Design,Synthesis And Photovoltaic Performance Of Porphyrin And Organic Sensitizers

In this work,we have designed and synthesized two major sensitizers:porphyrin and organic dyes.The influence of introducing different length of alkoxy at the meso-position,different additional into theπbridge at the meso-orβposition of porphyrins and different auxiliary acceptor at triphenylamine sensitizers on the optical and electrochemical properties and photovoltaic performance of PSCs based on these dyes are systematically investigated.（1）Three novel zinc porphyrin dyes which are featured with a D-π-A structure have been designed and synthesized for use in dye-sensitized solar cells（DSCs）.The influence of different hydrophobic hydrocarbon chains（none,butoxyl,octyloxy groups）attached at the porphyrin meso-aryl group on the molecules’photophysical and electrochemical properties,as well as on the photovoltaic performance of the corresponding DSCs were investigated systematically.The best performance was realized for the cell sensitized with the porphyrin（LP-2）containing a n-butoxyl chain on the meso-aryl group and with the acrylic acid as the acceptor.The optimized power conversion efficiency of LP-2 reaches 6.04%with open-circuit voltage of 730 mV,a short-curcuit current density of 11.67 mA·cm^-2,and a fill factor of 0.71.（2）The introduction of an additional acceptor to a typical donor-πbridge-acceptor（D-π-A）type porphyrin sensitizer results in a D-A-π-A featured porphyrin.Two porphyrins containing an additional acceptor with different electron-withdrawing abilities such as 2,3-diphenylquinoxaline（DPQ）for LP-11 and2,1,3-benzothiadiazole（BTD）for LP-12 between the porphyrin core and the anchoring group have been synthesized for use as sensitizers in dye-sensitized solar cells（DSCs）.Compared to LP-11,LP-12 with the stronger electron-withdrawing additional acceptor BTD possesses better light harvesting properties with regards to red-shifted Q-band absorption and a broader IPCE spectrum,resulting in a greater short circuit photocurrent density(J_sc)output.Interestingly,the steric hindrance of the DPQ group is favorable for suppressing dye aggregation,leading to a larger open-circuit voltage(V_oc)value for LP-11-based cell.The optimized cell based on LP-12 achieves the better performance with a J_sc of 15.51 mA·cm^-2,a V_oc of 674 mV,a fill factor（FF）of 0.7 and an overall power conversion efficiency（PCE）of 7.37%.（3）The β-functionalized porphyrin containing an additional electron-withdrawing unit,2,3-diphenylquinoxaline（DPQ）for LP-5 or2,1,3-benzothiadiazole（BTD）for LP-6 with different electron-withdrawing abilities,between the porphyrin core and the anchoring group and the reference porphyrin dye（LP-4）have been designed and synthesized for DSCs.Compared with LP-4,the introduction of additional electron-deficient unit at the porphyrinβπ-linker in LP-5and LP-6 decreases the lowest unoccupied molecular orbital（LUMO）energy levels,resulting in the broader absorption spectrum and significantly improved IPCE spectrum in the region 350-500 nm,which ensures the better light-harvesting properties and the higher short-circuit current density(J_sc).On the other hand,the introduction of additional acceptors of LP-5 and LP-6 induces dye aggregation and reduces the lifetime of the charge–separated states,which decreases the open–circuit voltage(V_oc).Interestingly,the decreased V_oc was overcompensated by a gain in J_sc,resulting in the superior performance,the optimized PCE of LP-6 reached to 6.14%with a V_oc of 710 mV,a J_sc of 11.47 mA·cm^-2 and a FF of 0.74.（4）Three β-functionalized porphyrin dyes with different auxiliary acceptors（2,1,3-benzoxadiazole（BOD）for LP-13,2,1,3-benzothiadiazole（BTD）for LP-14and 2,1,3-benzoselenadiazole（BSD）for LP-15）between the porphyrin core and the anchoring group have been designed and synthesized for dye-sensitized solar cells（DSCs）.It is found that increasing the electron withdrawing capability decrease the HOMO and LUMO levels and the energy gap,resulting in the little red-shifted absorption spectrum.Compared to LP-13 and LP-15,LP-14 based DSCs showed enhanced IPCE values at 350-650 nm and higher J_sc values due to the relatively higher molar extinction coefficient,and remarkably improved open circuit voltage as the effective suppression of charge of recombination,therefore leading to best photovoltaic performance.The LP-14 based DSC gave the best photovoltaic performance with a J_sc of 12.62 mA·cm^-2,a V_oc of 730 mV,a FF of 0.68 and an overall PCE of 6.25%.（5）Three new D–A–π–A configuration organic dyes（LC-6,LC-7 and LC-8）based on triphenylamine as the electron donors,benzoselenadiazole（BSD）as the auxiliary acceptor,thiophene and benzene as theπspacers and cyanoacetic acid as the anchoring group have been designed and synthesized for dye-sensitized solar cells.Introduction of octyloxy chain at triphenylamine could redshift the absorption spectrum and suppress the dye aggregation.Besides,using the benzene instead of thiophene asπspacers and CDCA coadsorbent could help to improve the cells’J_sc and V_oc.LC-8 based DSC with CDCA gave the best photovoltaic performance with a J_scof 13.21 mA·cm^-2,a V_oc of 734 mV,a FF of 0.69 and an overall PCE of 6.72%.（6）In this work,the BTZ structure is finely tuned by fusing another electron deficient ring to the benzo unit leading to a series of BTZ-based electron-withdrawing heterocyclic rings such as BBT,BTQ and BTT,which together with BTZ are incorporated into theπ-conjugated bridge,respectively,as an auxiliary acceptor that connects the triphenylamine electron donor and the cyanoacrylic acid anchoring group for the construction of D–A–π–A type metal-free organic sensitizers（LC-2,LC-3,LC-4 and LC-5）.Encouragingly the BBT,BTQ and BTT units show greatly increased electron-withdrawing capability.The corresponding sensitizers LC-3,LC-4and LC-5 exhibit broadened spectruml response range and significantly enhanced J_scvalues compared to the reference BTZ-based dye LC-2 due to the decreased HOMO-LUMO energy gap.However,the electrochemical impedance analysis suggests the electron lifetime and charge recombination resistance are decreased due to the introduction of enhanced conjugation and planarity units BBT,BTQ and BTT,resulting in a slight loss of V_oc.The LC-3 based DSCs shows the highest efficiency of8.56%with a short-circuit photocurrent density(J_sc)of 17.83 mA·cm^-2,an open-circuit photovoltage(V_oc)of 683 mV and a fill factor（FF）of 0.70.