1. Project Title: POLYMER: FULLERENE SOLAR CELLS Name: FAIZZWAN FAZIL (101180216) Project supervisor: DR. MUKHZEER MOHAMAD SHAHIMIN School of Microelectronic Engineering, University Malaysia Perlis (UniMAP) 01000, Kangar, Perlis, Malaysia Phone:+604-9798386 Fax:+6049798305

2. INTRODUCTION A lot of development had been made in order to obtain high reliability, green energy source with a reasonable capital cost. By replacing the non-renewable electrical generating source such as fuel, charcoal and nuclear energy, photovoltaic device also known as solar cell has been introduced which is operating to generate and dissociate EHP by harvesting photon shined by the sun. The organic solar cells is totally different compared to inorganic semiconductor solar cells in terms of structure. For the working principal, both are approximately the same. The inorganic solar cells are using the P-N junction with the valance and conduction band while the organic solar cells are using Donor Acceptor with the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital). Organic solar cell also provide one biggest advantage which is it can be fabricate on the flexible substrate. The capital cost also is reasonable compare to the inorganic solar cell. founded with a single active layer, followed by bilayer active layer, and recently, bulk heterojunction active layer (conjugated polymer: fullerene).

3. AIMS AND OBJECTIVES the absorption of photon, exciton (electron-hole pair) diffusion, exciton (electron-hole pair) dissociation, and the electron and hole mobility towards electrodes are the elements that need to be put as main priority are determined. The objectives of my project: is to relate the the relation between the thicknesses obtained using the Atomic Force Microscopy (AFM) after the application of those various spin speed with the absorbance which is obtained by using the UV- Vis Spectroscopy and the PCE obtained using the SPA which provide several reading for PCE evaluation purpose. To study the outcomes of using the Titanium Oxide (TiO2) as the holes blocker and the usage of Indium Tin Oxide (ITO) coated glass as cathode by replacing the Aluminum (Al). To review and study another parameter of the required material has to be fixed for the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT: PSS) solvent deposition, the temperature with annealing duration, and the weight ratio of polymer: fullerene with suitable dilution. To investigate and apply proper solution is required to overcome the failure

4. PROJECT CHALLENGES The active layer material itself is highly sensitive to H20 and O2. It might degrade either during fabrication process or inspection process where the samples are exposed to the mentioned causes. PEDOT: PSS might be dissolved with the H2O left on the ITO surface if the cleaned ITO coated glass is not dried thoroughly. The holes transportation potential of its will be degraded and will affect the performance of power conversion to be weakened. The application of the ITO as a substituent for Al also might result a poor result due to its electron mobility is not as good as Als. The insufficient material of PCBM is the biggest challenge in this experiment. The amount of PCBM purchased is just 1g for a bottle. The price is about RM900 and the shipping of the item takes a lot of time.

5. The Working Principle of Polymer: Fullerene Solar Cells The donor and acceptor material are blended together as an active layer for this device. This morphology will enrich the generated excitons to reach the interface of donor- acceptor if the length of the blend is same as the diffusion length of excitons. The efficiency of electron hole pairs (EHP) might be improved. The excitons in the bulk which are created by the absorption of photon, are everywhere. To observe the dissociation of exciton, we considered the exciton placed close to the interface between Donor and Acceptor. The absorbed exciton reaching the interface of Donor- Acceptor and the electron is separated from the exciton due to the binding energy broken by the electric field occurred in the interface, excited to the LUMO of Donor. The electron transported from LUMO of Donor to the LUMO of Acceptor then diffused and captured by the cathode. The remaining hole will be transported towards anode and captured.

6. Device Structure

7. 1. ITO coated glass :- i) the best choice for the substrate due to its high electrical properties and transparency ii) composed by 90% Indium Oxide (In2O3) and 10% Tin Oxide (SnO2). iii) acts as both electrode in this device. iv) it is providing high refractive index which is N = 2.35046 for ITO layer which mean it could trap incoming light besides being the electrode v) Al has been replaced with the ITO as the anode of this device 2. PEDOT:PSS :- i) the roughness of ITO it can has good contact with polymer in spin coating process so it is used to overcome this problem by smoothing the ITO surface ii) has good hole collecting (efficient electron blocking) iii) has a good transparency to allow light to travel into 3. Polymer: Fullerene (MEH PPV: PCBM) :- i) MEH PPV is one type of conductive polymer ii) act as electron donor (majority charge carrier is electron) iii) PCBM is one type of fullerene iv) act as electron acceptor 4. CdTe (additive material in actve layer) :- i) is in form of powder and can be diluted but it still can be mix together in the solvent ii) providing nano-scale interpenetrating network between the interface of Donor- Acceptor material 5. TiO2 :- i) used as an optical spacer to gain greater light absorption (it has high refractive index) by breaking the symmetry and blocking hole (electron collecting) ii) It is placed between ITO and active layer to comprise phase separated blend.

8. METHODOLOGY

9. Fabrication Process Cleaning and ITO etch Process a piece of ITO coated glass is cut to 100 samples with 2cm x 2cm unit area. all the samples are cleaned using acetone or deconex which mean they are dipped in a beaker filled with that solutions. After 20 to 30 minutes, all the glass substrates are rinsed using DI water and dried using an air pump 3/4 of the ITO layer which means 1.5cmx1.5cm unit area need to be etched The mixture of acid nitric and HCL with volume ratio 1: 3 can be used for this process. The part to be etched is partially dipped and hold in a beaker filled with those solution for 5min to 10min. Then the glass substrate is cleaned using DI water and dried using an air pump.

10. The PEDOT:PSS is coated on the substrate using a Spin Coater with speed of 2500rpm and 20ul a drop. Before that, the 1/8 which is approximately 0.25cm of ITO layer which is located from the right edge of the bottom glass is covered or pasted using a sellotape. The covered area is approximately 0.5cmx0.5cm. After depositing, the sellotape is removed. After finishing that process for all the required sample, all the deposited substrates are baked with 90oC for 5min PEDOT : PSS Deposition

11. Solvents and Paste preparation The weight ratio of MEH PPV:PCBM:CdTe has been determined as 1:4:3 and dissolved using chloroform or chlorobenzene which means 1mg of MEH PPV, 4mg of PCBM and 3mg of CdTe are diluted together in 1ml of chloroform or chlorobenzene. This weight ratio is going to be used in the experimental and is fixed. 20ul each drop. The PEDOT:PSS is already in the form of chemical so it doesnt require a dilution process. The TiO2 is in form of powder so it requires to be diluted. 9ml of acid acetic nitric is required to dissolve 6 gram of TiO2. The process need to be done little by little which means 1ml is dropped into beaker filled with 6g of TiO2 at the same time the mixture is grinded slowly but heavily pressed. This process is continuously done till the 9ml of acid acetic nitric is finished. After that, the complete mixture need to be anneal on the hot plate with 300oC for 10min to20min (until the colour change to brown and back to white)

12. MEH PPV: PCBM Deposition The active layer solvent prepared will be deposited at the varied spin speeds which are 1000rpm, 2000rpm, 3000rpm and 4000rpm. But before that, the area covered with the sellotape again get covered. A drop of active layer solvent which is 20ul is dropped on the substrate. Once it finishes, the sellotape is removed and annealed on the hot plate with 90oC for 5min

13. TiO2 Pasting The TiO2 is pasted on the etched area using a cotton bud and rod. Then annealed with 90oC for 5min. Before annealing process, the applied sellotape need to be removed.

14. Sandwiching with the top substrate the top glass is sandwiching the bottom glass by using the epoxy as a paste

15. Device Characterization the Atomic Force Microscopy (AFM) is used to obtain the thickness and surface roughness of each device. Four pieces of 4cm x 4cm ITO coated need to be etched using the Acid Acetic Nitric. all the substrates are deposited with the prepared active layer solvent using 1000RPM, 2000RPM, 3000RPM and 4000RPM . Spin Speed 1000RPM 2000RPM 3000RPM 4000RPM Thickness of MEH PPV: PCBM 40nm 30nm 20nm 10nm Surface Roughness

16. UV-Visible Evaluation The Lamda UV/Vis Spectrometer is used to observe the relation of the thickness of the active layer with the absorption to the fixed value of the wavelength which is from 250nm to 800nm. 250nm to 375nm is under UV spectrum, 375nm to 745nm is under Visible (Vis) spectrum, and 745nm to 800nm is under Near Infra-Red (NIR) spectrum. The figure shows the electromagnetic spectrum as a reference to evaluate the trend of the Absorbance versus wavelength graph.

17. UV-Visible Graph The graph is evaluated based on the changing of the graphs trend and the value of absorbance (A) and wavelength are obtained via the median position of each trend changing. maximum A is obtained, A=2.35 =300nm (boundary between UV and Vis region) by all devices. 330nm to 530nm, the trend of graph drops (concave-curve-shaped). 4000rpm shows A=0.4 =380nm, 3000rpm shows A=0.35, 2000rpm shows A=0.25, and 1000rpm shows A=0.23. (in visible region) 530nm to 570nm, the trend of graph drops (convex-curve-shaped). 4000rpm shows A=0.3 =550nm, 3000rpm shows A=0.27, 2000rpm shows A=0.24, and 1000rpm shows A=0.22. (in the middle of visible region) 570nm to 800nm, the trend of graph drops slightly. 4000rpm shows A=0.25 =685nm, 3000rpm shows A=0.22, 2000rpm shows A=0.18, and 1000rpm shows A=0.1. (Near Infra Red region) This evaluation shows the 1000rpm device is the best device at trapping the EM wave/ light due to its lowest absorbance

18. I-V Curve Evaluation The Semiconductor Parametric Analyzer (SPA) is used to obtain the electrical characteristic (I-V curve graph) and record the Isc (short cicuit current), Voc (open circuit voltage), Vmax (maximum voltage), Imax (maximum current) and FF (Fill Factor) for PCE (power conversion efficiency) evaluation purpose. the graph cross at the 0 of x-axis which is Voltage axis, the Isc value could be obtained. (refer to figure) the graph cross at the 0 of y-axis which is current axis, the Voc value could be obtained. (refer to figure) The Voc supposedly occur at the positive voltage region of the graph and the Isc supposedly occur at the negative current region of the graph. FF also supposedly obtained as a positive value because the FF indicates the ratio of the maximum power from the solar cells to the product of Voc and Isc.

19. Fill Factor (FF) evaluation: Power Conversion Efficiency:

20. I-V Curve of each device

21. The Recorded and Evaluated results Refer to the graph obtained, obviously the huge error could be observed. The trend of the graph is absolutely different compared to the typical one All the Vocs occur at negative voltage region (left side of the graph) All the Iscs occur at positive current region (upper side of the graph) This is because the solar cells operate in reverse bias The best power conversion efficiency is obtained by 1000rpm device (PCE percentage = 1.2484488 x 10-5) Others sample show worse performance especially the 4000rpm sample. attenuated by the reduced of Donor- Acceptor interface number (thin active layer obtained). The degradation could occur during illumination and in the dark, also could happen during fabrication process. the exposure of the active layer material towards the oxygen and water (H2O) PEDOT: PSS also can be dissolved with water The problem occur during pasting the TiO2 process (easily dry and hard to adhere to the substrate) Spin Speed (RPM) Voc (V) Isc (A/cm2) FF Plight (mW) PCE (%) 1000 (40nm) -32.0000E-3 80.1478E-9 -48.6776E+0 1000 1.2484488 x 10-5 2000 (30nm) -10.0000E-3 48.9684E-9 -251.0371E+0 1000 1.2293 x 10-5 3000 (20nm) -48.0000E-3 135.4012E-9 -14.6055E+0 1000 1.075441 x 10-5 4000 (10nm) -26.0000E-3 19.4816E-12 -121.8237E+0 1000 6.17063 x10 -11

22. Introduction of Carbon as counter electrode (cathode) Carbon was introduced to replace the TiO2 (idea obtained by viewing the application of carbon as counter electrode in dye sensitize solar cells structure)

23. I-V Curve of applied Carbon device (1000rpm)

24. The comparison of upgraded device with the previous devices Higher Voc is obtained Higher Isc is obtained Higher PCE is obtained The graph similar to the typical one The mobility of the electrons toward cathode have been enhanced The potential of the electrons to be captured by cathode also have been improved as well Spin Speed (RPM) Voc (V) Isc (A/cm2) FF Plight (mW) PCE (%) 1000 (40nm) -32.0000E-3 80.1478E-9 -48.6776E+0 1000 1.2484488 x 10-5 2000 (30nm) -10.0000E-3 48.9684E-9 -251.0371E+0 1000 1.2293 x 10-5 3000 (20nm) -48.0000E-3 135.4012E-9 -14.6055E+0 1000 1.075441 x 10-5 4000 (10nm) -26.0000E-3 19.4816E-12 -121.8237E+0 1000 6.17063 x10 -11 1000 (40nm) Carbon 2.2000E-3 180.1765E-6 173.0310E+0 1000 6.8588 x10-3

25. CONCLUSION The target to fabricate the device using CdTe as additive in active layer cant be achieved due to insufficient PCBM. The application of the TiO2 also didnt achieve the target. All the devices applied with the TiO2 degraded. But by using the Carbon as a counter electrode, the mobility and the power conversion efficiency are enhanced. Eventhough the results obtained werent good enough, but still the idea of applying ITO substrate glass as a replacement for Al is worthwhile because of the ability of absorbing the photon from both side of the device. (top and bottom)

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