Increased Optoelectronic Quality and Uniformity of Hydrogenated p‐InP Thin Films

The thin-film vapor−liquid−solid (TF-VLS) growth technique presents a promising route for high quality, scalable, and cost-effective InP thin films for optoelectronic devices. Toward this goal, careful optimization of material properties and device performance is of utmost interest. Here, we show that exposure of polycrystalline Zn-doped TF-VLS InP to a hydrogen plasma (in the following referred to as hydrogenation) results in improved optoelectronic quality as well as lateral optoelectronic uniformity. A combination of low temperature photoluminescence and transient photocurrent spectroscopy was used to analyze the energy position and relative density of defect states before and after hydrogenation. Notably, hydrogenation reduces the relative intragap defect density by 1 order of magnitude. As a metric to monitor lateral optoelectronic uniformity of polycrystalline TF-VLS InP, photoluminescence and electron beam induced current mapping reveal homogenization of the grain versus grain boundary upon hydrogenation. At the device level, we measured more than 260 TF-VLS InP solar cells before and after hydrogenation to verify the improved optoelectronic properties. Hydrogenation increased the average open-circuit voltage (VOC) of individual TF-VLS InP solar cells by up to 130 mV and reduced the variance in VOC for the analyzed devices. ■ INTRODUCTION InP is not only used in photocathodes, photodetectors, and lasers, but it is also an attractive absorber material for thinfilm solar cells due to its suitable optoelectronic properties, such as direct band gap, low unpassivated surface recombination velocity, and high electron mobility. Its band gap of 1.34 eV ideally matches the terrestrial solar spectrum which translates into a theoretically maximum solar conversion efficiency of 31% (under terrestrial irradiation using a single p−n junction). Our recently developed thin-film vapor− liquid−solid (TF-VLS) growth platform presents a promising route for the cost-effective fabrication of high quality InP. In a first demonstration of device applications using the TF-VLS process, as grown InP was doped p-type with Zn by an ex-situ doping process and fabricated into solar cells. Promising efficiencies of up to 12.1% and a VOC of 695 mV were obtained using a n-TiO2/p-InP heterojunction architecture. 8 Despite these promising results, the VOC is less than what has been reported for a InP wafer-based device, which displays a VOC of 785 mV. Received: March 29, 2016 Revised: June 8, 2016 Article