High efficiency small molecular acceptors based on novel O-functionalized ladder-type dipyran building block

Molecular electronic structure plays a vital role in the photovoltaic performances in nonfullerene polymer solar cells (PSCs) due to their influences on light-harvesting, charge carrier transfer, p-p stacking and morphology tuning etc. Oxygen as an electron donating atom, which has been incorporated into the nonfullerene acceptors (NFAs) system in the alkoxy forms at central, terminal or pi bridged moieties, could effectively tune the electron donating ability, absorption spectra, energy levels of the highest occupied molecular orbital (HOMO) & lowest unoccupied molecular orbital (LUMO) etc. In this contribution, a novel centrosymmetric pyran based ladder-typed conjugated building block (DTDP) has been designed and synthesized via a facile route and it was applied as the donor (D) core in acceptor-donor-acceptor (A-D-A) type NFAs design to evaluate its potential usage in polymer solar cells. This incorporated-O in the conjugated backbones effectively elevated the electron donating ability for the D moiety and wide absorption behaviors are observed. Although slight bent appears at the central part of the DTDP block from the geometry quantum optimization, strong crystallinity are found for both molecules. Photovoltaic performances of the solar cell devices are systematically investigated with poly[(2,6-(4,8-bis(5-(2-ethylhexyl) thiophen-2-yl) benzo[1,2-b:4,5-b'] dithiophene)-co-(1,3-bis(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl) benzo[1,2-c:4,5-c] dithiophene-4,8-dione)] (PBDB-T) as the polymer donor and a high power conversion efficiency (PCE) of 9.39% (V-oc = 0.80 V, J(sc) = 16.82 mA/cm(2) and FF = 69.95%) could be afforded by DTDP-MIC after the preliminary optimization, demonstrating the great promise in PSCs. Meanwhile, the morphologies as well as the carrier mobilities of the blend films are gleaned to assist understanding the structure-property relationships. Overall, the study in this work not only provides a promise ladder-typed dipyran donor unit for NFAs PSCs, it also paves a new way to use oxygens in small molecule NFAs design.