In the field of photovoltaics (PV), an important trend is to increase the aesthetic and creative design aspects of solar cells towards more attractive and customized devices for integration in for instance architecture (e.g. Building Integrated Photovoltaics BIPV). Recent evolutions in this domain are mainly situated in the class of emerging PV such as organic solar cells (OPV), dye sensitized solar cells (DSSC) and perovskite solar cells. These solar cell technologies provide additional degrees of design freedom, as they can be processed by printing and allow the realization of semi-transparent solar cells and the use of various colors. Here we aim to go a step further. In this contribution we report on our aim to develop semi-transparent solar cells with integrated images (photographs, paintings, geometric and graphical patterns, text,..) that generate electricity when illuminated, a concept we termed as “Photovoltaic Photographs”. The proposed concept consists of semi-transparent solar cells with an integrated image (functional as photoactive layer), allowing creative applications such as photovoltaic photographs, paintings, posters, etc. The approach proposed here to obtain a patterned 2D-photoactive layer, is by using direct photo-induced patterning process, i.e. one-step photolithography (with mask / without resist) and direct light writing (i.e. maskless / without resist). Encouraged by our recent realization of a proof-of-principle demonstrator using dye sensitized solar cells, in this project we pursue a thorough understanding and control of the proposed light-induced patterning processes and underlying physicochemical (“bleaching”) mechanisms, and their effects on nanoscale material properties, device characteristics, and stability. These insights will also help us in the exploration of combining this concept and these processes with other PV technologies.
KEYWORDS: Organic photovoltaics, Solar cells, Dielectric spectroscopy, Dielectrics, Heterojunctions, Renewable energy, Chemical engineering, Physics, Current controlled current source
Organic photovoltaics (OPV) show strong potential for a number of renewable energy applications because of some specifically appealing features (light weight, flexibility, color, …). Over the past decade, the power conversion efficiencies of organic solar cells have strongly risen to values surpassing the 10% threshold, mainly due to strong efforts in chemical engineering of the photoactive components, architectural device optimization and acquisition of fundamental insights in the underlying device physics. As part of the device optimization, the use of conjugated polyelectrolyte (CPE) interfacial layers has been introduced as a popular and powerful way to boost the inherent I-V characteristics. In the presented work, we applied impedance spectroscopy to probe the dielectric permittivity of a series of polythiophene-based CPE interlayer materials as a means to postulate design rules toward novel generation interfacial layers. The presence of ionic pendant groups grants the formation of a capacitive double layer, boosting the charge extraction and device efficiency. A counteracting effect is that the material’s affinity with respect to the underlying photoactive layer diminishes. To enhance the interlayer-photoactive layer compatibility, copolymer structures containing a certain amount of non-ionic side chains are found to be beneficial.
Ultrasonic spray coating is currently proven to be a reliable, flexible and cost efficient fabrication method for printed electronics [1-2]. Ultrasonic nozzles are by design especially well-suited to deposit nano-suspension dispersions. Due to the ultrasonic vibration of the nozzle, droplets having a median diameter of 20 μm are created in a homogeneous droplet cloud and directed towards the substrate. When one prepares an ink having the right wetting properties, thin and homogeneous layers, fully covering the surface, can be achieved. Together with conjugated polymer nanoparticles (NPs), emerging as a new class of nanomaterials, [3] it opens possibilities towards eco-friendly roll-to-roll processing of state-of-the-art organic bulk heterojunction solar cells.
A ultrasonic spray coater was used to print the conjugated polymer NP layers under different conditions. A first optimization of the spray coater settings (flow rate, spray speed and temperature) and the ink formulation (water and co-solvent mixture and NP content) was performed for polystyrene particles dissolved in a water-ethanol mixture. As a next step, the low bandgap donor polymer poly[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophene-diyl] (PCDTBT) [4] and the fullerene acceptor phenyl-C71-butyric acid methyl ester (PCBM[70]) were combined in a water-based blend NP dispersion which was prepared using the mini-emulsion technique. [5,6] Optical Microscopy, profilometry and Scanning Electron Microscopy (SEM) are performed to study the roughness, surface structure, thickness and coverage of the spray coated layers. Finally the printed NP layers are integrated in organic bulk heterojunction solar cells and compared to spin coated reference devices.
Pi-conjugated polymer light emitting devices have the potential to be the next generation of solid state lighting. In order to achieve this goal, a low cost, efficient and large area production process is essential. Polymer based light emitting devices are generally deposited using techniques based on solution processing e.g.: spin coating, ink jet printing. These techniques are not well suited for cost-effective, high throughput, large area mass production of these organic devices. Ultrasonic spray deposition however, is a deposition technique that is fast, efficient and roll to roll compatible which can be easily scaled up for the production of large area polymer light emitting devices (PLEDs). This deposition technique has already successfully been employed to produce organic photovoltaic devices (OPV)1. Recently the electron blocking layer PEDOT:PSS2 and metal top contact3 have been successfully spray coated as part of the organic photovoltaic device stack. In this study, the effects of ultrasonic spray deposition of polymer light emitting devices are investigated. For the first time – to our knowledge -, spray coating of the active layer in PLED is demonstrated. Different solvents are tested to achieve the best possible spray-able dispersion. The active layer morphology is characterized and optimized to produce uniform films with optimal thickness. Furthermore these ultrasonic spray coated films are incorporated in the polymer light emitting device stack to investigate the device characteristics and efficiency. Our results show that after careful optimization of the active layer, ultrasonic spray coating is prime candidate as deposition technique for mass production of PLEDs.
When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups.
In this Proceedings paper, we report on the synthesis of a family of polythiophene-based conjugated polyelectrolytes, both homopolymers and random copolymers varying in the building block ratio and counter ions, toward a better fundamental understanding of the structure-property relations of these ionic derivatives in organic photovoltaics. One of the ionic homopolymers was successfully implemented as a donor material in fully solution-processed efficient bi-layer solar cells (up to 1.6% PCE in combination with PC71BM) prepared by the low impact meniscus coating technique. On the other hand, these imidazolium-substituted polythiophenes were also applied as materials for electron transport layers (ETLs), boosting the I-V properties of PCDTBT:PC71BM solar cell devices up to average PCE values of 6.2% (~20% increase), which is notably higher than for previously reported ETL materials. Advanced scanning probe microscopy techniques were used to elucidate the efficiency enhancing mechanism.
Optimizing the post-production annealing conditions of polymer:fullerene bulk heterojunction solar cells is vitally
important, not only for fine-tuning the morphology - thus increasing the efficiency - but also for retaining the desired
morphology during long-term operation. However, optimal conditions for annealing temperatures and times can only be
chosen, once thermal transition temperatures and annealing kinetics of the blends are well-known. For instance, for
systems with glass transition temperatures (Tg) lower than the maximum device operation temperature of 80°C, the
mobility needed for morphology coarsening is present, leading to efficiencies decreasing in the course of time. Using
advanced fast-scanning thermal analysis techniques, the formation of nuclei and growth of crystals during heating or
cooling can be reduced or avoided, and thus, the fast crystallization processes occurring during annealing of the
polymer:fullerene blends can be followed. In this study, non-isothermal and isothermal crystallization kinetics of the
P3HT:PCBM (poly(3-hexyl thiophene: [6,6] -phenyl C61 - butyric acid methyl ester) and P3HT:bis-PCBM blends are
investigated and compared by using Rapid Heating Cooling Calorimetry (RHC).
Poly-3-AlkylThiophenes (P3ATs) with an n-alkyl chain length varying from C3 till C9 were synthesized by
using the Rieke method. Subsequently, these materials were used to make P3AT/PCBM blends which were investigated
in bulk heterojunction (BHJ) solar cells. The phase diagram of a P3H(exyl)T:PCBM blend was measured by means of
standard and modulated temperature differential scanning calorimetry (DSC and MTDSC). A single glass transition is
observed for all compositions. The glass transition temperature (Tg) increases with increasing PCBM concentration: from
12 °C for pure P3HT to 131 °C for pure PCBM. The observed range of Tg's defines the operating window for thermal
annealing and explains the long-term instability of both morphology and photovoltaic performance of P3HT:PCBM solar
cells. All regioregular P3ATs allow for efficient fiber formation in several solvents. The fibers formed are typically 15 to
25 nm wide and 0.5 to >4 μm long and mainly crystalline. By means of temperature control the fiber content in the
casting solution for P3AT:PCBM BHJ solar cells is controlled while keeping the overall molecular weight of the polymer
in the blend constant. In this way, fiber isolation and the use of solvent mixtures are avoided and with P3HT nanofibers,
a power conversion efficiency of 3.2 % was achieved. P3AT:PCBM BHJ solar cells were also prepared from P3B(utyl)T,
P3P(entyl)T and P3HT using the good solvent o-dichlorobenzene and a combination of slow drying and thermal
annealing. In this way, power conversion efficiencies of 3.2, 4.3, and 4.6 % were obtained, respectively. P3PT is proved
to be a potentially competitive material compared to P3HT.
Control of morphology is a key issue in order to improve the performance of organic bulk heterojunction solar
cells. Solar cells consisting of a blend of regioregular P3HT (poly(3-hexylthiophene)) and PCBM ([6-6]-phenyl
C61 butyric acid methyl ester) have demonstrated the highest efficiencies until now (up to 5 %). This
performance was achieved by applying a post-production annealing, which is considered to induce a dual
crystallization behavior. In order to control and tune the morphology, the phase behavior needs to be
described in terms of the underlying fundamental thermodynamics. Hence, it is essential to obtain a phase
diagram of the blend. In this study, the state diagram of P3HT:PCBM blends is measured by means of
standard and modulated temperature differential scanning calorimetry (DSC). For the first time, the glass
transition (Tg) of PCBM could be determined. All blends evidenced a single Tg, indicating an homogeneous
blend is formed. Phase separation is thus only induced from crystallization and no "intrinsic" phase separation
is occurring in the blend.
This paper investigates the thermal stability of organic bulk heterojunction solar cells, with a special focus on the thermal ageing of both photovoltaic parameters and morphology of the active layer. The photovoltaic parameters of a set of bulk heterojunction solar cells were determined by IV-characterization and their bulk morphology was investigated with transmission electron microscopy (TEM). A link could be made between the degradation of the short circuit current under a thermal treatment and the corresponding change in bulk morphology. A possible improvement of the thermal stability of bulk heterojunction solar cells is presented through the use of a polymer with higher glass transition temperature.
A new precursor route towards conjugated polymers is presented. Whereas difficulties occurred for the preparation of poly(2,5-thienylene vinylene) (PTV) derivatives via the existing precursor routes, PTV has been synthesised via a new developed "dithiocarbamate route" in good yields and satisfactory molecular weight. Structural characterisations of the conjugated polymers reveal an optical band gap around 1.7 eV. Organic field effect transistors and organic based photovoltaic devices were made and the results are discussed. Solar cells were produced using a blend of the precursor polymer and PCBM at various ratios. The conversion of the precursor polymer towards the conjugated polymer was performed in situ in film spin-coated from the blend. Promising energy conversion efficiencies were observed which were still improved by thermal annealing of the device at 70°C.
Optical absorption phenomena and in particular sub band gap absorption features are of great importance in the understanding of processes of charge generation and transport in organic pure and composite semiconductor films. To come towards this objective, an alternative and high sensitive spectroscopic approach is introduced to examine the absorption of light in pure and compound organic semiconductors. Because sub band gap absorption features are typically characterized by very low absorption coefficients, it is not possible to resolve them using common transmission and reflection measurements and high sensitive alternatives are needed. Therefore, a combination of photocurrent (Constant Photocurrent Method CPM/Fourier Transform Photocurrent Spectroscopy FT-PS) and photothermal techniques (Photothermal Deflection Spectroscopy PDS) has been used, increasing sensitivity by a factor of thousand, reaching detectable absorption coefficients ((E) down to 0.1 cm-1. In this way, the dynamic range of measurable absorption coefficients is increased by several orders of magnitude compared to transmission/reflection measurements. These techniques have been used here to characterize ground state absorption of thin films of MDMO-PPV, PCBM and a mixture of both materials in a 1:4 ratio, as typically used in a standard active layer in a fully organic solar cell. The spectra reveal defect related absorption phenomena and significant indication of existing interaction in the ground state between both materials, contrary to the widely spread conviction that this is not the case. Experimental details of the techniques and measurement procedures are explained.
The microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells was studied by Atomic Force Microscopy (AFM) and Kelvin Force Microscopy (KFM) to image the surface morphology and by means of Transmission Electron Microscopy (TEM) to reveal images of the film's interior.
By introducing KFM, it was possible to demonstrate that phase separated domains have different local electrical properties than the surrounding matrix. Since blend morphology clearly influences global electrical properties and photovoltaic performance, an attempt to control the morphology by means of casting conditions was undertaken. By using AFM, it has been proven that not only the choice of solvent, but also drying conditions dramatically influence the blend structure. Therefore, the possibility of discovering the blend morphology by AFM, KFM and TEM makes them powerful tools for understanding today's organic photovoltaic performances and for screening new sets of materials.
Current state-of-the-art bulk hetero-junction organic photovoltaic devices will be discussed based on poly(2-methoxy-5-(3',7'-dimethyl-octyloxy))-p-phenylene vinylene, (MDMO-PPV), as an electron donor and (6,6)-phenyl-C61-butric-acid (PCBM)(a soluble C60 derivative) as electron acceptor. A brief review will be provided summarizing recent results on efficiency enhancement on morphological investigations. A significant increase in power conversion efficiency has been demonstrated for devices based on so-called 'sulphinyl' synthesized MDMO-PPV (ηAM1.5 = 2.9%) in comparison with devices based on 'Gilch' synthesized MDMO-PPV (ηAM1.5 = 2.5%). In order to understand the higher efficiency values obtained using a different solvent or a different MDMO-PPV-material, electrical and morphological investigations are being performed. Concerning the latter, it has been shown with various analytical techniques that the morphology of the blended photoactive films and also the power conversion efficiency of the corresponding photovoltaic devices are both simultaneously influenced by preparation conditions such as choice of the solvent and drying conditions.
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