Efficient Organic Ternary Solar Cells Employing Narrow Band Gap Diketopyrrolopyrrole Polymers and Nonfullerene Acceptors

Abstract

Currently, high-performance organic solar cells (OSCs) are mainly composed of narrow band gap (NBG) nonfullerene acceptors and medium band gap (MBG) polymer donors, whereas the solar cells based on NBG polymer donors and MBG nonfullerene acceptors were much less successful. Herein, we report a new diketopyrrolopyrrole (DPP) polymer (PffBT-DPP) with a band gap of 1.33 eV for use in nonfullerene OSCs. When blended with a fullerene acceptor [6, 6]-phenyl C71 butyric acid methyl ester ([70]PCBM) and an MBG nonfullerene acceptor MeIC, the binary OSCs offered a power conversion efficiency (PCE) of 6.8 and 2.0%, respectively. Interestingly, a much higher PCE of 9.0% was achieved for the PffBT-DPP/[70]PCBM/MeIC ternary OSC, which is the sum of the two binary OSCs. The phase-separated morphology in this ternary OSC was characterized and correlated to the device performance. Moreover, the PCE of the ternary OSC is the best result for OSCs employing polymer donors with an optical band gap of <1.40 eV and nonfullerene acceptors. This work demonstrates the potential of constructing high-performance OSCs by employing NBG polymer donors and MBG nonfullerene acceptors.

Developing Halogen‐Free Polymer Donors for Efficient Nonfullerene Organic Solar Cells by Addition of Highly Electron‐Deficient Diketopyrrolopyrrole Unit

“…Diketopyrrolopyrrole (DPP) seems a suitable third‐component unit due to its accessible synthetic procedures and precise control of the polymer processability by alkylation of side chains at two N positions. Arguably, the DPP moiety features a highly planar π ‐backbone and large crystallinity, leading to strong optical absorption, whereas its exceptional electron‐withdrawing capability enables to deepen the lowest unoccupied molecular orbital (LUMO) energy level and result in a narrower optical bandgap ( E g ) in polymer donors, [ 43–45 ] both of which aid to improve light harvesting and carrier transport. However, the intrinsic drawback of the high‐lying HOMO level of DPP can be neglected by a slight incorporation of DPP content in the resultant terpolymer donors.…”

Analysis of the Performance of Narrow-Bandgap Organic Solar Cells Based on a Diketopyrrolopyrrole Polymer and a Nonfullerene Acceptor

“… 24 − 26 Two recent examples provide a more promising perspective. 27 , 28 In the first example, a polymer (PBDTT-DPP) consisting of thiophene-flanked DPP units alternating with alkylthiophene-substituted benzodithiophenes (BDTT) gave a PCE of 9.66% with a NFA. 27 In the second example, a polymer (PffBT-DPP) consisting of a DPP unit, two alkyl-substituted bithiophene units, and a difluorobenzothiadizole (ffBT) in the repeat unit gave a moderate PCE of 2.0% with a NFA in binary blend but up to 9.0% in a ternary blend with a fullerene acceptor.…”

“… 27 In the second example, a polymer (PffBT-DPP) consisting of a DPP unit, two alkyl-substituted bithiophene units, and a difluorobenzothiadizole (ffBT) in the repeat unit gave a moderate PCE of 2.0% with a NFA in binary blend but up to 9.0% in a ternary blend with a fullerene acceptor. 28 Despite this recent progress, it is currently unclear what causes the efficiency of organic solar cells consisting of DPP-based polymers and NFAs to be moderate in general.…”

High-Performance Ternary Organic Solar Cells Enabled by Synergizing Fullerene and Non-fullerene Acceptors

“…Therefore, the promising ternary strategy (containing three photovoltaic materials in a device) of using fullerenes and nonfullerenes as acceptors can be a simple way of substituting for the shortcomings of FA-or NFA-based binary OPVs and preserving their advantages for optimal device performance. 34 Besides, ternary OPVs also shows its potential in large-scale devices for commercialization. 35 A previous work includes a systematic and detailed summary of the ternary OPV from different aspects.…”