Cell Processing

Approximately 80% of today’s silicon solar cells industrially manufactured worldwide apply screen printing for the metallization of the silver front and aluminium rear contacts. In production, conversion efficiencies of ~18–18.5% are achieved using monocrystalline silicon wafers. A baseline process has been implemented at ISFH that is very similar to the industry-standard process, displaying conversion efficiencies of up to 18.5%. An analysis of the solar cells reveals that the conversion efficiency is limited in particular by the shadowing loss due to the silver front-side metallization, as well as infrared light being absorbed in the aluminium rear-side metallization. This paper summarizes recent developments at ISFH that resulted in a 19.4% efficient large-area screen-printed solar cell, when applying a print-on-print silver front-side metallization and an SiO2/SiNx rear-surface passivation.

In a multicrystalline silicon (mc-Si) cell production process, acid texturing is the most popular way of carrying out surface texturing. In general, the surface reflectivity and etch depth are the criteria used for quantifying the texture quality. In this study, four groups of cells were created with different etch depths of 2.82μm, 3.83μm, 4.41μm and 5.92μm. It was found that the etch depth had a notable effect on the efficiency of a cell. Also, the best texture was obtained with an etch depth of 4.41μm, at which there was a balance between a low reflectance and the removal of the saw-damage layer. As the etch depth increased, the film deposition thickness and the front bus-bar tensile strength were seen to increase. However, no linear relationship was found to exist between the diffusion sheet resistance and the etch depth.

Phosphorus dopant pastes are an attractive alternative to the conventional phosphorus oxychloride (POCl3) dopant source for emitter processing in solar cells, as they allow the fabrication of selective emitters on an industrial scale. In this paper it is demonstrated that single-sided uniform screen-printed emitters, processed with phosphorus dopant pastes, can getter multicrystalline silicon (mc-Si) wafers more effectively than conventional double-sided uniform POCl3 emitters. This result is confirmed by minority carrier lifetime measurements with the quasi-stead-state photoconductance (QSSPC) method. Solar cells with selective emitters were processed using phosphorus dopant pastes on mc-Si wafers and were subsequently characterized. The current-voltage (I-V) results are improved compared to uniform POCl3 emitter solar cells and an increased internal quantum efficiency (IQE) for selective emitter solar cells is demonstrated.

Technology computer-aided design (TCAD) is pervasive throughout research, development and manufacturing in the semiconductor industry. It allows very low-cost evaluation of process options and competing technologies, guides process development and transfer to production and supports more efficient process improvement with minimal down time in the factory environment. This paper reviews the use of TCAD in the semiconductor industry and shows, with some illustrative examples, its important enabling role for the PV industry.

Processing silicon substrates for PV applications involves texturing, cleaning and/or etching wafer surfaces with chemical solutions. Depending on the cleanliness of the industrial equipment and the purity of the chemical solutions, surface contamination with metals or organic residues is possible [1]. The presence of trace contamination at PV junctions leads to both mid-level traps and photonic defects, which ultimately cause reduced efficiency and rapid cell degradation. Metallic impurities have a greater impact on PV cell lifetime due to their deeper energy levels in the silicon band gap [2]. On the other hand, non-metallic impurities may modify the electrical activity of PV cells because these species involve complex interactions with the host silicon lattice and its structural defects. In other words, very small amounts of contamination can result in poor PV efficiency. This paper presents an overview of the effects of adding a biodegradable complexing agent in cleaning and rinsing baths to minimize surface contamination and thereby enhance solar cell efficiency.

This paper reviews the status of solar cell technology based on n-type crystalline silicon wafers. It aims to explain the reasons behind the strong and increasing attention for n-type cells, including the inherent advantages of n-type base doping for high diffusion length, and for the industrialization of designs with good rear-side electronic and optical properties. The focus will be on cells using diffused junctions.

Quality assurance and process control are becoming increasingly important in the industrial production chain to the manufacturing of silicon solar cells. There are a number of relevant wet chemical processes for the fabrication of standard screen-printed industrial solar cells, mainly for texturization and cleaning purposes. While one-component systems like pure HF for oxide-removal are easy to monitor, i.e., by conductivity measurements, typical texturization processes are much more complex due to the number of constituents. For acidic texturization of multicrystalline silicon wafers, typical mixtures involve amounts of hydrofluoric acid (HF), nitric acid (HNO3) and water. It has also been documented that mixtures can be found where additional additives like phosphoric acid (H3PO4), acetic acid (HOAc) and sulphuric acid (H2SO4) have been used [1, 2]. In alkaline random pyramid texturization for monocrystalline wafers, a base like potassium hydroxide (KOH) or sodium hydroxide (NaOH) and organic additives like 2-propanol (IPA) are used [3]. In addition to these processes, recently developed high-efficiency cell concepts require several additional wet chemical process steps like advanced cleaning processes, chemical edge isolation or single side oxide removal processes [4]. In order to obtain continuously stable and reproducible process results and to overcome process operations based on operator experience, a reliable monitoring of the bath concentrations is essential. Such quality control has the potential for significant cost reductions due to optimized durations between replacements of bath mixtures or shortening of processing times. In this context, the application of on-line analytical methods, either by means of chemical, optical or electrical measurement techniques, is of particular interest.

Laser-doped selective emitter (LDSE) technology, invented and patented by the University of New South Wales (UNSW), is presently generating considerable interest in the photovoltaics industry due to its low cost, high efficiency, and suitability for mass production. The excellent results achieved to date – as high as 19.7% on small area laboratory test devices [1], and 19.0% on industrial large-area 156mm wafers [2] – are attracting a similarly impressive array of commercial partners. Nearly 10 companies are at various stages of implementation of LDSE technology variants into production and pilot production. This paper takes a closer look at the potential for mass production of LDSE-based solar cells.

The aim of this work is to study the effects of dark lines on the face of polycrystalline silicon solar cells. The formative processes of dark lines were observed by laser scanning microscopy. Following the initial appearance of a few etch pits on the surface of the cells, extending the etching time saw these etch pits increase in size, eventually merging to form a single line, known as a ‘dark line’. Dark lines are lines that are linked together by a series of contiguous dislocation outcrops and have the potential to reduce silicon wafer lifetime, adversely affect both the electroluminescence and the quantum efficiency of a solar cell, and have resulting negative effects on the cell’s electrical properties.

This paper focuses on the latest developments from research on MWT (metal wrap-through) solar cells at Fraunhofer ISE. An overview of the current cell results for mc-Si and Cz-Si material with both Al-BSF and passivated rear side is presented. Recent progress in cell technology and challenges to reaching efficiencies of 20% for industrially processed large-area MWT solar cells are also discussed. Up to recently, MWT cell efficiencies of up to 19% for Cz-Si and up to 17.5% for mc-Si have been reached with industrially feasible processing. Improvements to the design of the MWT cell to increase cell efficiency and to allow an easy module assembly are also presented in this paper, as are first calibrated IV measurements of MWT solar cells.