Better Solar Cells with a ZnSnO Buffer
This is a summary of a scientific article.
For the best CZTS-type solar cells, the main limitation in efficiency has been a restricted maximum open-circuit voltage. One explanation is that the electron energy level in the conduction band of the buffer layer, CdS, does not match the conduction band energy of the CZTS layer, which leads to recombination at the interface between them. To investigate this, temperature-dependent current-voltage analysis (JVT) is often used to determine the activation energy of recombination. If this equals the bandgap of the absorber layer, recombination is interpreted as mainly occurring in the bulk of the material, while a lower activation energy indicates dominant interface recombination. For most CZTS solar cells, the activation energy has indicated recombination at interfaces. In this study, ZnSnO (ZTO) was investigated as a buffer layer; research on CIGS has shown that its bandgap varies depending on the deposition temperature, and the study examined how ZTO affected recombination and solar-cell performance.
CZTS solar cells were fabricated on glass substrates with the structure Mo/CZTS/buffer layer/ZnO/ZnO:Al/contact, using ZTO as the buffer layer and CdS as a reference. For ZTO cells, the buffer layer was deposited at 80, 95, and 120 °C. Cells deposited at 80 °C showed limited efficiency because the energy level in ZTO was too high and created a barrier for current, due to the increased bandgap at lower temperature. ZTO cells deposited at 120 °C had efficiency similar to the reference cells; studies on CIGS have shown that at this temperature the energy level is comparable to CdS, which likely explains the result. Cells deposited at 95 °C showed the highest efficiency — higher than the reference cells — indicating the best energy-level alignment.
For these cells, recombination was also analyzed using JVT. The activation energy was higher than in the reference cells but still slightly lower than the expected bandgap of CZTS. This can nevertheless be interpreted as recombination being reduced at the interface and shifted toward the bulk of the material, or at least that the barrier to interface recombination increased. Overall, ZTO as a buffer layer reduces recombination through improved energy alignment and appears to be a more advantageous material for CZTS than CdS.