Syntheses Of Novel Fullerene Derivatives Toward Applications As Interfacial Layer Of Solar Cells

Fullerene C60 has good electron accepting ability and its derivatives have attracted extensive attention in the fields of polymer solar cells as acceptors and perovskite solar cells as interfacial materials.Applications of pure C60 in solar cells is limited because of its poor solubility in common organic solvents and unmatched energy level.This can be solved by chemical modification of fullerenes via different reactions,because a lot of C=C double bonds distributed on the surface of fullerenes enabling similar reactivity of fullerenes to that of olefins.Furthermore,the attachment of functional groups can not only improve the solubility of fullerene derivatives,but also tune their LUMO levels,thus fullerene derivatives are more suitbale for photovoltaic devices.In this dissertation,aiming to extending the applications of fullerene materials in solar cells,we focused on designing and synthesizing a series of novel fullerene derivatives which were applied as interfacial layers of either polymer solar cells or perovskite solar cells,and carried out the following works:(1)We functionalized C60 with ethanolamine(ETA)via a feasible N-H nucleophilic addition.The ETA group has excellent solubility in alcohols,thus,unlike the alcohol-insoluble C60,the obtained ETA-functionalized fullerene(C60-ETA)is soluble in alcohols.The chemical structure of C60-ETA was studied by FT-IR,XPS spectroscopies and element analysis,and the average molecular formula of C60-ETA is estimated to be C60(NHC2H4OH)8(H)8.By incorporating C60-ETA as electron transport layer(ETL)into bulk heterojunction inverted polymer solar cell(BHJ-iPSC)devices based on different active layer systems including PTB7-Th:PC71BM,PBDTTT-C:PC71BM and P3HT:PC61BM,the best power conversion efficiencies(PCEs)reach 9.55%,6.50%and 4.18%,respectively,which are all higher than those of the corresponding reference devices based on ZnO ETL,indicating that C60-ETA rivals popularly used ZnO in terms of ETL performance.The PCE enhancement primarily originated from the increases of Jsc,which is attributed to the smoothened ETL surface and the increase of electron mobility,facilitating electron transport from the active layer to ITO cathode.(2)Based on the excellent performance of C60-ETA as electron transport layer,we further functionalized C60 with aminoethylene glycol bearing different length of alkoxy chain via a similar N-H nucleophilic addition,and synthesized two novel alcohol-soluble fullerene derivatives with different ethylene glycol units,namely C60-PEGA4 and C6o-PEGA1000.We then applied both ethylene glycol-functionalized C60 derivatives as ETL of BHJ-iPSC devices,based on PTB7-Th:PC7,BM affording the best PCEs of 9.25%and 8.62%for C60-PEGA4 and C60-PEGA1000,respectively,and the former one was better than that of the devices based on ZnO ETL whereas the latter one was comparable to ZnO ETL.The PCE enhancement of C60-PEGA4 device was originated from the higher electron mobility and more effective electron transport between acceptor and ITO cathode.(3)We synthesized a novel pyridine-functionalized fullerene derivative(C60-MPy)via a classic Prato reaction,which has good solubility in chlorobenzene.The molecular structure of C60-MPy was studied by 1H NMR and MALDI-TOF mass spectrum.The LUMO level of C60-MPy(-3.70 eV)calculated from cyclic voltammetry data was slightly higher than that of PC61BM(-3.71 eV).We then applied C60-MPy as ETL of p-i-n perovskite solar cells substituting PC61BM,and obtained a best PCE of 16.3%which was higher than that based on PC61BM ETL.The XPS spectroscopic results indicated that the pyridine group of C60-MPy can coordinate with the Pb in CH3NH3PbI3 perovskite.Meanwhile,the incorporation of C60-MPy ETL is beneficial to decrease the roughness of the underneath perovskite film,consequently C60-MPy could help to inhibit the charge recombination at the interface of cathode.Moreover,similar to the case of PC61BM ETL,the devices based on C60-MPy ETL showed no obvious J-V hysteresis.