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Recent progress in colloidal quantum dot photovoltaics
Xihua WANG
Front. Optoelectron.. 2015, 8 (3): 241-251.
https://doi.org/10.1007/s12200-015-0524-9
The development of photovoltaic devices, solar cells, plays a key role in renewable energy sources. Semiconductor colloidal quantum dots (CQDs), including lead chacolgenide CQDs that have tunable electronic bandgaps from infrared to visible, serve as good candidates to harvest the broad spectrum of sunlight. CQDs can be processed from solution, allowing them to be deposited in a roll-to-roll printing process compatible with low-cost fabrication of large area solar panels. Enhanced multi-exciton generation process in CQD, compared with bulk semiconductors, enables the potential of exceeding Shockley-Queisser limit in CQD photovoltaics. For these advantages, CQDs photovoltaics attract great attention in academics, and extensive research works accelerate the development of CQD based solar cells. The record efficiency of CQD solar cells increased from 5.1% in 2011 to 9.9% in 2015. The improvement relies on optimized material processing, device architecture and various efforts to improve carrier collection efficiency. In this review, we have summarized the progress of CQD photovoltaics in year 2012 and after. Here we focused on the theoretical and experimental works that improve the understanding of the device physics in CQD solar cells, which may guide the development of CQD photovoltaics within the research community.
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Cu2ZnSn(S,Se)4 thin film solar cells fabricated with benign solvents
Cheng ZHANG,Jie ZHONG,Jiang TANG
Front. Optoelectron.. 2015, 8 (3): 252-268.
https://doi.org/10.1007/s12200-015-0539-2
Cu2ZnSn(S,Se)4 (CZTSSe) is considered as the promising absorbing layer materials for solar cells due to its earth-abundant constituents and excellent semiconductor properties. Through solution-processing, such as various printing methods, the fabrication of high performance CZTSSe solar cell could be applied to mass production with extremely low manufacturing cost and high yield speed. To better fulfill this goal, environmental-friendly inks/solutions are optimum for further reducing the capital investment on instrument, personnel and environmental safety. In this review, we summarized the recent development of CZTSSe thin films solar cells fabricated with benign solvents, such as water and ethanol. The disperse system can be classified to the true solution (consisting of molecules) and the colloidal suspension (consisting of nanoparticles).Three strategies for stabilization (i.e., physical method, chemical capping and self-stabilization) are proposed to prepare homogeneous and stable colloidal nanoinks. The one-pot self-stabilization method stands as an optimum route for preparing benign inks for its low impurity involvement and simple procedure. As-prepared CZTSSe inks would be deposited onto substrates to form thin films through spin-coating, spraying, electrodeposition or successive ionic layer adsorption and reaction (SILAR) method, followed by annealing in a chalcogen (S- or Se-containing) atmosphere to fabricate absorber. The efficiency of CZTSSe solar cell fabricated with benign solvents can also be enhanced by constituent adjustments, doping, surface treatments and blocking layers modifications, etc., and the deeper research will promise it a comparable performance to the non-benign CZTSSe systems.
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ITO surface modification for inverted organic photovoltaics
Mingzhang DENG,Weina SHI,Chen ZHAO,Bingbing CHEN,Yan SHEN
Front. Optoelectron.. 2015, 8 (3): 269-273.
https://doi.org/10.1007/s12200-015-0531-x
The work function (WF) of indium-tin-oxide (ITO) substrates plays an important role on the inverted organic photovoltaic device performance. And electrode engineering has been a useful method to facilitate carrier extraction or charge collection to enhance organic photovoltaic (OPV) performance. By using self-assembly technique, we have deposited poly(dimethyl diallylammonium chloride) (PDDA) layers onto ITO coated glass substrates. The results indicate that the surface WF of ITO is reduced by about 0.3 eV after PDDA modification, which is attributed to the modulation in electron affinity. In addition, the surface roughness of ITO substrate became smaller after PDDA modification. These modified ITO substrates can be applied to fabricate inverted OPVs, in which ITO works as the cathode to collect electrons. As a result, the photovoltaic performance of inverted OPV is substantially improved, mainly reflecting on the increase of short circuit current density.
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Blue fluorophores comprised of tetraphenylethene and imidazole: aggregation-induced emission and electroluminescence
Jiayun XIANG,Han NIE,Yibin JIANG,Jian ZHOU,Hoi Sing KWOK,Zujin ZHAO,Ben Zhong TANG
Front. Optoelectron.. 2015, 8 (3): 274-281.
https://doi.org/10.1007/s12200-015-0522-y
By melting tetraphenylethene (TPE) and 1,2,4,5-tetraphenyl-1H-imidazole (TPI) units together through different linking positions, three new fluorophores are synthesized, and their optical, electronic and electroluminescence (EL) properties are fully studied. Owing to the presence of TPE unit(s), these fluorophores are weak emitters in solutions, but are induced to emit strongly in the aggregated state, presenting typical aggregation-induced emission characteristics. The experimental and computational results reveal that different connection patterns between TPE and TPI could impact the molecular conjugation greatly, leading to varied emission wavelength, fluorescence quantum yield and EL performance in organic light emitting diodes (OLEDs). The fluorophore built by attaching TPE unit to the 1-position of imidazole ring shows bluest fluorescence, and its EL device emits at deep blue region (445 nm; CIE= (0.16, 0.15)). And the device based on the fluorophore by linking TPE to the 2-position of imidazole ring shows EL at 467 nm (CIE= (0.17, 0.22)) with good efficiencies of 3.17 cd·A-1, and 1.77%.
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Improved gas sensor with air-core photonic bandgap fiber
Saeed OLYAEE,Hassan ARMAN
Front. Optoelectron.. 2015, 8 (3): 314-318.
https://doi.org/10.1007/s12200-015-0447-5
The propagation loss of a fiber can be increased by coupling core mode and surface mode which will deteriorate the performance of photonic bandgap fiber (PBGF). In this paper, we presented an air-core PBGF for gas sensing applications. By designing Λ = 2.63 µm, d = 0.95 Λ, and Rcore= 1.13 Λ, where Λ is the distance between the adjacent air holes, the fiber was single-mode, no surface mode was supported with fiber, and more than 90% of the optical power was confined in the core. Furthermore, with optimizing the fiber structural parameters, at wavelength of λ = 1.55 µm that is in acetylene gas absorption line, significant relative sensitivity of 92.5%, and acceptable confinement loss of 0.09 dB/m, were simultaneously achieved.
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Lens distortion correction based on one chessboard pattern image
Yubin WU,Shixiong JIANG,Zhenkun XU,Song ZHU,Danhua CAO
Front. Optoelectron.. 2015, 8 (3): 319-328.
https://doi.org/10.1007/s12200-015-0453-7
This paper proposes a detection method of chessboard corner to correct camera distortions –including radial distortion, decentering distortion and prism distortion. This proposed method could achieve high corner detection rate. Then we used iterative procedure to optimize distortion parameter to minimize distortion residual. In this method, first, non-distortion points are evaluated by four points near image center; secondly, Levenberg-Marquardt nonlinear optimization algorithm was adopted to calculate distortion parameters, and then to correct image by these parameters; thirdly, we calculated corner points on the corrected image, and repeated previous two steps until distortion parameters converge. Results showed the proposed method by iterative procedure can make the impact of slight distortion around image center negligible and the average of distortion residual of one line is almost 0.3 pixels.
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Analysis and modeling of ridge waveguide quarterly wavelength shifted distributed feedback laser with three rate equations
Abbas GHADIMI,Alireza AHADPOUR SHAL
Front. Optoelectron.. 2015, 8 (3): 329-340.
https://doi.org/10.1007/s12200-015-0476-0
In this paper, ridge waveguide quarterly wavelength shifted distributed feedback (RW-QWS-DFB) laser was modeled and analyzed. In this behavioral model, some characteristics of the device, such as threshold current, line width, power of output wave, spectrum of output wave, and laser stability in high powers, were investigated in accordance with different physical and geographical parameters such as sizes and structures of the layers. Considering a new proposed algorithm, the analysis of the mentioned structures was performed using transfer matrix method (TMM), the solution of coupled waves and carrier rate equations. The results showed the advantages of some parameters in this structure.
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