April Jeffries

X-ray fluorescence at nanoscale resolution for multicomponent layered structures: a solar cell case study

The study of a multilayered and multicomponent system by spatially resolved X-ray fluorescence microscopy poses unique challenges in achieving accurate quantification of elemental distributions. This is particularly true for the quantification of materials with high X-ray attenuation coefficients, depth-dependent composition variations and thickness variations. A widely applicable procedure for use after spectrum fitting and quantification is described. This procedure corrects the elemental distribution from the measured fluorescence signal, taking into account attenuation of the incident beam and generated fluorescence from multiple layers, and accounts for sample thickness variations. Deriving from Beer-Lamberts law, formulae are presented in a general integral form and numerically applicable framework. The procedure is applied using experimental data from a solar cell with a Cu(In,Ga)Se2 absorber layer, measured at two separate synchrotron beamlines with varied measurement geometries. This example shows the importance of these corrections in real material systems, which can change the interpretation of the measured distributions dramatically.

Low-Temperature Drop-on-Demand Reactive Silver Inks for Solar Cell Front-Grid Metallization

Formation of high-conductivity metal contacts at low temperatures expands optoelectronic device opportunities to include thermally sensitive layers, while reducing expended thermal budget for fabrication. This includes high-efficiency silicon heterojunction solar cells with intrinsic amorphous silicon layers. Efficiencies of these cells are limited by series resistance; the primary cause of this is the relatively high resistivity of the low-temperature silver paste used to form front-grid metallization. In this paper, we report the formation of highly conductive features by drop-on-demand printing of reactive silver ink (RSI) at a low temperature of 78 °C, resulting in media resistivities of 3-5 μΩ·cm. When used as a front grid on a silicon heterojunction solar cell, RSI fingers give cell series resistance of 1.8 Ω·cm2 (without optimization of the process), which is impressively close to 1.1 Ω·cm2 for our commercially available screen-printed low-temperature silver paste metallization. We present here the promising first results of RSI as metallic finger for photovoltaics, which upon optimization of design parameters has the potential to outperform the screen-printed low-temperature silver paste counterpart.

Structural and optical investigations of GaN-Si interface for a heterojunction solar cell

In recent years the development of heterojunction silicon based solar cells has gained much attention, lead largely by the efforts of Panasonics HIT cell. The success of the HIT cell prompts the scientific exploration of other thin film layers, besides the industrially accepted amorphous silicon. The band gap, mobilities, and electron affinity of GaN make it an interesting candidate to solve problems of parasitic absorption while selectively extracting electrons. Using a novel MBE based growth technique, thin films of GaN have been deposited at temperature significantly lower than industry standards. Crystalline measurements and absorption data of GaN are presented. Additionally, effects of deposition on the silicon wafer lifetimes are presented.

Characterization of encapsulated solar cells by x-ray topography

Solar panels reliability studies focus mainly on the properties of the encapsulating such as gel content and transmittance, while ignoring the impact of encapsulation process on the solar cells themselves. The harsh lamination conditions apply high temperature and pressure on the wafers, which can induce increased stress, deformation and defects. The investigation of solar cells sealed inside modules calls for a non-destructive method. In this paper, we demonstrate that transmission X-ray topography (XRT) can be used as an accurate method to evaluate bending feature of encapsulated wafers and present in detail the experimental methods from capturing diffraction data to the data analysis.

Innovative Methods for Low-Temperature Contact Formation For Photovoltaics Applications

Interest in silicon heterojunction with intrinsic thin layer and newly proposed wider bandgap carrier selective contact solar cells in recent years motivates the investigation of low temperature contact formation in order to preserve the order of electronic quality of these layers as well as the chemical surface passivation provided by hydrogenated passivation layers. The realization of low temperature contacts may also broaden solar cell and other optoelectronic devices opportunities, e.g. to use thermally sensitive materials, such as flexible polymer substrates, while at the same time reducing the thermal budget expended on device fabrication. In this work, two methods for low-temperature ohmic contact formation are investigated. The first is a rapid localized annealing technique using electromagnetic induction and the second a deposition method using inkjet printing of reactive silver inks. These techniques are evaluated for use in solar cell devices (not only silicon-based) by comparing demonstrated properties to those targeted for front contacts to solar cells, i.e. finger width, aspect ratio, resistivity, specific contact resistance, and apparent adhesion.

“Thin silicon solar cells: A path to 35% shockley-queisser limits”, a DOE funded FPACE II project

Crystalline silicon technology is expected to remain the leading photovoltaic industry workhorse for decades. We present here the objectives and workplan of a recently launched project funded by the U.S. Department of Energy through the Foundational Program to Advance Cell Efficiency II (FPACE II), which aims at leading crystalline silicon to an efficiency breakthrough. The project will tackle fundamental approach of materials design, defect engineering, device simulations and materials growth and characterization. Among the main novelties, the implementation of carrier selective contacts made of wide bandgap material or stack of materials is investigated for improved passivation, carrier extraction and carrier transport. Based on an initial selection of candidate materials, preliminary experiments are conducted to verify the suitability of their critical parameters as well as preservation of the silicon substrate surface and bulk properties. The target materials include III-V and metal-oxide materials.

Sensitivity analysis of materials availability for terawatt PV deployment

The road map for cost competitive large-scale PV deployment is ever changing and with PV grade poly silicon prices averaging 16 U

Adhesion of reactive silver inks on indium tin oxide

D/kg and companies focusing on using less materials the cost effectiveness of readily available materials for solar cell applications should be revisited. In this work we analyze to which extent the extraction cost of the absorber layer material plays a role in the overall cost of generating electricity, taking into account the potential for light trapping and the non-power producing component costs. Our calculations show that nearly all presently used materials have fundamental cost and availability potential which are well below a level at which material availability is a dominant consideration. Instead, constraints on parameters such as the amount of copper for wiring or substrate material for module fabrication become dominant issues.

Quantitative Mapping of Deflection and Stress on Encapsulated Silicon Solar Cells

Many emerging photovoltaic technologies, such as silicon heterojunction (SHJ) cells and perovskites, are temperature sensitive and are not compatible with the high sintering temperatures required for commercial screen-printed metallization pastes. Newer, low-temperature reactive silver inks exhibit good electrical conductivity and are compatible with temperature-sensitive substrates. However, preliminary investigations showed that the adhesion and reliability of these metallizations could vary dramatically with ink composition. This work evaluates the adhesion performance of printed reactive inks on indium tin oxide-coated SHJ cells to show that puckering phenomena originating from the porous nature of the printed reactive inks are responsible for lowering the as-printed adhesion strength. Adhesion performance was qualitatively determined using 180° peel test followed by optical imaging to quantify the amount of adhesive failure. Post-print scanning electron microscopy was used to observe the surface morphology. Diluting the reactive ink to reduce silver ion concentration decreased the observed puckering phenomenon and improved adhesion performance. This new understanding enables a more systematic design of reactive inks for novel photovoltaic applications.

Reactive silver ink as front contacts for high efficiency silicon heterojunction solar cells

The lamination process of photovoltaic modules relies on the application of high temperatures and pressures, which inherently introduces different amounts of expansion and shrinkage of the individual layers including glass, solar cells, polymeric encapsulants, and backsheet. There is no doubt that these effects translate into the cell in the form of deflection and stresses. Thus far though, only the consequences of this have been tracked in terms of failure modes—microcracks, delamination, stress points, etc. The general approaches have been applied to optimize processes with a focus on encapsulant properties, such as degree of cross-linking, moisture permeability, and their long-term lifetime, overlooking their effect on the solar cells. Module reliability is a major driver to lower the levelized cost of electricity and the bankability of projects, and more effort needs to be placed in predictive failure analysis and the optimization of the module components from the point of view of the active components—the cells. In this paper, we propose an in-house X-ray based technique as a novel approach to assess the state of the solar cell under polymeric encapsulation inside a fully assembled module. This gives access for the first time not only to the evaluation of cracks and microdefects, but also to the cell deflection and stress distribution inside the encapsulation.