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
Outline Why PV Why OPV Enhancing OPV efficiency Conclusion
Why PV?
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
Why OPVs: Advantages low cost tailoring of optoelectronic properties large areas flexible processing from solution roll to roll manufacturing light weight transparent low cost
Why OPVs: Problems
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
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)
Enhancing devices efficiency Better understanding of the physics Optimize the materials properties Optimize the morphology
Enhancing devices efficiency Better Understanding of the Physics of Organic Semiconductors: Photophysics Photoconductivity Thin film characterization Nanoscale microscopy (AFM,STM...)
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.
The importance of Photophysics In the ground state, F8T2 undergoes a temperature induced transition between differently ordered conformations.
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
Enhancing devices efficiency Optimize the materials properties Relative position of the energy levels of the donor and acceptor Better match with the solar spectrum.
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)
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).
Enhancing devices efficiency Optimize the morphology microscopic phase separation ( exciton diffusion length ~ 5 – 10 nm ) presence of carrier paths between the two electrodes
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
Project AIM Develop and engineer new materials that promote efficient extraction of electron and hole from single exciton.
OPVs: Designing The Future
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)