Organic Solar cells are low cost energy production. Organic cells were recently become of great activeness for photovoltaic applications, because of their potential to use high output. Organic solar cells will take to low cost electricity output. Organic photovoltaics (OPV) are low cost cell production with the aim of organic and inorganic material, these cells are Hybrid solar cells. The material preparing and execution characterization steps were generally successful. Hybrid solar cells have the potency to accomplish high power conversion efficiencies (PCE), in hybrid solar cells inorganic material used as the electron acceptor, peculiarly the electronic structure is important to the performance of the device. One multi-celled slide with partial practically fabricated and had open circuit voltages. Full I-V curve was measured for a silicon reference cell. It exists an best electronic structure design for inorganic acceptor. Four major materials have been examined, state cadmium compounds, metal oxide nanoparticles, silicon and low band gap nanoparticles. Presently, cadmium Sulfide (CdS) quantum dots stand for the state of the art, giving up of greater than 4%. This report gives the reader with a brief synthesis of the present state of the art for bulk heterojunction (BHJ) organic-in organic hybrid solar cells. The effect of hardening process on the performance of photovoltaic devices based on BHJ of poly (3-hexy1thiophene) (P3HT) and PCBM. The result indicates P3HT in the compounds film step by step gets an ordered structure with hardening.
Solar cells were invented in 1950s and first exploit in the 1960s for use in space programs. Since then there have been rapid advances in the ratio and dependability of these cells. As a result the photovoltaic industry has been growing rapidly. Solar photovoltaic power generators, also known as solar cells, it converts sunlight directly into electricity. Organic photovoltaic (PVs) is same as that of Inorganic PVs in the external behaviour, the mechanism by which the voltage and current are generated is quite various. PV material is not crystalline, so there are not good bands for the electrons, nor is there an electric field to propulsion them. Organic solar cells (OSCs) as compared to inorganic solar cells (silicon based solar cells) use organic semiconducting substances with better and tuneable properties. For silicon-based solar cells the plastic solar cells appear as a promising cost-effective option. The important asset of these cells includes ease of processing, mechanical flexibility and low cost of fabrication. Still, the efficiency of the plastic solar cell is too low for practical use at present. For photovoltaic conversion Bulk organic heterojunction solar cells based on polymers have become great interest, due to their altering efficiencies and to their ability of being processed by relatively low cost ways. BHJ solar cells acquired by blending conducting polymer (donor) and nanoparticles (acceptor) within a bulk (G.Yu et al 1995, J. GAO et al 1995;), output photovoltaic power conversion efficiency (PCE) up to 4-5% ( L.S.Roman et al 1997; G.Li et al 2005). In BHJ cells the cells from regioregular poly (P3HT) as the electron donor and (PCBM) as the electron acceptors have shown the highest conversion efficiency.
PCBM P3HT Solar cells
The above figures are P3HT and PCBM. P3HT poly (3-hexylthiophene) is Donor and PCBM-[6, 6]-phenyl--butyric acid methyl ester is Acceptor. These both chemical structures used in polymer/fullerene bulk heterojunction (BHJ).
Working principles of OSCs:
The basic principle of operation of OSCs, are as follows:
Photon absorption, which leads to the formation of coulumbically bound electron-hole pairs (excitons),
Exciton, by the electrostatic Coulomb force exciton is attracted to each other of an electron and hole in a bound state. It can transport energy without transporting net electric charge, and it is regarded as an elementary excitation of condensed matter.
Fig (a) Fig (b)
Fig (a) is the design of a BHJ OSC and Fig (b) is Operation process in an OSC.
For absorbing light we use solar cells in photoactive layer, we can found in all solar cells but with different panels. The photoactive layer is made of different materials. Inorganic layers are built from inorganic materials such as Si. The film has a layer arrangement of counter electrode and electrodes with organic layers in between the two, and the layer is found beneath the glass, reflection coating and adhesive.
An electron in the highest occupied molecular orbital (HOMO) of an organic material absorbs a photon and is excited into the lowest unoccupied molecular orbital (LUMO) by creating an exciton.
Characterisation of OSC device:
The PCE is given from the following relationship;
Jsc is the short circuit current density (current divided by area)
Voc is the open circuit voltage
Jm and Vm are the current density and voltage at the maximum power point and FF is the device's fill factor which is related to the quality of the device.
To fabricate an organic solar cell and test it, i.e., obtain its electrical properties. The device will be fabricated and tested in a nitrogen (N2) filled glove box with conditions of 0.1 ppm O2 and H2O.
The device will be tested by measuring the I-V (current- voltage) characteristics in the dark and under light with an illumination intensity of 1 sun (100 mW/).
Indium tin oxide (ITO) coated glass substrate (25mm*25mm)
Poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate), PEDOT: PSS
Active layer solution - a blend of donor P3HT, and acceptor PCBM in the ratio of 1:1 (12.5 mg of each dissolved in 1mL of dichlorobenzene, ODCB)
Aluminium wire for cathode deposition
Keithley 2400 source meter
Auto 500 deposition system
Fig.1. Flat energy band diagrams for BHJ (a) PB:PCBM. (b) P: PCBM and (c) PB: P: PCBM ternary BHJ. Δφ denotes the offset energy between the HOMO of electron donor (PB or P) and the LUMO of the electron acceptor (PCBM).
EXPERIMENTAL :( Sample preparation and device fabrication)
To determine the ITO coated side (conductive) of the glass substrate we use a multimeter in resistance mode. The resistance should measure less than 20â„¦ in a conductive side.
The conducting side of the ITO glass by using a multimeter to measure resistance. The conducting side will have a resistance in the order of 20-50 ohms. In this case the display reads a voltage of 4.00v and 0.08 Amp when in contact.
In ITO side facing up in a glass breaker, the glass substrate is cleaned in deionised water, isopropanol and acetone, sequentially in an ultrasonic bath. We need to dry cleaned substrate on a hot plate at a temperature of -120C for 10 minutes, and we should keep that in to the glove box.
Hole transporting layer (PEDOT: PSS) deposition
We need to filter Ì´1.5mL of PEDOT: PSS solution using 0.45µm filter into a glass phial.
In the glove box, we need to secure the substrate on the spin coater, which has been programmed to operate in two stages. i.e. stage 1. Run at 350 rpm (for 15 seconds), followed by stage 2. Run at 5,000 rpm (for 20 seconds). For the entire area of the substrate use pipette Ì´1mL of the filtered PEDOT: PSS solution and spin to obtain a PEDOT: PSS film of thickness Ì´40 - 60nm.
Next we need to place the PEDOT: PSS coated substrate on a hot plate and broil for 10 minutes at a temperature of Ì´120 minutes.
Active layer (P3HT: PCBM of 1:1 wt % ratio) deposition
We have to secure the substrate coated with PEDOT: PSS from the above method from 2.c on the spin coater, because we are using for the purpose of this deposition will be programmed to operate in two stages, i.e., Stage 1: 350 rpm ( for 15 seconds), followed by Stage 2: 900 rpm ( for 20 seconds). Using a micropipettor, pipette Ì´150µL of active layer solution on to the substrate, and spin.
After doing the above process (i.e., 3.a) we need to take off substrate from spin coater, place in a desiccators, and leave it to dry.
Once it gets dry, by using a cotton bud dabbed in dichlorobenzene, wipe a strip of the active layer off, for electrode contact.
Now transfer the spin coated substrate to the other section of the glove box where cathode deposition will be done.
Next the auto 500 electrode deposition system should have been switched on, warmed up and system ready for deposition. In the deposition chamber about 4 or 5 aluminium wire ( Ì´ 15 mm long) are mounted on the filament.
In the active area of the completed devices, the substrate coated with the active is loaded on to the shadow mask.
In the deposition chamber the shadow mask with the substrate is mounted in the deposition system and the chamber door is closed, and subsequently pumped down to fine vacuum.
Now start deposition at a vacuum of Ì´ mbar.
It's the important step we need to be very attention, monitor the developing thickness of the aluminium coating via the film thickness monitor, and stop deposition when desired thickness is reached. Now at last allow completed device to cool down in the chamber.
Take out the fabricated device from the deposition chamber of the auto 500, and mount on to the testing board.
Now estimate (or measure) the I-V characteristics using the Keithley 2400 source mater controlled by the LabVIEW programme(Lab Tracer), in the dark, and under illumination by placing the device under a light source(solar simulator) which has been calibrated to 100mW/, using a Si-refernce cell.
Results and Discussions:
In this thesis we characterize an ideal figure for organic materials in a solar cell. Many organic materials turnout with high fluorescent and demonstrate that non- radiative recombination is fewer of a problem than in inorganic materials. Between absorber and collector the large interface area and a aftermath of the small exciton scattering length would be steep with inorganic materials and it is because of increased non-radiative recombination at compound states. In future solar energy will be an important part of a credible energy, only if current challenges can be overcome. In atmospheric conditions advance solar technology will be fairly stable. Basic characterization was successfully developed and the process of spin coating on various layers and evaporating a custom pattern of electrodes. One cell is partial functionality was fabricated and had open circuit voltages. Now we should get a cell working then try it to optimize its performance to at least some decent level. To advice speed the assuming process a mechanical support could be machined that would create electrical connections by simply placing the slide in the holder. The data can be analysed in added detail by using Lab View analysis software. Finally OPV with a grapheme electrode would be expressive achievement and should be aim in future work.