Thin-film silicon triple junction solar cell for solar fuels

Abstract

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 and η = 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.