Effects of doping methods and dopant sizes on the performance of solar cells constructed with anchor-guided photoelectrochemical polymerization of thiophene

Solar cells constructed with well-ordered polythiophene (PT) or other π-conjugated polymer films are known to yield better photovoltaic characteristics. We recently constructed a PT-based solar cell (PTSC) wherein the photoactive PT film was grown photoelectrochemically along a dye anchor, 3-{5-[N,N-bis(4-diphenylamino)phenyl]thieno[3,2-b]thiophen-2-yl}-2-cyano-acrylic acid (C207) pre-adsorbed onto a compact TiO2 layer. In an effort to understand the effects of other important factors contributing to the PTSC performance, we compared ex-situ and in-situ doping of ordered PT films with three different alkylammonium cations, tetrabutylammonium (TBA+), tetraethylammonium (TEA+), and tetramethylammonium (TMA+). The PT films that have undergone such ex- and in-situ doping both show higher open-circuit voltage (Voc) and short-circuit current density (Jsc) than their pristine counterparts. The optimal conversion efficiency (η) of 7.53 ± 0.58% was achieved via in-situ PT doping with TMA+. The incident photon-to-electron conversion efficiency (IPCE), electrochemical impedance, and electron lifetime measurements all indicate that in-situ doping is less affected by the cation size and can lead to a higher doping level than the ex-situ doping method. The IPCE also benefits from the use of a thin TiO2 layer, as more light reaches the PT film to generate photohole/electron pairs. With the combined use of a thin layer of TiO2 and an ordered PT film containing a large number of dopant ions, the photohole/electron recombination is significantly suppressed.