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Dead zones in colloidal quantum dot photovoltaics: evidence and implications

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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.

©2010 Optical Society of America

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Figures (5)

Fig. 1
Fig. 1 (a) Cross-sectional SEM image of a device prior to top contact deposition. The scale bar (bottom left) is 500 nm. (b) Electron energy band diagram of a depleted heterojunction (DH) device. (c) Internal quantum efficiency profile in the PbS layer for three DH devices with PbS thicknesses of 60nm, 120nm, and 230nm.
Fig. 2
Fig. 2 Carrier generation profile in the PbS layer of an FTO/TiO2/PbS CQD/ITO device that is 60 (top), 120 (middle), or 230 (bottom) nm thick, illuminated from the top (left) or bottom (right).
Fig. 3
Fig. 3 Measured (a-c) and calculated (d-f) EQE spectra, for illumination from either the top or bottom, for devices with PbS layers that are 60nm (a,d), 120nm (b,e), and 230nm (c,f) thick. The fitting parameters were: depletion width: 50nm, diffusion length: 10nm, dead zone width: 40nm, IQE: 0.5.
Fig. 4
Fig. 4 Carrier extraction profile for top (a-c) and bottom (d-f) illumination, for devices with a PbS layer that is 60nm (a,d), 120nm (b,e), or 230nm (c,f) thick. The devices have a depletion width of 60nm, diffusion length of 10nm, dead zone width of 40nm, and an IQE of 0.5.
Fig. 5
Fig. 5 I-V curves of FTO/TiO2/PbS CQD/(ITO or Au/ITO) devices under 975nm illumination at 100 mW/cm2.
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