A-Si:H/c-Si heterojunctions: a future mainstream technology for high-efficiency crystalline silicon solar cells ?

In this contribution, we shortly review the main features of amorphous/crystalline silicon heterojunction (SHJ) solar cells, including interface defects and requirements for high quality interfaces. We show how a process flow with a limited number of process steps leads to screen printed solar cells of 2x2cm(2) with 21.8% efficiency and of 10x10cm(2) with 20.9% efficiency (n-type FZ). We show that the devices work in high injection conditions of 3x10(15)cm(-3) at the maximum power point, a factor two higher than the base doping. Several research labs and companies can now produce large area 6 '' cells well over 20% on CZ wafers and some of the critical cost factors, such a metallization can be overcome with suitable strategies. Based on the high quality coating tools and processes developed for thin films used for flat panel display or thin film solar cell coatings, the deposition of the layers required to make SHJ cells has the potential to be performed in a controlled way at low cost. Considering the few process steps required, the high quality n-type Cz wafers that can be obtained by proper crystal growth control, SHJ technology has several assets that could make it become a widespread PV technology.

TEM characterization of textured silicon heterojunction solar cells

Christophe Ballif, Aïcha Hessler-Wyser, Christian Monachon

Increasing the efficiency of crystalline silicon (c-Si) solar cells requires the reduction of both bulk and interface recombination. Even if bulk recombination is almost suppressed, the symmetry of the crystal lattice is disturbed at the surface and hence, due in particular to non-saturated bonds (dangling bonds), a large density of defects exists. Thus, in this case, the free-carrier lifetimes are no longer limited by the quality of the bulk c-Si, but by its surface. To keep the recombination losses at the c-Si surface at minimal levels, the surface must be electronically well passivated. An efficient way to obtain passivation is to use low temperature grown (typically 200°C) hydrogenated amorphous silicon (a-Si:H) [1]. In the case of photovoltaic applications, the passivation of both c-Si wafer surfaces (i.e., the emitter and the rear surface) is of crucial importance for good performances. In this work, the 3-7 nm a-Si:H based passivation layers of the device are grown by VHF-PECVD in a single chamber. The solar cells consist of a multilayered structure: Al back contact / DC-sputtered ITO (Indium Tin Oxyde) / n/i a-Si:H back surface field (BSF) / n-type c-Si substrate with a resistivity of 1-3 Ωcm / intrinsic a-Si:H / i/p a-Si:H emitter / and a front contact made out of DC-sputtered ITO via a shadow mask to define the cells (diameter = 4.5 mm). Such devices are called heterojunction (HJ) silicon solar cells. Surface recombination losses are a major concern for all c-Si solar cells. In particular, mastering of HJ emitter and back surface field formation on textured c-Si is crucial for high-performance HJ solar cells. High Resolution Transmission Electron Microscopy (HRTEM) is necessary to identify the key microstructural features of the a-Si:H/c-Si interface, and TEM micrographs of HJ interfaces on flat and textured high-performance devices are shown for the first time and discussed with respect to the resulting solar cell electrical performances. TEM micrographs of our flat high-efficiency HJs show abrupt c-Si/a-Si:H/μc-Si:H interfaces for emitter and BSF formation. Whereas on the pyramidal facets of the textured substrate the growth is identical to the flat substrate interface, we observed unexpected epitaxial growth at the bottom of the pyramid valleys. We have identified these local epitaxial domains as an efficient surface recombination path, and by consequence, as responsible for the observed decreased VOC on textured HJ cells. When minimizing the density of epitaxial domains at the c-Si/a-Si:H interface by adapting the deposition conditions, a solar cell VOC of 660 mV is obtained. An additional modification of the textured c-Si surface morphology leads to VOCs as high as 700 mV.

Springer2008

High-Quality Microcrystalline Silicon for Efficient Thin-Film Solar Cells

Grégory Bugnon

This thesis investigates the link between the plasma deposition conditions and microcrystalline silicon (μc-Si:H) material quality for thin-film silicon photovoltaic applications. The role of interfaces and the μc-Si:H material quality on the device performance are analyzed in detail. The low absorption of μc-Si:H at long wavelengths requires the deposition of absorber layers with thicknesses of typically a few micrometers for use in multi-junction TF Si solar cells. The growth typically takes place on highly textured surfaces, which provide increased light absorption—often called light trapping—but which potentially induce structural defects in the film during its growth. Therefore, to further improve the TF Si technology, one of the main challenges is the identification of the determinant plasma deposition parameters that result in the growth of very high-quality μc-Si:H at an increased deposition rate on textured substrates that guarantee efficient light trapping. As a first approach to better understand the plasma conditions necessary for the growth of high-quality μc-Si:H, the roles of both the silane depletion fraction and the deposition pressure are studied in an industrial-type large-area KAI reactor. With increasing pressure and silane depletion, the μc-Si:H defect density is significantly lowered leading to improved solar cell performance. An estimation of the average energy with which ions impinge on the substrate supports the hypothesis that ion bombardment is mainly responsible for the observed differences. Then, a fundamental aspect of μc-Si:H deposition on highly textured substrates is highlighted: two different phases of μc-Si:H material contribute to the overall solar cell efficiency, both of which can drive cell performance. The first phase relates to the bulk material and dominates the performance of cells on flat substrates. However, on rough morphologies, substrate-induced defective localized nanoporous regions—the second phase—develop and are found to be significantly more sensitive to the plasma process conditions and substrate morphology than the bulk phase. The relative importance of this secondary defective phase is shown through the use of new damp-heat experiments. Silicon oxide doped layers are demonstrated to mitigate the influence of these nanoporous regions on the solar cell performance. Next, a comparative study of the plasma excitation frequencies of 13.56 MHz (RF) and 40.68 MHz (VHF) shows that, while both allow for the growth of very good-quality bulk material, the efficiency of VHF-prepared cells is always poorer compared to that of RF-prepared cells within the range of our study for growth rates below 5 Å s−1. This decrease in solar cell performance is related to a higher density of nanoporous regions in the VHF-prepared cells as evidenced by damp-heat experiments, leading to strong open-circuit voltage instabilities. Still, the use of VHF is shown to be beneficial at increased deposition rates, thanks to reduced ion bombardment and improved bulk material quality. The crucial interplay between μc-Si:H growth rate and substrate morphology with regard to the formation of nanoporous regions is further discussed for regimes with high deposition rates of around 10 Ås−1. It is shown that high-silane-depletion regimes with significantly reduced H2 flow rate or increased pressure lead to a denser μc-Si:H material but are associated with increased secondary gas-phase reactions and powder formation. The use of a reduced interelectrode distance is demonstrated to allow for the growth of μc-Si:H with significantly improved bulk material quality at higher growth rates. Plasma simulations performed in collaboration with the University of Patras are presented and suggest that improvements observed in the μc-Si:H material quality are related mainly to an increased contribution of less reactive silane monoradicals, such as SiH3 and Si2H5, to the growing film, as compared to highly reactive ones such as SiH2 and SiH. Then, in an effort to better understand the formation of these two distinct μc-Si:H phases, the intrinsic stress within μc-Si:H i-layers is studied and correlated with the bulk defect density. Further improvements to both μc-Si:H and a-Si:H solar cells are obtained by introducing a novel intrinsic silicon oxide buffer layer at the p -i interface. For μc -Si:H solar cells, all electrical parameters are improved unless the i -layer is significantly more amorphous-rich and high quality, in which case an improvement only in carrier collection in the blue region is observed. In a-Si:H solar cells the silicon oxide buffer is shown to lower light-induced degradation, which is one of the weak points of TF Si technology. Furthermore, for both a-Si:H and μc-Si:H solar cells, the buffer can also act as an efficient barrier to boron cross-contamination, eliminating the need for additional time-consuming processing steps such as a water flush for single-chamber processes. Overall, this work contributes to a better understanding of the μc-Si:H material requirements for PV applications and how they relate to the plasma deposition conditions. Based on all the aforementioned developments, significant progress has been made in the understanding and the fabrication of thin-film silicon solar cells based on μc-Si:H. An outstanding single-junction μc-Si:H solar cell of 10.9% was attained; to our knowledge this is the highest reported in the literature. This work also contributed to the development of very high-efficiency tandem and triple-junction thin-film silicon solar cells at PV-Lab.

EPFL2013

Towards Very Low-Cost Mass Production of Thin-film Silicon Photovoltaic (PV) Solar Modules on Glass

Arvind Shah

Production volume of PV modules increases at > 35% per year, but one is yet far from making a global impact on energy supply. One of the obstacles is given by the present high production costs of PV modules. A possibility to reduce costs are thin-film PV modules on glass. The specific option of thin-film silicon is considered. The combination of amorphous and microcrystalline silicon thin films within a tandem solar cell corresponds to a theoretical optimum. In practice, stabilized efficiencies of 10% to 12% have so far been obtained in the laboratory with such tandem solar cells. Silicon being a material with an indirect band gap, its absorption coefficient is relatively low, and therefore light management in the solar cell has to be further optimized. Thin-film silicon can be deposited by plasma-enhanced CVD, as used for AM-LCD displays. The use of modified fabrication equipment from the AM-LCD Display Industry is therefore a promising way to implement low-cost mass production. © 2005 Elsevier B.V. All rights reserved.