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J. Phys. Chem. C 2008, 112, 9912–9916
Organic/Inorganic Polymer Solar Cells Using a Buffer Layer from All-Water-Solution Processing Qiquan Qiao,*,† Yu Xie,† and James T. McLeskey, Jr‡ Center for AdVanced PhotoVoltaics, Department of Electrical Engineering, South Dakota State UniVersity, Brookings, South Dakota 57006 and Energy ConVersion Systems Laboratory, Department of Mechanical Engineering, Virginia Commonwealth UniVersity, Richmond, Virginia 23284 ReceiVed: December 10, 2007; ReVised Manuscript ReceiVed: March 22, 2008
We present an environmentally friendly polymer solar cell based on a hybrid organic/inorganic device structure from an all-water-solution processing and fabricated using a blend of the hydrophilic conjugated polymer PTEBS and water-dispersible TiO2 nanocrystals. Generally, the low viscosity and high surface tension of the water-based solution inhibits the formation of a homogeneous PTEBS/TiO2 composite film on the transparent electrode (FTO). To overcome this, a TiO2 buffer layer was first spin coated onto the FTO surface to modify and improve the PTEBS/TiO2 film formation. The results show that solar cell performance has been significantly improved by the buffer layer. The fill factor for devices fabricated using the buffer layer is improved by a factor of 2 over those without the buffer layer, and the external energy conversion efficiency is increased by a factor of 4 as a result of improved interfacial contact between the absorbing layer and the electrodes, as well as the improved film morphology. The hydrophilic conjugated polymer and the water dispersible buffer TiO2 are both characterized by UV-vis spectroscopy, which reveals that the polymer has a high absorption coefficient (around 105 cm-1). The buffer TiO2 is transparent in the visible-near-infrared region so that the light will pass through and be harvested by the polymer. X-ray diffraction shows that the TiO2 buffer films primarily consist of anatase crystalline structure. This device offers few negative environmental consequences because no organic solvents will be used. Introduction Conjugated polymers are of interest for applications in optoelectronic devices such as light emitting devices and solar cells due to the prospect of low-cost, large area, solution-based manufacturing via spin coating,1 doctor blading,2 ink jet printing, screen printing,3 and reel-to-reel processing.4 The use of polymers as the active material in these devices offers other advantages as well, including flexibility, elasticity, ease of handling, and moldability. These polymers are semiconducting due to their conjugated structures of alternating single and double carbon-carbon bonds. Most conjugated polymers have a band gap between the LUMO (lowest unoccupied molecular orbitals) and the HOMO (highest occupied molecular orbitals) in the range of 1.5-3 eV and a high absorption coefficient of ∼105 cm-1. This makes them well suited for absorbing visible light in photovoltaic devices. Often, the band gap and ionization potential can be tuned to the desired energies by modifying the chemical structure.5 In these polymers, however, photogenerated excitons are strongly bound with a typical binding energy of about 0.4 eV,6 and these excitons resist dissociation into separate charges. The diffusion range of singlet excitons in most conjugated polymers is approximately 5-15 nm, and their radiative or nonradiative decays take place in a time of 100∼1000 ps.5 Furthermore, most conjugated polymers have poor charge carrier mobilities (usually
