Vikram Kuppa School of Energy, Environmental, Biological and Medical - PowerPoint PPT Presentation

Vikram Kuppa School of Energy, Environmental, Biological and Medical Engineering College of Engineering and Applied Science University of Cincinnati vikram.kuppa@uc.edu

Fei Yu Yan Jin Andrew Mulderig d ld i Greg Beaucage Greg Beaucage

― Renewable ― Potential for High coverage ― Low emission 1 US DOE Energy Information Administration (2012), Annual Energy Review 2011 U. Mich., Center for Sustainable Systems. 2012. “U.S. Renewable Energy Factsheet”. Pub No. CSS03 ‐ 12

Organic Photovoltaics (OPVs) Solar power offers unique advantages no mech. parts relatively expensive ― ― flexible & customizable storage ― ― IIIa generation ‐ Si & Ge cells are efficient but expensive OPVs are of IIIb type: low to moderate efficiency yp y processed at lower T low efficiency ― ― versatile manufacturing versatile manufacturing inadequate spectral coverage inadequate spectral coverage ― ― ― distinctive mechanical & optical properties poor mobility of charges ― ― tunability y ― cheap ―

Functioning of OPVs h ν ( − +) − +) ( − +) ( − ( ) Incident radiation produces e ‐ h pairs (excitons) Exciton motion length & time scales ~ 100 ps, 5 ‐ 20 nm + Morphology of active − material is KEY

OPV Materials ― Blend of polymer(s) and/or additive – bulk heterojunction (BHJ) ― Traditional BHJs have about 50% of polymer, and 50% PCBM (fullerene derivative) (fullerene derivative) ― PCBM only for charge conduction and exciton dissociation ― Critical Issues Critical Issues ― Increase fraction of conjugated polymer ― Helps capture more sunlight ― Improves efficiency ― Improve charge transport

Importance of interfaces in OPV devices D-A interface LUMO Donor 0.3eV 0 3eV LUMO Acceptor LUMO E Donor E Acceptor e - h HOMO Donor + HOMO Acceptor � D-A interface facilitates exciton dissociation � Electron transfer from donor(semiconducting polymer) to acceptor polymer) to acceptor � Exciton dissociation is energetically favorable � Exciton diffusion length(~10 nm) g ( ) � D-A interfacial area is determined by morphology 7

Typical OPVs + PCBM P3HT + P3HT F8TBT + + F8BT P3HT McNeill & Greenham, Adv. Mater. 2009 21 , 3840 Kim et al., Chem. Mater 2004 16 (23), 4813

Polymer Blend OPVs ― Mix of semiconducting polymers ― Both components active & capture sunlight ― Morphology control is again key ― Critical Issues ― Poor charge mobilities persist g p ― Greater recombination losses ― Crystallization of polymers and blend miscibility ? Crystallization of polymers and blend miscibility ? ― Free charge formation and transport ― Voltage V l Solution ?

Pristine graphene ― Excellent conductivity and high aspect ratio ― Percolation paths at very low concentration ― OPVs with chemically modified graphenes were reported* ― Functionalized sheets show poor electronic behavior F ti li d h t h l t i b h i TEM image of pristine graphene flake t=0.35 nm lateral~200-500nm Scale bar=50nm *Liu, Z. et al. , Adv. Mater., 2008 20 (20), 3924 Yu, D. et al. , ACS Nano, 2010 4 (10), 5633; Yu, D. et al. , J. Phys. Chem. Lett., 2011 2 (10), 1113

Graphene ‐ based OPVs + − P3HT(~90.99%) PCBM(~9%) PCBM(~9%) Graphene(~0.01%) ― Three ‐ fold enhancement in efficiency ― Increase in current – better mobility ― Novel device physics Novel device physics Yu, Bahner & Kuppa, Key Engr. Mater. 2012 21 , 3840 Yu & Kuppa, Mater. Lett. 2013 99 , 72

Current focus ‐ ternary blends + − P3HT (59.9%) F8BT (39.9%) Graphene (~0.2%)

Device Fabrication Anode Anode ― Patterned ITO as bottom electrode ― PEDOT:PSS by spin coating PEDOT PSS b i ti ― Active layer with graphene by spin Active layer with graphene by spin coating ― LiF and Aluminum LiF and Aluminum Cathode ― Fabricated and annealed in N 2

Solar Cell Parameters 14 Deibel and Dyakonov, Rep. Prog. Phys., 2010 73 (9), 096401

Cell Performance Open circuit voltage ‐ V oc 1.5 1.4 V) Voc (V 1.3 1.2 1.1 0.00 0.02 0.04 0.06 0.08 0.10 graphene concentration (mg/ml)

Cell Performance Short circuit current ‐ J sc 0.6 0.5 mA/cm 2 ) 0.4 Jsc (m 0.3 0.2 0 1 0.1 0 0.05 0.1 0.15 0.2 graphene concentration (mg/ml)

Cell Performance Fill factor ‐ FF 0.21 0.20 F FF 0.19 0 18 0.18 0.00 0.02 0.04 0.06 0.08 0.10 graphene concentration (mg/ml)

Cell Performance 0.16 0.14 (%) fficiency ( 0.12 0.1 ef 0.08 0.06 0 04 0.04 0 0.05 0.1 0.15 0.2 graphene concentration (mg/ml)

Cell Performance External quantum efficiency ‐ EQE PF10 PF10 G0 025 PF10 G0.025 4 PF10 G0.05 PF10 G0.1 PF10 G0.2 3 %) EQE (% 2 1 0 300 350 400 450 500 550 600 650 700 Wavelength (nm)

Device physics ‐ recombination 0.0 alpha=0.73 alpha=0.67 -0.2 alpha=0.62 2 )) -0.4 alpha=0.66 mA/cm J sc ~ I α alpha=0.74 p -0 6 0.6 ent density(m -0.8 alpha=0.90 G 0 G 0 Log (Curre -1.0 alpha=0.91 G 0.025 alpha=0.84 G 0.05 -1.2 G 0.1 alpha=0.93 G 0.2 G 0 2 -1.4 Linear Fit alpha=0.99 -1.6 1.0 1 0 1.2 1 2 1 4 1.4 1 6 1.6 1.8 1 8 2 0 2.0 2 2 2.2 2.4 2 4 2 6 2.6 (a) 2 )) Log (Intensity(mW/cm α = 1 for geminate α = 0.5 for bimolecular

graphene dependence of α 1.00 part 1 alpha part 2 alpha part 2 alpha 0.95 0.90 J sc ~ I α 0.85 Alpha 0.80 A 0.75 0.70 0.65 0.60 0.00 0.05 0.10 0.15 0.20 (b) Graphene concentration (mg/ml)

Role of Graphene E Extrinsic i i I Intrinsic i i Charge transport Morphology of blend Morphology of blend Mobility Structure of P3HT & F8BT Recombination Crystallization & Aggregation

UV ‐ VIS of thin films P3HT/F8BT 6/4 P3HT/F8BT/G 6/4/0.05 Peaks at 550 ‐ 600nm w/ P3HT/F8BT/G 6/4/0.1 P3HT/F8BT/G 6/4/0.2 Increasing [Gr] Increasing [Gr] Absorption P3HT crystallites Normalized Nucleating agent ? 350 400 450 500 550 600 650 700 Wavelength (nm)

Concentration dependence 0 5 0 8 0.15 ‐ 0.18 0.18 0.12 ‐ 0.15 0.09 ‐ 0.12 0.15 iency (%) 0.06 ‐ 0.09 0.12 0.03 ‐ 0.06 effici 0.09 0.06 10 10 0.03 8 0.1 [P3HT+F8BT] (mg/ml) 0.05 6 0.025 0 [graphene] (mg/ml)

Thickness dependence 0 18 0.18 0.15-0.18 0.15 (%) 0.12-0.15 efficiency 0.12 0 12 0.09-0.12 0.09 0.06-0.09 0.03-0.06 0.03 0.06 0 06 0.06 e 0.03 0.2 0.1 0.1 0.05 0.025 graphene concentration (mg/ml) 0 film thickess (nm) ( )

New paradigms in OPV BHJs − Graphene enhances charge transport ‐ high J sc , FF and η − Cells with majority active layer are now viable j y y − Better harnessing of solar energy − Improved mobility − Morphology of blend is altered – enhanced crystallization − Intrinsic and extrinsic effects are observed − Complex influence of thickness & concentration Complex influence of thickness & concentration − Synergistic role of high ‐ aspect ratio graphene additives Jin, Yu and Kuppa, (manuscript in preparation)

� Choice of donor and acceptor materials : band gap and miscibility � Choice of solvent : polymer chain packing � Donor-acceptor ratio : domain size � Annealing conditions : reorganize polymer chains, crystallization � Other post-production treatments : DC voltage during annealing for ordered structure * Morphology Performance 28 Pictures source: Dennler, Scharber and Brabec, Adv. Mater. 2009, 21 (13): p. 1323-1338. * Padinger, Rittberger and Sariciftci, Adv. Funct. Mater., 2003. 13 (1): p. 85-88.

BHJ features BHJ features Polymer:Fullerene BHJ device Polymer:Fullerene BHJ device � High interfacial area for exciton dissociation � Bicontinuous network for charge transport g p � 50:50 w/w P3HT:PCBM for optimum performance � Increase P3HT ratio to capture more solar energy p gy P3HT PCBM

F t Future Work W k � Better dispersed and oriented graphene via morphological control morphological control � Increase FF by reducing interfacial roughness � Increase FF by reducing interfacial roughness � St bilit � Stability and device encapsulation d d i l ti FY and VKK thank UC and the URC for funding and support