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The local breakdown behavior may be harmful to solar cells and could possibly permanently damage the cell. Therefore, understanding the breakdown mechanisms in commercially competitive photovoltaic devices such as monocrystalline silicon (Si) solar cells is of great importance. Here, by using the reverse-biased electroluminescence (ReBEL) imaging technique, we observed three types of breakdown phenomena in monocrystalline Si solar cells: defect-induced breakdown, avalanche breakdown, and early breakdown. We have applied a variety of methods to diagnose each breakdown mechanism. The positions of defect-induced breakdown were first determined by combining EL and ReBEL imaging. An innovation method, the distributed circuit modeling was further introduced to trace the formation of different defect-induced breakdown sites. It is firstly applied this approach in the analysis of the breakdown mechanism. Then, avalanche breakdown was demonstrated by the temperature coefficient. The origin of its emission spectra was analyzed by the Si energy band structure combined with Baraff theory. Moreover, the characteristic of early breakdown was found to be consistent with the Zener effect, which may be caused by the metal stains such as aluminum (Al) during the manufacturing process.

In this paper, Sb2S3 thin-film solar cells are fabricated by the vapor transport deposition (VTD) method. The effect of the source-substrate proximity on the performance of Sb2S3 thin-film solar cells has been investigated and comparative studies of different source-substrate proximity are carried out. The device efficiency is improved from 0.83 to 3.02% by optimizing the source-substrate proximity with the augment of open-circuit voltage, short-circuit current density and fill factor. X-ray diffraction and scanning electron microscopy studies indicate that the deposited Sb2S3 films can achieve optimal grain orientation, high crystallinity, and compact morphology. Moreover, the current transport mechanism is analyzed in detail from dark current density–voltage (J-V) measurements and shows the optimal sample to be least affected by Shockley-Read-Hall recombination and space-charge-limited current (SCLC). Meanwhile, temperature and light intensity-dependent open-circuit voltage measurements reveal the carrier recombination rates are lowest for the optimal cell in all regions, including the CdS/Sb2S3 interface, the space-charge region (SCR), and the quasi-neutral region (QNR). These can account for the efficiency enhancement of the optimal cell and can be used to facilitate the further development of Sb2S3 thin-film solar cells.

Despite many successful realizations of laser operations in various micro/nanostructured lead halide perovskites (LHPs), the electron–hole plasma (EHP) lasing dynamics has only rarely been reported...

The combination of lasers and solar cells has potential applications in many areas. Therefore, investigating the laser induced damage and defect formation in such cells is crucial. In this study, a Cu(In, Ga)Se2 solar cell was irradiated with a 532 nm continuous wave laser beam. The damage threshold power density with irradiation for 10 s was found to be approximately 5000 W/cm2 by analyzing the light current density–voltage curves and the variation of the solar cell efficiency after laser exposure. Further, absolute electroluminescence (EL) imaging with distributed circuit modeling was used to determine the laser induced defects. By simulating the measured injection current dependent EL intensities of the defect spots, the defects created by exposure to laser power densities less than 5000 W/cm2 could be mainly attributed to increased series resistance or increased transparent conductive oxide layer resistance, whereas those created by exposure to laser power densities greater than 5000 W/cm2 could be mainly attributed to decreased shunt resistance.

Current defect inspection methods for photovoltaic (PV) devices based on electroluminescence (EL) imaging technology lack juggling both labor-saving and in-depth understanding of defects, restricting the progress towards yield improvement and higher efficiency. Herein, we propose an adaptive approach for automatic solar cell defect detection and classification based on absolute EL imaging. Specifically, we first develop an unsupervised algorithm to automatically detect defects referring to the defect features in EL images. Then a diagnosis approach is proposed, which statistically classifies the detected defects based on the electrical origin. To the best of our knowledge, the proposed method is the first effort to integrate automatic defect detection with fine-grained classification. Experimental results on multiple types of solar cells show that the proposed method can achieve the average uncertainty of 5.15% at the minimum, with by up to 98.90% optimization ratio compared with two conventional methods. The proposed method is expected to provide more guiding feedback in both practical design and reliability diagnosis of the PV industry.

In this study, Sb2(S,Se)3 thin films are fabricated using the vapor transport deposition (VTD) method, with Sb2Se3 and Sb2S3 mixed powders as the evaporation source. The performance of the corresponding glass/ITO/CdS/Sb2(S,Se)3/Au solar cells are found to be correlated to the mass ratio between Sb2S3 and the overall powder mixture. The properties of the Sb2(S,Se)3 thin films and cell devices adopting four different Sb2S3 mass ratios (x = 0.1, 0.25, 0.5 and 0.75) are compared. Further, the electrical properties – from dark and light J-V measurements; structural properties – from X-ray diffraction and scanning electron microscope measurements; and carrier-recombination rates at the buffer/absorber interface in the space-charge region (SCR) and in the quasi-neutral region – from temperature-illumination-dependent open-circuit voltage (VOC) measurements – are compared. It is found that a Sb2(S,Se)3 solar cell with a Sb2S3 mass ratio of 0.25 had optimal crystallinity, the lowest density of deep traps and the smallest carrier-recombination rates at the interface, leading to a high efficiency of 7.31%.

The effect of Σ3 generation on random grain boundaries (R-GBs) were investigated using multicrystalline Si (mc-Si) grown from the microcrystalline template with random orientation. There existed three cases for the contrast variation of electron-beam-induced current (EBIC) on R-GBs after Σ3 generation from them, namely decrease, increase and constant cases. No clear tendency of EBIC contrast variation was found at the initial growth stage. On the other hand, the constant case became dominant at the steady state. This result indicates that Σ3 generation does not affect the electrical activity of R-GBs.

Upconversion (UC) emissions in the hexagonal phase of NaYF 4 :10%Er 3+ /NaYF 4 core/shell nanocrystals have been observed by off-resonant excitation at 1620 nm, whose energy is lower than that corresponding to the lowest f–f transition of Er 3+ ( 4 I 15/2 → 4 I 13/2 ). The UC emission spectrum consists of five main peaks at around 1540 ( 4 I 13/2 → 4 I 15/2 ), 980 ( 4 I 11/2 → 4 I 15/2 ), 800 ( 4 I 9/2 → 4 I 15/2 ), 660 ( 4 F 9/2 → 4 I 15/2 ) and 540 nm ( 4 S 3/2 → 4 I 15/2 ). The infrared absorption spectrum shows a weak peak at around 1600 nm which is separated from the 4 I 15/2 → 4 I 13/2 absorption peak by 315 cm −1 , and phonon bands of NaYF 4 has been observed at about 300 cm −1 in the Raman spectrum. From these results, the UC emission is interpreted to involve a phonon-assisted excitation process. The UC emission intensity under off-resonant excitation at 1620 nm exhibits a much stronger temperature dependence than that under resonant excitation at 1550 nm, which is consistent with the phonon-assisted interpretation.

A novel synthesis method for hexagonal (β)-phase NaYF 4 :Er nano-crystals (NCs) which showed up-conversion (UC) from infrared to visible spectral region was developed. The NaYF 4 :Er NCs were synthesized in oleic acid (OA) and 1-octadecene (ODE) with Y 2 (CO 3 ) 3 · x H 2 O, Er 2 (CO 3 ) 3 · x H 2 O, Na 2 CO 3 and NH 4 F as precursors. This proposed method was simple and less toxic compared with generally used method so far. The XRD results showed that the molar ratio of OA/ODE and the temperature were key factors for phase control of NaYF 4 :Er NCs. The UC emission spectra were obtained with the emission wavelength at about 980 nm ( 4 I 11/2 → 4 I 15/2 ), 800 nm ( 4 I 9/2 → 4 I 15/2 ), 660 nm ( 4 F 9/2 → 4 I 15/2 ) and 540 nm ( 4 S 3/2 → 4 I 15/2 ) from Er 3+ ions, by excitation wavelength of 1550 nm. The slope values, n , in the pump-power dependence, showed that the emission at 980 and 800 nm were generated by 2-step UC and at 660 nm and 540 nm were 3-step UC. The optical process for the UC excitation was discussed.

We have developed a microturbidostat for long time bacterial culture at constant density controlled by optical detection and integrated pneumatic valves. The device was fabricated by multilayer soft lithography and in-situ formation of an agarose filter. The culture chamber of bacteria was connected in one side to a single bacterial input-output channel and in another side to a nutrient channel in which the agarose filter was formed to ensure the diffusion of nutrients and metabolites without bacterial loss. The bacterial number in the culture chamber was determined by measuring the fluorescence intensity of GFP proteins of the bacteria and the redundant bacteria could be exported automatically through the input-output channel with integrated micro-valves. In order to optimize the operation performance, we investigated the bacterial exportation efficiency with different input-output channel widths. As expected, the bacterial sorting coefficient was proportional to the input-output channel width. The results also showed that with a 20 microm channel-width, a long time culture was possible with a constant bacterial number in the chamber in the range from 400 to 700.

We present a simple technique for cell loading, culturing, and phenotypic study in a multi-chamber microfluidic device made of polydimethylsiloxane (PDMS). This technique is based on the use of degassing induced aspiration of PDMS which allows loading cells into micro-cavities within 1 min. A large number of triangle cavities are patterned aside main flow channels with narrow connections so that cells can be loaded by aspirating into each cavity. In our device, high throughput and long-term monitoring can be done with minimum shear force of the flow. As a demonstration, we show a controlled loading at single cell level and the phenotypic variation of gene expression of the yeast strain w303 as a function of copper ion concentration of the medium.

Temperature dependent time resolved photoluminescence (TR-PL) measurements have been performed in similar structured kesterite Cu2ZnSn(S,Se)4 and chalcopyrite Cu(In,Ga)Se2 thin film absorbers. It is investigated that at 26 K the measured lifetime values for a set of CZTSSe samples are more than one order magnitude higher than at room temperature whereas in CIGS this difference is not significant. At room temperature the significantly lower photoluminescence emission and minority carrier lifetime for CZTSSe comparing to CIGS suggest that the dominant non-radiative recombination processes are associated with CZTSSe than CIGS and it can limit the efficiency of CZTSSe based solar cells.

Precise doping control is of critical importance in power device fabrication using widegap materials. In this study, we characterize pn-junctions in GaN and SiC by cathodoluminescence. To improve spatial resolution, the specimens were polished using cross-sectional polisher. GaN specimens were successfully characterized with line profile of CL spectra across the cross-section. On the other hand, CL data could not be used straightforwardly for characterization of pn-junctions in the case of SiC.

Defect properties of Cu 2 ZnSn(S x ,Se 1−x ) 4 (CZTSSe) were investigated by admittance spectroscopy (AS). Two defect states (labeled E A1 and E A2 ) were observed in CZTSSe (x=0.15) with different Cu/Sn ratio. When the Cu/Sn ratio increased from 1.75 to 1.95, the activation energy of E A1 and E A2 decreased and the defect densities increased. The capture cross sections of E A1 and E A2 defects are in the order from 10−16 cm2 to 10−18 cm2, indicating that these two defects possibly do not impact on device performance.

Room-temperature two-wavelength excited photoluminescence (PL) measurements have been performed in the kesterite Cu 2 ZnSnS 4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) thin film absorbers. A defect level at 0.8 eV from the valence band and its properties are investigated. Two light sources of 635 nm and 1550 nm diode lasers, respectively, were used for above bandgap and 0.8 eV defect level excitation. The two-wavelength excited PL intensity was stronger than that only above-gap laser irradiation for the CZTS specimen. This phenomenon strongly suggests that the 0.8eV defect level acts as recombination center at room temperature. On the other hand, this defect may act as a trap in lower gap CZTSSe.

A comparative study with focusing on carrier recombination properties in Cu2ZnSn(S,Se)4 (CZTSSe) and the CuInGaSe2 (CIGS) solar cells has been carried out. For this purpose, electroluminescence (EL) and also bias-dependent time resolved photoluminescence (TRPL) using femtosecond (fs) laser source were performed. For the similar forward current density, the EL-intensity of the CZTSSe sample was obtained significantly lower than that of the CIGS sample. Primarily, it can be attributed to the existence of excess amount of non-radiative recombination center in the CZTSSe, and/or CZTSSe/CdS interface comparing to that of CIGS sample. In case of CIGS sample, TRPL decay time was found to increase with the application of forward-bias. This can be attributed to the reduced charge separation rate resulting from the reduced electric-field at the junction. However, in CZTSSe sample, TRPL decay time has been found almost independent under the forward and reverse-bias conditions. This phenomenon indicates that the charge recombination rate strongly dominates over the charge separation rate across the junction of the CZTSSe sample. Finally, temperature dependent VOC suggests that interface related recombination in the CZTSSe solar cell structure might be one of the major factors that affect EL-intensity and also, TRPL decay curves.

To determine the minority carrier lifetime, room temperature time-resolved photoluminescence (TR-PL) measurements have been performed on a set of Cu2ZnSn(S,Se)4 (CZTSSe) samples with different Cu/Sn ratios of 1.65, 1.75, and 1.85. TR-PL measurements were carried out on the bare CZTSSe thin films, CdS covered CZTSSe films and on solar cell structure using a femtosecond laser. The sample containing high Cu/Sn ratio of 1.85 shows the lowest lifetime, while films with Cu/Sn ratios of 1.65 and 1.75 show almost equal lifetime. The difference in lifetime between the CdS covered and solar structure samples is not remarkable. This demonstrates domination of recombination than charge separation by electric field. The bare films show extremely small lifetime. To examine surface quality, TR-PL emission spectra of uncovered CZTSSe and Cu(In,Ga)Se2 (CIGS) films were measured with two different excitation wavelengths of 420 and 750 nm, which generate excess carriers at different depths in absorbers. This comparison confirms the dominant surface recombination by CZTSSe than CIGS.

Cu2ZnSn(S,Se)4 (CZTSSe) thin films with various Cu/Sn ratio in the films have been investigated to study the effect of compositional variation over the electrical, optical, and structural properties of the film. Surface morphology and grain size were found to be significantly influenced by the Cu/Sn ratio in the films and grain size was found better for the samples with moderate Cu/Sn ratio of 1.75. Irrespective of the growth condition and compositional variation, all the CZTSSe crystals show that grains are oriented along (112) direction as evident from the room temperature XRD data. Dark current-voltage (I-V) curve reveals that that sample with Cu/Sn = 1.75 exhibits lowest leakage current, while sample with Cu/Sn = 1.85 has the highest leakage current along with larger ideality factor indicating larger recombination centers in this film. Series resistance was also found to be higher in the sample with higher Cu-content. An anomaly in the optical band-gap has been explained with the presences of impurity phases and compositional inhomogeneities in the CZTSSe materials.