Direct observation of UV-induced charge accumulation in inverted-type polymer solar cells with a TiOx layer: Microscopic elucidation of the light-soaking phenomenon

Abstract

The mechanism of light-soaking phenomenon in inverted-type organic solar cells (IOSCs) with a structure of indium-tin-oxide/TiOx/P3HT:PCBM/Au was studied by electron spin resonance (ESR) spectroscopy. Charge accumulation in the cell during UV-light irradiation was observed using ESR, which was clearly correlated with the light-soaking phenomenon. The origin of the charge accumulation is clarified as holes that are deeply trapped at p-type P3HT polymer-chain ends with bromine after hole transfer from the band excitation in the TiOx layer. The holes are considered to be electrostatically attracted to trapped electrons in the TiOx layer after the band excitation. These accumulated charges are the origin of the light-soaking phenomenon. Our results strongly suggest that passivation of the residual OH groups in the TiOx layer is needed to avoid the light-soaking phenomenon by preventing electron trappings, a step that is indispensable in the operation of highly stable IOSCs without UV-light irradiation based on a low-cost and low-temperature device fabrication process using flexible plastic substrates.

Direct Observation of Radical States and the Correlation with Performance Degradation in Organic Light‐Emitting Diodes During Device Operation

“…To attain a high signal‐to‐noise (SN) ratio of the ESR signal by increasing the active area of the device, we utilized a rectangular device structure (3 × 20 mm 2 ) in an ESR sample tube with an inner diameter of 3.5 mm and a length of 270 mm . Figure shows the device structure of indium‐tin‐oxide (ITO) (150 nm)/NPB (60 nm)/Alq 3 (60 nm)/LiF (0.5 nm)/Al (100 nm) used in this study.…”

“…In addition, halogens are generally used for the NPB synthesis and may possibly remain in NPB molecules. Actually, it has been reported that the g factor of the ESR signal of a conducting polymer poly(3‐hexylthiophene) (P3HT) becomes larger by residual bromines at polymer‐chain ends of P3HT . To examine the possibility of residual halogens, we performed the quantitative analysis of the content of halogens in the NPB material used in this study.…”

Investigation of Charge Accumulation States in Polymer Solar Cells using Light-Induced Electron Spin Resonance Spectroscopy

“…Also, an intrinsic reversible deterioration mechanism during device operation has been discussed for a polymer solar cell with blend films of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) [16,17]. The deterioration has been ascribed to an accumulation of photogenerated charges during device operation [16][17][18][19][20].…”

“…This ESR method has clarified the charge formation in pentacene/C 60 layered thin-film solar cells during device fabrication [25,26]. For the P3HT:PCBM polymer solar cells, the molecules with the charge accumulation during device operation have been identified as P3HT using the ESR method [18][19][20]. However, the detailed charge accumulation states in P3HT have not yet been completely clarified.…”

An Increase in Open-circuit Photo-voltage of Inverted Polymer Solar Cells by Pre-treatment Applying Forward Bias Under UV-cut Light Irradiation

“…We had also investigated the mechanism of the both lightsoaking effect observed in the polymer solar cells using both ac impedance spectroscopic (IS) and electron spin resonance (ESR) measurements. 17,18,20 We have found that these light soaking effects are caused by favorable charge accumulations for suppressing the recombination and disappearing the charge-injection barrier between the electron collection electrode and the organic active layer. The expression of the effects needs to excite the electron collection layer such as n-type semi-conductive metal oxide by UV-light irradiation.…”