Study On Surface Morphology Optimization And Electrode Interfacial Layer Modification In Polymer Solar Cells

Polymer solar cells (PSCs) have attracted growing interests over the last decades due to their unique advantages of light-weight, low-cost, and flexible manufacturing. Great efforts have been made to break through the power conversion efficiency (PCE) bottleneck. However, there are still challenges to achieve further efficiency improvements in PSCs based on conjugated polymers and fullerene derivatives. The device performance of PSCs is mainly governed by the following four aspects:light absorption proprieties of the photoactive materials, charges separation and transfer characteristics between the donor and acceptor, charges transport efficiencies in the photoactive layer, and charges extraction and collection of the electrode. From the perspective of photoactive materials, developing and designing new donors and acceptors for the active layer and optimizing the morphology are undisputed the most original and common driving forces among all approaches to improve the PCEs. Apart from this, from the perspective of photovoltaic devices, applying interfacial layers to optimize the energy level alignment of the device, adjust the light distribution within the photoactive layer and decrease the interfacial defects, optimize the surface morphology and enhance the light absorption have also proven to be critical in achieving high performance PSCs. In this paper, we mainly investigate adding processing additives and introducing cathode buffer layer to optimize the energy levels alignment as well as improve photovoltaic performance.The morphology of the photoactive layer critically affects on the performance of the bulk heterojunction polymer solar cells (PSCs). To control the morphology, we introduced a hydrophobic fluoropolymer polyvinylidene fluoride (PVDF) as nonvolatile additive into the poly(3-hexylthiophene (P3HT):[6,6]-phenyl C61 butyric acid methylester (PCBM) active layer. The effect of PVDF on the surface and the bulk morphology were investigated by atomic force microscope (AFM) and transmission electron microscopy (TEM), respectively. Through the repulsive interactions between the hydrophilic PCBM and the hydrophobic PVDF, much more uniform phase separation with good P3HT crystallinity is formed within the active layer, resulting enhanced light harvesting and improved photovoltaic performance in conventional devices, which lead to 28% increased in PCE. The PVDF distribution within the active layer was investigated by secondary ion mass spectroscopy (SIMS), confirming a bottom distribution of PVDF. Therefore, inverted device structure was designed, and the PCE was improved from 2.81% to 3.45% with PVDF additive.Inserting appropriate interfacial buffer layer between the photoactive layer and the contact electrodes makes great impact on the performance of polymer solar cells. Ideal interfacial buffer layers could minimize the interfacial traps and the interfacial barriers caused by the incompatibility between the photoactive layer and the electrodes. In this work, we utilized solution-processed hafnium acetylacetonate (Hf(acac)4) as an effective cathode buffer layer (CBL) in PSCs to optimize the energy level alignment between the photoactive layer and the cathode contact, with the short-circuit current density (Jsc), open-circuit voltage (VOC) and fill factor (FF) all simultaneously improved with Hf(acac)4 CBL, leading to enhanced power conversion efficiencies. Ultraviolet photoemission spectroscopy (UPS) and scanning Kelvin probe microscopy (SKPM) were performed to confirm that the interfacial dipoles were formed with the same orientation direction as the build-in potential between the photoactive layer and Hf(acac)4 CBL, benefiting the exciton separation and electron transport/extraction. In addition, the optical characteristics and surface morphology of the Hf(acac)4 CBL were also investigated through a series of measurements such as AFM, impedance epectra and electron mobility, which gave the evidence of optimized surface morphology, increased the electron mobility and decreased the charges recommobination.