Interlayer energy and electron transfer from thiophene derivative to a porphyrin– fullerene dyad NGS-NANO meeting 10.– Kimmo Kaunisto Tampere University of Technology Department of Chemistry and Bioengineering

Background (1/6) Most of the world’s energy demand is covered with non- renewable energy sources (ca. 85%) Combustion of the fossil fuels (coil, oil, etc.) has many harmful effects on humans and environment »Global warming (greenhouse effect) »Acid rains »Smog »Human illnesses, etc.

Background (2/6) Limited supply of the fossil fuels together with their harmful side-effects have created demand for new and clean energy sources (renewable) »Solar energy (photovoltaics) »Wind power »Hydroelectric power »Geothermal

Background (3/6) Energy statistics: 15 TW, annual energy demand of the world TW, energy emitted by the Sun onto the Earth »0.02% light energy harvesting efficiency would be sufficient to replace all the other energy sources with the solar power »Sun is the ultimate energy source However, only TW was produced by photovoltaics in 2007 “the photovoltaic effect used in solar cells allows direct conversion of light energy emitted by the Sun into electricity”

Background (4/6) Lack of inexpensive and efficient technology restrains use of photovoltaics as energy source »Conventional semiconductor based solar cells are not cost-efficient enough (high light conversion efficiency, 15%) »Dye-sensitized cells have efficiencies about 11%, but stability and liquid electrolyte are the major problems »Light conversion efficiency of organic solar cells is only about 2% at date, also the stability is a major problem

Background (5/6) Conventional solar cells are approaching their theoretical efficiency limit of 33% and are unlikely to get more cost- efficient (expensive material costs) Organic electronic materials are attractive for photovoltaic applications (low production costs, high efficiency, flexibility, thinness, etc.) »However, fundamental physics of organic solar cells is poorly understood Need for basic research of organic photovoltaic principles!

Background (6/6) All photovoltaic systems are based on photoinduced charge separation In organic photovoltaics eletron transfer (ET) from a donor to an acceptor compound * Photophysical interaction between the donors and acceptors must be defined detailed in order to construct devices with ultimate performance hv ET * hv ET – + or DonorAcceptor e–e–

Aim of the study (1/2) Porphyrin–fullerene dyad, P-F Phenyl vinyl thiophene, PVT3 M= Zn or 2H The compounds and the used film structures are studied as possible candidates for organic solar cells To study photoinduced energy and electron transfer in solid state PVT3 acts as light absorbing and secondary electron donor layer Porphyrin–Fullerene dyad acts as energy and secondary electron acceptor layer »Primary ET from porphyrin to fullerene »Interlayer energy and electron transfer from PVT3 to P-F?

Aim of the study (2/2) hv * Energy transfer * Primary electron transfer Secondary electron transfer – + + Substrate ≈ 2 nm ≈ 15 nm Direction of electron movement substrate PVT3|P-FP-FPVT3 70% PVT310% P-F ODA (octadecylamine) substrate λ ex = 355 nm 430 nm 355 nm Studied film structures are deposited by the Langmuir–Blodgett (LB) method

Research methods Time-resolved vertical electron transfer inside the active layer after the pulsed laser excitation Nano- to millisecond timescale Number of ET states (N CS ), charge displacement (d) Electric method (photovoltage)Optical method (flash-photolysis) Time-resolved absorption change after the pulsed laser excitation (ΔA) Transient states of the compounds after the excitation Nano- to millisecond timescale [mV]

Photovoltage results 3-times higher photovoltage amplitude with the bilayered film Fast decay right after the excitation in the bilayered film Increased signal halftime for the bilayered film »Increased charge displacement »Charge migration in PVT3 and fullerene networks Electron transfer from PVT3 to P-F Possibility of the energy transfer λ ex = 430 nm

Flash-photolysis results Long-lived transient state exists in the bilayered film despite the fast decay right after the excitation »Fast decay: energy transfer »Long-lived tail: interlayer electron transfer from PVT3 to P-F Transient absorption bands of porphyrin cation when only PVT3 is excited »Energy transfer from PVT3 to P-F Stronger electron transfer from PVT3 to porphyrin cation if both P-F and PVT3 are excited λ ex = 355 nm λ ex = 430 nm P+P+ PVT3 + 3 PVT3

Reaction scheme

Summary The interlayer energy transfer from excited PVT3 to the porphyrin– fullerene dyad The secondary electron transfer from PVT3 to the porphyrin cation of the dyad In the final transient state of the PVT3|P–F film the positive charges are located at the PVT3 layer and the negative charges in the fullerene network Both compounds promising candidates for organic solar cells

Related references 1. K. Kaunisto et al., J. Phys. Chem. C 2008, 112, K. Kaunisto et al., Chem. Phys. Lett. 2008, 460, H. Lehtivuori et al., J. Am. Chem. Soc. 2006, 128, T. Vuorinen et al., Langmuir 2005, 21, V. Chukharev et al., J. Phys. Chem. B 2004, 108, 16377

Acknowledgements NGS-NANO for funding Tampere University of Technology, Department of chemistry and bioengineering, Chemistry laboratory Prof. Helge Lemmetyinen (supervisor) Prof. Nikolai Tkachenko (secondary supervisor) Dr. Vladimir Chukharev (instrumentation) Dr. Alexander Efimov (synthesis)