Enhanced Air Stability of Polymer Solar Cells with a Nanofibril-Based Photoactive Layer

In spite of the rapid increase in the power conversion efficiency (PCE) of polymer solar cells (PSCs), the poor stability of the photoactive layer in air under sunlight is a critical problem blocking commercialization of PSCs. This study investigates the photo-oxidation behavior of a bulk-heterojunction (BHJ) photoactive film made of single-crystalline poly­(3-hexlythiophene) (P3HT) nanofibrils and fullerene derivatives [phenyl-C₆₁-butyric methyl ester (PCBM), indene-C 60 bisadduct (ICBA)]. Because the single-crystalline P3HT nanofibrils had tightly packed π–π stacking, the permeation of oxygen and water into the nanofibrils was significantly reduced. Chemical changes in P3HT were not apparent in the nanofibrils, and hence the air stability of the nanofibril-based BHJ film was considerably enhanced as compared with conventional BHJ films. The chemical changes were monitored by Fourier-transform infrared (FT-IR) spectroscopy, Raman spectroscopy, and UV–vis absorbance. Inverted PSCs made of the nanofibril-based BHJ layer also showed significantly enhanced air stability under sunlight. The nanofibril-based solar cell maintained more than 80% of its initial PCE after 30 days of continuous exposure to sunlight (AM 1.5G, 100 mW/cm²), whereas the PCE of the conventional BHJ solar cell decreased to 20% of its initial PCE under the same experimental conditions

Effect of PEDOT Nanofibril Networks on the Conductivity, Flexibility, and Coatability of PEDOT:PSS Films

The use of poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) in electrodes and electrical circuits presents a number of challenges that are yet to be overcome, foremost amongst which are its relatively low conductivity, low coatability on hydrophobic substrates, and decreased conductivity at large strains. With this in mind, this study suggests a simple way to simultaneously address all of these issues through the addition of a small amount of a nonionic surfactant (Triton X-100) to commercial PEDOT:PSS solutions. This surfactant is shown to considerably reduce the surface tension of the PEDOT:PSS solution, thus permitting conformal coatings of PEDOT:PSS thin film on a diverse range of hydrophobic substrates. Furthermore, this surfactant induces the formation of PEDOT nanofibrils during coating, which led to the high conductivity values and mechanical stability at large strains (ε = 10.3%). Taking advantage of the superior characteristics of these PEDOT:PSS thin films, a highly flexible polymer solar cell was fabricated. The power conversion efficiency of this solar cell (3.14% at zero strain) was preserved at large strains (ε =7.0%)

Self-Seeded Growth of Poly(3-hexylthiophene) (P3HT) Nanofibrils by a Cycle of Cooling and Heating in Solutions

In spite of the recent successes in transistors and solar cells utilizing poly­(3-hexylthiophene) (P3HT) nanofibrils, systematic analysis on the growth kinetics has not been reported due to the lack of analytical tools. This study proposed a simple spectroscopic method to obtain the crystallinity of P3HT in solutions. On the basis of the analytical approach, we found that the crystallinity hysteresis upon temperature is a simple function of the solubility parameter difference (Δδ) between the P3HT and the solvents. When Δδ ≥ 0.7, a cooling (−20 °C)-and-heating (25 °C) process allowed the preparation of solutions including 1D crystal seeds dispersed in the solution. Simple coating of the seeded solutions completed the growth of the seeds into long nanofibrils at the early stage of the coating and thereby achieved almost 100% crystallinity in the resulting films without any postannealing process. The existence of PCBM for bulk-heterojunction (BHJ) solar cells did not affect the nucleation and growth of the nanofibrils during the cooling-and-heating process. The solar cells prepared from the solutions with Δδ ≥ 0.7 had solar conversion efficiencies higher than the conventional thermally annealed cells

Highly Bendable Large-Area Printed Bulk Heterojunction Film Prepared by the Self-Seeded Growth of Poly(3-hexylthiophene) Nanofibrils

Applying conventional printing technologies to fabricate large-area flexible bulk heterojunction (BHJ) solar cells is of great interest. Achieving this task requires (i) large tolerance of the maximum photoconversion efficiency (PCE) to the film thickness, (ii) fast hole transport in both the thickness and lateral directions of the BHJ layer, and (iii) improved stability against bending and heat. This paper demonstrates that a P3HT:PCBM BHJ layer made of long P3HT nanofibrils of almost 100% crystallinity can be an excellent approach to achieve large-area printed solar cells. We applied a cool-and-heat (C&H) process with a P3HT/PCBM <i>m</i>-xylene solution to generate P3HT:PCBM nanofibril composite films. We found that the hole transport of the nanofibril composite was 2.6 times faster in the thickness direction and 6.5 times more conductive in the in-plane direction compared with conventionally annealed composites. The fast hole transport in the thickness direction led to negligible dependence of the PCE on the thickness of the composite layer. The improved conductivity in the in-plane direction prevented the sharp drop of the PCE as the active area increased. Taking advantage of the unique characteristics, we employed a roll-printing method to fabricate large-area unit solar cells in air. In addition, the curved contour path of the nanofibrils provided excellent stability against large bending strains, allowing the production of highly bendable organic solar cells