Research group journal club talk.

1. WMD Journal Club
Suzanne Wallace
18th August 2015

2.  Advances in understanding of
PV-FE effects
 Recent progress in improving
device performance
 Integrating FE into organic
heterojunction PV devices

3.  Advances in understanding of
PV-FE effects
 Recent progress in improving
device performance
 Integrating FE into organic
heterojunction PV devices

4. FE-PV effect

5. Typical p-n junction
• Absorbed photons pump e- from
VBCB
• Holes left in VB
• e- quickly separated by built-in electric
field inside p-n junction
• Band gap sets theoretical limit for VOC
In FE-PV materials  photovoltages measured in un-doped single
crystals (no p-n junction present) + photovoltages a few orders of
magnitude larger than the band gap

6. FE-PV effect: BPE, APE,
both?
BPE: photovoltage measured in undoped single crystal
(no p-n junction present to produce asymmetry in
electrostatic potential in the conventional way)
APE: photovoltages measured that are orders of
magnitude larger than the band gap
FE-PV effect  both? Or BPE is also APE?
Unique feature of FE-PV: exptl observations that output
photovoltage is proportional to the magnitude of electric
polarization and electrode spacing.

7. Factors influencing measured
photovoltage
• Distance between two opposite electrodes
• Light intensity
• Electrical conductivity
• Remnant polarization of FE crystal
• Crystallographic orientation
• Dimension/ size of crystal
• Domain walls
• FE/ electrode interface

8. Theories to explain FE-PV
effects
1. Shift current model
2. Non-linear dielectric model
3. Domain wall theory
4. Schottky-junction effect
5. Depolarization field model
(or screening effect)
First two theories to
explain APE, involve BPE
 two phenomena
intrinsically linked?
Additional contributions
to photovoltage output
 Most relevant for thin-films

9. Shift current model (2.1.1)
• FE material acts as current source
• Related to non-centrosymmetric crystal
• Transition probability of e- jump from state of moment k
to state with momentum k’ may be different to
probability of reverse process
asymmetric momentum distribution of photogenerated
charge carriers
 steady photocurrent
 Predicts larger VOC under stronger light intensity
Total current through FE
Dark conductivity and
photoconductivity
(typically very low for FE)

10. Non-linear dielectric model
(2.1.1 last paragraph)
• Another theory for APE based on BPE
• Large observed photovoltage output caused by
non-linear response of polarization density to the
E field of incident light
• Leads to effective DC E field throughout FE
material

11. Domain wall theory (2.1.2)
• Exptl observation of photovoltage in
BiFeO3 film increasing linearly with no. of
domain walls along net polarization
direction (perpendicular to domain walls)
• Intrinsic potential drop at domain walls
 huge electric field  dissociation of
photogenerated exciton
• Illuminated domain walls act as nanoscale
photovoltage generators connected in
series
• Generated photocurrent is continuous
• Photogenerated voltage accumulates along
direction of net polarization

12. Schottky-junction effect
(2.1.3)
• When FE semiconductor forms Schottky contact with metal
electrodes
• Photocurrent under illumination driven by local electrical field,
which is caused by band bending near electrode
• Photocurrent dependent upon Schottky barrier height and
depletion region depth
• But photovoltage is still limited to band gap of FE material
• Effect originally ignored because much smaller than APE
• Becomes significant in thin-film PV because of small
photovoltage of these devices
• Additional photovoltage contribution absent if devices have
same electrode contacts (cancel out because of opposite
polarization of two Schottky-junctions)

13. Depolarization field model/
screening effect (2.1.4)
• High densities of polarization charges on surfaces of polarized FE
films
• Induce huge E field inside FE layer if not screened
• E field imperfectly screened by free charges in metal or
semiconductor in contact with FE layer
• Imperfect because CoG of polarization charge and free compensation
charge not coincident
• Results in depolarization field
• Believed to be dominating force for separation of photogenerated
charge carrier-pairs
• Screening depends on:
1. Remnant polarization of FE
2. Free charge density
3. Dielectric constant
4. Thickness of FE layer (thinner  larger depolarization field)
(larger depol. field for semiconductors
than metals due to weaker screening
because of lower free charge carriers
and higher dielectric const)

14. Organic semiconductors
Very strong absorption in the visible or near
infrared range
Poor charge separation (low carrier mobility in
existing polymers)
Progress in integrating FE in
OPV
FE materials:
Large band gaps
Can generate huge permanent electric field to assist separation of
electron-hole pairs in OPV
 Use FE in OPV as interfacial layer between active layer and electrodes,
between donor and acceptor layers or be blended in bulk films
+
+
-
-

15. The verdict
Very useful for my current project!
+ nice pictures and descriptions of models!
Bit of ambiguity over APE, BPE and general FE-PV effect
Summary section a bit naff?

16. The verdict
Very useful for my current project!
+ nice pictures and descriptions of models!
Bit of ambiguity over APE, BPE and general FE-PV effect
Summary section a bit naff?

17. The verdict
Very useful for my current project!
+ nice pictures and descriptions of models!
Bit of ambiguity over APE, BPE and general FE-PV effect
Summary section a bit naff?