Polymer solar cell by blade coating

Polymer bulk hetero-junction solar cells of poly(3-hexylthiophene) (P3HT) donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) acceptor are fabricated by blade coating in toluene solution. Desired donor–acceptor self-organization is achieved without the slow drying process and high boiling point solvent. Power conversion efficiency is 3.8%, much higher than the 2.6% obtained by spin coating in toluene solution. The blade coating method has nearly 100% material usage and can be integrated in the roll-to-roll process with high throughput production. 2009 Elsevier B.V. All rights reserved. Recently conjugated polymer solar cells have attracted great interests due to their unique properties such as easy solution process for large-area, light weight, and mechanical flexibility. The polymer bulk hetero-junction cells have interpenetrating networks of electron donors and acceptors, resulting in an efficient exciton dissociation and high photo-currents with the power conversion efficiency (PCE) about 5% [1,2]. Although the efficiency is not as high as that of inorganic solar cells, the potential of low-cost and rollto-roll process on flexible substrates makes the polymer solar cells attractive as the solution to the serious energy challenges. However, to date most of the efficient polymer solar cells are made by spin coating, which causes serious material waste and raises the cost dramatically. In addition, the spin coating process is not easy to scale up to very . All rights reserved. eng). large sizes up to meters and is incompatible with the rollto-roll process for high throughput production. Furthermore one major problem of the spin coated polymer solar cell is that a very toxic high boiling point organic solvent like dichlorobenzene (180.5 C) or chlorobenzene (131 C) is necessary for high efficiency. The high boiling point allows a slow drying process where the donor and acceptor molecules self-assemble into an interpenetrating network [1]. The high toxicity of dichlorobenzene and chlorobenzene makes the mass production environmentally unfriendly, and the slow drying process delays the production throughput. Although there have been researches on fabrication processes, including ink-jet printing [3], screen printing [4], spray coating [5], and blade coating[6], the development of alternative solution coating methods compatible with the low-cost and environmentally friendly mass production remains a crucial problem [7]. Since there is no necessary for organic layers to be Fig. 1. (a) Schematic working principle of blade coating. The polymer wet film is formed by moving the blade coater. The thickness of polymer wet film is defined by the gap of the blade coater. (b) The schematic device structure of the bulk hetero-junction cells. Fig. 2. Statistical results of the seven series of devices: (a) the power conversion the fill factor. The horizontal lines in the box denote the 25th, 50th, and 75th per open square inside the box denotes the mean value. 742 Y.-H. Chang et al. / Organic Electronics 10 (2009) 741–746 structured by printing in organic solar cells, blade coating for large area fabrication has been proved to be the better way [6]. The film thickness by blade coating can be reduced to nanometer scale by carefully controlling the fabrication parameters such as the solution concentration, the blade gap, and the blade coating speed. Recently we verified the feasibility of blade coating for high-efficiency polymer light-emitting diodes [8]. Unlike spin coating, the area can be easily scaled up and the material usage is almost 100% in blade coating. In this work, blade coating is applied to poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl-C61butyric acid methyl ester (PCBM) blend in the toluene solution which has a lower boiling point (110 C) and is less toxic. High efficiency is achieved from toluene solution without the slow solvent evaporation process. Fig. 1a shows the schematic working principle of blade coating. The thickness of the wet polymer film is defined by the gap. The polymer dry film thickness is tuned by the polymer concentration in solution and the gap of the blade coater. The polymer wet film is deposited by dragging the blade coater at a certain speed about 15 cm/s. We first focus on solar cells with P3HT:PCBM blend dissolved in toluene. Device performances are compared for efficiency, (b) the short-circuit current, (c) the open-circuit voltage and (d) centile values. The error bars denote the 5th and 95 percentile values. The Fig. 3. Current–voltage (J–V) relations of the devices in this work. (a) Devices made by spin coating (device A, solid square), blade coating (device B, empty square), blade coating at 60 C (device C, solid circle), and blade and spin coating (device D, empty circle) in toluene solution, blade and spin coating with bladed PEDOT:PSS (device G, solid star). (b) Devices made by blade and spin coating in toluene solution (device D, empty circle), chlorobenzene solution (device E, solid triangle), and dichlorobenzene solution (device F, empty triangle). Y.-H. Chang et al. / Organic Electronics 10 (2009) 741–746 743 different coating methods including spin coating, blade coating, blade coating on a hot plate, as well as blade and spin coating. For the blade and spin coating the polymer wet film is deposited by blade coating which is followed by spinning until the dry film is formed. In all cases the weight ratio of P3HT and PCBM is 1:1. The device structure is ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al shown in Fig. 1b. ITO is indium tin oxide and PEDOT:PSS is poly-(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (Baytron PVP AI 4083). A 40 nm PEDOT:PSS layer is spin coated on a patterned ITO substrate and baked at 200 C in nitrogen for 15 min. Seven series of devices with six devices in each are made to study the different coating processes with different solvents statistically. Among them, five series are made with toluene solution to compare the coating processes, including spin coating (series A), blade coating (series B), blade coating on a hot plate at 60 C (series C), as well as blade and spin coating (series D). In addition, the devices with the PEDOT:PSS layer by blade coating at 100 C and P3HT:PCBM by blade and spin coating from toluene solution is made to study the feasibility of bladed PEDOT:PSS layer (series G). The other two series made from two conventional high boiling point solvents, chlorobenzene (series E) and dichlorobenzene (series F), are compared with those from toluene by blade and spin coating. After coating all the P3HT:PCBM layers are annealed at 140 C for 20 min in nitrogen. The Ca(35 nm)/Al(100 nm) cathode is deposited by thermal evaporation. The active area of the device is 0.04 cm. All the devices are packaged in the glove box and measured in the ambient environment. The power conversion efficiency is measured by the solar simulator (PEC-L11, Peccell Technologies) under AM1.5G irradiation. The incident photon-to-electron conversion efficiency (IPCE) is measured by the spectral response measurement system (SR300, Optosolar GMBH). The morphology of P3HT:PCBM is monitored by atomic force microscope (AFM, Dimension 3100, Digital Instruments). Fig. 2 shows the statistical results of the seven series of devices. Among the series based on toluene solution (series A, B, C, D, and G), the series of devices by blade coating have the higher efficiencies except the series with bladed the PEDOT:PSS layer. Because the open-circuit voltages Voc are about the same, the high performances of series B, C, and D result from the high short-circuit currents Jsc and fill factor. Among the series by blade and spin coating (series D, E, and F), the series of devices from toluene solution and dichlorobenzene have the higher performances. While the Jsc of series E and series F are about the same, thehighperformance of series F results from the relatively high fill factor. For further discussion, the best devices in each series are chosen to show the advanced device properties. Fig. 3a shows the current–voltage (J–V) curves of five devices in toluene solution made by different active layer coating processes. The short-circuit currents Jsc made by blade coating (device B, C, and D) are larger than that of the device made by conventional spin coating (device A). Using blade coating the Jsc increases from 9.3 mA/cm 2 with spin coating to 11.5 mA/cm. The fill factor rises from 47% to 55% and the open-circuit voltage Voc remains the same. The efficiency, which is proportional to Jsc , Voc , and fill factor as a whole, is improved from 2.6% (device A) by spin coating to 3.8% (device C) by blade coating on a hot plate. It is believed that in order to get a high efficiency in bulk hetero-junction polymer solar cell the microscopic morphology of the active layer needs to be well controlled to achieve an ordered structure by certain annealing processes such as slow solvent evaporation [1] and postproduction heat treatment [2,9]. Such annealing promotes molecular self-organization and makes the polymer chains more ordered in its domains. In spin coating for low boiling point solvents such as toluene (110 C), high volatility leads to the fast drying of the active layer and may limit the self-assembly as well as the power conversion efficiency. However, the polymer films made by blade coating could be more ordered than those by spin coating due to the fact that the polymer chains are relatively free to move in the absence of centrifugal force. Therefore even without the slow drying process the donors and acceptors quickly