Three‐Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

Abstract

Here, an investigation of three‐dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post‐treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (ηIQE ) of the devices. To investigate the 3D BHJ morphology, novel time‐of‐flight secondary‐ion mass spectroscopy studies are employed along with conventional methods, such as UV‐vis absorption, X‐ray diffraction, and high‐resolution transmission electron microscopy studies. The ηIQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high‐performance P3HT:PCBM solar cells using the optimized solvent‐annealing method is reported, and these cells show a mean power‐conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm−2.

On the morphology of polymer‐based photovoltaics

“…61 Loos and coworkers used transmission electron microscopy tomography and found that solvent annealing resulted in an interpenetrating nanofibrillar morphology with fibrils of P3HT forming a crystalline network with a mixture of P3HT and PCBM in the interfibrillar regions. 62 Kim et al used the ToF-SIMS to determine the composition of the components normal to the surface of the films and found that solvent annealing caused PCBM to migrate to the surface, 63 which is similar to the findings of Nelson et al and is desirable for enhancing device efficiency. 48 Li et al used a slow drying process to control the P3HT crystallization and phase separation.…”

Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend

“…[ 5 , 9 ] Some suggest that this increase in performance is due to crystallization of disordered P3HT, resulting in the diffusion of the fullerene component out of the polymer matrix thereby forming effi cient charge transport pathways between the electrodes. [ 12 ] Others speculate that the increase in the polymer crystal size during thermal annealing inhibits the evolution of large fullerene domains, thus forming nano-scale crystalline domains of P3HT and PCBM. [ 13 ] Even though insight into this system has grown tremendously, the understanding of the evolution of the BHJ morphology and improvement in photoconversion efficiency with thermal annealing remains poorly understood.…”

Sequential Processing for Organic Photovoltaics: Design Rules for Morphology Control by Tailored Semi‐Orthogonal Solvent Blends

“…We believe that use of this co-solvent blend increases PTB7 diffraction because the solvent re-dissolves part of the polymer and then recrystallizes it upon drying, similar to solvent annealing. [ 90,91 ] Regardless, such dramatic changes in fi lm structure run counter to the ideas of SqP, where device reproducibility is guaranteed by the fact that polymer fi lms swell without dramatic restructuring of their morphology. As a result, for SqP we would like to avoid significant changes in polymer morphology upon fullerene intercalation like those shown in Figure 5 b.…”