Chemically deposited lead sulfide and bismuth sulfide thin films and Bi2S3/PbS solar cells

Solar cells with a short-circuit current density (Jsc) of 6 mA/cm2, an open circuit voltage (Voc) of 280 mV and a conversion efficiency of 0.5% under a 1000 W/m2 solar radiation were prepared by sequential chemical deposition of Bi2S2 (160 nm) and PbS (400 nm) thin films. The optical band gap (Eg) of Bi2S3 (160 nm) decreased from 1.67 to 1.61 eV upon heating the as-deposited film at 250 oC in air for 15 min to make it crystalline, but also reduced its thickness to 100 nm. Photoconductivity of this film is 0.003 (Ω cm)-1. The Eg of PbS film (200 nm) deposited at 25 oC (24 h) is 0.57 eV, and is 0.49 eV for the film deposited at 40 oC. The electrical conductivity of the latter is 0.48 (Ω cm)-1. The photo-generated current density for a Bi2S3(100 nm)/PbS(300 nm) absorber stack is above 40 mA/cm2 under AM 1.5 G (1000 W/m2) solar radiation. However, the optical losses in the cell structure reduces the Jsc. Spectral sensitivity of the external quantum efficiency of the cell establishes the contribution of Bi2S3 and PbS to Jsc. The energy level diagram of the cell structure suggests a built-in potential of 470 mV for the present case. Six series-connected cells gave the Voc of 1.4 V and Jsc of 5 mA/cm2.