Photovoltaic cells should pay back the energy invested in their production during their operational lifetime, and for crystalline cells this implies repaying a relatively high thermal budget used in the production of the basic wafers. The two approaches to ensuring a positive energy balance are to reduce the amount of crystalline material and to increase power conversion efficiency. Thin-film cells take the most extreme route to reducing material quantity but often have concomitant reductions in efficiency; high efficiency cell compositions are sometimes combined with low cost optics to increase their effective area by concentrating direct sunlight. The manufacturers of crystalline Si cells aim to both reduce wafer thickness and increase cell efficiency without optical concentrators. State-of-the-art Si cells such as those with efficiencies of 25% (from Green’s research at the University of New South Wales) may use a variety of means to improve optical absorption and to reduce carrier recombination. One feature is to recess the top contact grid in laser cut slots, which maintains a high conductor cross-section without shadowing too much of the cell surface (“laser grooved buried contact cells”). Reducing the wafer thickness will increase the effect of carrier recombination at the rear surface, as light will penetrate further towards this interface. Modelling shows that halving wafer thickness from the usual 300µm requires an improved rear surface passivation method in order to retain high efficiency. The “Highpoint” research project at Heriot-Watt University and Narec, Northumberland, supported by TSB (TP/8/LOW/6/I/Q3033K), seeks an optimised plasma-enhanced chemical vapour deposition (PECVD) process for such a passivation layer. Both amorphous silicon and silicon nitride chemistries, using either RF or microwave excitation, have been investigated, resulting in films with a sufficiently low rear surface recombination rate that solar cell efficiency should reach 20% for 150µm wafers.
