Roll-to-Roll Fabricated Polymer Solar Cells: Towards Low Environmental Impact and Reporting Consensus

Abstract

The sun is by far the largest source of renewable energy available; consequently solar cells, which are able to convert light into electricity, have the technical potential to cover the global energy needs. Polymer solar cells (PSCs) on flexible plastic substrate have a low embodied energy and can be processed by fast roll-to-roll (R2R) methods, using earth abundant materials, and thus deliver the prospects to fulfil this potential. A strong polarization in PSC research efforts have led to diverging and non-comparable results: While very high power conversion efficiencies (PCEs) above 10% have been demonstrated for small area devices prepared by batch processing, the demonstration of PSCs fabricated in large quantities using high thoughput R2R fabrication of large are solar cells have presented much lower PCEs. This thesis primarily focuses on lowering the cost and environmental impact of polymer solar cell mass fabrication, by the substitution of toxic solvents for water, and eliminating the use of rare earth indium which accounts for 90 % of the embodied energy in state-of-the-art R2R fabricated devices based on ITO. Secondly, a scheme to potentially raise the efficiency of PSCs is explored, through R2R processing of tandem PSCs. A final focus area of the thesis is the investigation into the extrinsic variability in standard J-V characterizations done on PSCs, and towards ways to minimize it. Organic solvents are predominant process solvents used for fabricating the active layer of a PSC. In this thesis, aqueous dispersions of polymer:PCBM blend nanoparticles are fabricated by the miniemulsion method, and utilized as active layer inks in both small area devices and fully R2R processed large area polymer solar cells. An aqueous dispersion of P3HT:PCBM blend nanoparticles is also employed in conjunction with PEDOT:PSS in R2R double slot-die coating, a process that demonstrates the simultaneous formation of a P3HT:PCBM/PEDOT:PSS bilayer on a substrate comprising PET/ITO/ZnO. Devices are subsequently completed with a metal electrode demonstrating working solar cells. A third way of utilizing the aqueous nanoparticle inks is demonstrated in fully R2R processed polymer tandem solar cells, in which the second junction is coated from the nanoparticle dispersion. The use of water as an orthogonal solvent is shown to be necessary when upscaling the process from small glass-based devices to large area devices based on flexible PET. Also described in this thesis, is the development of an all-solution processed alternative to ITO as transparent conductor in PSCs. In its simples form the electrode consist of high conductive PEDOT:PSS R2R coated on a PET substrate. To enable functional devices, the completed solar cells are exposed to a short burst of high voltage in a R2R post process. The working mechanism of the ‘switching’ is found to be an in-situ formation of a charge-selective interface layer. To enable scalability the PEDOT:PSS is combined with several types of silver grids and utilized in large area PSCs. The use of a flexographically (flexo) printed grid is shown to be superior to both embedded grids and ink jet printed grid, especially in terms of processing speed. The scalability of the PEDOT:PSS/flexo grid electrode, the flextrode, is tested, and shown to be superior to ITO in terms of both performance and processing, demonstrating >1% efficiency on the total module area and a >50% fill-factor on a >100 cm2 module, while also demonstrating a significant reduction in materials cost and processing time. Finally the thesis includes two examples of round robin studies, one conducted in Europe and a second one across China. These studies investigate the extrinsic variations in PCE values obtained under standardized test conditions. The first study demonstrate how the round robin method can be used to evaluated and obtain consensus values on the PCEs of high efficiency devices. The second round robin included 15 laboratories in China. The inter-laboratory variations led to an overall relative standard deviation in PCE of 12%, primarily owing from variations in the current.