Coupled optoelectronic simulation of organic bulk- heterojunction solar cells: Parameter extraction and sensitivity ana

1. Coupled optoelectronic simulation of organic bulk-heterojunction solarcells: Parameter extraction and sensitivity analysis Speaker: Yu-Chih Cheng Advisor:Peichen Yu R. Häusermann,1,a E. Knapp,1 M. Moos,1 N. A. Reinke,1 T. Flatz,2 and B. Ruhstaller1,2,b 1Institute of Computational Physics, Zurich University of Applied Sciences, Wildbachstrasse 21, 8401 Winterthur, Switzerland 2Fluxim AG, Dorfstrasse 7, 8835 Feusisberg, Switzerland

2. Outline • INTRODUCTION • DESCRIPTION OF THE NUMERICAL DEVICE MODEL • ESTIMATION OF THE DISSOCIATION RATE • SENSITIVITY ANALYSIS • CONCLUSION

3. Outline • INTRODUCTION • DESCRIPTION OF THE NUMERICAL DEVICE MODEL • ESTIMATION OF THE DISSOCIATION RATE • SENSITIVITY ANALYSIS • CONCLUSION

4. INTRODUCTION • Organic Photovoltaic advantages • Planar heterojunction devices and bulk-heterojunctionBHJ devices • The incouplingof light into a multilayer structure • The extraction of charges needs electrical model

5. D A

6. Outline • INTRODUCTION • DESCRIPTION OF THE NUMERICAL DEVICE MODEL • ESTIMATION OF THE DISSOCIATION RATE • SENSITIVITY ANALYSIS • CONCLUSION

7. A :Optical modeling AM 1.5 spectrum is used for I0

8. Absorbance k stands for the complex part of the refractive index

9. B. Electrical modeling • Charge-transfer excitongeneration and dissociation • Charge drift and diffusion • Charge extraction at the electrodes Three things need to be considered

10. Excitons (bound electron-hole pairs) Wannier exciton (typical of inorganic semiconductors) Frenkel exciton (typical of organic materials) treat excitons as chargeless particles capable of diffusion, also view them as excited states of the molecule MOLECULAR PICTURE SEMICONDUCTOR PICTURE Charge Transfer (CT) Exciton (typical of organic materials) GROUND STATE FRENKEL EXCITON GROUND STATE WANNIER EXCITON binding energy ~10meV radius ~100Å binding energy ~1eV radius ~10Å Electronic Processes in Organic Crystals and Polymers by M. Pope and C.E. Swenberg

11. 1. Charge-transfer-exciton dissociation • S(x):CT-excitondensity • - recombination term of free charge carrier pairs generates a CT exciton. • - absorption profile • - photon-to-CT-exciton conversion efficiency • - decay of a CT state • - dissociation of a CT state

12. Processes for CT-exciton modeling

13. Dissociation probability Pby Onsager–Braun theory • Key point: and the pair binding energy iscalculated under the assumption that CT excitons have the same dependence of the binding energy on the separation distance as ion pairs.

14. 2. Drift-diffusion modeling ( : 1D Poisson’ eq ) current equation for electrons Einstein relation r(x)stands for the Langevin recombination

15. 3. Built-in voltage • debate on the nature of the open-circuit voltage Voc: • E • the energy of the charge transfer absorption • work function of the electrodes • Light intensity • Temperature D A LUMO HOMO

16. 4. Charge extraction • This model considers the barrier reduction at an organic-metal interface due to the electric field and the image charge potential and calculates the net injection current.

17. 5.Validation of the simulator • Voc increased slightly with and also depends on the mobility, not equal to the Vbi • Fill factor influence by recombination , mobility and . • Jscdependslinearly on until recombination losses take over. • These results correspond to experimental observations.

18. Outline • INTRODUCTION • DESCRIPTION OF THE NUMERICAL DEVICE MODEL • ESTIMATION OF THE DISSOCIATION RATE • SENSITIVITY ANALYSIS • CONCLUSION

19. Parameters extraction • The two mobility measured the constant mobilitiesof electrons and holes in a P3HT:PCBM BHJ solar cell depending on the annealing temperature.

20. Estimation of unknown parameters • decay rate • the pair separation distance a • the photon-to-CT-excitonconversion efficiency

21. Simplify model • Assumes that absorbed photons directly generate free e-hole pairs . • Recombining charges are lost and not fed into the continuity eq. Reduced model:

22. varied between 1 and 0.01to check the influence of electron • geff=0.66 is consistent with the analysis • Fig suggests that recombination efficiency reff in the simplified model is 10% or lower. • (simple model) corresponds to (full model), P must be 90% or higher. The dissociation probability P

23. acan be determined under the assumption that is set to • P must be 90% or higher • The best fit ahas been chosen to be 1.285 nm • Dissociation probability according to the Onsager–Braun theory depending on the electrical field for several initial pair separation distances a

24. The best fit ahas been chosen to be 1.285 nm by comparing experimental current-voltage curves with simulated curves for an active layer thickness of 70 nm

26. Outline • INTRODUCTION • DESCRIPTION OF THE NUMERICAL DEVICE MODEL • ESTIMATION OF THE DISSOCIATION RATE • SENSITIVITY ANALYSIS • CONCLUSION

27. A. Thickness dependent sensitivity

28. B. Current-voltage curve sensitivity

29. Outline • INTRODUCTION • DESCRIPTION OF THE NUMERICAL DEVICE MODEL • ESTIMATION OF THE DISSOCIATION RATE • SENSITIVITY ANALYSIS • CONCLUSION

30. CONCLUSION • photon to CT-excitonconversion efficiency geff= 66%. • lower limit for the CT-excitondissociation efficiency of 90% • Adding the measured current-voltage curve to the numerical analysis and assuming that is set to • the influence of the two exciton parameters and the electron mobility are linearly dependent in the current-voltage curve and photocurrent thickness scaling