Effects of the Addition of Tin Powder to Perovskite Precursor Solutions on Band Bending at PEDOT:PSS/Perovskite Interfaces in Mixed-Cation Mixed-Halide Tin Perovskite Solar Cells.

Using electron spin resonance (ESR) spectroscopy, we investigated the effects of the addition of tin (Sn) powder to perovskite layers on band bending at the perovskite surface near poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-transport layers in perovskite solar cells (PSCs) involving formamidinium (FA)-methylammonium (MA)-mixed-cation I-Br-mixed-halide tin perovskites. We performed dark ESR spectroscopy measurements of a PEDOT:PSS/FA0.75MA0.25Sn(I0.75Br0.25)3 stack and of a PEDOT:PSS/Sn-powder-added FA0.75MA0.25Sn(I0.75Br0.25)3 stack. The results indicate that FA0.75MA0.25Sn(I0.75Br0.25)3 layers have significant downward band bending near PEDOT:PSS layers. Such downward band bending is unfavorable for hole selectivity and surface passivation at the interface. However, the addition of Sn powder to the tin perovskite precursor solution was found to significantly prevent the downward band bending and rather cause upward band bending, which can improve the hole selectivity and field-effect passivation quality. This can be due to prevented oxidation of perovskite layers by Sn powder addition. These findings are crucial for developing highly efficient and stable tin perovskite solar cells.

Open-Circuit-Voltage Improvement Mechanism of Perovskite Solar Cells Revealed by Operando Spin Observation.

Organic-inorganic hybrid perovskite solar cells have attracted much attention as important next-generation solar cells. Their solar cell performance is known to change during operation, but the root cause of the instability remains unclear. This report describes an investigation using electron spin resonance (ESR) to evaluate an improvement mechanism for the open-circuit voltage, VOC, of inverted perovskite solar cells at the initial stage of device operation. The ESR study revealed electron transfer at the interface from the perovskite layer to the hole-transport layer not only under dark conditions but also under light irradiation, where electrons are subsequently trapped in the hole-transport layer. An electron barrier is enhanced at the perovskite/hole-transport-layer interface, improving field-effect passivation at the interface. Thereby, the interface recombination velocity is reduced, and thus the VOC improves. These findings are crucially important for elucidating the mechanisms of device performance changes under operation. They reveal a relation between charge transfer and performance improvement, which is valuable for the further development of efficient perovskite solar cells.

Operando ESR observation in thermally activated delayed fluorescent organic light-emitting diodes.

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 MoO3 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.

Polarization-Type Potential-Induced Degradation in Front-Emitter p-Type and n-Type Crystalline Silicon Solar Cells.

For SiO2 layers underneath the SiN x antireflection/passivation layers of front-emitter p-type c-Si solar cells, this paper presents an investigation into their effects on polarization-type potential-induced degradation (PID), in addition to a comparison of polarization-type PID behavior in front-emitter p-type c-Si cells and front-emitter n-type c-Si cells. After PID tests with a bias of +1000 V, p-type c-Si cells without SiO2 layers underneath the SiN x layers showed no degradation, although p-type c-Si cells with approx. 10 nm thick SiO2 layers showed polarization-type PID, which is characterized by a reduction of the short-circuit current density and the open-circuit voltage. This result implies that highly insulating layers such as SiO2 layers play an important role in the occurrence of polarization-type PID. Comparison of polarization-type PID in p-type and n-type c-Si cells with SiO2 layers indicated that degradation in the n-type cells is greater and saturates in a shorter time than in the p-type cells. This result is consistent with an earlier proposed model based on the assumption that polarization-type PID is caused by charge accumulation at K centers in SiN x layers. The findings described herein are crucially important for elucidating polarization-type PID and verifying the degradation model.

Operando direct observation of spin-states and charge-trappings of blue light-emitting-diode materials in thin-film devices.

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.

Facile light-initiated radical generation from 4-substituted pyridine under ambient conditions.

Photochemical reactions that generate stable radical species under ambient conditions find unique applications in materials science. Here we present a facile photogeneration of a stable radical species from a 4-substituted pyridine derivative in the presence of water and air at room temperature. The radical generation reaction accompanies a visible colour change to green and is repeatable multiple times.

Molecular Oriented Charge Accumulation in High-Efficiency Polymer Solar Cells as Revealed by Operando Spin Analysis.

A low band-gap polymer, PTB7-Th, is one of the typical p-type semiconductors among the next-generation solar-cell materials that have achieved power conversion efficiencies of over 10%. However, the internal deterioration mechanism of high-efficiency polymer solar cells such as PTB7-Th-based cells is still an open issue and has been extensively studied. Here, we report a study with operando electron spin resonance (ESR) spectroscopy for PTB7-Th polymer solar cells with an n-type semiconductor PC71BM to clarify the internal deterioration mechanism at a molecular level. We have directly observed ambipolar charge accumulation with a face-on molecular orientation in the cells under simulated solar irradiation using an operando light-induced ESR technique. Moreover, we have found a clear correlation between the charge accumulation and performance deterioration of the cells. The charge accumulation sites have been clarified by the ESR analysis and density functional theory calculation. The prevention of such charge accumulation on the basis of the present finding would be important for the commercialization of high-efficiency polymer solar cells.

Motional narrowing under Markovian and non-Markovian hopping transitions in inhomogeneous broadened absorption line shape.

Inspired by recent experiments showing a minimum of electron spin or paramagnetic resonance (ESR and EPR) line width as a function of inverse temperature, we studied the motional narrowing effect by considering a combined model of carrier transitions and static dispersion of the angular frequency giving rise to an inhomogeneous broadening in the spectrum. The dispersion of the angular frequency results from the distribution of the local field. The transition between the sites under inhomogeneous static local field induces adiabatic relaxation of the spin. We also considered the on-site inherent (nonadiabatic) relaxation of the spin. We obtained the exact solution of the spin correlation function by explicitly considering transitions between two sites for both Markovian and non-Markovian transition processes. The absorption line shape is expressed in terms of the Voigt function, which is a convolution of a Gaussian function and a Lorentzian function. Using the known properties of the Voigt function, we discuss the correlation between the change in the full-width at half-maximum and the change in line shape, both of which are induced by motional narrowing. By assuming thermal activation processes for both the hopping transition and the on-site inherent relaxation, we show that the minimum of the width appears as a function of inverse temperature as observed experimentally in organic materials. Contrary to the general belief, we also show that the narrowing of the Gaussian line shape under a local random field did not necessarily lead to a Lorentzian line shape in particular under the presence of heavy tail property in the waiting time distribution of hopping transitions.

Operando Direct Observation of Charge Accumulation and the Correlation with Performance Deterioration in PTB7 Polymer Solar Cells.

Polymer solar cells are one of the promising energy sources because of the easy solution-processable production with large area at a low cost without toxicity. Among the polymer materials, a donor-acceptor conjugated copolymer PTB7 has been extensively studied because of the typical high-performance polymer solar cells. Here, we show operando direct observation of charge accumulation in PTB7:PC71BM blend solar cells from a microscopic viewpoint using electron spin resonance spectroscopy. The accumulation of ambipolar charges in the PTB7-based cells is directly observed for the first time, which shows a clear correlation with the performance deterioration during device operation. The sites of the ambipolar charge accumulation are elucidated at the molecular level, whose information would be useful for improving the cell durability in addition to the performance improvement.

Mechanism of Light-Soaking Effect in Inverted Polymer Solar Cells with Open-Circuit Voltage Increase.

In this study, we present novel insights into the light-soaking effect of inverted polymer solar cells (PSCs), where the open-circuit voltage (V oc) of the cells improves over time under light irradiation. The effect was investigated by electron spin resonance (ESR) studies of bare indium tin oxide (ITO) and piperazine derivative-modified ITO/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) substrates. These results were combined with alternating current impedance spectroscopy (IS) measurements of inverted PSCs based on the above substrates. In ESR experiments with the substrates under white light irradiation, with a UV light component, many P3HT•+ radical cations were observed in the bare-ITO/P3HT:PCBM substrate. The number of radical cations was considerably suppressed in the ITO/P3HT:PCBM substrates with ITO modified by piperazine derivatives. This is because adsorbed oxygen molecules on the ITO acted as acceptor dopants for photoexcited P3HT, and the amount of adsorbed oxygen was decreased by modifying the ITO with piperazine derivatives. In IS measurements of the inverted PSCs under white light irradiation, a decrease in the electric capacitance (CPE2) of an electric double layer formed at the ITO/P3HT:PCBM interface was observed. A strong correlation was observed between the decrease of CPE2 and the increase of V oc. From these results, the light-soaking behavior was attributed to the removal of an electron injection barrier formed between ITO and PCBM, under white light irradiation.

Direct observation of electrically induced Pauli paramagnetism in single-layer graphene using ESR spectroscopy.

Graphene has been actively investigated as an electronic material owing to many excellent physical properties, such as high charge mobility and quantum Hall effect, due to the characteristics of a linear band structure and an ideal two-dimensional electron system. However, the correlations between the transport characteristics and the spin states of charge carriers or atomic vacancies in graphene have not yet been fully elucidated. Here, we show the spin states of single-layer graphene to clarify the correlations using electron spin resonance (ESR) spectroscopy as a function of accumulated charge density using transistor structures. Two different electrically induced ESR signals were observed. One is originated from a Fermi-degenerate two-dimensional electron system, demonstrating the first observation of electrically induced Pauli paramagnetism from a microscopic viewpoint, showing a clear contrast to no ESR observation of Pauli paramagnetism in carbon nanotubes (CNTs) due to a one-dimensional electron system. The other is originated from the electrically induced ambipolar spin vanishments due to atomic vacancies in graphene, showing a universal phenomenon for carbon materials including CNTs. The degenerate electron system with the ambipolar spin vanishments would contribute to high charge mobility due to the decrease in spin scatterings in graphene.

2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion.

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.

Dynamical valence fluctuation at the charge-density-wave phase boundary in iodide-bridged Pt compound [Pt(chxn)2I]I2.

We synthesized a novel iodo-bridged linear chain platinum compound, having the quasi-two-dimensional charge-density-wave (CDW) ground state and the smallest band gap. In this compound, we discovered an anomalous valence state in the boundary region at which the CDW phase alternates in the crystal by means of ESR, X-ray diffuse scattering, STM, and electrical resistivity. This anomalous state can be explained by the fast fluctuation between Pt(IV)-I...Pt(II) and Pt(II)...I-Pt(IV) in the double well potential. This is the first observation of the dynamical fluctuation of the CDW phase among the quasi one-dimensional halogen-bridged complexes.

Spatial extent of wave functions of gate-induced hole carriers in pentacene field-effect devices as investigated by electron spin resonance.

An electron spin resonance (ESR) method is applied to a pentacene field-effect device to investigate gate-induced hole carriers in such devices. Clear field-induced ESR signals are observed, demonstrating that all of the field-injected carriers have S = 1/2 spins. Anisotropic ESR signals due to unpaired pi electrons show the molecular orientation at the interface in the devices. The spatial extent of the spin density distribution (wave function) of the carriers is evaluated from the ESR linewidth, accounting for the hyperfine structure, to be of the order of 10 molecules.

Carrier doping effect into quasi-one-dimensional bromo-bridged Ni(III) complexes with strong electron-correlation by Cu(II) ions, [Ni1-xCux(chxn)2Br]Br2-x.

This communication will describe the electron doping effect into Ni(III) complexes by Cu(II) ions, [Ni(1-x)Cu(x)(chxn)(2)Br]Br(2-x) (x = 0.038 and 0.101) by using an electrochemical oxidation method. A drastic increase of electrical conductivity as well as a new absorption band around 0.5 eV in single crystal reflectance spectra was observed by doping Cu(II) ions, indicating the electron doping was successfully made. An ESR result shows unpaired electrons locate in the d(x2-y2) orbitals of Cu(II) and have almost no interaction with those of other ions.

Tuning of Charge Density Wave Strengths by Competition between Electron-Phonon Interaction of Pd(II)-Pd(IV) Mixed-Valence States and Electron Correlation of Ni(III) States in Quasi-One-Dimensional Bromo-Bridged Ni-Pd Mixed-Metal MX Chain Compounds Ni(1)(-)(x)()Pd(x)()(chxn)(2)Br(3).

A series of single crystals of quasi-one-dimensional bromo-bridged Ni-Pd mixed-metal MX chain compounds Ni(1)(-)(x)()Pd(x)()(chxn)(2)Br(3) (chxn = 1(R),2(R)-diaminocyclohexane) have been obtained by electrochemical oxidation methods of the mixed methanol solutions of parent Ni(II) complex [Ni(chxn)(2)]Br(2) and Pd(II) complex [Pd(chxn)(2)]Br(2) with various mixing ratios. To investigate the competition between the electron correlation of the Ni(III) states (or spin density wave states) and the electron-phonon interaction of the Pd(II)-Pd(IV) mixed-valence states (or charge density wave states) in the Ni-Pd mixed-metal compounds, IR, Raman, ESR, XP, and Auger spectra have been measured. The IR, resonance Raman, XP, and Auger spectra show that the Pd(II)-Pd(IV) mixed-valence states are influenced and gradually approach the Pd(III) states with the increase of the Ni(III) components. This means that in these compounds the electron-phonon interaction in the Pd(II)-Pd(IV) mixed-valence states is weakened with the strong electron correlation in the Ni(III) states.