Synergetic enhancement of organic solar cell thermal stability by wire bar coating and light processing

Abstract

† Electronic Supplementary Information (ESI) available: SC and WBC drying kinetics measurements. UV-vis absorption spectra of SC and WBC PCDTBT:PC60BM blend layers. PL spectra of SC and WBC blended layers and their respective PCDTBT neat layers. EQE of blend devices deposited by SC and WBC techniques. Optical microscopy images of the blend layer deposited by SC and WBC on SiOx (as cast) and PEDOT:PSS (at 80°C and 140°C thermal annealing temperature). SeeWe demonstrate that organic solar cells can exhibit different morphological and performance stability under thermal stress depending upon the processing technique employed, without compromising initial device efficiency. In particular, we investigate benchmark PCDTBT:PC60BM solar cells fabricated by wire bar coating (a technique attractive for commercial manufacture) and the more widely employed, lab scale, technique of spin coating. For this system, wire bar deposition results in superior device stability, with lifetime improvements in excess of 20-fold compared to spun cast devices. Neutron reflectivity reveals that the enhanced PC60BM segregation to the top interface in the slower, wire bar, casting process is likely responsible for the hindered PC60BM nucleation at tens of nm length scale, characterized by atomic force microscopy (AFM), and thus enhanced morphological stability. Modest light exposure of the active layer (at approximately 10 mWcm -2 ), known to reversibly photo-oligomerize fullerenes and thus impart morphological stability, is found to further improve device stability by a factor of 10. The combined effects of wire bar coating and light processing are highly synergetic, resulting in solar cells which are overall 200 times more stable than devices prepared by spin casting without light processing.

Stability of Polymer:PCBM Thin Films under Competitive Illumination and Thermal Stress

“…The morphological stability of polymer/fullerene solar cells to thermal stresses and light exposure was investigated with model PS:PCBM thin films. Our work builds upon numerous reports of C 60 and PCBM phototransformations, dimerization, and burn‐in effects in organic solar cell stability, by our group and several others . To the best of our knowledge, however, this is the first time that the coupling of illumination, known to induce fullerene dimerization, and thermal stress, known to induce decomposition (or dedimerization), were simultaneous investigated.…”

“…This stabilization effect was also observed upon addition of chemically (rather than photochemically) synthesized fullerene dimers, which is likely related to the combined frustration of the fullerene crystallization processes and their reduced mobility. Substrate segregation and deposition methods, combined with light exposure, were found to synergetically prolong the lifetime of PCDTBT:PC 60 BM in excess of 200 times, compared to a reference device. However, other reports suggest detrimental or neutral effects to PCE upon illumination.…”

From fullerene acceptors to non-fullerene acceptors: prospects and challenges in the stability of organic solar cells

“…8,97 The versatile morphological stabilisation observations hold for other polymer:C 60 and polymer:PC 61 BM systems; 8,30,94,95 other substrates; 8,97 and other deposition methods. 98 While fullerene dimerisation is general to many donor polymers and has a significant impact on their thermal stability, the photoinduced fullerene dimers can be thermally reversed to its pristine monomeric form especially at sufficiently elevated temperatures, see Fig. 5(c).…”

“…51 Whilst wire-bar coating can affect vertical phase separation and film nucleation, likely correlated with the increased drying times, and can lead to improved stabilities when compared to spin-coated devices. 98 The interaction between interlayers and the active layer can also affect morphological stability, for example, by controlling the nucleation and film formation processes, 97 and the driving force for vertical phase separation. 160 Selection of a better matched interlayer can effectively address these issues.…”

Impact of solution phase behaviour and external fields on thin film morphology: PCBM and RRa-P3HT model system

“…The importance of the drying pathway has been recognised by several authors and has been found to considerably impact performances. [22][23][24]…”