Abstract
The first set of phenoxy BsubNc compounds, PhO-ClnBsubNc and F5-ClnBsubNc, was synthesized through an axial displacement reaction of Cl-ClnBsubNc with phenol and pentafluorophenol (respectively). Like their precursor, the products were found to be an alloyed mixture of phenoxylated ClnBsubNcs with random positioning in the solid state yet consistent frequency of bay position chlorination. The average bay position chlorine occupancy was determined to be 0.99 through single crystal diffraction of PhO-ClnBsubNc. Although the phenoxylation of Cl-ClnBsubNc did not influence the chromophore photophysical properties, the electrochemical behavior was found to be more stable. Phenoxylation yielded differences in organic photovoltaic (OPV) device metrics. Specifically, a significant increase in open circuit voltage (VOC) was observed, ultimately exceeding 1.0 V when phenoxylated ClnBsubNcs were paired with alpha-sexithiophene (α-6T) in planar heterojunction OPVs. Phenoxylation enabled the first example of BsubNcs incorporated into polymer-based bulk heterojunction (BHJ) OPVs through enhanced solubility. Phenoxylated ClnBsubNcs, when paired with poly-3-hexylthiophene, also showed high VOC in BHJ OPVs with broad spectral absorption up to 760 nm. In the BHJ case, simple phenoxy was shown to be a better axial substituent compared to pentafluorophenoxy. This study represents the first example of using ClnBsubNcs with nonchlorine axial substituents in OPVs and demonstrates that phenoxylation has a significant impact on device metrics while enhancing solubility to enable incorporation of ClnBsubNcs into BHJ OPVs.
Highlights
Boron subnaphthalocyanines (BsubNcs) are lower homologues of naphthalocyanines composed of three N-fused 1,3diiminobenzoisoindoline subunits around a single, central boron atom (Figure 1a)
We showed that the photo- and electrophysical properties as well as the planar heterojunction (PHJ) organic photovoltaic (OPV) device characteristics were found to be different among the Cl-ClnBsubNc samples with varying amounts of chlorination
Our findings strongly suggest that the Cl-BsubNc samples used in all previous reported OPV studies are likely not pure, consisting of a mixture of Cl-ClnBsubNcs with bay position chlorination.[16]
Summary
Boron subnaphthalocyanines (BsubNcs) are lower homologues of naphthalocyanines composed of three N-fused 1,3diiminobenzoisoindoline subunits around a single, central boron atom (Figure 1a). Like boron subphthalocyanines (BsubPcs), which are instead composed of 1,3-diiminoisoindoline subunits, BsubNcs are nonplanar, cone-shaped, and aromatic in structure.[1−6] Compared to BsubPcs, BsubNcs have a longer π-conjugation system, causing their absorption spectra to be heavily redshifted; the Q band is found in the mid and high 600 nm region in solution[2−6] and in thin solid films,[7] respectively This feature of capturing light in the red portion of the visible spectrum, combined with their high molar extinction coefficients, has made them desirable targets as a photosensitizing agent in photodynamic therapy.[5] and more relevant to our research group, Cl-BsubNc (Figure 1) has seen increasing interest as a photo- and electro-active material in organic photovoltaics (OPVs) since its debut in 2009.7−9 No other BsubNc derivative has been explored in this application. There are several reports on the use of Cl-BsubNc in OPVs.[7−16] In one of these studies, its inclusion resulted in an 8.4% efficient planar heterojunction (PHJ) OPV cell, a power conversion efficiency (PCE) value that is one of the highest within the space of PHJ OPVs.[14]
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