Nanophotovoltaics

Citation

(2012), "Nanophotovoltaics", Sensor Review, Vol. 32 No. 2. https://doi.org/10.1108/sr.2012.08732baa.007

Nanophotovoltaics

Article Type: Nanosensor update From: Sensor Review, Volume 32, Issue 2

As noted elsewhere in this issue, PV solar cells are increasingly being used to power sensors and many research groups are now investigating the role of nanomaterials in these. This emerging discipline, often termed “nanophotovoltaics”, involves the use of nanoscale materials and structures such as carbon nanotubes, semiconducting nanoparticles, quantum wells and dots, nanowires and nanoantennas. This research has several aims: to reduce the reliance on costly, crystalline silicon; to increase the energy conversion efficiency; to capture energy from longer (i.e. IR) wavelengths; and to develop lightweight and flexible rather than rigid cells. In terms of production devices, nanoparticles are presently the most widely used class of nanomaterials and an example of a company using these in its solar cells is Nanosolar, Inc. The cells are based on a proprietary ink formulation containing 20 nm diameter nanoparticles of copper indium gallium selenide which is printed onto a specially prepared aluminum alloy foil using a high throughput industrial printing technique. The resulting 2 km long roll of solar cells is completed by adding fingers and a back contact capable of carrying high currents with minimal optical and resistive loss. The foil is then slit and sheeted to form individual, flexible solar cells as shown in Figure 7.

Figure 7 Flexible solar cells incorporating nanoparticles of copper indium gallium selenide

A nanophotovoltaic technology being developed at the Lawrence Berkeley National Laboratory (LBNL) is based on radial p-n junctions fabricated from silicon nanowires. In this type of junction, a layer of n-type silicon forms a shell around a p-type silicon nanowire core. Each individual nanowire therefore acts as a PV device. As a result of this geometry, photo-excited electrons and holes travel much shorter distances to the electrodes, eliminating a charge-carrier bottleneck that often arises in a typical silicon solar cell. Photocurrent and optical transmission measurements revealed that the radial geometry array also greatly improves light trapping. Although the conversion efficiency was only around 5-6 per cent, the group was able to reduce both the quantity of, and the quality requirements for, silicon by using this junction configuration rather than conventional planar junctions. Allied work at the LBNL involves the fabrication of CdS-Cu₂S core-shell nanowires using a solution-based cation exchange reaction (Tang et al., Nature Nanotechnology, 6, pp. 568-72). The heterojunction prepared by this method is atomically well defined with low interface defects, enabling excellent charge separation and minimal minority carrier recombination. As a result, the device shows an excellent response to low light levels, compared to both planar solar cells and to previously reported nanowire cells.

Recent research at University of Texas at Austin and the University of Minnesota (Tisdale et al., Science, 328, pp. 1543-7) is representative of the work on quantum dots. This aims to increase the conversion efficiency of PV devices by capturing energy from a greater part of the electromagnetic spectrum. The work involved structures consisting of one or two monolayers of lead selenide (PbSe) quantum dots, ranging in diameter from 3.3 to 6.7 nm, deposited on atomically flat, single-crystal titanium dioxide (TiO₂). The films were chemically treated with either hydrazine or 1, 2-ethanedithiol to remove or substitute for the oleic acid present on the dots’ surfaces. It was shown that, when the PbSe quantum dots are excited by solar energy, the hot electrons can be transferred to the TiO₂ conductor. While several technical issues need to be resolved, this work illustrates the potential to capture hot electrons and if all of the energy of the hot carriers could be captured, conversion efficiencies could theoretically be increased to as much as 66 per cent.