Influence of the Crystalline Nature of Small Donors Molecules on the Efficiency and Stability of Organic Photovoltaic Devices

Abstract

Small‐molecule‐based organic photovoltaic (OPV) devices have gained attention because they offer more batch‐to‐batch consistency than their polymer‐based counterparts. Herein, we present two novel small molecules, 2,2′‐(((4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(4‐octylthiophene‐5,2‐diyl))bis(methaneylylidene))bis(1H‐indene‐1,3(2H)‐dione) (BDTTID) and 2,2′‐(((4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(3,3″‐dioctyl‐[2,2′:5′,2″‐terthiophene]‐5″,5‐diyl))bis(methaneylylidene))bis(1H‐indene‐1,3(2H)‐dione) (BDT3TID), which can be used as donors in OPV devices. These small molecules contain a benzo [1,2‐b:4,5‐b′] dithiophene (BDT) core and 1,3‐indandione (ID) terminal units with different numbers of thiophene flanking groups between the BDT core and the ID terminal units. The greater number of thiophene spacers in BDT3TID compared with BDTTID not only enhances the absorption coefficient of the small molecule but also broadens and redshifts its absorption spectrum. These properties may enable a high light‐harvesting efficiency, which is beneficial for achieving a high power conversion efficiency (PCE) with a high photocurrent. The highest PCE, i.e., 5.54%, is obtained from the optimized BDTTID‐based device treated with thermal annealing, which is attributed to the positive effects of its crystalline‐like nature. In contrast, the thermal annealing of BDT3TID‐based devices is found to considerably deteriorate the device performance due to its amorphous‐like structure. Moreover, the BDTTID‐based device is shown to possess superior thermal stability upon thermal treatment at 100 °C for 40 h. The PCE of the BDTTID‐based device remain above 66% of its initial PCE, whereas the PCE of the BDT3TID‐based device is dramatically reduced from 4.60 to 1.11%. These results show the importance of controlling the crystallinity when designing small donor molecules for efficient and stable OPV devices.