Lead tin sulfide (Pb_1-xSn_xS) nanocrystals: A potential solar absorber material

We present a new ternary semiconductor absorber material - Pb_1-xSn_xS - for solar cells. Pb_1-xSn_xS nanocrystals (NCs) were synthesized using the successive ionic layer adsorption reaction (SILAR) process. Energy-dispersive X-ray spectroscopy revealed the Sn ratio for a sample prepared with five SILAR cycles to be x=0.55 (i.e. non-stoichiometric formula Pb_0.45Sn_0.55S). The optical spectra revealed that the energy gap E_g of the Pb_1-xSn_xS NCs decreased with an increasing number of SILAR cycles n, with E_g=1.67eV for the sample with n=5. Liquid-junction Pb_1-xSn_xS quantum dot-sensitized solar cells were fabricated using the polysulfide electrolyte. The best cell yielded a short-circuit current density J_sc of 10.1mA/cm², an open circuit voltage of 0.43V, a fill factor FF of 50% and an efficiency of 2.17% under 1 sun. The external quantum efficiency spectrum (EQE) covered a spectral range of 300-800nm with a maximum EQE of ∼67% at λ=650nm. At the reduced light of 0.1 sun, the efficiency increased to 3.31% (with a normalized J_sc=17.7mA/cm²) - a respectable efficiency for a new sensitizer. This work demonstrates that Pb_1-xSn_xS shows potential as a solar cell absorber.