Ulrich Hörmann

Correlation between interface energetics and open circuit voltage in organic photovoltaic cells

We have used ultraviolet and inverse photoemission spectroscopy to determine the transport gaps (Et) of C60 and diindenoperylene (DIP), and the photovoltaic gap (EPVG) of five prototypical donor/acceptor interfaces used in organic photovoltaic cells (OPVCs). The transport gap of C60 (2.5 ± 0.1) eV and DIP (2.55 ± 0.1) eV at the interface is the same as in pristine films. We find nearly the same energy loss of ca 0.5 eV for all material pairs when comparing the open circuit voltage measured for corresponding OPVCs and EPVG.

Identification of different origins for s-shaped current voltage characteristics in planar heterojunction organic solar cells

We investigate different parameters influencing the occurrence of s-shaped current voltage (j-V) characteristics in planar heterojunction organic solar cells. It is shown how substrate modification, purity of the active organic material as well as variation of the top contact can affect the shape of the j-V curves. The studies are performed on vacuum-evaporated planar heterojunction solar cells with diindenoperylene (DIP) as electron donor and fullerene C60 as acceptor. The focus is on the fill factor and forward current being the most direct indicators for s-shapes in j-V curves. We find that the main effect of substrate heating during film growth can be assigned to changes in energy barriers rather than to the modification of morphology and crystallinity, which is also influenced by elevated substrate temperatures. The decisive role of the barrier height between the anode work function and the HOMO (i.e., highest occupied molecular orbital) level of the donor is approved by comparing hole-injection laye...

Solvent vapor annealing on perylene-based organic solar cells†

Diindenoperylene (DIP) and tetraphenyldibenzoperiflanthene (DBP) are two commonly used donor materials in organic solar cell devices. Despite their structural similarities, DIP films are crystalline, exhibiting good charge and exciton transport, whereas DBP films are amorphous and have lower carrier mobility and a short exciton diffusion length. However, DBP reveals a distinctly higher absorption due to the lying orientation of its transition dipole moments. In this paper, we investigate the influence of solvent vapor annealing (SVA) on the solar cell performance of both materials. In general, SVA induces a partial re-solubilization of the material leading to enhanced crystallinity of the treated layer. For DBP, extended annealing times result in a strong aggregation of the molecules, creating inhomogeneous layers unfavorable for solar cells. However, in DIP cells, SVA leads to an increase in fill factor (FF) and also a slight increase in short-circuit current density (JSC) due to interface roughening. The best results are obtained by combining solvent vapor annealed DIP layers with strongly absorbing DBP and C70 on top. Through this device architecture, we obtain the same increase in FF in addition to a higher gain in JSC, elevating the power conversion efficiency by a factor of 1.2 to more than 4%.

Organic heterojunctions: Contact-induced molecular reorientation, interface states, and charge re-distribution.

We reveal the rather complex interplay of contact-induced re-orientation and interfacial electronic structure – in the presence of Fermi-level pinning – at prototypical molecular heterojunctions comprising copper phthalocyanine (H16CuPc) and its perfluorinated analogue (F16CuPc), by employing ultraviolet photoelectron and X-ray absorption spectroscopy. For both layer sequences, we find that Fermi-level (EF) pinning of the first layer on the conductive polymer substrate modifies the work function encountered by the second layer such that it also becomes EF-pinned, however, at the interface towards the first molecular layer. This results in a charge transfer accompanied by a sheet charge density at the organic/organic interface. While molecules in the bulk of the films exhibit upright orientation, contact formation at the heterojunction results in an interfacial bilayer with lying and co-facial orientation. This interfacial layer is not EF-pinned, but provides for an additional density of states at the interface that is not present in the bulk. With reliable knowledge of the organic heterojunction’s electronic structure we can explain the poor performance of these in photovoltaic cells as well as their valuable function as charge generation layer in electronic devices.

Amorphous vs crystalline exciton blocking layers at the anode interface in planar and planar-mixed heterojunction organic solar cells

We compare the gain in power conversion efficiency (PCE) achieved by inserting either amorphous or crystalline exciton blocking layers at the anode interface for planar (PHJ) and planar-mixed heterojunction (PM-HJ) organic solar cells based on Tetraphenyldibenzoperiflanthene and fullerenes. For PHJ devices, there is a gain of more than 37% for both types of blocking layers, mainly due to an increase in photocurrent, indicating that this gain can be solely ascribed to the exciton blocking effect. A templating effect as proposed in literature for crystalline blocking layers cannot be affirmed. On the contrary, it is shown that there is a connection between the choice of acceptor (C60/C70) and the blocking effect on the anode side. Moreover, we can show that also for PM-HJ devices a remarkable efficiency enhancement is possible. The insertion of suitable blocking layers at the anode interface can alter the effective work function and thus the open-circuit voltage, leading to a maximum PCE of 5.8% in single j...

Temperature dependent competition between different recombination channels in organic heterojunction solar cells

A modification of the Shockley?Queisser theory for organic heterojunctions is presented with a special focus on constellations, where a linear extrapolation of the temperature dependence of the open circuit voltage results in the optical gap of the absorber rather than in the intermolecular charge transfer (CT) gap. We demonstrate that, depending on the electronic coupling strength between donor and acceptor molecules, either singlet or CT recombination is dominant in different temperature regimes. The different regimes are separated by a transition temperature that is usually well above room temperature (RT). However, in the case of small energy level offset and weak electronic coupling, it can be around 300 K or even below. We point out that a linear extrapolation of the open circuit voltage V oc towards 0 K for measured temperatures larger than the transition temperature results in a photovoltaic gap that is close to the optical gap, whereas for values below the transition temperature the CT gap will be extracted. We show that for ?-sexithiophene (6T)/diindenoperylene (DIP) solar cells heating the substrate during 6T deposition leads to a molecular configuration at the interface where the coupling between donor and acceptor molecules is strongly reduced. This leads to a transition temperature well below RT which is confirmed by temperature dependent electroluminescence measurements. By comparing the temperature dependent spectra of high temperature and RT grown 6T/DIP solar cells to the spectra of the individual materials, the different contributions from the CT gap and the optical gap are separated.

Microstructure and charge carrier transport in phthalocyanine based

The continuously growing and wide-spread utilization of blends of organic electron and hole conducting materials comprises ambipolar field-effect transistors as well as organic photovoltaic cells. Structural, optical and electrical properties are investigated in blends and neat films of the electron donor material Cu-phthalocyanine (CuPc) together with fullerene C 60 and Cu-hexadecafluorophthalocyanine (F 16 CuPc) as electron acceptor materials, respectively. The difference in molecular structure of the spherical C60 and the planar molecule CuPc leads to nanophase separation in the blend, causing charge carrier transport which is limited by the successful formation of percolation paths. In contrast, blends of the similar shaped CuPc and F16 CuPc molecules entail mixed crystals, as can be clearly seen by X-ray diffraction measurements. We discuss differences of both systems with respect to their microstructure as well as their electrical transport properties.

Charge Separation at Nanostructured Molecular Donor–Acceptor Interfaces

Planar and bulk heterojunctions of organic donor and acceptor molecules are used to understand elementary processes in photovoltaic cells. The electronic structure, interface and film morphology, excitonic behavior, device characteristics, and correlations between these properties are reviewed here using a wide range of material combinations.