Search

Plated Ni/Cu/Ag contacts are an industrially feasible metallization approach for high efficiency c-Si solar cells with low surface doping concentrations (10 18 cm -3 N D 20 cm -3 ). The 2d-simulations of this work define the minimum requirements on the contact resistivity of metal contacts in a high efficiency solar cell design. The following experimental study of the contact resistivity of plated Ni/Cu/Ag contacts on lowly doped phosphorus emitter demonstrates low contact resistivities in the mΩcm 2 regime, which enable solar cells with high fill factors. Furthermore, the paper analyzes the influence of the thermal silicidation process on pseudo-fill factor losses and on the mechanical contact adhesion. The contact adhesion is also studied with respect to the laser contact opening process. The results of this work demonstrate that the right choice of back-end processes enable plated Ni/Cu/Ag contacts with low contact resistivities in combination with high contact adhesions above 1 N/mm.

The tunnel oxide passivated contact (TOPCon) is a promising technology improving the efficiency of Si solar cells by cutting recombination losses and simplifying the solar cell design (1D junction design). The objective of this paper is to investigate possible contact materials having a high/low work function for passivated contacts thereby enabling the realization of a double-sided contact Si solar cell featuring n-type TOPCon on the front and p-type TOPCon at the rear side. The main part of this paper deals with the eligibility of thin atomic layer deposited (ALD) AZO (aluminum doped zinc oxide) interlayers sandwiched between n-TOPCon and ITO (tin doped indium oxide) in order to avoid sputter damage. It will be demonstrated that the insertion of ALD AZO improved the efficiency by 0.8% abs. and lead to a maximum efficiency of 20.2%. Finally, WO x and MoO x which have a high work function are identified as promising contact layers for p-TOPCon.

The present work deals with the attack of different chemicals used in nickel and copper plating electrolytes, on screen printed silver front side contacts, which lowers the adhesion and hinders industrial implementation. Dipping tests with four different self-made pastes in isolated electrolyte compounds show the dissolution of silver for pastes with glass content and acidic exposure conditions in chemical trace analysis (ICP-OES) of the dip solution. These results can be correlated to the macroscopic contact adhesion, measured with quantitative peel force tests. The results show that there is a link between the amount of silver that is found in the dipping solutions after the exposure of the contacts, the measured contact adhesion, and the presence of glass in the contact. The reaction between glass and silver during the firing process forms an oxidation layer between silver and glass which provides good adhesion. This interface seems to be dissolved by acidic plating chemicals. An additional SEM investigation shows the failure mechanism to take place exactly between the glass that covers the wafer after the firing process and the silver bulk, indicating a chemical attack at this interface.

Photoluminescence (PL) imaging is a promising characterization technique for rating and sorting of multicrystalline silicon (mc-Si) as-cut wafers concerning to their material quality. It is inline applicable and yields high resolution images showing recombination active defects from crystallization which influence solar cell performance. In this contribution the basic concepts in ongoing work concerning relevant defects, rating results, statistics, algorithms and general approaches for the rating are summarized. Since 2009 wafers are sorted into five quality classes at Fraunhofer ISE. Details on the rating criteria are given and three examples for application are presented: (i) the impact of edge contaminations on the solar cell results, (ii) a comparison of wafer suppliers based on random samples from a statistical basis of 10,000 wafers and (iii) the results of an advanced rating and sorting of wafers for the manufacturing of highly efficient MWT-PERC solar cells. The results confirm that PL imaging can be used for a very precise rating of material quality.

Carrier mobility in silicon plays a crucial role for photovoltaic applications. While the influence of doping on mobility in standard monocrystalline silicon is well understood, recent research has been focused on the effects of crystal defects in multicrystalline (mc) silicon and of the presence of both acceptors and donors in compensated silicon, both introducing additional scattering centers influencing carrier mobility. In this work measurements of the majority carrier mobility have been carried out in two blocks of compensated multicrystalline silicon. Confirming existing results we come to the conclusion that with increasing compensation level mobilities may be significantly lower than predicted by Klaassen's mobility model, which is basically suited for the description of mobilities in compensated silicon as it accounts for scattering on both acceptors and donors. However, as this model is based on mobility data from uncompensated silicon, a compensation-related reduction of screening is not taken into account sufficiently. To describe mobilities in compensated silicon, a modification of Klaassen's mobility model based on published mobility data in compensated silicon is suggested.

In this paper we measure, spatially resolved, the efficiency potential of multicrystalline material. The excess carrier lifetime in the bulk affects the open-circuit voltage, the short-circuit current and the fill factor of a solar cell differently as different injection conditions apply. Therefore, we measure the bulk lifetime injection-dependent and analyze each IV -parameter separately. Furthermore, multicrystalline wafers with laterally varying bulk recombination need a spatially resolved analysis to explore the impact of large grains or dislocation clusters on the efficiency. The crucial information we need is the spatially resolved injection-dependent bulk lifetime. For this purpose, we use lifetime calibrated photoluminescence imaging with a varying generation rate. Measuring a surface passivated wafer after the important high temperature steps of a solar cell process ensures that we attain the bulk recombination that is relevant for the solar cell. Combining this information with a cell simulation, we deduce the maximum efficiency for every image point. We also discuss how to obtain global values from these images and investigate the effect of charge currents flowing laterally through the emitter using Sentaurus Device and Spice network simulations. A comparison of these simulated global values to measured IV -parameters on finished solar cells shows good agreement between the two. A detailed analysis of two mc-wafers from different feedstock shows limiting IV -parameters in different regions, efficiency reductions due to dislocations and explains differences in the fill factor of two equally processed solar cells. The analysis shows that evaluating the material related efficiency limitation of silicon wafers has to be done at the efficiency level, as the lifetime at one generation level alone is not sufficient.

This paper is revisiting the problem of recombination on locally processed area (contacts, local doping …), based on the concept of point recombination rate (pLPA). The newly introduced effective point recombination rate (peff) can be easily determined from the effective surface recombination velocity. It also allows predictions and comparisons in most of the practical cases. Further analysis allows the separation of the influence of pLPA from the one of the transport of the carrier in a transparent way. Due to its simplicity, transparency and accuracy, the model proposed here is believed to be more suitable to recent local processing technology than the existing analytical models.

To enable a concentration control in the acidic texturing process we have closed the feedback loop from analytical data to the dosing mechanism of the used process tool. In order to analyze the process bath we used near-infrared spectroscopy in an online setup as well as ion chromatography as an inline method in a second approach. Using the developed dosing algorithm allows a concentration optimization of HF and HNO3 in dependence of the Si concentrations. This allows a further optimization of the acidic texturing process and leads to more homogeneous process results.

Well suited and reliable values of the optical and electrical properties of thin indium tin oxide (ITO) films are needed in order to choose the optimal deposition parameters and to perform reliable modeling for solar cells design. In this work, a new method will be presented to evaluate the ITO transparency directly on silicon substrates. The effects of each deposition parameter that influences the ITO transparency and conductivity force a trade-off in the frame of values useful for SHJ solar cells. The deposition of our optimized ITO film on a textured wafer yields a weighted average reflectance as low as 4.4±0.2%. The deposition of an MgF2/ITO double-layer anti-reflection coating (DL-ARC) on textured cells increases the efficiency from 17.9%, measured immediately after contacts have been added to the ITO, to 18.4% after the MgF2 deposition. An annealing step at 200 °C for 10 min proved to further increase the efficiency up to 18.9%, for a total gain of 1%.

Reliable process control or predictions of solar cell efficiencies from minority carrier lifetimes on silicon wafers require precise lifetime measurements. For inhomogeneous material quality this implies the necessity of adequately averaged spatially resolved lifetime measurements. Materials such as, e.g. multicrystalline upgraded metallurgical grade silicon frequently feature relatively low lifetimes, high trap densities, and several material parameters (charge carrier mobilities and net dopant concentration) that are not straightforwardly predictable or measurable. As this may substantially compromise conventional lifetime measurements, we present a solely luminescence based lifetime imaging technique, which requires virtually no a priori information about material parameters. Our approach is based on a calibration of a wafer's photoluminescence image through a precise lifetime determination of a part of this wafer via quasi-steady-state photoluminescence. Carrier mobilities, net dopant concentration, and surface morphology leave the determination of lifetime virtually unaffected, the injection dependence of lifetime is properly taken into account, and lifetimes down to the timescale of a microsecond can be reliably measured.