American Journal of Renewable and Sustainable Energy Vol. 4, No. 2, 2018, pp. 33-39

http://www.aiscience.org/journal/ajrse

ISSN: 2381-7437 (Print); ISSN: 2381-7445 (Online)

* Corresponding author E-mail address:

Indocyanine Green as a Sensitizer for Dye-Sensitized Solar Cell

Adrian Jones, William Ghann, Jamal Uddin*

Center for Nanotechnology, Department of Natural Sciences, Coppin State University, Baltimore, USA

Abstract

We report in this paper the photophysical characterization of Indocyanine green (ICG) dye and its application in dye sensitized

solar cells. ICG is a water soluble, tricarbocyanine dye which forms noncovalent fluorescent complexes with proteins and as a

result has been used for medical diagnostics through fluorescence imaging technologies. The ICG along with another cyanine

dye (Cy 4) was used in this study. The photophysical studies performed included UV-vis, fluorescence, and lifetime

measurements. The Cy 4 has two hydroxyl groups which serve as anchoring groups facilitating the attachment of the dye to the

titanium dioxide nanocrystalline surface. They are therefore well suited for use as sensitizing dye in dye sensitized solar cells.

There was no significant difference in the photophysical properties of the two different dyes deployed in the studies. The

samples were also characterized used Field Emission Scanning Electron Microscopy. The current and voltage characteristics

were measured and the short circuit current, open circuit voltage, fill factor, and the solar-to-electric power efficiency,

subsequently determined. The efficiency of the ICG was 0.08% whiles that of Cy 4 was 0.11%.

Keywords

Cyanine, Dye, Indocyanine Green, Solar Cell, Titanium Dioxide, DSSC

Received: April 19, 2018 / Accepted: May 29, 2018 / Published online: July 20, 2018

@ 2018 The Authors. Published by American Institute of Science. This Open Access article is under the CC BY license.

http://creativecommons.org/licenses/by/4.0/

1. Introduction

Dye Sensitized Solar Cells (DSSCs) have attracted

considerable attention in recent years owing to their low-cost

of production and ecofriendly nature. This type of solar cells,

also known as Gratzel cells were invented by Michael

Gratzel in 1991. [1] Fossil fuel shortage and increasing cost

of crude oil together with the impact of conventional energy

sources on the environment, has brought about a mounting

interest in renewable forms energy. DSSC provide a simple

means of converting a renewable energy source (sun) into

electricity and can have efficiencies in the upwards of 10%.

[2, 3] They convert visible light into electricity based on the

photosensitization of wide band-gap metal oxide

semiconductors, like TiO2. A dye sensitized solar cell is made

up a photoanode, a cathode and a redox couple electrolyte

that ensure the regeneration of the dye. [4, 5] When a

sensitizing dye absorb incident light, they are excited to

higher energy level where electrons are ejected into the

titanium dioxide semiconductor. The injected electron is

transported through the nanocrystalline layer to the

conductive side of the glass slide and subsequently through

an external circuit to the cathode. At the anode, the oxidized

dye species accepts an electron from the electrolyte and it is

reduced to the ground state. [6] The electrolyte is regenerated

by the acceptance of electron the counter electrode.

Various dyes, particularly, Ruthenium based dyes, have

frequently been used for the generation of electricity. [7]

Other forms of dyes often used in the fabrication of solar cell

include natural dyes, [8-13] porphyrin, [14-16] quantum dots

[17, 18] and cyanine dyes. [19-21]

Most of the dye that have been exploited in the fabrication of

American Journal of Renewable and Sustainable Energy Vol. 4, No. 2, 2018, pp. 33-39 34

dye sensitized solar cell have strong absorption in the Uv-

visible region and therefore have produced appreciable solar-

to-electric power efficiencies. There is however a growing

interest for near infrared sensitization and studies of their

application as dyes in dye sensitized solar cells.

Figure 1. Chemical structure of Indocyanine green and cyanine 4 used as

sensitizers in the dye sensitized solar cell.

The reason for this interest in cyanine dyes stems from

studies that point to the broadening of the absorption band of

the cyanine dyes upon adsorption onto the nanocrystalline

TiO2 electrode relative to that of a plain solution of the dye.

[22-24] The indocyanine green (ICG) is an infrared cyanine

dye that has a myriad of applications, predominantly in

biological systems. [25, 26} Some of the biomedical

applications include their use as contrast dye in biopsy. [27]

This research focuses on the application of ICG as sensitizer

for dye sensitized solar cells in comparison with cyanine 4

(Cy4) dye as displayed in Figure 1. The photophysical

properties of the dyes were determined and analyzed. The

electrochemical and photovoltaic properties of the fabricated

solar cells were also investigated.

2. Experimental Section

Absorption spectroscopy was carried out with UV-3600 Plus

from Shimadzu, MD, USA. Emission spectroscopy was

measured with RF-5301PC from Shimadzu, MD, USA.

Titanium dioxide paste was printed on FTO glass using WS-

650 Series Spin Processor from Laurell Technologies

Corporation, PA, USA. Carbon paint used in making cathode

slides was purchased from TED PELLA, INC, USA. The cell

performance was measured using 150 W fully reflective solar

simulator with a standard illumination of air-mass 1.5 global

(AM 1.5 G) having an irradiance of 100 mW/cm2

(Sciencetech Inc.), London, Ontario, Canada. Reference 600

Potentiostat/Galvanostat/ZRA from GAMRY Instruments

(Warminster, PA).

2.1. Preparation of Photo-anode

The TiO2 film was deposited on the FTO glass using a spin

coater as previously described. [27-31] The TiO2 suspension

was prepared using titanium dioxide powder, acetic acid, and

detergent. The acetic acid was applied to improve the

dispersion of the suspension. FTO glass slides with

dimension 2.5 cm by 2.5 cm were cleaned with detergent and

rinsed successively with water and ethanol solution to

remove any contamination on the surface of the glass slides.

The suspension was prepared specifically by mixing 2.4 g

TiO2 powder, 10 ml acetic acid and 1 ml detergent in a

porcelain mortar with a pestle. After grinding to obtain a

uniform mixture, the TiO2 paste was spin coated on the

conductive side of the FTO glass and sintered at 450 °C for

30 min to enhance the film compactness and crystallinity.

Upon cooling down to room temperature, they were

immersed into the dyes for 24 h.

2.2. Preparation of Counter Electrode

The counter electrode was prepared by depositing a layer of

colloidal graphite on an FTO glass. The graphite coated FTO

glass was subsequently dried at room temperature before use.

2.3. Construction of Solar Cell

The dye sensitized solar cell was constructed by placing the

photoanode on top of the graphite counter-electrode in a

sandwich type arrangement and adding drops of the

electrolyte solution into the space between the two

electrodes.

3. Results and Discussion

3.1. UV-Vis Studies

UV-visible spectroscopy was used to study the absorbance

characteristics of the Indocyanine green dye (ICG) and

Cyanine 4 (Cy4).

Uv-visible measurement was carried out in ethanol. The

wavelength of maximum absorption for ICG and Cy4

occurred at 787 nm and 782 nm, respectively. These peaks

are characteristic of all infrared dyes. On binding to the

titanium dioxide, the band expands both to lower wavelength

and higher wavelengths and this allow for the absorption of

more visible light. [22-24] Figure 2 shows UV-vis absorption

spectra of the indocyanine green dye and Cy 4 dye in ethanol.

The shapes of the peaks are very similar to each other.

35 Adrian Jones et al.: Indocyanine Green as a Sensitizer for Dye-Sensitized Solar Cell

Figure 2. Uv-Visible absorption spectra of ICG and Cy4 carried out in

ethanol solution.

3.2. Emission Studies

Fluorescence emission studies were carried out on the ICG

and Cy4 in ethanol solution. The emission spectra of these

two dyes are displayed Figure 3.

Figure 3. Emission spectra of ICG and Cy4 dye in ethanol solution.

The wavelength of the maximum absorption peak for ICG

was 830 nm while that of Cy4 was 869 nm. The difference in

emission of two emission peaks is thus 39 nm. Both dyes

were excited at 700 nm.

3.3. Lifetime Fluorescence Measurements

We studied the fluorescence lifetime of indocyanine green

and Cy4 in ethanol. The measurements were carried out

using excitation with Nanoled at 773 nm. Figure 4 shows the

time domain data obtained for the two dyes. IRF as shown in

the plot is the instrument response function in the absence of

any dye. The lifetime of ICG determined through our study

was 0.46 ns and that of the Cy4 was 4.8 ns. Thus Cy4 has a

slightly longer lifetime than the ICG. Different lifetimes have

been reported for ICG in different solvents, for instance:

DMSO (0.97 ns), [32] DMSO without any potential quencher

DMSO (3.5 ns), [33] and water (0.166 ns). [34] Table 1

display the lifetimes and the standard deviation of the two

dye which were analyzed.

Table 1. Fluorescence lifetime of ICG and Cy4 in ethanol.

Sample Name Lifetime (ps)

τ1(ps) +/-

ICG 4.6 0.012 Cy 4 4.8 0.058

Figure 4. Time domain data obtained from Cy4 and ICG in ethanol using

excitation with Nanoled® at 778 nm and emission at 800 nm. Prompt corresponds to the instrument response with a Ludox® solution containing

colloidal silica with no fluorophore present.

3.4. Field Emission Scanning Electron Microscopy Imaging

Field Emission Scanning Electron microscopy (FESEM)

images showing the surface morphology of the titanium

dioxide film and indocyanine green sensitized titanium

dioxide film were obtained (Figure 5 a & c). The films were

also characterized by Energy Dispersive X-ray Spectroscopy

(EDS) as displayed in Figure 5b and 5d. In the EDS

measurement, there is a significant carbon peak in the spectra

of the dye sensitized titanium dioxide film as shown in

Figure 5d, however in the bare titanium dioxide film (Figure

5b), the carbon peak is virtually absent in the spectra. This

difference originate from the carbon rich indocyanine green

dye in contrast with carbon free TiO2 nanocrystalline film.

The peaks of oxygen are almost the same for the two samples

since TiO2 contained in both samples are made up oxygen

and titanium atoms. The FESEM images reveal the

nanocrystallinity of the titanium dioxide that increases the

surface area for maximum adsorption of the dye which

ultimately increase the number of photons absorbed and the

amount of current produced. The FESEM and EDS

measurement were performed only for the ICG dye sensitized

TiO2 film since the two dyes have almost the same number of

carbon and oxygen atoms.

American Journal of Renewable and Sustainable Energy Vol. 4, No. 2, 2018, pp. 33-39 36

Figure 5. Field Emission Scanning Electron Microscopy (a & c) and Energy Dispersive X-ray spectroscopy spectra (b & d) of titanium dioxide film and dye

sensitized titanium dioxide film.

3.5. Photocurrent and Photovoltage Characteristics

The current and voltage (I-V) characteristic curve of the

fabricated ICG and Cy4 dye sensitized solar cells is

displayed in Figure 6. The I-V measurements were carried

out under standard illumination of AM 1.5G (air mass 1.5

globular).

The solar-to-electric power efficiency of the Cy4 dye

sensitized solar cell was slightly higher than that of the ICG

dye sensitized solar cell. The overall efficiency of the Cy4

was 0.11% with an open circuit voltage of 0.27 V, and short

circuit current of 1.02 mA/cm2. The fill factor of this device

was 0.38. The ICG dye sensitized solar cell on the other hand

had an overall efficiency of 0.08% with an open circuit

voltage of 0.30 V, short circuit current of 1.08 mA/cm2 and a

fill factor of 0.26. Thus, the fill factor of the ICG device was

much lower than that of the Cy4 device, which made a

difference in the efficiency of the solar cell. Table 2 shows

the efficiency of the solar cells along with all the relevant

parameters.

Figure 6. IV Characteristics of the ICG dye sensitized solar cell in

comparison with that of Cy4 DSSC.

Table 2. The performance of the ICG dye as potent photosensitizer via the current, voltage, fill factor and conversion efficiency measurement was assessed in comparison with that of Cy4 DSSC.

Dye ISC (mA/cm2) VOC (V) Imp (mA/cm2) Vmp (V) Fill Factor Efficiency (%)

ICG 1.02 0.30 0.067 0.12 0.26 0.08 Cy 4 1.08 0.27 0.70 0.16 0.38 0.11

37 Adrian Jones et al.: Indocyanine Green as a Sensitizer for Dye-Sensitized Solar Cell

3.6. Electrochemical Impedance Spectroscopy Measurements

Electrochemical impedance spectroscopy (EIS) studies were

conducted to investigate the kinetics and energetics of charge

transport and ionic diffusion in the ICG and Cy4 dye

sensitized solar cells.

Figure 7. Nyquist plot of the ICG dye sensitized solar cell in comparison

with that of Cy4 DSSC.

EIS studies shed light on the frequency response of the dye

sensitized solar cells and have often been used to characterize

dye sensitized solar cells. [35] The EIS is presented in

Nyquist plot (Figure 7) and Bode plot (Figure 8) The

Electrochemical Impedance Spectra were recorded in the

frequency range of 1 Hz and 100 KHz. The Nyquist plots

show two semi circles for the ICG and one circle for the Cy4.

There are also two distinct peaks in the Bode plot for the ICG

and one distinct peak for the Cy4. The first semi-circle in the

Nyquist corresponds to charge transfer processes at the

carbon/redox (I−/I3−) electrolyte and the second semicircle

correspond to the electron diffusion in the TiO2 film and the

electron back reaction at the TiO2/electrolyte. [36]

Figure 8. Bode plot of the ICG dye sensitized solar cell in comparison with

that of Cy4.

4. Conclusion

In this work, indocyanine green (ICG) along with another

cyanine dye (Cy4) was used in the sensitization of

nanocrystalline TiO2 for the fabrication of dye sensitized solar

cell. ICG has been extensively used as a fluorescent dye in

hepatic, cardiovascular, and ophthalmologic studies. In this

study, ICG is deployed as a sensitizing dye in the generation of

electricity. The dyes were first characterized using absorption

spectrometry, emission spectrometry, and fluorescence

lifetime measurements. The dye sensitized titanium dioxide

films were also characterized using field emission scanning

electron microscope and the Energy Dispersive X-ray

spectroscopy. The electrochemical in addition to the current

and voltage characteristics were additionally studied. An

overall conversion efficiency of 0.08% with a short circuit

current of 1.02 mA/cm2 and open circuit voltage 0.30 V was

achieved for the ICG while that of Cy4 dye was 0.11%, with a

short circuit current of 1.08 mA/cm2 and open circuit voltage

of 0.27 V. ICG and Cy4 were thus found to exhibit comparable

electrochemical and photovoltaic properties. The slightly

higher solar-to-electric energy of the Cy4 DSSC compared to

the ICG DSSC could be due to the presence of the two

hydroxyl groups on the Cy4 that allows for better interaction

of the dye with the nanocrystalline titanium dioxide electrode.

Acknowledgements

The work was financially supported by the University of

Maryland System Wilson E. Elkins Professorship (2485897),

Constellation, an Exelon Company, E2- Energy to Educate

grant program (163893) and Dept of Education, SAFRA Title

III Grant (2486005). The authors are also grateful to the

Institution of Advancement, Coppin State University, for

administrative help. The content is exclusively the

responsibility of the authors and does not necessarily

represent the official views of the funding agencies.

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