Effect of Incorporated Nitrogens on the Planarity and Photovoltaic Performance of Donor–Acceptor Copolymers

Abstract

Systematic control of the chemical structure of conjugated polymers is critically important to elucidate the relationship between the conjugated polymer structures and properties and to optimize their performance in bulk heterojunction (BHJ) polymer solar cell (PSC) devices. Herein, we synthesized three new copolymers, i.e., P0, P1, and P2; these copolymers contain the same benzodithiophene donor unit but have different acceptor units with different numbers of nitrogen atoms in the range of 0–2. The effects of the introduced nitrogen atoms on the structural, optical, electrical, and photovoltaic properties of the conjugated polymers were investigated; the structural properties of the polymers, in particular, were studied using both experimental (grazing-incidence X-ray scattering (GIXS) measurements) and computational methods (molecular simulation). As the number of introduced nitrogen atoms increased, the planarity of the main chain conformation increased in the order of P0 < P1 < P2. Additionally, the P0, P1, and P2 polymers showed increased interlayer domain spacings of 1.61, 1.72, and 1.78 nm, respectively, with increased intermolecular ordering. These results were in excellent agreement with the simulation results. In addition, the enhanced planarity resulted in a red-shifting at the onset of absorption in the polymer film from 544 to 585 nm, a downshift in the lowest unoccupied molecular orbital (LUMO) energy level from −3.02 to −3.26 eV, and an increase in the hole mobility from 2.33 × 10–6 to 3.78 × 10–5 cm2/(V s). As a result, we observed dramatically enhanced performance of the PSCs in the order of P0 < P1 < P2. For example, the P2:PC61BM device exhibited a 3.5-fold improvement in power conversion efficiency (PCE) compared to that of P0:PC61BM. The further optimization of P2 with PC71BM showed the PCE of 3.22%.

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…The smaller the organization energy is, the larger mobility of carriers is. A more planar conjugation structure is helpful to extend the conjugation length, strengthen the conjugation intensity, decrease the reorganization energy and enhance carrier mobility and/or OPV efficiency, as proved both in theory and experiments [1,38,81].…”

“…Firstly, extending the conjugation system and strengthening intra-chain and inter-chain interaction is a direct way to improve electronic properties. Based on the classic conjugated polymers such as P3ATs (P1), a straight strategy is inserting more conjugated moieties [3,82] such as carbon double bond (-HC=CH-, P2) [83][84][85], unsubstituted multiple or fused aromatic rings (i.e., thiophenes and thieno[3,2-b]thiophene, etc., P3-P7) [13,14,16,37,82,[86][87][88][89][90][91], other heterocyclic rings containing nitrogen(and sulfur) (P8 and P9) or selenium atoms (P10) [13,15,81,[92][93][94][95][96][97] and their combination (P11) [98]. Thus, the intra-chain conjugation is extended and strengthened by coupling with the more conjugated moieties, and the conjugation length is elongated due to the improved planarity.…”

“…Thus, the intra-chain conjugation is extended and strengthened by coupling with the more conjugated moieties, and the conjugation length is elongated due to the improved planarity. As a result, the inter-chain interaction of these extended conjugated chains becomes more intense to form more stable and ordered aggregates, resulting in the enhanced performance of devices [81]. Secondly, inserting acceptor moieties as n-type units [i.e., diketopyrrolopyrrole (DPP), naphthalene-bis(dicarboximide) (NDI), etc.]…”

Conventional and Inverted Photovoltaic Cells Fabricated Using New Conjugated Polymer Comprising Fluorinated Benzotriazole and Benzodithiophene Derivative

“…This is a typical π-π stacking distance in donor-acceptor conjugated polymers with good photovoltaic performances. 19 It can be concluded that the PTIPSBDT-DFDTBTz chains in the film are predominantly arranged with a face-on orientation to the substrate.…”

Theoretical Approach Towards Rational Design and Characterization of Benzo[1,2-b:5-B’]dithiophene (BDT)-Based (A-D-A) Small Molecules of Relevance for High Performance Solar Cells

“…Previous studies have demonstrated that the crystalline character of BDT based D-A copolymers and SMs can be adjusted by increasing the number of nitrogen atoms in the acceptor unit [32,33]. Benefited from the good properties of SM 2, we have subsequently designed SMs 3 and 4 by retaining the core acceptor unit of 2 and modifying the core donor BDTT-S by substituting some carbon atoms by nitrogen atoms.…”