Abstract
Growth approaches that limit the interface area between layers to nanoscale regions are emerging as a promising pathway to limit the interface defect formation due to mismatching lattice parameters or thermal expansion coefficient. Interfacial defect mitigation is of great interest in photovoltaics as it opens up more material combinations for use in devices. Herein, an overview of the vapor–liquid–solid and selective area epitaxy growth approaches applied to zinc phosphide (Zn3P2), an earth-abundant absorber material, is presented. First, we show how different morphologies, including nanowires, nanopyramids, and thin films, can be achieved by tuning the growth conditions and growth mechanisms. The growth conditions are also shown to greatly impact the defect structure and composition of the grown material, which can vary considerably from the ideal stoichiometry (Zn3P2). Finally, the functional properties are characterized. The direct band gap could accurately be determined at 1.50 ± 0.1 eV, and through complementary density functional theory calculations, we can identify a range of higher-order band gap transitions observed through valence electron energy loss spectroscopy and cathodoluminescence. Furthermore, we outline the formation of rotated domains inside of the material, which are a potential origin of defect transitions that have been long observed in zinc phosphide but not yet explained. The basic understanding provided reinvigorates the potential use of earth-abundant II–V semiconductors in photovoltaic technology. Moreover, the transferrable nanoscale growth approaches have the potential to be applied to other material systems, as they mitigate the constraints of substrate–material combinations causing interface defects.
Highlights
Performance in photovoltaic (PV) devices is often crippled by the presence of defects, to an extent by bulk defects, but most often interface defects are the limiting factor.[1−4] This may include dangling bonds due to incoherent interfaces, which facilitate charge recombination.[1−4] In addition, PV devices require several layers in order to effectively absorb the incoming light, to separate photogenerated electron−hole pairs, and to extract the charge carriers to perform work
By reducing two of the dimensions to nanometer lengths, one reduces the amount of strain energy build up at the interface due to lattice mismatch while activating additional elastic radial stress relaxation mechanisms.[7−9] various material systems have been explored in the form of nanowires, the bulk of the research is based on III−V nanowires grown by the vapor−liquid−solid (VLS) method.[10,11]
III−V nanowires have been of particular interest for photovoltaic applications and III−V integration on silicon for CMOS compatible optoelectronic applications.[12−15] With regard to PV applications, nanowires have been of particular interest as their nanophotonic properties, which allow for a reduction in the usage of critical raw materials, as a Special Issue: Exotic Materials and Innovative Concepts for Photovoltaics
Summary
Performance in photovoltaic (PV) devices is often crippled by the presence of defects, to an extent by bulk defects, but most often interface defects are the limiting factor.[1−4] This may include dangling bonds due to incoherent interfaces, which facilitate charge recombination.[1−4] In addition, PV devices require several layers in order to effectively absorb the incoming light, to separate photogenerated electron−hole pairs, and to extract the charge carriers to perform work. Single nanowire optical absorption measurements were performed on the zigzag nanowires in the 1.96−2.54 eV range using a suspended nanothermometer.[28] The zigzag nanowire showed a 5-fold greater optical absorptance when compared to gallium arsenide nanowires in this energy range This is explained through the large number of higher-order conduction bands creating a high density of states for above-bandgap energy transitions, increasing the absorptance and making it an ideal material for PV applications.[63,64] Further studies to discern the influence of stoichiometry and morphology on these properties are currently underway.
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