Search

Hydrogenated amorphous silicon (a-Si:H) has been used for decades—doped and as intrinsic absorber layers—in thin-film silicon solar cells. Whereas their effiency was improved for a long time by the deposition of higher quality absorber layers, recent improvements can be attributed to a better understanding of the interfaces, allowing for their specific engineering. In this review, we briefly resume the state-of-the-art of a-Si:H solar cell technology from growth and characterization of single layers to full solar cells and multijunction devices. Focusing on the absorber layer quality first, we highlight thereafter aspects of interface problematics and discuss the growth and role of doped microcrystalline silicon-oxide layers and approaches of 3D-solar-cell designs in more detail. Although the findings summarized in this review were obtained from thin-film solar cells, we show that a-Si:H is a very versatile material with properties that are of high interest for application in other devices such as heterojunction solar cells, detectors, or optoelectronic devices.

High reflectivity is essential when a metal is used as back contact and reflector in thin film silicon solar cells. We show that thermal annealing at 150 °C improves the reflectivity of silver films deposited by sputtering at room temperature on nanotextured substrates. The annealing provokes two interlinked effects: rearrangement of the silver layer with a modification of its morphology and an increase of up to 42 in the grain size of the polycrystalline film for the preferential orientation as measured by X ray diffraction. The main consequence of these two mechanisms is a large increase in the reflectivity of silver when measured in air. This reflectivity increase is also noticeable in devices: amorphous silicon thin film solar cells grown on annealed silver films yield higher internal and external quantum efficiencies compared to cells grown on as deposited silver. The morphology modification smoothes down the substrate which is revealed by a clear increase of the open circuit voltage and fill factor of the cells grown on top. An amorphous silicon cell with a 200 nm nominally thick i layer fabricated on a flexible plastic substrate yielded an initial efficiency close to 10 with 15.9 mA/cm 2 of short circuit current using highly reflective annealed textured silver. We also propose for industrial purpose the sputtering of thin silver layer (120 nm) under moderate substrate temperature (~150 °C) to increase the layer reflectivity which avoids lengthening of the back reflector fabrication. © 2011 Elsevier B.V. All rights reserved.

At the front contact of thin film silicon solar cells, the junction between the p-doped window layer and the n-type transparent electrode results in a barrier that must be surmounted by the charge carriers. The barrier height is governed by the work function difference of these two materials. For different compositions of the well known In2O3-SnO2 (ITO) system, we find that higher oxygen partial pressure during sputter deposition increases the work function of the deposited films. Over the same range of oxygen partial pressures, ITO electrodes with low tin content applied to n-i-p type thin film silicon solar yield a gain in Voc by up to 40 mV. *

We investigate the influence of the oxygen content in boron-doped nanocrystalline silicon oxide films (p-nc-SiOx) and introduce this material as window layer in n-i-p solar cells. The dependence of both, optical and electrical properties on the oxygen content is consistent with a bi-phase model which describes the p-nc-SiOx material as a mixture of an oxygen-rich (O-rich) phase and a silicon-rich (Si-rich) phase. We observe that increasing the oxygen content enhances the optical gap E-04 while deteriorating the activation energy and the planar conductivity. These trends are ascribed to a higher volume fraction of the O-rich phase. Incorporated into n-i-p a-Si: H cells, p-nc-SiOx layers with moderate oxygen content yield open circuit voltage (V-oc) up to 945mV, which corresponds to a relative gain of 11% compared to an oxygen-free p-layer. As a similar gain is obtained on planar and on textured substrates, we attribute the increase in V-oc to the higher work function of the p-nc-SiOx layer made possible by its wider band gap. These results are attained without changing the dilution ratio of the 250 nm thick intrinsic layer. We also observe an enhancement of 0.6mA cm(-2) in short circuit current density in the short wavelengths due to the higher transparency of the p-nc-SiOx layer. Finally, an initial efficiency of 9.9% for a single junction 250 nm a-Si:H n-i-p solar cell on plastic foil is achieved with the optimization of the p layer thickness, the doping ratio of the front transparent conductive oxide, and the optical properties of the back reflector. (C) 2011 American Institute of Physics. [doi:10.1063/1.3669389]

Tandem solar cells based on thin film silicon benefit from an intermediate reflector layer between the top and bottom cells since it enhances the absorption in the top cell. The top cell can thus be manufactured thinner and less prone to light induced degradation. Made from a thin layer of nanocrystalline silicon oxide, the interlayer provides a second functionality since it aids in the spatial separation of local shunts occurring in both sub-cells. Recently, the reflector morphology received attention since it can provide a third function; here, a substantial difference exists between the commonly used configurations, i.e. superstrate or substrate. In the former, the thin layer of nanocrystalline silicon oxide reproduces the morphology of the underlying top cell. Its surface may thus be too rough for the growth of the bottom cell. In the latter configuration, reflectors made from a thick layer of ZnO can yield an adequate texture for the top cell, but conductive ZnO loses the effect of shunt quenching. We present our recent progress with improved intermediate reflector layers in both cell types. For silicon oxide based interlayers, we introduce a smoothing lacquer layer with self-organized openings that allow current transport. For ZnO based interlayers, we demonstrate that a treatment in oxygen plasma is capable of tuning the in-plane resistivity.

Article history:Received 31 July 2011Received in revised form 23 January 2012Available online 8 March 2012Keywords:Thin film silicon solar cell;Off-stoichiometric silicon oxide;Open-circuit voltage;Built-in voltage The introduction of a nc-SiOx:H material as window layer in single junction a-Si:H n-i-p solar cell leads to aV oc enhancement of 80 mV compared to a μc-Si:H p-layer. According to numerical modeling of the V oc , boththe higher work function p-layer and the conduction band offset (CBO) at the i/p interface match well withthe experimental V oc increase with the oxygen content. Using the differential temperature method, the built-in voltage (V bi ) of the cells with the two different p-layers is measured to be similar, agreeing well with theCBO model. Thus we attribute the improvement of the V oc to the reduction of recombination at the i/p inter-face, as a consequence of the CBO in this region.© 2012 Elsevier B.V. All rights reserved. 1. IntroductionIn amorphous silicon solar cells, the open-circuit voltage (V

This paper focuses on our latest progress in n-i-p thin-micromorph solar-cell fabrication using textured back reflectors and asymmetric intermediate reflectors, both deposited by low-pressure chemical vapor deposition of zinc oxide. We then present microcrystalline bottom cells with high crystallinity, which yield excellent long wavelength response for relatively thin absorber thickness. In a 1.5-µm-thick µc-Si:H single-junction n-i-p solar cell, we thus obtain a short-circuit current density of 25.9 mA·cm−2, resulting in an initial cell efficiency of 9.1%. Subsequently, the roughness of the intermediate reflector layer is adapted for the growth of high-performance amorphous silicon (a-Si:H) top cells. Combining bottom cells with high current, an optimal intermediate reflector morphology and a 0.22-µm-thick a-Si:H top cell, we reach high initial open-circuit voltages of 1.45 V, and we obtain a stabilized cell with an efficiency of 11.1%, which is our best stable efficiency for n-i-p solar cells.