New buffer promises more eco-friendly solar cells

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Scientists have developed a new material for thin-film solar cells, which they say is more eco-friendly than existing varieties. The new buffer material reduces the pollution associated with producing cadmium sulfide (CdS) buffers, and is also less toxic – opening up a greater number of applications, such as solar windows

The team, from Incheon National University, in Korea, has published its findings in Nano Energy and made available online on August 10, 2020 (ahead of the final publication of the issue in December 2020).

Solar panels are composed of photovoltaic cells, whereby materials exposed to light generate excited electrons, an electric current. Modern thin-film solar cells are made up of micrometre- or sub-micrometre-thick layers of a photovoltaic material, allowing them to be integrated into flexible, lightweight panels for use in a variety of substrates. However, this process has some limitations. Professor JunHo Kim, who led the study, explained: 'Most thin-film solar cells include toxic and expensive elements, which may hinder the expansion of solar cell applications.'

Prof Kim and his team are working on the production of a solar cell using naturally abundant, eco-friendly materials, which are easy to extract and inexpensive to manufacture.

The scientists looked at eco-friendly cells made up of kesterite, a natural mineral that acts as a photon absorber. Most kesterite cells use a buffer layer made of cadmium sulfide (CdS) to optimise their performance. Despite their efficiency, the pollution associated with making these buffers and the toxicity of cadmium are not desirable traits in an eco-friendly solar cell. To deal with this issue, the researchers examined a promising alternative, called the 'ZTO buffer.'

To further improve the efficiency of the solar cell, the team aligned the energy levels of the electrons between the absorber layer (kesterite) and the buffer layer (ZTO). This allowed for a better circulation of electrons between the two layers, increasing the cell’s voltage and overall performance, with a power conversion efficiency of 11.22 per cent. To put things into perspective, current kesterite cells using CdS buffers have a maximum efficiency of 12.6 per cent, meaning that the proposed cell showed high efficiency. This technique is the first to yield such a high performance using solely eco-friendly, abundant, and inexpensive materials.

As the demand for solar panels grows, it is especially important to source its components in the most environmentally friendly and cheapest way possible, the researcher's say, making them more applicable to a wide range of applications. 'Eco-friendly thin-film solar cells could be installed on the roofs and walls of buildings and houses to produce electricity near us,' said Professor Kim. 'They could also be employed in ground vehicles (cars, buses, and trucks) and marine transportations (boats and long-range ships) to partially support electric power.'