Towards scalable fabrication of high efficiency polymer solar cells Hui Joon Park 2*, Myung-Gyu Kang 1**, Se Hyun Ahn 3, Moon Kyu Kang 1, and L. Jay Guo 1,2,3 1 Department of Electrical Engineering and Computer Science, 2 Macromolecular Science and Engineering 3 Mechanical Engineering The University of Michigan, Ann Arbor, MI 48109 Work supported in part by NSF, KACST, DOE EFRC University of Michigan
Issues to address for practical solar cells Power conversion efficiency Light absorption Low bandgap material Light trapping, plasmonics Charge separation and transport Optimize morphology Minimize recombination Transparent electrode Conductivity; Flexibility ~3 mm Fabrication of OPVs Spin-coating Annealing (long time)
All solution roll-to-roll processed polymer solar cells free from ITO and vacuum coating steps Frederik C. Krebs Organic Electronics 10 (2009) 761 768 PCE of Roll-to-roll processed device ~ <1%
Roll-to-roll processed polymer tandem solar cells partially processed from water Solar Energy Materials & Solar Cells 97 (2012) 43 49
Product integration of compact roll-to-roll processed polymer solar cell modules: methods and manufacture using flexographic printing, slot-die coating and rotary screen printing Krebs,et al. J. Mater. Chem., 2010, 20, 8994 9001
Nanoimprinted P3HT nanopillars 15 nm
ESSENCIAL Process to make OPV Pressure Modified PDMS Al LiF Modified PDMS P3HT : PCBM PEDOT : PSS ITO Substrate P3HT : PCBM PEDOT : PSS ITO Substrate P3HT : PCBM PEDOT : PSS ITO Substrate Evaporation of Solvent through Surface ENCapsulation with Induced ALignment (ESSENCIAL) of polymer chains by applied pressure Adv. Mater. 22, E247 (2010)
Advantage 1 enable high P3HT crystallinity UV absorbance More effective application of shear stress to the polymer chain across the entire depth during ESSENCIAL process P3HT crystallization: ESSENCIAL (a few seconds) Solvent assisted annealing (~hours) > Thermal annealing (tens of minutes) High crystallinity in short processing time Glass/ ITO / PEDOT:PSS (~45 nm) / P3HT:PCBM (~240 nm) / LiF (~1 nm) / Al (~80 nm)
10/48 Advantage 2 uniform D/A vertical distribution P3HT: Sulfur (S) PCBM: Carbon (C) XPS S 1 C C-C 6 -C= (resonance) 2 C-S 2 Uniform vertical distribution C C-C 4 -C= (resonance) 66 C-O 1 C-O = O 1 Sulfur (S) Carbon (C) Binding Energy Shift of Carbon C-O > C-S > C-C = O C-O -C= (resonance) The atomic sensitivity factor n 1 I 1 / S 1 = n 2 I 2 / S 2 I : Peak area S : Atomic sensitivity factor
ESSENCIAL Device Results AM 1.5G (100mWcm -2 )
Roll-to-roll application Adv. Mater., 2010, 22, E247-E253. IEEE J. Sel. Top. Quantum. Electron., 2010, 16, 1807
Thick film: Selected as BHJ (thermal) control device 100 270 320 380 100 270 320 380 P r o b l e m s Bulk Heterojunction D A Forrest et al. Nature Mater. 2005. 4. 37. Thick film Requires new structures/processing
High performance Bilayer-based Devices Park et al. manuscript in preparation From UCSB with Dr. Mates * ESSENCIAL process DSIMS Evp. time: ~10 min * Bilayer formation: Solvent Methlylene Chloride Thickness P3HT (~350 nm), PCBM (~100 nm) J sc (ma cm -2 ) V oc (V) FF (%) PCE (%) CBM diffuses within the film without affecting the crystal size, structure, or orientation of P3HT (diffusion occurs only through the disordered regions of P3HT) BHJ (TA) 9.84 ± 0.44 0.59 ± 0.00 58.96 ± 1.20 3.43 ± 0.17 Spin-Spin 5.57 ± 0.54 0.52 ± 0.02 47.87 ± 2.06 1.40 ± 0.17 Spin-Spin (TA) 6.88 ± 0.87 0.58 ± 0.00 47.28 ± 3.62 1.88 ± 0.31 Heat Similar PCE as BHJ Spin-ESS 12.41 ± 0.70 0.54 ± 0.03 60.98 ± 3.89 4.09 ± 0.30 ESS-ESS 15.10 ± 0.44 0.51 ± 0.01 67.70 ± 5.00 5.12 ± 0.32 J sc V oc (V) FF (%) PCE (%) Amorphous PCBM P3HT As-cast 5.85 ± 0.51 0.37 ± 0.02 48.00 ± 2 1.05 ± 0.20 PCBM diffuses into P3HT film through the P3HT amorphous Thermal 8.20 ± 0.30 0.60 ± 0.01 72.00 ± 2 3.50 ± 0.19 domains. Kramer, Hawker & Chabincy et al. Adv. Energy. Mater. 2011, 1, 82 Russell Lee et et al. al. Nano Adv. Mater. Lett. 2011, 2011, 11, 23, 2071 766
Improved Mobility t del = 40 μs Absorbance Mobility (μ h, 10-4 cm 2 /V*s) BHJ (Thermal) 0.35 Spin-ESS 0.61 ESS-ESS 1.58 * BHJ (thermal) : Limitation on high crystalline donor polymer Isolated donor & acceptor nanodomains Park et al. manuscript in preparation * In this work (Spin-ESS & ESS-ESS) : Maximal crystallinity Bicontinuous nanodomains better carrier transport
Better domain organization: facilitate transport & Reduce recombination ESSENCIAL Well organized nanodomain SEM Spin-coating AFM Reduced recombination
Looking ahead: R2R processed OPV Commercial materials Develop novel R2R coating process - Uniform thin film, crystallization, fast or no annealing Manufacture polymer PV without vacuum process Interface : how to deposit effective interfacial layer - cannot be too thin if done by non-vacuum process Need alternative/better transparent conductor - flexibility, trade-off between conductivity/transparency - new functionality
A roll-to-roll process to flexible polymer solar cells: model studies, manufacture and operational stability studies Krebs, et al. J. Mater. Chem., 2009, 19, 5442
P3HT/PCBM Cells by Coating Methods (no vacuum process) Device Jsc(mA/cm 2 ) Voc(V) FF(%) PCE(%) Conventional inverted cell (Evaporated Ag) 9.21 0.56 40.95 2.11 Film transferred 10.87 0.55 46.1 2.76
Average transmittance (%) Wire grid transparent conductors 100 Transparency vs. Conductivity 80 60 Au Al Cu 40 80nm thick 60nm thick 40nm thick 20 0 0 4 8 12 16 20 TME (Cu )with 70 nm line-width ITO Sheet resistance (ohm/square) Adv. Mater. 2007
Wire grid transparent electrodes for OPVs Adv. Mater. 2008, 20, 4408 Device structure Al (100nm) P3HT:PCBM 1:0.8 PEDOT:PSS (conductive) ITO or semitransparent Au Glass or PET <Nanoimprint mold> <STME with 120nm line-width>
Plasmon-enhanced OPV 35 % enhancement in efficiency as compared with ITO control devices using unpolarized light 2 J sc (ma cm -2 ) 0-2 -4 ITO device AgW device AgN device -6 0.0 0.2 0.4 0.6 Voltage (V) Adv. Mater. 2010, 22, 4378
Colored OPV & Energy-harvesting Color filters ACS Nano, 2011
Continuous R2R/R2PNIL nano patterning 2/16 Transferred Au Roll-to-Roll NIL Epoxysilicone Pattern Roll-to-Plate NIL 4 by 12, 350 nm gratings on PET 4 by 10.5, 350 nm gratings on glass Adv Mater, 2008 & ACS Nano, 2009
Transparent Electrode by Roller Phase-shift Lithography Nanotechnology, 2012
Wire grid electrode by R2R litho Nanotechnology, 2012
Thank you!
Continuous R2RNIL on 4 wide substrate ACS Nano, 2009
Simple roll coater with variable coating and temperature control for printed polymer solar cells Henrik F. Dam, Frederik C. Krebs Solar Energy Materials & Solar Cells 97 (2012) 191 196
Summary for BHJ optimization ESSENCIAL process High crystallinity Uniform distribution Photoluminescence (A. U.) 1.0 0.8 J (ma cm -2 ) 0.6 0.4 0.2 Efficient Efficient exciton charge diss. transport 10 4 10 3 TA SAA 10 2 ESSENCIAL 10 1 10 0 10-1 10-2 10-3 10-4 Further TA 0.0 10-5 10-6 600 700 800 Wavelength 0.1 (nm) 1.0 Bias (V)
Advantage 3 high carrier mobilities Hole-only devices Normal Electron-only devices CsCO 3 = 2.9 ev MoO 3 = 5.3 ev Method Hole mobility (μ e, 10-4 cm 2 V -1 s -1 ) Hole mobility (μ e, 10-4 cm 2 V -1 s -1 ) Before Further TA TA 1.57 SAA 3.29 3.29 2.20 ESSENCIAL 11.5 11.5 12.6 SCLC model J = Method 9 ε o ε r μ V2 8 L 3 μ e / μ h TA - SAA 1.50 ESSENCIAL 1.16 Method Electron mobility (μ e, 10-4 cm 2 V -1 s -1 ) Electron mobility (μ e, 10-4 cm 2 V -1 s -1 ) Before Further TA TA - SAA 4.95 4.95 - ESSENCIAL 3.61 x 10-3 3.61 x 10-3 14.6
Advantage 4 enable efficient exciton dissociation 12/48 PL (P3HT:PCBM blend) Photoluminescence Not annealed Annealed 95.2% RR 90.7% RR Y. Kim et al. Nature Mater. 2006, 5, 197 AFM phase image AFM phase image AFM phase image 50 nm 50 nm 50 nm Thermal annealing Solvent assisted annealing ESSENCIAL (Further TA)
Wire grid electrode High transparency by adjusting the line-width and period High conductance by adjusting the thickness Less dependency of transparency and conductance High flexibility