Characteristics and Electronic Band Alignment of a Transparent p-CuI/n-SiZnSnO Heterojunction Diode with a High Rectification Ratio.

Transparent p-CuI/n-SiZnSnO (SZTO) heterojunction diodes are successfully fabricated by thermal evaporation of a (111) oriented p-CuI polycrystalline film on top of an amorphous n-SZTO film grown by the RF magnetron sputtering method. A nitrogen annealing process reduces ionized impurity scattering dominantly incurred by Cu vacancy and structural defects at the grain boundaries in the CuI film to result in improved diode performance; the current rectification ratio estimated at ±2 V is enhanced from ≈106 to ≈107. Various diode parameters, including ideality factor, reverse saturation current, offset current, series resistance, and parallel resistance, are estimated based on the Shockley diode equation. An energy band diagram exhibiting the type-II band alignment is proposed to explain the diode characteristics. The present p-CuI/n-SZTO diode can be a promising building block for constructing useful optoelectronic components such as a light-emitting diode and a UV photodetector.

Analysis of Resistance According to Metal Ribbon Connection for Application to Interconnection of Shingled Photovoltaic Strings.

The Shingle Photovoltaic (PV) module is a new high power PV module technology manufactured by 'Dividing and ECA (Electrical Conductivity Adhesive) bonding' method for solar cell. In the case of a general PV module, a metal ribbon is soldered on the bus bar of the solar cell and connected to others. The dividing/ECA bonding technology connects the divided cells through bonding to manufacture a string. In order to make a module, the fabricated strings must be connected with Bus ribbon. The Shingled strings produced by the dividing and bonding method are not limited to the interconnection method by the metal ribbon. Also, it is not standardized for interconnections between strings. Therefore, we analyzed the characteristics of the shingled strings according to the soldering method. The characteristics of the string vary depending on the number of metal ribbons that contact the solar cell electrodes. Experimental results show that the series resistance increases significantly with fewer contacts. As a result, the efficiency of two-point contact decreased by 0.458%, four-point contact decreased by 0.048%, and eight-point contact decreased by 0.034%. This is because as the number of contacts increases, the resistance of the busbars becomes smaller and the contact resistance becomes smaller.

Improvement in Power of Shingled Solar Cells for Photo-Voltaic Module.

This paper presents a study on the effects of heat treatment conditions on electrically conductive adhesives. Among the advantages of the shingled solar cells include larger active area and smaller current density since one of the main factors of the power loss is due to a decrease in current density. Therefore, when there is a small current, there is a benefit in regards to the power loss. The advantage of this new technique of developing photovoltaic modules is the increase of module power using the same installed area. Electrically conductive adhesives play an important role in the manufacture of shingled solar cells and understanding the effects of its curing condition is necessary to maximize its output power. Through changing the curing time and temperature, the optimized curing conditions for electrically conductive adhesives and fabricated shingled strings for development of a module could be established. Finally, we demonstrated a 500 mm × 500 mm photovoltaic module with a conventional and the other using the shingled method for purposes of comparison and a shingled module showed about 29% increase in maximum output power compared to a conventional module with the same installed area.

Boosting Solar Cell Performance via Centrally Localized Ag in Solution-Processed Cu(In,Ga)(S,Se)2 Thin Film Solar Cells.

Fabrication of Cu(In,Ga)(S,Se)2 (CIGSSe) absorber films from environmentally friendly solutions under ambient air conditions for use in solar cells has shown promise for the low-cost mass production of CIGSSe solar cells. However, the limited power conversion efficiency (PCE) of these solar cells compared with their vacuum-processed counterparts has been a critical setback to their practical applications. This study aims to fabricate solution-processed CIGSSe solar cells with high PCEs by incorporation of Ag into the precursor layer of the CIGSSe absorber films. The results showed that Ag doping promoted grain growth by accelerating Se uptake, irrespective of the location within the CIGSSe film. Nevertheless, uniform Ag doping formed crevices that lowered the PCE of the cells, while centrally localizing the doped Ag prevented the formation of crevices, resulting in high PCEs up to 15.3%. Our results demonstrate that carefully doping Ag into a selected area of the precursor layer of the CIGSSe films can realize solution-processed chalcopyrite solar cells with high PCE.

Electrically Conductive Adhesives and the Shingled Array Cell for High Density Modules.

The shingled array of solar cells has the advantages of a larger active area and smaller current density than conventional solar cells. Because the power loss is mainly driven by the decrease in current density, this new method has the benefit of increasing module power with the same installed area as used in other methods. As the electrically conductive adhesive (ECA), CA3556HF was chosen and characterized by analysis of reflectance and sheet resistance. These analyzed data show consistent and relevant results for the cell efficiency of separated and serially connected cells fabricated by means of the shingled array method. We successfully demonstrated the increase of the high density module (HDM) power by 5.1% for a 30 cm×30 cm area and the fill factor also increased by 2% compared with conventional modules.

Electron Beam Irradiated Al Doped ZnO Thin Films as Efficient Tunnel Recombination Junction for Hydrogenated Amorphous Silicon/Cu(In,Ga)Se2 Tandem Solar Cells.

A tunnel recombination junction (TRJ) layer for hydrogenated amorphous silicon (a-Si:H)/ Cu(In,Ga)Se2 (CIGS) tandem solar cells is investigated. An Al-doped zinc oxide (AZO) thin film is applied to the TRJ, and the influence of electron beam (e-beam) irradiation on defects along the TRJ is investigated. The AZO thin films are prepared using radio frequency (RF) sputtering and the e-beam is irradiated at 200 W RF power and 2 keV DC power for 5 min. In the e-beam irradiated AZO thin film, the number of oxygen vacancies and Zn interstitials increases, which in turn strengthens the effect of defect-enhanced tunnel recombination.

Enhanced Photocurrents with ZnS Passivated Cu(In,Ga)(Se,S)2 Photocathodes Synthesized Using a Nonvacuum Process for Solar Water Splitting.

Chalcopyrite Cu(In,Ga)(Se,S)2 (CIGS) semiconductors are potential candidates for use in photoelectrochemical (PEC) hydrogen generation due to their excellent optical absorption properties and high conduction band edge position. In the present research, CIGS thin film was successfully prepared on a transparent substrate (F:SnO2 glass) using a solution-based process and applied for a photocathode in solar water splitting, which shows control of the surface state associated with sulfurization/selenization process significantly influences on the PEC activity. A ZnS passivation surface layer was introduced, which effectively suppresses charge recombination by surface states of CIGS. The CIGS/ZnS/Pt photocathode exhibited highly enhanced PEC activity (∼24 mA·cm-2 at -0.3 V vs RHE). The performances of our CIGS photocathode on the transparent substrate were also characterized under front/back light illumination, and the incident photon to current conversion efficiency (IPCE) drastically changed depending on the illumination directions showing decreased IPCE especially under UV region with back illumination. The slow minority carrier (electron) transportation is suggested as a limiting factor for the PEC activity of the CIGS photocathode.

Gravure-Offset Printed Metallization of Multi-Crystalline Silicon Solar Cells with Low Metal-Line Width for Mass Production.

The gravure offset method has been developed toward an industrially viable printing technique for electronic circuitry. In this paper, a roller type gravure offset manufacturing process was developed to fabricate fine line for using front electrode for solar cells. In order to obtain the optimum metallization printing lines, thickness of 20 µm which is narrow line is required. The main targets are the reduction of metallized area to reduce the shading loss, and a high conductivity to transport the current as loss free as possible out of the cell. However, it is well known that there is a poor contact resistance between the front Ag electrode and the n(+) emitter. Nickel plating was conducted to prevent the increase of contact resistance and the increase of fill factor (FF). The performance of n-Si/Ag (seed layer)/Ni solar cells were observed in 609 mV of open circuit voltage, 35.54 mA/cm2 of short circuit current density, 75.75% of fill factor, and 16.04% of conversion efficiency.

Improvement of Pre-Annealed Cu(In, Ga)Se2 Absorbers for High Efficiency.

We used a DC-sputtering method to deposit the precursor (Cu3Ga/In) onto Mo with 1 um thick/soda-lime glass (SLG). We moved it onto a graphite crucible for the pre-annealing process, and the pressure of the process tube was about 10 torr without Ar gas flow. The crucible in quartz tube was heated by halogen lamp to 250 degrees C for 30 min, and then raised to 550 degrees C for 10 min under a selenium atmosphere. To complete the solar cells, a buffer layer of 50 nm CdS was then deposited by chemical bath deposition (CBD), followed by a double layer (high resistivity/low resistivity) of RF sputtered i-ZnO/Al-ZnO thin films. The Al front contacts were deposited by thermal evaporator.

Effect of Reaction Temperature of CdS Buffer Layers by Chemical Bath Deposition Method.

This study investigated CdS deposition on a Cu(In,Ga)Se2 (CIGS) film via chemical bath deposition (CBD) in order to obtain a high-quality optimized buffer layer. The thickness and reaction temperature (from 50 degrees C to 65 degrees C) were investigated, and we found that an increase in the reaction temperature during CBD, resulted in a thicker CdS layer. We obtained a thin film with a thickness of 50 nm at a reaction temperature of 60 degrees C, which also exhibited the highest photoelectric conversion efficiency for use in solar cells. Room temperature time-resolved photoluminescence (TR-PL) measurements were performed on the Cu(In,Ga)Se2 (CIGS) thin film and CdS/CIGS samples to determine the recombination process of the photo-generated minority carrier. The device performance was found to be dependent on the thickness of the CdS layer. As the thickness of the CdS increases, the fill factor and the series resistance increased to 61.66% and decreased to 8.35 Ω, respectively. The best condition was observed at a reaction temperature of 60 degrees C, and its conversion efficiency was 12.20%.

Rapid Annealing of Cu-In-Ga-Se Precursors by Electron Beam Irradiation Method.

Cu-In-Ga-Se precursors were prepared by RF- and DC-sputtering methods and then irradiated with an in-situ electron beam irradiation unit. Ternary (In,Ga)Se2 and binary CuSe targets were simultaneously used for preparation of precursors. The electron dose and irradiation time were kept constant at 300 seconds and 200 W of RF power, respectively, while intensities of accelerated electrons were varied from 2.5 to 4.5 keV. The thickness of all e-beam irradiated CuInGaSe2 (CIGS) films decreased from 1,250 nm to 470 nm. The crystalline properties of e-beam irradiated CIGS films were clearly shown on all samples and the highest intensity of (112) peak at 3.5 keV. The compositional ratio of Cu/(In + Ga) in the e-beam sample irradiated at 3.5 keV was coincident with that of the precursors. The degree of Ga content on the depth of the e-beam sample irradiated at 3.5 keV was uniformly distributed between the TCO/CdS layer and Mo back contact. Electron beam irradiation onto Cu-In-Ga-Se precursors as a rapid annealing method could be an excellent candidate for crystallization to the Cu(In,Ga)Se2 films.

Characterization of Flexible CIGS Thin Film Solar Cells or Stainless Steel with Intrinsic ZnO Diffusion Barriers.

ZnO diffusion barrier layer was deposited by RF magnetron sputtering by using the same method as intrinsic ZnO layer. The CIGS solar cells were fabricated on stainless steel substrate. The 50-200 nm thin ZnO diffusion barriers effectively reduced the diffusion of Fe and Cr, from stainless steel substrates into the CIGS absorbers. The CIGS solar cells with ZnO diffusion barriers increased the J(sc) and FF, which resulted in an increase of cell efficiency from 5.9% up to 9.06%.

Thermally Stable Silver Nanowires-Embedding Metal Oxide for Schottky Junction Solar Cells.

Thermally stable silver nanowires (AgNWs)-embedding metal oxide was applied for Schottky junction solar cells without an intentional doping process in Si. A large scale (100 mm(2)) Schottky solar cell showed a power conversion efficiency of 6.1% under standard illumination, and 8.3% under diffused illumination conditions which is the highest efficiency for AgNWs-involved Schottky junction Si solar cells. Indium-tin-oxide (ITO)-capped AgNWs showed excellent thermal stability with no deformation at 500 °C. The top ITO layer grew in a cylindrical shape along the AgNWs, forming a teardrop shape. The design of ITO/AgNWs/ITO layers is optically beneficial because the AgNWs generate plasmonic photons, due to the AgNWs. Electrical investigations were performed by Mott-Schottky and impedance spectroscopy to reveal the formation of a single space charge region at the interface between Si and AgNWs-embedding ITO layer. We propose a route to design the thermally stable AgNWs for photoelectric device applications with investigation of the optical and electrical aspects.

Characteristics of Carrier Collection of Nanopillar-Patterned Silicon Solar Cells.

Nanopillar-patterned Si solar cells were investigated. Ag nanoparticles were coated on a polished Si substrate as an etching mask. Reactive ion etching caused Si nanopillars to replicate in a reverse fashion on the Ag nanoparticles over a large area. The nanopillar structures efficiently reduced the light reflection on the surface and effectively drove the incident light into a Si absorber. This induced a significant enhancement of the photogenerated-current with an improved solar cell efficiency of 16.07%. The Si nanopillar-patterned solar cells showed improved carrier collection for long wavelengths; however, the surface-defect induced recombination degraded the quantum efficiency at short wavelengths. We suggest that the reduction of recombination loss should be considered for efficient nanostructure solar cells.

Properties of Electron-Beam Irradiated CuInSe2 Layers by Multi-Step Sputtering Method.

Typically, CuInSe2 (CIS) based thin films for photovoltaic devices are deposited by co-evaporation or by deposition of the metals, followed by treatment in a selenium environment. This article describes CIS films that are instead deposited by DC and RF magnetron sputtering from binary Cu2Se and In2Se3 targets without the supply of selenium. As a novel method, electron beam annealing was used for crystallization of Cu2Se/In2Se3 stacked precursors. The surface, cross-sectional morphology, and compositional ratio of CIS films were investigated to confirm the possibility in crystallization without any addition of selenium. Our work demonstrates that the e-beam annealing method can be a good candidate for the rapid crystallization of Cu-In-Se sputtered precursors.

Preparation of Periodically Arrayed Silicon Microwires Using Simple Patterning Process.

The silicon (Si) microwires were fabricated by microsphere lithography using polystyrene (PS) beads monolayer. The Si wafer tailored into 40 x 40 mm2 was used as a substrate. The monolayer of 2.0 µm-sized PS beads was formed on substrates through convective assembly method. PS beads on substrates were tailored into smaller sizes by O2 plasma treatment using reactive ion etching (RIE). This controllable re-sizing process gave an opportunity to prepare the wire-array with various radii of Si microwires fabricated by using inductively coupled plasma (ICP)-etching. The convective assembly process was monitored in real-time through an optical microscope with a CCD camera. PS beads and structures of Si microwires were characterized using a scanning electron microscope (SEM) and the optical property was measured by the UV-Vis spectroscopy.

Direct metallization local Al-back surface field for high efficiency screen printed crystalline silicon solar cells.

In this paper, we present a detailed study on the local back contact (LBC) formation of rear-surface-passivated silicon solar cells, where both the LBC opening and metallization are realized by one-step alloying of a dot of fine pattern screen-printed aluminum paste with the silicon substrate. Based on energy dispersive spectrometer (EDS) and scanning electron microscopy (SEM) characterizations, we suggest that the aluminum distribution and the silicon concentration determine the local-back-surface-field (Al-p+) layer thickness, resistivity of the Al-p+ and hence the quality of the Al-p+ formation. The highest penetration of silicon concentration of 78.17% in aluminum resulted in the formation of a 5 microm-deep Al-p+ layer, and the minimum LBC resistivity of 0.92 x 10-6 omega cm2. The degradation of the rear-surface passivation due to high temperature of the LBC formation process can be fully recovered by forming gas annealing (FGA) at temperature and hydrogen content of 450 degrees C and 15%, respectively. The application of the optimized LBC of rear-surface-passivated by a dot of fine pattern screen(-) printed aluminum paste resulted in efficiency of up to 19.98% for the p-type czochralski (CZ) silicon wafers with 10.24 cm2 cell size at 649 mV open circuit voltage. By FGA for rear-surface passivation recovery, efficiencies up to 20.35% with a V(OC) of 662 mV, FF of 82%, and J(SC) of 37.5 mA/cm2 were demonstrated.

Investigation on properties of electron-beam treated Mo back contact layer prepared by DC sputtering method.

Molybdenum (Mo) has been used as back contact layer in chalcopyrite solar cells family (CulnSe2 and its alloys) because it showed the excellence of electrical properties such as low resistivity at the Mo interference. Generally, there are strong corrleations between working pressure and properites such as adhesion and conductivity during deposition of Mo layer. Electrical properites might be inversely proportional to adehsion between Mo layer and glass substrate. Several methods have been executed for improvement of electrical properties and ahesion. In this study, DC sputtered Mo back contact layers on sodalime glass were prepared with working pressure range of 1-10 mTorr, and then all samples were treated by rounded exposure of electron beam at DC power of 3 kV for 5 minutes. After electron-beam treatement, all samples showed the lower resisitivity and sheet resisitance than as-deposited Mo layers. As working pressure decreased, grain size of e-beam treated samples gradually increased up to 30.7 nm. E-beam treated Mo layer might be a good candidate as back contact layer of tandem structured CIS families thin film solar cells.

Effect on electron beam treatment of radio frequency sputtered i-ZnO thin films for solar cell applications.

Intrinsic ZnO (i-ZnO) thin films were prepared using radio frequency (RF) sputtering method with working pressure range of 1-20 mTorr and treated by electron beam (e-beam) irradiation unit with 300 W of RF power and 2.5 kV of DC power for 5 min. As working pressure increased to 20 mTorr, deposition rate of samples gradually decreased from 0.3 angstroms/sec to 0.18 angstroms/sec and grain size from 23.6 nm to 16.0 nm. After e-beam treatment on RF sputtered i-ZnO thin films with increasing of working pressure, thickness were totally declined by 10% and grain sizes were grown bigger. The electrical properties of e-beam treated samples were remarkably improved to be - 10(18) cm(-3) of carrier concentration, 2-7 cm2/Vs of Hall mobility and - 10(-1) omega x cm of resistivity. Transmittance of e-beam treated samples were up to -90% and optical bandgap increased to 3.27-3.31 eV, resulted from decline of thickness. The better properties of ZnO thin films as a buffer layer in thin film solar cells could be obtained by e-beam treatment method.

Effect on thickness of Al layer in poly-crystalline Si thin films using aluminum(Al) induced crystallization method.

The polycrystalline silicon (poly-Si) thin films were prepared by aluminum induced crystallization. Aluminum (Al) and amorphous silicon (a-Si) layers were deposited using DC sputtering and plasma enhanced chemical vapor deposition method, respectively. For the whole process Al properties of bi-layers can be one of the important factors. In this paper we investigated the structural and electrical properties of poly-crystalline Si thin films with a variation of Al thickness through simple annealing process. All samples showed the polycrystalline phase corresponding to (111), (311) and (400) orientation. Process time, defined as the time required to reach 95% of crystalline fraction, was within 60 min and Al(200 nm)/a-Si(400 nm) structure of bi-layer showed the fast response for the poly-Si films. The conditions with a variation of Al thickness were executed in preparing the continuous poly-Si films for solar cell application.