Advanced Functional Materials and Devices (AFMD) Group

Abstract:

We compare between two derivatives of dicyanovinyl-quinquethiophenes with different alkyl side chain lengths. Both materials show comparable open circuit voltages Voc in organic solar cells with fullerene C60 as acceptor, as expected since they have the same highest occupied molecular orbital energy. However, differences in the current-voltage-characteristics, particularly in the fill factor, are observed. We analyze both derivatives in hole-only devices and find a difference in the hole injection between the doped hole transport layer and the oligothiophenes. Additionally, we determine the hole mobility of the two materials and explain the different behaviors of the two materials in solar cells. © 2008 American Institute of Physics.

Abstract:

In this paper we focus on the current status of organic solar cells based on small molecules. Since their discovery, much progress has been made, and the main steps are highlighted that led to the current state-of-the-art devices. However, organic solar cells still need to be improved further, and the main strategies for improving the power conversion efficiency, namely raising the open circuit voltage V(oc) and increasing the short circuit current density J(sc), are discussed. In theory, power conversion efficiencies of around 15% should be possible with a single heterojunction; for higher efficiencies, stacked solar cell concepts have to be employed.

Abstract:

We show that ultrathin metal layers (Ag or Al/Ag) are feasible as transparent top contacts for zinc phthalocyanine: C60 bulk-heterojunction inverted organic solar cells thermally evaporated on glass substrates. Furthermore, it is demonstrated that the introduction of an organic capping layer drastically increases light incoupling and photon harvesting, in accordance with optical simulations. Proof of principle tin-doped indium oxide (ITO)-free solar cells employing a transparent metal contact and a capping layer reach efficiencies of 1.06%, compared to 0.69% without addition of the capping layer. © 2008 American Institute of Physics.

Abstract:

The aim of this article is to investigate the origin of the open circuit voltage (Voc) in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C60 as acceptor material and an oligophenyl-derivative 4, 4′ -bis-(N,N -diphenylamino)quaterphenyl (4P-TPD) as donor material. These photoactive materials are sandwiched between indium tin oxide and p -doped hole transport layers. Using two different p -doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the Voc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, Voc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, Voc is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orbital of C60 and the highest occupied molecular orbital of 4P-TPD. In planar heterojunctions, the open circuit voltage can exceed the built-in voltage. Furthermore, the investigations show that the efficiency of organic solar cells can be improved by using p -doped charge transport layers with optimized energy level alignment to the active materials. The optimized planar heterojunction shows a fill factor of up to 65.5% and a Voc of 0.95 V. For solar cells with insufficient energy level alignment between the photoactive layer system and the hole transport layer, a reduced Voc in bulk heterojunction cells and a characteristic S shape of the I-V characteristics in planar heterojunction cells are observed. © 2008 American Institute of Physics.