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0 前言
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太阳能作为一种绿色、清洁、可再生并广泛存在的新能源，充分利用太阳能是缓解能源短缺和解决环境污染的重要途径之一。目前，电能作为人类生产及生活的主要能源形式被广泛利用。因此，将太阳能转换为电能即构建太阳能电池器件是较为理想的方案。目前，太阳能电池已经发展到以钙钛矿和有机太阳能电池为代表的第三代新型电池。其中，钙钛矿太阳能电池器件（Perovskite solar cells，PSCs）的活性层为具有钙钛矿结构的 ABX3 型化合物，钙钛矿活性层（如 MAPbI3）凭借其较高的摩尔消光系数（～105）、载流子迁移率（66 cm2·V−1·s −1）、合成方法简单等特点得到迅猛发展[1-4]。经过十余年的发展，钙钛矿太阳能电池的光电转换效率（Power conversion efficiency，PCEs）已从最初的 3.8%一跃攀升至 25.7%[5-7]，成为光伏器件研究领域的新星。当前，深入探究钙钛矿活性层的大面积高效制备方法，并进一步优化器件的光电转换效率等方面的研究工作成为当前该领域的研究热点。
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制备高效钙钛矿太阳能电池器件的关键在于获得高纯度、高结晶度、大晶粒尺寸的钙钛矿晶体及较高基底覆盖率的光吸收层[8-12]。目前，钙钛矿活性层的制备方法主要分为溶液法[13-16]、真空蒸镀法[17-19]及气相辅助溶液法[20]等。其中，真空蒸镀法是利用一种或多种钙钛矿前驱体在真空环境中气化，并基于物理或化学沉积的方式制备钙钛矿薄膜，相比于溶液法易于获得高结晶度且分布均匀的致密活性层薄膜。不仅如此，真空蒸镀法制备过程规避了有机溶剂的使用，因而该方法可用于多层钙钛矿及钙钛矿同质结的制备，并在大面积连续化制备钙钛矿薄膜方面具有较大的优势、潜力及较高的试验复现性。
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基于真空蒸镀设备的蒸发源数量的不同，可分为单源、双源及多源三类设备。在双源和多源设备中，根据前驱体沉积将方法主要分为共蒸法和分步蒸镀法：
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（1）共蒸法是指在真空室体内同时加热两种或多种前驱体原料，通过调节加热源的加热功率来控制不同原料的蒸发速率，进而调控钙钛矿反应原料的化学计量比，最终获得钙钛矿薄膜。
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（2）分步蒸镀法是将不同的前驱体原料分批次沉积至基片表面，并通过各原料沉积的膜层厚度来调控化学计量比，来实现钙钛矿中各组分的精确配比。
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本文主要从真空蒸镀设备改进及蒸镀制备钙钛矿薄膜和太阳能电池工艺优化方面对相关研究现状进行综述。
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1 真空蒸镀介绍
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1.1 真空蒸镀基本原理
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真空蒸镀是指在一定的真空条件下，采用电阻、电子束、射频感应、电弧、激光等方法加热镀膜材料，使其气化为具有一定能量的气态粒子，由于空气气体的密度随着真空度的提升而减小，因此气态粒子在真空环境中具有更大的自由程，将自由、快速地运动至基片表面形成薄膜。在蒸镀过程中，一部分气态粒子吸附在基片表面上形成晶核，随着镀膜材料的持续蒸发，在基片表面沉积的晶核逐渐增大并连接起来，最终形成连续的薄膜，其余的气态粒子则与基片表面发生弹性碰撞，被反射后沉积在真空室体的内壁表面（如图1 所示）。
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图1 真空蒸镀原理示意图
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Fig.1 Schematic illustration of a vacuum deposition technology
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真空蒸镀过程对室体内的真空度具有较强依赖性，较低的真空度则易于导致气态粒子与室体内残余空气分子发生碰撞，难以形成连续均匀的薄膜，并导致膜层被污染或氧化。真空蒸镀法与其他沉积方法相比，具有薄膜纯度高、质量好、沉积厚度可控、成膜效率高、薄膜的生长机理单纯、工艺重复性高等特点。
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1.2 真空蒸镀体系制备钙钛矿太阳能电池的装备设计
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目前，用于钙钛矿薄膜制备的真空环境系统如图2 所示，主要由真空获得系统、热蒸发系统、电气控制系统，冷却及气压控制系统所组成，该系统可获得 10−3～10−5 Pa 的真空度，并具有清洁、可靠、获得目标真空度用时短的特点。
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1. Mechanical vacuum pump 2. Roots pump 3. Magnetic exchange valve4. Ionization gauge5. Resistance gauge6. Vacuum Chambers 7. High vacuum gate valve8. Molecular pump 9. Pressure sustaining valve10. Resistance gauge11. Magnetic exchange valv
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图2 真空蒸镀系统示意图
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Fig.2 Schematic illustration of a vacuum deposition system
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以双源共蒸的热蒸发系统为例（图3a），该系统配置了两个独立的热电阻蒸发源，在同一真空环境下同时实现两种不同膜料的蒸发，并在各自蒸发源的上方装备了膜厚监测仪，可以实时获得蒸发过程中膜料的蒸发速率，因而该热蒸发系统易于获得均匀、连续且结晶质量高的钙钛矿薄膜，LIU 等[17]构建的电池器件具有15.4 % 的光电转换效率。然而，该蒸镀过程需要严格控制两种膜料的蒸发速率，以确保不同的膜料按照既定的化学计量比同时被蒸镀至基片表面，从而形成钙钛矿薄膜。以 CH3NH3PbI3 （MAPbI3）为例，所采用的碘甲胺（MAI） 原料的分子量较小（158.9 g / mol），进而在蒸镀过程中对于 MAI 分子的发速率及沉积膜厚度难于实时精准测量[18]。为了能够避免共蒸过程中两蒸发源之间的相互热干扰，同时实现不同膜料沉积速率的精准监测， MALINKIEWICZ 等[21]首次在两个蒸发源之间架装挡板（挡板高度略高于蒸发源），并在基片等高处对于不同的膜料分别装备膜厚监测仪，为各组分膜料沉积效率的精准监测提供了有效的解决方案（如图3b 所示），大幅度降低了两蒸发源在膜厚仪监测过程中的相互干扰。进一步，通过调整蒸发源位置或加装自动控温设备来精确调控钙钛矿薄膜中各组分的化学计量比，可有效提升钙钛矿薄膜制备的稳定性和再现性[22]。例如，ONO 等[23]将 MAI 蒸发源装备于真空室体外部（如图3c 所示），并将正对 MAI 源喷头的膜厚监测仪探测头朝上放置，在共蒸过程中通过精确控制 MAI 蒸气进入真空室体内的流量来调节钙钛矿薄膜的各组分化学计量比（PbI2∶MAI），并且在精确控制 MAI 蒸气流量的条件下将更易于长期保持真空室体内的真空度，进而最终得到了均匀、致密且重现性良好的钙钛矿薄膜（5 cm×5 cm）。如图3d 所示，TEUSCHER 等[24]采用 PID（比例-积分微分）控制器，以蒸发源的加热功率作为被控变量，实现了对 PbI2 和 MAI 蒸发速率的精确控制，最终精确调节了沉积的钙钛矿膜中各组分的化学计量。在钙钛矿太阳能电池不断向产业化快速迈进的同时，实现高通量、连续性制备大面积且高效稳定的活性层薄膜成为当前研究面临的挑战之一。
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图3 双源共蒸系统示意图
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Fig.3 Schematic illustration of dual-source co-vacuum deposition system
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为此，需要对传统热蒸镀装备中的基片传动模块加以改进。如图4 所示，FENG 等[25]将目前普遍采用的旋转基片模块改为卷对卷式送片模块，并采用 300 cm2 的柔性基底，通过卷绕传动在收卷与放卷的过程中完成钙钛矿薄膜的蒸镀与传送，实现了大面积钙钛矿薄膜连续蒸镀，同时，通过真空低温退火工艺得到了高质量钙钛矿薄膜，并制备了光电转换效率高达 21.32 %的钙钛矿太阳能电池。
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综上所述，研究者们对真空蒸镀设备不断改进，钙钛矿太阳能电池在高效、稳定及大面积制备等研究领域取得了有目共睹的成果。在真空装备系统改进的基础上，对真空蒸镀过程的工艺优化将是钙钛矿太阳能电池在商业化进程中的核心环节，最终获得具有较高光电转换效率的大面积钙钛矿太阳能电池器件。
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图4 卷对卷式多源共蒸系统示意图[25]
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Fig.4 Schematic illustration of multi-source vacuum deposition system of roll-to-roll [25]
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2 真空蒸镀体系制备钙钛矿太阳能电池的工艺优化
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2.1 单源真空蒸镀工艺调控
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钙钛矿薄膜的单源蒸镀通常采用闪蒸的方式进行，即将前驱体放置在金属蒸发源上，在真空环境下对金属蒸发源施加大电流（30～100 A），使前驱体中有机及无机组分同时气化，并在相同时间内沉积至基板表面，进而完成薄膜制备[26-27]。
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例如，FAN 等[28]通过在 1 MPa 室体内压力条件下，将工作电流快速增加到 100 A，使 MAPbI3 晶体粉末迅速蒸发，获得约 400 nm 的 MAPbI3 薄膜；所构建的电池器件展现出 19.47 mA / cm2 的短路电流密度及几乎可以忽略的迟滞效应，但其光电转换效率仅有 10.9 %。为了进一步提高单源蒸镀 MAPbI3 钙钛矿光伏器件的光电转换效率，LONGO 等[29]在 10 Pa 的真空室体内对旋涂有 MAPbI3薄膜的金属钽基底施加约 30 A 电流，使 MAPbI3 瞬间气化并沉积在距离蒸发源 10 cm 的基底上，从而获得高纯度的 MAPbI3 薄膜，该电池器件获得了 1.067 V 的开路电压及 68.04 %的填充因子，其光电转换效率达到 12.2 %（如图5）。
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图5 单源蒸镀钙钛矿薄膜 XRD 及电池器件 J-V 曲线[29]
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Fig.5 XRD pattern of a MAPbI3 thin film and J-V curve of a MAPbI3 PSCs [29]
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然而，单源真空蒸镀工艺由于原料的蒸发速率较快，薄膜沉积的厚度难以实现精确调控。为了解决这一问题，LEE 等[30]对前驱体投料质量和沉积次数进行了优化调控，探究了前驱体投料量与蒸镀次数对晶粒尺寸和薄膜厚度的关系，即钙钛矿晶粒尺寸与前驱体投料质量为正相关，实现了沉积厚度可控，且在相同投料量下分批蒸镀可获得更厚的薄膜 （如图6）。
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图6 不同蒸发源投料量和蒸镀次数对 MAPbI3薄膜的影响[30]
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Fig.6 Effects of different source mass and deposited step for perovskite films [30]
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另一方面，OTALORA 等[31]探究了蒸发距离、时间及蒸镀压力对钙钛矿晶粒尺寸及薄膜覆盖率的影响。如图7 所示，蒸发源与基底之间的距离大于 1.5 cm 时可获得晶粒尺寸大于 1 mm 的钙钛矿薄膜，但薄膜对基底覆盖率不足 85 %，然而随着距离的增加晶粒尺寸明显减小，但薄膜对基底覆盖率逐渐增加至 100 %。与此同时，随着延长蒸镀时间和提高蒸镀压力均可增加晶粒尺寸和基底覆盖率。
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此外，WEI 等[32-34]提出利用激光诱导闪蒸印刷技术（Flash evaporation printing，FEP）制备 MAPbI3 薄膜的策略，并通过调整钙钛矿薄膜退火条件，进一步提高了钙钛矿薄膜的晶粒尺寸（如图8 所示），使电池器件的光电转换效率高达 16.8 %。然而，该闪蒸印刷技术对设备要求较高，增加了钙钛矿电池的制造成本，在钙钛矿太阳能电池的商业化应用领域仍面临挑战。
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图7 不同蒸镀参数对钙钛矿晶粒尺寸及薄膜覆盖度的影响[31]
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Fig.7 Effects of different evaporation parameters on grain size and film coverage of perovskite [31]
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图8 不同退火温度下钙钛矿薄膜的表面 SEM 图及电池器件 J-V 曲线[34]
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Fig.8 Surface SEM images of perovskite films after different annealing processes and J-V curves of PSCs [34]
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为了弥补有机-无机杂化钙钛矿长期稳定性不足的缺点，研究人员通过制备全无机的钙钛矿太阳能电池器件的方式进一步提升电池器件长期稳定性[35-36]。LIU 等[37]和 LI 等[38]分别通过加热坩埚中不同摩尔比的 CsBr 和 PbBr2 粉末（摩尔比分别为 2∶1 与 1.1∶1），并采用不同退火方式（马弗炉内退火与不退火），最终获得光电转换效率达到 7.81 % 与 8.65 %的电池器件。
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尽管单源真空蒸镀工艺可通过调节前驱体的化学组份制备不同种类的钙钛矿薄膜，例如 Yb3+ : CsPb（Cl1-xBrx）3、CH3NH3SnI3、Cs2NaBiI6 等钙钛矿薄膜[39-41]，但单源真空蒸镀较难对薄膜的各组分进行精确调控。因此，在单源真空蒸镀工艺基础上开发双源或多源真空蒸镀工艺，成为实现多种化学组分的钙钛矿薄膜高效率制备的研究热点。
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2.2 双源真空蒸镀工艺调控
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2013 年，基于双源真空蒸镀法制备钙钛矿太阳能电池的技术被首次报道[17]，相较于传统的溶液制备方法，该方法避免了有机溶剂的使用，并能够获得致密、均匀的钙钛矿薄膜。目前，有机-无机杂化结构的钙钛矿薄膜展现出较高的光电转换效率（如 MAPbI3，PCE 接近 21.8 %）[42]。但该类结构的钙钛矿薄膜需要精确控制前驱体的化学计量比，从而实现钙钛矿产物的高效生成及相应电池器件较高的光电转换效率。因此，精确控制双源真空蒸镀过程中前驱体沉积的化学计量比成为制备高结晶度的钙钛矿薄膜的关键因素之一[43-44]。本节依据双源真空蒸镀制备工艺的不同将其分为双源共蒸与分步蒸镀，并对其制备工艺及研究进展进行综述。
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在双源共蒸研究方面，DUAN 等[45]通过调整 CsBr∶PbBr2前驱体沉积速率比为 1∶1（0.5 Å / s）， N2 氛围下 300℃退火 20 min 获得 9.43 %的光电转换效率。HEINZE 等[46]采用延迟蒸镀 MAI 的方式，即在基底上先沉积一层 PbI2（约 20 nm）再进行双源共蒸，并通过调节室体内压力实现对 MAI 沉积速率的控制，最终在室体内压力为 7.5 MPa 下获得最高 14.6 %的光电转换效率（如图9）。
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图9 真空沉积压力对钙钛矿薄膜及电池器件的影响[46]
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Fig.9 Effect of vacuum deposition pressure on perovskite films and battery devices [46]
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关于双源共蒸沉积动力学的研究结果表明，基底上预先沉积的 PbI2 可作为钙钛矿晶体生长的中心，更有利于 MAI 的吸附和反应[47]。另外，LIU 等[17]采用 PbCl2 和 MAI 作为前驱体，在压力小于 1 MPa 的室体内同时对其加热蒸发，获得了致密且均匀的钙钛矿薄膜（MAPbI3-xClx），组装的电池器件达到 15.4 %的光电转换效率（如图10 所示）。然而双源共蒸需要对无机源（如 PbI2）和有机源（如 MAI）实现同步的蒸发速率精确控制，对于操作人员的要求较高，钙钛矿薄膜制备的重现性仍有待于提升。
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另外，LI 等[48-49]对基底形状进行调整，进一步减小串联电阻，制备了光电转换效率为 20.28 % （0.16 cm2）及 18.13 %（21 cm2）电池器件。同时，LI 等[50]通过改变电池器件结构（倒置结构），并提出在真空沉积过程中降低真空室体压力（1.3～0.16 MPa）来控制 MAPbI3沉积，从而制备不同能级的钙钛矿层，其中电池器件最佳光电转换效率达到 20.61 %。
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图10 真空沉积法与溶液法制备电池器件对比[17]
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Fig.10 Comparison between vacuum deposition method and solution method for preparing battery devices [17]
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在分步真空蒸镀研究方面，HUA 等[51]对 CsPbBr3 钙钛矿薄膜在不同压力（0.1～10 MPa）下进行退火处理，指出随着压力从 0.1 MPa 升高至 10 MPa，钙钛矿晶粒尺寸从 300～500 nm 增加到 1.5 μm 以上，其电池器件光电转换效率达到 7.22 %。通过改变退火条件，XIANG 等[52]采用马弗炉对 CsPbBr3 钙钛矿薄膜进行退火，控制升温速率为 15℃ / min，升温至 320℃保持 20 min，再降温到 300℃保持 40 min 后自然降温，获得的钙钛矿晶粒尺寸约为 2 μm，电池光电转换效率达到 9.35 %。
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NG 等[53]提出降低单次沉积的薄膜厚度（单层膜厚约 50 nm），并通过多次逐层沉积 PbI2 和 MAI 薄膜策略以有效减少 PbI2 的残留，相应的电池器件实现了 12.5 %的光电转换效率。另外，CHEN 等[18] 通过调控分步蒸镀过程中基底温度，延长 MAI 分子向 PbCl2 膜层扩散的距离，研究表明基底温度控制在 75℃时，MAI 可与厚度为 150 nm 厚的 PbCl2 薄膜完全反应，生成高纯度的钙钛矿薄膜，其电池器件的光电流密度可达到 20.9 mA / cm2，最终获得 15.4 %的光电转换效率（如图11 所示）。
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另外 TAVAKOLI 等[54-55]报道了以 PTAA 和 CuPc 分别作为空穴传输材料，采用分步真空沉积的方式制备倒置全蒸镀的 MAPbI3 太阳能电池器件，并分别取得 19.4 %及 20.3 %的光电转换效率。同时，ABZIEHER 等[56]及 ROß 等[57]研究了基底材料对钙钛矿形貌的影响，结果表明在 PTAA 等非极性基底表面，钙钛矿晶体优先在垂直于基底的方向生长，并在 MeO-2PACz 基底上制备了光电转换效率为 20.6 %的电池器件，尽管分步蒸镀方法降低了薄膜制备的操作技术难度，并有效解决了钙钛矿薄膜中无机前驱体的残留问题，但是薄膜中不同的前驱体之间的完全转化仍需较为复杂的退火、溶剂退火等后处理工艺，因而在高通量制备钙钛矿太阳能电池器件方面仍面临挑战。
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图11 双源分步真空沉积示意图及基底温度对钙钛矿薄膜与电池器件的影响[18]
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Fig.11 Schematic diagram of dual-source layer-by-layer vacuum deposition and the effect of substrate temperatures on perovskite thin films and PSCs [18]
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2.3 多源真空蒸镀工艺调控
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近年来，研究者们发现杂元素掺杂策略能够有效提升钙钛矿薄膜的光电性能，提高器件的光电转换效率。真空蒸镀法凭借着无溶剂、易于进行元素掺杂及可大面积制备等优势，受到广泛关注[58-59]。
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因此，在双源真空蒸镀工艺优化的基础上，通过增加蒸发源的数量来构建多源真空蒸镀体系，从而实现在蒸镀过程中对钙钛矿薄膜进行元素掺杂或加入添加剂，进一步提升其电池器件的光电转换效率。例如 ZHANG 等[60]引入 10 %的 PEAI 作为添加剂制备γ-CsPbI3 的电池器件，不仅将电池器件的光电转换效率从 3.17 %提升到 15.00 %，并且光电转换效率在 215 d 后基本保持不变。CHIANG 等[61]采用三源共蒸的方式制备光电转换效率为 18.2 %的 FA0.7Cs0.3Pb（I0.9Br0.1）3钙钛矿电池。BOLINK 课题组通过三源共蒸的方法分别制备了 MAPbI3-xBrx [62]、 MAPbI3-xClx [63]、FAxMA1-xPbI3 [64]薄膜，并获得了18.8 %的最佳光电转换效率。
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另外，多源蒸镀也可采用分步沉积的方式制备电池器件，FENG 等[25]通过三源分步真空蒸镀的方式，获得精确比例掺杂的 Cs0.15FA0.85PbI3 钙钛矿薄膜；并对钙钛矿薄膜在约 10 Pa 的 N2 氛围下进行退火处理，在 60℃退火条件下获得最佳电池器件，极大地增加了载流子的复合电阻（3 715 Ω）和降低缺陷密度（2.5×10−15 cm−3），并提高载流子迁移率 （1.33×10−2m2·V−1·s−1 ），极大提升了电池器件的光电转换效率（如图12）。类似地，LI 等[65]采用分步真空蒸镀的方式制备 Cs0.05FA0.95PbI3-xClx 钙钛矿薄膜。由于制备过程中引入 Cl−，进而利于有机卤化铵的扩散并促进钙钛矿的生成，并在获得结晶度高、缺陷密度低的高质量钙钛矿薄膜，其电池器件具有 1.152 V 的开路电压、25.92 mA / cm2 的短路电流密度、81.78 的填充因子及 24.42 %（0.1 cm2 ）的高光电转换效率（如图13）。
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图12 多源真空沉积制备钙钛矿电池器件示意图及表征结果[25]
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Fig.12 Schematic diagram and characterization of PSCs by multi-source vacuum deposition [25]
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图13 多源真空沉积制备 Cl 掺杂的钙钛矿太阳能电池[65]
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Fig.13 PSCs of Cl-doped by multi-source vacuum deposition [65]
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3 结论与展望
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相比于传统溶液法，真空蒸镀法能够制备更为致密、均匀及大面积的钙钛矿薄膜，并且可以避免薄膜制备过程中使用有机溶剂，减少对环境的污染，进而成为钙钛矿太阳能电池器件商业化生产中的有效方法之一。
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目前，真空蒸镀制备钙钛矿活性层的设备主要分为单源、双源及多源蒸镀，并对三种设备的优缺点进行对比，得出以下结论：
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（1）单源真空蒸镀虽然制备工艺相对简便，但由于蒸发速率较快容易导致薄膜的晶体质量不高，过快的膜料蒸发速率对于精确调控膜层的厚度及实现高结晶度的钙钛矿晶体带来诸多困难。
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（2）双源和多源真空蒸镀工艺具有相似之处，通过优化真空装备系统及改进蒸镀工艺，已经基本克服了无机前驱体残留和碘甲胺蒸汽分子沉积速率难以控制的问题，
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（3）多源真空蒸镀还能够实现多元素及多组分掺杂，从而提升钙钛矿薄膜的光伏性能，在今后的真空蒸镀大面积钙钛矿活性层薄膜领域极具潜力。
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然而，根据目前已有的报道，真空蒸镀法在制备钙钛矿太阳能电池器件中大部分局限于仅局限于钙钛矿活性层的制备工艺优化，对连续化高通量制备大面积钙钛矿太阳能电池器件仍然面临诸多困难。
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对真空蒸镀制备钙钛矿太阳能电池工艺的未来发展方向展望如下：
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基于多源真空蒸镀体系开发钙钛矿太阳能电池器件相关功能层（包括电子传输层，钙钛矿活性层，空穴传输层，电极）的高通量连续化蒸镀新工艺，并实现大面积制备具有高光电转换效率的钙钛矿太阳能电池器件，将成为今后重要的研究热点之一。
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此外，为了响应国家“碳达峰、碳中和”的号召，在保证钙钛矿电池器件高光电转换效率的基础上，开发低成本的真空蒸镀设备，简化器件制备工艺和电池器件结构，从而推进钙钛矿太阳能电池器件商业化，将成为未来的主流研究方向。
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摘要
钙钛矿太阳能电池器件因其优异的材料性能已经取得最高 25.7 %的光电转换效率。常见的钙钛矿薄膜的制备方法分为溶液法和真空蒸镀法。其中真空蒸镀法凭借其无溶剂化的特点，具有环境污染小、膜层致密性高、生产效率高，以及较为容易实现大面积连续化高通量制备等优点，在钙钛矿太阳能电池器件制备领域具有独特优势。针对真空蒸镀法制备高质量钙钛矿薄膜的技术，对真空蒸镀的基本工作原理及应用于钙钛矿薄膜制备的真空设备系统进行介绍，并以钙钛矿太阳能电池光电转换效率为切入点，介绍基于真空蒸镀技术制备钙钛矿太阳能电池器件及优化其光电转换效率的研究进展。探究真空蒸镀设备改进策略（温控系统和蒸发源设置）、蒸镀工艺条件（投料量、蒸镀距离、蒸镀时间、室体内压力、薄膜退火温度及时间）、基底材料极性对钙钛矿薄膜结晶度和晶粒尺寸的影响因素，为采用真空蒸镀技术制备具有高光电转换效率的钙钛矿太阳能电池器件提供重要的理论基础及研究思路。最后，总结真空蒸镀钙钛矿太阳能电池的工艺流程，从降低生产成本与提高生产效率的角度出发，提出连续化生产钙钛矿太阳能电池的构想，并对其商业化发展方向进行展望。
Abstract
Organic-inorganic halide perovskites have attracted considerable attention because of their excellent material characteristics, including an appropriate direct bandgap, high absorption coefficient, excellent carrier transport, and apparent tolerance to defects. The chemical structure of halide perovskites can be termed ABX3, where A, B, and C represent aromatic ammonium cations, divalent metal cations, and halogen anions, respectively. Owing to the significant efforts made by researchers, the certified power conversion efficiency (PCE) of perovskite solar cells (PSCs) has reached 25.7 %. Many processes, including solution- and vacuum-based methods, have been investigated for perovskite films. Among these, the solution-based method is simple; however, it is challenging to prepare PSCs on a large scale because of the excessive use of organic solvents, which results in environmental pollution. Vacuum deposition has many advantages in perovskite film preparation, including solvent-free, highly compact films, environmental friendliness, high productivity, and easy high-throughput large-area fabrication. Perovskite film preparation through the vacuum deposition technique mainly involves heating the precursors of organic (CH3NH3I) and inorganic (either PbI2 or PbCl2) in high-vacuum chambers to evaporate these precursors. To obtain organic-inorganic halide perovskite films with high quality and increased power efficiency, the proportion and evaporation rate of the precursors deposited on the target substrate need precise modulation. Additionally, the perovskite layer thickness significantly affects the final PCE of perovskite solar cell devices. Thicker perovskite film-based solar devices facilitate the harvest of more photons from sunlight; however, it makes separate transport of charge and holes more difficult. Therefore, the precise modulation of the layer thickness and stoichiometric ratio of the perovskite film is vital in vacuum deposition. However, it is challenging to monitor and control the deposition rate of organic precursors with low molecular weights (such as CH3NH3I) using quartz microbalance sensors. This can be attributed to the random diffusion of molecules inside the vacuum chamber and the difficulty in accumulating these precursors on the sensor surface. To obtain high-quality perovskite films using vacuum deposition, this review introduces the principle and basic structure of the vacuum deposition system. Vacuum deposition can be divided into single-, dual-, and multi-source processes based on the number of evaporation sources. Moreover, it can also be divided into flash evaporation, co-evaporation, and sequential evaporation processes. Recent advances in fabricating PSCs involved in vacuum deposition processes are discussed. For instance, the temperature-controlling system was optimized, and the evaporation source was installed in vacuum deposition equipment. Moreover, factors that affect the crystallinity, grain size, and final PCE performance of the prepared perovskite films were also investigated. These include precursor composition, the distance between the crucible and substrate, evaporation time, chamber pressure, and annealing temperature and time. Additionally, the surface polarity of the substrate material influences the nucleation, crystallization, and morphology in the vacuum deposition process. Furthermore, perovskite film composition is a significant factor that affects the PCE of PSCs and the long-term stability of PSCs. This review provides an important theoretical basis and research ideas for the preparation of PSCs with high PCE using vacuum deposition technology. Additionally, it introduces versatile fabrication methods that can be used to obtain high-performance and reproducible PSCs, which may push further improvements and commercialization of PSCs based on the vacuum deposition technique. Finally, the factors influencing vacuum deposition methods on the PCE of perovskite solar cell devices are comprehensively summarized, and the topic research is investigated in the field of manufacturing and commercially developing PSCs. In this review, the vacuum deposition of the perovskite, electronic and hole transport layers is proposed to achieve ceaseless, high-throughput, and large-area fabrication of PSCs with considerably lower cost, higher fabrication reproducibility, and excellent PCE.
