Shallow Defects and Ultralong Photoluminescence Decay Times up to 280 µs in Triple-Cation Perovskites

Abstract

Quantifying recombination in halide perovskites is a crucial prerequisite to control and improve the performance of perovskite-based solar cells. While both steady state and transient photoluminescence are frequently used to assess recombination in perovskite absorbers, quantitative analyses within a consistent model are seldomly reported. We use transient PL measurements with a large dynamic range of more than 10 orders of magnitude on triple-cation perovskite films showing long-lived photoluminescence transients featuring continuously changing decay times that range from tens of ns to hundreds of µs. We quantitatively explain both the transient and steady state PL with the presence of a high density of shallow defects and consequent significant rates of charge carrier trapping thereby showing that deep defects do not affect the recombination dynamics at all. The complex carrier kinetics caused by emission and recombination processes via shallow defects implies that reporting of only single lifetime values, as routinely done in literature, is meaningless for such materials. We show that the features indicative for shallow defects seen in the bare films remain dominant in finished devices and are therefore also crucial to understand the performance of perovskite solar cells.

Discerning rise time constants to quantify charge carrier extraction in perovskite solar cells

“…The large values between 1–1.3 eV followed by the transition at lower Δ E f has recently been interpreted as originating from SRH recombination via shallow traps. 50,51 The origin of the slope factor at large Δ E f is unclear, though we speculate it could be influenced by transport effects due to the position-dependent generation profiles of carriers in the perovskite at very early times. In the case of the glass/ITO/PTAA/Cs 0.05 FA 0.8 MA 0.15 PbI 2.25 Br 0.75 /C 60 /BCP stack, we observe a similar ‘S’-shaped evolution of the decay time (shown in Fig.…”