Decreasing Radiative Recombination Coefficients via an Indirect Band Gap in Lead Halide Perovskites.

High absorption coefficients imply also high radiative recombination coefficients due to detailed balance. It is therefore worthwhile to investigate whether a combination of two transitions with different absorption coefficients (such as a direct and an indirect band gap) could be used to reduce radiative recombination while at the same time retaining the high absorption coefficient. We show here that a longer radiative lifetime helping charge collection can indeed be achieved, while an increase in open-circuit voltage by adding an indirect band gap below the direct one is impossible. We also show that the absorption coefficient in CH3NH3PbI3 could indeed consist of a direct and an indirect contribution; however, the indirect one seems to dominate luminescence and therefore radiative recombination. Thus, the condition that the direct gap is mainly responsible for absorption and emission would not be valid for CH3NH3PbI3. Therefore, we would not expect any benefit of an indirect gap in the radiative limit. However, there may be benefits for charge collection but not open-circuit voltage if nonradiative recombination is dominant.