Optical and electronic proprieties of thin films based on (Z)-5-(4-chlorobenzylidene)-3-(2-ethoxyphenyl)-2 thioxothiazolidin-4-one, (CBBTZ) and possible application as exciton-blocking layer in heterojunction organic solar cells

Y Mouchaal,G.M Soto,A Djafri,M.A Del Valle,A Khelil,M Morsli,A Lakhdar Toumi,A Reguig,J.C Bernède,L Cattin,K Toubal,Y Lare,A.S Yapi

doi:10.1051/epjconf/20122900030

Abstract

In this work, organic thin film solar cells with structures based on CuPc/C60 bulk heterojunctions, have been fabricated and characterized. The effect of introducing an exciton blocking layer (EBL) between the active layer and the metal layer in the solar cell was investigated. For that (Z)-5-(4-chlorobenzylidene)-3-(2-ethoxyphenyl)-2- thioxothiazolidin-4-one, that we called (CBBTZ) has been synthesized, characterized and probed as EBL. It was shown that optimized structures containing EBLs resulted in an improvement in solar cell conversion efficiencies. The energy levels corresponding to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the CBBTZ have been determined from the first oxidation and reduction potential respectively, using cyclic voltametric (CV) measurements. From CV curves, CBBTZ in dichloromethane showed a one electron reversible reduction and oxidation waves. The values of its HOMO and LUMO have been estimated to be 6.42 eV and 3.42 eV respectively. Such values show that CBBTZ could be probed as EBL in organic solar cells based on the ED/EA couple copper phthalocyanine(CuPc)/fullerene (C60). The photovoltaic solar cells have been obtained by sequential deposition under vacuum of the different films where their thicknesses were measured in situ by a quartz monitor. When obtained, the averaged efficiency of the cells using the CBBTZ is higher than that achieved without EBL layer.

Highlights

The energy levels corresponding to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the CBBTZ have been determined from the first oxidation and reduction potential respectively, using cyclic voltametric (CV) measurements
The values of its HOMO and LUMO have been estimated to be 6.42 eV and 3.42 eV respectively. Such values show that CBBTZ could be probed as exciton blocking layer (EBL) in organic solar cells based on the ED/EA couple copper phthalocyanine(CuPc)/fullerene (C60)
In the present manuscript we studied the optical and electronic proprieties of CBBTZ ((Z)-5-(4chlorobenzylidene)-3-(2-ethoxyphenyl)-2-thioxothiazolidin-4-one) thin films and we have tested it as an exciton blocking layer in CuPc/C60 heterojunction solar cells

Summary

Introduction

The main challenges in organic solar cell research are currently to improve the device efficiency and lifetime [5,6]. Because metal electrodes are suspected to quench excitons in the electron acceptor layer (C60 in our case), exciton-blocking layers were introduced in devices between the metal cathode and the electron acceptor layer. These transparent spacer layers allow to shift the active layer toward the maximum of the electric-field intensity distribution of the light in these devices[11]. As a matter of fact, significant efficiency improvements of layered cells, based on an electron donor/electron acceptor junction, have been achieved through the introduction this buffer layer at the interface electron acceptor/cathode. In the present manuscript we studied the optical and electronic proprieties of CBBTZ ((Z)-5-(4chlorobenzylidene)-3-(2-ethoxyphenyl)-2-thioxothiazolidin-4-one) thin films and we have tested it as an exciton blocking layer in CuPc/C60 heterojunction solar cells

CBBTZ experimental characterization

Organic cells realisation and characterisation

CBBTZ characterization

Conclusion