Supplementary Information
Hybrid Pentacene/a-Silicon Solar Cells Utilizing Multiple
Carrier Generation via Singlet Exciton Fission
Bruno Ehrler, Kevin P. Musselman, Marcus L. Böhm, Richard H. Friend and Neil C.
Greenham*
Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
Methods
PbSe Nanocrystal Synthesis: PbSe nanocrystals were synthesized according to a
variation of a literature procedure[1]. In a three-neck flask, lead oleate
(Pb(OAc)2H2O, 3.44 mmol; 1.3 g) was degassed in a mixture of 1-octadecene (ODE;
75 mmol; 24 ml) and oleic acid (OA; 8.58 mmol; 2.7 ml) for 90 minutes at 70°C
under vacuum (10-2 mbar or better). The reaction mixture was then put under
nitrogen atmosphere and the temperature was increased to 160°C to complex the
lead oleate, leaving a clear, colorless solution. A second precursor solution was
prepared from selenium (Alfa Aeser, 10.8 mmol; 852.8 mg) and diphenylphosphine
(DPP; 15 μmol; 26.1 μl) in tri-n-octylphosphine (TOP; 24.2 mmol; 10.8 ml) and
stirred in a glovebox at 70°C until the selenium was dissolved. The selenium
precursor was rapidly injected into the flask at 100-120°C and the nanocrystals
were grown for 1-5 minutes. Cooling the reaction flask using a water bath swiftly
stopped the reaction. The nanocrystals were purified three times using 1-butanol
and methanol in a nitrogen-filled glovebox.
Device Fabrication: Trilayer solar cells were prepared on pre-patterned ITO
substrates that were cleaned by subsequent sonification in acetone and isopropanol
for 10 minutes. They were then treated with oxygen plasma for 10 minutes to
remove residual organics. Pentacene was evaporated where applicable in a thermal
evaporator (Kurt J. Lesker) at a pressure below 10-6 mbar. The rate was kept at or
below 0.1 Å/s. PbSe nanocrystal films were deposited using a layer-by-layer
technique reported earlier[2]. The spin coater was set to spin for 10 s at 1500 rpm
after a 3 s wait. First a layer of 1,3-benzenedithiol in acetonitrile (BDT, 0.02 M) was
spun followed by washing with pure acetonitrile. Then a thin layer PbSe
nanocrystals (12 – 25 mg/mL in octane) capped with oleic acid ligands was
deposited and crosslinked with BDT followed by washing with acetonitrile and
octane. This cycle was repeated until the desired thickness was achieved.
Amorphous silicon was sputtered using a DC magnetron sputtering system operated
at 330 V with argon as the sputtering gas (approximately 1.0 Pa). The sputtering
rate was 20 nm/min. The chamber was cooled with liquid nitrogen to prevent
heating of the substrates during sputtering and to improve the vacuum (base
pressure < 10-11 mbar). The amorphous nature of silicon was confirmed using XRD.
The samples were returned to nitrogen atmosphere within 5 minutes and
transferred into a thermal evaporator for deposition of the top contact. Lithium
fluoride and aluminum were evaporated at a rate of 0.05 – 0.5 nm/s. The devices
were then encapsulated by gluing a glass slide on the top using transparent epoxy
and measured in ambient atmosphere.
Solar Cell Measurement: External quantum efficiencies were measured with light
passing through an Oriel Cornerstone 260 monochromator. The current was
measured under short circuit. IV-curves were recorded under 1 sun illumination
from an Oriel 92250A solar simulator using a Keithley 2636A source measure unit.
The illumination was corrected for spectral mismatch.
[1]
W. W. Yu, J. C. Falkner, B. S. Shih, and V. L. Colvin. Chemistry of Materials 16,
3318 (2004).
[2]
A. G. Pattantyus-Abraham, I. J. Kramer, D. A. R. Barkhouse, X. Wang,
G. Konstantatos, R. Debnath, L. Levina, I. Raabe, M. K. Nazeeruddin, M. Grätzel, and
E. H. Sargent. ACS nano 4, 3374 (2010).