Design Principles for Photovoltaic Devices Based on Si Nanowires with Axial or Radial p–n Junctions

Abstract

Semiconductor nanowires (NWs) are a developing platform for electronic and photonic technologies, and many demonstrated devices utilize a p-type/n-type (p-n) junction encoded along either the axial or radial directions of the wires. These miniaturized junctions enable a diverse range of functions, from sensors to solar cells, yet the physics of the devices has not been thoroughly evaluated. Here, we present finite-element modeling of axial and radial Si NW p-n junctions with total diameters of ~240 nm and donor/acceptor doping levels ranging from 10(16) to 10(20) cm(-3). We evaluate the photovoltaic performance of horizontally oriented NWs under 1 sun illumination and compare simulated current-voltage data to experimental measurements, permitting detailed analysis of NW performance, limitations, and prospect as a technology for solar energy conversion. Although high surface-to-volume ratios are cited as detrimental to NW performance, radial p-n junctions are surprisingly insensitive to surface recombination, with devices supporting open-circuit voltages (V(OC)) of ~0.54 V and internal quantum efficiencies of 95% even with high surface recombination velocities (SRVs) of 10(5) cm/s. Axial devices, in which the depletion region is exposed to the surface, are far more sensitive to SRV, requiring substantially lower values of 10(3)-10(4) cm/s to produce the same level of performance. For low values of the SRV (<100 cm/s), both axial and radial NWs can support V(OC) values of >0.70 V if the bulk minority carrier lifetime is 1 μs or greater. Experimental measurements on NWs grown by a vapor-liquid-solid mechanism yield V(OC) of 0.23 and 0.44 V for axial and radial NWs, respectively, and show that axial devices are limited by a SRV of ~7 × 10(3) cm/s while radial devices are limited by a bulk lifetime of ~3 ns. The simulations show that with further development the electrical characteristics of 200-300 nm Si NWs are sufficient to support power-conversion efficiencies of 15-25%. The analysis presented here can be generalized to other semiconductor homo- and heterojunctions, and we expect that insights from finite element modeling will serve as a powerful method to guide the design of advanced nanoscale structures.

Semiconductor nanowire solar cells: synthetic advances and tunable properties

“…The ability to pattern arrays of NWs over large areas, at high density, and with robust control of their periodicity would constitute a breakthrough. Furthermore, NW design [70] and synthesis efforts should minimize the size and doping heterogeneity of NWs, because variations in such parameters limit the overall power conversion efficiency of the system [71]. Finally, though the systems cost of an assembled NW photovoltaic cell is difficult to quantify at present, synthetic advances, use of cheaper catalysts, and adoption of lower-temperature processes will drive cost reductions.…”

Semiconductor nanowires: a platform for exploring limits and concepts for nano-enabled solar cells

“…Recently, finite element method (FEM) simulations 96 showed that both axial and radial NW PVs are capable of achieving high V OC values of ~0.7 V. For axial NWs, simulations show that surface recombination near the exposed junction is primarily responsible for limiting V OC below 0.7 V. For radial NWs, bulk minority carrier lifetimes < 10 ns are primarily responsible for limiting V OC . Experimental and calculated results clarify the fundamental device physics limits that must be surmounted to achieve high V OC NW PVs.…”

“…Furthermore, to reduce packing defects during assembly of NW arrays, methods should maximize end-to-end registry of NWs and reject small NW fragments leading to voids. In addition, NW design 96 and synthesis efforts should minimize NW heterogeneity with regards to size and doping, because variations in these parameters can limit the power conversion efficiency of the system 97 . Finally, although the total systems cost of an assembled NW photovoltaic is difficult to quantify rigorously, synthetic advances including nanowire growth by aerotaxy 98 , use of cheaper metal catalysts and lower temperature processes 52 will likely be a primary driver of cost reductions.…”

Particle Suspension Reactors and Materials for Solar-Driven Water Splitting

“…Nano-scale particles present a shorter distance for collection of minority charge carriers, but require p-i-n doping distributions in order to support a space-charge region and assist in charge separation. [28][29][30][31] 10,32 Moreover, in comparison to smooth micron-sized particles, smooth nanoparticles present more surface area per particle volume which lessens cocatalyst requirements. However, micron-sized particles absorb more light, because the optical path length depends on particle size and micron-sized particles also more effectively scatter light likely increasing the number of times light passes through each particle.…”