Abstract
We present measurements of the outdoor stability of PCDTBT:PC71BM based bulk heterojunction organic solar cells for over the course of a year. We find that the devices undergo a burn-in process lasting 450 hours followed by a TS80 lifetime of up to 6200 hours. We conclude that in the most stable devices, the observed TS80 lifetime is limited by thermally-induced stress between the device layers, as well as materials degradation as a result of edge-ingress of water or moisture through the encapsulation.
Highlights
The past decade has witnessed a steady growth in the efficiency of organic photovoltaics (OPVs), with power conversion efficiencies (PCEs) currently exceeding 10%1–4; a value regarded as a significant step in the so-called 10/10 target for organic photovoltaics (10% efficiency and 10 years lifetime)
We find that when tested outdoors, PCDTBT:PC71BM devices undergo a burn-in stage characterized by an initial rapid loss in efficiency, followed by a period of slower, linear degradation - a result reported in laboratory testing[37]
Our study demonstrates that PCDTBT based OPVs have promising stability when tested under real-world conditions, this currently appears to be limited by the efficiency of our encapsulation materials and techniques with further refinements in testing protocols being required
Summary
The devices explored in this work were based on an ITO/HTL/Active layer/Ca/Al heterostructure as shown in Fig. 1(b), with each device containing 6 pixels. It can be seen that all the devices undergo a burn-in process during which the Voc and FF degrade rapidly, with the Jsc undergoing a relatively smaller reduction Such phenomena have been observed when PCDTBT:PC71BM OPVs have been aged under laboratory conditions[34]. We find that in summer, the temperature inside the chamber can change at a rate of 10°C/hour, reaching a maximum temperature of 75°C; a process that we believe causes thermally induced stress between the different layers in the device resulting in additional path-ways for the diffusion of oxygen or moisture into the film This may point to a thermal or UV-assisted break-down of the encapsulation-epoxy which results in enhanced photo-oxidation of the active organic layer. Enhanced device stability under real-world operation is expected through the use of more sophisticated encapsulation techniques, together with the use of UV400 filtration
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