Studies On The Performance Of Polymer Solar Cells Using PC₇₁BM Isomers And Formulation

Solar energy is one of the most promising solutions to energy issues we face today.Although the dominant photovoltaic technology is mainly based on inorganic materials,their wide applications are limited by the high cost of materials and manufacturing process.Because of development of low-cost photovoltaic technology,polymer solar cells(PSCs)are the promising devices for next generation solar energy.PSCs are composed of polymers donors and fullerene acceptors,which can be fabricated by solution processing technology.In recent years,with the rapid development of PSCs,their power-conversion efficiencies(PCEs)have been improved to 15%.Based on this,in order to commercialize,high power-conversion efficiency,good stability,and roll-to-roll fabrication technology are the leading research directions of PSCs.So far,the electron acceptor of bulk heterojunction(BHJ)PSCs has been dominated by C70 derivative[6,6]-phenyl-C71-butyric acid methyl ester(PC71BM),owing to its high electron mobility,large absorption coefficient in the visible region,and good ability to form favorable nanoscale network with various electron donors.However,fullerenes,especially high fullerenes,are usually a mixture of isomers.The coexistence of various isomers in fullerene derivatives will undoubtedly have an effect on the aggregation of fullerene molecules,thus changing its charge separation and electron transportation,finally leading to different performance.In addition,different structures of fullerene isomers can also influence their film-forming properties with donors,thereby affecting the performance and stability of devices.Therefore,obtaining accurate structure of the fullerene isomers can clarify the effect of isomer structure on photovoltaic performance and device stability,which is the basis for the design and synthesis of novel fullerene acceptor for high performence PSCs.In this thesis,three PC71BM isomers were isolated by high performance liquid chromatography(HPLC)and mixed with different donors to fabricate polymer solar cells.The relationship between the isomers structure and their photovoltaic performance as well as stability was studied.By adjusting the proportion of the three isomers,the synergistic effect of the three isomers was researched and the optimal devices were obtained.Firstly,three typical isomers of PC71BM(?-,?1-and ?2-PC71BM)have been separated and the isomer-dependent photovoltaic performance on change of ternary composition of them were established.The photovoltaic performance-composition triangular chart was drawn and the best power conversion efficiency of 7.67%can be achieved by a ratio of ?:?1:?2=8:1:1 for the PSCs with PTB7:PC71BM as photoactive layer(PTB7 =poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]).Even these three typical PC71BM isomers are sharing similar light absorption and LUMO energy level,they show starkly different photovoltaic performances.The average-performing PCE is between 1.28%and 7.44%because diverse self-aggregation phenomena are found in individual or mixed PC71BM isomers.Compare with an amorphous polymer PTB7 with narrow band gap,a crystalline polymer P3HT with wide band gap has different properties of blending and film-forming.Herein,we show a contour plot with different formulations of isomeric PC71BM as electron acceptors in poly(3-hexylthiophene)(P3HT):PC71BM polymer solar cells.The PCEs is changed between 0.87%and 4.01%upon altered fullerene formulation and the best PCE as high as 4.01%can be got by optimized formulation consisting of?80 wt%a-PC71BM,?10 wt%?1-PC71BM,and?10 wt%?2-PC71BM.This is because the optimum morphology of P3HT-fullerene heterojunction film can be achieved in this formulation.Therefore,the formulation of fullerene-based electron acceptors can seriously affect the photovoltaic performance of polymer solar cells and controlling morphology of PC71BM isomers is a promising strategy to improve the PCE of P3HT-based PSCs.As one of the research field of polymer solar cells,stability is the crucial to realize the commercial application.Mixing different fullerene isomers as electron acceptors with crystalline polymer P3HT and amorphous polymer PTB7,the relationship between the molecular structure of fullerene isomers with the thermal stability and photo-stability of the device were investigated.There are no rearrangement of fullerene molecules of suitable fullerene structure can be found in any kind of polymer under continuous heating,which have brought about good film morphology and stable device performance.The morphology of mix-PC71BM blend films and the thermal stability of the devices are determined by the maximum acceptor isomers,but each of these acceptor has influence on thermal stability more or less.Meanwhile,the photo-stability of the device is related to the chemical structure of polymer donor.The photo-stability of the devices using crystalline polymer is better than that of the amorphous polymer based devices.