Towards Quantum-Dot Solar Cells From Abundant Nontoxic Materials

Quantum confinement of electrons and holes in nanometer size crystals (quantum dots or QDs), endows them with properties that may be advantageous for efficient solar-to-electric energy conversion. First, electronic energy levels and optical absorption in QDs can be manipulated by changing their size.  This allows the optimization of their optical absorption for maximum overlap with the solar spectrum. Second, the ability to manipulate energy levels through size raises the possibility to make inexpensive multijunction solar cells by judiciously layering different size QDs. Third, it has been suggested that quantum confinement may slow energy dissipative electron and hole relaxation rates such that two new physical processes, 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.  For these reasons, solar cells based on QDs may have the potential to achieve high power conversion efficiencies at low cost and are promising candidates for third generation photovoltaic devices.

Most quantum dots made to date are based on PbSe and PbS QDs because we know the most about their photophysical properties and synthesis. Lead causes irreversible neurological, reproductive, renal and cardiovascular damage to humans and its use has been banned from many products. Quantum dots based on nontoxic and abundant materials are needed. Copper zinc tin sulfide (Cu₂ZnSnS₄ or CZTS) is emerging as a promising new sustainable semiconductor for photovoltaics. CZTS has a high absorption coefficient in the visible range of the electromagnetic spectrum and an electronic bandgap (~1.5 eV) that is ideal for photovoltaics.  We have developed a novel and facile synthesis method for making CZTS nanocrystals and achieved quantum confinement in nanocrystals with diameters less than 3 nm. Our synthesis method is based on decomposition of metal dithiocarbamates in an organic solvent such as dodecene. Presence of oleylamine in the reaction mixture reduces the decomposition temperature of copper, zinc and tin dithiocarbamates to a narrow temperature range (170-220 ^oC). Rapid injection of oleylamine and tin diethyldithiocarbamate into a hot mixture of copper and zinc dithiocarbamate in dodecene initiates a burst of nanocrystal nucleation followed by growth. We show that using this approach avoids the formation of binary and ternary sulfide impurities such as zinc sulfide or copper tin sulfide. Moreover, we have been able to control the sizes of CZTS nanocrystals from 2-7 nm by varying the experimental conditions such as the synthesis temperature and time. In this talk we will present our latest results towards making CZTS quantum dot solar cells.