DEPARTAMENT DE FÍSICA APLICADA I ÒPTICA Martí i Franquès 1, 08028 Barcelona STUDY OF ORGANIC SEMICONDUCTORS FOR DEVICE APPLICATIONS Marco Stella Programa de doctorat: Tècniques Instrumentals de la Física i la Ciència de Materials Bienni: 2004-2006 Tutor: Joan Esteve Pujol Directors: Jordi Andreu Batallé, Joaquim Puigdollers Gonzalez Memòria presentada per optar al grau de Doctor Barcelona, desembre 2009 Alla mia famiglia This work has been carried out in the Solar Energy Group of the Laboratory of Thin Film Materials of the Department of Applied Physics and Optics of the Universitat de Barcelona. The Solar Energy Group is a member of the Xarxa de Referència de Materials Avançats per a l’Energía (XaRMAE). The present work has been supervised by Dr. Jordi Andreu Batallé (Universitat de Barcelona) and Dr. Joaquim Puigdollers Gonzalez (Universitat Politècnica de Catalunya) in the framework of the project ENE2004-07376-C03-01 of the Spanish Government and with the support of a pre-doctoral grant from the Spanish Government (Beca FPI). Table of contents Acknowledgments ............................................................................................................ iii OUTLINE OF THIS THESIS ................................................................................................ v 1 INTRODUCTION .......................................................................................................... 1 1.1 Overview on photovoltaic technology .................................................................................. 1 1.2 Why investigate a new photovoltaic technology? ................................................................. 4 1.3 Natural processes of solar energy storage ............................................................................. 4 2 GENERAL REVIEW ON ORGANIC SEMICONDUCTORS THEORY .......................... 6 2.1 Materials and their chemical properties ................................................................................ 7 2.2 Basic working principles ..................................................................................................... 12 2.2.1 Molecular orbital theory............................................................................................... 12 2.2.2 Polyacetylene: band structure and primary excitations ................................................ 15 2.2.3 Polymers with non-degenerate ground state ................................................................ 18 2.2.4 Small molecules ........................................................................................................... 21 2.2.5 Experimental proofs and an alternative model ............................................................ 23 2.3 Optical properties ................................................................................................................ 25 2.4 Materials investigated in this work ..................................................................................... 27 3 TECHNOLOGICAL ASPECTS .................................................................................. 34 3.1 Doping mechanisms ............................................................................................................ 34 3.2 Current generation in an organic solar cell ......................................................................... 37 3.3 Deposition techniques ......................................................................................................... 39 3.4 Device structures ................................................................................................................. 43 3.4.1 Single layer device ....................................................................................................... 43 3.4.2 Bilayer heterojunction device ...................................................................................... 45 3.4.3 Bulk heterojunction device .......................................................................................... 46 3.4.4 Other device structures: hybrid solar cells ................................................................... 48 4 EXPERIMENTAL........................................................................................................ 50 4.1 Organic thin film deposition................................................................................................ 50 4.2 Optical characterization techniques ..................................................................................... 52 4.2.1 Photothermal Deflection Spectroscopy (PDS) ............................................................. 52 4.2.2 Optical Transmission Spectroscopy ............................................................................. 63 4.2.3 Calculation of optical gap and Urbach energy ............................................................. 64 4.2.4 Important aspects of PDS measurement ...................................................................... 66 4.3 Other characterization techniques ....................................................................................... 66 4.3.1 X-ray diffraction spectroscopy (XRD) ........................................................................ 66 4.4 Electrical characterization of the devices ............................................................................ 69 4.4.1 Thin film transistors ..................................................................................................... 70 4.4.2 Diodes .......................................................................................................................... 72 5 MATERIAL CHARACTERIZATION ........................................................................... 75 5.1 Thin film characterization ................................................................................................... 75 5.1.1 Pentacene ..................................................................................................................... 76 5.1.2 Copper phthalocyanine (CuPc) .................................................................................... 82 5.1.3 Fullerene (C60).............................................................................................................. 85 i 5.1.4 Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) ........................................... 89 5.1.5 N,N-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) ...................... 91 5.2 Effects of degradation on the optical properties .................................................................. 95 5.2.1 Pentacene ..................................................................................................................... 96 5.2.2 Copper Phthalocyanine (CuPc) .................................................................................. 100 5.2.3 Fullerene (C60)............................................................................................................ 103 5.2.4 Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) ......................................... 107 5.2.5 N,N-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) .................... 110 5.3 Conclusions about pure materials...................................................................................... 113 6 BULK HETEROJUNCTIONS ................................................................................... 116 6.1 Characterization of the bulk heterojunctions ..................................................................... 117 6.1.1 Pentacene-fullerene .................................................................................................... 117 6.1.2 Pentacene-PTCDA ..................................................................................................... 121 6.1.3 Pentacene-PTCDI-C13 ................................................................................................ 126 6.1.4 CuPc-fullerene ........................................................................................................... 131 6.1.5 CuPc-PTCDA............................................................................................................. 137 6.1.6 CuPc-PTCDI-C13 ....................................................................................................... 142 6.2 Annealing treatments ......................................................................................................... 146 6.2.1 Pentacene-fullerene .................................................................................................... 147 6.2.2 Pentacene-PTCDA ..................................................................................................... 148 6.2.3 Pentacene-PTCDI-C13 ................................................................................................ 149 6.2.4 CuPc-fullerene ........................................................................................................... 150 6.2.5 CuPc-PTCDA............................................................................................................. 153 6.2.6 CuPc-PTCDI-C13 ....................................................................................................... 155 6.3 Conclusions about the bulk heterojunctions ...................................................................... 159 CONCLUCIONS...............................................................................................................163 APPENDIX A....................................................................................................................165