Design, Synthesis and Modelling of Conjugated Polymers for Organic Photovoltaics

Secure, clean and renewable energy sources are believed to be the eventual solution for sustainable energy, especially by the direct utilization of solar energy. Organic photovoltaics offer such an option to convert solar energy into electricity based on solution-processed, lightweight, large-area, and potentially flexible devices. The current challenges for organic photovoltaics remain to further improve efficiency as well as durability and cost-effectiveness, to compete with traditional silicon-based solar cells. Material design through band gap and energy level tuning has been playing a key role in developing new donor materials for efficient polymer solar cells. Computationally driven material design can accelerate the search for optimal conjugated polymers, and the exploration of chemical methodologies is highly desirable in pushing the efficiency further toward the theoretical limit. This thesis deals with the design, synthesis, characterization, and computational modelling of π-conjugated polymers for bulk heterojunction organic solar cells. It focuses on material design of conjugated donor polymers through band gap and energy level engineering via structural modifications such as backbone manipulations, side-chain engineering, as well as incorporation of newly developed building blocks. This also establishes structure–property relationships of the polymer systems here studied, and explores potential chemical methodologies for future judicious material design

Synthesis and Electronic Properties of Diketopyrrolopyrrole-Based Polymers with and without Ring-Fusion

Diketopyrrolopyrroles (DPP) have been recognized as a promising acceptor unit for construction of semiconducting donor-acceptor (D-A) polymers, which are typically flanked by spacers such as thiophene rings via a carbon-carbon single bond formation. It may suffer from a decrease in the coplanarity of the molecules especially when bulky side chains are installed. In this work, the two N atoms in the DPP unit are further fused with C-3 of the two flanking thiophene rings, yielding a π-expanded, very planar fused-ring building block (DPPFu). A novel DPPFu-based D-A copolymer (PBDTT-DPPFu) was successfully synthesized, consisting of a benzo[1,2-b:4,5-b′]dithiophene (BDTT) unit as a donor and a DPPFu unit as an acceptor. For comparison, the unfused DPP-based counterpart PBDTT-DPP was also synthesized. Two dodecyl alkyl chains were attached to thiophene rings of DPP moieties to ensure good solubility of the DPPFu-based polymer. The influence of the ring-fusion effect on their structure, photophysical properties, electronic properties, molecular packing, and charge transport properties is investigated. Ring-fusion enhances the intermolecular interactions of PBDTT-DPPFu polymer chains as indicated by density functional theory calculation and analysis of electrostatic potential and van der Waals potential and results in significantly improved molecular packing for both the in-plane and out-of-plane directions as suggested by X-ray measurements. Finally, we correlate the molecular packing to the device performance by fabricating field-effect transistors based on these two polymers. The charge carrier mobility of the ring-fused polymer PBDTT-DPPFu is significantly higher as compared to the PBDTT-DPP polymer without ring-fusion, although PBDTT-DPPFu exhibited a much lower number-average molecular weight of 17 kDa as compared to PBDTT-DPP with a molecular weight of 108 kDa. The results from our comparative study provide a robust way to increase the interchain interaction by ring-fusion-promoted coplanarity

Low-bandgap nonfullerene acceptor based on thieno[3,2-b]indole core for highly efficient binary and ternary organic solar cells

A low-bandgap nonfullerene acceptor (NFA) TIT-2FIC based on thieno[3,2-b]indole-thiophenes core has been developed. Compared with the analogue NFAs DTC(4Ph)-4FIC and IT-4F, TIT-2FIC exhibited remarkably red-shifted absorption, and up-shifted HOMO energy level. In addition, TIT-2FIC showed interesting universal miscibility with the donors nonfluorinated PBDB-T and fluorinated PM6, therefore the corresponding organic solar cells achieved promising power conversion efficiencies (PCEs) of 11.80% and 13.00%, respectively, which are higher compared to the counterpart IT-4F based cells. Furthermore, the ternary PM6:TIT-2FIC:Y6 cell pronounced a high PCE of 17.22%, being significantly improved from that of 16.04% for the binary PM6:Y6 cell. Similar improvement in PCEs from 13.41% to 14.46% was also observed in the ternary PM6:TIT-2FIC:IT-4F cell with TIT-2FIC as the third component. These results indicated that TIT-2FIC is universally applicable as an acceptor with nonfluorinated or fluorinated polymer donor materials in both binary and ternary cells

Design, Synthesis and Modelling of Conjugated Polymers for Organic Photovoltaics

Secure, clean and renewable energy sources are believed to be the eventual solution for sustainable energy, especially by the direct utilization of solar energy. Organic photovoltaics offer such an option to convert solar energy into electricity based on solution-processed, lightweight, large-area, and potentially flexible devices. The current challenges for organic photovoltaics remain to further improve efficiency as well as durability and cost-effectiveness, to compete with traditional silicon-based solar cells. Material design through band gap and energy level tuning has been playing a key role in developing new donor materials for efficient polymer solar cells. Computationally driven material design can accelerate the search for optimal conjugated polymers, and the exploration of chemical methodologies is highly desirable in pushing the efficiency further toward the theoretical limit.This thesis deals with the design, synthesis, characterization, and computational modelling of π-conjugated polymers for bulk heterojunction organic solar cells. It focuses on material design of conjugated donor polymers through band gap and energy level engineering via structural modifications such as backbone manipulations, side-chain engineering, as well as incorporation of newly developed building blocks. This also establishes structure–property relationships of the polymer systems here studied, and explores potential chemical methodologies for future judicious material design

Computational modelling of donor–acceptor conjugated polymers through engineered backbone manipulations based on a thiophene–quinoxaline alternating copolymer

The rapid progress of bulk heterojunction organic photovoltaics has been boosted by (i) design and synthesis of novel conjugated donor materials, (ii) control and optimization of device fabrication, and (iii) the development of new device architectures such as tandem and ternary solar cells. Computationally driven material design has attracted increasing interest to accelerate the search for optimal conjugated photovoltaic materials, and the exploration of chemical methodologies is highly desirable in pushing the efficiency further towards the theoretical limit. Based on the motif of donor–acceptor polymers, over 50 comparable polymers were constructed and investigated, derived from an easily accessible thiophene–quinoxaline alternating polymer donor showing power conversion efficiency up to 7%. We performed a systematic density functional theory (DFT) study on the heteroatom effects of combining fluorine, nitrogen and chalcogen substitutions onto the donor/acceptor units as well as the effect of extending π-conjugation in the donor moiety, in order to gain insights into how structural modifications to the conjugated backbone can affect the molecular structure and electronic properties of a conjugated polymer. It is found that the trends in the energy levels and band gaps of these polymers correlate well to their structural modifications. Finally, by examining the systematically evaluated data in the energy diagram, we proposed three important ways of energy level modulation, showing potential chemical methodologies that can be applicable to further modify and optimize existing polymer backbones. Especially such energy level modulation can be applied to meet the particular requirements of different device architectures (including tandem and ternary solar cells) on the donor components, such as a prominent photocurrent or photovoltage combined with a high efficiency, to further maximize the overall performance of organic photovoltaics. This will provide valuable guidance and chemical methodologies for a judicious material design of conjugated polymers for solar cell applications with desirable photovoltaic characteristics

Conjugated polymers based on benzodithiophene and fluorinated quinoxaline for bulk heterojunction solar cells: thiophene versus thieno[3,2-b]thiophene as π-conjugated spacers

Two conjugated donor–acceptor copolymers based on a benzodithiophene donor unit and a fluorinated quinoxaline acceptor unit, spaced with either thiophene or thieno[3,2-b]thiophene π-bridges, were designed and synthesized. The effect of different π-bridges and of the processing conditions on optical, electrical, morphological and photovoltaic properties of the polymer:fullerene blend films were investigated. The polymer containing the thieno[3,2-b]thiophene π-bridge (PBDTFQ-TT) showed a red-shifted absorption and an enhanced charge carrier mobility, as compared to its analogue with the thiophene π-bridge (PBDTFQ-T), due to its narrower optical gap (by ∼0.1 eV) and stronger inter-chain interactions, favored by the structural planarity and increased linearity of the polymer backbone, as also supported by DFT calculations. The blend of PBDTFQ-TT and PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester), compared to the PBDTFQ-T:PC61BM one processed under the same conditions (by blade-coating technique), showed greatly enhanced photovoltaic performance, with more than doubled power conversion efficiency (PCE up to 5.60% for the best device) due to the increased short-circuit current density and fill factor. However, similar PCEs were also achieved for PBDTFQ-T:PC61BM-based devices by optimizing the processing conditions through the addition of 1,8-diiodooctane (DIO) as the solvent additive. Through morphological and electrical analysis of the films, produced with and without additive, it was observed that the addition of DIO greatly enhances the self-organization, and consequently the charge mobility, of the thiophene π-bridge-based polymer, while it was detrimental for the nanoscale morphology and photovoltaic performances of the thieno[3,2-b]thiophene π-bridge-based polymer in the corresponding blend

Improved performance and life time of inverted organic photovoltaics by using polymer interfacial materials

A previously published fluorene based interlayer polymer is here compared to three similar polymers where the fluorene monomer has been exchanged with monomers that have been reported to have a higher photo-chemical stability. The polymer interlayers have been studied in terms of their influence on device performance and stability on inverted devices with an active layer of P3HT:PC61BM. By acting as a hole-blocking layer the polymers are able to increase the efficiency of the devices with similar to 50% compared to devices with an ITO cathode. In addition, the polymers also improve the photo-stability of the devices, mainly as an effect of a reduced decrease in open-circuit voltage and fill factor. This indicates that solution processable polymer interlayers could be a way towards both higher efficiency and improved stability of inverted organic solar cells

Induced photodegradation of quinoxaline based copolymers for photovoltaic applications

We report here the synthesis and characterization of a series of p-type copolymers, which combine a fluorinated quinoxaline (FQ) acceptor unit either with a differently substituted benzodithiophene (BDT) or an unsubstituted thieno[3,2-b]thiophene (TT). The effect of the structural modifications on the photochemical stability of the resulting films is investigated and then correlated with the photovoltaic performance and lifetime measurements of corresponding photovolatic devices. To this end, we firstly studied the intrinsic stability of each polymer film by monitoring the UV-vis absorption decay, under simulated sunlight, as a function of ageing time. Bulk heterojunction solar cells, based on these polymers as donor materials, were fabricated and tested. Beside the initial values, we monitored the photovoltaic performance during prolonged light soaking in order to evaluate and compare the photostability of more complex systems such as working solar cells

Induced photodegradation of quinoxaline based copolymers for photovoltaic applications

We report here the synthesis and characterization of a series of p-type copolymers, which combine a fluorinated quinoxaline (FQ) acceptor unit either with a differently substituted benzodithiophene (BDT) or an unsubstituted thieno[3,2-b]thiophene (TT). The effect of the structural modifications on the photochemical stability of the resulting films is investigated and then correlated with the photovoltaic performance and lifetime measurements of corresponding photovolatic devices. To this end, we firstly studied the intrinsic stability of each polymer film by monitoring the UV–vis absorption decay, under simulated sunlight, as a function of ageing time. Bulk heterojunction solar cells, based on these polymers as donor materials, were fabricated and tested. Beside the initial values, we monitored the photovoltaic performance during prolonged light soaking in order to evaluate and compare the photostability of more complex systems such as working solar cells