From cells to laminate: probing and modeling residual stress evolution in thin silicon photovoltaic modules using synchrotron X‐ray micro‐diffraction experiments and finite element simulations

Fracture of silicon crystalline solar cells has recently been observed in increasing percentages especially in solar photovoltaic (PV) modules involving thinner silicon solar cells (<200 μm). Many failures due to fracture have been reported from the field because of environmental loading (snow, wind, etc.) as well as mishandling of the solar PV modules (during installation, maintenance, etc.). However, a significantly higher number of failures have also been reported during module encapsulation (lamination) indicating high residual stress in the modules and thus more prone to cell cracking. We report here, through the use of synchrotron X‐ray submicron diffraction coupled with physics‐based finite element modeling, the complete residual stress evolution in mono‐crystalline silicon solar cells during PV module integration process. For the first time, we unravel the reason for the high stress and cracking of silicon cells near soldered inter‐connects. Our experiments revealed a significant increase of residual stress in the silicon cell near the solder joint after lamination. Moreover, our finite element simulations show that this increase of stress during lamination is a result of highly localized bending of the cell near the soldered inter‐connects. Further, the synchrotron X‐ray submicron diffraction has proven to be a very effective way to quantitatively probe mechanical stress in encapsulated silicon solar cells. Thus, this technique has ultimately enabled these findings leading to the enlightening of the role of soldering and encapsulation processes on the cell residual stress. This model can be further used to suggest methodologies that could lead to lower stress in encapsulated silicon solar cells, which are the subjects of our continued investigations. Copyright © 2017 John Wiley & Sons, Ltd.     

Synergistic Roll‐to‐Roll Transfer and Doping of CVD‐Graphene Using Parylene for Ambient‐Stable and Ultra‐Lightweight Photovoltaics

A roll‐to‐roll (R2R) transfer technique is employed to improve the electrical properties of transferred graphene on flexible substrates using parylene as an interfacial layer. A layer of parylene is deposited on graphene/copper (Cu) foils grown by chemical vapor deposition and are laminated onto ethylene vinyl acetate (EVA)/poly(ethylene terephthalate). Then, the samples are delaminated from the Cu using an electrochemical transfer process, resulting in flexible and conductive substrates with sheet resistances of below 300 Ω sq^?1, which is significantly better (fourfold) than the sample transferred by R2R without parylene (1200 Ω sq^?1). The characterization results indicate that parylene C and D dope graphene due to the presence of chlorine atoms in their structure, resulting in higher carrier density and thus lower sheet resistance. Density functional theory calculations reveal that the binding energy between parylene and graphene is stronger than that of EVA and graphene, which may lead to less tear in graphene during the R2R transfer. Finally, organic solar cells are fabricated on the ultrathin and flexible parylene/graphene substrates and an ultra‐lightweight device is achieved with a power conversion efficiency of 5.86%. Additionally, the device shows a high power per weight of 6.46 W g^?1 with superior air stability.     

Shunting problems due to sub‐micron pinholes in evaporated solid‐phase crystallised poly‐Si thin‐film solar cells on glass

Recent progress in the metallisation of poly‐silicon thin‐film solar cells on glass, created by solid phase crystallisation (SPC) of evaporated amorphous silicon (EVA), revealed that shunting through sub‐micron holes (density 100–200 mm⁻²) in the films causes severe shunting problems when the air‐side metal contact is deposited onto these diodes, by creating effective shunting paths between the two highly doped layers of EVA cells. We present evidence of these pinholes by optical transmission and focussed ion beam (FIB) microscopic images and confirm the point‐like pinhole shunts using lock‐in thermographic images. The latter revealed that the Al rear electrode induces strong ohmic shunts below the grid lines and a high density of weak non‐linear shunts away from the grid lines. Two distinctly different approaches are shown to reduce the shunting problem to a negligible level: (i) to contact only a small fraction of the rear Si surface via a point contacting scheme, whereby the metal layer needs to be thin (<1 µm) and the fractional area coverage small (<5%), and (ii) to deposit line contacts in a bifacial interdigitated scheme, whereby a thick layer of metal is deposited followed by a wet‐chemical etching step that effectively reduces shunting by preferentially etching away the shunting paths. Test devices with an area of 1 cm² achieve pseudo fill factors ( pFF ) of above 75% and diode ideality factors of below 1·3, demonstrating that the proposed methods are well suited for the metallisation of the rear surface of EVA solar cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Interconnection of busbar‐free back contacted solar cells by laser welding

We are presenting the module integration of busbar‐free back‐junction back‐contact (BJBC) solar cells. Our proof‐of‐concept module has a fill factor of 80.5% and a conversion efficiency on the designated area of 22.1% prior to lamination. A pulsed laser welds the Al metallization of the solar cells to an Al foil carried by a transparent substrate. The weld spots electrically contact each individual finger to the Al foil, which serves as interconnect between different cells. We produce a proof‐of‐concept module using busbar‐free cell strips of 25 × 125 mm². These are obtained by laser‐dicing of a 125 × 125 mm² BJBC solar cell. The fill factor of this module is increased by 3.5% absolute compared with the initial cell before laser‐dicing. This is achieved mainly by omitting the busbars and reduction of the finger length. The improvement of the module fill factor results in an increase in the module performance of 0.9% absolute before lamination in comparison with the efficiency of the initial 125 × 125 mm² BJBC solar cell. Hence, this interconnection scheme enables the transfer of high cell efficiencies to the module. Copyright © 2014 John Wiley & Sons, Ltd.     

Development of back‐junction back‐contact silicon solar cells based on industrial processes

We have presented simplified industrial processes to fabricate high performance back‐junction back‐contact (BJBC) silicon solar cells. Good optical surface structures (solar averaged reflectance 2.5%) and high implied open‐circuit voltage (0.695 V) have been realized in the BJBC cell precursors through wet chemical processing, co‐diffusion, P ion implantation and annealing oxidation, as well as laser patterning and plasma enhanced chemical vapour deposition passivation processes. We have achieved a certified high efficiency of close to 22% on BJBC silicon solar cells with the size of 4.04 cm² by using screen printing and co‐firing technologies. The manufacturing process flow further successfully yields efficiency of around 21% BJBC silicon solar cells with enlarged sizes of 6 × 6 cm². The present work has demonstrated that the commercialization of low‐cost and high‐efficiency BJBC solar cells is possible because we have used processes compatible with existing production lines. Copyright © 2017 John Wiley & Sons, Ltd.     

Industrially feasible,dopant‐free,carrier‐selective contacts for high‐efficiency silicon solar cells

Dopant‐free, carrier‐selective contacts (CSCs) on high efficiency silicon solar cells combine ease of deposition with potential optical benefits. Electron‐selective titanium dioxide (TiO₂) contacts, one of the most promising dopant‐free CSC technologies, have been successfully implemented into silicon solar cells with an efficiency over 21%. Here, we report further progress of TiO₂ contacts for silicon solar cells and present an assessment of their industrial feasibility. With improved TiO₂ contact quality and cell processing, a remarkable efficiency of 22.1% has been achieved using an n‐type silicon solar cell featuring a full‐area TiO₂ contact. Next, we demonstrate the compatibility of TiO₂ contacts with an industrial contact‐firing process, its low performance sensitivity to the wafer resistivity, its applicability to ultrathin substrates as well as its long‐term stability. Our findings underscore the great appeal of TiO₂ contacts for industrial implementation with their combination of high efficiency with robust fabrication at low cost. Copyright © 2017 John Wiley & Sons, Ltd.     

Reducing shadowing losses with femtosecond‐laser‐written deflective optical elements in the bulk of EVA encapsulation

A new approach on decreasing the optical shadowing of the solar cell grid fingers is presented. The approach relies on a local change of the optical properties in the bulk of the photovoltaic module encapsulation material ethylene vinyl acetate (EVA). In particular, scattering and diffractive optical elements are locally generated within the volume of cross‐linked EVA encapsulation material by applying a femtosecond‐laser‐writing process. When these optical elements are located above the metal grid fingers, the optical shadowing of these grid fingers can be decreased. In an experimental proof of concept, the optical performance of this approach is demonstrated. The best results obtained so far indicate a decrease in optical shadowing by 17%. The material characteristics of the volume optics were investigated by applying confocal Raman microscopic characterisation, which indicates that the EVA material partially degraded upon the impact of the laser beam and is partly carbonised. Supplementary optical simulations show that the light deflection is caused by diffraction. However, parasitic absorption substantially deteriorates the optical performance of the deflective volume optics. Copyright © 2014 John Wiley & Sons, Ltd.     

Molecular‐Level Switching of Polymer/Nanocrystal Non‐Covalent Interactions and Application in Hybrid Solar Cells

Hybrid composites obtained upon blending conjugated polymers and colloidal semiconductor nanocrystals are regarded as attractive photo­active materials for optoelectronic applications. Here it is demonstrated that tailoring nanocrystal surface chemistry permits to control non‐covalent and electronic interactions between organic and inorganic components. The pending moieties of organic ligands at the nanocrystal surface are shown to not merely confer colloidal stability while hindering charge separation and transport, but drastically impact morphology of hybrid composites during formation from blend solutions. The relevance of this approach to photovoltaic applications is demonstrated for composites based on poly(3‐hexylthiophene) and lead sulfide nanocrystals, considered as inadequate until this report, which enable the fabrication of hybrid solar cells displaying a power conversion efficiency that reaches 3%. By investigating (quasi)steady‐state and time‐resolved photo‐induced processes in the nanocomposites and their constituents, it is ascertained that electron transfer occurs at the hybrid interface yielding long‐lived separated charge carriers, whereas interfacial hole transfer appears hindered. Here a reliable alternative aiming to gain control over macroscopic optoelectronic properties of polymer/nanocrystal composites by mediating their non‐covalent interactions via ligands' pending moieties is provided, thus opening new possibilities towards efficient solution‐processed hybrid solar cells.     

Silicon solar cells based on all‐laser‐transferred contacts

Crystalline silicon solar cells based on all‐laser‐transferred contacts (ALTC) have been fabricated with both front and rear metallization achieved through laser induced forward transferring. Both the front and rear contacts were laser‐transferred from a glass slide coated with a metal layer to the silicon substrate already processed with emitter formation, surface passivation, and antireflection coating. Ohmic contacts were achieved after this laser transferring. The ALTC solar cells were fabricated on chemically textured p‐type Cz silicon wafers. An initial conversion efficiency of over 15% was achieved on a simple cell structure with full‐area emitter. Further improvements are expected with optimized laser transferring conditions, front grid pattern design, and surface passivation. The ALTC process demonstrates the advantage of laser processing in simplifying the solar cell fabrication by a one‐step metal transferring and firing process. Copyright © 2013 John Wiley & Sons, Ltd.     

Laser ablation of SiO2 for locally contacted Si solar cells with ultra‐short pulses

We apply ultra‐short pulse laser ablation to create local contact openings in thermally grown passivating SiO₂ layers. This technique can be used for locally contacting oxide passivated Si solar cells. We use an industrially feasible laser with a pulse duration of τ_pulse ∼ 10 ps. The specific contact resistance that we reach with evaporated aluminium on a 100 Ω/sq and P‐diffused emitter is in the range of 0·3–1 mΩ cm². Ultra‐short pulse laser ablation is sufficiently damage free to abandon wet chemical etching after ablation. We measure an emitter saturation current density of J_0e = (6·2 ± 1·6) × 10⁻¹³ A/cm² on the laser‐treated areas after a selective emitter diffusion with R_sheet ∼ 20 Ω/sq into the ablated area; a value that is as low as that of reference samples that have the SiO₂ layer removed by HF‐etching. Thus, laser ablation of dielectrics with pulse durations of about 10 ps is well suited to fabricate high‐efficiency Si solar cells. Copyright © 2007 John Wiley & Sons, Ltd.     

FAPbI3‐Based Perovskite Solar Cells Employing Hexyl‐Based Ionic Liquid with an Efficiency Over 20% and Excellent Long‐Term Stability

Formamidinium lead triiodide (FAPbI₃)‐based perovskite materials are of interest for photovoltaics in view of their close‐to‐ideal bandgap, allowing absorption of photons over a broad solar spectrum. However, FAPbI₃‐based materials suffer from a notorious phase transition from the photoactive black phase (α‐FAPbI₃) to nonperovskite yellow phase (δ‐FAPbI₃) under ambient conditions. This transition dramatically reduces light absorbtion, thus, degrading the photovoltaic performance and stability of ensuring solar cells. In this study, 1‐hexyl‐3‐methylimidazolium iodide (HMII) ionic liquid (IL) is employed as an additive for the first time in FAPbI₃ perovskite to overcome the above‐mentioned issues. HMII incorporation facilitates the grain coarsening of FAPbI₃ crystal owing to its high‐polarity and high‐boiling point, which yields liquid domains between neighboring grains to reduce the activation energy of the grain‐boundary migration. As a result, the FAPbI₃ active layer exhibits micron‐sized grains with substantially suppressed parasitic traps with an Urbach energy reduced by 2 meV. Hence, the resulting perovskite solar cell achieves an efficiency of 20.6% with notable increase in open circuit voltage (V_OC) of 80 mV compared with HMII‐free cells (17.1%). More importantly, the HMII‐doped FAPbI₃‐based cells show a striking enhancement in shelf‐stability under high humidity and thermal stress, retaining >80% of their initial efficiencies at 60 ± 10% relative humidity and ≈95% at 65 °C.     

Investigation of laser ablation on boron emitters for n‐type rear‐junction PERT type silicon wafer solar cells

n‐type silicon wafer solar cells are receiving increasing attention for industrial application in recent years, such as the n‐type rear‐junction Passivated Emitter Rear Totally‐diffused (PERT) solar cells. One of the main challenges in fabricating the n‐PERT solar cells is the opening of the rear dielectric for localized contacts. In this work laser ablation is applied to locally ablate the rear dielectric. We investigate the laser damage to the emitter at the laser‐ablated regions using the emitter saturation current density, J_0e,laser, extracted by two approaches. J_0e,laser is observed to be injection dependent due to high J₀₂ recombination caused by laser damage to the space charge region. By using the optimized laser ablation parameters, n‐PERT solar cells with an efficiency of up to 21.0% are realized. Copyright © 2015 John Wiley & Sons, Ltd.     

Water‐Resistant and Flexible Perovskite Solar Cells via a Glued Interfacial Layer

Perovskite solar cells (PVSCs) are promising photovoltaic technologies for realizing power sources with outstanding power conversion efficiency (PCE) and low‐cost properties. However, the extraordinary photovoltaic performance can be maximized only if an extremely stabilized device structure is developed. Here, a novel glued poly(ethylene‐co‐vinyl acetate) (EVA) interfacial layer is introduced to fabricate highly efficient and stable PVSCs with excellent waterproofness and flexibility. This strategy can effectively passivate the perovskite surface, reduce defect density, and balance charge transfer, which leads to a champion PCE of 19.31% for a 0.1 cm² device and 11.73% for a 25 cm² solar module. More importantly, the formation of a glued EVA thin layer on the surface of perovskite can inhibit ionic migration to the Ag electrode, form favorable interfacial contact and adhesive interaction with the perovskite/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester to sustain mechanical bending, and produce significant waterproofness from moisture invasion, thus facilitating improvement in the operational stability of the PVSCs. The EVA‐treated PVSCs exhibit superior PCE values of 15.12% for a flexible device (0.1 cm²) and 8.95% for a flexible module (25 cm²), as well as over 85% retention after 5000 bending cycles, which opens up a new strategy for the practical application of PVSCs in portable and wearable electronics.     

Self‐Doping Fullerene Electrolyte‐Based Electron Transport Layer for All‐Room‐Temperature‐Processed High‐Performance Flexible Polymer Solar Cells

To achieve high‐performance large‐area flexible polymer solar cells (PSCs), one of the challenges is to develop new interface materials that possess a thermal‐annealing‐free process and thickness‐insensitive photovoltaic properties. Here, an n‐type self‐doping fullerene electrolyte, named PCBB‐3N‐3I, is developed as electron transporting layer (ETL) for the application in PSCs. PCBB‐3N‐3I ETL can be processed at room temperature, and shows excellent orthogonal solvent processability, substantially improved conductivity, and appropriate energy levels. PCBB‐3N‐3I ETL also functions as light‐harvesting acceptor in a bilayer solar cell, contributing to the overall device performance. As a result, the PCBB‐3N‐3I ETL‐based inverted PSCs with a PTB7‐Th:PC₇₁BM photoactive layer demonstrate an enhanced power conversion efficiency (PCE) of 10.62% for rigid and 10.04% for flexible devices. Moreover, the device avoids a thermal annealing process and the photovoltaic properties are insensitive to the thickness of PCBB‐3N‐3I ETL, yielding a PCE of 9.32% for the device with thick PCBB‐3N‐3I ETL (61 nm). To the best of one's knowledge, the above performance yields the highest efficiencies for the flexible PSCs and thick ETL‐based PSCs reported so far. Importantly, the flexible PSCs with PCBB‐3N‐3I ETL also show robust bending durability that could pave the way for the future development of high‐performance flexible solar cells.     

Reducible‐Shell‐Derived Pure‐Copper‐Nanowire Network and Its Application to Transparent Conducting Electrodes

The concept of using core Cu nanowires (CuNWs) that are conformally encapsulated by a reducible fugitive material for transparent conducting electrodes (TCEs) with high oxidation stability is presented. By the chemical reaction of an acid with surface oxide and hydroxide, a uniform surface shell layer is readily obtained on each CuNW upon adding lactic acid to the CuNW dispersion. The Cu lactate shell prevents the core CuNW from oxidizing during storage and film formation, enabling the core Cu nanowires to maintain their characteristic optoelectronic properties. Through simple thermal annealing under a nitrogen atmosphere, the Cu lactate shell is easily decomposed to expose the underlying pure Cu, providing an effective way to produce a pure‐CuNW‐network TCE with a sheet resistance of 19.8 Ω sq^?1 and an optical transmittance of 85.5% at 550 nm. The application of the CuNW‐based TCE to the transparent top electrode in organometallic halide perovskite solar cells is further demonstrated for the first time, yielding a power‐conversion efficiency 9.88% as compared to that of 13.39% for conventional perovskite solar cells with an indium–tin‐oxide electrode. This study proposes the high feasibility of these CuNWs as a vacuum‐free and noble‐metal‐free transparent‐window electrode in perovskite solar cells.     

Self‐Powered Electronics by Integration of Flexible Solid‐State Graphene‐Based Supercapacitors with High Performance Perovskite Hybrid Solar Cells

To develop high‐capacitance flexible solid‐state supercapacitors and explore its application in self‐powered electronics is one of ongoing research topics. In this study, self‐stacked solvated graphene (SSG) films are reported that have been prepared by a facile vacuum filtration method as the free‐standing electrode for flexible solid‐state supercapacitors. The highly hydrated SSG films have low mass loading, high flexibility, and high electrical conductivity. The flexible solid‐state supercapacitors based on SSG films exhibit excellent capacitive characteristics with a high gravimetric specific capacitance of 245 F g^?1 and good cycling stability of 10 000 cycles. Furthermore, the flexible solid‐state supercapacitors are integrated with high performance perovskite hybrid solar cells (pero‐HSCs) to build self‐powered electronics. It is found that the solid‐state supercapacitors can be charged by pero‐HSCs and discharged from 0.75 V. These results demonstrate that the self‐powered electronics by integration of the flexible solid‐state supercapacitors with pero‐HSCs have great potential applications in storage of solar energy and in flexible electronics, such as portable and wearable personal devices.     

Towards realization of a large‐area light‐emitting diode‐based solar simulator

Solar simulators based on light‐emitting diodes (LEDs) have shown great promise as alternative light sources for indoor testing of photovoltaic cells with certain characteristics that make them superior to the traditional solar simulators. However, large‐area uniform illumination more suitable for larger cells and module measurements still remain a challenge today. In this paper, we discuss the development and fabrication of a scalable large‐area LED‐based solar simulator that consists of multiple tapered light guides. We demonstrate fine intermixing of many LED light rays and power delivery in the form of a synthesized AM 1.5 spectrum over an area of 25 cm × 50 cm with better than 10% spatial nonuniformity. We present the spectral output, the spatial uniformity, and the temporal stability of the simulator in both the constant current mode and the pulsed‐mode LED operation, and compare our data with the International Electrotechnical Commission standards on solar simulators for class rating. Although the light intensity with our current design and settings falls short of the standard solar AM 1.5 intensity, this design and further improvements open up the possibility of achieving large‐area, high‐power indoor solar simulation with various desired spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

SHORT COMMUNICATION: Thin‐film free‐standing monocrystalline si solar cells with heterojunction emitter

We propose a novel approach to thin‐film silicon solar cells, namely the freestanding monocrystalline silicon layer transfer process with heterojunction emitter (FMS‐HJ). High crystallographic quality mono‐Si films were deposited on freestanding porous silicon (PS) films by chemical vapor deposition (CVD). These free‐standing mono‐Si (FMS) films were processed into solar cells by creating a‐a‐Si/c‐Si heterojunction. In our preliminary experiments a thin‐film FMS‐HJ solar cell with 9.6% efficiency was realized in a 20‐μμm‐thin active layer. Copyright © 2005 John Wiley & Sons, Ltd.     

Evidence for the Band‐Edge Exciton of CuInS2 Nanocrystals Enables Record Efficient Large‐Area Luminescent Solar Concentrators

Ternary I‐III‐VI₂ nanocrystals (NCs), such as CuInS₂, are receiving attention as heavy‐metals‐free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio‐imaging. The origin of the optical properties of CuInS₂ NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes‐shifted and long‐lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defect‐free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low‐bandgap NCs would be accompanied by increased re‐absorption losses. Here, by studying stoichiometric CuInS₂ NCs, it is revealed for the first time the spectroscopic signatures predicted for the free band‐edge exciton, thus supporting the VB‐structure model. At very low temperatures, the NCs also show dark‐state emission likely originating from enhanced electron‐hole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo ray‐tracing simulations. Based on the emerging device design guidelines, optical‐grade large‐area (30×30 cm²) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for large‐area devices.