Baojie Yan United Solar Ovonic LLC
Seiichiro Higashi Hiroshima University
Chuang-Chuang Tsai National Chiao Tung University
Qi Wang National Renewable Energy Laboratory
Helena Gleskova University of Strathclyde
National Renewable Energy Laboratory
United Solar Ovonic
A5: Poster Session: Solar Cells
Tuesday PM, April 26, 2011
Exhibition Hall (Moscone West)
A1: Solar Cells: Efficiency Improvement
Tuesday PM, April 26, 2011
Room 2002 (Moscone West)
9:00 AM - **A1.1
High Efficiency Amorphous and Microcrystalline Silicon Based Solar Cells.
Friedhelm Finger 1 , Tao Chen 1 , Andreas Lambertz 1 , Vladimir Smirnov 1 Show Abstract
1 IEK5-Photovoltaik, Forschungszentrum Juelich, Juelich Germany
The aim of the thin film silicon solar cell industry for production capacities of several Gigawatt in recent years is strongly related to the impressive development of microcrystalline silicon (µc-Si:H). The application of µc-Si:H as absorber layer in stacked solar cell devices has given new hope for highly stabilized efficiencies of thin film silicon solar modules. The quality and process technology of µc-Si:H have made considerable progress. As the µc-Si:H gets more advanced, new and old interest is in high efficiency & stable amorphous silicon (a-Si:H) material for top cells. Also any means which allow reducing the thickness of the top cell while still delivering sufficient current to match with the bottom cell are intensely investigated. Apparently all kinds of optical manipulations are of interest here unless one wants to reduce the total cell thickness and sacrifice some maximum efficiency against stability and production cost. We will present our latest developments in thin film silicon solar cells with new types of window layers, intermediate reflectors, anti-reflective coatings, all in connection with our high quality a-Si:H and µc-Si:H absorber layer materials.
9:30 AM - A1.2
Challenges in Optical Design of Thin-film Silicon Solar Cells to Achieve High Conversion Efficiencies above 20 %.
Janez Krc 1 , Marko Topic 1 , Miro Zeman 2 Show Abstract
1 , University of Ljubljana, Faculty of Electrical Engineering, Ljubljana Slovenia, 2 , Delft University of Technology - PVMD/DIMES, Delft Netherlands
The conversion efficiency of thin-film silicon (TF Si) solar cells needs to be raised up to achieve high level of competitiveness in the PV market. Challenging goals of the production expansion and the targeting high conversion efficiencies of TF Si solar cells up to 20 % by 2025 have been released . These goals require intensive R&D activities and breakthroughs on the material-, interface-, and the complete solar-cell device level.. Novel absorber layers, transparent conductive oxides (TCOs) with advanced nano-textures, oxide based doped layers, dielectric back reflectors and other innovative solutions are under investigation by several groups to improve the conversion efficiencies and decrease the production costs of TF Si solar cells and PV modules. However, clear directions and requirements for achieving high efficiency goals still need to be identified.In this contribution we investigate the requirements and define the directions for optical design of thin-film silicon solar cells to achieve high stabilized efficiencies above 20 %. Challenges concerning layers, interfaces and device structures by using ultra-thin absorber layers are identified. The investigation is focused on optical improvements, for the electrical parameters it is assumed that the values of currently achieved record solar cells can be preserved (or improved) in optically optimized device concepts. Detailed optical analysis and optimization of single-, double- and triple-junction devices are carried out by means of 1-D semi-coherent optical simulator SunShine  and other 2-D and 3-D simulation tools, which were well calibrated and verified on the existing state-of-the-art TF Si solar cells. Starting from the existing state-of-the art devices, considering realistic optical properties of layers and interfaces, we show step by step what is required to achieve the stabilized efficiencies of above 15 % for single-junction a-Si:H cell (with absorber thickness of only 100 nm) and above 20 % for tandem micromorph cell (a-Si:H of 100 nm and uc-Si:H of 800 nm). We demonstrate that the following optical improvements are required:- introduction of anti-reflecting coatings or nano-structures at front interfaces (air/glass, glass/TCO, TCO/p),- all front interfaces can be perfectly flat in our device design,- introduction of a special light scatterer at the back side (crucial requirement),- significantly reduced optical losses in the supporting layers (crucial requirement),- wavelength-selective intermediate reflector in multi-junction device.Possible directions towards realizations of the requirements are indicated and discussed. K. Kurokawa et al., “Accelerated and Extended Japanese PV Technology Roadmap “PV2030 +” released by NEDO in 2009. J. Krc et al., Prog. Photovot. Res. Appl. 11 (2003) 15.
9:45 AM - A1.3
High Efficiency, Large Area, Nanocrystalline Silicon Based, Triple-junction Solar Cells.
Arindam Banerjee 1 , Tining Su 1 , Jinyan Zhang 1 , Dave Beglau 1 , Ginger Pietka 1 , Frank Liu 1 , Baojie Yan 1 , Jeff Yang 1 , Subhendu Guha 1 Show Abstract
1 , United Solar Ovonic LLC, Troy, Michigan, United States
Hydrogenated nanocrystalline silicon (nc Si:H) has become a promising candidate to replace hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin film silicon solar cells. In view of its indirect bandgap, the nc-Si:H layer must be much thicker than its amorphous counterparts to effectively absorb the incident radiation. Typical thickness for a nc Si:H based multijunction cell is 2-5 µm, compared with <0.5 µm for a corresponding amorphous silicon (a-Si:H) and a-SiGe:H based device. For commercial viability, the nc-Si:H layer must be deposited at a high rate. In order to explore the limit of nc-Si:H technology, it is important to fabricate solar cells without constraint of manufacturing. In parallel, one must also determine the corresponding limit that incorporates manufacturing constraint of deposition time. Thus, one must investigate the highest efficiency attainable with and without manufacturing constraint of deposition time. In this paper, we report on this two-pronged strategy of fabricating large area, high efficiency a-Si:H/nc-Si:H/nc-Si:H solar cells at low and high rates.Triple-junction solar cells were fabricated in a large area batch reactor on Ag/ZnO back reflector coated stainless steel substrate, using a Modified Very High Frequency (MVHF) excitation process. For both low- and high-rate cases, we optimized the deposition parameters, such as pressure, gas flow, dilution, and power. The deposition rate for the nc-Si:H layers for the high-rate case was ~1.0-1.5 nm/s. We found that the deposition parameters were more relaxed for fabricating large-area solar cells for the low-rate case. We did SIMS analysis on the optimized films, and found the impurity concentrations were one order of magnitude lower than normally observed. In particular, the oxygen concentration is reduced to ~1018 cm-3. This is among the lowest oxygen concentration reported in literature. The low impurity content is attributed to superior cathode hardware and the optimized deposition recipe. We fabricated large area (aperture area 400-464 cm2) cells, and encapsulated the cells using standard flexible encapsulants. We have light soaked the high-rate cells, and are currently conducting light soak tests on the low-rate samples. The highest stable efficiency attained for the high rate cells is 10.6%, as confirmed by NREL. The initial efficiency of the low-rate samples is 11.8-12.2% as measured under our Spire solar simulator. We will present the details of the research done to develop the high- and low-rate devices.
10:00 AM - A1.4
Thin Film Silicon Solar Cells under Moderate Concentration.
L. Matthijs van Dam 1 , Wilfried G.J.H. van Sark 1 , Ruud E. Schropp 1 Show Abstract
1 Faculty of Science, Utrecht University, Utrecht Netherlands
Recently, in the quest for higher efficiencies for thin film solar cells, much emphasis is placed on light trapping or absorption enhancement techniques, such as the use of plasmonic or diffractive back contacts  and luminescent concentrators , to concentrate the incident light in cells with a thinner absorber layer or with a smaller area. Even in less advanced schemes, merely optical concentration of light can yield higher efficiencies. There are only very few reports on the effects of concentration in thin film silicon-based solar cells. Due to the presence of midgap states, a fast decline in fill factor was observed in earlier work. However, with the advent of more stable and lower defect density protocrystalline silicon materials as well as high quality micro-/nanocrystalline silicon materials, as well as the increasing concentration ratios obtained by novel light management techniques, it is worth revisiting the performance of cells with these absorber layers under moderately concentrated sunlight. We determined the behavior of the external J-V parameters of pre-stabilized substrate-type (n-i-p) amorphous and microcrystalline solar cells under moderate concentrations, between 1 sun and 20 suns, while maintaining the cell temperature at 25°C. It should be noted that these cells already comprise some sort of light concentration due to the use of textured surfaces. It was found that the cell efficiency of both the amorphous and the microcrystalline cells increased with increasing concentration, showing an optimum at approximately 5 suns. Furthermore, the enhancement in efficiency for the microcrystalline cells was larger than for the amorphous cells. From this we conclude that the carrier transport mechanism in microcrystalline cells is less of drift type than that in amorphous cells. We show that the Voc’s up to 0.63 V can be reached in microcrystalline cells while FF’s only decrease by 10%. The effects have also been computed using the device simulator ASA, showing qualitative agreement. We conclude that it is meaningful to design an optical concentration ratio of 5 to 10 suns for thin film silicon solar cells V.E. Ferry, M.A. Verschuuren, H.B.T. Li, E. Verhagen, R.J. Walters, R.E.I. Schropp, H.A. Atwater, A. Polman, “Light Trapping in Ultrathin Plasmonic Solar Cells”, Optics Express 18 102 (2010) A237. W.G.J.H.M. van Sark et al., “Luminescent solar concentrators – A review of recent results”, Optics Express16 (2008) 21773.
10:15 AM - A1.5
Effect of Bandgap Profiling on the Performance of a-SiGe:H Single Junction Thin-film Solar Cells.
Hung-Jung Hsu 1 , Chien-Ming Wang 1 , Cheng-Hang Hsu 1 , Chuang-Chuang Tsai 1 Show Abstract
1 Department of Photonics, National Chiao Tung University, Hsinchu Taiwan
Multi-junction solar cell is an effective approach toward high efficiency and improved stability in thin film solar cell applications. Hydrogenated amorphous silicon germanium (a-SiGe:H) has received much attention due to its high absorption coefficient and adjustable bandgap. The property of a-SiGe:H thin films has been characterized and optimized in our previous work . However, one of the major challenge is the bandgap discontinuity between the a-SiGe:H i-layer and the a-Si:H doped layer. Such discontinuity at both the p/i the i/n interfaces can be alleviated by graded interface . In this work, we applied the concept of bandgap profiling to our single-junction a-SiGe:H cells. Together with other cell optimization, the cell efficiency of 8.59% has been attained.The a-SiGe:H solar cells were deposited by a 27.12 MHz radio-frequency plasma-enhanced chemical vapor deposition (PECVD) system. The germane concentration and the hydrogen dilution ratio were varied during the deposition of the a-SiGe:H thin films. The bandgap was calculated from Tauc’s plot by analyzing the transmittance spectra measured in UV/VIS spectroscopy. The conductivity of the thin films and characteristics of the solar cells were measured by AM1.5G illuminated I-V measurement system.Our results show that the graded bandgap at both the p/i and i/n interfaces enhanced the cell efficiency as compared to cells with constant bandgap of 1.55 eV. According to the results, the increase of the efficiency was due to the enhanced Voc and FF, which were arising from a better short-wavelength absorption and carrier transport, respectively. To assess the effect of i/n grading, the width of the i/n region was systematically varied from 0 to 20nm while maintain the same absorber thickness of 200nm. As the thickness of i/n grading increases, the cell efficiency increases from 7.8% to 8.3% despite a slight reduction of Jsc. The graded i/n region might facilitate the transportation of deep holes; nevertheless, further increase in i/n grading deteriorates the cell efficiency, which may due to the suppression of long-wavelength absorption. Finally, the optimal thickness of p-layer, n-layer and grading structure were integrated for fabricating the solar cells. From our results, the a-SiGe:H cell efficiency of 8.59% was achieved with Voc = 748 mV, Jsc = 16.31 mA/cm2, FF = 70.38 %.This work was sponsored by the Center for Green Energy Technology at the National Chiao Tung University and the National Science and Technology Program for Energy.C.M. Wang, Y.T. Huang, K.H. Yen, H.J. Hsu, H.W. Zan and C.C. Tsai, Mat. Res. Soc. Symp. Proc., Spring meeting, San Francisco (2010)S. Guha, J. Yang, A. Pawlikiewicz, T. Glatfelter, R. Ross, and S. Ovshinsky, Appl. Phys. Lett. 54, 2330-2332 (1989)
10:30 AM - **A1.6
High-efficiency Microcrystalline Silicon and Microcrystalline Silicon-germanium Alloy Solar Cells.
Takuya Matsui 1 , Michio Kondo 1 Show Abstract
1 Research Center for Photovoltaics, AIST, Tsukuba Japan
The conversion efficiency of thin film silicon solar cells has been improved by employing the narrow-gap hydrogenated microcrystalline silicon (μc-Si:H) in combination with the wide-gap hydrogenated amorphous silicon (a-Si:H) based on a multijunction concept [1,2]. Since the μc-Si:H is an indirect band gap material, relatively thick absorber layer (>2 μm) is necessary for efficient infrared light absorption, which in turn requires the high-rate deposition technique for industrial production. To achieve high deposition rate and high efficiency, we have developed a novel deposition process using high-pressure depletion regime in plasma-enhanced chemical vapor deposition [3,4]. This technique allows growing μc-Si:H at high rates (> 2 nm/s) while preserving excellent film qualities in terms of denser microstructure and less post-oxidation behavior. As a result, efficiencies of 8-9% have been demonstrated for the μc-Si:H single junction solar cells at deposition rates between 2 and 3 nm/s.Despite the successful material combination of a-Si:H and μc-Si:H, the stabilized efficiency of the a-Si:H/μc-Si:H tandem solar cells is still limited as low as ~12%. The one of the major limitations of efficiency is the weak infrared absorption in μc-Si:H bottom cell. To extend the spectral sensitivities of solar cells into longer wavelengths, we proposed the application of hydrogenated microcrystalline silicon-germanium alloys (μc-Si1-xGex:H) as a narrower-gap bottom-cell absorber in multijunction structures such as a-Si:H/μc-Si1-xGex:H and a-Si:H/μc-Si:H/μc-Si1-xGex:H. In the previous work , we have demonstrated efficient (~7-8%) 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. Nevertheless, the photocarrier collection degrades severely when increasing either Ge content (x>0.2) or cell thickness (ti >1 μm). We attributed the inferior performance of such solar cells to the creation of the Ge-related native defect acceptors that strongly distort the built-in electric field in the p-i-n solar cells. Recently, we have developed a counter doping technique that compensates the Ge-related acceptor states for further improvement of μc-Si1-xGex:H solar cells.In this contribution, we review our research and progresses in μc-Si:H and μc-Si1-xGex:H thin film materials, deposition process and solar cell devices. Apart from the optimization of these materials, the improvements of light trapping and TCO layer, which are also crucial in boosting the photocurrent of the thin film solar cells, will be addressed. J. Meier et al., Solar Energy Material and Solar Cells, 66, 73 (2001). K. Yamamoto et al., Solar Energy, 77, 939 (2004). M. Kondo et al. J. Non-Cryst. Solids, 266-269, 84 (2000). T. Matsui et al. Jpn. J. Appl. Phys. Part 2, 42, L901 (2003). T. Matsui et al., Prog. Photovolt: Res. Appl. 18, 48 (2010).
A2: Growth Mechanism
Tuesday PM, April 26, 2011
Room 2002 (Moscone West)
11:30 AM - **A2.1
Control of Materials and Interfaces in µc-Si:H-based Solar Cells Grown at High Rate.
Yasushi Sobajima 1 2 , Chitose Sada 1 2 , Akihisa Matsuda 1 2 , Hiroaki Okamoto 1 2 Show Abstract
1 Department of Systems Innovation, Graduate School of Engineering Science, Oskaka University, Toyonaka, Osaka, Japan, 2 Japan Science and Technology Agency, CREST, Toyonaka, Osaka, Japan
Spatial distribution of dangling-bond defects in high-rate-growth microcrystalline silicon (µc-Si:H) thin films by plasma-enhanced chemical-vapor deposition (PECVD) has been investigated using high precision electron-spin resonance (ESR) system with specially designed TM-mode cavity where planar sample is measured without removing substrate. We have found that dangling-bond defect is distributed uniformly in the bulk region independent of crystallite size and high density dangling-bond is located at the surface region (~ 10-12 cm-2) in µc-Si:H films. Both the bulk-defect density and surface-areal-defect density are increased when increasing the growth rate (up to 6.7 nm/sec) through an elevation of electron temperature in the plasma during film growth. Presence of large number of surface defects is one of the crucial causes for deteriorating n-i-p type solar-cell performance through the photocarrier recombination as well as current leakage at the p/i interface. To overcome this issue in µc-Si:H based solar cells grown at high rate, we have attempted to use a novel interface-treatment method in fabrication process of n-i-p type solar cells, e. g., thin silicon layer with low defect density (compress layer) is deposited on the surface of µc-Si:H grown at high rate followed by thermal annealing. The compress layer deposition with post thermal annealing is a key to reduce surface-dangling-bond density, in which the areal dangling-bond density located at the surface is decreased down to less than 30% comparing with as-deposited state. Importance of starting procedure in µc-Si:H growth at high rate has also been indicated, since n/i-interface properties are determined at this moment in n-i-p type solar cell. We have found that SiH4-gas introduction-time constant into H2 plasma plays an important role in controlling the structural and optoelectronic properties of the n/i interface.Consequently, a high conversion efficiency of 9.27% has been demonstrated in µc-Si:H based n-i-p solar cells whose intrinsic layer is grown at high rate of 2.3 nm/sec thanks to the presence of 50 nm-thick compress layer with post thermal annealing together with the control of SiH4-gas-introduction scheme during the initial growth stage of intrinsic layer.
12:00 PM - A2.2
Real Time Spectroscopic Ellipsometry of Roll-to-roll Fabrication for Thin Film Si:H Solar Cells.
Lila Dahal 1 , Zhiquan Huang 1 , Dinesh Attygalle 1 , Michelle Sestak 1 , Carl Salupo 1 , Sylvain Marsillac 1 , Robert Collins 1 Show Abstract
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States
Real time spectroscopic ellipsometry (SE) has been developed to monitor cassette roll-to-roll deposition of thin film hydrogenated silicon (Si:H) n-i-p solar cells and their backreflector (BR) layers on flexible polymer substrates. Monitoring is performed at a single spot at the center of the substrate width using an SE range of 0.75 - 5 eV. The methodology is first demonstrated in growth studies from nucleation to final thickness for magnetron sputtered ZnO films on top of opaque Ag in the BR structure. The methodology is then extended to plasma-enhanced chemical vapor deposition (PECVD) of the i and p-layers in succession on the BR/n-layer stack. Roll-to-roll substrate motion is initiated first, followed by real time SE data collection; finally, the plasma is ignited so that film nucleation can be observed. The film thickness is observed to increase with time until a steady state is reached, after which the bulk layer thickness at the monitoring point is constant with time. This occurs when the elapsed deposition time equals the time required for the substrate to travel from the trailing edge of the deposition zone to the monitoring point. Although a constant substrate speed is selected such that the final film thickness is achieved in the time required for the substrate to move through the entire deposition zone, this speed does not permit study of film growth that occurs after the substrate passes the monitoring point. To address this deficiency, the substrate speed is reduced only over an initial length of the roll such that the final film thickness of interest is reached at the monitoring point. In this way, real time SE can be used to analyze the entire layer on an initial length of the roll before its full length is coated. Furthermore, the use of a moving substrate in real time SE enables new capabilities in process analysis. The thickness evolution of ZnO during sputtering shows reasonable agreement with a simulation assuming that the deposition flux varies in accordance with a simple inverse square of the target-substrate distance as the substrate moves through the deposition zone. The thickness evolution of the i- and p-layers during PECVD can be simulated by assuming constant deposition rate throughout the deposition zone. Fitting of the thickness evolution in sputtering can be further improved by introducing gas phase scattering of deposition species, whereas fitting in PECVD can be improved by assuming a non-zero decay length of reactive species beyond the cathode width. After the various multilayer fabrication steps, SE has also been applied for large area mapping of the coated polymer so as to determine the thickness uniformity of the layers across the width of the substrate. The overall goal of this work is to apply optical probes in order to develop optimum deposition procedures for thin film Si:H solar cell structures and to evaluate their uniformity in roll-to-roll multi-chamber deposition.
12:15 PM - A2.3
Monitoring of the Growth of Microcrystalline Silicon Deposited by Plasma-enhanced Chemical Vapor Deposition Using In-situ Raman Spectroscopy.
Stefan Muthmann 1 , Florian Koehler 1 , Markus Huelsbeck 1 , Matthias Meier 1 , Andreas Mueck 1 , Ralf Schmitz 1 , Wolfgang Appenzeller 1 , Reinhard Carius 1 , Aad Gordijn 1 Show Abstract
1 IEK-5 Photovoltaik, Forschungszentrum Juelich, Juelich Germany
The crystalline volume fraction of the intrinsic hydrogenated microcrystalline (µc-Si:H) absorber layer of a thin-film silicon solar cell is a crucial material parameter which strongly influences the performance of solar cells. To get a better understanding and control of the deposition process of µc-Si:H the implementation of in-situ diagnostic tools is of great importance. Raman spectroscopy is one of the most frequently used techniques used to obtain a measure of the crystalline volume fraction. However in a capacitively coupled plasma-enhanced chemical vapor deposition (PECVD) process the implementation of Raman spectroscopy is difficult to accomplish, particularly in large area systems. Due to the need of low angle optical access to the growing film it is necessary to pierce the electrode surface.We present a novel showerhead-electrode design that enables in-situ Raman measurements during PECVD deposition. In this paper the optical feed through was shielded electrically to guarantee the homogeneous deposition of a µc-Si:H thin film. We show that with this electrode it is possible to deposit homogenous intrinsic absorber layers and to measure the Raman crystallinity of the film in situ. To suppress the influence of the plasma emission on the recorded spectra a lock-in technique was used. With this setup, the signal-to-noise ratio is increased drastically.A high laser power density is favorable to get sufficiently large Raman signals but local heating also has to be avoided. The local temperature was simulated as a function of pulse intensity and length and the size of the laser spot. The optimum conditions enabling useful Raman signals at a minimum heating were applied experimentally to verify the reduced heating of the growing film. Using this setup it was possible to measure the evolution of crystallinity of the film during the growth with a time resolution of less than 20 seconds which corresponds to an additional film thickness of 5 nm up to 20 nm during each measurement depending on the studied deposition rate. By analyzing the ratio of the Stokes- and anti-Stokes-scattering intensity the thermodynamic temperature of the growing film was determined. An increase of film temperature due to plasma heating was observed for the deposition of µc-Si:H with an excitation frequency of 13.56 MHz and a growth rate of about 2.5 Å/s.
12:30 PM - A2.4
High Growth Rate Hot-wire CVD Epitaxial Silicon Absorber Layers for Film Crystal Silicon Solar Cells.
David Bobela 1 , Charles Teplin 1 , David Young 1 , Ina Martin 2 , Maxim Shub 1 , Howard Branz 1 , Paul Stradins 1 Show Abstract
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Physics, Case Western Reserve University , Cleveland, Ohio, United States
We have grown device-quality epitaxial silicon thin films at high growth rate (GR) up to 700 nm/min, using hot-wire chemical vapor deposition (HWCVD) from silane. These growth rates exceed those used for industrial amorphous and nanocrystalline thin-film Si solar cells by a factor of about 20. Such layers have potential as the absorber layer in film crystal silicon solar cells deposited on seed layers on inexpensive substrates such as display glass. To obtain high GR, we explored a parameter space suggested by our model of the GR dependence on the hot-wire to substrate distance (d), system pressure (p), and silane flow . In-situ spectroscopic ellipsometry (SE) was used to determine the growth rate and epitaxial quality in real time. The GR depends linearly on 1/d over a range of low silane pressures and sample-filament distances; however, deviations at high p and small d indicate that changes in the gas-phase and filament surface chemistries affect the production of growth radicals. In this regime, good quality epitaxy is possible as long as the substrate temperature is above about 620°C. For all deposition parameters we explored, the bulk epitaxy quality depends primarily on the initial moments of growth. Therefore, for our best test devices, we deposited about 50 nm of high quality epitaxial layer at a GR ~ 150 nm/min, before adjusting to the high GR conditions. To demonstrate layer quality in a high GR epitaxial solar cell, we deposited a 2.3 µm epitaxial silicon layer at 700 nm/min on a RCA-cleaned, (100) oriented, n+ (Si:As) silicon wafer. A simple mesa structure (wafer/epi Si/I a-Si/p+ a-Si:H/ITO) was processed into a solar cell by wet and dry etching techniques. The finished device had an open-circuit voltage of 0.424 V before any hydrogenation treatment. We will discuss further device improvements and strategies for increasing the GR beyond 1 micron/min.This work was supported by the U.S. DOE Solar Energy Technology Program under Contract No. DE-AC36-08GO28308.1. I. T. Martin, C. W. Teplin, J. R. Doyle, H. M. Branz, and P. Stradins. J. Appl. Phys. 107, 054906 (2010).
12:45 PM - A2.5
Deposition of P Type Nanocrystalline Silicon under High Pressure in a VHF-PECVD Single Chamber System.
Xiaodan Zhang 1 , Guanghong Wang 1 , Xinxia Zheng 1 , Shengzhi Xu 1 , Changchun Wei 1 , Jian Sun 1 , Xinhua Geng 1 , Shaozhen Xiong 1 , Ying Zhao 1 Show Abstract
1 , Institute of Photo-electronic Thin Film Devices and Technology of Nankai University, Tianjin China
Plasma-enhanced chemical vapour deposition (PECVD) of p-i-n type hydrogenated amorphous silicon (a-Si:H) based solar cells in a single plasma reactor offers advantages of low cost compared to multi-chamber processes that use separate reactors to deposit the p-, i- and n-layers, respectively. It is desirable that the deposition of each layer for solar cell prepared in a single chamber system has the same electrode distance, which requires the similar process pressures for all the layers. In addition, to reduce the manufacturing cost and to achieve high solar cell efficiency, it has been shown that microcrystalline silicon (μc-Si:H) intrinsic layer should be deposited under a high deposition pressure with a small electrode distance, which can have a high deposition rate. Therefore, it is very important to have an optimized p layer deposition condition under the high pressure.We will present a systematic study of boron-doped hydrogenated nanocrystalline silicon (nc-Si:H) films deposited using very high frequency-plasma enhanced chemical vapor deposition (VHF-PECVD) method under different deposition pressures. Electrical, structural and optical properties of the films were investigated. Dark conductivity as high as 2.75S/cm in a p-type nc-Si:H layer prepared at 2.5Torr pressure has been achieved at a deposition rate of 1.75Å/s for 25-nm thick films. By controlling boron and phosphor contaminations, single-junction μc-Si:H solar cells incorporated the p layers prepared under high pressure and low pressure, respectively, were deposited. It is shown that μc-Si:H solar cells with the p layer prepared under the high pressure have much better open circuit voltage, short circuit current density and then conversion efficiency than those with the low pressure p layer. By using the high pressure p layer and further optimizing the μc-Si:H bottom in a-Si:H/μc-Si:H micromorph solar cells, including application of ZnO/Al back reflector, an initial conversion efficiency of 10.59% has been achieved using an a-Si:H/μc-Si:H micromorph tandem solar cell (1.027cm2). The efficiency of the micromorph solar cell was confirmed by the National Renewable Energy Laboratory (NREL).
A3: Polycrystalline Films
Tuesday PM, April 26, 2011
Room 2002 (Moscone West)
2:30 PM - **A3.1
Polycrystalline Silicon Solar Cells Based on a Seed Layer Approach @ Imec: The Road to 14% Efficient Cells.
Dries Van Gestel 1 , Ivan Gordon 1 , Jef Poortmans 1 Show Abstract
1 , Imec, Leuven, 0, Belgium
From a cost perspective, thin-film silicon solar cells are still an interesting alternative to wafer-based solar cells, even with the recently decreased feedstock price. A technology based on polycrystalline-silicon (pc-Si) thin-films seems particularly promising since it combines the low-cost potential of a thin-film technology with the high efficiency potential of crystalline silicon. For this technology the challenge is to fabricate high quality coarse grained (grains size of 0.1-100μm) layer on foreign non-silicon substrates. In recent years, solid phase crystallization (SPC) and Aluminium Induced Crystallization (AIC) were investigated for obtaining pc-Si layer for PV applications resulting in energy conversion efficiencies of 10,5% and 8,5% respectively. To become economically viable, most likely energy conversion efficiencies of 14-15% need to be achieved. Today, the highest obtained open circuit voltage (Voc) for pc-Si cells is 553mV, the highest obtained short circuit current (Jsc) is 29,5mA/cm2 and the highest obtained fill factor (FF) is 75,5%. Combining these values reached for different cells into one cell would lead to an energy conversion efficiency of 12.3%. In this paper we will explain our road to >14% efficient pc-Si solar cells.We make pc-Si layers using a two-step approach of seed layer formation and epitaxial growth. This allows us to separate the crystallographic material properties from the electrical properties like doping. To obtain Jsc values above 30mA/cm2 we investigate superstrate configuration solar cells including seed layer formation on anti-reflective coatings and the use of nano-particles and photonic structures for advanced light trapping. So far the highest Jsc values in pc-Si solar cells were obtained with SPC layers (by CSG Solar AG). By comparing pc-Si solar cells with identical epitaxial growth and solar cell structure but different seed layers (namely AIC and SPC), we found that the AIC-based cells resulted in 3 mA/cm2 higher current densities. So far we reached 537 mV using an AIC seed layer, will probably well above 550mV need to be obtained. For this type of solar cells we found that the presence of intragrain defects (IGD) and impurities mainly limits our material quality at the moment. Experiments to lower the IGD and the contamination level in AIC seed layers are therefore ongoing. In parallel seed layers made by a new promising laser crystallization techniques namely mixed phase solidification (MPS), are also explored. MPS results in a drastically reduced IGD density compared to SPC and AIC. Finally we combine all this different types of seed layers with epitaxial growth of n-type pc-Si. N-type monocrystalline silicon seems to have some important benefits with respect to p-type. We investigate if this, due to the presence of grain boundaries and IGD, is still the case for pc-Si. All together we believe pc-Si solar cells with an energy conversion efficiency of >14% are possible in the future.
3:00 PM - A3.2
Solid Phase Crystallization of Amorphous Silicon: An in-situ XRD and Raman Studies.
Kashish Sharma 1 , Maria Adriana Creatore 1 , Mcm van de Sanden 1 Show Abstract
1 , Eindhoven University of Technology, Eindhoven Netherlands
The crystallization kinetics of thermally annealed amorphous silicon (a-Si:H) films have been extensively investigated for the past 2 decades. We have recently reported the development of large grains throughout the polycrystalline silicon (poly-Si) layer obtained by thermal annealing of a-Si:H1.By means of a classical model of nucleation and grain growth2, the following characteristics have been identified: incubation time, nucleation rate, grain growth and crystallization time. Many studies were carried out in the past to determine the effect of the structural properties of PECVD and HWCVD deposited a-Si:H on the crystallization kinetics2. The incubation time is thought to be affected by parameters such as the hydrogen content, the microstructure (structural order/disorder) of a-Si:H, i.e. the R* parameter 2. However, up to now, the hydrogen content and R* have been varied together in the above mentioned studies, which makes difficult to investigate the effect of hydrogen content independently from R* in terms of crystallization kinetics of a-Si:H. Therefore, in this contribution we report on the systematic crystallization study of expanding thermal plasma (ETP) deposited a-Si:H films by varying R* while keeping the hydrogen content constant and vice versa.1000 nm thick a-Si:H films characterized by an R* in the range of 0.05-0.55, were deposited by using the expanding thermal plasma technique3. Each R* value was obtained in an hydrogen content range of 3-14 at. % as determined by FTIR absorption spectroscopy. a-Si:H layers were annealed at 600 C on a heating stage coupled to XRD and Raman diagnostic tools. The crystallization kinetics of a-Si:H was followed as function of the annealing time and temperature.A relationship has been found to explain the complex crystallization process of a-Si:H. Better order regions on a medium range around the hydrogen in divacancies are identified as nucleation centers. Medium range order (as described by Mahan et al.2) of the order regions depends on hydrogen content and R*, increasing hydrogen content and R* leads to decrease in medium range order. Therefore, with increasing hydrogen content at a constant R*, an increase in incubation time is observed; similarly, an increase is observed for increasing R* at constant hydrogen content. A deeper understanding of the crystallization kinetics of a-Si:H can eventually lead to a significant improvement in polycrystalline silicon-based solar cells. Reference List1. A. Illiberi, K. Sharma, M. Creatore, and M. C. M. van de Sanden, Materials Letters 63, 1817 (2009).2. A. H. Mahan, T. N. Su, D. L. Williamson, L. M. Gedvilas, S. P. Ahrenkiel, P. A. Parilla, Y. Q. Xu, and D. A. Ginley, Advanced Functional Materials 19, 2338 (2009)(ref 2-14 therein).3. W. M. M. Kessels, R. J. Severens, A. H. M. Smets, B. A. Korevaar, G. J. Adriaenssens, D. C. Schram, and M. C. M. van de Sanden, Journal of Applied Physics 89, 2404 (2001).
3:15 PM - A3.3
Phosphorus- and Boron- doped Thin Polycrystalline Si Layers on Glass Prepared by Metal-induced Layer Exchange.
Tobias Antesberger 1 , Mehdi Kashani 1 , Michael Algasinger 1 , Christian Jaeger 1 , Thomas Wassner 1 , Martin Stutzmann 1 Show Abstract
1 Physics Department, Technische Universität München, Walter Schottky Institut, Garching, Bayern, Germany
Polycrystalline silicon thins film on low-cost substrates are of great interest for solar cells and large area electronic applications. Different approaches like solid phase or laser-induced crystallization suffer from very small crystallites or high process temperatures. A promising method to obtain large-grained high quality polycrystalline films by low-temperature crystallization of an amorphous precursor material is the aluminum-induced layer exchange (ALILE). Due to the intimate contact of the aluminum and the silicon, the ALILE-process results in highly p-type doped poly-Si layers with carrier concentrations up to 1019 cm-3. These high carrier concentrations are not suitable for most applications and have to be lowered by different post-process treatments. In the related AgILE-process, the aluminum layer is replaced by a silver layer leading to nominally undoped films. In this approach, an Ag/amorphous Si layer stack, separated by a thin diffusion barrier, is annealed at temperatures below the Ag-Si eutectic temperature of 1109 K, leading to a complete exchange of the positions of the initial Ag and Si layers and to the crystallization of the amorphous Si. The resulting polycrystalline silicon layers are intrinsic due to the low solubility of silver in silicon. By doping the amorphous silicon precursor layer with phosphorus or boron, the AgILE-process can be used to achieve a controlled n-type or p-type doping of the layers. We have studied the dynamics of the AgILE-process as well as the structural and electronic properties of resulting polycrystalline Si layers (20 nm – 500 nm) prepared on silica substrates. Optical microscopy shows grain like structures up to a size of about 100 µm. X-ray diffraction measurements show a preferential (100) and (111) orientation of the crystallites. Raman spectroscopy gives evidence of a good crystalline quality of the layers down to a layer thickness of 20 nm. Hall effect and conductivity measurements show tunable carrier concentration from intrinsic level up to 1019 cm-3 for both n-type (Phosphorus-doped) and p-type (Boron-doped) films. Furthermore, the influence of different diffusion barriers and substrates were studied, showing huge differences in the process dynamics and layer properties.This work is funded by “Dritte Patentportfolio Beteiligungsgesellschaft mbH & Co. KG”.
3:30 PM - A3.4
Flash-lamp-induced Lateral Solidification of Thin Si Films.
K. Omori 1 , G. Ganot 2 , U. Chung 2 , A. Chitu 2 , A. Limanov 2 , James Im 2 Show Abstract
1 Technical Development Department, THE JAPAN STEEL WORKS, LTD., Yokohama, Kanagawa, Japan, 2 Program in Materials Science and Engineering, Columbia University, New York, New York, United States
Using a flash lamp to heat and crystallize a-Si films can be recognized as an interesting and noteworthy technical procedure for a number of reasons: (1) it was demonstrated as a viable crystallization method already nearly thirty years ago, (2) it is an extremely flexible technique capable of being used for solid-phase as well as melt-mediated crystallization of a-Si films, and (3) the irradiation-system-related components are well developed as a consequence of the "flash lamp annealing" method being evaluated and developed for the semiconductor manufacturing industry.In this paper, we show that the approach can also be used effectively for inducing controlled lateral solidification of a-Si films (i.e., flash-lamp-induced controlled super-lateral growth (CSLG) of a-Si films [Im,et.al.,PSS,166,603(1998)], and that low-defect-density Si films are created in the process. We have chosen to utilize a Xenon-Arc-lamp-based approach as it can potentially lead to cost-effective and high-throughput processes and systems; these lamps possess established capability to deliver prodigious amounts of optical power over an extremely wide range of CSLG-suitable pulse durations. As such, a definite possibility exists here for developing a non-laser crystallization process that can capture the material-quality-related advantages that are typically associated with laser-based techniques, while avoiding the associated cost-related disadvantages.From the crystallization perspective, the most salient characteristic of the process is the exceptionally long lateral-growth distances (~10s to 100s of μm) that can be achieved in comparison to previously demonstrated pulsed-laser-based CSLG processes. This result, which is fully expected from thermal and kinetic considerations associated with the encountered experimental conditions (i.e., ~50 μsec to ~10 msec pulse duration range), does endow the approach with an unprecedented level of flexibility for generating various high-device-performance-enabling low-defect-density materials. In this paper, we will also present and discuss how shaping of the incident beam using a proximity/contact mask, and/or pre-patterning of the films using a photolithographic step can (1) effectively address a number of issues that are commonly associated with the present approach (e.g., cracking of the films, warpage of the substrate, the tendency of the films to agglomerate, and the formation of pronounced protrusions) and (2) readily satisfy the CSLG procedural requirement of inducing complete melting in, and only in, the pre-determined areas.
3:45 PM - A3.5
Poly-Si Thin Film Formation Using a Novel Low Thermal Budget Process.
Minghao Zhu 1 , Chen-Han Lin 1 , Yue Kuo 1 Show Abstract
1 Thin Film Nano & Microelectronics Research Lab, Texas A&M University, College Station , Texas, United States
The thin film a-Si solar cell has many advantages over the bulk solar cell such as requiring a very small amount of raw materials, low fabrication temperatures, almost unlimited supply of low-cost raw materials, and large-area capability [1,2]. Compared with the a-Si thin film solar cell, the poly-Si thin film solar cell is even more promising with respect to the high conversion efficiency and the long lifetime. However, currently the poly-Si thin film fabrication is limited to the high thermal-budget processes, such as the direct CVD deposition, the solid phase crystallization, or the metal induced crystallization, which require either a high temperature or a very long process time [3,4,5]. Recently, authors presented a novel low thermal-budget poly-Si thin film preparation process that is based on the principle of vertical crystallization using the pulsed rapid thermal annealing (PRTA) enhanced with an ultra-thin metal seed layer . In this paper, we will discuss new experimental results that include: 1) the dopant effect on the crystal structure, such as the volume fraction and crystal size; 2) the original a-Si film thickness effect on the crystal formation process, such as the minimum film thickness required for crystal formation; 3) the influence of the PECVD feed gas on the final poly-Si crystal structure, and 4) the PRTA process parameter effects on the n-i-p stack structure. It has been demonstrated that a 2 micrometer a-Si n-i-p Stack could be crystallized with a very low thermal budget PRTA process, e.g., 4 cycles of 1s 850°C heating and 5s cooling. W. G. J. H. M. van Sark, et al., Energ. Policy, 35, 3121 (2007).  Y. Kuo et al., Conf. Rec. IEEE Photovoltaic Spec. Conf. (2010).  N. H. Nickel, et al., Phys. Rev. B, 53, 12 (1996).  Matsuyama, et al., J. Non-Cryst. Solids 198-200, 940(1996).  S.-W. Lee, et al., IEEE T. Electron Dev. 17, 4 (1996)
A5: Poster Session: Solar Cells
Tuesday PM, April 26, 2011
Exhibition Hall (Moscone West)
6:00 PM - A5.1
Voc Saturation Effect in High-temperature Hydrogenated Polycrystalline Silicon Thin-film Solar Cells.
Hidayat Hidayat 1 2 , Per Widenborg 2 , Armin Aberle 2 1 Show Abstract
1 Electrical and Computer Engineering, National University of Singapore, Singapore Singapore, 2 , Solar Energy Research Institute of Singapore, Singapore Singapore
Polycrystalline silicon thin-film solar cells have the potential of achieving a conversion efficiency of more than 13% using a simple solar cell structure. The highest efficiency so far is 10.5%, achieved by CSG Solar. The technology also has the potential for low-cost fabrication. In this work, about 2 µm thick a-Si:H precursor diodes were deposited by PECVD onto 3 mm thick Borofloat glass substrates, followed by solid phase crystallization (SPC) to form a polycrystalline diode. The final sample structure is glass/70 nm SiN/100 nm n+ layer/ 2 µm p- layer/ 100 nm p+ layer. The sample is then heated to 900 °C for a short period of time (rapid thermal annealing, RTA) to activate the dopants and anneal crystal defects. Then, the sample is hydrogenated to passivate a large fraction of the remaining defects. We are using an AK800 system from Roth and Rau, Germany, for hydrogenation. The one-sun Voc of the devices is measured at room temperature, using the Sinton method . Five points are measured on each sample. The sample is then recycled for subsequent hydrogenation experiments, by baking it at 615 °C for 10 hours to drive out the hydrogen. Before each hydrogenation step, the sample is dipped in 5% HF to remove the oxide. Typically, after baking, the one-sun Voc is about 200 mV, whereas it is above 400 mV after hydrogenation.The relationship between the hydrogenation process temperature, T, and the one-sun Voc of the device was studied. Five different samples were studied, with two being planar (sample IDs 1578 and 188) and three being textured (2398, 788 and 888) by the aluminum induced texturing (AIT) method . The Voc starts to saturate at a hydrogenation temperature of about 450 °C. Increasing the hydrogenation temperature to 600 °C does not change the Voc significantly. The experimental data are fitted using a Boltzmann sigmoid statistical fit with 4 fit parameters. The Boltzmann fit is also used to extract the linear relationship between Voc and hydrogenation temperature. The activation energies were extracted for several samples, by plotting Voc/Vt against 1000/T, where Vt is the thermal voltage (25.7 mV at 300 K). The activation energies were found to lie in the range of 1.3-1.6 eV for the textured samples and 1.7-2.8 eV for the planar samples.The Voc saturation could be due to the out-diffusion of hydrogen balancing the in-diffusion of hydrogen at high temperature. It is also known that the hydrogenation process introduces defects (such as the formation of Si-H2) and this can possibly lead to the Voc degradation at very high hydrogenation temperature. The objective of this research is to better understand the factors that limit the Voc. With deeper insight, we possibly can reach a Voc of above 500 mV with this hydrogenation method. References1. R. Sinton, A. Cuevas, Applied Physics Letters 69, 2510 (1996).2. P. Widenborg, A. Aberle, Advances in OptoElectronics 7, (2007).
6:00 PM - A5.2
Enhanced Light-trapping in Thin-film Silicon Solar Cells Via Scattering from Embedded Nanoparticles.
James Nagel 1 , Michael Scarpulla 1 2 Show Abstract
1 Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah, United States
This research explores the potential light-trapping gains in thin-film silicon solar cells through the use of embedded dielectric nanoparticles. Such a concept has been experimentally demonstrated using tandem solar cells , but is not well-understood from a theoretical perspective. Using finite-difference time-domain (FDTD) simulations, we show that spherical nanoparticles of SiO2 embedded directly within the active layer of a 1.0 um silicon solar cell can increase the total absorption of AM 1.5 sunlight by 18 % relative to the same design without any particles. More complex particles utilizing metallic cores with dielectric coatings can also increase total solar absorption by as high as 29%. Using parametric sweeps, we also show that optimal conditions for light-trapping tend to occur when the particles are embedded near the surface of the cell rather than near the back contact. Larger diameter spheres are shown to be better at scattering light within the substrate, but also tend to displace the active material in which light is absorbed. An optimal balance between these effects occurs when the spherical particles are on the order of 200 nm in diameter.The special benefit to the embedded nanoparticle concept is its compatibility with anti-reflective coatings (ARC’s), which can readily improve light absorption by 37 % on their own without any light-trapping schemes. Light-injection and light-trapping can therefore be independently optimized such that total light absorption within thin films is maximized. This is in contrast with previous enhancement schemes that often place nanoparticles directly on the surface of solar cell . Such geometries can readily interfere with the performance of ARC’s , thereby hindering their practical performance.ReferencesS. Nunomura, A. Minowa, H. Sai, and M. Kondo, “Mie scattering enhanced near-infrared light response of thin-film silicon solar cells,” Applied Physics Letters, Vol 97 (6) 2010H. A. Atwater and A. Polman, "Plasmonics for improved photovoltaic devices," NatureMaterials, Vol 9 (3) 2010.J. R. Nagel and M. A. Scarpulla, “Enhanced absorption in optically thin solar cells by scattering from embedded dielectric nanoparticles,” Optics Express, Vol 18 (S2) 2010
6:00 PM - A5.3
Improvement of Single-junction a-Si:H Thin Film Solar Cells Toward 10% Efficiency.
Po-Hsiang Cheng 1 , Shin-Wei Liang 1 , Yi-Ping Lin 1 , Cheng-Hang Hsu 1 , Chuang-Chuang Tsai 1 Show Abstract
1 , Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu Taiwan
Hydrogenated amorphous silicon (a-Si:H) is one of the promising materials for thin film solar cell applications. At present, the a-Si:H still suffers certain degree of light-induced degradation (also known as Staebler-Wronski effect, SWE)  which leads to a reduction in efficiency. The large bandgap and high absorption coefficient make it suited for multi-junction solar cells, therefore, it is vital to continuously improve the a-Si:H solar cells.In this work, a-Si:H solar cells were fabricated on SnO2:F coated glass substrates by radio frequency (27.12 MHz) plasma-enhanced chemical vapor deposition (PECVD) system. A structure with a superstrate configuration and a back reflector were used. Electrical and optical measurements were executed to investigate the conductivity and the bandgap of each layer. The Fourier transform infrared spectroscopy (FTIR) was used to further examine the H-bonding configuration of the undoped a-Si:H film. The a-Si:H thin film solar cells were characterized by an I-V measurement system under AM1.5G illumination and a quantum efficiency (QE) instrument.In a typical a-Si:H solar cell, the boron-doped amorphous silicon carbide (a-SiC:H) has been widely used as the p-layer or the window layer due to its wide optical bandgap and reasonable conductivity. However, the band offset caused by the heterojunction between the p-layer and undoped layer (i-layer) induces defects at the p/i interface . To alleviate such effect, the CH4 flow rate was modulated to alter the carbon incorporation during the depositing of the a-SiC:H layer. Compared to the use of conventional buffer layer, our result showed that the application of the graded bandgap in buffer layer improves the Jsc from 11.84 mA/cm2 to 12.44 mA/cm2, leading to an increase of conversion efficiency from 7.95% to 8.45%. Furthermore, the doped layers were carefully optimized considering the trade-off between optical and electrical properties. The undoped layer was also improved by using hydrogen dilution in order to minimize the silicon bonding configuration, or the SiH2/(SiH+SiH2) ratio. Moreover, the hydrogen plasma treatment was applied at the surface to further refine the interfaces. The resulting short-circuit current was significantly improved from 13.30 mA/cm2 to 14.39 mA/cm2. Concerning the light absorption, electron-hole extraction and SWE, the best cell having an absorber layer of 300nm thickness exhibits an efficiency of 9.46%, with Voc=906mV, Jsc=14.42 mA/cm2 and FF=72.36%. Direction for further improvement will be discussed.This work was sponsored by the Center for Green Energy Technology at the National Chiao Tung University and National Science and Technology Program-Energy of Nation Science Council (no. 98-3114-E-009-004-CC2).1. D. Staebler and C. Wronski, Appl. Phys. Lett. 31, 292 (1977)2. S. Guha, J. Yang, A. Pawlikiewicz, T. Glatfelter, R. Ross and S. Ovshinsky, Appl. Phys. Lett. 54, 2330 (1989)
6:00 PM - A5.4
Temperature Dependent Charge Transport in Tandem a-Si:H/μc-Si:H Solar Cells.