Band gap tunable quaternary Pb_xCd_1−xS_1−ySe_y quantum dot-sensitized solar cells with an efficiency of 9.24% under 1% sun

We present a new solar absorber material - quaternary Pb_xCd_1−xS_1−ySe_y quantum dots (QDs) - with a band gap E_g tunable over a broad range of 1.5-2.4 eV. Pb_xCd_1−xS_1−ySe_y QDs were prepared using a two-step cation/anion co-alloying approach: first, Pb²⁺ was incorporated into the binary CdS host by cation exchange, leading to ternary Pb_xCd_1−xS QDs. Second, Se²⁻ replaced a fraction of S²⁻ in Pb_xCd_1−xS by anion exchange, leading to quaternary Pb_xCd_1−xS_1−ySe_y QDs. The reactions reduced the E_g from 2.42 (CdS, absorption onset 500 nm) to 1.91 (Pb_xCd_1−xS, 650 nm) to 1.55 eV (Pb_xCd_1−xS_1−ySe_y, 800 nm). The E_g of Pb_xCd_1−xS_1−ySe_y (1.55 eV) is close to the Shockley-Queisser gap. Solid-state Pb_xCd_1−xS_1−ySe_y QD solar cells (QDSCs) were fabricated with Spiro-OMeTAD as the hole transport material. The best cell (Pb_0.15Cd_0.85S_0.75Se_0.18) yielded J_sc = 13.71 mA cm⁻², V_oc = 0.59 V, FF = 54.8% and PCE = 4.43% under 1 sun. At low light intensities, the PCE increased to 7.18% under 10% sun and further to 9.24% under 1% sun. The co-alloying treatment improved the PCE from 4.49 (CdS) to 8.48 (Pb_xCd_1−xS) to 9.24% (Pb_0.15Cd_0.85S_0.75Se_0.18) under 1% sun. The cell with a PCE of 9.24% can be categorized as a high-efficiency QDSC, suggesting that Pb_xCd_1−xS_1−ySe_y can be an efficient solar absorber.