Optical Absorption Enhancement in Freestanding GaAs Thin Film Nanopyramid Arrays

Although III–V compound semiconductor multi-junction cells show the highest effi ciency 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 fi lms with advanced light management are desired. Here effective light trapping in freestanding thin fi lm 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 μ m thick planar fi lm. 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 fi lm, the overall number of photons absorbed is increased by about 100% at various incident angles due to signifi cant antirefl ection and light trapping effects. By implementing these nanopyramid structures, III–V material usage and deposition time can be signifi cantly reduced to produce high-effi ciency, low-cost thin fi lm III–V solar cells.