Conjugated Donor-Acceptor Terpolymers Toward High-Efficiency Polymer Solar Cells

The development of conjugated alternating donor-acceptor (D-A) copolymers with various electron-rich and electron-deficient units in polymer backbones has boosted the power conversion efficiency (PCE) over 17% for polymer solar cells (PSCs) over the past two decades. However, further enhancements in PCEs for PSCs are still imperative to compensate their imperfect stability for fulfilling practical applications. Meanwhile development of these alternating D-A copolymers is highly demanding in creative design and syntheses of novel D and/or A monomers. In this regard, when being possible to adopt an existing monomer unit as a third component from its libraries, either a D' unit or an A' moiety, to the parent D-A type polymer backbones to afford conjugated D-A terpolymers, it will give a facile and cost-effective method to improve their light absorption and tune energy levels and also interchain packing synergistically. Moreover, the rationally controlled stoichiometry for these components in such terpolymers also provides access for further fine-tuning these factors, thus resulting in high-performance PSCs. Herein, based on their unique features, the recent progress of conjugated D-A terpolymers for efficient PSCs is reviewed and it is discussed how these factors influence their photovoltaic performance, for providing useful guidelines to design new terpolymers toward high-efficiency PSCs.

Modified structure of two-dimensional polythiophene derivatives by incorporating electron-deficient units into terthiophene-vinylene conjugated side chains and the polymer backbone: synthesis, optoelectronic and self-assembly properties, and photovoltaic application

Molecular engineering on the conjugated side chains of two-dimensional (2D) conjugated polymers was conducted and its effect on the optical, electronic, self-assembly and photovoltaic properties was investigated.

Random terpolymer with a cost-effective monomer and comparable efficiency to PTB7-Th for bulk-heterojunction polymer solar cells

A new random terpolymer PTB7-Th-T2 was designed and synthesized for high performance PSCs by incorporating a significantly lower-cost 2,2′-bithiophene monomer to the famous PTB7-Th skeleton.

Regular terpolymers with fluorinated bithiophene units for high-performing photovoltaic cells

We demonstrate effective structural control of various electron-donating moieties containing bithiophene (BT) and naphthalene derivatives with 3,3′-difluoro-2,2′-bithiophene in a regular terpolymer system and compare the properties of these polymers with those of the three binary copolymers PDPPNp, PDPPBT, and PDPPFBT.

Enhancing the photovoltaic properties of low bandgap terpolymers based on benzodithiophene and phenanthrophenazine by introducing different second acceptor units

Two D–A type random terpolymers PBDTT-PPzIID and PBDTT-PPzDPP were synthesized by copolymerizing a donor and two acceptor units. The PBDTT-PPzDPP-based device shown a PCE of 5.91%.

2,1,3-Benzooxadiazole, thiophene and benzodithiophene based random copolymers for organic photovoltaics: thiophene versus thieno[3,2-b]thiophene as π-conjugated linkers

Influence of spacers on the optoelectronic properties of polymers.

Enhanced Performance of Polymer Solar Cells Comprising Diketopyrrolopyrrole-Based Regular Terpolymer Bearing Two Different π-Extended Donor Units

New regular and random diketopyrrolopyrrole (DPP)-based terpolymers (i.e., Reg-PBDPPT and Ran-PBDPPT, respectively) bearing DPP as an electron deficient unit and 2,2'-bithiophene and (E)-1,2-di(thiophen-2-yl)ethene as electron donating units were designed and synthesized, and their performance in photovoltaic cells was investigated precisely. The absorption properties and highest occupied molecular orbital (HOMO) of Reg-PBDPPT were found to be different from those of Ran-PBDPPT. The results of grazing incidence X-ray diffraction experiments revealed that Ran-PBDPPT typically had a predominantly edge-on chain orientation on the substrate, whereas Reg-PBDPPT showed mixed chain orientation both in pristine and thermally annealed films. Although Reg-PBDPPT exhibited a lower degree of edge-on chain orientation on the substrate, the corresponding TFTs showed a high hole mobility of 0.42-0.96 cm(2) V(-1) s(-1) and maintained a high current on/off ratio (>10(6)). A polymer solar cell (PSC) composed of Reg-PBDPPT and PC71BM exhibited power conversion efficiencies (PCE) of 5.24-5.45%, which were higher than those of the Ran-PBDPPT-based PSCs. The enhanced efficiency was supported by an increase in the short circuit current, which is strongly related to the unique internal crystalline morphology and pronounced nanophase segregation behavior in the blend films. These results obviously manifested that this synthetic strategy for regular conjugated terpolymers could be employed to control morphological properties to obtain high-performance PSCs.

Contrasting performance of donor-acceptor copolymer pairs in ternary blend solar cells and two-acceptor copolymers in binary blend solar cells

Here two contrasting approaches to polymer-fullerene solar cells are compared. In the first approach, two distinct semi-random donor-acceptor copolymers are blended with phenyl-C61-butyric acid methyl ester (PC61BM) to form ternary blend solar cells. The two poly(3-hexylthiophene)-based polymers contain either the acceptor thienopyrroledione (TPD) or diketopyrrolopyrrole (DPP). In the second approach, semi-random donor-acceptor copolymers containing both TPD and DPP acceptors in the same polymer backbone, termed two-acceptor polymers, are blended with PC61BM to give binary blend solar cells. The two approaches result in bulk heterojunction solar cells that have the same molecular active-layer components but differ in the manner in which these molecular components are mixed, either by physical mixing (ternary blend) or chemical "mixing" in the two-acceptor (binary blend) case. Optical properties and photon-to-electron conversion efficiencies of the binary and ternary blends were found to have similar features and were described as a linear combination of the individual components. At the same time, significant differences were observed in the open-circuit voltage (Voc) behaviors of binary and ternary blend solar cells. While in case of two-acceptor polymers, the Voc was found to be in the range of 0.495-0.552 V, ternary blend solar cells showed behavior inherent to organic alloy formation, displaying an intermediate, composition-dependent and tunable Voc in the range from 0.582 to 0.684 V, significantly exceeding the values achieved in the two-acceptor containing binary blend solar cells. Despite the differences between the physical and chemical mixing approaches, both pathways provided solar cells with similar power conversion efficiencies, highlighting the advantages of both pathways toward highly efficient organic solar cells.