Solution-processed High Performance Buffer Layers And Its Application In PDPP3T-based Polymer Solar Cells

Polymer solar cells（PSCs）as a new type of clean energy,due to their low cost,flexibility,simple preparation process and roll-to-roll manufacture properties,are receiving a major concern in recent years.At present,more and more materials are developed for the preparation of PSCs to improve the power conversion efficiency（PCE）.But the increase rate is unsatisfactory,there is still a gap to the large-scale industrial production standards.It is found that the solution-processed PSCs,due to the simple operation process,low equipment cost and batch experiments,very much in line with the industry standard,so it is the research hotspot in the field of PSCs,and the all-solution processed PSCs is a novel research area.The current materials of anode buffer layer for PSCs are just MoO₃ and Poly（3,4-ethylenedioxythiophene）:poly（styrene sulfonate）（PEDOT:PSS）traditional materials in generally,so this paper focuses on the properties of the solution-processed small molecule and polymer anode buffer layers,and improve the performance of PDPP3T:PC₆₁BM based PSCs.Here,the donor material PDPP3T employed in PSCs is a near-infrared absorbing donor material,easy to synthesis and more stable than the traditional P3HT.At the beginning of this paper,a solution-processed novel small molecule material 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile（HAT-CN）as anode buffer layer is employed in 5 wt.%PDPP3T:PC₆₁BM based PSCs for the first time.The solvent,concentration,thickness and annealing temperature of solution-processed HAT-CN（labeled as s-HAT-CN）are studied and optmized.Compared with conventional MoO₃ and PEDOT:PSS anode buffer layer materials,s-HAT-CN has a more regular surface morphology and higher hole transport ability.The PCE of 5 wt.%PDPP3T:PC₆₁BM based PSCs via s-HAT-CN as hole transport layer（HTL）could reach 1.24%,improved by 7.8%.Therefore,s-HAT-CN is proved to be an efficient and promising HTL material for solution-processed PSCs.In order to further optimize the performance of PSCs,the electron transport layers（ETLs）are also studied in this paper.Small molecular electron transport materials such as4,7-Diphenyl-1,10-phenanthroline（Bphen）and N-arylbenzimidazoles（TPBi）are used to prepared ETLs by solution method,and the results show that the solution-processed TPBi（labled as s-TPBi）based PSCs exhibit the best performance combined with s-HAT-CN,whose PCE could be improved from 1.15%（evaporation process）to 1.45%（solution process）.Therefore,a simple and efficient approach for preparing all solution-processed PSCs is provided in this paper.Secondly,some studies on the performance of traditional polymer transport material PEDOT:PSS by doping a series dopants are performed in this paper.Due to the dopants,PEDOT:PSS shows better performance,so as to improve the efficiency of PSCs based on PDPP3T:PC₆₁BM.Here,certain concentration of D-sorbitol doped into PEDOT:PSS could arouse molecules rearrange,which increases combination of PEDOT molecules and reduces the influence of insulation molecule of PSS,thus improves the conductivity of PEDOT:PSS and short current density(J_SC)of PSCs based on 20 wt.%PDPP3T:PC₆₁BM/BCP/Al structure.Compared with the traditional devices,the PCE is obviously increased from 2.50%to 3.62%.After some optimization experiments,the solar cells with structure of ITO/4 wt.%sorbitol doped PEDOT:PSS（30 nm）/20 wt.%PDPP3T:PC₆₁BM（130 nm）/LiF（1 nm）/Al（80 nm）could get the best power conversion efficiency（4.33%）,increased by 70%.In addition,a more efficient organic solar cell is prepared by using higher performance acceptor material PC₇₁BM instead of PC₆₁BM,and its efficiency could reach 4.56%.Finally,in order to further study on the effect of dopants into PEDOT:PSS,a series of alcohols with different chain length,including ethanol,glycerol,meso-erythritol,xylitol and D-sorbitol,are employed.And the performance of PSCs based on 20 wt.%PDPP3T:PC₆₁BM is also studied.The experimental results exhibit that the longer hydroxyl chain,the larger J_SC,the lower open circuit voltage(V_OC)and the fill factor（FF）,but the device based on meso-erythritol-doped PEDOT:PSS presents the best PCE totally.Then,the cathode buffer layer is optimized,and the performance of the device is further improved by replacing bathocuproine（BCP）with LiF.Finally,a series of batteries are fabricated on the flexible PET substrate to cater to the practical application of the PSCs.It is found that the longer the hydroxyl chain,the larger the PCE obtained,and the largest PCE is 1.06%.