Influence of Trap-Assisted Recombination on Polymer–Fullerene Solar Cells, Carsten Deibel et al, SPIE 2011 in San Diego

Inﬂuence of Trap-Assisted Recombination on Polymer–Fullerene Solar Cells C. Deibel, J. Lorrmann, A. Baumann, J. Gorenﬂot, A. Wagenpfahl, J. Rauh,* V. Dyakonov Julius-Maximilians-University of Würzburg, Germany SPIE Optics+Photonics* formerly known as Julia Schafferhans :-) 25th August 2011 in San Diego

Recombination Mechanisms: Overview PP diss V P Photo-! extract current P rec PEDOTbulk rec P surface! rec glass Surface! ‣ geminate recombination loss (PRL 103, 036402 (2009)) ‣ nongeminate recombinationinternal external ‣ surface recombination (PRB 82, 115306 (2010)) ‣ extraction at wrong electrode due to diffusion P = Polaron; PP = Polaron Pair (PRL 105, 266602 (2010)) 2

Nongeminate Recombination Disordered System: all charges localised, hopping Energy concentration: free << trapped LUMO trapped charges are immobile Etransport Etailstate traps ECT Edeep traps Etailstate traps Etransport HOMO a) b) c) d) e) DOS no recombination Trapped with Trapped (a)But: Free with Trapped Charges (b,c) , Free with Free (e) , or SRH (d) 3

Recombination OrderGeminate Recombination: concentration independent lifetime 1st orderNongeminate Recombination: bimolecular concentration dependent (effective) lifetime 2nd order typical for photogeneration w/out traps 1st order asymmetric doping or deep traps 4

Nongeminate RecombinationLangevin recombination (1) (2) (1) V RLangevin = np q = (µe + µh ) PEDOT ⇥ • 2nd order recombination? glass • Langevin? reduced? • inﬂuence of traps? 5

Experimental Method short Photo-CELIV delay 1.2 Photo-CELIV‣ ns laser pulse delay dependent @ T=150 K 1.0‣ delay time / P3HT:PCBM 1:0.8 j [x10 A/cm ] 2 recombination at V=Voff 0.8‣ charge extraction -3 0.6 delay time tdelay between laser and voltage pulse 0.4 ‣ mobility and ‣ carrier concentration 0.2 simultaneously 0.0 0.4 0.8 long -3 t [x10 s] delay and transient absorption for comparison 6

Bulk Recombination P3HT:PCBM (annealed) measured by photo-CELIV 125 K 22 RLangevin = np 10 q = (µe + µh ) ⇥ n [m ]-3 21 300 K 10 ‣ temperature dependence same as for mobility P3HT:PCBM 1:0.8 20 annealed 10 ‣ typical for Langevin recombination -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 tdelay [s]Andreas Baumann 7

Analysis: Fitting to the Continuity Equation P3HT:PCBM (annealed) measured by photo-CELIV 5 4 T=150K 3 2 P3HT:PCBM 1:0.8 22 10 next [m ]-3 6 experiment 5 4 3 2 21 10 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 tdelay [s] 8

Analysis: Fitting to the Continuity Equation Monomolecular Recombination? dn n = dt ⌧ 5 4 T=150K 3 2 P3HT:PCBM 1:0.8 22 10 next [m ]-3 6 experiment 5 -4 4 MR: τ = 6.6·10 s 3 2 21 10 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 tdelay [s]

Analysis: Fitting to the Continuity Equation Langevin Recombination? dn q 2 = · µ ·n dt ✏r ✏0 | {z } L 5 4 T=150K 3 2 P3HT:PCBM 1:0.8 22 10 next [m ]-3 6 experiment 5 -4 4 MR: τ = 6.6·10 s 3 Langevin 2 21 10 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 tdelay [s] 10

Analysis: Fitting to the Continuity Equation Reduced Langevin Recombination! dn q 2 = ⇣· · µ ·n dt ✏r ✏0 | {z } L 5 4 T=150K 3 2 P3HT:PCBM 1:0.8 22 10 next [m ]-3 6 experiment 5 -4 4 MR: τ = 6.6·10 s 3 Langevin 2 red. Langevin: ζ = 0.057 21 10 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 tdelay [s] 11

Analysis: Fitting to the Continuity Equation Recombination Order > 2 ? dn +1 = k +1 ·n dt 5 4 T=150K 3 2 P3HT:PCBM 1:0.8 22 10 next [m ]-3 6 experiment 5 -4 4 MR: τ = 6.6·10 s 3 Langevin 2 red. Langevin: ζ = 0.057 λ+1 21 kλ+1n with λ+1=2.41 10 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 tdelay [s] 12

Recombination Order P3HT:PCBM measured by photo-CELIV 3.4recombination order λ+1 P3HT:PCBM 1:0.8 3.2 pristine ‣ 2nd order or higher 3.0 annealed ‣ bimolecular 2.8 recombination 2.6 2.4 ‣ the more disordered 2.2 pristine sample: BR 2.0 ‣ higher rec. order 150 200 250 300 T [K] Andreas Baumann 13

For Comparison: Transient Absorption P3HT:PCBM (annealed) measured by transient absorption 7 ‣ P3HT:PCBM,Order of Decay λ+1 6 similar results as compared to 5 photo-CELIV 4 3 ‣ P3HT >140K due to polarons; 2 P3HT second order 1 P3HT:PCBM recombination! 50 100 150 200 250 300 ‣ neat P3HT: „no“ Temperature [K] phase separationJulien Gorenﬂot 14

Similar for „Intercalating“ Materials! 5.0 pBTCT-C 12:PC61BM photo-CELIV 1:1 4.5 1:4 TRMC 1:1 recombination order 4.0 1:4 3.5 or ﬁlms of pure pATBT, pBTTT, BM 8annealed at 120 °C. For 1000 3.0 ling at 165 °CpBTCT-C12:PC61BM 6 is also presented. 4 1:0 are given on the right. 2 1:1 1:4 2.5 100 ential of the polymer. Increas- 8 6 the polymeric donor is one 4 2.0 circuit voltage of the 2.0 nm 2 bulk related to the energy offset 108 1.5 ecular orbital (HOMO) of the 6 180 200 220 240 260 280 300 4 d molecular orbital (LUMO) 2.8 nmM.13,14 The polymers of interest 2 T/K2,5-bis(3-alkylthiophen-2-yl)- 2 3 4 5 6 an unsubstituted thieno[3,2-b]- 2Θ/° Adv. Funct. Mater. 21, 1687 (2011) 15 into the polymer backbone.

Causes for High Recombination Order?carrier concentration dependent mobility Shuttle et al, Adv. Funct. Mater 20, 698 (2010)inﬂuence of trapping Zaban et al, Chem. Phys. Chem. 4, 859 (2003) Nelson, PRB 67, 155209 (2003)inﬂuence of phase separation idea, but without change of order: Koster et al, APL 88, 052104 (2006) qualitatively: Baumann et al, Adv. Funct. Mater. 21, 1687 (2011) 16

Concentration Dependent Mobility? P3HT:PCBM (annealed) measured by photo-CELIV 4 3 2 -9 10µ [m /Vs] 6 5 ‣ at least, not for annealed2 4 P3HT:PCBM 1:0.8 3 annealed P3HT:PCBM solar cells 2 125 K -10 150 K 10 175 K 200 K 6 300 K 5 20 21 22 10 10 10 -3 n [m ] Andreas Baumann 17

Concentration Dependence of Rec. Prefactor P3HT:PCBM (annealed) measured by photo-CELIV -17 10 ‣ fit: -18 10 kBR [m /s] ‣ if it were pure Langevin: 3 -19 10 P3HT :PCBM 1:0.8 annealed -20 fit Langevin 10 125 K 175 K n and μ(n) from CELIV 300 K λ and kλ from fitting CELIV 20 21 22 10 10 10 -3 n [m ]Andreas Baumann 18

Concentration Dependence of Rec. Prefactor P3HT:PCBM (annealed) measured by photo-CELIV -17 10 ‣ consequence: -18 10 is not universal kBR [m /s] 3 -19 10 ‣ High recombination P3HT :PCBM 1:0.8 order in part due to annealed not all carriers being -20 ﬁt Langevin 10 125 K able to recombine with 175 K one another (no nt2) 300 K 20 21 22 10 10 10 -3 n [m ]Andreas Baumann 19

Trapping? P3HT:PCBM (annealed) by Thermally Stimulated Currents 21 6x10 T2 P3HT:PC61BM Trap density (Lower Limit) T1 PC61BM trap density (lower limit) [m ]-3 5 P3HT P3HT:PCBM: 6-8⋅1022 m-3 4 P3HT: 1⋅1022 m-3 3 T3 ‣ trapping in extrinsic traps 2 does occur 1 ‣ generally: in a hopping 0 system, trapping also within 100 200 300 400 intrinsic density of states activation energy [meV] APL 93, 093303 (2008), Org. Electron. 11, 1693 (2010),Julia Rauh Adv. Ener. Mater. 1, 655 (2011) 20

Phase Separation? Scenario Modelling Polymer et Fullerene but ! cannot be extracted delayed recombination due to emission from trap solving the continuity equation • trapping and release et • exponential DOS (intrinsic) => recombination order >2 • here: R ∝ nfree pfreeJens Lorrmann due to phase separation et 21

Recombination Order λ+1 even without phase separation ... but stronger with it! >10.0       Jens Lorrmann 22

Impact on Current–Voltage Characteristics free–free recombination free–free and free–trapped recombination 150 150 illum. σ dark illum. σ dark 100 25 meV 100 25 meV 50 meV 50 meVCurrent density [A/m ] Current density [A/m ] 75 meV2 2 75 meV 50 100 meV 50 100 meV 125 meV 125 meV 0 0 disorder -50 -50 disorder disorder -100 -100 -0.4 0.0 0.4 0.8 -0.4 0.0 0.4 0.8 Voltage [V] Voltage [V] unfortunately, probably a mixture of both in real devices: Alexander Wagenpfahl pure and intermixed phases 23

Conclusions: Nongeminate Recombinationcarrier concentration dependent mobility - CELIV: order > 2 also for cases with μ(n) = const - part of concentration dependence from R ≠ f(nt2)influence of trapping - significant trap concentration, nt >> nc => multiple-trapping-and-releaseinfluence of phase separation - 2nd order recombination in neat polymer indicates: phase separation plays role in blends‘ high orderorder > 2 from delayed bimolecular recombinationdue to trapping 24

Acknowledgments Thank You!deibel@physik.uni-wuerzburg.de Bayerische Akademie der Wissenschaftenwww.disorderedmatter.eu EP VI 25

Influence of Trap-Assisted Recombination on Polymer–Fullerene Solar Cells, Carsten Deibel et al, SPIE 2011 in San Diego

by disorderedmatter on Oct 17, 2011

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Influence of Trap-Assisted Recombination on Polymer–Fullerene Solar Cells, Carsten Deibel et al, SPIE 2011 in San Diego — Presentation Transcript