(581b) CVD-Fabricated Organic Active Layer and Hole Transport Material for Organic-Inorganic Mesoscopic Solar Cell

Authors: 
Nejati, S., Yale University
Lau, K. K., Drexel University

Unsubstituted polythiophene (PTh) was synthesized using a unique oxidative chemical vapor deposition (oCVD) approach. To enable oxidative polymerization of thiophene monomer, antimony pentachloride was used as an efficient oxidant. The deposited polymer was simultaneously doped to its conductive form. Upon dedoping, the polymer was found to absorb strongly at 510 nm. The optical band gap of the sample was determined to be around 1.9 eV and its LUMO orbital level was estimated to be 2.8 eV. Poly(3,4-ethylenedioxythiophene) (PEDOT) was synthesized by oCVD in the same way with antimony pentachloride as the oxidant. The deposited, doped film electrical conductivity was found to be non-linearly dependent on the inverse of polymer film thickness. Conductivity as high as ~1000 S/cm was estimated from sheet resistivity measured using 1 mA source current. The high conductivity of the film in the as deposited state is attributed to the presence of antimony complexes as dopant within the film, as exchanging these species with different ions including chloride and sulfate resulted in inferior conductivity. X-ray diffraction indicates that the PEDOT film is non-crystalline. The antimony complexes in the film were characterized using Auger electron and x-ray photoelectron spectroscopy. The antimony oxidation state was found to be +3. This along with the Raman spectroscopy results suggest that the PEDOT is in an over-oxidized state which could be the main reason behind the near metallic conductivity of the films. The conductivity in air and at room conditions (21 ˚C and ~30% RH) was monitored over 30 days and the loss in conductivity was found to be minimal. The oxidant complexes in the film were the main reason for the persistent conductivity, with a Sb/S ratio of ~ 1 yielding the most stable film conductivity for the period of testing. To fabricate an all solid-state organic-inorganic solar cell, a titania mesoporous electrode was deposited on a 50 nm compact layer of titanium dioxide which served as the electron transport layer. A thin layer of polythiophene (equivalent to ~5-8 nm thickness) was then deposited within the sample using oCVD and further dedoped by condensing methanol vapor within the film followed by annealing at 90 ˚C. The results were compared with the case where the samples were taken out and washed with methanol to remove all antimony complexes from the sample. We have found that the complete removal of antimony is essential for device performance. This can be attributed to the presence of antimony species at the interface which is known to be a strong electron scavenger. Upon drying, the samples were coated with PEDOT as a hole transport layer by oCVD until ~80 nm overlayer of PEDOT was achieved on the assembly. The assembled devices with a gold contact on the top were tested and a 0.8 % photoconversion efficiency without any further optimization was achieved.