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Electrodes

Dechan Angmo Michael Corazza

Metal electrode degradation

Generally, metals form at-least one of the electrodes in an organic photovoltaic cell irrespective of the architecture. In normal or conventional architecture, indium-tin-oxide (ITO) or other transparent conductors forms the electron-collecting electrode (cathode) and a low work function metal, mostly aluminum, forms the hole-collecting electrode (anode). In an inverted architecture, ITO or other transparent conductive oxides from the anode while a high work function metal such as Ag forms the cathode. In several ITO-free architectures, metals form both the cathode and the anode. Such metals that have been applied to OPV include copperDOI:10.1002/aenm.201300737, Chromium-aluminumDOI:10.1016/j.solmat.2008.12.022, titanium-aluminumDOI:10.1016/j.tsf.2005.06.006, Nickel-copperDOI:10.1039/C4NR01024H, etc. in both inverted and normal architectures. Here, we will mainly focus on aluminum which is mostly used in normal devices and compare it to inverted devices that employ Ag.

In normal architecture, the use of aluminum is a stability limiting factor. Aluminum is a low work-function metal (4.1 eV) which also means that it can be easily oxidized. Aluminum has an oxidation state of +3. Upon exposure of water or oxygen, it readily converts into aluminum oxide (Al2O3) which is an insulator. Hence, the photovoltaic performance of a normal architecture device with aluminum electrodes rapidly decay upon exposure to ambient conditions and the decay is observed in all photovoltaic parameters: JSC, VOC, and FF. Figure 1 demonstrates the degradation trend of the normal device when stored under ambient storage conditions (stored in a drawer or a laboratory cupboard for example).

Figure 1: Unencapsulated devices in conventional (normal) and inverted architecture stored under ambient conditions. From ref.DOI:10.1063/1.2945281.

Many studies have delved into understanding the degradation mechanisms in OPVs which has shed light into the role of aluminum in degradation. In a series of studiesDOI:10.1002/pip.794DOI:10.1016/j.solmat.2005.03.004DOI:10.1021/am800039w, normal devices were exposed to isotopically labelled water ($H_2^{18}O$) and oxygen ($^{18}O_2$) and their diffusion throughout the bulk of the active layer and into all the interfaces were recorded using the chemical probing method TOF SIMS. It was found that water diffuses into the device via the aluminum grains and leads to degradation/accumulation at all the interfaces, while oxygen diffuses mainly through the pin-holes in the aluminum film causing localized or inhomogeneous degradation. Not only will this lead to oxidation of aluminum interface with the organic material but it propagates degradation agents throughout the bulk of the photoactive layer. Further, it was found that aluminum species could diffuse into the device and can form organo-aluminum species with the active materials. Thus, when the aluminum species oxidizes, it can also lead to deterioration in the photovoltaic performance of the devices even when the active polymer is resistant against oxidationDOI:10.1002/pip.794.

Other studies have shown that PEDOT:PSS as a HTL buffer layer in the normal device stack significantly accelerates degradation of the aluminum/active interface due to hygroscopic nature of PEDOT:PSS which enhances water diffusion in the device causing oxidation of Al at the aluminum/active interfaceDOI:10.1002/aenm.201300737. Hence, by replacing the HTL to more stable materials such as MoO3 will lower the rate of degradation at the aluminum/metal interface. The use of a more stable interfacial buffer layer between aluminum and the active material such as chromium may also alleviate the degradation of normal architecturesDOI:10.1021/am100541d.

One method to achieve a device with higher stability is of course to eliminate the use of aluminum altogether. This is highly desired not only from a stability point of view but also from a processing point of view. Aluminum requires energy intensive processing such as vacuum and often glove box processing. An inverted architecture allows the avoidance of aluminum by incorporating a much stable albeit expensive material such as silver or gold. Since silver and gold are noble metals, they are resistive to oxidation. This means that the inverted architecture is significantly more stable compared to normal devices. One such comparison can be seen in Figure 1. Some studies have revealed that silver migration over time can also causes deterioration in device performance due to creating of local shorts in the deviceDOI:10.1016/j.solmat.2013.05.013.

Another method to enhance the stability is to of course prevent the device from the degradation agents such as water, oxygen, and UV light. Good encapsulation can therefore impart superior stability to a normal device. In a study, various types of devices in normal and inverted architectures were compared for stability. One type of device, labelled IAPP, is a normal device encapsulated with glass slides, getter material, and epoxy while the remaining are encapsulated without a getter material. These sealed devices were studied for stability under rigorous ageing conditions. The IAPP devices were found to be exceptionally stable among all the normal and inverted devices which was mainly attributed to the encapsulation of the IAPP devices (Figure 2). Only when the sealing was broken did the device start to degradeDOI:10.1039/C1RA00686J.

Figure 2: Evolution of several difference devices over time under constant 1 sun illumination (1000 W m-2, AM1.5G , 85 ± 5 °C). Stability of IAPP device, although in normal architecture with aluminum electrode, is attributed to its superior encapsulation.DOI:10.1039/C1RA00686J.

Although sealing/encapsulation takes precedence in comparison to the inherent stability of the architecture in defining the long-term stability of OPV cells, a compromise has to be found with regard to cost of sealing methods versus its benefit in enhancing the lifetime. This is generally a question of life-cycle and cost analyses. In short, costly encapsulation is only warranted if its cost is profitably recovered with the energy generated over lifetime of the solar cells.

ITO degradation

The most common material used as a transparent electrode is Indium Tin Oxide (ITO). ITO due to its good combination of high conductivity and high transparency to light is a perfect candidate for the task. However, it has a high cost and indium is in limited supply. ITO is used not only in OPV, but also in inorganic solar cells; it is typically deposited with sputtering.

Degradation of a solar cell with the structure: ITO/pentacene/C60/bathocuproine(BCP)/Al was studied by Sullivan and JonesDOI:10.1016/j.orgel.2010.09.014. They observed a fast degradation (within 70 minutes) of the cell starting with a kink in the IV curve. The given explanation was a photo-oxidation of the pentacene at the ITO interface that could, however, be delayed by the appliance of a UV filter. Another study by Schäfer showed a reduction of the VOC in an organic solar cell, which was caused by UV irradiation of ozone treated ITO; the studied claimed that the lowering of the VOC was caused by a change of the ITO work-functionDOI:10.1103/PhysRevB.83.165311. Using buffer layers between ITO and the active layer, such as MoO3 and Cs2CO3, it was possible to prevent or slow down such degradations.

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