Multiscale modeling of transport in silicon heterojunction solar cells

Abstract

Silicon based single junction solar cell technology continued to make significant strides in the past year with new world record module efficiencies being reported for the Panasonic heterojunction with thin intrinsic layer (HIT) module (23.8%) and the SunPower rooftop silicon module (24.1%). The HIT cell which is comprised of amorphous silicon (a-Si) and crystalline silicon (c-Si) currently holds the world record efficiency (25.6%) for a silicon based single junction solar cell. Further improvement in this technology requires a rigorous understanding of the underlying physics of the device. The device performance of a-Si and c-Si heterojunction solar cells depends heavily on the nature of transport at the hetero interface and defect assisted transport through the a-Si. Different microscopic processes dominate transport in different regions of the device and take place across widely varying time scales. In this work we present a multiscale model which utilizes different simulation methodologies to study physics in various regions of the device, namely, the Ensemble Monte Carlo (EMC), Kinetic Monte Carlo (KMC), and Drift Diffusion (DD) solvers. The EMC studies the behavior of the photogenerated carriers at the heterointerface; the KMC analyzes transport of the photogenerated carriers through the intrinsic amorphous silicon (i-a-Si) barrier layer; and the DD solver calculates current and other device properties in the low field regions of the cell. These solvers are then self consistently coupled to analyze device performance. Previously, our KMC simulations have shown that hopping is the main mode of transport through the i-a-Si, and the photogenerated carries are collected by defect emission rather that Poole - Frenkel emission or direct tunneling1. In addition, using EMC simulations we have shown that the photogenerated carriers exhibit non Maxwellian behavior at the heterointerface2. This work specifically describes the self-consistent coupling of the DD and EMC solvers. By adding the EMC solver to the multiscale solver we are able to capture the high field behavior of the photogenerated carriers, and its affect on device parameters such as JSC, VOC, FF and efficiency.