Synthesis And Photovoltaic Properties Of D-A Type Polymers

In recent years, polymer solar cells (PSCs) have attracted great attention and become a hot research topic due to their advantages, such as light-weight, large-area fabrication compatibility, low cost and mechanical flexibility. Because of limiting factors, for example, the low open-circuit voltage (Voc), low carrier mobility, mismatch between the absorption spectral response of the photoactive layer and the solar spectrum, there is a certain distance from the actual application for PSCs. Introducing of donor-acceptor (D-A) conjugated polymer photovoltaic materials can decrease the band-gap, enhance the absorption of infrared and near infrared light, which contributes to improve the power conversion efficiency (PCE) of photovoltaic device. Moreover, ladder-type polycyclic aromatic possesses several merits including deep-lying the highest occupied molecular orbital (HOMO) energy levels and high hole mobilities. If they can be chosen to prepare D-A conjugated polymers, the corresponding photovoltaic device may enhance the Voc and the short-circuit current density (Jsc) as well as PCE. This paper focuses on design and synthesis, characterization and photovoltaic properties research of dinaphtho-s-indacene derived D-A polymers.In Chapter 1, the progresses of PSCs are reviewed.In Chapter 2, a dinaphtho-s-indacene-based electron-donating monomer (Bo-DNI) is designed and synthesized, and is copolymerized with four electron-withdrawing monomers, including 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole, 4,7-bis(5-bromothiophen-2-yl)-5,6-bis(butoxy)benzothiadiazole, 5,6-difluoro-4,7-di(5-bromo-2-thienyl)-2,1,3-benzothiadiazole and 5,8-bis(5-bromothiophen-2-yl)-2,3-diphenylquinoxaline, respectively, to obtain a series of conjugated polymers (PF11, PF12, PF13 and PF14) via Suzuki coupling reaction. 1H NMR,13C NMR and high resolution mass spectra are recorded, and elemental analyser is used, to characterize the obtained products and determine their structure. The molecular weights and polydispersity indices of the polymers are measured by gel permeation chromatography. The optical and electrochemical properties of the target polymers are measured by UV/vis spectrophotometer and electrochemical work station, respectively. The results reveal that all four copolymers showed deep HOMO levels in the range of -5.59～5.72 eV, and optical band gaps of them are 1.96,2.00,2.02 and 2.09 eV, respectively. All these results indicate that the four synthesized polymers are promising candidates for photoactive materials employed in PSCs.In Chapter 3, the prepared dinaphtho-s-indacene derived copolymers are used as p-type semiconducting materials, and are mixed with 6,6-phenyl-C71-butyric acid methyl ester (PC71BM) to fabricate bulk heterojunction polymer photovoltaic devices. The weight ratio between polymer and PC71BM as well as the thickness of photoactive layer were finely tuned in order to optimize photovoltaic properties of the devices. Solar simulator system, atomic force microscopy and the space-charge-limited current model are employed to study relationships between structures of the target polymers and photovoltaic properties of the corresponding solar cells. The measurements show that the thin films fabricated by PF11:PC71BM and PF13:PC71BM bear higher hole mobilities, which are 5.14×10-5 and 1.45×10-4 cm2/(V/s), respectively. BHJ photovoltaic devices derived from these polymers have a relatively good stabilities and high Voc. In particular, the photovoltaic device based on PFll:PC71BM (1:4) exhibits the best PCE of 3.07%, with a high Voc of 0.99 V, a Jsc of 7.85 mA/cm2 and an fill factor of 39.5%.In Chapters 4 and 5, preliminary research about the synthesis and photovoltaic application of another two novel 4H-cy c lop enta[2,1-b:3,4-b’]dithiophene and indenofluorene-based electron-donating monomers are carried out.In Chapter 6, the main research results of this dissertation are summarized.