1. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.1 The molecular structure of polyethylene. Each carbon has four nearest neighbours and forms four bonds. Polyethylene is an insulator and has a wide energy gap in the ultraviolet energy range. Each carbon atom has an almost perfect tetrahedral bond symmetry even though it bonds to both carbon and hydrogen nearest neighbours

2. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.2 Polyacetylene is the simplest conjugated molecule. It is often thought of as a chain of single bonds alternating with double bonds although actually all the bonds are equal and are neither purely single nor purely double. It consists of a carbon chain with one hydrogen atom per carbon atom. Since only three of the four valence electrons of carbon are used for bonding, one π electron per carbon atom is available for electrical conduction and becomes delocalized along the carbon chain

3. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.3 Energy levels and bands in a few closely spaced organic molecules. Note the small energy barriers caused by the intramolecular bonding and the larger energy barriers caused by the intermolecular bonding

4. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.4 Molecular structures of well-known conjugated polymers. Many molecules contain a combination of linear and ring-type structures, the simplest example being poly paraphenylene vinylene (PPV). Reprinted from Li, Z., Meng, H., Organic Light-Emitting Materials and Devices, 157444-574X. Copyright (2006) with permission from Taylor & Francis

5. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.5 Poly para-phenylene vinylene (PPV) derivative forming a silicon-substituted soluble polymer

6. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.6 Absorption and emission of poly para-phenylene vinylene (PPV) and PPV derivatives. The energy gap determines the upper wavelength range of absorption as well as the lower wavelength range of emission. Here energy gaps from 1.9 eV ( ≅ 640 nm) to 2.5 eV ( ≅ 500 nm) result in these spectra. Reprinted from Li, Z., Meng, H., Organic Light-Emitting Materials and Devices, 157444-574X. Copyright (2006) with permission from Taylor & Francis

7. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.7 Structure of basic polymer OLED consisting of a glass substrate, a transparent ITO anode layer, an EL polymer layer and a low workfunction cathode layer

8. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.8 Energy diagram showing a high workfunction anode and a low workfunction cathode. With a constant vacuum energy the Fermi levels cannot be aligned (upper diagram) and this diagram is therefore not an equilibrium diagram, which invalidates the concept of Fermi energy. To resolve this problem an electric field forms between anode and cathode and an equilibrium diagram (lower diagram) having aligned Fermi energies is the result. This is the result of charge transfer

9. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.9 The upper π ∗ band and lower π band in a polymer EL layer. (a) The equilibrium condition. (b) The flat-band condition in which a positive voltage is applied to the anode. (c) Device in forward bias in which holes and electrons are injected and form molecular excitons, which annihilate to generate photons. The resulting hole injection barrier and electron injection barrier are shown

10. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.10 Typical luminance–voltage (L–V) and I–V characteristics of a polymer OLED. A well- defined threshold voltage is observed due to the sharp onset of carrier injection from the electrodes across the potential barriers at the electrode-EL polymer interfaces. Note the similarity between the shapes of the current and luminance curves

11. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.11 Small organic molecules used for small-molecule OLED devices. Holetransporting materials are TPD and NPD. Electron-transporting materials are PBD and Alq 3. Reprinted from Li, Z., Meng, H., Organic Light-Emitting Materials and Devices, 157444-574X. Copyright (2006) with permission from Taylor & Francis

12. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.12 Small-molecule OLED structure. The OLED includes a transparent substrate, transparent ITO anode, hole transport layer (HTL), electron transport layer (HTL) and cathode. HTL materials such as TPD or NPD and electron transport materials such as Alq 3 or PBD are suitable. A popular cathode is a two-layer Al/LiF structure as shown

13. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.13 A more optimized small-molecule OLED structure includes an electron injection layer, a hole injection layer and a light emitting material. The cathode includes the electron injection layer

14. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.14 Band diagram of a small-molecule OLED showing LUMO and HOMO levels for the various layers of the device. The band diagram is drawn without a bias applied. The accepted workfunctions of anode (ITO) and cathode (LiF/Al,) which are 4.7 eV and 3.6 eV respectively, are shown

15. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.15 OLED package includes front and back sheets, epoxy seal material on all edges, sacrificial dessicant or getter material, cathode and transparent anode having cathode contact areas for external connections. The rate of moisture penetration must be calculated to ensure a specified product life

16. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.16 Copper phthalocynanine, or CuPc, a widely used metal complex used for the HIL in small molecule OLEDs. Another HIL molecule is m-MTDATA, also shown. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

17. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.17 Organo-metallic complexes may also be used for the electron injection layer. Examples are shown consisting of some lithium-quinolate complexes. Liq, LiMeq, Liph and LiOXD. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

18. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.18 Two further examples of hole-conducting triarylamenes include TPA (triphenylamine) and TPTE (a tetramer of TPA). TPTE enables high-temperature OLED operation without crystallization. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

19. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.19 Phenylazomethines are formed by various arrangements of nitrogen-terminated six-carbon rings. These phenylazomethine molecules are thermally stable and are complexed with metal ions such as Sn ions introduced in the form of SnCl 2 molecules to form the HTL material. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and rancis

20. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.20 TPBI, ATZL and TPQ are members of imine-based molecules which are candidate electron transport layer (ETL) materials as well as light emission materials. Other candidate ETL materials include C 60. See Section 6.16. Chemical Structure reproduced from Organic lightemitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

21. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.21 Host-guest energy transfer. The energy transfer can occur due to three possible processes, which may be Fo¨ rster, Dexter or radiative energy transfer. Energy transfer within either host or guest molecule through intersystem crossing from singlet to triplet excited states can occur by spin-orbit interaction. Reprinted from Li, Z., Meng, H., Organic Light-Emitting Materials and Devices, 157444-574X. Copyright (2006) with permission from Taylor & Francis

22. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.22 Electron transport hosts Alq 3, BAlq, TPBI and TAZ1. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

23. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.23 Hole transport hosts CBP and CDBP. Chemical Structure reproduced from Organic light- emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

24. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.24 Coumarin-based green fluorescent dopant C-545TB and quinacridone-based dopant DMQA. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

25. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.25 The red fluorescent molecule DCJPP derived from the arylidene family of molecules and four variations of red fluorescent molecule DCDDC derived from the isophorone family of molecules. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

26. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.26 Distytylaraline family host DPVBI and dopant BCzVBI. Also shown are anthracene family host JBEM and dopant perylene. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

27. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.27 Phosphorescent iridium-based emitters: Red- Ir(thpy) 3, Green-Ir(ppy) 3, Blue-Ir(ppz) 3. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis

28. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.28 Single-layer organic solar cell consisting of a single organic semiconductor layer, a low workfunction cathode and a transparent anode. Device efficiency is well below 1%

29. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.29 Energy level diagram for single-layer organic solar cell. The absorption of light creates excitons through the promotion of molecular electrons from the HOMO level to the LUMOlevel. Electrode workfunctions are set to match theHOMOand LUMOlevels to facilitate the collection of the electrons and holes as shown

30. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.30 Organic planar heterojunction solar cell structure showing donor and acceptor organic layers

31. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.31 Heterojunction solar cell showing donor and acceptor LUMO and HOMO levels. Excitons are generated throughout the donor layer and these excitons are dissociated when they diffuse to the donor-acceptor interface. Finally the separated holes and electrons can drift to their respective electrodes

32. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.32 Bulk heterojunction organic solar cell. A number of small ( ∼ 10 nm) donor regions are organized within the bulk heterojunction layer and optimized to absorb sunlight and allow exciton diffusion to a nearby junction

33. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.33 (a) Bulk heterojunction structure showing a typical random structure of donor and acceptor materials. The dimension of one region within the heterojunction is about 10 nm. The problem is the connectivity of these regions to their appropriate contact materials. (b) Bulk heterojunction of vertically oriented stripes of donor and acceptor materials that enables the donor material to be in contact with the ITO electrode and the acceptor layer to be in contact with the aluminium electrode. The acceptor layer could be made using vertically oriented carbon nanotubes

34. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.33 (cont.)

35. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.34 Molecular structures of poly(3-hexylthiophene) (or P3HT) and PQT-12. Reprinted with permission from Organic Electronics, Efficient bulk heterojunction solar cells from regio-regular- poly(3,3  -didodecyl quaterthiophene)/PC70BM blends by P. Vemulamada, G. Hao, T. Kietzke and A. Sellinger, 9, 5, 661-666 Copyright (2008) Elsevier

36. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.35 Absorption spectra of P3HT and PQT-12. Reprinted with permission from Organic Electronics, Efficient bulk heterojunction solar cells from regio-regular-poly(3,3  -didodecyl quaterthiophene)/PC70BM blends by P. Vemulamada, G. Hao, T. Kietzke and A. Sellinger, 9, 5, 661-666 Copyright (2008) Elsevier

37. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.36 Molecular structures of the fullerene C 60 and its derivative [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). Reproduced from James Hugh Gervase Owen, Nanotubes and fullerenes for Quantum Computing, http://homepage.mac.com/jhgowen/research/nanotube_page/nanotubes.html. Copyright (2011) with permission from J. H. G. Owen

38. Principles of Solar Cells, LEDs and Diodes: The role of the PN junction, First Edition. Adrian Kitai. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd. Figure 6.37 Carbon nanotube. Courtesy of Dr. J.H.G Owen and the Oxford University QIPIRC