Enhanced Efficiency and Stability in P3HT:PCBM Bulk Heterojunction Solar Cell by using TiO2 Hole Blocking Layer
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0965-S11-04
Enhanced Efficiency and Stability in P3HT:PCBM Bulk Heterojunction Solar Cell by using TiO2 Hole Blocking Layer Osamu Yoshikawa, Akinobu Hayakawa, Takuya Fujieda, Kaku Uehara, and Susumu Yoshikawa Institute of Advanced Energy, Kyoto University, Uji, Kyoto, 611-0011, Japan
ABSTRACT Insertion of TiO2 layer between Al negative electrode and active layer of bulkheterojunction gave a high solar conversion efficiency and high stability. The TiO2 layer was prepared by spin-coating titanium(IV)isopropoxide (Ti(OC3H7)4) on active layer of blended P3HT:PCBM. The parallel resistance (Rp) is increased by inserting TiO2 layer leading to an increase in Voc. The TiO2 layer works as a hole blocking layer and prevents the physical and chemical damages by the direct contact between P3HT:PCBM active layer and Al electrode resulting in improvement of the parallel resistance and rectification. The efficiency of P3HT:PCBM bulk heterojunction cell with TiO2 hole blocking layer attained 4.05% with a Isc 9.72 mA/cm2, Voc 0.60 V, and FF 0.70 by using optimized TiO2 film thickness and ratio of P3HT:PCBM condition. INTRODUCTION Polymer solar cells are attracted all over the world because of its low cost and easy process in fabricating solar cells. Recently, intensive research on organic thin film solar cells have been devoted to improve the efficiency and to understand its mechanisms. The power conversion efficiency has been increased to 4~5% under A.M. 1.5 [1-4]. These high efficiencies have been achieved by bulk-heterojunction structure, which is contained micro-separated p-type semiconductor (polymer) and n-type semiconductor (fullerene) in order to compensate for the small depletion area of organic materials. Bulk-heterojunction structure is formed by only mixed p-type and n-type semiconductor in solvent and coated its solution on substrates. Therefore the fabrication process is very easy and its cost is very low. Recently, Kim et al. has reported polymer solar cell containing TiOx thin layer between Al and the active layer has been achieved 5% power conversion efficiency [5]. They reported TiOx layer in this device served as an optical spacer. Although, our group has investigated the same structure solar cell, we introduced another functional TiO2, which works as a hole blocking layer in the bulk-heterojunction solar cells. TiO2 is well known as the good electron-transporting material. LUMO and HOMO of TiO2 is 4.4 eV and 8.1 eV, respectively and the conduction band of TiO2 lies near the Fermi level of Al, as shown in Fig 1. Therefore TiO2 is expected to work as a hole and exciton-blocking layer because TiO2 has LUMO level near Fermi level of Al and a large band gap. Additionally, TiO2 layer between active layer and Al electrode may prevent the degradation of polymer solar cells. The main reasons for degradation of photocurrent are the reaction between the conjugated polymer and the Al electrode, diffusion of the Al from the electrode into the active layer, and oxidation by oxygen in air. In this work, regioregular poly[3-hex
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