Advanced Functional Materials and Devices (AFMD) Group

Abstract:

We report on a series of organic solar cells based on heterojunetions of oligothiophene derivatives with varying chain length and C60 fullerenes. Devices arc based on cither p-i-n or p-i-i structure. In the first the intrinsic photovoltaic active layer is sandwiched between a p-typc and n-typc doped organic wide-gap layer for hole and electron transport respectively. In the latter the electron transport layer is replaced by a thin layer of wide-gap material as exciton blocker. Through optimization of transport and absorber layers we are able to reach in devices with single heterojunetions an open circuit voltage Voc of about 1V, a short circuit current density Jsc of about 5.6mA/cm² and a fill factor FF above 50% under an AM1.5 illumination with 1000W/m². However, still only a small part of the available solar spectrum is used. Thus, based on these materials stacked solar cells have been made to further improve the light absorption. The thickness of each layer is optimized using optical simulations to match the currents delivered by each of the solar cells in the stack. Through the incorporation of a very efficient recombination zone between the stacked solar cells the resulting Voc nearly reaches the sum of the V oc of the two serially connected solar cells.

Abstract:

Alternatives for replacing the expensive ITO are explored and Poly(ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) is introduced as one possibility. We present the first small-molecule organic solar cells employing only PEDOT:PSS as transparent electrode. Solar cells on glass and on flexible plastic foil were prepared, using a p-doped hole transporting material, zinc phthalocyanine (ZnPc) and C60 as donor-acceptor hctcrojunction, and an exciton blocking layer. Different methods to structure the PEDOT:PSS electrodes were investigated and are presented. As proof of principle, non-optimized prototype cells with efficiencies of over 0.7% on glass and 0.9% on flexible plastic foil substrate were obtained.

Abstract:

The transparent electron transport material NTCDA (1,4,5,8- naphthalenetetracarboxylic dianhydride) was examined in order to find a suitable substitute for C60 which is today often used in small molecular organic solar cells as transport layer. Due to its wide band gap, NTCDA does not absorb in the visible range and is furthermore exciton blocking. By doping with AOB (acridine orange base), its conductivity was raised to about 1 · 10^_4S/cm. It can therefore simultaneously be used as electron transport material and optical spacer in p-i-n type solar cells, leading to power conversion efficiencies of up to 2.83%. Additionally, an investigation of the surface morphology using AFM was performed.

Abstract:

To reach higher performances in organic solar cells, each layer has to be optimised with respect to its purpose. In the case of a p-i-n structured solar cell, the layers are the absorber system, the doped electron and hole transport layers, and the bottom and top contacts. This work focuses on the investigation and characterisation of the transparent hole transport materials PV-TPD, PV-TPDoM, Di-NPB, and MeO-Spiro-TPD, as used in organic p-i-n solar cells. The motivation is to replace the hole transport material MeO-TPD, which has been used so far despite its morphological instability at elevated temperatures, with an energetically and morphologically more suitable material. The hole transport materials were investigated for dopability, hole mobility, absorption, reflection, cyclic voltammetry, and glass transition temperature. Further specific material properties were determined with simplified structures, e.g. m-i-p diodes, and the standard solar cells, consisting of the fullerene C 60 as acceptor and ZnPc as the donor material. The Di-NPB has turned out to be the best choice with respect to its intrinsic properties and device parameters. The deep lying HOMO, the high hole mobility of μ = 1.9 · 10^-4 cm²/Vs, the morphological stability of Tg = 158°C, and the excellent results of the C60:ZnPc bulk heterojunction solar cell makes the Di-NPB highly suitable for replacement of the MeO-TPD in organic solar cells.

Abstract:

We report on organic p-i-n homojunctions composed of differently doped vacuum-deposited pentacene layers. We observe a remarkably high built-in voltage of 1.65 V. An analysis of the current-voltage characteristics under dark and illuminated conditions reveals that the open-circuit voltage is directly related to the built-in voltage and that the recombination process is influenced by the distinct charge transport properties of electrons and holes in the pentacene film. By a comparison with p-i-p and n-i-n single-carrier homojunctions, deep trap states located around 0.63 eV below the electron transport level are shown to influence the properties. © 2008 The American Physical Society.