Dye-Sensitized Solar Cell Synthesis

Rishiraj B. Mathur1*

, Anthony J. Sauter2, Anna Charney

3, Tirthak Saha

4, Alex McBride

5, Yuriy

Smolin5, Dr. Kenneth Lau

5

1 Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia, PA

2 Department of Electrical and Computer Engineering, Drexel University, Philadelphia, PA

3 Department of Chemistry, Drexel University, Philadelphia, PA

4 Department of Electrical and Computer Engineering, Drexel University, Philadelphia, PA

5 Department of Materials Science and Engineering, Drexel University, Philadelphia, PA

*Corresponding Author

Abstract

The following paper describes the new methods and materials used to produce a traditional Dye-

Sensitized Solar Cell and displays and analysis the results produced. This team produced the test

cells using TiO2 as the substrate and Rhuthenium N719 as the dye. A platinum electrolyte was

added to the cell prior to the collection of current density and voltage data. The research was

conducted as part of the Drexel Smart House, which is a student run organization in Drexel

University aimed at sustainable and innovative inventions for potential use at Smart House.

Two kinds of TiO2 were used, handmade and magnetic stirred while two spreading techniques

were incorporated, namely, Doctor Blading and Spin Coating. The best efficiency found in this

study was 0.88% for a cell spin coated at 100 rotations per minute layered with a single layer of

magnetic stirred TiO2. The other efficiency increasing techniques employed were the use of

multiple layers of substrate. Double layered TiO2 cells were produced, the highest efficiency

amongst which was 0.743%, a little lower than the highest, 0.88%.

Introduction

“The dye-sensitized solar cells (DSC) provide a technically and economically credible

alternative concept to present day p–n junction photovoltaic devices. These offer the prospective

of very low cost fabrication and present attractive features that facilitate market entry.” [1] Since,

the invention of the DSSC in 1991, “TiO2 nanoparticles has always been used as a

photoelectrode material of DSSC. The TiO2 thin film commonly used in DSSC is made by TiO2

nanoparticles for commercial use. The gap between particles is smaller, so that the dye amount

absorbed is limited.” [2] DSSC have also been made from ZnO nanowires. The small surface

area of the nanowires limited the efficiencies of these cells. [3]

The DSSC technology has gained a huge interest from 1991 to around 1997 (as can be seen in

Figure 1). Major constituents like TiO2 nanoparticles and simple process stages such as sintering

make DSSC a cheap and less labor intensive alternative to capturing solar energy. Currently, the

DSSC stands at the highest efficiency of 13%. [4] By using a molecularly engineered porphyrin

dye to maximize electrolyte compatibility and improve light harvesting properties, a power

conversion efficiency of 13% was reached.

Figure 1: NREL Best Research-Cell Efficiencies [5]

The Drexel Smart House

The Drexel Smart House (DSH) is a student organization which has established itself as a launch

pad for research and technology at Drexel University, focusing on the renovation of a 19th

century urban home into an environmentally conscious, high performance, energy efficient

building to serve as a platform for innovation, model for the community, classroom and

residence. Figure 2 shows the envisioned design of the house. Among other projects, DSH is

involved in manufacturing techniques and improvement in efficiencies of DSSC technology.

With immense support from Dr. Kenneth Lau and his PhD. candidate, Yuriy Smolin, the DSSC

team has successfully achieved the highest efficiency of 0.88%. In the duration of about three

months, the conventional procedure of making a DSSC has been practiced and other

unconventional methods have also been undertaken. DSSCs have a very special property of

functioning in the presence of artificial light. Ergo, the long term goal of the DSH is to exploit

that property and incorporate DSSCs into indoor objects such as lampshades and window blinds.

Figure 2: DSH envisioned design [6].

Functioning of a DSSC

DSSCs operate through the excitation and transfer of electrons within collector dye molecules.

The anode is composed of a transparent mesoporous metal-oxide (in this case a layer of

nanocrystalline Titanium Dioxide, TiO2) matrix, sensitized with a molecular dye (Rhuthenium

N719). When light enters the system the excited dye-sensitizer injects an electron into the TiO2.

The electron is then conducted to a transparent electrically conductive substrate (Fluorine Doped

Tin Oxide, FTO) and flows out of the device. The oxidized dye molecule is reduced to its ground

state by a redox-couple present in a surrounding electrolyte (Pt). Re-entering the system at the

cathode, the electron flows through a FTO substrate and rejoins with the electrolyte. This design

allows for ambient light capture, as only a single photon is required to generate an exciton,

whereas other p-n junction strategies require charge buildup by a number of exciton pairs, before

energy conversion occurs.

Results

A conventional DSSC is made using TiO2, Rhuthenium N719 dye and Platinum electrolyte. The

TiO2 and the Pt electrolyte were synthesized in the lab while the dye was purchased. Fluorine

Doped Tin Oxide (FTO) electrically conducting glass substrates were used to in order to

complete the circuit of the DSSC. The FTOs were purchased as well. Figure 3 shows the

conventional schematic of the DSSC.

The TiO2 film was made using two different techniques. It was either handmade or made with

the use of magnetic stirrers. The TiO2 initially used was handmade. Both magnetic stirred and

handmade TiO2 were manufactured. Using the above paste, photoanodes can be produced by two

methods, Doctor Blading and Spin Coating.

Figure 3: General schematic of a DSSC [7]

Doctor Blading (DB) was the first method used for cell manufacture. It requires the manual

control deposition of TiO2 on the glass substrate and its subsequent sintering. The sintering

solidifies the TiO2 and creates a solid layer. The TiO2 layer is then finished by scratching the

deposited substrate into a smaller area (to be similar to the aperture of the light emitting testing

equipment and also reduce the amount of dye used). The glass substrates are then dipped in the

dye overnight.

Spin Coating, on the other hand involves the usage of a Sigma-Aldrich Spin Coater. The spin

coater rotates the glass substrate (with manually deposited known volume of TiO2) to a pre-

determined rotation per minute (RPM). The thickness of the TiO2 layer is proportional to the

RPM. The spin coated cells were subjected to 100, 300, 500, 600 and 700 RPM coats and the

100 RPM was found to be the most efficient with an efficiency of 0.88% while the 300 RPMs

were found to be the least efficient with an efficiency of 0.208%.

Although many more coating techniques have been devised, spin coating and doctor blading are

the most suited for small scale laboratory testing. Processes that involve roll-to-roll coating or

reel-to-reel coating (R2R) are used for high volume processing and were not chosen for this

sequence of samples. [8]

Figure 4: Comparison of different TiO2 coating methods

Discussion

As the DB method proved to produce significantly inferior results (low efficiency of 0.159), Spin

Coating was chosen to be the primary method of photoanode synthesis during this research.

Other cells were produced by double coating (DC) TiO2 onto the FTO. These cells were

promising the 500 RPM DC cells turned out to be the most efficient, giving an efficiency of

0.743%.

Figure 4 plots the data for 3 Doctor Bladed and 5 Spin Coated cells. As shown in Figure 4, the

cell performance is judged by its Current Density-Voltage (J-V) plot. The highest power

generated is given by the product of J and V at any instant. This is the reason, cells with plots

having a shallow curve can often be assumed to have a relatively mediocre power output.

Two unconventional techniques were incorporated with Spin Coating and Doctor Blading in

order to achieve larger efficiencies:

Multiple Layered DSSCs: Another idea was to create a second layer of TiO2 on the Photoanode.

This, theoretically, should increase the surface area available for the absorption of the dye thus

leading to a better electron ejection rate which, in turn, should churn out an increased efficiency.

These cells were termed Double Layered Photoanodes for obvious reasons. These DL

Photoanodes consisted of two subsequent layers of TiO2 as opposed to the single layer of paste

used in the previous versions of the cells. DL photoanodes were made using a unique method of

sintering two layers of TiO2 twice in succession with cooling time of 15 minutes in between. On

testing these cells, the 500 RPM DL cells were of the highest efficiency of 0.743%.

0

5

10

15

20

25

30

35

40

0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007

Cu

rre

nt

De

nsi

ty (

J)

Voltage (V)

Thousands

Cell Comparison

DB 1

DB 2

DB 3

SC DL @ 700

SL DL @ 500

SC DL @ 600

SC SL @ 100

SC SL @ 300

Furthermore, 500 RPM DL photoanodes were just below the 100 RPM SL cells in terms of

efficiency.

Magnetic Stirred TiO2 paste: The TiO2 paste used to create the initial photoanodes was handmade

and included the use of a mortar and pestle. This tends to produce a more uneven and thicker

TiO2 paste. The TiO2 powder is not dissolved and mixed in the paste thoroughly. This provides a

better option of magnetic stirring the mixture for a considerable amount of time before it can be

used. Magnetic stirred TiO2 creates paste that is thinner in consistency, well mixed with

minimum air bubbles. As TiO2 particles tend to clump together, having the paste well mixed is

important. In paste manufacturing, the clumping is avoided to a certain extent by adding Triton-

X and De-ionized water to the mixture.

Table 1: Efficiencies derived from cells

Spin Coated, SL 100 RPM 0.88 %

300 RPM 0.208 %

400 RPM 0.361 %

Spin Coated, DL 500 RPM 0.743 %

600 RPM 0.128 %

700 RPM 0.29 %

Doctor Bladed Cell 1 0.265 %

Cell 2 0.159 %

Cell 3 0.219 %

These results show that Doctor Blading method is inferior to Spin Coating. This is because it

involves a much thicker coat as compared to spin coated cells. Also, surprisingly enough, spin

coating at 100 RPM has proved to be the best. When the 100 RPM substrate was created, it was

noticed that the TiO2 took a considerable amount of time to form a layer on the FTO, due to the

very slow speed. Formation of the layer left a good amount of liquid TiO2 which had to be

evaporated over time. Table 1 shows the efficiencies reached for the manufacturing techniques

used. As can be seen, a thick TiO2 layer is more efficient that having a thinner one. Even for the

DL cells, it can be seen that the efficiency for a 500 RPM cell is considerably larger than the cell

coated at 700 RPM.

However, it was later realized that they have an advantage over the 100 RPM with regards to the

time taken to manufacture the cell. The 100 RPM ones, because of the low speed, require a

considerable amount of time before the TiO2 actually creates a solid layer on top of the FTO. The

extra time is crucial as the TiO2 shall remain liquid and would not form a rigid layer otherwise.

While the small difference in efficiency may not appear to be a significant advantage, it is

actually a lot when looked at in the perspective of relative percentage. The difference is of

approximately 15%.

Conclusions

The above results clearly indicate that the new approaches used to enhance the efficiency of the

DSSC show potential and can be very useful if applied. Avoiding in depth changes to the plan of

a DSSC and the ingredients to make one, the team has conjectured various ways to potentially

improve the efficiency of the cell. The proposed ways relate directly to the root of the theory of a

solar cell and, in particular, a DSSC and would be executed by the team as part of their future

work.

The highest efficiency reached was 0.88% pertaining to the TiO2 cell spin coated at 100 rotations

per minute. This cell was surprisingly not surpassed by the ones coated at larger RPMs.

Future Work

Based on our results, we identified several future directions for this work:

Increased TiO2 Surface Area: The main concept of DSSCs was understood to be dependent upon

the dye and the amount of dye that is absorbed by the TiO2. An idea was thought of which

involved the increase in the surface area of the TiO2 by creating conical wedges on its surface

using a microneedle. It was conjectured, that this will not be possible if a separate mould was

used to create depths on the TiO2 because of its thin consistency. This method is something the

DSH team would like to research more into and perform in the future.

Organic Dyes: A research conducted by Amaresh Mishra and Markus Fischer have scooped the

interest of researchers using organic dyes instead of the conventional Ruthenium based dyes [9].

They are mainly two kinds of dyes that are used in DSSCs, the first being functional Ruthenium

complexes and the second being metal free organic donor-acceptor (D-A) dyes. They have

attempted to explore more into the latter category and have subsequently been successful in

altogether skipping the arduous and expensive process of Ruthenium based dye synthesis and

obtained a high efficiency of 9%.

The DSH team hopes to replicate this to some extent in the near future to broaden our spectrum

of optimal efficiency to simultaneously achieve a cheaper product.

Flexible Substrates: Along with practicing the manufacturing of cells using FTO, many papers

were scrutinized for manufacturing flexible substrates. The ITO-PET substrate with Al2O3

blocking layer method below was compiled as a method to potentially create a flexible substrate

with a blocking layer. “The nanocrystalline TiO2 layers used in DSSC devices often contain

small holes that allow direct contact between the electrolyte and the conducting electrode and

result in the charge leakage. In order to prevent the carriage leak-age, a blocking layer has been

used between the con-ducting electrode and the nanocrystalline TiO2 layer.” [10]

This process can be used to create a flexible substrate DSSC using Indium Tin Oxide coated

Polyethylene Terephthalate (ITO-PET) as the substrate. This method involves using the ‘Lift off

process’ in order to create the TiO2 coat on the ITO-PET. The TiO2 cannot be applied straight to

the ITO-PET substrate due to the danger of melting and is, hence, sintered on top of a thin gold

sheet resting on a glass substrate, which is then lifted off of the gold sheet and placed on the

ITO-PET substrate. [11]

Upon illumination, electrons are injected into the conduction band of the monocrystalline oxide

film (in the case of TiO2) and are then absorbed through and collected by the FTO substrate.

Electrons are then donated to the sensitizer by the I-/I3

- electrolyte. There are many back

recombination pathways that can be followed to complete the circuit. The electrons can combine

with the oxidized dye and with the redox mediator at two locations: at the TiO2|electrolyte and

the FTO|electrolyte interfaces. To prevent any of the above, a thin non-conducting layer can be

applied. This layer is known as the blocking layer.

This method features a Al2O3 blocking layer which is made by dipping the TiO2 deposited

substrate into 7.5 mM Al(BuO)3-iso-propanol solution. The rinsing with DI water and sintering

will cause the hydrolysis of the same to produce a layer of Al2O3. [12]

New ideas and notions are being materialized by the DSH, like multiple layered photoanodes,

high surface area photoanodes. The multiple layered photoanodes and magnetic stirred TiO2 have

been tested to obtain fruitful results. In later days, the DSH hopes to come up with more such

ideas and apply them to our research to ultimately reach our goal of optimizing the efficiency of

DSSCs

References

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molecular engineering of porphyrin sensitizers.” Nature Chemistry. Feb. 2014. Accessed 5 Oct.

2015. Web

http://www.nature.com/nchem/journal/v6/n3/full/nchem.1861.html

[5] Figure 1 "Best Research-Cell Efficiencies" by NREL - US Department of Energy.

http://www.nrel.gov/ncpv/images/efficiency_chart.jpg

[6] “the House.” The Drexel Smart House. Drexel University.

http://www.drexelsmarthouse.com

[7] M. R. Jones (Original Work) [Public domain]. Wikimedia Commons.

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394-412.

[9] A. Mishra, M. K. R. Fischer and P. Bauerle, “Metal-Free Organic Dyes for Dye-Sensitized

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Volume 48. Issue 14. 23 Mar. 2009. Pg 2474-2499.

[10] L. Meng and C. Li. “Blocking Layer Effect on Dye-Sensitized Solar Cells Assembled with

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2011. Web 19 Oct. 2015.

[11] T. Yagamuchi et al. “Highly efficient plastic-substrate dye-sensitized solar cells with

validated conversion efficiency of 7.6%.” Solar Energy Materials and Solar Cells. 26 Dec. 2009.

Issue 94. Pg 812-816.

[12] S. A. Haque et al. “Flexible Dye-Sensitized Nanocrystalline Semiconductor Solar Cells”

Center of Electronic Materials and Devices, Department of Chemistry, Imperial College of

Science Technology and Medicine. Chemical Communications. Issue 24. 21, Oct. 2003.