Efficiency Gains: Advancements in solar PV technologies

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.