Singlet exciton fission-enhanced silicon photovoltaics: Interfacial engineering, device design and spectroscopic technique development

Abstract

The growing global energy demand combined with resource and space limitations necessitate enhancements in crystalline silicon solar cells, which are the current dominant solar technology. However, their efficiencies have only increased incrementally over the recent 20 years, as they are starting to approach the theoretical efficiency limit. The main source of loss is thermalization, where energy in excess of the bandgap absorbed by silicon is lost as heat. Singlet exciton fission in organic molecules has been proposed to reduce these losses. By having the organic layer absorb the high energy light and transferring the triplet excitons generated from the singlet fission process to silicon, the photocurrent in this spectral region can be doubled, with the potential of raising the efficiency from the traditional limit of 29.4 % to up to 42 %.

The greatest challenge with these devices has been to demonstrate an increase in the silicon photocurrent, a necessary condition to show that the technology is viable. Scientifically, there are three main components to this problem. The first is to successfully couple the triplet excitons to silicon. The second is that not much is understood regarding the exciton and charge carrier dynamics at this interface. Finally, the silicon solar cell architecture should also be considered to extract transferred carriers effectively.

This thesis tackles these three parts from an interfacial materials, device architecture and spectroscopy approach. Using tetracene as the singlet fission layer and n-doped silicon, we show that defect-induced states in a thin interlayer of hafnium oxynitride that lie near the band edge of silicon are beneficial for triplet exciton transfer. We also identify that triplet-induced electric field-effect passivation is beneficial for the triplet sensitization process of silicon, and design a new bilayer interface consisting of a zinc phthalocyanine donor layer that introduces preferential near- silicon band edge states, and an ultrathin oxide chemical passivation layer. We then study various device architectures, confirming the importance of using a device designed to extract surface charge carriers efficiently, demonstrating the first enhancements in single-junction silicon solar cell external quantum efficiencies and photocurrent from singlet fission. Finally, we build and use advanced spectroscopy techniques and numerical frameworks to study exciton and charge carrier dynamics in singlet fission-sensitized solar cell materials, confirming that the triplet excitons are contributing to all the positive effects observed in the devices.

These results have shown that singlet fission-sensitized silicon solar cells are a viable technology for enhancing silicon solar cell efficiencies beyond the conventional single-junction limit. This interface remains a rich area for fundamental scientific studies, involving coupling between molecular dark states to bulk silicon. We hope that the key findings can help direct research efforts towards scalable implementation of this technology, and stress that the fundamental understanding of the interface also has broad implications to other silicon technologies that can benefit from enhanced quantum yields, including photodetectors.