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Organic-inorganic hybrid perovskite solar cells have attracted much attention as one of the most important next-generation solar cells. The solar-cell performances are known to be changed under operation; however, the root cause of the instability has not yet been understood sufficiently. Here, we report improvement mechanism in the open-circuit voltage, VOC, of perovskite solar cells at the initial stage of device operation that were firstly investigated by electron spin resonance (ESR). ESR study reveals reverse electrons 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. Thereby, an electron barrier is enhanced at the perovskite/hole-transport-layer interface, improving field-effect passivation at the interface, and thus the VOC improves. These findings are crucial for understanding the mechanisms of device performance changes under operation, showing a relation between reverse electrons transfer and performance improvement, which is valuable for the development of further efficient perovskite solar cells.

Organic-inorganic hybrid perovskite solar cells have attracted much attention as one of the most important next-generation solar cells. The solar-cell performances are known to be changed under operation; however, the root cause of the instability has not yet been understood sufficiently. Here, we report an improvement in the open-circuit voltage, VOC, of inverted perovskite solar cells at the initial stage of device operation and its mechanisms that were first investigated by electron spin resonance spectroscopy. Operando ESR measurements indicate that electrons travel from the perovskite layer to the hole-transport layer under irradiation and are subsequently trapped there. Thereby, an electron barrier is formed at the perovskite/hole-transport-layer interface, and thus the VOC improves. These findings are crucial for understanding the mechanisms of device performance changes at the initial stage of operation and, additionally, demonstrate that electron spin resonance spectroscopy is a powerful tool for investigating perovskite solar cells.

Organic semiconducting materials that are optically transparent in the near-ultraviolet (NUV) region from 300 to 400 nm are needed for advanced perovskite devices such as bifacial semitransparent a...

CH3NH3PbI3 (MAPbI3) perovskite layers can be obtained by thermal annealing thin films of the intermediate dimethyl sulfoxide (DMSO) intercalated complex, MA2Pb3I8·2DMSO. In the present work, the fo...

We investigated a silicon nitride (SiN x ) film prepared by catalytic chemical vapor deposition as a gas barrier for further improving stability of mixed-cation-halide perovskite (PVK) Cs0.05FA0.85MA0.1PbI2Br under dark air conditions. The SiN x film can be formed on the PVK capped with carrier transport layers such as 2,2′,7,7′-tetrakis[N,N-di(4-methoxylphenyl)amino]−9,9′-spirobifluorene (Spiro-OMeTAD) and phenyl-C61-butyric-acid-methyl-ester (PCBM)/aluminum-doped-zinc-oxide (AZO) with less degradation in its optical transmittance property and crystal structure. The PVK/Spiro-OMeTAD encapsulated by SiN x exhibits a slower reduction in average carrier lifetime after storage for 330 h at room temperature with ∼65%RH. The PVK/PCBM/AZO covered with SiN x shows a small blue-shift (8–10 nm) in the absorption band-edge of PVK and with less decrease in the transmittance in the long wavelength range for ∼500 h at 85 °C with ∼2%RH. These results demonstrate the effect of SiN x in preventing the degradation of PVK due to the interactions with moisture and oxygen in the air.

Ultrastrong coupling (USC) between phonons and vacuum photons can result in fascinating quantum phenomena, though it is difficult to achieve due to the small dipole moments of phonons in solids. Here, we investigate the vacuum Rabi splitting by coupling phonons in perovskite CH₃NH₃PbI₃ films with photons in split ring resonators. As the gap size of the resonator decreases, the coupling strength η increases due to the enhanced vacuum field in the gap, reaching the USC regime (η∼0.24) at a gap size of 100 nm. Our results show that nanoresonators are an excellent platform for studies of vacuum-dressed phonon properties., テラヘルツ周波数帯の真空量子揺らぎと格子振動の超強結合状態の実現 --量子光を駆動力とする新規物性制御への挑戦--. 京都大学プレスリリース. 2021-07-30.

High efficiency lead halide perovskite solar cells employ spiro-OMeTAD or PTAA as a hole transporting material. This type of hole conductor requires dopants mainly to improve hole mobility. Although such doping has improved solar cell performance, in particular open circuit photovoltage and fill factor, the mechanism of the improvement has rarely been elucidated. Here, we demonstrate influence of dopants in spiro-OMeTAD on interfacial charge separation and recombination processes for a MAPbI3 perovskite film sandwiched by an m-TiO2 film and a spiro-OMeTAD layer by employing a series of transient absorption spectroscopies. The interfacial charge recombination time was significantly retarded by the doping. We propose that the retardation of the charge recombination originates from relatively longer distance of holes from the perovskite/spiro-OMeTAD interface owing to the increased hole mobility by the doping, potentially increasing Voc of the solar cell.

A series of two-dimensionally expanded azulene-core-based π systems have been synthesized with different alkyl chain lengths in the alkoxy moieties connected to the partially oxygen-bridged triarylamine skeletons. The thermal, photophysical, and electronic properties of each compound were evaluated to determine the influence of the alkyl chain length on their effectiveness as hole-transporting materials (HTMs) in perovskite solar cells (PSCs). All the synthesized molecules showed promising material properties, including high solubility, the formation of flat and amorphous films, and optimal alignment of energy levels with perovskites. In particular, the derivatives with methyl and n-butyl in the side chains retained amorphous stability up to 233 and 159 °C, respectively. Such short alkoxy chains also resulted in improved electrical device properties. The PSC device fabricated with the HTM with n-butyl side chains showed the best performance with a power conversion efficiency of 18.9 %, which compares favorably with that of spiro-OMeTAD-based PSCs (spiro-OMeTAD=2,2',7,7'-tetrakis[N,N-bis(p-methoxyphenyl)amino]-9,9'-spirobifluorene).

We investigate the impact of phonon excitations on the photoexcited carrier dynamics in a lead-halide perovskite CH3NH3PbI3, which hosts unique low-energy phonons that can be directly excited by terahertz pulses. Our time-resolved photoluminescence measurements reveal that strong terahertz excitation prolongs the cooling time of hot carriers, providing direct evidence for the hot-phonon bottleneck effect. In contrast to the previous studies where phonons are treated as a passive heat bath, our results demonstrate that phonon excitation can significantly perturb the carrier relaxation dynamics in halide perovskites through the coupling between transverse- and longitudinal-optical phonons., ハライドペロブスカイト半導体においてテラヘルツ励起によるホットキャリアの長寿命化を実現 --太陽電池材料のフォノン操作による高効率化への新たな指針--. 京都大学プレスリリース. 2021-02-19.

We measure the minority carrier lifetime of perovskite films by differential microwave photoconductivity decay (μ-PCD). Clear decay curves can be detected from bare and laminated methylammonium lead iodide (MAPbI3) films by the differential μ-PCD. The degradation of the bare and laminated MAPbI3 films under air exposure at room temperature is clearly observed as the continuous change of the decay curves. The differential μ-PCD can thus be a quick and non-destructive method for the characterization of the electrical quality of perovskite films and modules.

Lead halide perovskites have emerged as new optoelectronic materials owing to their outstanding optical properties. There has been increased interest in their temperature-sensitive optical properties and new optical applications have been proposed thereby. Here, we report the origin of the unusual negative thermo-optic coefficient of the halide perovskite $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{PbC}{\mathrm{l}}_{3}$, i.e., a decrease in the refractive index by an increase in temperature. From the temperature dependences of the absorption spectrum and the lattice constant and using the Lorentz oscillator model, we conclude that the negative thermo-optic coefficient below the absorption edge is predominantly determined by the large thermal expansion coefficient inherent to this soft material system. This work demonstrates that the negative thermo-optic coefficient is a distinctive phenomenon reflecting the unique electronic and lattice properties of halide perovskites.

Perovskite solar cells have rapidly been developed over the past several years. Choice of the most suitable solar cell structure is crucial to improve the performance further. Here, we attempt to determine an optimum cell structure for methylammonium lead iodide (MAPbI3) perovskite sandwiched by TiO2 and spiro-OMeTAD layers, among planar heterojunction, mesoporous structure, and extremely thin absorber structure, by identifying and comparing charge carrier diffusion coefficients of the perovskite layer, interfacial charge transfer, and recombination rates using transient emission and absorption spectroscopies. An interfacial electron transfer from MAPbI3 to compact TiO2 occurs with a time constant of 160 ns, slower than the perovskite photoluminescence (PL) lifetime (34 ns). In contrast, fast non-exponential electron injection to mesoporous TiO2 was observed with at least two different electron injection processes over different time scales; one (60–70%) occurs within an instrument response time of 1.2 ns...

Elucidation of interfacial charge separation and recombination mechanisms is crucial to improve performance of organic-inorganic metal halide perovskite solar cells. Here, we have investigated influence of initially populated electron and hole potential levels in a perovskite conduction band (CB) and valence band (VB), respectively, by altering an excitation wavelength on interfacial charge separation and recombination dynamics in a CH3NH3PbI3 perovskite film sandwiched by a mesoporous TiO2 structure as an electron transport material (ETM) and a spiro-OMeTAD film as a hole transport material (HTM). Multi-phasic electron injection reactions are observed over

Herein, the open-circuit voltage (V[OC]) loss in both polymer solar cells and perovskite solar cells is quantitatively analyzed by measuring the temperature dependence of V[OC] in order to discuss the difference in the primary loss mechanism of V[OC] between them. As a result, the photon energy loss for polymer solar cells is in the range of 0.7–1.4 eV, which is ascribed to temperature-independent and -dependent loss mechanisms while that for perovskite solar cells is as small as about 0.5 eV, which is ascribed to a temperature-dependent loss mechanism. This difference is attributed to the different charge generation and recombination mechanisms between the two devices. The potential strategies for the improvement of V[OC] in both solar cells are further discussed on the basis of the experimental data.

Organic–inorganic hybrid perovskite solar cells are attracting much attention due to their excellent photovoltaic properties. In these multilayered structures, the device performance is determined by complicated carrier dynamics. Here, we studied photocarrier recombination and injection dynamics in CH3NH3PbI3 perovskite solar cells using time-resolved photoluminescence (PL) and photocurrent (PC) measurements. It is found that a peculiar slowdown in the PL decay time constants of the perovskite layer occurs for higher excitation powers, followed by a decrease of the external quantum efficiency for PC. This indicates that a carrier-injection bottleneck exists at the heterojunction interfaces, which limits the photovoltaic performance of the device in concentrator applications. We conclude that the carrier-injection rate is sensitive to the photogenerated carrier density, and the carrier-injection bottleneck strongly enhances recombination losses of photocarriers in the perovskite layer at high excitation co...

Solution-processed organo-lead halide perovskite solar cells with deep pinholes in the perovskite layer lead to shunt-current leakage in devices. Herein, we report a facile method for improving the performance of perovskite solar cells by inserting a solution-processed polymer layer between the perovskite layer and the hole-transporting layer. The photovoltaic conversion efficiency of the perovskite solar cell increased to 18.1% and the stability decreased by only about 5% during 20 days of exposure in moisture ambient conditions through the incorporation of a poly(methyl methacrylate) (PMMA) polymer layer. The improved photovoltaic performance of devices with a PMMA layer is attributed to the reduction of carrier recombination loss from pinholes, boundaries, and surface states of perovskite layer. The significant gain generated by this simple procedure supports the use of this strategy in further applications of thin-film optoelectronic devices.

Performance of nanoporous TiO 2 based lead iodide perovskite solar cells was investigated under a series of light intensity up to 1.2 sun. The short circuit photocurrent increases linearly with the intensity increase, while the fill factor slightly decreases with the intensity increase. The analysis of logarithmic light intensity dependence of the open circuit voltage revealed a slope of 1.09 kT/q, indicating ideal function of the present solar cells with only bimolecular charge recombination and negligible leakage current. This analysis further suggests that the solar cell performance is improved by simply increasing incident light intensity up to 1.2 sun.

Organic-inorganic halide perovskite solar cells are attracting much attention from the photovoltaic community because of their high conversion efficiencies exceeding 20%. So far, intrinsic superior optoelectronic properties of this material class have been revealed through comprehensive studies on the thin films and single crystals [1,2]. For further improvement of the device architecture and conversion efficiency, the carrier recombination and transport dynamics in actual solar cell devices have to be clarified. The perovskite solar cell is usually implemented as a heterojunction structure consisting of a perovskite absorber layer and charge transport layers as selective contacts, and the carrier-injection properties at these heterointerfaces play a crucial role for the device performance. Time-resolved photoluminescence (PL) techniques are usually adopted to investigate carrier injection and transport properties [3]. However, PL is also additionally affected by traps and defects within the perovskite layer and also at the heterointerface. On the other hand, the photocurrent (PC) measurement can directly assess the net charge-carrier flow through the whole device. Therefore a combination of PL and PC enables us to investigate the details of the carrier injection. In addition, perovskite solar cells are prepared by a fast and cost-effective low-temperature solution-process, but this simple preparation method also causes a spatial nonuniformity in the optical and electrical properties [4]. Thus, the spatial imaging is invaluable for statistical evaluation of the solar cell characteristics.

Organic-inorganic hybrid perovskite solar cells are attracting much attention due to their excellent photovoltaic properties. In these multilayered structures, the device performance is determined by complicated carrier dynamics. Here, we studied photocarrier recombination and injection dynamics in CH

Although the interface between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and perovskite layers is one of the key factors to determine the open-circuit voltage of inverted perovskite solar cells, this interface remains poorly understood. We investigated the interface by electron spin resonance spectroscopy and found that the PEDOT undergoes dedoping in contact with the perovskite. The holes in PEDOT were reduced by electron transfer from the perovskite. Additional electron transfer occurred upon light irradiation. The electron transfer would be related to lower open-circuit voltages of the inverted structure compared to those of mesoporous and planar structures.

Charge carrier dynamics in perovskite CH3NH3PbI3 solar cells were studied by means of microscopic photoluminescence (PL) and photocurrent (PC) imaging spectroscopy. The PL intensity, PL lifetime, and PC intensity varied spatially on the order of several tens of micrometers. Simultaneous PL and PC image measurements revealed a positive correlation between the PL intensity and PL lifetime, and a negative correlation between PL and PC intensities. These correlations were due to the competition between photocarrier injection from the CH3NH3PbI3 layer into the charge transport layer and photocarrier recombination within the CH3NH3PbI3 layer. Furthermore, we found that the decrease in the carrier injection efficiency under prolonged light illumination leads to a reduction in PC, resulting in light-induced degradation of solar cell devices. Our findings provide important insights for understanding carrier injection at the interface and light-induced degradation in perovskite solar cells.

We investigate the performance degradation mechanism of CH

The fast-decaying component of photoluminescence (PL) under very weak pulse photoexcitation is dominated by the rapid relaxation of the photoexcited carriers into a small number of carrier-trapping defect states. Here, we report the subnanosecond decay of the PL under excitation weaker than 1 nJ/cm(2) both in CH3NH3PbI3-based heterostructures and bare thin films. The trap-site density at the interface was evaluated on the basis of the fluence-dependent PL decay profiles. It was found that high-density defects determining the PL decay dynamics are formed near the interface between CH3NH3PbI3 and the hole-transporting Spiro-OMeTAD but not at the CH3NH3PbI3/TiO2 interface and the interior regions of CH3NH3PbI3 films. This finding can aid the fabrication of high-quality heterointerfaces, which are required improving the photoconversion efficiency of perovskite-based solar cells.

Organic-inorganic hybrid perovskite materials, CH3NH3PbX3 (X = I and Br), are considered as promising candidates for emerging thin-film photovoltaics. For practical implementation, the degradation mechanism and the carrier dynamics during operation have to be clarified. We investigated the degradation mechanism and the carrier injection and recombination processes in perovskite CH3NH3PbI3 solar cells using photoluminescence (PL) and electroluminescence (EL) imaging spectroscopies. By applying forward bias-voltage, an inhomogeneous distribution of the EL intensity was clearly observed from the CH3NH3PbI3 solar cells. By comparing the PL- and EL-images, we revealed that the spatial inhomogeneity of the EL intensity is a result of the inhomogeneous luminescence efficiency in the perovskite layer. An application of bias-voltage for several tens of minutes in air caused a decrease in the EL intensity and the conversion efficiency of the perovskite solar cells. The degradation mechanism of perovskite solar cells under bias-voltage in air is discussed.

Two-dimensionally expanded π-systems, consisting of partially oxygen-bridged triarylamine skeletons that are connected to an azulene (1–3) or biphenyl core (4), were synthesized and characterized. When tetra-substituted azulene 1 was used as a hole-transporting material (HTM) in perovskite solar cells, the observed performance (power conversion efficiency = 16.5%) was found to be superior to that of the current HTM standard Spiro-OMeTAD. A comparison of the hole mobility, the ability to control the HOMO and LUMO levels, and the hole-collection efficiency at the perovskite/HTM interface in 1 with reference compounds (2–4 and Spiro-OMeTAD) led to the elucidation of key factors required for HTMs to act efficiently in perovskite solar cells.

We investigate the performance degradation mechanism of CH3NH3PbI3 perovskite solar cells under bias voltage in air and nitrogen atmospheres using photoluminescence and electroluminescence techniques. When applying forward bias, the power conversion efficiency of the solar cells decreased significantly in air, but showed no degradation in nitrogen atmosphere. Time-resolved photoluminescence measurements on these devices revealed that the application of forward bias in air accelerates the generation of non-radiative recombination centers in the perovskite layer buried in the device. We found a negative correlation between the electroluminescence intensity and the injected current intensity in air. The irreversible change of the perovskite grain surface in air initiates the degradation of the perovskite solar cells.