# The Opto-Electronic Functional Devices Based on Three-Dimensional Lead Halide Perovskites
## 5 Summary and Outlook  
In summary, the LHPs have been utilized in various functional devices such as lasers, photodetectors, modulators, and so on. The utilization of the material is due to its superior opto-electronic properties, as well as its low-cost fabrication and easiness to be combined with other materials. By now, LHP have been combined with semiconductors, metals, as well as polymers. By a careful material selection and device design, the device performance could be further enhanced. Due to the uniqueness of LHP, how to improve the working performance and lifespan are the two essential points concerned by researchers in the community. Certainly, the property of the corresponding LHP in use is the dominating factor. The interactions between particles including carriers, photons, and phonons, etc., could affect the working performance from one way or another. For instance, the lasing threshold in the LHP-lasers has been shown to be affected significantly when the LHP's crystalline quality or film morphology varies: when the number of scattering centers or grain boundaries inside LHP is reduced, a lower threshold is generally expected. A reduced trap density, on the other hand, could possibly lead to the ultrafast response in photodetectors. With the development in single-crystal LHP growth, LHPs with improved crystalline qualities as well as enhanced performance in the devices could be expected. Moreover, a better crystalline quality also tends to result in higher-level stability, which could be possibly ascribed by the higher thermal conductivity and thus faster heat dissipation. In some of the previous reports, LHPs with higher stability have been used to elongate the device lifespan but often introduced a trade-off of a deterioration in device performances at the same time (for instance, a narrower working bandwidth when switching from MAPbI\({}_{3}\) to CsPbI\({}_{3}\) in photodetectors). An improved crystal quality could provide the researchers with a higher degree of freedom when selecting the proper LHP materials, resulting in more robust devices as well. Currently, it is beneficial to realize that there is still much to be studied regarding the LHP's optical and electrical properties. It is necessary to not only characterize the important parameters but also to obtain an in-depth understanding on light-matter interactions inside LHP. The dynamics of photogenerated carriers, the behavior of traps, and so forth, could affect the device performance in one way or another. A deeper understanding regarding the perovskites' optical and electrical properties, could aid people in finding out the optimized working conditions to the LHP-devices. Moreover, since light is a necessity for most of the functional devices and leads to decomposition in LHP, efforts regarding the light-induced degradation mechanism will also be beneficial. With a better understanding of how the crystalline and electronic structure changes when exposed to light illumination, people will be able to determine the proper illumination condition for each type of functional devices and estimate the performance degradation with more certainty. The knowledge of light-induced degradation mechanism will also contribute to synthesis of new LHPs with improved stability. The design of device configuration is also a significant factor leading to development in LHP devices. Generally, the improvement is achieved by increasing the optical length (through cavity design) and/or enhancing the optical confinement in LHP (through design of material spatial layout or introduction of plasmonic effects). However, caution is needed when the optical field distribution and the degree of optical confinement is to be steered, since the degradation in LHP has been found to accelerate in light illumination with higher power density.