1. The cyclic lattice strain and induced fatigue failure of perovskite thin films in the temperature range of - 60 ℃ to 80 ℃ were studied by using synchrotron radiation GIWAXS
2. Using β- PV2F polymer material can improve the stability of perovskite film to temperature changes, stabilize the black phase of perovskite and improve its photoelectric properties.
3. The efficiency of the prepared trans-structure perovskite solar cell is up to 24.6% when the area is 18mm2, and 23.1% when the area is 1cm2
Introduction to achievements
Recently, Li Guixiang, Luyao Wang, Antonio Abate of the Helmholtz Center for Materials and Energy in Germany, Su Zhenhuang of the Shanghai Institute of Advanced Studies and Li Meng of Henan University published the research results of fatigue damage caused by perovskite under cyclic temperature changes in Science. Like metal fatigue, perovskite film will also produce fatigue damage under cyclic temperature changes. In order to solve this problem, the author uses an ordered dipole structure β- PV2F polymer material to improve the temperature fatigue resistance of its film, stabilize the black phase of perovskite and improve the photoelectric performance of perovskite solar cells. The trans-structure perovskite solar cell devices prepared by the author are 24.6% and 23.1% respectively when the area is 18mm2 and 1cm2. Under the conditions of 25 ° C and 75 ° C, the efficiency is maintained at 96% and 88% after 1000 hours of sunshine.
Effect of ambient temperature on properties of perovskite
In practical applications, changes in ambient temperature will limit the performance of solar cells, because perovskite will cause serious ion migration, phase change and temperature induced strain, resulting in the reduction of photoelectric conversion efficiency.
Cycling at different temperatures requires perovskite crystals to tolerate alternating tension and compression in the device structure. Therefore, the development of high-efficiency solar cells with thermal cycle stability is the key to promote the application of perovskite solar cells.
Results and discussion
Point 1: β- Role of pV2F
The effect of β-pV2F on film morphology and working mechanism is shown. From the top view and F1 cross-sectional scanning electron microscope (SEM) images (A to C), we can observe that the control perovskite film has obvious grain boundaries with an average grain size of about 400 nm. These defects create shunt pathways and nonradiative recombination centers. The grain boundaries of β-pV2F are reduced and the grain size is enlarged to ~480 nm (D to F). During the film formation process, the surface work function (WF) is caused to move upward after film formation (G); H shows that the work function of the target perovskite film increases to 300meV, which is beneficial to the extraction of interface charges and can enhance the device stability.
Point 2: Crystallization mechanism of thin films
In order to gain insight into the crystallization process of perovskite films, the authors performed synchrotron radiation-based GIWAXS to characterize the entire process of film formation. Comparing the GIWAXS patterns (A and B), the weakening of the diffraction signal during the first 60 s indicates that the initial mesophase of DMSO-DMF-PbX2 was suppressed. This effect can be attributed to the mesophase sequestered by long-chain β-pV2F molecules. The observed scattering features centered at q = ~10 nm-1 along the (001) crystal plane during film formation indicate that the colloid has solidified and transformed into a black phase. It is worth noting that the authors found that the black phase of the target film appeared earlier than that of the control film (Δtt>Δtc), implying that β-pV2F promoted the conversion of the intermediate phase to the perovskite black phase. The fast phase transition is associated with lower formation energy, possibly due to the rapid aggregation of dispersed PbX2 and organic salts by β-pV2F during the volatilization of DMSO and DMF. When crystallization is complete (stage t7), the target film has a stronger signal than the control film (c). This result indicates that the formed target perovskite film is more ordered. Therefore, β-pV2F controls the crystallization kinetics of perovskite by lowering the formation energy of perovskite, promotes the phase transition, and makes the crystal structure more ordered.
Point 3: Photovoltaic performance of the device
Photovoltaic performance of inverted p-i-n perovskite thin solar cells is shown. The photoelectric conversion efficiency of the control cell was 22.3%, the short-circuit current density (Jsc) was 24.7 mA/cm2, the Voc was 1.13 V, and the fill factor (FF) was 80.2%. The device performance was improved after using β-pV2F, Voc was 1.18 V, Jsc was 24.8 mA/cm2, FF was 84.3%, and PCE was 24.6%. The authors also documented a PCE of 23.1% for a device with a working area of 1 cm2 (B). From the external quantum efficiency (EQE) spectra (C), we calculated the integrated Jsc of 24.3 and 24.4 mA/cm2 for the control and target devices, respectively, which are comparable to the values extracted from the J-V curves. The steady power output at the maximum power point (MPP) is plotted in D. Under continuous 400-s 1-sun illumination, the control device showed a sustained efficiency decay. The tracked target device produced a highly stable power output and even a gradual increase in performance, which the authors attribute to the light-bubbling effect. The stability of the unencapsulated device under working conditions showed that the target PSCs retained 96% of their initial PCE after 1000 hours of continuous tracking of the MPP. In contrast, control PSCs decayed to 84% of their original PCE (E). After the heated device was heated to 75°C, 88% of the original was retained in the target device, compared with only 56% in the control group.
Point 4: Thermal cycle stability
The authors further evaluated the thermal cycling stability of the device, as shown. A and B are statistical PCE scores, indicating highly reproducible stabilization effects of β-pV2F. Subsequently, the unpackaged devices were aged (TC) under rapid thermal cycling between -60° and +80°C. As shown in C and D, after 120 thermal cycles, the control device suffered severe degradation at +80°C of 75.6% at +80°C and 63.0% at -60°C, while The target device retained 93.9% at 80°C and 88.7% at -60°C.
Point 5: Strain during temperature cycling
The difference in device performance originates from the β-pV2F used in the perovskite film. The authors characterized the crystal structure and strain of perovskite films undergoing thermal cycling to determine the effect of β-pV2F. This result indicates that temperature induces the degradation of the perovskite film, but the temperature-induced fatigue of the perovskite film is suppressed. The authors observed additional GIWAXS peaks formed after three thermal cycles (A). Specifically, in the second cycle, a degradation product appeared at q = 9.2 nm-1 which is the signal of PbI2. During the third thermal cycle, additional peaks ~8.2 and 8.6 nm-1 formed, corresponding to 4H and 6H of the non-perovskite phase. This result suggests that the control perovskite undergoes an irreversible phase transition. The generation of these phases may come from the lattice deformation of the grain boundary, where adjacent crystals with different orientations are squeezed against each other. However, this phenomenon is not observed in the target perovskite (B), indicating its high thermal cycle stability. Due to the difference in thermal expansion coefficient between the perovskite film and the substrate, temperature changes induce strain in the perovskite. The lattice strain evolution experienced by the control perovskite during thermal cycling (C).
The authors found that the strain of the perovskite drifted with temperature cycling, showing changes in the perovskite lattice parameters. In contrast, the target perovskite exhibits stable strain cycling within a narrow range (−0.06% to 0.38%), corresponding to a recoverable crystal structure and releasable lattice strain. The authors propose that due to the presence of ordered dipoles in the target perovskite, a self-assembled polymer layer is created that wraps the crystals in the perovskite film, reduces friction during thermal cycling, acts as a strain buffer and Lattice stabilization effect.
References:
Guixiang Li et al. Highly efficient p-i-n perovskite solar cells that endure temperature variations.
DOI: 10.1126/science.add7331
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