Dithiapyrannylidenes as Efficient Hole Collection Interfacial Layers in Organic Solar Cells

One inherent limitation to the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs) is the accumulation of positive charges at the anodic interface. The unsymmetrical charge collection of holes and electrons dramatically decreases the short-circuit current. Interfacial layers (IFLs) such as poly (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) have no effect on the unbalanced electron/hole transport across the BHJ. We report here on the use of dithiapyrannylidenes (DITPY), a new class of planar quinoid compounds, as efficient hole-transporting/electron-blocking layers in organic solar cells based on poly(3- hexylthiophene)/[6,6]-phenyl-C-61-butyric acid methyl ester (P3HT:PCBM) BHJs. Inserting a 15-nm-thick IFL of 4,4'-bis(diphenyl-2,6- thiapyrannylidene) (DITPY-Ph-4) between the indium-tin oxide electrode and the P3HT:PCBM BHJ prevents detrimental space charge effects and favors recombination limited currents. Current-sensing atomic force microscopy reveals a drastic increase of the hole-carrying pathways in DITPY-Ph-4 compared to PEDOT-PSS. In ambient conditions photovoltaic cells using DITPY-Ph-4 exhibit an 8% increase in the current density although the conversion efficiency remains slightly lower compared to PEDOT:PSS-based devices. Finally, we represent a detailed analysis of the photocurrent generation, showing that DITPY-Ph-4 IFLs induce a transition from unproductive space-charge-limited currents to recombination-limited currents.