Coupled optoelectronic simulation of organic bulk-heterojunction solar cells: Parameter extraction and sensitivity analysis 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 Speaker: Yu-Chih Cheng Advisor: Peichen Yu

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

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

INTRODUCTION Organic Photovoltaic advantages Planar heterojunction devices and bulk-heterojunction BHJ devices The incoupling of light into a multilayer structure The extraction of charges needs electrical model

D A

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

A :Optical modeling AM 1.5 spectrum is used for I0

Absorbance k stands for the complex part of the refractive index

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

Wannier exciton (typical of inorganic semiconductors) Frenkel exciton (typical of organic materials) Excitons (bound electron-hole pairs) SEMICONDUCTOR PICTURE MOLECULAR PICTURE treat excitons as chargeless particles capable of diffusion, also view them as excited states of the molecule GROUND STATE WANNIER EXCITON GROUND STATE FRENKEL 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 Charge Transfer (CT) Exciton (typical of organic materials)

1. Charge-transfer-exciton dissociation

Processes for CT-exciton modeling

Dissociation probability P by Onsager–Braun theory

2. Drift-diffusion modeling r(x) stands for the Langevin recombination

3. Built-in voltage LUMO HOMO D A

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.

5.Validation of the simulator

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

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

Estimation of unknown parameters

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:

The dissociation probability P

Dissociation probability according to the Onsager–Braun theory depending on the electrical field for several initial pair separation distances a

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

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

A. Thickness dependent sensitivity

B. Current-voltage curve sensitivity

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