Tining Su

High efficiency, multi-junction nc-Si:H based solar cells at high deposition rate

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.

Highly efficient charge transfer in nanocrystalline Si:H solar 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.

12.0% Efficiency on Large-Area, Encapsulated, Multijunction nc-Si:H-Based Solar Cells

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.

Advances in cell efficiency of a-Si:H and nc-Si:H-based multi-junction solar cells for space and near-space applications

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.

High efficiency large area multi-junction solar cells incorporating a-SiGe∶H and nc-Si∶H using MVHF technology

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. ∼12% small active-area (0.25 cm2) efficiency and 10.6–10.8% large aperture-area (≥ 400 cm2) 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.

Thermal activation of deep oxygen defect formation and hydrogen effusion in hydrogenated nanocrystalline silicon thin films

Deep oxygen related defects form in hydrogenated nanocrystalline silicon (nc-Si:H) as a consequence of thermal annealing, but their microscopic origins and formation mechanisms are not well understood. To gain insight to this behavior we intentionally drive-out hydrogen from nc-Si:H films by thermal annealing and monitor accompanying changes in the electronic and vibrational structure of the films with photoluminescence (PL) and Fourier transform infrared (FTIR) absorption spectroscopy. Hydrogen effusion (HE) data provide additional insight, because the annealing temperature range shown to induce a defect band, centered at ∼ 0.7 eV in PL studies, and that corresponding to the onset of thermally activated hydrogen desorption from grain boundaries, coincide. This coincidence suggests a probable link between the two processes. The activation energy obtained from correlated annealing-PL experiments, of ∼ 0.6 eV, for defect formation with thermal exposure, provides substantial insight regarding the mechanism.

Large area nanocrystalline silicon based multi-junction solar cells with superior light soaking stability

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.

High-efficiency Large-area Nanocrystalline Silicon Solar Cells Using MVHF Technology

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.

Effects of Grain Boundaries on Performance of Hydrogenated Nanocrystalline Silicon Solar Cells

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.

Magnetic Resonance in Hydrogenated Nanocrystalline Silicon Thin Films

We have investigated the localized electronic states in mixed-phase hydrogenated nanocrystalline silicon thin films (nc-Si:H) with electron-spin-resonance (ESR). The dark ESR signal most likely arises from defects at the grain boundaries or within the crystallites. With illumination with photon energies ranging from 1.2 eV to 2.0 eV, there is no evidence of photo-induced carriers trapped in the bandtail states within the amorphous region. Dependence of the light-induced ESR (LESR) upon the exciting photon energy reveals that, at different excitation photon energies, different regions dominate the optical absorption. This behavior may have potential consequences for understanding the light-induced degradation in nc-Si:H.