Dual Effect of ITO‐Interlayer on Inverted Top‐Illuminated Polymer Solar Cells: Wetting of Polyelectrolyte and Tuning of Cavity

Flexible inverted top‐illuminated polymer solar cells (IT‐PSCs) are fabricated by wetting of polyelectrolyte and designing a microcavity structure by laying an indium‐tin‐oxide (ITO) interlayer on top of an Ag reflector. The ITO‐coated Ag makes the surface hydrophilic, thereby improving wettability of polyethyleneimine (PEIE). This increased wettability of PEIE yields a reflective cathode with low work function of 3.73 eV. The ITO layer also tunes the light absorption spectrum in the active layer. Finite‐domain time‐difference simulation provides evidence that the ITO layer played a role in both the shift in resonant wavelength in the microcavity and confinement of the electric field to the active layer. Time‐dependent simulation suggests that the time to reach steady‐state light absorption is longer (6.6 fs) when a microcavity is present than when it is not present (3.8 fs); i.e., the microcavity increases light absorption in the active layer. The designed IT‐PSCs show a maximum photo‐conversion efficiency of 6.4% on plastic film and 6.1% on opaque copper foil; these are the highest values obtained by top‐illuminated PSCs on a metallic substrate. The IT‐PSCs have excellent mechanical flexibility and more stable in air than conventional normal structured devices.     

Realizing the Potential of ZnO with Alternative Non‐Metallic Co‐Dopants as Electrode Materials for Small Molecule Optoelectronic Devices

High performance indium tin oxide (ITO)‐free small molecule organic solar cells and organic light‐emitting diodes (OLEDs) are demonstrated using optimized ZnO electrodes with alternative non‐metallic co‐dopants. The co‐doping of hydrogen and fluorine reduces the metal content of ZnO thin films, resulting in a low absorption coefficient, a high transmittance, and a low refractive index as well as the high conductivity, which are needed for the application in organic solar cells and OLEDs. While the established metal‐doped ZnO films have good electrical and optical properties, their application in organic devices is not as efficient as other alternative electrode approaches. The optimized ZnO electrodes presented here are employed in organic solar cells as well as OLEDs and allow not only the replacement of ITO, but also significantly improve the efficiency compared to lab‐standard ITO. The enhanced performance is attributed to outstanding optical properties and spontaneously nanostructured surfaces of the ZnO films with non‐metallic co‐dopants and their straightforward integration with molecular doping technology, which avoids several common drawbacks of ZnO electrodes. The observations show that optimized ZnO films with non‐metallic co‐dopants are a promising and competitive electrode for low‐cost and high performance organic solar cells and OLEDs.     

Recent Advances in Optical Engineering of Light‐Emitting Electrochemical Cells

Since the first demonstration of light‐emitting electrochemical cells (LECs) in 1995, much effort has been made to develop this technology for display and lighting. A common LEC generally contains a single emissive layer blended with a salt, which provides mobile ions under a bias. Ions accumulated at electrodes facilitate electrochemical doping such that operation voltage is low even when employing high‐work‐function inert electrodes. The superior properties of simple device architecture, low‐voltage operation, and compatibility with inert metal electrode render LECs suitable for cost‐effective light‐emitting sources. In addition to enormous progress in developing novel emissive materials for LECs, optical engineering has been shown to improve device performance of LECs in an alternative way. Light outcoupling enhancement technologies recycle the trapped light and increase the light output from LECs. Techniques to estimate emission zone position provide a powerful tool to study carrier balance of LECs and to optimize device performance. Spectral tailoring of the output emission from LECs based on microcavity effect and localized surface plasmon resonance of metal nanoparticles improves the intrinsic emission properties of emissive materials by optical means. These reported optical techniques are overviewed in this review.     

金属诱导多晶硅电极用于液晶显示透反功能的设计

溶液法金属诱导晶化(S-MIC)的p型掺杂多晶硅薄膜,具有较好的电学特性和近似半透半反的光学特性,可作为透、反两用功能液晶显示器件(LCD)的像素电极材料.但MIC多晶硅薄膜的透射与反射在红.绿和蓝三色区存在着一定的差异,势必导致合成白光的"畸变".为此,作者在MIC(大晶畴)多晶硅材料制成的电极上,在制备并光刻TFT源、漏电极铝金属引线的同时,光刻出不同面积的铝反射片来平衡和补偿经过MIC多晶硅薄膜透过与反射的红、绿、蓝三基色光,有效地进行红、绿、蓝三基色出光光谱的校正.校正结果表明在可见光范围内,其红光、绿光和蓝光处的透射率和反射率基本符合白光平衡的要求;由此形成了具有透、反两用功能的LCD多晶硅像素电极技术.     

Efficient and ultraviolet durable inverted polymer solar cells using thermal stable GZO-AgTi-GZO multilayers as a transparent electrode

The optical and electrical properties of GZO/AgTi/AZO (GATG) multilayer transparent conducting films fabricated by magnetron sputtering method were investigated. The sheet resistance and maximum optical transmittance of GATG films are 5 Ω/sq and 86%, respectively. The sheet resistance of GATG still retains stable under annealing at 400 °C, which shows better thermal stability compared to GZO/Ag/AZO (GAG) film. The enhanced thermal stability of GATG is attributed to the formation of TiO_X in Ti doped Ag nanostructure film, which can inhibit Ag atom diffusion and aggregation. PTB7-TH:PC₇₁BM based inverted polymer solar cells (PSCs) with GATG electrode gave PCE of 9.20%, which is comparable to PCE (9.23%) of the control PSCs with ITO electrode. The PCE of PSCs with GATG and ITO electrodes respectively remain 59% and 23% of the original PCE values after UV exposure for 20 min with relativize humidity of 68% in air, indicating that PSCs with GATG show better UV durability. Our results suggest that GATG as an alternative to ITO electrode can obtain efficient inverted PSCs and have stronger anti-UV ability due to its low UV transparency.     

Spray‐Coated Silver Nanowires as Top Electrode Layer in Semitransparent P3HT:PCBM‐Based Organic Solar Cell Devices

Silver nanowire (Ag NW) thin films are investigated as top electrodes in semitransparent inverted organic solar cells. The performance of semitransparent poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) organic solar cells with Ag NW top electrode layers is found to match very closely the performance of reference devices based on thermally evaporated, highly reflective metal silver top electrodes. The optical losses of the semitransparent electrodes are investigated in detail and analyzed in terms of transmission, scattering, and reflection losses. The impact on an external back reflector is shown to increase the light harvesting efficiency of optically thin devices. Further analysis of transparent devices under illumination from the indium tin oxide (ITO) backside and through the Ag NW front electrode open the possibility to gain deep insight into the vertical microstructure related devices performance. Overall, Ag NW top electrodes are established as a serious alternative to TCO based electrodes. Semitransparent devices with efficiencies of over η = 2.0% are realized.     

Cavities of crystal light [photonic crystal microcavities]

This paper presents the characteristics of photonic crystal microcavity light sources. Microcavities with dimensions on the scale of the wavelength of light are being extensively investigated due to their ability to exhibit enhanced spontaneous emission, directional output, and single-mode operation. Photonic crystals, which are the optical analog of semiconductors in electronic devices, are capable of controlling the properties of light by confining photons in one, two, or three dimensions. The technology to fabricate photonic crystals at the optical-wavelength scale (i.e., feature sizes at the submicron scale) has only very recently been achieved. Single or multiple defects in the photonic crystals act as microcavities with dimensions on the order of the wavelength of light and have emerged as the preferred way to obtain defect-free optical microcavities. The authors have been investigating electrically injected photonic crystal microcavities, and these devices are described in this paper. Electrically injected microcavities offer the advantage of possible integration with current optoelectronic circuits and devices. Also, arrays of such devices can be fabricated when electrically controlled. Electrically injected photonic crystal microcavity light sources may also realize high-efficiency single-mode LEDs.     

Enhanced Light‐Harvesting by Integrating Synergetic Microcavity and Plasmonic Effects for High‐Performance ITO‐Free Flexible Polymer Solar Cells

In this work, a high‐performance ITO‐free flexible polymer solar cell (PSC) is successfully described by integrating the plasmonic effect into the ITO‐free microcavity architecture. By carefully controlling the sizes of embedded Ag nanoprisms and their doping positons in the stratified device, a significant enhancement in power conversion efficiency (PCE) is shown from 8.5% (reference microcavity architecture) to 9.4% on flexible substrates. The well‐manipulated plasmonic resonances introduced by the embedded Ag nanoprisms with different LSPR peaks allow the complementary light‐harvesting with microcavity resonance in the regions of 400–500 nm and 600–700 nm, resulting in the substantially increased photocurrent. This result not only signifies that the spectral matching between the LSPR peaks of Ag nanoprisms and the relatively low absorption response of photoactive layer in the microcavity architecture is an effective strategy to enhance light‐harvesting across its absorption region, but also demonstrates the promise of tailoring two different resonance bands in a synergistic manner at desired wavelength region to enhance the efficiency of PSCs.     

Highly transparent modulated surface textured front electrodes for high‐efficiency multijunction thin‐film silicon solar cells

To further increase the efficiency of multijunction thin‐film silicon (TF‐Si) solar cells, it is crucial for the front electrode to have a good transparency and conduction, to provide efficient light trapping for each subcell, and to ensure a suitable morphology for the growth of high‐quality silicon layers. Here, we present the implementation of highly transparent modulated surface textured (MST) front electrodes as light‐trapping structures in multijunction TF‐Si solar cells. The MST substrates comprise a micro‐textured glass, a thin layer of hydrogenated indium oxide (IOH), and a sub‐micron nano‐textured ZnO layer grown by low‐pressure chemical vapor deposition (LPCVD ZnO). The bilayer IOH/LPCVD ZnO stack guarantees efficient light in‐coupling and light trapping for the top amorphous silicon (a‐Si:H) solar cell while minimizing the parasitic absorption losses. The crater‐shaped micro‐textured glass provides both efficient light trapping in the red and infrared wavelength range and a suitable morphology for the growth of high‐quality nanocrystalline silicon (nc‐Si:H) layers. Thanks to the efficient light trapping for the individual subcells and suitable morphology for the growth of high‐quality silicon layers, multijunction solar cells deposited on MST substrates have a higher efficiency than those on single‐textured state‐of‐the‐art LPCVD ZnO substrates. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a‐Si:H/nc‐Si:H tandem solar cells with the MST front electrode, surpassing efficiencies obtained on state‐of‐the‐art LPCVD ZnO, thereby highlighting the high potential of MST front electrodes for high‐efficiency multijunction solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

3D‐printed external light trap for solar cells

We present a universally applicable 3D‐printed external light trap for enhanced absorption in solar cells. The macroscopic external light trap is placed at the sun‐facing surface of the solar cell and retro‐reflects the light that would otherwise escape. The light trap consists of a reflective parabolic concentrator placed on top of a reflective cage. Upon placement of the light trap, an improvement of 15% of both the photocurrent and the power conversion efficiency in a thin‐film nanocrystalline silicon (nc‐Si:H) solar cell is measured. The trapped light traverses the solar cell several times within the reflective cage thereby increasing the total absorption in the cell. Consequently, the trap reduces optical losses and enhances the absorption over the entire spectrum. The components of the light trap are 3D printed and made of smoothened, silver‐coated thermoplastic. In contrast to conventional light trapping methods, external light trapping leaves the material quality and the electrical properties of the solar cell unaffected. To explain the theoretical operation of the external light trap, we introduce a model that predicts the absorption enhancement in the solar cell by the external light trap. The corresponding calculated path length enhancement shows good agreement with the empirically derived value from the opto‐electrical data of the solar cell. Moreover, we analyze the influence of the angle of incidence on the parasitic absorptance to obtain full understanding of the trap performance. © 2015 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons, Ltd.     

Theory of Resonant Cavity-Enhanced Detection Applied to Thermal Infrared Light

The performances of thermal infrared light detector based on a model system of resonant semiconductor microcavities are theoretically investigated. An original transfer matrix formalism of cavity enhanced absorption is presented which makes use of the small thickness of the absorbing layer compared to the light wavelengths. This formalism yields exact expressions which take standing wave effects into account in a built-in way. Approximations lead to tractable expressions which allow deriving asymptotic behaviors and general trends. The tradeoff between large cavity absorption enhancement and reduction of the detector bandwidth is particularly studied, leading to a gain-bandwidth product analysis. Approximated expressions for detectors based on resonant (i.e type I quantum dots) and nonresonant (bulk or type II quantum wells) optical transitions are also derived, which are physically meaningful and may be conveniently used for engineering purposes. It is found that the limitations due to the gain-bandwidth product conservation can be overcome. However, these cavity enhancement effects are only important for very small quantum efficiency for which the finesse of the microcavity is not seriously deteriorated.     

Fine Tuned Nanolayered Metal/Metal Oxide Electrode for Semitransparent Colloidal Quantum Dot Solar Cells

Semitransparent photovoltaics have great potential, for example, in building‐integration or in portable electronics. However, the front and back contact electrodes significantly affect the light transmission and photovoltaic performance of the complete device. Herein, the use of a semitransparent nanolayered metal/metal oxide electrode for a semitransparent PbS colloidal quantum dot solar cell to increase the light transmission and power conversion efficiency is reported. The effect of the nanolayered electrode on the optical properties within the solar cells is studied and compared to a theoretically model to identify the origin of optical losses that lower the device transmission. The results show that the light transmission in the visible region and the photovoltaic performance are significantly enhanced with the nanolayered electrode. The solar cell shows an efficiency of 5.4% and average visible transmittance of 24.1%, which is an increase by 28.6% and 59.6%, respectively, compared to the device with a standard Au film as the electrode. These results demonstrate that the optical and electrical modification of transparent electrode is possible and essential for reducing the light reflection and absorption of the electrode in semitransparent photovoltaics, and, meanwhile the demonstrated nanolayered materials may provide an avenue for enhancing the device transparency and efficiency.     

Rationally Designed Donor–Acceptor Random Copolymers with Optimized Complementary Light Absorption for Highly Efficient All‐Polymer Solar Cells

Most of the high‐performance all‐polymer solar cells (all‐PSCs) reported to date are based on polymer donor and polymer acceptor pairs with largely overlapped light absorption properties, which seriously limits the efficiency of all‐PSCs. This study reports the development of a series of random copolymer donors possessing complementary light absorption with the naphthalenediimide‐based polymer acceptor P(NDI2HD‐T2) for highly efficient all‐PSCs. By controlling the molar ratio of the electron‐rich benzodithiophene (BDTT) and electron‐deficient fluorinated‐thienothiophene (TT‐F) units, a series of polymer donors with BDTT:TT‐F ratios of 1:1 (P1), 3:1 (P2), 5:1 (P3), and 7:1 (P4) are prepared. The synthetic control of polymer composition allows for precise tuning of the light absorption properties of these new polymer donors, enabling optimization of light absorption properties to complement those of the P(NDI2HD‐T2) acceptor. Copolymer P1 is found to be the optimal polymer donor for the fullerene‐based solar cells due to its high light absorption, whereas the highest power conversion efficiency of 6.81% is achieved for the all‐PSCs with P3, which has the most complementary light absorption with P(NDI2HD‐T2).     

Photocurrent enhancement in thin‐film silicon solar cells by combination of anti‐reflective sub‐wavelength structures and light‐trapping textures

Dielectric films with anti‐reflective sub‐wavelength structures are applied to thin‐film silicon solar cells to improve the light incoupling at the front surface. It is verified that modification of the refractive index of the incident medium using dielectric films with sub‐wavelength structures is beneficial to reduce the average reflectivity of Si solar cells with an anti‐reflective coating based on optical interference. It is also shown that the sub‐wavelength structure must be combined with a proper light‐trapping texture to enhance the absorption within thin‐film silicon solar cells. The effectiveness of dielectric films with sub‐wavelength structures is demonstrated by an increase of the short‐circuit current density of a microcrystalline silicon cell from 29.1 to 30.4 mA/cm² in a designated area of 1 cm². The optical interplay between the dielectric films and the light‐trapping textures is also discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

Enhanced Photoelectrochemical Energy Conversion in Ultrathin Film Photoanodes with Hierarchically Tailorable Mesoscale Structure

Highly porous electrode designs are often employed for photoelectrochemical energy conversion applications. “Inverse opal” structures generate high surface area electrodes to enhance light absorption in semiconductors with short carrier collection lengths, effectively increasing the optical depth of ultrathin film photoelectrodes. Here, the fabrication of hierarchically structured, “host–guest” photoelectrodes based on selective atomic layer deposition of ZnO in composite polystyrene–SiO₂ nanosphere films is described. Nanostructured scaffolds for ultrathin film photoanodes are prepared with a facile, continuously tunable solution‐phase synthesis. The characteristic length scales for absorption, carrier collection, and mass transport can be independently engineered into the electrode by choosing appropriate colloidal components for the composite scaffold. 20 nm ZnO photoanode layers based on the “host–guest” architecture exhibit roughly 500 times the photocurrent generated on an equivalent planar electrode and a 430% increase over a photoanode structured by a scaffold comprised of a close‐packed assembly of identical SiO₂ nanospheres. This results from an improved balance of reactant mass transport and the locus of light absorption throughout the electrode. This approach offers a facile route for preparing strategically nanostructured photoelectrodes based on strategies developed from more complex fabrication techniques.     

Design of color tunable thin film polymer solar cells for photovoltaics printing

Color tunable thin film polymer solar cells have demonstrated the potentials of a wide applications in photovoltaics printing, which is significant for ink pollution reduction and energy saving. This work presents a new effective approach to realize color-tuning photovoltaic cells with optical microcavity structures. Aluminum-doped zinc oxide is utilized as electron transport layer material. With its high electrical conductivity, the thickness tuning range can be quite large, which means the cavity length has a wide variation range. It thus provides sufficient space for optical thin film design to obtain multi colors. By the transfer matrix method, device reflection and absorption spectra are numerically investigated. Based on that, the optical principles for color tunability are explored. In further step, the relationship between device photovoltaics performance and reflective colors are also discussed. Finally, the color coordinates and luminosities are calculated. As results, the colors of the devices designed are capable to cover a relatively large region in Commission Internationale de l′Eclairage(CIE) 1931 x, y chromaticity diagram, which is available to be integrated into the advertisement poster boards, building wall printing and other display applications.     

Highly efficient ITO-free polymer solar cells based on metal resonant microcavity using WO3/Au/WO3 as transparent electrodes

Indium tin oxide (ITO)-free polymer solar cells (PSCs) with the structure of Glass/tungsten trioxide (WO₃)/Au/WO₃/PCDTBT: PC₇₀BM/LiF/Al was fabricated and studied. The multilayer structure of WO₃/Au/WO₃ is used as the potential transparent electrode to replace ITO. Metal resonant microcavity, which can enhance light harvesting of active layers, was constructed between Au and Al electrodes. According to the J–V and IPCE characterization with 70 nm active layer, power conversion efficiency (PCE) of the ITO-free microcavity device is approaching 4.55%, which is higher than that of the ITO-based device. However, PCE of the ITO-free device is much lower than that of the ITO-based device when the thickness of active layer increases to 130 nm. The opposite experimental tendency leads to theoretical research toward the simulation of light absorption and optical electric field and the calculation of maximum short circuit current density (J_{sc max}) as a function of active layer thickness based on ITO-free and ITO-based devices. The research results show that microcavity effect is closely linked to intrinsic absorption of active layers.     

Semitransparent,thin metal grid-based hybrid electrodes for polymer solar cells

A conducting polymer/thin Au grid hybrid electrode was investigated to replace an indium tin oxide (ITO) electrode in polymer solar cells (PSCs). Semitransparent, thin Au films were combined with transparent conducting PEDOT:PTS films (70 nm thickness, ~90% of transmission), to form Au grid/conducting polymer hybrid electrodes. The mixed self-assembled monolayers coated on the Au grids and glass substrate provided uniform and adherent coating of conducting polymer on the monolayer, achieving a low contact resistance of 0.6 Ω mm. This resulted in a robust PEDOT:PTS/Au grid hybrid structure.Theoretical calculation showed the dependence of figure of merits (FM) on the filling ratio (=grid width/(grid spacing+grid width)) and Au thickness. In addition, grid spacing had an effect on the surface morphology of the conducting polymer; decreasing the grid spacing produced more flat surface of the overlayers, leading to enhanced performance of PSCs. The fabricated PSCs based on these hybrid electrodes showed that the best efficiency of 3.54%, comparable to that of devices based on an ITO electrode, was obtained at the filling ratio of 0.5 for 15 nm-thick Au electrodes, which was different from that predicted from the theoretical calculation, probably due to the grid spacing effects on the charge collection efficiency.     

Fluorinated and Alkylthiolated Polymeric Donors Enable both Efficient Fullerene and Nonfullerene Polymer Solar Cells

In this work, four donor (D)–acceptor (A) copolymers based on benzodithiophene (BDT) and benzothiadiazole (BT) with different alkylthiolated and/or fluorinated side chains are developed for efficient fullerene and nonfullerene polymer solar cells (PSCs). The synergistic effect of sulfuration and fluorination on the optical absorption, energy level, crystallinity, carrier mobility, blend morphology, and photovoltaic performance is investigated systematically. By incorporating sulfur atoms onto the side chains, a little blueshifted but significantly increased absorption can be obtained for PBDTS‐FBT compared to PBDT‐FBT . On the other side, a little more blueshifted but much stronger absorption and much lower‐lying highest occupied molecular orbital (HOMO) level can be realized for PBDTF‐FBT when introducing fluorine atoms instead of sulfur atoms. With the combination of both fluorination and sulfuration strategies, PBDTS‐FBT exhibits the best absorption ability, lowest HOMO energy level, and highest crystallinity, which make PBDTSF‐FBT devices show the highest power conversion efficiency (PCE) of 10.69% in fullerene PSCs and 11.66% in nonfullerene PSCs. The PCE of 11.66% is the best value for PSCs based on BT‐containing copolymer donors reported so far. The results indicate that fluorination and sulfuration have a synergistically positive effect on the performance of D–A photovoltaic copolymers and their solar cell devices.     

Enhance external quantum efficiency of organic light-emitting devices using thin transparent electrodes

High-index transparent electrodes have been one major origin of light trapping and lower light extraction in organic light emitting diodes (OLEDs). In this work, influences of the bottom transparent electrode thickness on emission properties of OLEDs are systematically studied by both simulation and experiments. Simulation shows that with substantially decreasing the thickness of the high-index indium tin oxide (ITO) electrode, waveguided modes, that otherwise would be significantly induced in regular/thicker ITO devices, can be effectively eliminated. Consequently, the overall coupling efficiencies of OLED emission into substrates can be much enhanced. Through further effective light extraction from the substrate, green phosphorescent OLEDs with a high external quantum efficiency (EQE) of up to ≈57.5% were experimentally demonstrated by adopting the very thin (20 nm) ITO electrode and preferentially horizontal dipole emitters (with a horizontal dipole ratio of 76%). The simulation further predicts that very high optical coupling efficiencies into substrates and EQEs approaching 80% are possible with further adopting purely horizontal dipole emitters and/or low-index electron transport layer (ETL) to suppress surface plasmon modes. Overall, this study clearly reveals the potential of using thin transparent electrodes for highly efficient OLEDs.