Investigating electrodes degradation in organic photovoltaics through reverse engineering under accelerated humidity lifetime conditions

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...

Section: Discussionmentioning

confidence: 65%

mentioning

confidence: 99%

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

Züfle

Neukom

Altazin

et al. 2015

“…In contrast, PC As previously suggested Al metal is highly unstable and its oxidation is a primary reason for the degradation of several optoelectronic devices. [ 43 ] In addition, reports suggesting interaction of the perovskite iodide with the Al metal are another degradation mechanism of perovskite-based solar cells. [ 44 ] Since a thick AZO layer is in between the perovskite/PCBM layers and the Al we suggest that the delay in degradation of the p-i-n device when AZO is used could potentially be attributed to a delay in interaction between the perovskite and the top metal.…”

Section: Resultsmentioning

confidence: 99%

Improved Performance and Reliability of p‐i‐n Perovskite Solar Cells via Doped Metal Oxides

Savva

Burgués‐Ceballos

Choulis

2016

Self Cite

Perovskite photovoltaics (PVs) have attracted attention because of their excellent power conversion efficiency (PCE). Critical issues related to large‐area PV performance, reliability, and lifetime need to be addressed. Here, it is shown that doped metal oxides can provide ideal electron selectivity, improved reliability, and stability for perovskite PVs. This study reports p‐i‐n perovskite PVs with device areas ranging from 0.09 cm2 to 0.5 cm2 incorporating a thick aluminum‐doped zinc oxide (AZO) electron selective contact with hysteresis‐free PCE of over 13% and high fill factor values in the range of 80%. AZO provides suitable energy levels for carrier selectivity, neutralizes the presence of pinholes, and provides intimate interfaces. Devices using AZO exhibit an average PCE increase of over 20% compared with the devices without AZO and maintain the high PCE for the larger area devices reported. Furthermore, the device stability of p‐i‐n perovskite solar cells under the ISOS‐D‐1 is enhanced when AZO is used, and maintains 100% of the initial PCE for over 1000 h of exposure when AZO/Au is used as the top electrode. The results indicate the importance of doped metal oxides as carrier selective contacts to achieve reliable and high‐performance long‐lived large‐area perovskite solar cells.

“…[32][33][34][35][36] Especially in the case when there is no intermediate layer between the cathode and the photoactive layer (PAL), chemical modifi cations at the interface can further deteriorate the PAL and accelerate aging. [33][34][35] Conversely, Ag and Au are quite stable when exposed to the environment and appear to be less reactive towards organic materials. This is why it is commonly believed that inverted structures are inherently more stable compared to normal structures.…”

Section: Device Geometry: Normal Versus Invertedmentioning

confidence: 99%

Lifetime of Organic Photovoltaics: Status and Predictions

Gevorgyan

Madsen

Roth

et al. 2015

137

124

performance level, especially when considering large-scale mass-produced modules, is predicted to be signifi cantly lower. [ 8 ] Nevertheless, based on recent advances in the upscaling of OPV manufacture [9][10][11][12] it is reasonable to assume that the mass production of modules with an effi ciency of more than 5% is an achievable target today. Durability of OPV is however yet to be proven and demonstrated.Due to core architectural differences between OPV and inorganic technologies [ 13 ] the established testing standards of the latter are not applicable to the former. [ 1 ] Hence, for many years the stability testing of OPV has primarily been based on customized procedures that vary from one laboratory to another, generating incomparable data. [ 14 ] Moreover, due to the complexity of the OPV device architectures [ 15 ] and a multitude of aging mechanisms taking place at the same time [ 16 ] the aging curve of OPV often takes a complex shape, which is diffi cult to model, [ 14,17 ] thus making the identifi cation of a practical operational lifetime diffi cult or impossible. [ 18 ] As a result, even with a multitude of reported review articles discussing OPV stability [ 16,[19][20][21][22][23][24] at hand, it is still challenging to identify where the technology stands in terms of lifetime. To address this it is necessary to create a yardstick -a generic marker that allows accurate rating and comparison of the lifetime for OPV, and thus enabling the gauging of progress over time. An additional complication that has arisen due to the signifi cant improvements in OPV stability and durability in recent years is that the determination of OPV lifetimes has become an impractically long process, and establishing markers for both accelerated and real operational test conditions (if successful) would allow developing a prediction tool that could speed up the stability testing process and tackle this issue.The groundwork towards creating such a marker was laid in 2011 at the International Summit on OPV Stability (ISOS) where the ISOS testing guidelines were decided upon and described for OPV technologies. [ 18 ] These guidelines were primarily aimed at harmonizing testing procedures among different laboratories by offering a set of indoor and outdoor tests with controlled conditions. This has helped to reduce variations in the reported results, making the aging studies of different laboratories more comparable. [ 25 ] While the ISOS tests harmonized the test conditions, the questions of how to generically determine the lifetime from aging curves of diverse shapes, and how to build a technique for predicting the lifetime based on accelerated tests remained.The results of a meta-analysis conducted on organic photovoltaics (OPV) lifetime data reported in the literature is presented through the compilation of an extensive OPV lifetime database based on a large number of articles, followed by analysis of the large body of data. We fully reveal the progress of reported OPV lifetimes. Furthermore, a generic lifetime marker has...