Sep 2015, Volume 8 Issue 3

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    Jiang TANG
    Xihua WANG

    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.

    Cheng ZHANG,Jie ZHONG,Jiang TANG

    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.

    Mingzhang DENG,Weina SHI,Chen ZHAO,Bingbing CHEN,Yan SHEN

    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.

    Jiayun XIANG,Han NIE,Yibin JIANG,Jian ZHOU,Hoi Sing KWOK,Zujin ZHAO,Ben Zhong TANG

    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%.

    Taotao DING,Yu TIAN,Jiangnan DAI,Changqing CHEN

    In this paper, Bi2S3 nanorods were successfully synthesized via a facile one-pot hydrothermal method and characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. Then the Bi2S3 nanorods were deposited on Au interdigital electrodes by dip-coating to fabricate photodetectors. The photoresponse properties using Bi2S3 nanorods as a representative system showed a significantly enhanced conductivity and the current-voltage (I-V) characteristic exhibited about ca. 2 orders of magnitude larger than the dark current. The response and decay time was estimated to be ~371.66 and 386 ms, respectively, indicating Bi2S3 may be an excellent candidate for high speed and high-sensitivity photoelectrical switches and light sensitive devices.

    Sujata CHAKRABORTY,Nabin Baran MANIK

    In this work, we investigated the effect of single walled carbon nanotubes (SWCNT) on the electrical and photovoltaic properties of methyl red (MR) dye based photoelectrochemical cell (PEC). MR dye based PEC with LiClO4 as ion salt were fabricated with and without mixing SWCNT. The cells were characterized through electrical and optical measurements. The performance of the devices changed drastically in presence of SWCNT. The transition voltage and trap energy of the cells were estimated from the steady-state dark current voltage (I-V) analysis. The transition voltage and trap energy decreased for MR dye cell in presence of SWCNT. Open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and power conversion efficiency (η) increased due to the addition of SWCNT. Further measurement of the transient photocurrent showed that the growth and decay of photocurrent was quite faster in presence of SWCNT. The photocurrent decay with time was fitted for both the cells and found to follow a power law relation which indicates dispersive transport mechanism with exponential trap states distributed in between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels. Possible interpretation is done on the lowering of trap energy with the photocurrent. These results suggest that SWCNT lowers the trap energy of the cells by providing efficient percolation pathways for the conduction of charges. It is expected that due to lowering of trap energy the residing time of the free carriers within the traps decreases. In other words, it may also be said that the charge recombination decreases. These factors affect the overall conduction of charges and improve the electrical and photovoltaic properties.

    Qingsong LEI,Jiang LI

    Al doped zinc oxide (AZO) films were prepared by mid-frequency magnetron sputtering for silicon (Si) thin film solar cells. Then, the influence of deposition parameters on the electrical and optical properties of the films was studied. Results showed that high conductive and high transparent AZO thin films were achieved with a minimum resistivity of 2.45 × 10-4 Ω?cm and optical transmission greater than 85% in visible spectrum region as the films were deposited at a substrate temperature of 225°C and a low sputtering power of 160 W. The optimized films were applied as back reflectors in a-SiGe:H solar cells. A relative increase of 19% in the solar cell efficiency was achieved in comparison to the cell without the ZnO films doped with Al (ZnO:Al).

    Haobo CHENG,Jingshi SU,Yong CHEN,Hon-Yuen TAM

    This paper focuses on the process of electrorheological (ER) finishing optical surfaces. Experiments on K9 mirrors were conducted. In one experiment, the operating distance was varied over 0.5–0.8 mm with the voltage at 2000 V. The maximum peak-to-valley (PV) reduction was obtained at the distance of 0.5 mm, where the PV value was reduced from 58.71 to 25.03 nm. In another experiment, the voltage was varied over 1500–3000 V with operating distance at 0.5 mm. The final surface roughness (Ra) achieved was as low as 2.5 nm. A higher voltage produced a higher relative reduction of the Ra. These experimental results validated the process.

    Saeed OLYAEE,Hassan ARMAN

    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.

    Yubin WU,Shixiong JIANG,Zhenkun XU,Song ZHU,Danhua CAO

    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.


    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.