No abstract

This paper describes the use, within p–i–n‐ and n–i–p‐type solar cells, of hydrogenated amorphous silicon (a‐Si:H) and hydrogenated microcrystalline silicon (μc‐Si:H) thin films (layers), both deposited at low temperatures (200°C) by plasma‐assisted chemical vapour deposition (PECVD), from a mixture of silane and hydrogen. Optical and electrical properties of the i‐layers are described. These properties are linked to the microstructure and hence to the i‐layer deposition rate, that in turn, affects throughput in production. The importance of contact and reflection layers in achieving low electrical and optical losses is explained, particularly for the superstrate case. Especially the required properties for the transparent conductive oxide (TCO) need to be well balanced in order to provide, at the same time, for high electrical conductivity (preferably by high electron mobility), low optical absorption and surface texture (for low optical losses and pronounced light trapping). Single‐junction amorphous and microcrystalline p–i–n‐type solar cells, as fabricated so far, are compared in their key parameters (Jsc, FF, Voc) with the [theoretical] limiting values. Tandem and multijunction cells are introduced; the μc‐Si: H/a‐Si: H or [micromorph] tandem solar cell concept is explained in detail, and recent results obtained here are listed and commented. Factors governing the mass‐production of thin‐film silicon modules are determined both by inherent technical reasons, described in detail, and by economic considerations. The cumulative effect of these factors results in distinct efficiency reductions from values of record laboratory cells to statistical averages of production modules. Finally, applications of thin‐film silicon PV modules, especially in building‐integrated PV (BIPV) are shown. In this context, the energy yields of thin‐film silicon modules emerge as a valuable gauge for module performance, and compare very favourably with those of other PV technologies. Copyright © 2004 John Wiley & Sons, Ltd.

This letter reports on highly sensitive optical absorption measurements on organic donor-acceptor solar cells, using Fourier-transform photocurrent spectroscopy ͑FTPS͒. The spectra cover an unprecedented dynamic range of eight to nine orders of magnitude making it possible to detect defect and disorder related sub-band gap transitions. Direct measurements on fully encapsulated solar cells with an active layer of poly͓2-methoxy-5-͑3Ј ,7Ј -dimethyl-octyloxy͔͒-p-phenylene-vinylene:͑6,6͒-phenyl-C61-butyric-acid ͑1:4 weight ratio͒ enabled a study of the intrinsic defect generation due to UV illumination. Solar cell temperature annealing effects in poly͑3-hexylthiophene͒:PCBM ͑1:2 weight ratio͒ cells and the induced morphological changes are related to the changes in the absorption spectrum, as determined with FTPS.

Optical characterization methods were applied to a series of microcrystalline silicon thin films and solar cells deposited by the very high frequency glow discharge technique. Bulk and surface light scattering effects were analyzed. A detailed theory for evaluation of the optical absorption coefficient ␣ from transmittance, reflectance and absorptance ͑with the help of constant photocurrent method͒ measurements in a broad spectral region is presented for the case of surface and bulk light scattering. The spectral dependence of ␣ is interpreted in terms of defect density, disorder, crystalline/amorphous fraction and material morphology. The enhanced light absorption in microcrystalline silicon films and solar cells is mainly due to a longer optical path as the result of an efficient diffuse light scattering at the textured film surface. This light scattering effect is a key characteristic of efficient thin-film-silicon solar cells.

Absorption losses at a nanorough silver back reflector of a solar cell were measured with high accuracy by photothermal deflection spectroscopy. Roughness was characterized by atomic force microscopy. The observed increase of absorption, compared to the smooth silver, was explained by the surface plasmon absorption. Two series of silver back reflectors ͑one covered with thin ZnO layer͒ were investigated and their absorption related to surface morphology.

The spectral dependence of the optical absorption coefficient in thin films of hydrogenated microcrystalline silicon is measured over nine orders of magnitude in the subgap, defect-connected region, and in the above-the-band gap region. Transmittance, reflectance, and constant photocurrent method measurements are combined with Fourier-transform photocurrent spectroscopy (FTPS). Results are analyzed and interpreted as due to electron transitions from defects or interband electron transitions, all having direct relevance to the thin-film microcrystalline silicon solar cell performance. FTPS is a fast and sensitive quantitative method for quality assessment of microcrystalline silicon absorber in solar cells and can be used for quality monitoring in solar cell production.

We present an optical model for thin-film silicon solar cells (both single and multijunction) with nanorough surfaces/interfaces. For these cells, the optical absorptance within each layer and the total reflectance are computed taking into account roughness, angular distribution of scattered light, thicknesses, and optical constants of all layers. In the model, we combine coherent approach, scattering theory, and Monte Carlo tracing method. Results of the model are shown to be in good agreement with the experimentally measured spectral response and the total reflectance of solar cells. Some predictions of the ultimate solar cell performance based on the model are presented as well.

No abstract