Ai Shimazaki

Hole-Transporting Materials with a Two-Dimensionally Expanded π-System around an Azulene Core for Efficient Perovskite Solar Cells

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.

Charge Injection Mechanism at Heterointerfaces in CH3NH3PbI3 Perovskite Solar Cells Revealed by Simultaneous Time-Resolved Photoluminescence and Photocurrent Measurements

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 conditions. The physical origin of the bottleneck is discussed based on the result of numerical simulations.

Interfacial Charge-Carrier Trapping in CH3NH3PbI3-Based Heterolayered Structures Revealed by Time-Resolved Photoluminescence Spectroscopy

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.

Charge Injection at the Heterointerface in Perovskite CH3NH3PbI3 Solar Cells Studied by Simultaneous Microscopic Photoluminescence and Photocurrent Imaging Spectroscopy

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.

Optical characterization of voltage-accelerated degradation in CH 3 NH 3 PbI 3 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.

Origin of Open-Circuit Voltage Loss in Polymer Solar Cells and Perovskite Solar Cells

Herein, the open-circuit voltage (VOC) loss in both polymer solar cells and perovskite solar cells is quantitatively analyzed by measuring the temperature dependence of VOC to discuss the difference in the primary loss mechanism of VOC between them. As a result, the photon energy loss for polymer solar cells is in the range of about 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 VOC in both solar cells are further discussed on the basis of the experimental data.

Charge carrier injection at the heterointerface in CH 3 NH 3 PbI 3 perovskite solar cells studied by time-resolved photoluminescence and photocurrent imaging spectroscopy

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.

Carrier injection and recombination processes in perovskite CH3NH3PbI3 solar cells studied by electroluminescence spectroscopy

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.