Improving The Efficiency Of Polymer Solar Cells Via Modification Of Zno Cathode Interlayer And Treatment Of Solvent On The Active Layer

Polymer solar cells（PSCs）have been extensively studied because of their intrinsic merits of lightweight,low cost,and great potential for large size roll-to-roll production.Recently,the power conversion efficiencie（PCE）of single junction PSCs has exceeded 13%through synthesis of new materials,modification of interlayers,morphology control of active layers,and optimization of the device structure.In particular inverted polymer solar cells（i-PSCs）have drawn more attentions because of their higher PCE and longer lifetime than conventional device architectures.Usually,a cathode interlayer（CIL）is required to reduce the energy barrier and polish up contact between the active layer and indium tin oxide（ITO）in the i-PSCs.Zinc oxide（ZnO）has been regarded as a competitive CIL for i-PSCs because of its suitable energy level,low cost,environmental friendliness and so on.However,ZnO CIL also has several drawbacks.First,the inorganic ZnO has a poor interfacial contact with the organic active layer.Large series resistance（Rs）between them reduces the carrier transport efficiencies,resulting in a low short-circuit current density(J_SC)and fill factor（FF）.Second,the surface defects easily form during the fabrication of the ZnO film.These defects act as centers of interfacial recombination of photogenerated charge carriers,which seriously harm PCE and device stability.Finally,poor electrical conductivity of the solution-processed ZnO film would deteriorate the electron transport process,which inevitably compromises the performance of the device.To eliminate these adverse factors,it is very necessary to modify ZnO CIL with suitable materials.In addition,the methanol treatment on the active layer can redistribute the donor and acceptor in the active layer to form a better phase separation,resulting in improved performance of the PSCs.However,the volatilization of methanol is fast so that its working time for the redistribution of the donor and acceptor in the active layer is not long enough.Therefore,it is necessary to prolong the evaporation time of the methanol to make it more effectively to enhance the device performance.In this thesis,we firstly compared four methods of preparing ZnO to optimize out the best ZnO films for the i-PSCs and then modified ZnO with organic and inorganic materials to achieve high-efficiency i-PSCs.In addition,we also treated the active layer with methanol aqueous solution to improve the performance of PSCs.In the first part,we prepared four types of Zn O layer:ZnO（S1）and ZnO（S2）CBLs fabricated with sol-gel processing,ZnO（N1）and ZnO（N2）CILs derived from pre-fabricated ZnO nanoparticle suspensions.Then we compared the efficiency and stability of the PTB7:PC₇₁BM-based i-PSCs with different ZnO CILs.The results display that the device based on sol-gel ZnO（S1）has the best efficiency and stability.We find that the devices based on sol-gel ZnO（S1）have the highest built-in voltage,carrier concentration and mobility,exciton separation and carrier extraction efficiency,which can be attribute to the best interface contact quality between the sol-gel ZnO（S1）and the active layer.In addition,we investigated the effect of Zn O CILs thickness on the performance of the devices.The results show that the i-PSCs have the minimal PCE loss when the thickness of ZnO（S1）film increases from 30 nm to 90 nm.These results demonstrate that the sol-gel method for the synthesis of Zn O（S1）is a promising method.In the second part,we used alcohol-soluble isoindigo derivative IIDTh-NSB modifying ZnO to improve the PCE of i-PSCs.When IIDTh-NSB（0.2 mg/m L）was spin-coated on Zn O as CIL,PCE of the PTB7:PC₇₁BM based i-PSCs reached 8.88%,which is a 20%improvement of that of 7.40%for the device with the ZnO-only CIL.If ZnO was doped by IIDTh-NSB of 1.0 wt%,the PCE of 8.50%could be achieved in the i-PSCs.Combined measurements of capacitance-voltage characteristics,carrier mobility,and photocurrent density-effiective voltage characteristics revealed that incorporating IIDTh-NSB as the modifier of Zn O by coating or doping enhanced the built-in potential,charge carrier density and mobility,exciton dissociation,and charge carrier extraction in the i-PSCs because of the improved interfacial contact between the photoactive layer and ZnO as shown in water contact angle measurements and atomic force microscopy images.Finally,impedance spectroscopy investigation provided strong lines of evidence that incorporating IIDTh-NSB as the modifier of ZnO led to the great enhancement in short-circuit current density and fill factor.Furthermore,all the devices with IIDTh-NSB as a modifier of ZnO presented better stability than the device with ZnO-only.These findings suggest that IIDTh-NSB is an effiective material for modification of ZnO in the i-PSCs.In the third part,we used cheap lithium acetate（Li Ac）as a ZnO modifier to improve the performance of i-PSCs.The PCE of the i-PSCs based on PTB7:PC₇₁BM was 7.40%with pure ZnO as the cathode interface layer.However,when ZnO was doped by Li Ac of 5.0 wt%,the PCE could be increased to 8.65%.When LiAc（1.0mg/mL）was spin-coated on the ZnO film,the PCE also can be improved to 8.53%.Through capacitance-voltage（C-V）characterization,carrier mobility test and photocurrent density-effective voltage characterization,we find that LiAc as a dopant in ZnO or a coating layer on the ZnO film can effectively improve the built-in voltage,carrier concentration and mobility,exciton separation and carrier extraction efficiency in the i-PSCs.This is because incorporating Li Ac increases the interfacial contact between the active layer and ZnO.In addition,Impedance spectroscopy further revealed that LiAc as a dopant in ZnO or a coating layer on the ZnO film could effectively improve the J_SC and FF of i-PSCs.All of these results show that using inexpensive LiAc to modify ZnO interface is an effective method to improve the performance of i-PSCs.In the fourth part,significant improvement in the PCE of PSCs has been observed when the active layer was treated by a mixture of methanol and water（M:W）.For the PSCs based on ITO/PEDOT:PSS/PCDTBT:PC₇₁BM/Al,the PCE was6.58%when the surface of PCDTBT:PC₇₁BM film was treated by pure methanol.However M:W（6:1）treatment on the PCDTBT:PC₇₁BM could further improve the PCE to 7.44%which is even higher than the PCE values of the PSCs with LiF（6.00%）or PFN（6.88%）as a cathode interlayer.Similarly in the PSCs based on PTB7:PC₇₁BM,M:W（6:1）treatment on the surface of PTB7:PC₇₁BM can increase the PCE to 8.47%while the PCE values of the PSCs with methanol treatment and without any treatment on the PTB7:PC₇₁BM are 8.14%and 7.41%,respectively.Combined contact angle measurements and X-ray photoemission spectroscopy depth profiling demonstrated that the M:W treatments resulted in a more favorable redistribution of polymer and PC₇₁BM in the active layer because evaporation of the solvents drove PC₇₁BM to migrate from the interior of the blend film to the the top（air）surface.