Isomerically Pure Benzothiophene-Incorporated Acceptor: Achieving Improved V_oc and J_sc of Nonfullerene Organic Solar Cells via End Group Manipulation

Abstract

Benzene-based 1,1-dicyanomethylene-3-indanone (IC) derivatives have been widely utilized as the end-group to construct acceptor−donor−acceptor type nonfullerene acceptors (A−D−A type NFAs). The extension of the end-group conjugation of nonfullerene acceptors (NFAs) is a rational strategy to facilitate intermolecular stacking of the end-groups which are responsible for efficient electron transportation. A bicyclic benzothiophene-based end-group acceptor, 2-(3-oxo-2,3-dihydro-1H-benzo[b]cyclopenta [d]thiophen-1-ylidene)malononitrile, denoted as α-BC was designed and synthesized. The Knoevenagel condensation of the unsymmetrical 1,3-diketo-precursor with one equivalent of malononitrile selectively reacts with the keto group attached at the α-position of the thiophene unit, leading to the isomerically pure benzothiophene-fused α-BC. The well-defined α-BC with extended conjugation was condensed with three different laddertype diformylated donors to form three new A−D−A NFAs named BDCPDT-BC, DTCC-BC, and ITBC, respectively. The corresponding IC-based BDCPDT-IC, DTCC-IC, and ITIC model compounds were also synthesized for comparison. The incorporation of the electron-rich benzothiophene unit in the end-group upshifts the lowest unoccupied molecular orbital energy levels of the NFAs, which beneficially enlarges the V oc values. On the other hand, the benzothiophene unit in α-BC not also imparts an optical transition in the shorter wavelengths around 340−400 nm for a better light harvesting ability but also promotes the antiparallel π−π stacking of the end-groups for efficient electron transport. The organic photovoltaic cell devices using a PBDB-T polymer and BC-based NFAs all showed the improved V oc and J sc values. The BDCPDT-BC-and DTCC-BCbased devices exhibited a power conversion efficiency (PCE) of 10.82 and 10.74%, respectively, which outperformed the corresponding BDCPDT-IC-, and DTCC-IC-based devices (9.33 and 9.25%). More importantly, the ITBC-based device delivered the highest PCE of 12.07% with a J sc of 19.90 mA/cm 2 , a V oc of 0.94 V, and an fill factor of 64.51%, representing a 14% improvement relative to the traditional ITIC-based device (10.05%).

Efficient ternary bulk heterojunction organic solar cells using a low-cost nonfullerene acceptor

“…The technique adopted for the fabrication of organic solar cells and their characterization is described in the ESI. † 48 DTCCIC-4F, 49 DTCCIC, 49 DTCC-BC, 50 DTCC-IC, 50 H2FCN-C16, 51 ICz-Rd2, 52,53 ICz-RdCN2, 53 and SMDPP. 54 This selection is based on the best ones where the carbazole residue is employed as a sub-fragment of the nonfullerene acceptor.…”

“…Among the donating group, carbazole-containing residues have attracted significant attention. [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] Over the past two years, the molecular design of carbazole-based NFSMAs has evolved and several examples are presented in Fig. 1 along with the reported PCEs of their BHJ-PSCs (optimized values).…”

Recent advances of carbazole‐based nonfullerene acceptors: Molecular design, optoelectronic properties, and photovoltaic performance in organic solar cells

“…For example, DTCC‐BC , end‐capping the DTCC core with α‐BC, affords its best PCE of 10.74% with PBDB‐T . [ 22 ] Other DTTC‐based NFAs with the same end groups but different side chains, such as DTCCIC and DTCCIC‐4F , showed no better photovoltaic performance than DTCCIC‐C17 and DTC(4Ph)‐4FIC did. [ 23 ]…”

Recent Advances in Non-Fullerene Acceptors of the IDIC/ITIC Families for Bulk-Heterojunction Organic Solar Cells

“…In any case, photovoltaic efficiencies were over 10%. Last year, Chang et al [53] reported the synthesis of a novel NFA, b8, which contains a 2-(3-oxo2,3-dihydro-1H-benzo [b]cyclopenta[d]thiophen-1-ylidene) (α-BC) moiety as the electron-deficient end-capping group. B8 showed similar absorption properties with respect to b0.…”