1. C ARBON N ANOTUBE B ASED O RGANIC S OLAR C ELLS Arun Tej M. PhD Student EE Dept. and SCDT

2. Carbon Nanotubes Properties Useful for Solar Cells Efficiency Limiting Factors Nanotubes in Organic Solar Cells Results and Future Challenges 2 Arun Tej M, REACH - 2008 Outline

3. S. Iijima - MWNT (1990), SWNT (1993) Rolled graphene sheet with end caps Large aspect ratios Unique properties Finds applications in Conductive plastics and adhesives Energy storage Efficient heat conduits Structural composites Biomedical devices Numerous electronic applications www.applied-nanotech.com 3 Arun Tej M, REACH - 2008 Carbon Nanotubes

4. Arun Tej M, REACH - 2008 4 Nanotube Field Emission Display W.B. Choi, Samsung, APL, 1999

5. Thomas Rueckes, Nantero, 2000 5 Arun Tej M, REACH - 2008 Nanotube Random Access Memory Type of Memory Most Important Feature Applications DRAMHigh DensityComputer Operating Memory SRAM Flash Memory High Speed Non-volatility Cell Phones, Computer Caches PDAs, Cameras MRAMHigh Density High Speed Non-volatility All Uses NRAMHigh Density High Speed Non-volatility All Uses

6. Arun Tej M, REACH - 2008 6 Nanotube Liquid Flow Sensor A.K.Sood, IISc Bangalore, Science, 2003

7. 7 Arun Tej M, REACH - 2008 5 Stage Ring Oscillator on one SWNT Z.Chen, IBM, 2006 Nanotube Integrated Circuit

8. Arun Tej M, REACH - 2008 8 Nanotube Based Inorganic Solar Cell W.J.Ready, Georgia Tech, JOM, 2007

9. High carrier mobilities (~1,20,000 cm 2 V -1 s -1 ) Large surface areas (~1600 m 2 g -1 ) Absorption in the IR range (E g : 0.48 to 1.37 eV) Conductance - Independent of the channel length Enormous current carrying capability – 10 9 A cm -2 Semiconducting CNTs – Ideal solar cells Mechanical strength & Chemical stability 9 Arun Tej M, REACH - 2008 Nanotube Properties Useful for Solar Cells

10. Split-Gate device, Energy band diagram and I-V characteristics 10 Arun Tej M, REACH - 2008

11. Low Carrier Mobilities (~10 -5 cm 2 V -1 s -1 ) Low Exciton Diffusion Lengths (5-15 nm) Large Exciton Binding Energies (up to 1.5 eV) Large Energy Gaps (2-3 eV) Combine the advantages of Organics and SWNTs SWNTs Improve mobility SWNTs provide Large interfacial area SWNTs have Suitable energy levels SWNTs have Low energy gaps 11 Arun Tej M, REACH - 2008 Efficiency Improvement with SWNTs

12. Arun Tej M, REACH - 2008 12 Exciton dissociation sites As electron acceptors in bulk heterojunction solar cells Carrier transport Thin transparent films of m-SWNTs as electrodes Chhowalla et al, APL, 2005 Wu et al, Science, 2004 Nanotubes in Organic Solar Cells

13. Arun Tej M, REACH - 2008 13 Results (1) Photoluminescence Quenching Higher Efficiency Arun Tej M, S.S.K.Iyer, and B.Mazhari, IEEE INEC, 2008, Shanghai

14. Arun Tej M, REACH - 2008 14 Results (2) Trap filling behaviourTunneling behaviour Arun Tej M, S.S.K.Iyer, and B.Mazhari, IEEE PVSC, 2008, San Diego

15. Arun Tej M, REACH - 2008 15 High Open Circuit Voltages with Bulk Heterojunction Devices Results (3) Our Work To be published

16. Synthesis of stable organic compounds Separate semiconducting and metallic SWCNTs Aligned CNTs inside the semiconducting polymers give improved charge transport e-e- e-e- e-e- h+h+ h+h+ 16 Arun Tej M, REACH - 2008 e-e- h+h+ Future REACH (1)

17. Add nanoparticles, quantum dots, fullerenes etc to the side walls of SWNTs 17 Arun Tej M, REACH - 2008 e-e- h+h+ h+h+ e-e- e-e- h+h+ e-e- h+h+ e-e- Future REACH (2)

18. “A Solar Cell with Improved Light Absorption Capacity” S. Sundar Kumar Iyer and Arun Tej M. Patent Appln. No. 933/DEL/2006 Dt: 31 st March, 2006 New device structures 18 Arun Tej M, REACH - 2008 Future REACH (3)

19. Arun Tej M, REACH - 2008 19 Acknowledgements Faculty, Staff and Students, SCDT Prof. Ashutosh Sharma, Chemical Engineering

20. 20 Arun Tej M, REACH - 2008

21. Schematic and energy diagram of a typical polymer solar cell and its operation e-e- h+h+ Anode Cathode Donor Acceptor Exciton formation Exciton diffusion Exciton dissociation Carrier transport Charge collection 21 Arun Tej M, REACH - 2008 Organic Solar Cell

22. 22 Arun Tej M, REACH - 2008

23.  Conjugated polymers  Conduction due to sp 2 – hybridised carbon atoms   and  (p z -p z )bonds   electrons are delocalised in nature giving high electronic polarisability  High absorption in the UV-Visible range of the solar spectrum H.Hoppe and N.S. Sariciftci, 2004 23 Arun Tej M, REACH - 2008

24. M ETALLIC SWNT S 24 Arun Tej M, REACH - 2008

25. Conductance is independent of the channel length. 25 Arun Tej M, REACH - 2008

26. Conductance through a barrier with transmission probability T. Landauer Formula: With N parallel 1D channels (subbands): m-SWNTs: Only two subbands cross E F (N=2) Source of R: Mismatch in the number of conduction channels in the SWNT and the macroscopic metal leads. 26 Arun Tej M, REACH - 2008