Organic photovoltaic is under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here we show that for a series of regioregular-poly(3-hexylthiophene):fullerene bulk heterojunction organic photovoltaic devices with pinned electrodes, integer charge transfer states [1] present in the dark and created as a consequence of Fermi level equilibrium at the bulk heterojunction have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed and tested against a series of donor polymers [2]. To shed light on the spatial extent of the region at the bulk heterojunction containing dark integer charge transfer states, we use the original approach of building and characterizing multilayers composed of a well-defined number of polymer monolayers with the Langmuir-Shäfer method. This enables us to for the first time study energy level bending in polymer films of uniform and controlled thicknesses, avoiding the problems associated with spin-coating ultrathin films. We show that the disordered/amorphous films studied feature smaller, and in fact negligible, energy level bending compared to the more well-ordered films, in contradiction with existing models. Our results highlight that energy level bending depends on the oxidation/reduction energy distribution rather than the density of states of the neutral polymer chain in relation to the Fermi energy, taking into account the Coulomb energy associated with charging the polymer chain and transferring a charge across the interface. Based on this work, we can present a general model for energy level bending in conjugated polymer films in absence of significant doping-induced free charge density [3].
References [1] Slawomir Braun, et al, Adv. Mater. 21 1450 (2009) [2] Qinye Bao, et al, Adv. Funct. Mater. 24 6309 (2014) [3] Qinye Bao, et al, submitted.