Better solar cells with ZnSnO as buffer layer

Open-circuit voltage Voltage from a solar cell with an open circuit.
Recombination When the electron emits its energy and returns to the valence band.
Bandgap Energy difference between the valence band and the conduction band which the electron is excited between.

The best solar cells of CZTS-type have been limited in their efficiency largely due to a limited open-circuit voltage. An explanation has been that the energy level of the electrons in the conduction band of the buffer layer, CdS, does not match the energy for the conduction band of the CZTS layer which leads to recombination in the interface between them. To investigate this, the temperature dependent current- and voltage measurement (JVT) is often used. This will measure the activation energy for the recombination. The standard interpretation is that if it is equal to the bandgap the bulk recombination is dominant, while lower activation energy than the bandgap indicates that interface recombination is limiting the open-circuit voltage. For most CZTS-solar cells, the activation energy has indicated recombination at the interfaces. In this study, ZnSnO (ZTO) was used as a buffer layer, whose bandgap has been seen to change depending on deposition temperature, and it was investigated how this affects the recombination and the performance of the solar cell.

CZTS-solar cells were fabricated on a glass substrate with Mo/CZTS/buffer layer/ZnO/ZnO:Al/contact. ZTO was used as the buffer layer for the samples, and reference cells were also made with CdS as buffer layer. For solar cells with ZTO, the buffer layer was deposited at 80, 95 and 120°C. The solar cells that had been deposited at 80°C had a limited efficiency because the energy level in the ZTO was too high and created a barrier for the current, which is due to the increased bandgap at decreasing deposition temperatures. For ZTO-solar cells deposited at 120°C, the efficiency was similar to the reference cells. The bandgap for ZTO deposited at this temperature has in previous studies been shown to correspond to the bandgap of CdS, which is a probable explanation for the result. The solar cells deposited at 95°C had the highest efficiency, also higher than the reference cells, which indicated the best match of energy levels. For these cells, the recombination was also investigated using JVT. The activation energy was higher than for the reference cells, although still lower than the expected bandgap for CZTS. It can nevertheless be interpreted as if the interface recombination has decreased and shifted to be dominated by bulk recombination, or at least that the barrier for interface recombination has increased. Regardless, ZTO as a buffer layer has been shown to decrease the recombination with the matching energy levels and have advantages for CZTS-solar cells compared to CdS.

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