NIR-Absorbing Merocyanine Dyes for BHJ Solar Cells

Abstract

We have synthesized a series of new, polymethine chain extended merocyanine dyes 1−4 bearing varied acceptor units and an aminothiophene donor moiety. The optical and electronic properties of these new merocyanines have been studied in comparison with their corresponding lower homologues 5−8, which contain two methine groups less, by UV−vis and electrooptical absorption (EOA) spectroscopy and cyclic voltammetry. The absorption spectra of π-extended merocyanines are markedly redshifted, and their extinction coefficients are significantly increased compared to those of their lower homologues. The photovoltaic characteristics of these dyes have been explored in devices using them as donor and PC 61 BM fullerene as acceptor materials. Our detailed studies reveal that, despite more favorable absorption properties, the π-extended merocyanines exhibit lower short-circuit current densities (J SC ) as well as decreased open-circuit voltages (V OC ) and power conversion efficiencies (PCE) compared with those of their respective lower homologues. The unexpected decreased J SC values could be explained in terms of looser packing features of π-extended chromophores in the solid state as revealed by single-crystal X-ray analysis of two pairs (1/5 and 4/8) of these dyes. By optimization of device setup PCE of 2.3% has been achieved with the π-extended donor material 4.

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].…”

“…However, compared to the polymeric donors, small molecule based donors have several benefits such as simple preparation and purification, unique final product without batch-to-batch problem, easy tuning of the optoelectronic properties etc. Furthermore, device fabrication becomes more flexible as the small molecule organic solar cells (SMOSCs) could be fabricated using both the solution process and/or vacuum-deposition techniques [21][22][23]. Linear donor materials using in SMOSCs could roughly be divided into two broad categories with symmetric or asymmetric molecular skeleton.…”

Series of Quinoidal Methyl-Dioxocyano-Pyridine Based π-Extended Narrow-Bandgap Oligomers for Solution-Processed Small-Molecule Organic Solar Cells

“…Moreover, its quinoidal structure, as well-known in the D-A typed merocyanine dyes, can induce the molecular backbone a great tendency to adopt a significant π-electronic structure change, affording the merocyanine dye a high degree of zwitterionic character. [34][35][36] This feature accompanies with the strong push-pull driving forces in D-A typed merocyanine dyes to facilitate electron delocalization and give intense and red-shifting ICT absorption bands. 37,38 "Quinoidal" form is a possible resonant structure of the conjugated polymer donor molecules.…”

Vacuum-depositable thiophene- and benzothiadiazole-based donor materials for organic solar cells

“…In this study, we have designed and synthesized three donoracceptor-donor (D-A-D)-type organic electron donor molecules (NP, BT, and DT) to broaden the light absorption ranges, consisting of 2,1,3-benzothiadiazole as the electron-withdrawing core, 3,4 and benzothiophene, naphthalene, and thiophene as the electron-donating terminal groups. Two thiophene units were used as spacers between the electron-withdrawing core and the electron-donating terminal groups, because of the good hole/ electron transporting ability of oligothiophenes.…”