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

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

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

4. Low charge carrier mobility  large electric field (F) needed for extraction. so for low film thickness (d), can easily achieve required F. >100× thinner than c-Si Advantages associated with ‘thin film’. Positive temperature coefficient. d  200 nm Organic photovoltaics (OPV) – device architecture *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 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). Weak intermolecular interactions in molecular solids impart flexibility.

6. + - Mott-Wannier exciton  r =10-15 Binding energy < 25 meV Excitations in molecular semiconductors  Excitons = lattice site Crystalline inorganic semiconductor + - Frenkel exciton  r =2-4 Binding energy > 0.2 eV Molecular semiconductor Exciton diffusion length  10 nm 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 HOMO LUMO Electron acceptor Electron donor Tang, Appl. Phys. Lett. 48 (1986) 183. (Fermi level)  S +  H

8. SubPc C 60 -3.4 -4.2 -5.5 -6.2 Splitting excitons into free charge carriers Rapid photo-induced electron transfer (100 fs), long lived charge separated state (ms). e-e- e-e- SubPc C 60 SubPc F 6 -SubPc -3.6 -5.8 F 6 -SubPc e-e- Sullivan, et al., Advanced Energy Materials (2011) 1, 352–355. From donor to acceptor, by design:

9. Summary of fundamental processes 1.Light absorption to form an exciton. 2.Exciton diffusion to the heterojunction. 3.Exciton dissociation at the organic heterojunction. 4.Charge carrier transport to electrodes. 5.Charge carrier extraction.

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

11. 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) Processing (vacuum processing) Solar Simulator Solution Processing Area Evaporation Chamber

12. Spin coating Spray deposition Printing Spin-coating (wasteful) minimal loss & scalable to large area Processing (solution processing) 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: Alternating co-polymer EgEg EgEg Homo-polymer Semi-conducting polymers:

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

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

16. Photograph by P. Sullivan, University of Warwick. 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)

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 > 2 o C look likely, unless there is a real international appetite to change emission paths, and quickly.