Study On Conjugated Zwitterion As Cathode Interfacial Layer To Improve The Performance Of Organic Solar Cells

As one of the representatives of clean energy,organic solar cells show excellent photovoltaic performance,at the same time have excellent mechanical flexibility and simple preparation process,attracted the attention of many researchers.Now,the maximum efficiency of polymer solar cells（PSCs）has exceeded 17%,but there is still a certain gap compared to crystalline silicon solar cells.This dissertation mainly studies the novel conjugated zwitterions as cathode interfacial layer（CBL）to improve the performance of PSCs.The specific content can be divided into the following two parts:（1）A conjugated zwitterion ZW-Bu was designed and synthesized,ZW-Bu is amphiphilic and can be dissolved in polar solvent（methanol）and non-polar solvents（o-dichlorobenzene）.Therefore,the ZW-Bu film could be destroyed by solvents used by photoactive layer during the device preparation.However,since the NH group on the ZW-Bu molecule can form hydrogen bonding with oxygen ions on the ITO surface,this protects the ZW-Bu film from being completely cleaned and still can play an interface modification role.This characteristic makes ZW-Bu not need to satisfy the solvent orthogonal relationship with the photoactive layer.When applied to an inverted organic solar cell as a CBL,the device efficiency reached 3.92%in the P3HT:PC₆₁BM system.Compared to the standard device with ZnO CBL,which only showed a PCE of 3.51%.Moreover,thanks to the cleaning of the ZW-Bu CBL part when spin-coating the active layer,the initial thickness of ZW-Bu can be changed widely without sacrificing device efficiency.In order to study whether ZW-Bu CBL is universal,we also used ZW-Bu in high-efficiency photoactive layer systems PTB7-Th:PC₇₁BM and PTB7-Th:ITIC.Compared with standard devices,the efficiencies have also been significantly improved.（2）ZW-Bu CBL was prepared by self-assembly method.Different from the layer-by-layer preparation method,the self-assembly method（that is to dissolve the interface modification material into the solution of the photoactive layer.During the spin-coating process,the modification layer and the photoactive layer can generate spontaneous layering through molecular self-assembly）greatly simplifies the preparation process.ZW-Bu not only has amphiphilicity,but also has a large surface energy which makes it tend to move toward the ITO surface during the spin-coating process.When applied to the P3HT:PC₆₁BM device,the efficiency of the device prepared by the self-assembly method can reach 3.92%,which is similar to that of the layer-by-layer device,which proves the feasibility of the self-assembly method.However,in the PTB7-Th:PC₇₁BM system,although the efficiency of the device prepared by the self-assembly method（5.97%）is higher than that of the device without CBL（3.13%）,it is far lower than the device efficiency obtained by the layer-by-layer preparation method（9.03%）.We speculate that the lack of self-assembly time caused ZW-Bu to fail to move completely to the ITO surface in the PTB7-Th:PC₇₁BM system.We further designed and synthesized ZW-Bu derivatives（ZW-2）.The surface energy of ZW-2 is larger.It is hoped that a larger surface energy difference can provide a stronger separation driving force to make up for the problem of insufficient self-assembly time.The device efficiency of ZW-2:PTB7-Th:PC₇₁BM based on self-assembly method is 6.83%,which is significantly improved compared with the device using self-assembled ZW-Bu.The experimental results are close to our expectations.