Cost-benefit Photovoltaics by Hybridization of Dye-anchored Nano-crystalline TiO 2 and p-Type Organic Semiconductors
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1211-R06-01
Cost-benefit Photovoltaics by Hybridization of Dye-anchored Nano-crystalline TiO2 and p-Type Organic Semiconductors Shozo Yanagida Center for Advanced Science and Innovation, Osaka University, Yamada-oka, Suita, Osaka, Japan 567-0871 ABSTRACT Cost benefit analysis (CBA) and energy profit ratio (EPR) indicate that third generation PV should be high-throughput ones with higher efficiency than CdTe PV. The polymer photovoltaics (OPV) such as PCBM/P3HT, and the dye-sensitized solar cells (DSC), i.e. TiO2/dyes/iodide-iodine-electrolytes will be reviewed as printable PV to cope in near future with environmentally benign PV demand. The recent progress of OPV and DSC will be discussed in terms of diffusion length, and our recent studies on iodine-free DSC. The OPV-DSC-hybridized PV exemplified by solid-state Dye/TiO2/P3HT will become hopeful as printed thin-film solar cells, in particular when the conversion efficiency is further enhanced more than 10%.
INTRODUCTION Figure 1 shows the basic structure of Dye-staff photovoltaic, which is composed of electrode/electron transport material/dyes/hole transport material/ electrode and molecular structures of representative materials. In the case of OPV, C60 derivatives (PCBM) are used as electron transport materials, while DSC uses nano-crystalline (nc-TiO2).
Figure 1. Structure of Dye Photovoltaic using Representative Materials
The thickness of a typical DSC device, diffusion coefficient and electron lifetime, and diffusion length are compared with those of OPV in Table 1 [1]. In the case of OPV, DL is limited to 0.1~1.0 µm due to very short lifetime and low diffusion length of a few nm as exemplified by C60. This fact indicates superiority of nc-TiO2 as an electron transfer material. Table 1. Comparison of Device Thickness and Diffusion Length for DSC and OPV
MOTIVATION FOR HYBRIDIZATION OF OPV AND DSC, I.E., IODIDE/IODINEFREE DSC The I-/I3- redox electrolyte in DSC has oxidizing power, and then corrodes gradually metals like Ag (grid metal) and Pt (catalysts on cathode). The higher temperature accelerates the corrosion in the presence of invading water. When solvent with high boiling point or ionicliquid with high viscosity is employed for long-term-reliable DSC modules, significant loss in Jsc and Voc are often observed. This may be explained as due to concentration or gradient of the redox electrolyte’s I3- in vicinity of the dye molecules (I- in the vicinity of cathodes). Further, the reduction of the oxidized dye by the redox electrolyte’s I- ions occurs within about 10-8 seconds, which is slower than the electron injection (within about 10-12 seconds) to nc-TiO2 from the photo-excited dye molecules. In addition, the electrolytes (I-/I3-) absorb visible light (λ=~430nm), resulting in a loss of photo absorption that depends on concentration of iodine (~13 %). To innovate DSCs for cost-effective and robust solid-state PV using inexpensive metals like Al foil, we successfully introduced PEDOT as an alternative of the fluid I-/I3- redox electrolyte. The recent
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