Short-circuit current improvement in CdTe solar cells by combining a ZnO buffer layer and a solution back contact

Conventional CdTe solar cells have a CdS window layer, in which an absorption loss of photons with more than 2.4 eV occurs through the CdS layer. A thinner CdS layer was applied to enhance light transmission and a ZnO buffer layer with a band gap of 3.3 eV was introduced to suppress shunting through the thinner CdS window layer. A 100-nm thick ZnO layer sputter-deposited at 300 °C had uniform coverage on a transparent conductive oxide (TCO) after a subsequent high-temperature process. The ZnO layer was effective in preventing shunting through the CdS window layer so that the open-circuit voltage and fill factor of the CdTe solar cells were recovered and the short-circuit current was enhanced over that of the conventional CdTe solar cell. In the ZnO/CdS/CdTe configuration, the short-circuit current was further improved throughout the visible wavelength region by replacing the Cu-metal contact with a Cu solution contact. As a result the short-circuit current from 21.7 to 26.1 mA/cm2 and the conversion efficiency of the CdTe solar cell increased from 12 to 15% without antireflective coating. Our result indicates that the Cu solution back contact is a critical factor for achieving a higher cell efficiency in addition to ZnO buffer layer. •CdTe solar cell has low efficiency due to the blue-wave light absorption by CdS and Cu contamination by Cu back contact.•This paper systematically investigated by apply ZnO buffer in combination of thinner CdS layer and applying Cu solution back contact method.•First, the short-circuit current was improved from 21.7 to 25.1 mA/cm2 by the thinner CdS while the open-circuit voltage was maintained as before by the ZnO buffer layer as the increase of blue response in the QE curve.•The short-circuit current was further improved from 25.1 to 26.1 mA/cm2 by applying Cu solution contact in replacement of the Cu metal contact as the increase of the red response in the QE curve.•As result, the cell efficiency was improved from 12 to 15% without antireflective coating.•The physics behind the improvement of the short-circuit current was discussed in the paper.