Photoelectrochemical Cell Engineering for Solar Energy Conversion

Solar energy is the most abundant energy source, harnessing solar energy holds the solution to the challenge of increasing global energy demand and reducing our dependence on fossil fuels. Photovoltaics which directly convert solar energy into electricity offer a practical strategy to a clean and renewable energy matrix. Dye-sensitized solar cells (DSCs) are among the third generation photovoltaic technologies with distinct features including transparency, multicolor and low-cost fabrication. Recent progress have also shown their advantages under indoor lighting, making them very suitable to serve as power source for portable consumer devices and wearable electronics.To improve the performance of DSCs, we first designed and synthesized organic sensitizers through judicious molecular engineering. The dye MS5 in conjunction with a copper (II/I) based electrolyte enables a DSC to achieve a very high open-circuit voltage (Voc) of 1.24 V. The very low Voc deficit results from the retarded interfacial charge recombination. Co-sensitization of MS5 with a wide spectral-response dye XY1b produces a DSC with a power conversion efficiency (PCE) of 13.5% under standard AM1.5 G solar radiation and a record PCE of 34.5% under ambient lighting. Our work highlights the importance of molecular engineering of high-Voc co-sensitizers to improve the photovoltaic performance of DSCs.To further enhance the efficiency of DSC, we report a method of pre-adsorbing a monolayer of a hydroxamic acid derivative on the surface of titanium dioxide (TiO2) to improve the molecular packing of the dyes. The two co-adsorbed sensitizers are designed to have complementary absorption to harvest light across the entire visible region. The optimized solar cell reached a verified PCE of 15.2% under 1 sun and around 30% under ambient lighting over a wide range of light sources and intensities. Our findings provide a promising path to further advance the DSC performance.Converting solar energy to fuels is another desirable approach toward fulfilling the need for renewable energy, which circumvent the disadvantage of intermittent solar radiation and allow facile storage and transportation. Photoelectrochemical (PEC) cell based on organic semiconductors can potentially realize economical competitive and scalable green hydrogen production via water splitting.We first engineered the photocathode by replacing the conventional metal oxide hole transporting layer with an organic self-assembled monolayer (SAM) which can efficiently select holes while block electrons at the substrate/semiconductor interface. Through judicious organic semiconductor selection, the optimized photocathode delivered a photocurrent density of 4.6 mA cm-2 and exhibited good stability and pH compatibility. Together with a previously established photoanode, an organic semiconductor PEC tandem cell was for the first time successfully demonstrated for overall solar water splitting. Our work sets a new benchmark for the potential production of low-cost solar fuel.We further engineered the photoanode by using an organic n-type SAM molecule to replace the metal oxide electron transporting layer. The obtained photoanode with a simple structure showed higher photocurrent and comparable stability than what's reported in the literature. Further optimization would involve selection of suitable hole transporting layer as well as employment of organic semiconductor with complementary light absorption with photocathode.