Understanding Degradation Pathways in Organic Photovoltaics 

Enhanced Environmental Stability via Contact Selection

Matthew T. Lloyd,1 Dana C. Olson,1 Andres Garcia,1 Isaac Kauvar,2 Nikos Kopidakis,1 Matthew O. Reese,1 Joseph J. Berry,1 David S. Ginley1

1 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, CO 804012 Materials Science and Engineering, Stanford University, Stanford, CA 94305

Presented at the 2011 Photovoltaic Module Reliability Workshop, February 16-17, 2011,  Golden, Colorado

O i h t lt i (OPV )

Routes of Ingress for Devices Under OperationUsing a combinatorial degradation system we areable to compare the relative performance of 360Organic photovoltaics (OPVs)

require an offset in the workfunction of the anode and cathodein order to efficiently extractcharge from the active layer.Conventional devices rely on lowwork function contacts, such asLiF/Al or Ca/Al, the chemicalreactivity of which precludesfabrication or operation outside ofan inert atmosphere glovebox. Byemploying ZnO as the electronaccepting contact and usingPEDOT:PSS to collect holes, therelative stability of these materials

bl t d d ti i iCombinatorial degradation system designed 

d f b i d b M h R

able to compare the relative performance of 360diodes subjected to constant illumination. By varyingthe aspect ratio of the device active area we areable to observe an increase in device lifetime as therelative fraction for “unprotected edge” is decreased.For these small pixels, edge-in diffusion representsa significant route for degradation of the active layer.Generally, we find that the top contact acts asbarrier to prevent oxygen and water vapor transportthrough the contact. Hence, most aged devices arephotobleached around the active areas, but there islittle color loss under the electrode.

enables extended operation in airwithout an encapsulation layer.

and fabricated by Matthew Reese.

Lloyd et al., J. Mater. Chem., 19, 7638 (2009).

Impact of Oxygen and Moisture on the Active Layer

Device Lifetime Primarily Determined by Top Contact 

Contrary to conventionalwisdom, in an inertatmosphere the electronic

Inert atmosphere

We find that the thickness of the top contact is the first-order, rate limiting factor in determining device lifetime.The average (red), standard deviation (dark grey), andmax/min (light grey) behavior are shown (left) for deviceswith 50, 100, 200, and 400 nm electrodes. Reflectancemaps indicate that the thinner electrode eventuallybecome susceptible to oxygen or water vapor transport,indicated by loss of chromophores within the active layer.Tapping mode AFM images suggest that in addition tooffering shorter diffusion lengths, thinner top contacts alsodisplay a greater density of grain boundaries that assist in

50 nm  Ag electrodes

100 nm Ag electrodes

atmosphere the electronicand optical properties of thepolymer-fullerene blendprove to be inherentlystable. After 1000 hrsillumination in an inertatmosphere there is is nochange in thephotoconductivity or theUV-Vis absorption spectrumfor bare films ofP3HT:PCBM on glass (i.e.not functional devices).Outside of the glovebox,however, there is anappreciable loss in light

Ambient atmosphere

the transport of oxygen and water vapor.200 nm Ag electrodes

400 nm Ag electrodes

Reese et al. , Adv. Funct. Mater., 20, 3476 (2010).

Nucleation and Island Growth Suggests Pinhole Ingressappreciable loss in light-absorbing chromophores inpristine P3HT. Whenblended with PCBM,photobleaching of the activelayer system can bemitigated significantly.

The degree of stabilization imparted to P3HT by theelectron acceptor is intimately related to the D/A

Photocurrent mapping measurements displayregions of zero photocurrent, which nucleate andgrow radially as the device ages. This is stronglyindicative of point source ingress followed bylateral diffusion into the active layer. Thismechanism is also consistent with the dramaticincrease in device lifetime when thicker contactsare used. If pinhole ingress can be arrested byfilling or creating a tortuous path for diffusion, oneshould be able to see additional gains in devicelifetime. We show that Ag/Al compositeelectrodes although only 200 nm in combined

Lloyd, et al., Sol. Energy Mater. Sol. Cells (2011), doi:10.1016/j.solmat.2010.12.036

p ymorphology, exciton lifetime, and the energy levels thatdrive charge transfer. Preliminary data indicate that asmaller the LUMO-LUMO offset results in a higher rateof photobleaching in P3HT. We have also observed thistrend in D/A systems where the LUMO level of theacceptor can be systematically tuned via molecularfunctionalization.

electrodes, although only 200 nm in combinedthickness, extend the lifetime of these devices to~5000 hrs.

Conclusions

This work was funded through the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory DOE SETP program through the National Center for Photovoltaics. This poster does not contain any proprietary or confidential information. NREL is a national laboratory of the U.S. Department of Energy, Office of EnergyEfficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. • NREL/PO-5200-50739

• Degradation in conventional devices stems from the reactivity of low work function metals, not the active organic layers• In larger area devices, electrode thickness dependence studies indicate gas permeability in thin metal electrodes. Photocurrent mapping measurements show nucleation and growth of dark spots in the photocurrent, indicating pinhole ingress of oxygen and water vapor. • Multilayer electrodes yield surprisingly high stability demonstrating unecapsulated lifetimes of 5000 hours.

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