Optoelectronic properties of chemically deposited Bi_2S_3 thin films and the photovoltaic performance of Bi_2S_3/P3OT solar cells

摘要

Thin films of bismuth sulfide (Bi_2S_3), 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 ℃ for 30 min) to 1.5 eV for a 200 nm film deposited for 2 h. The electronegativity for Bi_2S_3 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 Bi_2S_3 film was deduced to be 4.5 eV. In the energy level analysis of ITO/Bi_2S_3/P3OT/Au structure, Bi_2S_3 was established as an electron acceptor. To produce ITO/Bi_2S_3/P3OT/Au solar cell structures, poly3-octylthiophene (P3OT), prepared in the laboratory was dissolved in toluene and was drop-cast on the Bi_2S_3 film and a gold film was thermally evaporated. Under 100 mW/cm~2 tungsten-halogen irradiation incident from the ITO-side, the cell using a Bi_2S_3 film with thickness of 50 nm has the highest open circuit voltage (V_(oc)) of 440 mV and short-circuit current density (J_(sc)) of 0.022 mA/cm~2. The addition of a CdS thin film (90 nm) between ITO and B_2S_3 would increase V_(oc)to 480 mV, and J_(sc) to 0.035 mA/cm~2. The role of surface morphology and optoelectronic properties of the Bi_2S_3 film in the photovoltaic performance of the junction is discussed.