Abstract
Rationally controlling the vertical component distribution within a photoactive layer is crucial for efficient polymer solar cells (PSCs). Herein, fine-tuning the surface free energy (SFE) of the titanium(IV) oxide bis(2,4-pentanedionate) (TOPD) cathode buffer layer is proposed to achieve a desired perpendicular component distribution for the PBDB-T:ITIC-M photoactive layer. The Owens-Wendt method is adopted to precisely calculate the SFE of TOPD film jointly based on the water contact angle and the diiodomethane contact angle. We find that the SFE of TOPD film increases as the annealing temperature rises, and the subtle SFE change causes the profound vertical component distribution within the bulk region of PBDB-T:ITIC-M. The results of secondary-ion mass spectroscopy visibly demonstrate that the TOPD film with an SFE of 48.71 mJ/cm2, which is very close to that of the ITIC film (43.98 mJ/cm2), tends to form desired vertical component distribution. Consequently, compared with conventional bulk heterojunction devices, the power conversion efficiency increases from 9.00 to 10.20% benefiting from the short circuit current density increase from 14.76 to 16.88 mA/cm2. Our findings confirm that the SFE adjustment is an effective way of constructing the desired vertical component distribution and therefore achieving high-efficiency PSCs.
Highlights
Stimulated by the need for a clean renewable energy source, there has been considerable interest in exploring polymer solar cells (PSCs) due to their unique properties of low cost, light weight, and flexibility (Krebs et al, 2010; Li G. et al, 2012; Zhao et al, 2017)
We found that the surface free energy (SFE) of TOPD changes as the annealing temperature increases, and the PBDB-T:ITIC-M photoactive layer with the desired vertical component distribution is obtained via fine controlling the SFE of the TOPD layer, leading to a power conversion efficiency (PCE) high up to 10.20% for inverted PSCs
Fine-tuning the SFE of the TOPD cathode buffer layer has been explored in this work, with the aim of unraveling the underlying mechanism and rational controlling the vertical distribution of the electron donor and acceptor
Summary
Stimulated by the need for a clean renewable energy source, there has been considerable interest in exploring polymer solar cells (PSCs) due to their unique properties of low cost, light weight, and flexibility (Krebs et al, 2010; Li G. et al, 2012; Zhao et al, 2017). We found that the SFE of TOPD changes as the annealing temperature increases, and the PBDB-T:ITIC-M photoactive layer with the desired vertical component distribution is obtained via fine controlling the SFE of the TOPD layer, leading to a PCE high up to 10.20% for inverted PSCs. The results of time-offlight secondary-ion mass spectroscopy (TOF-SIMS) visually present the optimized vertical concentration distribution, and the space-charge-limited current (SCLC) method elucidates that the rational vertical component distribution guarantees fully exciton dissociation and facilitates charge transportation.
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