Abstract
Understanding the phenomena at interfaces is crucial for producing efficient and stable flexible organic solar cell modules. Minimized energy barriers enable efficient charge transfer, and good adhesion allows mechanical and environmental stability and thus increased lifetime. We utilize here the inverted organic solar module stack and standard photoactive materials (a blend of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester) to study the interfaces in a pilot scale large-area roll-to-roll (R2R) process. The results show that the adhesion and work function of the zinc oxide nanoparticle based electron transport layer can be controlled in the R2R process, which allows optimization of performance and lifetime. Plasma treatment of zinc oxide (ZnO) nanoparticles and encapsulation-induced oxygen trapping will increase the absolute value of the ZnO work function, resulting in energy barriers and an S-shaped IV curve. However, light soaking will decrease the zinc oxide work function close to the original value and the S-shape can be recovered, leading to power conversion efficiencies above 3%. We present also an electrical simulation, which supports the results. Finally, we study the effect of plasma treatment in more detail and show that we can effectively remove the organic ligands around the ZnO nanoparticles from the printed layer in a R2R process, resulting in increased adhesion. This postprinting plasma treatment increases the lifetime of the R2R printed modules significantly with modules retaining 80% of their efficiency for ∼3000 h in accelerated conditions. Without plasma treatment, this efficiency level is reached in less than 1000 h.
Highlights
Several groups have demonstrated roll-to-roll (R2R) processes for flexible solar cells with the maturity level of high volume production increasing yearly.[1−3] Organic materials are one of the promising options for flexible solar cells, especially as novel high-performance organic photoactive materials are continuously emerging, showing power conversion efficiencies (PCE) above 14%.4 In addition to a cost-efficient processes and high performance, the lifetime of the solar cell modules needs to be at an acceptable level in order to meet market demands
As we have presented previously,[21] postprinting plasma treatment of the printed zinc oxide (ZnO) nanoparticle electron transport layer (ETL) will increase its adhesion on indium tin oxide (ITO) and improve the module performance due to removal of the organic insulating ligands around the nanoparticles
We can see that the pressure sensitive adhesives (PSA) encapsulation process has a negative effect on the performance, and especially in the plasma treated modules, we see an S-shape in the IV curve
Summary
Several groups have demonstrated roll-to-roll (R2R) processes for flexible solar cells with the maturity level of high volume production increasing yearly.[1−3] Organic materials are one of the promising options for flexible solar cells, especially as novel high-performance organic photoactive materials are continuously emerging, showing power conversion efficiencies (PCE) above 14%.4 In addition to a cost-efficient processes and high performance, the lifetime of the solar cell modules needs to be at an acceptable level in order to meet market demands. The stability is measured using standardized tests in accelerated conditions,[7] which allow estimates of real lifetimes for the modules.[8] In addition, OPVs have been tested in harsh outdoor conditions.[9] Lifetime of organic photovoltaics (OPV) has been improving,[10] e.g., due to more stable materials,[11−14] proper selection of solvents,[15] better encapsulation processes,[16,17] and transferring from the conventional unstable device structure to one with enhanced stability.[18] The degradation of wellencapsulated flexible solar cells is known to start from edges of the modules, where water and oxygen can diffuse into the device easiest.[9] Degradation mechanisms include photo-oxidization of Received: June 26, 2018 Accepted: October 8, 2018 Published: October 8, 2018
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