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为了更好地了解实验制备的不同TMDCs纳米片的晶体质量与结构以及TMDCs/CsPbBr3范德华异质结构的生长机理,文中利用TEM表征了TMDCs纳米片的形貌与结构。如图2(a)~(d)所示,通过液相剥离得到二维TMDCs成薄片状的结构。MoS2、MoSe2、WS2和WSe2纳米片的高分辨TEM(HRTEM)图像都呈现出有序的原子矩阵和清晰的晶格条纹(图2(e)~(f)),这都表明文中实验制备出了高质量的TMDCs纳米薄片。此外,从它们的HRTEM图像可以看出,MoS2、MoSe2、WS2和WSe2纳米片的(100)晶面的晶面距分别为2.70 Å、2.80 Å、2.79 Å和2.80 Å(1 Å=0.1 nm),这与以前的文献报道是一致的[13–16]。
图 2 TMDCs纳米片的TEM和HRTEM图。(a)、(e) MoS2纳米片,(b)、(f) MoSe2纳米片,(c)、(g) WS2纳米片和(d)、(h)WSe2纳米片的(a)~(d) TEM图与(e)~(h) HRTEM图
Figure 2. TEM and HRTEM image of TMDCs nanoflakes. (a)-(d) TEM pictures and (e)-(f) HRTEM pictures for (a), (e) MoS2 nanoflake, (b), (f) MoSe2 nanoflake, (c), (g) WS2 nanoflake, (d), (h) WSe2 nanoflake
为了研究CsPbBr3钙钛矿薄膜在TMDCs纳米片修饰的SnO2表面上的形貌演变,首先观察了不同ETL基底的表面形貌。图3为不同放大倍数的SnO2基底、SnO2/MoS2基底、SnO2/MoSe2基底、SnO2/WS2基底以及SnO2/WSe2基底的SEM图像。通过对不同TMDCs修饰后的基底对应的SEM图像与未修饰的基底对比可以看出,TMDCs纳米薄片修饰后基底表面更平滑且TMDCs分布均匀,这无疑有利于生长高质量的钙钛矿薄膜。此外,利用紫外-可见光分光光度仪测试了不同TMDCs修饰的基底在200~1100 nm波段的透过率。测试时首先将紫外-可见光分光光度仪在不放基底的情况下进行基线校准,然后放入不同的基底,分别测得其透过率曲线(图4)。对比未修饰的SnO2基底和不同TMDCs修饰的基底在可见光波段的透过率曲线可以看出,TMDCs修饰对基底的透过率影响不大。
图 3 不同TMDCs修饰后的SnO2基底的SEM俯视图。(a1)~(a3) SnO2基底,(b1)~(b3) SnO2/MoS2基底,(c1)~(c3) SnO2/MoSe2基底,(d1)~(d3) SnO2/WS2基底和(e1)~(e3) SnO2/WSe2基底在不同放大倍数下的SEM俯视图
Figure 3. Top-view SEM images of the SnO2 films with TMDCs modification. Top-view SEM images of the films with different magnifications, (a1)-(a3) bare SnO2, (b1)-(b3) SnO2/MoS2, (c1)-(c3) SnO2/MoSe2, (d1)-(d3) SnO2/WS2 and (e1)-(e3) SnO2/WSe2
图 4 不同TMDCs修饰的FTO/ETL基底在可见光波段的透过率曲线
Figure 4. Transmission curves of FTO/ETL substrates with different TMDCs modification at visible wavelengths
接着,研究分析了CsPbBr3钙钛矿薄膜在TMDCs上的生长机理。如图5(a)所示,CsPbBr3立方晶体中(100)平面的晶面距为5.83 Å(PDF#54–0752)[17],非常接近TMDCs的(100)晶面距的两倍(5.4~5.6 Å),这就使CsPbBr3晶体能在TMDCs纳米片上沿(100)方向自发外延生长[18]。因此,由于异质结界面处范德华相互作用的存在,CsPbBr3晶粒的生长过程将受到调控。钙钛矿前驱体在旋涂到TMDCs表面后,由于Pb和S、Se原子之间的相互作用,将实现自组装过程。CsPbBr3和TMDCs之间的面内范德华力耦合作用以及TMDCs无悬空键的(100)晶面为CsPbBr3在其上的面外取向生长提供了一个出色的模板,同时促进CsPbBr3形成大尺寸晶粒,并降低缺陷密度[18]。
图 5 在TMDCs修饰后的基底上生长的CsPbBr3钙钛矿薄膜生长示意图和SEM图像。(a) CsPbBr3钙钛矿在TMDCs上外延生长示意图;不同基底上生长的钙钛矿薄膜的SEM图:(b)未修饰TMDCs的SnO2基底,(c) SnO2/MoS2基底,(d) SnO2/MoSe2基底,(e) SnO2/WS2基底和(f) SnO2/WSe2基底
Figure 5. Schematic diagram and SEM images of CsPbBr3 perovskite grown on TMDCs. (a) Schematic diagram of CsPbBr3 perovskite grown on TMDCs; Top-view SEM images of CsPbBr3 perovskite films grown at the surface of (b) bare SnO2, (c) SnO2/MoS2, (d) SnO2/MoSe2, (e) SnO2/WS2 and (f) SnO2/WSe2
图5(b)~(f)是在不同基底上生长的CsPbBr3钙钛矿薄膜的俯视SEM图像。如图所示,无论是否有TM-DCs中间层,CsPbBr3薄膜都能均匀地覆盖住其下的ETL,没有任何针孔。仔细观察后可以发现,相对于对照组,在TMDCs上生长的CsPbBr3薄膜表现出明显的晶粒尺寸增大、晶界少、裂纹减少的现象,并且晶粒之间更加紧实,这表明TMDCs中间层在调节CsPbBr3结晶动力学方面具有积极作用。从CsPbBr3薄膜的XRD图也可以看出(图6),不同TMDCs修饰后的钙钛矿的(111)晶面的特征峰强度均明显增加,表明TMDCs中间层的界面调节促进了钙钛矿薄膜(111)晶面的生长,使其薄膜晶体质量得到改善。而良好的薄膜晶体质量可以降低钙钛矿层中的电荷陷阱密度,从而增强器件的光伏性能。
图 6 在不同基底上生长的CsPbBr3钙钛矿膜的XRD曲线
Figure 6. XRD patterns of the CsPbBr3 perovskite films grown at different surfaces
基于TMDCs优化的ETL界面,文中进一步评估了TMDCs中间层对全无机CsPbBr3 PSC光伏性能的影响。如图7所示,典型的全无机CsPbBr3 PSC器件结构为FTO/SnO2/TMDCs/CsPbBr3/C,包含FTO层、SnO2/TMDCs层(厚约60 nm)、CsPbBr3钙钛矿层(厚约500 nm)和碳电极层等。
图 7 基于TMDCs界面层的全无机CsPbBr3 PSC器件的截面SEM图
Figure 7. Cross-sectional SEM image of an all-inorganic CsPbBr3 PSC with TMDCs interlayer
由于TMDCs层的导带底的位置在SnO2层和钙钛矿层的导带底之间,因此可以形成中间能级,减小层间的能级差,从而提高了界面电子的提取效率[19]。文中在模拟阳光下(AM1.5,100 mW/cm2)测试了TM-DCs修饰前后的全无机PSC的光伏性能。图8(a)是不同TMDCs界面修饰器件在光照下的电流-电压(J-V)曲线,表1列出了不同器件的PCE、开路电压(Voc)、短路电流密度(Jsc)和填充因子(FF)等光伏数据。相比于未修饰的原始器件,不同TMDCs修饰后的器件效率均有提升,最高达到10.02%(WS2),远高于原始器件的7.94%效率。器件效率的提高主要得益于TMDCs界面调节后Jsc和Voc的显著提高,它们分别从原始器件的7.01 mA/cm2和1.474 V提高到界面调节后的7.97 mA/cm2和1.567 V。TMDCs修饰后器件性能的提升主要是由于修饰后钙钛矿薄膜质量的提升以及界面势垒的降低,减少了界面缺陷态密度,促进了载流子的分离与传输。为了进一步验证J-V测试的可靠性,在器件最大功率点的偏置电压下表征了其稳态功率输出。如图8(b)所示,与原始器件相比,经过TMDCs优化后的器件的效率均有明显增强,结果与J-V测量一致。此外,TMDCs/CsPbBr3范德华异质结的形成可以降低界面势垒,同时引导晶格重构,减少钙钛矿薄膜中的缺陷,加速电荷的提取和转移,这些都有利于减小器件正反扫迟滞。如图8(c)所示,WS2修饰后的器件的迟滞现象相比于原始器件明显减弱,修饰后的器件的迟滞因子从原始器件的32.4%减少到24.3%。最后,在温度为25 ℃,湿度为80%的环境条件下评估了器件的稳定性。如图8(d)所示,没有封装的FTO/SnO2/WS2/CsPbBr3/C器件的Voc、Jsc、PCE和FF在存储17天后可以保持初始值的90%以上。这种出色的器件稳定性除了是由于全无机CsPb-Br3钙钛矿本身的高稳定性外,还归因于其器件顶部碳电极层提供的额外的耐湿性。
图 8 不同TMDCs界面修饰PSC器件的光伏性能。不同器件的(a) J-V曲线和(b)稳定功率输出曲线;(c)原始器件和WS2修饰后器件的正反扫J-V曲线;(d) FTO/SnO2/WS2/CsPbBr3/C器件在25 ℃和80%湿度条件下的归一化Voc, Jsc, PCE and FF稳定性曲线
Figure 8. Photovoltaic performance of the PSCs with and without TMDCs modification. (a) J–V curves and (b) steady power output curves of different devices; (c) J-V curves of the pristine device and the device with WS2 modification under forward and reverse scan directions; (d) Normalized Voc, Jsc, PCE and FF stability curves of FTO/SnO2/WS2/CsPbBr3/C device under 25 °C and 80% humidity condition
表 1 不同TMDCs界面修饰全无机PSCs器件的Voc、Jsc、PCE和FF等光伏参数
Table 1. Photovoltaic parameters(Voc, Jsc, PCE and FF) for all-inorganic PSCs with and without TMDCs modification
All-inorganic PSC devices Voc/V Jsc/mA·cm−2 PCE FF FTO/SnO2/CsPbBr3/C 1.474 7.01 7.94% 76.9% FTO/SnO2/MoS2/CsPbBr3/C 1.525 7.13 8.26% 75.9% FTO/SnO2/MoSe2/CsPbBr3/C 1.562 7.51 9.26% 78.9% FTO/SnO2/WS2/CsPbBr3/C 1.567 7.97 10.02% 80.2% FTO/SnO2/WSe2/CsPbBr3/C 1.556 7.79 9.03% 74.5% 为了更好地理解器件中的电荷复合,文中通过暗态下的电容-电压(C-V)测试得到器件的Mott-Schottky曲线(图9),进一步分析TMDCs修饰后的全无机CsPbBr3 PSC性能增加的原因。根据Mott–Schottky方程[20]:
图 9 不同TMDCs界面修饰PSCs器件的Mott-Schottky曲线
Figure 9. Mott-Schottky curves of the PSCs with and without TMDCs modification
$$ {c}^{-2}=2\frac{{V}_{\rm bi}-V}{{A}^{2}e\varepsilon {\varepsilon }_{0}{N}_{A}} $$ 式中:c为器件电容;Vbi为器件的内建电势;V为施加的电压;A为有效电池面积;e为元电荷;ɛ为钙钛矿的相对介电常数;NA为界面的电荷密度。原始器件的Vbi为1.365 V,TMDCs修饰后器件的Vbi分别提升到1.403 V (MoS2)、1.452 V (MoSe2)、1.466 V (WS2)和1.407 V (WSe2),这表明TMDCs修饰后器件中电荷分离的驱动力增强,耗尽区扩大,因此抑制了电荷复合[20]。Mott-Schottky曲线中较大的斜率(图9中的虚线)也表明TMDCs修饰后器件的界面电荷密度较低[21]。这主要是由于TMDCs修饰提升了钙钛矿晶体的质量,减少了界面缺陷,而这些缺陷会成为复合中心阻碍载流子转移和提取。
器件优化后性能提升主要是得益于高质量的钙钛矿薄膜和界面能级匹配改善了载流子的传输并抑制了非辐射复合。器件的电子提取能力与其瞬态荧光寿命密切相关。图10(a)是钙钛矿薄膜在不同TMDCs修饰后的TRPL衰减曲线。衰减曲线可以使用如下双指数函数拟合[22]:
图 10 不同TMDCs修饰的钙钛矿薄膜的(a) TRPL衰减曲线和(b) PL曲线
Figure 10. (a) TRPL decay curves and (b) PL curves of perovskite films with and without TMDCs modification
$$ I=A {\rm e}^{-\left(t-t_0\right) / \tau_1}+B {\rm e}^{-\left(t-t_0\right) / \tau_2} $$ 式中:I为样品被激发的荧光光强;τ1为非辐射复合缺陷相关部分;τ2为辐射复合缺陷的相关部分。根据τave=(a1τ12+a2τ22)/(a1τ1+a2τ2)计算平均衰减时间τave,以评估器件的电子提取能力,其中a1和a2分别代表τ1和τ2的比例[23]。
表2列出了拟合得出的τ1、τ2和τave等相应参数。从表中可以看出,不同TMDCs修饰后的CsPbBr3钙钛矿薄膜的τave均有不同程度的降低,原始薄膜的τave为0.788 ns,TMDCs修饰后分别降低到0.738 ns (MoS2)、0.713 ns (MoSe2)、0.653 ns (WS2)和0.720 ns (WSe2)。由于TMDCs修饰后器件中ETL/钙钛矿界面用于电子提取的导带具有更好的能级匹配,同时界面缺陷更少,因此电荷提取更快,使CsPbBr3在不同的TMDCs修饰层上均表现出较短的载流子寿命。此外,钙钛矿薄膜的PL峰强度也高度依赖于电子-空穴的辐射复合。在将TMDCs引入界面时,钙钛矿薄膜的PL峰强度显著降低(图10(b)),这表明电子从钙钛矿到SnO2 ETL的提取能力增强,因此钙钛矿导带上光生电子的寿命无疑会缩短,这也与前面的TRPL测试结果一致。
表 2 不同TMDCs修饰的钙钛矿薄膜对应的TRPL拟合参数
Table 2. TRPL fitting parameters of perovskite films with and without TMDCs modification
Sample τ1/ns a1 τ2/ns a2 τave/ns Pristine 0.538 66.08% 8.349 33.92% 0.788 MoS2 0.572 75.69% 7.783 24.31% 0.738 MoSe2 0.468 64.09% 10.732 35.91% 0.713 WS2 0.537 80.64% 6.428 19.36% 0.653 WSe2 0.579 78.68% 6.875 21.32% 0.720
Control of photoelectric properties in all-inorganic CsPbBr3 thin films with two-dimensional interface modification
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摘要: 全无机CsPbBr3钙钛矿太阳能电池中的电荷传输层与钙钛矿层界面的载流子复合是制约其光电转换效率进一步提升的关键因素。文中采用带隙和带边可调控的二维过渡金属硫族化合物(MoS2、MoSe2、WS2和WSe2)材料作为电子传输层和钙钛矿层之间的界面修饰材料和载流子传输材料,利用晶格匹配的范德华外延生长高质量的钙钛矿薄膜,同时通过界面能级补偿和势垒消减,降低钙钛矿层与电子传输层之间的界面电荷损失,促进全无机CsPbBr3钙钛矿太阳能电池器件中载流子的提取与传输,使器件光电转换效率由初始的7.94%提高到10.02%,同时开路电压从1.474 V提升至1.567 V。该研究为制备高质量的钙钛矿薄膜同时实现界面能级匹配的高性能光电器件提供了一条新途径。Abstract:
Objective As a new generation photovoltaic technology, perovskite solar cells (PSCs) have achieved a comparable efficiency to commercial silicon-based solar cells, demonstrating great application potential. However, these photoactive layers based on organic-inorganic hybrid perovskites are very unstable, which seriously hinders their commercial application. Therefore, all-inorganic CsPbBr3 perovskite has attracted enormous attention due to its outstanding environmental tolerance to heat, moisture, oxygen and UV light. Unfortunately, the device efficiency based on CsPbBr3 perovskite is relatively lower compared to that of the PSCs device with organic-inorganic hybrid perovskites. The recombination of charge carriers at the interface between the charge transport layers and the perovskite layer in all-inorganic CsPbBr3 PSCs is the key factor that restricts the further improvement of its photoelectric conversion efficiency (PCE). In recent years, two-dimensional transition metal dichalcogenides (TMDCs) materials represented by MoS2, MoSe2, WS2 and WSe2 have attracted more and more attention due to their unique physical and chemical properties. With the advantages such as adjustable band gap and band edge, high carrier mobility, stable chemical properties and matching energy level with perovskite materials, two-dimensional TMDCs are regarded as effective interface modification materials to promote interface charge extraction in all inorganic CsPbBr3 PSCs. However, the current research on using two-dimensional materials as interface modification layers and charge carrier transport layers in all-inorganic PSC is still in its infancy. Methods Through interface engineering, various two-dimensional TMDCs (MoS2, MoSe2, WS2, and WSe2) materials are introduced at the interface between the perovskite layer and the electron transport layer of the all-inorganic CsPbBr3 PSCs with an FTO/SnO2/TMDCs/CsPbBr3/C structure. The two-dimensional TMDCs here act as both interface modification materials and carrier transport layers. Through interface level compensation and barrier reduction with TMDCs interlayers, the carrier extraction and transport in all-inorganic CsPbBr3 PSC devices are promoted (Fig.1). Moreover, by constructing TMDCs/CsPbBr3 van der Waals heterostructure, high-quality CsPbBr3 perovskite thin films with large and compact grains are grown via lattice matched van der Waals epitaxy (Fig.5). Results and Discussions The interface charge loss between the perovskite layer and the electron transport layer is reduced, and the carrier extraction in the all-inorganic CsPbBr3 PSC devices are enhanced, so that the PCE of the devices is increased from the initial 7.94% to 10.02%, and the open-circuit voltage is increased from 1.474 V to 1.567 V (Fig.8). In addition, Mott–Schottky curves are measured in the dark by performing capacitance-voltage characterization, which demonstrates an enhanced built-in potential, indicating the enlarged driving force for charge transportation with TMDCs/CsPbBr3 heterostructure (Fig.9). Finally, the time-resolved photoluminescence decay measurements are performed to investigate the photoluminescence decay lifetimes, which are closely related to the electron extraction ability. Due to better energy level matching of TMDCs with perovskite layers, CsPbBr3 perovskite layers show shorter carrier lifetimes on TMDCs interlayer (Fig.10), which further confirms the enhanced electron extraction ability from perovskite to the electron transport layer. Conclusions A novel strategy is developed to prepare high-quality perovskite films by constructing TMDCs/CsPbBr3 van der Waals heterostructure and achieve high-performance photoelectric devices through interfacial energy level matching, which provides a new way for the development of all-inorganic perovskite optoelectronic devices based on two-dimensional TMDCs materials. -
图 2 TMDCs纳米片的TEM和HRTEM图。(a)、(e) MoS2纳米片,(b)、(f) MoSe2纳米片,(c)、(g) WS2纳米片和(d)、(h)WSe2纳米片的(a)~(d) TEM图与(e)~(h) HRTEM图
Figure 2. TEM and HRTEM image of TMDCs nanoflakes. (a)-(d) TEM pictures and (e)-(f) HRTEM pictures for (a), (e) MoS2 nanoflake, (b), (f) MoSe2 nanoflake, (c), (g) WS2 nanoflake, (d), (h) WSe2 nanoflake
图 3 不同TMDCs修饰后的SnO2基底的SEM俯视图。(a1)~(a3) SnO2基底,(b1)~(b3) SnO2/MoS2基底,(c1)~(c3) SnO2/MoSe2基底,(d1)~(d3) SnO2/WS2基底和(e1)~(e3) SnO2/WSe2基底在不同放大倍数下的SEM俯视图
Figure 3. Top-view SEM images of the SnO2 films with TMDCs modification. Top-view SEM images of the films with different magnifications, (a1)-(a3) bare SnO2, (b1)-(b3) SnO2/MoS2, (c1)-(c3) SnO2/MoSe2, (d1)-(d3) SnO2/WS2 and (e1)-(e3) SnO2/WSe2
图 5 在TMDCs修饰后的基底上生长的CsPbBr3钙钛矿薄膜生长示意图和SEM图像。(a) CsPbBr3钙钛矿在TMDCs上外延生长示意图;不同基底上生长的钙钛矿薄膜的SEM图:(b)未修饰TMDCs的SnO2基底,(c) SnO2/MoS2基底,(d) SnO2/MoSe2基底,(e) SnO2/WS2基底和(f) SnO2/WSe2基底
Figure 5. Schematic diagram and SEM images of CsPbBr3 perovskite grown on TMDCs. (a) Schematic diagram of CsPbBr3 perovskite grown on TMDCs; Top-view SEM images of CsPbBr3 perovskite films grown at the surface of (b) bare SnO2, (c) SnO2/MoS2, (d) SnO2/MoSe2, (e) SnO2/WS2 and (f) SnO2/WSe2
图 8 不同TMDCs界面修饰PSC器件的光伏性能。不同器件的(a) J-V曲线和(b)稳定功率输出曲线;(c)原始器件和WS2修饰后器件的正反扫J-V曲线;(d) FTO/SnO2/WS2/CsPbBr3/C器件在25 ℃和80%湿度条件下的归一化Voc, Jsc, PCE and FF稳定性曲线
Figure 8. Photovoltaic performance of the PSCs with and without TMDCs modification. (a) J–V curves and (b) steady power output curves of different devices; (c) J-V curves of the pristine device and the device with WS2 modification under forward and reverse scan directions; (d) Normalized Voc, Jsc, PCE and FF stability curves of FTO/SnO2/WS2/CsPbBr3/C device under 25 °C and 80% humidity condition
表 1 不同TMDCs界面修饰全无机PSCs器件的Voc、Jsc、PCE和FF等光伏参数
Table 1. Photovoltaic parameters(Voc, Jsc, PCE and FF) for all-inorganic PSCs with and without TMDCs modification
All-inorganic PSC devices Voc/V Jsc/mA·cm−2 PCE FF FTO/SnO2/CsPbBr3/C 1.474 7.01 7.94% 76.9% FTO/SnO2/MoS2/CsPbBr3/C 1.525 7.13 8.26% 75.9% FTO/SnO2/MoSe2/CsPbBr3/C 1.562 7.51 9.26% 78.9% FTO/SnO2/WS2/CsPbBr3/C 1.567 7.97 10.02% 80.2% FTO/SnO2/WSe2/CsPbBr3/C 1.556 7.79 9.03% 74.5% 表 2 不同TMDCs修饰的钙钛矿薄膜对应的TRPL拟合参数
Table 2. TRPL fitting parameters of perovskite films with and without TMDCs modification
Sample τ1/ns a1 τ2/ns a2 τave/ns Pristine 0.538 66.08% 8.349 33.92% 0.788 MoS2 0.572 75.69% 7.783 24.31% 0.738 MoSe2 0.468 64.09% 10.732 35.91% 0.713 WS2 0.537 80.64% 6.428 19.36% 0.653 WSe2 0.579 78.68% 6.875 21.32% 0.720 -
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