Thin-film silicontriple junction solarcell for solar fuels

The ever increasing installed capacity of renewable, sustainable energy is essential in order to keep the earth habitable. However, the intermittent nature of solar and wind energy do not strictly follow human energy demands. Therefore an energy of sufficient magnitude buffer is essential to provide a constant supply of energy that matches the demand as closely as possible. Chemical energy shows high potential in terms of clean, long-term energy storage with efficient conversion to electricity. The DEMO project intends on manufacturing a monolithic photovoltaic device for the production of hydrogen gas and hydrocarbon products by the electrochemical splitting of water and carbon dioxide. an aSi:H/a-SiGe:H/nc-Si:H triple junction device is proposed to generate a combination of voltage and current to drive the electrochemical reactions efficiently, while simultaneously addressing scalability by using earthabundant materials. In this thesis, efforts were made to optimise the a-SiGe:H subcell as well as finding the optimal combination of materials to use as tunnel recombination junctions as intermediate layers between subcells of the multi-junction device. The best performing a-SiGe:H p-i-n single-junction device achieved a VOC = 719 mV, JSC =17.2 mA/cm2, F F = 0.63 and η = 7.85% as processed on an Asahi VU superstrate. This performance was observed show a strong decline with increasing intrinsic layer thickness and increasing deposition rate. Although a conclusive optimal device structure was not obtained, it is believed that bandgap profiling by adjusting the germanium content in the intrinsic layer as well as applying buffer layers can substantially improve the performance of the a-SiGe:H single-junction solar cell. To assess long-term stability, an a-SiGe:H single-junction device was subdued to 1000h light soaking. Meta-stable defects induced by recombination of photo-generated charge carriers resulted is a strong degradation in F F of 20% relative. VOC and JSC showed similar relative decreases in performance with 9.0% and 8.6%, respectively. The resulting relative drop in conversion efficiency for the degraded solar cell is observed to be 34%. The best tunnel recombination junction for an a-Si:H/a-SiGe:H double junction consists of a 5 nm highly doped n+-type nc-SiOx :H layer with a 2 nm nc-Si:H p-layer, placed between 25 nm nc-SiOx :H n-layer and a 16 nm nc-SiOx :H p-layer that resulted in good combination of F F , VOC and JSC by improved tunnelling, charge separation and better light management. The best tunnel recombination junction for an a-SiGe:H/nc-Si:H double-junction consists of a highly doped n+ nc-SiOx :H layer with a 2 nm nc-Si:H p-layer. These layers are sandwiched between an 50 nm ncSiOx :H n-layer, of which the first half has linearly increasing oxygen content, and a 16 nm nc-SiOx :H p-layer of the a-SiGe:H subcell and nc-Si:H subcell, respectively. These tunnel recombination junctions were used to further develop a p-i-n a-Si:H/a-SiGe:H/nc-Si:H triplejunction solar cell on a wet-etched glass superstrate with ZnO:Al as both sacrificial layer for texturing and as transparent conductive oxide. The best performing triple-junction device achieved a VOC = 1.96 V, JSC = 6.21 mA/cm2, F F = 0.63 andη= 7.63% with respective intrinsic layer thicknesses of 175 nm/120 nm/3000 nm. The conversion efficiency of this current mismatched device is throttled by the current limiting a-SiGe:H middle cell. Enhancing the a-SiGe:H material quality allows for thicker absorber layer to increase current generation without compromising the electrical performance, which can significantly improve the performance the triple-junction device. This device is able to achieve an estimated solar-to-fuels efficiency of 4.9% for producing hydrogen with a high performance electrode and an an IrOx counter electrode. Assuming similar electrode performance, the solar-to-fuels efficiency for producing both hydrogen and methane is expected to be 2.4%. Hydrocarbons with higher electrochemical potential are not likely to be produced with such a device due to the higher electrochemical potentials. Using the measured degraded performance of the a-SiGe:H subcell, a relative decrease in water splitting efficiency of 21% is expected for the triple-junction device.