Correlations of electronic and molecular structures of conducting polymers with device efficiencies in BHJ solar cells

Transcription

1 NF/NR Worshop on Key cientific and Technologicall Issues for Development of Next- Generation rganic olar Cells eptember 20 21, 2012, Arlington, VA Correlations of electronic and molecular structures of conducting polymers with device efficiencies in BHJ solar cells Lin. Chen Chemical ciences and Engineering Division, Argonne National Laboratory Department of Chemistry, Northwestern University upported by Basic Energy cience, ffice of ciences, U Department of energy

2 Jodi zarko, Brian Rolczynski, ylvia Lou, Jianchang Guo, ung Cho, Nicholas Jackson, Northwestern University/Argonne; Joseph trzalka, AP, Argonne National Lab. Luping Yu Group; University of Chicago: Tobin Marks, Robert Chang, Mark Ratner Groups;. Northwestern University and ANER Center.

3 utline Introduction Exciton and charge separation dynamics in isolated charge transfer (e.g., PTBF) polymers in solution: implication of materials design Exciton and charge separation dynamics in BHJ PV films (made from charge transfer polymers) Morphology requirements for high PCE and the role of DI additives ome challenges and concerns

4 Conjugated Donor-acceptor Copolymers in BHJ Materials e Dyad Triad

5 Electron Donor-Acceptor Characters in the Copolymers mall structural tuning, big PCE difference (PTBF0) Yu, U. Chicago Liang et al. JAC (2009), 131, ; Liang et al. Adv. Mater. (2010), 22, E135-E138; on et al. JAC (2011), 133,

6 Electron Donor-Acceptor Characters in the Copolymers mall structural tuning, big PCE difference Voc (V) Jsc (ma/cm 2 ) FF (%) PCE (%) PTB2(F0) PTB7(F1) PTBF PTBF on et al. JAC (2011), 133, PTB7 or PTBF1 promising materials for PVs with PCE ~8.5% in single junction devices (olamer).

7 Free Energy Fundamental Photophysical Processes in PV Materials Exciton 1 hν k IC k CT 1 Bound radical pair Polymer cation PCBM anion T 1 CT* C* k GR k C* 2 k CT-therm k C k BR Detecting transient optical signatures of different species as function of the time delay after the fs laser pulse excitation k C-therm 3 C ΔG C Free charge carriers 0 Exciton fs - ps >fs - ns >μs Pseudo Charge transfer or bound radical pair Charge separated state Intra-molecular and intermolecular processes

8 Exciton plitting Dynamics in Isolated Polymers in olution NIR transient absorption spectra at 10 ps delay PTBF1 C E PCT PTBF2 C E PCT PCT C E When isolated polymers in solution is excited in the absence of PCBM, PCT (or CT) and C species are generated within 150 fs of the excitation, with varying relative concentrations. Rolczynski, zarko, on, Liang, Yu, Chen, J. Am. Chem. oc. 134, (2012). 8

9 Exciton plitting Dynamics in Isolated Polymers in olution Red: polymer cation, C; Black: exciton; Blue: PCT or bound radical pair (solution spectra) PTBF0 PTBF1 PTBF2 BRP longer lived with more fluorines. PTBF3 Where is C (in < 150 fs) from in a single isolated polymer chain? Rolczynski, zarko, on, Liang, Yu, Chen, J. Am. Chem. oc. 134, (2012).

10 RBR RBR A urprising Correlations of Intramolecular Initial C/PCT in Isolated PTBFx with Device Performance 3 2 (a) 1 PTBF Device PCE 3 PTBF1 PTBF0 2 1 PTBF3 PTBF3 PTBF2 PTBF0 PTBF1 (b) FF RBR D D C PCT (0) (0) Larger local polarity (local dipole) More polar exciton Higher C efficiency ; Intramolecular C defrays some exciton binding energy; [ C] [ PCT ] Intramolecular interactions can be propagated to device performance; tronger pulling of electron density towards thienothiophene could further increase device PCE. Rolczynski, zarko, on, Liang, Yu, Chen, J. Am. Chem. oc. 134, (2012).

11 E Exciton plitting Dynamics in Isolated Polymers in olution A tug of war in local electron density F F F r h-e The local polarity favors the C state generation and lowers CT state population. Total electron density vs. no. F

12 A urprising Correlation of Intramolecular Initial C/PCT in Isolated PTBFx with Device Performance The local dipole creates a polar environment that favors a polarized exciton where a part of binding energy or Gcs has been defrayed by the precharge separation. +δ F +δ nsager theory How much local polarity is optimal? What are D and A in isolated polymers? What is the exciton diffusion dynamics?

13 PCE(%) A urprising Correlation of Intrinsic local dipole change and device PCE ge 2 2 [( gx ex) ( gy ey ) ( gz ez Equation y = a + b*x Weight No Weighti Residual um of quares Pearson's r Adj. R-quare Value tandard Er B Intercept B lope PTB2 PTBF1 ) 2 ] 1/ 2 Polym Δµ ge PCE er (D) (%) PTB PTBF PTBF PBB PBB3 PTBF ge (Debye) Carsten, zarko et al., J. Am. Chem. oc., (2011).

14 Exciton and Charge eparation Dynamics in Bulk Heterojunction PV Films e Rolczynski, zarko, on, Liang, Yu, Chen, to be submitted, zarko, Rolczynski et al., to be submitted.

15 Energy Dynamics of CT Detrapping in BHJ Films: a Probe for h e eparation (PTBF1:PC71BM) E r cs 2 e 4 k r 0 B T Polymer cation (C) peak shift Excitation 695 nm CT C Depth of the trap? Excitation 600 nm G Excess excitation energy helps detrapping of CT state and sustaining C population. Rolczynski, zarko, on, Liang, Yu, Chen, to be submitted, zarko, Rolczynski et al., to be submitted. kbt

16 Exciton Dynamics tudies: ummary Ultrafast dynamics for E, PCT (CT) and C states are measured for PTBF polymer series, and the RBR (C/CT) varies and linearly correlated with PCE in solution, neat films and BHJ films; Larger local dipole changes along the polymer backbone can facilitate polarized excitons, and defrays a part of the E binding energy in isolated polymers and in BHJ films, leading to high device PCE, but this phenomenological model needs to be evaluated in a longer segment; The detrapping of CT state is observed in BHJ films with a trap energy of ~kt for PTBF1 while traps are deeper for poorly performing polymers; Ultrafast excition delocalization and coherent coupling dynamics and long time scale C/CT dynamics need to be measured in the future.

17 Correlation of Interfacial π- stacking and Device Performance, and the Role of DI e - + -

18 An Interfacial rientation Favoring Charge Collection GIWA: π-π stacking q 2 d P3HT PTB1 zarko, Guo, Rolczynski, Chen, J. Mat. Chem. (Invited Highlight) 21, (2011).

19 The π-stack pacing vs. The Device Fill Factor ide group dependent stacking PTB1 PTB1 PTB2 PTB3 PTB4 Tighter packing larger FF PTB5 PTB6 PTB7 Better π-stacking at the electrode surface enhances charge collection efficiency, zarko, et al., Advanced Materials, 22, 5468 (2010)

20 Effects of additive on the active layer solution + Without DI + Diiodooctane (DI) 200 nm With DI How does the addition of DI affect the PTB7:PCBM blend in solution? V oc (V) J sc (ma/cm 2 ) FF (%) Eff. (%) PTB7:PC 71 BM without DI PTB7:PC 71 BM with DI

21 Effects of additive on the active layer solution olution small angle x-ray scattering traces of A) PTB7, B) PC 71 BM and C) PTB7:PC 71 BM (1:1.5 w/w) in chlorobenzene solution with and without 3% DI. Aggregate size of active layer materials determined by small angle scattering equation. Lou, zarko, u, Yu, Marks, Chen, JAC, (2011);

22 What else are special about PTB polymers? Norm. AB Current Density (ma/cm 2 ) PTB1-film PTB1-chlorobenzene Wavelength (nm) Media independent UV-vis absorption; Face down -stacking at the electrode interface; Annealing reduces cation initial population, speeds up its formation, but did not change the longer time kinetics PTB1:PCBM device PTB1/PCBM pristine PTB1/PCBM annealed Annealed Pristine Voltage (V) Cation signal in TA 200 nm Guo, Liang, zarko, Lee, on, Rolczynski, Yu, Chen, J. Phys. Chem. B114, 742 (2010).

23 ome Challenges and Concerns Photophysics of exciton formation, delocalization, coherence (size of exxitons) and splitting mechanisms are these important to device performance? The propagation of intrinsic/molecular properties to device performance from theoretical calculation to molecular design; Mechanisms of reducing charge germinate and bimolecular recombination h-e distance control; Material dependent optimized morphology: anneal or not anneal; amorphous vs. crystalline one morphology does not fit all; Re-evaluate/not reinvent conventional wisdom of PV materials/devices fabrication (domain size vs. exciton diffusion; charge transfer rate vs.h-e distance; NIR light harvesting vs. Voc; exciton polarity vs. oscillator strength; C driving force vs. band gap, etc.) 23

24 Thank you.