Sulfurization of electrodeposited CuInSe2-based solar cells

CuInSe₂ (CISe) thin films have been prepared by single-step electrodeposition on top of TCO/TiO₂ and TCO/TiO₂/In₂S₃ coated electrodes. TiO₂ and In₂S₃ have been deposited by spray–pyrolysis. The electrodeposition step is studied using cyclic voltammetry in an acidic electrolyte. Electrodeposited CISe is then subjected to two different thermal treatments. The first treatment is an annealing step under argon atmosphere, carried out to enhance the crystallinity of the film. The second consists of a sulfurization process, where sulfur is vaporized and mixed with the argon flux, leading to substantial changes in the composition of the chalcogenide. The crystallinity, morphology and stoichiometry of the annealed films are characterized by XRD, micro-Raman spectroscopy and SEM/EDX. Raman spectra and EDX show an almost complete replacement of the Se atoms by S atoms. Etching the films in KCN solution is a key step, enabling a final adjustment in the stoichiometry. The incorporation of In₂S₃ buffer layer in TiO₂/CuIn(SeS)₂ solar cells produces a marked improvement in the cell efficiency. Despite this improvement, the values of J_sc and the fill factor (FF) are relatively low, showing efficiencies below 1%, most likely associated to the resistances present in the multi-layered cell.     

All-chemically deposited Bi2S3/PbS solar cells

We report on solar cells with a cross-sectional layout: TCO/window/Bi₂S₃/PbS, in which a commercial SnO₂ transparent conductive oxide (TCO-PPG Sungate 500); chemically deposited window layers of CdS, ZnS or their oxides; n-type Bi₂S₃ (100 nm) and p-type PbS (360-550 nm) absorber films constitute the cell structures. The crystalline structure, optical, and electrical properties of the constituent films are presented. The open circuit voltage (V_oc) and short-circuit current density (J_sc), for 1000 W/m² solar radiation, of these solar cells depend on the window layers, and vary in the range, 130-310 mV and 0.5-5 mA/cm², respectively. The typical fill factors (FF) of these cells are 0.25-0.42, and conversion efficiency, 0.1-0.4%.     

Improved short-circuit current density of a-Si:H thin film solar cells with n-type silicon carbide layer

In this work, the performance of p–i–n hydrogenated amorphous silicon thin film solar cells by adopting n-type silicon carbide (n-SiC_x:H) layer was investigated. By varying CH₄/SiH₄ gas flow ratio, refractive index and electrical conductivity of n-SiC_x:H thin films were changed in the range of 3.4 to 3.8 and 1.48E?5 to 1.24 S/cm, respectively. Compared with solar cells with n-Si:H/Ag configuration, short-circuit current density (J _sc ) of solar cells with n-SiC_x:H/Ag configuration was improved up to 8.4%, which was comparable with that of solar cells with n-Si:H/ZnO/Ag configuration. Improved J _sc was related with enhanced spectral response at long wavelength of 500–800 nm. It was supposed that the decreased refractive index of n-SiC_x:H layer resulted in the increased back reflectance, which contributed to the improved J _sc. Our experiments demonstrated that n-SiC_x:H thin films were attractive choice because they functioned both as n-layer and interlayer in back reflector, and their deposition method was compatible with preparation process of solar cells.     

Optical enhancement by back reflector with ZnO:Al2O3(AZO) or NiCr diffusion barrier for amorphous silicon germanium thin film solar cells

Different composite films, including Al, Ag/NiCr/Al and Al₂O₃-doped ZnO (AZO)/Ag/NiCr (AZO)/Al, were utilized as the back reflectors for p-i-n hydrogenated amorphous silicon germanium (a-SiGe:H) thin film solar cells. The experimental results indicated that the AZO leyer between silicon layers and Ag/NiCr/Al back reflector was effect in improving solar cell performance, mainly owing to an increase in short-circuit current density (J_sc) of the solar cells. In addition, the thickness of AZO film could strongly affect the J_sc. The highest solar cell performance was achieved at the AZO thickness of about 90 nm. A nickel-chromium (NiCr) or AZO film was inserted between Ag and Al as a diffusion barrier against mutual diffusion of them, the similar performances of solar cells were achieved. So AZO/Ag/NiCr (AZO)/Al could be utilized as an advanced AZO/metal back reflector for p-i-n a-SiGe:H solar cells.     

The role of buffer layer between TCO and p-layer in improving series resistance and carrier recombination of a-Si:H solar cells

The properties of the window layer and transparent conducting oxide (TCO)/p interface in silicon based thin-film solar cells are important factors in determining the cell efficiency. As the potential barrier got larger at the interface, the transmission of photo-generated holes were impeded and the recombination of photo-generated electrons diffusing back toward the TCO interface were enhanced leading to a deterioration of the fill factor. In this paper different p-layers were studied. It was found that using p-type hydrogenated amorphous silicon oxide (a-SiO_x:H) layer as the window layer along with a 5 nm buffer layer which reduced the barrier at the fluorine doped tin oxide (SnO₂:F) TCO/p-layer interface, improved the cell efficiency. a-SiO_x:H was used as the buffer layer. With the buffer layer between TCO and p-type a-SiO_x:H, the potential barrier dropped from 0.506 eV to 0.472 eV. This lowered barrier results in increased short circuit current density (J_sc) and fill factor (FF). With the buffer layer, J_sc increased from 11.9 mA/cm² to 13.35 mA/cm² and FF increased from 73.22% to 74.91%.     

The potential of textured front ZnO and flat TCO/metal back contact to improve optical absorption in thin Cu(In,Ga)Se2 solar cells

The role of additionally textured front transparent conductive oxide − TCO (ZnO:Al) and flat TCO/metal contact on optical improvements in thin Cu(In,Ga)Se₂ (CIGS) solar cells are investigated by means of numerical simulations. A de-coupled analysis of two effects related to additional texturing of front surface of ZnO:Al TCO − (i) enhancement of light scattering and (ii) decreased total reflectance (antireflective effect) − reveals that the improvements in quantum efficiency, QE, and short-circuit current, J_SC, of the solar cell originate from an antireflective effect only. In order to improve optical properties of the back contact the introduction of a TCO layer (undoped ZnO) between CIGS and back metal contact is investigated from the optical point of view. In addition to ZnO/Mo, a highly reflective ZnO/Ag contact (ZnO is also assumed to work as a protection layer for Ag) is also included in simulations. Results show significant increase in reflectance related to introduced ZnO in front of Mo. Drastically increased reflectance is obtained if ZnO/Mo is substituted with ZnO/Ag. The improvements in QE and J_SC of a thin CIGS solar cell, related to ZnO/metal contacts are presented.     

High efficiency amorphous silicon solar cells with high absorption coefficient intrinsic amorphous silicon layers

This paper considers the intrinsic layer of hydrogenated amorphous silicon (a-Si:H) solar cells. The deposition temperatures (T_d) and electrode distances (between cathode and anode, E/S) are important factors for a-Si:H solar cells. Thus, this study examines the effects of deposition temperatures and electrode distances in the intrinsic layer of a-Si:H solar cells with regard to enhanced the short-circuit current density (J_sc) and thereby conversion efficiency. It is shown that the J_sc of a-Si:H solar cells can be increased by proper choice of T_d and E/S of the i-a-Si:H layers. The J_sc of the a-Si:H solar cells is largely dependent on light absorption of the i-a-Si:H layer. It is demonstrated that the absorption coefficient in an i-a-Si:H layer can be increased to provide higher J_sc under fixed thickness. Results show that the optimized parameters improve the J_sc of a-Si:H solar cells to 16.52 mA/cm², yielding an initial conversion efficiency of 10.86%.     

Investigation of low resistance transparent MoO3/Ag/MoO3 multilayer and application as anode in organic solar cells

Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. Thus doped In₂O₃ (ITO), ZnO, SnO₂ are commercially developed. However, the deposition process of these films need sputtering and/or heating cycle, which has negative effect on the performances of the organic devices due to the sputtering and heat damages. Therefore a thermally evaporable, low resistance, transparent electrode, deposited onto substrates room temperature, has to be developed to overcome these difficulties. For these reasons combination of dielectric materials and metal multilayer has been proposed to achieve high transparent conductive oxides. In this work the different structures probed were: MoO₃ (45 nm)/Ag (x nm)/MoO₃ (37.5 nm), with x = 5-15 nm. The measure of the electrical conductivity of the structures shows that there is a threshold value of the silver thickness: below 10 nm the films are semiconductor, from 10 nm and above the films are conductor. However, the transmittance of the structures decreases with the silver thickness, therefore the optimum Ag thickness is 10 nm. A structure MoO₃ (45 nm)/Ag (10 nm)/MoO₃ (37.5 nm) resulted with a resistivity of 8 × 10^− 5 Ω cm and a transmittance, at around 600 nm, of 80%. Such multilayer structure can be used as anode in organic solar cells according to the device anode/CuPc/C₆₀/Alq₃/Al. We have already shown that when the anode of the cells is an ITO film the introduction of a thin (3 nm) MoO₃ layer at the interface anode (ITO)/organic electron donor (CuPc) allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc [1]. This property has been used in the present work to achieve a high hole transfer efficiency between the CuPc and the anode. For comparison MoO₃/Ag/MoO₃/CuPc/C₆₀/Alq₃/Al and ITO/MoO₃/CuPc/C₆₀/Alq3/Al solar cells have been deposited in the same run. These devices exhibit efficiency of the same order of magnitude.     

Growth and characterization of transparent conducting nanostructured zinc indium oxide thin films

Transparent conductive oxide (TCO) films have been widely used in various applications, such as for transparent electrodes in flat-panel displays, and in solar cells, optoelectronic devices, touch panels and IR reflectors. Among these, tin doped zinc oxide (ZTO) and indium doped zinc oxide (ZIO) have attracted considerable attention. Particularly, IZO thin film is the best candidate for high-quality transparent conducting electrodes in OLEDs and flexible displays. In this work zinc indium oxide (ZIO) thin films were deposited on glass substrate with varying concentration (ZnO:In₂O₃ — 100:0, 90:10, 70:30 and 50:50 wt.%) at room temperature by flash evaporation technique. These deposited ZIO films were annealed in vacuum to study the thermal stability and to see the effects on the physical properties. The XRF spectra revealed the presence of zinc and indium with varying concentration in ZIO thin films, while the surface composition and oxidation state were analyzed by X-ray photoelectron spectroscopy. The core level spectra were deconvoluted to see the effect of chemical changes, while the valance band spectra manifest the electronic transitions. The surface morphology studies of the films using atomic force microscopy (AFM) revealed the formation of nanostructured ZIO thin films. The optical band gap was also found to be decreased for both types of films with increasing concentration of In₂O₃.     

Characterization of ZnO-In2O3 transparent conducting films by pulsed laser deposition

Transparent conducting oxide (TCO) films in the ZnO-In₂O₃ system were prepared by a pulsed laser deposition method. A target that consists of the mixture of ZnO and In₂O₃ powders was used. Influences of the target composition x (x = [Zn]/([Zn] + [In])) and heater temperature on structural, electrical and optical properties of the TCO films were examined. Introduction of oxygen gas into the chamber during the deposition was necessary for improvement in the transparency of the deposited films. The amorphous phase was observed for a wide range of x = 0.20-0.60 at 110 °C. Minimum resistivity was 2.65 × 10⁻⁴ Ω cm at x = 0.20. The films that showed the minimum resistivity had an amorphous structure and the composition shifted toward larger x, as the substrate temperature increased. The films were enriched in indium compared to the target composition and the cationic In/Zn ratio increased as the substrate temperature was increased.     

In2O3 : (Sn) and SnO2 : (F) films - application to solar energy conversion part II — Electrical and optical properties

Highly conductive and transparent thin films of SnO₂ : F and In₂O₃ : Sn have been prepared using the simple pyrolitic (spray) method. The electrical properties of these layers are studied in relation to their dopant concentrations and their stoichiometric deviation. Typically we obtained for In₂O₃ : Sn and SnO₂ : F layers having the best overall properties (higher transparency and lower sheet resistance), resistivities ranging between 4 and 6.10^?4 Ω cm with transparency exceding 85% over the visible and near infra-red range of the spectrum. Emphasis is put on the possible applications of these films in solar energy conversion systems (solar cell and flat plate collectors technology).     

Material properties and growth control of undoped and Sn-doped In2O3 thin films prepared by using ion beam technologies

Undoped (IO) and Sn-doped In₂O₃ (ITO) films have been deposited on glass and polymer substrates by an advanced ion beam technologies including ion-assisted deposition (IAD), hybrid ion beam, ion beam sputter deposition (IBSD), and ion-assisted reaction (IAR). Physical and chemical properties of the oxide films and adhesion between films and substrates were improved significantly by these technologies. By using the IAD method, non-stoichiometry and microstructure of the films were controlled by changing assisted oxygen ion energy and arrival ratio of assisted oxygen ion to evaporated atoms. Relationships between structural and electrical properties in ITO films on glass substrates were intensively investigated by using the IBSD method with changing ion energy, reactive gas environment, and substrate temperature. Smooth-surface ITO films (R_rms ≤ 1 nm and R_p-v ≤ 10 nm) for organic light-emitting diodes were developed with a combination of deposition conditions with controlling microstructure of a seed layer on glass. IAR surface treatment enormously enhanced the adhesion of oxide films to polymer substrate. The different dependence of IO and ITO films' properties on the experimental parameters, such as ion energy and oxygen gas environment, will be intensively discussed.     

Nanocrystalline SnO2 and In2O3 as materials for gas sensors: The relationship between microstructure and oxygen chemisorption

The correlations between microstructure of nanocrystalline TCO SnO₂ and In₂O₃ and parameters of oxygen chemisorption are analyzed. Nanocrystalline SnO₂ and In₂O₃ were prepared by wet chemical method. The sample's microstructure was characterized by TEM, XRD and low-temperature nitrogen adsorption. Electrical properties of TCO were studied at 200-400 °C depending on the oxygen partial pressure. Increase of TCO grain size leads to the increase of the fraction of atomic forms of chemisorbed oxygen at the fixed temperature. It could be due to the decrease of surface barrier resulting in the decrease of activation energy of dissociation of molecular ion O_2(ads)⁻.     

Analysis of single junction a-Si:H solar cells grown on different TCO’s

In consequence of previous investigation of individual transparent conductive oxide (TCO) and absorber layers a study was carried out on hydrogenated amorphous silicon (a-Si:H) solar cells with diluted intrinsic a-Si:H absorber layers deposited on glass substrates covered with different TCO films. The TCO film forms the front contact of the super-strata solar cell and has to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In this paper we focused our attention on the influence of using different TCO’s as a front contact in solar cells with structure as follows: Corning glass substrate/TCO (800, 950 nm)/p-type μc-Si:H (∼5 nm)/p-type a-Si:H (10 nm)/a-SiC:H buffer layer (∼5 nm)/intrinsic a-Si:H absorber layer with dilution R = [H₂]/[SiH₄] = 20 (300 nm)/n-type a-Si:H layer (20 nm)/Ag + Al back contact (100 + 200 nm). Diode sputtered ZnO:Ga, textured and non-textured ZnO:Al [3] and commercially fabricated ASAHI (SnO₂:F) U-type TCO’s have been used. The morphology and structure of ZnO films were altered by reactive ion etching (RIE) and post-deposition annealing.It can be concluded that the single junction a-Si:H solar cells with ZnO:Al films achieved comparable parameters as those prepared with commercially fabricated ASAHI U-type TCO’s.     

Properties of transparent conducting oxides formed from CdO alloyed with In2O3

The structure, optical and electrical properties of transparent conducting oxide films depend greatly on the methods of preparation, heat treatment, type and level of dopant. Thin films of (CdO)_1−x(In₂O₃)_x have been grown by electron beam evaporation technique for different concentrations of In₂O₃ (x = 0, 0.05, 0.1, 0.15 and 0.2). Increase of doping led to increased carrier concentration as derived from optical data and hence to increased electrical conductivity, which degraded the transparency of the films. An improvement of the electrical and optical properties of Cadmium indium oxide (CdIn₂O₄) has been achieved by post-deposition annealing. A resistivity value of 7 × 10^− 5 Ω cm and transmittance of 92% in the near infrared region and 82% in the visible region have been obtained after annealing at 300 °C for 90 min in air.     

Formation of semitransparent CuAlSe2 thin films grown on transparent conducting oxide substrates by selenization

Wide band-gap semiconductors have been studied for applications as buffer layers in thin film solar cells and as top cell in tandem devices. CuAlSe₂ (CAS) thin films were deposited onto bare and two different transparent conducting oxide (TCO)-coated glass substrates, In₂O₃:Sn (ITO) and ZnO:Al (AZO), by a two stage process consisting on the selenization of metallic precursor layers. Homogeneous and crystalline formation of CAS thin films is not trivial and it is strongly influenced by selenization conditions, type of substrate and the film thicknesses. Under certain conditions, polycrystalline CuAlSe₂ thin films with chalcopyrite structure and preferential orientation along the (112) plane were obtained onto bare glass susbtrates. However, formation and crystallization of homogeneous CAS thin films was promoted by transparent conducting oxides (ITO and AZO)-coated glass substrates and take place in a wide range of thicknesses and Se amounts with high degree of reproducibility. TCO-coated substrates promoted larger grains when the CAS compound was formed. The band-gap energy, preferential orientation, crystallite size and the average surface roughness varied depending on the film thickness and type of substrate.     

Design and fabrication of InxGa1 − xN/GaN solar cells with a multiple-quantum-well structure on SiCN/Si(111) substrates

The fabrication and characterization of In_xGa_1 − xN/GaN-based solar cells that use In_xGa_1 − xN multiple quantum wells (MQWs) and a SiCN/Si(111) substrate are reported. Solar cell operation with a low dark current density (J_d), a high open-circuit voltage (V_oc), a high short-circuit current density (J_sc), and a high fill factor (FF) is demonstrated. It was found that the proposed device and fabrication technology are applicable to the realization of solar cells with a low J_d of 2.14 to 8.88 μA/cm², a high V_oc of 2.72 to 2.92 V, a high J_sc of 2.72 to 2.97 mA/cm², and a high FF of 61.51 to 74.89%. The device performance with various quantum-well configurations was investigated under an air mass 1.5 global solar spectrum. A high photovoltaic efficiency of 5.95% in the MQW sample over the p-i-n sample was observed.     

The effects of the thickness of TiO2 films on the performance of dye-sensitized solar cells

Nanocrystalline anatase TiO₂ thin films with different thicknesses (0.5-2.0 μm) have been deposited on ITO-coated glass substrates by a sol-gel method and rapid thermal annealing for application as the work electrode for dye-sensitized solar cells (DSSC). From the results, the increases in thickness of TiO₂ films can increase adsorption of the N3 dye through TiO₂ layers to improve the short-circuit photocurrent (J_sc) and open-circuit voltage (V_oc), respectively. However, the J_sc and V_oc of DSSC with a TiO₂ film thickness of 2.0 μm (8.5 mA/cm² and 0.61 V) are smaller than those of DSSC with a TiO₂ film thickness of 1.5 μm (9.2 mA/cm² and 0.62 V). It could be due to the fact that the increased thickness of TiO₂ thin films also resulted in a decrease in the transmittance of TiO₂ thin films thus reducing the incident light intensity on the N3 dye. An optimum power conversion efficiency (η) of 2.9% was obtained in a DSSC with the TiO₂ film thickness of 1.5 μm.     

Photovoltaic properties of bulk heterojunction devices based on CuI-PVA as electron donor and PCBM and modified PCBM as electron acceptor

In this paper, we have investigated the bulk heterojunction organic solar cells based on CuI ?? polyvinyl alcohol (CuI-PVA) nanocomposite as electron donor and [6,6] ?? phenyl C₆₀ ?? butyric acid methyl ester (PCBM) or modified PCBM i.e. F as electron acceptor. The power conversion efficiencies (PCEs) of 0.46 % and 0.68 % were achieved for the photovoltaic devices based on as cast CuI-PVA:PCBM and CuI-PVA:F blend films, respectively. The higher PCEs of the organic solar cells based on F as electron acceptor resulted from the increase in both short circuit current (J _sc ) and open circuit voltage (V _oc ), due to the increased absorption of F in visible region and its higher LUMO level. After thermal annealing, the PCEs of the organic solar cells were further increased to 0.54 % and 0.80 % for CuI-PVA:PCBM and CuI-PVA:F blends, respectively. The increase in the PCEs was mainly due to the increase in J _sc , which has been attributed to the improvement in hole mobility and broadening of the absorption band in the longer wavelength region. The improved hole mobility resulted in more balanced charge transport in the devices based on the thermally annealed blends.     

Potential of thin-film silicon solar cells by using high haze TCO superstrates

Potential improvements in the performance of tandem amorphous silicon/microcrystalline silicon (a-Si:H/μc-Si:H) solar cells, related to the TCO superstrates with enhanced scattering properties are studied. In particular, optical effects of a high haze double textured (W-textured) SnO₂:F TCO superstrate are analyzed and compared to the properties of the pyramidal type SnO₂:F TCO superstrate. Solar cell with W-textured superstrate exhibits higher long-wavelength external quantum efficiency of the bottom μc-Si:H cell than the one with pyramidal type TCO superstrate. Optical simulations are employed to study the potential improvements of the solar cell performance if ideal haze parameter (H = 1) and/or a broad angular distribution function (Lambertian) of scattered light are applied to textured interfaces in the solar cell structure. Simulations reveal significant improvements in long-wavelength quantum efficiencies if a broad angular distribution function of scattered light is applied. Optical losses in the cells with enhanced scattering properties are analysed and evaluated in terms of short-circuit current losses in the supporting layers and losses due to reflected light.