Acceptor Gradient Polymer Donors for Non-Fullerene Organic Solar Cells

Abstract

In organic solar cells, maximizing the open-circuit voltage (V OC ) via minimization of the ionization energy or electron affinity offsets of the blended donor and acceptor often comes at the expense of achieving a considerable amount of short-circuit current (J SC ). To explore a hypothesis for the design of materials that may circumvent this tradeoff, eight structurally similar polymers were synthesized consisting of a fluorinated/non-fluorinated benzothiadiazole (BTDF/BTD) strong acceptor moiety, a thiophene ester (TE) weak acceptor, and various donor units composed of bithiophene (T2), biEDOT, and benzodithiophene (BDT) to form six acceptor gradient and two nongradient polymers. The acceptor gradient motif was designed and theorized to induce more facile exciton dissociation in low driving force solar cells by creating a further separated intramolecular charge-transfer state between the strong BTD acceptor and various donor units through a bridging TE component. Solar cells were fabricated using the eight polymers blended with phenyl-C 71 -butyric-acid methyl ester (PC 71 BM) to reveal two top performing isomeric polymers, T2-BTDF-(TE2) and TE2-BTDF-(T2), which were further tested with several non-fullerene acceptors (NFAs): EH-IDTBR, ITIC, and ITIC-4F. In order to fabricate optimally performing solar cells, a 0.2 eV ionization energy offset was found to be essential or the short-circuit current of the NFA cells diminished dramatically. Ultimately, optimized NFA solar cells were fabricated using ITIC-4F paired with each of the top performing polymers to produce an average PCE of 7.3% for TE2-BTDF-(T2) (nongradient) and 3.6% for T2-BTDF-(TE2) (gradient). The acceptor gradient effect was not shown to reduce the amount of charge recombination in NFA solar cells mainly due to the inability to fabricate solar cells, with minimal ionization energy or electron affinity offsets along with morphological complications. This work stresses the importance of acquiring accurate ionization energies and electron affinities when characterizing solar cell energetics, as differences as small as 0.1 eV in the offsets can make a significant impact on overall charge collection.

Conjugated Conductive Polymer Materials and its Applications: A Mini-Review

“…Recently, research has focused on non-fullerene acceptors (NFAs) in OSCs because of their advantages, including greater optical, electrochemical, and structural flexibility compared to their fullerene counterparts ( Lin et al, 2015 ; Nielsen et al, 2015 ; Hou et al, 2018 ; Zhang et al, 2018a ). In 2019, Reynolds’s et al reported a series of polymers by minimal structure modification with varying electron donors ( Figure 1D ) ( Jones et al, 2019 ). These polymers are consisted of a fluorinated/non-fluorinated benzothiadiazole strong acceptor moiety, a thiophene ester weak acceptor, and various donor units composed of bithiophene, biEDOT, and benzodithiophene to form six acceptor gradient and two non-gradient polymers.…”

Asymmetric Acceptors with Fluorine and Chlorine Substitution for Organic Solar Cells toward 16.83% Efficiency

“…[ 34–38 ] At present, focusing on designing well performed “acceptor‐donor‐acceptor (A‐D‐A)”‐type small molecular acceptors (SMAs), especially the A‐DA'D‐A structured SMAs, is the mainstream of breaking through the bottleneck of power conversion efficiencies (PCEs), for their photovoltaic properties can be easily tuned by simple synthesis steps. [ 39–45 ] One of the milestones of the SMA based OSC development is the PM6:Y6 systems reported by Zou's group recently, which yields a PCE as high as 15.7% for both conventional structure devices and inverter devices by utilizing the advantages of SMAs on absorption spectra and morphology. [ 46 ] Subsequently, a series of OSC systems based on Y6 or Y6 derivative acceptors have been developed, which further improved the PCE of the OSCs to over 16% rapidly.…”

Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

“…D-A polymer with alternating electron-donating (D) and electron-accepting (A) units are the most widely used polymer donors in OSCs because of the ease in fine-tuning frontier energy level and light absorption coverage. [15][16][17][18][19][20][21][22][23][24][25][26][27] Historically speaking, the creation of electron-accepting monomers play a crucial role in the evolution of…”