In this study, we present novel insights into the light-soaking effect of inverted polymer solar cells (PSCs), where the open-circuit voltage ( V oc ) of the cells improves over time under light irradiation. The effect was investigated by electron spin resonance (ESR) studies of bare indium tin oxide (ITO) and piperazine derivative-modified ITO/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C 61 butyric acid methyl ester (PCBM) substrates. These results were combined with alternating current impedance spectroscopy (IS) measurements of inverted PSCs based on the above substrates. In ESR experiments with the substrates under white light irradiation, with a UV light component, many P3HT •+ radical cations were observed in the bare-ITO/P3HT:PCBM substrate. The number of radical cations was considerably suppressed in the ITO/P3HT:PCBM substrates with ITO modified by piperazine derivatives. This is because adsorbed oxygen molecules on the ITO acted as acceptor dopants for photoexcited P3HT, and the amount of adsorbed oxygen was decreased by modifying the ITO with piperazine derivatives. In IS measurements of the inverted PSCs under white light irradiation, a decrease in the electric capacitance (CPE2) of an electric double layer formed at the ITO/P3HT:PCBM interface was observed. A strong correlation was observed between the decrease of CPE2 and the increase of V oc . From these results, the light-soaking behavior was attributed to the removal of an electron injection barrier formed between ITO and PCBM, under white light irradiation.

We present a novel insight on the light-soaking effect, in which the open-circuit voltage (V oc ) of inverted polymer solar cells improves with increase in light irradiation time. We previously have found that there are two kinds of the light soaking effects such as a short-circuit current density dependent type and a V oc dependent type. The expression of these effects needed to irradiate the light containing UV component. However, for 4-(triphenylphosphonio)butane-1-sulfonate (TPPBS)-modified indium tin oxide (ITO)/poly(3-hexylthiophene-2,5-diyl) (P3HT): [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS)/Au inverted cell, we first observed an increase in the V oc by irradiating white light not containing UV component. After pre-treatment by applying forward bias under UV-cut light, the V oc was improved even when the performance was evaluated by irradiating UV-cut light, that is, the V oc values of 0.36, 0.42, 0.46 and 0.54 V were obtained by holding at forward voltages of 0, 0.3, 0.6, and 1.0 V, respectively. We discussed about this pretreatment effect from the viewpoint of desorption of oxygen molecules adsorbing on ITO.

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Objective lenses are frequently employed in state-of-the-art ultrafast time-resolved microscopy techniques. While it enables tight spatial focusing, its dispersion causes a longer optical pulse duration. Since an objective lens is a combination of different lens materials, finding its correct dispersion can be challenging. In this paper, we propose a dispersion measurement method for an objective lens using white light interferometry. Our proposed method enables the experimental determination of the lens dispersion and improves the temporal resolution of ultrafast time-resolved microscopy techniques.

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