Doped Interlayers for Improved Selectivity in Bulk Herterojunction Organic Photovoltaic Devices

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is less selective for holes in an inverted-architecture organic photovoltaic (OPV) than it is in a conventional-architecture OPV device due to differences between the interfacial-PSS concentration at the top and bottom of the PEDOT:PSS layer. In this work, thin layers of polysulfonic acids are inserted between the poly(3-hexylthiophene) (P3HT):indene-C₆₀ bisadduct (ICBA) bulk heterojunction (BHJ) active layer and PEDOT:PSS to create a higher concentration of acid at this interface and, therefore, mimic the distribution of materials present in a conventional device. Upon thermal annealing, this acid layer oxidizes P3HT, creating a thin p-type interlayer of P3HT⁺/acid^- on top of the BHJ. Using X-ray absorption spectroscopy, Kelvin probe, and ellipsometry measurements, this P3HT⁺/acid^- layer is shown to be insoluble in water, indicating it remains intact during the subsequent deposition of PEDOT:PSS. Current density-voltage measurements show this doped interlayer reduces injected dark current while increasing both open-circuit voltage and fill factor through the creation of a more hole selective BHJ-PEDOT:PSS interface.