title: Enhanced charge transport and photovoltaic performance of PBDTTT-C-T/PC70BM solar cells via UV-ozone treatment.
abstract: In this work, the electron transport layer of PBDTTT-C-T/PC70BM polymer solar cells were subjected to UV-ozone treatment, leading to improved cell performances from 6.46% to 8.34%. The solar cell efficiency reached a maximum of 8.34% after an optimal 5 minute UV-ozone treatment, and then decreased if treated for a longer time. To the best of our knowledge, the mechanism behind the effects of UV-ozone treatment on the improvement of charge transport and cell performance is not fully understood. We have developed a fundamental understanding of the UV-ozone treatment mechanism, which explains both the enhancements in charge transport and photovoltaic performance at an optimal treatment time, and also the phenomenon whereby further treatment time leads to a drop in cell efficiency. Transient photocurrent measurements indicated that the cell charge transport times were 1370 ns, 770 ns, 832 ns, 867 ns, and 1150 ns for the 0 min, 5 min, 10 min, 15 min, and 20 min UV-ozone treatment times, respectively. Therefore the 5 min UV-ozone treatment time led to the shortest transport time and the most efficient charge transport in the cells. The 5 min UV-ozone treated sample exhibited the highest peak intensity (E2) in the Raman spectra of the treated films, at about 437 cm(-1), indicating that it possessed the best wurtzite phase crystallinity of the ZnO films. Further increasing the UV-ozone treatment time from 5 to 20 min induced the formation of p-type defects (e.g. interstitial oxygen atoms), pushing the ZnO Fermi-level further away from the vacuum level, and decreasing the wurtzite crystallinity.
authors.affiliation: Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer Sciences, South Dakota State University, Brookings, SD 57007, USA. Qiquan.qiao@sdstate.edu.
authors.forename: Prajwal
authors.initials: P
authors.lastname: Adhikary
authors.forename: Swaminathan
authors.initials: S
authors.lastname: Venkatesan
authors.forename: Nirmal
authors.initials: N
authors.lastname: Adhikari
authors.forename: Purna P
authors.initials: PP
authors.lastname: Maharjan
authors.forename: Olusegun
authors.initials: O
authors.lastname: Adebanjo
authors.forename: Jihua
authors.initials: J
authors.lastname: Chen
authors.forename: Qiquan
authors.initials: Q
authors.lastname: Qiao
mesh_terms: D004579:Electron Transport; D010126:Ozone / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry*; D012993:Solar Energy; D014466:Ultraviolet Rays; D015034:Zinc Oxide / Q000737:chemistry
chemical_list: D011108:Polymers; D010126:Ozone; D015034:Zinc Oxide