Air-processed inverted organic solar cells utilizing a 2-aminoethanol-stabilized ZnO nanoparticle electron transport layer that requires no thermal annealing

Highly stable ZnO nanoparticles (NPs) for use in organic solar cells (OSCs) have been synthesized in 2-aminoethanol, which acts as both a stabilizing ligand and a solvent. The ZnO NP films do not require thermal annealing and applying them in inverted P3HT: mix-PCBM OSCs fabricated almost entirely under ambient conditions show efficiencies >3%. We find that thermally annealing the ZnO NP films does not give rise to any significant differences in device performance up to 150 degrees C. Annealing the films at higher temperatures leads to reduced short-circuit current densities and fill factors due to the removal of 2-aminoethanol from the NP surface, as evidenced by X-ray photoelectron spectroscopy. Furthermore, to confirm that the post-annealing of P3HT: PCBM devices at 150 degrees C does not affect our results, we fabricated inverted small molecule OSCs using a squaraine donor and PC71BM acceptor that only requires a low-temperature thermal annealing step (70 degrees C). No substantial differences between annealed and non-annealed devices were observed, which demonstrates the applicability of these new ZnO NPs in flexible OSCs.