纳米技术  >> Vol. 10 No. 2 (May 2020)

尿素添加对钙钛矿太阳能电池光电性能的影响
Effect of Urea Addition on the Photovoltaic Performance of Perovskite Solar Cells

DOI: 10.12677/NAT.2020.102005, PDF, HTML, XML, 下载: 225  浏览: 768  国家自然科学基金支持

作者: 何艳伟, 谢 剑, 胡 笙, 施升志, 桃 李, 张 军*:湖北大学物理与电子科学学院;铁电压电材料与器件湖北省重点实验室,湖北 武汉

关键词: 钙钛矿尿素结晶度晶粒尺寸载流子复合Perovskite Urea Crystallinity Grain SizeCarrier Recombination

摘要: 钙钛矿太阳能电池以其成本低廉和优异的光电性能受到广泛的关注,其中钙钛矿光活性层的质量对组装电池的光电性能起着至关重要的作用。本文通过在钙钛矿前驱体溶液中添加路易斯碱——尿素(urea),详细研究了尿素添加量对钙钛矿薄膜微结构和组装电池光电性能的影响。研究发现,钙钛矿前驱液中添加尿素是一种有效的获得大晶粒、致密平整钙钛矿薄膜的策略。高质量的钙钛矿薄膜减少了晶界,抑制了电池中载流子的复合,因而提升了器件的能量转换效率。在尿素添加量为10%时,组装电池具有18.90%的最佳能源转换效率。
Abstract: Perovskite solar cells have attracted much attention due to their low cost and excellent photoe-lectric performance. The quality of the perovskite active layer plays a vital role in the performance of the solar cells. In this paper, urea as one of the Lewis bases has been added into the precursor solution of perovskite, and the effects of urea addition on the microstructure of perovskite thin films and the photovoltaic properties of the cells have been studied. It is found that the addition of urea in perovskite precursor is a promising strategy for obtaining compact and flat perovskite film with large grain size. The high quality of perovskite thin film reduces the grain boundaries of film, and suppresses the recombination of carriers. It therefore improves the photovoltaic performance of assembled perovskite solar cells. An optimum energy conversion efficiency of 18.90% was achieved in the cell with urea addition of 10% in the perovskite precursor.

文章引用: 何艳伟, 谢剑, 胡笙, 施升志, 桃李, 张军. 尿素添加对钙钛矿太阳能电池光电性能的影响[J]. 纳米技术, 2020, 10(2): 34-42. https://doi.org/10.12677/NAT.2020.102005

1. 引言

钙钛矿材料具有较强的光吸收、较长的电子–空穴扩散长度和较低的激子结合能等优良的光学性能 [1] [2] [3],因此在光电子器件领域引起了科研工作者的广泛关注和研究。基于钙钛矿薄膜组装的太阳能电池具有成本低、制备工艺简单、光电转换效率高等优点 [4],单节电池的能量转换效率在短短几年即从3.8%提升到了现在的23.2% [5],可以和传统的硅基电池和其它薄膜电池相媲美。钙钛矿层作为电池的光活性层,不仅起着吸收太阳光、产生光生载流子的作用,同时也起着分离和传导电荷的功能;钙钛矿层的成膜质量直接决定着组装电池的光电性能。结晶度高、晶界缺陷少、致密均匀的钙钛矿薄膜可以有效减少晶体或晶界处载流子复合,提高载流子的迁移率,促进电池光电性能的提升 [6] [7]。对钙钛矿层及其界面进行修饰以提高钙钛矿层的结晶度和优化与钙钛矿接触的界面与能带匹配等相关策略来提升电池性能有诸多报道 [8] [9]。由于钙钛矿中铅离子、碘离子具有良好的配位能力,在钙钛矿前驱液中引入添加剂,通过调控钙钛矿与基体的相互作用来调控钙钛矿形貌是最广泛采用的方法之一 [10] [11]。Jen等人证明了1,8-二碘辛烷(DIO)可以与Pb2+螯合,调节钙钛矿在结晶过程中的动力学 [12],Jiang等人将甲基氯化铵作为添加剂应用于介孔钙钛矿太阳能电池,获得了23.32%的效率 [13]。钙钛矿制备过程中引入添加剂不仅有助于提升组装钙钛矿太阳能电池的效率,也有利于消除器件的迟滞和提升器件稳定性 [14] [15]。由于钙钛矿中的铅离子可以作为路易斯酸,采用路易斯碱如尿素、硫脲等作为分子添加剂促进钙钛矿基体间的相互作用,也是获得高质量大晶粒钙钛矿的一种方法 [10] [11]。前期的研究 [10] [11] 主要聚焦于对路易斯碱添加剂种类的筛选上,实验发现路易斯碱的添加能调控钙钛矿结晶的速度,要获得高质量的钙钛矿薄膜和电池,不仅需要获得大晶粒的钙钛矿,还需要保持晶粒与晶粒间无缝隙和针孔,因而,进一步细化添加剂的含量来优化钙钛矿的成核与结晶速率,以获得致密、平整、大晶粒的钙钛矿薄膜也是至关重要的。本文在一步法制备钙钛矿过程中,在钙钛矿前驱液中引入尿素作为添加剂,详细研究了不同含量尿素添加对钙钛矿薄膜的结晶性、表面形貌和组装电池光电转换效率的影响与作用机理,大幅提高了钙钛矿太阳能电池的光电性能。

2. 实验部分

2.1. 材料合成

钙钛矿前驱液合成:将461 mg PbI2分为5份,分别加入一定量的尿素,配制成含0%、5%、10%、15%、20% 尿素(urea与PbI2的摩尔百分比比)的混合溶液,将159 mg MAI、75 μL DMSO和640 μL DMF溶液里混合,70℃搅拌2 h后也均分为5份,分别与上述掺杂PbI2溶液混合均匀,制备成CH3NH3PbI3前驱液备用。

空穴传导材料合成:采用锂掺杂Spiro-OMeTAD,将72.3 mg Spiro-OMeTAD、28.8 μL TBP、17.5 μL锂盐(0.52 g/mL锂盐的乙腈溶液)加入到1 mL氯苯溶液里混合,室温搅拌30 min备用。

2.2. 器件制备

取20 mm × 20 mm干净的ITO,依次采用去离子水、丙酮、酒精、去离子水分别超声清洗20 min,烘干备用。将SnO2浆料与去离子水按质量比1:4充分混合均匀,将其以3000 r/min转速旋涂在清洗干净的ITO衬底上,继以100℃热台上加热30 min在ITO衬底上获得致密的SnO2电子传导层。将前述配制好的CH3NH3PbI3前驱体溶液分别以4000 r/min的转速旋涂在干净的ITO衬底上35 s,在旋涂过程中滴加240 μL的乙酸乙酯作为反溶剂,将旋涂所得的薄膜样品放在100℃的热台上加热20 min,即可得到致密的钙钛矿薄膜。随后,将掺有锂盐的Spiro-OMeTAD溶液以4000 r/min的转速在制备好的钙钛矿薄膜上旋涂20 s获得空穴传导层。在10−4 Pa气压下蒸镀60 nm厚的金作为电极。

2.3. 表征测试方法

样品的形貌结构通过JSM-7100F型场发射扫描电子显微镜(SEM)表征;晶相结构通过X射线仪(XRD,德国Bruker,D8)测试;样品的光吸收性能采用UV-vis分光光度计(岛津,UV-3600)测试;电池的光电性能采用太阳电池J-V曲线测试系统(美国Newport Oriel光源,吉时利2402数据源表)进行测试,测试条件为标准光源AM1.5,强度100 MW/cm2;入射单色光子–电子转换效率(IPCE)通过CIMPS-2型光电化学测试系统(德国)测试;傅里叶红外光谱通过IR-960型傅里叶红外光谱仪(FTIR,天津瑞岸)测试;光致发光光谱通过PL600型智能激光粒度分析系统进行表征。

3. 结果与讨论

图1给出了添加不同浓度尿素的CH3NH3PbI3薄膜的SEM形貌图。对于未掺杂尿素的钙钛矿前驱液制备的CH3NH3PbI3薄膜,从图1(a)可以看出,其平均晶粒尺寸约300 nm,薄膜表面可以观察到明显的晶界,薄膜表面平整性较差。对尿素含量为5%的前驱夜,其制备的钙钛矿薄膜晶粒略有增大,如图1(b)所示;随着尿素含量提升到10%,钙钛矿晶粒尺寸出现明显增大,部分晶粒尺寸达到1.5 μm,薄膜表面也比较平整,晶粒与晶粒间结合致密,如图1(c)所示。当尿素的浓度达到15%和20%,晶界处会有明显的白色物质聚集,如图1(d)、图1(e)中圆圈标识处,结合图2中的XRD测试结果,这些晶界处的聚集物可能来自于钙钛矿薄膜中的PbI2晶粒的析出物 [16]。文献报道,钙钛矿晶界处的少量的PbI2残留有助于钝化晶界,但是如果PbI2残留较多,则可能成为载流子的复合位点,导致电池整体光电性能降低 [17]。图1(f)给出了添加10%尿素样品的钙钛矿太阳能电池的截面SEM图,可以看出钙钛矿层晶粒大,薄膜致密平整。

图2为掺有不同浓度尿素的钙钛矿前驱液制备的CH3NH3PbI3薄膜的XRD衍射图谱。由XRD衍射图可以发现,钙钛矿薄膜都具有较好的结晶性,在2θ为14.2˚,28.4˚处具有尖锐的衍射峰,分别对应于四方相钙钛矿CH3NH3PbI3的(110),(220)晶面衍射,这与文献报道一致 [18]。随着钙钛矿中尿素浓度的增加,四方相CH3NH3PbI3的(110)、(220)衍射峰的峰强逐渐增强,表明了尿素的添加有助于钙钛矿晶体获得更好的结晶性和取向性,这与图1中钙钛矿薄膜的SEM结果一致。然而,可以发现,当尿素含量超过15%时,在图2(d)和图2(e)中12.7°处可以观察到小的衍射峰,这可能来自于薄膜中残留的PbI2的(001)晶面 [19],而且,随着前驱液中尿素含量增大到20%,产物中PbI2(001)晶面的衍射峰也随之增强,说明随着尿素添加量的增加,薄膜中不仅钙钛矿晶体的晶粒增大,薄膜中的PbI2晶粒也有所增大。前期的报道表明,钙钛矿前驱液中添加尿素后,尿素可与MAI和PbI2等形成中间体,在一定程度上减缓了结晶速度,有利于较大晶粒的形成 [20]。

Figure 1. SEM images of perovskite thin films prepared with different contents of urea addition: (a) 0, (b) 5%, (c) 10%, (d) 15%, (e) 20%, and (f) a typical cross-sectional image of peroskite solar cells with 10% urea addition

图1. 不同浓度的尿素钙钛矿前驱液制备的钙钛矿薄膜的SEM图像:(a) 0,(b) 5%,(c) 10%,(d) 15%,(e) 20%,和(f) 添加10%尿素钙钛矿组装电池的典型截面图

Figure 2. XRD patterns of perovskite thin films prepared with different contents of urea addition: (a) 0, (b) 5%, (c) 10%, (d) 15%, (e) 20 %

图2. 添加不同浓度尿素制备的钙钛矿薄膜XRD图谱:(a) 0,(b) 5%,(c) 10%,(d) 15%,(e) 20%

图3给出了添加不同浓度尿素的钙钛矿前驱液制备的钙钛矿薄膜的紫外-可见光吸收谱。由吸收谱可以看出,与未添加尿素制备的钙钛矿薄膜相比,添加了10%浓度尿素制备的钙钛矿薄膜在350~500 nm范围内光吸收最强,这可能是因为添加10%尿素后,钙钛矿薄膜的晶体结晶度有所提高,晶粒尺寸较大,减少了晶界处的光损耗,因而展现了较强的光吸收 [21]。当添加尿素的浓度超过10%以后,随着尿素浓度的增加吸光度逐渐降低,这可能与薄膜产物中存在的PbI2有关,PbI2的吸光性能比CH3NH3PbI3钙钛矿低 [21],当薄膜产物中存在PbI2时,导致薄膜在350~500 nm范围内的吸光性能降低。所有样品在750~800 nm之间都观察到了明显的吸收边,根据Tauc分析,得到钙钛矿材料的禁带宽度在1.59 ± 0.03 eV附近,这与文献中报道的CH3NH3PbI3钙钛矿的禁带宽度较一致 [22]。

Figure 3. UV-Vis absorption spectrum of perovskite films prepared with different contents of urea addition

图3. 添加不同浓度尿素制备钙钛矿薄膜的紫外–可见光吸收谱

图4(a)和表1分别给出了添加不同浓度尿素的钙钛矿太阳能电池的J-V曲线和光伏性能参数。通过J-V曲线和光电转换效率参数可以看出,与未加尿素组装的电池相比,添加尿素组装的电池的短路电流密度(Jsc)和填充因子(FF)都有所提高,当添加尿素的浓度为10%时,组装的电池的光电性能最优,最高光电转换效率(PCE)达到18.90%,其对应的开路电压(Voc)、短路电流密度(Jsc)和填充因子(FF)分别为1.11 V、21.62 mA/cm2和78.60%,器件PCE相比未添加尿素电池增大了19.2%。这一方面是因为含10%的尿素的钙钛矿前驱液制备的钙钛矿薄膜晶粒尺寸更大,薄膜也更致密平整,有效降低了薄膜的缺陷态密度和载流子输运过程中的复合 [23];另一方面,含10%的尿素添加剂的钙钛矿薄膜在350~500 nm波段本身具有更强的光吸收,也有助于获得更高的短路电流密度。晶粒尺寸的增大和载流子复合概率的减少都有助于组装电池Voc的提高 [24],因而,随着钙钛矿前驱液中尿素含量的增多,组装电池的Voc先逐步上升,在尿素含量为10%时达到最大Voc (1.11V),而后,随着尿素含量进一步上升,薄膜中存在一定的PbI2析出,Voc稍有下降。图4(b)为未添加尿素和添加10%尿素电池的入射光电流效率(IPCE)光谱。添加10%尿素的钙钛矿电池在350~750 nm范围内具有更高的IPCE响应,这与图3的光吸收预期结果一致。由IPCE估算未添加与添加10%尿素的前驱液制备的钙钛矿组装的电池的Jsc分别达到 20.17 mA/cm2和21.33 mA/cm2,这与电池实际测得的Jsc值较一致。图4(c)给出了100个未加尿素制备的钙钛矿组装的电池组和100个添加10%尿素制备的钙钛矿组装的电池组的能量转换效率(PCE)统计图。由统计图可以看出,与未添加尿素的电池相比,尿素的添加明显提高了电池的能量转换效率。图4(d)为未添加尿素和添加10%尿素制备的钙钛矿组装的电池的效率的稳定性图。与未添加尿素的电池相比,尿素的添加有助于提高电池的稳定性。在未封装手套箱环境下存放60天后,添加尿素的电池效率保有初始值的90%左右,而未添加尿素的电池效率衰减到初始值的53%左右。添加适量尿素后,钙钛矿薄膜的晶界中存在一定的PbI2残留,可以起到钝化晶界的作用,有助于器件稳定性的提升;另一方面,添加尿素后,钙钛矿薄膜晶粒更大、薄膜更致密平整,也可能在一定程度上减少了外界水汽等从缝隙的进入,提升了器件稳定性。

Figure 4. Performance of perovskite solar cells without and with different contents of urea addition: (a) J-V curves, (b) incident photocurrent efficiency (IPCE) spectra, (c) statistic graph of the power conversion efficiency (PCE) (100 cells was counted for each), and (d) stability graph of the energy conversion efficiency

图4. 未添加尿素和添加不同浓度尿素的钙钛矿太阳能电池的(a) J-V曲线,(b) 入射光电流效率(IPCE)光谱,(c) 光电转换效率(PCE)统计图(各统计100个电池),(d) 效率稳定性曲线

Table 1. Photovoltaic performance parameters of cells prepared with different contents of ureaaddition

表1. 添加不同浓度的尿素制备的电池的光伏性能参数

图5(a)和图5(b)分别为室温下未添加尿素和添加10%尿素的钙钛矿薄膜(玻璃/钙钛矿)的稳态光致发光(PL)和时间分辨光致发光(TRPL)光谱。由图5(a)可以看出,添加10%尿素与未添加尿素制备的钙钛矿薄膜相比,稳态荧光光谱的峰值强度更强,说明添加尿素的前驱液制备的钙钛矿薄膜,其非辐射复合被明显抑制,这可能是由于适量尿素添加导致的钙钛矿薄膜结晶度提高、晶粒尺寸增大,薄膜缺陷减少导致的 [25]。添加尿素后,稳态PL峰和吸收边会有轻微的红移,这意味着窄禁带和宽吸收区,这可能是由于钙钛矿晶粒尺寸的增大引起的 [26]。由图5(b)钙钛矿薄膜的TRPL光谱,通过测试推算出添加尿素的钙钛矿薄膜的载流子的寿命由60.8 ns提高到112.2 ns,载流子的寿命提高了近一倍,这可能是由于大晶粒钙钛矿中尿素的添加抑制了晶体内的缺陷浓度 [27],因而载流子寿命显著延长,组装电池的性能也明显提升。

Figure 5. (a) Steady-state photoluminescence (PL) and (b) time-resolved photoluminescence (TRPL) of perovskite film on glass, respectively

图5. (a) 玻璃衬底上钙钛矿薄膜的稳态光致发光(PL),(b) 玻璃衬底上钙钛矿薄膜的时间分辨光致发光(TRPL)光谱

为了探明前驱液中尿素添加对钙钛矿薄膜的影响,我们对PbI2/MAI和urea/PbI2/MAI前驱液溶液进行了傅里叶变换红外光谱(FTIR)分析。图6(a)为两个样品的傅里叶红外透射光谱全谱, 可以观察到明显的C=O和S=O伸缩振动。图6(b)和图6(c)分别给出了C=O和S=O的伸缩振动图谱的局部图。当加入尿素后C=O的伸缩振动由1664 cm−1偏移到1658 cm−1 (图6(b)),S=O的伸缩振动频率也有轻微的偏移(图6(c))。在双原子谐波振动模型中,振动频率与力常数的平方根成正比 [28],加入尿素后,C=O伸缩振动和S=O伸缩振动向低频方向有轻微偏移,这表明尿素与钙钛矿之间存在一定的协调作用,与Han等前期的报道结果一致 [25]。Lee等报道,在没有DMSO溶剂的情况下,尿素与MAI·PbI2之间是弱结合,导致C=O的伸缩振动偏移不大,但是在尿素与MAI·PbI2和DMSO混合后,尿素和钙钛矿前驱液可以形成MAI·PbI2·DMSO·urea中间体(C=O伸缩振动偏移到1651 cm−1处),通过Urea与DMSO之间的强耦合作用,能有效减缓后续热处理过程中钙钛矿的结晶速度 [29],因而在PbI2中添加尿素并与MAI和DMSO等混合后,观察到了大晶粒钙钛矿薄膜的形成,组装电池的光电性能也得到了提升。

Figure 6. (a) Fourier transform infrared (FTIR) spectra of PbI2/MAI and urea/PbI2/MAI precursor solutions, respectively; (b) the C = O stretching vibration and (c) the S = O stretching vibration in FTIR spectra, respectively

图6. (a) PbI2/MAI和urea/PbI2/MAI前驱液的傅里叶红外透射(FTIR) (光谱);(b) C=O伸缩振动傅里叶红外透射光谱,(c) S=O伸缩振动傅里叶红外透射光谱

4. 结论

本文在一步法制备MAPbI3钙钛矿薄膜过程中,通过在PbI2前驱液中引入适量的路易斯碱–尿素作为添加剂,获得了大晶粒、致密平整的钙钛矿薄膜。研究发现,尿素引入PbI2溶液后,在与含有DMSO的MAI溶液混合的过程中,形成具有强耦合作用的中间体,有助于减缓钙钛矿晶体的结晶速度,形成结晶性良好的致密钙钛矿薄膜,从而减少了组装电池的晶界缺陷和载流子复合陷阱,提升了器件的光电性能和稳定性。在PbI2前驱液中添加10%尿素时制备的钙钛矿薄膜晶粒大、薄膜致密,组装钙钛矿薄膜太阳能电池具有最优的能量转化效率(18.90%),性能相比未添加尿素器件提升19.2%,电池的稳定性也得到大幅提升。这一方法为获得大晶粒钙钛矿,提升基于钙钛矿晶体的光电器件性能提供了较好的思路。

基金项目

本研究由国家自然科学基金(11374090)、武汉市科技局应用基础前研项目(2019010701011396)资助。

参考文献

NOTES

*通讯作者。

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