# Separate and combined effects of Gua+ and SCN− ions on charge carrier dynamics in mixed Sn–Pb perovskites

## TL;DR

Guanidinium and thiocyanate ions together improve the performance of tin-lead perovskite solar cells by reducing defects and enhancing charge transport.

## Contribution

The synergistic effect of Gua+ and SCN− ions on charge carrier dynamics in mixed Sn–Pb perovskites is revealed.

## Key findings

- SCN− suppresses tin oxidation and improves carrier transport by coordinating with Sn2+ and scavenging Sn4+.
- Combining Gua+ and SCN− with SnF2 results in large-grain, pinhole-free films with a 50 mV increase in pseudo-open circuit voltage.
- Excess GuaSCN forms 2D perovskites that do not enhance carrier dynamics.

## Abstract

Here, we investigated the separate and combined effects of guanidinium (Gua+) and/or thiocyanate (SCN−) ions on the opto-electronic properties of mixed Sn–Pb perovskites, which are used as absorber layers in photovoltaics. Therefore, we spin-coated Cs0.25FA0.75Sn0.5Pb0.5I3 thin films with GuaI, Pb(SCN)2 or GuaSCN, in the absence or presence of SnF2. By comparing the (micro)structural and opto-electronic properties of the perovskite films, we elucidated the functions of both ions. We found that SCN− suppresses tin oxidation and doping, reduces crystal defects and improves the carrier transport properties, regardless of SnF2 addition. We demonstrate that this is due to coordination with Sn2+ and scavenging of Sn4+ in the spin-coating solution, resulting in a pile-up of SnOx at the film surface. Gua+ is incorporated to a limited extent into the 3D cubic perovskite structure. Gua+ cannot suppress tin oxidation and doping, making this additive useless without SnF2. Conversely, when combined with SnF2, Gua+ enhances the carrier transport properties. Combining Gua+ and SCN− until a maximum addition of 4 mol% and SnF2 results in large grains and pinhole-free films with superior charge carrier transport properties, leading to a substantial increase in the pseudo-open circuit voltage of 50 mV. Addition of >4 mol% GuaSCN leads to the formation of Gua-based 2D perovskites, including GuaxFA2−xSnyPb1−yI4 and GuaxFA3−xSnyPb2−yI7, which do not improve the carrier dynamics. In short, we observe a synergistic effect on addition of Gua+ and SCN− ions, which leads to improved structural and opto-electronic properties, which is promising for implementation in solar cells.

Guanidinium and thiocyanate ions show a synergistic effect in mixed tin-lead perovskites, passivating crystal defects, enhancing carrier transport, and yielding pinhole-free films with large grains.