Charge carriers photogeneration. Maddalena Binda Organic Electronics: principles, devices and applications Milano, November 26-29th, 2013

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1 Charge carriers photogeneration Maddalena Binda Organic Electronics: principles, devices and applications Milano, November 26-29th, 2013

2 Charge carriers photogeneration: what does it mean? Light stimulus (CW, pulsed,...) FREE holes FREE electrons A/I = I and/or V PHOTOcurrent Other ways of generating carriers in organic semiconductors: -Injection from the electrodes -Doping -Thermal excitation PHOTOvoltage

3 Photogeneration: a multistep process PHYSICAL PHENOMENON LOSSES Light absorption Reflection and transmission abs PHOTOGENERATION Exciton generation Exciton splitting into free charges Excitons recombination ed Charge transport Charge collection at the electrodes Charges recombination Energy barriers at the electrodes cc Efficiency of photocurrent generation: = abs ed cc

4 Light absorption: a strength point of organics Reflection losses: * * normal incidence Transmission losses (or light absorption): We want low (n 1 -n 2 )... n air =1 n glass =1.5 Beer-Lambert law: n Si =3.2 n o ~1.7 abs

5 Exciton generation upon light absorption COULOMB MUTUAL INTERACTION INORGANIC SEMICONDUCTORS: Exciton Radius ~100Å Exciton Binding Energy (E B )~10meV High mobility (hundreds cm 2 /Vs) High (>10) H. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals and Polymers, 2nd ed. (Oxford, Univ. Press, 1999). kt(@300k)~25mev Thermal dissociation of the exciton into free charges ORGANIC SEMICONDUCTORS: Exciton Radius ~10Å Exciton Binding Energy (E B )~0.1 1eV Low mobility (< 1 cm 2 /Vs) (3 4) kt(@300k)~25mev Thermal dissociation of the exciton into free charges

6 Exciton generation upon light absorption LUMO Some kind of extra-energy is required! HOMO

7 How to obtain free charges from a strongly bound exciton? Molecular picture S3 S2 Internal conversion (~ps) S1 optical gap Fluorescence (~ns) S0 k IC >>k AI NOTE that: optical gap = electrical gap

8 How to obtain free charges from a strongly bound exciton? Molecular picture CT S3 S2 Internal conversion (~ps) + - S1 optical gap + - Fluorescence (~ns) CT (=charge transfer state) S0 k IC >>k AI NOTE that: optical gap = electrical gap

9 How to obtain free charges from a strongly bound exciton? Molecular picture CT S3 Internal conversion (~ps) S2 + - Autoionization CS (=charge separated state) S1 optical gap S0 + - Fluorescence (~ns) CT (=charge transfer state) electrical gap k IC >>k AI NOTE that: optical gap = electrical gap r

10 How to obtain free charges from a strongly bound exciton? Molecular picture CT S3 S2 Internal conversion (~ps) + - Autoionization CS (=charge separated state) S1 optical gap S0 k IC >>k AI + - Fluorescence (~ns) recombin CT (=charge transfer state) dissoc NOTE that: optical gap = electrical gap r c electrical gap kt Onsager-Braun model 4 q 0 2 kt r r c >10 nm (ideal system) r c ~few nm (energetic disorder) r q C irc i

11 How to obtain free charges from a strongly bound exciton? Molecular picture S3 S2 Internal conversion (~ps) Autoionization CS (=charge separated state) S1 optical gap S0 Fluorescence (~ns) recombin CT (=charge transfer state) dissoc r c electrical gap r Ē

12 How to obtain free charges from a strongly bound exciton? J. Appl. Phys. 106, , θ L. Onsager, J. Chem. Phys. 2 (1934) 599 C. L. Braun, J. Chem. Phys. 80, 4157, (1984). Onsager-Braun model, refined by Wojcik and Tachiya (M. Wojcik and M. Tachiya, J. Chem. Phys. 130, (2009)) r F

13 Photoinduced charge transfer: donor/acceptor interfaces D ΔE A <100 fs time scale Competitive with other relaxation phenomena Why is it different from interface between two molecules of the same kind? ΔE is a driving force to charge separation

14 Photoinduced charge transfer: donor/acceptor interfaces S2 D A S1 S0 <100 fs time scale CT geminate recombination (k rec ) CS CT binding energy up to hundreds of mev (a= 1nm, ε=3.5)

15 Photoinduced charge transfer: donor/acceptor interfaces S2 D A S1 S0 k diss k therm CT geminate recombination (k rec ) CS CT binding energy up to hundreds of mev (a= 1nm, ε=3.5) Dissociation from vibrationally «hot» CT : kdiss>ktherm Chemical Reviews, 2010, Vol. 110, No. 11 Dissociation of thermalized CT (kdiss>ktherm): via interfacial dipoles, entropy, energetic disorder, electric field, confined volume for geminate recombination, charge motion dinamics Arkhipov, V. I.; Heremans, P.; Bassler, H. Appl. Phys. Lett. 2003, 82, Nat. Commun. 4:2334 doi: /ncomms3334,(2013)

16 HOT dissociation: case of low bandgap polymer PCPDTBT PCPDTBT Donor PCBM Acceptor Grancini et al., Nature Materials, 12, 29-33, High energy excitation gives access to electronically hot CTs

17 Exciton deactivation In order to split up at the interface, exciton must survire till the interface is reached Sn Internal conversion (~ps) Conversion to a triplet state by spin orbit coupling S1 Intersystem crossing T1 Fluorescence (~ns) Phosphorescence (~μs-ms) - Singlet to triplet (ISC) - Singlet/singlet annihilation - Recombination with free charges S0

18 Exciton diffusion Radiative Non-radiative Mol A h Förster Dexter Electrostatic coupling Simoultaneous double charge transition Mol B C. Madigan, Phys. Rev. Lett. 96, 4, , Spectral overlap between A emission and B absorption required How? Mol A Long range <10nm SINGLET EXC. Mol B Mol A Mol B If both hole transfer and electron transfer are favorable: spatial wavefunctions overlap of A and B sites A nearest neighbor phenomenon <20Å TRIPLET EXC. Macromol. Rapid Commun. 2009, 30,

19 Exciton diffusion Radiative Non-radiative Mol A h Förster Dexter Electrostatic coupling Simoultaneous double charge transition Mol B C. Madigan, Phys. Rev. Lett. 96, 4, , Spectral overlap between A emission and B absorption required Mol A Long range <10nm SINGLET EXC. Mol B Mol A Mol B If both hole transfer and electron transfer are favorable: spatial wavefunctions overlap of A and B sites A nearest neighbor phenomenon <20Å TRIPLET EXC. Macromol. Rapid Commun. 2009, 30,

20 Exciton diffusion Radiative Non-radiative Mol A h Förster Dexter Electrostatic coupling Simoultaneous double charge transition Mol B C. Madigan, Phys. Rev. Lett. 96, 4, , Spectral overlap between A emission and B absorption required How far? Mol A Mol B Mol A Mol B If both hole transfer and electron Long L range <10 transfer nm are favorable: spatial <10nm D wavefunctions overlap of A and B sites SINGLET EXC. A nearest neighbor phenomenon <20Å TRIPLET EXC. Macromol. Rapid Commun. 2009, 30,

21 Donor/acceptor heterojunction based active materials acceptor (a) (b) L D BULK-HETEROJUNCTION Randomly mixed donor/acceptor molecules Maximized extension of donor/acceptor interface

22 Donor/acceptor heterojunction based active materials acceptor (a) (b) L D Free charge carriers BULK-HETEROJUNCTION Randomly mixed donor/acceptor molecules Maximized extension of donor/acceptor interface......but poor charge transport to the electrodes!

23 D/A heterojunction based active materials Photoluminescence (PL) competitive with exciton dissociation Strong PL quenching Exciton splitting efficiency (or energy transfer?) (a) Photocurrent onset (b) PL quenching DONOR: alkoxy-poly(p-phenylene vinylene)(ppv) ACCEPTOR: [6,6] phenyl C61 butyric acid methyl ester (PCBM) Charges percolation A. Aharony, D. Stauffer, Introduction to Percolation Theory, Taylor & Francis, London and New York, 2nd edition, 1992.

24 Morphology of D/A heterojunction Charge separation Charge separation Charge transport Charge transport (a) (b) Charge separation Charge transport Planar heterojunction Bulk-heterojunction Comb structure

25 Donor/acceptor materials: examples p-type, DONORS n-type, ACCEPTORS H. Hoppe and N. Serdar Sariciftci, J. Mater. Res., Vol. 19, No. 7, Jul 2004 Other dissociation interfaces: chemical defects, dopants, surface states, metal-semiconductor interface at the electrodes (C.J. Brabec and N.S. Sariciftci, Semiconducting polymers: VCH, 2000) Chemistry, physics and engineering, ch. 15, pp , Wiley-

26 To the electrodes... Charge Transport D. Natali Recombination: WITH OTHER CHARGE CARRIERS Between FREE hole and electron: WITH EXCITONS LANGEVIN Via CT states 1 Auger recombination: of an exciton with a free hole or electron but exciton lifetime ~10 100fs Between FREE hole(electron) and trapped electron(hole): very unlikely to occur Trap assisted Via tail 2

27 To the electrodes... Charge Transport Recombination: D. Natali WITH OTHER CHARGE CARRIERS Between FREE hole and electron: 1 LANGEVIN Via CT states LUMO r c R ( np 2 n i ) q( n p) q min( n, p ) HOMO U PG (1 P) R P=CT dissociation probability S. Zhou et al. / Physica B 415 (2013) R.A. Street, Phys. Review B 81, (2010)

28 To the electrodes... Charge Transport Recombination: D. Natali WITH OTHER CHARGE CARRIERS Between FREE hole(electron) and trapped electron(hole): Trap assisted (single trap) Shockley-Read-Hall (SRH): E T LUMO HOMO R C n C C n ( n p n N 1 ) t ( np C p n ( p 2 i ) p 1 ) 2 Cn,Cp=capture coeff Nt=traps density n1 exp( Et Ec) p1 exp( Ev Et) Via «tail» (distribution of traps) LUMO LUMO R N( E) R ( E) de HOMO SRH HOMO R TOT =R LUMO, tail +R HOMO, tail

29 To the electrodes... Charge Transport Recombination: D. Natali Reverse diffusive at the contacts: free carriers diffuse against the electric field and recombine at the metal contacts Energy Environ. Sci., 2011, 4,

30 Investigating recombination losses A. Geminate (CT recombination before splitting into free charges) B. Langevin (o CT mediated) C. Assisted by localized states (traps, tail states) D. Reverse diffusive at the contacts PHYSICAL REVIEW B 84, (2011) Review on 14 papers: 3 3 A 3 B 2 3 A+B C A+D Trap-dominated recombination should not be a surprise, because more carriers are trapped in localized states than are in the transport band under solar cell operating conditions R.A. Street

31 Investigating recombination losses Light intensity dependence (charge concentration) of R: R=kn order by transient photovoltage (TPV) and charge extraction (CE) measuerements at varying light intensity. TPV CE OC Voc +ΔVoc R=1/τ n CE τ n CE

32 Investigating recombination losses Light intensity dependence (charge concentration) of R: R=kn order order >2 experimentally?!? Geminate: bound hole/electron nexc order= 1 Langevin classic: free carriers holes/electrons (no disorder): nh, ne order=2 Trap-mediated: Trap states occupation (trap population) poorly dependent on charge concentration: nf Deep traps order =1 trapped population free carrier population (nf) Shallow traps (tail of the DOS) Trap states occupation dependent on charge concentration: nt=concentration of trapped hole(electrons) nt nf order =2

33 Investigating recombination losses If both carriers are trapped in the exponential tail, the recombination event will require at least one of them to be detrapped and become free but this is concentration dependent! R n t, h n f, e concentration dependent! Hp. exponential tail nt,h=nt,e nf,h=nt,fe n f n t ( Ech/ kt ) R n t n ( Ech/ kt ) (1 Ech/ kt ) t nt Ech>kT order >2!!!

34 To the electrodes... Collection at the electrodes Non-blocking contacts are required GENERAL REQUIREMENT: It s all about choosing suitable metals but... the choice of the metal contacts is driven by other requirements: leakage current, built-in electric field, availability and cost, stability, life at metal/semiconductor interface is far more complicated... D. Natali

35 Overall photogeneration efficiency? QUANTUM EFFICIENCY: EQE= Number of collected charges/s Number of incoming photons/s n c /s = = abs ed cc n v /s RESPONSIVITY: I λq λ[nm] R= = = EQE hc EQE 1240 P ott