We have developed a new method to dynamically measure the absolute intensity steady-state photoluminescence and the mean carrier diffusion length simultaneously in neat hybrid perovskite films. The measurements reveal four distinct regimes of material changes and show that photoluminescence brightening often coincides with losses in carrier transport, such as in degradation or phase segregation. The evolution of any perovskite film can be plotted parametrically with time on a graph of radiative efficiency versus diffusion length. We show how different environments affect the evolution of and coupling of material properties for MAPbI3 and high bandgap mixed halide perovskites.
We also investigate the origin of phase segregation and its implication for tandems with several mixed halide large-bandgap (∼1.75 eV) compositions. We show explicitly that MAPb(I0.6Br0.4)3 and (MA0.9,Cs0.1)Pb(I0.6,Br0.4)3, termed “MA” and “MACs”, respectively, rapidly phase segregate in the dark upon 1 sun equivalent current injection. This is direct experimental evidence that conduction band electrons or valence band holes are the culprit behind phase segregation. In contrast, (FA0.83,Cs0.17)Pb(I0.66,Br0.34)3, or “FACs,” prepared at only 75 °C resists phase segregation below 4 sun injection. FACs prepared at 165 °C yields larger grains and withstands higher injected carrier concentrations before phase segregation. Both the phase-stable FACs and phase-segregating MACs devices sustain near constant power output at 1 sun and do not affect the current output of a CIGS bottom cell when used as an incident light filter. Further, optimization of novel surface passivation method applied to the 1.75 eV bandgap FACs films resulted in an enhancement of the photoluminescence quantum yield (PLQY) of over an order of magnitude, an increase of 80 meV in the quasi-Fermi level splitting (to 1.29 eV), an increase in diffusion length by a factor of 3.5, and enhanced open-circuit voltage and short-circuit current in devices.
We compile our findings of quasi-Fermi level splitting and phase segregation to discuss routes toward development of phase-stable, high bandgap HPs with minimal voltage deficit. We will further elaborate on the role of composition in stability and quality of high bandgap HPs by presenting recent combinatorial data on thousands of other cation compositions.
Stoddard, R. J.; Eickemeyer, F. T.; Katahara, J. K.; Hillhouse, H. W., Correlation between Photoluminescence and Carrier Transport and a Simple In Situ Passivation Method for High-Bandgap Hybrid Perovskites. The Journal of Physical Chemistry Letters 2017, 8 (14), 3289-3298.
Braly, I. L.; Stoddard, R. J.; Rajagopal, A.; Uhl, A. R.; Katahara, J. K.; Jen, A. K.-Y.; Hillhouse, H. W., Current-Induced Phase Segregation in Mixed Halide Hybrid Perovskites and its Impact on Two-Terminal Tandem Solar Cell Design. ACS Energy Lett., 2017, 2 (8), pp 1841–1847