1 / 19

A bottom-up rationale for OPV architecture Fabrication Performance Challenges

Research Methods in PV: Organic photovoltaic devices (OPVs) Ross A. Hatton Department of Chemistry, University of Warwick. No emissions No noise No moving parts. A bottom-up rationale for OPV architecture Fabrication Performance Challenges Research opportunities.

bgettys
Download Presentation

A bottom-up rationale for OPV architecture Fabrication Performance Challenges

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Research Methods in PV: Organic photovoltaic devices (OPVs) Ross A. Hatton Department of Chemistry, University of Warwick. • No emissions • No noise • No moving parts • A bottom-up rationale for OPV architecture • Fabrication • Performance • Challenges • Research opportunities

  2. Organic semiconductors Chlorophyll Cu phthalocyanine Tang, Appl. Phys. Lett. 48 (1986) 183. • Earth abundant elements • (cheap, non-toxic) • Strong absorbers • Tuneable properties • (optical, electronic, processing) Meiss et al., Adv. Funct. Mater., 22 (2012) 405. ‘Dial-a -semiconductor’

  3. Organic semiconductors - a world of coulombic interactions Coulomb’s Law: r -q +q Hopping transport Vacuum Level LUMO Eg Energy HOMO Molecular Orbital (MO) Core Levels (AOs) Nuclei Isolated Molecule Molecular (VdW) Solid Vacuum Level – just outside solid surface where electron is at rest. HOMO = Highest occupied MO; LUMO = Lowest unoccupied MO

  4. Organic photovoltaics (OPV) – device architecture • Low charge carrier mobility  large electric field (F) needed for extraction. • so for low film thickness (d), can easily achieve required F. * d  200 nm • >100× thinner than c-Si • Advantages associated with ‘thin film’. • Positive temperature coefficient. *Figure from http://www.easac.eu/fileadmin/docs/Low_Carbon/KVA_workshop/Renewables/2013_09_Easac_Stockholm_Leo.pdf

  5. It’s good to be flexible • Weak intermolecular interactions in molecular solids impart flexibility. • Why is flexibility important? • Compatibility with roll-to-roll fabrication (rapid fabrication  low cost) • Compatible with light weight substrates (i.e. plastics) – typically flexible. • New applications possible (e.g. integration with fabrics).

  6. - + - + Excitations in molecular semiconductors  Excitons = lattice site Exciton diffusion length 10 nm Crystalline inorganic semiconductor Molecular semiconductor • Frenkelexciton • r =2-4 • Binding energy > 0.2 eV • Mott-Wannier exciton • r =10-15 • Binding energy < 25 meV Architecture must: (1) split excitons; (2) Overcome diffusion length limitation.

  7. Splitting excitons into free electrons and holes • Efficient exciton dissociation can be achieved at an organic heterojunction Tang, Appl. Phys. Lett. 48 (1986) 183. S +H LUMO (Fermi level) (Fermi level) Electron donor HOMO Electron acceptor

  8. Splitting excitons into free charge carriers e- e- -3.4 -4.2 SubPc SubPc F6-SubPc C60 -5.5 SubPc C60 -6.2 • Rapid photo-induced electron transfer (100 fs), long lived charge separated state (ms). • From donor to acceptor, by design: e- -3.6 -5.8 F6-SubPc Sullivan, et al., Advanced Energy Materials (2011) 1, 352–355.

  9. Summary of fundamental processes • Light absorption to form an exciton. • Exciton diffusion to the heterojunction. • Exciton dissociation at the organic heterojunction. • Charge carrier transport to electrodes. • Charge carrier extraction.

  10. Donor Acceptor The exciton diffusion bottleneck • Exciton diffusion length in organic semiconductors  10 nm. • Photoactive layer must be structured to accommodate this: Bi-layer Bulk-heterojunction (BHJ) Opaque electrode Transparent Electrode  Thin film (high electric field)  Organic hetero-junction (split excitons)  Interpenetrating heterojunction (overcome limitation in exciton diffusion length)

  11. Processing (vacuum processing) Solution Processing Area Solar Simulator Evaporation Chamber • High purity • High degree of control over layer thickness ( 0.1 nm) • Multi-layer architectures possible • High capital outlay • Energy intensive (high vacuum (10-6 mbar) needed)

  12. Processing (solution processing) Spin coating Spray deposition Printing * Spin-coating (wasteful) minimal loss & scalable to large area • Low cost equipment. • Low embodied energy (no vacuum required) / short payback time. • Difficult to make bilayer architectures. • Simple to make BHJ (spontaneous donor/acceptor phase separation). * Picture from Advancing spray deposition for low-cost solar cell production K. Xerxes Steirer, et al., 25 March 2009, SPIE Newsroom. DOI: 10.1117/2.1200903.1555

  13. Solution processable organic semiconductors • Chemical modification: • Semi-conducting polymers: Eg Eg Homo-polymer Alternating co-polymer

  14. Efficiency evolution Target: efficiency >15%, > 20yr lifetime and low cost Figure adapted from www.orgworld.de

  15. Challenges: Improving efficiency • Narrow band gap organic semiconductors. Donor Acceptor Maximum Voc • Semiconductors that can be processed from non-toxic solvents. • Materials amenable to rapid processing.

  16. Challenges: improving stability • Photo-stability of organic semiconductors • (Materials for OLEDS > 1 millions hours lifetime) • Interface stability (delamination at soft contacts) • Blocking water/oxygen ingress • (particularly challenging on flexible substrates) Photograph by P. Sullivan, University of Warwick.

  17. Challenges: Reducing materials cost • Need for low cost, transparent, flexible electrode. * Photograph from Hatton research group, University of Warwick.

  18. Concluding remarks • OPVs are an emerging thin film PV technology which is potentially very low cost and compatible with flexible substrates. • OPVs are fundamentally different from c-Si PV in the mode of operation and device architecture. • The challenges in this field of research are multi-faceted and inherently interdisciplinary. * Photograph by R. da Campo, Molecular Solar Ltd.

  19. Rises > 2oC look likely, unless there is a real international appetite to change emission paths, and quickly.

More Related