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.

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.

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.

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.

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.

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.

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.

Preparation of born-doped a-SiC:H thin films by ICP-CVD method and to the application of large-area heterojunction solar cells.

Hydrogenated amorphous silicon carbide (a-SiC:H) film has been widely used as an emitter p layer in solar cells. For the better p layer, wide optical bandgap, and high electrical conductivity should be obtained from the effective method. We prepared the boron-doped a-SiC:H thin films using inductively coupled plasma chemical vapor deposition (ICP-CVD) method and characteristics on the small-area (2 cm x 2 cm) as well as the large-area films (diameter of 100 mm) were shown on it. As a substrate, the n-type (100) oriented CZ c-Si (5.5 approximately 6.5 omega x cm, 650 microm) wafers were used and cleaned by using the reduced RCA method. A silane (SiH4) of 99.999% purity, H2 and 60% hydrogen diluted ethylene (C2H4) was used as source gas for the deposition of intrinsic a-SiC:H films, and then diborane (B2H6), as the doping gas, is added to C2H4 and SiH4/H2 during the deposition of films. The uniformity of thickness and optical bandgap from large-area as-dep. films was at 1.8% and 0.3%, respectively. Heterojunction solar cell with 2 wt%-AZO/p-a-SiC:H/i-a-Si:H/c-Si/Ag structure was fabricated and characterized with diameter of 152.3 mm in this large-area ICP-CVD system. Conversion efficiency of 9.123% was achieved with a practical area of 100 mm x 100 mm, which can show the potentials to the fabrication of the large-area solar cell using ICP-CVD method.

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.

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.

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.

Investigation on the surface passivation of intrinsic a-Si:H thin films prepared by inductively coupled plasma-chemical vapor deposition for heterojunction solar cell applications.

Intrinsic a-Si:H thin films, which can have passivation functions on the surface of crystalline Si, were deposited by inductively coupled plasma chemical vapor deposition (ICP-CVD). The properties of the films were investigated at deposition temperatures ranging from 50 to 400 degrees C. The Si--H stretching mode at 2000 cm(-1), which indicates good film quality, was found in the range of 150-400 degrees C, but the film quality was not good at deposition temperatures below 150 degrees C. The passviation quality was determined by measuring the effective carrier lifetime using the quasi-steady state photoconductance (QSS-PC) technique. Two, 5, 7.5 and 10 nm thick films were deposited at 150 degrees C and annealed at 200 degrees C for 1 hour. The carrier lifetime of these films was approximately 3 times higher than that observed before annealing. A p a-SiC:H/i a-Si:H/n c-Si hetero-structure solar cell with a 7.8% efficiency and approximately 85% quantum efficiency (QE) was obtained by inserting an intrinsic a-Si:H thin film (5 nm) between the interfaces. These results highlight the potential applications of a passivation layer to heterojunction solar cells.

Preparation of phosphorus doped hydrogenated microcrystalline silicon thin films by inductively coupled plasma chemical vapor deposition and their characteristics for solar cell applications.

Intrinsic and phosphorus-doped hydrogenated microcrystalline silicon (microc-Si:H) films were prepared using inductively coupled plasma chemical vapor deposition (ICP-CVD) method. Structural, electrical and optical properties of these films were studied as a function of silane concentration, ICP source power and PH3/SiH4 gas ratio. Characterization of these films from Raman spectroscopy and X-ray diffraction revealed that the conductive film exists in microcrystalline phase embedded in an amorphous network. The condition of electrical properties (sigma(d): approximately 10(-7) S/cm, sigma(ph): approximately 10(-4) S/cm) and activation energy (0.55 eV), satisfied with properties of intrinsic microc-Si:H, was obtained at 1200 W of ICP power and 2% of silane concentration, respectively. At PH3/SiH4 gas ratio of 0.09%, dark conductivity has a maximum value of approximately 18.5 S/cm and optical bandgap also a maximum value of approximately 2.39 eV. The deposition rate was not satisfactory (4.9 angstroms/s) at same condition.

Characterization of intrinsic a-Si:H films prepared by inductively coupled plasma chemical vapor deposition for solar cell applications.

The hydrogenated amorphous silicon (a-Si:H) films, which can be used as the passivation or absorption layer of solar cells, were prepared by inductively coupled plasma chemical vapor deposition (ICP-CVD) and their characteristics were studied. Deposition process of a-Si:H films was performed by varying the parameters, gas ratio (H2/SiH4), radio frequency (RF) power and substrate temperature, while a working pressure was fixed at 70 m Torr. Their characteristics were studied by measuring thickness, optical bandgap (eV), photosensitivity, bond structure and surface roughness. When the RF power and substrate temperature were 300 watt and 200 degrees C, respectively, optical bandgap and photosensitivity, similar to the intrinsic a-Si:H film, were obtained. The Si-H stretching mode at 2000 cm(-1), which means a good quality of films, was found at all conditions. Although the RF power increased up to 400 watt, average of surface roughness got better, compared to a-Si:H films deposited by the conventional PECVD method. These results show the potential for developing the solar cells using ICP-CVD, which have the relatively less damage of plasma.

Effect of aluminum on the formation of silver metal quantum dots in sol-gel derived alumino-silicate glass film.

The effect of aluminum incorporation on silver metal quantum dots formation in the alumino-silicate glass film processed by sol-gel process was investigated. The sol-gel derived glass was coated onto the silica glass plate by spin coating with the mixture solution of tetraethyl orthosilicate (TEOS), C2H5OH, H2O, AgNO3, Al(NO3)3. 39H2O, and HNO3 with the molar ratios of Ag/Si = 0.12 and Al/Si varying from 0 to 0.12. The formation of the silver metal quantum dots was confirmed by the measurements of the UV/VIS optical spectra, the X-ray diffraction patterns, and the transmission electron microscope images. While the radius of silver metal quantum dots increased with the increase of aluminum concentration, the concentration of the silver metal quantum dots decreased. The formation of the silver metal quantum dots was found strongly suppressed by incorporation of aluminum ions in the glass. The change in the glass structure due to the aluminum incorporation was investigated by the analysis of the Raman spectra. The silver ions in the glass contributed to form stable (Al:Ag)O4 tetrahedra by pairing with aluminum ions and thus clustering of silver metal quantum dots was hindered.

Fabrication and optical characteristics of a novel optical fiber doped with the Au nanoparticles.

Optical fibers containing gold metal nanoparticles were developed by modified chemical vapor deposition, in which Au(OH)3 and tetraethyl-orthosilicate (TEOS) was used via sol-gel process to incorporate gold metals by providing the reduction atmosphere. The absorption peak appeared near 490 nm was found to be due to the surface plasmon resonance of the gold nanoparticles incorporated in the fiber core.

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.