Tuning domain size and crystallinity in isoindigo/PCBM organic solar cells via solution shearing

Abstract

Despite having achieved the long sought-after performance of 10% power conversion efficiency, high performance organic photovoltaics (OPVs) are still mostly constrained to lab scale devices fabricated by spin coating. Efforts to produce printed OPVs lag considerably behind, and the sensitivity to different fabrication methods highlights the need to develop a comprehensive understanding of the processing-morphology relationship in printing methods. Here we present a systematic experimental investigation of a model low bandgap polymer/fullerene system, polyisoindigo thienothiophene/PC 61 BM, using a lab-scale analogue to roll-to-roll coating as the fabrication tool in order to understand the impact of processing parameters on morphological evolution. We report that domain size and polymer crystallinity can be tuned by a factor of two by controlling the temperature and coating speed. Lower fabrication temperature simultaneously decreased the phase separation domain size and increased the relative degree of crystallinity in those domains, leading to improved photocurrent. We conclude that domain size in isoindigo/PCBM is dictated by spontaneous phase separation rather than crystal nucleation and growth. Furthermore we present a model to describe the temperature dependence of domain size formation in our system, which demonstrates that morphology is not necessarily strictly dependent on the evaporation rate, but rather on the interplay between evaporation and diffusion during the printing process.

Surpassing 10% Efficiency Benchmark for Nonfullerene Organic Solar Cells by Scalable Coating in Air from Single Nonhalogenated Solvent

“…As shown in Figure 3b, we were able to achieve an average PCE greater than 10% when the coating temperature was 70-80 °C and the optimum performance of nearly 11% can be achieved at 70 °C, while the device FF was significantly lower when the coating temperature was below 70 °C or above 80 °C. Our data clearly suggest that the coating temperature is a critical parameter for scalable coating of the blend films, and this finding is in line with the previous reports of blade-coating polymer:fullerene OSCs [13,14,29] . Additionally, the film thickness can be readily tuned from 80 nm to 200 nm by altering the blade speed for a fixed solution concentration of 16 mg/ml in total.…”

The meniscus-guided deposition of semiconducting polymers

“…Other deposition parameters like substrate temperature are also important factors that affect the final deposited BHJ film. For example, higher temperatures result in increased phase-separated domain size and reduced polymer crystallinity in isoindigo/fullerene BHJ films 125 . Moreover, external fields (e.g., electrical 126 and magnetic) or UV light can be used to aid polymer association and aggregation, methods yet to be explored for OPV deposition.…”

Composition–Morphology Correlation in PTB7-Th/PC71BM Blend Films for Organic Solar Cells

“…With the demand for photovoltaic devices of low cost, light weight, environmental friendliness, ease of production, flexibility, and building-integrated potential, organic solar cells (OSCs) as a revolutionary green technology have gained immense research interest over the last decade. − In particular, the suitability for large-scale implementation in device fabrication like roll-to-roll printing promotes OSCs to have a great promise for practical applications. − Up to now, power conversion efficiencies (PCEs) of OSCs have been pushed well beyond 10%, − which are capable of competing with their inorganic counterparts. This great success is closely related to the introduction of the bulk heterojunction (BHJ) configuration, in which a blend of donor and acceptor materials is envisioned to have a co-continuous morphology with a local nanophase separation. − The active layers with BHJ configuration are typically manufactured with solution-based methods, which feature great complexity in morphology, crystallinity, and miscibility of the donor and acceptor materials since nonequilibrium morphologies are generated. − Moreover, these factors have a direct impact on the final device performance of the OSCs. Therefore, a close look into morphology of the BHJ configuration of the active layers is necessary .…”