Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells

Abstract

We investigate thin poly(3‐hexylthiophene‐2,5‐diyl)/[6,6]‐phenyl C61 butyric acid methyl ester (P3HT/PCBM) films, which are widely used as active layers in plastic solar cells. Their structural properties are studied by grazing‐incidence X‐ray diffraction (XRD). The size and the orientation of crystalline P3HT nanodomains within the films are determined. PCBM crystallites are not detected in thin films by XRD. Upon annealing, the P3HT crystallinity increases, leading to an increase in the optical absorption and spectral photocurrent in the low‐photon‐energy region. As a consequence, the efficiency of P3HT/PCBM solar cells is significantly increased. A direct relation between efficiency and P3HT crystallinity is demonstrated.

Influence of blend microstructure on bulk heterojunction organic photovoltaic performance

“…The growth of the PCBM clusters leads to the formation of percolation paths and, therefore, improvement of the photocurrent. Further studies by Erb et al 51 suggest that the significant increase in optical absorption is due to the aggregation state of P3HT, from amorphous (non-annealed) to crystalline (annealed). Grazing-incidence X-ray diffraction (XRD) measurements of P3HT:PCBM films reveal an increase in sharpness of the diffraction peak pertaining to the interchain, intra-lamellar ordering (a-axis) of P3HT upon thermal annealing Q4 ( Fig.…”

“…A substantial improvement in the PCE of P3HT:PCBM based solar cells has been obtained by thermal annealing at around 140 1C, and the favourable influence of annealing on blend film microstructure and solar cell performance is well documented. [50][51][52][53] The mechanism by which current density and PCE increase upon thermal annealing in P3HT:PCBM is not known precisely but it is believed to involve increased optical absorption and improved charge transport, both due to polymer crystallization, as well as improved charge pair separation efficiency, due to optimized phase segregation. Some of the literature is reviewed below.…”

“…The efficiency gain upon annealing is therefore more commonly attributed to reduced recombination losses, resulting from the increased phase segregation and molecular ordering, and increased spectral photocurrent generation in the near-IR than to improved transport. 51,56 Savenije and co-workers reported that annealing of P3HT:PCBM composites not only changes the morphology and optical absorption but can also change the redox and photoconductive properties of the blend layer. 57 Enhanced photoconductivity is mainly ascribed to the increase in the hole mobility.…”

Controlling PCBM aggregation in P3HT/PCBM film by a selective solvent vapor annealing

“…When represented by a two-dimensional model, it showed that a phase separated morphology with feature sizes smaller than ~50 nm could improve charge transport within and between the two phases and reduce bimolecular charge recombination [6]. In the case of P3HT/PCBM, the molecular ordering of rr-P3HT is concerned with photon absorption and hole transport [7][8][9], and the suitable size of phase separation is related to charge separation. In order to achieve an optimum morphology, many methods have been employed, including thermal annealing [10,11], solvent vapor annealing [12][13][14], mixed solvent [15,16], introduction of additives [17,18] and so on.…”

“…The scale of them was obviously much greater than the exciton diffusion length. Meanwhile, the diffusion of PCBM out of the polymer matrix to form aggregates enables the disordered P3HT to further crystallize [7], not disrupts the ordered lamellar …”