Supervisors: Dr. Ghazi Dr. Izadifard Organic Solar Cells Supervisors: Dr. Ghazi Dr. Izadifard presenter: Maryam Alidaie

Renewable Energy Consumption in the US Energy Supply, 2007 http://www.eia.doe.gov/cneaf/solar.renewables/page/trends/highlight1.html

Different generations of solar cells photovoltaics 1st generation Classic Silicon Poly crystal Single crystal 2nd generation Thin film Amorphous Silicon CdS CI(G)S(e) CdTe 3rd generation Organic small molecule Polymers DSSE

Organic Solar Cells Organic or plastic solar cells use organic materials (carbon-compound based) mostly in the form of small molecules and polymers, to convert solar energy into electric energy.

Advantages The lower material consumption of OSCs is enabled by the much higher absorption of the organic materials at a given wavelength. Active layer thicknesses of a few hundred nanometers Less energy-demanding purification steps of the raw materials The fast and easy R2R printing methods for large scale production Better environmental sustainability, their light weight, flexibility, and the possibility of transparency in different colors

Disadvantages The hopping transport mechanism gives organic semiconductors a rather low mobility Large band gap and small absorption range which lead to low absorption efficiency of photons in the long wavelength region Low stability, oxidation, low efficiency

History in the 1970s, OSCs with efficiencies of 10−5% were produced In the 1950s, the first investigations on the conductivity of organic materials were performed in the 1970s, OSCs with efficiencies of 10−5% were produced In the mid-1980s, Tang could increase the efficiency to around 1% In the mid-1990s, the concept of blend solar cells was developed In Bulk heterojunction architecture efficiency of 8.3% on a 1 cm2 single-junction device was demonstrated by the end of 2010

Efficiency evolution of OSCs

Materials for OSCs Active Materials for OSCs : {MEH-PPV:C60 } {MDMO-PPV:PCBM} {P3HT-PCBM} {CuPc:C60} {ZnPc:C60} Interfacial Materials PEDOT:PSS (hole-selective electrode ) Electrode Materials ITO, Ca, Al, Ag, or Au Solvent (for Solution Processing) Chlorobenzene, Toluene

Chemical Structure of Organic solar cell Donor and Acceptor (Active)Materials MDMO-PPV PCBM P3HT CuPc MEH-PPV PCBM MDMO-PPV C60 P3HT CuPc MEH-PPV

Device architecture Top electrode The difference of these architectures lays in the charge generation mechanism Active layer ITO glass Single layer Bilayer Bulk heterojunction

Organic or polymer single-layer PVs Disadvantage

device performance increases by more than five orders of magnitude Single-layer PVs Single crystalline pentacene iodine-doped (bromine-doped) About 7 mm2 AM 1.5, 100mW/cm2 η=1.9% (2.4%) 955 (970) mV = Voc 4.6 mA cm-2 (5.3 mA cm-2)= Jsc device performance increases by more than five orders of magnitude

bilayer PVs

Bilayer PVs Normal bilayer Inverted bilayer

Bulk heterojunction (BHJ) PVs Bulk heterojunction or blend Solar where active layer consists of a mixture of donor and acceptor materials.

Fabrication sequence for ITO-free bulk-heterojunction solar cells Heterojunction solar cells with a spray-coated PEDOT:PSS anode and a spray-coated P3HT:PCBM active layer

Bulk heterojunction (BHJ) PVs

Working principle of BHJ device 1. Incoming photons are absorbed ⇒ Creation of excitons on the Donor / Acceptor 2. Exciton is separated at the donor / acceptor interface ⇒ Creation of charge carriers 3. Charge carriers within drift distance reach electrodes ⇒ Creation of short circuit current ISC 1. The “photodoping” leads to splitting of Fermi levels ⇒ Creation of open circuit voltage VOC 2. Charge transport properties, module geometry ⇒ Fill factor FF

Polymer/PCBM interpenetrating system Voc = 0.82 V Jsc = 5.25 mA/cm2 FF = 0.61  AM1.5G = 2.5 % (under 80 mW/cm2) O M e MDMO-PPV PCBM < S. E. Shaheen, et al. 1998 > A D Metal electrode LiF Active layer PEDOT:PSS ITO glass Donor/Acceptor composite solution

Tandem solar cells

first tandem organic solar cell realized by Hiramoto et al (1990) Based on evaporated small molecules 50 nm of metal-free phthalocyanine (H2Pc) 70 nm perylene tetracarboxylic derivative (Me–PTC) In order to make ohmic contact between the two sub-cells, an ultra-thin (2 nm) Au interstitiallayer was evaporated 2 nm thick Au layer 2 nm thick Au layer Effective recombination center VOC = 0.78 V about twice the VOC of a single cell (0.44 V)

Tandem solar cells Yakimov and Forrest(In 2002) (CuPc) as a donor, (PTCBI) as acceptor An ultrathin (z5A° ) discontinuous layer of Ag clusters served as the charge recombination sites. (η) of the two and three HJ cells under one sun, η =2.5% and 2.3%, with VOC = 0.93 and 1.2 V (twice that of a comparable single junction cell)

Significant improvement in efficiency by stacking two bulk-heterojunctions, J. Xue(2004) They report an efficiency of up to 5.7%, (about 24% more efficient than the single CuPc/C60 devices) Thin layers of PTCBI and bathocuproine (BCP) were employed as ‘‘exciton blocking layer’’

Tandem organic solar cell realized by Maennig et al based on multiple stacked p–i–n structures (2004) Active region is sandwiched between two wide band gap layers. p-type(p-doped MeO–TPD) and the n-type C60 layers were the best choices. efficiency close to 2% in single cells. higher power efficiency of 2.4% for tandem P-i-n cells

Origin of open-circuit voltage (Voc) Which is the Voc? (Is it in the electrodes? (Voc (Is it in the bulk-heterojunction? (Voc

The specific case of organic solar cells It is shown experimentally that 0.3 eV is a minimum value below which the charge transfer may not occur.

Energy-level diagram showing the HOMO and LUMO energies of each of the component materials.

Production Methods Spin coating of thin layers Dip coating Doctor blade Spray coating Inkjet printing R2R production

Future Generation - Printable Cells

References Organic solar cells, materials and device physics(2013) Springer (Ebook) Plastic Solar Cells, L Sims, Comprehensive Renewable Energy, Volume 1 (2012) Angew. Chem. Int. Ed. (2012) 2020 Seok-In Na et al, Solar Energy Materials & Solar Cells 94 (2010) 1333 Tayebeh Ameri et al, Energy Environ. Sci, 2(2009)347 Jin Young Kim et al, Science 317 (2007) 222 Chih-Wei Chu , Appl. Phys. Lett , 86(2005) 243506 A. Yakimov and S. R. Forrest, Appl. Phys. Lett. 80 (2002), 1667 J. H. Schoen, Nature 403(2000)408 S. E. Shaheen, et al. Journal of Applied Physics, 84 (1998) 2324 c. W. Tang, Appl. Phys. Lett, 48 (1986) 183 www.pveducation.org http://www.eia.doe.gov/cneaf/solar.renewables/page/trends/highlight1.html