Functionalization Of Graphitic Carbon Nitride Toward Applications In Polymer Solar Cells And Photocatalysis

Polymer solar cells?PSCs?,as a promising renewable energy source featuring low-cost manufacturing,light-weight,high flexibility and easy roll-to-roll fabrication,have become an extremely popular option for energy production.Graphitic carbon nitride?g-C₃N₄?has been commonly used as photocatalyst with promising applications in visible-light photocatalytic water splitting.However,rare studies were reported in applying g-C₃N₄ in polymer solar cells.In this dissertation,we focused on functionalization of g-C₃N₄ toward applications in bulk heterojunction?BHJ?polymer solar cells and enhancing its photocatalytic activity,and carried out the following works:?1?For the first time we successfully applied g-C₃N₄ quantum dots?QDs?in bulk heterojunction polymer solar cells?BHJ-PSCs?toward efficiency enhancement,demonstrating the novel application of g-C₃N₄ in energy conversion other than the commonly used photocatalyst.In order to improve the solubility of C₃N₄ in o-dichlorobenzene solvent,solution-processable C₃N₄ QDs were prepared by acid treatment of bulk g-C₃N₄ followed by a solvothermal treatment.By doping C₃N₄ QDs in the P3HT:PC61BM,PBDTTT-C:PC₇₁BM or PTB7-Th:PC₇₁BM active layer,power conversion efficiency?PCEs?of the corresponding inverted BHJ-PSC devices reach 4.23%,6.36%and 9.18%,which are enhanced by?17.5%,11.6%and 11.8%respectively compared to those of the reference?undoped?devices.The PCE enhancement of the C₃N₄ QDs doped BHJ-PSC device is found to be primarily attributed to the increase of JSc.On the basis of the study of the effects of C₃N₄ QDs on the surface morphology,optical absorption and PL properties of the active layer film as well as the charge transport properties of the device,the mechanism of C₃N₄ QDs doping on the efficiency enhancement of the BHJ-PSC devices is proposed with the conjunct effects of the improved interfacial contact and the improved charge?hole and electron?transport.?2?In order to extend the application of g-C₃N₄ in polymer solar cells,we synthesized another type of C₃N₄ QDs with good solubility in polar solvent dimethylformamide?DMF?via changing the solvent used in solvothermal method.We then applied C₃N₄ QDs in BHJ-PSCs by modifying the ZnO electron transport layer,and the inverted BHJ-PSC devices based on PBDTTT-C:PC₇₁BM,PTB7:PC₇₁BM and PTB7-Th:PC₇₁BM active layer systems exhibited PCEs of 7.03%,8.47%and 9.29%,which were enhanced by 20.0%,12.2%and 11.1%respectively compared with the unmodified ZnO-based devices,.The efficiency enhancement was mainly.attributed to the increase of Jsc.On the basis of a series of studies of the effects C₃N₄ QDs on ZnO film,such as AFM,UPS and electron mobility,we proposed that modification of C₃N₄ QDs resulted in the improvement of electron transport and decreased work function of ZnO.?3?As another two-dimentional nanomaterials anologous to g-C₃N₄,graphenes have been extensively applied in polymer solar cells,and most reports were focused on applying graphenes as hole transport layers.In order to substitute the commonly used PEDOT:PSS hole transport layer which is acidic and may cause corrosion of the bottom transparent electrode,we developed a facile method to modify graphene oxide?GO?to improve its conductivity.We first modified GO by simply spin-coating a phosphorus oxide atop of the GO film,which can be applied as an effective hole transport layer as a PEDOT:PSS alternative,and the PCEs of P-GO HTL-based inverted BHJ-PSC devices reached 7.85%,6.56%and 3.75%for PTB7:PC₇₁BM,PBDTTT-C:PC₇₁BM and P3HT:PC61BM active layer systems,respectively.AFM morphology and water contact angle measurements demonstrated that the P-GO film was favorable for interfacial contact bewteen active layer and HTL.Ultraviolet photoelectron spectroscopy?UPS?,X-ray photoelectron spectroscopy?XPS?and Raman spectra revealed that phosphorus doping leads to a p-doping effect and consequently the increase of the work function of GO,affording an Ohmic contact between the HTL and active layer.As a result,the fill factor?FF?and open circuit voltage?Voc?were increased,contributing to the improvement of the performance of PSCs.?4?Due to the relatively large band gap of g-C₃N₄ and the existence of contact resistance between its nanosheets,the visible-light photocatalytic activity of g-C₃N₄ is limited.In order to improve the charge separation efficiency and conductivity of g-C₃N₄,we developed a facile method to functionalize g-C₃N₄ via covalently attaching fullerene C₆₀.We first synthesized a covalently bonded g-C₃N₄/C₆₀ hybrid by ball-milling g-C₃N₄ and C₆₀.The hybrid structure of g-C₃N₄/C₆₀ is confirmed by a series of characterizations including FTIR,Raman,XPS and XRD,and a possible conformation of g-C₃N₄/C₆₀ hybrid is proposed,featuring the direct bonding of C₆₀ onto the edges of g-C₃N₄ nanosheets via a four-membered ring of azetidine.The g-C₃N₄/C₆₀ hybrid was applied for H2 production from water splitting under visible light??>420 nm?in the presence of 5%triethanolamine as a hole scavenger,and a H2 production rate of 266 ?mol·h-1g-1 is obtained without using any noble metal cocatalyst such as Pt,which is about 4.0 times higher than that for the pristine g-C₃N₄ photocatalyst?67 ?mol·h-1g-1?.Upon covalent attachment of C₆₀,the improved photocatalytic activity of g-C₃N₄/C₆₀ hybrid photocatalyst is attributed to the conjunct effects of the enhanced absorption intensity in the vis-NIR region due to the decrease of the bandgap of g-C₃N₄,the facilitated separation of electron-hole pairs and photoinduced electron transfer from g-C₃N₄ to C₆₀.