(325h) Efficient Doping of Donor-Acceptor Polymers By Solution Processable High Electron Affinity Dopants

P-type molecular doping of organic semiconductors (OSC) is an important research area since doping enables tuning of OSC’s electrical conductivity for various device applications. Recently, chemists have introduced polymers with enhanced charge carrier mobilities and optical properties for device applications. Alternating push-pull co-polymers are an example of a new material that has generated significant recent interest. However, the high ionization energy for this class of polymers makes efficient molecular doping challenging due to limited availability of solution processable high electron affinity (EA) dopants. The P-type molecular dopant with highest EA reported to the date is CN6-CP (EA -5.87 eV) by Fukunaga et. al. and Karpov et. al. Doping with CN6-CP can only be achieved through thermal evaporation. This restricts development of technologies to mass-produce organic electronics, since scaled-up production of organic devices relies on solution processing. Here, we synthesized a series of CN6-CP analogs with one, two, or three nitrile groups replaced by mono-esters. These substitutions greatly increase the solution processability of the dopants with a marginal penalty in EA. We tested these new dopants by sequentially doping the p-type polymers; PDPP-4T, PDPP-3T, PDPP-2T, P(DPP₂-T₂F₄) and P3HT. We observed increased polaron density with increased doping as measured by UV/vis/NIR for all dopants and polymers. However, unlike previous studies with weaker dopants, we reach saturation of the doping sites and observe conductivities up to 100 S/cm for the highly doped polymers. The equilibrium between doped sites in a sequentially doped film and the counter ions in solution can be modeled using a Langmuir type of isotherm model. We quantify how the equilibrium constant for binding depends on electron affinity of the dopant and ionization of the polymer. Combined conductivity and UV/vis/NIR on the same film enables us to quantify the free hole density as a function of doping level. The near-IR /UV-Vis line shape is modelled using the Frenkel Holstein Hamiltonian expanded to include the coupling between Frenkel excitons and charge-transfer excitons on a linear donor-acceptor chain with repeat units represented by local HOMO and LUMO levels which vary periodically from donor to acceptor unit. The combined study is the first and only study comparing the driving energy for doping over multiple dopants and polymers. The achievement of high doping level and conductivity from solution processes is a large jump forward for the OSC field.