Epitaxial and polycrystalline CuInS2 thin films: A comparison of opto-electronic properties

Epitaxial and polycrystalline thin CuInS₂ (CIS) layers were grown by means of molecular beam epitaxy (MBE) on single crystalline silicon substrates of 4 inch diameter. Photoluminescence (PL) studies were performed to investigate the opto-electronic properties of these layers. For the epitaxial CIS, low-energy-hydrogen implantation leads to the passivation of deep defects and several donor-acceptor (DA) pair recombinations (from 1.034 eV to 1.439 eV) and two free-to-bound (FB) transitions (at 1.436 eV and 1.485 eV) become observable at low temperatures (5 to 100 K). Excitonic luminescence is completely absent for all investigated epitaxial CIS layers. This contrasts sharply with the PL of the polycrystalline films which is dominated by excitonic luminescence (1.527 eV). Also a donor-to-valence band transition at 1.465 eV (BF-1) and one donor-acceptor recombination at 1.435 eV (DA-1) were observed, while luminescence from deep levels is not present at all. Based on these data, a refined defect model for CuInS₂ with two donor and two acceptor states is presented. Under comparable growth conditions, the electronic quality of polycrystalline CIS is superior to epitaxially grown material.     

Distributed Bragg reflector enhancement of electroluminescence from a silicon nanocrystal light emitting device

We demonstrate distributed Bragg reflector (DBR) enhanced electroluminescence from a silicon nanocrystal-based light emitting device. An a-Si/SiO₂ superlattice containing silicon nanocrystals serves as the intrinsic layer in an n-i-n device that is embedded in a DBR cavity consisting of alternating layers of silicon and silicon dioxide. The entire structure, including DBR, superlattice and contact layers, is deposited by plasma-enhanced chemical vapor deposition. The photoluminescence, electroluminescence (EL) and optical output power are measured and compared to a reference device. The DBR is found to enhance the peak EL intensity by a factor of 25 and the external quantum and power conversion efficiencies by a factor of 2.     

Structural and Spectroscopic Characterizations of ZnO Quantum Dots Annealed at Different Temperatures

Here, we report the synthesis and characterizations of sol-gel derived zinc oxide （ZnO） quantum dots （QDs） using zinc acetate dihydrate （Zn（CH3COO）2.2H20） and lithium hydroxide monohydrate （LiOH.H20） as raw material. The as-prepared ZnO QDs was annealed at different temperature （400, 700, and 900 ℃） and the structural, optical properties were investigated by X-ray diffraction （XRD）, high resolution transmission electron microscopy （HRTEM）, UV-Vis and photoluminescence （PL） spectroscopy. The powder XRD patterns of the obtained samples showed the formation of single-phase wurtzite structure and the morphological changes have been observed with increasing annealing temperature. In the absorption spectra, the optical band gap of nanocrystalline ZnO QDs decreased from 3.18 to 3.11 eV and the particle size increased with increasing temperature. In the PL spectra, a broad green emission peak related to defect levels in the visible range of the spectra have been recorded.     

The effect of sodium doping to CuInSe2 monograin powder properties

The effect of sodium doping to the electrical and photoluminescence properties of CuInSe₂ monograin powders was studied. Sodium was added in controlled amounts from 5 × 10¹⁶ cm^− 3 to 1 × 10²⁰ cm^− 3. The photoluminescence spectra of Na-doped stoichiometric CuInSe₂ powders had two bands with peak positions at 0.97 and 0.99 eV. The photoluminescence bands showed the shift of peak positions depending on the Na doping level. Peak positions with maximum energy were observed if added sodium concentration was 1 × 10¹⁹ cm^− 3. This material had the highest carrier concentration 2 × 10¹⁷ cm^− 3. In the case of stoichiometric CuInSe₂ (Cu:In:Se = 25.7:25.3:49.0), Na doping at concentrations of 3 × 10¹⁷ cm^− 3 and higher avoided the precipitation of Cu-Se phase. Solar cells output parameters were dependent on the Na doping level. Sodium concentration 3 × 10¹⁸ cm^− 3 resulted in the best open-circuit voltage.     

Blue PL and EL emissions from Bi-activated binary oxide thin-film phosphors

The photoluminescent (PL) and electroluminescent (EL) characteristics in the thin films of various Bi-activated binary oxide phosphors have been investigated. La₂O₃:Bi, Gd₂O₃:Bi and Y₂O₃:Bi phosphor thin films were prepared on thick BaTiO₃ ceramic sheet substrates by r.f. magnetron sputtering depositions followed by postannealing. Intense blue PL emissions were observed from all Bi-activated binary oxide phosphor thin films postannealed at a high temperature. Blue, whitish blue-green or blue-green emissions were observed from thin-film electroluminescent (TFEL) devices fabricated with a La₂O₃:Bi, a Gd₂O₃:Bi or a Y₂O₃:Bi thin-film emitting layer, respectively. In addition, high luminance with good color purity in blue EL was obtained in a TFEL device using a La₂O₃:Bi thin film prepared under optimized conditions.     

Synthesis and photoluminescence properties of new NaAlSiO4:Eu phosphors for near-UV white LED applications

A new NaAlSiO₄:0.1Eu²⁺ phosphors were synthesized at different temperatures using a liquid phase precursor (LPP) technique. The XRD patterns indicate the presence of hexagonal nepheline phase for all the samples. The synthesized phosphors can be excited efficiently in the broad near-UV region. The PL emission spectra showed a broad emission peak at around 551 nm corresponding to 5d → 4f transition of Eu²⁺ ions. The synthesized phosphors showed better thermal stability when compared with the standard YAG:Ce³⁺ phosphor.     

Epitaxial and polycrystalline CuInS2 layers: Structural metastability and its influence on the photoluminescence

Thin epitaxial and polycrystalline CuInS₂ (CIS) films were grown on single crystalline Si(111) and Mo-coated Si substrates, respectively, by means of molecular beam epitaxy from elemental sources. Photoluminescence (PL) measurements were performed to investigate the optical properties of both, epitaxial and polycrystalline CIS films. Epitaxial CIS samples show defect related transitions only and the PL spectra are dominated by broad luminescence peaks of deep levels, while excitonic transitions are completely absent. This contrasts sharply with the PL of the polycrystalline films, which is dominated by excitonic luminescence. Contributions due to shallow defects are observed with a small intensity only. However, luminescence peaks of defects with electronic levels deep in the band gap are not present at all. This includes the broad PL lines around 1.2 or 1.3 eV which are typical for polycrystalline CIS solar-cell material. X-ray diffraction and selected area electron diffraction measurements were employed in order to study the crystal structure. The epitaxial CIS films show a coexistence of the metastable CuAu-type (CA) ordering with the ground-state chalcopyrite (CH) structure, while the polycrystalline layers crystallize exclusively in the ground-state CH ordering. Hence, the coexistence of the metastable CA ordering and the ground-state CH structure in the epitaxial films is accompanied by a high density of electrically active intrinsic defects with levels deep in the band gap.     

CuInS2量子点敏化太阳能电池材料的研究进展

CuInS_2量子点因低毒、良好的光电性能和热电稳定性而成为量子点敏化太阳能电池(QDSSC)极具前景的量子点敏化剂。首先简单介绍了CuInS_2量子点材料的光电性质和QDSSC中CuInS_2量子点的合成,然后重点总结了CuInS_2量子点敏化电极的制备及各电池材料对QDSSC性能的影响,最后指出了该电池目前存在的问题以及对未来的展望。     

MoS2/g-C3N4复合催化剂的制备及CdSe量子点敏化产氢性能研究

太阳能光催化分解水制氢被认为是从根本上解决能源与环境问题较为理想的途径之一。在以尿素为原料制得石墨相氮化碳(g-C_3N_4)的基础之上,采用简单的低温溶液反应法将二硫化钼(MoS_2)与石墨相氮化碳(g-C_3N_4)复合得到复合催化剂MoS_2/g-C_3N_4,并利用透射电子显微镜(TEM)、X射线衍射(XRD)、紫外-可见漫反射(DRS)、傅里叶变换红外光谱(FT-IR)和荧光光谱等对该复合光催化剂的组成、形貌和光物理性能进行了表征;进而以CdSe量子点为光敏剂,三乙醇胺(TEOA)为牺牲剂,构建了不含贵金属的三组分光催化产氢体系,并对体系pH值、CdSe量子点浓度等对产氢性能的影响进行了研究。结果表明:将MoS_2纳米颗粒负载到g-C_3N_4上可使g-C_3N_4的光催化产氢性能得到显著提高。当MoS_2负载量为7%(质量比)时,在最佳的条件下(pH=9.0,CdSe量子点的体积为25mL),最大产氢速率达到了141.74μmol·h-1,6h的产氢总量达到了212.61μmol。最后,结合荧光猝灭实验,推测了该体系的产氢机理。     

Thermodynamic and experimental study of chemical bath deposition of Zn(S,O,OH) buffer layers in basic aqueous ammonia solutions. Cell results with electrodeposited CuIn(S,Se)2 absorbers

This paper presents a thermodynamic study of Chemical Bath Deposition (CBD) of zinc sulphide based films in aqueous ammonia solutions. The aim is a better understanding of ammonia and temperature effects on the deposition conditions and films composition. The formation of solid phases has been predicted by means of the precipitation conditions of ZnO, Zn(OH)₂ and ZnS as a function of temperature between 298 and 333 K. Films have been deposited according to calculated diagrams and preliminary results on solar cells based on electrodeposited CuIn(S,Se)₂ layers have been demonstrated. Composition and thickness of the films have been extracted to link the theoretical study with experiments.     

Semiconductor-metal phase transition of iron disilicide by laser annealing and its application to form device electrodes

Phase transition of semiconducting β-FeSi₂ to metallic α-FeSi₂ has been realized by laser annealing (Nd:YAG laser, λ=1.064 μm) in order to form ohmic electrodes for β-FeSi₂ devices. The starting samples were 200-nm-thick β-FeSi₂ films formed on Si substrates by reactive deposition epitaxy method. XRD and micro-Raman spectroscopy measurements confirmed the successful phase transition. The electrical resistivity of the α-FeSi₂ film was 2.0×10⁻⁴ Ω·cm, three orders lower than that of the original β-FeSi₂ film (1.4×10⁻¹ Ω·cm). Cross-sectional TEM images and electron diffraction patterns indicated that the upper ∼100 nm of the annealed film was changed to α-FeSi₂ while the lower part remained to be β-FeSi₂. α-FeSi₂ layer had good ohmic contact with β-FeSi₂. A testing p-β-FeSi₂/n-Si diode with laser-annealed α-FeSi₂ as the electrode on β-FeSi₂ showed the same rectifying characteristic as that using Al-evaporated electrode.     

Preparation and characterization of Sr4Al2O7:Eu, Eu phosphors

A new composition of red strontium aluminate phosphor (Sr₄Al₂O₇:Eu³⁺, Eu²⁺) is synthesized using a solid state reaction method in air and in a reducing atmosphere. The investigation of firing temperature indicates that a single phase of Sr₄Al₂O₇ is formed when the firing temperature is higher than 1300 °C and that a Sr₃Al₂O₆ phase is formed as the main peak below 1300 °C. The effects of firing temperature and doping concentration on luminescent properties are investigated. Sr₄Al₂O₇ phosphors exhibit the typical red luminescent properties of Eu³⁺ and Eu²⁺. A comparison photoluminescence study with Sr₃Al₂O₆ phosphor shows that Sr₄Al₂O₇ has higher emission intensity than Sr₃Al₂O₆ as a result of the higher optimum doping concentration of Sr₄Al₂O₇ phosphor.     

Characteristics of stacked CuInS2 and CuGaS2 layers as determined by the growth sequence

CuInS₂ and CuGaS₂ thin films have been prepared sequentially from elemental evaporation sources onto conventional soda lime glass substrates heated at 350 °C during the deposition process. The gradient in the structure and composition of the stacked layers has been investigated for the two possible growth sequences. Structural depth profiling and crystallographic phase analysis were performed by grazing incidence X-ray diffraction. The atomic distribution in the films depth was analyzed by X-ray photoelectron spectroscopy combined with sputter etching. Formation of the quaternary compound CuIn_1 − xGa_xS₂, with a high Ga content x > 0.80, has been detected with different distribution depending on the growth sequence.     

Study on ZnGa2O4:Cr a.c. powder electroluminescent device

An a.c. powder electroluminescent (EL) device using ZnGa₂O₄:Cr³⁺ phosphor was fabricated by the screen printing method. Optical and electrical properties of the device were investigated. The fabricated device shows a red emission at 695 nm driven by the a.c. voltage. The emission is attributed to the energy transfer from hot electrons to Cr³⁺ centers via self-activated Ga-O groups. Luminance (L) versus voltage (V) matches the well-known equation of L = L₀exp(− bV ^− 1 / 2) and luminance increases proportionally with frequency due to the increase of excitation probability of host lattice or Cr³⁺ centers. The diagram of the charge density (Q) versus applied voltage (V) is based on a conventional Sawyer-Tower circuit. At 280 V and 1000 Hz, the luminance and the luminous efficiency of the fabricated powder EL device are about 1.0 cd/m² and 13 lm/W, respectively. And under the high field, the device fabricated with the oxide-based phosphor of ZnGa₂O₄:Cr³⁺ shows excellent stability in comparison with the conventional sulfide powder EL device.     

Radioluminescence and photoluminescence of hafnia-based Eu-doped phosphors

Crystal structures of hafnia are discussed and it is shown that addition of about 7 at.% of Lu to the HfO₂ host lattice enforces the mixed composition to crystallize in cubic structure even at room temperature. Without Lu HfO₂ crystallizes in monoclinic structure. Luminescence and luminescence excitation spectra of Hf_0.93Lu_0.07O_1.965 are presented and discussed for powders prepared at different temperatures (600–1000 °C) and with different content of Eu. It is shown that decay of the 595.4 nm luminescence is longer (2.5 ms) than the 610 nm (1.6 ms). Radioluminescence efficiency of the cubic Hf_0.93Lu_0.07O_1.965 is low and does not exceed 10% of the commercial Gd₂O₂S:Eu.     

Transfer of CuInS2 thin film by lift-off process and application to superstrate-type thin-film solar cells

Transfer of a CuInS₂ thin film grown on a Mo/soda-lime glass substrate was investigated using a lift-off process. The CuInS₂ thin film was flatly exfoliated, with preferential peeling occurring in the CuInS₂/MoS₂ interface vicinity. This suggests that the interfacial MoS₂ layer behaves as a sacrificial layer. The lift-off process was also applied to solar cell fabrication. A superstrate-type CuInS₂ thin-film solar cell was fabricated and exhibited no significant degradation of conversion efficiency compared with a substrate-type CuInS₂ thin-film solar cell. The lift-off process could therefore also be applied to fabricate the upper part of a tandem solar cell structure.     

Structure and phase relations in the 2(CuInS2)-Cu2ZnSnS4 solid solution system

The intermixing of roquesite (CuInS₂) and kesterite (Cu₂ZnSnS₄), i. e. Cu(In_x(ZnSn)_1−xS₂ was investigated by a combination of neutron and X-ray powder diffraction. Samples with 0 ≤ × ≤ 1 were synthesized by a solid state reaction of the pure elements in evacuated silica tubes at 800 °C and cooled with a 10 K/h rate after the final annealing. The structural parameters of CuIn_x(ZnSn)_1−xS₂ were determined by simultaneous Rietveld refinement of neutron and X-ray diffraction data. The microstructure and chemical composition of the samples were investigated by electron microprobe analysis. A broad miscibility gap exists in the region 0.4 ≤ × < 0.8 indicated by the coexistence of two phases, an In-rich (x ~ 0.77) and a Zn-Sn-rich (x ~ 0.33) phase. Cu(In_x(ZnSn)_1−xS₂ mixed crystals with 0 ≤ x < 0.4 crystallize in the kesterite type structure, and with 0.8 ≤ × ≤ 1.0 in the chalcopyrite type structure. In the latter In, Zn and Sn are disordered on the In site. In the mixed crystals the lattice constant a and c show a linear dependence on chemical composition, whereas the tetragonal deformation Δ = 1−c/2a varies nonlinearly. Moreover in the mixed crystal with x ~ 0.15 the tetragonal deformation is equal zero and thus its structure is characterized by a pseudo-cubic unit cell.     

Luminescence and energy transfer of Mn co-doped SrSi2O2N2:Eu green-emitting phosphors

Eu²⁺ and Mn²⁺ co-doped SrSi₂O₂N₂ green-phosphors, with promising luminescent properties (examined by their powder diffuse reflection, photoluminescence excitation and emission spectra) suitable for UV converted white LEDs, were produced by high temperature solid-state reaction method. The produced materials exhibited intense broad absorption bands at 220–500 nm and a broad emission band centered at ca. 530 nm, attributed to 4f–5d transitions of Eu²⁺. The emission intensity of Eu²⁺ ions was greatly enhanced by introducing Mn²⁺ ions into SrSi₂O₂N₂:Eu²⁺ due to the energy transfer from Mn²⁺ to Eu²⁺. The energy transfer probability from Mn²⁺ to Eu²⁺ depends strongly on the Mn²⁺ concentration, which is maximized at a Mn²⁺ concentration of 3 mol%. It drastically decreases for higher concentrations. The results indicated that SrSi₂O₂N₂:Eu²⁺, Mn²⁺ is a promising green-emitting phosphor for white-light emitting diodes with near-UV LED chips.     

Structures and light emission properties of nanocrystalline FeSi2/Si formed by ion beam synthesis with a metal vapor vacuum arc ion source

Thin layers of nanocrystalline FeSi₂ embedded in Si structures have been formed by Fe implantation using a metal vapor vacuum arc (MEVVA) ion source under various implantation and thermal annealing conditions. The microstructures were studied in details and correlated with the photoluminescence (PL) properties. It is found that higher lattice coherence between the FeSi₂ nanocrystals and the Si matrix is associated with better light emission efficiency. Multiple-cycle implantation schemes were introduced and it is shown that with appropriate process design the dose quenching effect can be suppressed to achieve light emission enhancement in higher dose samples. De-convolution of the PL spectra into two or three peaks was performed and their temperature and excitation power dependence were analyzed. The analysis results indicate that the 1.55-μm emission really originated from FeSi₂ and that the emission peaks are likely donor- or accepted-level-related. MOS structures with the incorporation of implanted nanocrystalline FeSi₂ were fabricated. Electroluminescence (EL) spectra from these devices showed two peak features of which one peak corresponds to FeSi₂ emission and the other corresponds to enhanced Si band-edge emission. Clear room-temperature EL signals from these device structures were observed. A model is proposed to qualitatively understand the temperature dependence of the EL spectra.     

Sulphurization of single-phase Cu11In9 precursors for CuInS2 solar cells

Using Rutherford backscattering (RBS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), the sulphurization of single-phase Cu₁₁In₉ precursors to be employed as light absorbing CuInS₂ (CIS) layers in CIS-CdS heterojunction thin-film solar cells has been investigated. The Cu₁₁In₉ precursor films were produced by DC-sputtering from a single-phase Cu₁₁In₉ target. The sulphurization at 500 or 300 °C was performed by adding different amounts of elemental sulphur with heating rate and sulphurization time as additional parameters. During sulphurization at 500 °C, up to 50% of the indium initially present in the precursor is lost. We relate the In-loss to the volatile In₂S compound, the formation of which is favoured by the phase transition of Cu₁₁In₉ to Cu₁₆In₉ at 307 °C. Consequently, the In-loss can be suppressed by employing a sulphurization temperature of 300 °C. At this temperature, a prolonged sulphurization time and a large sulphur excess are necessary in order to obtain stoichiometric CIS beneath a CuS_x surface phase.