Fabrication and Study of Photovoltaic Devices using Macrocyclic Organic Semiconductors

Abstract

During the last decade, different structures of photovoltaic (PV) cells fabricated from organic semiconductors have drawn tremendous economic and scientific interest due to their high optical absorption efficiency, low fabrication cost, lightweight, high mechanical flexibility and continuous growth of their power conversion efficiencies. Bulk heterojunction organic solar cells can be fabricated by simple processing techniques, such as, screen printing, spin casting, etc., and, therefore, are potential candidates for the mass production of flexible and cost-effective devices. In this dissertation, based on the soluble macrocyclic organic semiconductors 5,10,15,20- tetraphenyl-21H,23H-porphine zinc (ZnTPP) and copper (II) tetrakis (4-cumylphenoxy) phthalocyanine (Tc-CuPc) bulk heterjunction structures of ITO/PEDOT:PSS/ZnTPP:PCBM/Al and ITO/PEDOT:PSS/Tc-CuPc:PCBM/Al were fabricated employing spin casting and vacuum thermal evaporation techniques. The effect of donor to acceptor (D:A) mass ratio was investigated on photovoltaic properties of the ZnTPP:PCBM BHJ solar cell and the optimum D:A ratio was identified. Effect of the thickness and surface morphology of the active layer on the photovoltaic properties of this porphyrin-fullerne BHJ was also studied and the optimum active layer thickness was identified. Bulk and hybrid-bilayer heterojunctions of copper (II) tetrakis (4- cumylphenoxy) phthalocyanine (Tc-CuPc) and vanadyl 2,9,16,23-tetraphenoxy- 29H,31H-phthalocyanine (VOPcPhO) heterojunctions were also fabricated. Temperature dependent electrical properties of these devices and optical performance of the Tc- CuPc:PCBM bulk heterojunction was also investigated. Macrocyclic semiconductors e.g. metallo-phthalocyanines (MPcs) and metallo- porphyrins (MPPs), are restricted to dry processing techniques due to their insolubility in common organic solvents. Thus MPcs and MPPs are used in typical thermally evaporated donor-acceptor bi-layered solar cells. The performance of bi-layer solar cells is low due to small exciton diffusion length; only the excitons generated within 10 nm from the D/A interface are expected to contribute to photocurrent. This issue is resolved in BHJ architecture, in which the D-A interfacial area is distributed throughout the volume (bulk) viiiof the active layer. In other words the D-A interface is brought near to the exciton generation site. Soluble versions of MPcs and MPPs can be helpful to study their opto- electronic characteristics in BHJ solar cell architecture. In BHJ architecture there is a lot of flexibility and ease for rapid research e.g. changing solvent, varying concentration, using different D to A mass ratios, changing active layer thicknesses by spin coating at different speeds, incorporating different exciton-blocking layers etc. A lot of variations can be studied in a very short time and with less cost. However, as more and more research groups start studying soluble macrocyclic organic semiconductors in BHJ solar cells, further efficiency improvements and availability of new soluble macrocyclic materials are worthwhile. Since the microstructure of bulk heterojunction layer is strongly dependent on the donor to acceptor (D:A) mass ratio, an optimum D:A ratio improves device performance by improving the charge separation, transport and collection process. As one aspect of this research work, porphyrin-fullerne BHJ solar cells with ITO/PEDOT:PSS/ZnTPP:PCBM/Al structure were fabricated with different D:A mass ratios and their optical parameters were measured both under simulated solar spectrum AM 1.5G and monochromatic illuminations. The active layer thicknesses were extracted from their respective optical reflection and transmission measurements using average absorption coefficient. Scanning electron microscope (SEM) image of the device cross section was studied to verify layer thicknesses and examine the quality of layers. Effect of thermal annealing on performance of some devices was also studied. Incident photon- to-current conversion efficiency (IPCE) spectra at different D:A ratios was determined. On basis of the measured optical parameters such as fill factor FF, open circuit voltage V OC , short circuit current density J SC , and power conversion efficiency η , the optimum D:A ratio was identified. Maximum IPCE value of 21% was obtained for D:A ratio of 1:9 which lead to PCE of 0.21% which was 36 times better than a previously reported value in which C 60 was used as acceptor. Also incorporating PCBM as the acceptor instead of C 60 improved the open circuit voltage (V OC ) for all the D:A ratios. The reason for high V OC is greater difference between the ZnTPP HOMO and PCBM LUMO . Furthermore the 1:9 devices showed consistency in optical parameters when reproduced.