Mechanistic Investigation into the Light Soaking Effect Observed in Inverted Polymer Solar Cells Containing Chemical Bath Deposited Titanium Oxide

Abstract

In the glass-indium tin oxide (ITO) / titanium oxide (TiO x ) / regioregular poly(3-hexylthiophene) (P3HT) : [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) / poly(3,4-ethylenedioxylenethiophene) : poly(4-styrene sulfonic acid) (PEDOT:PSS) / Au cell (TiO x cell), which contains amorphous titanium oxide prepared by chemical bath deposition and dried at 150 °C, a light soaking effect has been observed upon irradiation with white light.In contrast, in ITO / titanium oxide (TiO 2 ) / P3HT:PCBM / PEDOT:PSS / Au cell (TiO 2 cell), which contains anatase titanium oxide prepared by heat treatment at 450 °C, the maximum power conversion efficiency was obtained just after irradiation with white light. The number of P3HT + cation radicals in the quartz-ITO/TiO x and TiO 2 /P3HT:PCBM substrates was estimated by ESR measurements at room temperature upon irradiation with white light. They increased gradually with an increase in irradiation time for the TiO x substrate, but increased only slightly just after light irradiation for the TiO 2 substrate. Upon irradiation with UV-cut light, the performance of the TiO x cell was inferior to that of the TiO 2 cell. This could be related to the resistances of the P3HT:PCBM layers which were estimated by alternating-current impedance spectroscopy. The resistance of the P3HT:PCBM layer in the TiO x cell was much larger than that in the TiO 2 cell, though the difference between the two cells was merely heat treatment temperature of titanium oxide films using as electron collection layers. That is, the concentration of photocarriers in the P3HT:PCBM of the TiO x 2 cell was significantly less than that in the P3HT:PCBM of the TiO 2 cell. From these experimental results, the light soaking effect could be reasonably explained by assuming the existence of charge recombination centers in the TiO x near the TiO x /P3HT:PCBM interface.

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

“…[3][4][5][6][7] One of the fundamental issues is the light-soaking phenomenon with respect to UV-light irradiation, which has been commonly observed in IOSCs with electron-collecting layers such as TiO x . 2,[8][9][10][11][12][13][14][15] Although high-temperature annealing treatments on the TiO x layer can prevent the phenomenon, such treatments are undesirable for low-temperature device fabrication using flexible plastic substrates. 15 Additionally, long-term UV-light irradiation on IOSCs can cause device degradation.…”

Significance of light-soaking effect in proper analysis of degradation dynamics of organic solar cells

“…19,22 On the other hand, the trap states, close to the ZnO/P3HT:PCBM, could change the induced interfacial dipoles. 23,24 Presented data do not consider which interface (ITO/ZnO or ZnO/P3HT:PCBM) plays the key role in the lightsoaking effect but highlight the need for taking into account the LS effect in the analysis. The impact of light-soaking effect is increasing in time and it should be considered for proper and reliable measurements and stability analysis.…”

“…19,22 In the second case, the key role in the light-soaking effect plays the interfacial dipole between the metal-oxide/organic interface. [23][24][25] Sometimes the devices should be stimulated under solar simulator for at least few minutes (∼10 min) at the 1000-W∕m 2 illumination intensity, for saturation of their photovoltaic properties. For the outdoor application, this illumination time could be equal to 1 to 2 h per day due to much lower solar irradiation intensity each morning.…”

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

“…However, such the increasing process is poorly understood, although several mechanisms have been suggested. [11][12][13][14][15] We previously have found that there are two kinds of light soaking effects such as a short-circuit current density (J sc ) dependent type [16][17][18] and an open-circuit voltage (V oc ) dependent type 19 as shown in Fig. 1.…”

“…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.…”