Jon Eggert Carlé

Photochemical stability of π-conjugated polymers for polymer solar cells: a rule of thumb

A comparative photochemical stability study of a wide range of π-conjugated polymers relevant to polymer solar cells is presented. The behavior of each material has been investigated under simulated sunlight (1 sun, 1000 W m−2, AM 1.5G) and ambient atmosphere. Degradation was monitored during ageing combining UV-visible and infrared spectroscopies. From the comparison of the collected data, the influence of the polymer chemical structure on its stability has been discussed. General rules relative to the polymer structure–stability relationship are proposed.

Scalable, ambient atmosphere roll-to-roll manufacture of encapsulated large area, flexible organic tandem solar cell modules

Inline printing and coating methods have been demonstrated to enable a high technical yield of fully roll-to-roll processed polymer tandem solar cell modules. We demonstrate generality by employing different material sets and also describe how the ink systems must be carefully co-developed in order to reach the ambitious objective of a fully printed and coated 14-layer flexible tandem solar cell stack. The roll-to-roll methodologies involved are flexographic printing, rotary screen printing, slot-die coating, X-ray scattering, electrical testing and UV-lamination. Their combination enables the manufacture of completely functional devices in exceptionally high yields. Critical to the ink and process development is a carefully chosen technology transfer to industry method where first a roll coater is employed enabling contactless stack build up, followed by a small roll-to-roll coater fitted to an X-ray machine enabling in situ studies of wet ink deposition and drying mechanisms, ultimately elucidating how a robust inline processed recombination layer is key to a high technical yield. Finally, the transfer to full roll-to-roll processing is demonstrated.

Upscaling from single cells to modules – fabrication of vacuum- and ITO-free polymer solar cells on flexible substrates with long lifetime

Fabrication of polymer solar cell (PSC) modules was done on a previously reported compact coating/printing machine and tested in a readily scalable roll process on flexible substrates without applying vacuum, ITO or spin coating. Our aim was to establish loss upon scaling from cells to small modules. We studied from single cells (1 cm2) to modules comprising four serially connected devices with a total active area of 8 cm2. Four different polymers (P3HT, PV-D4610, PDTSTTz-4 and PBDTTTz-4) were applied in the preparation of the modules and efficiencies of more than 3% were achieved which is comparable to single cell devices prepared using the same process. This proves that it is possible to scale up new materials in an ITO free device context to modules without having an efficiency drop, due to reliable and consistent processing. The main loss observed was due to the packaging using barrier materials. The photochemical stability of the materials was therefore studied using intense light along with the operational stability of the corresponding devices according to the ISOS-D-1 and ISOS-L-1 standards. For devices under constant illumination we found that barrier materials from Mitsubishi and 3M provide better operational stability over time, compared to the barrier foil obtained from Amcor, for all the polymers, which is ascribed to the cut-off at higher wavelengths thereby lowering the degree of UV light that reaches the device. When comparing the operational stability of the four polymers under constant illumination, P3HT generally retains its performance better with higher T80 values, while the polymer PV-D4610 shows the highest PCE (1.6%) after 300 hours of operation.

Photochemical stability of conjugated polymers, electron acceptors and blends for polymer solar cells resolved in terms of film thickness and absorbance

Photochemical degradation at 1 sun under AM1.5G illumination was performed on six conjugated polymers and five different electron acceptors. Additionally, the respective polymer:PC60BM and P3HT:electron acceptor blends were studied, and all degradations were resolved in terms of film thickness and absorbance. A fully automated degradation setup allowed for inclusion of in excess of 1000 degradations in this study to enable a discussion of reliability of the technique. Degradation rates were found to increase exponentially with decreasing film absorbance for all materials. The relative stabilities within each material group were found to vary for both the pure polymers and the blends. The stability ranking between the materials of the pure polymers was found to be similar to the ranking for their respective blends, implying that the photochemical stability of a pure polymer is a good measure of its associated blend stability. Different electron acceptors were found to stabilize P3HT decreasingly with decreasing donor–acceptor LUMO–LUMO gap. Destabilization of P3HT was observed in the case of the electron acceptor ICBA. Additionally, the decreased stabilization of P3HT by high LUMO electron acceptors poses a challenge to solar cell encapsulation if these materials are to be of commercial interest. The presented method is generally applicable to all types of organic materials to assess photochemical stabilities. The presented results of conjugated polymers demonstrate that this is a powerful tool for conjugated polymer stability assessment if the results are interpreted correctly.

High‐Volume Processed, ITO‐Free Superstrates and Substrates for Roll‐to‐Roll Development of Organic Electronics

The fabrication of substrates and superstrates prepared by scalable roll‐to‐roll methods is reviewed. The substrates and superstrates that act as the flexible carrier for the processing of functional organic electronic devices are an essential component, and proposals are made about how the general availability of various forms of these materials is needed to accelerate the development of the field of organic electronics. The initial development of the replacement of indium‐tin‐oxide (ITO) for the flexible carrier materials is described and a description of how roll‐to‐roll processing development led to simplification from an initially complex make‐up to higher performing materials through a more simple process is also presented. This process intensification through process simplification is viewed as a central strategy for upscaling, increasing throughput, performance, and cost reduction.

Comparative studies of photochemical cross-linking methods for stabilizing the bulk hetero-junction morphology in polymer solar cells

We are here presenting a comparative study between four different types of functionalities for cross-linking. With relatively simple means bromine, azide, vinyl and oxetane could be incorporated into the side chains of the low band-gap polymer TQ1. Cross-linking of the polymers was achieved by UV-light illumination to give solvent resistant films and reduced phase separation and growth of PCBM crystallites in polymer:PCBM films. The stability of solar cells based on the cross-linked polymers was tested under various conditions. This study showed that cross-linking can improve morphological stability but that it has little influence on the photochemical stability which is also decisive for stable device operation under constant illumination conditions.

Rapid flash annealing of thermally reactive copolymers in a roll-to-roll process for polymer solar cells

Light induced thermocleaving of a thermally reactive copolymer based on dithienylthiazolo[5,4-d]thiazole (DTZ) and silolodithiophene (SDT) in contact with the heat sensitive substrate the heat sensitive substrate polyethyleneterphthalate (PET) was effectively demonstrated with the use of high intensity pulsed light, delivered by a commercial photonic sintering system. Thermally labile ester groups are positioned on the DTZ unit of the copolymer that can be eliminated thermally for enhanced photochemical stability and advantages in terms of processing (solubility/insolubility switching). The photonic sintering system was successfully implemented in a full roll-to-roll process on flexible PET substrates and large-area polymer solar cell modules were prepared by solution processing of five layers under ambient conditions using the photonic sintering system for thermocleaving of the active layer. The PET foil did not show any deformation after exposure to the high intensity light only leaving the insoluble thermocleaved active layer. The active layer remained planar after light exposure thereby allowing the coating of supplementary material on top.

Polymers for organic photovoltaics based on 1,5-bis(2-hexyldecyloxy)-naphthalene, thiophene, and benzothiadiazole

Two new conjugated polymers consisting of the donors 1,5-bis(2-hexyldecyloxy)naphthalene, thiophene, or bithiophene and the acceptor benzothiadiazole has been synthesized and their optical and photovoltaic properties have been characterized. The two polymers were compared with earlier synthesized and characterized polymers containing benzene instead of naphthalene. The two polymers absorb light in the visible spectrum (400 to 700 nm). The naphthalene containing polymers had blueshifted absorption spectra compared to the benzene containing polymers and also higher band gaps. In photovoltaic devices the bithiophene containing polymer gave the best efficiency of 0.6%, whereas the single thiophene only showed efficiency of 0.005%. This is lower than the best benzene incorporated polymer that showed efficiency up to 2.2%.

In-line, roll-to-roll morphology analysis of organic solar cell active layers

We present the first comparative in situ small and wide angle X-ray scattering study of two polymers that are relevant for organic photovoltaics, during coating on a flexible substrate. From the obtained measurements we identified several differences between the drying of the two polymers. The polymer optimized for roll-to-roll coating attained its final morphological packing nearly instantly after deposition, and had the shortest drying profile. We therefore conclude that fast-drying polymers which are influenced less by drying temperature or substrate inhomogeneities are better suited for roll-to-roll coating, and that fundamentally, the kinetics of drying dominate the process in the case of roll-to-roll slot-die coating.

Conjugated Polymers Via Direct Arylation Polymerization in Continuous Flow: Minimizing the Cost and Batch-to-Batch Variations for High-Throughput Energy Conversion

Continuous flow methods are utilized in conjunction with direct arylation polymerization (DArP) for the scaled synthesis of the roll-to-roll compatible polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(4,7-di(thiophen-2-yl)-benzo[c][1,2,5]thiadiazole)] (PPDTBT). PPDTBT is based on simple, inexpensive, and scalable monomers using thienyl-flanked benzothiadiazole as the acceptor, which is the first β-unprotected substrate to be used in continuous flow via DArP, enabling critical evaluation of the suitability of this emerging synthetic method for minimizing defects and for the scaled synthesis of high-performance materials. To demonstrate the usefulness of the method, DArP-prepared PPDTBT via continuous flow synthesis is employed for the preparation of indium tin oxide (ITO)-free and flexible roll-coated solar cells to achieve a power conversion efficiency of 3.5% for 1 cm2 devices, which is comparable to the performance of PPDTBT polymerized through Stille cross coupling. These efforts demonstrate the distinct advantages of the continuous flow protocol with DArP avoiding use of toxic tin chemicals, reducing the associated costs of polymer upscaling, and minimizing batch-to-batch variations for high-quality material.