Instability of Dielectric Surface Passivation Quality at Elevated Temperature and Illumination

AbstractHydrogenated silicon nitride and aluminum oxide passivation layers were deposited on boron doped floatzone silicon wafers that underwent a high temperature firing step. The passivation quality was monitored during thermal treatment at 75°C, 150°C and 250°C in darkness or under illumination. It was found that the passivation quality of the specific layers under investigation is far from stable in the course of time showing both deterioration and improvement features on a time scale of minutes to weeks. Furthermore, it was found that these changes occur in both darkness and under illumination, whereupon (stronger) illumination accelerates the changes. Via corona charging and capacitance voltage experiments it could be shown that the observed changes in the short term are mainly caused by changes in the chemical passivation quality

Record Efficiency of PhosTop Solar Cells from n-type Cz UMG Silicon Wafers

AbstractHighly purified n-type Upgraded Metallurgical Grade (UMG) silicon carries a large potential for high efficiency low cost solar cells. In this study, the industrially producible “PhosTop” solar cell concept is employed to manufacture large-area n-type rear junction solar cells from such a Si material with a screen-printed Al-alloyed full-area emitter featuring a selective phosphorous front surface field (FSF) and a SiO2/SiNx:H passivation on the front.Since resistivity at the seed end is about seven times as high as at the tail end of the UMG Si ingot and carrier lifetime decreases from seed to tail end, a clear dependence of the solar cells’ IV characteristics on the original position of the corresponding wafers in the UMG Si ingot is observable. Maximum conversion efficiency is reached (on a wafer which has been taken out at about one fifth of the ingot's length distant from the seed end) by η = 19.0% being, to the authors’ knowledge, the highest efficiency so far reported on industrial type solar cells manufactured from 100% UMG Si

Relationships between diffusion parameters and phosphorus precipitation during the POCl3 diffusion process

The POCl3 diffusion process is still a common way to create the pn-junction of Si solar cells. Concerning the screen-printing process, it is necessary to find a compromise between low emitter recombination, low contact resistance and high lateral conductivity. The formation of a homogeneous emitter during the POCl3 diffusion process depends on several diffusion parameters, including duration, temperature and gas flow. This primarily controls the growth of the highly doped phosphosilicate glass (PSG) layer, which acts as a dopant source during the diffusion process. Detailed investigations of the PSG layer have shown a distinct correlation between the process gas flows and the composition of the PSG layer. Specifically, in this research we examine the influence of phosphorus precipitation at the PSG/Si interface. Furthermore, we show the influence of phosphorus precipitation during the pre-deposition phase on the passivation quality of the corresponding emitter. In a second step, we use the results to create emitters with a reduced density of phosphorus precipitates. In a last step, the optimized emitter structure was transferred to screen-printed solar cell processes, whereby efficiencies up to 19.4% abs. were achieved on monocrystalline p-type Cz material with full area Al-BSF rear side

INVESTIGATIONS OF HIGH REFRACTIVE SILICON NITRIDE LAYERS FOR ETCHED BACK EMITTERS: ENHANCED SURFACE PASSIVATION FOR SELECTIVE EMITTER CONCEPT (SECT)

ABSTRACT: The application of selective emitter solar cells via emitter etching has already shown its potential as an industrial type solar cell concept. Thereby the surface passivation of etched back emitters plays an important role. In this work the possibilities for enhanced surface passivation are investigated by using single and double layered high refractive PECVD SiN x layers. QSSPC, ellipsometry and FTIR measurements were performed on FZ wafers to determine the critical thickness of the high refractive SiN x for an improved hydrogen passivation. The studies were also focused on the firing stability of the passivation quality

Impact of extended contact co-firing on multicrystalline silicon solar cell parameters

During the temperature spike of the contact co-firing step in a solar cell process, it has been shown that the concentration of lifetime-killer dissolved metallic impurities increases, while adding an annealing after the spike getters most of the dissolved impurities towards the phosphorus emitter, where they are less detrimental. The contact co-firing temperature profile including the after-spike annealing has been called extended contact co-firing, and it has also been proposed as a means to decrease the emitter saturation current density of highly doped emitters, benefiting thus a wide range of materials in terms of detrimental impurity content. The aim of the present work is to determine the effect of performing this additional annealing on contact quality and solar cell performance, looking for an optimal temperature profile for reduction of bulk and emitter recombination without affecting contact quality. It presents the effect of the extended co-firing step on fill factor, series resistance and contact resistance of solar cells manufactured with different extended co-firing temperature profiles. Fill factor decreases when extended co-firing is performed. Series resistance and contact resistance increase during annealing, more dramatically when the temperature peak is decreased. Scanning Electron Microscopy (SEM) images show silver crystallites in contact with silver bulk before the annealing that allow a direct current path, and silver crystallites totally surrounded by glass layer (>100 nmthick) after annealing. Glass layer redistribution and thickening at low temperatures at the semiconductor-metal interface can be related to the series resistance increase. Degradation of series resistance during the temperature spike, when it is below the optimum one, can be also attributed to an incomplete silicon nitride etching and silver crystallite formation. To make full use of the beneficial effects of annealing, screen-printing metallic paste development supporting lower temperatures without thick glass layer growth is needed

Dissolution of electrically inactive phosphorus by low temperature annealing

In this study we investigate the dissolution of electrically inactive phosphorus complexes by low temperature annealing after the POCl3 diffusion process. This has the immediate consequence that the existing near-surface emitter volume SRH recombination can be reduced. Thereby, a significant reduction of emitter saturation current density j0E is achieved without driving the emitter further into the silicon substrate. For short-term temperature treatments well below the POCl3 diffusion temperature, a reduction of up to -60 fA/cm2 has been achieved. This study increases our understanding of the formation and dissolution of electrically inactive phosphorus complexes during post-annealing processes

Optimizing phosphorus diffusion for photovoltaic applications: Peak doping, inactive phosphorus, gettering, and contact formation

The phosphosilicate glass (PSG), fabricated by tube furnace diffusion using a POCl3 source, is widely used as a dopant source in the manufacturing of crystalline silicon solar cells. Although it has been a widely addressed research topic for a long time, there is still lack of a comprehensive understanding of aspects such as the growth, the chemical composition, possible phosphorus depletion, the resulting in-diffused phosphorus profiles, the gettering behavior in silicon, and finally the metal-contact formation. This paper addresses these different aspects simultaneously to further optimize process conditions for photovoltaic applications. To do so, a wide range of experimental data is used and combined with device and process simulations, leading to a more comprehensive interpretation. The results show that slight changes in the PSG process conditions can produce high-quality emitters. It is predicted that PSG processes at 860?°C for 60?min in combination with an etch-back and laser doping from PSG layer results in high-quality emitters with a peak dopant density Npeak?=?8.0?×?1018?cm?3 and a junction depth dj?=?0.4?m, resulting in a sheet resistivity?sh?=?380 ?/sq and a saturation current-density J0 below 10 fA/cm2. With these properties, the POCl3 process can compete with ion implantation or doped oxide approaches

Effect of electrically inactive posphorus versus electrically active phosphorus oniron gettering

In this study we investigate the efficacy of iron gettering as a function of electrically inactive phosphorus in the emitter in combination with low temperature annealing steps. To achieve different amounts of electrically inactive phosphorus in the emitter a highly doped PSG produced emitter with a large plateau depth of electrical active phosphorus is etched back stepwise by a wet-chemical procedure. Therewith we achieve a gradual reduction in electrically inactive phosphorus with small changes in electrically active phosphorus (?Rsh < 4 ?/sq). After this step, the wafers with different emitters have been annealed at 700 °C for 30 min and the content of Feiin the bulk has been measured using QSS-PC. The results show, (i) that for higher amounts of electrically inactive phosphorusa stronger iron gettering effect can be observed and (ii) that an additional annealing step leads to a significant change of Fei. This means, (i) that anelectricallyinactive phosphorus concentration dependence for iron gettering is observed and (ii) additional annealing steps, below the usual diffusion temperature of phosphorus, can be used to reduce interstitial iron in highly contaminated wafers further

RGS-Silicon - Material Analysis and Solar Cell Processing

In this work, RGS (Ribbon Growth on Substrate) a ribbon silicon material from Bayer AG is characterised, and solar cells from this material, which is presently under development, are processed. Due to the very fast casting process and the lack of kerf losses RGS could be a cost effective alternative to standard cast crystalline silicon wafers in the near future. RGS wafers are characterised with regard to diffusion lengths, crystal defects, transport properties and lifetimes in order to get a deeper inside in the material properties. A solar cell process is developed and optimised based on these results, which takes into account the special needs and properties of RGS silicon. Intensive gettering and H-passivation studies as well as a mechanical texture of the wafer surface led to cell efficiencies of 12 (2x2cm2), which is an overall record on this material

Status of selective emitter technology

Current industrial monocrystalline Cz Si solar cells based on screen-printing technology for contact formation and homogeneous emitter have an efficiency potential of around 18.4%. Apart from limitations at the rear side by the fully covering Al-BSF the front side is limiting e.g. by relatively high j0E values. This can be changed by selective emitter designs allowing a decoupling and separate optimization of the metalized and non-metalized areas. Several selective emitter concepts that are already in industrial mass production or close to it are presented, and their specialties and status concerning cell performance are demonstrated. Key issues to be considered are costeffectiveness, added complexity, additional benefits, reliability, and efficiency potential. The efficiency increase for best cells is around 0.5-0.6%abs and the current efficiency potential already demonstrated for all technologies is around 19.0%. Average efficiencies in industrial mass production for selected technologies are 18.5-18.6% for Cz and 17.1% for mc Si. The efficiency increase by selective emitter formation is higher for inline emitters, but selective emitters based on POCl3 show the highest absolute efficiency. By decreasing the phosphorous surface concentration, selective emitters are more sensitive to surface passivation and the use of a SiNx:H layer with a higher refractive index increases implied Voc values even further. Encapsulation under module glass and EVA reduces part of the gained jsc caused by a better blue response on cell level, but a calculation reveals that this extra loss for selective emitter solar cells is <0.1 mA/cm2 and therefore negligible. The full benefit of the improved front side in terms of a selective emitter structure will be achieved when local rear contacts will be used