Cadmium-free kesterite solar cell with 9.82% efficiency

Scientists from Chonnam National University in China have fabricated a heterojunction kesterite (CZTSSe) solar cell based on a zinc sulfide oxide Zn(O,S) buffer layer, which was used as an alternative to toxic cadmium sulfide (CdS), which is common in kesterite devices.

The researchers used a one-minute ammonium sulfide (NH4)2S treatment process for surface cleaning and passivation of the cell’s CZTSSe absorber. The process is intended at removing oxides and hydroxides from the absorber layer.

The Zn(O,S) films were deposited through atomic layer deposition (ALD) at a substrate temperature of 90 degrees Celsius. The CZTSSe-absorber layers were synthesized on a soda-lime glass (SLG) substrate coated with molybdenum (Mo).  Copper-tin-zinc (Cu-Sn-Zn) metal films were deposited on the SLG/Mo substrate by direct current (DC) sputtering. The CZTSSe-absorber layers were synthesized, again, by rapid thermal annealing in a graphite box and then exposed to potassium cyanide (KCN) etching for two minutes.

The performance of the device was assessed through J–V measurements under one-sun illumination and compared to that of a similar cell developed without the (NH4)2S treatment. The latter showed a power conversion efficiency of 7.46%, an open-circuit voltage of 0.483 V, a short-circuit current of 34.4 mA cm², and a fill factor of 0.45. The cell treated with (NH4)2S exhibited a much higher efficiency at 9.82% and a remarkable fill factor of 0.56, with the open-circuit voltage and short-circuit current being 0.496 V and 35.6 mA cm², respectively.

“Increasing the (NH4)2S treatment duration resulted in a gradual decrease in the FF and, thus, low cell efficiency,” the scientists noted, adding that the cell efficiency remained much higher than that of the cell without the treatment. “Further, it is worth noting that the cell efficiency obtained herein is the highest efficiency reported for the CZTSSe-based thin-film solar cells fabricated with the ALD-Zn(O,S) buffer layer.”

“The cell was so far developed with a small size and at the laboratory level,” the research main author, Jaeyeong Heo, told pv magazine. “It is currently hard to estimate its costs for production.” The research team claimed its findings are encouraging for the development of cadmium-free Zn(O,S) buffer layer-based kesterite solar cells fabricated by completely vacuum-based processes.

A thorough description of the cell and its fabrication process can be found in the paper CZTSSe/Zn(O,S) heterojunction solar cells with 9.82% efficiency enabled via (NH₄)₂S treatment of absorber layer, which was recently published in Progress in Photovoltaics.