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.

Ethanedithiol Treatment of Solution‐Processed ZnO Thin Films: Controlling the Intragap States of Electron Transporting Interlayers for Efficient and Stable Inverted Organic Photovoltaics

“…[14,[47][48] As reported by So and co-workers, UV-ozone treatment not only passivates the surface defects but also generates excess oxygen at the ZnO surfaces which may oxidize the polymers with higher-lying HOMO energies, such as P3HT, and hence degrade the device performance. [14] Upon successful attempts of EDT passivation on ZnO nanocrystal films, we further extended the EDT passivation approach to the ZnO films deposited from Sol-gel precursors.…”

Device and morphological engineering of organic solar cells for enhanced charge transport and photovoltaic performance

“…These defects push the ZnO Fermi level further away from the vacuum level and decrease the Wurtzite crystallinity, lowering the electron extraction efficiency of the layer and resulting in poor device efficiency. 6 The JV characteristics for three different UV ozone treatment conditions are shown in Fig. 6.…”

“…The donor and acceptor phase separation should be in the range of ∼10 to 20 nm with a continuous pathway so that free carriers can efficiently transport to their corresponding electrodes. 4 Examples of donor/acceptor composites include P3HT or poly(diketo-pyrrolopyrrole-terthiophene) (PDPP3T)-fullerene derivatives { [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 60 BM), [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 70 BM), Indene-C60 bisadduct} and other combinations using low-bandgap polymers.…”

“…[3][4][5][6][7][8] Bulk heterojunction (BHJ) polymer solar cells comprising an interpenetrating network of a photoactive conjugated polymer (an electron donor) and a fullerene derivative (an electron acceptor) provide an efficient interfacial area for exciton dissociation and a bicontinuous pathway for effective charge transport. Photons are absorbed in the conjugated polymer resulting in the generation of excitons that diffuse to the interface between the polymer and fullerene derivative.…”

Effect of UV-ozone process on the ZnO interlayer in the inverted organic solar cells

“…10,11 Moreover, V O defects inuence the work functions (WFs) of ZnONFs by decreasing the crystallinity, leading to a mismatch in the energy levels of the devices. 12,13 However, O i defects are a type of acceptor defects with level position close to the valence band of ZnO.…”