Low Substrate Temperature Encapsulation for Flexible Electrodes and Organic Photovoltaics

Encapsulation is of key importance to improve the stability and lifetime of organic conductors and devices, mainly in applications such as flexible electrodes or organic photovoltaics (OPV). Here, a single‐layer conformal encapsulation method is demonstrated via initiated chemical vapor deposition (iCVD) for organic conductor, poly (3,4‐ethylenedioxythiophene) (PEDOT). The accelerated degradation tests at 100 °C show that the conductivity of encapsulated PEDOT can be retained up to 17 times longer than that of the unencapsulated counterpart. PEDOT degradation and encapsulation mechanisms are also discussed. Furthermore, the versatility of the iCVD encapsulation on a top‐illuminated OPV architecture that can be used to produce low‐cost photovoltaics devices on various unconventional substrates (e.g., paper) is demonstrated. Unlike previous approaches of using solely water/oxygen barriers, the encapsulation effect of polymer films on OPV devices is improved by a thin layer capping of evaporated UV‐screening material, Cerium(IV) oxide (CeO₂) over the iCVD polymer layer. This bilayer encapsulation strategy efficiently slows the degradation of OPVs and represents a new method to encapsulate OPV and other organic devices.     

Efficient Polymer Solar Cells on Opaque Substrates with a Laminated PEDOT:PSS Top Electrode

Solution processed polymer:fullerene solar cells on opaque substrates have been fabricated in conventional and inverted device configurations. Opaque substrates, such as insulated steel and metal covered glass, require a transparent conducting top electrode. We demonstrate that a high conducting (900 S cm^?1) PEDOT:PSS layer, deposited by a stamp‐transfer lamination technique using a PDMS stamp, in combination with an Ag grid electrode provides a proficient and versatile transparent top contact. Lamination of large size PEDOT:PSS films has been achieved on variety of surfaces resulting in ITO‐free solar cells. Power conversion efficiencies of 2.1% and 3.1% have been achieved for P3HT:PCBM layers in inverted and conventional polarity configurations, respectively. The power conversion efficiency is similar to conventional glass/ITO‐based solar cells. The high fill factor (65%) and the unaffected open‐circuit voltage that are consistently obtained in thick active layer inverted geometry devices, demonstrate that the laminated PEDOT:PSS top electrodes provide no significant potential or resistive losses.     

Spray Deposition of Silver Nanowire Electrodes for Semitransparent Solid‐State Dye‐Sensitized Solar Cells

Transparent top electrodes for solid‐state dye‐sensitized solar cells (ssDSCs) allow for fabrication of mechanically stacked ssDSC tandems, partially transparent ssDSCs for building integration, and ssDSCs on metal foil substrates. A solution‐processed, highly transparent, conductive electrode based on PEDOT:PSS [poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)] and spray‐deposited silver nanowires (Ag NWs) is developed as an effective top contact for ssDSCs. The electrode is solution‐deposited using conditions and solvents that do not damage or dissolve the underlying ssDSC and achieves high performance: a peak transmittance of nearly 93% at a sheet resistance of 18 Ω/square – all without any annealing that would harm the ssDSC. The role of the PEDOT:PSS in the electrode is twofold: it ensures ohmic contact between the ssDSC 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)9,9′‐spirobifluorene (Spiro‐OMeTAD) overlayer and the silver nanowires and it decreases the series resistance of the device. Semitransparent ssDSCs with D35 dye fabricated using this Ag NW/PEDOT:PSS transparent electrode show power conversion efficiencies of 3.6%, nearly as high as a reference device using an evaporated silver electrode (3.7%). In addition, the semitransparent ssDSC shows high transmission between 700–1100 nm, a necessity for use in efficient tandem devices. Such an electrode, in combination with efficient ssDSCs or hybrid perovskite‐sensitized solar cells, can allow for the fabrication of efficient, cost‐effective tandem photovoltaics.     

Semi‐Transparent Tandem Organic Solar Cells with 90% Internal Quantum Efficiency

Semi‐transparent (ST) organic solar cells with potential application as power generating windows are studied. The main challenge is to find proper transparent electrodes with desired electrical and optical properties. In this work, this is addressed by employing an amphiphilic conjugated polymer PFPA‐1 modified ITO coated glass substrate as the ohmic electron‐collecting cathode and PEDOT:PSS PH1000 as the hole‐collecting anode. For active layers based on different donor polymers, considerably lower reflection and parasitic absorption are found in the ST solar cells as compared to solar cells in the standard geometry with an ITO/PEDOT:PSS anode and a LiF/Al cathode. The ST solar cells have remarkably high internal quantum efficiency at short circuit condition (～90%) and high transmittance (～50%). Hence, efficient ST tandem solar cells with enhanced power conversion efficiency (PCE) compared to a single ST solar cell can be constructed by connecting the stacked two ST sub‐cells in parallel. The total loss of photons by reflection, parasitic absorption and transmission in the ST tandem solar cell can be smaller than the loss in a standard solar cell based on the same active materials. We demonstrate this by stacking five separately prepared ST cells on top of each other, to obtain a higher photocurrent than in an optimized standard solar cell.     

High‐Performance Ta2O5/Al‐Doped Ag Electrode for Resonant Light Harvesting in Efficient Organic Solar Cells

A efficient indium tin oxide (ITO)‐free transparent electrode based on an improved Ag film is designed by introducing small amount of Al during co‐deposition, producing ultrathin and smooth Ag film with low loss. A transparent electrode as thin as 4 nm is achieved by depositing the film on top of Ta₂O₅ layer, and organic solar cells based on such ultrathin electrode are built, producing power conversion efficiency over 7%. The device efficiency can be optimized by simply tuning Ta₂O₅ layer thickness external to the organic photovoltaic (OPV) structure to create an optical cavity resonance inside the photoactive layer. Therefore Ta₂O₅/Al‐doped Ag films function as a high‐performance electrode with high transparency, low resistance, improved photon management capability and mechanical flexibility.     

Nanowire‐Based Three‐Dimensional Transparent Conducting Oxide Electrodes for Extremely Fast Charge Collection

A 3D transparent conducting oxide (3D‐TCO) has been fabricated by growing Sn‐doped indium oxide (ITO) nanowire arrays on glass substrates via a vapor transport method. The 3D TCO charge‐collection properties have been compared to those of conventional two‐dimensional TCO (2D‐TCO) thin films. For use as a photoelectrode in dye‐sensitized solar cells, ITO‐TiO₂ core‐shell nanowire arrays were prepared by depositing a 45 nm‐thick mesoporous TiO₂ shell layer consisting of ～6 nm anatase nanoparticles using TiCl₄ treatments. Dye‐sensitized solar cells fabricated using these ITO‐TiO₂ core‐shell nanowire arrays show extremely fast charge collection owing to the shorter electron paths across the 45 nm‐thick TiO₂ shell compared to the 2D TCO. Interestingly, the charge‐collection time does not increase with the overall electrode thickness, which is counterintuitive to conventional diffusion models. This result implies that, in principle, maximum light harvesting can be achieved without hindering the charge collection. The proposed new 3D TCO should also be attractive for other photovoltaic applications where the active layer thickness is limited by poor charge collection.     

Contact Doping with Sub‐Monolayers of Strong Polyelectrolytes for Organic Photovoltaics

Barriers to charge transfer at electrode‐semiconductor contacts are ubiquitous and limit the applicability of organic semiconductors in electronic devices. Molecular or ionic doping near contacts can alleviate charge injection or extraction problems by enabling charge tunneling through contact barriers, but the soft nature of organic materials allows for small molecule dopants to diffuse and migrate, degrading the performance of the device and limiting effective interfacial doping. Here, it is demonstrated that contact doping in organic electronics is possible through ionic polymer dopants, which resist diffusion or migration due to their large size. Sub‐monolayer deposition of non‐conjugated strong polyelectrolytes, e.g., sulfonated poly(sulfone)s, at the anode‐semiconductor interface of organic photovoltaics enables efficient hole extraction at the anode. The performance of contact‐doped organic photo­voltaics nearly matches the performance of devices composed of traditional hole transport layers such as poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The degree of sulfonation of the dopant polymer and the thickness of the ionic dopant layer is shown to be critical for optimizing doping and the efficiency of the device.     

Inverted Semi‐transparent Polymer Solar Cells with Transparency Color Rendering Indices approaching 100

Window‐ or building‐integrated semi‐transparent solar cells are particularly interesting applications for organic photovoltaic devices. In this work, we present an easy‐to‐process inverted device architecture comprising fully solution processable poly(3,4‐ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) bilayer top‐electrodes for efficient semi‐transparent organic solar cells. By incorporating dyes with a complementary absorption to the light harvesting polymer poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl] (PCDTBT) into the PEDOT:PSS electrode, we achieve fully color neutral transparency perception and a color rendering index approaching 100. This makes the devices suitable for applications such as window shadowing or the integration into overhead glazing.     

Thin‐Film Solar Cells with InP Absorber Layers Directly Grown on Nonepitaxial Metal Substrates

The design and performance of solar cells based on InP grown by the nonepitaxial thin‐film vapor–liquid–solid (TF‐VLS) growth technique is investigated. The cell structure consists of a Mo back contact, p‐InP absorber layer, n‐TiO₂ electron selective contact, and indium tin oxide transparent top electrode. An ex situ p‐doping process for TF‐VLS grown InP is introduced. Properties of the cells such as optoelectronic uniformity and electrical behavior of grain boundaries are examined. The power conversion efficiency of first generation cells reaches 12.1% under simulated 1 sun illumination with open‐circuit voltage (V_OC) of 692 mV, short‐circuit current (J_SC) of 26.9 mA cm^?2, and fill factor (FF) of 65%. The FF of the cell is limited by the series resistances in the device, including the top contact, which can be mitigated in the future through device optimization. The highest measured V_OC under 1 sun is 692 mV, which approaches the optically implied V_OC of ≈795 mV extracted from the luminescence yield of p‐InP.     

Improvement of Transparent Metal Top Electrodes for Organic Solar Cells by Introducing a High Surface Energy Seed Layer

We present highly transparent and conductive silver thin films in a thermally evaporated dielectric/metal/dielectric (DMD) multilayer architecture as top electrode for efficient small molecule organic solar cells. DMD electrodes are frequently used for optoelectronic devices and exhibit excellent optical and electrical properties. Here, we show that ultrathin seed layers such as calcium, aluminum, and gold of only 1 nm thickness strongly influence the morphology of the subsequently deposited silver layer used as electrode. The wetting of silver on the substrate is significantly improved with increasing surface energy of the seed material resulting in enhanced optical and electrical properties. Typically thermally evaporated silver on a dielectric material forms rough and granular layers which are not closed and not conductive below thicknesses of 10 nm. With gold acting as seed layer, the silver electrode forms a continuous, smooth, conductive layer down to a silver thickness of 3 nm. At 7 nm silver thickness such an electrode exhibits a sheet resistance of 19 Ω/□ and a peak transmittance of 83% at 580 nm wavelength, both superior compared to silver electrodes without seed layer and even to indium tin oxide (ITO). Top‐illuminated solar cells using gold/silver double layer electrodes achieve power conversion efficiencies of 4.7%, which is equal to 4.6% observed in bottom‐illuminated reference devices employing conventional ITO. The top electrodes investigated here exhibit promising properties for semitransparent solar cells or devices fabricated on opaque substrates.     

Enhancing the Open‐Circuit Voltage of Perovskite Solar Cells by up to 120 mV Using π‐Extended Phosphoniumfluorene Electrolytes as Hole Blocking Layers

Four π‐extended phosphoniumfluorene electrolytes (π‐PFEs) are introduced as hole‐blocking layers (HBL) in inverted architecture planar perovskite solar cells with the structure of ITO/PEDOT:PSS/MAPbI₃/PCBM/HBL/Ag. The deep‐lying highest occupied molecular orbital energy level of the π‐PFEs effectively blocks holes, decreasing contact recombination. It is demonstrated that the incorporation of π‐PFEs introduces a dipole moment at the PCBM/Ag interface, resulting in significant enhancement of the built‐in potential of the device. This enhancement results in an increase in the open‐circuit voltage of the device by up to 120 mV, when compared to the commonly used bathocuproine HBL. The results are confirmed both experimentally and by numerical simulation. This work demonstrates that interfacial engineering of the transport layer/contact interface by small molecule electrolytes is a promising route to suppress nonradiative recombination in perovskite devices and compensates for a nonideal energetic alignment at the hole‐transport layer/perovskite interface.     

Atomic Layer Deposition of Highly Transparent Platinum Counter Electrodes for Metal/Polymer Flexible Dye‐Sensitized Solar Cells

Atomic layer deposition (ALD) is used to deposit Pt nanoparticles at low temperature (25–150 °C) to fabricate highly transparent counter electrodes (CEs) for flexible dye‐sensitized solar cells (DSCs). The Pt nanoparticles (NPs) are deposited for different number of ALD cycles on indium tin oxide (ITO)/polyethylene naphthalate (PEN) substrates. Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM) are used to assess the Pt NP loading, density, and size. There is a trade‐off between transparency and catalytic activity of the CE, and the best cell performances of back‐side‐illuminated DSCs (≈3.7% efficiency) are achieved for Pt ALD at temperatures in the range of 100–150 °C, even though deposition at 25 °C is also viable. The best cell produced with ALD platinized CE (100 cycles at 100 °C) outperforms the reference cells fabricated with electrodeposited and sputtered Pt CEs, with relative improvements in efficiency of 19% and 29%, respectively. In addition, these parameters are used to fabricate a large area CE for a sub‐module (active area of 17.6 cm²), resulting in an efficiency of 3.1%, which demonstrates the scalability of the process.     

50% Sn‐Based Planar Perovskite Solar Cell with Power Conversion Efficiency up to 13.6%

In the present work, a Pb‐assisted two step method is successfully proposed to fabricate high‐quality CH₃NH₃Sn_0.5Pb_0.5I₃ (MASn_0.5Pb_0.5I₃) perovskite film on the indium tin oxide (ITO) glass/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) substrate. The film shows regular crystalline grains with a flat and compact morphology as well as full coverage on the planar PEDOT:PSS substrate. Remarkably, corresponding devices ITO/PEDOT:PSS/MASn_0.5Pb_0.5I₃/C₆₀/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline/Ag are fabricated with high reproducibility, achieving a high power conversion efficiency of 13.6%, which is, to the best of knowledge, the most efficient solar cell based on Sn‐based perovskite.     

Rapid Layer‐Specific Annealing Enabled by Ultraviolet LED with Estimation of Crystallization Energy for High‐Performance Perovskite Solar Cells

A rapid layer‐specific annealing on perovskite active layer enabled by ultraviolet (UV) light‐emitting diode (LED) is demonstrated and efficiency close to 19% is achieved in a simple planar inverted structure ITO/PEDOT:PSS/MAPbI₃/PC₇₁BM/Al without any device engineering. These results demonstrate that if the UV dosage is well managed, UV light is capable of annealing perovskite into high‐quality film rather than simply damaging it. Different in principle from other photonic treatment techniques that can heat up and damage underlying films, the UV‐LED‐annealing method enables layer‐specific annealing because LED light source is able to provide a specific UV wavelength for maximum light absorption of target film. Moreover, the layer‐specific photonic treatment allows accurate estimation of the crystallization energy required to form perovskite film at device quality level.     

Ultra‐High Voltage Multijunction Organic Solar Cells for Low‐Power Electronic Applications

Low power electronics are an ideal application for organic photovoltaics (OPV) where a low‐cost OPV device can be integrated directly with a battery to provide a constant power source. We demonstrate ultra‐high voltage small molecule multijunction devices with open circuit voltage (V_OC) values of up to 7V. Optical modelling is employed to aid the optimisation of the complex multi‐layer stacks and ensure current balancing is achieved between sub‐cells, and optimised multijunction devices show power conversion efficiencies of up to 3.4% which is a modest increase over the single junction devices. Sub‐cell donor/acceptor pairs of boron subphthalocyanine chloride (SubPc)/fullerene (C₆₀) and SubPc/Cl₆‐SubPc were selected both for their high V_OC in order to minimise the required number of junctions, but also for their absorption overlap to reduce the spectral dependence of the device performance. As a result, the devices are shown to directly charge a micro‐energy cell type battery under both low illumination intensity white light and monochromatic illumination.     

Work Function Control of Interfacial Buffer Layers for Efficient and Air‐Stable Inverted Low‐Bandgap Organic Photovoltaics

A water‐soluble cationic polythiophene derivative, poly[3‐(6‐{4‐tert‐butylpyridiniumyl}‐hexyl)thiophene‐2,5‐diyl] [P3(TBP)HT], is combined with anionic poly(3,4‐ethylenedioxythiophene):poly(p‐styrenesulfonate) (PEDOT:PSS) on indium tin oxide (ITO) substrates via electrostatic layer‐by‐layer (eLbL) assembly. By varying the number of eLbL layers, the electrode's work function is precisely controlled from 4.6 to 3.8 eV. These polymeric coatings are used as cathodic interfacial modifiers for inverted‐mode organic photovoltaics that incorporate a photoactive layer composed of either poly[(3‐hexylthiophene)‐2,5‐diyl] (P3HT) and the fullerene acceptor [6,6‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) or the low bandgap polymer [poly({4,8‐di(2‐ethylhexyloxyl)benzo[1,2‐b:4,5‐b′]dithiophene}‐2,6‐diyl)‐alt‐({5‐octylthieno[3,4‐c]pyrrole‐4,6‐dione}‐1,3‐diyl) (PBDTTPD)] and the electron acceptor [6,6‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM)]. The power conversion efficiency (PCE) of the resulting photovoltaic device is dependent on the composition of the eLbL‐assembled interface and permits the fabrication of devices with efficiencies of 3.8% and 5.6% for P3HT and PBDTTPD donor polymers, respectively. Notably, these devices demonstrate significant stability with a P3HT:PC₆₁BM system maintaining 83% of its original PCE after 1 year of storage and a PBDTTPD:PC₇₁BM system maintaining 97% of its original PCE after over 1000 h of storage in air, according to the ISOS‐D‐1 shelf protocol.     

Nanowire Interconnects for Printed Large‐Area Semitransparent Organic Photovoltaic Modules

Semitransparent organic photovoltaic (OPV) cells promise applications in various transparent architectures where their opaque counterparts cannot contribute. Realizing practical applications of this technology requires the manufacturing of large‐area modules without significant performance loss compared to the lab‐scale devices. In this work, efficient semitransparent OPV modules based on ultrafast laser patterning on both glass and flexible substrates are reported. Solution‐processed metallic silver nanowires (AgNWs) are used as transparent top electrodes. The efficient low‐ohmic contact of the interconnects between the top AgNWs and the bottom electrode in combination with high‐precision laser beam positioning system allow to fabricate semitransparent modules with high electrical fill factor of ≈63% and a remarkable geometric fill factor exceeding 95%, respectively. These results represent an important progress toward upscaling of high‐performance OPV modules with reduced production costs.     

ITO‐Free and Fully Solution‐Processed Semitransparent Organic Solar Cells with High Fill Factors

Organic photovoltaic (OPV) solar cells that can be simply processed from solution are in the focus of the academic and industrial community because of their enormous potential to reduce cost. One big challenge in developing a fully solution‐processed OPV technology is the design of a well‐performing electrode system, allowing the replacement of ITO. Several solution‐processed electrode systems were already discussed, but none of them could match the performance of ITO. Here, we report efficient ITO‐free and fully solution‐processed semitransparent inverted organic solar cells based on silver nanowire (AgNW) electrodes. To demonstrate the potential of these AgNW electrodes, they were employed as both the bottom and top electrodes. Record devices achieved fill factors as high as 63.0%, which is comparable to ITO based reference devices. These results provide important progress for fully printed organic solar cells and indicate that ITO‐free, transparent as well as non‐transparent organic solar cells can indeed be fully solution‐processed without losses.     

Foldable Semitransparent Organic Solar Cells for Photovoltaic and Photosynthesis

Semitransparent organic solar cells (ST‐OSCs) have attracted extensive attention for their potential greenhouse applications. Conventional ST‐OSCs are typically based on indium tin oxide (ITO) electrodes which suffer from mechanical brittleness. Therefore, alternatives for ITO are required for realization of foldable‐flexible ST‐OSCs (FST‐OSCs). Herein, flexible poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes are prepared as ITO alternatives via polyhydroxy compound (xylitol) microdoping and acid treatment. As a result, flexible opaque OSCs based on PBDB‐T‐2F:Y6 photoactive system yield a high efficiency of 14.20%. The desirable optical properties of modified PEDOT:PSS electrodes in the visible light region and PBDB‐T‐2F:Y6 photoactive layer in the near‐infrared region facilitate the fabrication of FST‐OSCs with over 10% efficiency and 21% average visible light transmittance. Those FST‐OSCs also display excellent mechanical stability against bending and folding due to the xylitol doping, where over 80% of the initial efficiency can still be maintained even after 1000 folding cycles. Meanwhile, parallel comparisons between plants grown under direct sunlight with a FST‐OSCs roof and those under direct sunlight yield very similar results in terms of branch sturdiness and hypertrophic leaves. The results pave the way for realizing high‐performing FST‐OSCs based on PEDOT:PSS electrodes that could utilize visible light for plant growth and infrared light for power generation.     

Charge Formation in Pentacene Layers During Solar‐Cell Fabrication: Direct Observation by Electron Spin Resonance

Microscopic characterization of charge carriers in solar cells is useful for high‐performance cell fabrication because the formation and accumulation of charges in cells greatly affect the device performance. Electron spin resonance (ESR) is suitable for such characterization because it can directly observe charge carriers with spins in these cells. In this work, the ESR method is applied to organic thin‐film solar cells to investigate charge formation in such devices. Heterojunction cells of indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS)/pentacene/C₆₀/bathocuproine (BCP)/Al are investigated. Clear ESR signals are observed by inserting a typical PEDOT:PSS hole buffer layer. From analysis of the dependence of the ESR characteristics on the external magnetic field direction, the bias voltage, and the duration of solar‐simulated irradiation, the charges (mobile holes) in pentacene layers are successfully identified and it can be deduced that these holes are formed at the PEDOT:PSS/pentacene interface during device fabrication. This ESR analysis provides useful knowledge for understanding device operation and improving device performance at the microscopic level.