Near-Infrared Harvesting Fullerene-Free All-Small-Molecule Organic Solar Cells Based on Porphyrin Donors

Fullerene-free all-small-molecule organic photovoltaic cells (SM-OPVs) have attracted considerable attention because of well-defined molecular structures with low batch-to batch variation. Porphyrin derivatives have recently emerged as one of the promising conjugated building blocks for the small molecule (SM) donors. Herein, we first report fullerene-free SM-OPVs employing porphyrin-based donors. Three zinc porphyrin (P-zn)-based SM donors, which have strong bimodal absorption in the visible region and near-infrared region, are synthesized. Constructing bulk-heterojunction (BHJ) active layers using the P-zn donors and a SM acceptor, IDIC, which have complementary absorption, achieved panchromatic photon-to-current-conversion from 400 to 900 nm. The manipulation of side chains in the Pin donors considerably influenced the molecular ordering and nanomorphology of the BHJ active layers. P-zn-based fullerene-free SM-OPV devices with promising power conversion efficiency of 6.13% were achieved, which also offers crucial guidance for developing fullerene-free OPVs using porphyrin derivatives

High-Performance Near-Infrared Absorbing n-Type Porphyrin Acceptor for Organic Solar Cells

While the outstanding charge transport and sunlight harvesting properties of porphyrin molecules are highly attractive as active materials for organic photovoltaic (OPV) devices, the development of n-type porphyrin-based electron acceptors has been challenging. In this work, we developed a high-performance porphyrin-based electron acceptor for OPVs by substitution of four naphthalene diimide (NDI) units at the perimeter of a Zn-porphyrin (P-zn) core using ethyne linkage. Effective pi-conjugation between four NDI wings and the Pan core significantly broadened Q-band absorption to the near infrared region, thereby achieving the narrow band gap of 1.33 eV. Employing a windmill-structured tetra-NDI substituted P-zn-based acceptor (P-zn-TNDI) and mid-band gap polymer donor (PTB7-Th), the bulk heterojunction OPV devices achieved a power conversion efficiency (PCE) of 8.15% with an energy loss of 0.61 eV. The PCE of our P-zn-TNDI-based device was the highest among the reported OPVs using porphyrin-based acceptors. Notably, the amorphous characteristic of P-zn-TNDI enabled optimization of the device performance without using any additive, which should make industrial fabrication simpler and cheaper

Guest-Induced Modulation of the Energy Transfer Process in Porphyrin-Based Artificial Light Harvesting Dendrimers

A series of dendritic multiporphyrin arrays (<b>P</b>_<b>Zn</b><b>Tz</b>-<i>n</i><b>P</b>_<b>FB</b>; <i>n</i> = 2, 4, 8) comprising a triazole-bearing focal zinc porphyrin (<b>P</b>_<b>Zn</b>) with a different number of freebase porphyrin (<b>P</b>_<b>FB</b>) wings has been synthesized, and their photoinduced energy transfer process has been evaluated. UV/vis absorption, emission, and time-resolved fluorescence measurements indicated that efficient excitation energy transfer takes place from the focal <b>P</b>_<b>Zn</b> to <b>P</b>_<b>FB</b> wings in <b>P</b>_<b>Zn</b><b>Tz</b>-<i>n</i><b>P</b>_<b>FB</b>’s. The triazole-bearing <b>P</b>_<b>Zn</b> effectively formed host–guest complexes with anionic species by means of axial coordination with the aid of multiple C–H hydrogen bonds. By addition of various anionic guests to <b>P</b>_<b>Zn</b><b>Tz</b> and <b>P</b>_<b>Zn</b><b>Tz</b>-<i>n</i><b>P</b>_<b>FB</b>’s, strong bathochromic shifts of <b>P</b>_<b>Zn</b> absorption were observed, indicating the HOMO–LUMO gap (Δ<i>E</i>_HOMO–LUMO) of <b>P</b>_<b>Zn</b> decreased by anion binding. Time-resolved fluorescence measurements revealed that the fluorescence emission predominantly takes place from <b>P</b>_<b>Zn</b> in <b>P</b>_<b>Zn</b><b>Tz</b>-<i>n</i><b>P</b>_<b>FB</b>’s after the addition of CN^–. This change was reversible because a treatment with a silver strip to remove CN^– fully recovered the original energy transfer process from the focal <b>P</b>_<b>Zn</b> to <b>P</b>_<b>FB</b> wings