Silicon-based Thin Film Solar Cells Research Articles

Research Progresses on High Efficiency Amorphous and Microcrystalline Silicon-Based Thin Film Solar Cells

AbstractThis paper reviews our research progresses of hydrogenated amorphous silicon (a-Si:H) and microcrystalline (μc-Si:H) based thin film solar cells. It coves the three areas of high efficiency, low cost process, and large-area proto-type multi-chamber system design and solar module deposition. With an innovative VHF power profiling technique, we have effectively controlled the crystalline evolution and made uniform μc-Si:H materials along the growth direction, which was used as the intrinsic layers of pin solar cells. We attained a 9.36% efficiency with a μc-Si:H single-junction cell structure. We have successfully resolved the cross-contamination issue in a single-chamber system and demonstrated the feasibility of using single-chamber process for manufacturing. We designed and built a large-area multi-chamber VHF system, which is used for depositing a-Si:H/μc-Si:H micromorph tandem modules on 0.79-m2 glass substrates. Preliminary module efficiency has exceeded 8%.

Simulation of nanorod structures for an amorphous silicon-based solar cell

Through the use of three-dimensional (3D) technology computer aided design (TCAD) numerical simulations, the authors have designed a nanorod solar cell that can solve the conflict between high light absorption and efficient carrier transport in most amorphous silicon-based thin film solar cells. This novel structure involves the use of an n-type amorphous silicon (a-Si:H) nanorod on the substrate, an a-Si:H i-layer and an a-SiC:H p-layer which are sequentially grown along the surface of each n-type a-Si:H nanorod. Sunlight is absorbed along the axial direction of the nanorods, while carrier transport is along the radial direction. The nanorod used in this study was long and therefore it could absorb a high amount of sunlight, while the solar cell was still thin enough for effective transport of photo-generated carriers.

Performance of superstrate multijunction amorphous silicon-based solar cells using optical layers for current management

The performance of multijunction amorphous silicon-based thin film solar cells has been reported using thin layers of TiO 2 and SiO x acting as refractive index matching optical layers for different interfaces of the superstrate device structure. Improvement of short-circuit current from the sub-cells of a-Si/μc-Si cells is demonstrated with TiO 2 as anti-reflection layer at TCO/Si interface and SiO x as intermediate-reflector layer between two sub-cells. An initial efficiency of 11.8% is achieved by applying both the TiO 2 and SiO x optical layers in a-Si/μc-Si solar cell.

Doping influence on intrinsic stress and carrier mobility of LP-MOCVD-deposited ZnO:B thin films

Textured boron-doped zinc oxide (ZnO:B) films, suitable as transparent and conductive layers in thin film silicon-based solar cells, have been obtained by low-pressure metalorganic chemical vapour deposition (LP-MOCVD) technique. The complex role of the boron doping in ZnO layers was examined and its influence on the morphological and structural properties was analysed. Furthermore, a correlation between such properties, intrinsic stress and carrier mobility in the film has been analysed. ZnO:B films have a polycrystalline structure with a columnar texture shape, they show a rough surface with pyramidal large grains. At the increase of the boron doping the pyramidal shape of the grains deteriorates and the average grain size reduces. Furthermore the a-lattice parameter decreases indicating that the boron incorporation introduces a deformation in the crystal. In addition to these large structural modifications, the decrease of the carrier mobility at the increase of the doping content is observed. At the same time, the boron content also plays a meaningful role on the intrinsic stress inside the film. Stress behaviour in ZnO films has been investigated by X-ray diffraction measurements using the sin 2 ψ method. Tensile stresses have been observed and the thermal and intrinsic components have been calculated. Respect to undoped ZnO films, boron incorporation on substitutional or interstitial sites increases the tensile stress by means of a lattice strain mechanism that reduces the d-spacing value.

Silicon-based thin-film solar cells fabricated near the phase boundary by VHF PECVD technique

The light-soaked and annealing behaviors for silicon (Si)-based thin-film single-junction solar cells fabricated near the phase boundary using a very-high-frequency plasma-enhanced chemical vapor deposition (VHF PECVD) technique are investigated. The hydrogen dilution ratio is changed in order to achieve wide band gap hydrogenated amorphous Si (a-Si:H) and narrow band gap hydrogenated microcrystalline Si (μc-Si:H) absorbers. Just below the a-Si:H-to-μc-Si:H transition, highly hydrogen-diluted a-Si:H solar cells with a good stability against light-soaking and fast annealing behavior are obtained. In contrast, the solar cell fabricated at the onset of the μc-Si:H growth is very unstable and its annealing behavior is slow. In the case of μc-Si:H solar cells with the crystal volume fraction of 43–53%, they show the lowest light-induced degradation among the fabricated solar cells. However, it is very difficult to recover the degraded μc-Si:H solar cells via thermal annealing.

Surface textured MF-sputtered ZnO films for microcrystalline silicon-based thin-film solar cells

Highly conductive and transparent aluminum-doped zinc oxide (ZnO:Al) films were prepared by reactive mid-frequency (MF) magnetron sputtering at high growth rates. By varying the deposition pressure, pronounced differences with respect to film structure and wet chemical etching behavior were obtained. Optimized films develop good light-scattering properties upon etching leading to high efficiencies when applied to amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon-based thin-film solar cells and modules. Initial efficiencies of 7.5% for a μc-Si:H single junction and 9.7% for an a-Si:H/μc-Si:H tandem module were achieved on an aperture area of 64 cm2.

Optical properties of silicon-based thin-film solar cells in substrate and superstrate configuration

Abstract The optical properties of microcrystalline silicon substrate-type solar cells with a front ZnO thickness of 500–1000 nm as required for monolithic series connection in PV-modules are investigated. The surface texture of the front ZnO was varied to study the possibility to reduce reflection losses and to improve light trapping. Before encapsulation, certain textures of the front ZnO exhibit improved light coupling for substrate solar cells. For substrate solar cells encapsulated with EVA and a glass cover, however, additional texture of the front ZnO has hardly any effect. Encapsulation also removes the antireflection conditions in case that the front ZnO was originally designed as ARC. So far, the quantum efficiency of encapsulated substrate solar cells does not show the high level as observed in superstrate solar cells possibly due to parasitic absorption in the front and back contact and the not optimized texture of the reflector (substrate).

Optical improved structure of polycrystalline silicon-based thin-film solar cell

This paper presents an n-i-p type solar cell structure consisting of polycrystalline silicon thin film as an absorber of incident radiation and a ZnO thin film for optical improvement. The characteristics of Si layers (thickness and doping level) are designed to assure a high value of collection efficiency for photogenerated carriers. The thin films of polycrystalline silicon are obtained by CVD at a temperature of around 620°C. ZnO thin film is prepared by thermal decomposition of Zn-acetylacetonate [Zn(C 5H 7O 2) 2] in a vertical reactor. It is used as AR coating and as contact electrode due to its properties of high transparency (>90%) and high conductivity (3×10 −4 Ω cm). Polycrystalline silicon and ZnO films have been investigated in terms of surface morphology and grain size by AFM and XRD.

98/00464 Novel light-trapping schemes involving planar junctions and diffuse rear reflectors for thin-film silicon-based solar cells

98/00464 Novel light-trapping schemes involving planar junctions and diffuse rear reflectors for thin-film silicon-based solar cells

Novel light-trapping schemes involving planar junctions and diffuse rear reflectors for thin-film silicon-based solar cells

Optical optimization of planar junction amorphous and microcrystalline silicon-based thin-film solar cells offers improved conversion efficiency. Planar junction solar cells with diffuse and selective angle rear reflectors were examined in detail. The refractive index of the diffuser material needs to be as close to that of silicon as possible. Insulating diffuse rear reflectors must be used in conjunction with conductive rear-window layers of the appropriate band gap, conductivity and refractive index. The refractive index of the aear window layer should be at least as large as that of the diffuser material. Rear windows comprised of ZnS, CdS and ZnSe (as well as other II–VI semiconductors and semiconductor alloys) used with a TiO 2 diffuse reflector could increase the current of optically thin silicon solar cells beyond that possible using rough TCO/glass substrates. Selective angle reflectors must also be comprised of either the solar cell semiconductor itself (not possible in many cases) or comprised of a material having properties similar to those of the conducting rear windows used with the diffuse reflectors.