lab module 3 of 4: polymer solar cell 2013/polymer... · lab module 3 of 4: polymer solar cell...

LAB MODULE 3 OF 4: POLYMER SOLAR CELL

Fabrication and Characterization of an Organic Photovoltaic Device

Created for the National Science Foundation CCLI Program (Grant No. 0633661, "Lab Teaching Modules on Organic Electronics and Liquid Crystal Displays")

Version 06 March 2010

Created by BL Conover and MJ Escuti North Carolina State University

Department of Electrical and Computer Engineering

Lab Module 3: Polymer Solar Cell Procedure

http://www.ece.ncsu.edu/oleg/wiki/NSF_Lab_Modules NSF CCLI Grant No. 0633661

Abstract In this lab experiment you will fabricate your own version of a Polymer Solar Cell, an

Organic Photovoltaic (OPV) device. You will start with a glass substrate coated with transparent and conductive indium tin oxide (ITO) as the anode, and then spin-cast two polymer layers on top. The polymers will be a blended solution of electron- and hole-transport polymers, PCBM:MDMO-PPV (methano[60]fullerene [6,6]-phenyl C61 butyric acid methyl ester : poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene]) on top of a second hole-transport polymer, PEDOT-PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)). Finally, you will employ a liquid metal (gallium-indium eutectic) on a second ITO-coated substrate to act as the cathode and you will seal the structure with optical adhesive. For characterization, you will measure and observe several aspects, including the current vs. voltage curve, the output created by different input illumination intensities, the power efficiency, and the absorbance spectrum.

Part 1 Part 2 Part 3 Part 4 Polymer Layers Create the Cathode

Substrate and the OPV Cell Device

Characterization Spectral

Characterization

Write-up Instructions Your lab report (Lab Notebook) will minimally consist of the following for each part:

A. Statement of experimental objective B. Sketches of experimental setup C. Record of all measurements D. All requested calculations

The following lab procedure will indicate specifically what to include and where. The purpose of this style of write-up is to force you to keep a technical record of your experiments in the way that many engineers and scientists are required to do (in industry and universities). The lab director(s) will provide you with blank technical notebook sheets in the lab (also available on the website). You are expected to follow the lab notebook guidelines introduced by the lab director(s) (also see the Appendix), and your lab grade will depend both on your experimental procedure and on how well you follow these guidelines. Note that the same pages you use during the lab experiment should also be the ones you complete at home and hand-in as your write-up — do not rewrite them.



A Brief History of Photovoltaics

Due in large part to the recognition of fossil fuels as a dwindling resource, renewable energies—especially photovoltaics (PVs)—have experienced significant annual growth over the past five years. PVs are unique among renewable energy sources in that they require no generators, are customizable by the end-users, and are flexible in terms of fabrication and scalable in terms of electrical power supply. However, PVs are currently limited by their relatively high production costs when compared to fossil fuels and even to other renewable resources.

Organic PV technologies currently proposed promise to decrease production costs while increasing maximum efficiencies. These include dye-sensitized nanostructured oxide solar cells (DSSCs), solid-state bulk-heterojunction devices (BHJs), and organic-inorganic composite devices [1]. Perhaps the most interesting is a BHJ based on a conjugated polymer (P3HT) as the donor and a fullerene (PCBM) as the acceptor. Such is the basis of the device built in this lab experiment.

Current OPV technologies present several advantages beyond lowered production costs including flexible substrates, continuous printing processes (e.g. inkjet), and easy integration into other commercial devices. However, solutions to increase efficiency and lifetime must be found in order to be competitive with inorganic technologies. Figure 1 presents an historical timeline of solar cell efficiencies (all types).

Figure 1. Solar cell efficiency over time [1]



Basic OPV Operation

A diode or rectifying junction (in general) can operate in several regimes. The most important are shown in Fig. 2. Quadrants (Q) I and III are both consuming power, corresponding to operation as a rectifying diode and photodetector, respectively. However, operation in Q IV produces appreciable power (negative power corresponds to generated power) that can drive current in an electrical circuit. The current-voltage curve shown in Fig. 2 is a typical for a solar cell under illumination. Increased illumination shifts the curve in the direction of the arrow.

An OPV cell can take on a bi-layer configuration. The photoactive region of such devices absorbs photons and then separates the resulting exciton and transports the electrons and holes to their respective electrodes, creating current as discussed above. The layout of this cell is depicted in Fig. 3(a). A conjugated polymer (the acceptor) and a fullerene material (the donor) can be utilized to perform the charge transport. Our hole transporter, i.e. the acceptor, will be the conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene] (MDMO-PPV), depicted in Fig. 3(b). This polymer is a variation of PPV, having additional alkyl groups attached to the basic phenylene ring, making it soluble in common solvents. Our fullerene-based material, i.e. the donor, will be methano[60]fullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM), depicted in Fig. 3(c), a soluble derivative of the fullerene C60 that can be spin-cast rather than evaporated. PCBM will be responsible for transporting electrons to the cathode, an ultrafast process with ~100% efficiency. By contrast, diffusion alone has an efficiency of ~10%.

Figure 2. General diode operating regimes (organic and inorganic).

Arrow indicates effect of increasing illumination.

Figure 3. (a) OPV device layout; (b) Chemical structure of MDMO-PPV; (c) Chemical structure of PCBM.

(a) (b) (c)

Dio

de



We will use transparent and conductive indium tin oxide (ITO) on a glass substrate as the anode. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) will be employed as a hole transport layer in order to ease injection of holes from the anode by lowering the intrinsic energy barrier between ITO and the donor material. A liquid metal alloy (gallium-indium (GaIn) eutectic, work function ~4.2 eV) on a second ITO-coated substrate will be the cathode. As described, PCBM and MDMO-PPV can be applied as separate layers. This process, however, tends to limit the exciton creation area to the interface between the materials. A better method is to mix them into a single solution and spin-cast a layer with the combination. This increases the surface area of the active region. A diagram of the resulting mixture is in Fig. 4.

Figure 4. OPV cell with blended PCBM:MDMO-PPV as the active layer [2]. Notice the increased surface area between the two materials in this configuration.

The spectra in Fig. 5(a) show the absorption of PCBM and MDMO-PPV. Recall that lower energies correspond to longer wavelengths and higher energies to shorter wavelengths. Therefore, in Fig. 5(a), the highest absorption for these materials occurs in the near-UV region—a very relevant wavelength regime for solar applications. The energy band diagram of a bi-layer OPV cell under illumination and at short circuit condition is depicted and described in Fig. 5(b).

Figure 5. (a) Visible-UV absorption spectra of [70]PCBM and MDMO-PPV [1]; (b) Energy band diagram of an OPV device under illumination at short circuit condition. Operation is as follows: (1) optical excitation, (2) exciton relaxation, (3) exciton dissociation by charge transfer, (4) charge collection. Excition diffusion toward the electrodes occurs continually.

PCBM

MDMO-PPV (a) (b)

ITO 4.8

eVPE

DOT -

PSS

5.0

eV Ga-In

4.2

eV

+++

-

--

1

2

3

4LUMO 3.7 eV

HOMO 6.1 eV

LUMO 3.0 eV

HOMO 5.3 eV PCBM

MDMO-PPV



Electro-Optical Properties

The electrical characteristics of an OPV are similar to those of an inorganic solar cell. For example, performance is measured under illumination and is based on the following parameters also labeled in the Current Density vs. Bias Voltage curve of Fig. 6: • ISC: short circuit current (density) • VOC: open circuit voltage • VMPP: voltage at maximum power point • IMPP: current (density) at maximum power point From these we can calculate the fill factor, a ratio of the actual maximum obtainable power to the theoretical power:

�

FF = VMPP ⋅ IMPP( ) VOC ⋅ ISC( ).

Using the power density curve, we can begin to characterize our device based on its efficiencies. Using the incident radiometric power density, POPT and the output electrical power density, PELEC , we can calculate the

overall efficiency of the OPV device:

�

η =PELEC( )maxPOPT

=VMPP ⋅ IMPP

POPT=VOC ⋅ ISC ⋅ FF

POPT .

Finally, the quantum yield, or external quantum efficiency (EQE) is the ratio as follows:

�

EQE = # external created charges/sec# incident photons/sec ,

where for monochromatic light,

�

EQE =

IMPPq

⎛ ⎝ ⎜

⎞ ⎠ ⎟

POPThcλ−1

⎛ ⎝ ⎜ ⎞

⎠ ⎟

=IMPP hcPOPTqλ

Figure 6. Example current-voltage curves for an organic solar cell (approximated for PCBM:MDMO-PPV materials).



Construction

In this module, we will fabricate an OPV, with the construction shown in Fig. 7. We will begin with two glass substrates coated with transparent and conductive ITO, one of which will act as the anode. Upon this substrate we will spin-cast two polymer layers, PEDOT-PSS and PCBM:MDMO-PPV. To the other substrate we will apply a small amount of GaIn eutectic, which will act as the cathode. The GaIn will be surrounded by a ring of optical adhesive in order to secure the top substrate to the bottom and to create an air- and water-tight seal.

Figure 7. OPV Structure

Anode [ITO]

Conducting Layer[PEDOT - PSS]

Photon-Absorbing Layer[PCBM:MDMO-PPV]

Substrate [Glass]

Cathode [GaIn]

Sealant /Adhesive

Substrate [Glass]

ConductiveFilm [ITO]



Experimental Procedure

Important Notes:

•The option exists to perform this lab within a Glovebox designed to effectively evacuate harmful vapors, to provide a controlled environment in which to construct devices, and to provide a constant flow of nitrogen when necessary. Such is a good alternative to a fume hood or other controlled environment. •Follow all of the instructions and precautions of your lab director(s) as variations from the procedures below may be in place. •Always hold your samples along the edges and keep the ITO side of the substrates facing up.

Part 1 –Polymer Layers EXPERIMENTAL OBJECTIVE: To apply the two polymer layers of the OPV Cell (PEDOT-PSS and PCBM:MDMO-PPV).

Procedure 1. Prepare your Lab Notebook

a. Fill in your lab notebook headings (Lab #, Station Name, Page #, Name, Date). b. Briefly record the objective of this experiment. c. Sketch the complete cross-section of the OPV we are creating in this lab.

2. Prepare Three Substrates (Two of ITO-Coated Glass and One of Clear Glass):

a. Use a Multimeter (or similar device) in resistance mode to find the conductive side of the ITO-coated glass. The ITO side should measure a resistance of below 1 kΩ.

b. Clean all substrates using an air gun and methanol. c. Transfer one ITO-coated substrate (the anode) and the clear glass substrate to a

hotplate set at 140 °C for ~10 minutes and the other ITO-coated substrate (the cathode) to the worktable.

3. Spin-Cast the Polymer Layers: a. Secure the anode substrate in the spin-caster and, using a syringe, place ~12 drops of

PEDOT-PSS solution onto the center of the substrate. b. Run Program D (see Appendix) unless instructed otherwise. c. When complete, place the substrate on a hotplate (140 °C) to dry for ~15 minutes and

repeat the PEDOT-PSS application with the clear glass substrate. d. Return the anode substrate to the spin-caster and, using a syringe, place ~10 drops of

PCBM:MDMO-PPV solution onto the substrate, attempting to cover the entire substrate.

e. Run Program E (see Appendix) unless instructed otherwise. f. When complete, place the substrate on a hotplate (140 °C) to dry for ~10 minutes and

repeat the PCBM:MDMO-PPV application with the clear glass substrate.



Part 2 — Create the Cathode Substrate and the OPV Cell EXPERIMENTAL OBJECTIVES:

(1) To prepare the cathode substrate consisting of GaIn eutectic and optical adhesive. (2) To adhere the cathode substrate to the anode substrate consisting of the polymer layers.

Procedure 1. Prepare your Lab Notebook as before.

2. Apply the GaIn Eutectic and Optical Adhesive:

a. Using a cotton swab applicator, apply a small quantity of GaIn Eutectic near the corner of the cathode substrate you earlier placed on the worktable.

b. Using the syringe filled with Optical Adhesive, loosely encircle the GaIn spot. c. You should now have a cathode substrate as drawn in Fig. 8 below.

3. Create the OPV Cell: a. Place the anode substrate on the worktable. b. Invert the cathode substrate and gently place it onto the anode substrate with enough

offset to attach alligator clips. c. Apply gentle pressure with a cotton swab applicator to spread the GaIn. d. Using the UV light, cure the optical adhesive for ~1 minute. Be sure to have on your

safety goggles in order to block the UV light from your eyes! e. You should now have a completed and cured OPV cell as drawn in Fig. 9.

GaIn Eutectic

Optical Adhesive

ITO-Coated Glass

Polymer-CoatedSubstrate

GaIn Eutectic

Optical AdhesiveITO-Coated Glass

Figure 8. GaIn and Optical Adhesive

applied to the cathode substrate. Figure 9. Cathode substrate inverted and

sealed onto the anode substrate.



Part 3 — Device Characterization EXPERIMENTAL OBJECTIVE: Characterize the OPV Cell by simultaneously measuring current and voltage while the cell is illuminated with a White LED from varying heights, i.e., varying intensities.

Procedure 1. Prepare your Lab Notebook as before, ensuring to sketch the characterization set-up.

2. Prepare the Multimeters and Connect the OPV Cell to the Characterization Set-Up:

a. Attach the RED clip to the anode substrate and the BLACK to the cathode substrate and place your OPV cell in the mount with the cathode substrate on the bottom.

b. Set one multimeter in parallel to your cell and set it to measure Volts. Set the other multimeter in series with your cell and set it to measure milliAmps.

c. Set the Load (the adjustable resistance box) to 1 MegaOhm.

3. Obtain Current and Voltage vs Load Data: a. Turn on the White LED and align it over the opening in the mount. Adjust the LED

mount so that the LED lens is approximately 15 mm from the OPV cell. b. Following the table below, begin reducing the Load value while recording the

Voltage and Current readings. Never have the resistance box set to 0 Ohms!! c. Repeat these measurements, this time adjusting the LED mount so that the LED lens

is approximately 75 mm from the OPV cell. d. Return the load to 1 MOhm, turn off both multimeters, and remove your OPV cell. e. Determine (measure/estimate) and record the actual area of the GaIn spot – the

effective cathode area. Load (Ohms) Voltage (V) Current (mA or uA)

1 MOhm 500 kOhm 100 kOhm 50 kOhm 25 kOhm 10 kOhm 5 kOhm 1 kOhm

800 Ohms 600 Ohms 400 Ohms 200 Ohms 100 Ohms 75 Ohms 50 Ohms 25 Ohms 10 Ohms 5 Ohms



Calculations/Questions (Part 3) These are to be written in your Lab Notebook.

You may make the following assumptions:

• The Power Density (POPT) of the White LED: 118 mW/cm2 at 15 mm; 36 mW/cm2 at 90 mm

• The White LED emits constant power across its output spectrum (not actually true!).

• The entire cathode area (the area of the applied metal alloy) is uniform, i.e., the thickness of the metal alloy layer is constant and charge carriers experience equal mobility at all points. Why do you think this might not be true?

For both datasets, i.e., both illumination distances, do the following:

1. Calculate and Plot the Current Density (vertical) vs. Voltage (horizontal). Graph both curves on the same plot. Do this on a computer, not freehand, and insert them into your Laboratory Notebook with tape or glue.

2. Calculate and Plot the Electrical Power Density (vertical) vs. Voltage (horizontal). Graph both curves on the same plot. Do this on a computer, not freehand, and insert them into your laboratory notebook.

3. Identify Short Circuit Current Density (IOC) and Open Circuit Voltage (VOC). 4. Determine Maximum Power Point and record the Current Density (IMPP) and Voltage

(VMPP) at that point. 5. Calculate Fill Factor (FF) of the device. 6. Calculate Efficiency (η) of the device. 7. Calculate External Quantum Efficiency (# of charge carriers created per second / # of

photons injected per second) at the maximum power point of the device. Assume a constant spectrum for the White LED from 425 nm to 650 nm.



Part 4 — Spectral Characterization

Note: This portion of the lab is intended to be performed with a portable spectrometer and light source, e.g., JAZ by Ocean Optics, Inc. However, any spectrometer or spectrophotometer may be used to perform these same tasks with minimal modification to the procedure.

EXPERIMENTAL OBJECTIVE: Measure the transmittance of the organic layers with a spectrometer.

Procedure 1. Prepare your Lab Notebook as before, ensuring to sketch the characterization set-up.

2. Prepare the Set-Up:

a. If necessary: Start the spectrometer software, Turn on the spectrometer, Select ‘New Transmission Measurement’, and Perform the required reference measurements.

b. Set the Zoom Range for the Transmission Measurement to Wavelength 400 nm – 800 nm for the x-axis and Transmission 70% - 100% for the y-axis.

c. Ensure that the sample holder is empty and positioned between the input and output optical fibers so that no part of the sample holder is blocking the path of the light.

3. Acquire Transmission Spectra:

a. Place your OPV cell in the sample holder and turn ON the Light Source. b. You should see transmission data similar to Fig. 4. If you do not, try repositioning

your OPV cell and ensure that the light is passing through the center of the cell. c. Save the transmission data to a file. d. Reposition your cell and repeat part (b) twice, saving the transmission data each time. e. Remove your OPV cell and ensure the set-up is ready for the next user.

Calculations/Questions (Part 4) These are to be written in your Lab Notebook.

1. Plot the Absorption (defined as = 1-T) of your organic layers for all datasets. Graph all curves on the same plot. Do this on a computer, not freehand, and insert them into your Laboratory Notebook with tape or glue.

a. Do you notice any major differences among the three datasets? Why or why not? b. Qualitatively discuss what the Absorption data reveals. Be as specific as possible. c. Based on the Absorption data, what can you say about the type or types of light

that would be most efficient in creating power in this OPV cell? 2. Locate and Record the Peak Absorption Wavelength, and the approximate range of

wavelengths that are most absorbed by the organic layers. 3. Research the output spectra of various sources (e.g. sun, lamp) and determine what type

of source will produce the highest external quantum efficiency with your OPV cell.



Appendix A: Lab Notebook Guidelines

1. Do record entries legibly, neatly, and in INK. 2. Do sign and date every page. 3. Do fill in headings completely (Lab #, Station Name, Page #). 4. Do record your experimental objective and describe your experiment. 5. Do record your experimental setup, data, and all calculations in such a way that someone

else could duplicate/verify your steps. 6. Do include extrinsic materials by tape or permanent glue (staples are acceptable but not

preferred). This includes all raw data from recording instruments (e.g., microscope photos), computer generated graphs, drawings, specification sheets, etc.

7. Do work in chronological order, i.e., Do Not skip parts unless specifically told to do so. 8. Do Not erase or remove material. If you mess up, simply cross it out and start again! This

is part of the experimental process. We will provide you with as many sheets as you need.

Appendix B: Spin-Caster Parameters

• The following programs were created on a Laurell Technologies Model WS-40B-6NPP/LITE Spin-Caster. Other spin-casters can be used but the lab director should ensure the parameters result in quality films.

Program D Single Stage: Speed 2000 rpm Acceleration 1870 rpm/second Time 30 seconds Program E Stage One: Speed 400 rpm Acceleration 255 rpm/second Time 10 seconds Stage Two: Speed 800 rpm Acceleration 1105 rpm/second Time 30 seconds



Appendix C: Material Recommendations

• 3 mL Syringes with Luer-Lok Tips, Catalog No. 14-823-41, distributed by Thermo Fisher Scientific, Inc., Pittsburgh, PA, USA.

• 23 Gauge Precision Dispense Tips, Part No. 5123-B-45, distributed by EFD, Inc., East Providence, RI, USA.

• UVS 91 Ultraviolet-Curable Optical Adhesive, distributed by Norland Products, Inc., Cranbury, NJ, USA.

• ITO-Coated Glass Substrates, Part No. CG – 40IN – 0115; 25 x 25 x 1.1 mm unpolished float glass, SiO2 passivated, indium tin oxide coated one surface, RS = 4 – 8 ohms; distributed by Delta Technologies, Ltd., Stillwater, MN, USA. Two substrates are needed for each OLED.

• PEDOT-PSS, Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), Item # 483095, 1.3 wt % dispersion in H2O, conductive grade, distributed by Sigma-Aldrich, Inc., St. Louis, MO, USA. A single 100 gram bottle of PEDOT:PSS is adequate for ~90 OLEDs (assuming 1 mL per substrate and some loss due to filtering). This material should be filtered through a 0.45 µm filter prior to use.

• MDMO-PPV, Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene], Item # 546461, Molecular Weight ~23,000, distributed by Sigma-Aldrich, Inc., St. Louis, MO, USA. A single 1 g bottle of MDMO-PPV will produce enough polymer solution for ~ OPVs.

• PCBM, [6,6]-Phenyl C61 butyric acid methyl ester, Item # 684449, distributed by Sigma-Aldrich, Inc., St. Louis, MO, USA. A single 100 mg bottle of PCBM will produce enough polymer solution for ~ OPVs.

• Dichlorobenzene, Anhydrous, 99%, Item # 240664, distributed by Sigma-Aldrich, Inc., St. Louis, MO, USA. A single 1000 mL bottle of Dichlorobenzene will produce enough polymer solution for ~ OPVs.

• Gallium-Indium, Eutectic: Item # 495425, 99.99+ % trace metals basis, distributed by Sigma-Aldrich, Inc., St. Louis, MO, USA. A single 5 gram bottle of Gallium-Indium is adequate for hundreds of OLEDs when used sparingly as suggested in the procedures.

• 5.0 µm Syringe Filters: Millex-LS 25mm Syringe Driven Filter Unit (PTFE), Catalog # SLLS025NS, distributed by Thermo Fisher Scientific, Inc., Pittsburgh, PA, USA.

• 0.45 µm Syringe Filters: Fisherbrand 25mm Syringe Filter (PTFE), Catalog # 09-719H, distributed by Thermo Fisher Scientific, Inc., Pittsburgh, PA, USA.

• Other Useful Materials Include: Methanol (for cleaning substrates), Acetone (for cleaning spin-caster, etc.), UV Lamp/UV LED Flashlight (for curing UV adhesive), White LED (for illuminating the OPV; white LED flashlights work well), Kimwipes (multitude of lint-free uses), Stirring Hot Plate and Ultrasonic Bath (for polymer preparation), Nitrogen Gun (for cleaning debris from substrates), Activated Charcoal Air Filter (for filtering hazardous solvent fumes), Electrical Tape, 20V Power Supply, Multimeters (for voltage and current measurements), Optical Power Meter, Decade Resistance Box, Assorted Hook-Up Cables, Spectrometer/Spectrophotometer (for Part 4).



Appendix D: Polymer Preparation Instructions

• NOTE: The instructions below prepare ~18 mL of polymer solution, enough for ~15 OPVs (assuming <1 mL of polymer solution per substrate and some loss due to filtering).

• The polymer blend described is intended to be used in conjunction with the spin-caster parameters in Appendix B. The lab director should ensure that alterations to ether the material preparation instructions or to the spin-caster parameters result in quality films and successful device fabrication.

PCBM:MDMO-PPV (3.5:1; 2.0 wt% in Dichlorobenzene) 1. Clean one 20 mL or 40 mL vial (amber or clear).

2. Add 75 mg PCBM to the vial.

3. Add 21.5 mg MDMO-PPV to the vial (ratio of PCMB to MDMO-PPV should be

approximately 3.5:1).

4. Add 4825 mg Dichlorobenzene to the vial (wt % of solids in solvent should be

approximately 2.0 %).

5. Secure the cap to the vial and wrap the seam with electrical tape.

6. Place the vial upright in an ultrasonic bath for 120 – 180 minutes or until well-mixed.

7. Total amount of prepared solution will be ~18 mL.

8. Prior to creating devices, solution should be processed through a 0.45 µm PTFE

syringe filter (it may be necessary to first use a 5.0 µm filter).

Appendix E: Adhesive Material Preparation Instructions

NOTE: A single 1 gram bottle of optical adhesive will be enough for thousands of OPVs. A syringe of the mixture as described below will be adequate for hundreds.

Adhesive 1. Fill a 3 mL syringe 1/4 - 1/3 full of adhesive and remove most of the air with the

plunger.

2. Screw a dispensing tip onto the syringe.

3. Wrap syringe with aluminum foil or electrical tape to prevent penetration by

ultraviolet light.

4. Adhesive will expire on date marked on the original optical adhesive bottle.



Appendix F: Relevant Sources (Books, Links, Papers, etc.)

1. G. Dennler, N.S. Sariciftci, and C.J. Brabec, “Conjugated polymer-based organic solar cells,” in Semiconducting Polymers: Chemistry, Physics and Engineering, 2nd edition, ed. by G. Hadziioannou and G.G. Malliaras, (Wiley-VCH, 2007), pp. 455 – 530.

2. P. Schilinsky, “Loss analysis of the power conversion efficiency of organic bulk heterojunction solar cells,” Ph.D. dissertation, Universität Oldenburg, Oldenburg, Germany, 2005.

3. Y. Gao, “Interface electronic structure and organic photovoltaic devices,” in Organic Photovoltaics: Mechanisms, Materials, and Devices, ed. by S.-S. Sun and N.S. Sariciftci, (CRC Press, 2005), pp. 421 – 451.

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