ABSTRACT
Understanding the degradation mechanisms in organic photovoltaics is crucial in order to develop stable organic semiconductors and robust device architectures. The rapid loss of efficiency, referred to as burn-in, is a major issue to be addressed. This study reports on the influence of the electron transport layer (ETLs) and UV light on the drop of open-circuit voltage (Voc) for P3HT:PC60BM-based devices. The results show that Voc loss is induced by the UV and, more importantly, that the ETL can amplify it, with TiOx yielding a stronger drop than ZnO. Using impedance spectroscopy (IS) and X-ray photoelectron spectroscopy (XPS), different degradation mechanisms were identified according to whether the ETL is TiOx or ZnO. For TiOx-based devices, the formation of an interface dipole was identified, resulting in a loss of the flat-band potential (Vfb) and, thus, of the Voc. For ZnO-based devices, chemical modifications of the metal oxide and active layer at the interface were detected, resulting in a doping of the active layer which impacts the Voc. This study highlights the role of the architecture and, more specifically, of the ETL in the severity of burn-in and degradation pathways.
ABSTRACT
The impact of side-chain variations on the photothermal stability of solar cells containing poly(benzodithiophene-diketopyrrolopyrrole) polymers are investigated in the absence of oxygen. Four different side-chains of benzodithiophene (BDT) are synthesized and copolymerized with diketopyrrolopyrrole (DPP) by Stille polymerization. The photothermal stability is measured as active layer blends with phenyl-C61-butyric acid methyl ester (PCBM) in encapsulated inverted photovoltaic cell architecture with zinc oxide and PEDOT: PSS as transport layers (ITO/ZnO/active layer/ PEDOT: PSS/Ag). Device degradation is correlated to the morphological behavior of the polymer:blend upon AM1.5 illumination (UV-visible light, 50 °C) and have been investigated by AFM, XRD, and UV-Vis. Once exposed to the light and to the temperature the BHJ stability is governed by two processes (i) PCBM crystallization and (ii) PCBM dimerization. Dimerization results in a rapid initial performance decrease followed by a more gradual decrease caused by a slower thermally activated crystallization. Depending on the blend morphology, dictated by the polymer's alkyl chain, the two processes occur to different extents thereby modulating the BHJ stability. Thus, of the polymer side-chains explored, linear alkyl side-chains stabilized the bulk heterojunction most effectively followed by no side-chain, alkoxy and branched side-chains. Lowering the concentration of fullerene in the active layer also reduces the rate of degradation across the polymers tested. This is a result of both the rate of crystallization and dimerization of fullerene being dependent on its concentration and the nature of the polymer side-chains. This approach appears to be a general strategy to increase the polymer:PCBM stability.
Subject(s)
Polymers/chemistry , Solar Energy , Electric Power Supplies , Molecular Structure , Polymers/chemical synthesis , SolubilityABSTRACT
A novel stable bisazide molecule that can freeze the bulk heterojunction morphology at its optimized layout by specifically bonding to fullerenes is reported. The concept is demonstrated with various polymers: fullerene derivatives systems enable highly thermally stable polymer solar cells.
