Local Background Hole Density Drives Non-Radiative Recombination in Tin Halide Perovskites

Tin halide perovskites, with the general formula ABX₃ [A = Formamidinium (FA), B = Sn, X = Br, I], offer a narrower bandgap (~1.3 eV) than their lead counterparts, making them suitable candidates for the light-absorbing semiconductor in single-junction photovoltaic devices. Moreover, this attribute makes tin halide perovskites integral to all-perovskite tandems, with the best-performing cells incorporating a 50-50 Sn-Pb composition in the low bandgap component. Nevertheless, record-breaking pure-Sn devices still exhibit severe losses in both open-circuit voltage (V_OC) and short-circuit current (J_SC), keeping power conversion efficiencies well below theoretical limits.<br/>Non-radiative voltage loss is directly linked to decreases in the photoluminescence quantum yield (PLQY), and thus the prevalence of non-radiative recombination.[1] This picture is complicated in Sn perovskites given that the presence of Sn⁴⁺ impurities in the precursor solution leads to both the introduction of background hole dopants (serving to <i>increase</i> the PLQY) and non-radiative recombination centers (serving to <i>decrease</i> the PLQY). [2] <br/>We use the observation of pseudo-first order photoluminescence (PL) decay kinetics in Sn perovskite films to establish a method for characterizing the hole dopant level and non-radiative recombination rate constant with combined time-correlated single photon counting (TCSPC) and photoluminescence quantum yield (PLQY) measurements. We find that untreated FASnI₃ films exhibit large hole doping concentrations of <i>p₀</i> &gt; 8.8 x 10¹⁸ cm^-3, which is reduced to <i>p₀ </i>~ 10¹⁶ cm^-3 after precursor sublimation and SnF₂ treatments. While it is well known that the radiative recombination rates are increased with <i>p₀</i>, we reveal that the non-radiative rate is also increased. Employing hyperspectral photoluminescence microscopy, we find that microscale <i>p</i>-type regions in untreated FASnI₃ films are centers for non-radiative recombination, which are diminished in films with <i>p₀</i> ~ 10¹⁶ cm^-3. We find significant PL heterogeneity even in FASnI₃ films with moderate dopant levels, suggesting that new strategies to eliminate deleterious defects in FASnI₃ should be developed. [3]<br/><br/>[1] Y. Shi <i>et al</i>, (3-Aminopropyl)Trimethoxysilane Surface Passivation Improves Perovskite Solar Cell Performance by Reducing Surface Recombination Velocity. ACS Energy Lett <b>2022</b>, 7 (11), 4081–4088. https://doi.org/10.1021/acsenergylett.2c01766.<br/>[2] D. Meggiolaro <i>et al</i>, Tin versus Lead Redox Chemistry Modulates Charge Trapping and Self-Doping in Tin/Lead Iodide Perovskites. Journal of Physical Chemistry Letters <b>2020</b>, 11 (9), 3546–3556. https://doi.org/10.1021/acs.jpclett.0c00725.<br/>[3] Westbrook <i>et al</i>, Local Background Hole Density Drives Non-Radiative Recombination in Tin Halide Perovskites, <i>Under Review</i>