Efficient and stable polymer solar cells with annealing-free solution-processible NiO nanoparticles as anode buffer layers

Annealing-free solution-processible nickel oxide (NiO) nanoparticles are prepared via sonochemical synthesis. In contrast to the common sol–gel route to prepare a NiOx film that had to undergo high-temperature post-annealing to be converted into NiO nanoparticles, herein, colloidal crystalline NiO nanoparticles with diameters of 4–8 nm are prepared with the metal salt at low temperature via ultrasonic irradiation. The colloidal crystalline NiO nanoparticles were directly used as the anode buffer layer in polymer solar cells (PSCs) without post-thermal annealing, which was compatible with flexible substrates and low-cost high-throughput fabrication. After O2-plasma treatment to the as-deposited buffer layer of NiO nanoparticles, the ITO/NiO contact work function was increased from −4.8 to −5.3 eV, providing superior contact with the highest occupied molecular orbital energy levels of polymer donors. Compared to PCDTBT:PC70BM PSCs with a PEDOT:PSS buffer layer, the power conversion efficiency of PSCs with an O2-plasma treated anode buffer layer of NiO nanoparticles was increased from 5.84% to 6.30% and the device stability was also enhanced.