| AVS 60th International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Monday Sessions |
| Session EN+AS+NS+SS-MoA |
| Session: | Interfacial Challenges in Nanostructured Solar Cells |
| Presenter: | M.E. Edley, Drexel University |
| Authors: | M.E. Edley, Drexel University H. Majidi, Drexel University G.W. Guglietta II, Drexel University L. Spangler, Drexel University J.B. Baxter, Drexel University |
| Correspondent: | Click to Email |
Solar cells can provide an abundant, clean, and sustainable source of electricity, but high costs have limited their implementation. The use of sensitized nanostructured architectures may enable both low-cost processing and high efficiency by decoupling the functions of light harvesting and charge transport into different materials. We report on extremely thin absorber (ETA) solar cells that use ZnO nanowire arrays coated with a thin CdSe layer and filled with a liquid electrolyte. CdSe absorbs visible light, and photoexcited electrons are injected into the ZnO while photoexcited holes oxidize the redox species. Nanowire arrays provide direct pathways for electron transport as well as sufficient surface area for sensitization. The CdSe coatings should be crystalline and conformal with well-controlled thickness. With this ETA architecture, interfacial recombination is the dominant loss process, so controlling the interfacial chemistry, morphology, and microstructure of the materials during processing is critical.
Our approach utilizes a combination of solar cell measurements and ultrafast transient absorption spectroscopy to understand the effects of CdSe thickness, annealing conditions, and interfacial treatments on the dynamics and efficiency of charge carrier separation, and ultimately on the solar-to-electric energy conversion efficiency. These studies provide guidelines for architecture design and materials selection for ETA solar cells. For example, we have found that an optimum thickness exists for planar cells that balances light absorption with photoexcited carrier collection, and that this optimum thickness depends on annealing conditions of the CdSe. Coatings on nanowires should be designed to use the largest thickness that gives the maximum internal quantum efficiency in planar cells, which is ~30 nm when annealing at 400 C. The nanowire geometry is then designed to achieve efficient light harvesting with a coating thickness of 30 nm. Such thin electrodeposited coatings are subject to pinholes and shunt pathways. However, these were overcome through the deposition of an ultrathin (<5 nm) CdS interfacial layer using successive ionic layer adsorption and reaction (SILAR), improving efficiency to above 2%. Ultrafast transient absorption spectroscopy shows that the lifetime of photoexcited carriers depends strongly on defect density, with lifetimes increasing from ~50 ps to ~500 ps upon annealing. Importantly, this lifetime is much longer than the characteristic time for electron transfer into ZnO (~2 ps) and hole transfer into the electrolyte (~100 ps), indicating that charge separation is very efficient.