Tensile strain in perovskite thin films, which originates from high-temperature annealing steps needed in the crystallization of the perovskite, is known to contribute appreciably to instability in these materials. This residual strain weakens bonds, favors the formation of defects, and lowers the activation energy for ion migration, accelerating perovskite degradation. Tensile-strain-induced instability is now pointed to widely as a major bottleneck toward the achievement of stable perovskite solar cells (PSCs).
Even worse, this instability is not overcome using conventionally extrinsic stabilization approaches such as encapsulation. Previously-reported strain regulation methods for PSCs have utilized substrates with high thermal expansion coefficients; however, those methods have led to inferior power conversion efficiencies (PCEs) as a result of the constraints on annealing temperatures.
In this work, we report a strain-compensation strategy that offsets the tensile strain in perovskite films by introducing an external compressive strain from the hole-transport layer (HTL). The HTLs suitable for our strategy would need to meet the following three criteria: (i) the top functional layer should have a higher thermal expansion coefficient compared with the perovskite, offering the possibility of compressive strain; (ii) the functional layer should have a strong interaction with the perovskite in order to anchor to the lattice and achieve strain offset; (iii) the top interface layer should be coated at high temperatures, inducing a compressive strain when cooling back to room temperature.
We employ a polymer-based HTL—poly[5,5-bis(2-butyloctyl)-(2,2-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene] (PDCBT)—with rich carbonyl anchoring groups that exhibit strong interactions with the perovskite surfaces, and thus we build a strong HTL:perovskite interface that transfers strain from the HTL to the perovskite active layer; we then balance the tensile/compressive strain transition by tuning the processing temperature and strain of the HTL.
Using this strategy, we achieve a PCE of 16.4% in the resultant compressively-strained CsPbI2Br solar cells. Notably, these retain 96% of their initial PCE following heating at 85oC for 1000 hours, and 95% after maximum power point tracking for 60 h, showing the most stable wide-bandgap perovskite (above 1.75 eV) cells reported so far.
This study provides a new approach to regulate the strain in perovskite films, and identifies as a result a new avenue to efficient and stable perovskite optoelectronic devices.
For more information, please see our recent publication in Nature Communications: “Regulating strain in perovskite thin films through charge-transport layers” (https://www.nature.com/articles/s41467-020-15338-1).