By Rabindra Kumar Satpathy, Executive Committee and Board Member, International Solar Energy Society, Germany
Solar photovoltaic (PV) module and cell technologies have been evolving rapidly over the years, with advancements aimed at improving efficiency, durability and cost-effectiveness. Some of the commonly known solar cell technologies are crystalline silicon (both monocrystalline and polycrystalline), thin-film solar cells such as cadmium telluride and copper indium gallium selenide, and emerging technologies such as perovskite solar cells.
The standard back surface field (BSF) cell architecture based on crystalline silicon has dominated the solar PV industry for more than a decade and has now been extended to include passivated emitter and rear cell (PERC) technology. The key feature of PERC technology is the incorporation of a rear side passivation scheme into standard BSF cell technology.
Various solar PV technologies
PERC cell: The PERC cell, now serving as the “workhorse” of the solar PV industry, faces the key challenge of maintaining its prominent role through continuous performance improvement and cost reduction. In terms of cost reduction, the production of PERC cells has the advantage that the entire supply chain is aligned and standardised to this technology. Increasing the throughput of tools and implementing automation are the primary avenues for reducing the cost of manufacturing. One of the most recent approaches involves increasing the wafer size to up to 210 mm, which presents enormous challenges not only in cell manufacturing but also in module design and assembly, and potentially module reliability. In terms of efficiency improvement, the task is becoming increasingly difficult, as production efficiency has already reached 23.8 per cent and the practical efficiency limit of this structure is about 24.5 per cent.
Today, PERC-type silicon cells dominate the solar PV market. According to the International Technology Roadmap for Photovoltaic Report from April 2021, the market share of PERC/PERL/PERT/TOPCon silicon solar cells will exceed 70 per cent in the next 10 years, with TOPCon emerging as a dominant technology. In terms of efficiency, most perspectives seem to favour silicon/perovskite tandem technology, with projected efficiencies as high as 29.5 per cent in the manufacturing process by 2031.
Silicon heterojunction solar cells (SHJ): Heterojunction solar cells, also known as HIT cells, utilise passivating contacts based on a layer stack of intrinsic and doped amorphous silicon. Due to their superior surface passivation quality, SHJ cells hold the record for open-circuit voltage at one sun, measuring 750 mV, and can achieve an efficiency of 26.3 per cent. For SHJ technology to become mainstream, it must overcome various challenges such as the higher cost of tools for cell production. Further, there is a need to reduce silver usage or replace it with copper by developing Cu-plating technology, and minimise indium usage in the transparent conductive oxide layer.
Polysilicon-based passivating contacts (TOPCon): TOPcon is a newer solar manufacturing concept that involves introducing a thin oxide layer to the silicon cell to reduce recombination losses and increase cell efficiency. TOPCon technology is fundamentally compatible with the conventional silicon solar cell process.
TOPCon technology involves the addition of a thin tunnelling silicon dioxide layer (about 1.5 nm) and a doped polysilicon layer between the silicon substrate and the rear metal contact. In the case of an n-type substrate, a phosphorus-doped polysilicon layer is utilised as the rear contact structure. With the implementation of TOPCon, the dark current is reduced. Fraunhofer ISE has demonstrated an n-type cell with a Voc of 718 mV, a current density (Js) of 42.5 mA per cm2, a fill factor of 82.8 per cent and an efficiency of 24.2 per cent.
Perovskites: Perovskite solar cells have gained significant attention due to their high efficiency potential and low production costs. These cells utilise a perovskite-structured compound as a light-absorbing material, which can be processed using low-cost techniques such as printing. While challenges related to stability and scalability remain, perovskite solar cells have shown remarkable progress in recent years.
Perovskite solar cells (PSCs) represent an emerging, revolutionary PV technology based on metal halide perovskites (MHPs) such as methylammonium (MAPbI3) or formamidinium lead iodide (FAPbI3). MHPs combine several preferred characteristics for a PV absorber, such as direct bandgap with strong absorption coefficient, long carrier lifetime and diffusion length, low defect density, and easy tuning of composition and bandgap. In 2009, MHPs were first reported as sensitisers in dye cell configurations based on liquid hole-conducting electrolytes. In 2012, the demonstration of around 10 per cent efficient PSCs based on a solid-state hole conductor has triggered explosive research in this field. Within a decade of research, the performance of a single-junction PSC has skyrocketed to a certified efficiency of 25.2 per cent.
Tandem perovskite-silicon solar cells: The technology combines the strengths of perovskite solar cells, which offer high efficiency, and silicon solar cells, which offer long-term stability. By stacking a perovskite solar cell on top of a silicon solar cell, the overall efficiency can be enhanced while maintaining the durability of silicon.
While single junction PV has nearly reached its practical efficiency limits, tandem cells provide headroom to achieve significantly higher efficiencies. Tandem cells have the potential to facilitate further PV cost reductions by increasing efficiency and thereby reducing the impact of balance of systems costs.
Bifacial solar cells and modules: Bifacial solar modules capture sunlight from both sides, increasing the overall energy yield by utilising reflected and diffused light. These solar PV modules have seen increasing deployment, and further developments may include improved system designs and enhanced performance under various lighting conditions. The bifacial solar cell is designed to absorb light energy from both sides. The power gains in bifacial modules range between 5 per cent and 30 per cent depending on the cell design, the intensity of the light falling on the rear side due to site albedo, and mounting conditions.
Bifacial modules are now considered one of the standard technologies for ground-mounted applications, whether using a fixed tilt structure, a tracker or even a vertical north-south orientation. The n-type PERT or TOPCon solar cells can reach 80–95 per cent bifaciality, while p-type PERC solar cells typically have 65-75 per cent bifaciality. The bifaciality factor for HJT cells is 90 per cent. The additional annual energy provided by bifacial modules is highly dependent on the reflectivity of the ground, which creates the albedo. It is a measure of the diffuse reflection of solar radiation and bifaciality. It varies from a few percentage points, typically 6 per cent for PERC and 9 per cent for TOPCon or SHJ cells, up to about 25 per cent in the best cases such as ground with very high reflectivity, like snow or white gravel.
Key trends in the solar PV space
Solar PV modules are now being designed with half-cells, which effectively reduce the loss caused by series resistance in cell interconnections by a factor of 4. This improves the power output by about 1.5 per cent under standard testing conditions. Meanwhile, the number of busbars has increased from 2 to 5, while maintaining the same shading factor. Moreover, there is a switch from flat ribbons to multi-busbars using round wires (9-15 wires) for interconnections.
Large-area solar cells have seen an increase in size from 125 mm square wafers in 2010 to 166 mm in 2020, and 182 mm and 210 mm wafers in 2021. Meanwhile, thinner wafers are being used with less kerf-loss in slicing. The diameter of wires used for slicing silicon wafers has decreased from about 150 ?m to less than 50 ?m over the past decade.
Moreover, bifacial solar modules using glass/ glass and glass/transparent back sheets and modules with large wattage capacities of up to 650 W are gaining popularity. In thin-film solar cell technology, CdTe and CIGS continue to thrive.
Emerging solar systems
Agrivoltaics refers to a method that allows the simultaneous use of agricultural land for food production and PV power generation. Solar panels mounted above a field can generate electricity while grain, fruit and vegetable crops grow underneath. This enables the dual use of land. Solar power deployment on open land can be substantially expanded without using valuable resources on fertile arable land. The yields from solar PV arrays and photosynthesis are optimised through targeted light management. These special PV systems offer farmers an additional source of income.
The integration of solar PV with energy storage systems, such as batteries, is gaining momentum. This combination allows for better utilisation of solar energy, increased self-consumption and improved grid stability, reliability and availability of solar power, especially during cloudy periods or at night.
Moreover, floating solar PV systems are becoming increasingly popular, particularly on waterbodies such as lakes, ponds and canals. Further, there is a growing trend in the utilisation of 31 per cent AMO-efficient inverted metamorphic type III-V compound-based multisolar PV cells for satellite applications.
Lastly, as solar energy continues to gain popularity, the design and appearance of solar panels are becoming increasingly significant. Researchers are working on developing solar cells that can be seamlessly integrated into building materials, such as roof tiles or facades, to enhance their visual appeal.