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CuInSe2 (CIS) has been proposed as an attractive bottom cell candidate in tandem solar cells. However, to justify the coupling with high-performance top cells (e.g., perovskites, GaAs), significant...

Humidity is widely accepted as a risk factor to the durability of photovoltaic modules. The ingress of moisture from humid environments into the module is linked to a host of module degradation modes, including contact corrosion, encapsulant yellowing, and delamination. While these degradation pathways are studied extensively, the exact moisture content within a module is typically a hidden variable, making it difficult to establish a quantitative relationship between module moisture content and degradation. We leverage our recently developed water reflectomery detection (WaRD) technique along with biased photoluminescence imaging to spatially quantify the moisture content and cell parameters of silicon modules subjected to various damp heat conditions for 2000 hours at a 1 millimeter resolution. This unique dataset reveals two modes of series resistance increase - finger interruptions and cell-wide “background” resistance - each showing varied sensitivity to moisture exposure depending on module architecture.

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.

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.

Growth of gallium nitride at low temperatures broadens the opportunity for its integration into optoelectronic devices that contain thermally sensitive substrates or active layers. As temperature is a very critical growth parameter, changes in crystallinity, defect density, optical, and structural properties are expected as temperatures fall below those typical of molecular beam epitaxy growth. In this contribution, energetic neutral atomic-beam lithography and epitaxy, a molecular beam epitaxy method that utilizes energetic neutral atomic nitrogen as the active nitrogen species, is used to grow gallium nitride directly on nitridized sapphire at temperatures between 800 and 200 °C. Photospectroscopy, photoluminescence, Raman spectroscopy, scanning electron microscopy and X-ray diffractometry are applied to determine changes in optical, morphological and structural properties induced by the unconventional low-temperature growth process. As anticipated, we observe that defect density, disorder, and light absorptance increase as growth temperature decreases. Interestingly, X-ray diffraction and photoluminescence reveal the presence of the cubic phase of gallium nitride in films grown at low temperatures under a nitrogen-rich regime, which differs from growth conditions reported by plasma-assisted molecular beam epitaxy and metalorganic molecular beam epitaxy. These discrepancies are presented in a critical review of several studies reporting the stabilization of the cubic phase over the energetically-favored hexagonal phase, with emphasis on relation to growth temperature, Ga/N flux ratio and surface kinetics during growth.

Silver-based metallizations in photovoltaic modules are susceptible to corrosion by acetic acid generated in ethylene vinyl acetate encapsulated modules, resulting in power losses over time. Here, three silver-based metallizations are exposed to diluted acetic acid, in concentrations representative of that found in field-exposed modules. Compositional, morphological, and structural changes of the metallizations are studied over 3000 h of exposure to diluted acetic acid using Raman spectroscopy mapping, X-ray diffraction, and scanning electron microscopy. The three metallizations studied are: 1) a commercial high-temperature fire-through Ag paste, commonly used for Si diffused junction solar cells; 2) a commercial low-temperature paste normally used for silicon heterojunction cells; and 3) a novel low-temperature reactive silver ink shown to be suitable for photovoltaic applications. We find distinct corrosion rates for the high-temperature silver paste and reactive silver ink in the presence of diluted acetic acid. On the other hand, the low-temperature silver paste appears to be more corrosion resistant, likely due to a polymer layer that protects the silver particles.

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.

Reactive silver ink (RSI) forms low-resistivity ( 3 COO on HT Ag Paste, dissolution of AgCH 3 COO from RSI, and LT Ag Paste remains relatively unchanged throughout 2936 h of exposure to diluted acetic acid.

Solar panel's 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.

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–Lambert's 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.

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.

In recent years the development of heterojunction silicon based solar cells has gained much attention, lead largely by the efforts of Panasonic's 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

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.

Cu(In,Ga)Se 2 (CIGS) solar cells were characterized in cross section using electron beam induced current (EBIC) and synchrotron based x-ray fluorescence (XRF) measurements. Samples with varying gallium ratios and growth methods were compared. A correlation was observed between the compositional gallium grading profile from XRF and carrier activity seen in EBIC through the thickness of the CIGS layer. Samples with steep back grading showed carrier activity isolated near the CIGS/CdS interface, whereas a more uniform grading resulted in carrier activity seen throughout the absorber layer. ‘Notch’ grading showed only slight variation in EBIC profile compared to a back graded sample with similar gallium ratios.

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

The vast majority of photovoltaic materials are highly sensitive to the presence of inhomogeneously distributed nanoscale defects, which commonly regulate the overall performance of the devices. The defects can take the form of impurities, stoichiometry variations, microstructural misalignments, and secondary phases - the majority of which are created during solar cell processing. Scientific understanding of these defects and development of defect-engineering techniques have the potential to significantly increase cell efficiencies, as well as provide a science-based approach to increase the competitiveness for the US PV industry on a dollar per installed kWh criterion. For the case of Cu(In, Ga)Se2 devices for example, the theoretically limit sits at 30.5% efficiency [1], thus, surpassing DOE's SunShot goals for cost-competitive solar power. However, to date, CIGS laboratory scale cells have been reported to achieve only 20.3% efficiencies and modules have not crossed the 15 % certified efficiency barrier. Recent reports have suggested that these record cells are limited by non-ideal recombination and, more specifically, by an increased saturation current that seems to originate from the particular defect chemistry at structural defects. In order to understand the severe efficiency limitations that currently affect solar cell materials, it is necessary to understand in detail the role of defects and their interactions under actual operating and processing conditions. In this work we propose to develop a high-temperature, in-situ stage for X-ray microscopes, with the capabilities of temperature and ambient control. Here, we provide insight into the design and preliminary testing at the Advanced Photon Source with beam sizes ≈100nm.