We investigate the effect of hydrogenation of grain boundaries on the performance of solar cells for hydrogenated nanocrystalline silicon (nc-Si:H) thin films. Using hydrogen effusion, we found that the amplitude of the lower temperature peak in the H-effusion spectra is strongly correlated to the open-circuit voltage in solar cells. This is attributed to the hydrogenation of grain boundaries in the nc-Si:H films.

We present the progress made in attaining high-efficiency large-area nc-Si:H based multi-junction solar cells using Modified Very High Frequency technology. We focused our effort on improving the spatial uniformity and homogeneity of nc-Si:H film growth and cell performance. We also conducted both indoor and outdoor light soaking studies and achieved 11.2% stabilized efficiency on large-area (≥400 cm 2 ) encapsulated a-Si:H/nc-Si:H/nc-Si:H triple-junction cells.

We demonstrate that in nanostructured films of nanocrystalline silicon imbedded in a hydrogenated amorphous silicon matrix, carriers generated in the amorphous region are transported out of this region and therefore do not recombine in the amorphous phase. Electron paramagnetic resonance (EPR) and photoluminescence (PL) measurements show that the EPR and PL from the amorphous phase are rapidly quenched as the volume fraction of Si nanocrystals exceeds about 30 vol. %. We propose the use of similar structures to dramatically increase the open circuit voltages in solar cell devices.

To effectively mitigate oxygen induced degradation of electronic properties in hydrogenated nanocrystalline silicon (nc-Si:H) the microscopic origins of the associated defects must be determined. This work elucidates the origin of a 0.7eV photoluminescence (PL) band observed in thermally annealed, oxygen contaminated nc-Si:H. Raman and PL spectroscopy data demonstrate the deep defects responsible for the 0.7eV PL reside in the crystalline phase. The electronic levels enabling the emission exist 0.4eV below the conduction band edge, or 0.4eV above the valence band edge, or both, with transitions into or out of extended states as opposed to band tail recombination. The defect formation energy is 0.6eV, which suggests that the formation involves a local reconfiguration of oxygen and hydrogen as opposed to bulk diffusion. Hydrogen plays multiple roles in the microstructural evolution and electronic activity of these centers. TEM micrographs show dislocations in the nanocrystalline regions, which suggests that oxygen gettering occurs. It seems in the absence of hydrogen dislocations in Si nanocrystals scavenge oxygen impurities from grain boundaries to become optically active, enabling emission at 0.7eV.

The Renewable Energy Materials Research Science and Engineering Center (REMRSEC) has made substantial progress in its research mission of identifying, launching and advancing innovative research directions in materials for renewable energy, and in its human resource mission of educating the next generation of renewable energy professionals, reaching out to students and the general community, and promoting diversity among faculty, research associates, staff, students, and future students. The Center has exploited the vibrant energy vision of the Colorado School of Mines (CSM), the extensive energy resources of the nearby National Renewable Energy Laboratory (NREL), and the growing wave of interest in energy as a national challenge to advance the global energy agenda. The Center has integrated forefront basic research with education and outreach, a powerful, timely and complementary mix. It has identified two promising and rich research directions, silicon nanostructures for next generation photovoltaics, and next generation ionic conducting membranes for battery, fuel cell and other applications. These themes effectively combine theory and experiment to drive new research frontiers. The Center’s seed program has been especially successful in exploring high-risk, high-payoff innovative research that can be integrated within the Center’s Interdisciplinary Research Groups (IRGs) or translated to independent funding. Four of the Center’s IRG or seed projects have leveraged additional funding, reflecting the vibrancy and fullness of its research agenda. The Facilities program has undergone a dramatic increase in breadth and depth, with significant advances in its Characterization Laboratory, Semiconductor Processing Laboratory, Synthesis Laboratory, Clean Room, and associated Atom Probe Development Laboratory. The new capabilities provided by these laboratories are a major boost to Center programs. The level of the technical staff of the Center, CSM and NREL has been raised with two Center hires that are joint with NREL, establishing a paradigm for leveraging cooperation and diversity. In three short years, the Center converted a vision for renewable energy into a vibrant and successful organization that engages NREL, CSM, the student body and the public in addressing a national priority, developing new materials for sustainable energy for the future.

We report the results of using n-type hydrogenated nanocrystalline silicon oxide alloy (nc-SiOx:H) in hydrogenated nanocrystalline silicon (nc-Si:H) and amorphous silicon germanium alloy (a-SiGe:H) single-junction solar cells. We used VHF glow discharge to deposit nc-SiOx:H layers on various substrates for material characterizations. We also used VHF glow discharge to deposit the intrinsic layer in nc-Si:H solar cells. RF glow discharge was used for the deposition of the doped layers and the intrinsic layer in a-SiGe:H solar cells. Various substrates such as stainless steel (SS), Ag coated SS, and ZnO/Ag coated SS were used for different cell structures. We found that by using nc-SiOx:H to replace the ZnO and the a-Si:H n-layer in nc-Si:H solar cells, the cell structure is greatly simplified, while the cell performances remain nearly identical to those made using the conventional n-i-p structure on standard ZnO/Ag BR's. Solar cells with nc-SiOx:H as the n layer directly deposited on textured Ag show similar quantum efficiency (QE) as the n-i-p cells on ZnO/Ag BRs. In both cases, QE is higher than that in the n-i-p cells made directly on Ag coated SS. This effect is probably caused by the shift of surface plasmon-polariton resonance frequency due to the difference in index of refraction of ZnO, nc-SiOx:H, and Si.

Because of its superior long-wavelength response and stability against light-induced degradation, 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 this paper, we report on the development of our proprietary high-frequency (HF) glow discharge deposition technology for nc-Si:H solar cells that has resulted in high-quality nc-Si:H materials with good spatial uniformity. We studied the HF-deposited nc-Si:H material using various analytical techniques. We fabricated a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells that are deposited on textured Ag/ZnO back reflectors. Large-area cells were fabricated and encapsulated using our proprietary lightweight flexible encapsulants. National Renewable Energy Laboratory (NREL) has confirmed (1) initial aperture-area efficiency of 11.8% on an 807.8-cm 2 encapsulated cell and (2) stable aperture-area efficiency of 11.2% on a 400-cm 2 encapsulated cell.

Summary form only given. 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 due to its superior long-wavelength response and stability against light-induced degradation. Due to the indirect band gap in crystalline silicon, the absorbing nc-Si:H layer needs to be much thicker than the corresponding a-SiGe:H layer. For nc-Si:H based solar cells to be commercially viable, the greatest challenge is to deposit the absorbing layers at a high rate with good spatial uniformity, while maintaining the same superior quality achieved at lower deposition rate. In this paper, we report on the development of our proprietary High Frequency (HF) glow discharge deposition technology to fabricate high efficiency, large area, a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells at a high deposition rate ≥1 nm/s. We have improved our nc-Si:H and a-Si:H processes to fabricate high performance component cells used in the triple-junction solar cells. We have fabricated small area cells (0.25 cm2) and mini module (1.2 cm2) cut out from the large deposited area. We have attained initial, active-area efficiency as high as ~14.0% and light-stabilized, active-area efficiency ~12.8% on these cells. SIMS analysis on the device show low impurity levels in the nc-Si:H absorbing layers. We have also fabricated large area encapsulated modules. We have attained initial aperture-area (~212 cm2) efficiency of ~11.8% on an encapsulated module. These are the highest values measured at United Solar for such high rate samples. Detailed results will be presented at the conference.

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 due to its superior long-wavelength response and stability against light-induced degradation. In this paper, we report on the development of our proprietary High Frequency (HF) glow discharge deposition technology for nc-Si:H solar cells that has resulted in high quality nc-Si:H materials with good spatial uniformity. We have studied the HF-deposited nc-Si:H material using various analytical techniques, such as X-ray diffraction, Secondary Ion Mass Spectrometry, and Glow Discharge Mass Spectrometry, and optimized the deposition parameters for best device quality. We conducted a systematic study of the quality and spatial uniformity of nc-Si:H solar cells. We fabricated and optimized a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells deposited on textured Ag/ZnO back reflectors on thin flexible stainless steel substrates using the optimized nc-Si:H component cells. Cells with aperture area ~400 cm2 and 807 cm2 were fabricated and encapsulated using our proprietary lightweight flexible encapsulants. We sent representative large-area samples to National Renewable Energy Laboratory (NREL) for confirmation of conversion efficiency. NREL has confirmed an initial aperture-area efficiency of 12.0% for cells with aperture area ~400 cm2. The highest initial efficiency for the encapsulated cells with aperture area ~807 cm2 is ~11.9% as measured at United Solar. We light soaked small-area and large-area cells to obtain stable performance. Detailed results will be presented at the conference.

We have fabricated large-area, thin-film multijunction solar cells based on hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) made in a large area batch reactor. The device structure consisted of an a-Si:H/nc-Si:H/nc-Si:H stack on Ag/ZnO back reflector coated stainless steel substrate, deposited using our proprietary High Frequency (HF) glow discharge technique. For the nc-Si:H films, we investigated two deposition rate regimes: (i) low rate <1 nm/s and (ii) high rate >1 nm/s. We optimized the deposition parameters, such as pressure, gas flow, dilution, and power. We did SIMS analysis on the optimized films, and found the impurity concentrations were one order of magnitude lower than the films made with the conventional RF process. In particular, the oxygen concentration is reduced to ~1018 cm-3. This value is among the lowest oxygen concentration reported in literature. The low impurity content is attributed to proprietary cathode hardware and the optimized deposition process. During the initial optimization and investigative phase, we fabricated small-area (0.25 cm2 and 1.1 cm2) cells. The information obtained from the initial phase was used to fabricate large-area (aperture area 400 cm2) cells, and encapsulated the cells using the same flexible encapsulants that are used in our commercial product. We have light soaked the low-rate and high-rate encapsulated modules. The highest initial efficiency of the low-rate modules is 12.0% as confirmed by NREL. The highest corresponding stable efficiency attained for the low-rate samples cells is 11.35%. For the high-rate small-area (1.1 cm2) cells, the highest initial active-area efficiency and corresponding stable efficiency attained are 13.97% and 12.9%, respectively. We present the details of the research conducted to develop the low- and high-rate cells and modules.

We report on the investigation of large area a-Si:H/a-SiGe:H double-junction and a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells prepared by our proprietary High Frequency (HF) glow discharge technique. For investigative purposes, we initially used the simpler double-junction structure. We studied the effect of: (1) Ge content, (2) cell thickness, and (3) SiH4 and GeH4 gas flow on the light-induced degradation of the solar cells. Our results show that the double-junction cells with different Ge concentration have open-circuit voltage (Voc) in the range of 1.62-1.75 V. Voc exhibits a flat plateau in the range of 1.65-1.72 V for both initial and stabilized states. The light-induced degradation for cells in this range of Voc is insensitive to the Ge content. In terms of thickness dependence of the intrinsic layers, we found that the initial efficiency increases with cell thickness in the thickness range 2000-4000 Å. However, light-induced degradation increases with increasing thickness. Consequently, the stabilized efficiency is invariant with cell thickness in the thickness range studied. The results of SiH4 and GeH4 gas flow on cell characteristics demonstrate that the deposition rate decreases by only 20% when the active gas flow is reduced to 0.25 times standard flow. The initial and stabilized efficiencies are similar. The information gleaned from the study was used to fabricate high efficiency, large area (~464 cm2) double- and triple-junction solar cells. The highest stable efficiency, as measured by NREL, was 9.8% and 11.0% for the double- and triple-junction structures, respectively.

We have developed thin film amorphous silicon alloy (a-Si:H) and nanocrystalline silicon (nc-Si:H) based multijunction solar cells on lightweight polymer substrate ~25 μm thick for space and near-space applications. The baseline cells use an a-Si:H/a-SiGe:H/a-SiGe:H structure deposited by conventional Radio Frequency (RF) plasma enhanced CVD using roll-to-roll deposition. The best initial performance for the baseline cells is aperture-area efficiency 9.84% and specific power ~1200 W/kg. The baseline cells are available to potential customers in large quantities. In order to increase the solar cell efficiency, we have pursued two new approaches. In the first, we use a Modified Very High Frequency (MVHF) technique to deposit the multijunction a-SiGe:H based cells. In the second, we have investigated nc-Si:H based multijunction cells. In this paper, we present the solar cell efficiency results on the three different device structures.

We have developed high efficiency large area a-Si:H and a-SiGe:H multi-junction solar cells using a Modified Very High Frequency (MVHF) glow discharge process. We investigated a-SiGe:H deposition rate dependence of cell performance, and optimized MVHF a-SiGe:H process at a deposition rate 2-4 times that of a typical RF deposition. We conducted a comparative study for different cell structures, and compared the initial and stable performance and light-induced degradation of solar cells made using MVHF and RF techniques. In additon to high initial efficiency, the MVHF cells also exhibit superior light stability, showing <10% degradation after 1000 hour of one-sun light soaking at 50 degrees C. We also studied light-induced defect level and hydrogen evolution characteristics of MVHF deposited a-SiGe:H films and compared them with the RF deposited films. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present our progress in attaining high efficiency nc-Si:H solar cells at high deposition rates with superior light soaking stability. We have focused our effort on three areas: (i) improving the spatial uniformity and homogeneous properties for nc-Si:H, such as crystallite grain size and volume fraction, (ii) optimizing nucleation and seed layer during the initial growth of the nc-Si:H film, and (iii) optimizing nc-Si:H bulk growth and grain evolution. We have conducted an extensive study of the effect of process parameters including hydrogen dilution profiling, VHF power, and substrate temperature on the nc-Si:H film properties and component cell characteristics. We also conducted light soaking tests both indoors and outdoors. The a-Si:H/nc-Si:H/nc-Si:H triple-junction cells incorporating the optimized nc-Si:H component cells show significantly higher performance, achieving an 11.2% AM1.5 stabilized efficiency for both encapsulated large-area (464 cm2) cells and inter-connected modules (2320 cm2). To the best of our knowledge, this is the highest stabilized efficiency for a large-area thin-film silicon module.

We report our investigations of large area multi-junction solar cells based on hydrogenated nano-crystalline silicon (nc-Si:H). We compared results from cells deposited by RF (13.56 MHz) at lower deposition rate (˜3 Å/s) and by Modified Very High Frequency (MVHF) at higher rate (= 10 Å/s). With optimized process conditions and cell structures, we have obtained ˜12% initial small active-area (˜0.25 cm2) efficiency for both RF and MVHF cells and 10˜11% large aperture-area (˜400 cm2) encapsulated MVHF cell efficiency for both a-Si:H/nc-Si:H double-junction and a-Si:H/nc-Si:H/nc-Si:H triple-junction structures on Ag/ZnO coated stainless steel substrate.

We fabricated five different types of a-SiGe:H and nc-Si:H based multi-junction solar cell structures using modified Very High Frequency (MVHF) technology. After optimization, all five structures reached similar initial cell performance, i.e. similar to 12% small active-area (0.25 cm(2)) efficiency and 10.6-10.8% large aperture-area (>= 400 cm(2)) efficiency after encapsulation. However, they showed quite different light soaking stability behavior, which can be attributed to the degradation of component cells. We conducted a comparative study between the MVHF deposited solar cells with those deposited by RF. Materials studies were also conducted to understand the mechanism responsible for better stability for the MVHF deposited a-SiGe:H solar cells. The best stable efficiency achieved for the large-area encapsulated cells is approaching 10% for both a-SiGe:H and nc-Si:H based multi-junction cells.

Utilizing the concepts of a critical crystallite size and local film inhomogeneity, it is shown that nucleation in thermally annealed hydrogenated amorphous silicon occurs in the more well ordered spatial regions in the network, which are defined by the initial inhomogeneous H distributions in the as-grown films. Although the film H evolves very early during annealing, the local film order is largely retained in the still amorphous films even after the vast majority of the H is evolved, and the more well ordered regions which are the nucleation center sites for crystallization are those spatial regions which do not initially contain clustered H, as probed by H NMR spectroscopy. The sizes of these better ordered regions relative to a critical crystallite size determine the film incubation times (the time before the onset of crystallization). Changes in film short range order upon H evolution, and the presence of microvoid type structures in the as grown films play no role in the crystallization process. While the creation of dangling bonds upon H evolution may play a role in the actual phase transformation itself, the film defect densities measured just prior to the onset of crystallization exhibit no trends which can be correlated with the film incubation times.

We have developed high efficiency large area a-Si:H and a-SiGe:H multi-junction solar cells using a Modified Very High Frequency (MVHF) glow discharge process. We conducted a comparative study for different cell structures, and compared the initial and stable performance and light-induced degradation of solar cells made using MVHF and RF techniques. Besides high efficiency, the MVHF cells also demonstrate superior light stability, showing <10% degradation after 1000 hour of one-sun light soaking at 50 °C. We also studied light-induced defect level and hydrogen evolution characteristics of MVHF deposited a-SiGe:H films and compared them with the RF deposited films.