Research Of Photovoltaic Properties Of Solar Cells Based On Polymer/Inorganic Hybrid System

Recently,organic polymer solar cells have attacted much attention due to their unique optical,electronic,low cost and flexible properties.However,it is difficult for polymer photovoltaics(PV) to realize commercialization resulting from the low efficiency.The limited factors for increasing efficiency are:the bound-energy of exciton is high,the disffusion length of exciton is short,the mobility of carrier is low and charge collection efficiency near the interface of polymer/electrode is uneffective. In this dissertation,we use polymer/inorganic hybrid systems to instead of single polymer systems in this work,and make much reseach on hybrid systems.The main investigating contents included:1 In order to increase dissociation interface of excitons and enhance transport of carriers,we introduce TiO₂-nanotubes to PV device and use the structure of ITO/ poly(3,4- ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS)/polymer:TiO₂/Al for fabricating bulk heterojunction devices.The obtained results are:PV device with the structure of ITO/PEDOT:PSS/poly[2-methoxy-5-(2'-ethylhexoxy-1,4-phenylenevinylene)MEH-PPV:TiO₂-nanotubes/Al.The experimental results show that photovoltaic characteristics of the device are improved with increasing concentration of TiO₂-nanotubes.PV device with the structure of ITO/PEDOT:PSS/MEH-PPV:C₆₀ (+TiO₂-nanotubes)/Al.The performance of device is dependent on the concentration of TiO₂-nanotubes,appropriate concentration of TiO₂-nanotubes can connect moles of C₆₀ which afford more direct band pathway for carrier transport especially electron transport.PV device with the structure of ITO/PEDOT:PSS/Poly(3-hexylthiophene)(P3HT): [6,6]-phenyl C61-butyric acid methyl ester(PCBM)(+TiO₂-nanotubes)/Al.SEM images show that TiO₂-nanotubes arrangement in the film is disordered;vertical arrangement and sideling arrangement of TiO₂-nanotubes afford direct pathways for carrier transport and reduce the numbers of carrier hopping,thus increase the conversion efficiency(η) of the device.Howevr,transverse arrangement of TiO₂ nanotubes afford recombination center for carriers and block the paveways for carrier transport resulting to lowerdηof the device. 2 In order to optimize the route of photogenerated carriers,reduce carrier recombination and enhance collection of electrons at the cathode,we use polymer: fullerence and inorganic materials-TiO₂ or cadmium sulfide(CdS) to fabricate polymer-inorganic hybrid PV device,and investigate the photovoltaic properties of this kind of PV devices.We design the devices with the structure of ITO/PEDOT:PSS/MEH-PPV:C₆₀/ TiO₂/LiF/Al.These devices show promising photovoltaic characteristics with higher I_sc and FF for introducing TiO₂ layer.I_sc reached to 2.35mA/cm²,FF reached to 0.284 andηincreased by 20%under the illumination of 500nm light with the intensity of 16.7mW/cm².Besides,both the thickness of TiO₂ layer and the weight ratios of MEH-PPV:C₆₀ can influence the photovoltaic properties of the device. When the thickness of TiO₂ film is 20nm and weight ratios of MEH-PPV:C₆₀ is 2:1, we can obtain the optimumη.For further clarifying that the increasedηcomes from TiO₂ layer,we also fabricate device with the structure of ITO/PEDOT:PSS/MEH-PPV/ TiO₂/LiF/Al.The results show that the introduction of TiO₂ increases I_sc and FF obviously.ITO/PEDOT:PSS/MEH-PPV:C₆₀/CdS/Al.The device shows a promising I_sc which reached to 4.67mA/cm² andηreached to 5.3%under the illumination of 500nm light with the intensity of 16.7mW/cm².So high I_sc not only come from the change of zone and amount of photogenerated carriers,but also the absorption of CdS in the visible light.The thickness of CdS layer also affectηof the device:the thiner the thickness is,the higher theηis in the range we are interested in.ITO/PEDOT:PSS/ P3HT:PCBM /CdS/Al.The device not only shows higher I_sc andη,but also improved FF because of the change of shunt resistance and series resistance.3 In order to investigating the effect of annealing on photovoltaic properties of solar cells and morphology of polymer.We investigate the influence of annealing on interfacial morphology.Besides,PV device with the structure of ITO/PEDOT:PSS/MEH-PPV:C₆₀/Al is annealed under presence of reverse bias.SEM image of P3HT:PCBM film show that roughness of the film increased by annealing,and the post-annealed device indicated the rougher interface than pre-annealed device.The increased interfacial roughness can enhance photocurrent and efficiency of the device.Under the presence of a reverse bias,the device with the structure of ITO/PEDOT:PSS/MEH-PPV:C₆₀/Al is treated at 120℃.After thermal treatment, I_sc andηof PV devices are improved more than 5 times.Attributing to the polarization in MEH-PPV layer,the resulting devices show improved photovoltaic properties.Additionally,the bias value and time also affect the photovoltaic properties of the device,the highestηcan be obtain when the bias is -6V and the time is 10min.4 We introduce Ag/Al hybrid electrode to the PV device for effective collection of electrons at the cathode.Ag/Al hybrid electrode prevents the interaction happening between Al and the polymer active layer,and play an active role in balancing carrier transport.Thus Ag/Al hybride electrode enhanced the FF of PV device.