TCO and light trapping in silicon thin film solar cells

Solar photovoltaic electricity: Current status and future prospects

In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

Tandem solar cells combining silicon and perovskite absorbers have the potential to outperform state-of-the-art high efficiency silicon single junction devices. However, the practical fabrication of monolithic silicon/perovskite tandem solar cells is challenging as material properties and processing requirements such as temperature restrict the device design. Here, we fabricate an 18% efficient monolithic tandem cell formed by a silicon heterojunction bottom- and a perovskite top-cell enabling a very high open circuit voltage of 1.78 V. The monolithic integration was realized via low temperature processing of the semitransparent perovskite sub-cell where an energetically aligned electron selective contact was fabricated by atomic layer deposition of tin oxide. The hole selective, transparent top contact was formed by a stack of the organic hole transport material spiro-OMeTAD, molybdenum oxide and sputtered indium tin oxide. The tandem cell design is currently limited by the photocurrent generated in the silicon bottom cell that is reduced due to reflectance losses. Based on optical modelling and first experiments, we show that these losses can be significantly reduced by combining optical optimization of the device architecture including light trapping approaches.

Monolithic perovskite/silicon-heterojunction tandem solar cells processed at low temperature

Thin-film solar cells

Intrinsic microcrystalline silicon: A new material for photovoltaics

Hydrogenated microcrystalline silicon (μc-Si:H) thin-film solar cells were prepared at high rates by very high frequency plasma-enhanced chemical vapor deposition under high working pressure. The influence of deposition parameters on the deposition rate (RD) and the solar cell performance were comprehensively studied in this paper, as well as the structural, optical, and electrical properties of the resulting solar cells. Reactor-geometry adjustment was done to achieve a stable and homogeneous discharge under high pressure. Optimum solar cells are always found close to the transition from microcrystalline to amorphous growth, with a crystallinity of about 60%. At constant silane concentration, an increase in the discharge power did hardly increase the deposition rate, but did increase the crystallinity of the solar cells. This results in a shift of the μc-Si:H∕a-Si:H transition to higher silane concentration, and therefore leads to a higher RD for the optimum cells. On the other hand, an increase in the t...

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Microcrystalline silicon solar cells deposited at high rates

We report on the development of high performance triple and quadruple junction solar cells made of amorphous (a-Si:H) and microcrystalline silicon (μc-Si:H) for the application as photocathodes in integrated photovoltaic–electrosynthetic devices for solar water splitting. We show that the electronic properties of the individual sub cells can be adjusted such that the photovoltages of multijunction devices cover a wide range of photovoltages from 2.0 V up to 2.8 V with photovoltaic efficiencies of 13.6% for triple and 13.2% for quadruple cells. The ability to provide self-contained solar water splitting is demonstrated in a PV-biased electrosynthetic (PV-EC) cell. With the developed triple junction photocathode in the a-Si:H/a-Si:H/μc-Si:H configuration we achieved an operation photocurrent density of 7.7 mA cm−2 at 0 V applied bias using a Ag/Pt layer stack as photocathode/electrolyte contact and ruthenium oxide as counter electrode. Assuming a faradaic efficiency of 100%, this corresponds to a solar-to-hydrogen efficiency of 9.5%. The quadruple junction device provides enough excess voltage to substitute precious metal catalyst, such as Pt by more earth-abundant materials, such as Ni without impairing the solar-to-hydrogen efficiency.

Multijunction Si photocathodes with tunable photovoltages from 2.0 V to 2.8 V for light induced water splitting

To further improve the stability of amorphous/microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cells, it is important to reduce the thickness of the a-Si:H top cell. This can be achieved by introduction of an intermediate reflector between the a-Si:H top and the μc-Si:H bottom cell which reflects light back into the a-Si:H cell and thus, increases its photocurrent at possibly reduced thickness. Microcrystalline silicon oxide (μc-SiOx:H) is used for this purpose and the trade-off between the material’s optical, electrical and structural properties is studied in detail. The material is prepared with plasma enhanced chemical vapor deposition from gas mixtures of silane, carbon dioxide and hydrogen. Phosphorus doping is used to make the material highly conductive n-type. Intermediate reflectors with different optical and electrical properties are then built into tandem solar cells as part of the inner n/p-recombination junction. The quantum efficiency and the reflectance of these solar cells are evaluated to find optical gains and losses due to the intermediate reflector. Suitable intermediate reflectors result in a considerable increase in the top cell current density which allows a reduction of the a-Si:H top cell thickness of about 40% for a tandem cell while keeping the current density of the device constant.

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Hydrogenated amorphous silicon oxide containing a microcrystalline silicon phase and usage as an intermediate reflector in thin-film silicon solar cells

Andreas Lambertz

Papers

Intrinsic microcrystalline silicon: A new material for photovoltaics

Microcrystalline silicon solar cells deposited at high rates

Multijunction Si photocathodes with tunable photovoltages from 2.0 V to 2.8 V for light induced water splitting

Hydrogenated amorphous silicon oxide containing a microcrystalline silicon phase and usage as an intermediate reflector in thin-film silicon solar cells

Microcrystalline silicon-oxygen alloys for application in silicon solar cells and modules