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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 study, nanoimprint processing was used to realize various multiscale textures on glass substrates for application in thin-film photovoltaic devices. The multiscale textures are formed by a combination of large and small features, which proofed to be beneficial for light trapping in silicon thin-film solar cells. Two approaches for the fabrication of multiscale textures are presented in this study. In the first approach, the multiscale texture is realized at the lacquer/transparent conductive oxide (TCO) interface, and in the second approach, the multiscale texture is realized at the TCO/Si interface. Various types of multiscale textures were fabricated and tested in microcrystalline thin-film silicon solar cells in p-i-n configuration to identify the optimal texture for the light management. It was found that the best light-scattering multiscale texture was realized using an imprint-textured glass substrate, which contains large craters, in combination with HF-etched TCO (ZnO:Al), which contains small features, on top of the imprint. With this structure (of the second approach), the short-circuit current density of the solar cell devices was improved by 0.6 mA/cm-2 using multiscale textures realized by nanoimprint processing.

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 structure of hydrogenated amorphous silicon films is investigated by Raman spectroscopy and X-ray diffraction. Raman spectroscopy probes the phonon density of states, whereas X-ray diffraction measures the distribution of the electron density. Yet, both methods can yield information on the microstructure of the material represented by certain parameters like, e.g., the position or the width of the transverse optical phonon or the width of the first scattering peak. Interdependences between these parameters are investigated and evaluated. A correlation was found between the structural disorder and the relative efficiency loss caused by the Staebler-Wronski effect for intrinsic films applied as absorbing layers in solar cells. This correlation could be used to estimate the solar cell degradation without time-consuming light-soaking experiments.

Single-chamber processes for the deposition of high efficiency thin-film silicon tandem cells of an a-Si:H p-i-n (top cell)/μc-Si:H p-i-n (bottom cell) structure involving short fabrication time are reported. An industry relevant reactor and an excitation frequency of 13.56 MHz were used. The conversion efficiency is found to be highly sensitive to dopant cross contamination into the μc-Si:H i-layer of the bottom cell and within the n/p-interface of the tunnel recombination junction (TRJ). Different reactor treatments at the p/i-interfaces of the top and bottom cells and at the n/p-interface of the TRJ were applied, aiming to prevent dopant cross contamination. The phosphorus and the boron concentrations were evaluated by secondary ion mass spectrometry measurements. Phosphorus cross contamination after TRJ n-layer deposition is found to result in significant n-type doping of the μc-Si:H i-layer of the bottom cell if no reactor treatment is applied. In situ reactor treatment via an Ar flush and pumping step of 15 min applied at the n/p-interface of TRJ results in reduction of the μc-Si:H i-layer phosphorus concentration to values below 1017 cm− 3. A conversion efficiency of 11.8% for such tandem cells is demonstrated. Shorter interface treatment time with phosphorus concentrations in the μc-Si:H i-layer of about 5 × 1017 cm− 3 results in lower conversion efficiencies of 10.6%, mainly due to the decrease of open-circuit voltage and fill factor.

In this work, we report on the fabrication of microcrystalline thin-film silicon solar cells on textured glass substrates. The development of transparent and conductive front contacts for these solar cells is presented. State-of-the-art random textures for light-trapping were replicated into a glass-like resist on glass substrates with an imprint process. We applied an industrial relevant soft polymer mold that gives excellent replication accuracy. The necessity of applying thin front contacts for enhanced incoupling of the incident light is shown. An increased series resistance of these thin front contacts caused a decrease of the fill factor of the solar cells. One way to surpass this decrease in fill factor by reducing the solar cell width is demonstrated. In addition, the light-trapping and the light-incoupling for solar cells deposited on three different types of random textures were compared.

In this work, we present a technology for a high precision nanostructure replication process based on ultraviolet nanoimprint lithography for the application in the field of thin-film photovoltaics. The potential of the technology is demonstrated by the fabrication of microcrystalline silicon thin-film prototype solar cells. The high accuracy replication of random microstructures made from sputtered and etched ZnO:Al, used to scatter the incident light in thin solar cells, is shown by local topography investigations of the same 7.5 × 7.5 µm2 area on the master and the replica. Different types of imprint resists and imprint moulds were investigated to find the optimal, high precision replication technology. Two types of thin-film silicon solar cells, in p-i-n and n-i-p configuration, were fabricated to study the potential of the imprint technology for different applications. It is shown that solar cells deposited on an imprinted glass hold similar performances compared with reference solar cells fabricated with a standard process on textured ZnO:Al. Thus, it is demonstrated that the replication of light scattering structures by using an imprint process is an attractive method to decouple the scattering properties from the layer forming the electrical front contact. Because a simple and cheap high throughput process is used, this study additionally proves the relevance for the industrial mass production in the field of photovoltaics. Copyright © 2013 John Wiley & Sons, Ltd.

The influence of oxygen and nitrogen impurities on the material properties of a-Si:H and μc-Si:H films and on the corresponding solar cell performances was studied. For intentional contamination of the i-layer the impurities were inserted by two contamination sources: (i) directly into the plasma through a leak at the chamber wall or (ii) into the gas supply line. The critical oxygen and nitrogen concentrations for silicon solar cells were determined as the lowest concentration of these impurities in the i-layer causing a deterioration of the cell performance. Similar critical concentrations for a-Si:H and μc-Si:H cells in the range of 4–6 × 1018 cm− 3 for nitrogen and 1–5 × 1019 cm− 3 for oxygen by applying a chamber leak are observed. Similar increase of conductivity with increasing impurity concentration in the a-Si:H and μc-Si:H films is found. A more detailed study shows that the critical oxygen concentration depends on the contamination source and the deposition parameters. For a-Si:H cells, the application of the gas pipe leak leads to an increased critical oxygen concentration to 2 × 1020 cm− 3. Such an effect was not observed for nitrogen. For μc-Si:H, a new deposition regime with reduced discharge power was found where the application of the gas pipe leak can also result in an increase of the oxygen concentration to 1 × 1020 cm− 3.

Boron (B) and phosphorus (P) cross-contamination for single-chamber deposited a-Si:H, μc-Si:H, and a-Si:H/μc-Si:H tandem solar cells has been investigated by studying their impact on the different layers of solar cells. To reduce the B and P cross-contamination into the i-layer and p-layer, respectively, to a tolerable level, for a-Si:H and μc-Si:H cells a 15' evacuation cycle prior to the i-layer deposition is applied. The effect of P cross-contamination into the i-layer is strongly reduced by the p-layer deposition and a 15’ evacuation cycle prior to the i-layer deposition. The p-layer is assumed to cover up or to fix (in form of P-B complexes) most of the P at the chamber walls. This leads to high quality μc-Si:H cells and a-Si:H cells with only slightly reduced performance. Here, a soft-start of the a-Si:H i-layer led to high quality cells, presumably due to reduced P recycling. Further, there is no need to clean the process chamber with, e.g. NF3, after each p-layer, as applied in many industrial processes. Instead, many cells are deposited without cleaning the process chamber. We established a single-chamber tandem cell process with 15' evacuation cycles prior to the μc-Si:H p-layer and to each i-layer with a cell efficiency of ~ 11.1%.

The continuous deposition of microcrystalline silicon has been monitored with in-situ Raman spectroscopy. The process and measurement settings were chosen such that one spectrum was taken during approximately 9 nm of layer growth. This allows observing the crystallinity in the initial growth phase of microcrystalline silicon absorber layers. The influence of different p-doped seed layers has been studied. Under constant deposition conditions, an initial decrease in crystallinity was observed over the first tens of nanometers. By profiling the process gas flows during the initial phase it was possible to reduce the amount of amorphous material that was detected during the initial phase of deposition.

Hydrogenated amorphous silicon (a-Si:H) is conventionally deposited using static plasma-enhanced chemical vapor deposition (PECVD) processes. In this work, a very high frequency (VHF) dynamic deposition technique is presented, on the basis of linear plasma sources. This configuration deploys a simple reactor design and enables continuous deposition processes, leading to a high throughput. Hence, this technique may facilitate the use of flexible substrates. As a result, the production costs of thin-film silicon solar cells could be reduced significantly. We found a suitable regime for the homogeneous deposition of a-Si:H layers for growth rates from 0.35–1.1 nm/s. The single layer properties as well as the performance of corresponding a-Si:H solar cells are investigated and compared with a state-of-the-art radio frequency (RF) PECVD regime. By analyzing the Fourier transform infrared spectroscopy spectra of single layers, we found an increasing hydrogen concentration with deposition rate for both techniques, which is in agreement with earlier findings. At a given growth rate, the hydrogen concentration was at the same level for intrinsic layers deposited by RF-PECVD and VHF-PECVD. The initial efficiency of the corresponding p–i–n solar cells ranged from 9.6% at a deposition rate of 0.2 nm/s (RF regime) to 8.9% at 1.1 nm/s (VHF regime). After degradation, the solar cell efficiency stabilized between 7.8% and 5.9%, respectively. The solar cells incorporating intrinsic layers grown dynamically using the linear plasma sources and very high frequencies showed a higher stabilized efficiency and lower degradation loss than solar cells with intrinsic layers grown statically by RF-PECVD at the same deposition rate. Copyright © 2012 John Wiley & Sons, Ltd.

This work presents as-grown textured ZnO:Al films by rf magnetron sputtering initiated by pre-treatment of glass substrate with mixed argon and oxygen ions. A 650 nm thick of this film exhibits surface texture features with lateral size around 500 nm; the resistivity is below 5 × 10 −4 Ω · cm and the transparency in the near-infrared spectral range is high (> 80% at 1000 nm). Microcrystalline silicon thin film solar cells grown on the textured glass exhibit excellent light trapping effect with a short circuit current density of 18.2 mA/cm².

In situ optical transmission measurements performed during thin-film silicon plasma-enhanced chemical vapor deposition (PECVD) are presented. Hereto, the plasma emission was used as light source. With this setup information about thickness, crystallinity and absorption characteristic of the growing intrinsic silicon thin film can be obtained. By integrating the intrinsic layers in solar cells with p-i-n configuration, the layer information gained in situ during the PECVD process can be directly correlated to the generated short-circuit current of the solar cell. The intention of this paper is to show that, by using these transmission measurements for the estimation of solar cell currents, an in situ current matching of stacked a-Si/μc-Si tandem devices is possible, which is a useful extension of the process control techniques.

For deposition of a-Si:H p–i–n solar cells, a single-chamber plasma enhanced chemical vapor deposition process at the frequency of 13.56 MHz is developed. A 40×40 cm² deposition chamber, which represents typical industry reactors equipped with showerhead electrodes is employed. Various methods are applied to reduce the boron-cross contamination from the boron-doped p-layer into the intrinsic layer, which is considered to reduce solar cell efficiency by losses especially in the short wavelength range. Three different device configurations and four different chamber treatment methods are studied, aimed to reach stable device efficiencies comparable to multi-chamber systems at minimum chamber treatment effort and treatment time. An ex-situ CO2–plasma treatment applied after deposition of the p-doped layer is found to be effective to reduce boron-cross contamination. However, this CO2-treatment is a time-consuming process step for production. We found a less time consuming treatment: by a chamber evacuation to 9×10−7 mbar subsequent to p-layer deposition. Initial and stable efficiencies of 10.2% and 7.7%, respectively, were obtained. This latter treatment results in a sharp drop of the boron concentration from ∼5×1020 cm−3 in the p-doped layer to ∼1017 cm−3 in the intrinsic layer. For comparison of different reactor geometries and their influence on the cross-contamination we used a small-area (10×10 cm2) lab-type reactor.

Thin‑film silicon solar cells based on hydrogenated amorphous silicon (a‑Si:H) and hydrogenated microcrystalline silicon (μc‑Si:H) absorber layers are typically deposited using static plasma-enhanced chemical vapor deposition (PECVD) processes. It has been found that the use of very‑high frequencies (VHF) is beneficial for the material quality at high deposition rates when compared to radio-frequency (RF) processes. In the present work a dynamic VHF‑PECVD technique using linear plasma sources is developed. The linear plasma sources facilitate the use of very-high excitation frequencies on large electrode areas without compromising on the homogeneity of the deposition process. It is shown that state-of-the-art a‑Si:H and μc‑Si:H single-junction solar cells can be deposited incorporating intrinsic layers grown dynamically by VHF-PECVD at 0.35 nm/s and 0.95 nm/s, respectively.

For different process conditions, series of hydrogenated amorphous silicon p-i-n solar cells with various oxygen concentrations in the intrinsic absorber layer were fabricated by plasma-enhanced chemical vapor deposition at 13.56 MHz using process gas mixtures of SiH 4 and H 2 . Oxygen was introduced into the gas phase during the deposition process by a controllable leak in the chamber wall and the amount of oxygen supply is characterized by the oxygen base pressure p b . It is found that for a certain deposition regime defined by silane and H 2 flows, deposition pressure and substrate temperature the oxygen incorporation follows an expected dependence on the ratio p b / r d with r d the deposition rate. This relation is not valid for the comparison of different deposition regimes. A high hydrogen flow is found to reduce the oxygen incorporation strongly. The photovoltaic parameters of the solar cells were measured in the initial state as well as after 1000 h of light-soaking. The critical oxygen concentration (i.e. the upper limit of incorporated oxygen not leading to a decay of the solar cell performance) was determined for each regime in the initial and light-soaked state. For all deposition regimes, the results show no difference in these critical oxygen concentrations for the initial and light-soaked state. The critical oxygen concentration, is found to differ for the different process regimes and turns out to be the highest (approximately 1×10 20 cm −3 ) for the deposition regime with the highest hydrogen flow rate, which interestingly is the regime with the lowest oxygen incorporation at a given p b / r d ratio. This combination makes the regime of high hydrogen gas flow suitable for depositing high-efficiency solar cells at high base pressure.

In this work we present a novel electrode design for the plasma enhanced chemical vapor deposition of microcrystalline silicon thin films that enables optical access to the growing layer under normal incidence. The optical access is realized by piercing the electrode with a conical feed through of 10 mm diameter at the electrode side facing the plasma. The influence of the feed through on deposition homogeneity is studied in different pressure regimes from 8 mbar to 24 mbar on intrinsic layers optimized for state of the art thin-film silicon solar cells. The homogeneity of the layers was determined by spatially resolved thickness measurements and evaluation of the crystalline volume fraction by Raman spectroscopy. With the aim to minimize the influence of the disturbance of the electrode surface the effect of different insets in the feed through on the homogeneity is studied. To find a maximum in optical transmission of the insets at optimal film homogeneity different designs of metallic grids were tested. We show that using the modified electrode it is possible to deposit microcrystalline silicon layers which are comparable in homogeneity to those fabricated with an unchanged standard electrode. This was achieved by covering only 19% of the area of the feed through by a metallic inset.

The performance and light-soaking behavior of hydrogenated amorphous silicon (a-Si:H) solar cells with absorber layers deposited under non-constant silane concentration (SC) – a measure of silane dilution in hydrogen – using plasma enhanced chemical vapor deposition (PECVD) are investigated. Constant SC values during deposition close to the amorphous to microcrystalline phase transition lead to the formation of crystallites after a certain thickness. To prevent this transition, SC is adjusted during growth to produce an amorphous material that is close to the microcrystalline phase transition without the inclusion of a detectable microcrystalline phase. By adjusting SC during deposition it was possible to achieve an increased open-circuit voltage that is up to 40 mV higher than that for a conventional amorphous silicon solar cell at initial efficiencies above 9%. The best solar cells produced with non-constant SC show improved stability against light induced degradation, which leads to a relative loss in fill factor of only 11.4%, resulting in a stabilized fill factor of 62.5%.

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.

Textured glass is prepared by ion beam treatment using wet chemically etched ZnO film as three dimensional etching mask. The shape and rms roughness of the textured glass can be adjusted by changing the initial ZnO thickness, wet-chemical etching duration and ion beam etching parameters. The maximum rms roughness we achieved is 228 nm, with lateral feature size larger than 2 μm. The glass texturing process allows to study scattering already at the glass/TCO interface to enhance the refractive index step from rough material to silicon. First microcrystalline silicon thin film solar cells grown on the textured glass reveal certain light trapping effect with a short circuit current density up to 20 mA/cm2. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Parameters for the structural short (SRO) and medium range order (MRO) of hydrogenated amorphous silicon (a-Si:H) films on the edge of the microcrystalline silicon (μc-Si:H) phase transition were studied with Raman spectroscopy. The observed samples were deposited using radio frequency plasma enhanced chemical vapor deposition. The studied films were grown with various constant and non-constant silane concentrations (SCs). A substrate dependent correlation of SC to the intensity ratio (I MRO ) of the transversal acoustical (TA) and the transversal optical (TO) phonon bands was found. A strong correlation between width and position of the (TO) phonon band was observed. These two easily accessible parameters show an increase of SRO when I MRO decreases.

Several approaches that lead to shorter production times of a-Si/ μc-Si tandem cells are combined in this paper: high-rate sputtering of aluminum-doped zinc oxide, high-rate 40 MHz plasma deposition of microcrystalline silicon and reduced i-layer thicknesses. On standard lab-type texture-etched ZnO:Al, 1 cm 2 a-Si:H/μc-Si:H tandem test cells on a deposition area of 30 x 30 cm 2 were made that showed an initial efficiency of 9.9%, whereas the total effective deposition time of intrinsic layers was only 22 min (15 min for the top cell and 7 min for the bottom cell). The silicon thickness is only 600 nm. On high-rate texture-etched ZnO:Al an efficiency of 9.4% initial was reached. Standard light-induced degradation experiments showed a degradation rate of only 5.5-7.9% after 1000 h. This regime of very short preparation times and relatively high-stabilized efficiencies is highly interesting from the production point-of-view.

Hydrogenated microcrystalline silicon has recently emerged as a promising material system for large-area electronic applications such as thin-film transistors and solar cells. In this paper, thin-film transistors based on microcrystalline silicon were realized with charge carrier mobilities exceeding 40 cm(2)/Vs. The electrical characteristics of the microcrystalline silicon thin-film transistors are limited by the influence of contact effects. The influence of the contact effects on the charge carrier mobility was investigated for transistors with different dimensions of the drain and source contacts. The experimental results were compared to an electrical model which describes the influence of the drain and source contact dimension on the transistor parameters. Furthermore, the Transmission Line Method was applied to investigate the contact effects of the thin-film transistors with different drain and source contact dimensions. Finally, optimized device geometries like the channel length of the transistor and dimension of the drain and source contacts were derived for the microcrystalline transistors based on the electrical model.

Fourier transform infrared absorption spectroscopy (FTIR), optical emission spectroscopy (OES), self-bias voltage and plasma impedance controls were applied as in situ process diagnostics during the deposition of amorphous silicon thin-films. The diagnostic abilities of OES and FTIR are compared. The FTIR in-situ direct measurement of silane concentration in exhaust line is more precise than OES control. All in situ process diagnostics clearly indicates the inconsistency of plasma properties and therefore of deposition conditions. The drifts are comparable with the film deposition time. The FTIR measurement of reactant concentration in the process chamber evidence that the strong silane concentration drop (about 50%) in a plasma is the cause of the short-term drift of OES signals (SiHlow asterisk emission), plasma impedance and self-bias voltage signals. The influences of the deposition chamber geometry and technological parameters on process drifts are considered. The decrease of the gas residence time in the reactor leads to a decrease of Initial Transient State phenomena. Finally, the improvement of solar cell performance based on thin silicon films is demonstrated when drifts are reduced.

This study addresses the potential of different approaches to improve the generated current in silicon thin-film solar cells and modules. Decreasing the carrier concentration in the front contact has proven to increase the quantum efficiency and the cell-current density significantly. Additionally, an optically improved ZnO/Ag back reflector and the optimized light incoupling by anti-reflection layers were studied. In this contribution, we show the potential of the different optical components and discuss combinations thereof in order to obtain a maximized cell-current density in silicon thin-film solar cells. Limitations of the cell-current density are discussed with respect to theoretical calculations.

Thin-film silicon solar cells need efficient light absorption to achieve high efficiencies. The standard light trapping approach consists of a randomly textured transparent substrate and a highly reflective back contact. In this case, light scattering at the rough TCO–silicon interface leads to a prolonged absorption path and consequently to an increased short circuit current. In this study, we will discuss a new approach based on silver nanoparticles to improve the light absorption in the thin-film silicon solar cells. Raman and SNOM measurements and theoretical investigations on systems with metallic nanoparticles indicate a strong increase of the electric field in their surrounding when they are irradiated by light. Moreover, nanoparticles with the proper diameter can enhance light scattering. In this study, we have investigated the influence of silver nanoparticles with different sizes on the optoelectronic properties of amorphous and microcrystalline silicon solar cells. The nanoparticles are located at the back contact of the thin-film solar cell deposited in a n–i–p layer sequence.

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 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.

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.

For the deposition of microcrystalline silicon it is important to increase the deposition rate and silane utilization rate. In the past, a method based on optical emission spectroscopy (OES) has been introduced to obtain the transition point from amorphous to crystalline growth in-situ, which is the point for optimum microcrystalline silicon solar cell conditions. The method is based on alternating deposition by a silane/hydrogen plasma and etching by a pure hydrogen plasma. This paper combines OES with Fourier transform infrared (FTIR) spectroscopy in the exhaust line to determine the growth rate in-situ. In this way, the multidimensional space of silane flow, deposition rate and gas utilization rate is determined in-situ in one deposition. It is aimed to increase the gas utilization rate towards 100%.

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.

Process control is very important in the fabrication of high quality thin-film silicon solar cells. Solar cell parameters like film thickness, crystalline volume fraction or conductivity are usually measured in the back end of an industrial production line using ex-situ techniques. At the back end of solar module production the most of the money has been spent already and detrimental effects on the system performance like process drifts during the fabrication have not been detected online. Measuring in-situ, during the deposition of the thin silicon layers, utilizes the advantage that the investments in the front end of the process are still at low level. Additionally, the possibility of an active process control and hence the optimization of the solar cell is given, by monitoring process parameters in real time. In recent studies we performed transmission measurements during silicon deposition, in which the plasma emission was used as light source. It was shown, that deposition rate and the crystalline volume fraction of microcrystalline silicon layers and the roughness of ZnO:Al substrates can be detected with high accuracy using only a single optical setup. Additionally, the setup convinces with its simplicity for the use as process control which makes it interesting for the industrial mass production. In this paper we show that using the transmission measurements the absorption characteristic of the growing silicon thin film can be estimated. It can be seen that a direct correlation between the measured absorption of intrinsic absorber layers and the resulting solar cell current is possible.

A sophisticated light-management is indispensable for silicon thin-film silicon solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. The optical properties of thin-film solar cells have a significant influence on the conversion efficiency. The topology of the nano-textured interfaces affects the optical path and absorption. A rough transparent conductive oxide (TCO) film leads to a high quantum efficiency and shortcircuit current density. Simulations of various geometries indicate the optimal texture. Therefore, we simulate 3-dimensional tandem thin-film solar cells with different interfaces. The roughness can be identified by atomic force microscope (AFM) scans. In order to accurately analyze all aspects of the light propagation in solar cells, numerical simulations of Maxwell's equations are needed. By standard simulation programs for solving Maxwell's equations, it is difficult to simulate realistic textures of the solar cell layers. Therefore, a simulation tool based on the finite difference time domain (FDTD) method and the finite integration technique (FIT) is developed, which is able to integrate AFM scan data. To incorporate the nanostructure of a relevant section in the AFM scans, high computational domains are needed. This leads to a large number of grid points in the resulting discretization. Parallel computations on high performance computers are needed to meet the large computational requirements. The simulations show that the light propagation in the investigated thin-film device is a complex phenomenon depending on the wavelength and phase of the incident light.

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]

We report on a direct comparison of the effect of the atmospheric contaminants on a-Si:H and μc-Si:H p-i-n solar cells deposited by plasma-enhanced chemical vapor deposition (PECVD) at 13.56 MHz. Nitrogen and oxygen were inserted by two types of controllable contamination sources: (i) directly into the plasma through a leak at the deposition chamber wall or (ii) into the process gas supply line. Similar critical concentrations in the range of 4-6×1018 cm-3 for nitrogen and 1.2-5×1019 cm-3 for oxygen were observed for both a-Si:H and μc-Si:H cells for the chamber wall leak. Above these critical concentrations the solar cell efficiency decreases for a-Si:H solar cells due to losses in the fill factor under red light illumination (FFred). For μc-Si:H cells the losses in FFred and in short-circuit current density deteriorate the device performance. Only for a-Si:H the critical oxygen concentration is found to depend on the contamination source. Conductivity measurements suggest that at the critical oxygen concentration the Fermi level is located about 0.05 eV above midgap for both a-Si:H and μc-Si:H.

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.

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.

An analysis of the monolithical series connection of silicon thin-film modules with metal back contact fabricated by high speed laser ablation will be presented. Optically pumped solid state lasers with wavelengths of 1064 nm and 532 nm were used for the patterning steps. The near infrared laser is applied to pattern the TCO (P1) while the green laser is used for the ablation of the silicon layer stack (P2) and the back contact layer stack (P3). The influence of various laser parameters on the performance of amorphous and microcrystalline silicon modules was studied. In particular the back contact patterning and the Si removal can significantly affect the module efficiency. Non-optimized patterning conditions for P2 can lead to a high contact resistance, while the ablation of the ZnO/Ag back contact system can introduce shunts at the laser scribed line. Therefore, a criterion for flakeless patterning will be briefly introduced and the influence of flakeless back contact patterning on the electrical behavior of silicon single junction cells will be discussed.

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.

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.

High performance transparent conducting oxides (TCOs), have significance for optimising PV performance. We have developed an advanced atmospheric pressure chemical vapour deposition process, by applying fast experimentation and using a combinatorial chemistry approach to aid the studies. The deposited films were characterised for crystallinity, morphology (roughness), haze and resistivity to aid optimisation of material suitable for solar cells. Optical measurements on these samples showed low absorption losses, less than 1% around 500 nm for one pass, which is much lower than those of industrially available TCOs. Selected samples were then used in manufacture of single junction a-Si : H solar cells, which showed high initial solar energy conversion efficiencies up to 9.3% and high short circuit current densities of 15 mA/cm². Compared with (commercially available) TCO CVD coated glasses, these TCO coatings show excellent performance resulting in a high quantum efficiency yield for a-Si : H solar cells.