Advanced Functional Materials and Devices (AFMD) Group

Abstract:

We investigate the role charge carrier mobility plays for loss mechanisms in organic bulk heterojunction solar cells. For this purpose, we perform drift-diffusion calculations for several recombination models and properties of the contacts. We show that in case of selective contacts, higher mobilities increase device efficiency, independent of injection barrier heights, energy level bending at the contacts, and the amount of background dark carriers in the device. Nonselective contacts provide a source of photocarrier loss at the "wrong" electrode. This is evident from a decrease of the open-circuit voltage (V oc) with an increased role of charge carrier diffusion, which originates from a higher mobility or from interface barriers reducing the built-in potential. In this case, V oc furthermore depends on the device thickness. Considering the effect of different recombination models, a too high mobility of one charge carrier decreases V oc significantly for Langevin recombination. That is why balanced mobilities are desirable for high efficiency in this case. In presence of recombination via CT states, V oc is mainly governed by the dynamics of the charge transfer state. Based on these differentiations we show that the existence of an optimum mobility derived from simulation depends strongly on the assumptions made for contact and recombination properties and obtain a comprehensive picture how charge carrier mobility influences the parameters of organic solar cells. © 2012 American Physical Society.

Abstract:

For high efficiencies, organic solar cells have to harvest a large part of the solar spectrum. However, in particular for small molecule based cells, efficient infrared absorbers are still rare. We investigate here two aza-bodipy dyes which have promising properties. Upon benzannulation of the pyrrole of difluoro-bora-1,3,5,7-tetraphenyl-aza-dipyrromethene (Ph 4-bodipy), the thin film absorption maximum is shifted by about 70773 nm compared to the non-annulated molecule. The thin film properties of both materials are investigated and vacuum-processed solar cells using a mip-architecture (metal intrinsic p-doped) are compared. With the new benzannulated difluoro-bora-bis- (1-phenyl-indoyl)-azamethine (Ph 2-benz-bodipy) as donor material, these planar heterojunction solar cells show an open-circuit voltage of 0.65 V, a fill factor of 65%, and an external quantum efficiency extending up to 860 nm. The mismatch corrected power conversion efficiency reaches 1.1%. © 2011 Elsevier B.V. All rights reserved.

Abstract:

We present a comprehensive investigation of morphological changes inside the active layer of an organic solar cell induced by substrate heating during layer deposition by thermal evaporation in ultra-high vacuum. To explore the trends observed in solar cell devices, we apply absorption and photoluminescence spectroscopy, atomic force microscopy, X-ray diffraction, and organic field effect transistor measurements. The material combination we use comprises unsubstituted dicyanovinyl end-capped quaterthiophene (DCV4T) as the donor material mixed with C60 as the acceptor. The solar cell power conversion efficiency decreases with increasing substrate temperature during film deposition due to changes in the crystalline structure of the oligothiophene phase, leading to a decrease in absorption strength. Photoluminescence measurements show that substrate heating increases the amount of phase separation between the donor and acceptor, and topology and structure investigations reveal large aggregates of polycrystalline DCV4T at the surface. However, the fill factor is increased for higher substrate temperatures due to better transport properties. The highest efficiency obtained with this material combination and stack design is 3.0% under AM1.5g illumination. © 2012 Elsevier B.V. All rights reserved.

Abstract:

An equivalent circuit to characterize energy distribution of trap states present in organic semiconductors by impedance spectroscopy is proposed. We analyze the impedance spectra of a small-molecule organic solar cell and observe the contribution of trap states at low frequencies. Starting from previously reported equivalent circuits and a theoretical model based on the integration of a general traps distribution, we develop an equivalent circuit, which is able to describe the energetic distribution of trap states typically observed in organic semiconductors. The experimental data can be reproduced by our equivalent circuit, and we estimate a density of trap states in a Zn-phthalocyanine:C60 bulk heterojunction to be about 1.9 0.6 10 16 cm -3 eV -1. © 2012 American Institute of Physics.

Abstract:

Recombination contacts play an important role in highly efficient organic tandem solar cells. We present a photoelectron spectroscopy study on contact systems that have previously been shown to work efficiently as recombination contacts. Here, the conversion of an electron current into a hole current is realized either by insertion of gold clusters or by a highly doped pn-junction. From the measured energy level alignments, we show that the working principles of these two approaches are significantly different. For gold clusters, the recombination current is promoted by an accumulation of charge carriers, while for doped pn-junctions, it is achieved by tunneling through a depletion layer. © 2012 American Institute of Physics.