合工大周儒/中科大陈涛/牛津大学Robert Hoye合作Adv. Mater.：晶粒工程实现低开路电压损失硫化锑薄膜太阳能电池
合工大周儒/中科大陈涛/牛津大学Robert Hoye合作Adv. Mater.：晶粒工程实现低开路电压损失硫化锑薄膜太阳能电池
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第一作者：刘新年、蔡志远
通讯作者：周儒、陈涛、Robert Hoye
单位：合肥工业大学、中国科学技术大学、牛津大学
【引言】
锑基硫属化合物是一类富有前景的新型光伏材料，其具有环境友好、储量丰富、稳定性优异、组分简单、光吸收系数高（10
4
~10
5
cm
-1
）以及带隙可调（1.10~1.70 eV）等理想特性。特别是，硫化锑（Sb
2
S
3
）禁带宽度约为1.70 eV，十分适合用于室内光伏器件以及硅基叠层太阳能电池；同时，低熔点（约500°C）和高饱和蒸气压使其可以实现低温制备柔性轻型器件，为低功耗物联网终端传感器供能。根据单结太阳能电池Shockley–Queisser极限理论，具有1.70 eV带隙光伏材料的最高性能参数是：开路电压（V
oc
）为1.402 V，短路电流密度（J
sc
）为22.46 mA/cm
2
，填充因子（FF）为91%，在一个太阳光照下（AM1.5G）的光电转换效率为28.64%。然而，目前Sb
2
S
3
太阳能电池的效率记录仍远远落后于此理论效率，其主要原因是器件V
oc
较低。在过去十年中，Sb
2
S
3
太阳能电池的V
oc
基本保持在550-750 mV之间，V
oc
损失超过900 mV，这显著高于其他具有类似带隙的光吸收材料体系（如CH
3
NH
3
PbI
3
、GaAs、CdTe等）。因此，亟需探索有效策略以提升Sb
2
S
3
太阳能电池器件的开路电压。研究表明Sb
2
S
3
太阳能电池较大的V
oc
损失主要源自器件内部的界面和体相缺陷。对于多晶半导体光电器件来说，晶界显著影响其薄膜光电特性，进而影响光伏器件性能。这是由于晶界处存在大量悬挂键，会增加电荷非辐射复合，且晶界会造成载流子散射，从而阻碍载流子输运。由于薄膜微观结构在很大程度上取决于初始成核和生长过程，因此，合理设计和调控Sb
2
S
3
的成核和生长过程十分重要。然而，由于溶液环境或气相系统中涉及复杂的反应过程，使薄膜生长过程调控较为困难。调控晶粒尺寸和晶界网络以制备大晶粒Sb
2
S
3
吸收层薄膜仍然具有较大挑战性。
【成果展示】
近日，合肥工业大学周儒课题组与中国科学技术大学陈涛、牛津大学Robert Hoye 课题组合作，在著名学术期刊Advanced Materials上发表题为“Grain engineering of Sb 2 S 3 thin films to enable efficient planar solar cells with high open-circuit voltage”的研究论文。该文章针对Sb 2 S 3 薄膜太阳能电池中V oc 损失较大的问题，通过薄膜晶粒尺寸调控实现器件V oc 提升，获得高效Sb 2 S 3 薄膜太阳能电池。在本工作中，作者通过在Sb 2 S 3 沉积前驱溶液中加入适量的镧系离子Ce 3+ ，使Sb 2 S 3 薄膜晶界密度大幅下降，从1068±40 nm μm -2 （最大晶粒约为5 μm）显著降低至327±23 nm μm -2 （最大晶粒>15 μm）。通过系统的材料结构、薄膜形貌和光电特性表征并辅以计算，揭示了Sb 2 S 3 薄膜晶粒尺寸增加和器件光伏性能改善的潜在机制。研究表明：晶界密度降低的关键因素之一是在CdS/Sb 2 S 3 界面处形成了超薄Ce 2 S 3 层，这会降低Sb 2 S 3 层与衬底之间的界面能并增加粘附功，进而促进Sb 2 S 3 薄膜在衬底上的异质成核及其侧向生长。通过降低晶界密度以及CdS/Sb 2 S 3 异质界面处的非辐射复合，改善异质结处的载流子传输性能，获得高效Sb 2 S 3 薄膜太阳能电池，光电转换效率为7.66%，开路电压达796 mV，这是目前Sb 2 S 3 光伏器件中的开路电压最大值。本研究提供了一种通过调节原位化学环境实现Sb 2 S 3 吸收层薄膜成核和生长调控的有效策略，该策略可以广泛应用于其他薄膜材料体系。
【图文导读】
1. 制备大晶粒尺寸Sb 2 S 3 薄膜
通过在Sb
2
S
3
薄膜沉积前驱溶液中添加镧系离子Ce
3+
能够实现有效晶粒尺寸调控，从而制备具有超低密度晶界网络的大晶粒Sb
2
S
3
吸收层薄膜。系统表征揭示：在CdS/Sb
2
S
3
界面形成超薄Ce
2
S
3
，可以调控成核和生长过程。对照组Sb
2
S
3
薄膜晶粒尺寸约为2.5-5.0 μm，通过引入一定量Ce
3+
到前驱体溶液中能够将薄膜晶粒尺寸显著提升至超过15 μm。Sb
2
S
3
薄膜表面的晶界密度则从对照组样品的1068±40 nm μm
-2
大幅下降至327±23 nm μm
-2
。FIB-TEM表征证实这些大晶粒确实为单晶晶粒。
Figure 1.
(a) Top-view scanning electron microscopy (SEM) images of Sb
2
S
3
thin films prepared without (
i.e.
, control) and with the addition of Ce(CH
3
COO)
3
salt, with molar ratios of Ce
3+
/Sb
3+
= 0.5%, 1%, 2% and 3%. The hydrothermal deposition time was 180 min in all cases. The grain boundaries (GBs) are highlighted to show more clearly the changes in the GB density. (b, c) Cross-sectional SEM images of the control Sb
2
S
3
and 1%Ce-Sb
2
S
3
films. (d) The dependence of the GB density on the Ce
3+
concentration in the precursor solution. Three samples were measured to determine the mean GB density values shown, and the error bars represent the standard deviation. (e) X-ray diffraction (XRD) patterns of the control Sb
2
S
3
film and Ce-Sb
2
S
3
films prepared with different Ce
3+
concentrations. (f) Texture coefficients of the (120), (130), (211) and (221) peaks, which dominate the diffraction patterns of the Sb
2
S
3
films.
Figure 2.
(a, b) SEM images of the 1%Ce-Sb
2
S
3
film used for making samples for transmission electron microscopy (TEM) characterization, and corresponding process of obtaining the lamella using a focused ion beam. (c) Cross-sectional TEM image of 1%Ce-Sb
2
S
3
sample deposited on FTO/SnO
2
/CdS substrate. (d) Selected area electron diffraction (SAED) pattern from Sb
2
S
3
crystals, with diffraction spots indexed. (e) Illustration of the crystal structure of Sb
2
S
3
, viewed from the [001] direction. (f-h) High-resolution (HR) TEM measurements performed at points A1, A2 and A3 (see part c), and the corresponding lattice fringes.
2. 大晶粒尺寸Sb 2 S 3 薄膜的生长机理
综合TEM、XRD、SIMS和XPS表征结果，能够排除Ce 3+ 在Sb 2 S 3 基体中发生取代或间隙掺杂的可能性，同时揭示在CdS/Sb 2 S 3 界面处形成超薄Ce 2 S 3 层。作者运用材料科学中的成核和生长理论来理解Sb 2 S 3 薄膜的微观结构演变过程，提出一个合理的机理解释：Ce 2 S 3 界面层促进了Sb 2 S 3 的异质成核，抑制了溶液中的均相成核；与CdS/Sb 2 S 3 异质界面相比，Ce 2 S 3 /Sb 2 S 3 异质界面的界面能降低、粘附功增加，导致Sb 2 S 3 生长模型由Volmer-Weber生长模型（岛状模型）向Stranski-Krastanov模型（层状+岛状模型）转变，促进了Sb 2 S 3 薄膜侧向生长。同时，Ce 2 S 3 界面层能够改善异质结质量，使Sb 2 S 3 与衬底之间结合更加紧密。
Figure 3.
(a) Secondary ion mass spectrometry (SIMS) depth profiles of the 180 min-deposited 1%Ce-Sb
2
S
3
thin film. (b) HRTEM image at the CdS/Sb
2
S
3
interface of the 180 min-deposited 1%Ce-Sb
2
S
3
thin film. (c-e) High resolution XPS spectra of Sb 3d, S 2p and Ce 3d core levels, along with peaks fitted to these spectra, for the 15 min- and 30 min-deposited 1%Ce-Sb
2
S
3
film samples.
Figure 4. (a) Illustration of the contact angle ( θ ) for heterogeneous nucleation, and the water contact angles of pristine CdS film, as well as CdS films that have undergone 15 min- and 30 min- hydrothermal deposition in Sb 2 S 3 precursor solutions (both without and with 1% Ce). (b) Histogram of the calculated interfacial adhesion work and interfacial energy for the heterointerfaces of CdS/Sb 2 S 3 and Ce 2 S 3 /Sb 2 S 3 . (c) Schematic illustrating conventional growth and Ce 2 S 3 -mediated growth of Sb 2 S 3 thin films.
3. Sb 2 S 3 薄膜太阳能电池的器件性能
作者进一步构筑了平面结构Sb
2
S
3
薄膜太阳能电池：FTO/SnO
2
/CdS/Sb
2
S
3
/Spiro-OMeTAD/Au。器件表现出良好的性能以及重复性。与对照组器件相比，引入Ce
2
S
3
界面层的Sb
2
S
3
光伏器件性能获得显著提升。最佳1% Ce-Sb
2
S
3
器件光电转换效率为7.66%，V
oc
为796 mV，J
sc
为16.67 mA cm
-2
，FF为57.72%。接近800 mV的V
oc
是迄今为止报道的Sb
2
S
3
光伏器件的最大值。
Figure 5 . (a) Illustration and (b) cross-sectional SEM image of the device structure, which have the configuration: FTO/SnO 2 /CdS/Sb 2 S 3 /Spiro-OMeTAD/Au. (c) The statistics of the performance parameters of the control Sb 2 S 3 device and Ce-Sb 2 S 3 devices obtained with the addition of different concentrations of Ce 3+ to the precursor solution. 20 devices were measured for each condition, and the performance metrics of each device is shown as individual data points. (d) J-V curves of the control Sb 2 S 3 and Ce-Sb 2 S 3 solar cells, measured under AM 1.5G (100 mW cm -2 ) illumination. (e) External quantum efficiency (EQE) curves of best-performing 1%Ce-Sb 2 S 3 solar cells. (f) Evolution in the record efficiency of Sb 2 S 3 solar cells. (g) V OC values of previous work on well-developed planar and sensitized Sb 2 S 3 solar cells.
4. 太阳能电池器件物理
阐明薄膜缺陷特性并建立缺陷与器件性能之间的关联十分重要。作者利用深能级瞬态谱（DLTS）探索了Sb 2 S 3 薄膜中的缺陷深度和密度。与对照组Sb 2 S 3 器件相比，1%Ce-Sb 2 S 3 器件内部吸收层薄膜中的缺陷密度降低，从而抑制了器件内部的电荷复合。超快瞬态吸收光谱（TAS）表明：与对照组样品相比，1%Ce-Sb 2 S 3 薄膜样品中的载流子寿命更长，揭示体相与（或）界面处的载流子复合获得有效抑制。光生少数载流子空穴的寿命延长将改善器件V oc 。此外，电荷密度差分析揭示：Ce 2 S 3 /Sb 2 S 3 异质界面的键合比CdS/Sb 2 S 3 异质界面更强，即Ce 2 S 3 界面层的存在有助于形成更理想的异质界面，改善Sb 2 S 3 光伏器件中异质结界面处的载流子传输特性。
Figure 6. (a) Deep-level transient spectroscopy (DLTS) signals from the control Sb 2 S 3 and 1%Ce-Sb 2 S 3 devices. (b) Arrhenius plots derived from DLTS signals. The data points were obtained by calculating internal transients included in DLTS signals with the discrete Laplace transform, and the solid lines are corresponding linear fits. H1 and H2 correspond to S Sb1 and S Sb2 anti-site defects, respectively. (c) The statistical histogram of calculated σ · N T for different hole traps in the control Sb 2 S 3 and 1%Ce-Sb 2 S 3 devices. (d, e) Schematic of band edge positions and defect levels of the control Sb 2 S 3 and 1%Ce-Sb 2 S 3 , respectively, including CB ( E C ) and VB ( E V ) edges, Fermi level ( E F ) and defect energy levels (H1, H2), relative to the vacuum level. (f) Illustration of S Sb1 and S Sb2 defects in a [Sb 4 S 6 ] n unit of Sb 2 S 3 crystal structure. (g) Space-charge limit current density (SCLC) measurements of the control Sb 2 S 3 and 1%Ce-Sb 2 S 3 based on the electron-only structure device of FTO/CdS/Sb 2 S 3 /PCBM/Au. (h, i) The dependence of V OC and J SC on the light intensity for the control Sb 2 S 3 and 1%Ce-Sb 2 S 3 solar cells.
Figure 7
.
(a, b) Transient absorption (TA) spectra obtained at 1, 10, 100, 1000, and 5000 ps pump-probe delay for control Sb
2
S
3
and 1%Ce-Sb
2
S
3
film samples. Excitation was with a 400 nm wavelength pulsed laser at a fluence of 251 μJ cm
-2
pulse
-1
and a repetition rate of 1000 Hz. (c, d) Transient kinetic decay (scatter) and corresponding bi-exponential curve fittings (solid line) monitored at 560 nm of the control Sb
2
S
3
and 1%Ce-Sb
2
S
3
films. Δ
A
is defined as the change in the absorption of the sample before and after pumping. (e, f) Diagram of the charge density difference analysis of the heterointerfaces of CdS/Sb
2
S
3
and Ce
2
S
3
/Sb
2
S
3
.
Xinnian Liu, Zhiyuan Cai, Lei Wan, Peng Xiao, Bo Che, Junjie Yang, Haihong Niu, Huan Wang, Jun Zhu, Yi-Teng Huang, Huimin Zhu, Szymon J. Zelewski, Tao Chen, Robert L. Z. Hoye, Ru Zhou, Grain Engineering of Sb 2 S 3 Thin Films to Enable Efficient Planar Solar Cells with High Open-Circuit Voltage, Advanced Materials, 2023.
https://doi.org/10.1002/adma.202305841
通讯作者简介
周儒 现任合肥工业大学电气工程与自动化学院副教授。于中国科学技术大学物理系获理学博士学位，曾赴美国华盛顿大学材料科学与工程系、英国牛津大学化学系等单位访问学习。目前主要从事下一代低成本、高性能太阳能电池材料与器件研究，包括锑基硫属薄膜太阳能电池、量子点太阳能电池、钙钛矿太阳能电池等。以第一作者/通讯作者在Adv. Mater.. Adv. Energy Mater., Adv. Funct. Mater., Nano Energy, Coordin. Chem. Rev., J. Mater. Chem. A, Sci. China Mater.等学术刊物发表研究论文50余篇，英文专著章节1部，授权发明专利10余项，主持国家自然科学基金面上/青年项目、安徽省自然科学基金面上/青年项目等科研项目10余项。
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