Design of injection and recombination in quantum dot sensitized solar cells

Semiconductor Quantum Dots (QDs) currently receive widespread attention for the development of photovoltaic devices due to the possibility of tailoring their optoelectronic properties by the control of size and composition. Here we show that it is possible to design both injection and recombination in QD sensitized solar cells (QDSCs) by the appropriate use of molecular dipoles and conformal coatings. QDSCs have been manufactured using mesoporous TiO ₂ electrodes coated with "in situ" grown CdSe semiconductor nanocrystals by chemical bath deposition (CBD). Surface modification of the CdSe sensitized electrodes by conformal ZnS coating and grafting of molecular dipoles (DT) has been explored to both increase the injection from QDs into the TiO₂ matrix and reduce the recombination of the QD sensitized electrodes. Different sequences of both treatments have been tested aiming at boosting the energy conversion efficiency of the devices. The obtained results showed that the most favorable sequence of the surface treatment (DT+ZnS) led to a dramatic 600% increase of photovoltaic performance compared to the reference electrode (without modification): V_oc = 0.488 V, j_sc = 9.74 mA/cm², FF = 0.34, and efficiency = 1.60% under full 1 sun illumination. The measured photovoltaic performance was correlated to the relative position of the CdSe conduction band (characterized by surface photovoltage measurements) and TiO₂ conduction band (characterized by the chemical capacitance, C_μ) together with recombination resistance, R_rec.