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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 design and operation of ambipolar transistors and inverters were studied. In order to gain insights in the operation of ambipolar inverters an analytical model was developed which describes the electrical behavior of ambipolar transistors and inverters. The model was compared to experimentally realized inorganic and organic ambipolar thin-film transistors and inverters. Furthermore, the influence of the transistor parameters on the voltage transfer characteristics and the static noise margin are discussed. The model provides a general description which is applicable to a variety of ambipolar transistor technologies based on different material systems.

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.

An etching procedure was applied to microcrystalline silicon (μc-Si:H) thin films in order to obtain a wedge-shaped profile for depth-resolved characterization. A microfluidic flow cell that merges deionized water with a potassium hydroxide solution (KOHaq) was utilized. The samples consisted of texture-etched ZnO:Al on a Corning Glass substrate, a microcrystalline p-doped layer serving as seed layer and the investigated intrinsic microcrystalline or amorphous silicon (a-Si:H). Along the etched profiles, microscopic Raman spectroscopy was used to estimate the crystalline volume fraction Xc for samples deposited with intentionally varied silane concentration to investigate the a-Si:H/μc-Si:H and μc-Si:H/a-Si:H transition.

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.

Atmospheric pressure Chemical Vapour Deposition has been used to deposit polycrystalline thin films of SnO2:F on aluminium foil. This foil is used commercially as a temporary substrate in the production of flexible solar cells. The resulting thin films were characterised and fabricated into a-Si:H/mc-Si:H tandem solar cells. These devices gave promising initial energy conversion efficiencies up to 7.5% and high current densities. Spectral response measurements showed good blue response indicating sufficient transparent conducting oxide/p-interface and excellent current values for the bottom cell as a result of the high quality of the SnO2:F.

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.

In this work we investigate and discuss the influence of a dynamic inline deposition process on the properties of microcrystalline silicon solar cells. A VHF-PECVD inline deposition system with linear plasma sources working at 81.36 MHz was employed for film preparation. We focused on the dynamic deposition of solar cells at different substrate carrier speeds. The solar cell properties as well as the crystallinity were determined and analyzed. Our best cells so far showed efficiencies of 6.3% at a static deposition rate of 0.43 nm/s. For increasing substrate carrier speeds no negative trend could be observed. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Silicon based thin tandem solar cells were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a 30 × 30 cm2 reactor. The layer thicknesses of the amorphous top cells and the microcrystalline bottom cells were significantly reduced compared to standard tandem cells that are optimized for high efficiency (typically with a total absorber layer thickness from 1.5 to 3 µm). The individual absorber layer thicknesses of the top and bottom cells were chosen so that the generated current densities are similar to each other. With such thin cells, having a total absorber layer thickness varying from 0.5 to 1.5 µm, initial efficiencies of 8.6–10.7% were achieved. The effects of thickness variations of both absorber layers on the device properties have been separately investigated. With the help of quantum efficiency (QE) measurements, we could demonstrate that by reducing the bottom cell thickness the top cell current density increased which is addressed to back-reflected light. Due to a very thin a-Si:H top cell, the thin tandem cells show a much lower degradation rate under continuous illumination at open circuit conditions compared to standard tandem and a-Si:H single junction cells. We demonstrate that thin tandem cells of around 550 nm show better stabilized efficiencies than a-Si:H and µc-Si:H single junction cells of comparable thickness. The results show the high potential of thin a-Si/µc-Si tandem cells for cost-effective photovoltaics. Copyright © 2010 John Wiley & Sons, Ltd.

Photovoltaic cell performance can be greatly improved by optimising the transparent conducting oxide used as the front contact. We have developed an advanced atmospheric pressure chemical vapour deposition (APCVD) process, by applying fast experimentation and using a combinatorial chemistry approach to aid the studies. By use of this process, F-doped SnO2 has been deposited using either monobutyl tin trichloride or tin tetrachloride with aqueous HF or trifluoro-acetic acid as the dopant source. The deposited films were characterised for crystallinity, morphology, resistivity and growth rate to aid optimisation of material suitable for solar cells. The results from use of the different tin precursors and dopants were compared. The most striking changes were related to resistivity and surface morphology. Compared with commercially available TCO CVD coated glasses, our coatings show excellent performance resulting in a high quantum efficiency yield for a-Si:H solar cells.

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.

Crystalline silicon carbide alloys have a very high potential as transparent conductive window layers in thin-film solar cells provided they can be prepared in thin-film form and at compatible deposition temperatures. The low-temperature deposition of such material in microcrystalline form (µc-Si:C:H) was realized by use of monomethylsilane precursor gas diluted in hydrogen with the Hot-Wire Chemical Vapor Deposition process. A wide range of deposition parameters has been investigated and the structural, electronic and optical properties of the µc-SiC:H thin films have been studied. The material, which is strongly n-type from unintentional doping, has been used as window layer in n-side illuminated microcrystalline silicon solar cells. High short-circuit current densities are obtained due to the high transparency of the material resulting in a maximum solar cell conversion efficiency of 9.2%.

The potential of thin film photovoltaic technologies in supporting sustainable energy policies has led to increasing interest in high performance transparent conducting oxides (TCOs), and in particular doped SnO2, as electrical contacts for solar cells. 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) and resistance 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 1 pass, which is much lower than those of industrially available TCOs. Selected samples were then used for manufacturing single amorphous silicon (a-Si:H) solar cells, which showed high solar energy conversion efficiencies up to 8.2% and high short circuit currents of 16 mA/cm2. Compared with (commercially available) TCO glasses coated by chemical vapour deposition, our TCO coatings show excellent performance resulting in a high quantum efficiency yield for a- Si:H solar cells.

In this paper we report the results of a study assessing a newly developed deposition process for Fdoped SnO2 films by CVD operating at atmospheric pressure (APCVD). The technology is designed to be compatible with industrial requirements such as high process speed, possible up-scaling to wide substrate widths and low costs. The optical and electrical properties of layers deposited on glass are found to be similar to those of commercially available lowpressure CVD. Optical absorptance below1% is achieved for films of around 0.8�mthick. Such transparent conductive oxide (TCO) is used with a-Si:H single junction p–i–n solar cells grown by PECVD. The cells are characterised by I–V measurements using AM1.5 spectra and by measuring the external quantum efficiencies (EQE). The initial efficiencies were up to 9.3% with FF = 73%. The TCO films demonstrated an enhanced performance in the EQE compared to commercially available TCO (Asahi-U).

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.

N-type microcrystalline silicon carbide layers prepared by hot-wire chemical vapor deposition were used as window layers for microcrystalline silicon n–i–p solar cells. The microcrystalline silicon intrinsic and p-layers of the solar cells were prepared with plasma-enhanced chemical vapor deposition at a very high frequency. Amorphous silicon incubation layers were observed at the initial stages of the growth of the microcrystalline silicon intrinsic layer under conditions close to the transition from microcrystalline to amorphous silicon growth. ‘Seed layers’ were developed to improve the nucleation and growth of microcrystalline silicon on the microcrystalline silicon carbide layers. Raman scattering measurement demonstrates that an incorporation of a ‘seed layer’ can drastically increase the crystalline volume fraction of the total absorber layer. Accordingly, the solar cell performance is improved. The correlation between the cell performance and the structural property of the absorber layer is discussed. By optimizing the deposition process, a high short-circuit current density of 26.7 mA/cm 2 was achieved with an absorber layer thickness of 1 μm, which led to a cell efficiency of 9.2%.

The influence of the crystalline volume fraction of hydrogenated microcrystalline silicon on the device performance of thin-film transistors fabricated at temperatures below 200 °C was investigated. Transistors employing microcrystalline silicon channel material prepared close to the transition to amorphous growth regime exhibit the highest charge carrier mobilities exceeding 50 cm2/V s. The device parameters like the charge carrier mobility, the threshold voltage and the subthreshold slope will be discussed with respect to the crystalline volume fraction of the intrinsic microcrystalline silicon material.

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.

Raman spectra of microcrystalline silicon layers have been recorded in-situ during growth. The spectra have been collected under realistic conditions for solar cell deposition. To enable these measurements an electrode with an optical feed through has been developed. By using a metallic grid to shield the feed through it is possible to achieve homogeneous deposition of µc-Si:H at a sufficient optical transmission. In-situ Raman measurements were carried out during the deposition of a layer with an intentionally introduced gradient in crystallinity that was seen in-situ as well in reference measurements performed on the same layer ex-situ. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 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.

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.