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A series of acceptor-donor-acceptor simple oligomer-like small molecules based on oligothiophenes, namely, DRCN4T-DRCN9T, were designed and synthesized. Their optical, electrical, and thermal properties and photovoltaic performances were systematically investigated. Except for DRCN4T, excellent performances were obtained for DRCN5T-DRCN9T. The devices based on DRCN5T, DRCN7T, and DRCN9T with axisymmetric chemical structures exhibit much higher short-circuit current densities than those based on DRCN6T and DRCN8T with centrosymmetric chemical structures, which is attributed to their well-developed fibrillar network with a feature size less than 20 nm. The devices based on DRCN5T/PC71BM showed a notable certified power conversion efficiency (PCE) of 10.10% under AM 1.5G irradiation (100 mW cm(-2)) using a simple solution spin-coating fabrication process. This is the highest PCE for single-junction small-molecule-based organic photovoltaics (OPVs) reported to date. DRCN5T is a rather simpler molecule compared with all of the other high-performance molecules in OPVs to date, and this might highlight its advantage in the future possible commercialization of OPVs. These results demonstrate that a fine and balanced modification/design of chemical structure can make significant performance differences and that the performance of solution-processed small-molecule-based solar cells can be comparable to or even surpass that of their polymer counterparts.

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The chemical structure–property relationships and mechanism for high performance organic photovoltaics of acceptor–donor–acceptor type molecules are discussed.

In the past few years, great progress has been made in organic photovoltaic (OPV) cells for an alternate of silicon semiconductorbased solar cells. OPV has the advantages of clean, low-cost, flexibility, and the possibility of roll-to-roll production. [1][2][3][4] Currently, most of the works have been focused on polymer donor molecules using bulk heterojunction (BHJ) architecture and [6,6]-phenyl-C61-butyric acid methyl ester (PC 61 BM) as the acceptor. [5,6] Indeed, in addition to the currently better OPV performance than small molecules, polymers have the advantages for such as better film forming quality and so on. [7] However, it cannot be denied that there are disadvantages for polymer-based OPV, such as batch to batch reproducibility, difficulty of purification, and so on. In contrast, small molecules intrinsically do not have such flaws; [8] additionally, their band structures could be tuned easily with much more choices of chemical modification. Furthermore, small molecules generally have higher charge mobility and open voltages. [9,10] However, even with these advantages, small-molecule-based OPV cells have not been investigated as intensively as that of their polymer counterparts because one of the major problems for small molecules is their generally poor film quality when using the simple solution spinning process. [11] This has been hampering their performance, and indeed their power conversion efficiencies (PCEs) (4%-5%) [12][13][14][15][16][17][18] are still significantly lower compared with that (>7%) [19][20][21][22][23][24][25] from polymers. It is thus expected that better PCE could be achieved when their intrinsic bad film quality and morphology in BHJ architecture could be improved combining with their other advantages. But to achieve this, careful molecule design has to be carried out to address many factors collectively, including their molar absorption, morphology compatibility with the acceptors for a better film quality, and so on.Previously, we have reported a small molecule (DCAO7T, Scheme 1), which gives rather high quality of solution spincoated film and a high 5.08% PCE. [12] But this molecule comes with an end unit of cyanoacetate group, which could not contribute too much for the overall solar absorption. So, we thought that the current density J sc might be able to be enhanced if structures with stronger molar absorption is used to replace such an end unit, in the conditions that overall molecular backbone and morphology will not be affected too much. Considering the generally high absorption coefficients and good OPV performance of many dye molecules, such as triarylamines, [16,26] acenaphthoquinoxaline, [27] diketopyrrolopyrroles (DPP), [17] squaraines, [18,28,29] merocyanine, [30,31] isoindigo, [32] pthalocyanine, [33] and dipyrromethene boron difluoride (BODIPY), [34][35][36] we thus introduce a new dye unit, 3-ethylrhodanine, into the targeted OPV molecule named as DERHD7T (Scheme 1) with a conjugated thiophene backbone. With such a design, DERHD7T is expected to have a str...