Conjugated Polymer Based on Polycyclic Aromatics for Bulk Heterojunction Organic Solar Cells: A Case Study of Quadrathienonaphthalene Polymers with 2% Efficiency

Abstract

Polycyclic aromatics offer great flexibility in tuning the energy levels and bandgaps of resulting conjugated polymers. These features have been exploited in the recent examples of benzo[2,1‐b:3,4‐b']dithiophene (BDT)‐based polymers for bulk heterojunction (BHJ) photovoltaics (ACS Appl. Mater. Interfaces 2009, 1, 1613). Taking one step further, a simple oxidative photocyclization is used here to convert the BDT with two pendent thiophene units into an enlarged planar polycyclic aromatic ring—quadrathienonaphthalene (QTN). The reduced steric hindrance and more planar structure promotes the intermolecular interaction of QTN‐based polymers, leading to increased hole mobility in related polymers. As‐synthesized homopolymer (HMPQTN) and donor–acceptor polymer (PQTN‐BT) maintain a low highest occupied molecular orbital (HOMO) energy level, ascribable to the polycyclic aromatic (QTN) moiety, which leads to a good open‐circuit voltage in BHJ devices of these polymers blended with PCBM ([6,6]‐phenyl‐C61‐butyric acid methyl ester; HMPQTN: 0.76 V, PQTN‐BT: 0.72 V). The donor–acceptor polymer (PQTN‐BT) has a smaller optical bandgap (1.6 eV) than that of HMPQTN (2.0 eV), which explains its current (5.69 mA cm−2) being slightly higher than that of HMPQTN (5.02 mA cm−2). Overall efficiencies over 2% are achieved for BHJ devices fabricated from either polymer with PCBM as the acceptor.

Heuristic approaches to the optimization of acceptor systems in bulk heterojunction cells: a computational study

“…These systems have been proposed by Brunetti [27]. Since the idea of bulk heterojunction (BHJ) organic solar cell emerged over one decade ago [28], there has been an intense activity in this field [29]. Organic solar cells based on BHJ are particularly attractive due to their mechanical flexibility and potential low cost [30].…”

“…Table 1). The energy E LUMOÀA and the total electric dipole moment of each individual are next estimated for the optimized geometry at B3LYP/3-21G level [29].…”

“…Therefore, we consider an additional constraint which imposes that the substituents must be placed symmetrically with respect to the in-plane molecular axes, which makes the resulting structure considerably easier to synthesize. All together, three cases are considered in this preliminary study: [29]. Below 0.3 eV the transfer from donor to acceptor is no more competitive with respect to other possible processes.…”

“…Below 0.3 eV the transfer from donor to acceptor is no more competitive with respect to other possible processes. The energy of E LUMOÀD for the P3HT polymer is set to -2.7 eV [29]. We have tested four different couples of crossover/ mutation probability p c ; p m equal to (0.8, 0.05) [44], (0.95, 0.01), (0.95, 0.05), and (0.95, 0.025), with and without the presence of elitism.…”

Poly(benzodithiophene) Homopolymer for High-Performance Polymer Solar Cells with Open-Circuit Voltage of Near 1 V: A Superior Candidate To Substitute for Poly(3-hexylthiophene) as Wide Bandgap Polymer

“…27,28 By fabricating the devices using a PBDTT:phenyl-C71-butyric acid methyl ester (PC71BM) (1:2, w/w) blend, the PCE value of 6.12% was achieved, which is the highest PCE value reported to date for homopolymer:PCBM-based PSCs. [24][25][26][29][30][31][32][33][34] Significantly, the optimized condition of the device was achieved without any solvent additive or thermal treatment. And, the PBDTT:PC71BM devices produced very high VOC values approaching 1 V, demonstrating that PBDTT has great potential for use as a substitute for P3HT in PSCs with single and tandem structures.…”