Critical Impact of Hole Transporting Layers and Back Electrode on the Stability of Flexible Organic Photovoltaic Module

There is a huge market potential for organic photovoltaics due to their attractive features which has led to considerable industrial efforts for their commercial production. However, the limitation factor to this commercialization is their stability and durability. Various electrode materials have been used as hole transporting layers in the organic photovoltaics which include poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate), molybdenum trioxide, as well as silver deposition. These electrodes have a profound effect on the stability of the organic photovoltaics depending on their materials and properties and therefore worth analyzing their degradation mechanism.

In a recent paper published in Advanced Energy Materials, researchers led by professor Hyoung Jin Son studied the impact of back electrodes and hole transporting layers on the stability of inverted organic photovoltaic modules encapsulated by flexible barriers, and have suggested 2 kinds of organic photovoltaic cell structures.

Organic photovoltaic modules are made up of organic cells that are laterally arranged and enclosed between films of poly(ethylene terephthalate). Ageing tests on modules were conducted under accelerated conditions, and an analysis on the interlayer modification by acceleration factors was used to examine the degradation mechanism in these organic photovoltaic modules.

From a comparison between the parameters of printed and evaporated silver electrodes with the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate), it was seen that the former exhibited a lower open-circuit voltage and short-circuit current. This difference is deduced to be as a result of factors such as the deposition method, process environment, and presence of additives, which may have induced severe degradation in the screen-printed silver electrodes. The major source of degradation in screen-printed silver electrode devices is the organic solvents in silver inks, which are of nanoparticle form and which are a probable channel for the diffusion of these solvents.

For the device with molybdenum trioxide, the open-circuit voltage was higher as compared with that having a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) layer, which is probably due to the high quality of the interface between molybdenum trioxide and the active layer making it a stable hole transporting material. The poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) degradation is related to the properties of intrinsic hygroscopic materials which impedes current flow.

The degradation of modules of organic photovoltaic devices in damp heat conditions, is majorly due to reduction of the short-circuit current. Although devices with evaporated silver electrodes exhibit a prolonged lifetime as compared with the ones screen-printed, the short-circuit current shows unstable features which is due to the poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) hygroscopic properties. In comparison, molybdenum trioxide based devices exhibited high stability under the exposure of light which indicates the good functionality of its other layers.

From spectroscopy analysis, the authors noted diminished sulphur peaks of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) in the devices having screen-printed silver electrodes, which shifted towards higher binding energy as compared with evaporated silver electrodes. The ratio of poly(styrenesulfonate) to poly(3,4-ethylenedioxythiophene) in evaporated silver electrode devices, was approximately 2.5 as compared with 0.5 for the screen-printed silver electrode devices. Further analysis showed that chemical changes do not occur with a 2.5 poly(styrenesulfonate) ratio, and are not affected by heat, moisture and active layers. Also, the photostability of the used commercial poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) device was enhanced, with a substantial stabilizing effect on the open-circuit voltage, clearly indicating a pathway for stability improvement.