Reducing nonradiative recombination is crucial for minimizing voltage losses in metal-halide perovskite solar cells and achieving high power conversion efficiencies. Photoluminescence spectroscopy on complete or partial perovskite solar cell stacks is often used to quantify and disentangle bulk and interface contributions to the nonradiative losses. Accurately determining the intrinsic loss in a perovskite layer is key to analyzing the origins of nonradiative recombination and developing defect engineering strategies. We study perovskite films on glass and indium tin oxide-covered glass substrates, functionalized with a range of different molecules, using absolute and transient photoluminescence. We find that grafting these substrates with 1,6-hexylenediphosphonic acid effectively reduces the nonradiative losses in pristine perovskite films for a series of perovskite semiconductors with widely different bandgaps.^[1] The results suggest that perovskites processed on HDPA-functionalized substrates suffer the least from nonradiative recombination and thus approach the properties of a defect-free semiconductor.

Still, remaining shallow defects dominate charge recombination in metal-halide perovskites. \ Shallow defects can be modified using perovskite bulk additives or top surface treatments and can impact efficiency. We studied the shallow defect properties in metal-halide perovskite films with passivation and electron transport layers on top. By measuring transient photoluminescence (tr-PL), we confirm that the tr-PL decay is dominated by shallow defects. Interestingly, lowering the temperature changes the trap energy landscape, making the shallow defects shallower until they vanish into the conduction or valance bands at cryogenic temperature. This result is corroborated by an increase in PL quantum yield and is explained by noting that the shallow defects are intrinsic to the perovskite and formed in a thermally activated process.  Choline chloride as passivating agent, C₆₀ as electron transport layer, or a combination of both on top of the perovskite can induce significant nonradiative recombination losses at room temperature, but cooling to cryogenic temperature makes the charge recombination dynamics almost identical across all treatments. At low temperature nonradiative recombination losses become less dominant because interfacial recombination velocities are reduced. The results provide a new insight into perovskite shallow defects and recombination losses and can help us to better understand the effect of surface treatments on shallow defect properties.