Realized High Efficient Polymer Solar Cells Through Optical Engineering

Polymer solar cells(PSCs), which can directly converteabsorbed sunlight to electricity, consists of stacked thin films with thickness from tens to hundreds of nanometers. Since from their invention, PSCs have emerged as a promising candidate for affordable, clean, and renewable energy source. In the past few years, a rapid increase in the power conversion efficiency(PCE) of PSCs toward 10 % milestone has been achieved, mainly through comprehensive optimizations via the design and synthesis of novel electron donor/acceptor materials, the control over the thin-film morphology in nanoscale, and device structure and interfacial and optical engineering. The PSCs are expected to find many practial applications in the near future, such as flexible solar modules, semitransparent solar cells for window curtain, and indoor building applicationswhere light intensity is weak.So far, indium tin oxide(ITO) is the most widely used transparent electrode for PSCs owing to its high optical transparency(>85 % in the visible spectrum) and low sheet resistance(10-20 ohm/sq for 100-200 nm thick film). However, ITO films are brittle, thus may limit their applications as electrode for flexible substrates. Furthermore, indium is a rare metal on the earth and is very expensive, thus limits their applications for mass production and large area size devices. Therefore, replacing ITO transparent electrodes with cheaper alternatives for PSCs is in urgent and thus receives intense research interest.On the other side, owing to an intrinsic short exciton diffusion length(~10-20 nm) and the disordered nature of organic semiconductors and the concomitant low carrier mobility, the design of PSC devices face a dilemma, that the realization of highle efficient devices should rely on optically thick and electrically thin device structures. Besides, there exists a trade-off between light harvesting and short circuit density(Jsc) and the open circuit voltage(Voc) in PSCs, making a gain in overall device efficiency an difficult challenge.In this thesis, the Ph.D candidate successfully demonstrated highly efficient, low-cost ITO-free PSCs from an ultrathin copper(Cu) film, which is prepared by simple thermal evaporation. The average optical transmittance of the 10 nm Cu coated poly[(9,9-bis(3‘-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN) in the visible region of the spectrum was found to be around 70.6 %, while a maximal transmittance of 75.6 % was observed at 680 nm. Upon the use of the PFN interfacial layer, the Cu electrode can form Ohmic contact with photoactive layer and facilitate charge transport and extraction. When the blend of poly[[2,6’-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b] dithiophene] [3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7-Th) and [6,6]-phenyl C71-butyric acid methyl ester(PC71BM) blend was utilized as photoactive layer, a remarkable PCE of 8.21 % was achieved, demonstrating the thermally evaporated Cu thin film electrode can be promising candidate to replace ITO for highly efficient PSCs.In the third chapter, the candidate described the use of sub-micron thick film as photoactive layer for highly efficient PSCs with either conventional or inverted-type device structure. The loss mechanism for photo-genearated charge carriers in the thick-film PCDTBT:PC71BM solar cells have been systematically investigated. For PSCs with specic thickness in the photoactuive layer, the low Jsc and hence PCEs can be mainly attributed to a low optical absorption efficiency. As the thickness increase up to 200 nm, the space charge limited photocurrent regime is reached, leading to a reduced charge collection efficiency as a result of undesired distribution of photo-generated charge carriers accross the device, unbalanced charge mobility and the concomitant space-charge accumulation. We demonstrated that the device performance of the device with thick active layer can be effectively improved through the use of solvent vapor annealing. The best solar cells showed a PCE of 7.1%, which was among one of the highest values for PCDTBT:PC71BM system. This strategy was further sucessfully applied to other promising donor material systems with high charge mobility, leading to a remarkable PCE of 11.0% in an inverted-type devices with active layer thick up to 300 nm.In the last chapter, parallel-like bulk heterojunction(BHJ) polymer solar cells has been proposed and successfully demonstrated. The method eliminates the need for careful design and precise control of the intermediate layers in tandem cells, thereby significantly reducing the complexity of the device fabrication. More importantly, unlike conventional multi-blend systems, which incorporate only small amounts of additional donor materials as sensitizers, parallel-like system enables the effective use of multiple donors with much improved light absorption and conversion efficiency. Thus, parallel-like system combines the advantages of both tandem cells and conventional multi-blends. Meantime, The open-circuit voltage(Voc) can be regulated by changing the weight ratio of multi-donors. In the parallel-like systems, a noticeably enhanced overall efficiency(PCE up to 9.06%) of was obtained, obviously higher than that of the optimum single-junction binary BHJ devices.Though the detailed working mechanism is still unclear, we believe that parallel-like systems open a new avenue to accelerate improvement in the efficiency of polymer solar cells.