Low Temperatures Reduce Shallow Defect Energies and Interfacial Recombination Losses in Triple-Cation Lead-Halide Perovskites

Understanding the origin and mechanisms of defect-induced voltage losses and recombination in metal-halide perovskites aids in the development of new strategies to achieve higher solar cell efficiencies. Photoluminescence (PL) spectroscopy provides insight into charge recombination at defects in the perovskite and at interfaces with passivation and electron transport layers. Measuring the time-resolved PL as a function of temperature for a triple-cation mixed-halide perovskite (CsFAMA) reveals that the PL decay is dominated by shallow defects and that lowering the temperature changes the trap-energy landscape, making the shallow defects shallower until they vanish into the conduction or valence bands at 90 K. Depositing fullerene <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mo stretchy="false">(</a:mo> <a:msub> <a:mi mathvariant="normal">C</a:mi> <a:mn>60</a:mn> </a:msub> <a:mo stretchy="false">)</a:mo> </a:math> on top of the CsFAMA perovskite induces significant nonradiative recombination losses at room temperature, which can largely be reduced by applying interfacial passivation. Interestingly, cooling to 90 K makes the charge-recombination dynamics almost identical for CsFAMA and <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mtext>CsFAMA</f:mtext> <f:mo>/</f:mo> <f:msub> <f:mi mathvariant="normal">C</f:mi> <f:mn>60</f:mn> </f:msub> </f:math> films, irrespective of interfacial passivation treatments. At such low temperatures, nonradiative recombination losses become less dominant because interfacial recombination velocities are reduced. The results provide new insight into shallow defects and recombination losses in CsFAMA perovskites and help to better understand the effect of surface treatments on shallow-defect properties.