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Plastic solar cells

Plastic solar cells. M. A. Loi Zernike Institute for Advanced Materials University of Groningen, The Netherlands e-mail M.A.Loi@rug.nl. Overview. 1 st hour Solar cells in general Solar Radiation p-n junction The organic version 2 nd hour Improving plastic solar cells

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Plastic solar cells

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  1. Plastic solar cells M. A. Loi Zernike Institute for Advanced Materials University of Groningen, The Netherlands e-mail M.A.Loi@rug.nl

  2. Overview 1st hour • Solar cells in general • Solar Radiation • p-n junction • The organic version 2nd hour • Improving plastic solar cells • Low band-gap polymers • Charge transfer states is detrimental?

  3. Solar Cells I • Long duration power supply • Satellites • Space vehicles • Remote locations on earth • Valid alternative to fossil fuels • Pollution free

  4. Solar Cells II • Photovoltaic effect • Becquerel (1839) • Fritts {Selenium} (1883) • Ohl {semiconductor junction solar cell}(1946) • Chapin, Fuller, Person {Silicon p-n junction solar cells} (1954)

  5. Motivations • ENERGY Increasing energy need Exhaustion of fossil fuels Diversification of energy sources Energy for all (2 billion people without electricity) • ECOLOGY Pollution of environment CO2 Responsible Climate change • ECONOMY Energetically independent

  6. Solar Radiation • Every second in the sun 6 x 1011 kg H2 → He + 4 x 1020 J ☼ • At the average distance of the earth the solar radiation is 1353 W/m2 • The atmosphere attenuates the solar radiation • Absorption water - IR • Absorption Ozone – UV • Scattering Air Mass

  7. Air Mass • Air mass = the path length of the light from a celestial source relative to that at the zenith at sea level. • increases as the angle between the source and the zenith increases (AM38 at the horizon). • Out of the atmosphere AM0 • On earth surface with sun at the zenith AM1 • Average for terrestrial applications - 45˚ from the zenith AM1.5 AM= sec qq = zenith angle

  8. Solar spectrum

  9. Solar cells – inorganic case • Single bandgap material • Photons with hn<Eg lost energy • Photons with hn=Eg used energy • Photons with hn>Eg (hn-Eg) lost energy Illuminated p-n junction

  10. P-n junction solar cells

  11. Ideal solar cell IL current produced by solar radiation Is diode saturation current RL load resistance Shockley diode equation A device area

  12. IV characteristics Short circuit current Open circuit voltage

  13. Ideal solar cell

  14. IV characteristics-realistic Series resistance Junction, impurity concentration Shunt resistance – leakage current

  15. IV characteristics-realistic Rs in Si solar cells 0.7-0.4 W The effect RSH is negligible

  16. Conversion efficiency • FF; IL; Voc should be maximized for efficient solar cells! Fill factor Conversion efficiency EQE or IQE, quantum efficiency-percentage of photon converted in carriers (ISC)

  17. Ideal efficiencies

  18. Real efficiency

  19. Plastic Solar cells 2000 2008

  20. Pro & con Advantages • tailoring of opto-electronic properties • large areas • low temperatures (RT) • processing from solution • roll to roll manufacturing • light weight • transparent • low cost…….maybe… Power paint?

  21. Pro & con Problems • low ambient stability • strongly bound excitons (Frenkel like) • Exciton diffusion length rather short 5-20 nm. • low mobility of charge carriers • μn (c-Si) > 1000 cm2/Vs • μh (polymer) ≈ 0.1 cm2/Vs • difficult to obtain low band-gap materials

  22. Nevertheless http://www.konarka.com Disposable low-end applications!

  23. To start - photoexcitations Charge Transfer exciton Frenkel exciton Stronghly bound (0.4 eV in PPV); radius  5 Å Molecola + - - + Polarons • Molecular semiconductors • coulomb interaction • elettron-phonon coupling + -

  24. Intermolecular excitons Frenkel excitons non radiative states Triplet excitons Non radiative-emission Fluorescence Phosphorescence Ground state

  25. The first examples • Early works inspired by nature (photosynthesis) • Porphyrins, phthalocyanines, perylenes (xerography), merocyanines • Organic heterojunction devices: p-type / n-type organic semiconductors • – 1970’s until 1995: organic heterojunction bilayers • – 1985 Tang cell: PTCBI (45 nm) and CuPc (25 nm) • 1% efficiency

  26. The Kodak approach Tang et al., APL 2005

  27. The polymer approach! • Active layer: bulk heterojunction - hole conducting material - electron conducting material • Operation principle: • Exciton photoexcitation • Diffusion of the excitons towards the organic-organic interface • Charge separation/electron transfer • Transport of charge carriers towards the electrodes

  28. Photoinduced Charge Generation An ultra-fast e- transfer occurs between Conjugated Polymer / Fullerene composites upon illumination. The transition time is less than 40 fs. Back transfer very slow! ms - ms ACCEPTOR DONOR exciton MDMO PPV 3,7 - dimethyloctyloxy methyloxy PPV PCBM 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C61 N. S. Sariciftci et al., Science 258, 1474 (1992)

  29. The driving force! • Electron affinity fullerene derivatives! -3.5 eV -4.2 eV -5.2 eV -6 eV PCBM Polymer

  30. Bulk Heterojunctions e- Al Electrode Al Electrode e- e- e- P+ e- e- e- e- P+ ITO on Glass / Plastic MDMO-PPV PCBM hn

  31. P-Solar Cells - FILM PREPARATION Spin Casting is a easy coating technique for small areas. Material loss is very high. Doctor Blade Technique was developed for large area coating Doctor Blade has no material loss

  32. Production - Large Area Large Area Thin Film Production using Doctor/Wire Blading a) b)

  33. Plastic Solar Cells - CONTACTING The cathode electrode is applied by evaporation. Different electrodes are used for different applications. Sealing is absolutely necessary for an increased life time of plastic solar cells.

  34. Characterization under A.M. 1.5

  35. Bulk Heterojunctions e- Al Electrode Al Electrode e- e- e- P+ e- e- e- e- P+ ITO on Glass / Plastic MDMO-PPV PCBM hn

  36. The morphology issue… 2,5% < 1% S. E. Shaheen, Appl. Phys. Lett., 78, 841–843 (2001)

  37. Now.. Organic solar cells performances depend on the material properties and microscopic structure of the bulk heterojunction! P3HT 4,5-5.0 % > 60 polymers checked last 5 years!

  38. Optimization heff = Isc * Voc * FF / Iinc Isc Tuning of the Transport Properties - Mobility VocTuning of the Electronic Levels of the Donor Acceptor Systems FF Tuning of the Contacts and Morphology Iinc Tuning of the Spectral Absorbance/Absorbing more light (low bandgap)

  39. The future?

  40. Intermezzo!

  41. Organic Solar cells bulk heterojunction 3D heterostructure hole conducting material + electron conducting material bulk heterojunction PCBM (acceptor) Polymer (donor) Power conversion efficiency ~ 5 - 6%

  42. Remember-Organic Solar Cells • Working mechanism-steps • Excitons photoexcitation • 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 • the microscopic structure of the bulk hetero-junction

  43. The driving force! • Donor and acceptor LUMO energy offset! -3.5 eV -4.2 eV -5.2 eV -6 eV PCBM Polymer Ultrafast phenomena!

  44. Enhancing devices efficiency • Optimize the materials properties • Matching solar spectrum! NIR materials • Relative position of the energy levels of the donor and acceptor • optimal offset between LUMO (D) – LUMO(A) for electron transfer at least 0.3 – 0.5 eV • P3HT:PCBM: LUMO (D) –LUMO(A) ~1.1 eV • Optimize the morpholog • microscopic phase separation ( exciton diffusion length ~ 5 – 7 nm ) • presence of a percolation pathway

  45. Remember-Solar cells parameters • JSC– short-circuit current • Jph– photocurrent • FF– fill factor: • VOC – open circuit voltage LUMO (D) 3 LUMO (A) 4 Donor Voc • h - power conversion efficiency Energy (eV) Acceptor HOMO (D) 5 6

  46. bisPCBM Power conversion efficiency ~ 4.5 % !!! 3 P3HT:PCBM 4 Donor Voc • LUMO offset ~ 1.0 eV • Voc~ 0.73 V Acceptor 5 • power conversion efficiency ~ 3.8 % • LUMO offset ~ 1.1 eV • Voc~ 0.59 V 6 The reduction of the LUMO offset Energy(eV) M. Lenes et al, Adv. Mater.2008, 20, 2116

  47. PL of thermally annealed films The devices performance: P3HT:PCBM – 3% P3HT:bisPCBM – 3.6% P3HT:bisPCBM –tPL ≈ 60 ps P3HT:PCBM –tPL ≈ 41 ps  electron transfer is more efficient for P3HT:PCBM

  48. PL of solvent annealed films The devices performance: P3HT:PCBM – 3.8% P3HT:bisPCBM – 4.6% P3HT:bisPCBM –tPL ≈ 38 ps P3HT:PCBM –tPL ≈ 31 ps  electron transfer is more efficient for P3HT:PCBM

  49. AFM measurements P3HT:PCBM P3HT:bisPCBM • surfaces is smoother for samples prepared by thermal annealing • difference in RMS roughness between P3HT:PCBM and P3HT:bisPCBM 3.9 nm 4.6 nm spin coated 10.7 nm 12.4 nm slow dried 10x10mm

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