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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.

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...

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.

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.

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.

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.

We investigate the performance degradation mechanism of CH

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.