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We report fabrication of random nanometer-scale radial p-n junction solar cells (SCs). Nanoholes were fabricated on the Si wafer by combining silver film annealing and metal catalyzed electrochemical etching (MECC) of the material. The dimension of the holes can be adjusted by varying the annealing conditions and the thickness of the Ag film. Systematic investigations on the effects of the nanohole size and the doping conditions on the cell efficiency were performed.
The SLIM-Cut process is a kerf-free wafering technique to obtain silicon substrates as thin as 50μm. The quality of the resulting material must be assessed to ensure that this innovative Si-foil approach does not jeopardize the potential efficiency of the final solar cell in terms of electronic activity, defect density and location. For that reason, we performed Microwave-Detected Photoconductance Decay (MW-PCD), Deep-Level Transient Spectroscopy (DLTS) and optical inspections after defect etching of the foils surface. Analyses indicate that SLIM-Cut generates crystallographic defects which create deep level traps that have a negative impact on the lifetime of the silicon foil. Nonetheless, a decrease of the thermal budget will lead to a reduction of plasticity and hence lower the amount of defects and increase the foil quality.
Molybdenum back contact deposition is a bottleneck in high volume manufacturing due to the current state of art where multi layer molybdenum film needs to be deposited to achieve the required properties. In order to understand and solve this problem experiments were carried out. The effect of working distance (distance between the target and the substrate) on film properties was studied and is presented in this work. Earlier work carried out at Florida Solar Energy Center reflected on the effect of the sputtering power and working gas pressure on the film properties. This work is continuation of that effort in understanding effects of various sputtering parameters and determining the possible route to develop single layer molybdenum films with the required properties of near zero stress, low resistivity and good adhesion to substrate.
Layers that enhance light scattering and Raman-scattering-based spectral modification for solar cell applications were investigated. Titanium-oxide based rear diffuse reflector were found to increase the long wavelength response of crystalline solar cells. Also particle within the Titanium-oxide produce a far greater Stokes and anti-Stokes shift when compared to bulk crystal counterparts. The anti-Stokes to Stokes shift ratio in these particle systems is also greater and increased with increasing probe or bias light intensity. When applied to solar cells these layers extend the red response and thereby increase the overall performance.
Polycrystalline thin-film CdS/CdTe PV cells nearly always require “activation” with vapors containing chlorine and oxygen near 400 oC in order to realize the highest cell performance, even when growth occurs near 600 oC. In this study we have used film growth near 270 oC by magnetron sputtering in an oxygen-free ambient and have studied the effects of post-deposition heat treatments for 20 minutes at 400, 425 and 450 oC without CdCl2 in a dry air ambient. The heat treatments enhanced grain growth and produced re-crystallization of the CdTe film at all three temperatures, but 450 oC was required to reach the best electrical performance. Grain size increased from a couple of hundred nanometers to more than a micron as the preferred (111) growth orientation decreased. Efficiencies up to 11.6% were achieved with no CdCl2 compared to ~13% with activation at 387 oC in the presence of CdCl2 vapors. X-ray diffraction and quantum efficiency measurements show interdiffusion of CdS and CdTe at 450 oC comparable with a standard CdCl2 treatment at 387 oC. The results are discussed in terms of CdSTe alloy gradients and minority-carrier diffusion lengths.
We have developed a prototype spectroscopic ellipsometer for imaging/mapping purposes requiring only one measurement cycle (one rotation period of a polarizer or analyzer) for the acquisition of a two-dimensional array of data points. Our new measurement technique serves as a novel form of imaging ellipsometry, using a divergent (uncollimated, diffuse) source system and a detection system consisting of an angle-of-incidence-sensitive pinhole camera. By incorporating broad-band sources and wavelength dispersion optics, the instrument provides continuous high-resolution spectra along a line image of the sample surface. As a result, information on multilayer photovoltaics stacks can be obtained over large areas (several dm2) at high speed. The technique can be expanded to even larger areas by scaling-up the optical geometry. The spatial resolution of the line image is limited by the minimum resolved-angle as determined by the detection system. Small-aperture polarizers (25 mm diameter) are incorporated into the instrument, which reduces its cost. Demonstration mapping measurements have been performed ex situ on a multilayer sample deposited on a polymer substrate, including an intentionally graded 80-350 nm thick hydrogenated amorphous silicon (a-Si:H) layer and an intended uniform 400-500 nm thick transparent conducting ZnO:Al layer, both on opaque silver. Alternative commercial instruments for ex situ SE mapping must translate the sample in two dimensions. Even a 15 x 15 cm2 sample requires > 200 measurements with cm-resolution and at least 15 min. By collecting ex situ data in parallel along one dimension through imaging, the divergent-beam system can measure with similar spatial resolution in < 2 min. In situ measurements on both roll-to-roll polymer and rigid glass will be possible in the future.
Alternatives to ITO are under heavy investigation. Organic and inorganic transparent conducting materials are compared based on their transparency versus sheet resistance characteristics. Although graphene has advanced recently, TCOs are still superior in performance and can only be surpassed by the combination of transparent materials with a metal grid. Results on modeling and design optimization using a monolithically integrated CIGS cell configuration as case showed that considerable efficiency enhancement (up to 17% in power output compared to single TCOs) can be achieved for metal grid/TCO combinations. Conductivity improvement has been experimentally verified by four point probe measurements. on both commercial ITO coated PET foil as well as on ZnO coated glass with electrochemically deposited metal grids Sheet resistances as low as 0,1 Ohm/sq were reached and 80 times and 400 times conductivity improvements were obtained at a transparency loss of only 3% and 6%, respectively. It was also found that electrochemical deposition results in more conductive grids than obtained by Ag-ink screen printing due to higher aspect ratios and bulk-like conductivity of the first. Simultaneously, nanopatterning allows optimal grid width of 20 μm, as determined by modeling.
Nanoscale silver crystals at the interface of silver thick film contacts on n-type silicon carry the current across the contact and therefore control the contact resistance, which is a main performance limiting parameter for semiconductor devices. The silver crystals are located in pits at the silicon surface. The shape of the pits is different on Si-(111) and Si-(100). During contact formation, these pits form before the silver crystals. Hence they determine the crystal size and shape. Consequently, the pits with the crystals influence the contact resistance. We investigate these pits experimentally by scanning electron microscopy. We are the first to simulate the mechanism of pit formation at a contact interface by considering a model that is based on the removal probability of silicon surface atoms. This model leads to good agreement between experimental and simulated data. It enables the prediction of pit formation for arbitrary process parameters like temperature and duration for silver thick film contact formation on silicon.
One of the primary objectives of the global photovoltaic research community is to effect significant manufacturing cost reductions, either by reducing material and processing costs or by increasing solar cell efficiency. One very promising technology for achieving both of these goals is Sliver technology, which offers potential for a 10- to 20-fold reduction in the consumption of purified silicon, while at the same time achieving very high cell efficiencies by fully exploiting the advantages of mono-crystalline silicon.
Sliver solar cells are thin, mono-crystalline silicon solar cells fabricated using a combination of micro-machining techniques and standard silicon device fabrication technologies. Rather than fabricating a single solar cell on the surface of a wafer, many hundreds to several thousand individual Sliver solar cells are fabricated within a single wafer. The dimensions of a Sliver cell depend upon wafer size, wafer thickness, and the micro-machining method employed.Cells typically have a length of 5 – 12cm, a width of 0.5 – 2mm, and a thickness of 20 – 60 micron. 20% efficient Sliver solar cells using standard cell processing methods and a robust processing sequence, have been fabricated at ANU. Current research efforts are directed towards developing and establishing new fabrication techniques to further simplify the fabrication sequence and to improve cell efficiency.
This paper presents an overview of Sliver technology. The fabrication method and some key challenges in producing Sliver cells is presented along with the measured performance of cells fabricated in the ANU solar research laboratory.
The fabrication of nanoporous aluminum oxide (Al2O3) membranes for large scale production of nanowires is performed at room temperature by a two-step anodization of commercially available aluminum foil tapes. During the anodization process, an oxide barrier layer is formed at the interface with aluminum. In the present work, the removal of the barrier is performed by (i) ramping down the voltage with a rate in the range of 0.5 V per 60s to 2 V per 60s and (ii) immersing the substrate in 50% phosphoric acid for up to 5 minutes. Depending on the removal conditions, several morphologies at the oxide-aluminum interface are observed by Scanning Electron Microscopy (SEM). Ramping down the voltage at less than 0.3 V per 15s combined by immersion in 50% phosphoric acid for less than 3 minutes is found to open the barrier layer of the nanopores. The pores have root-like structure with branches as small as few nanometers due to the slow voltage ramping. Several amorphous anodized Al2O3 (AAO) templates with pore diameter ranging from 30 to 40 nm and with length up to 25 μm were prepared by two-step anodization for the cathodic electrodeposition of photoactive nanowire semiconductors such as copper indium diselenide and cadmium sulfide.
Atomic Layer Deposition (ALD) is a gas phase deposition technique for depositing very high quality thin films with an unsurpassed conformality. The main drawback of ALD however is the very low deposition rate (~ 1 nm/min). Recently, record deposition rates for alumina of up to 1 nm/s were reached using spatial ALD, while maintaining the typical assets regarding film quality as obtained by conventional, slow ALD . This allows for ALD at high throughput numbers.
One interesting application is passivation of crystalline silicon solar cells. Applying a thin alumina layer is reported to increase solar cell efficiency and enables the use of thinner wafers, thus reducing the main cost factor . In this paper we report on the latest progress made by SoLayTec that delivered a working prototype of a system realizing full area single sided deposition of alumina on 156 x 156 mm2, mono- and multi crystalline silicon wafers for solar cell applications. The alumina layers showed excellent passivation. Based on this concept, a high-throughput ALD deposition tool is being developed targeting throughput numbers of up to 3000 wafers/hr, making ALD ready for mass production. This will bring on new opportunities in other applications.
Developing a durable and scalable transparent conductor (TC) as an electrode with high optical transmission and low sheet resistance is a significant opportunity for enabling next generation solar cell devices. High performance fibrous composite materials based on a carrier polymer with embedded functional nanostructures have the potential to serve as a TC with high surface area that can be deposited by the novel and scalable process of electrospinning. This work presents the development of a fibrous TC, where polyacrylonitrile (PAN) is used as a carrier polymer for multi-walled carbon nanotubes (MWCNT) to create electroactive nanofibers 200-500nm in diameter. Once carbonized, thin layers of this material have a low sheet resistance and high optical transmission. It is shown that in a two stage carbonization process, the second stage temperature of above 700C is the primary factor in establishing a highly conductive material and single layers of nanofibers are typically destabilized at high temperatures. A high performance TC has been developed through optimizing carbonization rates and temperatures to allow for single nanofiber layers fabricated by electrospinning MWCNT/PAN solutions onto quartz. These TCs have been optimized for concentrations of MWCNTs less than 20% volume fraction with well above 90% transmissivity and sheet resistances of between .5-1kohm/square. The required MWCNT loading is well below that for TCs based on random networks of MWCNTs.
A new magnetron sputtering strategy is introduced that utilizes high plasma density (~5mA.cm-2) to avoid or reduce high temperature processing. The technique uses magnetrons of opposing magnetic polarity to create a “closed field” in which the plasma density is enhanced without the need for high applied Voltages. A batch system has been used which employs a rotating vertical drum as the substrate carrier and a symmetrical array of linear magnetrons. The magnetrons are fitted with target materials for each of the thin films required in the photovoltaic (PV) stack including the CdTe absorber layer, CdS window layer, metal contact using the conventional superstrate configuration. The “closed field” sputtering technology allows scale up not only for larger batch system designs but it is also configurable for “in-line” or “roll to roll” formats for large scale production. The morphology of each of the layers is characterized using a variety of structural and optical techniques including Field Emission Gun SEM and X-ray diffraction (XRD).
Cu(In,Ga)Se2 (CIGS) is one of the most advanced absorber materials with conversion efficiencies reaching up to about 20%. Electrodeposition of CIGS precursors is highly attractive due to its low cost, efficient utilization of raw materials and scalability to high-volume manufacturing, however, a strict chemistry control of the plating baths is required in a manufacturing environment to ensure a consistent plating process with high yields. In the present study, we tested the use of ion chromatography (IC), for the quantitative analysis of both the cationic and anionic species in a variety of aqueous alkaline electroplating solutions we developed for the fabrication of CIGS precursors. Using ion chromatography we were able to precisely determine the concentrations of several key anions commonly employed in the plating baths including chloride, sulfate, selenite, selenate, tartrate, citrate, gluconate, and ethylenediaminetetraacetate. Our results indicated IC might not be a suitable method to determine the cationic concentrations for Cu, In, Ga ions when complexing species, such as ethylenediaminetetraacetate, are present in the electroplating solutions. We determined that inductively coupled plasma optical emission spectroscopy (ICP-OES) could be used instead for the precise determination of the cationic concentrations.
Selective emitter structure has long been regarded as a good and relatively simple approach to improve the energy conversion efficiency of Si wafer-based single-junction photovoltaic (PV) cells. Recently emerged double printing method, on the other hand, potentially has the capability of improving the efficiency with no requirement for device structure modification. The manufacturability of these two approaches has been studied on a mass-production platform at JA Solar recently with large scale sampling. The experimental results collected from over two hundred thousand cells demonstrated that both approaches are capable of achieving significant conversion-efficiency gain in a cost-effective way with high yield rate on the PV industry commonly used mass production platform currently adopted by the vast majority of cell manufacturers
Wire shading during thin film deposition is a promising approach to low-cost, high volume manufacturing of flexible thin film photovoltaic modules. This contribution demonstrates successful patterning of a transparent conducting oxide layer by wire shading during dynamic web coating. Continuous sputter deposition of Al-doped ZnO on a 30 cm wide polymer foil and simultaneous wire shading form 1 cm wide and 300 cm long front contact stripes for thin film photovoltaic modules. Analysing the distribution of lateral shunt resistances after separating the initial 28 stripes into 1323 pieces, yields a patterning success of 97.3 %. Thus the technique seems well suited for flexible modules from organic solar cells.
Deoxyribonucleic acids provide exciting opportunities as templates in self assembled architectures and functionality in terms of optical and electronic properties. In this study, we investigate the effects of metalized DNA sequences in organic bulk-heterojunction solar cells. These effects are characterized via optical, quantum efficiency and current-voltage measurements. We demonstrated that by arranging the band energy structure of the devices via placing metalized deoxyribonucleic acid sequences on the hole collection side of the active layer lead to a 20% increase in the power conversion efficiency.
Exposure to highly focussed flash light (photonic flash sintering) has been developed as a technology to successfully cure printed metal inks on temperature sensitive plastic substrates. In contrast to the traditional approach of thermal oven sintering, conductivities up to 30 % of the value of bulk silver can be achieved within a few seconds without foil deformation. The compatibility of this technology with R2R production has been demonstrated with line speeds up to 5 m/min. As a consequence, our approach is expected to enable the high throughput fabrication of current collecting grids for organic solar cells in order to replace transparent electrodes based on metal oxides such as ITO. Additionally, our new sintering technology has enabled us to process a new generation of conductive inks, based on copper complexes, which cannot be processed by oven sintering.
Optimum quality polycrystalline AgGaSe2 thin films were deposited on H-terminated n-Si substrates by controlled thermal evaporation method. The film deposition conditions were varied to optimize the structure and optoelectronic properties of AgGaSe2 thin films. X-ray diffraction (XRD) studies showed that all AgGaSe2 films were of chalcopyrite structure and while the films deposited at room temperature (300 K) had random grain orientation, the films deposited at higher substrate temperature (≥ 450K) showed preferred (112) orientation. The composition of the films were analyzed by electron probe microanalysis (EPMA) deposited at different substrate temperatures. The ultraviolet-visible (UV-Vis) spectra showed the optical bandgap of 1.80 eV, with sharper band edge for the films deposited at higher temperature. The films were p-type and the resistivities of the as deposited films at 300 and 650K were ~5×103 and ~200 Ω.cm respectively. p-AgGaSe2/n-Si heterojunction solar cells, having an active area of 0.18 cm2 without any antireflection coating were designed and fabricated. It was observed that the films deposited at 650K produced heterojunctions with significantly improved photovoltaic properties. The evidence of the barrier height modifications have been provided by C-V measurements. Under solar simulator AM1 illumination, the improved junction exhibited an efficiency of 5.2%, whereas the AgGaSe2 films deposited at 300K showed a lower efficiency of 2.1%.