Search

8th World Conference on Photovoltaic Energy Conversion; 123-127, Various amorphous silicon (a-Si:H) deposition methods for heterojunction solar cells were investigated regarding their suitability for effective wafer passivation and high solar cell efficiencies. Among them, radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) is the standard approach for low temperature a-Si:H fabrication, while very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) and hot-wire chemical vapor deposition (HW-CVD) are emerging technologies to achieve softer deposition conditions. High effective carrier lifetimes of about 9 ms and high efficiencies of about 22 % for heterojunction solar cells were achieved with all deposition methods. A better passivation with soft VHF-PECVD and HW-CVD could not be verified. However, the latter deposition methods enable higher deposition rates but therefore require elaborate power coupling compared to state-of-the-art RF-PECVD. For reference reasons, a high temperature a-Si:H fabrication process, namely lowpressure chemical vapor deposition (LPCVD), was also applied and the layer properties were compared to the standard low-temperature approaches.

38th European Photovoltaic Solar Energy Conference and Exhibition; 358-362, The catalytic chemical vapor deposition technology (CAT-CVD), also called Hot-Wire CVD (HW- CVD), is a commonly used technique for the production of various thin-film materials in many branches of the thin- film industry. The focus of this work has been placed on the process development of the hydrogenated amorphous silicon thin-film (a-Si:H) deposition used for fabrication of highly efficient heterojunction solar cells with intrinsic thin layer (HIT). The advantages of the HW-CVD technique are high deposition rates and a lack of plasma damage which is a significant feature in the production of the thin-film layers. The main subject of this work is the development of a high-rate dynamic deposition technique, which enables the realization of a process on large area substrates and in roll- to-roll systems.

37th European Photovoltaic Solar Energy Conference and Exhibition; 290-294, The main invention in this work is an application of HW-CVD and PECVD technique in one dynamic deposition system, utilizing the advantages of both methods. The focus here has been placed on the compatibility of both deposition technologies in the production of layer stacks used in the solar cell fabrication. The process parameters were developed in such a way, that different combinations of i-, p- and n- layers using PECVD and HW-CVD were deposited during a single in-line dynamic process run. Electrical parameters of fabricated thin films, as well as the passivation properties of the layer stacks, and characteristics of the HIT solar cells, give a clear picture of the quality of the combined deposition procedure. The latest results clearly show that the passivation layers with high lifetime values (14 ms) can be achieved for both deposition methods. Although, during the fabrication of the solar cells using stacks of HW-CVD i-layer and PECVD n/p-layers a degradation of the open-circuit voltage (Voc) was observed. One possible explanation for these phenomena can be that the ion bombardment during the PECVD process degrades the properties of i-HW-CVD layer. Thus, for better understanding, the electrical and structural properties of different conducting and intrinsic materials and their stacks were analyzed. Finally, complex simulation studies of HW-CVD and PECVD processes were performed to explain the principle differences between these two deposition methods.

36th European Photovoltaic Solar Energy Conference and Exhibition; 533-537, In this study a first direct comparison of three fabrication methods for high-rate fabrication of amorphous silicon (a-Si:H) in the same deposition system will be carried out. High-efficiency heterojunction solar cells with intrinsic thin-layer (HIT) are fabricated with various techniques: radio frequency (RF-), and very high frequency (VHF-) plasma enhanced chemical vapor deposition (PECVD) and hot wire chemical vapor deposition (HW-CVD) [1-4]. In this study the evaluation of process parameters for each technology will be presented. Furthermore, the properties of an in-line deposition system containing a RF-, VHF-PECVD and HW-CVD chamber for static and dynamic fabrication will be discussed. The a-Si:H layers fabricated with varied silane concentrations and deposition rates on polished (100) and textured wafers (111) will be studied. The passivation quality for each process will be evaluated by means of the effective carrier lifetime (eff). Finally, the individual steps taken to develop the a-Si:H passivation process with high carrier lifetimes and high open-circuit voltage (Voc > 720 mV) values for high efficiency solar cells (>22 %) fabricated with PECVD and HW-CVD technology will be presented.

35th European Photovoltaic Solar Energy Conference and Exhibition; 672-675, In this study, the properties of the intrinsic amorphous silicon (a-Si:H) and p-doped microcrystalline silicon (μc-Si:H) materials were investigated in a wide range of the deposition rates using the plasma enhanced chemical vapour deposition (PECVD) technique at the very high excitation frequency (VHF) and standard radio-frequency (RF). Thereby, the high-rate and low-rate regimes were compared. The material properties of the passivation layers as well as carrier lifetime were studied. Furthermore, the new p-type μc-Si:H electrodes were introduced. The initial growth process of the p-layer on top of the a-Si:H material was developed. Additionally, the influence of the ion bombardment energy on the deposition process is studied using simulation methods. At the final stage of the development, the dynamic VHF-PECVD process for the passivation and electrode layers has been introduced and applied for a highly productive fabrication of the heterojunction silicon solar cells (HIT).

35th European Photovoltaic Solar Energy Conference and Exhibition; 665-667, The plasma enhanced chemical vapor deposition (PECVD) process is used for the deposition of thin-film silicon. By varying the deposition conditions, it is possible to produce silicon layers with different material structures. The increase of the excitation frequency leads to a decrease of the ion bombardment energy on the growing layer and a reduction of the surface damage during the deposition is possible [1]. The recombination losses at the interface between absorber and emitter layer strongly affects the heterojunction solar cells (HIT) and excellent interfacial passivation is required [2]. For this purpose, the application of very high plasma excitation frequencies (VHF) for the deposition of passivation layers in comparison with standard radio frequency (RF) is investigated. The VHF-process enables a highly productive deposition of a-Si:H with a significant enhancement of the deposition rate. Thus, the influence of the deposition rate on layer properties for 13.56 and 140 MHz frequency will be presented.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 745-748, Intrinsic hydrogenated amorphous silicon layers (a-Si:H) for heterojunction solar cells with intrinsic thin layers (HIT) are deposited by means of radio frequency (RF) and very high frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD). The VHF plasma excitation frequency used here (140 MHz) is much higher than typically used in the VHF range. It is widely known, that the ion energy in the plasma decreases with increasing plasma excitation frequency. Thus, it can be expected that the reduced ion bombardment energies in VHF plasmas results in a better passivation of the wafer surface. The properties and the passivation potential of intrinsic a-Si:H layers fabricated with a plasma excitation frequency of 140 MHz and 13.56 MHz are compared. It is demonstrated, that the effective carrier lifetimes remain higher for VHF a-Si:H layers grown near the onset of epitaxial growth as compared to RF a-Si:H layers. This leads to a higher degree of freedom for the fabrication of passivation layers by VHF-PECVD with high hydrogen dilution ratios in the plasma. High carrier lifetimes up to 13 milliseconds are achieved. The static and dynamic deposition mode of a-Si:H passivation layers is compared. A drawback of the dynamic deposition method could not be observed. Instead, a slightly increased carrier lifetime for the dynamic deposition is demonstrated. High solar cell efficiencies above 20 % are achieved for heterojunction devices with intrinsic thin a-Si:H layers.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2577-2579, A concept for large-area high-rate VHF-PECVD (60 MHz) of a-Si:H and μc-Si:H on moving substrates is developed and evaluated. Up to 40x40 cm² substrates can be used. The deposition plasma is sustained between linear electrodes and a moving substrate. Due to the gas flow geometry and the high degree of source gas depletion, the moving substrate "sees" different silane concentrations when passing the electrodes. These effects are smeared out and state-ofthe- art solar cells are produced by this reactor. Initial solar cell efficiencies reached up to 9.8% which is the same as for the reference cells deposited by RF-PECVD. However, the deposition rates of a-Si are higher for VHF (~5 Å/s) than for RF (~2 Å/s). For μc-Si solar cells with an efficiency of 7.6% a deposition rate of 9.5 Å/s has been obtained. Standard μc- Si solar cells deposited by RF-PECVD showed efficiencies up to 8.6 % at growth rates of 5.4 Å/s.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2558-2561, Improvement in the layer quality is one of the main topics in increasing efficiency of thin film silicon solar cells. Therefore, process control is discussed in order to reduce effects of initial transition states. In this work we present our progress in installing real-time process control into an existing deposition system. By monitoring different depositions with optical emission spectroscopy (OES) it becomes clear that optical emission signals are sensitive to SiH4 flow, power and pressure changes. H/SiH and H/SiH intensity ratios correlate with Raman crystallinity of the deposited layers, when SiH4 flow is changed. Deposition processes can be stabilized by using real-time process control, where setpoints and actual values of mass flow control are modified depending on OES signals. Hence, the H/SiH is stable during the complete deposition.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 2983-2986, An alternative concept for the dynamic deposition of amorphous and microcrystalline silicon is presented in this paper. The aim of our work is to combine Very High Frequency PECVD with a linear plasma source in order to demonstrate high efficiency cells and high deposition rates at the same time. Due to the design of the linear plasma source the homogeneity has to be guaranteed in the direction perpendicular to the substrate movement. In movement direction the homogeneity is achieved by stable plasma conditions throughout the deposition. So far amorphous solar cells with initial efficiencies of 9.2 % and microcrystalline solar cells with an efficiency of 6.6 % have been produced. The static deposition rates were 0.56 nm/s and 0.43 nm/s respectively.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3032-3035, In this paper the activities of the photovoltaics group at the EWE research centre – NEXT ENERGY is described. The focus of the work lies on thin film silicon solar cells. A baseline process has been implemented allowing the deposition of state-of-the-art micromorph tandem solar cells. This is done with the goal to substantionally increase the efficiency of this kind of cells in the future and to build up the base for new concepts like triple junction solar cells. Detailed simulations were performed to get physical insight into the fundamental mechanisms in the solar cell and to extract further potential of efficiency enhancement. 3-dimensional optical simulations of structures with realistic topography will be presented. First experiments with ps-laser scribing have been performed for TCO and a-Si:H ablation At the end an outlook about the triple junction activities will be given.