Advanced Functional Materials and Devices (AFMD) Group

Abstract:

We study the influence of the molecular energy levels on doped organic layers, using four different combinations of two amorphous hosts (MeO-TPD and BF-DPB) and two efficient p-dopants (F6-TCNNQ and C 60F36). Conductivity and Seebeck studies are performed in situ, varying the doping concentration over more than two orders of magnitude. Whereas trends of doping are in agreement with the hosts' energy levels, trends deviate from the expectation based on the dopants' energy levels. A lower limit for the mobility can be derived from conductivity data, which for samples of F6-TCNNQ increases with doping, even exceeding the measured OFET-mobility of intrinsic MeO-TPD. © 2013 Elsevier B.V. All rights reserved.

Abstract:

The open-circuit voltage (VOC) of an organic solar cell is limited by the donor-acceptor material system. The effective gap E geff between the electron affinity of the acceptor and the ionization potential of the donor is usually regarded as the upper limit for VOC, which is only reached for T → 0 K. This relation is confirmed for a number of small-molecule bulk heterojunction p-i-n type solar cells by varying the temperature and illumination intensity. With high precision, the low temperature limit of VOC is identical to E geff. Furthermore, the influence of the hole transport material in a p-doped hole transport layer and the donor-acceptor mixing ratio on this limit V0 is found to be negligible. Varying the active material system, the quantitative relation between V0 and E geff is found to be identity. A comparison of V 0 in a series of nine different donor-acceptor material combinations opens a pathway to quantitatively determine the ionization potential of a donor material or the electron affinity of an acceptor material. The effective gap of a photovoltaic donor-acceptor system equals the open-circuit voltage extrapolated to temperature zero. The extrapolation is independent of the illumination intensity, and material variations in the doped transport layers do not affect the measurement result. This is shown for bulk-heterojunction devices with different mixing ratios and with small-molecular materials from various classes. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.