Amorphous Silicon Pin Solar Cells Research Articles

Property control of expanding thermal plasma deposited textured zinc oxide with focus on thin film solar cell applications

Property control of expanding thermal plasma deposited textured zinc oxide is demonstrated considering intrinsic, i.e. bulk, and extrinsic transparent conducting oxide quality relevant for application in thin film amorphous silicon pin solar cells. Particularly the interdependence of electrical conductivity, film composition and film morphology, i.e. structure, feature shape and roughness of the surface, is addressed. Control of film composition is mainly governed by plasma production and gas phase chemistry inherently inducing a significant contribution to film morphology, whereas control of film morphology solely is governed by near-substrate conditions. Especially the ratio of zinc to oxygen and the reactor chamber pressure appear to be determinative in obtaining zinc oxide exhibiting the appropriate intrinsic and extrinsic quality, i.e. a high electrical conductivity, a high transmittance, a textured rough surface morphology and a strong hydrogen plasma resistance. The solar cell performance of appropriate undoped and aluminium doped textured zinc oxide inherently obtained during deposition is comparable with respect to Asahi U-type fluorine-doped tin oxide.

Structural, Optical and Electrical Properties of Natively Textured ZnO Grown by PECVD for Thin-Film Solar Cell Applications

Natively textured ZnO layers have been deposited between 150 and 350 °C with an expanding thennal plasma created by a cascaded arc. As shown by Scanning Electron Microscopy, Atomic Force Microscopy, X-ray Diffraction, and optical measurements, the crystallite size and surface roughness of the films increase with increasing substrate temperature. At the same time, columnar textured growth becomes less pronounced, and a change to granular structure is observed.The ZnO films exhibit low sheet resistance ( 80%) and effective light scattering properties. Preliminary amorphous silicon pin solar cells deposited on ZnO show an initial efficiency approaching 10 %, close to the value obtained on Asahi U-type Sn02:F.

Amorphous silicon solar cells on natively textured ZnO grown by PECVD

Natively textured ZnO layers deposited by the expanding thermal plasma CVD technique between 150 and 350°C at a deposition rate between 0.65 and 0.75 nm/s have been investigated with respect to their suitability as front electrode material for amorphous silicon pin solar cells in comparison to reference SnO 2:F (Asahi U-type). At higher substrate temperature and with growing thickness, the surface roughness of the ZnO films increases. Layers with electrical (sheet resistance<10 Ω/□), optical (transmittance >80%) and morphological (surface texture) properties comparable to Asahi U-type SnO 2:F have been obtained. Preliminary solar cells deposited on ZnO show an efficiency approaching 10%.

Upscaling of texture-etched zinc oxide substrates for silicon thin film solar cells

Large area (320×400 mm 2) glass/ZnO-films were prepared by high-rate d.c. magnetron sputtering from ceramic targets and compared to lab-type r.f.- and m.f.-sputtered ZnO. The very uniform and initially smooth films exhibit excellent electrical and optical properties (resistivity ≤5×10 −4 Ωcm, transmission >80% for visible light and 1500-nm thick films). Upon etching in diluted hydrochloric acid they develop a surface texture. Independent of sputter technique (d.c. or r.f.) and substrate size, higher substrate temperatures and lower sputter gas pressures have a similar influence on the film structure and lead to more robust and etch-resistant films. Showing excellent light scattering properties, amorphous silicon pin solar cells prepared on these large area glass/ZnO samples exhibit initial efficiencies up to 9.2%, proving the viability of sputtered and texture-etched ZnO as TCO-substrate for industrial solar module production.

Infrared Electroabsorption Spectra in Amorphous Silicon Solar Cells

AbstractWe report measurements of the infrared spectrum detected by modulating the reverse-bias voltage across amorphous silicon pin solar cells and Schottky barrier diodes. We find a band with a peak energy of 0.8 eV. The existence of this band has not, to our knowledge, been reported previously. The strength of the infrared band depends linearly upon applied bias, as opposed to the quadratic dependence for interband electroabsorption in amorphous silicon.The band's peak energy agrees fairly well with the known optical transition energies for dangling bond defects, but the linear dependence on bias and the magnitude of the signal are surprising if interpreted using an analogy to interband electroabsorption. A model based on absorption by defects near the n/i interface of the diodes accounts well for the infrared spectrum.

The Time-of-Flight Technique Applied to Amorphous Silicon Pin Solar Cells

ABSTRACTFast transient photocurrent measurements have been performed on a-Si:H based pin diodes with thicknesses down to 0.5 μm. On a thick sample drift mobilities derived in either the small-signal or the space-charge limited current mode are compared. On real solar cell devices in SCL voltage (integral) Mode the electron lifetimes τ are determined from the extraction time of the photo-generated charge in dependence on the degradation state of the sample. A clear and reproducible correlation between lifetime and degradation state has experimentally been established. Lifetimes τ, for which an intensity dependence has been observed in the SCL voltage Mode, are compared to the μτ-product from a small-signal charge collection Measurement. The influence of the real absorption profile on the measurements is discussed.

Defect distribution in a-Si:H-pin solar cells before and after degradation

Amorphous silicon pin solar cells suffer from a strong performance degradation due to light soaking. It is widely accepted that the microscopic cause of this effect is the creation of metastable defects. However, it is not yet clear whether the additional defects are created predominantly in the bulk of the i-layer or at the pi- and in-interface. Numerical modeling can be a very significant means to distinguish between these opposite assumptions, but absolutely needs an optimization of its parameters in order to produce reliable results. After doing this first step very carefully, we have modeled the effect of different spatial defect distributions in a series of model solar cells, which are compared with state of the art pin solar cells with varying i-layer thicknesses. The best coincidence for the annealed state is found if the defect density is computed selfconsistently with the varying position of the fermi level in analogy to a simplified defect pool model. We do not introduce an energy variance of the defects themselves. Furthermore, we find clear evidence that the light induced degradation leads to a homogeneous rise of the defect density in the bulk of the i-layer and not only at the pi- (and in-)interface.

Enhanced Stability of Amorphous Silicon Pin Solar Cells by Doping Profiles

Abstractpin cells with the light entering the n-layer (pinITO) or the p-layer (SnO2p(C)in) were prepared taking into account the ‘basic’ boron profile in the i-layer. Their efficiencies (up to 7.4% and 10.1%) show a light-induced degradation. This can be removed by doping the i-layer of pinITO cells with a decreasing boron profile. The initial efficiency is only slightly affected. Cells containing fluorine due to prior plasma-etching require higher amounts of boron for stability. For the cell type SnO2p(C)in, phosphorus profiles, however increasing towards the n-layer, lead to reduced cell degradation without affecting the efficiency. The results of profiling were interpreted in terms of a recombination model.