Polythiophene derivatives solar cells and effect of charge transport layer on enhancement of power conversion efficiency

In the last a few decades, polymer solar cells have aroused great interest within both academic and industrial organizations due to their great potential applications in sustainable energy systems. In comparison to silicon-based solar systems, polymer solar cells offer advantages of low cost, light weight, high flexibility and room temperature processability. However, they have not been commercialized up to date due to their relatively low power conversion efficiency and short operation lifetime. It is essential to explore and investigate new materials, unique architectures, optimized fabrication process and fundamentals for greatly improvement of the device power conversion efficiency and long life time for practical applications. The PhD dissertation presented here focuses on exploring new thiophene-based polymers for solar cell applications and investigation of the charge-transporting layer in polymer solar cell devices to enhance power conversion efficiency. Polymer photovoltaic solar cells using poly (2, 6-bis (3-alkylthiophen-2-yl) dithieno-[3, 2-b;2', 3’-d] thiophene) (PBTDT), a new P type polymer with a very high hole mobility, as the donor is demonstrated. The UV absorption and band structure show that PBTDT is a promising donor material for fabricating high power conversion efficiency polymer solar cells. By adjusting the ratio of PBTDT to PCBM and optimizing the annealing temperature, the optimized PBTDT based polymer solar cells shows a PCE of 0.42% under 100 mW/cm2 AM1.5G simulated sunlight. Poly(2,6-bis(3-dodecylthiophen-2-yl)-N-alkyl-dithieno[3,2-b:2’,3’-d]pyrrol e) (PDTPBT) with two different side chain lengths as donors and PCBM as the acceptor are also investigated in polymer solar cells. After optimization with thermal treatments, the device made from PDTPBT with a longer alkyl side chain (Pl) delivers a power conversion efficiency (PCE) of 1.06%, which is much higher than that made from PDTPBT with a shorter side chain (0.26%). Atomic force microscopy (AFM) and modeling results show that the polymer with longer side chain produces much better uniform nanostructure with less pinhole than the polymer with a short side chain, resulting in lower interfacial resistance for higher short circuit current and reduced cathode penetration into the active layer for lower charge recombination rate, and further leading to higher power conversion efficiency. An insoluble polymer, poly(2,6-bis(thiophene-2-yl)-3,5-ditridecyl-dithieno [3,2-b;2’,3’-d]thiophene) (PBTDTT-13) is used to fabricate PBTDTT-13: PCBM solar devices, demonstrating a power conversion efficiency of 1.82%, which indicates that an insoluble conjugated polymer can be used in polymer solar cells. The microscopy images of the PBTDTT-13 films reveal that the insoluble polymer has micro-size particles in the film, and a mechanism based on the large particles structure is proposed to explain the behavior of PBTDTT-13: PCBM solar devices. To investigate the effect of charge transporting layer, V2O5 nano-belt structure is synthesized with a solution process and further used as an anode buffer layer in polymer solar cells, resulting in significantly improved power conversion efficiency (PCE of2.71%) much higher than that of devices without the buffer layer (PCE of 0.14%) or with V2O5 powder as the buffer layer (1.08%).XRD result indicates that the V2O5 nano-belt structure has better phase separation in the P3HT: PCBM active layer to enhance short circuit current. The measured impedance spectrum shows that the V2O5 nano-belt structure has faster charge transport than the powder material. This work clearly demonstrates that V2O5 nano-belt has great potential as a substitute of the conventionally used PEDOT-PSS buffer layer for high performance devices. The effect of PEDOT-PSS charge-transporting layer is studied by solvent treatment. Dimethylformamide (DMF), an organic solvent was used to treat the PEDOT-PSS layer in P3HT: PCBM polymer solar cells, resulting in significant enhancement of photocurrent and PCE improved by 70%. Analyses of I-V characteristics reveal that the change in the active layer rather than that of the PEDOT-PSS buffer layer is ascribed to performance improvement. AFM images indicate that the roughness of PEDOT-PSS layer has been increased after the treatment. I argue that the protrudent PEDOT-PSS could serve as the centers for an initial crystallization of P3HT chains leading to alignment of P3HT: PCBM domains for greatly enhanced photocurrent. In summary, this work involved an investigation of the application of new thiophene-based polymers in solar cell devices and provided guidelines on the future synthesis of polymer for solar cell applications. In addition, it involved the study of the function of the charge-transporting layer to research mechanisms to enhance power conversion efficiency and obtain a thorough understanding of the charge-transporting layer.​Doctor of Philosophy (SCBE