Ralf Preu

Very low surface recombination velocity on p-type c-Si by high-rate plasma-deposited aluminum oxide

Aluminum oxide layers can provide excellent passivation for lowly and highly doped p-type silicon surfaces. Fixed negative charges induce an accumulation layer at the p-type silicon interface, resulting in very effective field-effect passivation. This paper presents highly negatively charged (Qox=−2.1×1012 cm−2) aluminum oxide layers produced using an inline plasma-enhanced chemical vapor deposition system, leading to very low effective recombination velocities (∼10 cm s−1) on low-resistivity p-type substrates. A minimum static deposition rate (100 nm min−1) at least one order of magnitude higher than atomic layer deposition was achieved on a large carrier surfaces (∼1 m2) without significantly reducing the resultant passivation quality.

High-Efficiency c-Si Solar Cells Passivated With ALD and PECVD Aluminum Oxide

Ultrathin (7 nm) atomic layer deposited Al2O3 layers and high-deposition-rate plasma-enhanced chemical vapor deposited AlOx layers have been applied and characterized as rear-surface passivation for high-efficiency silicon solar cells. The excellent efficiency values (up to 21.3%-21.5%) demonstrate that both aluminum oxide deposition processes have a very high potential comparable to the reference cells with SiO2 passivation. The high voltages ( 680 mV), the excellent long-wavelength quantum efficiency, and the high short-circuit currents of these cells (~40 mA/ cm2) are a proof for the low rear-surface recombination velocity and excellent internal rear-surface reflection.

Comprehensive analytical model for locally contacted rear surface passivated solar cells

For optimum performance of solar cells featuring a locally contacted rear surface, the metallization fraction as well as the size and distribution of the local contacts are crucial, since Ohmic and recombination losses have to be balanced. In this work we present a set of equations which enable to calculate this trade off without the need of numerical simulations. Our model combines established analytical and empirical equations to predict the energy conversion efficiency of a locally contacted device. For experimental verification, we fabricate devices from float zone silicon wafers of different resistivity using the laser fired contact technology for forming the local rear contacts. The detailed characterization of test structures enables the determination of important physical parameters, such as the surface recombination velocity at the contacted area and the spreading resistance of the contacts. Our analytical model reproduces the experimental results very well and correctly predicts the optimum cont...

Evaluating luminescence based voltage images of silicon solar cells

In this paper we give a mathematical derivation of how luminescence images of silicon solar cells can be calibrated to local junction voltage. We compare two different models to extract spatially resolved physical cell parameters from voltage images. The first model is the terminal connected diode model, where each pixel is regarded as a diode with a certain dark saturation current, which is connected via a series resistance with the terminal. This model is frequently used to evaluate measurement data of several measurement techniques with respect to local series resistance. The second model is the interconnected diode model, where the diode on one pixel is connected with the neighbor diodes via a sheet resistance. For each model parameter at least one image is required for a coupled determination of the parameters. We elaborate how also the voltage calibration can be added as an unknown parameter into the models, and how the resulting system of equations can be solved analytically. Finally the application of the models and the different ways of voltage calibration are compared experimentally.

Surface passivation of crystalline silicon by plasma-enhanced chemical vapor deposition double layers of silicon-rich silicon oxynitride and silicon nitride

Excellent surface passivation of crystalline silicon (c-Si) is desired for a number of c-Si based applications ranging from microelectronics to photovoltaics. A plasma-enhanced chemical vapor deposition double layer of amorphous silicon-rich oxynitride and amorphous silicon nitride (SiNx) can provide a nearly perfect passivation after subsequent rapid thermal process (RTP) and light soaking. The resulting effective minority carriers’ lifetime (τeff) is close to the modeled maximum on p-type as well as on n-type c-Si. Restrictions on the RTP of passivated surfaces, typical of other common passivation schemes (e.g., amorphous Si), are relieved by this double layer. Harsher thermal treatments can be adopted while still obtaining salient passivation. Furthermore, characterization of the same, such as, surface photovoltage, capacitance voltage, and electron paramagnetic resonance, enables the reproducibility and the understanding of the passivation scheme under test. It is shown that the strong quality of surf...

n-type silicon - enabling efficiencies > 20% in industrial production

In the first part of this paper we estimate the efficiency potential of crystalline silicon solar cells on conventionally pulled p-type boron-doped Czochralski-grown silicon with typical oxygen concentrations. Taking into account an industrial high-efficiency cell structure featuring fine-line metallization, shallow and well-passivated emitter and a rear surface structure with dielectric passivation and local laser-fired point contacts, the maximum achievable efficiency is around 20%. The main limitation of such a cell is due to the rather low bulk lifetime after light-induced degradation. Even when avoiding the metastable boronoxygen defect by using Gallium-doped or magnetic Cz-silicon, it has to be kept in mind that the detrimental impact of metal contaminations on p-type silicon is greater than on n-type silicon. A potential strategy to reduce this loss is the use of n-type silicon. Therefore, the second part of the paper discusses different architectures for solar cells on n-type silicon substrates and shows the latest results achieved at Fraunhofer ISE in this field.

Towards 19% efficient industrial PERC devices using simultaneous front emitter and rear surface passivation by thermal oxidation

Higher solar cell efficiencies enable a reduction of the cost per watt ratio, if production effort is maintained at an acceptable level. A proven high-efficiency concept is the passivated emitter and rear cell (PERC) [1]. However, the transfer of this solar cell structure from demonstrator level to industrial application is challenging. We present a simple approach for the industrial fabrication of PERC solar cells which utilizes the simultaneous passivation of the front emitter and the rear surface by a thin layer of thermally grown oxide. This Thermal Oxide Passivated All Sides (TOPAS) structure represents an industrially feasible implementation of the PERC concept.

High-efficiency silicon solar cells for low-illumination applications

At Fraunhofer ISE the fabrication of high-efficiency solar cells was extended from a laboratory scale to a small pilot-line production. Primarily, the fabricated cells are used in small high-efficiency modules integrated in prototypes of solar-powered portable electronic devices such as cellular phones, handheld computers etc. Compared to other applications of high-efficiency cells such as solar cars and planes, the illumination densities found in these mainly indoor applications are significantly below 1 sun. Thus, special care was taken to keep the cell efficiency level high even at very low illumination levels. For this reason, particularly the cell border was analyzed and optimized carefully. The excellent cell characteristics achieved at low illumination densities increase the benefit of a solar power supply for such devices by an order of magnitude if compared to standard solar cells.

20% Efficient Passivated Large-Area Metal Wrap Through Solar Cells on Boron-Doped Cz Silicon

We present metal wrap through passivated emitter and rear solar cells (MWT-PERC) on monocrystalline p-type silicon featuring laser-doped selective emitter structures in combination with either screen-printed (SP) or more advanced dispensed front side contacts. Thermally grown silicon oxide layers serve as emitter and rear surface passivation. Laser-fired contacts connect the SP aluminum rear contact to the silicon base. The rear side features solder contacts for both polarities. Conversion efficiency values of 20.6% for float-zone and 20.1% for Czochralski-grown silicon (not stabilized) are achieved on large-area cells with 149 wafer size. These are within the highest values reported for large-area p-type silicon solar cells to date. Analytical modeling enables a consistent description of the devices and allows for determining the dominating loss mechanisms.

PECVD-ONO: A New Deposited Firing Stable Rear Surface Passivation Layer System for Crystalline Silicon Solar Cells

A novel plasma-enhanced chemical vapour deposited (PECVD) stack layer system consisting of a-:H, a-:H, and a-:H is presented for silicon solar cell rear side passivation. Surface recombination velocities below 60 cm/s (after firing) and below 30 cm/s (after forming gas anneal) were achieved. Solar cell precursors without front and rear metallisation showed implied open-circuit voltages values extracted from quasi-steady-state photoconductance (QSSPC) measurements above 680 mV. Fully finished solar cells with up to 20.0% energy conversion efficiency are presented. A fit of the cells internal quantum efficiency using software tool PC1D and a comparison to a full-area aluminium-back surface field (Al-BSF) and thermal is shown. PECVD-ONO was found to be clearly superior to Al-BSF. A separation of recombination at the metallised and the passivated area at the solar cells rear is presented using the equations of Fischer and Kray. Nuclear reaction analysis (NRA) has been used to evaluate the hydrogen depth profile of the passivation layer system at different stages.