All solution roll-to-roll processed polymer solar cells free from indium-tin-oxide and vacuum coating steps

A roll-to-roll process enabling fabrication of polymer solar cells comprising five layers on flexible substrates is presented. The device geometry is inverted and allow for fabrication on both transparent and non-transparent flexible substrates. The process is illustrated in this work by formation of a bottom electrode comprising silver nanoparticles on a 130 micron thick polyethyleneternaphthalate (PEN) substrate. Subsequently an electron transporting layer of zinc oxide nanoparticles was applied from solution followed by an active layer of P3HT-PCBM and a hole transporting layer of PEDOT:PSS. These first four layers were applied by slot-die coating. The final electrode was applied by screen printing a grid structure that allowed for transmission of 80% of the light. The materials were patterned into stripes allowing for formation of a single cell device and serially connected modules comprising 2, 3 and 8 stripes. All five layers in the device were processed from solution in air and no vacuum steps were employed. An additional advantage is that the use of indium-tin-oxide (ITO) is avoided in this process. The devices were tested under simulated sunlight (1000 W m^?2, AM1.5G) and gave a typical performance 0.3% in terms of power conversion efficiency (PCE) for the active layer. The low PCE was due to poor transmission of light through the back electrode.     

Large area roll-to-roll sputtering of transparent ITO/Ag/ITO cathodes for flexible inverted organic solar cell modules

We fabricated solution-processed flexible inverted organic solar cell (IOSC) modules (10 cm × 10 cm) on roll-to-roll (RTR) sputtered ITO/Ag/ITO multilayer cathodes. By using a pilot-scale RTR sputtering system equipped with mid-range frequency power for dual ITO targets and direct current power for the Ag target, we were able to continuously deposit a high-quality ITO/Ag/ITO multilayer on PET substrate with a width of 700 mm and length of 20,000 mm as a function of Ag thickness. At the Ag thickness of 12 nm, the ITO/Ag/ITO multilayer had a very low sheet resistance of 3.03 Ohm/square and high transmittance of 88.17%, which are better values than those of amorphous ITO film. A strip-type ITO/Ag/ITO cathode was successfully patterned using a RTR wet etching process. Successful operation of flexible IOSC modules on RTR sputtered ITO/Ag/ITO cathodes indicate that the RTR sputtering technique is a promising coating process for fabrication of high-quality transparent and flexible cathodes and can advance the commercialization of cost-efficient flexible IOSCs.     

From Groundwork to Efficient Solar Cells: On the Importance of the Substrate Material in Co-Evaporated Perovskite Solar Cells

Vacuum-based deposition of optoelectronic thin films has a long-standing history. However, in the field of perovskite-based photovoltaics, these techniques are still not as advanced as their solution-based counterparts. Although high-efficiency vacuum-based perovskite solar cells reaching power conversion efficiencies (PCEs) above 20% are reported, the number of studies on the underlying physical and chemical mechanism of the co-evaporation of lead iodide and methylammonium iodide is low. In this study, the impact of one of the most crucial process parameters in vacuum processes—the substrate material—is studied. It is shown that not only the morphology of the co-evaporated perovskite thin films is significantly influenced by the surface polarity of the substrate material, but also the incorporation of the organic compound into the perovskite framework. Based on these studies, a selection guide for suitable substrate materials for efficient co-evaporated perovskite thin films is derived. This selection guide points out that the organic vacuum-processable hole transport material 2,2″,7,7″-tetra(N,N-di-p-tolyl)amino-9,9-spirobifluorene is an ideal candidate for the fabrication of efficient all-evaporated perovskite solar cells, demonstrating PCEs above 19%. Furthermore, building on the insights into the formation of the perovskite thin films on different substrate materials, a basic crystallization model for co-evaporated perovskite thin films is suggested.     

High Power Conversion Efficiency of 13.61% for 1 cm2 Flexible Polymer Solar Cells Based on Patternable and Mass-Producible Gravure-Printed Silver Nanowire Electrodes

With the aim of developing high-performance flexible polymer solar cells, the preparation of flexible transparent electrodes (FTEs) via a high-throughput gravure printing process is reported. By varying the blend ratio of the mixture solvent and the concentration of the silver nanowire (AgNW) inks, the surface tension, volatilization rate, and viscosity of the AgNW ink can be tuned to meet the requirements of gravure printing process. Following this method, uniformly printed AgNW films are prepared. Highly conductive FTEs with a sheet resistance of 10.8 Ω sq⁻¹ and a high transparency of 95.4% (excluded substrate) are achieved, which are comparable to those of indium tin oxide electrode. In comparison with the spin-coating process, the gravure printing process exhibits advantages of the ease of large-area fabrication and improved uniformity, which are attributed to better ink droplet distribution over the substrate. 0.04 cm² polymer solar cells based on gravure-printed AgNW electrodes with PM6:Y6 as the photoactive layer show the highest power conversion efficiency (PCE) of 15.28% with an average PCE of 14.75 ± 0.35%. Owing to the good uniformity of the gravure-printed AgNW electrode, the highest PCE of 13.61% is achieved for 1 cm² polymer solar cells based on the gravure-printed FTEs.     

Optical properties of thin‐film silicon solar cells with grating couplers

The effect of grating couplers on the optical properties of silicon thin‐film solar cells was studied by a comparison of experimental results with numerical simulations. The thin‐film solar cells studied are based on microcrystalline silicon (μc‐Si:H) absorber layers of thickness in the micrometer range. To investigate the light propagation in these cells, especially in the red wavelength region, three‐dimensional power loss profiles are simulated. The influence of different grating parametres—such as period size, groove height, and shape of the grating—was studied to gain more insight into the light propagation within thin‐film silicon solar cells and to determine an optimized light trapping scheme. The effect of the TCO front and TCO back side layer thickness was investigated. The calculated quantum efficiencies and short‐circuit current densities are in good agreement with the experimental data. The simulations predict further optimization criteria. Copyright © 2006 John Wiley & Sons, Ltd.     

Efficient and Flexible Thin Film Amorphous Silicon Solar Cells on Nanotextured Polymer Substrate Using Sol–gel Based Nanoimprinting Method

The mechanical flexibility of substrates and controllable nanostructures are two major considerations in designing high‐performance, flexible thin‐film solar cells. In this work, we proposed an approach to realize highly ordered metal oxide nanopatterns on polyimide (PI) substrate based on the sol‐gel chemistry and soft thermal nanoimprinting lithography. Thin‐film amorphous silicon (a‐Si:H) solar cells were subsequently constructed on the patterned PI flexible substrates. The periodic nanopatterns delivered broadband‐enhanced light absorption and quantum efficiency, as well as the eventual power conversion efficiency (PCE). The nanotextures also benefit for the device yield and mechanical flexibility, which experienced little efficiency drop even after 100,000 bending cycles. In addition, flexible, transparent nanocone films, obtained by a template process, were attached onto the patterned PI solar cells, serving as top anti‐reflection layers. The PCE performance with these dual‐interfacial patterns rose up to 8.17%, that is, it improved by 48.5% over the planar device. Although the work was conducted on a‐Si:H material, our proposed scheme can be extended to a variety of active materials for different optoelectronic applications.     

Progress in flexible perovskite solar cells with improved efficiency

Perovskite solar cell has emerged as a promising candidate in flexible electronics due to its high mechanical flexibil-ity,excellent optoelectronic properties,light weight and low cost.With the rapid development of the device structure and mater-ials processing,the flexible perovskite solar cells (FPSCs) deliver 21.1％ power conversion efficiency.This review introduces the latest developments in the efficiency and stability of FPSCs,including flexible substrates,carrier transport layers,perovskite films and electrodes.Some suggestions on how to further improve the efficiency,environmental and mechanical stability of FPSCs are provided.Specifically,we considered that to elevate the performance of FPSCs,it is crucial to substantially improve film quality of each functional layer,develop more boost encapsulation approach and explore flexible transparent electrodes with high conductivity,transmittance,low cost and expandable processability.     

Efficient flexible organic photovoltaics using silver nanowires and polymer based transparent electrodes

Planarization and filling voids between wires are key issues when using nanowire electrodes in flexible solar cells such as organic photovoltaics (OPV). For this purpose, we use poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) which leads to an electrically well connected silver nanowire (AgNW) network. Furthermore, the use of water based PEDOT: PSS leads to humidity assisted AgNW fusing, resulting in a maximum processing temperature of only 120 °C. OPV cells using this AgNW/PEDOT: PSS transparent electrodes exhibit power conversion efficiencies up to 7.15%. Moreover, OPV devices on PET substrates with an alumina encapsulation and barrier adhesive show excellent mechanical flexibility.     

Nano-gap micro-electro-mechanical bulk lateral resonators with high quality factors and low motional resistances on thin silicon-on-insulator

The design, fabrication and experimental investigation of 22–25 MHz fragmented-membrane MEM bulk lateral resonators (BLR) with 100 nm air-gaps on thin (1 and 6 μm) silicon-on-insulator (SOI) are reported. Quality factors as high as 120,000 and motional resistances of as little as 60 kΩ are measured under vacuum at room temperature, with 12 V DC bias and low AC power. The temperature influence on the resonance frequency and quality factor is studied and discussed between 80 K and 320 K. Significant quality factor increase and motional resistance reduction are reported at cryogenic temperature. The paper shows that high-quality factor MEM resonators can be integrated on partially depleted thin SOI, which can be a substrate of choice for the fabrication of future integrated hybrid MEMS–CMOS integrated circuits for communication applications.     

Influence of flexible substrates on inverted organic solar cells using sputtered ZnO as cathode interfacial layer

Zinc oxide (ZnO) has recently shown to be of considerable interest for the development of interfacial buffer layers in inverted organic solar cells (OSCs). High quality ZnO thin films can indeed be prepared on large-area ITO-coated flexible substrates, using low temperature deposition techniques such as sputtering, a compatible technique with roll-to-roll process. However, further studies are still needed for a better understanding of the influence of the flexible substrate properties on the photovoltaic performances of those devices. In this work, ZnO films have been sputtered on ITO-coated flexible (PEN) substrates and annealed at different temperatures. The role of the surface morphology and the crystalline quality of ZnO films has been investigated. In the window of flexible compatible process, we found that moderate annealing temperatures of ZnO (?180 °C) lead to improved structural properties and performances. Interestingly, we achieve optimal performances for an annealing temperature of 160 °C, resulting in power conversion efficiency (PCE) equivalent to the highest performances usually achieved on rigid cells.     

Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors

The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S–D) electrodes. The S–D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9 Ω/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current–voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine exhibits a high transmission rate of ∼75% in the range of visible light from 400 to 550 nm, a threshold voltage of −6.0 V, an on–off drain current ratio of 8.4 × 10⁵, and a field-effect mobility of 1.71 cm²/V s, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers.     

8.2% pure selenide kesterite thin‐film solar cells from large‐area electrodeposited precursors

Cu₂ZnSnSe₄ solar cell absorbers are synthesized by large‐area electrodeposition of metal stack precursors followed by selenization. A champion solar cell exhibits 8.2% power conversion efficiency, a new record for Cu₂ZnSnSe₄ solar cells prepared from electrodeposited metallic precursors. Significant improvements of device performance are achieved by the application of two etching procedures and buffer layer optimization. These results validate electrodeposition as a credible alternative to vacuum processes (sputtering, co‐evaporation) for earth‐abundant thin‐film solar cell fabrication at low cost. Copyright © 2015 John Wiley & Sons, Ltd.     

Flexible CdTe solar cells on polymer films

Lightweight and flexible CdTe/CdS solar cells on polyimide films have been developed in a ‘superstrate configuration’ where the light is absorbed in CdTe after passing through the polyimide substrate. The average optical transmission of the approximately 10‐μm‐thin spin‐coated polyimide substrate layer is more than ∼75% for wavelengths above 550 nm. RF magnetron sputtering was used to grow transparent conducting ZnO:Al layers on polyimide films. CdTe/CdS layers were grown by evaporation of compounds, and a CdCl₂ annealing treatment was applied for the recrystallization and junction activation. Solar cells of 8·6% efficiency with V_oc = 763 mV, I_sc = 20·3 mA/cm² and FF = 55·7% were obtained. Copyright © 2001 John Wiley & Sons, Ltd.     

Efficient and Thermally Stable Wide Bandgap Perovskite Solar Cells by Dual-Source Vacuum Deposition

Wide bandgap perovskites are being widely studied in view of their potential applications in tandem devices and other semitransparent photovoltaics. Vacuum deposition of perovskite thin films is advantageous as it allows the fabrication of multilayer devices, fine control over thickness and purity, and it can be upscaled to meet production needs. However, the vacuum processing of multicomponent perovskites (typically used to achieve wide bandgaps) is not straightforward, because one needs to simultaneously control several thermal sources during the deposition. Here a simplified dual-source vacuum deposition method to obtain wide bandgap perovskite films is shown. The solar cells obtained with these materials have similar or even larger efficiency as those including multiple A-cations, but are much more thermally stable, up to 3500 h at 85 °C for a perovskite with a bandgap of 1.64 eV. With optimized thickness, record efficiency of >19% and semitransparent devices with stabilized power output in excess of 17% are achieved.     

Flexible High-Performance Photovoltaic Devices based on 2D MoS2 Diodes with Geometrically Asymmetric Contact Areas

Optoelectronic performance of 2D transition metal dichalcogenides (TMDs)-based solar cells and self-powered photodetectors remain limited due to fabrication challenges, such as difficulty in doping TMDs to form p–n junctions. Herein, MoS₂ diodes based on geometrically asymmetric contact areas are shown to achieve a high current rectification ratio of ≈10⁵, facilitating efficient photovoltaic charge collection. Under solar illumination, the device demonstrates a high open-circuit voltage (V_oc) of 430 mV and a short-circuit current density (J_sc) of −13.42 mA cm⁻², resulting in a high photovoltaic power conversion efficiency (PCE) of 3.16%, the highest reported for a lateral 2D solar cell. The diodes also show a high photoresponsivity of 490.3 mA W⁻¹, and a large photo detectivity of 4.05 × 10¹⁰ Jones, along with a fast response time of 0.8 ms under 450 nm wavelength at zero bias for self-powered photodetection applications. The device transferred on a flexible substrate shows a high photocurrent and PCE retentions of 94.4%, and 88.2% after 5000 bending cycles at a bending radius of 1.5 cm, respectively, demonstrating robustness for flexible optoelectronic applications. The simple fabrication process, superior photovoltaic properties, and high flexibility suggests that the geometrically asymmetric MoS₂ device architecture is an excellent candidate for flexible photovoltaic and optoelectronic applications.     

Photovoltaic performance of Organic Photovoltaics for indoor energy harvester

Wireless sensor network becomes widespread into home and offices to keep them comfort and save the energy. The battery-less wireless sensor nodes need the high performance indoor solar cells for stable and sustainable operation. Organic Photovoltaics (OPV) has great indoor photovoltaic performance because ultra-thin organic layer has strong absorption against the UV–visible spectrum that is good spectral matching with indoor lightings. In this study, OPV module has 8 cells in series and same size as the conventional amorphous silicon solar cells (a-Si) for indoor light harvesting. OPV and a-Si are measured their photovoltaic performance under the fluorescent light and demonstrated for energy harvester of wireless sensor network. The output power of OPV and a-Si is 43.4 μW cm⁻² and 28.5 μW cm⁻² at fluorescent light 1000lux respectively. The data transmission rate of the wireless sensor node driven by OPV is 30–40% improved under the dim light condition compared to a-Si.     

Using the acetylacetonates of zinc and aluminium for the Metalorganic Chemical Vapour Deposition of aluminium doped zinc oxide films

Metalorganic Chemical Vapour Deposition is a promising method for the growth of thin aluminium doped zinc oxide films (ZnO:Al), a material with potential application as transparent conducting oxide (TCO), e.g. for the use as front electrode in solar cells. For the low-cost deposition, the choice of the precursors is extremely important. Here we present the deposition of quite homogeneous films from the acetylacetonates of zinc and aluminium that are rather cheap, commercially available and easy to handle. A user-made CVD-reactor activating the deposition process by the light of halogen lamps was used for film deposition. Well-ordered films with an aluminium content between 0 and 8% were grown on borosilicate glass and Si(100). On both types of substrate, the films are crystalline and show a preferred orientation along the (002)-direction. The 0.3 to 0.5 μm thick films are highly transparent in the visible region. The best films show a low electric resistivity between 2.4 and 8 mΩ cm.     

Tailoring Multifunctional Self-Assembled Hole Transporting Molecules for Highly Efficient and Stable Inverted Perovskite Solar Cells

The self-assembled hole transporting molecules (SAHTMs) bearing anchoring groups have been established as the hole transporting layers (HTLs) for highly efficient p–i–n perovskite solar cells (PSCs), yet their stability and engineering at the molecular level remain challenging. A topological design of highly anisotropic aligned SAHTM-based HTLs for operationally stable PSCs that exhibit exceptional solar-to-electric power conversion efficiencies (PCEs) is demonstrated. The judiciously designed multifunctional self-assembled molecules comprise the donor–acceptor subunit for hole transporting and the phosphonic acid group for anchoring, realizing face-on π-stacking parallel to the transparent conductive oxide substrate. The high affinity of SAHTMs to the multi-crystalline perovskite thin film benefits passivating the perovskite buried interface, strengthening interfacial contact while facilitating interfacial hole transfer. Consequently, highly efficient p–i–n PSC devices are obtained with a champion PCE of 23.24% and outstanding operational stability toward various environmental factors including long-term full sunlight soaking at evaluated temperatures. Perovskite solar modules with a champion efficiency approaching 20% are also fabricated for an active device area above 17 cm².     

Flexible and Semi‐Transparent Ultra‐Thin CIGSe Solar Cells Prepared on Ultra‐Thin Glass Substrate: A Key to Flexible Bifacial Photovoltaic Applications

For applications to semi‐transparent and/or bifacial solar cells in building‐integrated photovoltaics and building‐applied photovoltaics, studies are underway to reduce the processing cost and time by decreasing the thickness of Cu(In_1?x,Ga_x)Se₂ (CIGSe) absorber to the ultra‐thin scale (≤500 nm). To dynamically and affordably meet the growing demand for electric power, daylighting, and architectural aesthetics of buildings in urban area, flexible semi‐transparent ultra‐thin (F‐STUT) CIGSe solar cells are proposed on flexible ultra‐thin glass (UTG) and compared with rigid semi‐transparent ultra‐thin (STUT) CIGSe solar cells fabricated on soda‐lime glass (SLG). At all the tested deposition temperatures of CIGSe, the F‐STUT CIGSe solar cells exhibit superior performance compared to the rigid STUT CIGSe solar cells. Furthermore, through realistic measurement under ≈1.3‐sun illumination, maximum bifacial power conversion efficiency of 11.90% and 13.23% are obtained for SLG and UTG, respectively. The major advantages of using UTG instead of SLG are not only the intrinsic characteristics of UTG, such as flexibility and high transmittance, but also collateral benefits such as the larger CIGSe grain size at the deposition temperature, better CIGSe crystalline quality, more precise controllability of the alkali element, and reduced thickness of the interfacial GaO_x layer, which enhance the photovoltaic parameters.     

Super‐ and Ultrathin Organic Field‐Effect Transistors: from Flexibility to Super‐ and Ultraflexibility

Physically flexible electronics offer a wide range of benefits, including the development of next‐generation consumer electronics and healthcare products. The advancement of physical flexibility, typically achieved by the reduction of the total device thickness, including substrates and encapsulation layers, shows great promise for skin‐laminated electronics. Organic electronics—devices relying on carbon‐based materials—offer many advantages over their inorganic counterparts, including the following: significantly lower fabrication temperatures resulting in alternative fabrication techniques, including inkjet and roll‐to‐roll printing, enabling low‐cost and large‐area fabrication; biocompatibility; and spectacular physical flexibility. This article presents a review, spanning the last two decades, of organic field‐effect transistors with the total thickness of just a few microns as well as devices demonstrated in this decade with a total thickness of few hundred of nanometers. A handful of demonstrations of other organic electronic thin film devices are also presented.