【作者】 陈卓；

【导师】 臧剑锋；

【作者基本信息】 华中科技大学 ， 微电子学与固体电子学， 2021， 博士

【摘要】 钙钛矿纳米材料由于具有高的荧光量子产率（PLQY）和色纯度,窄的发射峰以及发光波长在可见光区域可调节等优点,在发光二极管（LED）照明和显示等领域获得了极大的关注。然而,由于钙钛矿材料对温度、水、光照以及热的极度敏感,易发生分解,这种结构的不稳定极大地限制了其实际的应用。因此,要实现钙钛矿未来在照明和显示等领域的实际应用,解决其稳定性问题是至关重要的。本论文主要围绕钙钛矿纳米材料的稳定性,从其生长机制出发,提出了低温两步法工艺及掺杂工艺,获得了零维、一维和二维的钙钛矿纳米结构,并探究了其稳定的机制。具体研究如下:（1）低温两步法直接合成钙钛矿相的CsPbI₃纳米线针对CsPbI₃结构不稳定,容易从带隙窄的钙钛矿相（本文为γ-CsPbI₃）转变为带隙更宽的正交相（δ-CsPbI₃）的问题,本论文采用较低温度下两步热注入的方法,直接合成稳定的钙钛矿相的CsPbI₃纳米线而无需进行额外的退火处理。系统地研究了反应温度对于CsPbI₃纳米线的结晶、形态、结构和光学性能的影响。研究表明低的反应温度、配体保护以及延长的生长时间是成功制备钙钛矿相CsPbI₃纳米线的关键。进一步地,通过XRD等测试手段,证明这种低温制备的纳米线溶液具有较好的稳定性,在惰性气体中90天内仍然保持了钙钛矿相而未发生结构变化。这种低温溶液法制备的钙钛矿相结构的纳米线由于具有较窄的带隙和良好的光学特性,展示了在光电器件中的应用潜力。（2）铜离子掺杂提高CsPbI₃纳米晶的稳定性针对上述（1）中提及的钙钛矿相CsPbI₃不稳定易发生相变的特性,进一步地降低材料的维度,在空气中较低温度下制备CsPbI₃纳米晶,并进行了铜离子掺杂处理。研究表明经过铜离子掺杂后,CsPbI₃纳米晶薄膜和溶液的稳定性均有了很大的提升。特别是薄膜,掺杂之后的CsPbI₃纳米晶薄膜可以在空气中保持红色的荧光达35天而不发生相变,而未掺杂的薄膜在几天之后就变成了不发光的正交相。针对这种稳定性的提升,采用基于密度泛函理论的第一性原理计算,结果表明这种稳定性的增强可归因于掺杂后CsPbI₃体系形成能的增加（未掺杂时CsPbI₃的形成能为-14.0965 e V,3.125%铜掺杂后形成能为-14.2609 e V）。进一步地,把这种掺杂后的CsPbI₃纳米晶作为发光层,应用于发光二极管QLED,获得的QLED的器件性能如发光亮度和外量子效率EQE均有了很大的提升。掺杂之后的QLED器件的开启电压为1.6 V,相比未掺杂的器件（3.8 V）明显地降低,而且发光亮度达到1270 cd/m²,是未掺杂的两倍多（542 cd/m²）。这种掺杂后器件的性能还可通过优化器件结构和提纯方法进一步提高,展示了今后在LED器件应用的潜力。（3）锰掺杂发光CsPbCl₃纳米片的制备及稳定性研究针对二维CsPbCl₃纳米片发光效率较低以及稳定性较差的问题,本论文采用了一种大气环境中较低温度下制备锰掺杂CsPbCl₃纳米片。研究了温度、反应时间和锰掺杂比例对于CsPbCl₃纳米晶形态的影响。结果表明可以通过控制上述因素,进而控制合成高发光效率的二维的锰掺杂的CsPbCl₃纳米片。在室温下,制备的锰掺杂纳米片在空气中放置60天后,仍然保持了较强的荧光,展示了较好的稳定性。掺杂之后不仅稳定性有了很大的提升,这种掺杂的纳米片的发光效率也有了很大的提升,可达到53.76%,是目前报道的Mn掺杂二维全无机铅卤钙钛矿的最高值。由于这种掺杂纳米片具有较好的稳定性以及较高的发光效率,进一步地,首次将其应用于橙光LED和白光LED,表明了锰掺杂的橙光纳米片在发光领域具有较大的应用潜力。此外,通过将这种掺杂的纳米片溶液与PDMS等柔性高分子材料混合得到复合发光薄膜,在空气中放置6个月后,发光薄膜仍然保持了较强的荧光。表明掺杂的发光纳米片通过与高分子材料复合,可以很好地提升稳定性,这对于其未来在柔性显示领域的应用具有重要的意义。更多还原

【Abstract】 Perovskite nanomaterials have been attracted great attention in the fields of display and light-emitting diodes（LED）due to high photoluminescence quantum yields（PLQYs）,high color purity,narrow band-width,and a widely tunable emission spectrum from ultraviolet to visible.However,their stabilities are highly sensitive to temperature,water,light,and heat,and are prone to decompose,which is one of the major barriers for practical applications.Therefore,it`s very crucial to improve the stability of the perovskites for future applications.In this thesis,we focus on the stability of perovskite nanomaterials.Starting from its growth mechanism,we put forward a low temperature two-step process and doping process to obtain zero-dimensional,one-dimensional and two-dimensional perovskite structure,and explore its stability mechanism.The research results are as follows:（1）Direct synthesis of perovskite phase CsPbI₃ nanowires via two-step injection at low temperatureIn this work,regarding that the CsPbI₃ is unstable and can be easily transformed from the perovskite phase into the orthorhombic（δ）CsPbI₃ with an undesired wider band gap（2.82 e V）,we report a simple approach to directly grow highly stabilized perovskite phase CsPbI₃ nanowires via low temperature two-step injection method.We systemically investigate the effect of reaction temperature on crystallization,morphologies,structures,and optical properties of CsPbI₃ nanowires.We find that low reaction temperature,capping ligand protection,and a second growth opportunity are key to the successful growth of perovskite phase CsPbI₃ nanowires.The X-ray diffraction（XRD）,photoluminescence（PL）,and UV-vis absorption results verify that these perovskite phase nanowires maintain excellent stability at room temperature for 90 days in an inert atmosphere.The excellent optical properties and narrow band gap of the phase stable CsPbI₃ nanowires offer great potential in the field of optoelectronic devices.（2）Enhancing stability of CsPbI₃ perovskite nanocrystals through copper substitutionIn this work,due to the fact that perovskite phase CsPbI₃ is unstable,we report a low-temperature and atmospheric approach to synthesize Cu²⁺-doped CsPbI₃ nanocrystals with enhanced stability.Results show that the stability of solution and film both have been improved after Cu²⁺doping.In particular,the Cu²⁺-doped CsPbI₃ film can maintain red luminescence for 35 days in air while the undoped ones transformed into the nonluminescent yellow phase in several days.Furthermore,first-principles calculations verify that the enhanced stability is ascribed to the increased formation energy due to the successful doping of Cu²⁺in CsPbI₃.The formation energy of undoped CsPbI₃ is-14.0965 e V,while the formation energy of 3.125%Cu²⁺doped CsPbI₃ is-14.2609 e V.Benefiting from such an effective doping strategy,the as-prepared Cu²⁺-doped CsPbI₃ as an emitting layer shows much better performance compared with that of the undoped counterpart.The turn-on voltage of the Cu²⁺-doped quantum-dot light-emitting diode（QLED）（1.6 V）is significantly reduced compared with that of the pristine QLED（3.8 V）.In addition,the luminance of the Cu²⁺-doped QLED can reach 1270 cd/m²,which is more than twice that of the pristine CsPbI₃QLED（542 cd/m²）.The device performance is believed to be further improved by optimizing the purification process and device structure,shedding light on future applications.（3）Synthesis and improve the stability of CsPbCl₃ nanoplatelets via Mn²⁺dopingIn this work,considering that the two-dimensional CsPbCl₃ nanoplatelets have low PLQYs and poor stability,we report a low-temperature and atmospheric approach to synthesize highly luminescent Mn-doped CsPbCl₃ nanoplatelets.Results show that the moderate temperature,approximate growth time,and appropriate Mn-Pb feed ratio are the key to the successful growth of Mn-doped CsPbCl₃ nanoplatelets with high PLQYs.The nanoplatelets exhibit excellent stability at room temperature in air（over 60 days）.Apart from the enhanced stability,the PLQY of our Mn-doped CsPbCl₃ nanoplatelets is also enhanced,up to 53.76%,which is the highest reported value in Mn-doped two dimensional all-inorganic halide perovskites.Due to the enhanved stability and high PLQYs,for the first time,both orange-red and warm white light-emitting diodes（LEDs）have been achieved through the nanoplatelets as the color conversion materials.Moreover,flexible composite fluorescent polymer films based on the nanoplatelets are demonstrated with high luminescence and stability（over 6 months）,which indicate prominent potential in flexible displays.更多还原