN radical anions largely accumulate in the film, which will cause the large degradation of the molecule and devices. The result will give deeper understanding of blue OLEDs and be useful for developing high-performance and durable devices.

Perovskite solar cells are attractive because of their remarkably improved power conversion efficiency. In view of their application, however, it is important not only to increase the power conversion efficiency, but also to elucidate the deterioration mechanism. Here, we show operando direct observation of spin states in the cells using electron spin resonance, thereby investigating the operation and deterioration mechanisms from a microscopic viewpoint. By simultaneous measurements of solar cell characteristics and electron spin resonance, the spin states in the hole transport material spiro-OMeTAD are demonstrated to change in accordance with the device performance variation under operation. These variations are ascribed to the change of hole transport and to interfacial electric dipole layers. Reverse electron transfer from TiO 2 to the hole transport material layer is demonstrated under ultraviolet light irradiation, which decreases hole doping. Conducting such operando microscopic investigation will be useful to obtain further guidelines for improving the device performance and durability. Perovskite solar cells have seen a strong improvement in power conversion efficiency, but their intrinsic degradation is yet to be elucidated. Here, operando electron spin resonance is used to probe the number of spin states and relate its variation with the device performance under operation.

Organic light-emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) materials have advantages over OLEDs using conventional fluorescent materials or high-cost phosphorescent materials, including higher efficiency and lower cost. To attain further high device performance, clarifying internal charge states in OLEDs at a microscopic viewpoint is crucial; however, only a few such studies have been performed. Here, we report a microscopic investigation into internal charge states in OLEDs with a TADF material by electron spin resonance (ESR) at a molecular level. We observed operando ESR signals of the OLEDs and identified their origins due to a hole-transport material PEDOT:PSS, gap states at an electron-injection layer, and a host material CBP in the light-emitting layer by performing density functional theory calculation and studying thin films used in the OLEDs. The ESR intensity varied with increasing applied bias before and after the light emission. We find leakage electrons in the OLED at a molecular level, which is suppressed by a further electron-blocking layer MoO 3 between the PEDOT:PSS and light-emitting layer, resulting in the enhancement of luminance with a low-voltage drive. Such microscopic information and applying our method to other OLEDs will further improve the OLED performance from the microscopic viewpoint.

Ternary polymer solar cells based on a thiazolothiazole-based polymer donor (PTzBT) and a fullerene acceptor (PC 61 BM) have attracted attention because they show high efficiency and stability by addition of a non-fullerene acceptor (ITIC). However, the performance improvement mechanism is not completely elucidated. Here, we show the stability improvement mechanism due to less charge accumulation in the PTzBT cells with ITIC using operando electron spin resonance from a microscopic viewpoint. We observed two correlations between device performance and number of spins ( N spin ) under solar irradiation. One correlation is the decrease in short-circuit current and the N spin increase in electrons on PC 61 BM and holes in PTzBT, where the ITIC addition causes the less these N spin . The other correlation is the increase in open-circuit voltage and the N spin decrease in holes in ZnO. These findings explain the stability improvement mechanism, showing the correlation between less charge accumulation and higher stability, which is valuable for the development of further efficient and stable polymer solar cells.

Sn-based perovskite solar cells (PSCs) have attracted attention because of their low environmental impact. Unfortunately, the readily occurring oxidation of Sn 2+ inhibits further improvement of their efficiency and stability. Ruddlesden–Popper (RP) Sn-based perovskites are considered promising candidates as absorbers that improve the performance and stability of Sn-based PSCs. However, microscopic understanding of performance-enhancing mechanisms remains insufficient. For this study, electron spin resonance (ESR) spectroscopy measurements were taken of RP Sn-based PSCs with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-transport layers and (BA 0.5 PEA 0.5 ) 2 FA 3 Sn 4 I 13 perovskite layers to clarify the space-charge region formation mechanism at the PEDOT:PSS/(BA 0.5 PEA 0.5 ) 2 FA 3 Sn 4 I 13 interface. These results indicated electron-barrier formation in the (BA 0.5 PEA 0.5 ) 2 FA 3 Sn 4 I 13 layer near the PEDOT:PSS layer. Moreover, the electron barrier was found to be enhanced during device operation. The enhanced interface band bending reduces interface recombination and thereby improves the device's performance. These findings might provide important progress in practical applications of PSCs and might advance the realization of a carbon-neutral society.

Potential‐induced degradation (PID) has been identified as a central reliability issue of photovoltaic (PV) cell modules. Several types of PID depend on the cell structure. Among those types, polarization‐type PID, which is characterized by reductions in short‐circuit current density (JSC) and open‐circuit voltage (VOC), is the fastest PID mode. Additionally, polarization‐type PID occurs readily at room temperature or at markedly low magnitudes of electric potential difference. Therefore, polarization‐type PID is a severe difficulty affecting silicon PV modules. Recently, degradation behavior, preventive measures, and mechanism have been investigated. As described herein, mechanistic aspects of polarization‐type PID are specifically examined and details of a recently proposed model involving a charge accumulation process at K centers in SiNx dielectric layers: the K‐center model are discussed. The K‐center model consistently explains previously reported results of experimentation, which indicates the validity of this model. Discussions presented herein are expected to improve the mechanistic understanding of polarization‐type PID in the PV community and to stimulate further discussions and verifications of the model. Potential‐induced degradation (PID) is a central issue related to photovoltaic module reliability. Several types of PID depend on the cell structure. Among them, polarization‐type PID is the fastest PID mode. This study examines mechanistic aspects of polarization‐type PID, with detailed discussion of the recently proposed model involving a charge accumulation process at K centers in SiNx layers: the K‐center model.

Perovskite solar cells have attracted much attention as next-generation solar cells. However, a typical hole-transport material, spiro-OMeTAD, has associated difficulties including tedious synthesis and high cost. To overcome these shortcomings, an easily synthesized and low-cost hole-transport material has been developed: HND-2NOMe. Although HND-2NOMe has high local charge mobility because of the quasi-planar structure, its lower device performance is a weak point, the cause of which has not yet been clarified. Here, we analyse the source of the lower performance by clarifying the internal states from a microscopic viewpoint using electron spin resonance. We observe hole diffusion from perovskite to HND-2NOMe under dark conditions, indicating hole barrier formation at the perovskite/HND-2NOMe interface, leading to lower performance. Although such a barrier is formed, less hole accumulation for the HND-2NOMe-based cells under solar irradiation occurs, which is related to the stable performance. The sources of the lower but stable performance are crucially important for providing guidelines for improving the device performance.Hole-transport materials possessing high charge mobility are important in perovskite solar cells but the source of lower performance remains a mystery. Here, the microscopic mechanism for low but stable perovskite solar cell performance using these materials is analysed using electron spin resonance.

Tin-based perovskite solar cells (PSCs) are the most promising alternatives to toxic lead-based ones. However, the loss in open-circuit voltage ( V OC ) remains an important issue. Improvement of V OC has been achieved by using a fullerene derivative, indene-C 60 bisadduct (ICBA), as the electron transporting layer (ETL). For further V OC improvement, the V OC improvement mechanisms must be clarified. Herein, we show, at a molecular level, V OC improvement mechanisms by an ICBA ETL in tin-based PSCs. Electron spin resonance spectroscopy reveals that electron diffusion from perovskite to ETL occurs at perovskite/ETL interfaces, producing unfavorable upward band-bending of perovskite. Employing ICBA with a shallower LUMO level suppresses the upward band-bending as well as reduces the energy offset with the conduction band minimum of perovskite. Suppressing this unfavorable upward band-bending reduces interface recombination at perovskite/ETL interfaces and contributes to V OC improvement. These insights support efficient optimization of the charge-transporting layer for additional improvement of V OC .

Light-emitting electrochemical cells (LECs) are next-generation devices that are flexible, emit light and have several advantages over organic light-emitting diodes, such as a simpler structure and lower cost. However, the operation mechanism of LECs remains unknown from a microscopic viewpoint. Here, we perform an operando microscopic investigation of LECs with Super Yellow, a typical light-emitting material, by observing the spin states of electrically doped charges using electron spin resonance. The operando electron spin resonance and light emission increase as the voltage applied to the LECs increases. Through density functional theory, we determine that the origin of the electron spin resonance increases to be from electrochemically doped holes and electrons in Super Yellow. We find that the doping progress correlates with the luminance increase, suggesting that electrochemically doped charges are distributed over the light-emitting layer as the operation mechanism. Moreover, we deduce the molecular orientation of electrochemically charge-doped Super Yellow. Light-emitting electrochemical cells are next-generation light-emitting devices but the operation mechanism is still not well understood microscopically. Here, the operation mechanism of light-emitting electrochemical cells is microscopically investigated by operando observation of spin states.