# The Opto-Electronic Functional Devices Based on Three-Dimensional Lead Halide Perovskites
## 2 The Properties of the LHP Materials
### Intrinsic Instability of LHP  
Although LHPs have novel properties leading to outstanding working performances in opto-electronic industry, the massive usage of the materials in practical occasions are mainly hindered by the severe instability of LHPs. As shown in Figure 3, moisture, oxygen, elevated temperature, and light radiation are found to induce material degradation quite easily [46; 47; 48]. As for moisture and oxygen which is generally not necessary for device function, the corresponding degradation could be generally dealt with by insulation. For instance, it is required that the preparation of LHP-based solar cells should be in a relatively low humidity level of \(<\)1% atmosphere [49]. By careful encapsulation or introduction of hydrophobic contacts [50], device lifespan could be improved considerably. Unlike moisture and oxygen, heat is likely to accompany the device operation in some applications. The degree of heat-induced degradations is found to be dependent on chemical composition. For instance, while MAPbI3 will degrade at 85 \({}^{\circ}\)C [51], CsPbI3 could endure much higher temperature, showing stability up to 300 \({}^{\circ}\)C [52]. Figure 3: Instability of LHP induced by different environmental factors. (**a**) External quantum efficiency (EQE) as a function of exposure time to humid or dry air. Reproduced with permission from [53]; published by Wiley, 2016. (**b**) Evolution of the normalized power conversion efficiency (PCE) of FTO/cp-TiO2/mp-TiO2/MAPbI3/spiro-OMeTAD/Au devices in various environments. Illumination was performed with white LEDs equipped with ultraviolet (UV) filters. Reproduced with permission from [47]; published by The Royal Society of Chemistry, 2016. (**c**) I-V curves of a MAPbI3 Perovskite Solar Cells as a function of temperature. Reprinted with permission from [54]. Copyright 2016 American Chemistry Society. (**d**) PL intensities of the spin-coated MAPbI3 sample over time, under laser excitations at 0.2 and 0.65 W/cm2, respectively. Reproduced with permission from [55]; published by Wiley, 2016. (**e**) The PCE evolution with time under 1-sun illumination and after resting the device in dark, when the device is stressed at point A (red circles, J = 0, V = Voc) or at point B (blue squares, J = JSC, V = 0) between each data point. Reproduced with permission from [47]; published by Springer Nature, 2016. Light-induced degradation of LHP may be the most intractable problem, since light radiation is a necessity for most of the LHP-based devices to function properly. Researchers have found that the detailed degradation depend significantly on factors including illumination time, power density, and wavelength. When the device is exposed to relatively low power density for a short time, the so-called photo-curing effect is likely to occur, leading to substantial enhancements in PL intensity as well as devices' working performances [56; 57]. However, the enhancements could not last long. With the further extension of illumination time, apparent degradation could be observed [47; 58; 59]. When the illumination power density increases even higher, the degradation accelerates, and structure transformation could be easily exhibited by characterization techniques such as Raman spectroscopy [60; 61; 62; 63; 64]. LHP may recover after being placed in dark for a certain period of time; however, the recovery reported varies from different research groups [47; 58]. The illumination wavelength has been also found to have apparent influence on LHP's degradation. Both ultraviolet (UV) [65; 66; 67; 68] and infrared (IR) light [58] has been found to accelerate the degradation of solar cells, attributed to different physical mechanisms. By now, people have proposed explanations such as ion-migration and light-induced trap states [69; 70; 12; 71; 72; 73; 74; 75; 76; 77; 78; 79; 11], but no census has been reached yet. The light-induced degradation turns out to be a very complex process, and it is possible that the degradation involves more than one contributing mechanism simultaneously.