By Soumitra Satapathi, Professor, Indian Institute of Technology Roorkee
The ever-increasing demand for power and global concerns about climate change have prompted the scientific community to explore emerging solar cells beyond silicon photovoltaics (PV). Conventional silicon solar cells are gradually reaching their theoretical efficiency limit, demanding exploration for novel materials and new processing technology for building PV cells. Thanks to the unprecedented growth in materials research, the development of the most efficient and cost-effective solar cells is dominated by novel materials. This has led to a variety of third-generation or emerging solar cell technologies.
The emerging third-generation solar cells include dye-sensitised solar cells (DSSCs), organic solar cells (OSCs), quantum dot-based solar cells (QSCs) and perovskite solar cells (PSCs). In DSSCs, also commonly known as Grätzel cells, a light-absorbing dye is incorporated into porous inorganic titanium oxide (TiO2) and placed between two electrodes. Upon photo absorption by dye, an electron-hole pair is generated. Electrons then move from the TiO2 semiconductor to the outer circuit. As they reach the counter electrode, they generate a current. An electrolyte ensures the continuous repetition of this process. DSSCs have been used in indoor PV, building-integrated PV and small-scale power devices. Recently, we used naturally occurring, ultra-low-cost jamun fruit as a dye molecule in DSSCs, successfully generating power from it. These solar cells are commonly known as Jamun solar cells.
Another promising emerging solar cell technology is OSC, in which two organic molecules – usually a conducting polymer and a small molecule – are blended together in an appropriate ratio to form a thin film. The thin film is placed between two conductive electrodes, with suitable charge extraction layers placed between the organic active layer and respective electrodes. This type of geometry is commonly called bulk heterojunction solar cells. Due to their flexibility, lightweight nature and solution processability, OSC technology has been used in defence applications, electronics, internet of things sensors and medical devices.
Another promising third-generation technology is QSC, which uses the intriguing characteristics of quantum dots to improve energy conversion and light absorption. Quantum dots are nanoscale semiconductor particles measuring 1-10 nanometres in size and they behave differently at this scale than they do in bulk, which results in distinct optical and electrical properties. Lead sulfide or cadmium selenide are common examples of quantum dots. These nanoscale quantum dots function as charge-carrying and light-absorbing components in a QSC. These cells employ several layers of different-sized quantum dots in place of conventional solar cells, which use a single semiconductor material to collect light energy and transform it into power. Because each layer is made to absorb various light wavelengths, the solar spectrum may be used more effectively. QSCs are increasingly used in flexible and compact power applications.
Of these third-generation solar cell technologies, PSCs are the most promising due to their excellent optoelectronic properties such as direct and tunable band gaps, ambipolar charge transport, long carrier diffusion length, and light absorption in the visible range along with low-cost fabrication. The power conversion efficiency (PCE) of single-junction PSCs at the lab scale has increased from 3.8 per cent to 26.1 per cent over the past decade, approaching the PCE values of crystalline silicon. This demonstrates the potential of PSCs for commercialisation. In PSCs, an organic-inorganic metal halide hybrid perovskite is spin-coated or printed on an organic or inorganic charge transport layer, with another charge transport layer put on top of it. The resulting devices are placed between two electrodes. Various research groups across the world are actively working on this solar cell technology, mainly to improve efficiency and stability. In the Satapathi Lab at IIT Roorkee, we are taking multiple approaches to improve the PCE and stability of PSCs. We have demonstrated a PCE of 23 per cent at the lab scale and 15 per cent at a larger scale. We are also actively working on transparent solar glasses made from PSCs. We have reported a 15 per cent PCE and 35 per cent transmittance in these semi-transparent solar cells, which is a notable achievement. These semi-transparent PSCs have the potential for use in building-integrated PV. More recently, we have demonstrated silicon-perovskite tandem solar cells with a PCE of 28 per cent, showcasing their commercial potential.
As the progress in materials properties and processing techniques continues, these emerging PV technologies will continue to evolve. In India, various educational institutions and research labs are actively working on various emerging PV technologies. With suitable government policy and research infrastructure, India will soon emerge as a global leader in the PV industry.
