Organic/Polymer Solar Cells: Active Layer Structure, Morphology And Photovoltaic Performance

The economic growth in many parts of the world during the past decade was able to be sustained because of the support of energy. The dependence on oil and electricity has made energy a vital component of our everyday needs. As the energy demand for oil and gas and the attendant global warming from the fossil fuel greenhouse gases continue to build, it is becoming clear that we have to seek environmentally clean alternative energy resources. One major option that is at our disposal to tackle the energy problem is the development of renewable energy. Renewable energy can be tapped from the available resources: hydroelectric resource, from all tides & ocean currents, geothermal integrated over all of the land area, globally extractable wind power, and solar energy striking the earth. Among these options solar energy stands out as the most viable choice to meet our energy demand. Harvesting energy directly from the sunlight using photovoltaic (PV) technology is being widely recognized as an effective ways to utilize solar energy.Until very recently, the development of solar photovoltaic systems has been essentially related to inorganic semiconductors, in particular polycrystalline silicon. While the maximum yields approach 25%, the high cost of fabrication of the raw materials yields only limited commercial developments. In parallel, the use of organic semiconductors developed during the 1970s and 1980s was envisaged as a possible route. There are many foreseeable advantages in commercializing organic and polymer-based PV systems, including ease of fabrication and manipulation, flexibility, low weight and low cost. The superior material properties of organic and polymers (plastics) combined with a large number of cheap processing techniques has made organic/polymer based PV cells attract great attention in recent years.Organic and polymer solar cells compose of anode, cathode and active layer (donor and acceptor). The active layer is a key part of the solar cell where the process of converting light into electric current is accomplished by four consecutive steps: (1) Absorption of a photo leading to the formation of an excited state, the electron-hole pair (exciton). (2) Exciton diffusion to a region, where (3) the charge separation occurs. (4) Finally the charge transport to the anode (holes) and cathode (electrons). All these four steps depend on the structure and morphology of the active layer. Meanwhile, the light harvesting, energy levels and molecular structure of the materials play an important role in determining the morphology of the active layer and the device performance. Consequently, the molecular structure, the morphology of active layer and the device performance are closely related, indivisible.In this thesis, we have mainly discussed how the molecular structures of materials, the structure and morphology of active layer affect the photovoltaic performance of solar cells. We prepared planar-diffused photovoltaic devices by a simple solution process, and investigated the relationship between the structure of active layer and the device performance; We introduced a symmetrical D-π-A-π-D organic small molecule DADP as donor material, acceptor material and sensitizer to bulk heterojunction (BHJ) solar cells, and investigated the relationship between the molecular structure of DADP, morphology of active layer and the device performance; Three D?A alternating copolymers consisting of a same electron-accepting unit coupled to different electron-donating units were used as donor materials for photovoltaic application. We investigate the effect of molecular structure on the morphology of active layer and device performance. More details are listed below,1. A planar-diffused photovoltaic device based on MEH-PPV and PCBM has been successfully prepared by a simple solution process, in which the PCBM organic solution was spin-coated onto the underlying MEH-PPV layer to fabricate the MEH-PPV/PCBM planar-diffused active layer. Investigation of the effects of solvents and active layer thicknesses on the performance of planar-diffused photovoltaic devices indicates that, the degree of PCBM penetrating to the underlying MEH-PPV layer and the surface roughness of the active layer are affected by the solvents, in which the PCBM was dissolved. Good power conversion efficiency is obtained for the device prepared by spin-coating nonaromatic chloroform solution of PCBM onto MEH-PPV layer, rather than device prepared by spin-coating the mixed solution of chloroform and chlorobenzene or aromatic chlorobenzene solution of PCBM onto MEH-PPV layer. With increasing the underlying MEH-PPV layer thickness, a gradual transition from absolutely penetrated to planar-diffused active layer structure occurs, and the best photovoltaic performance was obtained from the device where the active layer thickness and the degree of PCBM penetrating to the underlying MEH-PPV layer coordinate well. Moreover, it is found that the lower open circuit voltage is due to the formation of continuous PCBM phase between cathode and anode.2. (a) A symmetrical D-π-A-π-D organic small molecule DADP as donor material and PCBM as acceptor material were used for bulk heterojunction (BHJ) solar cells. The optimized device exhibits an open-circuit voltage around 1V and power conversion efficiency of 1.5% under the illumination of AM 1.5G simulated solar light (100mW/cmP2P). Thermal annealing treatment on the DADP:PCBM solar cell almost has no influence on the improvement of device efficiency. (b) BHJ solar cells based on DADP as acceptor material and P3HT as donor material were fabricated and its efficiency can increase from 0.093% to 0.64% upon heating the device at 120°C for 10 min. The improvement of the efficiency is attributed to the occurrence of vertical phase separation of P3HT and DADP in the active layer during the thermal treatment, which results in a perfect DADP-top and P3HT-bottom bilayer structure. (c) Dye-sensitized BHJ solar cells were fabricated using DADP as a sensitizer mixed with a blend of P3HT and PCBM. The open-circuit voltage is almost constant but the short-current density increases greatly, as compared to a device based on P3HT and PCBM without thermal treatment. Although the bad miscibility of P3HT and DADP leads to a significant phase separation in P3HT:DADP:PCBM ternary blend films, the efficiency still reach 1.14% under the illumination of AM 1.5 simulated solar light (100mW/cm3. Three D?A alternating copolymers, PFTMT, PPTMT and PDTTMT, consisting of a same electron-accepting group coupled to different electron-donating groups, have been used for photovoltaic application. The absorption, molecular geometry and electronic structure could be tuned through attaching different electron-donating groups to the polymer bones. These three polymers were blended with PCBM to serve as active layers for BHJ solar cells. The effects of extrinsic (blend ratio and solvent) and intrinsic factors (donor materials) on the morphologies of this series of active layers were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The results highlight the crucial role of polymer-PCBM interactions in determining the final film morphology, that is, films with large-scale phase segregation are favored by strong polymer-PCBM repulsion. Moreover, extrinsic factors involved in device fabrication such as the blend ratio and solvent also play important roles in optimizing the morphology of the active layer for use in organic solar cells.