Structural and optoelectronic properties of hybrid bulk-heterojunction materials based on conjugated small molecules and mesostructured TiO2

Improved hybrid bulk-heterojunction materials was fabricated by spin-casting a benchmark conjugated small molecule, namely, 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP(TBFu)2), into mesostructured TiO2. Due to both a reduced molecular size and less hydrophobic nature of the conjugated molecules (relative to conjugated polymers), homogeneous and improved infiltration into the mesoporous TiO2 are achieved without the need for pre-treatment of the TiO2. Remarkably, this small molecule can realize loadings of up to 25% of the total pore volume—2.5× the typical loadings achieved for conjugated polymers. The small molecule loading was determined using dynamic secondary ion mass spectroscopy and absorption spectroscopy. Further characterization such as charge transfer and nanoscale conducting atomic force microscopy helps to demonstrate the promise and viability of small molecule donors for hybrid optoelectronic devices.