Advanced Functional Materials and Devices (AFMD) Group

Abstract:

We present semitransparent small-molecule organic solar cells (OSC) deposited by thermal evaporation onto indium tin oxide (ITO)-coated glass substrates. The devices employ ITO-free ultrathin metal layers as top electrodes, containing 1 nm metal surfactant interlayer for improved morphology. Using a bulk heterojunction of zinc phthalocyanine and C60, sandwiched in between doped dedicated transport layers for efficient charge carrier extraction, power conversion efficiencies comparable to conventional OSC with an intransparent thick back electrode and similar device layout are achieved: the semitransparent OSC yield power conversion efficiencies well above 2% with external quantum efficiencies above 30%-40%. Organic light incoupling layers improve the transmission to up to 50% in the visible part of the optical spectrum. © 2009 American Institute of Physics.

Abstract:

We have investigated the ageing of ZnPc/C60 solar cells in terms of changes of the charge carrier mobility and potential barrier height. The initial fast degradation of device performance within several hours was followed by a much slower one. The mobility decreases during degradation by about 30 to 50 %. Moreover, an increase of the effective barrier height at ZnPC/C60 interface from ∼0.55 eV up to ∼0.65 eV was observed. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV to about 0.34 eV after ageing. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

Abstract:

Thin metal films are a desirable alternative to indium tin oxide for utilization in organic solar cells (OSC).We describe background and processing parameters for thin metal films and show examples of top-illuminated OSC employing metal electrodes. Simulations are introduced as tool for OSC fabrication; several pitfalls are presented which must be considered for successful numerical treatment of thin-film layers at coalescence thicknesses and for coherent treatment. © 2009 SPIE.

Abstract:

For organic solar cells, effective absorption over a wide wavelength range is important. A simple donor-acceptor pair is usually not sufficient to reach this goal. Thus, it would be desirable to utilize multiple photoactive materials in a single cell. In this work, two hole conducting materials, pentacene and zinc phthalocyanine (ZnPc), and electron conducting C60 are chosen to construct three-component heterojunctions aiming at improved effective photon harvesting in organic solar cells. It is found that in pentacene/ZnPc/ C60 multiple heterojunctions, part of the excitons in pentacene reach the ZnPc/ C60 interface, where efficient exciton separation occurs and contributes to the photocurrent (PC). Triplet excitons are confirmed to be the major origin of PC by transient PC response measurements, suggesting that triplet-to-triplet energy transfer from pentacene to ZnPc is responsible for the improved PC of pentacene/ZnPc/ C60 multiheterojunctions. Furthermore, exothermic energy transfer from ZnPc to the lower lying triplet levels of pentacene is employed for extending the absorption range and enlarging the absorption intensity. To realize such a structure, an ultrathin ZnPc layer is embedded in the pentacene film in pentacene/ C60 single heterojunctions, leading to an enhanced quantum efficiency in the long wavelength range compared to the reference cell. These findings pave a way to efficient photovoltaic cells with a wide photoresponse ranging from near UV through the visible to the near infrared. © 2009 American Institute of Physics.