Vol.1 No.4 2012 Scientific Research Journal of India 63

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All-Oxide Solar Cells: The Way of the Future

Akshay Vijay Dongarwar*

Abstract: We as a world are looking at our globe depleting of its natural

resources. The quantity of coal presently available can lead us through for twenty

more years at maximum considering the growing demand for high quality coal

and natural resources and to suffice the growing population and bettering

lifestyle. Again, on one side we have cut throat technological advancement in the

silicon valley and the mobile world and on other, we have fairly advanced

technologies for bringing in better, faster, more efficient and cheaper solutions to

the environmental concerns. The question is basically inspired from this ever

daunting situation. Can’t we have a cheap and highly effective solar energy

treatment plant which can actually reach poor countries and help them get over

their energy crisis without undergoing high-end processing in posh labs like is

done for silicon cells? Even in one of the fastest growing economies of world,

India, silicon processing is not done by any industry commercially to make solar

cells. All the pre-processed cells are imported and further distributed because of

the complexity in the process. Also, being cheap and easily available, it must have

a huge life like silicon cells have. So, it should possess the best of silicon while

eliminating the negatives. Can we find an alternative to conventional solar cells

that can reach out to everyone?

Keywords: All Oxide Solar Cell

THE QUESTION We as a world are looking at our globe

depleting of its natural resources. The

quantity of coal presently available can

lead us through for twenty more years at

maximum considering the growing

demand for high quality coal and natural

resources and to suffice the growing

population and bettering lifestyle. Again,

on one side we have cut throat

technological advancement in the silicon

valley and the mobile world and on other,

we have fairly advanced technologies for

bringing in better, faster, more efficient

and cheaper solutions to the

environmental concerns. The question is

basically inspired from this ever daunting

situation.

Can’t we have a cheap and highly

effective solar energy treatment plant

which can actually reach poor countries

and help them get over their energy crisis

without undergoing high-end processing

in posh labs like is done for silicon cells?

Even in one of the fastest growing

economies of world, India, silicon

processing is not done by any industry

commercially to make solar cells. All the

pre-processed cells are imported and

further distributed because of the

complexity in the process. Also, being

cheap and easily available, it must have a

huge life like silicon cells have. So, it

should possess the best of silicon while

eliminating the negatives. Can we find an

alternative to conventional solar cells that

can reach out to everyone?

HYPOTHESIS

A cavity of metal m2 (W2) with thin

polish of metal m1 (W1, W1<W2) on

inner surface, with a pin hole is kept at the

focus of the solar concentrator coinciding

the pinhole and focus. Pinhole is covered

with transparent glass to protect inner

polish of cavity from atmospheric reaction.

Such cavity behaves as metal-metal

junction solar cell (termed M-M cavity

solar cell).

But using nanowires and nanotubes

increases the functionality further as

diffraction light rays occurs. Again, using

metal oxide makes further sense as they

are chemically under thermodynamic

equilibrium. Another approach is used

which is of titanium dioxide for photo-

sensitization.

RESEARCH

The main challenge with producing a solar

cell with whole new materials is the

availability of photo sensitive materials

and their production. I had prepared a

project for the prestigious “KVPY”

scholarship, where I tried to theoretically

explain the use of metal-metal junction

cavity cell for emitting electrons. The

same research is used here, but with some

changes to make it further effective and to

eliminate short-comings. Here, I present

an all-oxide solar cell fabricated from

vertically oriented zinc oxide nanowires

and cuprous oxide nanoparticles. It

consists of vertically oriented n-type zinc

oxide nanowires, surrounded by a film

constructed from p-type cuprous oxide

nanoparticles. The idea behind using

metal oxides is to eliminate the effects of

atmosphere. Oxides being benign, are safe

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from environmental contamination. The

use of cuprous oxide as solar cells is a

very well studied since last 20 years.

Adding another metal-oxide film seemed

difficult at first as the oxides are already

in stable states and to make use of metal-

metal junction films, we had to change the

physical properties to excite them. But,

with the knowledge of photo-electricity

(diffusion) that I had acquired in the

recent months made me think a step

further and the idea of using nanowires

and nano-particles that respond better to

incident light seemed possible.

In the second part, I used titanium dioxide,

another successful oxide to take in the

solar light and convert them into

electricity (Research done by Dr. M.

Graetzel ). The cell was not taken as it

was. I just used pure titanium dioxide dust

here as polyphyrine derivatives. I did not

use dyes as is done in Graetzel cell but

instead let the oxide in white colour. Its

property of reflecting back visible range

light was later used and sorted out with

design. Being from an engineering

background, I designed a model, that

could make use of both these oxide films

efficiently and expected to get a desired

output of >12% efficiency.

EXPERIMENT

Zn oxide film preparation:

5 mM solution of zinc acetate dihydrate in

absolute ethanol was prepared. Two drops

of this solution were placed onto an

indium tin oxide (ITO) coated glass

substrate (Thin Film Devices, ~40-50

Ω/square). The substrate was then rinsed

with absolute ethanol and blown dry with

nitrogen. The dropcasting, rinsing and

drying was repeated four times per

substrate. The substrates were then

annealed in air at 350°C for 30 minutes,

converting the Zn(OAc)2 into ZnO, and

then cooled to room temperature. This

process was then repeated a second time

to ensure a conformal layer of ZnO.

The nanowires were then grown by

placing the seeded substrate in an aqueous

solution containing 25 mM zinc nitrate

hexahydrate, 25 mM

hexamethylenetetraamine, and 5 mM

polyethyleneimine at 90°C. The substrate

was suspended upside-down to prevent

any larger ZnO aggregates from

accumulating on the surface. Typical

growth times ranged from 30-60 minutes,

yielding wires that averaged from 400-

1000 nm in length and 30-50 nm in

diameter. After the growth, the nanowire

arrays were rinsed thoroughly with

deionized water, then annealed at 400°C

for 30 minutes to remove any residual

organics on the nano wire surface.

The Cu2O nanoparticles (NPs) were

prepared as follows:

A solution of copper (I) acetate (0.5 g),

trioctylamine (15 mL) and oleic acid (Alfa

Aesar, 99%, 4 mL) was flushed with

nitrogen, then rapidly heated to 180°C

under nitrogen flow. The solution was

maintained at this temperature for 1 hour,

then was quickly increased to 270°C and

held for one additional hour, ultimately

producing a burgundy colloidal solution,

which are metallic copper nanoparticles.

The solution was cooled to room

temperature, at which point absolute

ethanol was added to precipitate the

nanoparticles. The supernatant was

removed and the nanoparticles were

redispersed in hexane and then exposed to

air. After 12 hours, the burgundy solution

turned into deep green, indicating the

oxidation of the copper nanoparticles into

Cu2O. The Cu2O nanoparticles underwent

further cleaning by repeated precipitation

with ethanol. Finally, the nanoparticles

were dispersed in toluene for dropcasting

onto the ZnO nanowire arrays.

The processing required no posh research

labs and could be done without much

efforts.

The titanium oxide film is prepared the

usual Graetzel cell way. Except, we do not

use dye. The main motto was to simplify

the process. Dying induces lot of

complexity and we want the process to

remain easy.

DATA

The complete experiment was done by

using the available technologies at

disposal. Instead of using the paraboloid

sun-tracking reflector concentrator, a fine

beam of SODIUM VAPOUR LAMP was

used to create a similar effect. The metal-

metal oxide junction solar cell and the

titanium oxide cell were tested over a long

period of time to get accurate readings.

The cuprous oxide-zinc oxide junction

cells were studied first as they formed the

key research. A fine layer of the junction

nanoparticles was taken and placed in a

small glass box. The glass was designed in

such a way that it didnt let the incident

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light get out and caused multiple internal

reflections, thereby reproducing the effect

as we see in the model. The input currents

and output currents were first measured

for a silicon solar cell of known efficiency.

It gave the total losses caused due to

atmosphere and other resistances in the

wire. Considering the same,the silicon

solar cell was replaced by the meta-metal

junction cell. Calculating the output

currents for same input current given t

sodium vapour lamp and subtracting the

effects of losses previously calculated, the

efficiency was calculated to be

The details of the experiment are given as

follows

There were mainly 2 methods employed

to double check the results

1) V-A meter, where voltage of input was

noted and then the output current. Thus

the power of cell was measured.

2) A solar cell's energy conversion

efficiency (eff), is the percentage of

incident light energy that actually ends up

as electric power. This is calculated at the

maximum power point, Pm, divided by

the input light irradiance (E, in W/m2)

under standard test conditions (STC) and

thesurface area of the solar cell (Ac in m2).

eff=P/EA

Similar procedure was carried out for

Titanium dioxide cell.

The net efficiency was found out as

12.2374%

OBSERVATION

The observations of the experiment that I

performed are listed below

For the Metal-metal oxide junction cell:

Sr No

Voltage (V)

Output Current (mA)

1 11.5 100

2 10.6 99

3 11.4 100

Max power point 1.146 w

Light irradiance 1000 w/m^2

Area is 12*8cm^2 or 0.0096 m^2

Efficiency=11.9374

For the titanium dioxide cell

Sr No

Voltage (V)

Output current (mA)

1 10.2 100

2 10.3 100

3 10.2 99

Max power point 1.0243

Light irradiance 1000w/m^2

Area is 12*8 cm^2 or 0.0096 m^2

Efficiency= 10.6697

Now, we observe that the efficiency of the

proposed cells with the given design

comes out to be quite more than that of the

silicon cells. Thus, one coupling the cells,

the efficiency will increase further.

Here, an interesting trend observed is that

the maximum power point doesn't change

much for a considerable change of input

voltage in case of metal-metal oxide

junction cells. The reason is unknown.

CONCLUSION

Thus, as the results showcase, using some

of the most common oxides and some

simple primary treatment processes

coupled with engineering ideas, we were

able to increase the efficiency of solar

energy harnessing devices by an

outstanding ~6-7% (results show 4.3% but

that is under lab conditions).

Thus, the basic idea of trying to use the

metal oxides arising from a simple urge to

use environmentally inert materials turns

out to be a revolutionary alternative for

the conventional silicon solar cells. The

trait that make the idea highly successful

is that the processing is very easy and can

be done on a commercial level with some

material engineering guidance. Also, it

turns out to be a relief for countries like

India and other developing countries as

importing silicon cells was never cheap.

Hence, here, with technologically

advanced institutes in the nation like IITs

and NITs the implementation and

bettering the scope of the idea can be done.

A major issue was designing.

• How could we make most of the

sunlight. The answer came with

the paraboloid concentrator.

• How could we use it at all times

during the day? The secret lied

with the solar tracking device

which had become pretty common.

• How would we place the cells to

get output from both? The design

came to me by instincts. After a

host of designs, the most suitable

and easy to construct was used.

• Titanium di-oxide reflects back the

visible light. I offered a solution in

the design.

• At some places, the solar energy is

directly used for heating purposes.

Thus a band filter can be employed

to filter out the harmful ultraviolet

and infra-red light.

CORRESPONDENCE:

*29, Nelco Housing Society, Near Nagarjuna Trust Hospital, Khamla-Nagpur-440025. Contact- +91

9175017645, Email-id: adongarwar@gmail.com