In this research, a haptacyclic carbazole-based dithienocyclopentacarbazole (DTCC) ladder-type structure was formylated to couple with two 1,1-dicyanomethylene-3-indanone (IC) moieties, forming a new nonfullerene acceptor DTCCIC-C17 using a bulky branched 1-octylnonayl side chain at the nitrogen of the embedded carbazole and four 4-octylphenyl groups at the sp-carbon bridges. The rigid and coplanar main-chain backbone of the DTCC core provides a broad light-absorbing window and a higher-lying LUMO energy level, whereas the bulky flanked side chains reduce intermolecular interactions, making DTCCIC-C17 amorphous with excellent solution processability. The DTCCIC-C17 as an acceptor is combined with a medium band gap polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T) as the donor in the active layer to obtain suitable highest occupied molecular orbital/lowest unoccupied molecular orbital energy alignments and complimentary absorption. The devices with an inverted configuration (ITO/ZnO/active layer/MoO/Ag) without using an aqueous poly(3,4-ethylenedioxythiophene) polystyrene sulfonate layer were fabricated for better device stability. When the diiodooctane-treated PBDB-T:DTCCIC-C17 active layer was thermally annealed at 50 °C for 10 min, the device achieved the highest efficiency of 9.48% with a high V of 0.98 V, a J of 14.27 mA cm, and an FF of 0.68.
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%).
A new thieno[3,2-b]thiophene-incorporated acceptor
TTC has been developed. The TTC acceptor was installed in a haptacyclic
ladder-type core (BDCPDT) to furnish an n-type BDCPDT-TTC. The standard
PBDB-T:BDCPDT-IC device showed a PCE of 9.33% with a V
oc of 0.86 V and a J
sc of
16.56 mA/cm2. By molecular engineering of the acceptor
unit, the BDCPDT-TTC:PBDB-T-based device delivered an enhanced efficiency
of 10.29% with a simultaneously enhanced V
oc of 0.94 V and J
sc of 17.72 mA/cm2. Incorporation of the electron-donating thieno[3,2-b]thiophene unit into the acceptor moiety decreases the
electron-accepting strength, thereby upshifting the HOMO/LUMO energy
levels to decrease the ΔE
HOMO and E
loss, achieving a larger V
oc. Second, the extended conjugated bicyclic thieno[3,2-b]thiophene ring beneficially induces an additional optical
transition at short wavelengths, leading to improvement of J
sc. Alternatively, BDCPDT-FIC installed with
the fluorinated acceptor shows more red-shifted absorption to achieve
a high J
sc of 19.12 mA/cm2.
A formylated benzodi(cyclopentadithiophene) (BDCPDT) ladder-type structure with forced coplanarity is coupled with two 1,1-dicyanomethylene-3-indanone (IC) moieties via olefination to form a non-fullerene acceptor, BDCPDT-IC. The BDCPDT-IC, as an acceptor (A) with broad light-absorbing ability and excellent solution processability, is combined with a second PCBM acceptor (A) and a medium band gap polymer, PBDB-T, as the donor (D) to form a ternary blend with gradient HOMO/LUMO energy alignments and panchromatic absorption. The device with the inverted architecture using the D:A:A ternary blend has achieved a highest efficiency of 9.79% with a superior J of 16.84 mA cm.
An angular-shaped 4,9-didodecylnaphthodithiophene-based octacyclic ladder-type structure was developed. This new ladder-type donor (LD) was coupled with electronwithdrawing IC and FIC units to form two A-LD-A type nonfullerene acceptor materials, NCIC and NCFIC. NCIC and NCFIC as the n-type acceptors are blended with a p-type donor PBDB-T to form the complementary absorption and suitable energy level alignments. The binary-blend PBDB-T:NCIC and PBDB-T:NCFIC devices achieved an efficiency of 7.3% and 7.5%, respectively. PC 71 BM was incorporated to form D 1 :A 1 :A 2 ternary blends which further strengthen the absorption at shorter wavelengths. Introduction of PC 71 BM not only efficiently improves the absorption but also provides multiple channels for exciton dissociation and electron transport to dramatically improve J sc . It is interesting to find that the V oc of the ternary-blend devices is reversely in proportional to the added amount of PC 71 BM. The device using the PBDB-T:NCIC:PC 71 BM (1:1:1 in wt %) showed an improved PCE of 8.32%. Moreover, the optimized device using the PBDB-T:NCFIC:PC 71 BM (1:1:1.5 in wt %) blend achieved the highest efficiency of 9.18%.
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