High performance organic solar cells : from molecular design to morphology optimization

Abstract

Organic solar cells (OSCs) consisting of bulk-heterojunction (BHJ) architecture have attracted great attention over the past two decades owing to their unique advantages, including mechanical flexibility, light weight, large area and low-cost fabrications. To date, OSCs devices have achieved power conversion efficiencies (PCEs) that exceed 12%. Much of this success has come with the great efforts on optimizing the light absorption spectrum, molecular energy levels, crystallinity, and charge carrier mobilities of donor polymers. On top of the well-known fact that thick-film devices are not only able to enhance the absorption strength of the solar cell and thus OSC efficiency, but also are important for the industrial application of OSCs. However, state-of-the-art PTB7 family donor polymers did not perform well in thick-film devices due to their relatively low crystallinity, low hole mobility and impure polymer domains. To further improve the deice performance and enable OSCs commercially viable, there is a strong demand for the design of new donor polymers that can achieve optimal blend morphology with high carrier mobilities even in blend films with high thickness. My major research focus on developing and understanding of a new family of conjugated polymers which exhibits strong temperature-dependent aggregation (TDA) property at room temperature. We found that the strong aggregation property is crucial for the formation of optimal BHJ morphology both in fullerene and non-fullerene based OSCs. This unique aggregation property that enables fine morphology control during solution processing, leading to high PCEs even when the active layer is as thick as 300 nm. By studying the structure–property relationship of the donor polymers and their impacts on polymer:fullerene and polymer:non-fullerene blend morphology and device performance, it is found that the design rationale of donor polymers for non-fullerene acceptors is different from that for fullerene-based OSCs. Our preliminary results demonstrate that those polymers with strong lamellar stacking and high crystallinity may be better matches for fullerene acceptors, while the non-fullerene-based OSCs prefer TDA polymers with slightly reduced polymer crystallinity. For chapter 2, we report a series of difluorobenzothiadizole (ffBT) and oligothiophene-based polymers with the oligothiophene unit being quaterthiophene (T4), terthiophene (T3), and bithiophene (T2). We demonstrate that a polymer based on ffBT and T3 with an asymmetric arrangement of alkyl chains enables the fabrication of 10.7% efficiency thick-film polymer solar cells without using any processing additives. By decreasing the number of thiophene rings per repeating unit and thus increasing the effective density of the ffBT unit in the polymer backbone, the HOMO and LUMO levels of the T3 polymers are significantly deeper than those of the T4 polymers and the absorption onset of the T3 polymers is also slightly red-shifted. For the three T3 polymers obtained, the positions and size of the alkyl chains play a critical role in achieving best OSC performances. The T3 polymer with a commonly known arrangement of alkyl chains yields poor morphology and OSC efficiencies. Surprisingly, a T3 polymer with an asymmetric arrangement of alkyl chains enables the best-performing OSCs. Morphological studies show that the optimized ffBT-T3 polymer forms a polymer:fullerene morphology that differs significantly from that obtained with T4-based polymers. The morphological changes include a reduced domain size and a reduced extent of polymer crystallinity. The change from T4 to T3 comonomer units and the novel arrangement of alkyl chains in our study provide an important tool to tune the energy levels and morphological properties of donor polymers, which has an overall beneficial effect and leads to enhanced OSC performance. For chapter 3, a series of isoindigo (ID) and quaterthiophene (T4)-based donor-acceptor copolymers are synthesized and compared. Our results show that by changing the positions of fluorine substitution from the acceptor to the donor unit, the temperature-dependent aggregation and electronic properties of the polymer can be significantly changed. The polymer with fluorination on the donor unit exhibits the strongest extent of temperature-dependent aggregation, which leads to a higher hole mobility for the polymer and OSCs with efficiencies up to 7.0% without using any processing additives. Our results provide important insights into how fluorination affects the aggregation properties and performance of isoindigo-based polymers, which is an important class of low-bandgap conjugated polymers. For chapter 4, non-fullerene OSCs based on polymer donors and non-fullerene small molecular acceptors (SMAs) have recently attracted considerable attention due to their promising power conversion efficiency. Although much of the progress was driven by the development of novel SMAs, the donor polymer also plays an important role in achieving efficient non-fullerene OSCs. However, it is far from clear how the polymer donor choice influences the morphology and performance of the SMAs and the non-fullerene blends. In addition, it is challenging to carry out quantitative analysis of the morphology of polymer:SMA blends, due to the low material contrast and overlapping scattering features of the π-π stacking between the two organic components. Here we study a series of non-fullerene blends based on ITIC-Th blended with five different donor polymers. Through quantitative analysis, we characterize the (010) coherence length of the SMA and establish a good correlation between the coherence length of the SMA and the device fill factor and photocurrent density. It is well-known fact that the donor polymer can affect the hole mobility of the blend, our study reveals that the donor polymer can also significantly change the molecular ordering of the SMA and thus the electron mobility and fill factor of the devices.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology