A ZnS and metal hydroxide composite passivation layer for recombination control in high efficiency quantum dot sensitized solar cells

Abstract

Charge recombination at the photoanode/electrolyte interface is one of the priority factors limiting the photovoltaic performance of quantum dot sensitized solar cells (QDSCs). Exploring interfacial engineering with the use of a proper passivation layer around the photoanode is an efficient way to control the charge recombination process and to promote the performance of the resultant cell devices. Herein, an easily synthesised and effective passivation layer making use of a ZnS and metal hydroxide composite material was developed for suppressing charge recombination and therefore improving the photovoltaic performance of a model CdSeTe QDSC. This novel composite passivation layer was formed by mixing the target metal ion with a Zn(OAc)₂ aqueous solution during a successive ionic layer adsorption and reaction (SILAR) procedure for overcoating ZnS layers on photoanodes. The influence of the different metal ions (including Be²⁺, Mg²⁺, Ca²⁺, Al³⁺, and Ga³⁺) was investigated and photovoltaic measurement results indicate that the formed ZnS and metal hydroxide composite passivation layer contributed to a considerable improvement in the photovoltage and power conversion efficiency of the resultant cell devices in comparison with a plain ZnS layer. The average efficiency was improved from 8.88% to 9.50% and a champion efficiency of 9.64% (J_sc = 21.20 mA cm⁻², V_oc = 0.702 V, FF = 0.648) was achieved under AM 1.5G full one sun irradiation for the ZnS/Ga(OH)₃ composite passivation layer based cells. Impedance spectroscopy (IS) and open-circuit voltage-decay (OCVD) measurements further confirmed that the ZnS and metal hydroxide composite passivation layer outperforms the reference plain ZnS layer in suppressing charge recombination.