This has been achieved with an innovative, silicon nanorod based concept. The amount of active photovoltaic material (Si) can be significantly reduced by growing light-trapping nanorod "forests" (thickness from <1 µm to a few µm at most) on cheaper substrates such as glass or flexible foils. This has already led to over 9 % energy conversion efficiencies with very good long-term stabilities of cells. Due to their effective 3D geometry, the nanorod forests have high active surface area which enables efficient light absorption – much more efficient than in conventional 2D thin film solar cells. Also, the location of the p-n junction is much closer to the surface than in normal solar cells, which radically improves minority carrier charge transport and thus the amount of electricity that can be extracted from the cell.
Due to the micrometer/sub-micrometer dimensions of the nanorod forests (dense packing, rod diameters of typically few hundreds of nm and lengths <1 µm) ALD has proven to be ideal technique for manufacturing some of the most crucial cell components. To prevent recombination losses in the active photovoltaic layer, and thus cell efficiency decrease, a recombination barrier i.e., passivation layer needs to be coated on the rods’ surface. An ultrathin ALD-deposited Al2O3 film serves ideally for this purpose, and the gas-phase, surface-controlled and self-limiting nature of the ALD process ensures that even the deepest and narrowest nooks and crannies between-the-rods will be reliably covered with 100 % uniform, conformal and pinhole- and defect-free passivation film.
Another central cell component where ALD has shown its indispensability is the transparent conductive oxide (TCO) layer that works as the current collector on the top of the cell. Different TCO deposition methods were investigated in the course of the project, and ALD turned out to be the ideal method regarding both the TCO film quality and the scalability of the technique, due to Picosun’s fast, efficient and easy-to-use HVM (High Volume Manufacturing) batch ALD system, which was developed specifically during the project ROD-SOL.
"Solar photovoltaics still remains one of the fastest growing industries in the world. To enable more efficient utilization of this free, clean energy, the efficiencies of the solar cells have to increase and their manufacturing costs decrease. ROD-SOL’s silicon nanorod cell concept shows promising potential to this, and we at Picosun have been especially satisfied with the ALD’s central role in realizing this novel, innovative, high efficiency solar electricity converter", states Picosun’s Managing Director Juhana Kostamo.
For further information: www.picosun.com.