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.

To decrease the extent of degradation of a‐Si:H/μc‐Si:H tandem solar cells it is important to reduce the thickness of the a‐Si:H component cell. A silicon oxide intermediate reflector between top and bottom cell reflects the light back into the a‐Si:H top cell and thus increases its current. For this purpose the reflector material has to fulfill certain optical and electrical requirements. Phosphorous doped silicon oxide films (a‐SiOx:H<n>) with a certain volume fraction of microcrystalline silicon (μc‐Si:H<n>) are used for this purpose and the trade‐off between the material's optical and electrical properties is studied in detail. A suitable intermediate reflector is implemented in a tandem solar cell resulting in an increase in top cell current density by 1.3 mA/cm2 with similar FF and Voc compared to cells without intermediate reflector. An efficiency of 11.5% is achieved at a top cell absorber layer thickness of only 250 nm (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic transport and paramagnetic defects detected by Electron Spin Resonance (ESR) in both intrinsic and <n>‐type silicon oxide prepared by PECVD were investigated. The structure and alloy composition of the material were varied all the way from microcrystalline silicon (µc‐Si:H) to amorphous silicon oxide (a‐SiOX:H). The transition‐phase‐mixture material is called “microcrystalline silicon oxide” (µc‐SiOX:H). In undoped samples we find a strong reduction of the dark conductivity from 10‐3to 10‐12 S/cm and an increase of the spin density from1017 to 3×1019 cm‐3 as the crystallinity decreases from 80% to 0%. The variation of the dark conductivity in phosphorous doped samples was even higher from 101 to 10‐12 S/cm. ESR spectra of intrinsic material consist of a single featureless line with g‐values in the range of 2.0043…2.005 depending on the structure and alloying. The spectra of <n>‐type material exhibit a broader range of g‐values of 1.998…2.0043 due to strong variations of the Fermi level over the entire crystallinity range. The results are discussed within the frame of current understanding of µc‐SiOX:H as a phase mixture of µc‐Si:H crystallites embedded in a‐SiOX:H matrix (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)