Efficient Polymer Solar Cells Fabricated by Simple Brush Painting

Abstract

Since the report of a molecular thin-film organic solar cell (OSC) by Tang, organic materials have increasingly become attractive candidates for the fabrication of cost-efficient and flexible photovoltaic cells. In particular, polymer bulkheterojunction (BHJ) solar cells based on interpenetrating networks of an electron donor and an acceptor, with a largearea donor and acceptor interface, resulting in an efficient photo-induced charge separation, have gained considerable interest. Despite of their relatively low efficiency in comparison to conventional inorganic solar cells, the potential of roll-to-roll processing on low-cost and flexible substrates makes polymer solar cells (PSCs) so attractive as a cost-effective solution to the energy problems we are facing today. Among the various BHJ systems, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) networks produced by spin-coating a blend combined with a preor post-heat treatment to improve the degree of ordering of P3HT have shown the highest efficiencies up to ca. 4–5 % under 80 or 100 mW cm illumination under AM 1.5 G condition. Because the electrical and optical properties of conjugated polymers are strongly dependent on the structural order of polymers, the processing methods and conditions, which determine the degree of organization of polymers, have a critical influence on the performance of electrical and optoelectrical devices based on these polymers. In addition, considering efficiencies of ca. 5 %, the development of novel solution processes that are compatible with low-cost mass production is one of the crucial requirements for practical device applications. Here, we report on novel solution-processed high-efficiency polymer solar cells based on a blend of P3HT and PCBM, resulting from improved organization of the P3HT. By directly brush painting a blend of P3HT and PCBM on appropriately temperature-controlled substrates, enhanced ordering of the polymers, induced by the shear stress in the direction parallel to the brushing direction, was achieved. This highly ordered active layer facilitates charge transport separated at the interface of the P3HT and PCBM, leading to an increased efficiency, in particularly an improved fill factor. In addition, this novel solution process can be considered a promising method for the fabrication of flexible and large-area polymer solar cells based on high-throughput roll-to-roll manufacturing, which would make the realization of low-cost PSCs possible. First, two types of devices were fabricated by conventional spin-coating and brush painting, respectively, without any preor post-heat treatment, which are usually performed to stabilize a nanoscale interpenetrating network with a crystalline order, resulting in an increase in overall conversion efficiency. Figure 1 shows the schematic of the brushing method and the resulting I–V curves for the two types of devices. The brush painting was performed on a poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrene sulfonate) (PSS)-coated indium tin oxide (ITO) substrate on a hot plate with a temperature of 50 °C. A general paintbrush made of nylon fibrils was used and the active layer was coated with a speed of ca. 1.5 cm s. It took ca. 2 s to prepare a complete film on the ITO substrate with a size of 1.5 cm × 1.5 cm by brush painting twice. During the brush painting process, an appropriate temperature was necessary to make a smooth and uniform active layer with a high degree of ordering, which is related to the evaporation rate of the solvents. In our case chlorobenzene was used, for which good-quality films could not be obtained when the brush painting was carried out below 50 °C because of too slow evaporation of the solvent at low temperatures. To prepare active layers with the same thickness, the blend was spin-coated at room temperature on the PEDOT:PSScoated ITO substrate at 2000 rpm and the thickness was determined to be ca. 90 nm by means of a surface profiler (Kosaka ET-3000i). The surface morphologies determined by atomic force microscopy (AFM) for the surfaces produced by the brushing technique (with a rms roughness of ca. 0.91 nm) were similar to that of a conventional spin-coated active layer (not shown here). As shown in Figure 1b, the performance was improved when the active layer was prepared by the brush-painting process. In the case of a spin-coated device, a poor performance was observed with VOC (opencircuit voltage) = 0.6 V, ISC (short-circuit current density) = 3.59 mA cm, FF (fill factor) = 32.5 %, and ge (power conversion efficiency) = 0.7 %, which is similar to previous C O M M U N IC A TI O N