Atomic layer deposited ZnO:Al for nanostructured silicon heterojunction solar cells

For transparent front contacts of nanostructured silicon solar cells aluminum doped zinc oxide (ZnO:Al), deposited by atomic layer deposition (ALD), is investigated. For this purpose it is crucial that the ZnO:Al layer covers the nanostructures conformally. ZnO:Al deposition at a temperature of 225 °C, compatible with the underlying solar cell structures, yields a resistivity of 1.2×10−3 Ω cm and 85% mean optical transmittance in the VIS–NIR range (<1300 nm). The complex dielectric function of the ALD–ZnO:Al is determined by fitting optical spectra with a multi-oscillator model. An investigation of the layer structure shows a preferential growth in the c-direction of the hexagonal ZnO crystal and 100–200 nm long wedge-shaped crystallites. I–V measurements on planar ZnO:Al/a-Si:H(n/p)/c-Si(n+/p+) test structures reveal the nature of the ZnO:Al contact to both n- and p-type a-Si:H. Simple planar solar cells exhibited an excellent rectification behavior and open circuit voltages VOC=620–640 mV.The feasibility of nanostructure silicon heterojunction solar cells is demonstrated by showing the conformal coating of deep Si nanowire structures.

► ALD–ZnO:Al has a resistivity of 1.2 m Ω cm at a-Si:H compatible temperatures.
► Nanowires and Black silicon can be coated uniformly by ALD.
► ALD–ZnO:Al/a-Si:H(p/n) contacts are Ohmic if the a-Si:H doping is sufficiently high.
► ALD offers the fabrication of nanostructure silicon heterojunction solar cells.