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.