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

Overview

Introduction

Background

Objectives

Synthesis of small molecules

Physical properties (DSC, TGA, UV-vis, CV)

Device fabrication and testing

Morphological Study

Conclusion

An hour sunshine is enough to power the world for twenty years.

1.33 octillion (1027 ) Btu solar energy per hour reaches theearth.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 GMT3. R. Po and J. Roncali, J. Mater. Chem. C, 2016, 4, 3677–3685.

( )

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.

Considerations for the synthesis photoactive organic materials.

Quinoid formation compounds withreduced 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.

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.

Objectives

Synthesise and purify novel low bandgap semiconductingorganic molecules.

Determine the physical properties of molecules (both theoryand experiment).

Fabricate and test organic solar cells using synthesisedmolecules as photoactive units.

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 50020

40

60

80

100

We

ight

%

Temperature / oC

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-2TEsmall molecules.

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

Ab

so

rba

nce

/ a

.u.

Wavelength / nm

DRH-2TE

DECA-2TE

DIN-2TE

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

b

Ab

so

rba

nce

/ 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.

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), recordedat a scan rate of 100 mV s-1.

-2 0 2

DECA-2TE

DRH-2TE

DIN-2TE

Cu

rre

nt

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 andDECA-2TE small molecules.

Device Fabrication

𝑷𝑪𝑬 =𝑱𝒔𝒄 × 𝑭𝑭 × 𝑽𝒐𝒄

𝑷𝒊𝒏

Device performance

DEVICEJsc

(mA cm-2)

Voc

(V)FF

PCE

(%)

DRH-2TE: PC71BM

(1:3) a3.04 0.64 0.30 0.63

DRH-2TE: PC71BM

(1:3) a c5.60 0.68 0.35 1.36

DECA-2TE: PC71BM

(1:4) b2.96 0.85 0.41 1.03

DECA-2TE: PC71BM

(1:4) b c2.99 0.90 0.39 1.05

-0.3 0.0 0.3 0.6 0.9

-8

-4

0

4

8

Cu

rre

nt

de

nsity/

mA

cm

-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

Cu

rre

nt

de

nsity /

mA

cm

-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 optimisedphotovoltaic 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.

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.

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.