Pb-Sn Halide Perovskites for Photovoltaics—Understanding Carrier Extraction with a Doped Active Layer

The possibility of extending the absorption of lead based metal halide perovskites (LHPs) to the near-infrared region by partially substituting Pb with Sn has attracted great attention for the possible applications in tandem devices¹ and single junction solar-cells². In addition to the nonlinear reduction of the band gap, the gradual substitution of lead with tin also changes significantly the photophysical properties of the material.^3,4 Despite the optoelectronic quality of tin halide perovskites (THPs) being inherently comparable to LHP⁵, device efficiency and stability are still limited.^6,7<br/>With a combination of steady-state, time-resolved and pump-probe spectroscopy techniques we address the interplay of radiative and non-radiative processes occurring in tin-halide perovskites FA_0.85Cs_0.15Sn_xI₃. Recombination dynamics are explored as a result of electronic and chemical doping, oxidation and density of surface to bulk trap states. Carrier dynamics simulations and density functional theory calculations of defects are adopted to understand experimental results and identify the dominant processes occurring in different regimes of photoexcitation. We demonstrate that when the material is highly doped Auger recombination plays a fundamental role in limiting performances even at low photoexcitation density.^8,9<br/>We also investigate the fundamental optoelectronic properties of THPs when integrated in a device. The currently used architecture of lead-based solar cells, where an intrinsic absorber is sandwiched between two organic extraction layers (pin), is intrinsically not suitable for THPs as it would result in a p-p-n structure (as long as the THP remains intrinsically p doped). The short carrier lifetimes deriving from the intrinsic p-doping and the upshifted valence and conduction bands result in a reduced extraction capability. The bulk-free carrier dynamics in the presence of extraction layers are probed with transient absorption spectroscopy from the fs to time scales. We find changes in the extraction of electrons and holes in different temporal regimes and identify the presence of long-lived trap states. This information is correlated with spatial and time-resolved photoelectron spectroscopy (tr-PEEM)^10,11 at the interface of THPs with an evaporated extraction layer. The band alignment, surface trap distribution and charge transfer are probed for mixed FA_0.85Cs_0.15Pb_1-xSn_xI₃: as the lead-based perovskite is alloyed with tin, its doping density, valence band position and bandgap can be gradually tuned, resulting in a modified energetic landscape of photogenerated carriers affecting carrier extraction.<br/><br/><b>REFERENCES</b><br/>1. Yang, Z.; Yu, Z.; Wei, H. ; <i>et al.</i>; <i>Nat Commun</i>10, 4498 (2019).<br/>2. Nasti, G.; Abate, A.; <i>Adv. </i><i>Energy Mater. </i>10, 1902467 (2020).<br/>3. Klug, M. T.; Milot, R. L.; Patel, J. B.; Green, T.; <i>et al.; </i><i>Energy Environ. Sci</i><i>. </i>13, 1776–1787 (2020).<br/>4. Savill, K. J.; Ulatowski, A. M.; Herz, L. M.; <i>ACS Energy Lett. </i>6, 2413–2426 (2021).<br/>5. Poli, I.; Kim, G.; Wong, E. L.; Treglia, A.; <i>et al.</i>; <i>ACS Energy Letters</i>, <i>6</i>, 609–611 (2021).<br/>6. Jiang, X.; Wang, F.; Wei, Q.; Li, H.; <i>et al. ;</i> <i>Nat Commun</i> 11, 1245 (2020).<br/>7. Nishimura, K.; Kamarudin, M. A.; Hirotani, D.; Hamada, K.; <i>et al.; Nano Energy</i>, <i>74</i>, 104858 (2020).<br/>8. Treglia, A.; Ambrosio, F.; Martani, S.; Folpini, G.; <i>et al</i>.; <i>Mater. Horiz</i>.,9, 1763-1773 (2022).<br/>9. Poli, I., Ambrosio, F., Treglia, A., Berger, F. J.; <i>et al</i>.;<i> Adv. Sci.</i>, 2202795 (2022).<br/>10. Man <i>M.; </i>Margiolakis, A.; Deckoff-Jones, S.; Harada, T.; <i>et al.; </i><i>Nat. Nanotechnol. </i>2017, <i>12</i>, 36.<br/>11. Doherty, T.A.S., Winchester, A.J., Macpherson, S. <i>et al.;</i> <i>Nature </i>2020, <i>580</i>, 360.