Widening the Gap – Optimizing Donor and Acceptor Materials for Organic Photovoltaics

Abstract

Silicon-based photovoltaics (Si-PV) currently dominate the solar energy market due to their high efficiency, long lifespan, and cost-effectiveness. However, their inherent limitations such as rigidity, lack of flexibility, and energy-intensive production restrict their use for applications that demand lightweight, conformal, or aesthetically adaptable solutions. Organic photovoltaics (OPV) offer a compelling alternative for the more niche and emerging energy markets where SiPV falls short. With their adaptability, low-cost manufacturing, and ability to tailor absorption to specific parts of the solar spectrum, OPV addresses many of the challenges posed by traditional photovoltaic technologies. As a result, OPV technology has gained significant traction, leading to the emergence of pioneering companies like Heliatek, Epishine, and Dracula technologies, which are driving With notable recent efficiency improvements up to 15% and enhanced morphological stability, single-component organic solar cells (SC-OSCs) based on conjugated block copolymers (CBCs) show potential to revolutionize the OPV field by enabling long-term stable OPVs. However, as discussed in Chapter 5, CBC materials face issues in terms of structural control and characterization. Furthermore, preliminary results of a droplet-flow synthesis protocol to create individual blocks with precise chain-end functionalities are shared in this chapter. Expected to improve reproducibility and control of CBC materials, this method will assist in the identification of structure-property relationships, which are of crucial importance to the advancement of SC-OSC technology. advancement of the OPV field and commercialization. Very important over the last decade was the development of non-fullerene acceptors (NFAs). With enhanced and tuneable optoelectronic properties, these materials played a crucial role in the efficiency surge toward 19% for outdoor OPV. Proceeding on this huge leap forward and the rise of indoor OPV, Chapter 2 covers the development of high open-circuit voltage Voc (indoor) OPVs (IOPVs) by pairing wide-gap NFAs with the state-of-the-art polymeric donor PM6. While all examined NFAs exhibited the desired optical gaps (1.8–2.0 eV) and elevated lowest unoccupied molecular orbital (LUMO) energy levels (~-4 eV), overall device performances varied significantly. Within the series, the novel wide-gap material IO-4F emerged as the best-performing NFA under indoor conditions, achieving a balance of increased short-circuit current density (Jsc) and a decent fill factor (FF), outperforming the reproduced benchmark PM6:IO-4Cl devices. Reduced FF and Jsc values in other blends, including the novel acceptor IDT-4Cl, limited overall performance, and also a ternary device strategy failed to raise efficiencies. Despite the challenges, the study confirms the potential of wide-gap NFAs combined with PM6 to achieve high-Voc IOPV devices, setting a foundation for continued exploration and improvement. Complementary to the wider-gap NFAs in Chapter 2, three donor polymers with deep highest occupied molecular orbital (HOMO) energy levels (< -5.5 eV) and wide optical gaps (~2.0 eV) were synthesized in Chapter 3. Under standard AM1.5G conditions, P-BDTT(F)-Tc-TT-Tc:IT-4F achieved a Voc of 0.90 V and a power conversion efficiency (PCE) of 7.4%, though it fell short of the 9.8% PCE observed for the benchmark PM6:IT-4F devices, primarily due to reduced Jsc and FF values. However, under indoor lighting, P-BDTT(F)-Tc-TT-Tc:IT-4F outperformed PM6:IT-4F, delivering a PCE of 18.8%, attributed to a higher Voc and FF, and reduced voltage losses under indoor lighting conditions. Ternary blends incorporating P-BDTT(F)-Tc-TT-Tc, IT-4F, and IO-4Cl further improved performance under both solar and indoor conditions. Whereas the PCE under AM1.5G illumination reached 8.6%, it remained below that of PM6:IT-4F. However, under indoor lighting, the ternary blend achieved a PCE just below 20%, with one of the highest indoor Voc values (for a IT-4F device) and impressive low additional voltage losses. Synthesis, characterization, and evaluation of other high-gap donor copolymers incorporating 3,4-dicyanothiophene was described in Chapter 4. The literature material PB3TCN-C(BO)H and the prepared halogenated derivatives exhibited optical gaps similar to PM6 and remarkably deep HOMO energy levels (< -5.65 eV). Unfortunately, the very deep energy levels of the donor materials, in combination with the limited optical gap, presented challenges in finding suitable acceptor materials, especially for the chlorinated derivative PB3TCN-C(HD)Cl. A blend of the fluorinated polymer PB3TCN-C(BO)F and IT-4F resulted, as envisaged, in a high-Voc device under both AM 1.5G and indoor lighting conditions. Nevertheless, overall efficiency stayed limited because of a relatively low FF and only matched the performance of the reference blend PM6:IT-4F. Since only minor optimization was conducted, further refinement of the blend morphology and device architecture is expected to unlock the full potential of the PB3TCN-C(BO)F material.