Trap-Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics (original) (raw)

2014, Advanced Functional Materials

1 wileyonlinelibrary.com optimize and improve the effi ciencies so as to enable their successful commercialization, signifi cant efforts are made to increase two particular photovoltaic parameters: short circuit current density ( J sc ) and open circuit voltage ( V oc ). The energy difference between the holetransporting level of the donor and the electron-transporting level of the acceptor heavily infl uences the V oc and can be seen as an upper limit to what can be achieved in the device. Strategies to increase the V oc typically have focused on synthesis of new polymers and/or new acceptor/fullerene derivatives so as to achieve optimal donor (D)-acceptor (A) energy level offsets. More recently, a signifi cant infl uence of photogenerated donor-acceptor charge transfer (CT) complexes on V oc has been demonstrated, but strategies for V oc (and overall effi ciency) improvement based on this effect are less explored. In general, the V oc is found to be proportional to the incoming light intensity I such that ( ) ∝ ln oc s eV n kT I , where n s is a prefactor (sometimes referred to as the light ideality factor), usually 1 < n s < 2. Ultraviolet photoelectron spectroscopy (UPS), inverse photoemission spectroscopy (IPES) and cyclic voltammetry (CV) are typically used to measure the energies of the hole-and electron transporting levels, with CV being most commonly used due to its relative simplicity and low cost. Knowledge of the (bulk) transport levels does not enable the determination of electrode and BHJ energetics, however, as a potential step is often formed at metal/organic and organic/organic interfaces modifying the relative position of the energy levels at either side of the interface, even for weakly interacting physisorbed interfaces such as those typically found in a BHJ solar cell. It's proposed that the energy level alignment at weakly interacting metal/organic and organic/organic interfaces and in multilayer stacks can be predicted by the Integer Charge Transfer (ICT) model where the relation between the original Fermi level of a surface and the so-called pinning energies ( E ICT+,− ) of the organic semiconductor (OS) overlayer plays a key role. The E ICT+ ( E ICT-) energy of the positive (negative) ICT state relates to the smallest energy required to take away one electron (the largest energy gained from adding one electron) from (to) the OS molecule at an interface producing a fully relaxed state, where screening from the environment and the Coulombic interaction with the opposite charge across the Organic photovoltaics are under intense development and signifi cant focus has been placed on tuning the donor ionization potential and acceptor electron affi nity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene):fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ 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 affi nity energy difference at the donor-acceptor interface, even though there is a large variation in electron affi nity 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.