Using Solution-Processed Small Molecules

8 9 E nergy remains a critical issue for the survival and prosperity of human 10 civilization. Many experts believe that the eventual solution for sustain11 able energy is the use of direct solar energy as the main energy source. 12 Among the options for renewable energy, photovoltaic technologies that 13 harness solar energy offer a way to harness an unlimited resource and 14 minimum environment impact in contrast with other alternatives such as 15 water, nuclear, and wind energy. 16 Currently, almost all commercial photovoltaic technologies use Si-based 17 technology, which has a number of disadvantages including high cost, lack of 18 flexibility, and the serious environmental impact of the Si industry. Other 19 technologies, such as organic photovoltaic (OPV) cells, can overcome some of 20 these issues. Today, polymer-based OPV (P-OPV) devices have achieved power conversion efficiencies (PCEs) that exceed 9%. 21 Compared with P-OPV, small molecules based OPV (SM-OPV) offers further advantages, including a defined structure for more 22 reproducible performance, higher mobility and open circuit voltage, and easier synthetic control that leads to more diversified 23 structures. Therefore, while largely undeveloped, SM-OPV is an important emerging technology with performance comparable to 24 P-OPV. 25 In this Account, we summarize our recent results on solution-processed SM-OPV. We believe that solution processing is 26 essential for taking full advantage of OPV technologies. Our work started with the synthesis of oligothiophene derivatives with an 27 acceptor donor acceptor (A-D-A) structure. Both the backbone conjugation length and electron withdrawing terminal groups 28 play an important role in the light absorption, energy levels and performance of the devices. Among thosemolecules, devices using 29 a 7-thiophene-unit backbone and a 3-ethylrhodanine (RD) terminal unit produced a 6.1% PCE. With the optimized conjugation 30 length and terminal unit, we borrowed from the results with P-OPV devices to optimize the backbone. Thus we selected BDT 31 (benzo[1,2-b:4,5-b0]dithiophene) and DTS (dithienosilole) to replace the central thiophene unit, leading to a PCE of 8.12%. In 32 addition to our molecules, Bazan and co-workers have developed another excellent system using DTS as the core unit that has also 33 achieved a PCE greater than 8%.