Electronic structure of molybdenum-oxide films and associated charge injection mechanisms in organic devices

Abstract

Abstract. We report on the electronic structure of freshly evaporated and air-exposed Molybdenum tri-oxide (MoO 3 ) and the energy-level alignment between this compound and a holetransport material [e.g., N,N -diphenyl-N,N -bis (1-naphthyl)-1,1 -biphenyl-4,4 -diamine (α-NPD)]. Ultraviolet and inverse photoelectron spectroscopy show that freshly evaporated MoO 3 exhibits deep-lying electronic states with an electron affinity (EA) of 6.7 eV and ionization energy (IE) of 9.7 eV. Air exposure reduces EA and IE by ∼1 eV, to 5.5 and 8.6 eV, respectively, but does not affect the hole-injection efficiency, which is confirmed by device studies. Thus, MoO 3 can be applied in low-vacuum environment, which is particularly important for low-cost manufacturing processes. Our findings of the energy-level alignment between MoO 3 and α-NPD also leads to a revised interpretation of the charge-injection mechanism, whereby the hole-injection corresponds to an electron extraction from the organic highest-occupied molecular orbital (HOMO) level via the MoO 3 conduction band. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

Selective contacts drive charge extraction in quantum dot solids via asymmetry in carrier transfer kinetics

“…This semiconductor (not presented herein) aligns with the VB (HOMO) of the organic layers, while the energy difference is taken by an interface dipole. Even though it is n-type, MoO 3 forms a selective contact to holes in organic layers, by electron extraction through the CB of the oxide, which is situated only 0.6-0.7 eV above the HOMO of the organic film 35 . These examples indicate that although general rules are useful for basic understanding of tendencies of data of different materials, interfaces must be separately investigated when the type of injection and energy alignment mechanism needs to be really established.…”

Photovoltaic properties of cadmium selenide–titanyl phthalocyanine planar heterojunction devices

“…MoO 3 /Ag bilayer, commonly used as an anode in organic solar cells [1,2,10,13,14], serves the role of a high work function electrode in the considered system, while charge carrier transport through the MoO 3 layer is realized via states localized in the energy band gap of this material. It is worth noticing that different values of energy levels for this material can also be found [16]. All samples were prepared on glass substrates partially covered by ITO (sheet resistance of 40 X/square, purchased from PGO), cleaned in an ultrasonic bath (in acetone and isopropanol) and dried in a hot air flow.…”

Bilayer Interdiffused Heterojunction Organic Photodiodes Fabricated by Double Transfer Stamping

“…[23][24][25] A simple and reliable means to suppress the reverse-biased dark current, therefore, is to have only the acceptor material in contact with the cathode, while only donor material contacts the anode. [4,10] In the inverted BHJ in Figure 1a, the hole injection barrier at the cathode is ΔE hole = 1.4 eV (equal to the difference between the highest occupied molecular orbital (HOMO) energy of P3HT at 5.0 eV [26] and Φ ITO/PEIE = 3.6 eV [27] ), and the electron injection barrier at the anode is ΔE electron = 2.6 eV (equal to the difference between Φ MoO3 6.9 eV [28] and the lowest unoccupied MO (LUMO) energy of PCBM of 4.3 eV [26] ). A significant increase of the barrier height is obtained by the formation of the bilayer HJ in Figure 1b.…”

“…A significant increase of the barrier height is obtained by the formation of the bilayer HJ in Figure 1b. Then, the hole injection barrier at the cathode/PCBM interface is ΔE hole = 2.5 eV (equal to the difference between the HOMO energy of PCBM of 6.1 eV [26] and Φ ITO/PEIE ) and the electron injection barrier at the anode/P3HT layer is ΔE electron = 3.9 eV (equal to the difference between Φ MoO3 [28] and the LUMO energy of P3HT of 3.0 eV [26] ). We expect that the increased ΔE hole and ΔE electron at cathode/anode interfaces reduce the dark leakage current, while the lower energetic barrier (e.g., ΔE hole ) encourages current injection.…”