Layer-by-layer CdTe nanoparticle absorbers for ZnO nanorod solar cells - the influence of annealing on cell performance
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Layer-by-layer CdTe nanoparticle absorbers for ZnO nanorod solar cells - the influence of annealing on cell performance Joe Briscoe1, Diego E. Gallardo2 and Steve Dunn3 1 Nanotechnology Centre, Cranfield University, MK43 0AL, UK. 2 Now at: Optoelectronics Group, Department of Physics, Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge, CB3 0HE, UK. 3 Materials Department, School of Engineering and Materials, Queen Mary, University of London, E1 4NS, UK. ABSTRACT The conformal coating of ZnO nanorods with CdTe nanoparticles using layer-by-layer (LbL) processing produces a quantum dot-sensitised solar cell. As the number of CdTe layers increases the absorption of incident light increases below the absorption onset of the nanoparticles (650 nm). Photoluminescence investigations of the CdTe-ZnO composite structure suggest a transfer of photoexcited electrons from the CdTe nanoparticles the ZnO nanorods. Filling of the semiconductor composite structure with CuSCN provides the solar cell with a ptype semiconductor to collect the photogenerated holes from the system. Annealing the CdTepolymer coated nanorods lowers the series resistance of the cell by removing the polymer component of the film. A cell annealed at 350 °C has a Jsc of 0.12 mAcm-2, and a Voc of 49 mV under 0.25 mW/cm2 illumination. INTRODUCTION Since the first dye-sensitised solar cell (DSSC) produced by O’Regan and Grätzel in 1991 [1] demonstrated that a nanostructured TiO2 substrate could absorb 780 times more lightabsorbing dye than a planar substrate there has been significant interest in developing readily manufacturable analogous systems. The extra-thin absorber (eta) solar cell can be seen as a solid-state version of the DSSC where a semiconductor absorber layer replaces the dye layer of the DSSC, and the electrolyte is replaced with a solid-state hole conductor such as the p-type semiconductor CuSCN [2] or polymers such as PEDOT:PSS [3], spiro-OMeTAD [4] and MEHPPV [5]. Materials used for absorber layers include thin films of CdS [6], In2S3 [7] and Sb2S3 [8], and PbS [9] nanoparticles. While TiO2 has gained significant interest as a material for DSSCs, alternative materials have also been investigated. One such material is ZnO nanorods as they offer a number of advantages over porous TiO2. The nanorods provide a direct current pathway for electrons to the front contact [10] with few grain boundaries. ZnO has a higher electron mobility than TiO2 [11], with a more open morphology that can allow precursors to readily reach the ZnO surface for processes where the absorber is formed in-situ [12]. A disadvantage is the lower surface area of ZnO nanorods compared to porous TiO2, giving lower efficiencies than TiO2-based DSSCs due to low light absorption [13]. However, it has been shown that high levels of light-harvesting can be achieved using ZnO nanorods with semiconductor absorber coatings; the thickness of such coatings can be increased much more than organic dyes, and by adjusting the coating thickness to balance l
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