Application Of Two-dimensional Materials In Organic Solar Cells

Organic solar cells（OSCs）as a new conversion sunlight into electricity technology have been concerning because of its inexpensive price,low weight,roll-to-roll,printing fabrication.To date,the photovoltaic conversion efficiency（PCE）of single-junction has been exceeded19%.Despite the great progress in research on various novel organic photosensitive materials,many important obstacles remain to be overcome.In the active layers,the poor charge transport,short-lived exciton and difficult to control morphology and other factors of the organic material have limited their progress towards higher performance.In transport layer,metal oxides（ZnO and MoO₃,etc.）mainly relies on high-temperature annealing treatment or vacuum processing,which in turn makes the fabrication process is expensive and complex as well as difficult to scale up for large area manufacturing.While the PEDOT:PSS due to its highly hygroscopic and acidic,this will greatly limit the improvement of solar cell performance and the extension of device lifetime.two-dimensional（2D）materials have excellent optoelectronic properties,stable mechanical and physical properties,as well as unique morphologies and structures,which are ideal materials for improving the performance of OSCs.Furthermore,large-scale 2D materials can be produced by solution-processing methods.Therefore,incorporation of 2D materials into the active layer is an effective method for improving carrier mobility,morphology of the active layer,and stability of the device.Moreover,the transport layer can be doped or replaced by 2D materials to improve stability and conductivity.Today,due to the rapid development of flexible transparent and three-component OSCs,2D materials are sure to find more applications.Therefore,the research on this hot area is great significance.Therefore,we focus on the application of various 2D materials in the active layers and transport layer layers of organic solar cells.The main content of the survey includes the following six parts:Firstly,we introduce 2D Bi₂O₂S nanosheets into the fullerene system active layer of PTB7:PC₇₁BM to improve light harvesting and carrier mobility.By adding 1 wt%Bi₂O₂S,the devices obtain a PCE of 10.71%that is dramatically increased by 21.8%in comparison with binary device with a PCE of 8.97%.Introduction of 2D Bi₂O₂S nanocrystals into the PTB7:PC₇₁BM system can enhance the absorption of light in the OSCs,effectively enhance the crystallinity of PTB7 and optimize the surface morphology of photoactive layer,which leads to more efficient photon harvesting,enhances exciton dissociation,accelerates charge carrier transport and reduces charge recombination.Moreover,the addition of appropriate amounts of2D Bi₂O₂S nanocrystals into PTB7:PC₇₁BM can effectively improve the lifetime of PSCs.Secondly,we introduce 2D Bi₂OS₂ nanosheets to the non-fullerene system active layer of PBDB-T:ITIC to optimize the phase separation and crystallinity of the active layer.The addition of 2D Bi₂OS₂ nanomaterial can effectively enhance the carrier mobility,improve the exciton dissociation,and reduce the charge recombination in the OSCs.The PCE of ternary PSCs（adding 1 wt%Bi₂OS₂）increases by 17%（PCE=12.31%）compared with the control binary device with a PCE of 10.51%.Moreover,the introduction of 2D Bi₂OS₂ nanocrystals into PBDB-T:ITIC also can effectively enhance the stability of the OSCs.Thirdly,we add 2D Bi₂O₂Se nanosheets with better carrier mobility to the active layers of PBDBT-T:ITIC and PM6:Y6 systems to improve the carrier mobility and investigate the mechanism of its effect on device stability.The 2D Bi₂O₂Se nanoflakes,when introduced into the active layer,not only provide new interface between donor and acceptor and efficient charge transfer pathways but also induce crystallization of photosensitive layer and form the continuous interpenetrating networks,which promotes the exciton separation and charge transfer in photosensitive layer.As a result,the PCE of device based on PBDB-T:ITIC is increased from 10.09%（0 wt%）to 12.22%（2 wt%）.Meanwhile,the PCE of device based on PM6:Y6 is also increased from 14.59%for binary device to 16.28%for optimized ternary device（2 wt%）.Moreover,the optimized ternary device shows excellent air stability by suppressing the mixing of the two phases.Fourthly,we use alkali and annealing treatment to regulate the surface functional groups in Nb₂CT_x and then adjust its work function（WF）.After KOH treatment,-F group is replaced by-OH group,leading to a decrease in WF from-4.62 e V(Nb₂CO_1.2OH_0.6F_0.2)to-4.32 e V(Nb₂CO_1.2OH_0.8).After annealing,-OH is transformed into-O,resulting in a WF increase to-5.03 e V(Nb₂CO_1.36OH_0.2).These Nb₂CT_xare for the first time applied as the ETL and HTL in PM6:Y6 based OSCs with an excellent PCE of 15.22%（ETL）and 15.03%（HTL）.Meanwhile,PM6:BTP-e C9:PC₇₁BM based OSCs also achieve a PCE of 17.64%（ETL）and 17.51%（HTL）.These efficiencies are comparable to those gained using conventional charge transport layers.Fifthly,the HfO₂ are formed on HfX₂（X=S,Se）surfaces by oxygen plasma treatment to passivate surface defects of HfX₂（X=S,Se）,which results in the reduction of defects and the increase of conductivity in HfX₂（X=S,Se）film.Moreover,the HfO₂ induces an interface dipole layer at the HfX₂（X=S,Se）interface and forms a p-type doping of HfX₂（X=S,Se）,which leads to an increase of HfX₂（X=S,Se）WF.When the HfX₂/HfO₂（X=S,Se）films are used as HTL,the OSCs based on PM6:BTP-e C9:PC₇₁BM get a higher PCE of 17.76%（HfS₂/HfO₂）and 17.83%（HfSe₂/HfO₂）respectively,which is superior to those based on PEDOT:PSS HTL.Moreover,the OSCs based on HfX₂/HfO₂（X=S,Se）show greater stability than the OSCs based on PEDOT:PSS.Sixthly,Ti₃C₂T_x was modified with amino acids to tune its WF and passivate the vacancies of surface defects,thereby improving the electrical properties of Ti₃C₂T_x.On the one hand,the charge transfer from-NH₂ of amino acid to the Ti₃C₂T_x surface and the formation of interface dipole led to the decline of WF in Ti₃C₂T_x.On the other hand,the positively charged-NH₂ of amino acids passivate the negatively charged Ti vacancies of Ti₃C₂T_x,while the negatively charged-COOH of amino acids passivate the positively charged C vacancies of Ti₃C₂T_x,thereby enhancing the conductivity of Ti₃C₂T_x.Finally,in PM6:BTP-EC9 system active layer,based on Ti₃C₂Tx/Gly,Ti₃C₂T_x/D-Ala,Ti₃C₂T_x/L-Cys,Ti₃C₂T_x/L-Lys and Ti₃C₂T_x/L-Arg achieve a PCE of 17.09%,17.07%,17.03%,17.16%and 17.00%,respectively,which is comparable to the ZnO-based devices（17.18%）.