Suppressing photooxidation of conjugated polymers and their blends with fullerenes through nickel chelates

Abstract

Conjugated polymer semiconductors offer unique advantages over conventional semiconductors but tend to suffer from electro-optic performance roll-off, mainly due to reduced photofastness. Here, we demonstrate that the commodity nickel chelate nickel(II) dibutyldithiocarbamate, Ni(dtc) 2 , effectively inhibits photooxidation across a wide range of prototypical p-conjugated polymer semiconductors and blends. The addition of 2-10 wt% of Ni(dtc) 2 increases the resilience of otherwise quickly photobleaching semiconducting thin films, even in the presence of detrimental, radical forming processing additives. Using electron spin resonance spectroscopy and sensitive oxygen probes, we found that Ni(dtc) 2 acts as a broadband stabilizer that inhibits both the formation of reactive radicals and singlet oxygen. The mechanism of stabilization is of sacrificial nature, but contains non-sacrificial contributions in polymers where singlet oxygen is a key driver of photooxidation. Ultrafast pump-probe spectroscopy reveals quenching of triplet excited states as the central mechanism of non-sacrificial stabilization. When introduced into the active layer of organic photovoltaic devices, Ni(dtc) 2 retards the short circuit current loss in air without affecting the sensitive morphology of bulk heterojunctions and without major sacrifices in semiconductor properties. Antioxidants based on nickel complexes thus constitute functional stabilizers for elucidating degradation mechanisms in organic semiconductors and represent a cost-effective route toward organic electronic appliances with extended longevity. Broader contextOrganic semiconductors based on conjugated polymers bear exceptional opportunities for many disruptive technologies, including light and power generation, sensor technology and electronic circuitry, which could potentially be realized in a sustainable fashion using highly efficient and low-cost approaches. However, conjugated polymers suffer from photo-oxidation-induced performance loss and need to be carefully encapsulated, compromising both applicability and cost benefits. In the present work, we introduce a nickel chelate, nickel(II) dibutyldithiocarbamate, as a universal antioxidant for increasing the longevity of conjugated polymers. We carried out a mechanistic study that quantitatively describes the stabilization of conjugated polymer moieties with technological relevance for OLEDs, OPVs and OFETs. We introduce for the first time a figure of merit (FOM) as a stabilization metric for antioxidants in conjugated polymers. This FOM serves likewise to distinguish between sacrificial and non-sacrificial protective mechanisms. We draw wide ranging conclusions that reflect on how conjugated polymer and polymer:fullerene blends photo-oxidize and how the underlying mechanisms can be significantly suppressed in the presence of nickel chelates. Finally, we demonstrate its beneficial use in a broad range of organic photovoltaic devices.

The role of the third component in ternary organic solar cells

“…The mechanistic pathways for photo-oxidation involve two main processes. One process involves exothermic energy transfer from the triplet state of either the donor or the acceptor (formed from intersystem crossing of photoexcited singlet excitons) to molecular oxygen, which creates a highly reactive singlet oxygen species 124 . Singlet oxygen can then undergo a range of chemical reactions within the active layer, leading to degradation of the conjugated electronic structure of the organic molecules.…”

Beyond Ternary OPV: High‐Throughput Experimentation and Self‐Driving Laboratories Optimize Multicomponent Systems

“…Interestingly, complex photochemical interactions between donor, acceptor and oxygen can suppress or enhance photooxidation in D:A blends 26,27. Further, adding a third component to the active layer may increase thermal and light stability 28,29. However, the degradation behaviors of higher dimensional multicomponent systems may depend on various interactions between the components.…”

Laser Systems and Networks with Organic Nanowires and Nanofibers

“…Photostability of the ASE and lasing emission is another issue for many organic compounds. Photo‐oxidation processes can activate quenching paths for the excited states, decreasing the overall emission quantum yield . Approaches to increase ASE and lasing operational lifetimes include the use of additives, and the encapsulation of active materials in transparent, protecting layers made by drop casting or 3D printing .…”

“…Photo‐oxidation processes can activate quenching paths for the excited states, decreasing the overall emission quantum yield . Approaches to increase ASE and lasing operational lifetimes include the use of additives, and the encapsulation of active materials in transparent, protecting layers made by drop casting or 3D printing . A long operational lifetime (>10 6 pumping pulses at 10 Hz repetition rate) has been recently reported for ASE of PS films doped with carbon‐bridged oligo( p ‐phenylenevinylene)s …”