Synergistically boosting performances of organic solar cells from dithieno[3,2-b]benzo[1,2-b;4,5-b′]dithiophene-based copolymers via side chain engineering and radical polymer additives

Abstract

As a notable analogue of benzo[1,2-b:4,5-b′]dithiophene (BDT), dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) is expected to be a more promising building block for polymer photovoltaic donor materials due to its larger coplanar core and extended conjugation length as well as a similar electron donor ability to BDT. However, the performance of organic solar cells (OSCs) from DTBDT-based copolymers is much lower than that of OSCs from BDT-based copolymers, which is attributed to the higher voltage loss of the OSCs from DTBDT-based polymers as compared to that from BDT-based polymers. In this study, approaches such as increasing the donor (D) and acceptor (A) spacing by lengthening the side chains of the polymer donors and use of radical conjugated polymer additives are synergistically employed in OSCs from 2-alkyl-3-chlorothiophene flanked DTBDT-alt-1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) polymers paired with Y6. Compared to the OSCs from the DTBDT-alt-BDD polymer with 2-ethylhexyl side chains (PBDT-Cl) paired with Y6, the power conversion efficiencies (PCEs) of the counterpart OSCs from the DTBDT-alt-BDD polymer with 2-butyloctyl side chains (PDBT-Cl-BO) increased from 12.67% to 14.58%, with a remarkable improvement of the open circuit voltage (V_OC). The reduction of non-radiative energy loss of the OSCs from PBDT-Cl-BO:Y6, ascribed to the increase of the DA spacing by lengthening the side chains, is supported through detailed studies such as Fourier-transform photocurrent spectroscopy external quantum efficiency (FTPS-EQE), electroluminescence (EL), electroluminescence external quantum efficiency (EQE_EL), and molecular dynamics simulations (MD). Afterwards, the PCEs of the OSCs from the blends of PDBT-Cl-BO:Y6 were further improved from 14.58% to 15.93% with a notable improvement of short circuit densities (J_SCs) and fill factors (FFs), along with a small improvement in V_OC upon the addition of the radical conjugated polymer GDTA as an additive. For comparison, the PCEs of the OSCs from the blends of PDBT-Cl:Y6 remained almost unchanged upon the addition of GDTA. This work suggests a wise strategy to synergistically utilize side-chain engineering and radical conjugated polymer additives to reduce the non-radiative energy loss, thus improving the performance of OSCs from DTBDT-based polymer donors.