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Microcrystalline silicon fabricated by plasma-enhanced chemical vapour deposition (PECVD) is commonly used as an absorber material in thin-film tandem solar cells. The source gases used in the μc-Si:H PECVD process are silane and hydrogen. One way to further increase the production efficiency of solar modules is to increase the gas utilization during deposition of the silicon absorber layer. In this work this is achieved by reducing the hydrogen flow. These deposition conditions are known to promote powder formation in the plasma, which can be detrimental for the solar cell’s conversion efficiency as well as for the maintenance of the system. Therefore, an easily applicable approach to determine powder formation in-situ during the PECVD process is presented. Both the self-bias-voltage and the ratio of the optical emissions from SiH* to Hβ as function of the gas residence time in the plasma is used to determine the onset of powder formation. Furthermore, a clear link between the precursor gas residence time in the plasma to the onset of powder formation is shown independent of the chosen pressure.

In this work, we report on the development of thin-film silicon solar cells on textured glass substrates. The textured glass substrates are fabricated by ion beam etching using a wet-chemically textured three-dimensional etching mask. The development of transparent and conductive front contact ZnO:Al films on textured glass is presented. The optimum of the front contact layer thickness was found to be 60 nm. For this thickness deteriorating reflection maxima are avoided, which occur due to the interferences in the front contact layer. The glass texture is adjusted to achieve better light trapping in the near infrared range. In addition, ITO instead of ZnO:Al film was investigated to surpass the decreased fill factor of solar cells deposited on ZnO:Al thin front contacts.

The intrinsic microcrystalline absorber layer growth in thin-filmsilicon solar-cells is investigated by in-situ Raman spectroscopy during plasma enhanced chemical vapor deposition. In-situ Raman spectroscopy enables a detailed study of the correlation between the process settings, the evolution of the Raman crystallinity in growth direction, and the photovoltaic parameters η(solar cell conversion efficiency), JSC (short circuit current density), FF (fill factor), and VOC (open circuit voltage). Raman spectra were taken every 7 nm of the absorber layer growth depending on the process settings. The Raman crystallinity of growing microcrystalline silicon was determined with an absolute error of approximately ±5% for total absorber layer thicknesses >50 nm. Due to this high accuracy, inherent drifts of the Raman crystallinity profiles are resolvable for almost the entire absorber layer deposition. For constant process settings and optimized solar celldevice efficiency Raman crystallinity increases during the absorber layer growth. To compensate the inhomogeneous absorber layer growth process settings were adjusted. As a result, absorber layers with a constant Raman crystallinity profile — as observed in-situ — were deposited.Solar cells with those absorber layers show a strongly enhanced conversion efficiency by ∼0.5% absolute. However, the highest FF, VOC, and JSC were detected for solar cells with different Raman crystallinity profiles. In particular, fill factors of 74.5% were observed for solar cells with decreasing Raman crystallinity during the later absorber layer growth. In contrast, intrinsic layers with favorable JSC are obtained for constant and increasing Raman crystallinity profiles. Therefore, monitoring the evolution of the Raman crystallinity in-situ provides sufficient information for an optimization of the photovoltaic parameters with surpassing depth resolution.

We present a study on the performance and analysis of hybrid solar cells comprising a planar heterojunction between between a conjugated donor polymer, P3HT or PCPDTBT, and hydrogenated amorphous silicon (a-Si:H). A comparison of the modeled absorption spectra of the layer stack with the measured external quantum efciency is used to investigate the contribution of the inorganic and organic material to the photocurrent generation in the device. Although both materials contribute to the photocurrent, the devices exhibit poor quantum e ciencies and low short circuit currents. Bandstructure simulations of the hybrid layer structure reveal that an unfavorable electric eld distributionwithin the planarmultilayer structure limits the performance. Using electroabsorption measurements we can show that the electric eld is extremelyweak in the amorphous siliconbut strong in the organicmaterial. The situation changes drasticallywhen the conjugated polymer is p-doped. Doping not only increases the conductivity of the organic material, but also restores the electric eld in the amorphous silicon layer. Optimized hybrid solar cells comprising thin doped P3HT layers exhibit energy conversion e ciencies (ECE) up to 2.8 %. || S. Schaefer, S. Albrecht, D. Neher: Universitat Potsdam, Institute of Physics and Astronomy, Soft Matter Physics, D-14476 Potsdam, Germany T. F. Schulze, E. Conrad, L. Korte, B. Rech: Department of Silicon Photovoltaics, Helmholtz Center Berlin for Materials and Energy, Kekulestr. 5, D-12489 Berlin, Germany J. Wordenweber, A. Gordijn: IEK5-Photovoltaik, Forschungszentrum Julich, D-52425 Julich, Germany U. Scherf, I. Dumsch: Bergische Universitat Wuppertal, Macromolecular Chemistry and Institute for Polymer Technology, GaussStrasse 20, D-42097 Wuppertal, Germany

In this study optical transmission measurements were performed in-situ during the growth of microcrystalline silicon (μc-Si:H) layers by plasma enhanced chemical vapor deposition (PECVD). The stable plasma emission was used as light source. The effective absorption coefficient of the thin μc-Si:H layers which were deposited on rough transparent conductive oxide (TCO) surfaces was calculated from the transient transmission signal. It was observed that by increasing the surface roughness of the TCO, the effective absorption coefficient increases which can be correlated to the increased light scattering effect and thus the enhanced light paths inside the silicon. A correlation between the in-situ determined effective absorbance of the μc-Si:H absorber layer and the short-circuit current density of μc-Si:H thin-film silicon solar cells was found. Hence, an attractive technique is demonstrated to study, on the one hand, the absorbance and the light trapping in thin films depending on the roughness of the substrate and, on the other hand, to estimate the short-circuit current density of thin-film solar cells in-situ, which makes the method interesting as a process control tool.

We present a meaningful characterization method for tandem solar cells. The experimental method allows for optimizing the output power instead of the current. Furthermore, it enables the extraction of the approximate AM1.5g efficiency when working with noncalibrated spectra. Current matching of tandem solar cells under short-circuit condition maximizes the output current but is disadvantageous for the overall fill factor and as a consequence does not imply an optimization of the output power of the device. We apply the matching condition to the maximum power output; that is, a stack of solar cells is power matched if the power output of each subcell is maximal at equal subcell currents. The new measurement procedure uses additional light-emitting diodes as bias light in theJVcharacterization of tandem solar cells. Using a characterized reference tandem solar cell, such as a hydrogenated amorphous/microcrystalline silicon tandem, it is possible to extract the AM1.5g efficiency from tandems of the same technology also under noncalibrated spectra.

For hydrogenated amorphous silicon (a-Si:H) solar cells, the critical concentration of a given impurity defines the lowest concentration which causes a decay of solar cell efficiency. Values of 2-5 x 10(19) cm(-3) are commonly found for the critical oxygen concentration (C-O(crit)) of a-Si: H. Here we report a dependence of C-O(crit) on the contamination source. For state-of-the-art a-Si: H solar cells prepared at the same plasma deposition conditions, we obtain with a (controllable) chamber wall leak C-O(crit) similar to 2 x 10(19) cm(-3) while for a leak in the gas supply line a higher C-O(crit) of similar to 2 x 10(20) cm(-3) is measured. No such dependence is observed for nitrogen. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3357424]

Etch product detection by in situ optical emission spectroscopy is used to detect the phase transition from amorphous to microcrystalline silicon. In this contribution it is demonstrated that a calibrated version of this technique can be used to determine the absolute hydrogen flux under state-of-the-art silicon thin film deposition conditions.

The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (mu c-Si:H) has been systematically investigated. Single mu c-Si:H layers and complete mu c-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of similar to 1.2 mu m thick mu c-Si: H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2 x 10(19) to 2 x 10(19) cm(-3); in the case of nitrogen contamination the critical impurity level is lower ([N](critical)=6 x 10(18)-8 x 10(18) cm(-3)). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of mu c-Si:H films. It was observed that the both contaminant types act as donors. Efficiency losses due to oxygen or nitrogen impurities are attributed to fill factor decreases and to a reduced external quantum efficiency at wavelengths of >500 nm. In the case of an air leak during the mu c-Si:H deposition process, the cell performance drops at an air leak fraction from 140 to 200 ppm compared to the total gas flow during i-layer deposition. It is demonstrated that oxygen and nitrogen impurities close to the p/i-interface have a stronger effect on the cell performance compared to impurities close to the n/i-interface. Moreover, thick mu c-Si:H solar cells are found to be more impurity-sensitive than thinner cells. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3104781]

Exploiting the higher etch probability for amorphous silicon relative to crystalline silicon, the transiently evolving phase composition of silicon films in the microcrystalline growth regime was probed in situ by monitoring the etch product (SiH4) gas density during a short H-2 plasma treatment step. Etch product detection took place by the easy-to-implement techniques of optical emission spectroscopy and infrared absorption spectroscopy. The phase composition of the films was probed as a function of the SiH4 concentration during deposition and as a function of the film thickness. The in situ results were corroborated by Raman spectroscopy and solar cell analysis. (C) 2007 American Institute of Physics.

Microcrystalline silicon carbide (mu c-SiC) films were prepared using hot wire chemical vapor deposition at low substrate temperature. The mu c-SiC films were employed as window layers in microcrystalline silicon (mu c-Si:H) n-i-p solar cells. Quantum efficiency (QE) and short circuit current density (J(SC)) in these n-side illuminated n-i-p cells were significantly higher than in standard p-i-n cells. A high QE current density of 26.7 mA/cm(2) was achieved in an absorber layer thickness of 2 mu m. The enhanced J(SC) was attributed to the wide band gap of the mu c-SiC layer and a sufficiently high hole drift mobility in mu c-Si:H absorber layer. (C) 2007 American Institute of Physics.

In order to reduce the costs of thin-film silicon solar cell production, a manufacturing concept in which amorphous silicon (a-Si) solar cells are produced in a roll-to-roll manner on a temporary metal substrate (temporary-superstrate process) has been introduced in the Helianthos Project. Later in the process the device is transferred to a permanent plastic substrate on which cells are monolithically series connected, resulting in PV modules. In the Helianthos pilot line, modules with efficiencies above 7% are obtained on 1-foot-wide foil. On 30times30 cm/sup 2/ modules, initial efficiencies of 5.7% have been reached, while 60 cm/sup 2/ modules cut from the 35 cm wide foil have reached stabilized efficiencies of 5.8%. Lab modules with the deposition of a-Si in a batch process on roll-to-roll deposited SnO/sub 2/, have reached stabilized efficiencies of 6.7%. In order to increase efficiency, amorphous/microcrystalline (a-Si/muc-Si) tandem solar cells have been implemented in the temporary-superstrate process to fabricate flexible, lightweight tandem modules with monolithic series integration. A first module with an aperture area efficiency of 8.6% has been presented earlier. At present, an initial aperture area efficiency of 9.4% has been reached on a 60 cm/sup 2/ module

Hydrogenated microcrystalline silicon (mu c-Si:H) is a promising candidate for thin-film transistors (TFTs) in large-area electronics due to high electron and hole charge carrier mobilities. We report on ambipolar TFTs based on mu c-Si: H prepared by plasma-enhanced chemical vapor deposition at temperatures compatible with flexible substrates. Electrons and holes are directly injected into the mu c-Si: H channel via chromium drain and source contacts. The TFTs exhibit electron and hole charge carrier mobilities of 30-50 cm(2)/V s and 10-15 cm(2)/V s, respectively. In this work, the electrical characteristics of the ambipolar mu c-Si: H TFTs are described by a simple analytical model that takes the ambipolar charge transport into account. The analytical expressions are used to model the transfer curves, the potential and the net surface charge along the channel of the TFTs. The electrical model provides insights into the electronic transport of ambipolar mu c-Si: H TFTs. (C) 2011 American Institute of Physics. [doi:10.1063/1.3531990]

The switching behavior of hydrogenated microcrystalline silicon thin-film transistors (TFTs) was examined and switching frequencies exceeding 20 MHz were measured for short channel devices. The microcrystalline silicon TFTs were prepared by plasma-enhanced chemical vapor deposition at temperatures compatible with plastic substrates. The realized microcrystalline silicon transistors exhibit high electron charge carrier mobilities of 130 cm2/V s. The switching frequency is limited by the contact resistances and overlap capacitances between the gate and the drain/source electrodes. Switching frequencies larger than 20 MHz were measured for transistors with a channel length of 5 μm. The high switching frequencies facilitate the realization of radio-frequency identification tags operating at 13.56 MHz.

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The influence of atmospheric contaminants oxygen and nitrogen on the performance of thin-film hydrogenated amorphous silicon (a-Si:H) solar cells grown by plasma-enhanced chemical vapor deposition at 13.56 MHz was systematically investigated. The question is addressed as to what degree of high base pressures (up to 10(-4) Torr) are compatible with the preparation of good quality amorphous silicon based solar cells. The data show that for the intrinsic a-Si: H absorber layer exists critical oxygen and nitrogen contamination levels (about 2 x 10(19) atoms/cm(3) and 4 x 10(18) atoms/cm(3), respectively). These levels define the minimum impurity concentration that causes a deterioration in solar cell performance. This critical concentration is found to depend little on the applied deposition regime. By enhancing, for example, the flow of process gases, a higher base pressure (and leak rate) can be tolerated before reaching the critical contamination level. The electrical properties of the corresponding films show that increasing oxygen and nitrogen contamination results in an increase in dark conductivity and photoconductivity, while activation energy and photosensitivity are decreased. These effects are attributed to nitrogen and oxygen induced donor states, which cause a shift of the Fermi level toward the conduction band and presumably deteriorate the built-in electric field in the solar cells. Higher doping efficiencies are observed for nitrogen compared to oxygen. Alloying effects (formation of SiOx) are observed for oxygen contaminations above 10(20) atoms/cm(3), leading to an increase in the band gap. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3009384]

The H flux to Si growth flux ratio is experimentally determined under state-of-the-art silicon thin-film deposition conditions by employing the recently introduced etch product detection technique. Under the technologically relevant high-pressure depletion conditions and for different process parameter settings such as pressure, SiH4 concentration, rf power, and excitation frequency, it was demonstrated that the microcrystalline to amorphous silicon phase transition is uniquely and reactor independently determined by the flux ratio of H and Si growth species. (C) 2008 American Institute of Physics.