A Semi-transparent Plastic Solar Cell Fabricated by a Lamination Process†

Polymer solar cells have attracted broad research interest because of their advantageous solution processing capability and formation of low-cost, flexible, and large area electronic devices. However, the efficiency of polymer solar cells is still low compared to that of inorganic solar cells. Therefore, it is a challenge to find a polymer that has all the required properties for high efficiency devices, such as strong and broad absorption, high carrier mobility, and appropriate energy levels. One possible solution to avoid the strict material requirements is to stack two or more devices with different spectral responses, which enables more efficient utilization of solar energy. Such a solution would require a semitransparent solarcell device with high efficiency in its absorption wavelength range, while high transparency would be required in the complementary wavelength range. Semitransparent solar cells are also interesting for other appealing applications, such as energy-generating color window glasses. It is desirable that such solar cell devices can be fabricated using a low-cost strategy, such as the roll-to-roll fabrication process. One critical issue in this fabrication process is how to form the active-layer/cathode mechanic and electronic contacts. The lamination process is one very promising technique to fulfill this requirement owing to its simplicity and low cost. It has been reported to produce two-layer heterojunction solar cells; however, the method is not applicable to bulk heterojunction solar cells, nor compatible with roll-to-roll fabrication process. In this Communication, an electronic glue-based lamination process combined with interface modification is presented as a one-step process for semitransparent polymer solar-cell fabrication. The finished device is metalfree, semitransparent, flexible, self-encapsulated, and highly efficient (with a maximum external quantum efficiency of 70 % and power efficiency of 3 % under AM 1.5 global 1 sun solar illumination conditions with spectral mismatch correction). This approach represents a critical step towards the ultimate goal of low-cost polymer solar cells. The device fabrication process is illustrated in Figure 1, and can be described by the following steps. In Step I, two transparent substrates coated with a transparent conductor such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), or a high conductivity polymer, etc., are selected. In Step II, one substrate is coated with a very thin buffer layer (Cs2CO3 ) to act as the low-work-function cathode, followed by coating of the active polymer layer. Step III involves the coating of conductive polymer glue to the other transparent substrate. We used modified conducting polymer poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the electronic glue, which was spin-coated to form the adhesive anode. Step IV is the lamination process: after drying both the substrates, they are laminated together by exerting force so that the two substrates are tightly glued together. During this lamination, a plastic rod with proper hardness rolls the plastic substrate to remove air bubbles. Both substrates are heated to a temperature of 105–120 °C during the lamination process, and the finished devices are then kept on the hotplate for 5–10 min for the final heat treatment. The PEDOT:PSS was purposely modified to become adhesive, so that the two separate films formed good contact at the interface, both electronically and mechanically. In this work, this adhesive and conductive PEDOT:PSS layer was obtained by doping D-sorbitol or volemitol into PEDOT:PSS, as has been successfully demonstrated in polymer light emitting diodes. However, the efficiency of such a device is too low for application. The polymer blend used in this work is regioregular poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (RRP3HT:PCBM) in 1:1 w/w ratio. The 200 nm thick polymer blend film was deposited by the slow-growth method (or solvent annealing) to enhance device efficiency. Either glass or plastic can be used as the transparent substrate. Figure 1b shows a picture of an all-plastic solar cell. The device area is ca. 40 mm. With both cathode and anode being transparent, a semitransparent polymer solar cell is formed. The transparency (T%) of the device is shown in Figure 1c, together with the solar illumination spectrum. A transparency of around 70 % was obtained in the wavelength range where polymer/PCBM has no absorption, which makes this device suitable for application in stacking devices to make full use of the solar spectrum. This device fabrication method has many advantages over the regular procedure. First of all, no thermal evaporation process is involved in the process, and each layer is coated by a low-cost and easy solution process. Second, in contrast to the reactive metal cathode in regular devices, the cathode in C O M M U N IC A IO N