Optimization of a High Work Function Solution Processed Vanadium Oxide Hole-Extracting Layer for Small Molecule and Polymer Organic Photovoltaic Cells

Abstract

We report a method of fabricating a high work function, solution processable vanadium oxide (V2O x(sol)) hole-extracting layer. The atmospheric processing conditions of film preparation have a critical influence on the electronic structure and stoichiometry of the V2O x(sol), with a direct impact on organic photovoltaic (OPV) cell performance. Combined Kelvin probe (KP) and ultraviolet photoemission spectroscopy (UPS) measurements reveal a high work function, n-type character for the thin films, analogous to previously reported thermally evaporated transition metal oxides. Additional states within the band gap of V2O x(sol) are observed in the UPS spectra and are demonstrated using X-ray photoelectron spectroscopy (XPS) to be due to the substoichiometric nature of V2O x(sol). The optimized V2O x(sol) layer performance is compared directly to bare indium–tin oxide (ITO), poly­(ethyleneoxythiophene):poly­(styrenesulfonate) (PEDOT:PSS), and thermally evaporated molybdenum oxide (MoO x ) interfaces in both small molecule/fullerene and polymer/fullerene structures. OPV cells incorporating V2O x(sol) are reported to achieve favorable initial cell performance and cell stability attributes.

Band offset of vanadium-doped molybdenum oxide hole transport layer in organic photovoltaics

“…According to literature [25,26], MoO 3 is considered as a direct bandgap material and n is set as 0.5. V 2 O 5 -MoO 3 [27] and V 2 O 5 [28] are both reported as indirect bandgap materials and n shall be set as 2. In this study, we set n of MoO x and V 0.05 MoO x as 0.5, because of low doping concentration of V. For V 0.2 MoO x , V 0.5 MoO x and VO x samples, we set n = 2 for Tauc plot.…”

Room-temperature and solution-processed vanadium oxide buffer layer for efficient charge injection in bottom-contact organic field-effect transistors

“…A VO x layer was formed below the Au S/D electrodes because the field-induced current in OFETs mainly flows within a few monolayers of the pentacene molecules. X-ray photoelectron spectroscopy (XPS) data from previous reports [15,16] showed that both thermal evaporation of vanadium oxide and the hydrolysis of vanadium precursor are likely to form vanadium sub-oxide (VO x ) instead of vanadium pentoxide (V 2 O 5 ) due to oxygen vacancies. The formation of VO x with solution-process was verified by comparing the UVeVis absorption spectrum with that of the thermally evaporated VO x film, as shown in Fig.…”

Flexible ITO-free organic solar cells applying aqueous solution-processed V2O5 hole transport layer: An outdoor stability study

“…Such is the case of NiO, 9 MoO 3 , 10,11 CuO, 12 or V 2 O 5 . 13,14 For V 2 O 5 , one of its most widespread fabrication methods is the application of sol-gels made from vanadium (V) oxitriisopropoxide (V 2 O 5 -i) [14][15][16][17][18] or (NH 4 ) 3 VO 4 . 19,20,25 These precursors when used for the fabrication of flexible organic solar cells must be annealed at temperatures not exceeding 150…”

“…We developed in-house software to control 21 As already mentioned, the applied fabrication methods are those obtained from sol-gels made from vanadium (V) oxytriisopropoxide (ViPr) or (NH 4 ) 3 VO 4 , among others. [14][15][16][17][18][19][20] In these cases, the precursor materials are selected for the fabrication of flexible solar cells where the sintering temperatures cannot exceed 150…”