Study On Heterojunction Solar Cells Fabricated By Novel Structure

Our society is based on coal,oil and natural gas,but these fossil fuels will be depleted someday in the future because they are limited.Carbon dioxide is produced in the combusition has affected the consequence of climate,resulting in the global warming effect.Under these circumstances,interest in photovoltaic solar cells is increasing rapidly as an alternative and clean energy source.Photovoltaic solar cells provide clean electrical energy because the solar energy is directly converted into electrical energy without emitting carbon dioxide.The solar energy is not limited,free of charge and distributed uniformly to all human beings. Up to now, the photovoltaic cells based on inorganic materials (mainly silicon) have been proved to convert sunlight to electricity efficiently. But the high cost for manufacture limits them to be widely used. Organic and polymer solar cells, based on organic and polymer materials as active layer, possess the advantage of light weight, plenty of choices for active layer, flexibility, low cost and easily being large-scale etc., which attracts great attention in recent years.Under illumination, light is converted to electricity by polymer heterojunction solar cells through five processes sequentially, (i) photons are absorbed by the active layer and exitons form; (ii) excitons diffuse in the active layer; (iii) charge transfer when excitons reach donor/acceptor interface and electron-hole (e-h) geminate pair forms; (iv) e-h geminate pairs dissociate with field assisted and free charge carriers are produced; (v) finally, the free carriers are transported through their respective phases to the electrodes in order to be extracted. The photovoltaic performance of devices is strongly dependent on the light harvesting, energy levels of materials, the morphology of thin films, buffers near electrodes, and so on.This thesis is mainly discussing how the above factors such as device structure, morphology of thin films photovoltaic performance of organic/inorganic thin films heterojunction solar cells and inverted bulk-heterojunction solar cells. We used different polymer materials and organic small molecular CuPc materials, and investigated the relation between absorption spectrum of materials and device performance; We doped polyethylene glycol(PEG) of different concentration in TiO2 sol and investigated the relation between morphology of thin films and device performance. We evaporated CuPc as a buffer near gold electrode, and investigated the relation between the thickness of CuPc and device performance. More details are now listed below,1. For organic/inorganic thin film heterojunction solar cells, we used sol-gel method to fabricate nano crystalline TiO2 thin films by sintering as electron acceptor and used P3HT, water soluble PEDOT as electron donor. We make a research on exciton creating mechanism, electron-hole dissociation, charge collection, energy level matching of materials and so on. We compared the photovoltaic performance of different materials, and optimized the thin film thickness.We used different solvent ,including to Chlorobenzene, CHCl3, Xylence, to dissolve P3HT, and fabricated the solar cells. It is found that the performance of the device with P3HT dissolved with Chlorobenzene is better than that of two. AFM is used to compare the surface morphology, and the film of P3HT dissolved with Chlorobenzene is smoother than that of two.2. We doped polyethylene glycol(PEG) of different concentration in TiO2 sol, after sintering, SEM and AFM are used to observe the surface morphology. The thin film is porous.The contact area between TiO2 thin film and P3HT is bigger than that of without PEG, so there are more exciton can be generated and separated in the interface. The photocurrent and power conversion efficiency was improved .3. We fabricated the photovoltaic device of sandwich structure. CuPc is a kind of P type small material, which is applied widely in organic light emitting diodes. a new type of hybrid nanocrystalline solar cells was fabricated. The device structure is indium tin oxide (ITO)/ titanium oxide (TiO2)/ copper phthalocyanine (CuPc) / poly (3-hexylthiophene) (P3HT) /Au. In this architecture, TiO2 was designed to act as the electron acceptor, and P3HT was the electron donor. CuPc was used as a sensitizer to enhance the photon absorption. The results showed that by inserting CuPc layer between P3HT and TiO2 layers, the light absorption, excitons separation and photocurrent under white light were dramatically enhanced. The device has a short current density(JSC) of 1.15 mA/cm2 and power conversion efficiency(PCE) of 0.28% without CuPc layer. However, JSC and PCE turn to be 2.22mA/cm2 and 0.66% respectively with a 20nm thickness CuPc layer under white light illumination with an intensity of 100 mW/cm2.The performance improvement can be attributed to the higher carrier mobility and the stronger photon absorption using CuPc layer.4. A new type of inverted polymer solar cells was fabricated. The device structure was indium tin oxide(ITO)/ titanium dioxide (TiO2)/ regioregular poly-(3-hexylthiophene)(RR-P3HT): [6,6]-phenyl C60 butyric acid methyl ester (PCBM)/ copper phthalocyanine (CuPc)/Au. In this architecture, TiO2 was designed as an electron selective layer. CuPc, known as a sort of P type semiconductor material, was inserted between the active layer and top electrode to improve hole collection and transport. Without CuPc,short-circuit current of 4.22 mA/cm2, open-circuit voltage of 0.48V, filling factor is 0.48 and power conversion efficiency of 0.97% were obtained. Using an optimized CuPc layer of 10nm thickness, short-circuit current of 5.86 mA/cm2, open-circuit voltage of 0.54 V, filling factor is 0.53 and power conversion efficiency of 1.65% were obtained.