High-efficiency silicon solar cells with evaporated front contacts and an oxide-passivated rear require post metallization annealing (PMA). In an industrial environment the evaporated front contacts ...are replaced by screen printed contacts for fast processing and cost reasons. The PMA conditions necessary for optimum rear side passivation can be inferior to such a front side metallization. In order to design a PMA supporting contact in future, this paper investigates what mechanism deteriorates the contact resistance of screen printed front side metallization during nitrogen PMA. Scanning electron microscopy (SEM) on samples with increased contact resistance reveals an altered microstructure at the silver–silicon contact interface that is proposed to impede current flow and hence increases the contact resistance. We present a model that describes the mechanism of contact deterioration during nitrogen PMA.
We present progress of the analysis of multicrystalline silicon and of improvements of our understanding of material related performance limits of solar cells. Aspects covered are advanced numerical ...simulation, experimental results on carrier lifetime distributions, detailed analysis of the defect luminescence spectrum, and findings about breakdown behaviour. Modelling of impurity redistribution, specifically iron, with respect to high temperature steps enlightens mechanisms governing the phosphorus diffusion and aluminium gettering in heterogeneous systems. Evidence of outdiffusion of metallic impurities from grain boundaries, after high temperature steps, is given by analysing spatially resolved carrier lifetime measurements. In addition to lifetime, we present defect luminescence spectra measured by photoluminescence spectroscopy, including an interpretation of the origin of a remarkably temperature stable emission line at 838
meV. Finally, defect luminescence is connected to the origin of pre-breakdown of solar cells at reverse bias, a topic which is of high interest especially in context with the increased usage of purified metallurgical grade silicon.
The passivation mechanisms and qualities of Al2O3, SiNx, SiO2 and a-Si:H(i) on p- and n-type silicon are investigated by quasi-steady-state photoluminescence measurements. This technique allows ...effective lifetime measurements in an extremely large injection range between 1010cm-3 and 1017cm-3. The measurements are discussed focusing on injections below 1012cm-3 in order to determine the most effective passivation layer for solar cells arranged for indoor applications. Fixed negative charges in the passivation layer cause field-effect passivation due to band bending leading to either accumulation or inversion at the passivation layer/silicon interface. Accumulation causes a stable passivation quality at low level injection. Inversion leads to effective lifetime losses similar to the losses in the space charge region. On p-type silicon the most effective surface passivation at low injections is provided by Al2O3 or a-Si:H(i). The n-type silicon samples passivated with a-Si:H(i) show the best effective lifetimes. SiNx and SiO2 show lifetimes one order of magnitude below a-Si:H(i). Al2O3 on n-type is the most effective passivation at high injections around 1015cm-3. Due to inversion losses at low level injections the passivation quality decreases more than two orders of magnitude for injections around 1010cm-3.
We have identified at least five different local breakdown mechanisms according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under reverse bias. ...These are (1) early pre-breakdown (strongly negative TC, low slope), (2) edge breakdown (positive TC, low slope, no EL), (3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550 nm defect luminescence), (4) strong defect-induced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and (5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at high reverse bias. In addition to the local breakdown sites there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Since defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication, we propose defect level-induced avalanche instead of trap-assisted tunneling to be responsible for this breakdown mechanism.
We present a solar cell based on a titanium dioxide / p-type silicon charge carrier separating heterojunction with an efficiency of 10.1%, on a cell area of 0.9 cm 2 . The solar cells were fabricated ...using industrial feasible processes like laser fired contacts on the rear and silicon nitride antireflection coating on the front side. The ultra-thin TiO 2 layers (2.5 to 5.0 nm) were deposited on p-type Si wafers by means of Atomic Layer Deposition (ALD), using Tetrakis-dimethyl-amido-titanium (TDMAT) and water at a process temperature of 150°C. A current density of 35 mA/cm 2 was achieved with 2.5 nm TiO 2 . By means of Suns-F oc measurements, the loss in fill factor was identified to be due to the high series resistance. Using the series resistance free pseudo fill factor the current solar cell design achieves a pseudo efficiency of 13.4%.
We present a qualitative method for series resistance imaging that generates lateral current flow in solar cells without electrical contacting. The applicability of the method is demonstrated on a ...multicrystalline silicon solar cell, and the results are briefly discussed with respect to lifetime artifacts and injection dependence of the series resistance distribution. The method overcomes one of the major drawbacks of state-of-the-art series resistance imaging methods, which require electrical contacting for current injection or extraction. It allows for all-contactless measurements, reduces the cell breakage rate, and simplifies the measurement setup and sample handling.
We have identified at least five different kinds of local breakdown according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under a reverse bias. ...These are 1) early prebreakdown (negative TC, low slope), 2) edge breakdown (positive TC, low slope, no EL), 3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550-nm defect luminescence), 4) strong defect-induced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and 5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at a high reverse bias. The defects leading to the etch pits are investigated in detail. In addition to the local breakdown sites, there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication. It has been found recently that it is caused by metal-containing precipitates lying in grain boundaries.
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