Modelling of Interfaces for High Performance Solar Cells – HiperSol

This project ended Nov. 30, 2012

HiperSol is an ambitious project with the main objective to build up a multi-scale modelling framework based on first principles, and to use this framework to describe and understand the interfaces between the metal contact and passivation layer with the silicon substrate in solar cell devices (Figure 1). A significant part of the electronic losses in today’s solar cells are attributed to effects at these interfaces. Through model predictions, HiperSol aims to provide a better understanding of these loss mechanisms and thereby enable substantial decreases in the cost/efficiency ratio. Even though the modelling framework developed in HiperSol is specifically designed for contacting and passivation interfaces in solar cells, the generic character of the methodology spreads the impact other fields within materials science and engineering.

Figure 1. Solar cell with the contact and passivation interfaces marked.

Project context and objectives

In order to bridge the gap between what is currently possible and what is needed for accurate modelling of the properties of the contacting and passivating interfaces, the project is organised in six scientific work packages, as illustrated in Figure 2. The geometric and electronic structures are determined on the atomistic level in WP1 and WP2. On top of these, specific models for the contacting and passivation interfaces are developed and applied in the remaining work packages. The objectives for the individual work packages are as follows:

WP1: The main purpose of this workpackage is to provide realistic descriptions for the contacting and passivation interfaces at the atomic scale using a range of simulation methods validated and enhanced by experimental data. These models will then be used as input to the other work packages 

Figure 2. Scientific work packages.

WP2: Ab initio calculations are nowadays able to predict highly accurate band structures for small to medium sized systems. This is achieved by a hierarchy of methods. At the highest and most accurate level are quasiparticle methods, such as the GW methods, which are only applicable to small systems containing 20-100 atoms. At the second level, ab-initio density functional theory methods can be found, which are nowadays applicable to about 1000 atoms. For applications relevant to solar cell modelling, however, we often need to consider models containing from several thousand up to 100.000 atoms, in particular if transport properties and scattering at defects and interfaces need to be taken into account. To enable such applications is the main goal of the work-package.

Specifically, we want to be able to predict the band structures and band alignments across interfaces and the scattering of electron and hole wave functions at impurities and interfaces between silicon and silicon-nitride, silicon and glass (PbO/SiO2), and silicon and Ag. To this end we will develop a single program package that seamlessly merges high level methods (such as GW), density functional theory methods, and screened semiempirical pseudopotential descriptions.

WP3: The main objective is to develop a multi-scale modelling environment to describe and predict the properties of contact interfaces in silicon solar cells, consisting of the following parts:

• First-principles (FP) calculations
• Dynamic and static force field calculations.
• Continuum calculations
• Validation of and input to the models from advanced characterisation tools
• Appropriate interaction between the above-mentioned parts.

WP4: We will develop and apply a multi-scale modelling framework to accurately describe the passivation interface, and provide input and feedback to WP6 to improve passivation.

WP5: The multi-scale modelling scheme from WP3 will be used to improve understanding of and suggest improvements to contacting interfaces. This may lead to adjustments in the techniques being used in WP1, 2 and 3, whenever the modelling results do not correspond to experiment.

WP6: This will use the methodology developed in WP4 to improve passivation schemes. There will be interplay between modelling efforts and experimental work. Both the improvement of existing schemes and development of novel materials and structures will contribute to this goal. The output of the model will be compared to the experimental results using both test samples and complete solar cells with the goal to improve surface passivation using new technologies.