Transient Photovoltage in Perovskite Solar Cells: Interaction of Trap-Mediated Recombination and Migration of Multiple Ionic Species

It is highly probable that perovskite solar cells (PSCs) are mixed electronic-ionic conductors, with ion migration being the driving force for PSC hysteresis. However, there is much that is not understood about the interaction of ion migration with other processes in the cell. The key question is: what factors of a PSC are influenced when ions are free to move? In this contribution, we employ a numerical drift-diffusion model of PSCs to show that the migration of both anions and cations in interaction with trap-mediated recombination in the bulk and/or at the surfaces of the perovskite absorber can manifest both current–voltage hysteresis and unusual nonmonotonic PSC photovoltage transients. We identify that a key mechanism of this interaction is the influence of the net ionic charge throughout the perovskite bulkwhich varies as the ions approach new steady-state conditionson the distribution of electrons and holes and subsequently the spatial distribution of trap-mediated recombination modeled after Shockley Read Hall (SRH) statistics. Relative to intrinsic recombination mechanisms, SRH recombination can be highly sensitive to local asymmetries of the electron–hole population. We show that this sensitivity is key to replicating nonmonotonic transients with multiple time constants, the forms of which may have suggested multiple processes. This work therefore supports the conceptualization of the hysteretic behavior of PSCs as dominated by the interplay between ion migration and trap-mediated recombination throughout the perovskite absorber

Ultralow Absorption Coefficient and Temperature Dependence of Radiative Recombination of CH₃NH₃PbI₃ Perovskite from Photoluminescence

Spectrally resolved photoluminescence is used to measure the band-to-band absorption coefficient α_BB(ℏω) of organic–inorganic hybrid perovskite methylammonium lead iodide (CH₃NH₃PbI₃) films from 675 to 1400 nm. Unlike other methods used to extract the absorption coefficient, photoluminescence is only affected by band-to-band absorption and is capable of detecting absorption events at very low energy levels. Absorption coefficients as low as 10^–14 cm^–1 are detected at room temperature for long wavelengths, which is 14 orders of magnitude lower than reported values at shorter wavelengths. The temperature dependence of α_BB(ℏω) is calculated from the photoluminescence spectra of CH₃NH₃PbI₃ in the temperature range 80–360 K. Based on the temperature-dependent α_BB(ℏω), the product of the radiative recombination coefficient and square of the intrinsic carrier density, <i>B</i>(<i>T</i>) × <i>n</i>_<i>i</i>², is also obtained