Performance Enhancement Of Polymer Solar Cells By Electron Buffer Layer Optimization

Over the past several decades, Polymer solar cells (PSC) based on bulk-heterojunction (BHJ) concept are considered to be a promising candidate for the properties of light weight, potentially low production cost, mechanical flexibility and compatibility with roll-to-roll manufacturing. However, the power conversion efficiency (PCE) of PSCs is still not comparable with those of inorganic solar cells. The need to improve the PCEs requires the implementation of optimize the performance of devices with the structure, materials and processes. A general challenge in organic electrical devices is the properties at the interfaces between electrode and organic material. Therefore, the optimization of interfaces between metal contacts and organic semiconductors are critical for efficiency of polymer-based photovoltaic (PV) devices.A variety of interfacial buffer layers have been applied to improve the performance of PSC in term of charge exchange. LiF and ZnO is the common n-buffer layer in traditional normal device and inverted device, respectively. An annealed thin layer of LiF in normal device will sacrifice the performance of devices which bring trouble on optimization of device as thermal annealing is a common process to improve the performance of OPVs. Moreover, the disadvantages of ZnO, such as relatively low conductivity, nanoparticles are unstable in solution and hardly keep uniformity, not only limit the PEC of inverted polymer solar cells, but also imposes a major challenge for the manufacturing of polymer solar cells by an industrial scale production. To solve this and understand the origin, strategies aimed at addressing these restrictions in P3HT:PCBM organic solar cells have emerged in the literatures.Firstly, we depict that the strong influence of different thermal annealing processes on the morphology and interfacial behavior of the LiF layer between Al electrode and active layer. The origin of the efficiency improvement in pre-annealed P3HT/PCBM solar cells with LiF/Al electrode is examined by comparing different thermal annealing processes on the basis of ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al device structure. A nanoscopic difference of interfacial behavior and morphology of active layer between LiF/Al pre-annealed and post-annealed devices. LiF will diffuse into the active layer in the period of post-annealing process and no definite LiF layer structure can be observed, which makes LiF lose its actions of dipole moment, tunneling injection, and block electron-hole recombination. Moreover, due to a penetration of LiF into the active layer, the crystalline of P3HT and the morphology of the active layer where contacted with LiF/Al layers are detracted. These investigations have important implications for optimizing the performance of organic photovoltaic device based upon the LiF/Al electrode.Then, a simplified preparation of uniform sol-gel derived aluminum-doped zinc oxide with relatively low annealing temperatures as a buffer layer is investigated, which optimize inverted devices by improve nanoparticles stability and uniformity, achive nano-ridge structures to provided an effective hole blocking layer and also an increased interfacial area for electron collection, enhance the conductivity of ZnO buffer layer and allowed a thicker thickness for buffer layer which save the challenge of the polymer solar cells manufacturing by an industrial scale production. The performance of PSCs was observed to increase by doping Al in pure ZnO buffer, especially in Jsc, and has a maximum PCE in3at.%aluminum doping layer since a higher doping concentration increase the leakage current. All of these improve the electron transport of ZnO buffer layer and this play an important role in enhancement of PSCs performance with an inverted device structure.