Ultrafast Transient Absorption Spectroscopy Investigation of Photoinduced Dynamics in Poly(3-hexylthiophene)-block-oligo(anthracene-9,10-diyl) (Oligo-ANT-b-P3HT) Jacob Strain*, Hemali Rathnayake, Jinjun Liu 2017-06-21 The 72nd International Symposium on Molecular Spectroscopy

P3HT: Poly-3-hexylthiophene PCBM: 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 Outline Background Introduction to Poly(3-hexylthiophene)-block-oligo(anthracene-9,10-diyl) --Will be referred to as “Oligo-ANT-b-P3HT or block polymer” for the remainder of the presentation-- Experimental Setup Oligo-ANT-b-P3HT:PCBM Blend Transient Absorption Spectra Control: P3HT:PCBM Blend Discussion Concluding Remarks Full name of PCBM? Figures of PCBM and P3HT?

NREL chart A zoom-in to show PCE of organic solar cells in recent year. Your audience won’t be able to see the details without it. I think you used a zoom-in last year? I did but I think when we rehearsed it on the bigger screen it looked okay and the blow up image was super grainy and not at all useful.

Why Organic Solar Cells? Advantages: Low cost Lightweight Large Area Flexible Simple Solution Processing: Printing Spray Coating Remove the hyphens in “Low-cost” and “Large-area”.

Bulk Heterojunction Cell Schematic hn + - Donor Phase Bulk Heterojunction Cell Schematic Acceptor Phase Active Layer - PEDOT:PSS OR + - Cathode Anode hn ET HT Processes Involved in Photovoltaic Conversion: Light absorption Monomolecular deactivations of singlet excitons Singlet exciton diffusion to the interface Charge transfer Geminate recombination of bound radical pair Charge dissociation from bound radical pair Charge collection of free carriers + Al ITO Glass Substrate Use abbreviation for journal names. For example, J. Am. Chem. Soc. For “Journal of the American Chemical Society”. You can find list of journal abbreviations here: https://www.library.caltech.edu/journal-title-abbreviations Light Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Journal of the American Chemical Society 2010, 132, 6154.

Coulombically bound electron and hole pair Chunk of Energy What is an Exciton? Coulombically bound electron and hole pair Chunk of Energy Quasi-particle LD: distance exciton can physically travel

The Active Layer: Processes Involved in Photovoltaic Conversion: Light absorption (hn) Monomolecular deactivations of singlet excitons (kF) Singlet exciton diffusion to the interface (kED) Charge transfer (kCT) Geminate recombination of bound radical pair (kGCR) Charge dissociation from bound radical pair (kCD) Charge collection of free carriers (kCC) Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Journal of the American Chemical Society 2010, 132, 6154.

Internal Quantum Efficiency (IQE): numbers of charge carriers formed CT = ~100%: Charge Transfer CD: Charge Dissociation CC: Charge Collection ED: Exciton Diffusion, can be estimated from pristine polymer and polymer:fullerene time constant (tED = 6-16 ps); where ED ̴ kED higher kED -> higher ED ->higher IQE ->higher PCE! Has PCE been defined (full name)? Simply say “IQE is an indicater of the efficiency of a solar cell” How is IQE experiment performed? IQE = electrons per second / photons absorbed per second EQE = electrons per second / photons per second IQE always larger than EQE

Visual for Exciton Diffusion’s Impact on PCE PCBM Phase ~20 nm Excitons able to reach interface + - I rearranged the slide. The original one is a little bit misleading because the righ arrow in the legend is pointing toward the figure. Excitons unable to reach interface PCBM Phase Polymer Phase

Visual for Exciton Diffusion’s Impact on PCE ~10 nm PCBM Phase Excitons able to reach interface + - Rearranged. Excitons unable to reach interface PCBM Phase Polymer Phase

Twisted Geometry: Oligo-ANT P3HT Oligo-ANT-b-P3HT 50 nm Ananthakrishnan, S. J.; Strain, J.; Neerudu Sreeramulu, N.; Mitul, A.; McNamara, L. E.; Iefanova, A.; Hammer, N. I.; Qiao, Q.; Rathnayake, H. Journal of Polymer Science Part A: Polymer Chemistry 2016, 54, 3032.

The 0-0 peak is due to interchain interactions between polymers Steady State Spectra The 0-0 peak is due to interchain interactions between polymers

Add lines to show where you cut the slices. t1 = 0.603 ps t2 = 8.93 ps t3 = 110 ps t4 = long

t1 = 0.834 ps t2 = 12.75 ps t3 = 160 ps t4 = long

What are these peaks in polymer:fullerene films? The GSB peak around 610 nm generated from interchain interactions in the polymer is almost absent in Oligo(ANT-b-P3HT) at 1 ps. What are these peaks in polymer:fullerene films? Polymer GSB Delocalized Localized Polymer Polarons

What does the time constants mean? P3HT:PCBM (ps) Oligo(ANT-b-P3HT):PCBM (ps) Assignment t1 0.834 0.603 polaron generation at the polymer:fullerene interface t2 12.75 8.93 polaron generation in the polymer t3 160 110 hole transfer from crystalline polymer to amorphous t4 long charge carriers Add an example of multiexponential decay fitting. Add one more column to show “GSB(-)” and “ESA(+)”. These four terms have components that are negative and positive. Attempting to indicate them as such would be very misleading. What does “charge carrier” mean? Why photophysical process of “charge carriers”? The actual current generated 25% decrease in lifetime or 25% increase in rates Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Journal of the American Chemical Society 2010, 132, 6154. Paraecattil, A. A.; Banerji, N. Journal of the American Chemical Society 2014, 136, 1472. .

Discussion: Marcus Theory, a Decrease in Reorganization Energy Higher crystalline order (and slightly smaller domains) caused by the Oligo-Ant moieties decrease the reorganization energy (l) thus increasing k DA DA P3HT:PCBM Block:PCBM l D+A- D+A- l Gibbs Energy DGǂ DGǂ DG° DG° Reaction Coordinate ~50% less Brédas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chemical Reviews 2004, 104, 4971.

Conclusions The presence of Oligo-ANT in the P3HT polymer causes higher crystalline order. Because of the higher crystalline order the rates of charge separation increases. Furthermore, because of the smaller phase domains in the Oligo(ANT-b-P3HT):PCBM blend a higher amount of surface area present between the donor and acceptor domains allowing more possible sites for charge dissociation.

Advisors Funding: Dr. Jinjun Liu Dr. Hemali Rathnayake Acknowledgements University of Louisville Lab Group: Advisors Dr. Jinjun Liu Dr. Hemali Rathnayake Funding: NSF MRI CHE 1338072 KCF-KSEF-144-401-13-062 Department of Energy