Alvin Kwiram Symposium Seattle, June 24, 2003 Plastic Solar Cells: current status and future prospects Bernard Kippelen, Neal R. Armstrong, and Seth Marder Optical Sciences Center, and Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA NREL, ONR, NSF
Collaborators Prof. David O’Brien, deceased
Organic Electronics Low temperature processing of organic semiconductors, metals and dielectrics on flexible substrates: low cost ($0.01) Metal deposition on plastics from solution, micro-size features using soft lithography and transfer
OLEDs OFETs PV • High mobility • Very thin OK • No condition on optical absorption • High mobility • High photo-generation efficiency • Absorb visible spectrum • Photo-stability • Light emission • High luminescence efficiency • Low mobility OK • Photo-stability Memories A convergence of new material technologies for the development of Application Specific Integrated Plastic Chips (ASIPC) Electro-optics Lasers and amplifiers Light-weight flexible substrate, barrier to O2 and H2O A Complementary Material Platform Light-weight, high versatility, low cost, large area
Efficiency Commercial Military Low cost Flexibility Consumer Technology Opportunities • Low cost scanners • Optical isolators • Devices that take advantage of the integration of photodetectors on light-weight flexible substrates
Introduction to photovoltaic technologies Organic excitonic solar cells Requirements for conversion with high efficiency An approach based on self-assembly Outline
LUMO Electrode HOMO Electrode Semiconductor
I V Solar cell parameters • short-circuit current ISC • open circuit voltage VOC • fill factor FF
Evolution of PV Technologies [A.M.: air mass; G: global, direct + scattered; angle of 48.2, zenith angle (sec(48.2) = 1/cos(48.2) = 1.5)] AM 1.5 G, 25 C, 1 sun = 100mW/cm2
State-of-the-art in organic photovoltaics • Grätzel cell (liquid electrolyte, solid) • Small molecules (bi-layers) • Polymer blends (interpenetrated networks) • Hybrid approaches (Inorganic sc doped in organic matrix)
The Challenge Harvesting the solar spectrum and…
…maintain simultaneously high open circuit voltage and high fill factor • optimize absorption, charge generation, charge collection: photocurrent • optimize relative energy levels: built-in voltage • optimize electrical characteristic: fill factor
Step #1: Achieve efficient dissociation of excitons in organic materials vacuum vacuum LUMO electrode HOMO electrode Double layer Single layer Overcome exciton binding energy
Optimum harvesting: 400 – 900 nm (1.4 eV) Maximum efficiency: = 28% x FF Ansatz: the maximum value for Voc is the smallest band gap minus the exciton binding energy (0.5 eV)
Equivalent Circuit Model Understanding Key Factors for Efficient Organic Photovoltaic Cells • Finite conductance of materials and contact resistance : nonzero Rs • Leakage path : finite Rp (J=0) (V=0) OPEN-CIRCUIT VOLTAGE SHORT-CIRCUIT CURRENT DENSITY What determines the fill factor ?
Effects of Rs A need for high mobility materials
Self-Assembly: a Path for Controlled Morphology in Wet Processed Materials Adapted from D. Haarer
Number of atoms (C,O,N) in the core Existing other approaches Choice of hexabenzocoronene (HBC) driven by large core that can lead to large mobility John Warman, Adv. Mater. 13, 130 (01) Voc = 0.69 V; FF = 0.4; Jsc = 30 A/cm2 Saturation for illumination > 1 mW/cm2 Mixtures of HBC and perylene K. Müllen, R. Friend et al. Science, 293, 1119, (01)
Our material’s choice: phthalocyanines • Good Thermal stability • Strong Molar absorptivity • Good Light stability Alkoxy substituted Pc known to form discotic hexagonally ordered mesophases. Problems: KI > 350°C, difficult to align, no photocurrent when combined with PTCDI Skoulios et al. J. Am. Chem.Soc. 1982, 104, 5245-5247
Molecular design Metal: Tuning of spectroscopic and electronic properties Core: Provides large core for strong -orbitals coupling and cohesive forces through Van der Waals interactions Arms: Influence the solid-to-mesophase (KDh) and mesophase-to-isotropic liquid (DhI) transition temperatures
Molecular optimization O-Et-O-Bz CuPc S-Et-O-Bz CuPc KDh 134°C; DhI 320°C; easy to process into thin films by spin-coating (chloroform) KDh 111°C; DhI > 400°C; difficult to process
d d d Substrate Substrate Substrate Material Characterization Small-angle X-ray scattering Data show that Pc form three different types of crystalline phases; dependent on surface treatment
Before annealing AFM studies Possibility to form nanostructured surfaces by thermal annealing to create high area networks for improved exciton dissociation After annealing Spin-coated at 4000 rpm on PEDOT:PSS/ITO; 180°C for two hrs.
Device Configuration C60 DLC-CuPc * Energy scale in eV w.r.t vacuum
Experimental Results • Annealing of DLC-CuPc film resulted in 3.7-fold increase in Jsc. • Estimation of RsA values by inverse slopes of J-V curves at V »Voc suggests increase of mobility in annealed device. • Reduction of Voc is considered due to creation of pinholes in DLC-CuPc film caused by dewetting while being annealed. Result for device with ITO/PEDOT:PSS (30nm) /DL-CuPc (20nm)/C60 (40nm)/BCP (10nm)/Al, under 50mW/cm2 (AM1.5Direct illumination)
Self-assembled electron transport materials “Star-like” discotic LC oxadiazole materials with good electron mobility
TOF experiments N2 laser, 337 nm, 6 ns R = 102 –104, C = 10 pF, RC <<
Cyclicvoltammetry of Discotic LCs 0.6 volt shift in reduction potentional
OE Testing Facilities Fully automated high vacuum deposition system with four organic sources and two high power sources for metals and oxides (co-deposition capabilities). Integrated with double glove box (one dry and one wet with integrated spin-coater).
Conclusions and future work • Transport properties of organic semiconductors often limit power conversion efficiency in organic solar cells. High mobility required in materials that can be processed from solution. • DLC-CuPc is solution-processible, and we demonstrated that its transport property can be improved in the discotic liquid crystalline phase. • Photocurrents reaching mA/cm2, significant improvement over HBC-based devices • Development of discotic electron-transport oxadiazole-based materials. • Optimization of parameters will require control of interfaces, relative orbital energies, control of morphology through use of self-assembly.
Stabilization of geometry and patterning Photo-crosslinking between adjacent side chains through cyclobutane links; > 50 % conversion of styryl groups 254 nm
Effects of Rs and Rp on Fill Factor in High Photocurrent Regime Minimize Rs and Maximize Rp
Vacuum level EA E Ip E HTL ETL Excitonic Solar Cells: Energy Level Engineering B) A) C) D) Band offset < exciton binding energy Band offset > exciton binding energy Working hypothesis:the maximum value for Voc is the smallest band gap minus the exciton binding energy (0.5 eV)