Research Background
Antimony based sulfur compounds are a promising new type of photovoltaic materials, which have ideal characteristics such as environmental friendliness, abundant reserves, excellent stability, simple composition, high light absorption coefficient (104-105 cm-1), and tunable bandgap (1.10-170 eV). Especially, the bandgap width of antimony sulfide (Sb2S3) is about 1.70 eV, making it very suitable for indoor photovoltaic devices and silicon-based stacked solar cells; At the same time, its low melting point (about 500 ° C) and high saturation vapor pressure enable it to prepare flexible lightweight devices at low temperatures, providing power for low-power IoT terminal sensors. According to the Shockley Queisser limit theory of single junction solar cells, the highest performance parameters of photovoltaic materials with a 1.70 eV bandgap are: open circuit voltage (Voc) of 1.402 V, short circuit current density (Jsc) of 22.46 mA/cm2, fill factor (FF) of 91%, and photoelectric conversion efficiency of 28.64% under one solar irradiation (AM1.5G). However, the current efficiency record of Sb2S3 solar cells still lags far behind this theoretical efficiency, mainly due to the lower device Voc. In the past decade, the Voc of Sb2S3 solar cells has remained basically between 550-750 mV, with a Voc loss of over 900 mV, which is significantly higher than other light absorbing material systems with similar bandgaps (such as CH3NH3PbI3, GaAs, CdTe, etc.). Therefore, it is urgent to explore effective strategies to improve the open circuit voltage of Sb2S3 solar cell devices. Research has shown that the significant Voc loss in Sb2S3 solar cells is mainly due to interface and bulk defects within the device. For polycrystalline semiconductor optoelectronic devices, grain boundaries significantly affect their thin film optoelectronic properties, thereby affecting the performance of photovoltaic devices. This is due to the presence of numerous dangling bonds at grain boundaries, which increase non radiative charge recombination and ca.
Achievement Display
Recently, the Zhou Ru research group of Hefei University of Technology, together with Chen Tao from the University of Science and Technology of China and Robert Hoye from the University of Oxford, published a research paper titled "Grain engineering of Sb2S3 thin films to enable effective planar solar cells with high open circuit voltage" in the renowned academic journal Advanced Materials. This article addresses the issue of significant Voc loss in Sb2S3 thin-film solar cells, and achieves efficient Sb2S3 thin-film solar cells by controlling the grain size of the thin film to improve device Voc. In this work, the author significantly reduced the grain boundary density of Sb2S3 thin films from 1068 ± 40 nm by adding an appropriate amount of lanthanide ion Ce3+to the precursor solution of Sb2S3 deposition μ M-2 (maximum grain size is about 5) μ m) Significantly reduced to 327 ± 23 nm μ M-2 (maximum grain size>15) μ m) . By characterizing the material structure, film morphology, and optoelectronic properties of the system, supplemented by calculations, the potential mechanisms for increasing the grain size of Sb2S3 thin films and improving the photovoltaic performance of the device were revealed. Research has shown that one of the key factors for the decrease in grain boundary density is the formation of an ultra-thin Ce2S3 layer at the CdS/Sb2S3 interface, which reduces the interface energy between the Sb2S3 layer and the substrate and increases the adhesion work, thereby promoting the heterogeneous nucleation and lateral growth of Sb2S3 thin films on the substrate. By reducing the grain boundary density and non radiative recombination at the CdS/Sb2S3 heterojunction, the carrier transport performance at the heterojunction is improved, resulting in an efficient Sb2S3 thin film solar cell with a photoelectric conversion efficiency of 7.66% and an open circuit voltage of 796 mV, which is currently the highest open circuit voltage in Sb2S3 photovoltaic devices. This study provides an effective strategy for nucleati
Introduction to graphic and textual reading
Key point 1: Preparation of Sb2S3 thin films with large grain size
By adding lanthanide ions Ce3+to the precursor solution of Sb2S3 film deposition, effective grain size control can be achieved, thereby preparing large grain Sb2S3 absorption layer films with ultra-low density grain boundary networks. System characterization reveals that the formation of ultra-thin Ce2S3 at the CdS/Sb2S3 interface can regulate nucleation and growth processes. The grain size of Sb2S3 thin film in the control group is approximately 2.5-5.0 μ m. By introducing a certain amount of Ce3+into the precursor solution, the grain size of the thin film can be significantly increased to over 15 μ M. The grain boundary density on the surface of Sb2S3 thin film decreased from 1068 ± 40 nm in the control group sample μ M-2 significantly decreased to 327 ± 23 nm μ M-2. FIB-TEM characterization confirms that these large grains are indeed single crystal grains.
Figure 2 (a, b) SEM images of the 1% Ce Sb2S3 film used for making samples for transmission electron microscopy (TEM) characterization, and corresponding process of observing the lamblla using a focused ion beam (c) Cross sectional TEM image of 1% Ce Sb2S3 sample deposited on FTO/SnO2/CdS substrate (d) Selected area electric diffusion (SAED) pattern from Sb2S3 crystals, with diffusion spots indexed (e) Illustrations of the crystal structure of Sb2S3, 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 fries
Key point 2: Growth mechanism of Sb2S3 thin films with large grain size
Based on the characterization results of TEM, XRD, SIMS, and XPS, the possibility of substitution or interstitial doping of Ce3+in the Sb2S3 matrix can be ruled out, while revealing the formation of an ultra-thin Ce2S3 layer at the CdS/Sb2S3 interface. The author applies nucleation and growth theories in materials science to understand the microstructure evolution process of Sb2S3 thin films and proposes a reasonable mechanism explanation: the Ce2S3 interface layer promotes heterogeneous nucleation of Sb2S3 and suppresses homogeneous nucleation in solution; Compared with the CdS/Sb2S3 heterojunction interface, the interface energy of Ce2S3/Sb2S3 heterojunction interface decreases and the adhesion work increases, leading to a transition of the Sb2S3 growth model from the Volmer Weber growth model (island model) to the Stranski Krastanov model (layered+island model), promoting the lateral growth of Sb2S3 thin films. Meanwhile, the Ce2S3 interface layer can improve the quality of the heterojunction, making the bonding between Sb2S3 and the substrate more compact.
Figure 4 (a) Illustrations of the contact angle（ θ) For heterogeneous nucleation, and the water contact angles of pristine CdS film, as well as CdS films that have underground 15 minutes - and 30 minutes - hydraulic deposition in Sb2S3 precursor solutions (both without and with 1% Ce) (b) Histogram of the calculated interfacial adhesion work and interfacial energy for the heterointerfaces of CdS/Sb2S3 and Ce2S3/Sb2S3. (c) Schematic illustrating conventional growth and Ce2S3 mediated growth of Sb2S3thin films
Key point three: Device performance of Sb2S3 thin film solar cells
The author further constructed a planar structure Sb2S3 thin film solar cell: FTO/SnO2/CdS/Sb2S3/Spiro OMeTAD/Au. The device exhibits good performance and repeatability. Compared with the control group devices, the performance of Sb2S3 photovoltaic devices with Ce2S3 interface layer was significantly improved. The optimal 1% Ce Sb2S3 device has a photoelectric conversion efficiency of 7.66%, Voc of 796 mV, Jsc of 16.67 mA cm-2, and FF of 57.72%. The Voc close to 800 mV is the highest value reported so far for Sb2S3 photovoltaic devices.
Figure 5 (a) Illustration and (b) cross sectional SEM image of the device structure, which has the configuration: FTO/SnO2/CdS/Sb2S3/Spiro OMeTAD/Au (c) The statistics of the performance parameters of the control Sb2S3 device and Ce Sb2S3 devices observed with the addition of different concentrations of Ce3+to the precursor solution 20 devices were measured for each condition, and the performance metrics of each device are shown as individual data points (d) J-V curves of the control Sb2S3 and Ce Sb2S3 solar cells, measured under AM 1.5G (100 mW cm-2) illumination (e) External quantum efficiency (EQE) curves of best performing 1% Ce Sb2S3 solar cells (f) Evolution in the record efficiency of Sb2S3solar cells (g) VOC values of previous work on well developed planar and sensed Sb2S3solar cells
Key point four: Physics of solar cell devices
It is important to elucidate the characteristics of thin film defects and establish the correlation between defects and device performance. The author used deep level transient spectroscopy (DLTS) to explore the depth and density of defects in Sb2S3 thin films. Compared with the control group Sb2S3 device, the defect density in the internal absorption layer thin film of the 1% Ce Sb2S3 device is reduced, thereby suppressing the charge recombination inside the device. Ultra fast transient absorption spectroscopy (TAS) shows that compared with the control group sample, the carrier lifetime in the 1% Ce Sb2S3 thin film sample is longer, revealing effective suppression of carrier recombination at the bulk and/or interface. Extending the lifetime of photo generated minority carrier holes will improve the device's Voc. In addition, charge density difference analysis reveals that the bonding at the Ce2S3/Sb2S3 heterojunction interface is stronger than that at the CdS/Sb2S3 heterojunction interface, indicating that the presence of the Ce2S3 interface layer helps to form a more ideal heterojunction interface and improve the carrier transport characteristics at the heterojunction interface in Sb2S3 photovoltaic devices.
Figure 7. (a, b) Transient absorption (TA) spectra obtained at 1, 10, 100, 1000, and 5000 ps pump-probe delay for control Sb2S3 and 1%Ce-Sb2S3 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 Sb2S3 and 1%Ce-Sb2S3 films. ΔA 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/Sb2S3 and Ce2S3/Sb2S3.
【Article link】
Grain Engineering of Sb2S3 Thin Films to Enable Efficient Planar Solar Cells with High Open-Circuit Voltage
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*
Adv. Mater., 2023, 2305841
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