ELE 208 - Arvind P. Ravikumar

ELE 208
Electronic and Photonic Devices
Laboratory procedures
Organic solar cells
Spring 2014
Week 1: Substrate preparation, Oxide hydrolysis
Lab instructors: 1. Arvind Ravikumar ([email protected])
2. Sabbir Liakat ([email protected])
In this lab, we would fabricate and measure a bulk heterojunction organic solar cell. You will come across
multiple questions dispersed throughout this manual. As you make your devices, try to answer those
questions on your own and make a record of it in your lab notebook. Your lab grade will also take into
account your answers to these questions. The questions are lettered and starred for your convenience.
You lab notebook should also contain lab observations regarding how your sample looked under an
optical microscope and what, if any, irregularities you noticed.
A typical organic solar cell consist of a transparent electron – Indium tin oxide (ITO, anode)
being the most commonly used material, a buffer layer (TiOx or PEDOT:PSS), the photo-active
layer and a top electrode (cathode), usually gold. Figure 1 gives the typical structure of an
organic solar cell.
P3HT/PCBM
Buffer layer
Indium Tin Oxide
The organic solar cells that we will make today belong to a specific configuration called as
inverted solar cells. Traditionally, the ITO acts as the anode while the top metal contact acts as
the cathode. However, we will make ITO as the cathode contact while a higher work function
metal like gold will act as the anode. Such a configuration has been experimentally proven to be
resistant to atmospheric conditions resulting in a slower rate of degradation.
P3HT/PCBM
TiOx
Indium Tin Oxide
P3HT and PCBM are the components of the active layer that responds to sunlight. In the
simplest of terms, P3HT is an electron donor and PCBM is an electron acceptor. When light with
sufficient energy falls on the active layer, it removes an electron from the P3HT which migrates
to a PCBM molecule. In effect, P3HT has an extra hole and PCBM has an extra electron – an
electron-hole pair, called an exciton. These electrons and holes move to the respective
electrode and conduct current.
In order to start making the solar cell, we would first have to pattern the ITO on glass electrode.
*A) Why would you need to pattern the ITO? In other words, what will happen if you try to operate the
solar cell without patterning the ITO?
1. Sample Clean
a. Take 200 ml of acetone in a clean glass beaker and completely immerse the sample into
it. Ultrasonicate this sample in acetone for 10 minutes.
b. Take 200 ml of IPA in a clean glass beaker and quickly transfer the sample from the
acetone bath. Again, ultrasonicate this for 10 minutes.
c. Blow dry using nitrogen
2. Pattern ITO on glass.
a. Using AZ 5214 as the photoresist, transfer the given mask pattern on to the ITO coated
glass substrate using photolithography. Refer to your common manual for the procedure
on lithography.
*B) You baked the sample on a hot plate at 100 C for 1 minute after spin coating the photoresist. Explain
what will happen if you baked for (i) much less (ii) or much more than 1 minute.
*C) You exposed the sample under UV light for 9 sec and developed the pattern using the developer.
Explain what will happen if the exposure time had been (i) much shorter or (ii) much longer than 40 sec.
*D) Discuss how high humidity will affect the lithography process, and what steps would you take in such
a scenario to get reliable pattern transfer.
3. Wet-etch ITO
a. This step is performed to transfer the pattern from the photoresist on to the ITO. As
detailed in the common manual, use conc. HCl for 3 to 4 minutes to etch the ITO.
Things to note: Make sure the ITO is completely etched along the sides using a multimeter
to check its resistance.
b. Remove the photoresist from the sample and clean the wafer as outlined in Step 1.
You now have in your hands, a patterned ITO coated glass substrate. You would now have to
spin coat the respective organic active layers on to this patterned substrate.
ITO
Lithography
4. Spin-coat Titanium oxide
a. Spin coat the TiOx precursor on the ITO coated glass at 3000 rpm for 60 seconds. Ask
your TAs on how to program the spinners. The TiOx precursor was prepared in a 1 wt%
solution in iso-propyl alcohol (1 ml of the precursor in 120 ml of IPA). The precursor,
Titanium (IV) isopropoxide, was obtained from Sigma Aldrich (Product No. 37799625ML).
b. Leave the sample exposed to the atmosphere for 1 hour. The precursor that you just
spin coated is titanium isopropoxide. Exposing this to the atmosphere for an hour results
in a hydrolysis reaction that converts this compound to titanium oxide, the electron
blocking layer.
Week 2: Active cells, measurements
c. Bake the TiOx sample you spin coated last week on a hot plate at 150 C for 10 minutes.
Ideally, the active solution of Poly (3-hexylthiophene) (P3HT: Sigma Aldrich: Product No.
698997-1G) and a fullerene derivative of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM:
Sigma Aldrich: Product No. 684449-500MG) have to be prepared 12 hours prior to the lab and
left standing overnight in order for the organic compounds to dissolve in the chlorobenzene
solution. We use a 2.4 wt% concentration in 1:1 ratio of P3HT and PCBM (12 mg of each
compound dissolved in 1 ml of Chlorobenzene).
5. Spin coat active material
a. Prior to spin coating the active material on to your sample, heat the P3HT/PCBM mixture
for 2 minutes at 100 C to improve solubility.
b. Using a syringe and a 0.45 µm filter, pour enough active material to cover your sample
completely and spin at 300 rpm for 60 seconds.
c. Bake the sample at 170 C for 1 minute.
The sample is now ready with all the active organic ingredients and you just need a top metal
contact, which will be evaporated using electron-beam deposition. Consult the general manual
for more details on electron beam evaporation.
6. E-beam evaporation
Using the shadow mask given to you, evaporate 50 nm of gold to form the top contact.
You now have in your hand, a (working?) organic solar cell which will be used for further
measurements. It would be interesting to compare the efficiency of this to that of a silicon solar
cell. Your device should now look like this – the ITO will now act as the anode while the top gold
contact will be the cathode.
7. Measurements
Measure the current-voltage (I-V) characteristics of the solar cell you just made under dark and
illuminated conditions. Refer to the silicon solar cell manual for a description of the
measurement and various quantities that you need to get from the I-V curves.
As you take your data, please be sure to note down the following for all the devices that you
measure. You need reliable data from at least 2 devices on your sample.
For the following questions, assume that the power density of the lamp that you use is about
100 W/m2.
*E) What is the short circuit current of your solar cell?
*F) What is the open circuit voltage of your solar cell?
*G) What is maximum power generated by your solar cell?
*H) What is the power conversion efficiency of your solar cell?
*I) Plot the current-voltage curve of your solar cell under dark and illuminated conditions.
*J) Plot the power-voltage curve of your solar cell under dark and illuminated conditions.
*K) A typical silicon solar cell can have an efficiency between 5% and 10%. Compare that to the
efficiency of the organic solar cell that you just measured. Why are the two numbers very different?
*L) In spite of the large difference in efficiency, why do you think organic solar cells are a viable
alternative to silicon solar cells? Give an answer from an economic standpoint as well as a technical
standpoint.

Download Report