Advanced Functional Materials and Devices (AFMD) Group

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.

Abstract:

A nondestructive method for assessing the thickness of the photoactive layer in poly(3-hexyl-thiophene):1-(3-methoxy-carbonyl)propyl-1-phenyl- (6,6) C61 (P3HT:PCBM) solar cells is reported. In the approach the absorption spectrum of the solar cell as derived by optical simulations is fitted to the corresponding measured spectrum, varying only the P3HT:PCBM layer thickness. Within the 50-250 nm thickness range, a linear correlation between the position of a certain spectral minimum and the P3HT:PCBM layer thickness is shown, based on simulated absorption spectra. As an initial application, absorption spectra for 240 P3HT:PCBM solar cells prepared at four different spin-coating speeds were recorded, and the average P3HT:PCBM layer thickness estimated for each spin-coating speed. The simulated fraction of light absorbed in the P3HT:PCBM layer of the solar cells is compared with the P3HT:PCBM absorption spectra measured for films spin coated on simpler substrate types. The latter spectra cannot account for the light harvested in the photoactive layer of P3HT:PCBM solar cells because of substantial optical interference in the solar cells. The measured short circuit current densities Jsc for the solar cells vary with the spin-coating speed in a manner confirmed by optical simulations of the maximal short circuit current densities. The measured efficiencies follow the same pattern. On average the measured Jsc is 1-2 mA cm2 below the simulated maximal short circuit current densities. Based on the resemblance of the measured and simulated absorption spectra such difference can be attributed to recombination exclusively. © 2007 American Institute of Physics.

Abstract:

The current-voltage characteristic and the performance of organic bulk-heterojunction solar cells are very sensitive to small variations in the production steps or environmental influences. In our experiments, we found a large variation of the short-circuit current, which does not correspond to the device thickness as one might expect. The fill factor of some devices is below 25% under illumination, while the best devices have a fill factor of about 70%. Electrical impedance spectroscopy can provide information about the conductivity of different regions within the device. In earlier measurements, it was observed that devices with a thick absorber layer might consist of a conductive bulk region and a very poorly conductive depletion region at the metal contact. Using a standard semiconductor device model, it is shown in this paper that this reduces the charge collection efficiency under short-circuit conditions, as there is no electrical field in the bulk region, supporting the charge separation. For devices with the low fill factor, a thin-current limiting layer under forward bias can be identified by electrical impedance spectroscopy and is suggestive of a corroded metal contact. © 2006 Elsevier B.V. All rights reserved.