Quantum dot sensitized solar cell based on bifunctional linker modified ZnO nanorod array

Abstract

Summary form only given.Recently, there is an increased interest in quantum dot sensitized solar cell (QDSSC), i.e., applying semiconductor quantum dots (QDs) in dye-sensitized solar cells [1]. However, QDSSCs have shown comparatively lower efficiencies than expected. One possible reason for the poor efficiency of QDSSC is the difficulty of assembling the QDs on ZnO surface to obtain a well-covered QDs monolayer. In this paper, we shall assemble CdS QDs by CBD method on bare and mercaptopropionic acid (MPA) modified ZnO nanorod films. We shall also compare the performance of these as-prepared QDSSCs. The method for fabrication ZnO nanorod has been reported elsewhere. The ZnO nanorod was grown by the hydrothermal method with the length of 2 μm. ZnO was immersed into 1M MPA/acetonitrile solution for 12 hours. The CBD process were described elsewhere [2], which involved dipping ZnO in a 0.5 M Cd(N03)2 ethanol solution for 15 min, rinsing it with ethanol, and then dipping it for another 15 min in a 0.5 M Na2S methanol solution, and rinsing it again with methanol. The two-step dipping procedure is considered as one CBD cycle. The photoelectrodes of ZnO/CdS, and ZnO/MPA/CdS prepared by their optimal CBD cycles were named as sample A and B. The carbon nanotubes screen-printed on ITO glass were used as counter electrodes. Fig. 1(a) and (b) shows the UV-vis absorption spectra of CdS-sensitized ZnO photoelectrodes prepared by various CBD cycles performed on bare and MPA-modified ZnO electrodes. It can be seen that the absorption range enlarged and steadily shifted to the longer wavelength with the increased CBD cycles. The absorbance of ZnO/MPA/CdS is higher that of ZnO/CdS electrode at the same CBD cycle, indicating that higher amount of CdS QD was deposited on MPA-modified ZnO. CdS QDSSCs were prepared by various CBD cycles performed on bare and MPA-modified ZnO electrodes. Fig. 2(a) and (b) shows the SEM images of ZnO nanorod modified with MPA before and after sensitizing CdS QDs. From Fig. 2a it can be seen that, ZnO nanorods stand vertically on the substrate with 2 μm in length. After assembling with CdS QDs, the surface of ZnO nanorods is well covered by nanopaticles [in Fig. 2b]. It was reported that ZnO nanorod helps to improve the electron transport by avoiding particle-to-particle hopping that occurs in the Ti02 network. Fig. 3(a) and (b) show the I-V characteristics and the IPCE spectra of sample A and B at simulated one Sun (AM 1.5G, 100 mW/cm ). It can be seen that Jsc and Voc of ZnO/CdS and ZnO/MPA/CdS samples are 3.24 and 4.03 mA/cm2, and 0.532 and 0.601 V respectively. The FF and PCE of ZnO/MPA/CdS (28.8%, 0.70%) are higher than those of ZnO/CdS (23.4%, 0.41%). The details will be reported in the full manuscript.