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

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