Microcrystalline Silicon Bottom Cell Research Articles

Significant light absorption enhancement in silicon thin film tandem solar cells with metallic nanoparticles

Enhancing the light absorption in microcrystalline silicon bottom cell of a silicon-based tandem solar cell for photocurrent matching holds the key to achieving the overall solar cell performance breakthroughs. Here, we present a concept for significantly improving the absorption of both subcells simultaneously by simply applying tailored metallic nanoparticles both on the top and at the rear surfaces of the solar cells. Significant light absorption enhancement as large as 56% has been achieved in the bottom subcells. More importantly the thickness of the microcrystalline layer can be reduced by 57% without compromising the optical performance of the tandem solar cell, providing a cost-effective strategy for high performance tandem solar cells.

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.

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).

Anodic degradation of ZnO on soda-lime glass

We introduce a novel degradation effect of the transparent conductive oxide zinc oxide (ZnO). The effect is observed for ZnO layers deposited on commercially available soda-lime glass. When an electric field is applied, the potential induced anodic degradation causes a massive sheet resistance increase of the ZnO. We show that the increase in resistance strongly affects the power output of a small non-encapsulated thin-film photovoltaic module, in which ZnO was used as the front electrode. For the module, an amorphous silicon top and a microcrystalline silicon bottom cell were used in the tandem configuration. We found that the evolution of gaseous oxygen at the anode (ZnO) might be responsible for the increased resistance of the ZnO. A model that describes our observation is presented and solutions to suppress the effect are proposed.

Smoothening intermediate reflecting layer for tandem thin-film silicon solar cells

We introduce the concept of smoothening intermediate reflecting layers (IRLs) in Micromorph thin-film silicon tandem solar cells in the superstrate configuration. The aim of such structures is not only to provide a selective reflection of light for optimal light management in both sub-cells but also to tune the morphology of the surface on which the microcrystalline silicon bottom cell is grown. This novel type of IRL is shown to enable the use of rough front electrodes with excellent light scattering properties in Micromorph devices without impacting the electrical property of the microcrystalline material. A low-index (1.5), highly transparent and insulating UV-curable lacquer is used as IRL. The electrical contact between both sub-cells is ensured by a partial covering of this lacquer layer: the bottoms of the structure are efficiently filled whereas the tips are unveiled by an etching step. This results in an efficient smoothening of the surface of the top cell, validated by a Voc boost of up to 50mV compared to a standard IRL. A strong top cell current increase is also shown, with up to 2.3mA/cm2 (20% relative) current gain compared to a cell with no IRL. Adjusting the volume of lacquer composing this advanced interlayer enables a fine tuning of its opto-electrical properties. The introduction of a smoothening IRL is shown, with simple calculations, to be a key element towards the obtaining of 13.5% stable-efficiency Micromorph devices using present state-of-the-art single junctions.

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.

Micromorph tandem solar cells: optimization of the microcrystalline silicon bottom cell in a single chamber system

We report on the development of single chamber deposition of microcrystalline and micromorph tandem solar cells directly onto low-cost glass substrates. The cells have pin single-junction or pin/pin double-junction structures on glass substrates coated with a transparent conductive oxide layer such as SnO2 or ZnO. By controlling boron and phosphorus contaminations, a single-junction microcrystalline silicon cell with a conversion efficiency of 7.47% is achieved with an i-layer thickness of 1.2 μm. In tandem devices, by thickness optimization of the microcrystalline silicon bottom solar cell, we obtained an initial conversion efficiency of 9.91% with an aluminum (Al) back reflector without a dielectric layer. In order to enhance the performance of the tandem solar cells, an improved light trapping structure with a ZnO/Al back reflector is used. As a result, a tandem solar cell with 11.04% of initial conversion efficiency has been obtained.

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.

Microcrystalline Single-Junction and Micromorph Tandem Thin Film Silicon Solar Cells

ABSTRACTHigher open circuit voltages of the microcrystalline silicon bottom cell have a direct impact on the efficiency of the micromorph (μc-Si:H/a-Si:H) tandem cell. In this paper it is shown that open circuit voltages over 500 mV can be achieved leading to gc-Si:H cell efficiencies of 8.5 %. The behaviour of such cells is characterised both by the illuminated and the dark I-V characteristics in function of cell temperature. Microcrystalline cells with Voc-values higher than 500 mV and micromorph tandems possess in general a lower value of the temperature coefficient of the fill factor and thus of the efficiency, when compared to c-Si. Temperature-dependent dark I-V measurements suggest that the dominant recombination mechanism in lgc-Si:H cells is different from that prevailing in a-Si:H solar cells.

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.