Optoelectronic properties of chemically deposited Bi2S3 thin films and the photovoltaic performance of Bi2S3/P3OT solar cells

Thin films of bismuth sulfide (Bi2S3), prepared on conductive tin-doped indium oxide (ITO)-glass substrates by chemical deposition showed a variation of optical band gap with thickness: 1.8 eV for a 50 nm film (deposited at 40 °C for 30 min) to 1.5 eV for a 200 nm film deposited for 2 h. The electronegativity for Bi2S3 compound is 5.3 eV, as estimated from the ionization energy and electron affinity of elemental Bi and S, and thus the electron affinity of chemically deposited Bi2S3 film was deduced to be 4.5 eV. In the energy level analysis of ITO/Bi2S3/P3OT/Au structure, Bi2S3 was established as an electron acceptor. To produce ITO/Bi2S3/P3OT/Au solar cell structures, poly3-octylthiophene (P3OT), prepared in the laboratory was dissolved in toluene and was drop-cast on the Bi2S3 film and a gold film was thermally evaporated. Under 100 mW/cm2 tungsten-halogen irradiation incident from the ITO-side, the cell using a Bi2S3 film with thickness of 50 nm has the highest open circuit voltage (Voc) of 440 mV and short-circuit current density (Jsc) of 0.022 mA/cm2. The addition of a CdS thin film (90 nm) between ITO and B2S3 would increase Voc to 480 mV, and Jsc to 0.035 mA/cm2. The role of surface morphology and optoelectronic properties of the Bi2S3 film in the photovoltaic performance of the junction is discussed.

► The electronegativity (EN) of bulk Bi2S3 was estimated from reported data.
► The electron affinity (χ) of Bi2S3 films was deduced from Eg and EN of bulk Bi2S3.
► Voc values of Bi2S3/P3OT cells depend on χ and morphology of Bi2S3 films.
► Jsc values of the same devices depend on the photoconductivity of Bi2S3 films.