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Thermally-induced tensile strain that remains in perovskite films following annealing results in increased ion migration and is a known factor in the instability of these materials. Previously-reported strain regulation methods for perovskite solar cells (PSCs) have utilized substrates with high thermal expansion coefficients that limits the processing temperature of perovskites and compromises power conversion efficiency. Here we compensate residual tensile strain by introducing an external compressive strain from the hole-transport layer. By using a hole-transport layer with high thermal expansion coefficient, we compensate the tensile strain in PSCs by elevating the processing temperature of hole-transport layer. We find that compressive strain increases the activation energy for ion migration, improving the stability of perovskite films.
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We achieve an efficiency of 16.4% for compressively-strained PSCs; and these retain 96% of their initial efficiencies after heating at 85 °C for 1000 hours—the most stable wide-bandgap perovskites (above 1.75 eV) reported so far. The power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have increased from 3.8% to a certified 25.2%, approaching those of crystalline silicon solar cells. Continued progress in stability remains a topic of importance along the path to industrial application. Encapsulating PSCs to protect against oxygen and moisture is a straightforward method to stabilize these devices,; however, some important sources of instability are not overcome using extrinsic stabilization approaches such as encapsulation.Residual tensile strain in perovskite films is a source of instability, and stems from the thermal expansion mismatch between perovskites and substrates during the annealing steps required in the formation of perovskites. Specifically, thermally-induced tensile stresses in perovskite films can reach 50 MPa, sufficient to induce the deformation of copper. These tensile stresses weaken bonds, decreasing the formation energy of defects and lowering the activation energy for ion migration. These mechanisms are obstacles to stability at elevated temperatures, such as those required in accelerated lifetime testing.The correlation between stress ( σ) and thermal expansion mismatch is quantified as follows.
(1)where E p is the modulus of the perovskite, υ p is Poisson’s ratio in the perovskite, α s and α p are the thermal expansion coefficients of the substrate and the perovskite, respectively, and Δ T is the temperature gradient during cooling from the annealing temperature of the perovskite film to room temperature. Several strategies have been reported to reduce this detrimental tensile strain in perovskite films based on the above equation. They can be divided into two categories: (i) lowering the formation temperature of perovskite films to reduce Δ T; (ii) using plastic substrates with thermal expansion coefficients that are similar to that of the perovskite to decrease Δ α. Although these strategies have been shown to diminish the residual tensile stress in perovskite films, they also lower device efficiency. Room-temperature-prepared CH 3NH 3PbI 3-based solar cells exhibit a PCE of 17.1%, a result of the lower quality of perovskite films fabricated by low-temperature processing. It is imperative to control the residual stress to move further in the direction of high efficiency and increased stability of PSCs.Here we report a strain-compensation strategy that reduces the tensile strain in perovskite films with the aid of hole-transport layer (HTL).
We use an HTL possessing numerous 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. The resultant compressively-strained photovoltaic devices retain 95% of their initial PCEs of 16.4% after maximum power point tracking for 60 h, and 96% after heating at 85 °C for 1000 h, showing the most stable wide-bandgap perovskite (above 1.75 eV) cells reported so far.
The improved stability of compressively-strained material stacks is explained through the higher activation energy (from 0.547 to 0.794 eV) for ion migration compared to that of tensile-strained films. Residual strain in perovskite filmsBased on Eq. , the difference in thermal expansion coefficients ( α) between the perovskite and the contacting layers provides one source of stress; and the requirement of high annealing temperatures to form crystalline perovskites also contributes due to a large Δ T. Regarding the thermal expansion mismatch, PSCs typically consist of a stack of multilayers including a substrate coated with a transparent conducting oxide electrode (TCO) layer, followed by the electron-transport layer (ETL), the perovskite layer, the hole-transport layer (HTL), and another electrode layer.
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The thermal expansion coefficient varies greatly with each functional layer (Fig. ). The widely used ITO-coated glass and metal oxide charge-transport layers possess low values of α in the range of 0.37 to 1 × 10 −5 K −1. In contrast, perovskites have much higher α values ranging from 3.3 to 8.4 × 10 −5 K −1, approximately an order of magnitude higher than that of substrates or ETLs. Such a large thermal expansion difference is the main reason for the formation of tensile strain as perovskite films cool to room temperature. As shown in Fig., the perovskite films used for high-efficiency perovskite solar cells typically require annealing at temperatures over 100 °C for improved crystallinity and minimization of defects,. Notably, when comparing with hybrid organic-inorganic perovskites, all-inorganic perovskites require even higher temperatures to stabilize the black cubic perovskite phase. In particular, CsPbI 3 requires annealing temperatures varying from 180 to 330 °C.
Therefore, inorganic perovskite films processed at high temperatures suffer from an even larger tensile strain.When a perovskite film is deposited on a layer with lower α, the contact formed between the two layers during the high-temperature annealing process constrains the perovskite from contracting when it cools back to room temperature, introducing tensile strain along the in-plane direction (Fig. ). Conversely, when using a layer with higher α, the perovskite film contracts more, resulting in compressive strain. Therefore, tensile or compressive strain in perovskite films can be adjusted by using adjacent device layers with lower or higher α compared with that of perovskite.We propose a strain-management approach to reduce residual tensile strain in perovskite films—since tensile strain is more often produced in these layer stacks than compressive strain. When using a substrate with lower α, we are able to compensate the tensile strain through an external compressive strain that is induced by the top contacting HTL with higher α, leading to a non-strained or even compressively-strained perovskite film. Strain-compensated perovskite filmsTo evaluate this strain-compensation strategy experimentally, we focused on all-inorganic cesium lead halide perovskites primarily due to their superior thermal stability compared to their organic-inorganic hybrid counterparts. This high thermal stability enables all-inorganic perovskites to withstand higher annealing temperatures (Fig. ), while also inducing a large amount of stress considering the soft lattice of perovskites that can endure large strain (Fig.
And Supplementary Fig. ), thereby facilitating the investigation of strain control.
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