Abstract
Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Current record efficiency solar cells nearly all use advanced heterojunction contacts that simultaneously provide carrier selectivity and contact passivation. One remaining challenge with heterojunction contacts is the tradeoff between better carrier selectivity/contact passivation (thicker layers) and better carrier extraction (thinner layers). Here we demonstrate that the nanowire geometry can remove this tradeoff by utilizing a permanent local gate (molybdenum oxide surface layer) to control the carrier selectivity of an adjacent ohmic metal contact. We show an open-circuit voltage increase for single indium phosphide nanowire solar cells by up to 335 mV, ultimately reaching 835 mV, and a reduction in open-circuit voltage spread from 303 to 105 mV after application of the surface gate. Importantly, reference experiments show that the carriers are not extracted via the molybdenum oxide but the ohmic metal contacts at the wire ends.
Highlights
Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device
Our results demonstrate that the nanowire geometry allows for a traditional heterojunction layer to act as a surface gate, increasing the local hole concentration and thereby providing excellent carrier selectivity by changing the effective doping concentration, without changing the impurity doping level at the contact
We have demonstrated a contact geometry where surface layers traditionally used as heterojunctions can be placed next to, instead of underneath, the metal contact to improve carrier
Summary
Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Nanowire photovoltaics can in principle decouple the carrier selectivity and extraction functions of the heterojunction by using the extreme surface sensitivity to control electron and hole concentrations in the vicinity of the contact Such an approach is commonly used in electronics where an electrostatic gate voltage can drastically alter the carrier concentration in a narrow surface channel adjacent to electrical contacts, causing accumulation, depletion or even inversion without the need for an interfacial layer in between the contact and channel. Our results demonstrate that the nanowire geometry allows for a traditional heterojunction layer to act as a surface gate, increasing the local hole concentration and thereby providing excellent carrier selectivity by changing the effective doping concentration, without changing the impurity doping level at the contact. In contrast to traditional heterojunction contacts, the surface gate approach does not require conduction through the often resistive heterojunction contact material itself, making it possible to use very thick surface gate layers without introducing a charge carrier extraction barrier
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