Electrodeposition of Thin Film Semiconductors for Large and Small Area Solar Cell Applications

To increase the uptake of photovoltaic modules we must increase their efficiency in converting light to electricity whilst making them cheaper. Thin film solar cell technologies based on Cu(In,Ga)Se2 and CdTe absorber layers, already need less raw materials and less energy to fabricate than their crystalline silicon counterparts. Currently commercially, these semiconductors are synthesized using vacuum vapor deposition techniques. A more materially efficient and possibly lower energy cost method is to electrodeposit the components of the semiconductor and then anneal them in a chalcogen vapor. Electrodeposition involves the reduction of solvated ions to form deposited atoms on the surface of a conductive substrate. For the example of Cu(In,Ga)Se2, where composition control of the elements is critical, Cu can easily be deposited from aqueous solution, whereas In and Ga are more difficult to deposit due to a competing hydrogen evolution reaction. This can be overcome by using an ionic liquid in order to electrodeposit any In or Ga composition, free of competing reactions [1]. In fact, special ionic liquids where the cation only consists of ions to be reduced may be designed to achieve the deposition of metal layers in seconds [2]. Since deposits only form where there is contact between the electrolyte and the conducting substrate, it is possible to make small deposits either by shaping the solvent inside a macroscopic polymer gel [3] or by templating the substrate with a non-conducting material. Combined with the near 100% material utilization, this allows the material efficient deposition of semiconductors for concentrator micro-solar cells photovoltaic applications. These solar cells are designed to be used with concentrated light pushing their efficiency above normal limits imposed by standard 1 sun illumination. Bringing the solar cell size to the micro scale advantageously enables normal operating temperatures and a thin module design, similar to the regular flat plate module. Our initial results for 200 micron diameter sized solar cells show a near 5 % power conversion efficiency, similar to micro devices fabricated by vapor selective area deposition methods[4]. [1] J. C. Malaquias et al. Phys. Chem. Chem. Phys., vol. 16, no. 6, 2014. [2] M. Steichen et al. Chem. Commun., vol. 53, no. 5, 2017. [3] A. Malyeyev et al. publication in preparation [4] D. Correia et al. Results Phys., vol. 12, pp. 2136–2140, 2019.