Material efficiency measures in solar cells and novel concepts
Based on the arguments presented in the previous chapter, it is clear that further improving resource efficiency in renewable technologies is vital in order to ensure their sustainability and cost-effective wide-scale implementation. Resource efficiency in photovoltaics can be achieved via direct or indirect measures. A direct material efficiency measure can be the improvement in the production process of PV products, like solar cells and modules. An example of this is the reduction of kerf losses which occur from the slicing of silicon ingots into thin wafers. By 2018, almost the entire c-Si PV industry switched from the slurry-based wafer sawing to the diamond wire sawing, which reduces the amount of silicon consumed per wafer by approximately 15% (ITRPV 2017). The further material reduction can be expected by using kerfless watering technologies, thinner diamond wires as well as increasing the rate of recycling for the silicon residue during the sawing process. In addition to that, efforts are undertaken in order to reduce the use of silver in cell metallisation since it is currently — and will most probably remain — the most expensive metal in a crystalline solar cell. Besides reducing the amount of silver via improved screen printing and cell interconnection, feasible ways of substituting it with a mixture of copper and nickel are already available, which allows for comparable efficiency.
Material efficiency can also be indirectly achieved by improving cell efficiency, which reduces the specific demand for all relevant materials, as well as reduction of cell thickness, which improves the material efficiency of silicon. The most common cell thickness of a silicon-based solar cell is 180 pm.
Nonetheless, efforts are being carried out to reduce this, so that thickness as low as 120 pm can already be expected by 2030 for mono c-Si solar cells. In terms of efficiency, the maximum achievable efficiency for c-Si solar cells is limited at 29.4% (Polman et al. 2016). The current maximum recorded c-Si solar cell efficiency is 26.7%, which shows that there is still slight room for improvement (Green et al. 2019). Furthermore, the module efficiency, which lies currently around 17—20%, can also be improved further. For example, losses in cell interconnections can be reduced by utilising shingles solar cells whereas shading losses can be reduced by using frameless modules.
Improved concepts such as the Silicon heterojunction (SHJ) solar cell can help in further reducing specific material content of solar cells due to their high efficiency. SHJ has very high conversion efficiency due to distinctive surface passivation. Compared to other silicon-based solar cells such as Passivated Emitter and Rear Cell (PERC) or Back Surface Field (BSF), the main advantages of SHJ include the simple and low-temperature manufacturing processes, which decrease the thermal budget and thus the cost of the cell (Louwen et al. 2016). However, the production facility is not yet available on a large scale as main players in the industry often opt to optimise their current portfolio and respective products rather than adopting a new cell concept and corresponding production line. However, this is most likely to change in the future once the cost advantages increase, driven by significant learning effects.
A further increase in efficiency above 30% can be achieved by the transition from one to two or more p-n-junctions, named tandem or multijunction solar cells. In terms of tandem solar cells, one of the most widely discussed concepts is the perovskite cell in combination with Si-cells. This solar cell has gained much attention in the past few years due to the enormous increase in efficiency in a very short period of time, as well as the utilisation of inexpensive materials. Nonetheless, the main challenges faced by perovskite cells is the instability issue and the use of lead which is toxic and faces the uncertainty of being banned in the EU Directive for Hazardous Substances (RoHS). Research efforts are already being undertaken to substitute lead with other metals such as bismuth. Nonetheless, the power conversion efficiency is still low compared to the requirement for large scale commercial application. Another promising multijunction concept is the monolithic application of III-V solar cells onto a Si bottom cell. A triple junction configuration has demonstrated to reach efficiencies beyond 37% for terrestrial applications, which clearly offers an enormous material-saving potential (NREL 2019).