High-Crystallinity π-Conjugated Small Molecules Based on Thienylene-Vinylene-Thienylene: Critical Role of Self-Organization in Photovoltaic, Charge-Transport, and Morphological Properties

Photoisomerization of dicyanorhodanine-functionalized thiophenes

π-Conjugated oligomers functionalized with the popular dicyanorhodanine (RCN) electron acceptor are shown to be susceptible to photo-induced Z/E isomerization. The stereochemistry of two model RCN-functionalized thiophenes is confirmed by single crystal X-ray analysis and 2D NMR, and shown to be the thermodynamically stable Z form. Relative energies, Z/E configurations, and conformational preferences are modelled using density functional theory (DFT). The photophysical properties of the model compounds are explored experimentally and computationally; the Z and E isomers display similar absorption profiles with significant spectral overlap and are inseparable upon irradiation to a photostationary state. The well-behaved photoisomerization process is routinely observable by thin-layer chromatography, UV-vis, and NMR, and the photochemical behavior of the two RCN-functionalized thiophenes is characterized under varying wavelengths of irradiation. Ultraviolet (254 nm) irradiation results in photostationary state compositions of 56/44 and 69/31 Z-isomer/E-isomer for substrates functionalized with one thiophene and two thiophenes, respectively. Ambient laboratory lighting results in excess of 10 percent E-isomer for each species in solution, an important consideration for processing such materials, particularly for organic photovoltaic applications. In addition, a photoswitching experiment is conducted to demonstrate the reversible nature of the photoreaction, where little evidence of fatigue is observed over numerous switching cycles. Overall, this work showcases an approach to characterize the stereochemistry and photochemical behavior of dicyanorhodanine-functionalized thiophenes, widely used components of functional molecules and materials.

High-Performance Organic Energy-Harvesting Devices and Modules for Self-Sustainable Power Generation under Ambient Indoor Lighting Environments

Organic photovoltaics (OPVs) that perform more efficiently under artificial indoor lighting conditions than they do under sunlight are attracting growing interest as they can potentially serve as ambient energy harvesters for powering low-power electronics and portable devices for the Internet of Things. Herein, solution-processed small-molecule OPVs are demonstrated to exhibit high power conversion efficiencies exceeding 16% under white LED illumination, delivering high output power densities of up to 12.4 and 65.3 μW cm^-2 at 200 and 1000 lx, respectively. Increasing the open-circuit voltage ( V_oc) of OPVs is a critical factor for achieving higher indoor photovoltaic performance. Toward real applications, this small-molecule OPV system is adopted to fabricate six series-connected modules with an active area of ∼10 cm² that are capable of generating a high output power surpassing 100 μW and a high V_oc of over 4.2 V even under dimly lit indoor conditions of 200 lx. These results indicate that OPVs are promising as indoor electric power sources for self-sustainable electronic devices.