1. O Futuro é Orgânico?
O Papel dos Polímeros Condutores no Desenvolvimento de Materiais Solares Fotovoltaicos
Palmira F. Silva
Centro de Química Estrutural
Instituto Superior Técnico
Universidade Técnica de Lisboa

2. Outline
Why PV
Why OPV
Enhancing OPV efficiency
Conclusion

3. Why PV?

4. Why PV? ENERGY Increasing energy need ECOLOGY Pollution
Human total energy use: 4.0x1014 kWh/year
Incoming solar radiation (on Earth): 5.5x1017 kWh/year
ENERGY Increasing energy need
Exhaustion of fossil fuels
Diversification of energy sources
Energy for all (16% world population without electricity)
ECOLOGY Pollution
CO2 Responsible Climate change
ECONOMY Energetically independent

5. Why OPVs: Advantages low cost tailoring of optoelectronic properties
large areas
flexible
processing from solution
roll to roll manufacturing
light weight
transparent
low cost

6. Why OPVs: Problems

7. Why OPVs: Problems low ambient stability low efficiencies
strongly bound excitons
exciton diffusion length rather short (5-10 nm).
low mobility of charge carriers
μn (c-Si) > 1000 cm2/Vs
μh (polymer) ≈ 0.1 cm2/Vs
Organic Solar Cells

8. OPVs: Solving Problems
Working mechanism-steps
Excitons formation
Diffusion of the excitons towards the interface
Charge separation/electron transfer
Transport of charge carriers towards the electrodes
Organic solar cells performances depend on
the material properties
morphology (structure of the bulk hetero-junction)

9. Enhancing devices efficiency
Better understanding of the physics
Optimize the materials properties
Optimize the morphology

10. Enhancing devices efficiency
Better Understanding of the Physics of Organic Semiconductors:
Photophysics
Photoconductivity
Thin film characterization
Nanoscale microscopy (AFM,STM...)

11. The importance of Photophysics
Conjugated polymers frequently show complex fluorescence decays in films (and solution), and knowledge of what happens in polymer excited states deactivation is important to develop strategies to improve exciton splitting.

12. The importance of Photophysics
In the ground state, F8T2 undergoes a temperature induced transition between differently ordered conformations.

13. Photophysics Aim
Determine the factors controlling the efficiency and energetics of charge transfer
Fundamental understanding of photophysical and kinetic properties will allow us to design systems for efficient photovoltaic generation and separation of charges

14. Enhancing devices efficiency
Optimize the materials properties
Relative position of the energy levels of the donor and acceptor
Better match with the solar spectrum.

15. Optimize the materials
Optimal offset LUMO (D) – LUMO(A) for electron transfer
at least 0.3 – 0.5 eV
P3HT:PCBM:
LUMO (D) – LUMO(A) ~1.1 eV
bulk heterojunction
Power conversion efficiency ~ 5 - 6%
PCBM
(acceptor)
P3HT
(donor)

16. Optimize the materials
Improve the absorption of the solar radiation broadening the UV/Vis spectra using a polymer blend instead of a single polymer
P3HT
F8BT
UV-Vis absorption spectra of P3HT/F8BT blend films P3HT:F8BT - 0:100 (a), 20:80 (b), 40:60 (c), 60:40 (d), 80:20 (e), and 100:0 (f).

17. Enhancing devices efficiency
Optimize the morphology
microscopic phase separation
( exciton diffusion length ~ 5 – 10 nm )
presence of carrier paths between the two electrodes

18. Optimize the morphology
synthesis of donor and acceptor crosslinkable polymers to prepare segregated D/A columnar- like domains, at the micro- or submicrometer dimensions
ArcoCell PTDC/CTM/64127/2006

19. Project AIM
Develop and engineer new materials that promote efficient extraction of electron and hole from single exciton.

20. OPVs: Designing The Future

21. Fast Kinetics Lab – CQE/IST António Maçanita (PhD)
PTDC/CTM/58767/2004
PTDC/CTM/64127/2006
Fast Kinetics Lab – CQE/IST
António Maçanita (PhD)
Palmira F. Silva (PhD)
Roberto di Paolo (Pos Doc)
Rita Rodrigues (pHD Student)
Bruno Ferreira (Research Student)
Technology & Materials Group – IT/IST
Jorge Morgado (PhD)
Ana Charas (PhD)
Quirina Ferreira (Pos Doc)
Luís Filipe Morgado (PhD Student)
Joana Farinhas (PhD Student)
Tânia Braz (PhD Student)