Photoelectric properties and charge dynamics for a set of solid state solar cells with Cu4Bi4S9 as the absorber layer

In this work, many single-crystalline, orthorhombic phase Cu4Bi4S9 nanoribbons were fabricated by a facile solvothermal method. Four types of three-dimensional solar cells prepared with In2O3, ZnO, TiO2 and SnO2 thin films as the working electrodes, In2S3 as the buffer layer and Cu4Bi4S9 as the inorganic dye and hole collector were tested using standard techniques. Under the same conditions, the I–V characteristics and photocurrent spectra of the cells were systematically measured. It is shown that the four types of cells all have good photoelectric properties. However, the In2O3 and TiO2 cells show better performance than the other two cells, and the highest efficiencies are about 6.2% (In2O3), 5.5% (TiO2), 4.8% (ZnO) and 3.9% (SnO2). The overall efficiencies were also compared with various oxide film thicknesses and the best performance was for films of 6 μm thickness for each type of cell. Using steady state and electric field-induced surface photovoltage spectroscopy, we further studied the charge dynamics and the transport mechanism in the sensitized electrodes. It is suggested that the matching degree of energy levels between two conduction bands (Cu4Bi4S9 and the oxides) has an obvious effect on the transport of the photogenerated charges. For good performance, we should pay more attention to alternative materials with matching degrees of energy levels, besides the optical stability, film structure, substrate thickness and the carrier mobility for solid state solar cells in the future.