Depositing microcrystalline intrinsic silicon films is an important step for the production of thin silicon tandem junction solar cells. Due to the high cost of capital equipment, it is becoming increasingly important to improve the processing speed of thin silicon films for continued commercial viability. In this work, a combination of the excitation frequencies 13.56MHz + 27.12 MHz was used for thin silicon film deposition. According to the electrical asymmetry, the DC self bias on the RF electrode was varied by adjusting the phase between the two applied frequencies. A single junction microcrystalline cell with above 5.5% efficiency was deposited in a Gen5 PECVD process using the Electrical Asymmetry Effect (EAE). The deposition rate was higher than 0.8 nm/s. A similar increase of the deposition rate in a pure 13.56 MHz discharge led to a strong degradation of the mu c-Si:H quality and the single junction cell performance fell to 4% efficiency. It was found that layers deposited using the EAE have a better uniformity compared to layers deposited in a pure 27.12 MHz discharge. In comparison to traditional RFPECVD processes, electrically asymmetric discharges allow to achieve a regime of plasma conditions with low ion energies and high electron densities. (C) 2012 Elsevier Ltd. All rights reserved.

The deposition of amorphous and microcrystalline silicon is an important step in the production of thin silicon film solar panels. Deposition rate, layer uniformity and material quality are key attributes for achieving high efficiency in such panels. Due to the multilayer structure of tandem solar cells (more than 6 thin silicon layers), it is becoming increasingly important to improve the uniformity of deposition without sacrificing deposition rate and material quality. This paper reports the results of an investigation into the influence of the electrical asymmetry effect (EAE) on the uniformity of deposited layers. 13.56MHz+27.12MHz excitation frequencies were used for thin silicon film deposition in a Gen5 reactor (1100×1400mm). To change the plasma properties, the DC self bias voltage on the RF electrode was varied by adjustment of the phase angle between the two frequencies applied. It was found that the layers deposited by EAE method have better uniformity than layers deposited in single frequency 27.12MHz discharge. The EAE provides additional opportunities for improvement of uniformity, deposition rate and material quality.

A postdeposition thermal treatment has been applied to sputtered Al-doped zinc oxide films and shown to strongly decrease the resistivity of the films. While high temperature annealing usually leads to deterioration of electrical transport properties, a silicon capping layer successfully prevented the degradation of carrier concentration during the annealing step. The effect of annealing time and temperature has been studied in detail. A mobility increase from values of around 40 cm2/Vs up to 67 cm2/Vs, resulting in a resistivity of 1.4×10−4 Ω cm has been obtained for annealing at temperatures of 650 °C. The high mobility increase is most likely obtained by reduced grain boundary scattering. Changes in carrier concentration in the films caused by the thermal treatment are the result of two competing processes. For short annealing procedures we observed an increase in carrier concentration that we attribute to hydrogen diffusing into the zinc oxide film from a silicon nitride barrier layer between the zinc oxide and the glass substrate and the silicon capping layer on top of the zinc oxide. Both are hydrogen-rich if deposited by plasma-enhanced chemical vapor deposition. For longer annealing times a decrease in carrier concentration can occur if a thin capping layer is used. This can be explained by the deteriorating effect of oxygen during thermal treatments which is well known from annealing of uncapped zinc oxide films. The reduction in carrier concentration can be prevented by the use of capping layers with thicknesses of 40 nm or more.

Silicon based thin tandem solar cells were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a 30 x 30cm(2) reactor. The layer thicknesses of the amorphous top cells and the microcrystalline bottom cells were significantly reduced compared to standard tandem cells that are optimized for high efficiency (typically with a total absorber layer thickness from 1.5 to 3 mu m). The individual absorber layer thicknesses of the top and bottom cells were chosen so that the generated current densities are similar to each other. With Such thin cells, having a total absorber layer thickness varying from 0.5 to 1.5 mu m, initial efficiencies of 8.6-10.7% were achieved. The effects of thickness variations of both absorber layers on the device properties have been separately investigated. With the help of quantum efficiency (QE) measurements, we could demonstrate that by reducing the bottom cell thickness the top cell current density increased which is addressed to back-reflected light. Due to a very thin a-Si:H top cell, the thin tandem cells show a much lower degradation rate under continuous illumination at open circuit conditions compared to standard tandem and a-Si:H single junction cells. We demonstrate that thin tandem cells of around 550 nm show better stabilized efficiencies than a-Si:H and mu c-Si:H single junction cells of comparable thickness. The results show the high potential of thin a-Si/mu c-Si tandem cells for cost-effective photovoltaics. Copyright (C) 2010 John Wiley & Sons, Ltd.

The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (μc-Si:H) has been systematically investigated. Single μc-Si:H layers and complete μc-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of ∼1.2 μm thick μc-Si:H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2×1019 to 2×1019 cm−3; in the case of nitrogen contamination the critical impurity level is lower ([N]critical=6×1018–8×1018 cm−3). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of μc-Si:H films. It was observed that the both contaminant types act as donors. Efficiency losses due to oxygen or nitrogen impurities are attributed to fill factor decreases and to a reduced external quantum efficiency at wavelengths of >500 nm. In the case of an air leak during the μc-Si:H deposition process, the cell performance drops at an air leak fraction from 140 to 200 ppm compared to the total gas flow during i-layer deposition. It is demonstrated that oxygen and nitrogen impurities close to the p/i-interface have a stronger effect on the cell performance compared to impurities close to the n/i-interface. Moreover, thick μc-Si:H solar cells are found to be more impurity-sensitive than thinner cells.

Fourier transform infrared absorption spectroscopy (FTIR), optical emission spectroscopy (OES), self-bias voltage and plasma impedance controls were applied as in situ process diagnostics during the deposition of amorphous silicon thin-films. The diagnostic abilities of OES and FTIR are compared. The FTIR in-situ direct measurement of silane concentration in exhaust line is more precise than OES control. All in situ process diagnostics clearly indicates the inconsistency of plasma properties and therefore of deposition conditions. The drifts are comparable with the film deposition time. The FTIR measurement of reactant concentration in the process chamber evidence that the strong silane concentration drop (about 50%) in a plasma is the cause of the short-term drift of OES signals (SiH⁎ emission), plasma impedance and self-bias voltage signals. The influences of the deposition chamber geometry and technological parameters on process drifts are considered. The decrease of the gas residence time in the reactor leads to a decrease of Initial Transient State phenomena. Finally, the improvement of solar cell performance based on thin silicon films is demonstrated when drifts are reduced.

Etch product detection by in situ optical emission spectroscopy is used to detect the phase transition from amorphous to microcrystalline silicon. In this contribution it is demonstrated that a calibrated version of this technique can be used to determine the absolute hydrogen flux under state-of-the-art silicon thin film deposition conditions.

The H flux to Si growth flux ratio is experimentally determined under state-of-the-art silicon thin-film deposition conditions by employing the recently introduced etch product detection technique. Under the technologically relevant high-pressure depletion conditions and for different process parameter settings such as pressure, SiH4 concentration, rf power, and excitation frequency, it was demonstrated that the microcrystalline to amorphous silicon phase transition is uniquely and reactor independently determined by the flux ratio of H and Si growth species.