Solution-processed semiconductors combine ease of processing, scalable fabrication, and compatibility with flexible substrates – compelling properties for next-generation optoelectronic devices. However, their lifetimes when aged, or operated, in room ambient – and also when exposed to elevated temperatures and humidity – have not yet fulfilled the multi-thousand-hour requirements demanded for many industrial applications.
Inorganic cesium lead halide perovskites, an otherwise promising material for solar photovoltaics, suffer from an undesired phase transition in the vicinity of room temperature – a consequence of their flexible lattice. Structurally robust colloidal quantum dots (CQDs), prized for their tunable bandgap, are prone to aggregation and surface oxidization at high temperatures as a consequence of incomplete surface passivation. Previously, researchers have investigated methods to improve the stability of each material system separately. Until now, the stability under demanding accelerated lifetime conditions has yet to prove satisfactory.
Recent studies have shown the growth of perovskite around CQDs and the formation of heterojunctions thereby. CsPbX3 perovskite and PbS quantum dots have similar crystal structures and lattice parameters. This encouraged us to attempt to keep the perovskite in a desired crystal phase by anchoring it using robust, embedded CQDs. We further sought evidence that the lattice-matched perovskite could in turn passivate the CQDs efficiently.
We therefore pursued to design a lattice-anchored hybrid structure to address the stability issues present in the two constituent materials. We prepared hybrid films using CsPbX3 solutions combined with pre-treated CQDs (Figure 1). To achieve the strain-free epitaxial growth of perovskite, we tuned its composition to ensure a near-zero lattice mismatch (ε).
Figure 1. Schematic depicting an atomistic model of CQD:perovskite lattice-anchored hybrid materials system.
Remarkably, the newly-stabilized CsPbX3 achieves a longevity of greater than six months when stored in ambient; and it remains stable following five hours’ exposure to 200 oC in air. This is an order of magnitude longer than for films that are missing the CQD stabilizers (Figure 2). The benefits of this new material also address CQDs’ challenges: the perovskite fully passivates the CQDs, rendering them stable against oxidation and reducing – by a factor of five – their propensity to fuse under 100 oC accelerated ageing. The new lattice-anchored hybrid materials demonstrate superior charge transport properties, including efficient carrier transfer and high photoluminescence quantum yield.
Figure 2. Stability of the lattice-anchored CsPbBrI2 perovskite with different ratio of CQDs.
This work showcases the benefits of a new solution-processed hybrid material system – one that overcomes the limited stability of inorganic perovskites and CQDs and presents superior optoelectronic properties.
For more details, please check out our paper “Lattice anchoring stabilizes solution-processed semiconductors” in Nature: https://doi.org/10.1038/s41586-019-1239-7