Magneto-Optical tudies on Internal Photovoltaic Processes in Organic olar Cells Bin Hu Department of Materials cience and Engineering University of Tennessee, Knoxville Wu Han National Laboratory for Optoelectronics Huazhong University of cience and Technology
Content: I. Overview II. Recent progress III. Perspective Topics: Excited states Inter-molecular interface Electrode interface
ITO Metal Interface issues in organic solar cells Inter-molecular interface Exciton D:A interface dissociation Exciton Device interface OCH 3 O C 60 PCBM Polymer chain Polymer chain Binding energy at D:A interface Charge collection at electrode interface
Inter-molecular interface: Overview Two necessary conditions: 1. Interfacial electrical polarization to break excitons 2. Interfacial energy offsets to facilitate exciton dissociation Two key questions: 1. At D:A interface, are electrons and holes bound? 2. What control the binding energy of e-h pairs at D:A interface?
Experimental tools to study D:A interface Light absorption Exciton Charge-transfer complex LUMO Polymer chain C 60 HOMO Donor CT complex LUMO HOMO Acceptor PA Ab PL EL MFE PC O. Inganäs. JAC. 131, 11819, 2009 B. Hu, Adv. Func. Mater. 18, 2611, 2008
Photocurrent change (%) Our experimental tool Magnetic field effects of photocurrent: Jsc changes with B. Bin Hu, Adv. Mater. 21, 1500, 2009 Z. Xu & B. Hu, Adv. Func. Mater. 18, 2611, 2008 Magnetic field effects of photocurrent to show internal OPV processes B etup N Light ITO PV film Al olar cell Inside polymer Low field (< 200 mt) Dissociation D:A interface High field (> 200 mt) Increase inglet ratio Increase Jsc(Dissociation: e + h) Decrease triplet ratio Decrease Jsc (Charge reaction:t + C e + h) Experimental evidence: inglet MEHPPV: Only increasing component Triplet P3HT: Both increasing and decreasing components 1.5 1.0 0.5-0.5-1.0 ITO/polymer/Al MEHPPV Triplet charge reaction P3HT inglet dissociation 0 40 80 120 160
Isc change (%) Photocurrent change (%) Photocurrent change (%) Our experimental tool Magnetic field effects of photocurrent: Jsc changes with B. ITO/polymer/Al Exciton 1.5 CT MEHPPV 1.0 0.5 Triplet charge reaction P3HT -0.5 inglet dissociation -1.0 PCBM 0 40 80 120 160 Polymer chain MEHPPV: Triplet 1.2% P3HT: Triplet = high H. D. Burrows, JAC, 2003 ITO/PEDOT/P3HT+(x%)PCBM/Al 1 0-1 -2-3 0% 1% >5% 0 40 80 120 160 Low PCBM doping ignature of CT complexes 0.6 P3HT:PCBM 0.5 0.4 0.3 0.2 0.1 0 200 400 600 800 1000 High PCBM doping-real solar cell
Dissociation in polymer and D:A interface Isc P3HT:PCBM P3HT 150 mt B Inside polymer At D:A interface
Isc change (%) Isc change (%) Low-field (< 200 mt): dissociation within PV polymer New polymer versus P3HT 1.0 P3HT 0.5 PTB -0.5 R 1 OOC 0 40 80 120 160 R 2 R 2 Y. Liang, et.al. JAC. 131, 56, 2009 n 1.2 0.8 0.4-0.4-0.8 P3HT:PCBM x=0% x=5% 0 40 80 120 160 Meganetic field (mt) n-c 6 H 13 P3HT n 5% PCBM doping in P3HT is equivalent to pure PTB.
Isc change (%) Isc change (%) Photocurrent (ma/cm 2 ) Dissociation with new acceptor ICBA Information from IV curves High Voc Large Isc ITO/PEDOT/P3HT:ICBA(x)/Ca/Al 1.0 P3HT 0.5-0.5 5% ICBA 0 50 100 150 ITO/PEDOT/P3HT:x/Ca/Al 6 FF=62.4% =3.2% PCBM 0 FF=63.9% ICBA -6 =5.1% -12-0.8-0.4 0.4 0.8 Voltage (V) ITO/PEDOT/P3HT:PCBM(x)/Ca/Al 1.0 P3HT 0.5 5% PCBM -0.5 0 50 100 150 ICBA Our information:dissociation with ICBA can be further improved by 50% ICBA:partial dissociation Collaboration with Prof. Yongfang Li PCBM:complete dissociation
Isc change (%) Photocurrent change (%) PCBM doping effects on dissociation at D:A interface Exciton CT ITO/PEDOT/P3HT+(x%)PCBM/Al 1 0-1 -2-3 0% 1% >5% 0 40 80 120 160 Low PCBM doping Polymer chain PCBM ignature of CT complexes 0.6 P3HT:PCBM 0.5 0.4 0.3 0.2 0.1 0 200 400 600 800 1000 High PCBM doping: real solar cell
Binding energy at D-A interfaces Magnetic field + electric field Exciton CT Isc P3HT P3HT:PCBM - + 150 mt B Polymer chain PCBM Low field: dissociation in P3HT High field: dissociation at D:A interface Electric field dissociates CT states at D-A interfaces. + Magnetic field effects of photocurrent detect existence of CT states Binding energy of CT states
Isc (ma/cm 2 ) Isc change (%) Isc change (%) Binding energy at D:A interface in organic solar cells 2.0 1.5 1.0 0.5 PTB2:PCBM=1:1 0V -2V 0 300 600 900 Low binding energy R 1 OOC Huidong Zang, et.al., Adv. Energy. Mater. 1, 923, 2011 R 2 R 2 n 0.4 0.3 0.2 0.1 ITO/PEDOT/Polymer:PCBM/Ca/Al 10 0-10 -20 P3HT:PCBM PTB:PCBM 0.4 0.8 Voltage (V) P3HT:PCBM=1:0.8 0V -2V Annealed 0 300 600 900 Magnatic field (mt) High binding energy n-c 6 H 13 n
Perspective: D:A binding energy W A Anode Donor F D Vacuum F A Acceptor F D W C Cathode Built-in electric field e-h capture radius r Two forces: e K 2 B T Coulomb attraction + Drifting r Mobilities Energy U 1 e 4 r 2 1 2 m v e 2 e 1 2 m h v 2 h Columb interaction Kinetic Energy
Current (ma/cm 2 ) Isc (ma/cm 2 ) Interface-enhanced charge collection ITO/PEDOT/P3HT:PCBM/Ca/Al 32 24 16 8-8 0-16 0.1un FF 1 un 72 71 68 65 62-1.0-0.5 0.5 1.0 Voltage (V) 5 0-5 -10-15 -20 (a) Inverted cell (PCE:7.8%) Normal cell (PCE:6.0%) Gold MoO 3 PTB7:PC 70 BM PTB7:PC 70 BM TiOx ITO-Glass 0.2 0.4 0.6 0.8 Bias (V) Aluminum Calcium PTB7:PC 70 BM PTB7:PC 70 BM PEDOT:P ITO-Glass Interface increases Jsc: increasing charge collection.
Capacitance (nf) Capacitance (nf) Interface effects on charge accumulation Photoinduced impedance studies Gold MoO 3 PTB7:PC 70 BM PTB7:PC 70 BM Aluminum Calcium PTB7:PC 70 BM PTB7:PC 70 BM 25 20 15 10 5 0 (a) TiOx ITO-Glass Norma cell 0.30V 1 sun 0.1sun -1.0-0.5 0.5 1.0 Bias (V) More accumulation 40 30 20 10 0 (b) PEDOT:P ITO-Glass Inverted cell 6V 1 sun 0.1 sun -1.0-0.5 Bias (V) 0.5 1.0 Less accumulation
Capacitance (nf) Perspective: Electrode interface 12 11 10 9 8 7 6 Inverted cell Normal cell 0 20 40 60 80 100 Light intensity (mw/cm 2 ) An interfacial layer Electrical polarization Charge tunneling Decreasing traps
Acknowledgement NF-ECC project for organic solar cells: Magneto-Optical tudies of Charge Dissociation, Transport, and Collection in Organic olar Cells The research has been collaborated with Dr. Ilia Ivanov at ORNL Prof. Luping Yu at University of Chicago Prof. Tzung-Fang Guo at NCKU Prof. Yongfang Li at Institute of Chemistry (China) Prof. Guanghua Wei at Taiwan Jiaotong University
Mingxing Li Qing Liu Michael tanford The research was done by Huidong Zang, Yu-Che Hsiao, Qing Liu, Mingxing Li, and Michael tanford. Huidong Zang Yu-Che Hsiao