Magneto-Optical Studies on Internal Photovoltaic Processes in Organic Solar Cells

Transcription

1 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

2 Content: I. Overview II. Recent progress III. Perspective Topics: Excited states Inter-molecular interface Electrode interface

3 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

4 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?

5 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

6 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 ITO/polymer/Al MEHPPV Triplet charge reaction P3HT inglet dissociation

7 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 Triplet charge reaction P3HT -0.5 inglet dissociation -1.0 PCBM Polymer chain MEHPPV: Triplet 1.2% P3HT: Triplet = high H. D. Burrows, JAC, 2003 ITO/PEDOT/P3HT+(x%)PCBM/Al % 1% >5% Low PCBM doping ignature of CT complexes 0.6 P3HT:PCBM High PCBM doping-real solar cell

8 Dissociation in polymer and D:A interface Isc P3HT:PCBM P3HT 150 mt B Inside polymer At D:A interface

9 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 R 2 R 2 Y. Liang, et.al. JAC. 131, 56, 2009 n P3HT:PCBM x=0% x=5% Meganetic field (mt) n-c 6 H 13 P3HT n 5% PCBM doping in P3HT is equivalent to pure PTB.

10 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 % ICBA ITO/PEDOT/P3HT:x/Ca/Al 6 FF=62.4% =3.2% PCBM 0 FF=63.9% ICBA -6 =5.1% Voltage (V) ITO/PEDOT/P3HT:PCBM(x)/Ca/Al 1.0 P3HT 0.5 5% PCBM ICBA Our information:dissociation with ICBA can be further improved by 50% ICBA:partial dissociation Collaboration with Prof. Yongfang Li PCBM:complete dissociation

11 Isc change (%) Photocurrent change (%) PCBM doping effects on dissociation at D:A interface Exciton CT ITO/PEDOT/P3HT+(x%)PCBM/Al % 1% >5% Low PCBM doping Polymer chain PCBM ignature of CT complexes 0.6 P3HT:PCBM High PCBM doping: real solar cell

12 Binding energy at D-A interfaces Magnetic field + electric field Exciton CT Isc P3HT P3HT:PCBM 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

13 Isc (ma/cm 2 ) Isc change (%) Isc change (%) Binding energy at D:A interface in organic solar cells PTB2:PCBM=1:1 0V -2V Low binding energy R 1 OOC Huidong Zang, et.al., Adv. Energy. Mater. 1, 923, 2011 R 2 R 2 n ITO/PEDOT/Polymer:PCBM/Ca/Al P3HT:PCBM PTB:PCBM Voltage (V) P3HT:PCBM=1:0.8 0V -2V Annealed Magnatic field (mt) High binding energy n-c 6 H 13 n

14 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 m v e 2 e 1 2 m h v 2 h Columb interaction Kinetic Energy

15 Current (ma/cm 2 ) Isc (ma/cm 2 ) Interface-enhanced charge collection ITO/PEDOT/P3HT:PCBM/Ca/Al un FF 1 un Voltage (V) (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 Bias (V) Aluminum Calcium PTB7:PC 70 BM PTB7:PC 70 BM PEDOT:P ITO-Glass Interface increases Jsc: increasing charge collection.

16 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 (a) TiOx ITO-Glass Norma cell 0.30V 1 sun 0.1sun Bias (V) More accumulation (b) PEDOT:P ITO-Glass Inverted cell 6V 1 sun 0.1 sun Bias (V) Less accumulation

17 Capacitance (nf) Perspective: Electrode interface Inverted cell Normal cell Light intensity (mw/cm 2 ) An interfacial layer Electrical polarization Charge tunneling Decreasing traps

18 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

19 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