Invited Paper EN-MoA6
Solar Cells Based on Semiconductor Quantum Dots and Nanowires

Monday, October 18, 2010, 3:40 pm, Room Mesilla

Solar cells based on colloidal semiconductor nanocrystals, or quantum dots (QDs) may have the potential to achieve high power conversion efficiencies at low cost. Quantum confinement of electrons and holes in these nanometer-size crystals endows them with properties that may be advantageous for efficient solar-to-electric energy conversion. For example, varying the QD size changes the electronic energy levels and optical absorption in QDs. This allows the optimization of their optical absorption for maximum overlap with the solar spectrum. In addition, QDs can be prepared in large quantities as stable colloidal solutions under mild conditions and deposited as thin films using inexpensive, high-throughput coating processes to form solar cells.

After a brief review of the literature on QD solar cells, I will focus on a new type of QD solar cell based on heterojunctions between PbSe QDs and thin ZnO films designed to improve on the current state-of-the-art. These QD solar cells were fabricated by depositing thin films of ZnO and PbSe QDs onto a glass substrate coated with conductive indium-tin-oxide (ITO), which forms the bottom contact of the device. Absorption of light produces electron–hole pairs in the QDs that dissociate, either at a QD–electrode interface or within the QD film and generate photocurrent. Specifically, electrons lower their energy by transferring into the ZnO film, which forms a type-II heterojunction with the PbSe QDs. These electrons move across the ZnO film and are collected at the ITO contact while the positive charges are transported to and collected at a top gold electrode.

Under simulated sunlight, the QD solar cells exhibit short-circuit currents as high as 15 mA/cm² and open-circuit voltages up to 0.45 V. The solar cell open-circuit voltage depends on the QD size and increases linearly with the QD effective band gap energy. Charge collection in these devices can be increased further by using nanostructured interfaces between PbSe QDs and ZnO. Specifically, the ZnO film can be replaced with a vertical array of ZnO nanowires, and infiltrating this array with colloidal PbSe QDs. These nanowire–quantum-dot solar cells exhibited power conversion efficiencies of 2%, nearly three times higher than that achieved with thin-film ZnO devices constructed with the same amount of QDs. Supporting experiments using field-effect transistors made from these QDs also show that the QDs’ electrical properties are strongly influenced by the presence of nitrogen and oxygen atmospheric gases. Such results have important implications with respect to the assembly, characterization, and exposure of QD-based solar cells to an ambient environment.