Andrey E. Rudenko

Polymer-Based Solar Cells: State-of-the-Art Principles for the Design of Active Layer Components

Abstract The vision of organic photovoltaics is that of a low cost solar energy conversion platform that provides lightweight, flexible solar cells that are easily incorporated into existing infrastructure with minimal impact on land usage. Polymer solar cells have been a subject of growing research interest over the past quarter century, and are now developed to the point where they are on the verge of introduction into the market. Towards the goal of continuing to improve the performance of polymer solar cells, a number of avenues are being explored. Here, the focus is on optimization of device performance via the development of a more fundamental understanding of device parameters. The fundamental operating principle of an organic solar cell is based on the cooperative interaction of molecular or polymeric electron donors and acceptors. Here the state-of-the-art in understanding of the physical and electronic interactions between donor and acceptor components is examined, as is important for understanding future avenues of research and the ultimate potential of this technology.

Influence of β-linkages on the morphology and performance of DArP P3HT-PC61BM solar cells.

Direct arylation polymerization (DArP) has emerged as a greener and more atom-efficient alternative to Stille polymerization. Despite the attractiveness of this method, DArP is known to produce β-linkages in polymers, which have β-protons available for activation. Here, we report the influence of the β-defect content in DArP poly(3-hexylthiophene) (P3HT) on the performance of bulk-heterojunction solar cells and the morphology of pristine polymers and their blends with PC61BM in thin films and compare with Stille P3HT containing 0% β-defects as a reference point. The optical and electrochemical properties as well as the hole mobilities of pristine polymers remain virtually the same when the amount of β-defects is limited to 0.75% or lower, as evidenced by UV-visible absorption spectra, cyclic voltammetry and space-charge-limited current (SCLC) mobility measurements. However, an increase of β-defect concentration to 1.41% significantly affects the oxidation onset, UV-visible absorption profile and hole mobility of P3HT. The key result of this study is that the photovoltaic performance of DArP P3HT with 0% β-defects is remarkably close to that of Stille P3HT, whereas the performance of DArP P3HT with 0-0.75% β-defects does not differ dramatically from that of Stille P3HT and could potentially be improved upon by individual optimization of the processing conditions.

Influence of selenophene on the properties of semi-random polymers and their blends with PC61BM

In an effort to broaden the absorption of conjugated polymers, atomistic bandgap control was applied to the semi-random polymer architecture. Here, we report the physical properties of semi-random polyselenophenes as compared to analogous polythiophenes. In order to examine the effect of the selenium heteroatom on the optical properties of the polymers, UV-vis spectra were studied and it was found that all polyselenophenes exhibit lower bandgaps and higher absorption coefficients in thin films. Further, differential scanning calorimetry and grazing incidence x-ray diffraction results indicate that semi-random polyselenophenes are semicrystalline polymers and their (100) interchain distances are shorter than in the case of semi-random polythiophenes, which may be responsible for higher absorption coefficients. To probe the effect of the selenium heteroatom on the nano-organization of these polymers and their blends with PC61BM, thin films were studied by transmission electron microscopy (TEM). The TEM images show a segregation between more densely packed areas from less densely packed areas in the pristine polymer films, which is more pronounced for polyselenophenes than for polythiophenes. The blends of polyselenophenes with PC61BM do not show the well-defined segregation observed for the polythiophene analogues. However, the broadened and extended absorption of semi-random polyselenophenes translates into an extended photocurrent response in the photovoltaic devices, as evidenced by external quantum efficiency measurements.

Photochemical stability of random poly(3-hexylthiophene-co-3-cyanothiophene) and its use in roll coated ITO-free organic photovoltaics

Abstract. The photochemical stability of the active layer blend for organic solar cells was explored by introducing electron withdrawing cyano groups into the backbone of poly-3-hexylthiophene (P3HT). Random copolymerization of 2-bromo-3-hexyl-5-trimethylstannylthiophene and 2-bromo-3-cyano-5-trimethylstannylthiophene enabled introduction of the cyanogroups along the polythiophene backbone. The percentage of the cyano groups was 10%. The photochemical stability of poly(3-hexylthiophene-co-3-cyanothiophene) (CN-P3HT) was shown to be significantly better than pristine P3HT and the addition of CN-P3HT to P3HT also increased the photochemical stability of the blend. The photochemical stability of bulk heterojunction mixtures of the polymers and their blends with the fullerene phenyl-C61-butyric acid methyl ester ([60]PCBM) were then studied and it was found that [60]PCBM had a significantly more stabilizing effect on P3HT than CN-P3HT and that the stabilization of the bulk heterojunction mixture was dominated by the fullerene. The mixture comprising both fullerene and CN-P3HT, however, demonstrated the highest degree of photochemical stability supporting earlier observations that the stabilizing effects are additive. Finally, the blends were explored in fully printed flexible ITO-free roll coated inverted devices (with an active area of 0.8  cm2) using two different back PEDOT:PSS electrode compositions and the operational stability of the devices was studied under ISOS-L-2 conditions. The pure P3HT:PCBM devices were found to be the most stable in operation demonstrating that photochemical stability alone is not necessarily the dominant factor for overall device stability.