Doping In Photoactive Layer And A Novel Transparent Electrode:Applications In Polymer Solar Cells

As a new type of thin-film organic solar cells, polymer solar cells (PSCs) have been receiving great attention because of its advantages in terms of the simple fabrication, low-cost, light-weight, and flexibility. Most recent studies of PSCs have been focused on using novel donor and acceptor photovoltaic materials, optimizing microphase separation structure and morphology of the photoactive layers, incorporating the buffer layers, and developing new electrode materials. To improve the power conversion efficiency (PCE) is the key issue of current PSC research. In this dissertation, we focused on enhancing the PCE of PSCs by doping in photoactive layer and developing a novel transparent electrode to substitute ITO, and carried out the following works The results are as follow:(1) By doping Fe3O4magnetic nanoparticles (NPs) into P3HTPCBM bulk heterojunction (BHJ) PSCs for the first time, the PCE of the OA-Fe3O4P3HTPCBM BHJ-PSC device is enhanced by～18%at the optimum OA-Fe3O4NPs doping ratio of1%. This enhancement is primarily due to the increase of short-circuit current (Jsc) by～14%, which is attributed to the magnetic field effect originated from the superparamagnetism of Fe3O4NPs, resulting in an increase of the the rate of intersystem crossing from the photogenerated singlet to the trip let state. Consequently the population of triplet excitons increases, leading to an increase of dissociated charge carriers and the photocurrent.(2) For the first time an amphiphilic surfactant oleamide was doped into P3HTPCBM photoactive layer, resulting in the formation of a novel cathode buffer layer (CBL) due to self-assembly of oleamide molecules, thus the PCE of P3HTPCBM PSC was enhanced dramatically. At the optimum oleamide doping ratio of2.5%, the incorporation of oleamide CBL leads to the PCE enhancement by～28%. The enhancement of PCE is primarily caused by the increase of FF. The microstructures and surface morphologies of the oleamide-incorporated P3HTPCBM photoactive films was studied by TEM, AFM, and SKPM, revealing that oleamide molecules initially doped in P3HTPCBM layer may undergo self-assembly and migrate to the surface of the P3HTPCBM layer, leading to the formation of a CBL that functioned as an interfacial dipole layer between the photoactive layer and Al electrode. Thus the energy level offset between the work function of Al and the LUMO level of the PCBM acceptor was decreased, facilitating the electron extraction by the Al cathode.(3) By spin-coating a glycerol monostearate (GMS) surfactant layer onto PEDOTPSS film, a PEDOT:PSS/GMS bilayer transparent electrode was fabricated, which can be used to substitute ITO. The highest conductivity of the as-prepared PEDOTPSS film reached1019Scm-1with a sheet resistance of98Ω1sq-1,and the PEDOTPSS/GMS bilayer films exhibited transparency of around80%in the visible range. The conductivity improvement was proposed to result from the GMS-induced segregation of PSS chains and the conformational change of the conductive PEDOT chains within PEDOT:PSS. When GMS in methanol was spin-coated onto PEDOTPSS film, due to the swelling effect of methanol, GMS molecules may diffuse into the PEDOTPSS film and interact with PEDOTPSS, leading to the phase separation between PEDOT and PSS chains. As a result PEDOT chains would take reorientation by getting away from the PSS chains’coils, allowing more inter-chain interactions among the PEDOT components. Thus, the energy barrier for inter-chain and inter-domain charge hopping would be lowered and charge transfer among the PEDOT chains would become easier, leading to a tremendous enhancement of the conductivity of PEDOT:PSS film. Using Clevios PH1000/GMS bilayer films as the transparent anodes substituting ITO, high-efficiency ITO-free BHJ-PSC devices based on PTB7:PC71BM systems exhibit highest PCE of7.06%, which is comparable to the corresponding devices based on the traditional ITO anode. The obtained PCE of7.06%is the highest one for ITO-free BHJ-PSC devices reported up to now. The universality of our new bilayer anode for BHJ-PSC devices based on versatile active layer materials promises its potential in the large-area high-efficiency ITO-free BHJ-PSCs.