A Multiscale Modeling Approach to Study Transport in Silicon Heterojunction Solar Cells
Authors: Muralidharan Pradyumna , Bowden Stuart , Goodnick Stephen M. , Vasileska Dragica 
Published: April 2016

Single junction solar cells based on Silicon continue to be relevant and commercially successful in the market due to their high efficiencies and relatively low cost processing. Heterojunction solar cells based on crystalline (c-Si) and amorphous (a-Si) silicon (HIT Cells) have paved the way for devices with high VOC's (&gt;700 mV) and high efficiencies (&gt;20%) [1]. Panasonic currently holds the world record efficiency of 25.6% for its trademark a-Si/c-Si HIT cell [2].
The novel structure of the device precludes the usage of traditional methods (such as drift diffusion) to accurately understand the nature of transport. Theoretical models used by commercial simulators make a variety of assumptions that simplifies the transport problem (assumes a Maxwellian distribution of carriers) and thus lacks the sophistication to study defect transport. In this work we utilize a combination of Ensemble Monte Carlo (EMC) simulations, Kinetic Monte Carlo (KMC) simulations and traditional drift - diffusion (DD) simulations to study transport in the heterojunction solar cell.
The device performance of an amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell depends strongly on the interfacial transport properties of the device [3]. The energy of the photogenerated carriers at the barrier strongly depends on the strength of the inversion at the heterointerface and their collection requires interaction with the defects present in the intrinsic amorphous silicon buffer layer [4]. In this work we present a multiscale model which can bridge the gap in time scales between different microscopic processes to study the transport through the interface by coupling an ensemble Monte Carlo (EMC) and a kinetic Monte Carlo (KMC). The EMC studies carrier properties such as the energy distribution function (EDF) at the heterointerface whereas the KMC method allows us to simulate the interaction of discrete carriers with discrete defects [5]. This method allows us to study defect transport which takes place on a time scale which is too long for traditional ensemble Monte Carlo's to analyze. We analyze the injection and extraction of carriers via defects by calculating transition rates for different processes. By using the principles of SRH recombination, this method can also be extended to study recombination processes at the interface and in the amorphous bulk which are crucial parameters for solar cell performance. Therefore, by using the multiscale approach all important processes can be studied rigorously to evaluate device performance.
Our simulations indicate that a phonon assisted emission process from a defect is the most favored extraction mechanism and both Poole-Frenkel emission (&lt;2%) and thermionic emission (&lt;1%) were not significant. We extended our simulation methodology to study recombination at the interface and in the buffer layer of the device to find that the device performance is mainly interface recombination limited and that defect densities in the buffer layer have to be really high (&gt;1018 cm-3) in order to degrade device performance.