Tuning the Morphology and Performance of Low Bandgap Polymer:Fullerene Heterojunctions via Solvent Annealing in Selective Solvents

Abstract

5129www.MaterialsViews.com wileyonlinelibrary.com fl exibility, ease of processing, and straightforward thin fi lm formation. A typical bulk heterojunction (BHJ) active layer is formed by casting a thin fi lm from a solution mixture of the donor polymer and acceptor fullerene, where nanoscale interpenetrating networks are ideally formed upon solvent evaporation. [1][2][3][4][5][6][7][8] One of the most studied OPV BHJ blends is the mixture poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), where the as-cast devices often exhibit poor photovoltaic effi ciencies. Subsequent processing, such as thermal annealing or solvent vapor annealing are therefore required to allow the morphology of the mixture to evolve towards a structure with improved photovoltaic efficiency. [9][10][11][12][13][14][15][16][17] Moreover, low bandgap (LBG) polymer/fullerene systems are the most promising conjugated polymer fullerene systems for OPVs. Unfortunately, thermal annealing after deposition does not improve or often decreases the device performance. [18][19][20][21] Therefore, the use of additives during fi lm deposition has become a popular method to modify the morphology of LBG/fullerene system in order to enhance the device performance. These additives usually have a higher boiling temperature than the host solvent and selective solubility to PCBM. One of the most studied LBG system is the poly[2,6-(4,4-bis-(2,1,3-benzothiadiazole)] (PCPDTBT):PCBM system, often deposited with a solvent additive. The power conversion efficiency (PCE) of PCPDTBT:PCBM BHJ system has been shown to increase from 1.7% to 3.45-4.62% with the addition of a solvent additive. [ 18 ] To more fully understand the role of the additive, Russell and coworkers studied the morphology changes of the thin fi lm of a PCPDTBT:PCBM mixture that is deposited from solution with additives, where they found that PCBM aggregation and an increase of PCPDTBT ordering occurred with the presence of solvent additives. They further ascribe these structural changes to an improvement in charge transport, which translates to improved device performance. [ 18 ] While the addition of solvent additives has shown some performance improvement of LBG based organic photovoltaic, the morphology of the as-cast LBG:fullerene fi lm, with or without solvent additives, is locked in from the non-equilibrium deposition process. Therefore, there is still a pressing need to Low bandgap polymer (LBG):fullerene mixtures are some of the most promising organic photovoltaic active layers. Unfortunately, there are no post-deposition treatments available to rationally improve the morphology and performance of as-cast LBG:fullerene OPV active layers, where thermal annealing usually fails. Therefore, there is a glaring need to develop postdeposition methods to guide the morphology of LBG:fullerene bulk heterojunctions towards targeted structures and performance. In this paper, the structural evolution of PCPDTBT:PCBM mixtures with solvent annealing (SA) is examined, focus...

Morphology of a Ternary Blend Solar Cell Based on Small Molecule:Conjugated Polymer:Fullerene Fabricated by Blade Coating

“…As shown in Figure a,b, the scattering curves were found to be well fitted with Schulz sphere model, and from the fitting, average radius of pure PCBM domains ( R a ), volume fraction of pure PCBM phase (Φ ag ), polydispersity of the size of pure PCBM domains ( P ), and specific interfacial area ( S / V ) between pure PCBM phase and its surrounding matrix were obtained and tabulated in Table . The accuracy of volume fraction of pure PCBM phase obtained from fitting can be self‐consistent checked with calculated pure PCBM concentration with knowledge of the SLD of the surrounding matrix and the composition of the film . The results showed that for p‐DTS(FBTTh 2 ) 2 :PCBM binary blends, the volume fraction of pure PCBM phase increased from 11.9% to 26.9%, R a increased from 5.3 to 10.1 nm, polydispersity of the size of pure PCBM domains increased from 0.42 to 0.54, and S / V first increased and then decreased with an increase of blade coating speed.…”

Removal of Residual Diiodooctane Improves Photostability of High-Performance Organic Solar Cell Polymers

“…Indeed, UV–visible absorption spectra of this film show no signature of DIO-accelerated photobleaching (Figure S7). However, we note that thermal annealing generally does not improve OPVs utilizing low bandgap push–pull copolymers. , …”

Improvement of Device Performance of Organic Photovoltaics via Laser Irradiation

“…Meanwhile, from the fitting, the concentration of pure PCBM domains in the blends (Φ ag ), size of pure PCBM domains ( R a ), polydispersity of the size of pure PCBM domains ( P ), and specific interfacial area ( S / V ) between pure PCBM domains and their surrounding matrix can be determined and listed in Table . More detailed analysis and self-consistent check of the accuracy of volume fraction of pure PCBM phase can be found in our previous work . The results showed that the volume fraction of pure PCBM phase was 23.3% for TA samples, while it decreased to 20.9% for LA sample with a laser intensity of 55 mJ/cm 2 .…”

“…More detailed analysis and self-consistent check of the accuracy of volume fraction of pure PCBM phase can be found in our previous work. 38 The results showed that the volume fraction of pure PCBM phase was 23.3% for TA samples, while it decreased to 20.9% for LA sample with a laser intensity of 55 mJ/cm 2 . A further increase of laser intensity suppressed PCBM phase separation and Φ ag decreased to 13.2% with a laser intensity of 75 mJ/cm 2 .…”