Simple EDOT based photo-active molecules have been synthesised by fewer synthetic steps. The molecules separately acted as donor units in organic solar cells fabrications. Best device efficiency was 1.36%.

1. Acceptor-Donor-Acceptor small molecules based on
derivatives of 3,4-ethylenedioxythiophene for solution
processed organic solar cells
Boniface Y. Antwi (AMRSC)
PhD Chemistry Candidate (Year 4)
University of Ghana, Legon –Accra.
Supervisors
Prof. Robert Kingsford-Adaboh
Prof. Peter J. Skabara (FRSC)
Dr. Richard Boakye Owoare

2. Overview
 Introduction
 Background
 Objectives
 Synthesis of small molecules
 Physical properties (DSC, TGA, UV-vis, CV)
 Device fabrication and testing
 Morphological Study
 Conclusion

4. An hour sunshine is enough to power the world for twenty years.
1.33 octillion (1027 ) Btu solar energy per hour reaches the
earth.1
0.82 quintillion (1018 ) Btu global energy demand by 2040.2
Organic solar cells (OSC) have unique properties.3
flexible
easy to process
light weight
wide area applicability
1. A. Mishra and P. Bauerle , Angew. Chem. Int. Ed. 2012, 51, 2020 – 2067.
2. http://www.eia.gov/todayinenergy/detail.cfm?id=12251; 13.06.2016; 12:08 GMT
3. R. Po and J. Roncali, J. Mater. Chem. C, 2016, 4, 3677–3685.

5. ( )
Mechanism
Photon induced exciton
generation.
Exciton diffusion to donor
acceptor interface.
Charge separation at
interface
Charge transport to
electrodes (Electrons to
cathode and holes to anode)
+
-
-
( )
-
++
-
-
-
+++
-
Donor AcceptorD/A
Scheme 1. Operational mechanism of OSC.

7. Considerations for the synthesis photoactive organic
materials.
Quinoid formation compounds with
reduced aromaticity.
Reasonable Increase in Conjugation.
Incorporation of,
 Electron Donating Group.
 Electron Withdrawing Group5.
5. C. Yen-Ju , Y. Sheng-Hsiung, and H. Chain-Shu. Chem. Rev. 2009, 109, 5868–5923.

8. Architecture of Interest
Well defined Structures
Less batch to batch variation
Versatile Chemical Structure leading to
easier energy level control6.
ACCEPTOR DONOR ACCEPTOR
6. W. Ni, X. Wan, M. Li, Y. Wang and Y. Chen. Chem. Commun., 2015, 51, 4936—4950.

10. Objectives
Synthesise and purify novel low bandgap semiconducting
organic molecules.
Determine the physical properties of molecules (both theory
and experiment).
Fabricate and test organic solar cells using synthesised
molecules as photoactive units.

14. Thermal Stability
Thermal Properties DIN-2TE DRH-2TE DECA-2TE
Melting point (DSC) or Tg / °C 173 249 236
5% weight loss Temp (TGA) 360 362 363
200 300 400 500
20
40
60
80
100
Weight%
Temperature / o
C
DECA-2TE
DRH-2TE
DIN-2TE
Figure 2. TGA curve of DIN-2TE, DR2TE and DECA-2TE
measured at 10 °C/min under Argon.
Table 1. Thermal properties of DIN-2TE, DRH-2TE and DECA-2TE
small molecules.

15. Optical behaviour
Solution
Narrow Peaks
Absorption at long wave length
Thin film
 Broad peaks
Bathochromic shift (Planner backbone,
aggregation)
300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
a
Absorbance/a.u.
Wavelength / nm
DRH-2TE
DECA-2TE
DIN-2TE
300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
b
Absorbance/a.u.
Wavelength/ nm
DRH-2TE
DECA-2TE
DIN-2TE
𝞹-𝞹 interaction-
shoulder peaks
Best HOMO-LUMO energy gap-(1.57 eV, DIN-2TE thin film)
Figure 3. Normalised absorption spectra of DIN-2TE, DRH-2TE, and DECA-2TE (a) in solution and (b) drop cast film.

16. Electrochemical properties
Figure 4 Cyclic voltammograms of DIN-2TE, DRH-2TE,
and DECA-2TE in dichloromethane solution (10-4 M)
with Bu4NPF6 supporting electrolyte (0.1 M), recorded
at a scan rate of 100 mV s-1.
-2 0 2
DECA-2TE
DRH-2TE
DIN-2TE
Current
Potential / V vs Fc/Fc+
Fc/ Fc+
DIN-2TE DRH-2TE DECA-2TE
Potential
(V)
Reversible
(E1/2)
+0.33, and
+0.72.
+0.66, +1.15
and -1.50
Irreversible +0.69, +1.16,
and -1.62
+1.14, and -
1.64.
HOMO (eV) -5.49 -5.13 -5.46
LUMO (eV) -3.18 -3.16 -3.30
Eg (eV) 2.31 1.97 2.16
Table 2. Electrochemical properties of DIN-2TE, DRH-2TE and
DECA-2TE small molecules.

19. Device Fabrication

20. 𝑷𝑪𝑬 =
𝑱 𝒔𝒄 × 𝑭𝑭 × 𝑽 𝒐𝒄
𝑷𝒊𝒏

21. Device performance
DEVICE
Jsc
(mA cm-2)
Voc
(V)
FF
PCE
(%)
DRH-2TE: PC71BM
(1:3) a
3.04 0.64 0.30 0.63
DRH-2TE: PC71BM
(1:3) a c
5.60 0.68 0.35 1.36
DECA-2TE: PC71BM
(1:4) b
2.96 0.85 0.41 1.03
DECA-2TE: PC71BM
(1:4) b c
2.99 0.90 0.39 1.05
-0.3 0.0 0.3 0.6 0.9
-8
-4
0
4
8
Currentdensity/mAcm-2
Voltage / V
DRH-2TE:PC71
BM_without DIO
DRH-2TE:PC71
BM_with 1% DIO
(a)
-0.4 0.0 0.4 0.8
-3
-2
-1
0
Currentdensity/mAcm-2
Voltage / V
DECA-2TE:PC71
BM_without DIO
DECA-2TE:PC71
BM_with 1% DIO
(b)
a60 °C and b90 °C annealing temperatures for 20 mins, c1 %
diiodooctane.
Table 3. Summary of the average optimised
photovoltaic performance for DRH-2TE and DECA-
2TE devices. AM 1.5G illumination.
Figure 5. Current–voltage curves of optimised (a) DRH-2TE and
(b) DECA-2TE bulk-heterojunction devices without and with 1%
DIO additive under AM 1.5 G illumination.

23. Figure 5: Tapping mode AFM height images of best performing DECA-2TE device without DIO (left) and with 1% DIO
(right). 1:4 D/A weight ratio, annealed at 60°C.
Figure 6: Tapping mode AFM height images of best performing DRH-2TE device without DIO (left) and with 1% DIO
(right). 1:3 D/A weight ratio, annealed at 90°C.

25. Three novel low bandgap A-D-A small molecules have
been synthesised.
Power conversion efficiencies, 1.36% and 1.05% have
been recorded for DRH-2TE and DECA-2TE based BHJ
organic solar cells respectively.
DIN-2TE was unsuitable for solution processable BHJ
OSC application, due to its poor solubility.