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In this work, in-situ transmission measurements using plasma as light source are presented for the determination of growth rate and crystallinity during silicon thin-film growth. The intensity of distinct plasma emission lines was measured at the backside of the transparent substrates on which silicon films, ranging from amorphous to microcrystalline, were deposited. Using this configuration, the growth rate of thin-films was determined with high accuracy. In addition, we show that the crystallinity of the films can be monitored in the most critical range (between 40% and 80%) for microcrystalline silicon solar cells by evaluating the intensity ratio of two transmitted wavelengths in-situ. The gradual change in the absorption behaviour of the films during the phase transition is reflected by this ratio of two wavelengths as demonstrated by the good correlation with the crystallinity fraction determined by ex-situ Raman spectroscopy. This approach of in-situ transmission spectroscopy provides an easy-to-implement monitoring and control system for the industrial deposition of thin-film silicon solar cells, as critical material properties can be determined real-time during the deposition process even on rough substrates that are optimized for light trapping in solar cells. (C) 2011 Elsevier B.V. All rights reserved.

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.

In this work, the authors present a 7.5% efficient hybrid tandem solar cell with the bottom cell made of amorphous silicon and a Si-PCPDTBT:PC70BM bulk heterojunction top cell. Loss-free recombination contacts were realized by combing Al-doped ZnO with either the conducting polymer composite PEDOT:PSS or with a bilayer of ultrathin Al and MoO3. Optimization of these contacts results in tandem cells with high fill factors of 70% and an open circuit voltage close to the sum of those of the sub-cells. This is the best efficiency reported for this type of hybrid tandem cell so far. Optical and electrical device modeling suggests that the efficiency can be increased to similar to 12% on combining a donor polymer with suitable absorption onset with PCBM. We also describe proof-of-principle studies employing light trapping in hybrid tandem solar cells, suggesting that this device architecture has the potential to achieve efficiencies well above 12%. (C) 2014 Elsevier B.V. All rights reserved.

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.

The increase of deposition rate of microcrystalline silicon absorber layers is an essential point for cost reduction in the mass production of thin-film silicon solar cells. In this work we explored a broad range of plasma enhanced chemical vapor deposition (PECVD) parameters in order to increase the deposition rate of intrinsic microcrystalline silicon layers keeping the industrial relevant material quality standards. We combined plasma excitation frequencies in the VHF band with the high pressure high power depletion regime using new deposition facilities and achieved deposition rates as high as 2.8 nm/s. The material quality evaluated from photosensitivity and electron spin resonance measurements is similar to standard microcrystalline silicon deposited at low growth rates. The influence of the deposition power and the deposition pressure on the electrical and structural film properties was investigated.

The electrical stability of high-mobility microcrystalline silicon (μc -Si:H) thin-film transistors (TFTs) was investigated and compared to amorphous silicon (a-Si:H) TFTs. Under prolonged bias stress the microcrystalline silicon TFTs exhibit an improved electrical stability compared to amorphous silicon TFTs. The microcrystalline silicon TFTs were prepared by plasma-enhanced chemical vapor deposition at temperatures compatible with flexible substrates. The realized microcrystalline silicon transistors exhibit electron charge carrier mobilities exceeding 30 cm2/V·s. Prolonged operation of the transistors leads to a shift of the threshold voltage towards positive and negative gate voltages depending on the gate biasing conditions (positive or negative gate voltage). The shift of the threshold voltage increases with increasing positive and negative gate bias stress. The behavior is fundamentally different from the behavior of the amorphous silicon TFTs, which exhibit only a shift of the threshold voltage towards positive gate voltages irrespective of the polarity of the gate bias stress. The threshold voltage shift of the microcrystalline silicon TFTs saturates after a few minutes to a few hours, depending on the gate voltage. After prolonged bias stress, a recovery of the initial threshold voltage is observed without any thermal annealing or biasing of the transistors, which is not the case for the measured amorphous silicon TFTs.

For microcrystalline silicon based p–i–n solar cells the effect of deposition conditions on the critical oxygen concentration was investigated. All solar cells were prepared by 13.56 MHz plasma-enhanced chemical vapour deposition. The critical oxygen concentration defines the lowest oxygen concentration in the intrinsic absorber layer causing a deterioration of the solar cell performance. For intentional contamination of ∼1.2–1.3 µm thick i-layers, the oxygen was inserted by a controllable leak at the process gases supply line, i.e. by a gas pipe leak. For µc-Si:H deposited at a discharge power of 0.53 W/cm2 we find a critical oxygen concentration of 1–2 × 1019 cm–3 in agreement with values commonly reported in literature. However, changing the deposition conditions, we find that the critical oxygen concentration in µc-Si:H cells is not fixed. At reduced power of 0.20 W/cm2 a much higher value for the critical oxygen concentration of 1 × 1020 cm–3 is observed. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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]

Hydrogenated microcrystalline silicon (μc-Si:H) has recently attracted significant attention as a promising candidate for thin-film transistors (TFTs) in large-area electronics due to high electron and hole charge carrier mobilities. We report on top-gate ambipolar TFTs based on μc-Si:H prepared by plasma-enhanced chemical vapor deposition at temperatures below 200 °C. Electrons and holes are directly injected into the μc-Si:H channel via chromium drain and source contacts. The TFTs exhibit electron and hole charge carrier mobilities of 30-50 cm2/Vs and 10-15 cm2/Vs, respectively. In this work, the electrical characteristics of the top-gate ambipolar μc-Si:H TFTs are described by a simple analytical model that takes the ambipolar 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 μc-Si:H TFTs (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrogenated microcrystalline silicon (mu c-Si:H) has lately attracted considerable attention as a promising candidate for thin-film transistors (TFTs) in large area electronic applications due to its superior charge carrier mobility. Here, we present ambipolar TFTs and inverters based on microcrystalline silicon prepared by plasma-enhanced chemical vapor deposition at low deposition temperature of 160 degrees C. The electrical parameters of the ambipolar microcrystalline silicon TFTs and inverters will be described. The influence of contact effects on the operation of ambipolar microcrystalline silicon TFTs was investigated. Furthermore, the influence of the ambipolar transistor characteristics on the performance of the ambipolar inverter will be discussed. (C) 2009 Elsevier Ltd. All rights reserved.

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.

Hydrogenated microcrystalline silicon (µc-Si:H) has recently received significant attention as a promising material for thin-film transistors (TFTs) in large area electronics due to its high electron and hole charge carrier mobilities. We report on ambipolar TFTs based on microcrystalline silicon prepared by plasma-enhanced chemical vapor deposition at temperature of 160 °C with high electron and hole charge carrier mobilities of 40 cm 2 /Vs and 10 cm 2 /Vs, respectively. The ambipolar microcrystalline silicon TFTs provide a simple route in realizing large area integrated circuits at low cost. The electrical characteristics of the ambipolar microcrystalline silicon TFTs will be described and the first results on ambipolar inverters will be presented. The influence of the ambipolar TFT characteristics on the performance of the inverter will be also discussed.

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]

Thin-film transistors (TFTs) are core elements of novel display media for large-area electronic applications. Microcrystalline-silicon TFTs prepared at low temperatures (150 degrees C-200 degrees C) have recently gained much attention as potential elements for such applications due to their high charge carrier mobilities exceeding 10 cm(2)/V . s. Understanding the relationship between structural properties and charge transport is the key in realizing transistors with high charge carrier mobility at low temperatures. In this paper, we investigated the correlation between the structural properties of microcrystalline silicon and the performance of high-mobility microcrystalline-silicon TFTs. Transistors with high electron and hole charge carrier mobilities exceeding 50 and 12 cm(2)/V . s, respectively, were realized near the transition to the amorphous-growth regime. The results reveal that electronic defects at the grain boundaries of silicon crystallites are passivated by the amorphous phase. The results contradict the commonly believed assumption that the highest charge charier mobility can only be achieved for films with high or very high crystalline-silicon volume fraction. The crystalline volume fraction of the material will be correlated to the device parameters of transistors. Furthermore, the first results of microcrystalline-silicon-TFT-based complementary-metal-oxide-semiconductor inverters with high voltage gains exceeding 22 will be presented.

A detailed analysis of the monolithical series connection of silicon thin-film modules with metal back contact fabricated by high-speed laser ablation will be presented. In this study, optically pumped solid-state lasers with wavelengths of 1064 nm and 532 nm were used for the patterning process. The influence of various laser parameters on the performance of amorphous and microcrystalline silicon modules will be discussed. In particular, the line-scribing parameters for a TCO and Ag back contact system was analyzed in detail, since it is the most critical patterning step. A detailed description of the back contact ablation process will be presented and a criterion for flakeless patterning was defined. Finally the influence of the back contact patterning on the electrical behavior of silicon single junction cells was studied. The dark current density versus back-contact patterning line length was analyzed by means of a developed SPICE (simulation program with integrated circuit emphasis) simulation model. Copyright © 2007 John Wiley & Sons, Ltd.

Thin-film transistors (TFTs) are core elements of novel display media on rigid or flexible substrates, radio-frequency identification tags on plastic foils, and other large area electronic applications. Microcrystalline silicon TFTs prepared at temperatures compatible with flexible substrates (150-200 degrees C) have gained much attention as potential elements for such applications due to their high charge carrier mobilities. Understanding the relationship between the structural properties and the charge transport is essential in realizing TFTs with high charge carrier mobility at low temperatures. In this study, top-gate staggered microcrystalline silicon TFTs were realized by plasma-enhanced chemical vapor deposition at maximum temperature of 180 degrees C. We investigated the correlation between the structural properties of the microcrystalline silicon channel material and the performance of the microcrystalline silicon TFTs. Transistors with the highest charge carrier mobility, exceeding 50 cm(2) /V s, were realized near the transition to amorphous growth. The results reveal that electronic defects at the grain boundaries of the silicon crystallites are passivated by the amorphous phase near the transition to amorphous growth. The crystalline volume fraction of the channel material will be correlated with the transistor parameters such as charge carrier mobility, threshold voltage, and subthreshold slope. (C) 2008 American Institute of Physics.

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.

Encapsulated and series-connected amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) based thin film silicon solar modules were developed in the superstrate configuration using an aluminum foil as temporary substrate during processing and a commodity polymer as permanent substrate in the finished module. For the development of μc-Si:H single junction modules, aspects regarding TCO conductivity, TCO reduction, deposition uniformity, substrate temperature stability and surface morphology were addressed. It was established that on sharp TCO morphologies where single junction μc-Si:H solar cells fail, tandem structures consisting of an a-Si:H top cell and a μc-Si:H bottom cell can still show a good performance. Initial aperture area efficiencies of 8.2%, 3.9% and 9.4% were obtained for fully encapsulated amorphous silicon (a-Si:H) single junction, microcrystalline silicon (μc-Si:H) single junction and a-Si:H/μc-Si:H tandem junction modules, respectively.