Monolithic Interconnection Research Articles

Impact of interlayer application on band bending for improved electron extraction for efficient flexible perovskite mini-modules

The development of highly efficient lightweight flexible perovskite solar cells (PSCs) opens the way to high-throughput roll-to-roll manufacturing processes and new applications such as building integration and mobile products. Flexible PSCs are generally realized on small areas (< 0.2 cm2), far from technology commercialization where modules-scale is necessary. In this work, we demonstrate highly efficient n-i-p PSCs grown on flexible substrates by proper interface engineering for improved electron extraction. We compared spin coated PEIE and vacuum deposited LiF as interlayers between Al-doped ZnO, as transport conductive oxide (TCO), and thermally evaporated C60, as electron transport layer (ETL). Once interlayers are applied, we observed a favorable band bending at TCO interface which results in enhanced charge extraction and lower recombination losses. We achieved flexible PSCs with stabilized efficiencies of 14.8%, both with PEIE and LiF interfacial modifications.In addition, we developed a flexible perovskite mini-module with stabilized efficiency of 10.5% onto an aperture area larger than 10 cm2. The monolithic interconnections are entirely obtained by highly accurate and reliable laser scribing methods. A geometric fill factor as high as ~ 94% is achieved, with a dead area width of ~ 250 µm.

Up-scalable sheet-to-sheet production of high efficiency perovskite module and solar cells on 6-in. substrate using slot die coating

Scalable sheet-to-sheet slot die coating processes have been demonstrated for perovskite solar cells and modules. The processes have been developed on 6in. × 6in. glass/ITO substrates for two functional layers: the perovskite photo-active layer and the Spiro-OMeTAD hole transport layer. Perovskite solar cells produced using these slot die coating processes demonstrate device performances identical to the spin coated devices. All manufactured devices illustrate a high level of reproducibility. The developed slot die coating processes were also used for the manufacturing of perovskite PV modules. Large area modules of 12.5 × 13.5cm2 were realized by slot die coating on 6in. × 6in. substrates in combination with newly developed laser ablation processes for conventional P1-P2-P3 monolithic cell interconnections. The modules demonstrate power conversion efficiencies above 10%, with a power output of 1.7W. This achievement is an important milestone in the development of up-scalable manufacturing technologies for perovskite PV modules.

On-chip optical interconnect using visible light

We propose and fabricate a monolithic optical interconnect on a GaN-on-silicon platform using a wafer-level technique. Because the InGaN/GaN multiple-quantum-well diodes (MQWDs) can achieve light emission and detection simultaneously, the emitter and collector sharing identical MQW structure are produced using the same process. Suspended waveguides interconnect the emitter with the collector to form in-plane light coupling. Monolithic optical interconnect chip integrates the emitter, waveguide, base, and collector into a multi-component system with a common base. Output states superposition and 1×2 in-plane light communication are experimentally demonstrated. The proposed monolithic optical interconnect opens a promising way toward the diverse applications from in-plane visible light communication to light-induced artificial synaptic devices, intelligent display, on-chip imaging, and optical sensing.

Revealing and Identifying Laser-Induced Damages in CIGSe Solar Cells by Photoluminescence Spectroscopy

Laser-based patterning for serial interconnection of chalcopyrite (i.e., Cu(In ${}_{\rm{x}},\text{Ga}_{\rm{1-x}}$ )Se2 or CIGSe) solar cells was obtained by 1) laser ablation using picosecond (ps) pulses, 2) local phase transformation using nanosecond (ns) laser pulses, and 3) conventional needle-based patterning. All three patterning approaches cause a modification of the material properties in the vicinity of the actual P2 scribing lines, which affects and limits the electrical functionality of the interconnection, and thus has to be considered for positioning the P3 scribe. Thus, the extension and the properties of the affected zone aside the P2 scribe was investigated through spectral and spatial photoluminescence (PL). From the depletion of the PL intensity when approaching the scribing line and a peak shift analysis it is concluded that the laser-affected zone is distinctively larger than visual inspections suggest. Even putatively ultrashort, nonthermal ps pulses cause material modifications which might be facilitating recombination losses and thus limiting solar cell efficiencies. For ps laser patterning the affected area is even larger than for the ns laser patterning, due to a modification of the band structure and to thermal decomposition. Evolving subpeaks at the low energy tails are found to originate from Cu-related flat defect levels, i.e., Cu vacancies ( $V_{{\rm{Cu}}}$ ) and antisites (Cu In), created upon laser impact. These findings provide insights into laser-based material modification and provide beneficial information for minimizing the dead area resulting from laser-based monolithic interconnection.

$1.3~\mu $ m Optical Interconnect on Silicon: A Monolithic III-Nitride Nanowire Photonic Integrated Circuit

A feasible optical interconnect on a silicon complementary metal-oxide-semiconductor chip demands epitaxial growth and monolithic integration of diode lasers and optical detectors with guided wave components on a (001) Si wafer, with all the components preferably operating in the wavelength range of 1.3-1.55 μm at room temperature. It is also desirable for the fabrication technique to be relatively simple and reproducible. Techniques demonstrated in the past for having optically and electrically pumped GaAs and InP-based lasers on silicon include wafer bonding, selective area epitaxy, epitaxy on tilted substrates, and use of quantum dot or planar buffer layers. Here, we present a novel monolithic optical interconnect on a (001) Si substrate consisting of a III-nitride dot-in-nanowire array edge emitting diode laser and guided wave photodiode, with a planar SiO 2 /Si 3 N 4 dielectric waveguide in between. The active devices are realized with the same nanowire heterostructure by one-step epitaxy. The electronic properties of the InN dot-like nanostructures and mode confinement and propagation in the nanowire waveguides have been modeled. The laser, emitting at the desired wavelength of 1.3 μm, with threshold current ~350 mA for a device of dimension 50 μm × 2 mm, has been characterized in detail. The detector exhibits a responsivity ~0.1 A/W at 1.3 μm. Operation of the entire optical interconnect via the dielectric waveguide is demonstrated.

Local photocurrent mapping and cell performance behaviour on a nanometre scale for monolithically interconnected Cu(In,Ga)Se2 solar cells.

The local efficiency of lamellar shaped Cu(In,Ga)Se2 solar cells has been investigated using scanning near-field optical microscopy (SNOM). Topographic and photocurrent measurements have been performed simultaneously with a 100 nm tip aperture. The lamellar shaped solar cell with monolithic interconnects (P scribe) has been investigated on a nanometre scale for the first time at different regions using SNOM. It was found that, the cell region between P1 and P2 significantly contributes to the solar cells overall photocurrent generation. The photocurrent produced depends locally on the sample topography and it is concluded that it is mainly due to roughness changes of the ZnO:Al/i-ZnO top electrode. Regions lying under large grains of ZnO produce significantly less current than regions under small granules. The observed photocurrent features were allocated primarily to the ZnO:Al/i-ZnO top electrode. They were found to be independent of the wavelength of the light used (532 nm and 633 nm).

Morphology and topography of perovskite solar cell films ablated and scribed with short and ultrashort laser pulses

The unique properties of halide perovskites are suitable for low-cost high-efficiency photovoltaic applications. For commercialization of this technology, it is pivotal to upscale towards solar modules. Monolithic interconnection of solar cells is a necessary step for realization of thin-film solar modules and the laser scribing of the constituent layers with well-defined profiles of high accuracy is a promising approach for high speed processing. Here the laser ablation and scribing of methylammonium lead iodide perovskite (CH3NH3PbI3: MAPbI3) layers are investigated. Nanosecond (ns) and picosecond (ps) laser pulses were used to ablate and scribe MAPbI3 films on FTO/glass by irradiation from the film- and the glass-side. Depending on the irradiation configuration laser ablation or lift-off delamination was determined to be the dominating mechanism of thin-film removal. Various surface modifications such as film smoothening and decomposition of the MAPbI3 have been observed, especially when nanosecond laser pulses are used. The complete removal of the MAPbI3, film without damaging the FTO/substrate, has been achieved for all studied laser sources.

Printed interconnects for photovoltaic modules

Film-based photovoltaic modules employ monolithic interconnects to minimize resistance loss and enhance module voltage via series connection. Conventional interconnect construction occurs sequentially, with a scribing step following deposition of the bottom electrode, a second scribe after deposition of absorber and intermediate layers, and a third following deposition of the top electrode. This method produces interconnect widths of about 300µm, and the area comprised by interconnects within a module (generally about 3%) does not contribute to power generation. The present work reports on an increasingly popular strategy capable of reducing the interconnect width to less than 100µm: printing interconnects. Cost modeling projects a savings of about $0.02/watt for CdTe module production through the use of printed interconnects, with savings coming from both reduced capital expense and increased module power output. Printed interconnect demonstrations with copper-indium-gallium-diselenide and cadmium-telluride solar cells show successful voltage addition and miniaturization down to 250µm. Material selection guidelines and considerations for commercialization are discussed.

Laser-induced local phase transformation of CIGSe for monolithic serial interconnection: Analysis of the material properties

The change of electrical conductivity in chalcopyrite (i.e., Cu(Inx, Ga1−x)Se2 or CIGSe) solar cells induced by nanosecond laser pulses is investigated as a function of the elemental composition and its spatial distribution. The underlying laser induced phase transformation process, which results in a decomposition of the CIGSe semiconductor and a modification of its elemental composition, is utilized to form the monolithic series interconnection between front and back contact in CIGSe based thin film solar cells. The results show a dependence of the composition of the CIGSe layer and the resulting series resistance on the applied laser fluence. Lower series resistance is primarily related to an enhanced fraction of copper, gallium and zinc in the laser transformed zone resulting from selective vaporization of absorber elements. For intermediate laser fluences (~0.36J/cm2) a patterning process is established that allows reliable and high-quality series interconnection. Both, lower and higher laser fluences result in high series resistances due to incomplete phase transformation or damages of the back contact, respectively.

Variation of P2 series interconnects electrical conductivity in the CIGS solar cells by picosecond laser-induced modification

Cu-chalcopyrite based solar cells, such as Cu(In,Ga)Se2 (generally called CIGS) have been established as the most efficient thin-film technology in converting sunlight into electricity. High efficiency, flexibility and small weight make this technology attractive for future developments. Large scale production of these devices requires innovative technological solutions including the laser scribed monolithic interconnects. Laser scribing is needed to maintain module efficiency by dividing large scale device to smaller cells interconnected in series. Serious challenges in laser scribing technology have to be solved, including the laser induced thermal modification of the CIGS absorber layer. CIGS layer is thermally sensitive material, and laser modification can induce local structural changes and phase transitions to the metallic state. That is undesirable for the P3 scribing since superior isolating properties are needed. However, this effect can be used for the P2 process – interconnection of the adjacent cells. In this study, we investigated the picosecond laser modification of the CIGS active layer to form the series interconnect. The P2 laser process was optimized relying on the scribe electrical resistivity measurements with the best value of 3.5Ω·cm. The EDS analysis revealed the increase of Cu/(Ga+In) ratio in laser treated areas while Raman measurements indicated changes in main CIGS peak and the formation of the Cu-rich CuGaSe2 phase. Therefore, this resulted in a significant electrical conductivity increase in laser-treated areas which is acceptable for the cell serial interconnection.

Monolithic Interconnection of Silicon Based Thin-Film Solar Cells on Aluminium Substrates

Monolithic Interconnection of Silicon Based Thin-Film Solar Cells on Aluminium Substrates

Validation of monolithic interconnection conductivity in laser scribed CIGS thin-film solar cells

Over the last few years, thin-film based CIGS solar cell technology became even more attractive for producers and end-users. Implementation of laser based tools for module scribing still face serious challenges in terms of the laser damage to the CIGS active layer. Shunt formation near the scribing zone can lead to module efficiency losses, therefore optimization of laser based processes according to the cell electrical behavior is needed. We investigate two methods for cell parallel resistance measurement, including the linear laser scribing technique (LLST) and the nested circular laser scribing technique (NCLST). Also, we introduce simple cell conductance simulation. We found that for high conducting samples the series resistance of the top-contact layer plays significant role in the measurement of the structure parallel conductance, therefore careful measurement interpretation is needed. We also found, that he LLST technique provided more reliable measurement in these conditions. It was possible to optimize the scribing geometry and minimize measurement errors caused by the AZO film series resistance. The in-process laser-induced shunt evaluation is a useful method for scribing process optimization in the same material, although using both LLST and NCLST techniques, the fitting results are more reliable for low conducting samples.

Quadruple-junction solar cells and modules based on amorphous and microcrystalline silicon with high stable efficiencies

Quadruple junction solar cells and modules are presented, which consist of hydrogenated amorphous (a-Si:H) and microcrystalline silicon (µc-Si:H) in the a-Si:H/a-Si:H/µc-Si:H/µc-Si:H configuration. The highest measured conversion efficiency of a mini-module with an aperture area of 61.44 cm2 was 13.4% before and 12.0% after more than 1000 h of light soaking, respectively. In this paper, we discuss the advantages of the quadruple junction design over the common tandem design, which is ascribed mainly to the fact that the total absorber thickness can be increased while electronic properties and stability are maintained or even improved. The role of the µc-SiOx:H intermediate reflector is highlighted and an optimization of the doping concentration in this layer is presented. Furthermore, the advantage of the high maximum power voltage for the monolithic cell interconnection laser design of modules is shown.

Monolithically interconnected lamellar Cu(In,Ga)Se2 micro solar cells under full white light concentration

AbstractThin film solar cells already benefit from significant material and energy savings. By using photon management, the conversion efficiency and the power density can be enhanced further, including a reduction of material costs. In this work, micrometer‐sized Cu(In,Ga)Se2 (CIGS) thin film solar cells were investigated under concentrated white light illumination (1–50×). The cell design is based on industrially standardized, lamellar shaped solar cells with monolithic interconnects (P‐scribe). In order to characterize the shunt and serial resistance profiles and their impact on the device performance the cell width was reduced stepwise from 1900 to 200 µm and the P1‐scribe thickness was varied between 45 and 320 µm. The results are compared to macroscopic solar cells in standard geometry and dot‐shaped microcells with ring contacts. Under concentrated white light, the maximal conversion efficiency could be increased by more than 3.8% absolute for the lamellar microcells and more than 4.8% absolute in case of dot‐shaped microcells compared to their initial values at 1 sun illumination. The power density could be raised by a factor of 51 and 70, respectively. But apparently, the optimum concentration level and the improvement in performance strongly depend on the chosen cell geometry, the used contact method and the electrical material properties. It turns out, that the widely used industrial thin film solar cell design pattern cannot simply be adapted to prepare micro‐concentrator CIGS solar modules, without significant optimization. Based on the experimental and simulated results, modifications for the cell design are proposed. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Demonstration of the monolithic interconnection on CIS solar cells by picosecond laser structuring on 30 by 30 cm2 modules

AbstractIn this paper, we present the selective structuring of all three patterns (P1, P2 and P3) of a monolithic interconnection of CIS (Cu(In,Ga)(S,Se)2) thin film solar cells by picosecond laser pulses at a wavelength of 1064 nm. We show results for single pulse ablation threshold values and line scribing of molybdenum films on glass (P1), CIS on molybdenum (P2) and zinc oxide on CIS (P3). The purposes of these processes are the p‐type isolation (P1), cell interconnect (P2) and n‐type isolation (P3), which are required for complete cell architecture. The half micron thick molybdenum back electrode can be structured with a process speed of more than 15 m/s at about 15 W average power without detectable residues and damage by direct induced laser ablation from the back side (P1). The CIS layer can be structured selectively down to the molybdenum at process speeds up to 1 m/s at about 15 W average power, due to the precision of direct laser ablation in the ultrashort pulse regime (P2). The ZnO front electrode layer is separated by clean trenches with straight side walls at process speeds of up to 15 m/s at about 10 W average power, as a result of indirect induced laser ablation (P3). A validation of functionality of all processes is demonstrated on CIS solar cell modules (30 × 30 cm2). By replacing one state‐of‐the‐art process by a picosecond laser process at a time, solar efficiencies could be increased for P1 and P2 and stayed on a similar level for P3. After an optimization of the patterning processes in the R&amp;D pilot line of AVANCIS, we achieved a new record efficiency for an all‐laser‐patterned CIS solar module: 14.7% as best value for the aperture area efficiency of a 30 × 30 cm2 sized CIS module was reached. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Printed Monolithic Photovoltaic Interconnects

Monolithic interconnects in photovoltaic modules connect adjacent cells in series, and are typically formed sequentially involving multiple deposition &amp; scribing steps. Interconnect widths on order of about 500 &#x3BC;&#x3BC;m every 10 mm result in about 5 % dead area, which does not contribute to power generation in an interconnected solar panel. This work introduces an alternative interconnection method capable of producing interconnect widths less than 100 &#x3BC;m, which can be accomplished in a single pass after deposition of active layers and electrodes. This alternative method can be used for all types of thin film photovoltaics. Voltage addition using printed interconnects and ongoing efforts to optimize performance of modules with printed interconnect are discussed.

Ultra-short laser patterning of thin-film CIGS solar cells through glass substrate

A comparative analysis and a novel method involving a monolithic interconnection process (scribing) of creating thin film CIGS solar cells are presented based on experimental approaches and models. The boundaries of the delamination and ablation of molybdenum film in the P1 process of low-intensity picosecond (ps) laser irradiation through a substrate (substrate-side processing) are investigated. The observed ablation is explained in two steps based on the boundary formation observed and the deformation of the molybdenum layer resulting in the ablation of the delaminated layer. A novel idea of the P2 process through the substrate using a ps laser is then proposed and tested based on these explanations. The new P2 process can ablate nearly 2 microns of an absorber layer of a CIGS sample at a laser pulse energy of 5.3 μJ (with a 36 micron (1/e2) spot diameter) without signs of thermal effects while the conventional film-side processing for P2 requires 78 μJ and exhibits a typical laser induced heat-affected zone. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) observations show the advantages of the new process, which can prevent melting and other heat effects on the absorber and molybdenum layers.

Picosecond-laser structuring of amorphous-silicon thin-film solar modules

Laser scribing with nanosecond (ns) diode pumped solid-state laser sources is the industry standard in the fabrication of silicon-based thin-film photovoltaic (TFPV) modules. Reducing the interconnection area is one of the on-going challenges for the next generation of TFPV modules. In this regard, replacing ns laser sources by picosecond (ps) laser sources is one of the logical steps. Ps-laser pulses reduce the heat-affected zones compared to ns pulses, and thus enable a reduction of the interconnection zone. This work describes the substrate-side ablation of fluorine-doped tin oxide, amorphous silicon (a-Si:H) and a-Si:H with an aluminum layer on top, using a 10-ps laser with a wavelength of 1064 nm. The investigation of single-pulse ablation and trench scribing demonstrates that the complete monolithic interconnection can be achieved at the fundamental wavelength. In addition, the evaluation of the ablation efficiency shows that the best trench quality is achieved at the efficiency maximum.

Measurement of the Open-Circuit Voltage of Individual Subcells in a Dual-Junction Solar Cell

Dual-junction solar cells have the potential for higher conversion efficiencies than single junctions, thanks to a better utilization of the solar spectrum. However, their optimization is more complex because the monolithic interconnection makes the individual subcell electrical measurement difficult or impossible. This study presents a new technique to measure the open-circuit voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> of each individual subcell of a dual-junction device with a method that circumvents the need for completely selective illumination. The method is based on the description of the total device V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> as a function of two independent variables determined by two distinct illumination spectra. Without completely selective illumination, one can measure only a limited range of this function; however, an unambiguous extension gives values corresponding to the conditions of ideal selectivity, which correspond in turn to the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> of the individual subcells at 1-sun illumination. This method does not rely on any specific electrical or optical model of solar cell. Method is successfully tested with a simple measurement setup based on commercially available luminescence diodes and voltmeters in combination with specially prepared three-terminal dual-junction thin-film silicon solar cells.

Analysis of laser scribes at CIGS thin-film solar cells by localized electrical and optical measurements

Laser patterning of thin-film solar cells is essential to perform external serial and integrated monolithic interconnections for module application and has recently received increasing attention. Current investigations show, however, that the efficiency of thin-film Cu(In,Ga)Se2 (CIGS) modules is reduced due to laser scribing also with ultrashort laser pulses. Hence, to investigate the reasons of the laser-induced material modifications, thin-film CIGS solar cells were laser-scribed with femto- and picosecond laser pulses using different scribing procedures and laser processing parameters. Besides standard electrical current voltage (I–V) measurements, additional electrical and optical analysis were performed such as laser beam-induced current (LBIC), dark lock-in thermography (DLIT), and electroluminescence (EL) measurements to characterize and localize electrical losses due to material removal/modifications at the scribes that effecting the electrical solar cell properties. Both localized as well as distributed shunts were found at laser scribe edges whereas the laser spot intensity distribution affecting the shunt formation. Already laser irradiation below the ablation threshold of the TCO film causes material modification inside the thin film solar cell stack resulting in shunt formation as a result of materials melting near the TCO/CIGS interface that probably induces the damage of the pn-junction.