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The photovoltaic behavior of Schottky barrier devices consisting of a single diindenoperylene (DIP) layer sandwiched between an indium tin oxide and Ag electrode has been investigated. Correlating the spectral dependence of the photocurrent and the absorption coefficient, we estimated the exciton diffusion length in DIP to ~100  nm along the c direction. X-ray structural analysis yielded this length to be in agreement with the average crystallite size, thereby, revealing domain boundaries to be the limiting effect on the exciton transport. The corresponding exciton diffusion constant of 5×10⁻³  cm²/s resembles that of highly ordered single crystals of polyaromatic hydrocarbons. ©2008 American Institute of Physics

Contents

• BODY OF ARTICLE
• REFERENCES
• FIGURES
• FOOTNOTES

Organic semiconductors have received considerable attraction by their low-temperature processibility, compatibility with various substrates, and functionality. In case of organic light emitting diodes, these advantages have resulted in first commercially available devices admitting the efficient conversion of electrical energy into light.^1,2 In contrast, conversion of optical excitation into electrical energy by organic materials is highly desirable for two reasons: their broad variety enables optimized matching to the solar spectrum and the excitons formed upon light absorption are quite stable due to their strong binding energies of about 0.5  eV.^3,4 Therefore, various innovative device concepts implementing organic thin films are currently under investigation.^5,6,7,8,9,10 Among these concepts, photovoltaic elements based on exciton generation, diffusion to the donor-acceptor interfaces, and subsequent dissociation, are the most promising ones. This is mainly caused by the variety of absorption layers and the adjustable positions of energy levels at the organic-organic interfaces which are required for effective exciton dissociation.¹¹ However, optimizing the thickness of the exciton diffusion layer requires a trade-off between optical and electronic properties.¹² On the one hand, for molecular materials with typical absorption coefficients of ~1  µm⁻¹, a thicker film provides better absorption and, thereby, generation of more excitons. On the other hand, the total amount of structural and chemical inhomogeneities, both leading to effective exciton trapping, increases with film thickness. As a result, the exciton diffusion length measured on crystalline films of low-weight polyaromatic molecules covers a range from 3  nm for 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCB1), 30  nm for ZnPc up to 68  nm for CuPc, making thicker films inefficient for exciton transport.^12,13,14 Therefore, the envisaged target is to understand the relation between the film properties, viz., the structural coherence of the transport layer, and the exciton diffusion length, and to search for molecular compounds with diffusion lengths approximating the optical absorption length.

In this study, we focussed our attention on the organic semiconductor diindeno(1,2,3,-cd,1,2,3-lm)perylene (DIP) which consists of a perylene core with two indeno endgroups resulting in a long molecular axis of 1.8  nm [Fig. 1(a)]. DIP has shown a balanced charge carrier transport along the c direction in single crystals.¹⁵ A particular material property is the strong tendency of self-ordering along the long molecular axis, presumably sustained by the molecular motif.¹⁶ On weakly interacting substrates such as SiO₂ or indium tin oxide (ITO), molecules align almost upright, thereby, offering voids at the indeno endgroups for the next layer to interdigitate [Fig. 1(b)].

Figure 1.

Using Schottky barrier cells employing DIP single films, we have investigated in as much this long range order favors exciton diffusion along the surface normal. The devices comprised of a 100–300  nm thick DIP layer embedded between ITO and a semitransparent Ag top contact [Fig. 1(c)]. The molecular films were thermally deposited under high vacuum at substrate temperatures of 200  K and deposition rates of 0.1–4.0  nm/min. Prior to top-contact deposition, the samples had to be transferred in atmosphere, which lasted for at most 20  min, with the samples shielded against light. Due to its planar conjugated -system, we expect no significant degradation of DIP in the course of transfer.¹⁵ Using shadow masks, 16 individual top electrodes with an area of 0.2  cm² each, were manufactured [Fig. 1(c)].

The out-of-plane sample structure was examined by ex situ x-ray Bragg diffraction (_Cu  K=0.1542  nm). By small angle x-ray reflectivity, we determined the ITO thickness to 80  nm at a roughness of 1  nm. Reference measurements revealed thicknesses of the Ag top contact of 10–20  nm. Photocurrents were measured in short-circuit geometry (resolution 0.1  pA) between 200 and 700  nm under spectral illumination by a 150  W XBO light source. Using a feedback loop controlled gray filter, the photon density was kept constant at 5×10¹²  s⁻¹  cm⁻². Light of higher diffraction order was suppressed by a 630  nm edge filter placed in the optical path if necessary. Absorption spectra were recorded by a Perkin Elmer Lambda 16 UV/visible spectrometer.

To characterize the structural properties of the exciton transport layer, we performed x-ray diffraction on DIP reference samples on glass, prepared simultaneously to the photovoltaic cells and providing similar substrate roughness as ITO. A representative DIP diffraction spectrum is shown in Fig. 2. The wide-angle spectrum exhibits Bragg peaks up to the sixth order which indicate the high structural coherence of the (00l) planes along the surface normal, i.e., along the c direction. As shown in the inset, the intensity of the first order Bragg peak is modulated by Laue oscillations, which by modeling yielded a DIP lattice spacing of 1.66±0.01  nm along the c direction and a total number of 113±1 lattice planes. This renders an average thickness of 187±2  nm for the DIP crystallites along the surface normal, i.e., along the direction of exciton transport. Comparing this value with the nominal film thickness of 220  nm, the average DIP crystallite size proves to be of the same order. The tendency of forming single crystalline grains with out-of-plane extension comparable to the total film thickness has been observed for most of the DIP layers prepared under similar conditions and is attributed to the tapered indeno endgroups supporting vertical stacking. As an additional indicator of the structural quality, we have estimated the average tilting of the crystallites with respect to the surface normal by means of rocking scans. The experimental value of ±0.08° indicates a small mosaicity, i.e., a good alignment of crystalline domains along the direction of exciton transport.

Figure 2.

To correlate film structure and optoelectronic properties, the spectral photocurrent of several Ag/DIP/ITO Schottky barrier cells has been measured. A representative curve for a 142  nm thick DIP layer cell is shown in Fig. 3. Together with the photocurrents obtained by illuminating the ITO or the Ag electrode, the DIP absorption over the same wavelength range is displayed by the blue curve. Obviously, both photocurrents resemble the characteristics of the absorption coefficient (symbatic behavior). Considering the film thickness and the optical absorption length in DIP, this symbatic behavior indicates the photocurrent to be governed mainly by exciton diffusion in the absorption layer. Furthermore, the absolute photocurrent under ITO illumination is slightly smaller than that obtained by illuminating the Ag contact. As the silver layer already absorbs 50% of the initial photons, this interface turns out to dissociate excitons more efficiently than that formed by ITO. We therefore consider, without loss of generality, Ag/DIP to be the active interface and ITO/DIP to be the inactive interface.

Figure 3.

Correlating spectral photocurrent and optical absorption via the exciton diffusion model by Ghosh and Feng, we deduced the exciton diffusion length in DIP.¹⁷ According to this approach, both contact interfaces are expected to be exciton sinks, i.e., the density renders zero, whether due to dissociation or geminate recombination. The absorption competes with the exciton diffusion length ⁻¹ which under the assumption that the layer thickness d exceeds the diffusion length, i.e., exp(−d)0, results in an inverse proportionality between reciprocal absorption coefficient and photocurrent at the active and passive electrodes:

The experimental correlation between the inverse photocurrent and the absorption length ⁻¹ is displayed exemplarily in Fig. 4 for the 142  nm thick DIP layer (see Fig. 3). The linear behavior of Eqs. (1a),(1b) can be reproduced between 260 and 600  nm for the active Ag/DIP interface as well as for the inactive ITO/DIP interface, the latter weighted by the absorption exp(−d). From the linear slope, an exciton diffusion length in DIP of 89  nm has been estimated for illumination through the Ag contact and of 103  nm for the ITO contact. From the intersection with the ordinate, the efficiency of free charge carrier generation, defined by the product of the probabilities for exciton creation upon photon absorption ₁ and the collection efficiency upon exciton dissociation ₂ can be adjusted to 5.2% for the active Ag electrode and to 3.0% for the inactive ITO electrode. These results confirm the previous assumption that the photocurrent in our Schottky barrier cells is governed by exciton diffusion rather than by dissociation at the particular interfaces.

Figure 4.

Applying the approximated diffusion model described above, the following aspects have to be carefully considered. At first, Eqs. (1a),(1b) were derived under the assumption that d1, i.e., that the film thickness exceeds the exciton diffusion length. However, regarding the obtained values, this might be no longer valid as the deduced exciton diffusion length amounts to 73% of the DIP layer thickness. Therefore, we have cross-checked the linear approximation by modeling the spectral photocurrent with the full spatial dependence of the exiton density.¹⁷ We proceeded this way by imposing the limiting condition that the exciton diffusion length ⁻¹ matches the full DIP layer thickness d (Fig. 4). Under this constraint, we deduced an exciton diffusion length of about 190  nm and an internal quantum efficiency of 8.0% for Ag and 6.8% for ITO. This diffusion length will be discussed together with the x-ray data below.

At second, inclusion of interference effects in the model would reduce the estimated diffusion lengths by at most 20% for the DIP thickness range studied.^18,19 However, it has to be mentioned that the surface roughness of ~10  nm estimated by atomic force microscopy might compensate for these Ag/DIP interface effects and renders the cited length reduction to be overestimated.

Finally, metal penetration into the organic layer might play a crucial role for the characteristic length scales.²⁰ As has been demonstrated by radiotracer studies, significant penetration of Ag into DIP thin films occurs upon thermal metal deposition.²¹ Presumably, metal diffusion proceeds preferentially along grain boundaries, while in our DIP cells, we might probe effectively the out-of-plane extension of the first layer of crystalline domains.

Taking into account these uncertainties, the calculated exciton diffusion length resembles the out-of-plane extension of crystalline domains in the DIP layer. This corroborates the idea that within the crystalline domains, excitons can diffuse within their lifetime without significant trapping, whereas at the grain boundaries reflection, trapping and recombination of excitons occur, assisted also by the penetrated metal. Therefore, the value of 100  nm estimated by the linear approximation [Eqs. (1a),(1b)] might be considered as the lower limit for the exciton diffusion length in DIP and could be even enhanced in case of extending the crystalline domains in the organic film and hampering the metal penetration. This conclusion seems to be further supported by photoluminescence quenching studies on DIP films, which are currently in progress and which indicate similar or even higher exciton diffusion lengths.

We analyzed the spectral photocurrents and the absorption of DIP single layers sandwiched between ITO and Ag electrodes. By correlating these quantities, we estimated an exciton diffusion length of at least ~100  nm in accordance to the spatial extension of the crystalline DIP grains. For singlet lifetimes of ~10  ns measured on DIP, this length translates into an exciton diffusion constant of 5×10⁻³  cm²/s, which is in accordance to values reported for oligoacene single crystals.^22,23 The obtained exciton diffusion length is one of the largest reported so far for organic materials and, in combination with the balanced transport properties, proposes DIP to be an interesting candidate for thin film photovoltaic device applications.

T. Roller and T. Schuon are acknowledged for helpful discussion and J. Brill, Chair of Display Technology, University of Stuttgart, for providing the ITO substrates.

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Full figure

Fig. 1. (Color online) (a) Structure and dimensions of the diindenoperylene (DIP) molecule. (b) Scheme of the out-of-plane stacking induced by the DIP motif. (c) Single DIP layer Schottky barrier cell with ITO bottom and Ag top contact. First citation in article

Full figure

Fig. 2. (Color online) X-ray scan along the surface normal measured on a nominally 220  nm thick DIP film on ITO. Diffraction peaks up to the sixth order and Laue oscillations around the first order Bragg peak (see inset) indicate the high structural quality of the DIP layer along the c direction. First citation in article

Full figure

Fig. 3. (Color online) Spectrally resolved photocurrent measured on a 142  nm thick DIP Schottky barrier device illuminated at a constant photon rate of 5×10¹²  s⁻¹  cm⁻². Comparing the DIP absorption characteristics (blue curve) with the photocurrent for illumination through the Ag and ITO contacts, a symbatic behavior can be established. First citation in article

Full figure

Fig. 4. (Color online) Correlation between photocurrent and absorption length for a 142  nm DIP cell illuminated via the active Ag (upper part) and inactive ITO electrodes (lower part). From the linear slope, the exciton diffusion length is estimated to be 89  nm for Ag/DIP and 103  nm for ITO/DIP. The internal quantum efficiency ₁₂ results to 5.2% (Ag/DIP) and 3.0% (ITO/DIP), respectively. In addition, the full diffusion model has been fitted under the assumption that the exciton diffusion length is equal to the film thickness (black curves). First citation in article

^aElectronic mail: j.pflaum@physik.uni-stuttgart.de.

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