Showing papers by "Baojie Yan published in 2010"
TL;DR: In this article, a-Si:H was used as a low bandgap absorber material to substitute for aSiGe:H alloys in multi-junction solar cells.
Abstract: This paper reviews our progress of using nc-Si:H as a low bandgap absorber material to substitute for a-SiGe:H alloys in multi-junction solar cells. We have focused on three topics: (1) high deposition rate, (2) large area uniformity of thickness and material properties, (3) high solar cell and module efficiencies. Initially, we investigated various deposition methods, including RF, VHF, and microwave glow discharges. After several years of systematic studies, we have been convinced that VHF glow discharge is an applicable method to attain high rate and large-area uniform nc-Si:H depositions. We also studied the stability of nc-Si:H solar cells and observed various metastability phenomena in nc-Si:H solar cells. We have reported an initial active-area cell efficiency of 15.4% using an a-Si:H/a-SiGe:H/ nc-Si:H triple-junction structure. Subsequently, we have increased the deposition rates to around 1.0-1.5 nm/s and achieved an initial active-area efficiency of 13.4% using an a-Si:H/nc-Si:H/nc-Si:H triple-junction structure. Recently, a stable total-area efficiency of 12.5% was measured at NREL on our a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cell. We have also developed large-area VHF deposition systems and demonstrated encouraging module efficiencies using a-Si:H/ nc-Si:H/nc-Si:H triple-junction structures. Initial and stable aperture-area (400 cm 2 ) efficiencies of 11.0 and 10.1 % have been achieved with fully encapsulated modules with a-Si:H/nc-Si:H/nc-Si:H triple-junction structures. In this paper, various aspects of nc-Si:H solar cells are discussed, including the material structures, device design, light trapping, metastability, high efficiency solar cell optimization, and large-area deposition.
59 citations
19 Aug 2010
TL;DR: In this paper, an optimized Ag/ZnO back reflector for nc-Si:H solar cells was constructed with moderately textured Ag and thin ZnO layers, which achieved an initial active area efficiency of 14.5% with a total current discrepancy exceeding 30 mA/cm2.
Abstract: We report our progress in the optimization of Ag/ZnO back reflectors (BR) for a-Si:H and nc-Si:H solar
cells. Theoretically, a BR with a smooth metal surface and a textured dielectric surface would be more desirable. A
smooth metal/dielectric interface reduces the plasmonic resonance loss and parasitic losses due to light trapped in
sharp angles; a textured dielectric/semiconductor interface provides scattering for light trapping. In order to obtain
sufficient light scattering at the ZnO/silicon interface, a highly textured ZnO layer is normally used. However, a
highly textured ZnO surface causes deterioration of nc-Si:H material quality. In addition, to make a highly textured
ZnO surface, a thick ZnO layer is needed, which could introduce additional absorption in the bulk ZnO layer and
reduce the photocurrent density. Therefore, Ag/ZnO BR structures for nc-Si:H solar cells needs to be optimized
experimentally. In this study, we found that an optimized Ag/ZnO BR for nc-Si:H solar cells is constructed with
textured Ag and thin ZnO layers. Although a textured Ag layer might cause certain losses resulting from plasmonic
absorption, the enhanced light scattering by a moderately textured Ag layer makes it possible to use a thin ZnO
layer, where the absorption in the ZnO layer is low. With such a BR, we achieved a short-circuit current density of
over 29 mA/cm2 from a nc-Si:H single-junction solar cell. Using the high performance nc-Si:H cell in an a-Si:H/nc-
Si:H/nc-Si:H triple-junction structure, we achieved an initial active-area efficiency of 14.5% with a total current
density exceeding 30 mA/cm2.
20 citations
TL;DR: In this article, structural and compositional properties of a range of hydrogenated nanocrystalline films were investigated using Raman spectroscopy for crystallinity and time-of-flight secondary ion mass spectroscopic (TOF-SIMS) for impurity characterizations.
Abstract: We report measurements of the structural and compositional properties of a range of hydrogenated nanocrystalline films. We employed Raman spectroscopy for crystallinity and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) for impurity characterizations. The crystalline volume fractions and impurity levels are correlated with the deep state densities determined by drive level capacitance profiling. Those defects were found to have a thermal emission energy of 0.65±.05 eV. We found that the overall crystallinity correlated reasonably well with the density of such defect states and also found a strong correlation between the defect density and the levels of oxygen impurities. Possible origins of these defects are discussed.
20 citations
20 Jun 2010
TL;DR: In this article, a thin film amorphous silicon alloy and nanocrystalline silicon (nc-Si:H) based multijunction solar cells on a polymer substrate were developed for space and near-space applications.
Abstract: 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.
9 citations
20 Jun 2010
TL;DR: In this article, the authors measured the absorption coefficients as a function of wavelength and found that no good linearity was observed on the Tauc plot for nc-Si:H films.
Abstract: Hydrogenated nanocrystalline silicon (nc-Si:H) has attracted a great deal of attention in solar cell applications. However, the material properties have not been very well understood because of the complexity of the structure. The objective of this paper is to find the optical bandgap by measuring the absorption coefficients as a function of wavelength. We found that no good linearity was observed on the Tauc plot for nc-Si:H films. It means that one cannot obtain the optical bandgap of nc-Si:H from the Tauc plot. Instead, a plot of (αhυ)1/5 versus hυ shows a straight line for a wide range of photon energies. The intersection on the h?-axis is around 1.1 eV, which is coincidentally the same as the bandgap of c-Si. The simplest explanation of the 1/5 power could be a superposition of absorptions from nanocrystallites and amorphous tissues. We also measured the dark current versus voltage characteristics as a function of temperature for nc-Si:H solar cells. The pre-factor of the diode characteristics shows a thermal activation energy of 0.55–0.65 eV. If the Fermi level is at the middle of the bandgap, the mobility bandgap of nc-Si:H is around 1.1–1.3 eV.
8 citations
TL;DR: In this paper, hydrogenated nanocrystalline silicon (nc-Si:H) solar cells were prepared at different deposition rates and achieved an initial active area efficiency of 10.2%.
Abstract: We report recent progress on hydrogenated nanocrystalline silicon (nc-Si:H) solar cells prepared at different deposition rates. The nc-Si:H intrinsic layer was deposited, using a modified very high frequency (MVHF) glow discharge technique, on Ag/ZnO back reflectors (BRs). The nc-Si:H material quality, especially the evolution of the nanocrystallites, was optimized using hydrogen dilution profiling. First, an initial active-area efficiency of 10.2% was achieved in a nc-Si:H single-junction cell deposited at ~5 A/s. Using the improved nc-Si:H cell, we obtained 14.5% initial and 13.5% stable active-area efficiencies in an a-Si:H/nc-Si:H/nc-Si:H triple-junction structure. Second, we achieved a stabilized total-area efficiency of 12.5% using the same triple-junction structure but with nc-Si:H deposited at ~10 A/s; the efficiency was measured at the National Renewable Energy Laboratory (NREL). Third, we developed a recipe using a shorter deposition time and obtained initial 13.0% and stable 12.7% active-area efficiencies for the same triple-junction design.
7 citations
TL;DR: In this article, the microscopic structure and solar cell performance in hydrogenated mixed-phase thin-film silicon (Si:H) solar cells were studied with RF glow discharge with different hydrogen dilution profiles.
Abstract: Microscopic structure and solar cell performance in hydrogenated mixed-phase thin film silicon (Si:H) solar cells are studied. The samples were made with RF glow discharge with different hydrogen dilution profiles. The material properties were measured with Raman, X-TEM, AFM, and C-AFM. Several interesting phenomena are observed. First, the cone-structured nanocrystalline aggregations were formed when a constant hydrogen dilution was used. Second, no uniform block-like (or cylinder-like) structured nanocrystalline clusters were observed even when hydrogen dilution profiling was optimized for this purpose. Instead, tree-like structured nanocrystalline clusters were formed and embedded in the intrinsic layer. Third, the magnitude of light-induced Voc increase was reduced by hydrogen dilution profiling. When the dilution profiling was sufficiently steep, no light-induced Voc increase was observed. Instead, the Voc decreased after light-soaking regardless of the crystalline volume fraction. In addition, AFM and C-AFM showed that this type of mixed-phase material has hill-like surface structure, where the hills correspond to nanocrystalline clusters. The local current density in hill-like areas was much higher in the samples made with constant hydrogen dilution than those using hydrogen dilution profiling. For the samples with a very steep hydrogen dilution profiling, the local forward current density is very low. Based on our previous model, the light-induced Voc increase depends on the formation of the current path in the nanocrystalline cluster areas. When a steep hydrogen dilution profiling is used, the tree-like nanocrystalline clusters are isolated and embedded in the intrinsic layer, therefore, no high current paths are formed and no light-induced Voc increase is observed. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
5 citations
TL;DR: In this article, the authors developed high efficiency large area aSi:H and a-SiGe:H multi-junction solar cells using a Modified Very High Frequency (MVHF) glow discharge process.
Abstract: 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 °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 (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
4 citations
TL;DR: The photoluminescence band in nc-Si:H is centered at ∼ 0.7 eV, which emerges in response to annealing at an onset temperature of ∼ 200-300 °C as discussed by the authors.
Abstract: Hydrogenated nanocrystalline silicon (nc-Si:H), a mixture of nanometer sized crystallites and amorphous silicon tissue, demonstrates a photoluminescence band centered at ∼ 0.7 eV, which emerges in response to annealing at an onset temperature of ∼ 200–300 °C. This temperature range correlates well with hydrogen effusion spectroscopy studies, and evidence suggests thermal liberation of hydrogen from grain boundary regions allows oxidation of crystallite surfaces during annealing. We tentatively attribute the 0.7 eV PL in nc-Si:H to deep donor defect states related to oxygen precipitates, and argue for the possible involvement of dislocations inside of crystallites to accompany these precipitates.
4 citations
TL;DR: In this article, the authors measured the quantum efficiencies of nc-Si:H solar cells and normalized photoconductance spectra in SOI photdetectors with and without silver nanoparticle layers.
Abstract: Quantum efficiency measurements in (nc-Si:H) solar cells deposited onto textured substrates indicate that these cells are close to the “stochastic light-trapping limit“ proposed by Yablonovitch in the 1980s. An interesting alternative to texturing is “plasmonic“ light-trapping based on non-textured cells and using an overlayer of metallic nanoparticles to produce light-trapping. While this type of light-trapping has not yet been demonstrated for nc-Si:H solar cells, significant photocurrent enhancements have been reported on silicon-on-insulator devices with similar optical properties to nc-Si:H. Here we report our work on the measurerements of quantum efficiencies in nc-Si:H solar cells and normalized photoconductance spectra in SOI photdetectors with and without silver nanoparticle layers. As was done previously, the silver nanoparticles were created by thermal annealing of evaporated silver thin films. We observed enhancement in the normalized photoconductance spectra of SOI photodetectors at longer wavelengths with the silver nanoparticles. For nc-Si:H solar cells, we have not yet observed significant improvement of the quantum efficiency with the addition of annealed silver films.
4 citations
20 Jun 2010
TL;DR: In this article, the spatial uniformity and homogeneous properties of nc-Si:H solar cells were optimized to achieve high efficiency at high deposition rates with superior light soaking stability.
Abstract: 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.
TL;DR: In this paper, the authors present results of extremely short carrier lifetime measurements on a series of hydrogenated nanocrystalline silicon (nc-Si:H) thin films by a novel, non-destructive, noncontact method.
Abstract: We present results of extremely short carrier lifetime measurements on a series of hydrogenated nanocrystalline silicon (nc-Si:H) thin films by a novel, non-destructive, non-contact method. Transmission modulated photoconductive decay (TMPCD) is a newly developed technique which appears to have high enough sensitivity and time resolution to measure the extremely short carrier lifetimes on the order of a nanosecond. As a proof of this, we measure various nc-Si:H samples of varying crystalline volume fraction as well as a fully amorphous sample. To ascribe an effective lifetime to the materials, we use a simple model which assumes a single exponential decay. By using this model, effective lifetimes can be deconvoluted from our pump beam giving nanosecond lifetimes. Lifetimes of between 1.9 and 0.9 nanoseconds are reported and trend to decreasing lifetimes as crystalline volume fraction is increased.
TL;DR: In this paper, the effect of hydrogenation of grain boundaries on the performance of solar cells for hydrogenated nanocrystalline silicon (nc-Si:H) thin films was investigated and the amplitude of the lower temperature peak in the H-effusion spectra was strongly correlated with the open-circuit voltage in solar cells.
Abstract: 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.
20 Jun 2010
TL;DR: In this article, the decay of photoexcited carriers in nanocrystalline silicon as a function of generation rate was investigated using a noncontact method based on the conduction of a 500 MHz RF field called transmission modulated photoconductive decay (TMPCD).
Abstract: We investigate the decay of photoexcited carriers in nanocrystalline silicon as a function of generation rate. We use a non-contact method based on the conduction of a 500 MHz RF field called transmission modulated photoconductive decay (TMPCD). In order to interpret the results we employ a combination of theories based on the direct recombination of free-carriers and the effects of multiple trapping of carriers which exist in disordered materials. Our results show that free-carrier recombination can, to some extent, describe the decay dynamics, which occur at times shorter than our nanosecond pulse length. We establish this interpretation through its measured generation rate dependence. Also, it is shown that the theory of multiple trapping can accurately describe the behavior of the photoconductive decay that exists at times following the pulse even though this is not preceded by an establishment of a steady-state photoconductivity.
TL;DR: In this paper, the authors presented the progress made in attaining high efficiency large-area nc-Si:H based multi-junction solar cells using Modified Very High Frequency technology.
Abstract: 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.
TL;DR: In this article, a series of a Si,Ge:H alloy devices were examined using both RF and VHF glow discharge in two configurations: SS/n+/i (a-SiGe-H)/p + /ITO nip devices and SS/m+/m (a -SiGe):H)/Pd Schottky contact devices over a range of deposition rates.
Abstract: We have examined a series of a Si,Ge:H alloy devices deposited using both RF and VHF glow discharge in two configurations: SS/n+/i (a-SiGe:H)/p + /ITO nip devices and SS/n+/i (a-SiGe:H)/Pd Schottky contact devices, over a range of deposition rates. We employed drive-level capacitance profiling (DLCP), modulated photocurrent (MPC), and transient junction photo-current (TPI) measurement methods to characterize the electronic properties in these materials. The DLCP profiles indicated quite low defect densities (mid 1015 cm-3. to low 1016 cm-3 depending on the Ge alloy fraction) for the low rate RF (∼1A/s) deposited a-SiGe:H materials. In contrast to the RF process, the VHF deposited a-SiGe:H materials did not exhibit nearly as rapid an increase of defect density with the deposition rate, remaining well below 1017 cm-3. up to rates as high as 10A/s. Simple examination of the TPI spectra on theses devices allowed us to determine valence band-tail widths.. Modulated photocurrent (MPC) obtained for several of these a-SiGe:H devices allowed us to deduce the conduction band-tail widths. In general, the a-Si,Ge:H materials exhibiting narrower valence band-tail widths and lower defect densities correlated with the best device performance.
TL;DR: In this article, a colloidal silver nanoparticle monolayer was used on nanocrystalline silicon solar cells with conducting oxide top layers to increase the photoconductance at shorter wavelengths (500-750 nm).
Abstract: A layer of silver nanoparticles created by thermal annealing of evaporated silver films can increase the photocurrents in silicon-on-insulator (SOI) devices by fivefold or more, but significant enhancements have been restricted to wavelengths greater than 800 nm. Here we report a significant enhancement of photoconductance at shorter wavelengths (500-750 nm) by using a monolayer of silver nanoparticles transferred from a colloidal suspension. Photocurrents on SOI increased in the 500-750 nm spectral range with the addition of silver nanoparticles, with enhancements more than two times; enhancements at longer wavelengths were small, in contrast to results with annealed silver films. We prepared similar colloidal silver nanoparticle monolayers layers on nanocrystalline silicon solar cells with conducting oxide top layers. There is an overall decrease in the quantum efficiency of these cells with the deposition of silver nanoparticles. We attribute these effects to the substantial substrate-mediated changes in the localized surface plasmon resonance frequencies of the differing nanoparticle configurations.