An Effective Area Approach to Model Lateral Degradation in Organic Solar Cells

Abstract

tooxidation of the poly mer, giving rise to absorption loss and to oxygen doping. [9][10][11] Water ingress can also strongly affect the electrodes and lead to their oxidation or to other chemical reactions. [12][13][14] In solar cells with classical geo metry, degradation often takes place at the organic/cathode interface, [ 15 ] where it has been shown that low-workfunction metals are most susceptible to oxidation. This degradation process can be prevented effectively by either using an inverted geometry [ 16 ] or by replacing the hygroscopic poly(3,4-ethylenedioxythiophene):polysty rene sulfonate (PEDOT:PSS) with alternative hole transport materials (HTM).In a recent study Voroshazi et al. [ 12 ] investigated the process of cathode oxidation in more detail and revealed an important interaction between the lowworkfunction cathode and the widely used PEDOT:PSS hole transport layer (HTL). It was observed that the hygroscopic nature of PEDOT:PSS causes a water ingress from the edges of the device toward the central part and therefore accelerates the oxidation of the Al cathode signifi cantly. The oxidized organic/electrode interface hinders charge carrier extraction and can be observed as a loss in J sc . This mechanism was confi rmed recently by a study of Feron et al. [ 17 ] who used laser beam induced current (LBIC) to spatially resolve the degradation process caused by water ingress either through pinholes or device edges.In order to show that the water-related degradation is strongly promoted by the PEDOT:PSS layer we replaced this layer with other nonhygroscopic hole transport materials based on solution-processed metal-oxides. Furthermore we investigated the stability of the unencapsulated devices under different stress conditions.In order to assess the physical process of degradation in these devices, we investigated them with a multitude of electrical steady-state, transient and impedance characterization techniques, and also employed drift-diffusion modeling with Setfos for deeper insights into the cell operation. [ 24,52 ] The combination of techniques not only allows for a more reliable parameter extraction, [ 18 ] but also helps to identify the governing physical processes by their specifi c signatures in the various techniques. Thus, even by restricting to nondestructive optoelectrical techniques, the degradation mechanisms can be monitored, as will be shown below.In standard unencapsulated poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester solar cells exposed to humid air, the oxidation of the aluminum cathode is known to be a key degradation mechanism. Water that enters the device at the edges and through pinholes diffuses to the organicelectrode interface. The forming oxide acts as a thin insulating layer that gives rise to an injection/extraction barrier and leads to a loss in the device current. In order to understand this behavior in detail various steady-state, transient, and impedance measurement techniques are performed in combination with drift-diffusion simulations. With thi...

Impact of P3HT materials properties and layer architecture on OPV device stability

“…Qualitative comparison of nominally identical devices, and between fresh and aged cells may contribute to the understanding of the underlying degradation processes. Current-voltage (IV), charge extraction by linearly increasing voltage (CELIV), transient photocurrent (TPC), transient photovoltage (TPV), double injection transient (DoIT), impedance spectroscopy, capacitancevoltage curves (C-V), and further optoelectrical characterization methods were performed at Fluxim AG using their All-in-one characterization platform Paios for Solar Cells 73,74 . For this purpose, Fluxim received a batch of fresh devices as well as aged cells from the L2 and O1 tests.…”

Toward Environmentally Robust Organic Electronics: Approaches and Applications

“…After exposure to water, Al electrodes were found to exhibit pinholes, which led to the penetration of more water . Second, an insulating metal‐oxide layer can form at the active‐layer/electrode interface as a result of water penetration, resulting in a reduction of the effective active interface area and in inefficient charge extraction . Finally, the position of the fullerene acceptor in the well‐studied polymer solar cell blend system P3HT:[6,6]‐phenyl‐C 61 ‐butyric acid methyl ester (PCBM) is rearranged by the introduction of water into the active layer, inducing further aggregation of the fullerene.…”

The use of charge extraction by linearly increasing voltage in polar organic light-emitting diodes

“…30,40 Since in the MIS-regime of the polar device only holes are present, the CELIV transients can be fitted with the hole mobility as the only fitting parameter. Using the drift-diffusion simulation software Setfos, we obtain a value of 1.2 Â 10 À4 cm 2 /Vs, in very good agreement with the measurement.…”

“…37 The electrical transient and impedance measurements reported here have been performed by the all-in-one characterization platform Paios developed and commercialized by Fluxim AG, Switzerland. 39 Drift-diffusion simulations to calculate the CELIV currents and charge density profiles have been generated using the commercial tool Setfos 4.4 by Fluxim AG, 40 which was already successfully used to characterize OSCs 23,30,41 and OLEDs. 38,42 Hereby, the macroscopic polarization was taken into account as demonstrated in our previous publication.…”

“…28 The most general modelling approach for the bipolar case still is the dual-carrier drift-diffusion model, ideally coupled with a fitting routine, which in the end allows for a real quantitative extraction of both charge carrier mobilities. 23,29,30 II. APPROACH So far, it seemed impossible to use fully operating bipolar devices for a valid CELIV analysis.…”