1. LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

2. LBNL 9/15/06 Giant Single Molecule Cell (bulk collection)  record =24. 7 % 1-D “nano” cell (depleted collection)  record = 13% (triple - UniSolar) 3-D “nano” cell (dye or QM sensitized)  record = 12.6% (Graetzel, wet) From Macro to Nano

3. LBNL 9/15/06 THE DEVICE VIEW  J sc V oc FF J sc V oc

4. LBNL 9/15/06 Short Circuit Current J sc

5. LBNL 9/15/06 BAND GAP Black body: Thermal emission from cell via above bandgap photons.

6. LBNL 9/15/06 J sc AM 1.5 Theoretical Upper Limit versus bandgap.

7. LBNL 9/15/06 J sc  1. Incident energy flux reduction Reflection (Si ~ 10%) Contact shading Insufficient absorption ( d <  )

8. LBNL 9/15/06 J sc  2. Failure to generate electron hole pairs Sub bandgap photons (excluding two photon processes*) Bulk free carrier absorption, “Auger” Free carrier absorption due to  V across cell Frank Keldysh effect * investigated in Si for two photon processes

9. LBNL 9/15/06 3. Failure of e-h to contribute to current Bulk recombination (impurities, structural defects, ) Surface recombination Non-contacted surface Contacted surface J sc 

10. LBNL 9/15/06 J sc  4. Remedies Bulk Losses: Clean Starting Materials Gettering Thin bulk ! => Light trapping design ! Collecting from fully depleted layers with uniform (!) electric fields. (No “hang through” !)

11. LBNL 9/15/06 Surface Recombination a. Non contacted area Small Area ! (Implication for “nano” !) Factors influencing S: Capture cross section of surface states (in Si >>for electrons than holes) Hole and electron concentration at surface (V output dependent as c minority increases with V out) Charge of passivating layer J sc 

12. LBNL 9/15/06 J sc  Contact Areas: Small area ! (Nano, again ) Heavy doping => “ Back surface field” (Graetzel Patents on TiO 2 )

13. LBNL 9/15/06 Open circuit voltage V oc

14. LBNL 9/15/06 V oc Small n i, Long lifetimes (Diffusion distance) 1. “ Junction Leakage “

15. LBNL 9/15/06 Green, semi-empirical Kiss & Rehwald (thermodynamic) V oc Good a:Si-H (Roca, Meillaud et al.)

16. LBNL 9/15/06 Additional contribution(s) to J o 1. Generation current due to bulk “midgap” states 2. Generation current to due surface states. V oc n, diode ideality factor, function of operating conditions. Sum over exponentials commonly expressed as

17. LBNL 9/15/06 Efficiency tends to improve with illumination level.

18. LBNL 9/15/06 2. Junction Defects (shorts, partial shorts, more general: Spatial inhomogeneity) 1. Scale with junction area (nano !) 2. Difficult to diagnose 3. The bane of the multicrystalline cell

19. LBNL 9/15/06 FF Empirical relation (Green) Additional Effects Series Resistance : Contacts & Leads Parallel Resistance : Shunts !

20. LBNL 9/15/06 Green:

21. LBNL 9/15/06 Module Efficiency CdTe modules are much less efficient than CdTe cells ! Cells must be uniform ! Cost Not just materials but processing (CIS) Stability a-Si:H, DSC..

22. LBNL 9/15/06 PERL cell  = 24 % PERL => Passivated Emitter, Rear contact Locally diffused GIANT SINGLE MOLECULE DEVICE

23. LBNL 9/15/06 200 nm 100 nm M. Schmidt, A. Schoepke, O. Milch, Th. Lussky, W. Fuhs A-Si:H 1-D “NANO” N(E)  as d 

24. LBNL 9/15/06 A(luminum) Induced C(rystallization) I(on) A(ssisted) D(eposition) poly-Si cell (M.Green) 9…12% (Pinnacle of optical engineering) Si reappears via M. Green

25. LBNL 9/15/06 QM dots 3-D NANO

26. LBNL 9/15/06 NANO CELLS: Insufficient absorption => Multiple stacking large surface area => junction leakage increased lead length => resistance, recombination Contact shading => transparent electrodes e.g.TiO 2 surface recombination, parasitic resistance,  E drops !  n mismatch => multiple bounce

27. LBNL 9/15/06 Need to match 2 semiconductors + 1 Redox couple

28. LBNL 9/15/06 QD dots sensitized Higher absorption than dyes. More corrosion resistant. Layer 3 is the insulator (TiO 2 ) between organic p conductor and layer 2, the F doped SnO contact layer. Potential large area problem.

29. LBNL 9/15/06 J sc  Absorption tuning by PbS QD growth

30. LBNL 9/15/06 J sc  J sc  at QD size below optimal optimum. Monochromatic Light  max ~ 10% Trade off between coverage (1x), QM size tuning, and transport loss.

31. LBNL 9/15/06 Summary Challenges: Transition from single giant molecule cell to nano cell 1. Large increase in junction area.. junction defects. 2. Surface states 2. Transparent conductors…. bandgap matching. 3. Large contact area … shunts Rewards : 1. Bandgap tuning 2. Easier implementation of “sub bandgap” and “hot carrier”... ;.. Conversion. 3. Large area for “in situ” chemical conversion !

32. LBNL 9/15/06 Appendix

33. LBNL 9/15/06...

34. LBNL 9/15/06