The electronic structure and hole-injection properties of ambient contaminated molybdenum trioxide (MoO3) surfaces are studied by ultraviolet and inverse photoemission spectroscopy, and current-voltage measurements. Contamination reduces the work function (WF), electron affinity (EA) and ionization energy by about 1 eV with respect to the freshly evaporated film, to values of 5.7 eV, 5.5 eV, and 8.6 eV, respectively. However, the WF and EA remain sufficiently large that the hole-injection properties of MoO3 are not affected by contamination. The results are of particular importance in view of potential applications of transition metal oxides for low-cost manufacturing of devices in low-vacuum or nonvacuum environment.

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...

No abstract

No abstract

No abstract