Small Molecules Based on Thieno[3,4‑c]pyrrole-4,6-dione for High Open-Circuit Voltage (V OC ) Organic Photovoltaics: Effect of Different Positions of Alkyl Substitution on Molecular Packing and Photovoltaic Performance Yoon Suk Choi, † Tae Joo Shin, ‡ and Won Ho Jo* ,† † Department of Materials and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea ‡ Pohang Accelerator Laboratory, Kyungbuk, Pohang 790-784, Republic of Korea * S Supporting Information ABSTRACT: Two different thienopyrroledione (TPD)-based small molecules (SMs) with different alkyl substitution positions were synthesized, and their photovoltaic properties are measured and compared to examine the effect of the alkyl substitution position on their optical, electrochemical, and photovoltaic properties. The use of TPD as an electron-accepting unit in conjugated SMs effectively lowers the highest occupied molecular orbital (HOMO) energy levels of the conjugated SMs and leads to high open-circuit voltage (V OC ). The two SMs with n-hexyl group substituted at different positions exhibit almost identical optical and electrochemical properties in the pristine state. However, the crystallographic and morphological characteristics of the two SMs are significantly different, because they are blended with PC 71 BM. The SM in which n-alkyl groups are substituted at the central accepting unit exhibits a power conversion efficiency (PCE) of 6.0% with V OC = 0.94 V, which is among the highest PCE values of TPD-based SM devices, whereas the SM with n-alkyl groups being substituted at the chain ends shows a moderate PCE value of 3.1%. KEYWORDS: thienopyrroledione, organic solar cells, small molecules, high V OC , alkyl chain position 1. INTRODUCTION Recently, the power conversion efficiencies (PCEs) of solution- processed small molecule (SM)-based organic solar cells (OSCs) have steadily been increased, with efficicncies close to those of polymer solar cells (PSCs). 1-4 However, the overall performances of SM-based OSCs are still inferior to those of polymer counterparts. For high-performance SM-based OSCs, the strategies used for the design of high-performance conjugated polymers could also be applied to the molecular design of SMs. Therefore, the electron donor-acceptor (D- A)-type architecture that has been proven as the most effective method to achieve high-performance PSCs can also be utilized for the design of high-performance SMs. 5-12 Thieno[3,4-c]pyrrole-4,6-dione (TPD) has been a promising moiety as an A unit 13-18 in D-A-type conjugated polymers, because its relatively strong electron accepting power leads to low frontier orbital energy levels of corresponding conjugated polymers, which is required for high open-circuit voltage (V OC ) in bulk heterojunction (BHJ) PSCs. Although it is generally accepted that the highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level of D-A-type conjugated molecules are governed mainly by the electronic properties of D and A units, respectively, exceptional but interesting results have been reported when a TPD unit is used as an A unit in D-A-type conjugated backbone: Both HOMO and LUMO levels are lowered when TPD is used as an A unit in D-A-type conjugated molecules. For instance, it has been reported that a low-bandgap polymer (PTB7), composed of benzoditihophene (BDT) and thieno[3,4-b]thiophene (TT) as the D and A units, respectively, exhibits HOMO and LUMO energy levels of -5.15 and -3.31 eV, respectively. 19 When the TT unit in PTB7 is replaced by a stronger electron accepting unit (TPD), the polymer (PBDTTPD) exhibits lower-lying HOMO and LUMO energy levels of -5.56 and -3.75 eV, respectively. 20 Another polymer (PDTSTPD) composed of dithieno[3,2- b:2′,3′-d]silole (DTS) as the D unit and TPD as the A unit also exhibits deep HOMO and LUMO energy levels of -5.57 and -3.88 eV, respectively. 21 It should be mentioned here that TPD-based molecules exhibit deeper HOMO energy levels, Received: August 20, 2014 Accepted: October 21, 2014 Research Article www.acsami.org © XXXX American Chemical Society A dx.doi.org/10.1021/am505608s | ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX