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In the PV industry, innovative manufacturing processes, new materials, solar cells, and modules designs are constantly being developed to increase the device performance and lower the final energy cost. One such innovative technology claiming an ever growing share of the solar panel market is heterojunction solar cells. Heterojunction solar cells (HJT), variously known as Silicon heterojunctions (SHJ) or Heterojunction with Intrinsic Thin Layer (HIT), are a family of photovoltaic cell technologies based on a heterojunction formed between semiconductors with dissimilar band gaps. They are a hybrid technology, combining aspects of conventional crystalline solar cells with thin-film solar cells.
Heterojunction solar cells include two different technologies in one cell: a crystalline silicon cell sandwiched between two layers of amorphous “thin-film” silicon. This enables increased efficiency of the panels and more energy to be harvested as compared to conventional silicon solar panels. Thin-film silicon is amorphous. Unlike crystalline silicon, commonly used in panels, amorphous silicon does not have a regular crystalline structure. Instead, the silicon atoms are randomly ordered. As a result, manufacturing this type of solar cell is less expensive.

Lower cost and flexibility in the type of materials that amorphous silicon can be deposited on are among the greatest advantages of thin-film solar cells. In heterojunction solar cells, a conventional crystalline silicon wafer has amorphous silicon deposited on its front and back surfaces. This results in a couple of layers of thin-film solar that absorb extra photons that would otherwise not get captured by the middle crystalline silicon wafer.

Now (2023), scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) managed to enhance the efficiency of their solar cells by means of re-designing the materials in the cell stack.

With the help of both computational and experimental studies, the scientists made a gallium arsenide (GaAs) heterojunction solar cell using dynamic hydride vapor phase epitaxy (D-HVPE) with a certified efficiency of 27%. III-V solar cells get their name from the position of the elements used to make them on the periodic table of elements and are widely used to power space-faring technologies. D-HVPE offers the potential to be a lower-cost method of synthesizing these cells compared to incumbent techniques.

Alongside the GaAs base layer, the solar cell included an emitter layer of gallium indium arsenide phosphide (GaInAsP). The two different layers made up the heterojunction. To get a better understanding of possible efficiencies, the researchers modelled the effect of varying the zinc doping density and bandgap of the emitter layer. This was achieved by varying the relative concentrations of gallium, indium, arsenic, and phosphorus during layer growth, on cell efficiency. The researchers then designed cells drawing on the results of the modeling and achieved model-predicted efficiency enhancements. The rear heterojunction solar cell that served as a baseline used an emitter consisted of GaInP and had a reported efficiency of 26%. Reducing the doping in the emitter and changing its composition from GaInP to the lower bandgap GaInAsP Helped increase the efficiency to 27% even though the rest of the device was exactly the same.

In view of the ongoing energy transition heterojunction solar cell research is among the chief interests of governments and energy companies. In 2022, scientists created silicon heterojunction cells with improved rear contact consisting of a p-type doped nanocrystalline silicon and a tailored transparent conductive oxide. Because of the low contact resistance of hole-selective contacts (< 5 mΩ·cm² ), a high power conversion efficiency of 26.74% as well as a record filling factor (FF) of 86.48% were achieved on industrial-grade silicon wafers (274 cm² , M6 size). The electrical properties of the modified silicon heterojunction cells were thoroughly analyzed in comparison with the normal p-type transporting layer counterparts ( i.e. , amorphous silicon), and the improved charge carrier transport were also fully shown.

Image: Optical Performance

Source: Hao Lin, Miao Yang, Xiaoning Ru, Genshun Wang/ 26.7% efficiency silicon heterojunction solar cells achieved by electrically optimized nanocrystalline-silicon hole contact layers/ Research Square, December 2022/ DOI:10.21203/rs.3.rs-2402141/v1/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2023, scientists managed to boost the PCEs of tandem devices with front-side flat Si wafers. By using 2,3,4,5,6-pentafluorobenzylphosphonic acid (pFBPA) in the perovskite precursor ink that non-radiative recombination near the perovskite/C60 interface was suppressed, and through using SiO₂ nanoparticles under the perovskite film the enhanced number of pinholes and shunts introduced by pFBPA was reduced, while [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid was employed as a hole transport layer. By means of integrating a perovskite cell featuring these developments with a Si cell, reproducible PCEs of 30±1% and a certified maximum of 30.9% were reached for an active area of 1cm².

Image: Pentafluorobenzylphosphonic acid (pFBPA) as an additive. a, Chemical structure of 2,3,4,5,6-pentafluorobenzylphosphonic acid (pFBPA). b, Open circuit voltage (Voc), fill factor (FF) c, pseudo-FF (pFF) and the series resistance near the maximum power point (Rs) for single-junction 0.1 cm2 perovskite solar cells on glass/ITO/2PACz substrates with varying pFBPA concentration in the perovskite precursor ink.  d, Depth profiles of F-, CF- and P- in finished cells with 5 mM pFBPA in the precursor, as measured by secondary ion mass spectroscopy (SIMS). e-f, X-ray photoelectron spectroscopy (XPS) spectra of the Pb 4f core level (e) and the energy levels of the vacuum (Evac), conduction (Ec) and valence band (Ev) (f) of bare perovskite films with and without pFBPA. g, Non-radiative recombination loss (qVnon-rad) in half-finished perovskite cells on glass/ITO substrates extracted by steady-state photoluminescence quantum yield measurements. h, Scanning electron microscopy images of perovskite absorbers with and without pFBPA on glass/ITO/2PACz substrates viewed from the top side. The yellow arrow indicates the location of a pinhole as an example

Source: Deniz Turkay, Kerem Artuk, Xin-Yu Chin, Daniel Jacobs/ High-efficiency (>30%) monolithic perovskite-Si tandem solar cells with flat front-side wafers/ Research Square, June 2023/ DOI:10.21203/rs.3.rs-3015915/v1/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

The new gallium-arsenide solar cells hold several benefits: The research shows up a road map to improving the performance of solar cells via optimization of the doping and bandgap of a device layer called the “emitter” to minimise the impact of defects on device efficiency. Using a heterojunction structure, with carefully designed emitter properties, helps reduce the adverse impact of these defects on efficiency, even though nothing has been done to reduce their concentration. The relative efficiency improvement balances defect concentration. The results could be applied to materials beyond III-Vs that use heterojunctions such as silicon, cadmium telluride, or perovskites.

The benefits of heterojunctions are generally known, although experimental demonstrations of III-V heterojunctions are limited to a handful of combinations. The research gave a roadmap to future heterojunction solar cell design which will enable cheaper and mor efficient solar cells.

By the Editorial Board