Enhanced Near‐Bandgap Response in InP Nanopillar Solar Cells

Abstract

the performance of photocathodes for solar hydrogen production. [ 9 ] Compared to a planar reference, the implementation of nanopillars improved both photocurrent and onset potential, but the exact mechanism for this improvement remained unclear.Here we investigate the effect of nanopillar texturing on the performance of InP solar cells. On the optical level, nanopillars minimize the refl ectance over a broad spectral range. Mapping the current generated by an electron beam across the cell cross-section, we further show that texturing increases the effective minority carrier collection length leading to improved photocurrent and open-circuit voltage and a power conversion effi ciency of 14.4%. Results and DiscussionNanopillars are fabricated via maskless, lithography-free reactive ion etching (RIE). The entire device fabrication avoids the use of expensive metal-organic precursor gases necessary for the fabrication of III-V solar cells via traditional chemical vapor deposition. In addition, we recently introduced a thinfi lm vapor-liquid-solid growth technique for the growth of high-quality InP thin fi lms (1-3 µm) on cheap non-epitaxial substrates, [ 10 ] enabling future elimination of the thick (350 µm) expensive InP wafers used in this work which, together with the precursor gases, represent the dominant cost drivers for III-V photovoltaics. [ 11 ] A sketch of the fabrication fl ow is shown in Figure 1 a. Zndoped p-type InP wafers with a carrier density of 3 × 10 17 cm −3 are etched in hydrochloric (HCl) acid (6%) for 30 s to remove the native oxide. An ohmic backcontact is formed by sputtering a thin layer of Zn (20 nm) capped with a Au layer (100 nm) and subsequent annealing at 420 °C in forming gas (H 2 :N 2 = 5%) for 45 min. The front side is wet etched again in HCl (6%) and dry etched in a H 2 :CH 4 :Ar plasma (40 sccm: 10 sccm: 10 sccm) at a pressure of 110 mTorr, a power density of 0.5 W cm −2 and a bias voltage of 500-550 V to form the nanopillars. Following oxidation in a O 2 plasma [ 12 ] and etching in HCl (3%) for 30 s to remove undesired metallic indium residues and the defective InP layer (see discussion below) formed during the RIE process at the pillar surface, a conformal transparent conductive indium tin oxide electrode (ITO, In 2 O 3 :SnO = 90%:10%, 35 nm) is deposited by RF sputtering at room temperature atThe effect of nanopillar texturing on the performance of InP solar cells is investigated. Maskless, lithography-free reactive ion etching of InP nanopillars improves the open-circuit voltage, reduces refl ectance over a broad spectral range, and enhances the near-bandgap response compared to a fl at, non-textured cell with comparable refl ectance in the infrared. Electron-beam induced current measurements indicate an increased effective minority carrier collection length. The response at short wavelengths decreases due to the formation of a defective surface layer with strong non-radiative recombination. Plasma oxidation and wet etching partially restore the blue response resulting in a ...

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