Andreas Lambertz

Microcrystalline silicon solar cells deposited at high rates

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...

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

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 evaluat...

Development of highly efficient thin film silicon solar cells on texture-etched zinc oxide-coated glass substrates

Abstract ZnO films prepared by magnetron sputtering on glass substrates and textured by post-deposition chemical etching are applied as substrates for p–i–n solar cells. Using both rf and dc sputtering, similar surface textures can be achieved upon etching. Excellent light trapping is demonstrated by high quantum efficiencies at long wavelengths for microcrystalline silicon solar cells. Applying an optimized microcrystalline/amorphous p-layer design, stacked solar cells with amorphous silicon top cells yield similarly high stabilized efficiencies on ZnO as on state-of-the-art SnO 2 (9.2% for a-Si/a-Si). The efficiencies are significantly higher than on SnO 2 -coated float glass as used for module production.

A constructive combination of antireflection and intermediate-reflector layers for a-Si/μC-Si thin film solar cells

This device design approach combines sputter-deposited TiO2 antireflection layer (ARL) and plasma-enhanced chemical vapor deposition-grown SiOx intermediate-reflector layer (IRL) in superstrate a-Si∕μc-Si thin film solar cell. The loss of current from either the component cells with individual application of ARL and IRL has been recovered with their combined application. With both ARL and IRL in a-Si∕μc-Si cell, (a) the top cell current and (b) the sum of top and bottom cell current increases. An initial efficiency of 11.8% [Voc=1.42V, FF=0.74, Jsc (top)=11.5mAcm−2, Jsc (bottom)=11.2mAcm−2] is achieved from such an a-Si∕μc-Si cell with a total Si layer thickness less than 2μm.

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

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.

Relationships between structure, spin density and electronic transport in `solar-grade' microcrystalline silicon films

Abstract Microcrystalline silicon (μc-Si:H) sfilms produced by very high frequency (95 MHz) plasma enhanced chemical vapour deposition (VHF-PECVD) under deposition conditions used to obtain best solar cells are investigated. The relationship between structure, spin density and conductivity is studied for films deposited under different silane–hydrogen gas compositions and different substrate temperatures. The transition from microcrystalline to amorphous structure induced by the increase in silane concentration is clearly reflected in the spin and transport properties. Within the microcrystalline growth regime, both the spin density and the conductivity show the lowest values close to the transition to amorphous growth, i.e. for material which shows the best performance in solar cells. Also with variation of the substrate temperature a clear correlation between the spin density and maximum solar cell efficiency is found with an optimum temperature of 250 °C.

Changes in electric and optical properties of intrinsic microcrystalline silicon upon variation of the structural composition

Device-grade microcrystalline silicon prepared by plasma enhanced chemical vapor deposition is investigated with respect to its structural, electronic and optical properties in order to identify material parameters with relevance to the solar cell efficiency. All sample series show a similar increase of dark conductivity with increasing crystalline volume fraction, suggesting a close relationship between electrical transport and the structural details of the material. Conditions yielding the highest solar cell performance, i.e. close to the transition to amorphous growth, are characterized by the lowest dark conductivity values together with the maximum photosensitivity in the whole crystalline range.

Development of Thin Film Amorphous Silicon Tandem Junction Based Photocathodes Providing High Open-Circuit Voltages for Hydrogen Production

Hydrogenated amorphous silicon thin film tandem solar cells (a-Si:H/a-Si:H) have been developed with focus on high open-circuit voltages for the direct application as photocathodes in photoelectrochemical water splitting devices. By temperature variation during deposition of the intrinsic a-Si:H absorber layers the band gap energy of a-Si:H absorber layers, correlating with the hydrogen content of the material, can be adjusted and combined in a way that a-Si:H/a-Si:H tandem solar cells provide open-circuit voltages up to 1.87 V. The applicability of the tandem solar cells as photocathodes was investigated in a photoelectrochemical cell (PEC) measurement set-up. With platinum as a catalyst, the a-Si:H/a-Si:H based photocathodes exhibit a high photocurrent onset potential of 1.76 V versus the reversible hydrogen electrode (RHE) and a photocurrent of 5.3 mA/cm2 at 0 V versus RHE (under halogen lamp illumination). Our results provide evidence that a direct application of thin film silicon based photocathodes fulfills the main thermodynamic requirements to generate hydrogen. Furthermore, the presented approach may provide an efficient and low-cost route to solar hydrogen production.

N-side illuminated microcrystalline silicon solar cells

Thin-film microcrystalline silicon solar cells illuminated through the n layer were studied and compared with classical p-layer illuminated cells. To investigate the corresponding charge carrier extraction properties, variation of the intrinsic absorber layer thickness was carried out. It was found that the J–V characteristic and the quantum efficiency of the n- and p-side illuminated cells are almost identical in the thickness range investigated, up to 7 μm. No differences in the collection of photogenerated electrons or holes are observed. Hence, the illumination side of μc-Si:H single junction solar cells of conventional thickness may be randomly chosen without adverse effect on their performance.

Advancing tandem solar cells by spectrally selective multilayer intermediate reflectors

Thin-film silicon tandem solar cells are composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell, stacked and connected in series. In order to match the photocurrents of the top cell and the bottom cell, a proper photon management is required. Up to date, single-layer intermediate reflectors of limited spectral selectivity are applied to match the photocurrents of the top and the bottom cell. In this paper, we design and prototype multilayer intermediate reflectors based on aluminum doped zinc oxide and doped microcrystalline silicon oxide with a spectrally selective reflectance allowing for improved current matching and an overall increase of the charge carrier generation. The intermediate reflectors are successfully integrated into state-of-the-art tandem solar cells resulting in an increase of overall short-circuit current density by 0.7 mA/cm(2) in comparison to a tandem solar cell with the standard single-layer intermediate reflector.