The Role of Fullerene Mixing Behavior in the Performance of Organic Photovoltaics: PCBM in Low‐Bandgap Polymers

Abstract

This manuscript reports the mixing behavior, interdiffusion, and depth profi le of 1-[3-(methoxycarbonyl)propyl]-1-phenyl-[6,6]C 61 (PCBM):low-bandgap (LBG) polymer thin fi lms that are formed by thermally annealing initial bilayers. The extent of mixing of PCBM is higher in polymers that include the 2,1,3-benzothiadiazole (BT) unit than in polymers that incorporate the 2,1,3-benzooxadiazole (BO) unit. This difference is ascribed to the enhanced mixing behavior of PCBM with the benzothiadiazole functionality than with benzooxadiazole functionality, which is attributed to preferred intermolecular interactions. The increased polymer/fullerene mixing is found to be crucial for optimal device performance. A decrease of polymer/fullerene mixing reduces the donor/acceptor interface, which lowers the probability of exciton dissociation and charge generation. Moreover, low PCBM mixing provides limited pathways for electron transport out of a miscible region, due to long distances between adjacent PCBM in such a miscible phase. This inhibits electron transport and increases the recombination of free charge carriers, resulting in a decrease in short circuit current and device performance. These results further exemplify the importance of the thermodynamic mixing behavior of the polymer:fullerene pair in designing next-generation conjugated polymers for organic photovoltaic (OPV) applications, as this controls the fi nal morphology of the OPV active layer.

Comparative Study of Thermal Stability, Morphology, and Performance of All-Polymer, Fullerene–Polymer, and Ternary Blend Solar Cells Based on the Same Polymer Donor

“…While the increase of the BHJ domain size from 60 to 69 nm is not significant after annealing for 12 h, the peak intensity increased greatly after annealing, indicating increased domain purity, which is also evident in the TEM image in Figure e. These observations are consistent with the diffusion of PCBM from the polymer:PCBM intermixed domains into PCBM-rich phases during thermal annealing. ,− A more noticeable change in the RSoXS data due to annealing was the appearance of a new high-intensity peak at q = 0.0012 Å –1 ( d = 523 nm) in the PCBM-PSC film annealed for 6 h, attributed to the formation of PCBM crystallites with several hundred nanometers. After further annealing (12 h), the d spacing decreased to 273 nm ( q = 0.0023 Å –1 ), but the peak intensity increased greatly.…”

Synthesis and photovoltaic properties of two new alkoxylphenyl substituted thieno[2,3-f]benzofuran based polymers

“…As shown in Table S1 (ESI †), an optimal donor/acceptor weight ratio was obtained as 1 : 2 for both PTBFP-BT and PTBFP-BO based devices. 44 As shown in Fig. The higher FF of the devices based on PTBFP-BT is consistent with the better balanced hole and electron mobility.…”

Optimizing the Performance of Conjugated Polymers in Organic Photovoltaic Cells by Traversing Group 16

“…In a more recent study on fullerene mixing behavior with DA copolymers, Bazan et al report that BO/CPDT and BO/DTS copolymers show reduced donor/acceptor interfaces when compared with similar BT-based copolymers as a result of poorer mixing of BO copolymers with PC 71 BM. 166 A similar trend holds for OPVs using copolymers of thiophene and alkoxy-functionalized BO (427) and BT (428) with PC 61 BM, where the devices of 427 have a higher V OC by 0.15 eV, but those composed of 428 have better overall performance resulting from greater J SC and better FF. 167,168 One of the few examples of heteroatom substitution in BCs that compares the effects of oxygen, sulfur, and selenium was done by studying OPVs consisting of copolymers of hexylthiophene-flanked BCs and hexylphenothiazine by Lin et al 169 In this particular set of copolymers, there was not a significant difference in the HOMO levels, with only a 0.05 eV drop from 431 to 430 and again from 430 to 429, resulting in a negligible difference between the V OC of the subsequent OPVs based on a 1:1 w/w blend polymers and PC 61 BM.…”