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无机材料学报, 2023, 38(12): 1387-1395 DOI: 10.15541/jim20230098

研究论文

超长稳定的混合阳离子钙钛矿太阳能电池性能优化研究

马婷婷,1,2, 汪志鹏1,2, 张梅1,2, 郭敏,1,2

1.北京科技大学 钢铁冶金新技术国家重点实验室, 北京 100083

2.北京科技大学 冶金与生态工程学院, 北京 100083

Performance Optimization of Ultra-long Stable Mixed Cation Perovskite Solar Cells

MA Tingting,1,2, WANG Zhipeng1,2, ZHANG Mei1,2, GUO Min,1,2

1. State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China

2. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China

通讯作者: 郭 敏, 教授. E-mail:guomin@ustb.edu.cn

收稿日期: 2023-02-27   修回日期: 2023-06-21   网络出版日期: 2023-06-28

基金资助: 国家自然科学基金(52172137)
国家自然科学基金(51772023)
国家自然科学基金(51572020)

Corresponding authors: GUO Min, professor. E-mail:guomin@ustb.edu.cn

Received: 2023-02-27   Revised: 2023-06-21   Online: 2023-06-28

Fund supported: National Natural Science Foundation of China(52172137)
National Natural Science Foundation of China(51772023)
National Natural Science Foundation of China(51572020)

摘要

钙钛矿太阳能电池(PSCs)发展迅速, 其能量转换效率(PCE)被一再刷新, 但长期稳定性还有待提高。目前大部分高效率钙钛矿太阳能电池在惰性气体环境中完成制备, 成本高且操作空间有限, 不利于产业化应用。本研究成功在空气中制备了具有超长稳定性的混合阳离子钙钛矿太阳能电池, 系统探究了A位阳离子掺杂对钙钛矿微观结构、光电性能以及稳定性的影响。实验结果表明, 掺杂FA+和Cs+可以提高钙钛矿薄膜质量, 优化钙钛矿/SnO2的能级排列, 抑制载流子复合, 显著提高器件的光电转换效率、长期以及湿热稳定性。Cs0.05MA0.35FA0.6PbI3电池的最佳PCE为19.34%, 在(20±5) ℃, 相对湿度<5%的黑暗环境中放置242 d后, 仍保持初始效率的85%。MAPbI3电池在同样测试条件下放置112 d后, 效率下降为初始值的30%。掺杂FA+和Cs+也显著提高了电池的抗热和抗湿性。Cs0.05MA0.35FA0.6PbI3电池分别在(85±5) ℃、相对湿度20%~30%和(20±5) ℃、相对湿度80%~90%的黑暗环境中放置96 h后, PCE分别为初始值的99%和84%, 而MAPbI3在同样条件下的PCE仅为初始值的70%和56%。本研究为在空气环境制备高效、超长稳定的混合阳离子钙钛矿太阳能电池提供了参考。

关键词： 钙钛矿太阳能电池; 混合阳离子; 长期稳定性; 全空气环境制备

Abstract

Perovskite solar cells (PSCs) are developing rapidly and their power conversion efficiency (PCE) has been repeatedly refreshed, but their long-term stability still needs to be improved. At present, most of the preparation of high-efficiency PSCs is completed in the inert gas, with high cost and limited operating space, which is not conducive to its industrial application. Here, perovskite solar cells with mixed cation, displaying ultra-long stability, were successfully prepared in the air. Effects of A-site cation doping on the microstructure, optoelectronic properties and stability of the perovskite were systematically investigated. The experimental results show that FA+ and Cs+ co-doping improves the quality of perovskite films, modulates the energy level arrangement of perovskite/SnO2, suppresses carrier complexation, and significantly improves the PCE, long-term, wet and thermal stability of the cell. The optimal PCE of Cs0.05MA0.35FA0.6PbI3 cells is 19.34%, maintaining 85% of the initial efficiency after reserving for 242 d in dark environment at (20±5) ℃ and <5% relative humidity. In contrast, the PCE of the MAPbI3 cell decreased to 30% of the initial value after reserving for 112 d under the same test conditions. FA+ and Cs+ co-doping also significantly improved the thermal and moisture resistance of the cells. Cs0.05MA0.35FA0.6PbI3 PSCs remain 99% and 84% of initial PCE after aging for 96 h at (85±5) ℃ and 20%-30% relative humidity, (20±5) ℃ and 80%-90% relative humidity in the dark, respectively. In contrast, PCEs of MAPbI3 PSCs under the same conditions remain only 70% and 56%. This study provides a reference for the preparation of highly efficient and ultra-long stable mixed cation solar cells in the air.

Keywords： perovskite solar cell; mixed cation; long-term stability; full-air environment preparation

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马婷婷, 汪志鹏, 张梅, 郭敏. 超长稳定的混合阳离子钙钛矿太阳能电池性能优化研究. 无机材料学报, 2023, 38(12): 1387-1395 DOI:10.15541/jim20230098

MA Tingting, WANG Zhipeng, ZHANG Mei, GUO Min. Performance Optimization of Ultra-long Stable Mixed Cation Perovskite Solar Cells. Journal of Inorganic Materials, 2023, 38(12): 1387-1395 DOI:10.15541/jim20230098

钙钛矿太阳能电池(Perovskite solar cells, PSCs)为光伏产业提供了广阔的发展空间和前景[1], 在过去十几年中, 单结PSCs的能量转换效率(PCE)已从2009年的3.8%[2]迅速提高到25.7%, 接近单晶硅的最高效率(26.7%)。其优异的性能归因于钙钛矿材料的光吸收能力强[3]、缺陷密度低[4]、电荷传输特性优良[5]和电荷载流子寿命长[6]等。然而, 钙钛矿材料的长期稳定性一直是制约钙钛矿太阳能电池发展的瓶颈[7]。

为了提高钙钛矿材料的稳定性, 科学家进行了许多探索。其中, 混合阳离子工程[8⇓-10]是一种提高效率和稳定性的有效方法。典型钙钛矿的化学组成为ABX3型[11], A阳离子一般为+1价的CH3NH3+、CH(NH2)2+或Cs+; B离子一般为+2价金属阳离子, 如Pb2+、Sn2+、Ge2+等; X离子一般为−1价Cl−、Br−、I−等卤素离子。在铅基钙钛矿骨架中, Pb2+位于[BX6]4−八面体晶格的中心, A阳离子占据八面体间隙[12], X离子占据八面体顶角。容差因子$\text{Goldschmidt}\left( t=\frac{{{R}_{\text{A}}}+{{R}_{\text{B}}}}{\sqrt{2}({{R}_{\text{B}}}+{{R}_{\text{X}}})} \right)$可用于预测晶体结构的稳定性, 其中RA、RB和RX分别是A、B和X的离子半径。0.8<t<1.0条件下钙钛矿结构可以维持稳定[8], t越接近1.0, 钙钛矿结构越稳定。最经典的MAPbI3(MA: 甲胺)钙钛矿材料具有钙钛矿材料独有的低陷阱密度、长载流子寿命等优点, t为0.91[13], 结构较为稳定, 室温下呈四方相。但MAPbI3的带隙为1.57 eV[14], 主要吸收太阳光中800 nm波长以下的可见光部分, 红外和近红外光吸收能力弱, 限制其电流密度。此外, 由于MA离子的挥发特性, MAPbI3的热稳定性较差[15]。与MAPbI3相比, FAPbI3(FA：甲脒)具有更合适的带隙(1.48 eV)[16], 其吸光范围更广, 电流密度更大; 半径大的FA+不易挥发, 因此FAPbI3具有优异的热稳定性。然而, FAPbI3的t为1.10, 在室温下相不稳定, 立方相(α相)将逐渐转变为宽带隙的六方相(δ相)[17], 降低器件效率。相比于MA+, FA+与空气中的H2O能形成更强的氢键, 因此FA基钙钛矿对湿度更加敏感。纯无机钙钛矿CsPbI3的容差因子约为0.8, 其室温下为不具备光活性的正交相, 不适合用于光伏器件; 具有光伏效应的黑色立方相CsPbI3带隙较宽(1.73 eV), 限制了其电流密度, 但CsPbI3的热稳定性和抗湿性很高[18]。

单一阳离子钙钛矿难以同时满足稳定性和高效性的要求, 通过A位阳离子掺杂可以解决这个问题。据报道[19-20], 向MAPbI3中引入部分FA+有利于薄膜结晶, 提高器件的效率和稳定性, 在A位添加Cs+可以进一步抑制δ相的形成[21-22], 优化器件性能, 提高重现性。尽管已经有大量学者研究掺杂FA+和Cs+对钙钛矿的影响, 但对性能改善机制和稳定性的研究还不够系统, 且目前绝大部分高效率的钙钛矿电池都是在手套箱等低湿低氧的惰性气体环境中进行的, 成本高且不利于工业化大规模生产, 空气中制备超长稳定钙钛矿太阳能电池的研究还有待进一步优化。

本研究采用简单温和且成本低廉的两步旋涂法在全空气环境中制备了钙钛矿薄膜并组装成器件。同时系统研究了掺杂FA+和Cs+对MAPbI3钙钛矿吸收层微观形貌、物相组成、PSCs光伏性能和长期稳定性的影响并进行机理分析。

1 实验方法

实验过程均在空气环境(相对湿度30%~40%)中完成, 实验流程如图1所示。

图1

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图1   在空气中两步旋涂法制备钙钛矿薄膜及器件制造的流程图

Fig. 1   Flow chart for the preparation of perovskite films and device manufacturing by two-step spin coating in the air


1.1 SnO2电子传输层的制备

将FTO玻璃依次用洗涤剂、去离子水、乙醇等分别超声清洗10 min, 随后放入紫外臭氧清洗机处理40 min。取60 μL SnO2(质量分数10%)胶体水溶液旋涂(5000 r/min, 30 s)在FTO基底上, 150 ℃退火30 min, 得到致密SnO2电子传输层。

1.2 钙钛矿薄膜的制备

MAPbI3钙钛矿薄膜(MP): 将PbI2溶于DMF和DMSO的混合溶剂(VDMF∶VDMSO=9∶1)中, 105 ℃下搅拌1 h, 制成1.3 mol/L的PbI2前驱体溶液。称取适量MAI和MACl药品溶解在异丙醇(IPA)中得到混合(0.4 mol/L MAI, 0.07 mol/L MACl)溶液。取30 μL PbI2溶液旋涂(3000 r/min, 30 s)在涂有致密SnO2电子传输层的基底上, 随后取110 μL MAI溶液旋涂 (5000 r/min, 10 s)在PbI2薄膜上, 空气环境中(相对湿度30%~40%)在105 ℃退火10 min, 得到结晶良好的MAPbI3薄膜, 标记为MP。

MA0.4FA0.6PbI3钙钛矿薄膜(MFP): 用适量FAI取代MAI溶解于异丙醇中, 制备MAFA混合有机盐溶液, 与PbI2反应生成MA0.4FA0.6PbI3, 最后在150 ℃退火30 min, 其它制备条件与MAPbI3相同, 得到的薄膜标记为MFP。

Cs0.05MA0.35FA0.6PbI3钙钛矿薄膜(CMFP): 添加适量1.0 mol/L CsI溶液(DMSO为溶剂)到PbI2溶液中, 其他制备条件与MA0.4FA0.6PbI3钙钛矿相同, 得到的薄膜标记为CMFP。

1.3 空穴传输层和背电极的沉积

在钙钛矿薄膜表面旋涂30 μL的Spiro- OMeTAD溶液(4000 r/min, 20 s), 得到空穴传输层, 然后将基片置于干燥器中遮光老化12 h。再利用热蒸发仪在Spiro-OMeTAD层上蒸镀一层80 nm厚的金电极。

2 结果与讨论

2.1 A位阳离子掺杂对钙钛矿薄膜结构和形貌的影响

钙钛矿光吸收层是电池的核心, 钙钛矿薄膜的质量对于电池的性能至关重要。为表征薄膜质量, 分别对三种薄膜进行X射线衍射(XRD)分析和扫描电子显微镜（SEM）表征。如图2(a)所示, 位于2θ= 14.24°, 28.25°的衍射峰归属于MAPbI3钙钛矿晶体结构的(110)和(220)晶面[23]; 加入FA+和Cs+后, 衍射峰逐渐增强。从图2(b)可以看出, FA+取代MA+后, 位于2θ=14.24°的衍射峰向低角度偏移(2θ= 14.02°)。根据布拉格方程2dsinθ=nλ(其中d为晶面间距, θ为入射X射线与相应晶面的夹角, λ为X射线的波长, n为衍射级数), 加入Cs+基本没有改变衍射峰角度(相比MFP), 可能是因为Cs+的掺杂量较少。

图2

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图2   不同样品的晶体结构和微观形貌

Fig. 2   Crystal structures and morphologies of different samples

(a) XRD patterns; (b) Locally magnified XRD patterns in the range of 2θ=12.8°-15°; (c, e, g) SEM images and (d, f, h) Statistical distributions of grain diameter for (c, d) MP, (e, f) MFP, and (g, h) CMFP


不同样品的SEM照片如图2(c, e, g)所示, 三种钙钛矿薄膜都均匀且致密, 掺杂FA+和Cs+使薄膜的晶粒尺寸逐步增大。图2(d, f, h)中, 掺杂FA+薄膜的平均晶粒尺寸从413 nm增大到733 nm, 进一步掺杂Cs+, 薄膜的平均晶粒尺寸增大到941 nm, 说明加入FA+和Cs+可以促进钙钛矿薄膜结晶, 增大晶粒, 减少晶界。

2.2 A位阳离子掺杂对器件PCE的影响及机理分析

2.2.1 A位阳离子掺杂对器件PCE的影响

为了研究A位阳离子掺杂对器件光电转换效率的影响, 测试器件的J-V参数。图3(a~d)统计了 50个PSCs的J-V参数的分布, 图3(e)是三种PSCs中性能最佳的J-V曲线, 相应的结果总结在表1和表S1中。

图3

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图3   PSCs的光电性能

Fig. 3   Photovoltaic performances of PSCs

Statistical diagram of (a) PCE, (b) Jsc, (c) Voc, and (d) FF; (e) J-V curves; (f) EQE spectra. Colorful figures are available on website


表1   三种钙钛矿太阳能电池的光电性能参数

Table 1  Photovoltaic parameters of three types of perovskite solar cells

SampleJsc /(mA•cm-2)Voc/VFFPCE/%PCEmax/%
MP21.32±0.990.95±0.030.74±0.0215.11±0.8616.31
MFP22.94±0.980.99±0.040.76±0.0317.18±0.6118.87
CMFP22.24±0.771.03±0.030.78±0.0217.83±0.3319.34

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图3(a~d)和表1显示, 掺杂FA+使MFP平均Jsc由21.32 mA•cm-2(MP)提高到22.94 mA•cm-2, 而平均Voc和FF分别从0.95 V和0.74增大到0.99 V和0.76, PSCs的平均PCE从15.11%提高到17.18%。掺杂Cs+使CMFP的平均PCE进一步提高到17.82%, 这主要归因于电池的Voc和FF得到了提高, CMFP的最大PCE为19.34%。图3(f)是三种电池的外量子效率(EQE)测试曲线, 可以用于校准短路电流密度。通过EQE图谱(图3(f))得到的MP、MFP和CMFP电池的积分电流密度分别为21.87、22.71和22.37 mA·cm-2, 与J-V测试(图3(e))得到的Jsc(22.87、23.48、23.30 mA·cm-2)相差不大, 说明测试结果较准确。

2.2.2 A位阳离子掺杂提升器件PCE的机理

一般来说, PSCs的PCE与Jsc、Voc和FF密切相关。因此, 探索A位阳离子掺杂对这三个参数的影响对于揭示器件光电性能的机理至关重要。Jsc由以下公式[23]决定。

${{J}_{\text{sc}}}\propto {{\eta }_{\text{lh}}}\times {{\eta }_{\text{inj}}}\times {{\eta }_{\text{ec}}}~$

其中, ${{\eta }_{\text{lh}}}$为PSCs对太阳光的捕获效率, 取决于钙钛矿层的带隙宽度; ${{\eta }_{\text{inj}}}$为电子的注入效率, 与钙钛矿层的缺陷密度以及电子传输层/钙钛矿层间的能级匹配有关; ${{\eta }_{\text{ec}}}$为透明电极对电子的收集效率。

因为三种PSCs使用相同的透明电极, 所以${{\eta }_{\text{ec}}}$保持一致, 本研究主要聚焦于阳离子掺杂对${{\eta }_{\text{lh}}}$和${{\eta }_{\text{inj}}}$的影响。

为了探索阳离子掺杂对薄膜${{\eta }_{\text{lh}}}$的影响, 本研究测试了紫外-可见光谱图和稳态光致发光(PL)谱图。图4(a)表明, 掺杂FA+和Cs+增强了薄膜在550~810 nm范围内的吸光度且吸收边出现了明显的红移。根据紫外-可见光谱图计算[24]得到样品的Tauc图(图4(b)), 相应的带隙Eg列于其中。MP的带隙为1.61 eV, MFP和CMFP的带隙均为1.55 eV, 说明FA+掺杂可以降低薄膜的带隙。由于Cs+的掺杂量较少, 因此基本不影响薄膜的Eg。薄膜的光致发光(PL)谱图如图4(c)所示, MP(λmax=774 nm)的发射峰红移了27 nm(MFP和CMFP的λmax=801 nm), 说明掺杂FA+和Cs+拓宽了薄膜的吸光范围。带隙越小, 其吸光范围越大, 与图4(a, c)结果一致, 综合得到三种薄膜的${{\eta }_{\text{lh}}}$大小顺序为MP<MFP<CMFP。

图4

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图4   三种钙钛矿薄膜的光电特性和能级结构

Fig. 4   Photoelectric properties and energy levels of three perovskite films

(a) UV-Vis absorption spectra; (b) Tauc plots; (c) PL and (d) TRPL spectra excited from the perovskite layer;(e) Energy level schematics of three samples


薄膜的${{\eta }_{\text{inj}}}$是影响Jsc的另一重要因素, 首先从钙钛矿一侧激发薄膜, 测试PL和时间分辨光致发光(TRPL)图谱, 定性表征其薄膜内部缺陷诱导的非辐射性复合。如图4(c)所示, 三种薄膜的PL强度大小顺序为MFP<CMFP<MP, 表明掺杂FA+增强了薄膜非辐射复合, 而Cs+在一定程度上可以抑制这种非辐射复合。为了更直观地反映钙钛矿薄膜中缺陷密度和载流子复合行为, 使用双指数函数(式(2))[25] 拟合TRPL谱(图4(d)), 结果列于表S2:

$y={{y}_{0}}+{{A}_{1}}\exp \left( -\frac{x}{{{\tau }_{1}}} \right)+{{A}_{2}}\exp \left( -\frac{x}{{{\tau }_{2}}} \right)$

式中, τ1表示钙钛矿表面和界面处的光生载流子寿命(快过程), ns; τ2表示钙钛矿晶粒内部的光生载流子寿命(慢过程), ns; A1和A2分别表示这两个过程的相对振幅, y0表示常数。

平均载流子寿命τmean可以用来衡量薄膜整体的缺陷密度, ${{\tau }_{\text{mean}}}={{B}_{1}}{{\tau }_{1}}+{{B}_{2}}{{\tau }_{2}}$。其中, ${{B}_{i}}=\frac{{{A}_{i}}{{\tau }_{i}}}{{{A}_{1}}{{\tau }_{1}}+{{A}_{2}}{{\tau }_{2}}}$ (i=1,2), B1和B2分别表示对应过程的相对贡献值。如图4(d)所示, 三种PSCs的缺陷密度大小顺序为MP<CMFP<MFP, 说明掺杂Cs+在一定程度上抑制了薄膜缺陷, 与PL测试结果(图4(c))一致。此外, ${{\eta }_{\text{inj}}}$还与电子传输层/钙钛矿层间的能级匹配有关, 因此对薄膜进行紫外光电子能谱(UPS)测试(图S1)。由图S1(a~d)计算得到各薄膜的功函数(Φ), 由图S1(e, f)计算得到薄膜相对于真空的价带位置($E_{\text{VB}}^{\text{F}})$, 结果列于表S3。

如图4(e)所示, MFP具有最高的功函数(Φ= 4.02 eV)以及最低的ECB(–3.96 eV), 因此MFP钙钛矿与SnO2之间的能级势垒(ΔE)最小, 表明引入FA+可以改善电荷收集, 增大${{\eta }_{\text{inj}}}$, 而掺杂Cs+略微增大了能级势垒ΔE, 因此三种电池的电子传输层与光吸收层间的能级势垒ΔE大小顺序为MFP<CMFP< MP。一般能级势垒越小, 器件电流密度越大, 因此三种钙钛矿太阳能电池的Jsc大小顺序为MP<CMFP<MFP, 这可能是因为电子传输层/钙钛矿层间的能级匹配在电流密度的影响因素中占据重要地位。以上结果表明, 掺杂FA+有利于提高PSCs的Jsc, Cs+掺杂会略微降低电池的Jsc。

开路电压Voc可以被定义为[26]:

${{V}_{\text{oc}}}=({{E}_{\text{Fermi}}}-{{E}_{\text{N}}})/q$

其中, EFermi表示电子传输材料的准费米能级, EN为空穴传输层的能斯特电势。电子传输层中收集的电子越多, EFermi越高, 则Voc越大。因此Voc的大小与界面间的能级匹配、电子的界面传输性质以及器件的复合阻抗(Rrec)密切相关。而填充因子FF本质上代表了抽取光伏器件中光生载流子的难易程度, 串联电阻(Rs)减小和并联电阻(Rrec)增大都有利于提高器件的填充因子。

图4(e)是三种钙钛矿薄膜能带排列情况, CMFP的ECB和Φ相比MFP均向上移动, 这种能带结构有利于减少电荷复合, 因此Cs+掺杂可以增大电池的开路电压Voc。

透过FTO激发进行PL和TRPL测试(图5(a, b)), 拟合结果(表S4)能够反映电子从钙钛矿层传输到SnO2的速率, 即电子的抽取速率。CMFP、MFP和MP的$\tau _{1}^{'}$分别为8.21、35.40和75.22 ns, 一般$\tau _{1}^{'}$越小表明电子从钙钛矿传输到SnO2的速率越快, 因此电子抽取速率的大小顺序为MP<MFP<CMFP。根据公式(3), SnO2导带中收集的电子越多, EFermi越高, 因此掺杂FA+和Cs+均有利于提高电池的Voc。

图5

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图5   PSCs的界面传输和载流子复合特性

Fig. 5   Interface transmission and carrier recombination characteristics of PSCs

(a) PL and (b) TRPL spectra excited from FTO layer; (c) Dark-state EIS profiles of the device at 0.8 Vbias with inset showing an equivalent circuit diagram


为了探究整个PSCs中的载流子复合行为, 测试了三种电池的暗态电化学阻抗谱(EIS), 结果如图5(c)所示。MP、MFP和CMFP的Rrec分别为4932、7261和10890 Ω·cm2, 表明引入FA+和Cs+有利于抑制PSCs的载流子复合行为, 降低电压损耗[27], 从而提高电池的Voc, 由于CMFP电池的Rs最小、Rrec最大, 因此CMFP电池的Voc和FF最大。

综上所述, 掺杂FA+可以全面提高器件的Voc、Jsc和FF, 从而将电池的平均PCE从15.11%提高到17.18%; 掺杂Cs+有利于进一步优化器件的Voc和FF, 最终将电池的平均PCE提高到17.83%。

2.3 A位阳离子掺杂对器件稳定性的影响

长期稳定性测试结果如图6所示。为了直观观测电池性能, 用相对数值表征各参数的变化, 即后续的测试结果与器件最初相应参数的比值。例如, 将测试过程中得到的Voc与该器件的初始Voc的比值, 记为Vnoc。

图6

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图6   三种钙钛矿太阳能电池在(20±5) ℃, 相对湿度<5%, 黑暗条件下的长期稳定性

Fig. 6   Long-term stabilities of three perovskite solar cells at (20±5) ℃, relative humidity <5% in the dark


表S6给出了长期稳定性测试结束后各PSCs的J-V相对参数值, MP PSCs放置112 d后PCEn为0.30, MP PSCs的FF和Jsc显著下降, 尤其是FFn为0.54。MFP PSCs放置242 d(5808 h)后, PCEn为0.81, 表明掺入FA+后器件的稳定性可以得到显著改善。CMFP PSCs在测试结束时, PCEn最高, 为0.85, 主要原因是加入Cs+明显延缓了Voc、FF和Jsc衰减。

图7给出了各PSCs在(20±5) ℃, 相对湿度<5%条件下老化前后的暗态EIS图谱, 利用图5(c)中的等效电路对其进行拟合, 结果列于表S5。Rrec减小主要与钙钛矿层以及各界面间缺陷增加有关[28], 在稳定性测试结束后, MP器件的阻抗降低幅度最大, 意味着电池的缺陷产生速率最快, PCE下降最严重。Rrec衰减速率的大小顺序为CMFP<MFP<MP, 这也与电池效率降低速率的大小顺序(图6)相一致, 说明掺杂FA+和Cs+有利于抑制载流子复合, 提高电池的长期稳定性。

图7

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图7   (a) MP、(b) MFP、(c) CMFP钙钛矿太阳能电池在(20±5) ℃, 相对湿度<5%条件下老化前后的暗态EIS图谱

Fig. 7   Dark state EIS profiles of (a) MP, (b) MFP, (c) CMFP PSCs before and after aging at (20±5) ℃ and relative humidity<5%


热稳定性测试结果如图8所示, 相对数值列于表S7。由图8可以看出, 各电池的J-V参数均明显下降, 但下降的幅度有所不同。三种电池性能衰减速率的大小顺序为CMFP<MFP<MP。表S7显示, 放置96 h后, MP、MFP和CMFP的PCEn为0.70、0.93和0.99, MP的性能下降最快, 器件的Voc、Jsc、FF同时下降, 尤其是FFn降到0.81。

图8

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图8   三种PSCs在(85±5) ℃, 相对湿度20%~30%, 黑暗条件下的热稳定性

Fig. 8   Thermal stability of three PSCs at (85±5) ℃ and relative humidity 20%-30% in the dark


从薄膜的热稳定性测试的归一化XRD变化图谱(图S2(a))中可以看到从24 h开始, MP薄膜开始出现PbI2特征峰(2θ=12.7°), 而且该特征峰的相对强度随加热时间延长而迅速增大, 表明MP膜的受热分解速率较快。然而图S2(b, c)显示, 从加热开始至结束, MFP和CMFP均未在2θ=12.7°处出现明显的特征峰, 表明MFP和CMFP薄膜在整个加热过程中并未发生分解, 这是由于掺杂FA+和Cs+可以显著提高钙钛矿器件的抗热稳定性。

湿度稳定性测试结果如图9所示, 相对数值列于表S8。图9显示, 与热稳定性测试结果相似, 所有PSCs性能均有不同程度的下降。表S8显示, CMFP电池的湿稳定性最佳, 放置96 h后其PCE较原器件仅下降了16%; 而MP和MFP电池的PCE则分别下降了44%和22%。从图S3(a)中可以看出, 放置12 h以后, MP在2θ=12.7°处开始出现PbI2特征衍射峰, 随着在高湿度环境中放置时间延长, 该特征峰的相对强度逐渐增大。到测试结束时其PbI2衍射峰已经非常明显, 薄膜分解现象严重, 表面发黄。图S3(b, c)显示MFP在老化实验60 h时出现了PbI2特征峰, 而从高湿度老化实验开始到结束, CMFP在2θ=12.7°处均未产生PbI2的特征衍射峰。这意味着CMFP薄膜在整个老化过程中均未发生分解, 说明掺杂FA+和Cs+也可以显著提高钙钛矿器件的抗湿稳定性。

图9

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图9   三种PSCs在(20±5) ℃, 相对湿度80%~90%, 黑暗条件下的湿稳定性

Fig. 9   Wet stability of three PSCs at (20±5) ℃ and relative humidity 80%-90% in the dark


综上所述, 掺杂FA+和Cs+可以全面提高钙钛矿太阳能电池的抗热、抗湿和长期稳定性。

3 结论

本研究采用两步旋涂法在空气环境中制备了混合阳离子钙钛矿薄膜并组装成电池, 系统研究了A位阳离子掺杂对薄膜微观结构及电池性能的影响。掺杂FA+和Cs+可以增大薄膜晶粒尺寸, 减少晶界, 有利于提高器件的PCE。这是因为掺杂FA+有利于增大薄膜吸光能力, 降低钙钛矿/SnO2的能级势垒, 抑制载流子复合, 从而全面提高电池的Jsc、Voc和FF。而掺杂Cs+有利于降低薄膜的缺陷密度, 提高电子的抽取速率, 抑制非辐射复合, 提高Voc和FF, 使CMFP的最佳PCE为19.34%。此外, 掺杂FA+和Cs+可以增强钙钛矿晶体结构的稳定性, 改善界面接触, 显著提高电池的长期稳定性, CMFP在(20±5) ℃, 相对湿度<5%的黑暗环境中放置超过5808 h(242 d), 效率仍保持初始值的85%。掺杂FA+和Cs+使器件的抗湿和抗热稳定性也得到了全面提高。

补充材料

本文相关补充材料可登录 https://doi.org/10.15541/jim20230098查看。

超长稳定的混合阳离子钙钛矿太阳能电池性能优化研究

马婷婷1,2, 汪志鹏1,2, 张 梅1,2, 郭 敏1,2

(北京科技大学1. 钢铁冶金新技术国家重点实验室; 2. 冶金与生态工程学院, 北京100083)

S1 实验方法

S1.1 实验试剂

碘化铅(PbI2, 99.99%)、碘化甲铵(MAI, 99.90%)、碘化甲脒(FAI, 99.90%)、碘化铯(CsI, 99.90%)、氯化甲铵(MACl, 99.90%)和Spiro-OMeTAD(99.50%), 均购自西安宝莱特光电科技有限公司; N, N-二甲基甲酰胺(DMF, 99.90%)、二甲基亚砜(DMSO, 99.80%)和异丙醇(IPA, 99.80%)购自阿拉丁试剂有限公司; SnO2胶体溶液(质量分数15%)购自阿法埃莎(中国)化工有限公司。FTO导电玻璃(14 Ω·cm-2)购自武汉晶格太阳能科技有限公司; 金颗粒(99.999%)购自中诺新材(北京)科技有限公司。

S1.2 实验表征及测试

在AM 1.5G模拟太阳光的照射下, 采用电化学工作站(CHI660C, CH Instruments, Inc., 美国)测量PSCs的J-V曲线。使用标准硅电池校准光强为 100 mW·cm-2, 以短弧氙灯(CHF-XM-500W, 中国)为光源, 在0~1.2 V范围内, 以100 mV·s-1的扫速线性扫描得到测试结果。电化学阻抗谱(Electrochemical Impedance Spectroscopy, EIS)测试在黑暗中(频率范围：0.1 Hz~0.1 MHz, 0.8 V)进行。采用X射线衍射仪(X-ray Diffraction, XRD, Rigaku Dmax2500, 日本)表征样品的物相结构, 使用Cu Kα1(1.5056 Å)和40 kV高压稳定电源, 扫速为10 (°)/min。使用Supra-55(德国)高分辨率扫描电子显微镜(Scanning Electron Microscope, SEM)研究样品的微观形态, 工作电压为10 kV。外量子效率(External Quantum Efficiency, EQE)测试采用QER-3011 (Enlitech)测试系统测试电池的单色光光电转换效率, 测试波长范围300~900 nm, 步长10 nm。使用紫外-可见分光光度计(TU-1901, Beijing Persee)获得紫外-可见吸收光谱, 扫描范围300~850 nm。采用紫外光电子能谱仪(Thermo Scientifc NEXSA)表征样品的电子结构(Ultraviolet Photoelectron Spectroscopy, UPS), 气压为5×10-9 mbar。使用稳态瞬态荧光光谱仪(FLS980, 爱丁堡)获得稳态光致发光(Photoluminescence, PL)和时间分辨光致发光(Time Resolved Photoluminescence, TRPL)光谱, 激发波长和发射波长分别为470和770 nm(810 nm)。

长期稳定性测试：将三种未封装的电池置于干燥器中((20±5) ℃, 相对湿度<5%), 避光保存, 每隔12 d测试电池的J-V性能。

热稳定测试性：将三种未封装的电池置于85 ℃加热板上加热12 h(空气中), 然后在室温下放置1 h冷却, 为一次循环, 测试电池的J-V曲线。

湿稳定性测试：将三种未封装的电池放在密封容器((20±5) ℃, 80%~90% RH)内, 并置于避光处。每隔12 h, 测试电池的J-V性能。

图S1

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图S1   三种钙钛矿薄膜的UPS图谱

Fig. S1   UPS profiles of three perovskite films

(a) UPS full spectra; UPS spectra corresponding to the secondary electron cutoff region for (b) MP, (c) MFP, (d) CMFP;(e) UPS valence band spectra; UPS spectra of the valence band top region with respect to the Femi level for (f) MP, (g) MFP, (h) CMFP


图S2

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图S2   在85 ℃, 相对湿度20%~30%条件下, 三种钙钛矿薄膜随时间变化的XRD图谱

Fig. S2   XRD patterns of three perovskite thin films over time at 85 ℃, 20%-30% RH

(a) MP; (b) MFP; (c) CMFP


图S3

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图S3   在(20±5) ℃, 相对湿度80%~90%条件下三种钙钛矿薄膜随时间变化的XRD图谱

Fig. S3   XRD patterns of three perovskite thin films over time at (20±5) ℃, 80%-90% RH

(a) MP; (b) MFP; (c) CMFP


表S1   三种性能最佳的PSCs的性能参数(图3(e)和图S1)

Table S1  Performance parameters of three best performing PSCs (Fig. 3 (e) and Fig. S1)

SampleJsc /(mA•cm-2)Voc/VFFPCEmax/%Intergrated current density/(mA•cm-2)
MP22.870.970.7416.3421.87
MFP23.481.050.7618.6622.71
CMFP23.301.060.7819.3422.37

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表S2   从钙钛矿一侧激发瞬态光谱(图4(d))的拟合结果

Table S2  Fitting results of transient spectra (Fig. 4(d)) excited from the perovskite side

Sampleτ1/nsτ2/nsB1B2τmean/ns
MP33.6496.210.450.5568.05
MFP31.7081.410.680.3247.61
CMFP26.5177.120.220.7865.99

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表S3   由 UPS图谱(图S1)计算得到的三种钙钛矿薄膜的EVB和ECB

Table S3  EVB and ECB of three perovskite films calculated from UPS profiles (Fig. S1)

SampleФ/eV$E_{\text{VB}}^{\text{F}}$/eVEVB/eVEg/eVECB/eV
MP3.741.59-5.331.61-3.72
MFP4.021.49-5.511.55-3.96
CMFP3.931.47-5.401.55-3.85

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表S4   从FTO一侧激发瞬态光谱(图5(b))的拟合结果

Table S4  Fitting results of transient spectra (Fig. 5(b)) excited from the FTO side

Sample${{\tau }_{1}}^{\prime }$/ns${{\tau }_{2}}^{\prime }$/ns${{B}_{1}}^{\prime }$${{B}_{2}}^{\prime }$
MP75.22422.180.470.53
MFP35.421.420.380.62
CMFP8.2134.520.760.24

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表S5   EIS(图7)数据的拟合结果

Table S5  Fitting results of EIS (Fig. 7) data

Long-term stabilityRs/(Ω·cm2)Rrec/(Ω·cm2)Rnrec
Before testingMP32.404932—
MFP27.187261—
CMFP21.8410890—
After testingMP47.9914620.30
MFP39.8854300.75
CMFP34.1691730.84

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表S6   三种PSCs经长期稳定性测试后的J-V相对参数值

Table S6  Relative J-V values of three PSCs after long-term stability testing

SampleFFnVnocJnscPCEn
MP0.540.850.630.30
MFP0.900.940.920.81
CMFP0.880.960.900.85

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表S7   三种PSCs经热稳定性测试后的J-V相对数值

Table S7  Relative J-V values of three PSCs after thermal stability testing

SampleFFnVnocJnscPCEn
MP0.810.890.870.70
MFP0.940.940.920.93
CMFP0.950.960.950.99

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表S8   三种PSCs经湿稳定性测试后的J-V相对数值

Table S8  Relative J-V values of three PSCs after wet stability testing

SampleFFnVnocJnscPCEn
MP0.720.860.880.56
MFP0.890.920.900.78
CMFP0.910.960.920.84

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We fabricated a perovskite solar cell that uses a double layer of mesoporous TiO2 and ZrO2 as a scaffold infiltrated with perovskite and does not require a hole-conducting layer. The perovskite was produced by drop-casting a solution of PbI2, methylammonium (MA) iodide, and 5-ammoniumvaleric acid (5-AVA) iodide through a porous carbon film. The 5-AVA templating created mixed-cation perovskite (5-AVA)x(MA)1- xPbI3 crystals with lower defect concentration and better pore filling as well as more complete contact with the TiO2 scaffold, resulting in a longer exciton lifetime and a higher quantum yield for photoinduced charge separation as compared to MAPbI3. The cell achieved a certified power conversion efficiency of 12.8% and was stable for >1000 hours in ambient air under full sunlight. Copyright © 2014, American Association for the Advancement of Science.

LEIJTENS T, BUSH K A, PRASANNA R, et al.

Opportunities and challenges for tandem solar cells using metal halide perovskite semiconductors

Nature Energy, 2018, 3(10): 828.

DOI      [本文引用: 1]

JIN Y K, LEE J W, JUNG H S, et al.

High-efficiency perovskite solar cells

Chemical Reviews, 2020, 120(15): 7867.

DOI      PMID      [本文引用: 1]

With rapid progress in a power conversion efficiency (PCE) to reach 25%, metal halide perovskite-based solar cells became a game-changer in a photovoltaic performance race. Triggered by the development of the solid-state perovskite solar cell in 2012, intense follow-up research works on structure design, materials chemistry, process engineering, and device physics have contributed to the revolutionary evolution of the solid-state perovskite solar cell to be a strong candidate for a next-generation solar energy harvester. The high efficiency in combination with the low cost of materials and processes are the selling points of this cell over commercial silicon or other organic and inorganic solar cells. The characteristic features of perovskite materials may enable further advancement of the PCE beyond those afforded by the silicon solar cells, toward the Shockley-Queisser limit. This review summarizes the fundamentals behind the optoelectronic properties of perovskite materials, as well as the important approaches to fabricating high-efficiency perovskite solar cells. Furthermore, possible next-generation strategies for enhancing the PCE over the Shockley-Queisser limit are discussed.

VALADI K, GHARIBI S, TAHERI-LEDARI R, et al.

Metal oxide electron transport materials for perovskite solar cells: a review

Environmental Chemistry Letters, 2021, 19(3): 2185.

DOI      [本文引用: 1]

AVA T T, MAMUN A A, MARSILLAC S, et al.

A review: thermal stability of methylammonium lead halide based perovskite solar cells

Applied Sciences, 2019, 9(1): 188.

DOI      URL     [本文引用: 1]

Perovskite solar cells have achieved photo-conversion efficiencies greater than 20%, making them a promising candidate as an emerging solar cell technology. While perovskite solar cells are expected to eventually compete with existing silicon-based solar cells on the market, their long-term stability has become a major bottleneck. In particular, perovskite films are found to be very sensitive to external factors such as air, UV light, light soaking, thermal stress and others. Among these stressors, light, oxygen and moisture-induced degradation can be slowed by integrating barrier or interface layers within the device architecture. However, the most representative perovskite absorber material, CH3NH3PbI3 (MAPbI3), appears to be thermally unstable even in an inert environment. This poses a substantial challenge for solar cell applications because device temperatures can be over 45 °C higher than ambient temperatures when operating under direct sunlight. Herein, recent advances in resolving thermal stability problems are highlighted through literature review. Moreover, the most recent and promising strategies for overcoming thermal degradation are also summarized.

ZHANG C, WANG Y, LIN X, et al.

Effects of A site doping on the crystallization of perovskite films

Journal of Materials Chemistry A, 2021, 9(3): 1372.

DOI      URL     [本文引用: 2]

GONG J, GUO P, BENJAMIN S E, et al.

Cation engineering on lead iodide perovskites for stable and high-performance photovoltaic applications

Journal of Energy Chemistry, 2018, 27(4): 1017.

DOI      [本文引用: 1]

Perovskite solar cells (PSCs) based on methylammonium lead iodide (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) have shown unprecedentedly outstanding performance in the recent years. Nevertheless, due to the weak interaction between polar CH<sub>3</sub>NH<sub>3</sub><sup>+</sup> (MA<sup>+</sup>) and inorganic PbI<sub>3</sub><sup>-</sup> sublattices, CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> dramatically suffers from poor moisture stability, thermal decomposition and device hysteresis. As such, strong electrostatic interactions between cations and anionic frameworks are desired for synergistic improvements of the abovementioned issues. While replacements of I<sup>-</sup> with Br<sup>-</sup> and/or Cl<sup>-</sup> evidently widen optical bandgaps of perovskite materials, compositional modifications can solely be applied on cation components in order to preserve the broad absorption of solar spectrum. Herein, we review the current successful practices in achieving efficient, stable and minimally hysteretic PSCs with lead iodide perovskite systems that employ photoactive cesium lead iodide (CsPbI<sub>3</sub>), formamidinium lead iodide (HC(NH<sub>2</sub>)<sub>2</sub>PbI<sub>3</sub>, or FAPbI<sub>3</sub>), MA<sub>1-<em>x</em>-<em>y</em>-<em>z</em></sub>FA<sub><em>x</em></sub>Cs<sub><em>y</em></sub>Rb<sub><em>z</em></sub>PbI<sub>3</sub> mixed-cation settings as well as two-dimensional butylammonium (C<sub>4</sub>H<sub>9</sub>NH<sub>3</sub><sup>+</sup>, or BA<sup>+</sup>)/MA<sup>+</sup>, polymeric ammonium (PEI<sup>+</sup>)/MA<sup>+</sup> co-cation layered structures. Fundamental aspects behind the stabilization of perovskite phases <em>&alpha;</em>-CsPbI<sub>3</sub>, <em>&alpha;</em>-FAPbI<sub>3</sub>, mixed-cation MA<sub>1-<em>x</em>-<em>y</em>-<em>z</em></sub>FA<sub><em>x</em></sub>Cs<sub><em>y</em></sub>Rb<sub><em>z</em></sub>PbI<sub>3</sub> and crystallographic alignment of (BA)<sub>2</sub>(MA)<sub>3</sub>Pb<sub>4</sub>I<sub>13</sub> for effective light absorption and charge transport will be discussed. This review will contribute to the continuous development of photovoltaic technology based on PSCs.

KNIGHT A J, BORCHERT J, OLIVER R D J, et al.

Halide segregation in mixed-halide perovskites: influence of A-site cations

ACS Energy Letters, 2021, 6(2): 799.

DOI      PMID      [本文引用: 1]

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(BrI) and FACsPb(BrI) films. We report evidence for low-barrier ionic pathways in MAPb(BrI), which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FACsPb(BrI) lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.© 2021 The Authors. Published by American Chemical Society.

YANG Y, GAO J, CUI J R, et al.

Research progress of perovskite solar cells

Journal of Inorganic Materials, 2015, 30(11): 1131.

DOI      URL     [本文引用: 1]

CHU Z Y, LI G L, JIANG Z H, et al.

Recent progress in high- quality perovskite CH3NH3PbI3 single crystal

Journal of Inorganic Materials, 2018, 33(10): 1035.

DOI      URL     [本文引用: 1]

WELLER M T, WEBER O J, CHARLES B.

Phase behaviour and composition in the formamidinium-methylammonium hybrid lead iodide perovskite solid solution

Journal of Materials Chemistry A, 2016, 4(40): 15375.

DOI      URL     [本文引用: 1]

YIN W J, YANG J H, KANG J, et al.

Halide perovskite materials for solar cells: a theoretical review

Journal of Materials Chemistry A, 2015, 3(17): 8926.

DOI      URL     [本文引用: 1]

CONINGS B, DRIJKONINGEN J, GAUQUELIN N, et al.

Intrinsic thermal instability of methylammonium lead trihalide perovskite

Advanced Energy Materials, 2015, 5(15): 1500477.

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LU H, LIU Y, AHLAWAT P, et al.

Vapor-assisted deposition of highly efficient, stable black-phase FAPbI3 perovskite solar cells

Science, 2020, 370(6512): 74.

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EPERON G E, STRANKS S D, MENELAOU C, et al.

Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells

Energy & Environmental Science, 2014, 7(3): 982.

[本文引用: 1]

YU B, SHI J, TAN S, et al.

Efficient (>20%) and stable all- inorganic cesium lead triiodide solar cell enabled by thiocyanate molten salts

Angewandte Chemie International Edition, 2021, 60(24): 13436.

DOI      URL     [本文引用: 1]

PELLET N, GAO P, GREGORI G, et al.

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Angewandte Chemie International Edition, 2014, 53(12): 3151.

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JASON J, GABKYUNG, CHUA R M, et al.

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Nature, 2021, 590(7847): 587.

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SALIBA M, MATSUI T, SEO J Y, et al.

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Energy & Environmental Science, 2016, 9(6): 1989.

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SUSANA R, FRANCISCO I J, DIEGO B, et al.

Insight into the role of guanidinium and cesium in triple cation lead halide perovskites

Solar RRL, 2021, 5(12): 2100586.

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WANG Z, LI R, HUO X.

Effcient and stable TiO2 nanorod array structured perovskite solar cells in air: co-passivation and synergistic mechanism

Ceramics International, 2022, 48(12): 17950.

DOI      URL     [本文引用: 2]

LEE J W, SEOL D J, CHO A N, et al.

High-efficiency perovskite solar cells based on the black polymorph of HC(NH2)2PbI3

Advanced Materials, 2014, 26(29): 4991.

DOI      URL     [本文引用: 1]

KOH T M, FU K, FANG Y, et al.

Formamidinium-containing metal-halide: an alternative material for near-IR absorption perovskite solar cells

Journal of Physical Chemistry C, 2014. 118(30): 16458.

DOI      URL     [本文引用: 1]

GUO M, ZHANG M, LI R, et al.

Nb2O5 coating on the performance of flexible dye sensitized solar cell based on TiO2 nanoarrays/upconversion luminescence composite structure

Journal of Inorganic Materials, 2019, 34(6): 590.

DOI      URL     [本文引用: 1]

HAGFELDT A, GRAETZEL M.

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Chemical Reviews, 1995, 95(1): 49.

DOI      URL     [本文引用: 1]

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Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%

Nature Energy, 2016, 1(10): 16142.

DOI      [本文引用: 1]

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