FINAL Understanding the Role of Charge Mobility and Recombination
1. Understanding the Role of Charge
Mobility and Recombination in Organic
Photovoltaics
David Lam1
Advised by Professor Michael D. McGehee2
1. Department of Physics, Stanford University
2. Department of MaterialsScience and Engineering, Stanford University
May 18, 2015
2. EnergyConsumption[1015
BTU]
Year
Why Solar?
1
Energy consumption increase
by 56% in 30 years1
Ex: In 2003, 13.1 x 1015 BTU
used in U.S.2
With 15% efficiency, only
need 100 km x 100 km
1. International Energy Outlook, U.S. EIA, 2013
2. Shaheen et al., MRS Bulletin, 2005
3. Why Organic Photovoltaics?
2 Image from popupcity.net, Solar Cells for Cheap, Cheap Soon
Solution processed
Low temperatures
Inexpensive substrates
Large-scale, wet-processing
production
Blade-coating
Roll-to-roll printing
15. H1 Experimental vs Simulated Data
14
Fill Factor Efficiency
Average: 𝒌 = 𝟔. 𝟓 × 𝟏𝟎−𝟏𝟎 𝒄𝒎 𝟐
𝑽∗𝒔
Range: 𝒌 = 𝟏. 𝟖 to 1𝟎 × 𝟏𝟎−𝟏𝟎 𝒄𝒎 𝟐
𝑽∗𝒔
16. Conclusion
15
Fill factor of P3HT:PCBM devices sensitive to
thickness and charge mobility
Bimolecular recombination can be decreased by:
Decreasing k
Increasing charge mobility
Simulation captures device physics
Rising energy usage
Solar good avenue
Example
OECD: Organization for Economic Cooperation and Development
Inorganic PVs bad because of costly purification
OPVs good because solution processed
Inexpensive, large scale production
Photon absorption: polymer absorbs a photon, which excites an electron.
Creates ehp, which is tightly bound due to low dielectric constant in OSC
Diffusion
Diffuses to heterojunction
CT
Energy difference splits the Exciton
Charge Drift
Difference in wf of contacts leads to built in electric field
Sweeps out charge carriers to contacts for collection
Talk about recombination
EQE Related to Jsc, affected by absorption and extraction. Higher charge mobility should mean better extraction. Thicker samples should mean more absorption.
FF affects the max power, higher FF means higher P. FF affected by recombination
Goals to hit PCE of 15%: 90% EQE (high!)
Increase thickness
Optimized at 70-100 nm, but if we increase to 300 nm (thick active layer), absorbs all light
0.8 FF (high!)
FF decreases when we make devices thicker! Actually hurts performance
Why?
Note: Pick 90% eqe because ITO para absorb, 0.8 FF because highest recorded
How can we improve performance?
What causes poor FF? We think it’s recombination and charge mobility.
Can we simulate this?
P3HT conjugated polymer, light absorber, electron donor
PCBM fullerene C60 derivative, electron acceptor
Vary charge mobility of P3HT:PCBM by annealing devices
Low temperatures: 25 C uniformly low before 100 nm, then shoots up above 100 nm
Intermediate temp:
71 C FF low initially, then shoots up around 150 nm
111 C FF starts off higher and rises
Both approach same FF at 60 nm
High Temp: Constant at 0.7 FF
To figure out what kind of recombination in our devices
Bimolecular is k -> constant, n and p, electron and hole density
Bimolecular recombination dependent on light intensity squared
We see fill factor go up as a fcn of light intensity
Combine bimolecular recombination expression with current expression
If we maintain same J (good approx. since Jsc is same order of magnitude):
Increase charge mobility
N, p must decrease to balance out the products. R decreases
Decrease thickness means increase in E field
Again, n p must decrease to balance out the sum. R decreases.
Inorganic PVs have many simulators that augment research
Allows them to determine performance from measured quantities
OPVs don’t have as many simulators because of complicated heterojunctions
-> Effective medium?
Measure then obtain
Focus on k
Compare trends
Captures device physics!
Compare FF trends.
Also agrees!
One important thing to note is that simulated predicts 0.8 FF for intermediate and high temp: pinholes shunt resistance hurt experimental performance
Compare simulated and literature k
Double check by simulating another system:
H1: Small molecule
PCBM fullerene
Vary weight fraction of H1 to change charge mobility. Experimental data shown on right. Max FF of 0.7
Comparing FF and efficiency: for the most part, they agree.
Compare average k and range of k. They fall within same order of magnitude.
Acknowledge VPUE Major Grant
Alex:
Have the whole phrase on slide 4 and 9
Talk slower
Black on Brown
No white covering graphs
Don’t say you’re done at the end
![EnergyConsumption[1015
BTU]
Year
Why Solar?
1
Energy consumption increase
by 56% in 30 years1
Ex: In 2003, 13.1 x 1015 BTU
used in U.S.2
With 15% efficiency, only
need 100 km x 100 km
1. International Energy Outlook, U.S. EIA, 2013
2. Shaheen et al., MRS Bulletin, 2005](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)