Publication date: 1st April 2013
Quantum dots(QDs) based on compound semiconductors have been gaining attention as promising materials forsolar cellssincethe position of their exciton absorption bandscan be tuned by selecting suitable semiconductorcompositions, QD sizesand so on, thereby coveringthe entire solar spectral region. In this presentation, two types of PbS-QD solar cells were fabricated: the one is an FTO/planar ZnO/PbS/Au solar cell (Planar type cell, Fig.1(a)), and the other an FTO/ZnO nanowire array:PbS/Au solar cell (NW-type cell, Fig.1(b)). The photovoltaic performances of the solar cells were investigated.
The planar ZnO layers were formed on FTO glass substrates by a spray pyrolysis method. The ZnO nanowire arrays with different thicknesses were grown on the 0.15-µm-thick planar ZnO layer by a liquid-phase growth method [C. Xu et al., J. Phys. Chem. C, 114, 125(2010)]. PbS QDs with the first exciton absorption band centered at approximately 1.05 mm were synthesized. The PbS-QD layers of the planar-type cells were formed on the ZnO layers by a spin coating method [J. Tang, et al., Nature Mater., 10, 765(2011)]. In the NW-type cells, PbS QDs fill the inner space of the ZnO nanowire array and form a 0.3-µmm-thick layer on top of the array. A gold layer with a thickness of 0.2 µmm was deposited on top of the PbS-QD layer by thermal evaporation to complete the cell fabrication. Solar cell performances were evaluated by measuring current-voltage characteristics under the AM1.5G illumination (100 mW/cm2), and IPCE action spectra.
Short-circuit photocurrent densities (JSCs) of the planar-type cells increased with increasing PbS-QD layer thickness and saturated when the layer got thicker than 0.3 mm. The power conversion efficiency (PCE) of the cell was then 0.92% (JSCF7.0mA/cm2CVOCF0.34 VCFFF0.39). In contrast, the PbS-QD layer of the NW-type cells could be thickened without Jsc’s leveling off or decreasing, and gave a maximum JSCof 25 mA/cm2 when the layer was 1.5 µmm thick. This is mainly because the ZnO nanowire array structure allows the exciton created in the PbS-QD region to find the PbS/ZnO interface within its diffusion length. The PCE of the NW-type cell with a 1.2-µmm-thick QD-layer reached 3.3% (JSCF22.3mA/cm2CVOC F0.36 VCFFF0.41) accordingly. More importantly, the IPCE of the cell gave a maximum of approximately 45% at 1.04 µmm), and reached 80% at 0.5 µmm.
Fig. 1. QD solar cell structures.
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