Transcript dT/T - IPS Meeting 2015

Institute of Materials Research and Engineering
Investigating Polaron Generation in Organic Solar Cells Using
Photoinduced Absorption Spectroscopy
Wei Peng Goh1, Zi En Ooi1, Evan L. Williams1, Joel K.W. Yang1, Wee Shing Koh2, Subodh Mhaisalkar3
1Institute
2Institute
3Energy
of Materials Research & Engineering, 3 Research Link, Singapore 117602
of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis North, Singapore 138632
Research Institute @ Nanyang Technological University, Research Techno Plaza, Level 5, 50 Nanyang Drive, Singapore 637553
Photoinduced Absorption (PIA) Spectroscopy is used to investigate the photogeneration of charge carriers in an
organic photovoltaic (OPV) cell. In this study, poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester
(P3HT:PCBM) is used as the active layer. Relative strength of polaron signals arising at ~1.25 eV in the PIA spectrum
can be interpreted as a measure of charge generation efficiency. Through the use of PIA spectroscopy, we aim to
understand the effects on photocurrent generation when incorporating plasmonic nanostructures in a P3HT:PCBM film.
Aim:
To investigate polaron generation in poly(3hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester
(P3HT:PCBM) using Photoinduced Absorption (PIA)
spectroscopy.
Preliminary Results:
Introduction:
•PIA is a pump-probe technique
•Excited species are created by the pump.
•Photoexcited states and free charge carriers exhibit additional
absorption bands1, which are not present or observed in the
ground state absorption spectrum. These states are probed
after excitation by a pump source. This will cause a change in
the optical transmittance of the sample.
•Photoexcited samples of P3HT:PCBM are known to exhibit a
characteristic absorption around 1.25 eV which is attributed to
polaron species2.
•Relative strength of the peak is a measure of charge carrier
density
PIA spectra of P3HT and P3HT:PCBM blend. Data not corrected for photoluminescence.
•Film thickness ~200 nm for P3HT and P3HT:PCBM films.
•dT/T, which is the change in transmittance, is plotted as a
function of energy.
•P3HT has a generally flat signal. P3HT film does not allow for
polaron generation; photoexcited states remain as excitons
Experimental setup:
•P3HT:PCBM blend has a characteristic dip at ~1.25 eV. PCBM
functions as an electron acceptor, allowing for exciton
dissociation and polaron production upon photoexcitation.
•Due to extra absorption by polarons, dip corresponds to an
increase in opacity of film.
Conclusions & Future Work:
•Successful demonstration of a PIA setup
•~1.25 eV dip is the signal which is characteristic of polaron
generation.
Schematics of a PIA setup
•Quartz Tungsten Halogen functions as the probe; 470 nm
pulsed LED as the pump
•InGaAs detector measures T and dT in the 720 – 1220 nm
range; dT is the change in transmittance while T is the reference
transmittance
Lock-in signal = (Probe intensity) x Detector responsivity x dT
dT
=
T
Source meter signal = (Probe intensity) x Detector responsivity
x (T + dT)
•Aim to understand the effects on photocurrent generation
when incorporating plasmonic nanostructures in a P3HT:PCBM
film
Acknowledgement:
This research work is supported by funding from the Agency for Science, Technology and Research
(A*STAR)
References:
1) I. Hwang et al., J. Phys. Chem. C 2008, 112, 4350-4354
2) Abhishek P. Kulkarni et al., Nano Lett. 2010, 10, 1501–1505
where T + dT ≈ T
3 Research Link, Singapore 117602
Email: [email protected] Website: www.imre.a-star.edu.sg
Tel: (65) 6874 8111 Fax: (65) 6872 0785
Feb 2013