Development of Benzodithiophene (BDT)-Based Wide Bandgap Donor-Acceptor (D-A) Polymers Containing Novel Thiophene and Benzene Acceptor Building Blocks for Organic Solar Cells

Abstract

Organic solar cells (OSCs) are an emerging renewable energy technology with advantages of low cost, flexibility, lightweight, and easy processability. High performing OSCs often consist of a wide bandgap polymer donor and a small molecule non-fullerene acceptor (NFA) for complementary absorption. These materials are usually mixed in solution to form a bulk heterojunction (BHJ) structure to achieve efficient charge transfer. Currently, leading materials can reach a power conversion efficiency (PCE) greater than 18 %. However, several challenges remain before OSCs can be commercially viable including further enhancement of the PCE, long-term stability, large area device fabrication, low-cost materials development, and environmentally friendly synthesis. It is critical to target solution-processable polymer donors, with well-matched opto-electronic properties and morphological compatibility with NFAs through effective side chain engineering for further progress in this field. This thesis work targets low-cost polymer donor materials through relatively easy synthesis routes and low-cost starting materials. Side chains will be selected to target good solubility, low EHOMO levels (to match with high performance NFAs like Y6), and good morphology to achieve high performing polymer donor materials. In this work, several novel donor polymers are developed, which have a donor-acceptor (D-A) structure. D-A polymer building blocks have become increasingly important as donor materials in OSCs since the energy levels and bandgaps are easily tunable via intramolecular charge transfer. Four novel series of polymers: ethynyl thiophene-benzodithiophene (BDT), triazole thiophene-BDT, tetrafluorobenzene-BDT, and acetal thiophene-BDT, with a variety of electron withdrawing group (EWG) side chains were designed, synthesized, and characterized as wide bandgap polymer donors for OSC applications. An ethynyl series of polymers were designed with the goal of extending the conjugation into the side chain. This can effectively polarize the polymers and increase the exciton lifetime, resulting in improved performance. The ethynyl series included the following side chains: a trimethyl silyl group (PSETBDT), an unsubstituted benzene ring (PBETBDT), and an alkyloxime-substituted benzene ring (POBETBDT). The solar cell devices based on PSETBDT:Y6 and PSETBDT:IT-4F had a PCE of 1.44 and 0.77 %, respectively. The poor performance was attributed to low, unbalanced mobility, and a high polydispersity index (PDI), indicating that cross-linking occurred. Solubility issues and low molecular weight were experienced with polymer PBETBDT; therefore, this polymer was only preliminarily tested for opto-electrochemical measurements. POBETBDT had a PCE of 3.65 % with Y6, while only achieving an open circuit voltage (Voc) of 0.71 and a fill factor (FF) of 0.33. Low mobility and the bulky side chain are potential issues with this material. Future work will look to assess the surface roughness and domain size of these polymers using atomic force microscopy (AFM) to further assess morphological issues. Similarly, a triazole series of polymers were designed with the goal of extending the conjugation into the side chain. The triazole series involved converting an aldehyde to a triazole ring, which was then substituted at the middle nitrogen with an alkyl chain (PTTBDT), and a carbamate chain (PCTBDT). Polymer PTTBDT achieved a PCE of 5.00 % with Y6, while PCTBDT only achieved 3.29 % due to lower short circuit current density (Jsc), Voc, and FF. Both polymers suffered from low mobility, while PCTBDT had exceptionally low hole mobility (10-7 cm2V-1s-1). DFT calculations indicated these polymers suffer from backbone and side chain twisting, which can negatively affect the charge transfer. Additionally, PCTBDT has a thermally removable side chain that could result in extensive hole trapping, which would limit charge extraction. Future work will look to assess the surface roughness via AFM and further optimize PTTBDT:Y6 devices by altering the donor:acceptor (D:A) ratio to improve the charge mobility. A tetrafluorobenzene-BDT (PFBBDT) polymer was designed to improve upon the EHOMO and co-planarity characteristics of an unsubstituted benzene-BDT polymer by adding fluorine atoms as an electron withdrawing group (EWG). This can help energy level matching with NFAs such as Y6 and allow for good charge transfer. PFBBDT achieved a PCE of 5.14 %; however, the performance was limited by low molecular weight and strong lamellar stacking interactions. The latter is thought to cause potential aggregation in the active layer and contribute to the low and unbalanced mobility observed. The lamellar stacking indicates this material has potential for future transistor applications. Previously, synthesis of a polymer based on BDT and a formaldehyde-substituted thiophene was attempted by our group but was unsuccessful. A polymer (PATBDT) was designed with an acetal side chain substituted thiophene to facilitate an acid-catalyzed post polymerization modification to obtain a soluble formaldehyde-substituted thiophene-BDT polymer (PXTBDT). DFT calculations indicated that both these materials had potential for good organic photovoltaic performance (OPV). When PATBDT was paired with Y6, a PCE of 8.20 % was achieved. Low electron mobility resulted in unbalanced charge transfer and low FF for this material. PXTBDT had a quite low EHOMO of -5.67 eV but still worked well with Y6, achieving a better PCE of 9.97 %, mainly due to higher Voc and FF. The low FF of these materials leaves room for process optimization to improve performance. Furthermore, future work will explore other D units, such as bithiophene and thienothiophene, to pair with acetal-protected aldehyde A units to investigate if this simple chemistry can produce any other high performing materials.