TPA-DPP-based extended π-conjugated SMDs: Central core regulation on photovoltaic performance

Small molecule donors (SMDs) have made great progress in the field of organic semiconductor materials due to their unique characteristics. The extended π-conjugation SMDs system combines the advantages of wide absorption and effective carrier mobility. However, the new structure of active layer material and the power conversion efficiency (PCE) of bulk-heterojunction (BHJ) organic solar cells (OSCs) are critical issues. Herein, a series of new SMDs, (TPA-DPP)₂-CZ, (TPA-DPP)₂-PTZ and (TPA-DPP)₂-PhFLU, were constructed based on an extended D-A-D′-A-D structural skeleton using triphenylamine-diketopyrrolopyrrole (TPA-DPP) as the terminal electron-donating group and π-bridge electron-withdrawing linker. The strong push-pull effect of TPA-DPP leads to wide and strong light absorption, which is one of the reasons for the high short-circuit current density (J_SC) of bulk heterojunction organic solar cells (OSCs). Furthermore, taking carbazole (CZ), phenothiazine (PTZ), and alkoxyphenyl substituted fluorene (PhFLU) as the central D′ units respectively, the effects of structural control strategies on the photovoltaic properties of the materials were discussed. It should be emphasized here that the introduction of alkoxyphenyl groups into the central core generates significant push-pull effects, promotes intermolecular charge transfer (ICT) processes, reduces energy loss, and facilitates charge transport in photovoltaic devices. As a result, the compounds (TPA-DPP)₂-CZ, (TPA-DPP)₂-PTZ and (TPA-DPP)₂-PhFLU show high molar extinction coefficients ranging from 1.12 × 10⁵ M⁻¹ cm⁻¹ to 1.68 × 10⁵ M⁻¹ cm⁻¹, which is helpful for devices based on these materials to obtain high J_SC. Among the three compounds, (TPA-DPP)₂-PhFLU obtained the lowest HOMO^CV energy level of −5.20 eV and the narrowest band gap of 1.65 eV, which will be conducive to the generation of high open circuit voltage (V_OC) and J_SC. The PCE of SMDs/PC₇₁BM-based OSCs ranged from 4.66 % to 7.44 %. It is worth noting that the (TPA-DPP)₂-PhFLU/PC₇₁BM-based device obtained the highest PCE of 7.44 % with a V_OC of 0.94 V, and a high J_SC of 19.71 mA cm⁻² without further optimization. The measured photocurrent and PCE of the device is one of the best performances for TPA-DPP-based SMDs available so far, which fills the gap of TPA-based long conjugated molecules in the field of organic photovoltaics. The better photovoltaic performance is attributed not only to the relatively weak electron-donating ability of the central core unit PhFLU of this molecule but also to the higher and more balanced carrier mobility of the device. This work demonstrates that the extended D-A-D′-A-D molecules based on the TPA-DPP skeleton are a type of promising photovoltaic material that can be used to improve J_SC and PCE by combining the regulation of the central core.