Sophisticated Cells

Emerging PV technologies focus on higher efficiencies at lower prices

 

The solar PV segment has come a long way both in India and globally. Its acceptance has increased significantly owing to the decline in costs. As a result, it has acquired a higher market share, especially in countries like India that have undergone rapid capacity expansion in the recent past. The manufacturing of solar PV cells is at an all-time high, mainly in countries like China, Japan and South Korea. Technological innovations in solar PV, however, have taken longer to reach the market. While there have been advancements in PV technology focused on increasing the energy yield per solar cell, these are largely restricted to pilot projects and laboratory tests. Over the next few years, as the solar PV market stabilises and the demand increases, the deployment of advanced technologies is expected to pick up pace. These new technologies provide solutions for many challenges plaguing the market such as those pertaining to land availability, greater energy demand and efficient energy generation. Renewable Watch analyses some of the upcoming technologies in the Indian and global solar PV space…

Bifacial PV

One of the most promising technological advancements in this space is the development of bifacial modules, which can produce energy from both sides of the panel. The energy generation per unit module increases significantly as the solar irradiation falls over two module faces. This doubles the effective surface area even as the total module area remains the same.

The bifacial modules are fitted with transparent backsheets whereas mono-facial modules have an opaque backsheet. The transparent sheet allows sunlight to pass through the module. The sunlight reflects on the surface underneath and falls on the other face of the module. In this way, the power is generated from both sides. The reflected irradiance, also known as albedo, is a primary performance determiner for bifacial modules. The additional output of these modules ranges from 5-50 per cent depending on the albedo, which varies with the surface on which the solar plant is installed. Albedo is the highest over snow-covered surfaces and lowest on water surfaces. Another determining factor is the conversion efficiency of cells on the rear side.

Bifacial modules increase power generation by an average of 20 per cent. Studies are currently being undertaken to understand the yield gain of vertically mounted bifacial modules in comparison to latitude-tilted solar plants. Several companies including LONGi, LG, Prism Solar and Sunpreme are manufacturing bifacial modules.

PERC

Advanced passive emitter and rear cell (PERC) technology is an innovative step beyond the traditional mono-facial solar modules. PERC involves the coming together of back surface passivation, front surface advanced passivation and anti-light induced degradation technologies. PERC technology works on the concept of greater light absorption by solar cells with the help of chemicals applied to the rear surface of the cell in the form of a dielectric passivation film. The construction of a PERC is similar to that of a monocystalline PV cell with an additional passivation layer. Energy is generated when sunlight falls on the front side of the cell. With PERC, the scattered photons are reflected back by the passivation sheet to the surface resulting in greater generation. The passivation layer allows the unabsorbed light a second chance to be absorbed by the cell, thereby increasing the yield. Moreover, the PERC prevents electrons from recombining and causing a blockage in the free movement of electrons through the solar cell. With reduced electron recombination, PERC helps improve cell efficiency. It also reflects specific wavelengths of light (greater than 1,180 nanometres), thereby lowering the temperature of the cell. The light at these wavelengths is typically absorbed by the metal backsheet in traditional solar cells. It increases the temperature, thus reducing the efficiency.

With PERC technology, the energy density of solar installations increases. As a result, fewer PERC cells are required to produce the same amount of energy generated using traditional solar cells. Such technology advancements are important, particularly in the Indian context where even limited land or rooftop space can be utilised to generate greater amounts of energy. According to a research, PERC will replace polycrystalline cells to reach around 160 GW by 2022 globally.

Heterojunction technology

Among crystalline silicon solar PV cells, silicon-based heterojunction technology (Si-HJT) has reported a record efficiency energy conversion of up to 26.6 per cent. This was achieved by Kaneka Corporation. According to Germany-based semiconductor technology major Meyer Burger, the HJT cell is developed by applying thin layers of doped and intrinsic amorphous silicon on both sides of n-type silicon wafers along with transparent, conductive oxide layers to absorb the power thus generated. As a result, the efficiency rates are high and the temperature coefficient is lower than that of conventional solar cells. One of the most important advantages of HJT is that the wide band-gap film used in these cells helps in the displacement of highly recombination-active contacts from the crystalline surface. The amorphous silicon films have the thickness of a few nanometres, more than that of traditional crystalline silicon cells. HJT is one among the emerging technologies that holds significant promise in the solar PV industry worldwide. In fact, work is in progress to make the technology affordable and mainstream. Singulus Technologies in association with the Solar Energy Research Institute of Singapore is working to develop cost-effective methods for the gigawatt-scale production of the technology. HJT can be used in both mono- and bifacial solar cells.

Perovskites

Perovskites have emerged as a low-cost alternative to crystalline silicon technology. There has been significant research and development in perovskite-based solar cells, resulting in efficiencies of over 20 per cent in lab-based PV devices. Perovskites are composed of organometal halides and have optoelectronic properties similar to traditional cells. These include a direct optical band gap, long carrier diffusion lengths and low exciton binding energy, as per a paper published by Amlan J. Pal. Moreover, these low-temperature solutions allow ease of processing. The processing requires low-cost materials. Meanwhile, the flexibility of substrates and the sequential formation of multiple layers make perovskites a suitable PV module technology, according to the paper.

Research is underway to improve the processes and efficiencies. Given the efficiency gains and cost effectiveness of perovskites, it can be used in large-scale module manufacturing. The research also explores the possibility of stacking perovskites over existing thin-film technologies for both electrical and optical compatibility. However, the degradation of materials is a challenge in the large-scale deployment of perovskites. This is a result of chemical reactions of the material in different atmospheric conditions. Research suggests that perovskites are susceptible to oxygen and moisture, ultraviolet radiation and high temperature. The reactions lead to severe irreversible degradation during the manufacturing process. Thermal decomposition of perovskites is another issue being faced by researchers.

These are some of the primary factors that impede the commercial development of this technology. It is expected that the technological roadblocks will be removed in due time with the implementation of the ongoing research.

Conclusion

As the solar power sector gains scale, PV technologies will be further upgraded. However, what remains to be seen is how and when these technologies will achieve commercialisation. Meanwhile, it is important to understand the domestic manufacturing processes, the cost of these technologies, and the economics involved if imported from other countries. These PV technologies have the potential to enable large-scale solar power adoption and achieve higher efficiencies at lower prices.

By Ashay Abbhi

GET ACCESS TO OUR ARTICLES

Enter your email address