Harnessing the solar energy incident on earth to provide clean energy to approximately 10 billion people is one of the grand challenges of the 21st century. Many of the novel strategies for solar-to-electric conversion utilize the following sequence of physical processes: (i) a light-absorbing material generates an electron-hole pair, (ii) the electron and hole separate quickly into two different phases, and (iii) the carriers are transported in these respective phases to opposing electrodes. During each of these steps, charge recombination must be minimized to achieve high power conversion efficiencies. In this presentation, we report a new kind of quantum-dot-sensitized solar cell which combines CdSe quantum dots adsorbed on ZnO nanowires to convert light into electrical current flow. Specifically, an array of ZnO nanowires was grown vertically on a transparent and conducting glass substrate in aqueous solutions of methenamine and zinc nitrate at ~95 °C. Nanometer-size CdSe quantum dots were synthesized separately and capped with mercaptopropionic acid. Following, the quantum dots were attached to the surface of the nanowires to form a photosensitized anode where the ZnO nanowire surfaces were decorated with a monolayer of CdSe quantum dots. A platinized transparent conducting oxide photocathode and the nanowire photoanode were positioned face-to-face and separated by 25 micron thick spacers. The space between the anode and the cathode was filled with an electrolyte containing I^3-/I^-. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot-nanowire interface into the ZnO nanowires. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the transparent conducting oxide anode. After the electron is injected into the ZnO, the positively-charged CdSe quantum dot was reduced through an electrochemical reaction with I^- in the electrolyte to form I^3- which in turn was reduced at the photocathode by an electron that has traveled across the load to complete the circuit. These solar cells exhibited short-circuit currents ranging from 1–2 mA/cm² and open-circuit voltages of ~0.6 V when illuminated with 100 mW/cm² simulated AM1.5 spectrum. The internal quantum efficiency (IQE) of this QDSSC in the spectral range covering the first excitonic transition (~500–600 nm) was 50–60%. For this particular solar cell architecture, the IQE is the product of the efficiency of electron injection from the QD to the nanowire and the collection efficiency of photoinjected electrons. Thus, 50–60% is also a lower bound on the electron injection efficiency. These remarkably high and promising internal quantum efficiencies also indicate that the overall conversion efficiency is limited by the availability of ZnO nanowire surface area for quantum dot adsorption. Further improvement on the efficiency and stability of these devices will require replacement of the electrolyte with an alternative hole transport medium because CdSe quantum dots were unstable in the electrolyte and solar cell performance decayed on a time scale of hours to days.

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