Photovoltaics Lecture 7 Organic Thin Film Solar Cells Photonics - Spring 2017 dr inż. Aleksander Urbaniak
Barcelona, Spain
Perpignan train station, France
source: pinterest
Why organic solar cells? 1. Low weight and flexibility of the PV modules. 2. Semitransparency. 3. Easy integration into other products. 4. New market opportunities, e.g. wearable PV. 5. Significantly lower manufacturing costs compared to conventional inorganic technologies. 6. Manufacturing of OPV in a continuous process using state of the art printing tools. 7. Short energy payback times and low environmental impact during manufacturing and operations.
DSSC Dye Sensitized Solar Cells Michael Graetzel 1988
Mechanism of operation Imitation of photosynthesis Electron hole pairs are generated in dye particles organic dye Excited electrons then diffuse into titanium dioxide TiO 2 (semiconductor)
Definitions RED (reduction) accepting an electron (X X - + h) ~ acceptors in semiconductors OX (oxidation) donation an electron (X X + + e - ) ~ donors in semiconductors HOMO Highest Occupied Molecular Orbital ~ valence band in semiconductors LUMO Lowest Unoccupied Molecular Orbital ~ conduction band in semiconductors
The mechanism 1. Photon absorption in photosensitive dye 2. Dye particles excited from the ground state (S) do the excited state (S ). Excited electrons move to the condution band in TiO2. Dye lose electrons (its particles become oxidized S + ) S + hν S S S + + e (TiO 2 ) 3. Electrons in TiO 2 conduction band diffuse to the external circuit 4. Oxidized dye particles (S+) accept electrons from the electrolyte (I ). Dye partcilees return to the ground state (S) while electrolyte is further oxidized up to I 3- S + + e S 5. Oxidized electrolyte (I 3- ), diffuse to the back electrode and is reduced to I I 3 + 2e 3I \
nature.com solaronix.com
Conjugated polymers Delocalization of p and d orbitals single bond double bond cinamone aldehyde current beta carotene
Conjugated polymers benzene
Bonds to bands H 2 molecule 1,3 butadiene
The idea V OC E g 1. charge excitation A 2. diffusion transfer e to acceptor material t 100 fs 3. collection 4. recombination D
First (reasonable) organic solar cell eff. = 0.95% CuPc copper phthalocyanine PV perylene tetracarboxylic derivcative source: C.W. Tang, Appl. Phys. Lett. 48, 183 (1986)
Bulk heterojunction cencept Bilayer organic cells fail due to low diffusion lengths (max. 10 nm), much lower than the thickness of the absorber (min. 100 mn) BH solar cell features a distributed junction between donor and acceptor materials polymer fullerene blend or conjugated molecules
Efficiencies Yokohama, source: Mitsubishi Chemicals Laboratory cells, around 1 cm 2, Modules around 3 % efficiency Roll to roll processes Mitsubishi Chemicals + 3M
Donor and acceptor materials are mixed together in a solution Excitons are strongly bonded (around 0.5 ev>> E T @RT) Electron accepting molecules have to added Bulk heterojunction
Bulk heterojunction cencept A - D : potential barrier acting as a driving force for electrons carriers must be generated within the diffusion length from D-A interface a) good charge separation but poor transport a) poor charge generation b) ideal c) real
Donor and acceptor materials
Bulk heterojunction device E(H)TL electron (hole) transport layer HTLs : poly-mer like PEDOT:PSS (poly(3,4ethylenedioxythiophene)poly(styrenesulfonate)) or a thin oxide layer (e.g. MoO 3 ) ETL materials are often oxides like zinc oxide or titanium dioxide b) allows non-vacuum processing
Perovskite- based solar cells rapid growth in last 10 years simplicity in production efficiencies competitive with Si and thin film solar cells small areas problems with stability
A perovskite A perovskite is any material with the same type of crystal structure as calcium titanium oxide (CaTiO 3 ), known as the perovskite structure, or XII A 2+VI B 4+ X 2 3 with the oxygen in the face centers The red spheres are X atoms (usually oxygens), the blue spheres are B-atoms (a smaller metal cation, such as Ti 4+ ), and the green spheres are the A-atoms (a larger metal cation, such as Ca 2+ ) source: wikipedia
Perovskites in photovoltaics Crystal structure of CH 3 NH 3 PbX 3 perovskites (X=I, Br and/or Cl). The methylammonium cation (CH 3 NH 3+ ) is surrounded by PbX 6 octahedra optical bandgap between 1.5 and 2.3 ev depending on halide content source: wikipedia
A perovskite solar cell
Tandem cells source: http://pvlab.epfl.ch
Tandem cells source: Nature Energy 2, Article number: 17009 (2017)