Effect of 2D delocalization on charge. R. Österbacka

Effect of 2D delocalization on charge transport and recombination in bulkheterojunction solar cells R. Österbacka

The people! Drs: H. Majumdar, S. Majumdar, T. Mäkelä, T. Remonen ÅA PhD students: H. Aarnio, N. Kaihovirta, D. Tobjörk, F. Jansson, N. Björklund, M. Nyman, S. Sanden, F. Petterson M.Sc. Students. E. Holm, M. Pesonen, A. Ylinen K.-M. Källman (lab engineer), ÅA Left the group: J. Lin (China), J. Baral (India), A. Pivrikas (Linz), T. Bäcklund (Merck), H. Sandberg (VTT), M. Westerling (Perkin- Elmer), H. Stubb (emeritus) G. Juška, G. Sliauzys, K. Arlauskas, N. Nekrasas, K. Genevicius/Vilnius Univ A. Pivrikas, A. Mozer and N.S. Sariciftci, LIOS G. Dennler and M. Scharber, Konarka Austria D. Vanderzande, Universiteit Hasselt, Belgium

Outline 1. Motivation for plastic solar cells Langevin recombination 2. Effect of morphology on transport Treated/untreated bulk-heterojunction solar cells 3. 3D vs 2D Langevin recombination Drift model Full drift-diffusion model 4. Importance of the interface Cw-PIA on treated and untreated solar cell blends 2D polarons generated Hybrid interfaces

Why Solar Energy? Available solar power: 20 MW p.p. (total earth surface) Human energy consumption: range 100 W - 10 kw p.p. average 2.5 kw p.p. (NL: 6 kw) at 10% overall efficiency: surface needed 1400x1400 km 2 to cover energy needs in 2050 (~1500 EJ) How to store and distribute? Source: ECN, Slide courtesy of P.W.M. Blom

The challenge for plastics Source: NREL/Wikipedia 7.87%, Solarmer Energy Inc, November 2009

Motivation High power conversion efficiencies in solar cells of lowmobility materials require high carrier densities. Higher carrier densities leads to shorter lifetimes [ ] (0) 1 τ = n β where β is the bimolecular recombination coefficient To understand charge transport and recombination is crucial for making more efficient solar cells!

Langevin Recombination Expected in all low-mobility (µ<1 cm 2 /Vs) materials Necessary condition: The carrier mean free path is much smaller than the Coulomb capture radius r c, i.e. a << r C. r c = 2 e 4 πεε kt 4 0 19nm Langevin recombination is determined by the probability for the charge carriers to meet in space, independent of the subsequent fate of the carriers dp dt = dn dt = β np = L β L n 2 β e( µ + µ ) n p L = µ f εε 0 ( F, T ) P. Langevin, Ann. Chim. Phys. 28, 433 (1903).

Consequences of Langevin recombination Langevin recombination is the time reversed process to Onsager-type generation Photogenerated charges will be bound within the Coulomb radius, r r c Field dependent generation of free carriers due to lowering of barrier Clarifying the recombination mechanism yields also information about generation! r c r kt

Effect of morphology l Untreated Treated 1.6.2010 Beal et al., Macromolecules 43, 2343 2348 (2010) Page 9

3D vs 2D Langevin recombination Homogeneous (3D) Lamellar structure (2D) l f 3D = β e( µ + µ ) L dn dt n εε 0 p = β L n 0 n 2 e ( µ n + µ p) = εε f 2D = 3 π e( µ n + µ n) 4 εε dn dt 0 = γ 2 3 2 ( ln) 3/ = γ n n 5/ 2 2D γ 2D π = l n = 6 10 β 4 L 3 3/ 2 1/ 2 3 2D ( * ) for l = 1.6 nm n=10 16 cm -3 Juska et al., APL 95 013303 (2009) (*)Shuttle et al., APL 92, 093311 (2008)

Including diffusion in 2D Greenham and Bobbert, PRB 68, 245301 (2003) Page 11

Inclusion of diffusion important in 2D! Neglecting diffusion underestimates the recombination time! V.I. Nenashev, F. Jansson et al., APL in press. Sivu 12

Importance of the interface 1,2 OD (normalized) 1,0 0,8 0,6 0,4 0,2 P3HT P3HT:PCBM 4:1 P3HT:PCBM 1:1 PCBM 0,0 300 400 500 600 700 800 900 λ [nm] 754 nm K. Vandewal et al. / Thin Solid Films 516 (2008) 7135 7138

Cw-PIA in RR-P3HT 514 nm 754 nm - T/T [10-3 ] 1.2 PA P3HT OS2100 IN PA 1.0 OUT 514 nm 0.8 0.6 0.4 0.2 0.0-0.2 T=80K, f=133hz -0.4 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 E [ev] - T/T [10-3 ] 0.006 0.004 0.002 0.000-0.002-0.004 P3HT 785 nm T=80K, f=133hz PA IN PA OUT -0.006 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 E [ev] Above gap excitation Below gap excitation H. Aarnio et al., manuscript in preparation Page 14

Cw-PIA in P3HT:PCBM 514 nm 754 nm 1.5 1.0 P3HT:PCBM 1:1 514 nm 0.06 P3HT:PCBM 1:1 785 nm - T/T [10-3 ] 0.5 0.0-0.5-1.0 PA IN PA OUT T=80K, f=133hz -1.5 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 E [ev] - T/T [10-3 ] 0.04 0.02 0.00-0.02 Signs of 2D-delocalized polarons Also with sub-gap excitation! T=80K, f=133hz 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 E [ev] PA IN PA OUT H. Aarnio et al., manuscript in preparation Page 15

Use of nanostructured TiO 2 for hybrid solar cells Gold P3HT PCBM Dye h + e - h + e - h + e - TiO 2 TiO 2 TiO 2 ITO Nanocratermonolayer Access to substrate through TiO 2 pores Charges can dissociate at: P3HT:TiO 2 P3HT/PCBM Dye:TiO 2 interface Extraction can take place directly to ITO or through TiO 2 Effective charge screening? Possibility to expand the absorption by using an IR-dye S. Sanden, Q. Xu, J.-H. Smått, et al.,

300 K, V off = 0,05 V 180 K, V off = 0,14 V j [ma/cm 2 ] 2,0 1,5 1,0 0,5 30 µs 40 µs 60 µs 90 µs 150 µs 300 µs 600 µs 1 ms 2 ms 4 ms 8 ms dark j [ma/cm 2 ] 0,4 0,3 0,2 0,1 40 µs 60 µs 90 µs 150 µs 300 µs 600 µs 1 ms 2 ms 4 ms 8 ms 16 ms dark 0,0 0,0-0,5 0 100 200 300 0 500 1000 1500 t [µs] t [µs] j [ma/cm 2 ] 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0-0,1 240 K, V off = 0,07 V 30 µs 40 µs 60 µs 90 µs 150 µs 300 µs 600 µs 1 ms 2 ms 4 ms 8 ms dark -100 0 100 200 300 400 500 600 700 j [ma/cm 2 ] 0,09 0,08 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0,00 120 K, V off = 0,15 V 60 µs 90 µs 150 µs 300 µs 600 µs 1 ms 2 ms 4 ms 8 ms 16 ms 32 ms dark -0,01-1000 0 1000 2000 3000 4000 5000 6000 7000 t [µs] t [µs]

µ [cm 2 /Vs] 10-3 300 K 270 K 240 K 10-4 10-5 10-6 10-7 10 1 10 2 10 3 10 4 t del [s] 210 K 180 K 150 K 120 K (dn/dt)/n 2 [cm 3 /s] 10-11 10-12 10-13 10-14 10-15 10-16 10 10-5 10-4 10-3 10-2 10-1 10-17 300 K 270 K 240 K 210 K 180 K 150 K 120 K t del + t max [s] n(t) [cm -3 ] 10 17 300 K 270 K 240 K 210 K 10 16 180 K 150 K 120 K 10 15 10 14 10-5 10-4 10-3 10-2 10-1 10 0 t del +t max [s] β/µ [cm 3 /s] 10-6 10-7 10-8 10-9 10-10 10-11 300 K 270 K 240 K 210 K 180 K 150 K 120 K 10-5 10-4 10-3 10-2 10-1 t del + t max [s]

Measurements on Sn:Sn Devices Au 3,0 ev LUMO 3,7 ev 4,4 ev 4,4 ev 5,1 ev HOMO 6,3 ev ITO Sn The work function of Sn lies between the LUMO of PCBM and the HOMO of P3HT The low conductivity of Sn is problematic

Measurements on Sn:Sn Devices 1E15 P3HT:PCBM 1:1 (Sn:Sn contacts) τ = 7,5 µs β = 1,33E-10 β/β L = 0,37 N0 =1E15 N -3 ext [cm ] 1E14 τ = 30 µs β = 1,45E-10 β/β L = 0,24 N0=2,3E14 Untreated Treated Bimolecular fit 1E13 1E-6 1E-5 1E-4 1E-3 Time [s] Mathias Nyman In collaboration with Fraunhofer ISE and Linköping niversity

Summary We have measured charge tranport and recombination in bulk heterojunction solar cells We found greatly reduced recombination in annealed RR-P3HT/PCBM Demixing and formation of lamellar structures in P3HT seems to be very important 2D Langevin model suggested Neglecting diffusion underestimates the recombination time in 2D Photoinduced absorption shows that 2D delocalized polarons are generated even with sub-gap excitation Hybrid interfaces offer new challenges

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