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.