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In this work, we proposed a novel CuGa-NaF (CuGa:Na) target route to improve Ga distribution and device efficiency of flexible Cu(In,Ga)(S,Se) (CIGSSe) solar cells. Analysis involved characterizing the basic material properties of two absorber layers fabricated using sputtering targets of CuGa:Na or Mo: Na. We assessed the uniformity of Ga and Na distribution in the absorber layers. We also examined the effects of various sputtering parameters and sought to optimize the Na content. Finally, we compared the conversion efficiency of the solar cells fabricated using the two Na addition routes (targets of CuGa:Na or Mo:Na target) with the following structure: metal foil/Cr/Mo/CIGSSe/CdS/i-ZnO/ITO/Al. Experimental results show that the CuGa:Na target improved the Ga distribution on the surface of the CIGSSe absorber compared to the Mo:Na target. When applied at a CuGa:Na/In thickness ratio of 300 nm/300 nm, the conversion efficiency of the two devices was as follows: CIGSSe solar cell with Mo: Na-derived absorber layer (7.5%) and CIGSSe solar cell with CuGa: Na-derived absorber layer (9.81%).

This paper explores the flexible ITO/nano-Ag/ITO multilayer electrodes grown on polyethylene terephthalate (PET) substrates and processed by a continuous roll-to-roll (R2R) sputtering system at room temperature used for flexible electrochromic device (ECD) applications. The effect of the nano-Ag interlayer thickness on the electrical and optical properties of the flexible ITO/nano-Ag/ITO multilayer was thoroughly investigated. By using R2R-sputtered at an Ag DC power of 0.2 kW, we were able to achieve optimal ITO/nano-Ag/ITO multilayer that exhibits a high optical transmittance of 87.19% and the best figure of merit value (30.93 × 10−3 Ω−1). The EC performance and stability of the flexible devices were tested by a cathodic WO3 coloration. Coloring and bleaching tests show that ITO/nano-Ag/ITO multilayers are highly effective conductors, indicating that the R2R sputtering technique is a promising continuous sputtering process in preparing for the fabrication of optical devices and flexible electronics industries.

Ink-printing method emerges as a viable way for manufacturing large-scale flexible Cu(In,Ga)Se2 (CIGS) thin film photovoltaic (TFPV) devices owing to its potential for the rapid process, mass production, and low-cost nonvacuum device fabrication. Here, we brought the femtosecond laser annealing (fs-LA) process into the ink-printing CIGS thin film preparation. The effects of fs-LA treatment on the structural and optoelectronic properties of the ink-printing CIGS thin films were systematically investigated. It was observed that, while the film surface morphology remained essentially unchanged under superheating, the quality of crystallinity was significantly enhanced after the fs-LA treatment. Moreover, a better stoichiometric composition was achieved with an optimized laser scanning rate of the laser beam, presumably due to the much reduced indium segregation phenomena, which is believed to be beneficial in decreasing the defect states of InSe, VSe, and InCu. Consequently, the shunt leakage current and rec...

Most thin-film techniques require a multiple vacuum process, and cannot produce high-coverage continuous thin films with the thickness of a few nanometers on rough surfaces. We present a new "paradigm shift" non-vacuum process to deposit high-quality, ultra-thin, single-crystal layers of coalesced sulfide nanoparticles (NPs) with controllable thickness down to a few nanometers, based on thermal decomposition. This provides high-coverage, homogeneous thickness, and large-area deposition over a rough surface, with little material loss or liquid chemical waste, and deposition rates of 10 nm/min. This technique can potentially replace conventional thin-film deposition methods, such as atomic layer deposition (ALD) and chemical bath deposition (CBD) as used by the Cu(In,Ga)Se₂ (CIGS) thin-film solar cell industry for decades. We demonstrate 32% improvement of CIGS thin-film solar cell efficiency in comparison to reference devices prepared by conventional CBD deposition method by depositing the ZnS NPs buffer layer using the new process. The new ZnS NPs layer allows reduction of an intrinsic ZnO layer, which can lead to severe shunt leakage in case of a CBD buffer layer. This leads to a 65% relative efficiency increase.

Ink-printing method emerges as a viable way for manufacturing large-scale flexible Cu(In,Ga)Se

We have successfully demonstrated a great advantage of plasmonic Au nanoparticles for efficient enhancement of Cu(In,Ga)Se2(CIGS) flexible photovoltaic devices. The incorporation of Au NPs can eliminate obstacles in the way of developing ink-printing CIGS flexible thin film photovoltaics (TFPV), such as poor absorption at wavelengths in the high intensity region of solar spectrum, and that occurs significantly at large incident angle of solar irradiation. The enhancement of external quantum efficiency and photocurrent have been systematically analyzed via the calculated electromagnetic field distribution. Finally, the major benefits of the localized surface plasmon resonances (LSPR) in visible wavelength have been investigated by ultrabroadband pump-probe spectroscopy, providing a solid evidence on the strong absorption and reduction of surface recombination that increases electron-hole generation and improves the carrier transportation in the vicinity of pn-juction.

This study investigates how to apply space-charge-limited (SCL) current to describe shunt leakage current in a CIGS solar cell. Possible factors inducing SCL current have been observed through conductive atomic force microscopy (C-AFM), which supports the SCL current theory, describing the shunt current of a CIGS solar cell. In simulations derived from experimental data, deviation of dark IV curves is due to flaws in the real device. These flaws are absent in simulation, but investigation verifies the characteristics of SCL current component's experimental IV curves within shunt leakage current. A device with a metal/CIGS/metal structure could simulate SCL current and confirm its characteristics. Such a simulated structure, representing flaws inserted into a CIGS solar cell, generates the same dark-current behavior revealed in experimental dark IV curves. This study investigates the response of dark current to varying sizes of the flaw within the CIGS solar cell.

Chalcopyrite CuInGaSe 2 (CIGS) based thin-film solar cells are regarded as most promising candidates for film power modules due to high efficiencies and low cost. Aluminum doped zinc oxide (ZnO:Al, AZO) is a common material for transparent conducting films used as a front contact layer on CIGS solar cells. In this study, AZO thin films were deposited on soda-lime glass (SLG) by conventional RF-magnetron sputtering using a commercial ceramic target. The dependence of the electrical and crystalline properties on the substrate to target position at different substrate temperature was investigated. The result shows that adjusting the substrate temperature could develop better transparent conducting thin films. However, the plasma ion-bombardment producing from the magnetron sputtering method makes significant effects on the distribution of electric conductive uniformity. When the substrate is placed on the appropriate position and the deposition process is carried out at higher substrate temperature, highly conductive AZO films with the average transmittance > 90% would be obtained. The transparent conducting AZO films are also utilized as the front contact to fabricate (Cu(In,Ga)Se 2 , CIGS) thin-film solar cells at different substrate temperature. The results find that the CIGS solar cells show better efficiencies at lower temperature even if high conductive AZO films are obtained at higher substrate temperature. It shows that the substrate temperature significantly affects the properties of CIGS solar cells.

The result of an extensive research on this material is the achievement of approaching 20% efficiency by the co-evaporation of copper, indium, gallium and selenium elements. Recently, photoluminescence (PL) spectra have been studied on Cu (In,Ga) Se2 (CIGS) thin films and CIGS solar cells, to clarify the carrier recombination process. The CIGS layers were grown on the Mo-coated soda-lime glass substrate by the three stage process and four sources co-evaporation of constituent elements onto a heated substrate. It has found that the structural and optical properties of the CIGS thin film was influenced by the Cu/Ga ratio (RCu/Ga) of the CIGS thin film compositional variation. The X-ray diffraction and PL spectra were used to characterize the structure property and carrier recombination mechanism of CIGS thin film.

Large-grain, copper-poor CuInGaSe 2 (CIGS) films are favored in the fabrication of highly efficient solar cells. However, the degradation of cell performance caused by residual copper selenide (Cu 2− x Se) remains a problem. This work studies the formation and behavior of excess Cu x Se and further compares the cell performance of typical copper-poor with that of copper-rich solar cells. Since excess Cu 2− x Se cannot be exhausted during the growth, it fully surrounds the polycrystalline CIGS grains. Excess Cu 2− x Se in the CIGS film produces serious shunt paths and causes the pn junction to be of poor quality. A short circuit in copper-rich CIGS solar cells is attributable to the conductive Cu 2− x Se. The best way to ensure high-efficiency of the cells is to exhaust Cu 2− x Se during growth. Otherwise, a dense, chemically treated CIGS film is required to prevent the negative effects of excess Cu 2− x Se.

We present a spectroscopic study of single quantum-dot molecules (QDMs) formed by two closely stacked In0.5Ga0.5As/GaAs layers. It was found that the interdot coupling and directional energy transfer between the two adjacent dots can be controlled by temperature tuning. Direct and indirect excitons, as well as charged excitons in single QDMs were classified and identified by excitation-power dependent, excitation-energy dependent and polarization-resolved micro-photoluminescence measurements. With the increasing temperature, the direct-exciton intensity decreases while the indirect-exciton intensity increases. A rate equation model considering phonon mediated processes has been developed. The directional energy transfer in QDMs is explained in terms of the phonon-assisted tunnelling of hole between the two adjacent dots. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the preparation of low density self-assembled InGaAs on GaAs grown by metal–organic chemical vapour deposition for single photon sources. Through using a set of optimized growth parameters, including the arsine partial pressure, total coverage of quantum dots, and growth temperature, high optical quality quantum dots with density as low as 5 × 106 cm−2 have been obtained. Using local optical excitation through a sub-micron aperture of a single quantum dot, its spectral lines associated with the exciton, biexciton, multi-exciton, and charged exciton have been resolved and identified. Photon correlation measurements show that the single quantum dot can successfully emit antibunched photons.

In this paper, we investigate the effects of growth temperature and growth rate on the formation of InAs/GaAs quantum dots (QDs) grown by metalorganic chemical vapor deposition. These QDs are formed with large InAs coverage (3.0 MLs) and periodic growth interruption via the Stranski–Krastanow epitaxial growth mode. The photoluminescence (PL) spectra at 300 K exhibit a red shift in peak wavelength by decreasing the InAs growth temperature from 540°C to 500°C. As the growth rate increases from 0.05 to 0.2 ML/s at a growth temperature of 500°C, the PL linewidth decreases and the PL intensity increases. These results are related to the In clusters and uniformity of InAs/GaAs QDs, which are observed by scanning electron microscopy (SEM). Finally, the room-temperature PL spectrum of InAs/GaAs QDs directly capped with GaAs layer shows an emission wavelength at 1.35 μm and a narrow linewidth of 30.8 meV.

A novel scheme for hybridizing inkjet-printed thin film Cu(In,Ga)Se2 (CIGS) solar cells with self-assembled clusters of nanocrystal quantum dots (NQDs), which provides a 10.9% relative enhancement of the photon conversion efficiency (PCE), is demonstrated. A non-uniform layer of NQD aggregates is deposited between the transparent conductive oxide and a CdS/CIGS p-n junction using low cost pulsed-spray deposition. Hybridization significantly improves the external quantum efficiency of the hybrid devices in the absorption range of the NQDs and in the red to near-IR parts of the spectrum. The low wavelength response enhancement is found to be induced by luminescent down-shifting (LDS) from the NQD layer, while the increase at longer wavelengths is attributed to internal scattering from NQD aggregates. LDS is demonstrated using time-resolved spectroscopy, and the morphology of the NQD layer is investigated in fluorescence microscopy and cross-sectional transmission electron microscopy. The influence of the NQD dose on the PCE of the hybrid devices is investigated and an optimum value is obtained. The low costs and limited material consumptions associated with pulsed-spray deposition make these flexible hybrid devices promising candidates to help push thin-film photovoltaic technology towards grid parity.

This study demonstrates the single photon emissions form site-controlled InGaAs quantum dots (QDs) grown on an apex plane of a GaAs multifaceted structure by pattern-growth techniques, multifaceted structure comprises the top (001) plane, and {111} and {011} side wall facets. The QDs formed on the apex plane rather than the side wall facet because the growth rate of apex plane is exceeded that of side wall facet. Microphotoluminescence (μ PL), μ PL excitation (μ PLE) and theoretical calculations are adopted to estimate the growth rate of the {111} ({011}) facets as ∼82% (∼69%) lower than that of the (001) plane. Since the (001) plane growth rate is higher than that of the side wall facets, the QDs can be carefully controlled. This method is effective in the fabrication of a single photon source. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast carrier dynamics in Cu(In,Ga)Se₂ films are investigated using femtosecond pump-probe spectroscopy. Samples prepared by direct sputtering and co-evaporation processes, which exhibited remarkably different crystalline structures and free carrier densities, were found to result in substantially different carrier relaxation and recombination mechanisms. For the sputtered CIGS films, electron-electron scattering and Auger recombination was observed, whereas for the co-evaporated CIGS films, bandgap renormalization accompanied by band filling effect and hot phonon relaxation was observed. The lifetime of defect-related recombination in the co-evaporated CIGS films is much longer than that in the direct-sputtered CIGS films, reflecting a better quality with higher energy conversion efficiency of the former.

This work investigates the single-photon emissions from self-assembled InGaAs quantum dots that are grown on an apex plane of a GaAs pyramid-like multifaceted structure. The number of QDs on a multifaceted structure is estimated by scanning electron microscopy. Single-exciton emissions from individual quantum dots are examined by micro-photoluminescence and by making photon correlation measurements. This experiment demonstrates the improvement of the single-photon extraction efficiency as quantum dots are grown on a reduced apex plane of a multifaceted structure.

CuInSe 2 /CuIn(Se x S 1−x ) 2 structures have been fabricated on Mo/Glass substrate using Cu-poor or Cu-rich CuInSe 2 thin film by RTP sulfurization at high temperature. The surface morphologies of CuInSe 2 thin films could be changed with varied S amounts or ratio of Se/S during sulfurization. In our experimental results, the CuInSe 2 /CuIn(Se x S 1−x ) 2 structure was easy formed based on Cu-rich CuInSe 2 thin film and the Cu x Se phase was helpful to produce CuIn(Se x S 1−x ) 2 film onto CISe film. But, the second phase (CuxSe) must be etching in KCN solution after sulfurization of CuInSe 2 /CuIn(Se x S 1−x ) 2 structures. The high quality and high open circuit voltage of chalcopyrite absorber had been made by co-evaporation and RTP sulfurization in this study.

Large grain and copper-poor CuInGaSe 2 (CIGS) film is preferred to fabricate high efficiency solar cells. However, the degradation caused by excess copper selenide (Cu 2−x Se) is still a problem. This study investigates the formation and behavior of excess CuxSe and further compares the cell performances between typical copper-poor and copper-rich solar cells. As the excess Cu 2−x Se can not be exhaust during the growth, the excess Cu 2−x Se fully surrounded by the polycrystalline CIGS grain. The CIGS film with excess Cu 2−x Se would result in serious shunt paths and poor PN junction. A short circuit in the copper-rich CIGS solar cells can be attributed to the conductive Cu 2−x Se. For high efficiency cells, the best way is to exhaust Cu 2−x Se during the growth. Or otherwise a dense CIGS film with chemical treatments is necessary to avoid the negative effects of the excess Cu 2−x Se.

We present a spectroscopic study of single quantum dot molecules (QDMs) formed by two closely stacked ${\text{In}}_{0.5}{\text{Ga}}_{0.5}\text{As}$ layers. The exciton fine structures as well as direct and indirect excitonic species associated with QDMs were identified by power dependent and polarization resolved microphotoluminescence measurements. As the temperature was increased, a directional energy transfer between the direct and indirect excitons in single QDMs was observed. A rate-equation model was developed to explain our data. We show that a phonon-assisted nonresonant tunneling of the hole between the two adjacent dots is responsible for such directional energy transfers in QDMs.

An efficient single‐photon source based on low‐density InGaAs quantum dots in a photonic‐crystal nanocavity is demonstrated. The single‐photon source features the effects of photonic band gap, yielding a single‐mode spontaneous emission coupling efficiency as high as β=92% and a linear polarization degree up to p=95%. This appealing performance makes it well‐suited for practical implementation of polarization‐encoded schemes in quantum cryptography.

An efficient single-photon source based on low-density InGaAs quantum dots in a photonic-crystal nanocavity is demonstrated. The single-photon source features the effects of a photonic band gap, yielding a single-mode spontaneous emission coupling efficiency as high as $\ensuremath{\beta}=92%$ and a linear polarization degree up to $p=95%$. This appealing performance makes it well suited for practical implementation of polarization-encoded schemes in quantum cryptography.

A steady‐state thermal model based on a set of rate equations is proposed in this article. The carrier dynamics of the main components in quantum dot (QD) systems, including the intersublevels in InAs QDs, the wetting layer, and the GaAs barriers, are all described by the equations. The QD sizes distribution, carrier capture, carrier relaxation, thermal escape and retrapping are all taken into account in this model. Two InAs QD samples with different size distributions and uniformities were grown. The measured temperature dependent photoluminescence (PL) spectra of the samples are consistent with the computational results. Furthermore, we also analyze the relationships between the carrier dynamics and the full width at half maximum (FWHM) for the samples.

Ultrafast carrier dynamics in Cu(In,Ga)Se₂ films are investigated using femtosecond pump-probe spectroscopy. Samples prepared by direct sputtering and co-evaporation processes, which exhibited remarkably different crystalline structures and free carrier densities, were found to result in substantially different carrier relaxation and recombination mechanisms. For the sputtered CIGS films, electron-electron scattering and Auger recombination was observed, whereas for the co-evaporated CIGS films, bandgap renormalization accompanied by band filling effect and hot phonon relaxation was observed. The lifetime of defect-related recombination in the co-evaporated CIGS films is much longer than that in the direct-sputtered CIGS films, reflecting a better quality with higher energy conversion efficiency of the former.

This work explores the origins of nonzero multiphoton emission probability for quantum dots embedded in photonic crystal nanocavities using different excitation energies to inject excitons into either the GaAs barrier or the quantum-dot excited state. The detected multiphoton events are established to arise from both the recapture of excitons and background emissions from the wetting layer tail states. The exciton emission is analyzed using rate-equation calculations, which suggest that multiphoton emission is dominated by the recapture effect.

The authors report an effective way to enhance the optical efficiency of InAs quantum dots (QDs) on GaAs emitting at the wavelength of 1.5μm. It is found that the loss of holes from QDs to their proximity via the high indium composition InGaAs overgrown layer, which is necessary for achieving long wavelength emission, is the origin of photoluminescence intensity degradation at high temperature. Inserting a 4nm thick Al0.45Ga0.55As layer, acting as a carrier blocking layer, into the GaAs capping matrix can improve the room temperature photoluminescence peak intensity by five and two times for the ground and first excited states, respectively.

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The first demonstration of InAs∕GaAs quantum-dot (QD) resonant-cavity light-emitting diode (RCLED) operating at 1.32μm at room temperature is reported. A single-layer InAs QDs inserted in GaAs matrix as the active medium was grown by metalorganic chemical vapor deposition. The bottom and top mirrors of QD RCLEDs were fabricated by employing epitaxial AlGaAs∕GaAs pairs and one dielectric SiO2∕Si3N4 pair as distributed Bragg reflectors (DBRs), respectively. As compared to the nonresonant QD LEDs, the RCLEDs exhibit a forward voltage of 1.13V at 20mA, a peak wavelength of 1.318μm, a narrower full width at half maximum in the electroluminescent spectrum of 14meV at 20mA, a high Q factor of 73.9, a low redshift rate with injection current of 0.033nm∕mA, and a higher light-output power of 28μW at 100mA.

We report a comparative study on the growth of InAs quantum dots (QDs) on GaAs by metalorganic chemical vapor deposition using triethylgallium (TEGa) and trimethylgallium (TMGa) for the GaAs cap layer. QDs exhibit 1.3μm photoluminescence (PL) at room temperature, as the GaAs cap layer is directly grown on the QDs. The PL emission can be extended to 1.49μm when an In0.25Ga0.75As overgrown layer is inserted between the cap layer and the InAs QDs. The use of TMGa or TEGa for the growth of the GaAs cap layer is essential for a further increase in the emission wavelength of the InAs QDs. Strong PL emission at 1.55μm with a linewidth of 28meV can be obtained as the GaAs cap layer is grown by TEGa, while the optical properties degrade severely when using TMGa.

The optical properties of InGaAsN QW grown on AlGaAs∕GaAs cladding layers were investigated. Al contamination in the quantum well due to the memory effect is responsible for the degradation of optical quality. We propose a growth method to reduce the Al contamination by growth interruption and dimethylhydrazine passivation. This approach improves the optical properties as well as simplifies the growth processes significantly as compared to the reported methods.

We report selective growth of InAs self-assembled quantum dots (QDs) on nano-ridges (30–50nm) formed by wet chemical etching and epitaxial growth processes. The QDs formed on the ridges exhibit distinctive characteristics from those on the planar region between the ridges in terms of physical shape as well as optical property. The QDs, which align one by one on the top of the ridges, are of an asymmetric shape along the [011] and [011] directions. By defining the width of ridge and the monolayers of QDs, we are able to control the height and the width of these QDs. Compared to the QDs on the planar region between the ridges, the luminescence from QDs on the ridges exhibits a shorter wavelength, which is attributed to higher compressive strain in these QDs.