Loss Mechanisms In Non-Fullerene Organic Solar Cells

Abstract

Photovoltaics are one of the most important sustainable energy sources in the 21st century. Among photovoltaics, organic solar cells (OSCs) offer many advantages such as the ease of processing, light weight, the potential for flexible devices, and tunable properties. Their unique nature and complexity present a fascinating charm, attracting many researchers. Thanks to researchers' efforts, the power conversion efficiency (PCE) of OSCs has been boosted from below 1% to 19% over the last three decades. Despite the exciting PCE, some problems remain unsolved, for example, the large voltage loss and unsatisfying long-term stability. This thesis aims to understand the fundamental physics of the state-of-the-art OSCs, especially the loss mechanisms. Ultimately, properly understanding the mechanisms will serve as the basis of OSC's further improvements and commercialization. The beginning of this thesis introduces basic concepts about semiconductor physics and donor-acceptor OSCs, explaining the generation of electricity from light and the fundamentals of organic electronics. Subsequently, the detailed balance in a solar cell is reviewed, which is the basis of voltage loss analysis. In this section, we see how the generation, recombination, and output of charges reach a balance. Then, the way to determine the voltage loss is shown, and the most recent understandings in reducing this loss are reviewed. The fill factor, as a measure of the quality of a solar cell, is a complex parameter, especially in OSCs. The latter part of this thesis starts from the photophysical processes in an OSC, and then relates intrinsic parameters to the fill factor. Figure of merits have been employed to express the fill factor analytically. Finally, experimental methods and basic principles for the previous analysis are introduced, including Fourier transform infrared spectroscopy, external quantum efficiency of photovoltaics, spectrograph for electroluminescence or photoluminescence, transient absorption, and time-delayed collection field. Overall, this thesis combines thermal dynamics and charge dynamics to analyse voltage losses and fill factor losses. The author hopes this work can contribute to a better understanding of the loss mechanisms in OSCs.

Kinetic Monte Carlo Modelling of Organic Photovoltaic Devices

“…The main attraction of using the DD model is computational effort, which is small, to study the device physics in less time. The model yields a good understanding at a macroscopic level [134][135][136] but can't provide a significant insight into the microscopic, internal properties, or track the particle dynamics/thermalization.…”

“…Figure 5.1: Schematic illustration of the SQ limit as a function of band gap a) theoretical maximum efficiency, b) theoretical maximum VOC (which is lower than the band gap shown in red dotted line), c) theoretical jSC and d) theoretical fill factor. Ref[134,135] …”