The Application Of Interface Modification In Polymer Solar Cells

It is important to look for economical and environmentally friendly energy sources as the cost of fossil energy is increasing.One of the most attractive energy sources is solar energy.Organic polymer solar cells are a promising alternative to inorganic ma-terials because of their advantages of low cost,light weight,good flexibility and large area production by roll-to-roll processing.At present,laboratory polymer solar cells have achieved high energy conversion efficiency.However,low light absorption,short life of carrier and carrier recombination are still problems that must be solved.Espe-cially,the carrier recombination seriously restricts device performance.In order to op-timize the device performance,new polymer materials and electrode materials were developed to match the work function and effectively reduce the carrier recombination.However,the non-ideal contact between layers of organic solar cells,intrinsic defects of transport layer and other problems still affect carrier transport,resulting in interface recombination and light soaking effect.Interface modification has been paid more and more attention for its advantages of easy operation and good performance.The interface work function matching can be improved effectively by introducing appropriate buffer layer.On the one hand,the effective contact between layers is improved,the interface defects are filled,and the recombination in the process of carrier transmission is reduced.On the other hand,the speed of response is improved,carrier collection rate is increased,even light absorption can be enhanced by good structure of device.It is a good way to improve efficiency of polymer solar cells by interface modification.In this paper,high-performance polymer solar cells were developed based on the modification of the cathode interface and anode interface respectively.The interface modification can decrease the interface defect,adjust the work function,change the surface energy and optimize the carrier transfer,leading to the improvement of photo-electric conversion efficiency of polymer solar cells.The relationship between interface modification and device performance was explored through a series of photoelectric performance tests.The relationship between device performance improvement and modification of transport layer,electrode and active layer was discussed.The main research contents include:（1）By introducing PEI and PFN organic modification layers into the upper and lower interfaces of SnO₂ electron transport layer,the organic/inorganic/organic-OIO composite electron transport layer is constructed,and the device performance is opti-mized.Devices with structure of ITO/PEI/SnO₂/PFN/PCDTBT:PC₇₁BM/Mo O₃/Ag were fabricated.On the one hand,the organic buffer layer provides better growth con-ditions for SnO₂ electron transport layer and PCDTBT:PC₇₁BM active layer to achieve better interface contact and film morphology,leading to the reduction of defect states.On the other hand,the introduced PEI and PFN can form an energy ladder between the active layer and the electrode through the interface dipole,optimizing the electron transport and extraction capacity.The（short-circuit current density）J_scof the optimized device increased from 12.19 m A/cm² to 14.02 m A/cm²,and the（power conversion effi-ciency）PCE increased from 5.51%to 7.18%.This method provides an effective way to optimize the application of inorganic transport layer in polymer solar cells.（2）In order to achieve good contact between transport layer and active layer,organic material PEI was used as electron transport layer in this study.By introducing PC₇₁BM into interface between the PEI and the active layer in the device with structure of ITO/PEI/PTB7:PC₇₁BM/Mo O₃/Ag,the vertical separation of donor and acceptor materials of the active layer was optimized.In this situation,PTB7 is closer to the hole transport layer while PC₇₁BM is closer to the electron transport layer,thus optimizing the carrier transport in the active layer,reducing exciton recombination and improving the efficiency of the device.Meanwhile,the active layer film has better morphology in the optimized device,and the current density is increased by optimizing the interface contact and reducing the interface recombination.The current density-voltage（J-V）characteristic test results show that the J_sc of the optimized device increased from 16.47m A/cm² to 19.15 m A/cm²,and the device efficiency increased from 7.42%to 9.17%.The optimization of device performance mainly attributed to the increase of J_sc.This study provides further optimization method for the application of organic transport layer in polymer solar cells.（3）It is found that PFN was not supposed to be used as an electron transport layer alone because of light soaking effect.A polymer solar cell with the structure of ITO/Al Ac₃/PFN/PTB7:PC₇₁BM/Mo O₃/Ag was prepared by introducing Al³⁺at the in-terface between ITO and PFN by spinning Al Ac₃ alcohol solution,which can suppress the light soaking effect and improve the response speed of the device.The light soaking effect of solar cells based on PFN electron transport layer leads to kink shape or S-shape of J-V curves and decrease of photovoltaic response speed.Continuous illumination is required to achieve stable parameters.After the introduction of Al³⁺,the defects in PFN at ITO/PFN interface are passivated by Al³⁺.Meanwhile,the electron injection barrier and transmission resistance are reduced.Finally,the stable J-V characteristic parame-ters were achieved under the initial light with a normal curve shape,which means the light soaking effect was obviously suppressed.The（open-circuit voltage）V_oc,J_sc,fill factor（FF）and PCE of the optimized device are 0.73 V,17.88 m A/cm²,65.50%and8.50%respectively under initial illumination.After 120 s of continuous illumination,the PCE of the device only increased by 2%,while the PCE of the device without opti-mization increased by 70.87%,indicating that the light soaking effect of the device was significantly inhibited.This method can obviously suppress the light soaking effect of the device,and provides an effective solution for the application of PFN as electron transport layer in polymer solar cells.（4）Based on polymer solar cells with good performance,the Ag/Ge/Ag compo-site semitransparent electrode was prepared by introducing 1 nm Ge,and the poor con-tinuity of single layer semi-transparent Ag electrode affecting device performance was improved without interrupting the vacuum environment to fabricate electrode.The op-timum device with structure of ITO/PEI/PTB7:PC₇₁BM/Mo O₃/Ag/Ge/Ag was fabri-cated and explored.The growth mode of Ag was affected by Ge and the activation en-ergy for growth and diffusion of Ag islands was changed.Smaller and denser nucleation points are formed in early stage of film formation,thus denser and smoother electrodes with continuous and uniform distribution of elements was formed after increasing thick-ness.At the same time,Ag/Ge/Ag semitransparent electrode has smaller roughness,fewer defects,better carrier collection capacity and higher conductivity.When the thickness of Ag is the same,the PCE of device with Ag/Ge/Ag electrode is 34%higher than that of 15 nm single Ag electrode.The device with Ag/Ge/Ag semitransparent electrode has good optical characteristics and good color rendering index with im-proved performance.The device with electrode structure of Ag（1 nm）/Ge（1 nm）/Ag（14 nm）achieved the best performance with an average light transmittance of 25%,which can meet the standard of the application of windows.