Theoretical Investigation On The Effects Of Dye Molecules And Interface Properties On DSSC Photovoltaic Performance

Theoretical investigation of dye molecules and interface is very important for enhancing dye sensitized solar cell（DSSC）efficiency.In this work,theoretical calculations have been performed to disclose the effects of dye molecules and interfacial properties on the photovoltaic performance of DSSC with Zn-porphyrin（Zn P）dyes and metal-free organic dyes as sensitizers.New strategies of molecular design and interfacial modification for enhancing DSSC J_SC and V_OC have been proposed based on the calculation and analysis of molecular structures,interfacial configurations,and doped Ti O₂ surfaces.i)In the analysis of vertical excitation process of dyes,Natural Transition Orbitals（NTO）analysis and Natural Population Analysis（NPA）were employed to determine the excited states with significant intra-molecular charge separation（ICS）and to analyze the charge transfer indicated by the product of the oscillator strength and the amount of charge collected at the acceptor group,∣f×Δq _AC∣.The calculation results are consistent with the experimental trends of J_SC of the DSSC with A1-A3 as sensitizers.Four new dyes A4-A7,with diphenylquinoxaline（DPQ）,benzothiadiazole（BTD）,benzobitriazole（BBT）and benzotriazolethiadiazole（BBT）as auxiliary acceptor（AA）and thiophene（Thi）asπbridge（π）,were designed and investigated using NTO,NPA and Density of States（DOS）analysis.According to the calculation results,the auxiliary acceptor moiety with strong electron-withdrawing capacity,small size and good symmetry could enhance the light harvesting ability and ICS,and increase the J_SC of DSSC.The Zn P dyes with DPQ+Thi and BBT+Thi could be good sensitizers due to more significant ICS and good alignment of the energy between LUMO and 3d orbitals of Ti atoms,which is beneficial for electron injection.ii)In order to analyze the effects of substitution position on the dyes’photovoltaic performance,seven newβ-substituted Zn P dyes（B1-B7）with the same donor（D）,AA,πand acceptor（A）as meso-substituted Zn P dyes（A1-A7）were designed and investigated.Based on the calculations of ground state properties,vertical excitation,electron-hole separation,NTO,NPA and DOS of dye-Ti O₂,it is concluded that despite the good coplanarity and significant charge-hole separation ofβ-Zn P,the high LUMO energy and big energy gap weaken the light harvesting ability and reduce the J_SC.In the excitation ofβ-Zn P,the weak effects of electron pushing of D and electron pulling of A,due to the different directions of push and pull forces,reduce the amount of electron shifting to acceptor,which is unfavorable for electron injection.In order to enhance the performance ofβ-Zn P,AA with stronger electron-withdrawing ability would be needed.According to the analysis,B7,with BBT+Thi moiety,could be good sensitizer among the sevenβ-Zn P dyes.iii)Dye-Ti O₂ systems with stoichiometric and defective Ti O₂（101）surface for Zn P dyes（T1-T3）withα-cyanoacrylic acid as acceptor were constructed to investigate the effects of molecular structure,interfacial configuration and–CN group on the J_SC and V_OCof DSSC.In the system with stoichiometric surface and the same interfacial configuration,T2 molecule with iminodibenzyl group（IDB）as donor have the highest J_SC and V_OC due to the fast IET rate and large dipole moment perpendicularly oriented to the Ti O₂ surface.Tridentate interfacial configuration with N involved in interfacial bonding could increase the J_SC and V_OC of DSSC.After the O_2c vacancy was introduced into Ti O₂ surface,the Zn P withα-cyanoacrylic acid as acceptor could form the dye-D_O-Ti O₂ system,where one O atom of–COOH could insert into O vacancy and the other O atom could be bonded to the adjacent Ti_5c atom.Dye-D_O-Ti O₂ system could enhance the V_OC while maintaining relatively high J_SC.On the other hand,if–CN does not participate in interfacial bonding,it could cause the downshift of LUMO energy to the conduction band minimum（CBM）of Ti O₂ and even below the CBM,which is unfavourable for electron injection.iv)Graphene oxide（GR）and graphdiyne oxide（GD）doped Ti O2 surface were constructed to invested the effects of doped modes,interfacial interactions,and C/O atom ratio on DSSC performance based on the IET simulation,charge dispersion analysis,band structure（BS）and DOS analysis.It is concluded that when GR layer is placed between Ti O2and dyes,the doped system could not enhance the IET efficiency,while cause interfacial electron recombination.When the dyes and GR layer or GD layer coadsorbed on Ti O2surface,GR and GD could insert some unoccupied electron states in the region of Ti O2CBM,leading to the interaction between p orbitals of C atoms and the 3d orbitals of Ti atoms,which is favourable for electron dispersion in Ti O2,enhancing the IET efficiency.When the interaction between GR or GD and Ti O2 is chemisorption,forming Ti-O-C bond,the doped system has the highest IET efficiency,and GD is the most effective dopant.Increasing the C/O atom ratio of GD and GR would shorten IET time.However,if the C/O atom ratio is too low,the GR and GD would accumulate large amount of charges,which is unfavourable for electron dispersion and could reduce JSC.GD doped system could cause the upshift of Ti O2 CBM,and hence enhance the VOC.