CHARGE CARRIERS PHOTOGENERATION. Maddalena Binda Organic Electronics: principles, devices and applications Milano, November 23-27th, 2015

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1 CHARGE CARRIERS PHOTOGENERATION Maddalena Binda Organic Electronics: principles, devices and applications Milano, November 23-27th, 2015

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 and 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: = abs ed cc * We want low (n 1 -n 2 )... n air =1 n glass =1.5 * normal incidence n Si =3.2 Transmission losses (or light absorption): n o ~1.7 Beer-Lambert law: abs

5 = abs ed cc 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: Low mobility (< 1 cm 2 /Vs) (3 4) Exciton Radius ~10Å Exciton Binding Energy (E B )~0.1 1eV kt(@300k)~25mev Thermal dissociation of the exciton into free charges

6 = abs ed cc Exciton generation upon light absorption CB VB

7 = abs ed cc 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: Low mobility (< 1 cm 2 /Vs) (3 4) Exciton Radius ~10Å Exciton Binding Energy (E B )~0.1 1eV kt(@300k)~25mev Thermal dissociation of the exciton into free charges

8 = abs ed cc Exciton generation upon light absorption LUMO EB Some kind of extra-energy is required! HOMO

9 How to obtain free charges from a strongly bound exciton? = abs ed cc E Molecular picture Sn Only EXTRINSIC photogeneration! S2 S1 CS (=charge separated state) optical gap Fluorescence (~ns) S0 electrical gap r NOTE that: optical gap = electrical gap A. Kohler, H. Bassler Electronic processes in organic semiconductors, Wiley-VCH 2015, Germany. epfd ISBN:

10 How to obtain free charges from a strongly bound exciton? = abs ed cc E Molecular picture Sn Autoionization CT S2 S1 Internal conversion (~ps) CS (=charge separated state) optical gap Fluorescence (~ns) S0 electrical gap r NOTE that: optical gap = electrical gap

11 How to obtain free charges from a strongly bound exciton? = abs ed cc E Molecular picture Sn S2 Autoionization CT S1 optical gap S0 CT (=charge transfer state) electrical gap CS (=charge separated state) r NOTE that: optical gap = electrical gap

12 = abs ed cc E How to obtain free charges from a strongly bound exciton? Molecular picture CT Sn S2 S1 optical gap S0 Autoionization CT (=charge transfer state) electrical gap CS (=charge separated state) r NOTE that: optical gap = electrical gap

13 = abs ed cc E How to obtain free charges from a strongly bound exciton? Molecular picture CT Sn Autoionization S2 S1 optical gap S0 + - CS (=charge separated state) + - CT (=charge transfer state) dissoc recombin electrical gap r c Onsager-Braun model 2 2 q q kt rc 4 0 irc 4 0 ikt r NOTE that: optical gap = electrical gap

14 = abs ed cc E How to obtain free charges from a strongly bound exciton? Molecular picture CT Sn rc S2 S1 optical gap S0 Autoionization CT (=charge transfer state) recombin dissoc electrical gap NOTE that: optical gap = electrical gap r c CS (=charge separated state) kt Onsager-Braun model 2 2 q q rc 4 0 irc 4 0 ikt r c >10 nm (ideal system) r c ~few nm (energetic disorder) r

15 On the excess energy Sn S2 S1 Charge generation efficiency (net of light absorption ) Increasing extention of excited state wavefunction for higher energy electronic states S0 DONOR/ACCEPTOR COPOLYMERS (PCDTBT, PCPDTBT, ) Confirmed by quantum mechanical calculations! Primary excitation already has strong CT character D A Adv. Funct. Mater. 2015, 25, Nature Materials, 2013, 12, 29-33

16 = abs ed cc E How to obtain free charges from a strongly bound exciton? Molecular picture CT Sn rc Autoionization S2 S1 optical gap S0 + - CS (=charge separated state) + - CT (=charge transfer state) dissoc recombin electrical gap r c E r θ r NOTE that: optical gap = electrical gap

17 How to obtain free charges from a strongly bound exciton? = abs ed cc J. Appl. Phys. 106, , 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))

18 Photoinduced charge transfer: donor/acceptor interfaces = abs ed cc D ΔE A <100 fs time scale Competitive with other relaxation phenomena ΔE is a driving force to charge separation

19 Photoinduced charge transfer: donor/acceptor interface = abs ed cc E Sn D A S2 S1 <100 fs CS (=charge separated state) optical gap S0 ns electrical gap r

20 Photoinduced charge transfer: donor/acceptor interface = abs ed cc D E Sn A S2 S1 optical gap S0 geminate recombination (k rec ) electrical gap CS (=charge separated state) CT binding energy up to hundreds of mev (a= 1nm, ε=3.5) r

21 Photoinduced charge transfer: donor/acceptor interface = abs ed cc E Sn S2 S1 optical gap S0 k therm k rec or k diss Dissociation from vibrationally «hot» CT: kdiss>ktherm Chemical Reviews, 2010, 110, 11 Dissociation of thermalized CT: kdiss<ktherm CS via interfacial dipoles, entropy, energetic disorder, electric field, confined volume for geminate recombination, charge motion dinamics Appl. Phys. Lett. 2003, 82, 4605; Nat.? Commun. 2013, 4, 2334 electrical gap r

22 = abs ed cc On the excess energy S Only If excess energy (LUMOD/LUMOA offset, high energy photon) gives access to a higher ELECTRONIC CT level, it can be exploited for dissociation. «Excess VIBRATIONAL energy has virtually no effect» «It is the spatial extention of the wavefunction of the excited state that determines the yield of dissociation» Kohler - Adv. Funct. Mater. 2015, 25, See also: Nature Materials, 2013, 12, 29-33

23 = abs ed cc Exciton deactivation Sn Internal conversion (~ps) - Radiative/non-radiative decay to the ground state (mediated by chemical defects, dopants, surface states) - Singlet to triplet (ISC) - Singlet/singlet annihilation S1 S0 Intersystem crossing T1 Fluorescence (~ns) Phosphorescence (~μs-ms) - Recombination with free charges (Auger recombination) - Quenching at the metal/organic interface

24 = abs ed cc 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,

25 = abs ed cc 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,

26 = abs ed cc 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,

27 Donor/acceptor heterojunction: morphology = abs ed cc acceptor (a) (b) L D BULK-HETEROJUNCTION Randomly mixed donor/acceptor molecules Maximized extension of donor/acceptor interface

28 = abs ed cc Morphology of D/A heterojunction Charge separation Charge transport Charge separation Charge transport (a) (b) Charge separation Charge transport Bulk-heterojunction Planar heterojunction Comb structure

29 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

30 To the electrodes... = abs ed cc 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

31 To the electrodes... = abs ed cc 1 Between FREE hole and electron: LANGEVIN R ( np 2 n i ) q( ) p n LANGEVIN in BHJ: R BHJ ( n A p D n A i p D i ) A D q( ) n p Via CT states: R CT N CT / CT R CT ( n A p D n A i p D i ) 1 / ' 1/ 1/ CT CT sep exp( CT 2 CT ) CT=std dev of CT state energetic distribution J. Phys. Chem. C, 2015 DOI: /acs.jpcc.5b08936; Phys. Review B 2010, 81,

32 To the electrodes... 2 Between FREE hole(electron) and trapped electron(hole): Trap assisted (single trap) R SHR C n Shockley-Read-Hall (SRH) E T C C n p ( n n N 1 t ) C LUMO ( np n p HOMO 2 i ) ( p p 1 ) = abs ed cc n 1 p p 1 1 Cn,Cp=capture coeff Nt=traps density exp(( Et Ec) / kt) exp(( Ev Et) / kt) n 1 2 n i Via «tail» (distribution of traps) LUMO HOMO LUMO R N( E) R ( E) de HOMO SRH RSRH=recombination rate per energy state at a specific energy Phys. Rev. Lett. 2011, 107,

33 To the electrodes... = abs ed cc Reverse diffusive at the contacts: free carriers diffuse against the electric field and recombine at the metal contacts Energy Environ. Sci., 2011, 4,

34 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) = abs ed cc 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

35 = abs ed cc 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

36 Overall photogeneration efficiency? EXTERNAL 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