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    SPECIAL TOPIC — Emerging photovoltaic materials and devices

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    Surface modulation of halide perovskite films for efficient and stable solar cells
    Qinxuan Dai(戴沁煊), Chao Luo(骆超), Xianjin Wang(王显进), Feng Gao(高峰), Xiaole Jiang(姜晓乐), and Qing Zhao(赵清)
    Chin. Phys. B, 2022, 31 (3): 037303.   DOI: 10.1088/1674-1056/ac1fe0
    Abstract183)   HTML1)    PDF (3510KB)(134)      
    As the main distribution place of deep-level defects and the entrance of water, the interface is critical to determining both the power conversion efficiency (PCE) and the stability of perovskite solar cells (PSCs). Suitable interface design can dramatically passivate interface defects and optimize energy level alignment for suppressing the nonradiative recombination and effectively extracting the photogenerated carriers towards higher PCE. Meanwhile, a proper interface design can also block the interface diffusion of ions for high operational stability. Therefore, interface modification is of great significance to make the PSCs more efficient and stable. Upon optimized material choices, the three-dimensional halide perovskite graded junction layer, low-dimensional halide perovskite interface layer and organic salt passivation layer have been constructed on perovskite films for superior PSCs, yet a systematic review of them is missing. Thus, a guide and summary of recent advances in modulating the perovskite films interface is necessary for the further development of more efficient interface modification.
    Applications and functions of rare-earth ions in perovskite solar cells
    Limin Cang(苍利民), Zongyao Qian(钱宗耀), Jinpei Wang(王金培), Libao Chen(陈利豹), Zhigang Wan(万志刚), Ke Yang(杨柯), Hui Zhang(张辉), and Yonghua Chen(陈永华)
    Chin. Phys. B, 2022, 31 (3): 038402.   DOI: 10.1088/1674-1056/ac373a
    Abstract286)   HTML0)    PDF (4879KB)(179)      
    The emerging perovskite solar cells have been recognized as one of the most promising new-generation photovoltaic technologies owing to their potential of high efficiency and low production cost. However, the current perovskite solar cells suffer from some obstacles such as non-radiative charge recombination, mismatched absorption, light induced degradation for the further improvement of the power conversion efficiency and operational stability towards practical application. The rare-earth elements have been recently employed to effectively overcome these drawbacks according to their unique photophysical properties. Herein, the recent progress of the application of rare-earth ions and their functions in perovskite solar cells were systematically reviewed. As it was revealed that the rare-earth ions can be coupled with both charge transport metal oxides and photosensitive perovskites to regulate the thin film formation, and the rare-earth ions are embedded either substitutionally into the crystal lattices to adjust the optoelectronic properties and phase structure, or interstitially at grain boundaries and surface for effective defect passivation. In addition, the reversible oxidation and reduction potential of rare-earth ions can prevent the reduction and oxidation of the targeted materials. Moreover, owing to the presence of numerous energetic transition orbits, the rare-earth elements can convert low-energy infrared photons or high-energy ultraviolet photons into perovskite responsive visible light, to extend spectral response range and avoid high-energy light damage. Therefore, the incorporation of rare-earth elements into the perovskite solar cells have demonstrated promising potentials to simultaneously boost the device efficiency and stability.
    High-throughput computational material screening of the cycloalkane-based two-dimensional Dion—Jacobson halide perovskites for optoelectronics
    Guoqi Zhao(赵国琪), Jiahao Xie(颉家豪), Kun Zhou(周琨), Bangyu Xing(邢邦昱), Xinjiang Wang(王新江), Fuyu Tian(田伏钰), Xin He(贺欣), and Lijun Zhang(张立军)
    Chin. Phys. B, 2022, 31 (3): 037104.   DOI: 10.1088/1674-1056/ac4036
    Abstract275)   HTML0)    PDF (3192KB)(74)      
    Two-dimensional (2D) layered perovskites have emerged as potential alternates to traditional three-dimensional (3D) analogs to solve the stability issue of perovskite solar cells. In recent years, many efforts have been spent on manipulating the interlayer organic spacing cation to improve the photovoltaic properties of Dion—Jacobson (DJ) perovskites. In this work, a serious of cycloalkane (CA) molecules were selected as the organic spacing cation in 2D DJ perovskites, which can widely manipulate the optoelectronic properties of the DJ perovskites. The underlying relationship between the CA interlayer molecules and the crystal structures, thermodynamic stabilities, and electronic properties of 58 DJ perovskites has been investigated by using automatic high-throughput workflow cooperated with density-functional (DFT) calculations. We found that these CA-based DJ perovskites are all thermodynamic stable. The sizes of the cycloalkane molecules can influence the degree of inorganic framework distortion and further tune the bandgaps with a wide range of 0.9—2.1 eV. These findings indicate the cycloalkane molecules are suitable as spacing cation in 2D DJ perovskites and provide a useful guidance in designing novel 2D DJ perovskites for optoelectronic applications.
    An n—n type heterojunction enabling highly efficientcarrier separation in inorganic solar cells
    Gang Li(李刚), Yuqian Huang(黄玉茜), Rongfeng Tang(唐荣风), Bo Che(车波), Peng Xiao(肖鹏), Weitao Lian(连伟涛), Changfei Zhu(朱长飞), and Tao Chen(陈涛)
    Chin. Phys. B, 2022, 31 (3): 038803.   DOI: 10.1088/1674-1056/ac4022
    Abstract170)   HTML1)    PDF (1002KB)(55)      
    Carrier separation in a solar cell usually relies on the p—n junction. Here we show that an n—n type inorganic semiconductor heterojunction is also able to separate the exciton for efficient solar cell applications. The n—n type heterojunction was formed by hydrothermal deposition of Sb2(S,Se)3 and thermal evaporation of Sb2Se3. We found that the n—n junction is able to enhance the carrier separation by the formation of an electric field, reduce the interfacial recombination and generate optimized band alignment. The device based on this n—n junction shows 2.89% net efficiency improvement to 7.75% when compared with the device consisted of semiconductor absorber—metal contact. The study in the n—n type solar cell is expected to bring about more versatile materials utility, new interfacial engineering strategy and fundamental findings in the photovoltaic energy conversion process.
    Reveal the large open-circuit voltage deficit of all-inorganicCsPbIBr2 perovskite solar cells
    Ying Hu(胡颖), Jiaping Wang(王家平), Peng Zhao(赵鹏), Zhenhua Lin(林珍华), Siyu Zhang(张思玉), Jie Su(苏杰), Miao Zhang(张苗), Jincheng Zhang(张进成), Jingjing Chang(常晶晶), and Yue Hao(郝跃)
    Chin. Phys. B, 2022, 31 (3): 038804.   DOI: 10.1088/1674-1056/ac464b
    Abstract198)   HTML1)    PDF (1390KB)(56)      
    Due to excellent thermal stability and optoelectronic properties, all-inorganic perovskite is one of the promising candidates to solve the thermal decomposition problem of conventional organic—inorganic hybrid perovskite solar cells (PSCs), but the larger voltage loss (Vloss) cannot be ignored, especially CsPbIBr2, which limits the improvement of efficiency. To reduce Vloss, one promising solution is the modification of the energy level alignment between the perovskite layer and adjacent charge transport layer (CTL), which can facilitate charge extraction and reduce carrier recombination rate at the perovskite/CTL interface. Therefore, the key issues of minimum Vloss and high efficiency of CsPbIBr2-based PSCs were studied in terms of the perovskite layer thickness, the effects of band offset of the CTL/perovskite layer, the doping concentration of the CTL, and the electrode work function in this study based on device simulations. The open-circuit voltage (Voc) is increased from 1.37 V to 1.52 V by replacing SnO2 with ZnO as the electron transport layer (ETL) due to more matching conduction band with the CsPbIBr2 layer.
    TiO2/SnO2 electron transport double layers with ultrathin SnO2 for efficient planar perovskite solar cells
    Can Li(李灿), Hongyu Xu(徐宏宇), Chongyang Zhi(郅冲阳), Zhi Wan(万志), and Zhen Li(李祯)
    Chin. Phys. B, 2022, 31 (11): 118802.   DOI: 10.1088/1674-1056/ac8349
    Abstract93)   HTML1)    PDF (2844KB)(24)      
    The electron transport layer (ETL) plays an important role on the performance and stability of perovskite solar cells (PSCs). Developing double ETL is a promising strategy to take the advantages of different ETL materials and avoid their drawbacks. Here, an ultrathin SnO2 layer of ~ 5 nm deposited by atomic layer deposit (ALD) was used to construct a TiO2/SnO2 double ETL, improving the power conversion efficiency (PCE) from 18.02% to 21.13%. The ultrathin SnO2 layer enhances the electrical conductivity of the double layer ETLs and improves band alignment at the ETL/perovskite interface, promoting charge extraction and transfer. The ultrathin SnO2 layer also passivates the ETL/perovskite interface, suppressing nonradiative recombination. The double ETL achieves outstanding stability compared with PSCs with TiO2 only ETL. The PSCs with double ETL retains 85% of its initial PCE after 900 hours illumination. Our work demonstrates the prospects of using ultrathin metal oxide to construct double ETL for high-performance PSCs.
    Sputtered SnO2 as an interlayer for efficient semitransparent perovskite solar cells
    Zheng Fang(方正), Liu Yang(杨柳), Yongbin Jin(靳永斌), Kaikai Liu(刘凯凯), Huiping Feng(酆辉平), Bingru Deng(邓冰如), Lingfang Zheng(郑玲芳), Changcai Cui(崔长彩), Chengbo Tian(田成波), Liqiang Xie(谢立强), Xipeng Xu(徐西鹏), and Zhanhua Wei(魏展画)
    Chin. Phys. B, 2022, 31 (11): 118801.   DOI: 10.1088/1674-1056/ac67c5
    Abstract91)   HTML0)    PDF (3698KB)(21)      
    SnO2 is widely used as the electron transport layer (ETL) in perovskite solar cells (PSCs) due to its excellent electron mobility, low processing temperature, and low cost. And the most common way of preparing the SnO2 ETL is spin-coating using the corresponding colloid solution. However, the spin-coated SnO2 layer is sometimes not so compact and contains pinholes, weakening the hole blocking capability. Here, a SnO2 thin film prepared through magnetron-sputtering was inserted between ITO and the spin-coated SnO2 acted as an interlayer. This strategy can combine the advantages of efficient electron extraction and hole blocking due to the high compactness of the sputtered film and the excellent electronic property of the spin-coated SnO2. Therefore, the recombination of photo-generated carriers at the interface is significantly reduced. As a result, the semitransparent perovskite solar cells (with a bandgap of 1.73 eV) based on this double-layered SnO2 demonstrate a maximum efficiency of 17.7% (stabilized at 17.04%) with negligible hysteresis. Moreover, the shelf stability of the device is also significantly improved, maintaining 95% of the initial efficiency after 800-hours of aging.
    Could two-dimensional perovskites fundamentally solve the instability of perovskite photovoltaics
    Luoran Chen(陈烙然), Hu Wang(王虎), and Yuchuan Shao(邵宇川)
    Chin. Phys. B, 2022, 31 (11): 117803.   DOI: 10.1088/1674-1056/ac693e
    Abstract84)   HTML2)    PDF (3119KB)(18)      
    The high efficiency and low production cost enable the halide perovskite solar cells as a promising technology for the next generation photovoltaics. Nevertheless, the relatively poor stability of the organic-inorganic halide perovskites hinders their commercial applications. In the past few years, two-dimensional (2D) perovskite has emerged as a more stable alternative to the three-dimensional (3D) counterparts and attracted intense research interests. Although many attempts and advances have been made, it is still ambiguous that whether the 2D perovskites could bring closure to the stability issue. To answer this essential question, a systematic study of the nature of 2D halide perovskites is necessary. Here, we focus on the stability investigations of 2D perovskites from different perspectives, especially light, heat, ion migration and strain. Several remaining challenges and opening problems are also discussed. With further material and device engineering, we believe that the 2D perovskites would promote perovskite solar cells to a promising future.
    A silazane additive for CsPbI2Br perovskite solar cells
    Ruiqi Cao(曹瑞琪), Yaochang Yue(乐耀昌), Hong Zhang(张弘), Qian Cheng(程倩), Boxin Wang(王博欣), Shilin Li(李世麟), Yuan Zhang(张渊), Shuhong Li(李书宏), and Huiqiong Zhou(周惠琼)
    Chin. Phys. B, 2022, 31 (11): 110101.   DOI: 10.1088/1674-1056/ac7c01
    Abstract110)   HTML0)    PDF (1406KB)(26)      
    Adding additives into peroskite precursor solution has been proven as a simple and efficient strategy to improve the quality of peroskite films. In this work, we demonstrate an effective additive strategy to improve the quality of all-inorganic perovskite films by adding a novel silazane additive heptamethyldisilazane (HDMS). The power conversion efficiency (PCE) of the optimized devices is enhanced from 14.55% to 15.31% with an open-circuit voltage over 1.26 V due to the higher quality perovskite films with lower trap density after the incorporation of HDMS. More interestingly, the HDMS devices exhibit superior humidity and thermal stability compared with the control ones. This work provides a simple and efficient strategy to enhance the device performance and stability of all-inorganic perovskite solar cells, which could facilitate its commercialization.
    Defect physics of the quasi-two-dimensional photovoltaic semiconductor GeSe
    Saichao Yan(闫赛超), Jinchen Wei(魏金宸), Shanshan Wang(王珊珊), Menglin Huang(黄梦麟), Yu-Ning Wu(吴宇宁), and Shiyou Chen(陈时友)
    Chin. Phys. B, 2022, 31 (11): 116103.   DOI: 10.1088/1674-1056/ac685f
    Abstract111)   HTML1)    PDF (990KB)(22)      
    GeSe has recently emerged as a photovoltaic absorber material due to its attractive optical and electrical properties as well as earth abundancy and low toxicity. However, the efficiency of GeSe thin-film solar cells (TFSCs) is still low compared to the Shockley-Queisser limit. Point defects are believed to play important roles in the electrical and optical properties of GeSe thin films. Here, we perform first-principles calculations to study the defect characteristics of GeSe. Our results demonstrate that no matter under the Ge-rich or Se-rich condition, the Fermi level is always located near the valence band edge, leading to the p-type conductivity of undoped samples. Under Se-rich condition, the Ge vacancy (VGe) has the lowest formation energy, with a (0/2-) charge-state transition level at 0.22 eV above the valence band edge. The high density (above 1017 cm-3) and shallow level of VGe imply that it is the p-type origin of GeSe. Under Se-rich growth condition, Sei has a low formation energy in the neutral state, but it does not introduce any defect level in the band gap, suggesting that it neither contributes to electrical conductivity nor induces non-radiative recombination. In addition, Gei introduces a deep charge-state transition level, making it a possible recombination center. Therefore, we propose that the Se-rich condition should be adopted to fabricate high-efficiency GeSe solar cells.