Asymmetric photocurrent extraction in semitransparent laminated flexible organic solar cells

Abstract

1Scalable production methods and low-cost materials with low embodied energy are key to success for organic solar cells. PEDOT (PSS) electrodes meet these criteria and allow for low-cost and all solution-processed solar cells. However, such devices are prone to shunting. In this work we introduce a roll-to-roll lamination method to construct semitransparent solar cells with a PEDOT(PSS) anode and an polyethyleneimine (PEI) modified PEDOT(PSS) cathode. We use the polymer:PCBM active layer coated on the electrodes as the lamination adhesive. Our lamination method efficiently eliminates any shunting. Extended exposure to ambient degrades the laminated devices, which manifests in a significantly reduced photocurrent extraction when the device is illuminated through the anode, despite the fact that the PEDOT(PSS) electrodes are optically equivalent. We show that degradation-induced electron traps lead to increased trap-assisted recombination at the anode side of the device. By limiting the exposure time to ambient during production, degradation is significantly reduced. We show that lamination using the active layer as the adhesive can result in device performance equal to that of conventional sequential coating.npj Flexible Electronics (2018) 2:4 ; doi:10.1038/s41528-017-0017-6 INTRODUCTIONThere is an urgent need to replace the present fossil-based energy production with sustainable energy sources. Thin-film solar cells and organic photovoltaics (OPV) in particular are technologies with a large potential to contribute to the energy transition due to very short energy payback times 1 and scalable production methods.2 The best devices approach 12% power conversion efficiency (PCE) 3 and due to improved OPV performance under low light-intensity conditions the yearly-average energy output is improved, reducing the competitive gap between OPV and silicon photovoltaics. Nevertheless, the OPV technology is not intended to compete with the high PCE of silicon-based solar cell installations but is instead targeted for entry markets that require the unique features of OPV, such as curved form factors enabled by flexible substrates, color tunability, semitransparency, low weight and the uniquely low energy payback times.To realize these advantages materials and production methods must be scalable with high throughput, such as roll-to-roll printing under ambient atmosphere. Vacuum-based processing, commonly used to deposit reflective (such as aluminum/chromium or silver) or transparent metal electrodes (indium tin oxide (ITO)), should ideally be avoided due to their high energy input.4 Indium in ITO is in addition too scarce to be a scalable alternative. Solution-processed electrodes have a significantly lower embodied energy and allow for the high speed printing needed for scalable production. To fully utilize the benefits of solution processing all layers in the device should be coated or printed.The use of semitransparent Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), PEDOT(PSS) electrodes for both the anode

Large‐Area, Semitransparent, and Flexible All‐Polymer Photodetectors

“…Upon illumination from the anode side, more generated electrons and holes are close to the anode and far away from the cathode. If the electron transport is less efficient than the hole transport, the electrons may not reach the cathode but recombine in this thick active layer . As depicted in Figure c,d, the corresponding EQE agreed well with the photocurrent measurements at the bias voltages.…”

“…After that, active layers were spin coated on top of both the PEDOT: PSS and PEDOT: PSS/PEI films. Finally, the two stacks were laminated by using a roll laminator with roll temperature of 120 °C . The device configuration was PET/PEDOT: PSS (200 nm)/PEI (5 nm)/Active layer (350 nm)/PEDOT: PSS (200 nm)/PET.…”

Organic Photovoltaics over Three Decades

“…[32,197,198] This method was used to build optical microcavity devices for lasers, [124] and Magnus Granström with a PhD from my group, later went for a postdoc with Richard Friend in Cambridge to use lamination for OPV devices. [200] Laminated devices are our choice in making use of the currently developing plethora of high performance materials for OPV (Figure 12). In recent work, our large area www.advmat.de www.advancedsciencenews.com modules are produced by lamination of the active layer to the active layer.…”

“…To correlate the drying process to the function of the deposited material, in terms of the photocurrent generation, we developed new imaging tools for inline imaging [230] and for off-line imaging of recombination processes. [232] Here we first printed the semitransparent electrode, and on top printed the active layer. Deposition of the same material at every point is a serious challenge, as are the difficulties of handling printing and lamination in atmosphere.…”

All‐Polymer High‐Performance Photodetector through Lamination

“…Compared to polymer/fullerene OPDs, all polymer OPDs has advantages of mechanical robustness and more flexibility of spectral coverage. Combined with roll‐to‐roll compatible lamination method,20,21 all polymer BHJ or bilayer OPDs can be easily fabricated.…”