Generation and Recombination in Organic Solar Cells

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Presentation transcript:

Generation and Recombination in Organic Solar Cells Lior Tzabari, Dan Mendels, Nir Tessler Nanoelectronic center, EE Dept., Technion

Outline Macroscopic View of recombination P3HT:PCBM - Exciton Annihilation as the bimolecular loss Generalized Einstein Relation (one page)

What about recombination in P3HT-PCBM Devices Let’s take a macroscopic look and decide on the relevant processes. What experimental technique would be best? Picture taken from: http://blog.disorderedmatter.eu/2008/06/05/picture-story-how-do-organic-solar-cells-function/ (Carsten Deibel)

Mobility Distribution Function or Spatially Dispersive Transport Different time-scales Different Populations (PV is a CW device ) N. Rappaport et. al., APL, 88, 252117, 2006 N. Rappaport et. al., JAP, 99, 064507, 2006 N. Rappaport, et. al., Phys. Rev. B 76 (23), 235323 (2007). L. S. C. Pingree, et.al., Nano Lett. 9 (8), 2946-2952 (2009).

Exciton recombination (Intra, Inter, “pairs”,…) QE as a function of excitation power (If Undoped) Only Loss Mechanism Is Exciton recombination (Intra, Inter, “pairs”,…) Free-Charge Generation Efficiency Other Losses Kick in HOMO Glass ITO PEDOT:PSS Ca Al Cell Efficiency Generating Power (mWcm-2) N. Tessler and N. Rappaport, JAP, vol. 96, pp. 1083-1087, 2004. N. Rappaport, et. al., JAP, vol. 98, p. 033714, 2005.

QE as a function of excitation power Langevin /Bimolecular loss Charge generation rate Photo-current Bimolecular recombination-current No re-injection Smaller Bimolecular Coefficient Signature of bi-molecular Loss N. Tessler and N. Rappaport, Journal of Applied Physics, vol. 96, pp. 1083-1087, 2004. N. Rappaport, et. al., Journal of Applied Physics, vol. 98, p. 033714, 2005.

QE as a function of excitation power SRH (trap assisted recombination) loss LUMO Bimolecular dEt Mid gap HOMO Monomol Doped  Traps already filled Intrinsic (traps are empty) Nt – Density of traps. dEt - Trap depth with respect to the mid-gap level. Cn- Capture coefficient L. Tzabari, and N. Tessler, Journal of Applied Physics 109, 064501 (2011)

QE as a function of excitation power SRH (trap assisted recombination) loss Fewer Traps LUMO Traps Mid gap Deeper Traps HOMO L. Tzabari, and N. Tessler, Journal of Applied Physics 109, 064501 (2011)

What can we learn using simple measurements (intensity dependence of the cell efficiency) Bi- Molecular SRH (trap assisted) L. Tzabari, and N. Tessler, Journal of Applied Physics 109, 064501 (2011)

Recombination in P3HT-PCBM 10 -2 2 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 4 min Anneal , - Experiment , - Model Intensity [mW/cm2] Normalized QE 4min 1.5e-12 Kb[cm3/sec] Kb – Langevin bimolecular recombination coefficient In practice detach it from its physical origin and use it as an independent fitting parameter צריך קבוע יותר גדול – הגיוני כי הניידות גדלה. אבל!!! מצד שני שינוי הניידות בלבד יביא לתוצאות הפוכות מאחר והגדלת הניידות מגדילה גם את יכולת האיסוף – וברגע שיכולת האיסוף גדלה כמות נושאי המטען האפקטיבית בהתקן קטנה ומנגנוני האיבוד נכנסים מאוחר יותר. => יכול להיות שינוי תלוי מורפולוגיה בלבד. 190nm of P3HT(Reike):PCBM (Nano-C)(1:1 ratio, 20mg/ml) in DCB PCE ~ 2%

Recombination in P3HT-PCBM 4 min 10 min 10min 4min 8e-12 1.5e-12 Kb[cm3/sec] צריך קבוע יותר גדול – הגיוני כי הניידות גדלה. אבל!!! מצד שני שינוי הניידות בלבד יביא לתוצאות הפוכות מאחר והגדלת הניידות מגדילה גם את יכולת האיסוף – וברגע שיכולת האיסוף גדלה כמות נושאי המטען האפקטיבית בהתקן קטנה ומנגנוני האיבוד נכנסים מאוחר יותר. => יכול להיות שינוי תלוי מורפולוגיה בלבד. , - Experiment , - Model

Shockley-Read-Hall Recombination , - Experiment , - Model 4 min 10 min LUMO dEt Mid gap HOMO Intrinsic (traps are empty) I. Ravia and N. Tessler, JAPh, vol. 111, pp. 104510-7, 2012. (P doping < 1012cm-3) L. Tzabari and N. Tessler, "JAP, vol. 109, p. 064501, 2011.

The dynamics of recombination at the interface is both Shockley-Read-Hall + Langevin The dynamics of recombination at the interface is both SRH and Langevin 10min 4min 1.2e17 1.9e17 Nt [1/cm3] 0.371 0.435 dEt [eV] 0.5e-12 Kb[cm3/sec] 4 min 10 min LUMO , - Experiment , - Model Mid gap dEt HOMO

Exciton Polaron Recombination M. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals., 1982. Neutrally excited molecule (exciton) may transfer its energy to a charged molecule (electron, hole, ion). As in any energy transfer it requires overlap between the exciton emission spectrum and the “ion” absorption spectrum.

Quenching of Excitons by Holes in P3HT Films A. J. Ferguson, N. Kopidakis, S. E. Shaheen and G. Rumbles, J Phys Chem C 112 (26), 9865, 2008 In neat P3HT ramping the excitation power results in exciton-exciton annihilation Generated Charge Density (at t=0) Add 1% PCBM and losses become dominated by Exciton-Polaron recombination. Kep=3x10-8 cm3/s Excitation Density

Exciton Polaron Recombination 4 minutes 10 minutes , - Experiment , - Model Sensitivity 10min 4min 1.05e17 1.9e17 Nt [1/cm^3] 0.015 0.365 0.435 dEt [eV] 1.08e-8 1.6e-8 Kep[cm^3/sec] Nt – Density of traps. dEt - trap depth with respect to the mid-gap level. Kep – Exciton polaron recombination rate. Kd– dissociation rate 1e9-1e10 [1/sec] A. J. Ferguson, et. al., J Phys Chem C, vol. 112, pp. 9865-9871, 2008 (Kep=3e-8) J. M. Hodgkiss, et. al., Advanced Functional Materials, vol. 22, p. 1567, 2012. (Kep=1e-8)

Traps or CT states are stabilized during annealing 4 minutes 10 minutes Sensitivity 10min 4min 1.05e17 1.9e17 Nt [1/cm^3] 0.015 0.365 0.435 dEt [eV] 1.08e-8 1.6e-8 Kep[cm^3/sec] T. A. Clarke, M. Ballantyne, J. Nelson, D. D. C. Bradley, and J. R. Durrant, "Free Energy Control of Charge Photogeneration in Polythiophene/Fullerene Solar Cells: The Influence of Thermal Annealing on P3HT/PCBM Blends," Advanced Functional Materials, vol. 18, pp. 4029-4035, 2008. (~50meV stabilization)

Bias Dependence 10 minutes anneal - Generation - Recombination Charge Generation - Generation - Recombination - Mobility

What does it all mean (summary, conclusions,…) The “geminate” recombination occurs through “defect sites” and their availability limits the recombination. “Defect sites” or “Traps” act like stabilized charge transfer states. At high enough density (depending on morphology) a new channel opens up and Losses become Bi-molecular. Bi-molecular = electron-hole or exciton-polaron? Charge generation requires some field and this is observed at very low light intensities

Degenerate (gas) Pressure  Pressure = Enhanced Diff. Disordered hopping systems degenerate semiconductors Y. Roichman and N. Tessler, APL, vol. 80, pp. 1948-1950, Mar 18 2002. To describe the charge density/population one should use Fermi-Dirac statistics and not Boltzmann Degenerate Degenerate (gas) Degenerate (gas) Pressure  Pressure = Enhanced Diff. White Dwarf It’s effect is in basic thermodynamics texts. Astronomy: Degenerate gas pressure. Fluidics: Osmosis Drift Diffusion Diffusion Seebeck Streaming Streaming Enhanced Diffusion In Semiconductors: D. Mendels and N. Tessler, J. Phys. Chem. C 117 (7), 3287-3293 (2013).

Thank You Ministry of Science, Tashtiyot program Helmsley project on Alternative Energy of the Technion, Israel Institute of Technology, and the Weizmann Institute of Science Israeli Nanothecnology Focal Technology Area on "Nanophotonics for Detection"

Charge recombination is activated

Why Generalized Einstein Relation does not affect the Ideality Factor of PN Diode Long Diode [N. Tessler and Y. Roichman, Org. Electron. 6 (5-6), 200-210 (2005)] In Amorphous semiconductors: Exponential DOS Short Diode [Y. Vaynzof, Y. Preezant and N. Tessler, Journal of Applied Physics 106 (8), 6 (2009)]