Dye-sensitized solar cells (DSSCs) and quantum dot-sensitized solar cells (QDSSCs) are two promising alternative, cost-effective concepts for solar-to-electric energy conversion that have been offered to challenge conventional Si solar cells over the past decade. The configuration of a DSSC or a QDSSC consists of sintered TiO2 nanoparticle films, ruthenium-based dyes or quantum dots (QDs) (i.e., sensitizers), and electrolytes. Upon the absorption of photons, the dyes or QDs generate excitons (i.e., electron-hole pairs). Subsequently, the electrons inject into the TiO2 photoanode to generate photocurrent; scavenged by a redox couple, holes transport to the cathode. The overall power conversion efficiency (PCE) of a DSSC or QDSSC is dictated by the light harvest efficiency, quantum yield for charge injection, and charge collection efficiency at the electrodes. The goal of our research is to understand the fundamental physics and performance of DSSCs and QDSSCs with improved PCE at the low cost based on rational engineering of TiO2 nanostructures, sensitizers, and electrodes through an integrated experimental and modeling study. In this presentation, I will discuss three aspects that I have accomplished over the last several years.

(1) Effects of surface treatment and structural modification of photoanode on the performance of DSSCs. First, our research indicates that the surface treatment with both TiCl4 and oxygen plasma yields the most efficient dye-sensitized TiO2-nanoparticle solar cells. A maximum PCE is achieved with a 21 µm thick TiO2 film; the PCE further increases to 8.35% after TiCl4 and O2 plasma treatments, compared to the untreated TiO2 (PCE = 3.86%). Second, we used a layer of TiO2 nanoparticle film coated on the FTO glass, and a bilayer of TiO2nanoparticle/freestanding TiO2 nanotube film deposited on the FTO glass as photoanodes. The J~V parameter analysis acquired by equivalent circuit model simulation reveals that nanotubular structures are advantageous and impart better charge transport in nanotubes. However, the photocurrent generation is reduced due to the small surface area, which in turn results in low dye loading. Third, we fabricate ZnO and TiO2 nanoflowers by the chemical bath deposition (CBD) method. The PCEs of DSSCs crafted with ZnO and TiO2 nanoflowers are low comparing to those with TiO2 nanoparticles.

(2) The use of earth abundant, environmentally friendly quaternary Copper Zinc Tin Sulfide (CZTS) as a low-cost alternative to noble metal Pt as the counter electrode (CE). With a simple wet chemistry synthesis of CZTS and a viable spin-coating fabrication of CE, the resulting CZTS film after selenization exhibits an impressive electrocatalytic performance as CEs to promote the regeneration of iodide from triiodide in electrolyte, yielding an impressive PCE of 7.37%, remarkably comparable to that with the Pt CE (PCE = 7.04%). The use of CZTS as CE may expand the possibilities for developing low-cost and scalable DSSCs, thereby dispensing with the need for expensive and rare Pt.

(3) Simulation of the light harvesting ability of TiO2 nanotube solar cells coated with CdSe and PbSe QDs and the charge injection at the interfaces of TiO2 substrate and quantum dots. We find that for short nanotubes, there is a diffractive photonic effect where the absorption is maximized for the lattice pitch close to the wavelength of light being absorbed. The ab initio simulation results reveal appreciable overlaps of the wave-functions in the QDs and the TiO2 substrate, which render the electron transfer on a time scale shorter than the electron-hole recombination time in the QDs.