Carbon: The Ultimate Electrode Choice for Widely Distributed Polymer Solar Cells

As mass‐produced, low‐cost organic electronics enter our everyday lives, so does the waste from them. The challenges associated with end‐of‐life management must be addressed by careful design and carbon‐based electrodes are central to these developments. Here, the reproducible production of vacuum‐, indium tin oxide (ITO)‐, and silver‐free solar cells in a fully packaged form using only roll‐to‐roll processing is reported. Replacing silver with carbon as electrode material significantly lowers the manufacturing cost and makes the organic photovoltaic (OPV) modules environmentally safe while retaining their flexibility, active area efficiency, and stability. The substitution of silver with carbon does not affect the roll‐to‐roll manufacturing of the modules and allows for the same fast printing and coating. The use of carbon as electrode material is one step closer to the wide release of low‐cost plastic solar cells and opens the door to new possible applications where silver recycling is not manageable.     

Combinatorial Screening of Polymer:Fullerene Blends for Organic Solar Cells by Inkjet Printing

Polymer:fullerene blends were screened in a combinatorial approach using inkjet printing thin film libraries for photovoltaic devices. The application of inkjet printing enabled a fast and simple experimental workflow from film preparation to the study of structure‐property‐relationships with a very high material efficiency. Inkjet printing requires less material for the preparation of thin film libraries in comparison to other dispensing techniques, like spin‐coating. Two polymers (PCPDTBT, PSBTBT) and two fullerene derivatives (mono‐PCBM, bis‐PCBM) were investigated in various blend ratios, concentrations, solvent ratios, and film thicknesses. Morphological and optical properties of the inkjet printed films were investigated and compared with spin‐coated films. This study shows the principle of an experimental setup from solution preparation to film characterization for the combinatorial investigation of large polymer:fullerene libraries.     

The Roles of Poly(Ethylene Oxide) Electrode Buffers in Efficient Polymer Photovoltaics

The role of poly(ethylene oxide) polymer is investigated as an effective buffer with Al electrodes to markedly improve the electrode interface and enhance the open‐circuit voltage (V_OC) and the power conversion efficiency (PCE, η) of poly(3‐hexylthiophene) (P3HT):[6,6]‐phenyl C61‐butyric acid methyl ester (PCBM)‐based bulk‐heterojunction (BHJ) solar cells. A unique process is developed by thermally co‐evaporating the poly(ethylene glycol) dimethyl ether (PEGDE, Mn ca. 2000) polymer with Al metal simultaneously at different ratios in vacuum (10^?6 Torr) to prepare the electrode buffers. The instant formation of a carbide‐like junction at the ethylene oxide/Al interface during the thermal evaporation is of essential importance to the extraction of electrons through the Al electrode. The performance of P3HT:PCBM‐based solar cells can be optimized by modulating the co‐evaporation ratios of the PEGDE polymer with Al metal due to the changes in the work functions of the electrodes. The V_OC and η for devices fabricated with Al electrode are 0.44 V and 1.64%, respectively, and significantly improve to 0.58 V and 4.00% when applying the PEGDE:Al(2:1)/Al electrode. This research leads to a novel electrode design – free of salts, additives, complicated syntheses, and having tunable work function – for fabricating high‐performance photovoltaic cells.     

Upscaling of Perovskite Solar Cells: Fully Ambient Roll Processing of Flexible Perovskite Solar Cells with Printed Back Electrodes

A scaling effort on perovskite solar cells is presented where the device manufacture is progressed onto flexible substrates using scalable techniques such as slot‐die roll coating under ambient conditions. The printing of the back electrode using both carbon and silver is essential to the scaling effort. Both normal and inverted device geometries are explored and it is found that the formation of the correct morphology for the perovskite layer depends heavily on the surface upon which it is coated and this has significant implications for manufacture. The time it takes to form the desired layer morphology falls in the range of 5–45 min depending on the perovskite precursor, where the former timescale is compatible with mass production and the latter is best suited for laboratory work. A significant loss in solar cell performance of around 50% is found when progressing to using a fully scalable fabrication process, which is comparable to what is observed for other printable solar cell technologies such as polymer solar cells. The power conversion efficiency (PCE) for devices processed using spin coating on indium tin oxide (ITO)‐glass with evaporated back electrode yields a PCE of 9.4%. The same device type and active area realized using slot‐die coating on flexible ITO‐polyethyleneterphthalate (PET) with a printed back electrode gives a PCE of 4.9%.     

Square‐Centimeter‐Sized High‐Efficiency Polymer Solar Cells: How the Processing Atmosphere and Film Quality Influence Performance at Large Scale

Organic solar cells based on two benzodithiophene‐based polymers (PTB7 and PTB7‐Th) processed at square centimeter‐size under inert atmosphere and ambient air, respectively, are investigated. It is demonstrated that the performance of solar cells processed under inert atmosphere is not limited by the upscaling of photoactive layer and the interfacial layers. Thorough morphological and electrical characterizations of optimized layers and corresponding devices reveal that performance losses due to area enlargement are only caused by the sheet resistance of the transparent electrode reducing the efficiency from 9.3% of 7.8% for PTB7‐Th in the condition that both photoactive layer and the interfacial layers are of high layer quality. Air processing of photoactive layer and the interfacial layers into centimeter‐sized solar cells lead to additional, but only slight, losses (<10%) in all photovoltaic parameters, which can be addressed to changes in the electronic properties of both active layer and ZnO layers rather than changes in layer morphology. The demonstrated compatibility of polymer solar cells using solution‐processed photoactive layer and interfacial layers with large area indicates that the introduction of a standard active area of 1 cm² for measuring efficiency of organic record solar cells is feasible. However electric standards for indium tin oxides (ITO) or alternative transparent electrodes need to be developed so that performance of new photovoltaic materials can be compared at square centimeter‐size.     

Slot‐Die Coating of a High Performance Copolymer in a Readily Scalable Roll Process for Polymer Solar Cells

Copolymers based on dithieno[3,2‐b:2′,3′‐d]silole (DTS) and dithienylthiazolo[5,4‐d]thiazole (TTz) are synthesized and tested in an all‐solution roll process for polymer solar cells (PSCs). Fabrication of polymer:[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) solar cells is done on a previously reported compact coating/printing machine, which enables the preparation of PSCs that are directly scalable with full roll‐to‐roll processing. The positioning of the side‐chains on the thiophene units proves to be very significant in terms of solubility of the polymers and consequently has a major impact on the device yield and process control. The most successful processing is accomplished with the polymer, PDTSTTz‐4 , that has the side‐chains situated in the 4‐position on the thiophene units. Inverted PSCs based on PDTSTTz‐4 demonstrate high fill factors, up to 59%, even with active layer thicknesses well above 200 nm. Power conversion efficiencies of up to 3.5% can be reached with the roll‐coated PDTSTTz‐4 :PCBM solar cells that, together with good process control and high device yield, designate PDTSTTz‐4 as a convincing candidate for high‐throughput roll‐to‐roll production of PSCs.     

Critical Impact of Hole Transporting Layers and Back Electrode on the Stability of Flexible Organic Photovoltaic Module

Properties of hole transporting layers (HTLs) and back electrode are very critical to the stability of inverted bulk heterojunction organic photovoltaic (OPV) modules. Here, various deposition methods for back electrodes and materials of HTLs are examined by applying to inverted organic solar cells with a structure of indium tin oxide/ZnO/photoactive layer/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Ag. The experiment is performed on encapsulated modules with flexible barrier films under accelerated conditions. The OPV modules with screen‐printed Ag electrodes are shown to be electrically unstable with a reduction of the current density under damp heat condition at 85 °C/85% RH. Optical images for the active layer/PEDOT:PSS interface reveal that a reaction between the solvent from the Ag electrode and the underlying layers is the major cause for the degradation. In comparison with materials of the HTLs, the PEDOT:PSS layer shows low stability compared to the MoO₃ layer under the accelerated conditions. Unusual chemical changes in the PEDOT:PSS film are observed through X‐ray photoelectron spectroscopy and this is further addressed by correlating the stability of the OPV devices.     

Matrix Organization and Merit Factor Evaluation as a Method to Address the Challenge of Finding a Polymer Material for Roll Coated Polymer Solar Cells

The results presented demonstrate how the screening of 104 light‐absorbing low band gap polymers for suitability in roll coated polymer solar cells can be accomplished through rational synthesis according to a matrix where 8 donor and 13 acceptor units are organized in rows and columns. Synthesis of all the polymers corresponding to all combinations of donor and acceptor units is followed by characterization of all the materials with respect to molecular weight, electrochemical energy levels, band gaps, photochemical stability, carrier mobility, and photovoltaic parameters. The photovoltaic evaluation is carried out with specific reference to scalable manufacture, which includes large area (1 cm²), stable inverted device architecture, an indium‐tin‐oxide‐free fully printed flexible front electrode with ZnO/PEDOT:PSS (poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate), and a printed silver comb back electrode structure. The matrix organization enables fast identification of active layer materials according to a weighted merit factor that includes more than simply the power conversion efficiency and is used as a method to identify the lead candidates. Based on several characteristics included in the merit factor, it is found that 13 out of the 104 synthesized polymers outperformed poly(3‐hexylthiophene) under the chosen processing conditions and thus can be suitable for further development.     

Inkjet‐Printed Micrometer‐Thick Perovskite Solar Cells with Large Columnar Grains

Transferring the high power conversion efficiencies (PCEs) of spin‐coated perovskite solar cells (PSCs) on the laboratory scale to large‐area photovoltaic modules requires a significant advance in scalable fabrication methods. Digital inkjet printing promises scalable, material, and cost‐efficient deposition of perovskite thin films on a wide range of substrates and in arbitrary shapes. In this work, high‐quality inkjet‐printed triple‐cation (methylammonium, formamidinium, and cesium) perovskite layers with exceptional thicknesses of >1 µm are demonstrated, enabling unprecedentedly high PCEs > 21% and stabilized power output efficiencies > 18% for inkjet‐printed PSCs. In‐depth characterization shows that the thick inkjet‐printed perovskite thin films deposited using the process developed herein exhibit a columnar crystal structure, free of horizontal grain boundaries, which extend over the entire thickness. A thin film thickness of around 1.5 µm is determined as optimal for PSC for this process. Up to this layer thickness X‐ray photoemission spectroscopy analysis confirms the expected stoichiometric perovskite composition at the surface and shows strong deviations and inhomogeneities for thicker thin films. The micrometer‐thick perovskite thin films exhibit remarkably long charge carrier lifetimes, highlighting their excellent optoelectronic characteristics. They are particularly promising for next‐generation inkjet‐printed perovskite solar cells, photodetectors, and X‐ray detectors.     

All‐Inkjet‐Printed,All‐Air‐Processed Solar Cells

The prospective of using direct‐write printing techniques for the manufacture of organic photovoltaics (OPVs) has made these techniques highly attractive. OPVs have the potential to revolutionize small‐scale portable electronic applications by directly providing electric power to the systems. However, no route is available for monolithically integrating the energy‐harvesting units into a system in which other components, such as transistors, sensors, or displays, are already fabricated. Here, the fabrication and the measurement of inkjet‐printed, air‐processed organic solar cells is reported for the first time. Highly controlled conducting and semiconducting films using engineered inks for inkjet printing enable good efficiencies for printed OPVs between ≈2 and 5% power conversion efficiency. The results show that inkjet printing is an attractive digital printing technology for cost‐effective, environmentally friendly integration of photovoltaic cells onto plastic substrates.     

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.     

Loss Mechanisms in Thick‐Film Low‐Bandgap Polymer Solar Cells

Polymer bulk heterojunction solar cells based on low bandgap polymer:fullerene blends are promising for next generation low‐cost photovoltaics. While these solution‐processed solar cells are compatible with large‐scale roll‐to‐roll processing, active layers used for typical laboratory‐scale devices are too thin to ensure high manufacturing yields. Furthermore, due to the limited light absorption and optical interference within the thin active layer, the external quantum efficiencies (EQEs) of bulk heterojunction polymer solar cells are severely limited. In order to produce polymer solar cells with high yields, efficient solar cells with a thick active layer must be demonstrated. In this work, the performance of thick‐film solar cells employing the low‐bandgap polymer poly(dithienogermole‐thienopyrrolodione) (PDTG‐TPD) was demonstrated. Power conversion efficiencies over 8.0% were obtained for devices with an active layer thickness of 200 nm, illustrating the potential of this polymer for large‐scale manufacturing. Although an average EQE > 65% was obtained for devices with active layer thicknesses > 200 nm, the cell performance could not be maintained due to a reduction in fill factor. By comparing our results for PDTG‐TPD solar cells with similar P3HT‐based devices, we investigated the loss mechanisms associated with the limited device performance observed for thick‐film low‐bandgap polymer solar cells.     

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.     

Electrode Considerations for the Optical Enhancement of Organic Bulk Heterojunction Solar Cells

In this paper, we consider the optical effects in conventional and inverted bulk heterojunction organic solar cells associated with various electrodes, and perform a systematic study on the anode and cathode buffer layers commonly used in high performance devices. In the devices produced here, we determine that parasitic absorption by low work function metals such as calcium reduces photocurrent by 25%, and that parasitic reflection at interfaces between the transparent conducting oxide and metal oxide buffer layers can reduce photocurrent by more than 10%. We also quantify the impact of an optical spacer, and determine that this accounts for only a fraction of the improvement that can be gained through alternative electrode optimization routes. It is therefore our intention that this study serves as a guideline for the optimization of the electrodes of organic thin film photovoltaic devices.     

Current Collecting Grids for ITO‐Free Solar Cells

Indium‐tin‐oxide (ITO) free polymer solar cells prepared by ink jet printing a composite front electrode comprising silver grid lines and a semitransparent PEDOT:PSS conductor are demonstrated. The effect of grid line density is explored for a large series of devices and a careful modeling study enabling the identification of the most rational grid structure is presented. Both optical and light beam induced current (LBIC) mapping of the devices are used to support the power loss model and to follow the evolution of the performance over time. Current generation is found to be evenly distributed over the active area initially progressing to a larger graduation in areas with different performance. Over time coating defects also become much more apparent in the LBIC images.     

Roll‐to‐Roll Printed Large‐Area All‐Polymer Solar Cells with 5% Efficiency Based on a Low Crystallinity Conjugated Polymer Blend

The challenge of continuous printing in high‐efficiency large‐area organic solar cells is a key limiting factor for their widespread adoption. A materials design concept for achieving large‐area, solution‐coated all‐polymer bulk heterojunction solar cells with stable phase separation morphology between the donor and acceptor is presented. The key concept lies in inhibiting strong crystallization of donor and acceptor polymers, thus forming intermixed, low crystallinity, and mostly amorphous blends. Based on experiments using donors and acceptors with different degree of crystallinity, the results show that microphase separated donor and acceptor domain sizes are inversely proportional to the crystallinity of the conjugated polymers. This methodology of using low crystallinity donors and acceptors has the added benefit of forming a consistent and robust morphology that is insensitive to different processing conditions, allowing one to easily scale up the printing process from a small‐scale solution shearing coater to a large‐scale continuous roll‐to‐roll (R2R) printer. Large‐area all‐polymer solar cells are continuously roll‐to‐roll slot die printed with power conversion efficiencies of 5%, with combined cell area up to 10 cm². This is among the highest efficiencies realized with R2R‐coated active layer organic materials on flexible substrate.     

Improving piezoelectric cell printing accuracy and reliability through neutral buoyancy of suspensions

The sedimentation and aggregation of cells within inkjet printing systems has been hypothesized to negatively impact printer performance. The purpose of this study was to investigate this influence through the use of neutral buoyancy. Ficoll PM400 was used to create neutrally buoyant MCF‐7 breast cancer cell suspensions, which were ejected using a piezoelectric drop‐on‐demand inkjet printing system. It was found that using a neutrally buoyant suspension greatly increased the reproducibility of consistent cell counts, and eliminated nozzle clogging. Moreover, the use of Ficoll PM400 was shown to not affect cellular viability. This is the first demonstration of such scale and accuracy achieved using a piezoelectric inkjet printing system for cellular dispensing. Biotechnol. Bioeng. 2012; 109: 2932–2940. © 2012 Wiley Periodicals, Inc.     

High Efficiency (>17%) Si‐Organic Hybrid Solar Cells by Simultaneous Structural,Electrical, and Interfacial Engineering via Low‐Temperature Processes

Highly efficient organic–inorganic hybrid solar cells of Si‐poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) have been demonstrated by simultaneous structural, electrical, and interfacial engineering with low processing temperature. Si substrate has been sculpted into hierarchical structure to reduce light reflection loss and increase interfacial junction area at the same time. Regarding the electrical optimization, highly conductive organic PEDOT:PSS layer has been formulated with low sheet resistance. It is argued that the sheet resistance, rather than conductivity, is the primary parameter for the high efficiency hybrid cells, which leads to the optimization of thickness, i.e., thick enough to have low sheet resistance but transparent enough to pass the incident sunlight. Finally, siloxane oligomers have been inserted into top/bottom interfaces by contact‐printing at room ambient, which suppresses carrier recombination at interfaces and reduces contact resistance at bottom electrode. Contrary to high‐temperature doping (for the formation of front surface or back surface fields), wet solution processes or vacuum‐based deposition, the contact‐printing can be done at room ambient to reduce carrier recombination at the interfaces. The high efficiency obtained with low processing temperature can make this type of cells be a possible candidate for post‐Si photovoltaics.     

Versatile N‐Doped MXene Ink for Printed Electrochemical Energy Storage Application

Printing is regarded as a revolutionary and feasible technique to guide the fabrication of versatile functional systems with designed architectures. 2D MXenes are nowadays attractive in printed energy storage devices. However, owing to the van der Waals interaction between the MXene layers, the restacking issues within the printed electrodes can significantly impede the ion/electrolyte transport and hence handicap the electrochemical performances. Herein, a melamine formaldehyde templating method is demonstrated to develop crumpled nitrogen‐doped MXene (MXene‐N) nanosheets. The nitrogen doping boosts the electrochemical performances of MXene via enhanced conductivity and redox activity. Accordingly, two types of MXene‐N inks are prepared throughout the optimization of the ink viscosity to fit the 2D screen printing and 3D extrusion printing, respectively. As a result, the screen printed MXene‐N microsupercapacitor delivers an areal capacitance of 70.1 mF cm^?2 and outstanding mechanical robustness. Furthermore, the 3D‐printed MXene‐N based supercapacitor manifests an areal capacitance of 8.2 F cm^?2 for a three‐layered electrode and readily stores a high areal energy density of 0.42 mWh cm^?2. The approach to harnessing such versatile MXene‐N inks offers distinctive insights into the printed energy storage systems with high areal energy density and large scalability.     

The Impact of Atmosphere on Energetics of Lead Halide Perovskites

Solar cells based on metal halide perovskites have emerged as a promising low‐cost photovoltaic technology. In contrast to inert atmospheres where most of the lab‐scale devices are made to date, large‐area low‐cost production of perovskite solar cells often involves processing of perovskites in various atmospheres including ambient air, nitrogen, and/or vacuum. Herein, the impact of atmosphere on the energy levels of methylammonium lead halide perovskite films is systematically investigated. The atmosphere is varied to simulate the typical fabrication process. Through a comprehensive analysis combining the Fermi level evolution, surface photovoltage, photoluminescence properties, photovoltaic performance, and device simulation, an overall landscape of the energy diagram of the perovskite layer is able to be determined. The findings have direct implications for real‐world devices under typical atmospheres, and provide insights into the fabrication‐process design and optimization. Furthermore, a universal Fermi level shift under vacuum for lead halide‐based perovskites revealed in this study, urges a refreshed view on the energetics studies conducted without considering the atmospheric effect.