Identifying an Optimum Perovskite Solar Cell Structure by Kinetic Analysis: Planar, Mesoporous Based, or Extremely Thin Absorber Structure

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 and the other (30–40%) on nanosecond time scale, while most of hole injection (85%) completes in 1.2 ns. Analysis of the slow charge injection data revealed an electron diffusion coefficient of 0.016 ± 0.004 cm² s^–1 and a hole diffusion coefficient of 0.2 ± 0.02 cm² s^–1 inside MAPbI₃. To achieve an incident photon-to-current conversion efficiency of >80%, a minimum charge carrier diffusion coefficient of 0.08 cm² s^–1 was evaluated. An interfacial charge recombination lifetime was increased from 0.5 to 40 ms by increasing a perovskite layer thickness, suggesting that the perovskite layer suppresses charge recombination reactions. Assessments of charge injection and interfacial charge recombination processes indicate that the optimum solar cell structure for the MAPbI₃ perovskite is a mesoporous TiO₂ based structure. This comparison of kinetics has been applied to several different types of photoactive semiconductors such as perovskite, CdTe, and GaAs, and the most appropriate solar cell structure was identified

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/cm² 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 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

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

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 (<b>1</b>–<b>3</b>) or biphenyl core (<b>4</b>), were synthesized and characterized. When tetra-substituted azulene <b>1</b> 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 <b>1</b> with reference compounds (<b>2</b>–<b>4</b> and Spiro-OMeTAD) led to the elucidation of key factors required for HTMs to act efficiently in perovskite solar cells

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

Herein, the open-circuit voltage (<i>V</i>_OC) loss in both polymer solar cells and perovskite solar cells is quantitatively analyzed by measuring the temperature dependence of <i>V</i>_OC to discuss the difference in the primary loss mechanism of <i>V</i>_OC 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 <i>V</i>_OC in both solar cells are further discussed on the basis of the experimental data

Highly Efficient and Stable Perovskite Solar Cells by Interfacial Engineering Using Solution-Processed Polymer Layer

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