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2021 , Vol. 9 >Issue 4: 342 - 350

DOI: https://doi.org/10.3969/j.issn.2095-560X.2021.04.010

Study on Solar Cells Based on Different All-Inorganic Perovskite Materials

  • Chao SUO ,
  • Xiao-lin LIU , † ,
  • Jia LIN
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  • College of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 201306, China

Received date: 2020-12-17

  Revised date: 2021-01-07

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版权所有 © 《新能源进展》编辑部
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Abstract

With the development of emerging photovoltaic cells, halide perovskite materials have been paid much attention. Among them, all-inorganic perovskites hold a great promise in the fields of optoelectronic and photovoltaic devices because of their good thermal stability, high absorption coefficient, adjustable band gap, and simple preparation process. The highest photoelectric conversion efficiency of all-inorganic perovskite solar cells has reached 20.4%. In this paper, the photoelectric conversion efficiency and stability of the solar cells were summarized based on all-inorganic perovskite materials such as ABX3, A2BX6, A2B1+B3+X6 and perovskite-type. The key factors that affect the efficiency and stability, and the optimization methods were mainly analyzed. At last, challenges of the all-inorganic perovskite solar cell materials in the future were prospected.

Key words： all-inorganic perovskite; perovskite solar cell; phase stability; double perovskite; perovskite-type

Cite this article

Chao SUO , Xiao-lin LIU , Jia LIN . Study on Solar Cells Based on Different All-Inorganic Perovskite Materials[J]. Advances in New and Renewable Energy, 2021 , 9(4) : 342 -350 . DOI: 10.3969/j.issn.2095-560X.2021.04.010

0 引言

卤族钙钛矿太阳能电池因其高吸光系数、高光电转换效率、制备工艺简单等优势,在太阳能电池领域脱颖而出,光电转换效率由最初的3.8%在短短十几年中提高到现在的25.5%[1],和目前占光伏产业化市场约90%主导地位的晶硅太阳能电池效率（单晶硅效率27.6%[1]）相当,成为最具应用潜力的高效新型太阳能电池之一[2,3]。其中有机-无机混合钙钛矿材料ABX3中A位通常是甲胺离子CH3NH3+（MA+）、甲脒离子NH2-CH=NH2+（FA+）等有机官能团,其结构不稳定,对环境较为敏感,造成了器件的制备及封装工艺条件更加苛刻[4,5]。为解决这一问题,科研工作者利用全无机A位元素（例如Cs元素）替代有机阳离子,既保证了载流子传输性与有机-无机杂化钙钛矿相似[6],在湿热或光照条件下又不会产生挥发性分解产物[7],目前的最高效率已经超过了20%[8],因此全无机钙钛矿材料逐渐成为目前的研究热点之一。
全无机钙钛矿材料不仅具有较高的量子产率、高激子结合能、高耐缺陷性、发射波长可调节、在带隙附近有较大光吸收系数等优点,更重要的是具备优异的热稳定性和与生物的兼容性,使得相关光电器件具备可稳定长期使用的基础[9,10]。但相较于有机-无机钙钛矿太阳能电池,全无机钙钛矿太阳能电池在效率上有着明显差距。本综述从全无机钙钛矿太阳能电池结构入手,总结基于不同结构全无机钙钛矿材料的太阳能电池的光电转换效率,并分析了影响材料稳定性的因素,进而寻求可能的优化及改良方案。

1 全无机钙钛矿太阳能电池结构

目前钙钛矿太阳能电池的结构是以染料敏化太阳能电池的结构衍生而来,主要分为介孔结构和平面结构两大类,主要区别是电子传输层除了致密层外,是否包括介孔层作为骨架支撑结构,如图1a所示。介孔结构由于介孔层如二氧化钛的存在,可以更有效地收集钙钛矿层所产生的光生电子,但平面结构的制备方法简单,可以低温制备,适用性强,经济成本更低,因而应用更为广泛。目前常见的平面结构包括正置n-i-p结构[11]和反置p-i-n结构[12]（如图1b和图1c）。
Fig. 1 (a) TiO2 mesoporous structure; (b) planar normal-structure; (c) planar invers-structure; (d) energy level diagram of each layer of inorganic perovskite solar cell

图1 （a）TiO2介孔结构;（b）平面正置结构;（c）平面反置结构;（d）无机钙钛矿太阳能电池中各层材料能级图

各层材料的选择对全无机钙钛矿太阳能电池的性能至关重要,除能级匹配外,对于n-i-p型结构,电子传输层应具有高光透过率,空穴传输层应具有高稳定性等优点。图1d列举了目前全无机钙钛矿太阳能电池各层较常用材料的能级结构示意图,图中给出了各种材料的最高电子占据轨道（highest occupied molecular orbital, HOMO）能级和最低电子未占据轨道（lowest unoccupied molecular orbital, LUMO）能级。根据能级匹配原则,界面处的自由电子会流向更低LUMO能级的材料,而自由空穴会流向更高HOMO能级的材料。因此通过对各层材料进行合适的选择,可以有效改善开路电压等参数性能,获得更高的光电转换效率。
除此之外,考虑到钙钛矿材料具有独特的双极性性质,既可实现光电转换,又可作为空穴电子的导体。因此在该基础上可以进一步简化制备工艺,降低成本,发展出无空穴传输层或无电子传输层的钙钛矿太阳能电池。例如,LIANG等[13]基于介孔光伏电池结构,利用碳浆材料作为导电极,以CsPbBr3为吸光层制备FTO/c-TiO2/m-TiO2/CsPbBr3/碳电极无空穴传输层器件,最终光电转换效率为6.7%。KE等[14]通过臭氧处理氧化氟化锡（fluorinated tin oxide, FTO）导电玻璃并且调控钙钛矿层组分,制备的无电子传输层器件也获得13%的光电转换效率。

2 不同结构全无机钙钛矿材料的应用

不同结构全无机钙钛矿材料主要指的是晶体结构为ABX3、A2BX6（2-1-6）、A2B1+B3+X6（2-1-1-6）、类钙钛矿如A3B2X9（3-2-9）的钙钛矿材料,具体晶体结构如图2所示。ABX3型材料（图2a）是传统的三维正八面体结构（A位于八面体间隙中,B位于八面体体心,X位于八面体各个顶点位置）,具备高对称性,结构的理化性质优异。而A2BX6和A2B1+B3+X6结构都属于双钙钛矿结构,A2BX6（图2b）通过在B位交替形成一个空位和一个 +4阳离子维持电中性条件,但空位的形成会降低原有的三维钙钛矿结构的维度,影响材料的光电性能;为了弥补A2BX6这一缺陷,A2B1+B3+X6（图2c）通过一个+1价阳离子和 +3价阳离子交替排列维持电中性条件,在一定条件下也能维持三维结构,但是由于离子半径和键长差异导致材料结构畸变,进而造成光电性能的改变。除此之外,A3B2X9结构是类钙钛矿结构（图2d和图2e）,通常是零维或者二维层状结构,因此诸多性能与ABX3型也有较大差别。下面分别介绍不同类型材料及其太阳能电池性能。
Fig. 2 Crystal structures of different perovskite materials

图2 不同钙钛矿材料晶体结构

2.1 基于ABX3型钙钛矿的太阳能电池

CsBX3（B = Pb2+, Sn2+; X = I-, Br-, Cl-）是目前全无机钙钛矿材料研究最为广泛的结构。CsBX3具有传统3D的正八面体结构（如图2a所示）使得晶相结构更加稳定,并且不含有机官能团,表现出对光照、高温、水分、氧气更好的耐受性等优点,并且可以通过调控I、Br和Cl的掺杂比例,在一定程度上实现对钙钛矿材料吸收光谱的连续调整[15]。目前常见的CsBX3有CsPbI3、CsPbI2Br、CsPbIBr2、CsPbBr3、CsSnX3等。CsPbCl3应用较少,主要是由于CsPbCl3的缺陷形成能低,导致晶体结构中易产生大量缺陷,增加非辐射跃迁概率,特别是在蓝紫光区域的光致发光量子产率（photoluminescence quantum yield, PLQY）低于10%,极大地限制了CsPbCl3晶体材料在短波区域的应用[16]。如图3所示,基于不同ABX3型钙钛矿材料的太阳能电池效率的总结,可见CsPbI3是目前效率最高的,也是研究较为广泛的一种材料。
Fig. 3 Efficiency characteristics of ABX3 type inorganic perovskite materials in different device configurations

图3 ABX3型无机钙钛矿材料在不同器件构型的效率特性

利用Goldschmidt容忍因子（tolerance factor, t）准则,如公式（1）,计算CsPbI3晶体结构,发现由于Cs+ 半径较小,使得CsPbI3钙钛矿晶体的容忍因子为0.807,在室温、大气环境下晶体结构具有热力学不稳定的性质,正八面体极易受环境影响发生倾斜,导致黑色钙钛矿相结构易受环境影响转变为光电性能较差的黄色非钙钛矿相结构;另外,碘空位缺陷易造成界面能级匹配度变差。以上两点在一定程度上限制了太阳能电池光电转换效率的提高,因此对于该材料的研究,主要集中在利用降维合成、组分工程、添加剂改性处理等几种方式有效阻止其由钙钛矿相转变为非钙钛矿相、提高相稳定性,以及降低全无机钙钛矿薄膜缺陷等方面。
$t=\frac{{{R}_{A}}+{{R}_{X}}}{\sqrt{2}\left( {{R}_{A}}+{{R}_{X}} \right)}$ (1)
降维合成策略可有效提高全无机钙钛矿稳定性。例如SWARNKAR等[17]在以CsPbI3为吸光材料的基础上制备了α-CsPbI3量子点电池,光电转换效率达到10.77%,在空气中放置数月仍具有良好的稳定性,利用降维合成策略还可以制备纳米线,纳米柱等表面能高的低维纳米结构,使得晶相结构更加稳定。
以元素掺杂和替代为主的组分工程也可以有效改善CsPbI3稳定性、钝化表面缺陷和提高载流子运输能力[18]。例如引入小半径的Br- 提高结构的容忍因子,可以使四方相钙钛矿结构转变为立方相结构,提高晶体热稳定性,但Br- 的引入会使碘基钙钛矿材料的带隙拓宽,溴的含量越多,带隙也越宽。引入离子半径更小的Cl- 能影响钙钛矿薄膜形貌和晶格取向,使得钙钛矿的电学特性产生明显的改变,而对于光学带隙的影响很微小[19,20];并且由于Cl- 和I- 半径的巨大差异,Cl- 可被纳入碘基钙钛矿晶格的最大含量受到极大限制[21,22]。孟庆波课题组通过两步法制备表面致密均匀、高结晶性和大尺寸晶粒的CsPbI2Br钙钛矿薄膜,制成器件的光电转换效率为13.27%,并表现出较好的环境稳定性[23]。
添加剂可细化分为界面添加剂和晶体表面添加剂工程,即钝化材料表面和界面的缺陷和陷阱态,并且包覆保护材料免受外部环境（如水、氧等）的影响。例如赵一新课题组采用三甲基苯基氯化铵（trimethylphenylammonium chloride, PTACl）来钝化CsPbI3钙钛矿的表面,经高温退火处理获得高质量薄膜,电池器件最终获得目前最高的光电转换效率,为19.03%[24]。WANG等利用碘化胆碱（choline iodide, CHI）改性剂处理钙钛矿表面,在很大程度上改善了钙钛矿层表面裂纹和针孔对薄膜性能的影响,增加了电荷载流子寿命,提高了CsPbI3光吸收层同其他功能层间的能级匹配程度,器件的最终光电转换效率也达到了18.4%[25]。
针对碘空位对电池的影响,WANG等[26]通过在制备CsPbI3钙钛矿时使用HPbI3,有效降低了黑色相的结晶能垒。并且在CsPbI3钙钛矿表面用聚醚酯乙酰胺（polyetherester acetamide, PEAI）层修饰,发现PEAI在晶格表面形成了功能有机分子层而不是进入钙钛矿晶格形成PEA2PbI4二维钙钛矿,可以有效钝化表面的Cs+ 和I- 空位以及其他缺陷,降低表面能,提高能级匹配度,进而提高钙钛矿的效率及其相稳定性。
另外,考虑到Pb的毒性,研究者们一直尝试利用Sn替换Pb,可是从图3可以看出基于Sn2+ 的效率始终很低。虽然Sn2+ 形成的ASnX3无机钙钛矿光电性能较好,但其稳定性较差,Sn2+ 极易被氧化成 Sn4+,形成自掺杂形式的Sn空位,导致器件的性能较差。因此出现了利用Sn4+ 或者双原子替代Pb的双钙钛矿材料。

2.2 基于双钙钛矿材料的太阳能电池

A2BX6构型的钙钛矿结构也被称为双钙钛矿构型或缺陷型钙钛矿,晶体结构如图2b所示。这种双钙钛矿型材料通常被认为通过移除一半的B位阳离子得到。根据电中性原则,移除留下的为空位,剩余一半B位阳离子应处于 +4价态。另一种情况是一半B位离子为 +1价态,则需在另一半B位掺杂 +3价态元素确保电荷中性要求,由此产生了A2B1+B3+X6构型（如图2c）。不论A2BX6型,还是A2B1+B3+X6型,均是在无铅环保理念条件下产出的。例如Cs2SnX6这类锡基缺陷型钙钛矿的最大优势在于,化合物的B位点是Sn4+ 而不是易氧化的Sn2+,这使其性质上更加稳定,器件对空气和水分体现出更高的耐受性[45]。另外,对于新型无机双钙钛矿材料,例如Cs2AgBiX6（X = Cl, Br）,因其载流子复合寿命长、毒性低、高稳定性等优点,已被理论和实验证实是光伏应用中的潜在候选材料[46,47]。
有科研工作者以A2BX6和A2B1+B3+X6（B = Sn; B1+ = Na, Ag; B3+ = Bi）构型展开了诸多新的研究探索,寻求其在光电转换效率上的突破。表1列出部分基于双钙钛矿结构太阳能电池器件的构型及光电转换效率。从表中可以看出,Cs2SnI6钙钛矿电池短路电流密度（short-circuit current density, JSC）较其他材料有着突出优势,这主要是由于Cs2SnI6的n型半导体性质可以防止早期电荷重组,提供比液体电解质更长的电荷扩散长度。并且高填充因子（fill factor, FF）弥补了准固态材料Cs2SnI6低开路电压（open-circuit voltage, VOC）的影响,最终光电转换效率（power conversion efficiency, PCE）为6.1%。而A2B1+B3+X6构型材料JSC普遍比较低,在很大程度上归结于A2B1+B3+X6钙钛矿成膜质量不佳,导致器件性能很难有所突破。由于A2BX6和A2B1+B3+X6这类双钙钛矿结构仍处于开发的早期阶段,薄膜质量较差,急需进一步改进制备工艺,并且相转变机理亦在探索中,因此基于双钙钛矿型太阳能电池的光电转换效率普遍较低;另一个限制卤化物双钙钛矿太阳能电池性能的主要因素是钙钛矿材料与其他功能层的能带结构不匹配、自身光学吸收波长窗口较窄等问题。
Table 1 Efficiency characteristics of A2BX6 and A2B1+B3+X6 inorganic perovskites in different device configurations

表1 A2BX6及A2B1+B3+X6型无机钙钛矿材料在不同器件构型中的效率特性

Structure type Device structure PCE / % JSC / (mA/cm2) VOC / V FF / % Ref.
A2BX6 FTO/TiO2/Cs2SnI6/PEDOT/FTO 6.10 14.10 0.62 70.0 [48]
A2B1+B3+X6 FTO/TiO2/Cs2AgBiBr6/Spiro-OMeTAD/Au 2.43 3.93 0.98 63.0 [49]
FTO/TiO2/Cs2AgBiBr6/PTAA/Au 1.26 1.84 1.02 67.0 [50]
FTO/TiO2/Cs2AgBiBr6/P3HT/Au 1.37 1.79 1.12 68.0 [51]
ITO/Cu-NiO/Cs2AgBiBr6/C60/BCP/Ag 2.23 3.19 1.01 69.2 [52]
ITO/TiO2/Cs2AgBiBr6/C60Spiro-OMeTAD/MoO3/Ag 2.51 3.82 1.01 65.0 [53]
ITO/TiO2/Cs2NaBiI6/Spiro-OMeTAD/Au 0.42 1.99 0.47 44.0 [54]
FTO/TiO2/Cs2TiBr6/P3HT/Au 3.28 5.69 1.02 56.4 [55]
因此,对于基于双钙钛矿材料的太阳能电池的设计重点,主要集中在寻找有效的合成路线,完善制备工艺,利用带隙工程、界面修饰等钝化缺陷,以及寻找与双钙钛矿材料能级匹配的电子传输层和空穴传输层新材料,改善能级匹配度,改进太阳能电池的结构等,拓展与其他钙钛矿结构形成多层串联太阳能电池等方面,均是双钙钛矿太阳能电池未来突破桎梏的契机。

2.3 基于类钙钛矿材料的太阳能电池

类钙钛矿材料,或钙钛矿衍生物。可以理解为从顶层的钙钛矿中衍生出来,其中A3B2X9构型（如图2d和图2e）是最常见的类钙钛矿材料,其结构为每三个ABX3去除一个B位阳离子。在缺失一个B位阳离子的情况下,要保证电中性原则,其他B位阳离子必须处于 +3氧化状态,因此B位阳离子通常为Bi3+、Sb3+。由于Bi3+ 的核外电子排布与Pb2+ 一致（6s2 6p0 ),离子半径差距小,电负性接近,因此基于Bi3+ 的钙钛矿材料的结构很稳定,而且在带隙可调及溶液加工等方面,与铅基钙钛矿相似,成为替代铅基钙钛矿的重要候选材料[56]。
鉴于离子尺寸的因素,不同的X位卤素元素会导致A3B2X9结构的改变,呈现零维或者二维结构。以Cs3Bi2X9（X = I, Br）为例,当X = I时,晶体结构如图2d所示,属于六方晶系,空间群P63/mmc,其基本的组成单元是由两个正八面体BiI6面面相接组成,由于其结构单元 [Bi2I9]3- 彼此独立,被称为零维钙钛矿。零维钙钛矿相比于传统钙钛矿降低了维数,这使其具有更好的稳定性来抵抗相变;当X = Br时,晶体结构变如图2e所示,属于三方晶系,空间群P3m1,正八面体B位原子共享三个顶点形成波纹层,也被称为二维钙钛矿,其代表材料还有Cs3Sb2I9（P3m1）、Rb3Sb2I9、Cs3Sb2Br9等。
BAI等[57]通过溶解-再结晶的方法制备了高质量的零维Cs3Bi2I9纳米薄片,并且进一步利用碘化亚铜（cuprous iodide, CuI）、2,2',7,7'-四[N,N-二(4-甲氧基苯基)氨基]-9,9'-螺二芴{2,2',7,7'-tetrakis [N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene, Spiro-OMeTAD}、聚三芳胺（polytriarylamine, PTAA）这三种不同的空穴输运材料（hole transport material, HTM）制备了基于Cs3Bi2I9的平面异质结结构太阳能电池,光电转换效率达到3.20%、1.77%、2.30%。这说明了A3B2X9吸光材料在光伏领域运用发展的可行性。除此之外,含Bi3+ 的其他无铅材料也是广泛研究的热点之一。表2列举了几种典型的基于无铅类钙钛矿材料的太阳能电池的性能。
Table 2 The performance and configuration of perovskite solar cells based on A3B2X9 and other typical lead-free perovskite analogs

表2 基于A3B2X9及其他无铅类钙钛矿材料的太阳能电池性能及构型

Structure type Device structure PCE / % JSC / (mA/cm2) VOC / V FF / % Ref.
0D FTO/TiO2/Cs3Bi2I9/CuI/Ag 3.20 5.78 0.86 64.38 [57]
FTO/TiO2/Cs3Bi2I9/Spiro-OMeTAD/Ag 1.77 4.45 0.79 50.34 [57]
FTO/TiO2/Cs3Bi2I9/PTAA/Ag 2.30 4.82 0.83 57.49 [57]
FTO/c-TiO2/m-TiO2/Cs3Bi2I9/Spiro-OMeTAD/Ag 1.09 2.15 0.85 60.00 [58]
2D ITO/PEDOT:PSS/Cs3Sb2I9/PCBM/Al 1.49 5.31 0.72 38.97 [59]
ITO/TiO2/Rb3Sb2I9/Spiro-OMeTAD/Au 1.37 4.25 0.55 59.50 [60]
ITO/NiOx/Cs3Sb2I6Br3/PCBM/C60/BCP/Ag 1.15 3.15 0.64 57.00 [61]
Other materials FTO/c-TiO2/m-TiO2/AgBiI4/PTAA/Au 2.20 5.24 0.67 62.09 [62]
FTO/c-TiO2/m-TiO2/Ag2BiI5/PTAA/Au 2.60 6.04 0.69 62.40 [62]
FTO/TiO2/Ag3BiI6/PTAA/Au 4.30 10.70 0.63 64.00 [63]
FTO/c-TiO2/m-TiO2/AgBi2I7/P3HT/Au 2.12 4.83 0.62 60.00 [64]
ITO/CuBiI4/Spiro/Au 1.11 7.18 0.38 28.67 [65]
FTO/TiO2/CsBi3I10/Carbon 1.51 4.75 0.46 69.10 [66]
从表2中可以明显看到,三价金属基钙钛矿的太阳能电池器件性能并不是很好。主要的原因是根据Shockley-Queisser极限,更宽的带隙导致可实现效率大幅降低[67];其次,Bi基类钙钛矿形成孤立的BiI6八面体框架（零维）,而不是传统的三维角共享的PbI6八面体（三维）结构,导致较差的光电特性,如高激子结合能、低载流子迁移率、高陷阱态密度等,造成明显的效率下降[68]。A3B2X9及其他典型无铅型太阳能电池显著的缺点表现在开路电压VOC上尤为突出。若想要获得高的电池光电转换效率,开路电压VOC应为0.9 ~ 1.1 V,而A3B2X9及其他类钙钛矿材料的电池开路电压比高效率的开路电压低了约0.2 ~ 0.4 V,这主要是由于A3B2X9类钙钛矿材料与用于ABX3构型太阳能电池中的传统电荷传输层材料彼此之间的能级匹配度较低以及界面处的电子与空穴复合速率大等导致的。此外填充因子FF较低,也能在一定程度上反映出各层接触电阻Rs较大,材料间匹配度较低的问题。这些因素导致三价金属基钙钛矿太阳能电池效率与ABX3型、有机-无机钙钛矿太阳能电池相比相差甚远,很可能因效率问题影响其后续的发展。
基于A3B2X9以及其他Bi基钙钛矿材料的无铅钙钛矿太阳能电池虽然光电转换效率较低,但为后续无铅型钙钛矿太阳能电池的发展提供了宝贵的经验。而上述这些问题可以通过以下手段来提升基于类钙钛矿材料的太阳能电池器件性能：（1）研究缺陷机理,利用材料工程进行缺陷钝化;（2）着重电池构型设计,寻求合适的能级匹配材料;（3）完善制备工艺、提高薄膜质量等。

3 总结与展望

基于CsPbX3型的全无机钙钛矿太阳能电池的光电转换效率已经超过了20%,说明全无机钙钛矿太阳能电池在效率上的发展前景广阔。相较而言,基于双钙钛矿材料以及类钙钛矿材料等无铅型全无机钙钛矿材料的太阳能电池在效率上还相差甚远,但独特的结构使其具备更好的稳定性,在要求环保无铅、稳定高效的光伏领域未来可期。目前基于全无机钙钛矿材料的太阳能电池仍然存在着诸多的问题,例如材料合成、薄膜制备以及器件组装等方面仍然缺乏深入的研究,尤其是双钙钛矿材料以及类钙钛矿材料的薄膜质量不佳,严重影响电池的效率,此外各功能层材料的选择仍较单一,很多是继续沿用有机-无机钙钛矿材料的电荷传输层材料,导致能级失配较严重。对于后期的研究,可将重点放在以下几个方向：提高材料的相稳定性;提高基于无铅型全无机钙钛矿材料的太阳能电池的光电转换效率,以及开发具有高效特性的全无机电荷传输层,实现真正意义的全无机太阳能电池。因此,深入研究材料内部电荷传输与复合的工作机理、完善双钙钛矿结构、开发与类钙钛矿材料能级相匹配的电荷传输材料等,将有助于提高太阳能电池的性能和稳定性。

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