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Figure 12 AFM scans of debond surfaces
Al side
ITO side
No annealing 150 °C, 30 min
Figure 10 Four point bend results. Average Gc determined from Gc values at criDcal load peaks
Figure 11 Plot of fracture energy vs. anneal Dme
Figure 8 Average Gc at different MoO3 deposiDon rate. Average Gc determined from three samples tested per cell
Cohesion and Reliability of Organic Bulk Heterojunc?on Solar Cells Ellen Tsay, Christopher Bruner, and Reinhold H. Dauskardt
Department of Materials Science and Engineering, Stanford University
• We chose to study P3HT:PC60BM bulk heterojuncDon (BHJ) cells because they offer high internal quantum efficiency and have been thoroughly researched
• Typically these cells fail cohesively within the BHJ, the weakest layer
• A rougher debond surface is associated with greater cohesion for several reasons: o Stress is distributed over a larger area and in
mulDple direcDons o More contact between asperiDes behind crack Dp Figure 4 Mechanical tesDng system used for four point bend test
Fabrica?ng the Solar Cells 1. 50 mm x 50 mm borosilicate glass substrates coated with ITO 2. Cleaning: ultrasonic baths with Extran, IPA and Ethanol and UV ozone 3. Low pressure thermal evaporaDon of MoO3 onto ITO 4. BHJ soluDon contained 1:1 weight raDo PCBM:P3HT and 50 mg of
solute per 1 ml of chlorobenzene solvent 5. Spin-‐coated BHJ onto the MoO3 at 900 rpm and 500 rpm/s for 45 s.
Devices slow dried overnight 6. Low pressure thermal evaporaDon of Ca and Al metal electrodes
Figure 3 A diced cell, ITO side up. Individual samples were detached and prenotched
1. We hypothesize that varying the deposiDon rate of a MoO3 hole transport layer (HTL) will influence its surface roughness and therefore cohesion. Since the BHJ is so thin, the failure path may follow the surface of the underlying MoO3 layer in order to stay cohesive within the BHJ layer.
2. A second hypothesis is that thermally annealing cells will increase cohesion. This has been shown in cells with a PEDOT:PSS HTL, and we aeempt to duplicate the result with MoO3 as the HTL.
• All devices failed cohesively within the bulk heterojuncDon • Varying the rate of MoO3 deposiDon had no significant effect on surface roughness, and there was no correlaDon between MoO3 surface roughness and cohesion
• Annealing produced a 70-‐112% increase in Gc, from 3.63 J/m2 ± 0.642 up to 7.71 J/m2 ± 0.162 • The opDmal annealing Dme at 150 °C was 30 min. Annealing results closely resembled those for PEDOT:PSS HTL cells
Introduc?on
Organic Bulk Heterojunc?on Solar Cells
Hypotheses
Device Prepara?on and Tes?ng Effect of Deposi?on Rate
Conclusions
References and Acknowledgements
Annealed four samples from the same cell at 150 °C in a glovebox for 5, 30, 60, 120 min
Preparing Cells for Fracture Tes?ng • A solar cell “sandwich” was created by adhering a second, idenDcal glass substrate to the top electrode with epoxy
• Cured sandwich for 30 min at 85 °C to harden the epoxy • Diced cells into individual samples (see Fig. 3) The four point bend test was used to measure the cohesion, or criDcal energy release rate, of the cells (see Fig. 4). The criDcal energy release rate, Gc, is the energy released per unit crack area and was calculated using the equaDon in Fig. 5.
P3HT a polymer
electron donor
+
PC60BM a fullerene derivaDve electron acceptor
=
Figure 1 Typical structure of a bulk heterojuncDon solar cell
Organic solar cells a r e p r om i s i n g a l t e r n aD ve s t o silicon solar cells, but in order to become a viable technology their m e c h a n i c a l reliability must be
improved. Cracks and defects shorten device lifeDmes and have a negaDve impact on producDon yields and cost. We study the effect of chemical deposiDon rate and thermal annealing on the mechanical strength of organic bulk heterojuncDon solar cells and whether these factors can be successfully manipulated to improve device reliability.
Brand, Bruner & Dauskardt. 2012. “Cohesion and device reliability in organic bulk heterojuncDon photovoltaic cells.” Solar Energy Materials & Solar Cells 99:182–189.
Brand, Levi, McGehee & Dauskardt. 2012. “Film stresses and electrode buckling in organic solar cells.” Solar Energy Materials & Solar Cells 103:80-‐85.
Dupont, Oliver, Krebs & Dauskardt. 2012. “Interlayer adhesion in roll-‐to-‐roll processed flexible inverted polymer solar cells.” Solar Energy Material & Solar Cells 97:171-‐175.
I would like to thank the Vice Provost for Undergraduate EducaDon and the Materials Science and Engineering Department for funding this project. Special thanks to Christopher Bruner, the Dauskardt Group, and the McGehee Group for their guidance and support.
Figure 9 Debonded devices
Figure 7 MoO3 surface roughness for different deposiDon rates. Rrms is the root mean square value of the surface topography
Figure 2 Structure of the tested solar cell “sandwiches”
Science Kn
owledge 2010
Molecule im
ages:
Brand, Brune
r & Dauskardt 2012
Figure 5 EquaDon for Gc, where Pc is the criDcal load, L is the moment arm length, E’ is the biaxial modulus and b and h are the specimen width and half-‐height
Brand, Brune
r & Dauskardt 2012
• Built three devices at MoO3 deposiDon rates of 0.5, 5, 20 Å/s
• Characterized roughness of MoO3 surfaces with AFM
Figure 6 XPS results for the ITO side of the cell annealed for 30 min
• Characterized debond surfaces with non-‐contact atomic force microscopy (AFM) and X-‐ray photoelectron spectroscopy (XPS)
• XPS results for all devices looked similar, showing carbon and sulfur peaks
Debond Path Characteriza?on
Effect of Thermal Annealing