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

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

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

Role of thermal SiO₂ on passivation of highly doped layer

In this paper we investigate the influence of thermal oxidation during and after POCl3 diffusion. The main focus here is on the question of how far the presence of a thermally grown oxide layer has an influence on the emitter passivation. For this purpose, characterization methods such as electrochemical capacitance-voltage measurements and QSSPC are used. The novel finding of this paper is that in the case of a stack of passivation layers, the presence of the thermal oxide does not make a significant contribution to improved emitter passivation.publishe

Diffusivity analysis of POCl3 emitter SIMS profiles for semi empirical parametrization in sentaurus process

The diffusion of phosphorus mediated by phosphorus oxychloride (POCl3) is extensively used in photovoltaics due to its higher controllability over phosphine. However, the POCl3 decomposition in the gas phase as well as the subsequent oxidation reaction between silicon and the resulting phosphorus oxide are not very well documented and increase the complexity of a realistic simulation of the POCl3 diffusion. This paper examines the possibility of modeling the POCl3 diffusion only by modifying the diffusion constants of a standard phosphine mediated diffusion simulation implemented in Sentaurus process. To this aim, the Boltzmann-Mattano Analysis (BMA) is applied to experimental SIMS profiles in order to extract a concentration dependent effective phosphorus diffusivity. The fitting of diffusivity profiles is then performed to determine the specific diffusion constants of the diffusion mechanisms believed to be responsible for the phosphorus diffusion in silicon. Through the study of specific artifacts appearing in the diffusivity profile because of the non ideal conditions for application of the BMA, it is believed that at low POCl3 flow, the surface concentration during the pre-deposition phase is low enough to induce an enhanced diffusivity at the early stage of the diffusion. Finally, the assumptions of this diffusivity fitting are reviewed and a model matching these assumptions is identified in Sentaurus Process for a direct implementation of the BMA results into Sentaurus Process

A novel approach for the evaluation of a phosphorus diffusion design of experiment

The emitter formation in industrial p-type silicon solar cells is generally done by phosphorus diffusion. To optimize the emitter in terms of high efficiency, series resistance and recombination losses of the emitter are balanced out. In this work, we present the results of a design of experiment (DoE) to optimize the main parameters of the P-diffusion. Therefore, we use a novel approach to evaluate the results of the DoE: We approximate the unavoidable correlation between the emitter saturation current density (j0e) and the emitter sheet resistance (ρsh) and create a new target variable (dj0e), which describes the deviation from this correlation. This allows us to understand the key parameters that can minimize emitter saturation current density while keeping emitter sheet resistance at a target value.publishe