Enhanced Charge‐Carrier Injection and Collection Via Lamination of Doped Polymer Layers p‐Doped with a Solution‐Processible Molybdenum Complex

Abstract

2197 www.MaterialsViews.com wileyonlinelibrary.comcan be straightforwardly controlled down to a few nanometers by co-evaporation of the host and the dopant molecules. [ 2,4,12,13 ] Carrier injection enhancement of several orders-of-magnitude in organic lightemitting diodes and transistors has been demonstrated. [14][15][16][17][18] Doping-induced enhancement of charge carrier collection at small-molecule organic solar cell contacts has also been demonstrated. [ 12,13,19 ] Achieving the same degree of spatial control of dopant profi le for polymerbased devices is more challenging: polymer fi lms are solution-deposited and, although doped fi lms can be obtained via deposition of a solution containing the host and dopant, spatial distribution of the dopants cannot be easily controlled. Sequential spin-coating of undoped and doped fi lms using orthogonal solvents is one possible approach; however, the polymer-solvent compatibility needed for forming high quality fi lms and interfaces is not assured. We present here a conceptually simple approach based on soft-contact transferlamination (SCTL) of a polymer homojunction, using a soft stamp, for example, polydimethylsiloxane (PDMS), as a transfer medium. SCTL has been extensively used in the past for the deposition of metal electrodes [ 20,21 ] or active layers in organic transistors [ 22,23 ] and photovoltaic cells. [24][25][26][27][28] In the present case, two polymer fi lms, one undoped and one doped, are prepared separately via spin-coating, and then joined via SCTL to realize doped hole-injection or -collection contacts for diodes and solar cells. We have previously demonstrated with two polymers, poly(3-hexylthiophene) (P3HT) ( Figure 1 ) and poly[(9-,9-dioctylfl uorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′-3}-thiadiazole)] (F8BT), that the lamination process of undoped fi lms leads to "transparent" homojunctions, that is, interfaces that are electronically and electrically seamless [ 29 ] as evidenced by the facts that i) no signifi cant energy shift of molecular levels occurs across the SCTL junction, and ii) the current injected from either electrode is independent of whether the middle layer consists of a single polymer fi lm of thickness "t", or of two laminated fi lms with the same total thickness "t' . These results demonstrate that SCTL is a viable method for assembling multilayer polymer devices from separately fabricated layers, thus enabling functionalization, doping, and other materials modifi cation of specifi c parts of a device structure. Though demonstrated to work effectively across metal-metal interfaces, this Poly(3-hexylthiophene) (P3HT) is p-doped by the new soluble dopant molybdenum tris[1-(methoxycarbonyl)-2-(trifl uoromethyl)-ethane-1,2-dithiolene] and investigated via photoemission spectroscopy and transport measurements.Soft-contact transfer lamination of thin layers of the doped P3HT on undoped polymer layers is used to create spatially-confi ned doped regions, which serve as hole-injection contacts on P3HT diodes. This strategy is then used to...

Elucidating the Role of a Tetrafluoroborate‐Based Ionic Liquid at the n‐Type Oxide/Perovskite Interface

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

“…9 shows 3 is somewhat surprising, as it is considerably bulkier than the planar structure of F4TCNQ, and is only sparingly soluble in most common solvents. 29,30,32 These results suggest that sequential doping is widely applicable to other polymer:dopant systems. In addition, after doping, these films are essentially insoluble in their casting solvents.…”

“…[20][21][22][23][24] Several groups have synthesized and studied alternative molecular dopants in an effort to increase the thermal stability of doped films. 21,[25][26][27][28][29][30][31][32] Finally, a less commonly discussed but equally problematic issue lies in solution processing doped films. Several groups have noted that doping often results in drastically reduced solubility of polymers; as a result, mixed polymer:dopant solutions must be kept at high temperatures and dilute concentrations, and tend to rapidly aggregate, forming gels or large particles.…”

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