Reversible Optical Control of Conjugated Polymer Solubility with Sub-micrometer Resolution

Abstract

Organic electronics promise to provide flexible, large-area circuitry such as photovoltaics, displays, and light emitting diodes that can be fabricated inexpensively from solutions.A major obstacle to this vision is that most conjugated organic materials are miscible, making solution-based fabrication of multilayer or micro-to nanoscale patterned films problematic. Here we demonstrate that the solubility of prototypical conductive polymer poly(3-hexylthiophene) (P3HT) can be reversibly "switched off" using high electron affinity molecular dopants, then later recovered with light or a suitable dedoping solution. Using this technique, we are able to stack mutually soluble materials and laterally pattern polymer films by evaporation or with light, achieving sub-micrometer, optically limited feature sizes. After forming these structures, the films can be dedoped without disrupting the patterned features; dedoped films have identical optical characteristics, charge carrier mobilities, and NMR spectra as as-cast P3HT films. This method greatly simplifies solution-based device fabrication, is easily adaptable to current manufacturing workflows, and is potentially generalizable to other classes of materials.

Comparison of solution-mixed and sequentially processed P3HT:F4TCNQ films: effect of doping-induced aggregation on film morphology

“…Therefore, we expect that only the disordered regions of the film are accessible to the dopant molecules, limiting the achievable doping level from an acetonitrile solution (or any poor solvent for P3HT). Previous work 7 has also established that P3HT films are completely insoluble after sequential doping from a 0.1 mg ml À1 F4TNCQ/acetonitrile solution. Despite their insolubility, these films still show a significant solvochromic absorption shift in typically good solvents for P3HT, such as CB, indicating that solvent is able to infiltrate crystallites in the film without causing dissolution.…”

“…4 Because acetonitrile is a poor solvent for P3HT, and specifically because P3HT films show no solvochromic shift when immersed in acetonitrile, 7 very little solvent infiltration into crystalline polymer domains is expected. Therefore, we expect that only the disordered regions of the film are accessible to the dopant molecules, limiting the achievable doping level from an acetonitrile solution (or any poor solvent for P3HT).…”

“…1a), have become the focus of intense study. [2][3][4][5][6][7][8] Molecular dopants such as F4TCNQ undergo groundstate charge transfer with their host semiconductor, yielding polarons or bipolarons on the semiconductor. 5,[8][9][10][11][12][13][14][15] The dopant anions remain in the film as counter-ions (radical anions in the case of F4TCNQ).…”

“…Our group recently used F4TCNQ to demonstrate reversible solubility control on thin films of P3HT. 7 We showed that doped films are completely insoluble in organic solvents, but chemical or optical de-doping methods can recover intrinsic material properties. This method, called doping-induced solubility control (DISC), can be used to stack mutually soluble materials or for sub-micron patterning.…”

The effect of thermal annealing on dopant site choice in conjugated polymers

“…F4TCNQ has also been investigated as a p-type dopant for polymer samples using solution-processing with many of these studies focused on the non-polar polymer P3HT [24e26]. More recently, F4TCNQ was used to control the solubility of P3HT for patterning purposes and is likely therefore to be incorporated into solution-processed devices in the near future [27]. Unfortunately, little attention has been paid to the question of whether the molecular dopants have preference for the polar layer or for the non-polar layer, which is of high interest from the device stability perspective.…”