Frontiers of Optoelectronics

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Self-trapped excitons in two-dimensional perovskites
Junze LI, Haizhen WANG, Dehui LI
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1051-x
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With strong electron–phonon coupling, the self-trapped excitons are usually formed in materials, which leads to the local lattice distortion and localized excitons. The self-trapping strongly depends on the dimensionality of the materials. In the three-dimensional case, there is a potential barrier for self-trapping, whereas no such barrier is present for quasi-one-dimensional systems. Two-dimensional (2D) systems are marginal cases with a much lower potential barrier or nonexistent potential barrier for the self-trapping, leading to the easier formation of self-trapped states. Self-trapped excitons emission exhibits a broadband emission with a large Stokes shift below the bandgap. 2D perovskites are a class of layered structure material with unique optical properties and would find potential promising optoelectronic. In particular, self-trapped excitons are present in 2D perovskites and can significantly influence the optical and electrical properties of 2D perovskites due to the soft characteristic and strong electron–phonon interaction. Here, we summarized the luminescence characteristics, origins, and characterizations of self-trapped excitons in 2D perovskites and finally gave an introduction to their applications in optoelectronics.

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A corona modulation device structure and mechanism based on perovskite quantum dots random laser pumped using an electron beam
Yan ZHU, Yining MU, Fanqi TANG, Peng DU, Hang REN
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1045-8
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Although laser pumping using electron beam (EB) has high transient power output and easy modulation based on perovskite quantum dot (PQD) film, its lasing emitting direction is the same as the pumped EB’s direction. Thus, realizing the conventional direct device structure through the film lasing mechanism is extremely difficult. Therefore, using the random lasing principle, herein, we proposed a corona modulation device structure based on PQDs random laser pumped using an EB. We discussed and stimulated the optimized designed method of the device in terms of parameters of the electronic optical device and the utilization ratio of output power and its modulation extinction ratio, respectively. According to the simulation results, this type of device structure can effectively satisfy the new random lasing mechanism in terms of high-speed and high-power modulation.

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Optical generation of UWB pulses utilizing Fano resonance modulation
Zhe XU, Yanyang ZHOU, Shuhuang CHEN, Liangjun LU, Gangqiang ZHOU, Jianping CHEN, Linjie ZHOU
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1010-6
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In this paper, we reported an integrated method to generate ultra-wideband (UWB) pulses of different orders based on a reconfigurable silicon micro-ring resonator-coupled Mach–Zehnder interferometer. Under proper operating conditions, the device can produce Fano resonances with a peak-to-valley extinction ratio of above 20 dB. UWB monocycle and doublet signals with picosecond pulse widths are produced when the micro-ring resonator is modulated by square and Gaussian electrical pulses, respectively. With our Fano resonance modulator on silicon photonics, it is promising to foresee versatile on-chip microwave signal generation.

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Dimensionality engineering of metal halide perovskites
Rashad F. KAHWAGI, Sean T. THORNTON, Ben SMITH, Ghada I. KOLEILAT
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1039-6
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Metal halide perovskites are a class of materials that are ideal for photodetectors and solar cells due to their excellent optoelectronic properties. Their low-cost and low temperature synthesis have made them attractive for extensive research aimed at revolutionizing the semiconductor industry. The rich chemistry of metal halide perovskites allows compositional engineering resulting in facile tuning of the desired optoelectronic properties. Moreover, using different experimental synthesis and deposition techniques such as solution processing, chemical vapor deposition and hot-injection methods, the dimensionality of the perovskites can be altered from 3D to 0D, each structure opening a new realm of applications due to their unique properties. Dimensionality engineering includes both morphological engineering–reducing the thickness of 3D perovskite into atomically thin films–and molecular engineering–incorporating long-chain organic cations into the perovskite mixture and changing the composition at the molecular level. The optoelectronic properties of the perovskite structure including its band gap, binding energy and carrier mobility depend on both its composition and dimensionality. The plethora of different photodetectors and solar cells that have been made with different compositions and dimensions of perovskite will be reviewed here. We will conclude our review by discussing the kinetics and dynamics of different dimensionalities, their inherent stability and toxicity issues, and how reaching similar performance to 3D in lower dimensionalities and their large-scale deployment can be achieved.

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Composition engineering to obtain efficient hybrid perovskite light-emitting diodes
Chuanzhong YAN, Kebin LIN, Jianxun LU, Zhanhua WEI
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1046-7
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Metal halide perovskites have received considerable attention in the field of electroluminescence, and the external quantum efficiency of perovskite light-emitting diodes has exceeded 20%. CH3NH3PbBr3 has been intensely investigated as an emitting layer in perovskite light-emitting diodes. However, perovskite films comprising CH3NH3PbBr3 often exhibit low surface coverage and poor crystallinity, leading to high current leakage, severe nonradiative recombination, and limited device performance. Herein, we demonstrate a rationale for composition engineering to obtain high-quality perovskite films. We first reduce pinholes by adding excess CH3NH3Br to the actual CH3NH3PbBr3 films, and we then add CsBr to improve the crystalline quality and to passivate nonradiative defects. As a result, the (CH3NH3)1−xCsxPbBr3 based perovskite light-emitting diodes exhibit significantly improved external quantum and power efficiencies of 6.97% and 25.18 lm/W, respectively, representing an improvement in performance dozens of times greater than that of pristine CH3NH3PbBr3-based perovskite light-emitting diodes. Our study demonstrates that composition engineering is an effective strategy for enhancing the device performance of perovskite light-emitting diodes.

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Surfactant-assisted doctor-blading-printed FAPbBr3 films for efficient semitransparent perovskite solar cells
Hangkai YING, Yifan LIU, Yuxi DOU, Jibo ZHANG, Zhenli WU, Qi ZHANG, Yi-Bing CHENG, Jie ZHONG
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1031-1
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Organic–inorganic hybrid perovskite solar cells have generated wide interest due to the rapid development of their photovoltaic conversion efficiencies. However, the majority of the reported devices have been fabricated via spin coating with a device area of <1 cm2. In this study, we fabricated a wide-bandgap formamidinium lead bromide (FAPbBr3) film using a cost-effective, high-yielding doctor-blade-coating process. The effects of different surfactants, such as l-α-phosphatidylcholine, polyoxyethylene sorbitan monooleate, sodium lauryl sulfonate, and hexadecyl trimethyl ammonium bromide, were studied during the printing process. Accompanying the optimization of the blading temperature, crystal sizes of over 10 mm and large-area perovskite films of 5 cm × 5 cm were obtained using this method. The printed FAPbBr3 solar cells exhibited a short-circuit current density of 8.22 mA/cm2, an open-circuit voltage of 1.175 V, and an efficiency of 7.29%. Subsequently, we replaced the gold with silver nanowires as the top electrode to prepare a semitransparent perovskite solar cell with an average transmittance (400–800 nm) of 25.42%, achieving a high-power efficiency of 5.11%. This study demonstrates efficient doctor-blading printing for preparing large-area FAPbBr3 films that possess high potential for applications in building integrated photovoltaics.

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Possible top cells for next-generation Si-based tandem solar cells
Shuaicheng LU, Chao CHEN, Jiang TANG
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1050-y
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Si-based solar cells, which have the advantages of high efficiency, low manufacturing costs, and outstanding stability, are dominant in the photovoltaic market. Currently, state-of-the-art Si-based solar cells are approaching the practical limit of efficiency. Constructing Si-based tandem solar cells is one available pathway to break the theoretical efficiency limit of single-junction silicon solar cells. Various top cells have been explored recently in the construction of Si-based tandem devices. Nevertheless, many challenges still stand in the way of extensive commercial application of Si-based tandem solar cells. Herein, we summarize the recent progress of representative Si-based tandem solar cells with different top cells, such as III-V solar cells, wide-bandgap perovskite solar cells, cadmium telluride (CdTe)-related solar cells, Cu(In,Ga)(Se,S)2 (CIGS)-related solar cells, and amorphous silicon (a-Si) solar cells, and we analyze the main bottlenecks for their next steps of development. Subsequently, we suggest several potential candidate top cells for Si-based tandem devices, such as Sb2S3, Se, CdSe, and Cu2O. These materials have great potential for the development of high-performance and low-cost Si-based tandem solar cells in the future.

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In vivo volumetric monitoring of revascularization of traumatized skin using extended depth-of-field photoacoustic microscopy
Zhongwen CHENG, Haigang MA, Zhiyang WANG, Sihua YANG
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1040-0
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Faster and better wound healing is a critical medical issue. Because the repair process of wounds is closely related to revascularization, accurate early assessment and postoperative monitoring are very important for establishing an optimal treatment plan. Herein, we present an extended depth-of-field photoacoustic microscopy system (E-DOF-PAM) that can achieve a constant spatial resolution and relatively uniform excitation efficiency over a long axial range. The superior performance of the system was verified by phantom and in vivo experiments. Furthermore, the system was applied to the imaging of normal and trauma sites of volunteers, and the experimental results accurately revealed the morphological differences between the normal and traumatized skin of the epidermis and dermis. These results demonstrated that the E-DOF-PAM is a powerful tool for observing and understanding the pathophysiology of cutaneous wound healing.

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