• 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

Chlorophyll

Organic semiconductors

N

N

N

N

N

N

N

NCu

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’

Organic semiconductors - a world of coulombic interactions

Coulomb’s Law: +q -qr

Eg

Eg

LUMO

HOMO

Molecular Solid

Hopping transport

20

21

4 rqqFr

Isolated Molecule

Vacuum Level

Molecular Orbital (MO)

Core Levels (AOs)Nuclei

Ener

gy

Vacuum Level – just outside solid surface where electron is at rest.

HOMO = Highest occupied MO; LUMO = Lowest unoccupied MO

• 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

dVE

*Figure from http://www.easac.eu/fileadmin/docs/Low_Carbon/KVA_workshop/Renewables/2013_09_Easac_Stockholm_Leo.pdf

*

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.

+

-

• 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

N

N

N

N

N

N

N

NCu

Molecular semiconductor

Exciton diffusion length 10 nm

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.

SubP

cC 60

-3.4

-4.2

-5.5-6.2

N N

NB

N

N

NCl

Splitting excitons into free charge carriers

• Rapid photo-induced electron transfer (100 fs), long lived charge separated state (ms).

e-e-

SubPc C60

SubP

cF 6-S

ubPcN N

N

N

N

N

B

ClF

F F

FF

F

-3.6

-5.8F6-SubPc

e-

Sullivan, et al., Advanced Energy Materials (2011) 1, 352–355.

•From donor to acceptor, by design:

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.

• 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)

•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

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

*

Solution processable organic semiconductorsOCH3

O• Chemical modification:

Alternating co-polymer

EgEg

Homo-polymer

• Semi-conducting polymers:

Figure adapted from www.orgworld.de

Efficiency evolution

Target: efficiency >15%, > 20yr lifetime and low cost

Challenges: Improving efficiency

• Semiconductors that can be processed from non-toxic solvents.

• Materials amenable to rapid processing. Do

nor

Acce

ptor

• Narrow band gap organic semiconductors.

Maximum Voc

6

5

4

3

2

1

0

Voc

/ V,

PC

E/%

605550454035302520151050Time/ d

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Jsc/ mA

cm-2,

FF

Burn-in time ~6d1.01 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)

Challenges: Reducing materials cost

• Need for low cost, transparent, flexible electrode.

* Photograph from Hatton research group, University of Warwick.

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.