| Summary: | In this study, a solution-processable compact vanadium oxide (V<sub>2</sub>O<sub>5</sub>) film with a globular nanoparticulate structure is introduced to the hole transport layer (HTL) of polymer bulk-heterojunction based solar cells comprised of PTB7:PC<sub>70</sub>BM by using a facile metal-organic decomposition method to replace the conventionally utilized poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). For this, a biocompatible structure-determining agent, polyethylene glycol (PEG, <i>M<sub>n</sub></i> 300), is used as an additive in the precursor to form the nanoparticulate compact V<sub>2</sub>O<sub>5</sub> (hereafter referred to as NP-V<sub>2</sub>O<sub>5</sub>) film, which possesses an outstandingly smooth surface morphology. The introduction of NP-V<sub>2</sub>O<sub>5</sub> HTL via the solution process with a neutral pH condition successfully improved the stability by preventing the decomposition of indium tin oxide (ITO) glass and the penetration of heavy-metal components and moisture, which are considered as the crucial drawbacks of using PEDOT:PSS. Over 1440 h (60 days) of the stability test, an organic solar cell (OSC) with NP-V<sub>2</sub>O<sub>5</sub> showed a significant durability, maintaining 82% of its initial power conversion efficiency (PCE), whereas an OSC with PEDOT:PSS maintained 51% of its initial PCE. Furthermore, due to the positive effects of the modified surface properties of NP-V<sub>2</sub>O<sub>5</sub>, the PCE was slightly enhanced from 7.47% to 7.89% with a significant improvement in the short-circuit current density and fill factor.
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