Comparative Studies on Optical, Redox, and Photovoltaic Properties of a Series of D–A–D and Analogous D–A Chromophores

Abstract

A series of new symmetrical donor-acceptor-donor (D−A−D) dyes based on s -indacene-1,3,5,7(2 H ,6 H )-tetraone as an acceptor unit containing varying electron donating moieties and analogous donor-acceptor (D−A) chromophores with indane-1,3-dione as an acceptor are synthesized. By employing these two sets of dyes, the infl uence of a scaffold change from unsymmetric push-pull (D−A) to symmetrical (D−A−D) systems on optical, electrochemical, and photovoltaic properties are explored. Detailed comparative studies reveal favorable optical characteristics and considerably decreased bandgaps for the D−A−D dyes compared to those of the reference D−A chromophores. Accordingly, the evaluation of the present dyes as donor materials in bulk heterojunction (BHJ) solar cells in combination with fullerene derivatives PC 61 BM or PC 71 BM as acceptors afforded signifi cantly improved performance for devices based on D−A−D blends (up to a factor of 4 compared to the respective D-A reference) with power conversion effi ciencies of up to 2.8%. In less polar solvents such as toluene, some of the novel D−A−D chromophores exhibit unexpectedly high fl uorescence quantum yields Φ em of up to unity, in striking contrast to their weakly fl uorescent D-A counterparts.

Merocyanines for vacuum-deposited small-molecule organic solar cells

“…Since the presence of the naphthyl ring only imposes steric bulkiness in the crystal without being involved into p-p interactions, this result imposes an irregularity in the lattice which might be responsible for lowering the hole mobility and J sc . It is obvious that the intermolecular arrangement was divergent observation on the study of merocyanine dyes by Würthner and Meerholz et al In their study, these merocyanine dyes were proposed to eliminate strong dipole in solid state by antiparallel stacking, which prevented the energetic disorder that has interfering impact on exciton transport (or charge-carrier transport) [22,24,25,30]. Our results basically agree with their observations and indicate the importance of molecular packing order in charge transport and thus the device performance.…”

“…Several strategies have been developed so far to resolve the problem of impeding charge hopping in dipolar system, one typical example being the centrosymmetric dimer found in merocyanine dyes. Two neighboring merocyanine molecules are arranged in an antiparallel fashion and this stacking is proposed to eliminate the dipole moments in the bulk state [22,24,25,30]. In this line, our previous reports also conform to the importance of antiparallel packing fashion that can lead to fine device performance [31][32][33][34].…”

“…Both the A-D-A and D-A-D systems have low molecular dipole moment which may facilitate charge transport in the bulk state [24]. Besides, as the symmetric systems usually have relatively larger p-conjugation, they often offer greater light-harvesting capability and thus enhanced photocurrent density in OPV device [25]. To date, the A-D-A systems in BHJ solar http cells have been reported to reach efficiency up to 9.8% [26] and 10.1% [27] for single-junction and double-junction tandem solar cells respectively.…”

π-Linkage effect of push-pull-structure organic small molecules for photovoltaic application

“…Alkene including vinylene, styrene and phenylacrylonitrile is a kind of common π-bridges [58][59][60][61][62][63][64][65][66][67][68]. Introducing the alkene into D-A typed molecular architecture not only enhance the conjugation degree but also strengthen the coplanarity of the molecule.…”

Unfused Nonfullerene Acceptors Based on Simple Dipolar Merocyanines

“…These results indeed are encouraging, because herein common MCs generally with p‐type semiconducting characteristic have been successfully transformed to be n‐type. Furthermore, as the NFA, the performance of Z2 outperforms most of solution‐processable MC:PCBM devices, [3a,4, 6a, 17, 25] and most of P3HT‐based nonfullerene OSCs, particularly with unfused NFAs [16] . We further measured the device photostability as shown in Figure S14, and the PCEs of Z1 and Z2‐based devices can remain 55 % and 68 % of the initial values after 100 h light illumination, suggesting fluorinatiom can improve the photostability of NFAs.…”