Kyu-Woong Yeom1,Nam-Gyu Park1
Sungkyunkwan University1
Kyu-Woong Yeom1,Nam-Gyu Park1
Sungkyunkwan University1
Here, we report on additive engineering in Sn<sup>2+</sup> and Pb<sup>2+</sup> bearing precursor solution to improve photovoltaic performance and stability of Sn-Pb alloyed perovskite solar cells. Additives of urea as O-donor and thiourea as S-donor are compared, where oxygen is relatively hard base than sulfur. Photovoltaic performance is higher for the additives than for the control without additive, where thiourea is found to be more effective than urea in enlarging grain size and carrier lifetime. As a result, power conversion efficiency (PCE) of perovskite solar cell employing FA<sub>0.5</sub>MA<sub>0.5</sub>Pb<sub>0.5</sub>Sn<sub>0.5</sub>I<sub>3</sub> is improved from 14.58% for control (without additive) to 16.00 % for urea and to 18.59% for thiourea. FTIR study on SnI<sub>2</sub> (or PbI<sub>2</sub>)-additive adduct shows that the C=S stretching frequency of thiourea is lowered from 729 cm<sup>-1</sup> to 721 cm<sup>-1</sup> and 708 cm<sup>-1</sup> upon interaction with PbI<sub>2</sub> and SnI<sub>2</sub>, respectively, which indicates that interaction of S with Sn<sup>2+</sup> is stronger than that with Pb<sup>2+</sup>. Shift to lower wavenumber is also observed for urea but the degree is marginal for both SnI<sub>2</sub> and PbI<sub>2</sub>. Stronger interaction of thiourea with Sn<sup>2+</sup> than Pb<sup>2+</sup> is due to increased covalency by interaction of smaller Sn<sup>2+</sup> ion with soft sulfur bearing thiourea. Thus, the significantly improved performance in the presence of thiourea is ascribed to underlying a kinetic control of crystal growth. X-ray photoelectron spectroscopic study reveals that oxidation of Sn<sup>2+</sup> is effectively suppressed by thiourea, which is beneficial to not only photovoltaic performance but also phase stability.