Amorphous Silicon Top Cell Research Articles

Amorphous silicon/crystal silicon heterojunction double-junction tandem solar cell with open-circuit voltage above 1.5 V and high short-circuit current density

Photovoltaic device requires light-absorbing materials that are highly efficient, low cost, lightweight, and stable during operation. Here we demonstrate the use of a hydrogenated amorphous silicon (a-Si:H) top cell and a crystal silicon heterojunction (HIT) bottom cell to form a double-junction solar cell with a high open circuit voltage (VOC), which is potentially functioned in the solar-to-hydrogen generation process and the replacement of chemical battery. Our photovoltaic device shows a high overall VOC of 1.561 V and a short current density of over 14 mA/cm2, which outperforms all other reported pure silicon-based double-junction solar cells. The newly developed a-Si:H/HIT double-junction solar cells hold the promise for some potential applications.

Characterization of thin film tandem solar cells by radiofrequency pulsed glow discharge – Time of flight mass spectrometry

Beside low production costs and the use of nontoxic and abundant raw materials, silicon based thin-film solar cells have the advantage to be built up as multi junction devices like tandem or triple junction solar cells. Silicon thin film modules made of tandem cells with hydrogenated amorphous silicon (a-Si:H) top cell and microcrystalline (μc) Si:H bottom cell are available on the market. In this work, the analytical potential of state-of-the art radiofrequency (rf) pulsed glow discharge (PGD) time of flight mass spectrometry (TOFMS) commercial instrumentation is investigated for depth profiling analysis of tandem-junctions solar cells on 2mm thick glass substrate with 1µm thick ZnO:Al. Depth profile characterization of two thin film tandem photovoltaic devices was compared using millisecond and sub-millisecond rf-PGD regimes, as well as the so-called “low mass mode” available in the commercial instrument used. Two procedures for sample preparation, namely using flat or rough cell substrates, were compared and the distribution of dopant elements (phosphorous, boron and germanium) was investigated in both cases. Experimental results obtained by rf-PGD-TOFMS as well as electrical measurements of the samples showed that a worse depth resolution of dopant elements in the silicon layers (e.g. distribution of boron in a thicker region that suggests a diffusion of this dopant in the coating of the sample) found using a rough sample substrate was related to a higher power conversion efficiency.

Material and solar cell research in high efficiency micromorph tandem solar cell

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchâtel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. This paper describes the use, within p–i–n- and n–i–p-type solar cells, of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (_c-Si:H) thin films (layers), both deposited at low temperatures (200_C) by plasma-assisted chemical vapour deposition (PECVD), from a mixture of silane and hydrogen. Optical and electrical properties of the i-layers are described. Finally, present performances and future perspectives for a high efficiency ‘micromorph’ (mc-Si:Hya-Si:H) tandem solar cells are discussed.

Comparison of LPCVD and sputter-etched ZnO layers applied as front electrodes in tandem thin-film silicon solar cells

Aluminum-doped zinc oxide (ZnO:Al) layers deposited by sputtering and boron-doped zinc oxide (ZnO:B) layers deposited by low-pressure chemical vapor deposition (LPCVD) are well-established materials for front electrodes in thin-film silicon solar cells. In this study, both types of front electrodes are evaluated with respect to their inherent properties and their surface textures in micromorph tandem solar cells in the superstrate configuration. The silicon layer stack investigated here consists of a 220-nm-thick amorphous silicon top cell, a 40-nm-thick intermediate reflector and a 1.1-µm-thick microcrystalline silicon bottom cell; for this specific silicon layer stack, the LPCVD ZnO:B provides higher power conversion efficiency than its sputtered ZnO:Al counterpart. The growth-friendly surface topography of ZnO:Al yields better electrical performance. However, tandem cells on ZnO:Al suffer from a fundamental optical limitation in terms of light trapping in the top cell. They also show a higher parasitic absorption linked to the relatively high doping concentration of the ZnO:Al layers used here. Detailed analysis of the experimental results allows us to clearly understand the opto-electrical behavior of both types of cells and envisage several possible upgrades to further improve their performance.

Triple-junction amorphous/microcrystalline silicon solar cells: Towards industrially viable thin film solar technology

The status of the development of triple-junction (amorphous silicon/microcrystalline/microcrystalline silicon) test cells fabricated with up-scalable PECVD processes is given. A first draft of novel rather flat scattering superstrates with an optimized high bandgap amorphous silicon top cell including a carbon-free buffer at the p/i interface is presented. The careful optimization of the interfaces (including sub-oxide doped layers and buffers) and of the bulk crystallinity of the middle and bottom microcrystalline sub-cells allowed to reach a stabilized efficiency of 13.5 % (after 1000h light-soaking, internal measurement). Sub-cells individual developments are discussed and their specific integration into the full triple-junction device with its optimization for AM1.5G illumination relying on a partly LED supported solar simulator characterization is presented.

High-Stable-Efficiency Tandem Thin-Film Silicon Solar Cell With Low-Refractive-Index Silicon-Oxide Interlayer

We report the recent advances and key requirements for high-efficiency tandem thin-film silicon solar cells composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell. The impact of inserting a low-refractive-index silicon-oxide (SiOx) film as intermediate reflecting layer (IRL) is highlighted. We show that refractive indexes as low as 1.75 can be obtained for layers still conducting enough to be implemented in solar cells, and without no additional degradation. This allows for high top-cell current densities with thin top cells, enabling low degradation rates. A micromorph cell with a certified efficiency of 12.63% (short-circuit current density of 12.8 mA/cm(2)) is obtained for an optimized stack. Furthermore, short-circuit current densities as high as 15.9 mA/cm(2) are reported in the amorphous silicon top-cell of micromorph devices by combining a 150-nm- thick SiOx-based IRL and a textured antireflecting coating at the air-glass interface.

Advancing tandem solar cells by spectrally selective multilayer intermediate reflectors.

Thin-film silicon tandem solar cells are composed of an amorphous silicon top cell and a microcrystalline silicon bottom cell, stacked and connected in series. In order to match the photocurrents of the top cell and the bottom cell, a proper photon management is required. Up to date, single-layer intermediate reflectors of limited spectral selectivity are applied to match the photocurrents of the top and the bottom cell. In this paper, we design and prototype multilayer intermediate reflectors based on aluminum doped zinc oxide and doped microcrystalline silicon oxide with a spectrally selective reflectance allowing for improved current matching and an overall increase of the charge carrier generation. The intermediate reflectors are successfully integrated into state-of-the-art tandem solar cells resulting in an increase of overall short-circuit current density by 0.7 mA/cm(2) in comparison to a tandem solar cell with the standard single-layer intermediate reflector.

Thin-Film Silicon Triple-Junction Solar Cells on Highly Transparent Front Electrodes With Stabilized Efficiencies up to 12.8%

High-efficiency thin-film silicon triple-junction solar cells in p-i-n configuration have been fabricated using amorphous silicon top cell absorber layers, as well as microcrystalline silicon middle and bottom cell absorbers. The triple-junction cells were fabricated on boron doped zinc oxide (ZnO) films with different surface morphologies. To this end, the naturally grown rough ZnO surfaces were flattened using an Ar plasma for three different treatment times. For the shortest time, we achieved a summed current density over 30 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and initial and stabilized conversion efficiencies of 13.5% and 12.5%, respectively. For the medium treatment time, we obtained the highest efficiencies (13.7% initial and 12.8% stable), whereas the longest treatment time led to the highest open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ) of 1.91 V but lower current densities, leading to efficiencies of 12.9% initial and 12.2% stable, respectively. These results were obtained by combining various recently developed features and approaches: first of all, we implemented high-quality μc-Si:H cells with novel buffer layers, leading to very high efficiencies. Second, we applied randomly textured pyramids on the front glass to improve light in-coupling, and finally, we used very thin (~140 nm) top cells that led to a low light-induced degradation (5%-7% relative loss in efficiency).

Enhanced recovery of light-induced degradation on the micromorph solar cells by electric field

The recovery of light-induced degradation of the tandem micromorph solar cell by applying reverse bias is compared with the single-junction amorphous silicon solar cell. The illuminated current density-voltage characteristics and external quantum efficiency show that the degradation of both the micromorph and the amorphous silicon cells can be recovered by applying sufficient reverse bias. The micromorph cell was recovered at smaller reverse bias than amorphous silicon cell. The abundant H in the microcrystalline silicon bottom cell of the micromorph cell can act as a reservoir to repair the defects in the amorphous silicon top cell at the reverse bias. This is responsible for small recovery bias of tandem cells.

Thin film solar cells incorporating microcrystalline Si1–xGex as efficient infrared absorber: an application to double junction tandem solar cells

AbstractWe have fabricated efficient (∼7–8%) hydrogenated microcrystalline Si1–xGex (µc‐Si1–xGex:H, x ∼ 0.1–0.17) single junction p‐i‐n solar cells with markedly higher short‐circuit current densities than for µc‐Si:H (x = 0) solar cells due to enhanced infrared absorption. By replacing the conventional µc‐Si:H with the µc‐Si1–xGex:H as infrared absorber in double junction tandem solar cells, the bottom cell thickness can be reduced by more than half while preserving the current matching with hydrogenated amorphous silicon (a‐Si:H) top cell. An initial efficiency of 11.2% is obtained for a‐Si:H/µc‐Si0.9Ge0.1:H solar cell with bottom cell thickness less than 1 µm. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Silicon Thin-Film Solar Cells

We review the field of thin-film silicon solar cells with an active layer thickness of a few micrometers. These technologies can potentially lead to low cost through lower material costs than conventional modules, but do not suffer from some critical drawbacks of other thin-film technologies, such as limited supply of basic materials or toxicity of the components. Amorphous Si technology is the oldest and best established thin-film silicon technology. Amorphous silicon is deposited at low temperature with plasma-enhanced chemical vapor deposition (PECVD). In spite of the fundamental limitation of this material due to its disorder and metastability, the technology is now gaining industrial momentum thanks to the entry of equipment manufacturers with experience with large-area PECVD. Microcrystalline Si (also called nanocrystalline Si) is a material with crystallites in the nanometer range in an amorphous matrix, and which contains less defects than amorphous silicon. Its lower bandgap makes it particularly appropriate as active material for the bottom cell in tandem and triple junction devices. The combination of an amorphous silicon top cell and a microcrystalline bottom cell has yielded promising results, but much work is needed to implement it on large-area and to limit light-induced degradation. Finally thin-film polysilicon solar cells, with grain size in the micrometer range, has recently emerged as an alternative photovoltaic technology. The layers have a grain size ranging from 1 μm to several tens of microns, and are formed at a temperature ranging from 600 to more than 1000∘C. Solid Phase Crystallization has yielded the best results so far but there has recently been fast progress with seed layer approaches, particularly those using the aluminum-induced crystallization technique.

Influence of microcrystalline silicon bottom cell on micromorph tandem solar cell performance

Micromorph tandem solar cells have been realized growing the intrinsic layers of both amorphous silicon top cell and microcrystalline silicon bottom cell by VHF-PECVD at 100 MHz at low substrate temperature (150 °C). The influence of the microcrystalline p-layer on the bottom cell have been studied by growing single microcrystalline devices. For the bottom absorber material two different regimes have been explored by setting the plasma power at 20 W and 33 W. The effect of the structural composition of the microcrystalline absorber layer on the electrical parameters of the device has been investigated. A wider amorphous to crystalline transition region has been found at 33 W, while larger V OC and FF values have been obtained at 20 W. The highest efficiency (11.1%) has been obtained at 20 W.

Deposition pressure effects on material structure and performance of micromorph tandem solar cells

Tandem solar cells represent an elegant way of overcoming the efficiency limits of single-junction solar cells and reducing the light-induced degradation of amorphous silicon films. Stacked structures consisting of an amorphous silicon top cell and a microcrystalline silicon bottom cell allow a good utilization of the solar spectrum due to the band gap values of the two materials. These devices, firstly introduced by the IMT research group, were designated as “micromorph” tandem solar cells. To better exploit this concept, it is important to tune parameters like the band gaps and the short-circuit currents. In this work, we have realized micromorph tandem solar cells on Asahi U-type TCO-covered glass substrates. The intrinsic layer of both the amorphous top cell and the microcrystalline bottom cell is grown by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at 100 MHz at low substrate temperature (150 °C). Finally, a ZnO reflector and a metal contact complete the structure. No intermediate optical mirror between the two cells is used at this stage. Undiluted a-Si:H, with reduced band gap when compared to H 2-diluted amorphous silicon, is used as absorber layer in the top cell. As for the bottom cell, the high-pressure–high-power regime (up to 267 Pa–80 W) has been explored aiming at growing high-quality microcrystalline silicon at large deposition rates. The effect of the structural composition of the microcrystalline absorber layer on the current–voltage characteristic and spectral response of tandem devices has been investigated. An efficiency of 11.3% has been obtained with short-circuit current densities around 13 mA/cm 2, open-circuit voltages ∼1.34 V and fill factors ∼66%.

Amorphous solar cells, the micromorph concept and the role of VHF-GD deposition technique

During the last two decades, the Institute of Microtechnology (IMT) has contributed in two important fields to future thin-film silicon solar cell processing and design: (1) In 1987, IMT introduced the so-called “very high frequency glow discharge (VHF-GD)” technique, a method that leads to a considerable enhancement in the deposition rate of amorphous and microcrystalline silicon layers. As a direct consequence of reduced plasma impedances at higher plasma excitation frequencies, silane dissociation is enhanced and the maximum energy of ions bombarding the growing surface is reduced. Due to softer ion bombardment on the growing surface, the VHF process also favours the formation of microcrystalline silicon. Based on these beneficial properties of VHF plasmas, for the growth of thin silicon films, plasma excitation frequencies f exc in the range 30–300 MHz, i.e. clearly higher than the standard 13.56 MHz, are indeed scheduled to play an important role in future production equipment. (2) In 1994, IMT pioneered a novel thin-film solar cell, the microcrystalline silicon solar cell. This new type of thin-film absorber material––a form of crystalline silicon––opens up the way for a new concept, the so-called “ micromorph” tandem solar cell concept. This term stands for the combination of a microcrystalline silicon bottom cell and an amorphous silicon top cell. Thanks to the lower band gap and to the stability of microcrystalline silicon solar cells, a better use of the full solar spectrum is possible, leading, thereby, to higher efficiencies than those obtained with solar cells based solely on amorphous silicon. Both the VHF-GD deposition technique and the “micromorph” tandem solar cell concept are considered to be essential for future thin-film PV modules, as they bear the potential for combining high-efficiency devices with low-cost manufacturing processes.

Microcrystalline silicon and micromorph tandem solar cells

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchâtel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. According to our present physical understanding microcrystalline silicon can be considered to be much more complex and very different from an ideal isotropic semiconductor. So far, stabilized efficiencies of about 12% (10.7% independently confirmed) could be obtained with micromorph solar cells. The scope of this paper is to emphasize two aspects: the first one is the complexity and the variety of microcrystalline silicon. The second aspect is to point out that the deposition parameter space is very large and mainly unexploited. Nevertheless, the results obtained are very encouraging and confirm that the micromorph concept has the potential to come close to the required performance criteria concerning price and efficiency.

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 10 16/cm 3 could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 Å/s for microcrystalline silicon could be achieved.

The “Micromorph” Cell: a New Way to High-Efficiency-Low-Temperature Crystalline Silicon Thin-Film Cell Manufacturing?

AbstractIn the past, microcrystalline silicon (μc-Si:H) has been successfully used as active semiconductor in entirely μc-Si:H p-i-n solar cells and a new type of tandem solar cell, called the “micromorph” cell, was introduced [1]. Micromorph cells consist of an amorphous silicon top cell and a microcrystalline bottom cell. In the paper a micromorph cell with a stable efficiency of 10.7 % (confirmed by ISE Freiburg) is reported.Among sofar existing crystalline silicon-based solar cell manufacturing techniques, the application of microcrystalline silicon is a new promising way towards implementing thin-film silicon solar cells with a low temperature deposition. Microcrystalline silicon can, indeed, be deposited at temperatures as low as 220°C; hence, the way is here open to use cheap substrates as, e.g. plastic or glass. In the present paper, the development of single and tandem cells containing microcrystalline silicon is reviewed. As stated in previous publications, microcrystalline silicon technique has at present a severe drawback that has yet to be overcome: Its deposition rate for solar-grade material is about 2Å/s; in a more recent case 4.3 Å/s [2] could be obtained. In the present paper, using suitable mixtures of silane, hydrogen and argon, deposition rates of 9.4 Å/s are presented. Thereby the dominating plasma mechanism and the basic properties of resulting layers are described in detail. A first entirely microcrystalline cell deposited at 8.7 Å/s has an efficiency of 3.15%.