Charge fluctuations at the Si-SiO2 interface and its effect on surface recombination in solar cells

The Si–SiO2 interface has and will continue to play a major role in the development of silicon photovoltaic devices. This work presents a detailed examination of how charge at or near this interface influences device performance. New understanding is identified on the effect of charge-induced potential fluctuations at the silicon surface. Such fluctuations have been considered in Si–SiO2 recombination models previously, where a universal value of electrical potential deviation was used to represent the effect. However, the approach disregards that the variation occurs in the charge concentration rather than the potential. We modify the models to accurately reflect fluctuations in external charge, allowing a precise representation of surface recombination velocity, with self-consistent Dit, δp, and δn parameters. Correctly accounting for these parameters can provide insights into the passivation mechanisms which can aid the development of future devices. Using the corrected model, we find that the effect of charge fluctuation at the Si–SiO2 interface is significant for the depletion regime to the weak inversion regime. This indicates that surface passivation dielectrics must operate with charge concentrations in excess of 2x1012 q/cm2 to avoid these effects. TCAD device simulations show that the efficiency of future PERC cells can improve up to 1% absolute when optimally charged dielectric coatings are applied both at the front and rear surfaces

Modelling of hydrogen transport in silicon solar cell structures under equilibrium conditions

This paper presents a model for the introduction and redistribution of hydrogen in silicon solar cells at temperatures between 300 and 700 °C based on a second order backwards difference formula evaluated using a single Newton-Raphson iteration. It includes the transport of hydrogen and interactions with impurities such as ionised dopants. The simulations lead to three primary conclusions: (1) hydrogen transport across an n-type emitter is heavily temperature dependent; (2) under equilibrium conditions, hydrogen is largely driven by its charged species, with the switch from a dominance of negatively charged hydrogen (H−) to positively charged hydrogen (H+) within the emitter region critical to significant transport across the junction; and (3) hydrogen transport across n-type emitters is critically dependent upon the doping profile within the emitter, and, in particular, the peak doping concentration. It is also observed that during thermal processes after an initial high temperature step, hydrogen preferentially migrates to the surface of a phosphorous doped emitter, drawing hydrogen out of the p-type bulk. This may play a role in several effects observed during post-firing anneals in relation to the passivation of recombination active defects and even the elimination of hydrogen-related defects in the bulk of silicon solar cells

2014 IEEE 40th Photovoltaic Specialist Conference, PVSC 2014

We present an approach to estimate multicrystalline silicon (mc-Si) passivated emitter rear contact (PERC) solar cell efficiencies from as-grown silicon bricks. The approach is based on interstitial iron concentration Fei measurements across an as-grown mc-Si brick. Numerical impurity gettering and phosphorus diffusion simulations are used to simulate possible cell fabrication process conditions. The results are then used for numerical device simulations to show the efficiency potential of PERC solar cells across the mc-Si brick. This approach shows a possible pathway to predict final solar cell efficiencies based on characterization of mc-Si materials prior to cell production with state of the art experimental and simulation techniques

Extracting band-tail interface state densities from measurements and modelling of space charge layer resistance

Dielectric-silicon interfaces are becoming ever more important to device performance. Charge inside a surface dielectric layer is neutralized in Si leading to an accumulation or inversion layer of free carriers. Additionally, states at the interface are occupied by charges via Shockley-Read-Hall carrier statistics. It is accepted that the density of interface charge near midgap, which can only reach a concentration as high as the density of states, Dit, has a minor effect on band bending compared to the charges in the dielectric for a well passivated interface. Here, we show that it is the state density near the band edge what plays the major role. We conclude this by comparing our measurements with device modelling of a Si/SiO2 interface. We measure the wafer sheet resistance while applying various amounts of positive charge to the passivating dielectric on an n-type Si wafer, and then reproduce the measured resistance values using simulations. This modelling indicates that Dit at midgap has indeed a minor effect on sheet resistance change, while the total amount of tail states has a significant impact on the distribution of induced carriers. We test this model to detect the amount of acceptor-like states at the band-tails of oxide passivated silicon with different processing. We discuss and analyse the limitations of this technique. While we report on the Si/SiO2 interface due to its relevance in photovoltaics, our method can be used to study the properties of other semiconductor-dielectric interfaces. As such this work is of importance across various optoelectronic devices

Imaging and quantifying carrier collection in silicon solar cells: a submicron study using electron beam induced current

In this work electron-beam-induced current (EBIC) is used to study the collection efficiency of emitters in industrial silicon solar cells. Laser-doped local emitters have been deployed industrially, yet in mas production they are designed wider than the screen-printed silver fingers to allow alignment tolerances. EBIC has allowed to image and quantify the laser-induced damage that occurs in these local emitter regions. A model is developed to account for such damage, so that losses in EQE could be quantified from the observed EBIC collection characteristics. The damaged regions present ~12% lower collection efficiency at short wavelength (300–500 nm) than the homogenous emitter. Sentaurus TCAD simulations reveal that eliminating such damage would improve cell efficiency by ~0.12%. Additional degradation is found in a region 1–2 µm wide adjacent to the silver fingers, which has not been detected before. It is also found that pulsed laser doping leads to ~15 µm long un-doped gaps, along the direction of laser movement. As laser doping becomes a key part of industrial cell fabrication, it is crucial to develop a better understanding of the potential pitfalls, and future improvements to the process. The versatility of EBIC imaging is also demonstrated using FIB milling to improve lateral resolution and study the depth profile of boron emitters in newly developed industrial i-TOPCon cells. EBIC imaging, in combination with advanced device simulations, have proven powerful tools to elucidate carrier collection characteristics and drawbacks, thus helping to understand and improve fabrication processes at industrial level

Alternative dielectrics for hole selective passivating contacts and the influence of nanolayer built-in charge

Highly passivating, hole selective contacts are required for future high efficiency silicon solar cells. This work investigates selected dielectrics as potential SiOx replacements to act as hole selective contacts. AlOx and SiNx were identified as good candidates due to their low valence band offsets to silicon and proven surface passivation capabilities. Simulated J-V curves show AlOx and SiNx maintain acceptable contact resistivities at thicknesses below 1.4 and 1.7 nm, respectively. The SiOx hole contact was found to become extremely resistive even at thicknesses <1 nm, suggesting that either pinholes dominate conduction, or the band offset parameters differ from those in real TOPCon structures. The passivation of the dielectrics was also simulated, with SiOx outperforming both AlOx and SiNx primarily due to the excellent interface. Additionally, the effect of nanolayer built in charge was investigated. Charges below 1012 q/cm2 were found to have little effect, while negative charges above 1012 q/cm2 resulted in reductions in the contact resistivity and recombination current. The calculated selectivity for a 1.4 nm layer of AlOx was 12.9, while a value of 13.8 was calculated for SiNx at typical intrinsic charge levels