Lecture delivered by Ross Hatton, University of Warwick. Super Solar Research Methods in PV, University of Liverpool, April 2016.

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
Eg
Eg
LUMO
HOMO
Molecular Solid
Hopping transport
2
0
21
4 r
qq
F
rεπε
=
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 (E) needed for extraction.
• so for low film thickness (d), can easily achieve required E.
• 100 x thinner than c-Si
• Advantages associated with ‘thin film’.
• Positive temperature coefficient.
d ≤ 200 nm
Organic photovoltaics (OPV) – device architecture
d
V
E =
*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
• εr =10-15
• Binding energy < 25 meV
Excitations in molecular semiconductors ⇒ Excitons
= lattice site
Crystalline inorganic
semiconductor
+
-
• Frenkel exciton
• εr =2-4
• Binding energy > 0.2 eV
Molecular
semiconductor
Exciton diffusion
length ∼ 10 nm

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.

8. SubPc
C60
-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-
SubPc C60
SubPc
F6-SubPc
-3.6
-5.8
F6-SubPc
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
⇒ Organic hetero-junction
⇒ Interpenetrating heterojunction (BHJ)
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
AreaEvaporation
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
Eg
Eg
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 Voc

16. 6
5
4
3
2
1
0
Voc/V,PCE/%
605550454035302520151050
Time/ d
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Jsc/mAcm-2,FF
Burn-in time ~6d
1.01 mA cm-2, 5.16 V, 0.52, 2.67 %
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.