No abstract

9We report on the development of high performance triple and quadruple junction solar cells 10 made of amorphous (a-Si:H) and microcrystalline silicon (µc-Si:H) for the application as 11 photocathodes in integrated photovoltaic-electrosynthetic devices for solar water splitting. We 12 show that the electronic properties of the individual sub cells can be adjusted such that the 13 photovoltages of multijunction devices cover a wide range of photovoltages from 2.0 V up to 14 2.8 V with photovoltaic efficiencies of 13.6 % for triple and 13.2 % for quadruple cells. The 15 ability to provide self-contained solar water splitting is demonstrated in a PV-biased 16 electrosynthetic (PV-EC) cell. With the developed triple junction photocathode in the a-17 Si:H/a-Si:H/µc-Si:H configuration we achieved an operation photocurrent density of 7.7 18 mA/cm 2 at 0 V applied bias using a Ag/Pt layer stack as photocathode/electrolyte contact and 19 ruthenium oxide as counter electrode. Assuming a faradic efficiency of 100 %, this 20 corresponds to a solar-to-hydrogen efficiency of 9.5 %. The quadruple junction device 21 provides enough excess voltage to substitute precious metal catalyst, such as Pt by more Graphical AbstractBias-free solar water splitting is demonstrated using thin film silicon based triple and quadruple junction solar cells with solar-to-hydrogen efficiencies up to 9.5 %.

Hydrogenated microcrystalline silicon ͑ c-Si:H͒ 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 ͑R D ͒ 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 c-Si:H/a-Si:H transition to higher silane concentration, and therefore leads to a higher R D for the optimum cells. On the other hand, an increase in the total flow rate at constant silane concentration did lead to a higher R D , but lower crystallinity. With this shift of the c-Si:H/a-Si:H transition at higher flow rates, the R D for the optimum cells decreased. A remarkable structure development along the growth axis was found in the solar cells deposited at high rates by a "depth profile" method, but this does not cause a deterioration of the solar cell performance apart from a poorer blue-light response. As a result, a c-Si:H single-junction p-i-n solar cell with a high efficiency of 9.8% was deposited at a R D of 1.1 nm/s.

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.

The development of microcrystalline silicon (mc-Si:H) for solar cells has made good progress with efficiencies better than those of amorphous silicon (a-Si:H) devices. Of particular interest is the absence of light-induced degradation in highly crystalline mc-Si:H. However, the highest efficiencies are obtained with material which may still include a-Si:H regions and lightinduced changes may be expected in such material. On the other hand, material of high crystallinity is susceptible to in-diffusion of atmospheric gases which, through adsorption or oxidation, affect the electronic transport. Investigations are presented of such effects concerning the stability of mcSi:H films and solar cells prepared by plasma-enhanced chemical vapour deposition and hot wire chemical vapour deposition.