Operation of lightly doped Si microwires under high-level injection conditions

Abstract

The operation of lightly doped Si microwire arrays under high-level injection conditions was investigated by measurement of the current-potential behavior and carrier-collection efficiency of the wires in contact with non-aqueous electrolytes, and through complementary device physics simulations. The currentpotential behavior of the lightly doped Si wire array photoelectrodes was dictated by both the radial contact and the carrier-selective back contact. For example, the Si microwire arrays exhibited n-type behavior when grown on a n + -doped substrate and placed in contact with the 1,1 0 -dimethylferrocene +/0 -CH 3 OH redox system. The microwire arrays exhibited p-type behavior when grown on a p + -doped substrate and measured in contact with a redox system with a sufficiently negative Nernstian potential. The wire array photoelectrodes exhibited internal quantum yields of $0.8, deviating from unity for these radial devices. Device physics simulations of lightly doped n-Si wires in radial contact with the 1,1 0 -dimethylferrocene-CH 3 OH redox system showed that the carriercollection efficiency should be a strong function of the wire diameter and the carrier lifetime within the wire. Small diameter (d < 200 nm) wires exhibited low quantum yields for carrier collection, due to the strong inversion of the wires throughout the wire volume. In contrast, larger diameter wires (d > 400 nm) exhibited higher carrier collection efficiencies that were strongly dependent on the carrier lifetime in the wire, and wires with carrier lifetimes exceeding 5 ms were predicted to have near-unity quantum yields. The simulations and experimental measurements collectively indicated that the Si microwires possessed carrier lifetimes greater than 1 ms, and showed that radial structures with micron dimensions and high material quality can result in excellent device performance with lightly doped, structured semiconductors. Broader contextStructuring semiconductors on the nano-and microscale is a promising route to enable exible, lightweight photovoltaics and efficient solar fuels devices. Wire devices with radial junctions should allow for both the efficient absorption of light along the axial dimension of the wire and radial collection of excited photocarriers, therefore enabling the fabrication of efficient devices based on inexpensive materials with low electronic quality. Prior systematic experimental and device physics modeling has largely focused on arrays of doped semiconductors, including Si radial p-n junction devices. The work described herein focuses on the device performance and device physics of lightly doped Si microwires, to allow for new device architectures and increased understanding of devices using other semiconductor materials with less controllable doping.

Physical Modeling of Photoelectrochemical Hydrogen Production Devices

“…There are some similarities, but also striking differences, to individual nanoparticles. The results indicate that high quantum yields (short circuit current) can be achieved even with short carrier diffusion lengths by reducing the wire radius, but too short a radius actually results in very low quantum yield 95,96 . This is contrary to the nanoparticles that show monotonic increase when particle size decreases 94 .…”

“…Because the high injection criterion depends also on the dopant density, increasing it allows high charge collection efficiencies, if a sufficient increase is possible within material constraints 44,96 . Near the substrate contact the collection efficiency remains high, because the majority carriers do not need to travel a long distance, so they can be collected before recombining 95 . This situation with a short collection path for both minority and majority carriers is similar to individual nanoparticles, but farther from the contact the longer collection path of majority carriers makes the collection efficiency more sensitive to charge transport and recombination 95 .…”

“…This is contrary to the nanoparticles that show monotonic increase when particle size decreases 94 . The yield is reduced when the wire radius becomes so small that the inversion layer (where minority carrier concentration is higher than majority carrier concentration) near the surface occupies a significant fraction of the total electrode volume, so that 31 recombination near the surface dominates device operation 95,96 . This situation corresponds to operation in high-level injection conditions, meaning that electron excitation rate (i.e.…”

“…This situation corresponds to operation in high-level injection conditions, meaning that electron excitation rate (i.e. light intensity) is sufficiently high that the concentrations of electrons and holes are higher than the dopant density and approximately equal to each other, which increases the recombination rate in the device 95,96 . This in turn is typically considered to reduce both current density and voltage significantly although it might be possible to achieve high currents and voltages with discrete selective contacts, analogously to point-contact solar cells 44,96 .…”

“…The operation of wire structures has been analyzed in detail with 2D simulations that included also the electric fields 44,95,96 . There are some similarities, but also striking differences, to individual nanoparticles.…”