Inverted Device Architecture Research Articles

Stability of bilayer trimethine cyanine dye/fullerene organic solar cells

The long-term (up to 100 days) stabilities of thin films of a trimethine cyanine dye (Cy3-P) and of Cy3-P/C60 bilayer solar cells with initial power conversion efficiencies of up to 3.6% in the regular and inverted device architecture were studied. For storage conditions under nitrogen at room temperature in the dark, Cy3-P films were stable and no decrease in performance was measured for inverted ITO/TiO2/C60/Cy3-P/MoO3/Ag solar cells. ITO/PEDOT:PSS and aluminum were found to be responsible for a slight inherent degradation of regular solar cells under inert storage conditions. Solar cells stressed at accelerated (80°C) full sun simulation showed a burn-in period of ~5 days with a performance loss of 75%. This initial performance decrease might be linked to the photopolymerization reaction of C60. For longer times, performances stabilized and arrived at 15% of the initial values after a test period of 17 days.

Inverted ITO- and PEDOT:PSS-free polymer solar cells with high power conversion efficiency

Highest published power conversion efficiencies of organic solar cells have mostly been achieved on substrates bearing a transparent indium tin oxide (ITO) electrode. However, the incorporation of ITO is not suited for future industrial production processes of organic solar cells, which will rely on a high-throughput of flexible substrates in order to achieve low cost of the final product. In this manuscript we present an alternative transparent electrode consisting of a layer stack of aluminum doped zinc oxide and a thin silver layer. Substrates with these electrodes have a transparency of above 75% in the wavelength range in which the photoactive layer absorbs light. Solar cells with a bulk-heterojunction of PTB7 and PC71BM in an inverted device architecture achieved a power conversion efficiency of 6.1%, which is the highest reported value for polymer solar cells free from both ITO and PEDOT:PSS. The sheet resistance of the novel electrodes increased only marginally after repeated bending which shows their full compatibility with future reel-to-reel processes or flexible products.

A Series of New Medium‐Bandgap Conjugated Polymers Based on Naphtho[1,2‐c:5,6‐c]bis(2‐octyl‐[1,2,3]triazole) for High‐Performance Polymer Solar Cells

A series of novel conjugated copolymers based on naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) are synthesized. These copolymers exhibit medium bandgaps of ≈1.9 eV. One of them demonstrates a high performance of up to 6.10% power conversion efficiency in a bulk-heterojunction (BHJ) solar-cell device. The performance can be further enhanced to 7.11% when applied in an inverted device architecture, using PF3 N-OX as an interfacial modifier.

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

AbstractWindow‐ 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.

Cathode buffer layers based on vacuum and solution deposited poly(3,4-ethylenedioxythiophene) for efficient inverted organic solar cells

Vacuum and solution processed versions of poly(3,4-ethylenedioxythiophene) (PEDOT) are used as cathode interlayers in inverted organic photovoltaic cells comprising tetraphenyldibenzoperiflanthene as the electron donor and C60 as the electron acceptor. Chemical treatment of the as-deposited PEDOT layers with tetrakis(dimethylamino)ethylene or cesium carbonate reduces the work function by up to 0.8 eV. Inserting these PEDOT layers at the indium tin oxide cathode results in improved electron collection and efficiencies of up to 2.3 ± 0.2%, approaching the 3.2 ± 0.3% of the conventional device. This illustrates the potential for efficient polymer cathode materials and inverted device architectures compatible with either solution or vacuum processing.

Highly Transparent and Conductive ZTO/Ag/ZTO Multilayer Top Electrodes for Large Area Organic Solar Cells

Transparent multilayer electrodes prepared by rf sputtering of zinc tin oxide (ZTO) and thermal evaporation of silver (Ag) as contact for organic electronics are reported. This multilayer has similar optical properties in the VIS range compared with ITO, though a significantly higher conductivity. Specifically we study the electrical and optical properties of the novel multilayer electrode and use it as bottom and top contact for semitransparent bulk hetero junction (BHJ) [poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester] solar cells with an inverted device architecture.

Charge‐Carrier Balance and Color Purity in Polyfluorene Polymer Blends for Blue Light‐Emitting Diodes

AbstractA study of an efficient blue light‐emitting diode based on a fluorescent aryl polyfluorene (aryl‐F8) homopolymer in an inverted device architecture is presented, with ZnO and MoO3 as electron‐ and hole‐injecting electrodes, respectively. Charge‐carrier balance and color purity in these structures are achieved by incorporating poly(9,9‐dioctylfluorene‐co‐N‐(4‐butylphenyl)‐diphenylamine (TFB) into aryl‐F8. TFB is known to be a hole‐transporting material but it is found to act as a hole trap on mixing with aryl‐F8. Luminance efficiency of ≈6 cd A−1 and external quantum efficiency (EQE) of 3.1% are obtained by adding a small amount (0.5% by weight) of TFB into aryl‐F8. Study of charge injection and transport in the single‐carrier devices shows that the addition of a small fraction of hole traps is necessary for charge‐carrier balance. Optical studies using UV–vis and fluorescence spectroscopic measurements, photoluminescence quantum yield, and fluorescence decay time measurements indicate that TFB does not affect the optical properties of the aryl‐F8, which is the emitting material in these devices. Luminance efficiency of up to ≈11 cd A−1 and EQE values of 5.7% are achieved in these structures with the aid of improved out‐coupling using index‐matched hemispheres.

Life cycle assessment of ITO-free flexible polymer solar cells prepared by roll-to-roll coating and printing

Indium is a scarce and expensive material that has been identified as a bottleneck for future organic electronics deployment in large scale. Indium is the main constituent of Indium Tin Oxide (ITO), which is the most successful transparent electrode in organic photovoltaics (OPV) so far. A new process, termed Hiflex, allows for manufacture of flexible OPV modules where the ITO electrode has been replaced by a sputtered Al/Cr electrode in an inverted device architecture with front illumination. This work presents a life cycle assessment of the Hiflex process, in order to compare the environmental impact of avoiding ITO as electrode. The new ITO-free process reduces some of the processing steps, leading to important reductions of the energy input during OPV module manufacturing in comparison to ITO-based modules. The environmental analysis reveals an Energy Pay-Back time (EPBT) of 10 years due to the high-energy consumption of Al/Cr roll-to-roll sputtering and to the relatively low efficiency of the Hiflex OPV modules (≈1%). An optimization of the active area fraction could easily reduce the EPBT to ≈5 years. A further enhancement of the efficiency to 5% would give rise to a promising EPBT of only 1 year. Our work highlights that vacuum processing steps should be avoided.

Polarizing Organic Photovoltaics

Polarizing organic photovoltaic (ZOPV) technology is demonstrated as a novel concept for energy harvesting and recycling technology. Novel, inverted quasi-bilayer device architecture is utilized to realize the ZOPV device. An anisotropic photovoltaic effect is obtained. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Efficient large area semitransparent organic solar cells based on highly transparent and conductive ZTO/Ag/ZTO multilayer top electrodes

We present transparent multilayer electrodes prepared by RF sputtering of zinc tin oxide (ZTO) and thermal evaporation of silver (Ag) as top contact for organic electronics. Specifically we study the electrical, optical and structural properties of the ZTO/Ag/ZTO (ZAZ) electrode. In the first step, we analyze the origin of high conductivity and high transparency by studying structural, electrical and optical properties of ultra thin Ag films. Secondly, we demonstrate that continuous and highly conductive Ag films can be deposited down to a thickness of 6 nm on top of ZTO. Furthermore we show, that owing to the stabilizing effect of the sputtered ZTO cap, ZAZ is substantially more robust than ZTO/Ag (ZA). As a first result we achieve a low sheet resistance of 5 Ω/sq and a transmissivity of 82% in the visible region of light. These values are compared to common transparent and semitransparent electrode materials deposited at room temperature like indium tin oxide (ITO). As an application we realized an optimized ZAZ structure as highly conductive and transparent top contact for large area semitransparent bulk hetero junction (BHJ) polymer solar cells (OSC) with an inverted device architecture. To expose the outstanding properties of the novel multilayer electrode we compare semitransparent OSCs using common ITO and ZAZ as top electrode cells with a large cell area >2 cm 2.

Effect of device architecture on hybrid zinc oxide nanoparticle:poly(3-hexylthiophene) blend solar cell performance and stability

Hybrid zinc oxide nanoparticle (ZnO np):poly(3-hexylthiophene) (P3HT) photovoltaic devices with a blend morphology in the active layer show up to a ninefold improvement in device efficiency above devices with a planar donor–acceptor interface. However, blend devices in a conventional architecture have very poor stability upon white light exposure. Blend devices in an inverted architecture have not been previously achieved because the commercially available poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole transport layer etches the ZnO when PEDOT:PSS is deposited on top of the ZnO np:P3HT blend. Here we report the successful demonstration of an inverted ZnO np:P3HT blend solar cells that is made possible through the use of a pH neutralized PEDOT:PSS hole transport layer, and show how the inverted device architecture leads to greatly improved device stability under white light exposure.

A Review on the Development of the Inverted Polymer Solar Cell Architecture

The increase in energy production costs for fossil fuels has led to a search for an economically viable alternative energy source. One alternative energy source of particular interest is solar energy. A promising alternative to inorganic materials is organic semiconductor polymer solar cells due to their advantages of being cheaper, light weight, flexible and made into large areas by roll-to-roll processing. However, the conventional architecture that is typically used for fabricating solar cells requires high vacuum to deposit the top metal electrode which is not suitable for roll-to-roll processing. Recently an inverted device architecture has been investigated as a suitable architecture for developing the ideal roll-to-roll type processing of polymer-based solar cells. This review will go over the recent advances and approaches in the development of this type of inverted device architecture. We will highlight some of the work that we have done to integrate materials, device, interface, and processing of the inverted device architecture platform to produce more idealized polymer-based solar cells.

Organic solar cells incorporating buffer layers from indium doped zinc oxide nanoparticles

Monodisperse, indium doped zinc oxide (IZO) nanoparticles were prepared via the polyol-mediated synthesis and incorporated into regular and inverted poly-(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C 61-butyric acid methyl ester organic photovoltaic devices as buffer layers between the active layer and the cathode. Efficient hole blocking at the particle buffer layers leads to an enhanced open-circuit voltage of the solar cells. This effect is even more pronounced for inverted device architectures. Device degradation studies revealed a solar cell performance reduction upon sample exposition to ambient atmosphere. However, this degradation is fully reversible under UV illumination. In addition, the n-doped IZO particles form suitable charge carrier transport layers for an efficient recombination in an intermediate recombination zone in tandem solar cells. Accordingly we have fabricated fully solution-processed tandem solar cells and investigated their optoelectronic properties.

Effect of Fabrication Parameters on Three-Dimensional Nanostructures of Bulk Heterojunctions Imaged by High-Resolution Scanning ToF-SIMS

Solution processable fullerene and copolymer bulk heterojunctions are widely used as the active layers of solar cells. In this work, scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to examine the distribution of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly(3-hexylthiophene) (rrP3HT) that forms the bulk heterojunction. The planar phase separation of P3HT:PCBM is observed by ToF-SIMS imaging. The depth profile of the fragment distribution that reflects the molecular distribution is achieved by low energy Cs(+) ion sputtering. The depth profile clearly shows a vertical phase separation of P3HT:PCBM before annealing, and hence, the inverted device architecture is beneficial. After annealing, the phase segregation is suppressed, and the device efficiency is dramatically enhanced with a normal device structure. The 3D image is obtained by stacking the 2D ToF-SIMS images acquired at different sputtering times, and 50 nm features are clearly differentiated. The whole imaging process requires less than 2 h, making it both rapid and versatile.

Flexible Polymer Photovoltaic Devices Prepared With Inverted Structures on Metal Foils

We have prepared top-illuminated organic photovoltaic devices on stainless-steel substrates, using molybdenum trioxide/indium tin oxide as the transparent top electrode. Incorporating an Al counter electrode grid having a shadow fraction of 10% effectively reduced the device resistance, thereby enhancing the power conversion efficiency to ca. 3%. This device exhibited superior mechanical flexibility. The inverted device architecture eliminated the need to use low-work-function metals, which are air sensitive and easily oxidized, thereby improving the air stability of the flexible devices.