Optical Absorption Enhancement in Freestanding GaAs Thin Film Nanopyramid Arrays

Although IIIV compound semiconductor multi-junction cells show the highest efficiency among all types of solar cells, their cost is quite high due to expensive substrates, long epitaxial growth and complex balance of system components. To reduce the cost, ultra-thin films with advanced light management are desired. Here effective light trapping in freestanding thin film nanopyramid arrays is demonstrated and multiple-times light path enhancement is realized, where only 160 nm thick GaAs with nanopyramid structures is equivalent to a 1 mu m thick planar film. The GaAs nanopyramids are fabricated using a combination of nanosphere lithography, nanopyramid metal organic chemical vapor deposition (MOCVD) growth, and gas-phase substrate removal processes. Excellent optical absorption is demonstrated over a broad range of wavelengths, at various incident angles and at large-curvature bending. Compared to an equally thick planar control film, the overall number of photons absorbed is increased by about 100% at various incident angles due to significant antireflection and light trapping effects. By implementing these nanopyramid structures, IIIV material usage and deposition time can be significantly reduced to produce high-efficiency, low-cost thin film IIIV solar cells.