Kesterite Offers A Pathway To Greener Solar Cells

Kesterite is a natural mineral with many important material properties, including a band gap that can be used as a light absorber material for next generation thin-film solar cells. Unlike their perovskite-based competitor technologies, Kesterite solar cells are natural and non-toxic, making them them a truly eco-friendly alternative to the fully or partially inorganic materials, silicon and perovskite respectively, currently dominating studies in the photovoltaic industry.

Although kesterite’s use for making solar cells has been constrained so far, due to its efficiency being stuck around 10%, scientists are pushing the boundaries of research to make it the most efficient, affordable, non-toxic, flexible successor to traditional silicon solar cells. In August 2018, Australian researchers achieved a 10% efficiency for a cell based on copper zinc tin sulfide, or sulfide kesterite. The world record for such cells is 12.6%, achieved by Japanese thin-film producer Solar Frontier in 2013.

In September last year, researchers from two leading Indian universities – SRM University and IIITDM, Kancheepuram- published  a paper entitled ‘Towards Quantum Efficiency Enhancement of Kesterite Nanostructured Absorber: A Prospective of Carrier Quantization Effect’ in the scientific journal Applied Physics Letter September. The paper argued that one way to overcome the challenge of low efficiency is to use multiple quantum wells with very thin layers of two types of kesterite material (Kesterite copper-zinc-tin-sulphide (CZTS) and chalcopyrite-type CuInS2 (CIS) ) to avoid high recombination rate. Using this approach, the team reported a possible power conversion efficiency of 25 percent from MQW Kesterite solar cells.

Two further developments in kesterite technology were reported in January this year. Scientists working at Incheon National University, Republic of Korea, claimed that they had developed a new eco-friendly ZTO buffer to replace expensive and toxic buffers currently used in solar panels. They aligned the energy levels of electrons between the absorber layer (kesterite) and buffer layer (ZTO) to improve solar cells’ efficiency. This enabled a better circulation of electrons between two layers, which helped enhance the cell’s voltage and overall performance. The kesterite solar cell equipped with ZTO buffer obtained a power conversion efficiency of 11.22%.

The same month, researchers from the University of Oldernburg, Germany, announced that they had built a kesterite copper zinc tin selenide (CZTSe) thin-film solar cell by using thin diffusion-barrier layers of silicon oxynitride (SiOxNy) to reduce the thickness of the molybdenum diselenide (MoSe2) interface layer, which has a negative impact on the cell’s performance. They applied two different types of back-contact structures using the SiOxNy which, the scientists claim, prevents the strong reaction of molybdenum and selenium from forming MoSe2 during annealing with a high temperature of around 520 C. “In cases of Mo/SiOxNy/Mo back contacts, the performance of CZTSe solar cells remain unchanged or slightly improved in the range of approximately 11% for the adoption of 10 nm SiOxNy layers,” they said.

In March this year, a paper entitled ‘Kesterite Solar Cells: Insights into Current Strategies and Challenges’, which was supported by Australian Renewable Energy Agency (ARENA), was published in the journal Advanced Science. The study offered a new direction to control the formation of detrimental intrinsic defects – which decrease kesterite material’s performance  – by engineering the local chemical environment and optimizing the absorber synthesis process during the growth of kesterite materials. Surface passivation to alleviate the defects, postdoping an additional passivator (e.g., alkali elements), and post-treatment (oxygen plasma treatment) were some of the recommended solutions. Therefore, integrating the bulk defects control with additional passivation steps will be crucial for delivering a step-change in kesterite’s performance, the researchers said.

The latest research in this long trail of studies came last month from scientists working at Fuzhou University, China, who published a paper entitled ‘Novel Symmetrical Bifacial Flexible CZTSSe Thin Film Solar Cells for Indoor Photovoltaic Applications’ in the journal Nature Communications. They claim to have developed a flexible bifacial kesterite (CZTSSe) PV device, suitable for applications in indoor and outer space photovoltaics. The bifacial solar cell achieved an efficiency of 9.3% on the front side, with an open-circuit voltage of 436 mV, a short-circuit current of 33.76 mA/cm2, and a fill factor of 63.2%. As for the backside, the cell reached a power conversion efficiency of 9.0%, an open-circuit voltage of 434 mV, a short-circuit current of 33.7 mA/cm2, and a fill factor of 61.7%. “The bifacial device could directly utilize all-directional lights and produce periodically variable currents by substrate rotation,” the scientists concluded.