Impact of Surfaces on Photoinduced Halide Segregation in Mixed-Halide Perovskites

Abstract

Photoinduced halide segregation currently limits the perovskite chemistries available for use in high-bandgap semiconductors needed for tandem solar cells. Here, we study the impact of post-deposition surface modifications on photoinduced halide segregation in methylammonium lead mixed-halide perovskites. By coating a perovskite surface with the electron-donating ligand trioctylphosphine oxide (TOPO), we are able to both reduce nonradiative recombination and dramatically slow the onset of halide segregation in CH 3 NH 3 PbI 2 Br films. This result, coupled with the observation that the rate of halide segregation can be tuned by varying the selective contact, provides a direct link between surface modifications and photoinduced trap formation. On the basis of these observations, we discuss possible mechanisms for photoinduced halide segregation supported by drift-diffusion simulations. This work suggests that improved understanding and control of perovskite surfaces provides a pathway toward stable and high-performance wide-bandgap perovskites for the next generation of tandem solar cells.

Photoinduced Anion Segregation in Mixed Halide Perovskites

“…Obtained E a,forward -values are consistent with prior studies of ion migration in perovskite materials, where Br -(I -) vacancy activation energies between 0.09 and 0.27 eV (0.08 and 0.58 eV) have been found [29,[45][46][47][48]. This suggests vacancy-mediated anion transport, corroborating prior suggestions made [20,[40][41][42]49]. The Arrhenius dependence leads to a functional k forward,abs expression of:…”

“…At the very least, the studies suggest that perovskite film surfaces (or photogenerated carrier proximity to surface defects/grain boundaries) influence halide segregation. This is supported by Belisle and colleagues [49] who have recently found that passivating MAPb(I 1-x Br x ) 3 surfaces using Lewis bases suppresses halide segregation. More about this study will be discussed shortly in the models section of this review.…”

2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four‐Terminal Tandems with Silicon and CIGS

“…However, a slight decrease of the absolute EQE with increasing bandgap can be recognized even at wavelengths around 430 nm for which the absorptance in the perovskite absorbers is the same for all bandgaps (Figure 2b; Figure S13, Supporting Information). This effect might be ascribed to poorer charge extraction properties in Br‐rich semitransparent PSCs due to energetic misalignment and/or enhanced carrier trapping, [ 83,84 ] thus, further limiting the achievable PCE for wider bandgaps.…”

“…[ 3,52,69,73,74 ] A common issue is photo‐induced phase segregation and subsequent funneling of charge carriers into iodide‐rich lower bandgap regions, [ 75 ] acting as radiative recombination centers. [ 68,69,74–80 ] In addition, a higher density of defect states [ 41,79,81,82 ] and stronger interfacial recombination (e.g., due to energy level offsets) [ 51,79,83–87 ] have been proposed to increase non‐radiative recombination losses. Finally, the concentration of vacancy defects and strength of phase segregation seem to be directly linked to each other.…”

“…[ 72,74,79,80,88–90 ] All these factors result in a larger open‐circuit‐voltage ( V OC ) deficit (defined by E g /q − V OC ) for wide‐bandgap PSCs (>≈0.5 V) as compared to state‐of‐the‐art low‐bandgap PSCs (≈0.35–0.4 V), negating a linear increase of V OC with increasing bandgap. [ 3,52 ] We would like to stress that the relative contribution of the above mentioned factors to the V OC deficit is a complicated function of the exact perovskite composition, [ 69,72,91 ] film surface nature, [ 78,84,90 ] defect density as well as the charge extraction layers employed. [ 79,86,87 ] One popular approach to reduce the V OC deficit is to deposit a large organic cation on the surface of a 3D perovskite film, which acts as a 2D passivation agent and/or improves the energetic alignment with the charge transport layer.…”