Abstract
In order to fabricate photovoltaic (PV) cells incorporating light-trapping electrodes, flexible foil substrates, or more than one junction, illumination through the top-contact (i.e.: non-substrate) side of a photovoltaic device is desirable. We investigate the relative collection efficiency for illumination through the top vs. bottom of PbS colloidal quantum dot (CQD) PV devices. The external quantum efficiency spectra of FTO/TiO₂/PbS CQD/ITO PV devices with various PbS layer thicknesses were measured for illumination through either the top (ITO) or bottom (FTO) contacts. By comparing the relative shapes and intensities of these spectra with those calculated from an estimation of the carrier generation profile and the internal quantum efficiency as a function of distance from the TiO₂ interface in the devices, a substantial dead zone, where carrier extraction is dramatically reduced, is identified near the ITO top contact. The implications for device design, and possible means of avoiding the formation of such a dead zone, are discussed.
Highlights
Colloidal quantum dots (CQDs) are a promising material for use in high efficiency, low-cost photovoltaics due to their size-effect tuneability and compatibility with solution-based deposition
By measuring the efficiency of photocarrier extraction of our devices as a function of illumination wavelength, during either top or bottom illumination, and comparing it with that expected from a simple model of the carrier generation and extraction in such a device, we are able to infer the presence of a substantial ‘dead zone’, where absorption remains strong but carrier extraction is limited
To determine whether the PbS CQD layer of our devices is damaged during top contact deposition, devices with both top (ITO) and bottom (FTO) transparent contacts were fabricated as described above
Summary
Colloidal quantum dots (CQDs) are a promising material for use in high efficiency, low-cost photovoltaics due to their size-effect tuneability and compatibility with solution-based deposition. The fact that their bandgap can be tuned from 700nm to beyond 1800nm means that they can be used as the absorber material in single junction, tandem, and triple junction photovoltaics. For many photovoltaic device architectures, it is desirable to deposit a transparent contact on top of a photoactive layer without damaging it This enables the use of light trapping substrates that could help overcome the transitional absorption/extraction tradeoff faced by all photovoltaic devices [1,2]. Such a dead zone has important implications for device design, especially devices where illumination through the top contact is required
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