Diana K. Susarova

Material solubility and molecular compatibility effects in the design of fullerene/polymer composites for organic bulk heterojunction solar cells

We report a systematic study of more than 100 bicomponent systems composed of 19 different fullerene derivatives blended with 9 different conjugated polymers (including previously investigated poly(3-hexylthiophene)). It was shown that short circuit current density (JSC) and light power conversion efficiency (η) of the fullerene/polymer photovoltaic devices depend on the solubility of the fullerene components in the solvent used for the blend film deposition (chlorobenzene). The revealed dependences have unusual “double branch” character because many fullerene derivatives possessing similar solubilities showed different photovoltaic performances. This behavior was related to the peculiarities of the molecular structures of the fullerene derivatives. Substituents attached to the cyclopropane ring fused with the fullerene cage in methanofullerenes affected both the morphology of their composites with conjugated polymers and their photovoltaic performance. It was demonstrated that variation of the fullerene component blended with a conjugated polymer might easily change its photovoltaic performance by a factor of 3–4. The obtained results proved that design of appropriate fullerene derivatives and novel conjugated polymers are equally important tasks on the way towards highly efficient organic photovoltaics.

PHOTOVOLTAIC PERFORMANCE OF PPE-PPV COPOLYMERS: EFFECT OF THE FULLERENE COMPONENT

Two conjugated PPE-PPV copolymers were studied as electron donor materials in bulk heterojunction organic solar cells in combination with a library of electron acceptor fullerene derivatives. It was shown that molecular structure and solubility of the fullerene counterpart significantly affect the photovoltaic performance of both polymers. Use of [60]PCBM as an electron acceptor material yielded quite moderate power conversion efficiencies. The best results were achieved when fullerene derivatives with suitable molecular structures and solubilities were applied. The obtained results suggest that every newly designed conjugated polymer should be evaluated in solar cells using a library of fullerene derivatives instead of just conventional PCBMs. We believe that only this combinatorial approach might bring the best performing donor/acceptor combinations for future generations of efficient organic solar cells.

Morphology evaluation of a polymer–fullerene bulk heterojunction ensemble generated by the fullerene derivatization

We investigated a series of bulk heterojunction solar cells and corresponding thin films by blending of an anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (PPE–PPV) copolymer with various fullerene derivatives. The so-called AnE-PVab copolymer was equipped with octyloxy side-chains at the PPE part and ethylhexyloxy side-chains at the PPV part, and has been chosen for this investigation due to its previously demonstrated superior performance in combination with phenylene-C60-butyric-acid-methyl-ester (PCBM). It is shown that the performance of bulk heterojunction solar cells strongly depends on the nature of the fullerene derivative, in particular the side-chain attached to the fullerene cage. In fact the intermolecular interaction between the polymer and fullerene derivative controls the blend morphology to a large extent, which has been interpreted on the basis of a novel approach for combining structural, topographical, photophysical, electrical and electro-optical characterizations.

Trannulenes: a new class of photoactive materials for organic photovoltaic devices

We report the first large-scale preparation of fluorinated trannulene C60F15[C(COOMe)3]3 (TMCMT) and its application as a photoactive material in organic solar cells. Two types of photovoltaic devices were fabricated: bulk heterojunction cells comprising blends of TMCMT with a polyfluorene polymer and bilayer cells based on a planar heterojunction of TMCMT with a perylene diimide. Photoluminescence quenching studies suggested that a photoinduced electron transfer occurs from polyfluorene and perylenediimide to TMCMT, therefore it behaves as photoinduced electron acceptor. Both types of solar cells comprising TMCMT as photoactive material showed clear photovoltaic effects. This proves for the first time the feasibility of the previously anticipated idea of using trannulenes as light-harvesting materials in photovoltaic devices.

Light-induced generation of free radicals by fullerene derivatives: an important degradation pathway in organic photovoltaics?

We present a systematic comparative study of the intrinsic photochemical stability of several fullerene-polymer systems under the natural outdoor conditions in the Negev desert. It has been shown, in particular, that light-induced dimerization of [60]fullerene derivatives is irrelevant to the degradation behavior of the solar cells incorporating these materials in the blends with PCDTBT. The conventional [60]PCBM was shown to undergo rapid and severe photodimerization, when exposed to sunlight, which, however, does not noticeably affect the PCDTBT/[60]PCBM device efficiency. In contrast, two novel fullerene derivatives, which were shown to be far more resistant towards the photodimerization, induced a dramatic failure of the solar cell performance. The application of electron paramagnetic resonance (EPR) spectroscopy allowed us to reveal an alternative photochemical degradation pathway of the fullerene derivatives resulting in the formation of persistent free radicals. The accumulation of free radicals in the active layer was shown to be a critical degradation mechanism, ruining the photovoltaic performance of the devices. These findings strongly suggest that the current understanding of the processes responsible for the “burn-in” failure of organic solar cells has to be reconsidered and additional studies should be performed to clarify the actual mechanisms of the relevant processes.

ITO modification for efficient inverted organic solar cells

We demonstrate a facile approach to designing transparent electron-collecting electrodes by depositing thin layers of medium and low work function metals on top of transparent conductive metal oxides (TCOs) such as ITO and FTO. The modified electrodes were fairly stable for months under ambient conditions and maintained their electrical characteristics. XPS spectroscopy data strongly suggested integration of the deposited metal in the TCO structure resulting in additional doping of the conducting oxide at the interface. Kelvin probe microscopy measurements revealed a significant decrease in the ITO work function after modification. Organic solar cells based on three different conjugated polymers have demonstrated state of the art performances in inverted device geometry using Mg- or Yb-modified ITO as electron collecting electrode. The simplicity of the proposed approach and the excellent ambient stability of the modified ITO electrodes allows one to expect their wide utilization in research laboratories and electronic industry.

Synthesis of novel conjugated polymers comprising modified cyclopentadithiophene units in the main chain

We have synthesized novel copolymers P1–P3 comprising fluorene units alternating with cyclopentadithiophene fragments modified at the bridge positions with cyanovinyl or imine groups. Optoelectronic properties of the polymers were investigated in solution and in solid films. A potential of using these materials in organic electronics was considered.