Addressing the light-soaking issue in inverted organic solar cells using chemical bath deposited fluorinated TiO_x electron transport layer

Abstract

The device lifetime of an inverted organic solar cell (IOSC) is significantly better compared to standard-architecture OSC under ambient conditions. However, various studies have shown that when an n-type oxide is used as a selective electron transport layer (ETL) in the IOSC, a reversible light-soaking treatment is required. This reversible treatment largely hampers the practicality of the device, especially in outdoor applications, in which the light-soaking time may take hours every morning. In this work, fluorinated TiOx (F-TiOx), prepared by low-temperature solution-processed chemical bath deposition technique, was used as the ETL to significantly reduce the light-soaking time for a P3HT:PCBM based IOSC. Without affecting the device efficiency, more than ten-fold reduction in light-soaking time was observed for fluorinated TiOx (F-TiOx) when compared with conventional sol-gel TiOx. Ultraviolet photoelectron spectroscopy (UPS) and UV photoconductivity measurements were used to understand the light-soaking time reducing mechanism. From the perspective of ITO/TiOx interface, shift in work function was observed in F-TiOx due to the partial filling of its defective sites by fluorine atoms. Consequently, this process reduces its intrinsic trap state density compared to sol-gel TiOx even before the light-soaking treatment. As a result, the trap filling action can be completed in a shorter time upon illumination, and thus significantly reduce the duration of the necessary light-soaking

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

“…[32][33][34][35][36][37] Some of these trap states, close to the ITO/ZnO, could be filled up during the light-soaking illumination. 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.…”

“…In both cases, the fabricated solar cells need to be exposed to light for a certain time to achieve their maximum photovoltaic properties. 19 This phenomenon, known as a light soaking (LS), occurs in the as-prepared solar cells, which exhibit low short-circuit current (J sc ) and a kink shape (or S-shape) of I-V curve and thus low fill factor (FF). 20,21 There are two main mechanisms behind the light-soaking effect.…”

“…In the first case, the filling of trap states upon illumination decreases the work function of the ETL, which reduces the potential barrier and thus improves the extraction of electron through the ITO/metal-oxide interface. 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. 19,26 Improvement of the stability of OSCs is crucial for their industrial applications, thus the proper methods to determine the stability that takes light-soaking effect into account should be considered. Stability testing International Summit on OPV Stability (ISOS) protocols for OSCs proposed by the ISOS committee describe both indoor and outdoor procedures for testing the solar cells.…”