(38e) Quantum-Dot Solar Cells

Solar cells based on colloidal semiconductor nanocrystals may have the potential to achieve high power conversion efficiencies at low cost and are promising candidates for third generation photovoltaic devices. 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 this reason, nanocrystals with sizes less than the Bohr radii of the charge carriers are also called quantum dots (QDs) and solar cells made from such nanocrystals are referred to as quantum-dot solar cells. There are several reasons for pursuing QD solar cells. First, 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 and the ability to manipulate energy levels through size raises the possibility to make inexpensive multijunction solar cells by judiciously layering different size particles. Second, it has been suggested that quantum confinement may slow energy dissipative electron and hole relaxation rates such that multiple exciton generation and hot electron extraction may now compete with relaxation and lead to higher photocurrents or higher photovoltages, respectively. Finally, QDs can be prepared in large quantities as stable colloidal solutions under mild conditions and deposited on surfaces of various planar or nanostructured substrates as thin films through inexpensive high-throughput coating processes to form photovoltaic devices. In this talk, we will describe a new type of solar cell based on heterojunctions between PbSe QDs and thin ZnO films. We fabricated QD solar cells by depositing thin films (80?100 nm) of ZnO and PbSe QDs (50-700 nm) onto a glass substrate coated with transparent and conducting indium tin oxide (ITO) which forms the bottom contact of the device. A 100 nm thick gold film is used as the top contact and is deposited by evaporation either directly onto the PbSe QD film or onto a thin (15-30 nm) α-NPD [N-N'-bis(1-naphtalenyl)-N-N'-bis(phenylbenzidine)] layer. Absorption of light produces photoexcited electron?hole pairs that are confined within the QDs. These electron?hole pairs may dissociate, either at a QD?electrode interface or within the QD film and generate photocurrent. Specifically, electrons can 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 the gold electrode. Under simulated sunlight (AM1.5), our QD solar cells exhibit short-circuit currents as high as 15 mA/cm² and open-circuit voltages up to 0.45 V. We show that the solar cell open circuit voltage depends on the QD size and increases linearly with the QD effective band gap energy. Thus, these solar cells resemble traditional semiconductor-semiconductor heterojunction photovoltaic devices but changing the size of the QDs can vary the band gap of one of the semiconductors and hence the cell's open circuit voltage. Overall power conversion efficiency of the best device to date is 1.6% but may be increased further using nanostructured interfaces between PbSe QDs and ZnO. Specifically, we show that the incident photon to current conversion and overall power conversion efficiencies can be increased by replacing the ZnO film with a vertically-oriented array of single-crystalline ZnO nanowires, and infiltrating this array with colloidal PbSe QDs. These QD-nanowire solar cells exhibited power conversion efficiencies approaching 2%, three times higher than that achieved with thin-film ZnO devices constructed with the same amount of QDs.