We report about solution-processing of a doped small molecule hole transport layer (s-HTL) comprising of N,N'-((diphenyl-N,N'-bis)9,9,-dimethyl-fluoren-2-yl)-benzidine (BF-DPB) as matrix and the p-dopant 'NDP9' in the non-halogenated solvent tetrahydrofuran (THF). We show that the doping process is already happening in solution and stays effective after coating. Conductivities achieved with this process are comparable to those reached by thermal co-evaporation under high vacuum, which is the usual deposition method for this material. Applied as planarization layer onto AgNW films with best performance values of 15 Omega/square and 83.5% total transmission including the substrate, the s-HTL is proving its ability to sufficiently smoothen the initially rough AgNW topography. We analyze the necessary lateral conductivity to bridge micrometer-large voids in the mesh, as they are inherent in nanowire network electrodes. In combination with zinc phtalocyanine:C60 organic solar cells, a s-HTL conductivity less than 1 x 10-4 S/cm can lead to decreased device performance with a loss greater than 10% for nanowires with 90 nni diameter and the associated mesh width. Furthermore, we demonstrate more efficient vacuum-deposited p-i-n solar cells with an oligothiophene (DCV5T-Me) as donor, C60 as acceptor. They exhibit power conversion efficiencies up to 4.4% on AgNW bottom electrodes with s-HTL, compared to 4.1% on ITO with s-HTL as reference device.
Bormann, Ludwig, Franz Selzer, Nelli Wei David Kneppe, Karl Leo, Lars Müller-Meskamp