MICROCRYSTALLINE SILICON SOLAR CELLS: EFFECT OF SUBSTRATE TEMPERATURE ON CRACKS

Single junction microcrystalline silicon solar cells presently reach confirmed efficiencies up to 10.1%. Further improvement on device quality is now necessary to continuously increase the electrical performances of the solar cells. Zones of porous material, called “cracks”, appear when the substrate, such as glass covered with zinc oxide (ZnO), is too “rough”. Previous works have demonstrated that these cracks have mainly detrimental effects on the Fill Factor (FF), and act as bad diodes with a high reverse saturation current. The number of cracks can be decreased with appropriate surface treatment, but then, the light scattering is reduced (lower roughness). This study presents an alternative/complementary way to decrease crack density by increasing the substrate temperature during deposition, which leads to an increase of FF

Kinetics of creation and of thermal annealing of light-induced defects in microcrystalline silicon solar cells

Single-junction microcrystalline silicon (mu c-Si:H) solar cells of selected i-layer crystalline volume fractions were light soaked (AM1.5, 1000 h at 50 degrees C) and subsequently annealed at increasing temperatures. The variations of subbandgap absorption during light soaking and during thermal annealing were monitored by Fourier transform photocurrent spectroscopy. The kinetics were shown to follow stretched exponential functions over long times such as 1000 h. The effective time constants appearing in the stretched exponential function decrease with decreasing crystalline volume fraction as well with increasing annealing temperature. Their Arrhenius-like dependence on temperature is characterized by a unique value of the activation energy. Furthermore, we demonstrate that the configuration of the solar cells (p-i-n or n-i-p) does not influence the degradation kinetics, as long as the average crystallinity of the intrinsic layer is of comparable value. (C) 2008 American Institute of Physics

Microcrystalline silicon solar cells: effect of substrate temperature on cracks and their role in post-oxidation

Microcrystalline silicon (mc-Si:H) cells can reach efficiencies up to typically 10% and are usually incorporated in tandem micromorph devices. When cells are grown on rough substrates, ‘‘cracks’’ can appear in the mc-Si:H layers. Previous works have demonstrated that these cracks have mainly detrimental effects on the fill factor and open-circuit voltage, and act as bad diodes with a high reverse saturation current. In this paper, we clarify the nature of the cracks, their role in postoxidation processes, and indicate how their density can be reduced. Regular secondary ion mass spectrometry (SIMS) and local nano-SIMS measurements show that these cracks are prone to local post-oxidation and lead to apparent high oxygen content in the layer. Usually the number of cracks can be decreased with an appropriate modification of the substrate surface morphology, but then, the required light scattering effect is reduced due to a lower roughness. This study presents an alternative/complementary way to decrease the crack density by increasing the substrate temperature during deposition. These results, also obtained when performing numerical simulation of the growth process, are attributed to the enhanced surface diffusion of the adatoms at higher deposition temperature. We evaluate the cracks density by introducing a fast method to count cracks with good statistics over approximately 4000 mm of sample cross-section. This method is proven to be useful to quickly visualize the impact of substrate morphology on the density of cracks in microcrystalline and in micromorph devices, which is an important issue in the manufacturing process of modules. Copyright#2010 John Wiley & Sons, Ltd

Silane depletion dependent ion bombardment and material quality of microcrystalline silicon deposited by VHF-PECVD

Microcrystalline silicon is a composite material embedding silicon nanocrystals in an amorphous matrix [1]. It has attracted much research efforts in the photovoltaic domain [2], because of its potential for integration in a tandem cell concept as bottom cell with an amorphous silicon top cell. Efficiencies of micromorph tandem cells and modules well above 10% have thus been demonstrated [3]. However, due to its complex structure that depends on deposition conditions [1, 4] and substrate properties [5], and due to the difficulty of characterizing plasma deposition regimes, the impact of these parameters on the microcrystalline material quality is still an open field of research. In this paper, microcrystalline silicon thin films are deposited in different conditions of silane depletion following a recent publication [6] and the material quality is investigated. It is shown that by simply reducing the hydrogen flow, the microcrystalline material quality can be greatly improved. This improvement is correlated with the reduced ion bombardment energy in high depletion regimes, leading to lower defect densities in the microcrystalline intrinsic layer

Influence of pressure and silane depletion on microcrystalline silicon material quality and solar cell performance

Hydrogenated microcrystalline silicon growth by very high frequency plasma-enhanced chemical vapor deposition is investigated in an industrial-type parallel plate R&D KAI (TM) reactor to study the influence of pressure and silane depletion on material quality. Single junction solar cells with intrinsic layers prepared at high pressures and in high silane depletion conditions exhibit remarkable improvements, reaching 8.2% efficiency. Further analyses show that better cell performances are linked to a significant reduction of the bulk defect density in intrinsic layers. These results can be partly attributed to lower ion bombardment energies due to higher pressures and silane depletion conditions, improving the microcrystalline material quality. Layer amorphization with increasing power density is observed at low pressure and in low silane depletion conditions. A simple model for the average ion energy shows that ion energy estimates are consistent with the amorphization process observed experimentally. Finally, the material quality of a novel regime for high rate deposition is reviewed on the basis of these finding

Internal electric field and fill factor of amorphous silicon (a-Si:H) solar cells

The electric field E within the i-layer of hydrogenated amorphous silicon (a-Si:H) solar cells strongly affects the cell performances, and, specifically, the fill factor FF. It governs the drift length Ldrift = μτE which is the crucial parameter limiting charge collection. Ideally, a constant electric field is assumed across the i-layer, whereas in real devices, it is deformed by charged band tail states and dangling bonds. If the i-layer is too thick or has a high density of charged defects, E is deformed and reduced. To determine theoretically the charge states of band tails and dangling bonds, we must know the carrier density profiles within the i-layer. Here, the SunShine program is used to determine carrier generation profiles within i-layers of pincells on TCO-covered glass substrates. A classical model for transport and electron/hole capture is employed to determine charge conditions of band tail states and dangling bonds. Results are: (a) charged dangling bonds are predominant for the electric field deformation, affecting the output performance of the cell; (b) this effect is very pronounced especially in degraded cells; (c) it is independent of light intensity; (d) it accounts for performance breakdown of thick, degraded a-Si:H cells. Calculated results are confronted with experimental observations (measurements of FF, collection voltage Vcoll and external quantum efficiency EQE) on pin-type solar cells of 100, 200, 300, and 400 nm thickness produced at IMT Neuchâtel, in initial and degraded state. Ldrift is evaluated via Vcoll, determined here with the method of variable intensity measurements (VIM). Trends observed are explained to full satisfaction

Influence of ion bombardment on microcrystalline silicon material quality and solar cell performances

Microcrystalline hydrogenated silicon growth with VHF-PECVD was examined in an industrial type parallel plate KAITM reactor. The influence of pressure on material quality was studied in single junction solar cells. Solar cells with their intrinsic layer prepared at higher pressures exhibit remarkable improvements, reaching 8.2% efficiency at 3.5 mbar. Further analyzes showed that μc- Si:H intrinsic layers grown at higher pressures have a significantly lower defect density. These results are attributed to lower ion bombardment energies due to higher working pressures, which improve the microcrystalline material quality. Layer amorphization with increasing power density is observed at low pressure. Calculations show that the average ion energy drops from roughly 20 eV to a few eV in the pressure range studied

Determination of Raman emission cross-section ratio in microcrystalline silicon

The determination of the crystalline volume fraction from the Raman spectra of microcrystalline silicon involves the knowledge of a material parameter called the Raman emission cross-section ratio y. This value is still debated in the literature. In the present work, the determination of y has been carried out on the basis of quantitative analysis of medium-resolution transmission electron microscopy (TEM) micrographs performed on one layer deposited by very high frequency plasma enhanced chemical vapor deposition (VHF- PECVD) close to the amorphous/microcrystalline transition. Subsequent comparison of these data with the crystallinity as evaluated from measured Raman spectra yields a surprisingly high value of y = 1.7. This result is discussed in relation to previously published values (that range from 0.1 to 0.9). © 2006 Elsevier B.V. All rights reserved