Silicon–organic pigment material hybrids for photovoltaic application

ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 91 (2007) 1873–1886 www.elsevier.com/locate/solmat

Silicon–organic pigment material hybrids for photovoltaic application T. Mayera,, U. Weilera, C. Keltingb, D. Schlettweinb, S. Makarovc, D. Wo¨hrlec, O. Abdallahd, M. Kunstd, W. Jaegermanna a

Institute of Materials Science, Darmstadt University of Technology, Petersenstreet 23, D-64287 Darmstadt, Germany b Institute for Applied Physics, Justus Liebig University Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany c Institute of Organic and Macromolecular Chemistry, University Bremen, Leobener Street NW II, D-28359 Bremen, Germany d Department Solar Energy, Hahn-Meitner-Institute, D-14109 Berlin, Germany Received 23 March 2007; accepted 3 July 2007 Available online 17 September 2007

Abstract Hybrid materials of silicon and organic dyes have been investigated for possible application as photovoltaic material in thin film solar cells. High conversion efficiency is expected from the combination of the advantages of organic dyes for light absorption and those of silicon for charge carrier separation and transport. Low temperature remote hot wire chemical vapor deposition (HWCVD) was developed for microcrystalline silicon (mc-Si) deposition using SiH4/H2 mixtures. As model dyes zinc phthalocyanines have been evaporated from Knudsen type sources. Layers of dye on mc-Si and mc-Si on dye films, and composites of simultaneously and sequentially deposited Si and dye have been prepared and characterized. Raman, absorption, and photoemission spectroscopy prove the stability of the organic molecules against the rough HWCVD-Si process. Transient microwave conductivity (TRMC) indicates good electronic quality of the mc-Si matrix. Energy transfer from dye to Si is indicated indirectly by luminescence and directly by photoconductivity measurements. FxZnPc pigments with x ¼ 0,4,8,16 have been synthesized, purified and adsorbed onto H-terminated Si(1 1 1) for electronic state line up determination by photoelectron spectroscopy. For x ¼ 4 and 8 the dye frontier orbitals line up symmetrically versus the Si energy gap offering similar energetic driving forces for electron and hole injection, which is considered optimum for bulk sensitization and indicates a direction to improve the optoelectronic coupling of the organic dyes to silicon. r 2007 Elsevier B.V. All rights reserved. Keywords: Inorganic–organic composites; Pigments; Silicon; Absorption; Charge transfer; Photoelectron spectroscopy; Raman spectroscopy; Photovoltaic devices

1. Introduction Thin film photovoltaics will play a decisive role in establishing a sustainable energy cycle [1]. Aside from compound semiconductor thin film cells like those based on CdTe or on representatives of the copper–indium–gallium–sulphide–selenide (chalkopyrite) family, cells based on Si thin films are one of the most promising candidates [2]. Si thin films are typically prepared by catalytic (hot wire, HW) or plasma-activated chemical vapor deposition (CVD) using a mixture of silane and hydrogen or by reactive sputtering [1–4]. Depending on the choice of preparation parameters, either amorphous (a-Si) or microCorresponding author. Tel.: +49 6151 165532; fax: +49 6151 166308

E-mail address: [email protected] (T. Mayer). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.07.004

crystalline (mc-Si) silicon films are obtained. Because of a higher mobility of charge carriers, prolonged lifetime of minority charge carriers and hence a decreased recombination probability, cells based on mc-Si generally have the higher potential to reach technically interesting conversion efficiencies [3,4]. The beneficial electronic properties of crystalline Si directly dwell on the indirect character of the Si band gap [5]. This in turn, however, leads to a rather weak optical absorbance of Si, in particular within the red part of the visible spectrum (Fig. 1), which leads to the need of a substantial film thickness of mc-Si in the cells. However, thin film solar cells of mc-Si are currently developed in the pilot plant state for commercialization [4]. Organic pigments or dye molecules, on the other hand, are designed to strongly absorb in the visible range (Fig. 1).

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Semiconducting pigment films exhibit interesting optoelectronic properties as e.g. electroluminescence in organic light emitting diodes (OLED) and photovoltaic energy conversion in organic solar cells (OSC) [6,7]. Purely organic thin film photovoltaic junctions, however, lead to rather poor performance because of a limited mobility of charge carriers and rapid recombination [8]. In OSC special measures have to be taken for effective charge separation because the optically generated excitons are strongly bound due to the low dielectric constant of organic semiconductors. Some of these problems can be overcome by the use of junctions with a maximized surface area and decreased average film thickness. Organic–organic [9] and organic–inorganic [10] bulk hetero-junctions of donor and acceptor phases have been developed, but reported efficiencies still lie well below 5% even for laboratory cells [11–14].

Absorption coefficient α/ cm-1

250000

β-PcZn

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150000

a-Si

100000

α-PcZn

50000 μ-Si 0 400

500

600 700 Wavelength / nm

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Fig. 1. Optical absorption coefficient a calculated from optical absorption spectra of a 50 nm thick amorphous Si film (a-Si), 2 mm thick microcrystalline Si (mc-Si), and 50 nm ZnPc deposited at room temperature (a-ZnPc) and 50 nm ZnPc deposited at a substrate temperature of 250 1C (b-ZnPc), all films deposited on glass.

Another approach to utilize the high absorptivity of organic dyes, and also (substituted) phthalocyanines is to sensitize wide gap semiconductor electrodes as TiO2 [15] and ZnO [16–18] in photoelectrochemical solar cells (PESC). Nanoporous oxide electrodes are used in order to increase the effective surface covered with a dye monolayer for light absorption. In these cells the organic absorber injects an electron from its excited state to the conduction band of the wide band-gap semiconductor and in turn has to be regenerated by a reaction with either a redox electrolyte or a hole–conductor penetrating to the pore system of the electrode. Whereas the liquid electrolytes lead to acceptable efficiencies above 10% at, however, technical difficulties to ensure stable operation of the cells, the hole conductors still lead to a considerably lower efficiency. In this paper we report about a new approach to utilize strongly absorbing organic dyes as sensitizers in an inorganic material of complementary optimized electrical properties (mc-Si) in order to reduce film thickness (Fig. 2) and therewith production time and cost. Functional hybrids based on inorganic/organic composites offer a wide range of possibilities to tailor their chemical and physical properties. Hybrid materials have been developed for optical applications as, e.g., coloured glass ware, dielectric mirrors and lasers [19]. Phthalocyanines and porphyrins appear most attractive candidates as photo sensitizers since they absorb throughout the visible region and into the near IR [9,20]. The silicon matrix shall be used as electron as well as hole conductor, following injection from the dye LUMO and HOMO states into the host semiconductor conduction and valence bands as sketched in Fig. 3a and b. Electron–hole pairs may be injected either subsequently or simultaneously by the socalled Dexter process or the energy may be transferred via dipole–dipole interaction by the so-called Fo¨rster process [21,22]. We propose a p-i-n device structure in which the

Fig. 2. Combination of the optical properties of organic dyes (high absorptivity) and electronic properties of mc-Si (high life time) is expected to result in an optimized photovoltaic material.

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Fig. 3. Sketch of the bulk composite absorber in the proposed p-i-n device structure: (a) steric structure and (b) electronic structure.

intrinsic sensitized matrix layer is sandwiched between p+ and n+ doped Si contact layers [23]. Electrons and holes will drift in opposite directions to the p+ and n+ doped Si contact layers. In order to create driving forces for electron as well as hole injection the frontier orbitals should be lined up as sketched in Fig. 3b: the HOMO below the valence band maximum of the Si matrix and the LUMO above the conduction band minimum. Variation of the molecular ionization energy by substitution is expected to result in variation of the energy alignment of the semiconducting organic HOMO and LUMO states versus the silicon substrate valence and conduction bands, respectively. Thereby a systematic engineering of the frontier orbital line-up versus the Si bands should be possible allowing for optimization of the energy transfer. Details of the deposition process and its development from an amorphous to a microcrystalline silicon matrix and optoelectronic measurements on Si-dye bilayers and bulk composites demonstrating the injection process have been published before [24,25]. The HOMO LUMO orbital line up of ZnPc, F16ZnPc, and ZnTPP adsorbed onto the hydrogen terminated Si(1 1 1) surface (Si(1 1 1):H) has been deduced from photoelectron spectroscopy and compared to electrochemical oxidation and reduction potentials, and the gap width has been discussed with respect to different transfer processes and measuring methods [26]. The concept of sensitization of direct semiconductors with organic molecules to a wider part of the solar spectrum in the sense of third generation solar cells has been presented [23]. Here we give a summary of the cooperative work on the deposition and characterization of silicon and zinc phthalocyanine bilayers and on bulk composite films that demonstrate the dye stability and the sensitization process. In addition the Fluorine substitution row FxZnPc (x ¼ 0,4,8,16) has been synthesized and purified and the calculated ionization potentials are compared to experimentally determined HOMO positions measured in adsorption model experiments on Si(1 1 1):H using photoelectron spectroscopy. The derived orbital line up pinpoints

F4ZnPc and F8ZnPc as more suitable sensitizer candidates than ZnPc and F16ZnPc. 2. Experimental The experimental setup for Si-dye composite deposition is sketched in Fig. 4. Silane diluted in hydrogen was activated by a remote HW for CVD of mc-Si and a-Si matrices. The films were prepared in an UHV chamber equipped with mass flow controllers for silane and hydrogen allowing for separate flux control. In addition the gas pressure in the reactor was adjusted by a throttle valve. A HW coiled tantalum 0.5 mm in thickness and approx. 7 cm in length was used. In variance to the common HW-CVD process the HW was shielded by a water cooled shroud. The HW temperature was read out online by a 2-colour pyrometer. The films were deposited on glass substrates allowing for transmission measurements. The substrate temperature was calibrated with a thermocouple installed on a reference sample. Zinc phthalocyanine was deposited via PVD from a homemade thermal evaporation source. It has been situated just next to the silicon source (Fig. 4). Optical absorbance spectra were performed in a (modular) Jobin Yvon spectrometer in the range of 350–1000 nm or with a Tec5 evaluation line diode array spectrometer in transmission measurement geometry. The spectra were normalized to the incident photon flux. Raman spectra were measured at the Hahn–Meitner Institute, Berlin with a confocal Dilor spectrometer using a Helium–Neon LASER (633 nm). For the photoconductivity experiments, silver electrodes (100 nm thickness) with a gap of 50 mm  10 mm were Ar+- RF- sputtered (Leybold Z 400, 103 mbar, 1500 V) on top of Si to contact the samples. The photoconductivity measurements were made in air with a Keithley 6517 electrometer at 10 V bias under illumination with a water-filtered beam of an Oriel 1000 W Xenon arc lamp at 100 mW cm2 and using interference filters (LOT Oriel; FWHM 10 nm). All reported photoconductivity values are steady state values. UPS measurements of the composites were carried out in a Physical Electronics 5600

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Fig. 4. Vacuum reactor for remote HW CVD of silicon/organic-dye composites. H2/SiH4 ratios were adjusted by two mass flow controllers. The pressure was kept constant by a feed back loop steering a throttle valve in the exhaust pumping line. The organic dye was deposited from a PVD source. Shielding by a water cooled shroud prevented damage of the organic molecules by the HW.

system with a helium discharge lamp. The samples were transported from the preparation chamber to the analysis system in a battery pumped transportable load lock without contact to ambient air. The variation of the HOMO line up of the F substitution series on Si(1 1 1):H was measured at the synchrotron BESSY at beamline U49/ 2 also using a helium discharge lamp in addition. Phthalocyanines were prepared by the cyclotetramerization of phthalonitriles and metal salts. In dependence of the kind of employed phthalonitrile different reaction conditions must be selected. Some examples of the preparations are given in Fig. 5 that contains the synthetic routes for the prepared phthalocyanine zinc(II) complexes. ZnPc is also commercially available but contains various impurities like small amounts of other metal ions and phthalic acid derivatives. Therefore ZnPc was synthesized by the reaction of sublimed phthalonitrile and zinc(II) acetate (99.5% purity, o0.01% total content of other metals) in dry pentanol in the presence of the strong non-nucleophilic base DBU under reflux. The isolated product was treated with water and methanol in a Soxhlet apparatus. For the synthesis of 3-F4ZnPc at first 3-nitrophthalonitrile was converted with dried tetramethylammonium fluoride in dimethylacetamide to 3-fluorophthalonitrile which was purified by column chromatography. Then 3-fluorophthalonitrile was reacted either in the bulk or in 1-chloronaphthalene with zinc acetate at higher temperature to 3-F4ZnPc. The residue was washed with water, treated with methanol and dissolved in

THF which was then evaporated. 4,5-F8ZnPc was prepared starting with 4,5-dichlorophthalonitrile. This phthalonitrile derivative was converted by the reaction with potassium fluoride in DMF in the presence of 18-crown-6 to 4,5difluorophthalonitrile which is purified by sublimation. Heating with zinc acetate in 1-chloronaphthalene resulted in 4,5-F8ZnPc which was purified by washing with water, DMF and methanol. For the synthesis of F16ZnPc sublimed tetrafluorophthalonitrile was reacted with Zinc(II) acetate in a vacuum-sealed glass tube for 1 h at 180 1C. The resulting dark blue powder was treated in a Soxhlet apparatus with water and light petroleum to remove inorganic and organic contaminants. The zinc(II) complex of meso-tetraphenylporphyrin 2a was used as received and purified by zone sublimation. The metal complexes were purified by zone sublimation at 106–107 mbar and 370 1C. The zone sublimation of the metal complexes is the most time consuming step: 0.1–0.2 g within 1 week. Energy levels of dye molecules in the gas phase have been calculated using the Hyperchem program, Release 4.5. The calculation method PM3, next lowest state in the RHF approximation, gradient 10exp-3, was employed. 3. Results and discussion 3.1. Bilayers of Si and ZnPc To reach the goal of a composite material consisting of mcSi with embedded molecular clusters of phthalocyaninatozinc

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CN

Zn(OAc)2 , DBU

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NO2

R

R R CN

4 R

CN

metal salt

R

(CH3)4N+F-

N

R M = Zn(II) R = -H, -F

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N R

F CN

Zn(OAc)2

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1-chloronaphthalene, 220 oC

CN

Zn(OAc)2

CN

o

bulk, 200 C

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pentanol-1, 136 C

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bulk, 200 C

F

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F

Fig. 5. Synthesis of fluoro-containing phthalocyanine zinc complexes

20000

b a

15000 Intensity

(ZnPc) the compatibility of the CVD of mc-Si and the physical vapor deposition (PVD) of ZnPc had to be ensured. Since the crystallinity of Si thin films generally can be increased easily at higher substrate temperatures, the upper limit of the thermal stability of solid ZnPc thin films was investigated. Films of ZnPc were deposited on Si wafers at room temperature and at elevated temperatures. Films could only be grown at a reasonable growth rate up to about 250 1C. The films were monitored by optical reflection spectra. At room temperature, films of the a-crystalline structure as indicated by absorbance spectra (Fig. 1) were obtained [27], while at 250 1C the films crystallized in the high-temperature b-phase [27]. These films were left in vacuum and reflection changes were monitored in-situ under increasing substrate temperatures. The films were stable up to 300 1C and even at this temperature only slowly evaporated at a rate of about 10 nm h1. This temperature can therefore serve as an upper limit of substrate temperatures during preparation of Si on ZnPc bilayer test devices. For composite preparation, however, 260 1C is the limit (see below) because of a lower sticking coefficient of individual molecules and rather slow crystallization kinetics. To analyse the chemical integrity of ZnPc under the conditions of the CVD process, about 1 mm Si was prepared in an CVD reactor with an open non-shielded HW (1700 1C) at a substrate temperature of 200 1C on top of an equivalent of 100 nm ZnPc that were PVD-deposited on glass in a prior step. Raman spectra were used to analyse both, the chemical structure of ZnPc and the crystalline character of the Si film. The results are summarized in Fig. 6. The crystallinity of the deposited Si film was shown by the clear Raman line at 520 cm1 [28] for the Si film deposited on glass as well as for Si deposited

10000

5000 c d

0 400

600

800

1000

Wavenumber / cm-1 Fig. 6. Raman spectra of a 100 nm thick ZnPc film on glass (a), the same film following the conditions of Si-CVD but without deposited Si (b), for Si on top of ZnPc (c) and for pure Si deposited next to the ZnPc film onto bare glass (d), layer thickness of Si about 1 mm.

on top of ZnPc. On the other hand, the Raman lines characteristic for ZnPc [29] were detected for the film as deposited, for the one that was present during Si deposition but not covered with Si and also for the film with Si deposited on top of it. So it is concluded that the chemical structure of ZnPc was not altered by the CVD process and also that mc-Si could be deposited on top of ZnPc, both fundamental prerequisites for the preparation of a composite material of the two constituents. Further, the Raman experiments indicated electronic interaction of ZnPc and mc-Si. The increase in the Raman spectra towards higher

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wave numbers is caused by a strong fluorescence background of the ZnPc [29]. This is observed for pure ZnPc before and after the CVD conditions, but not for the one with mc-Si deposited on top of it, a quite positive hint towards the desired sensitization of mc-Si by ZnPc. An open tungsten wire heated to 1700–1800 1C is generally used for the activation of the SiH4/H2 process gas in thin film silicon deposition. But contact with the HW would destroy the organic dye molecules and therefore has to be strictly avoided. Shielding with a water cooled shroud was mandatory, but with such a remote HW only amorphous Si was deposited before. Since also a-Si is used in photovoltaic devices it is interesting to characterize the a-Si ZnPc composite. The sensitization of a-Si by ZnPc was directly followed in photoconduction experiments starting with a pure a-Si thin film and observed following deposition of a-ZnPc. Fig. 7a shows the spectral dependence of the photoconduction in the visible range. The strong contribution of photons absorbed in Si was detected in the broad band centered on 500 nm. Photons absorbed in ZnPc also contributed to the observed photoconductivity as detected as a shoulder on the long wavelength part of the band between 600–700 nm proving the sensitization of Si by ZnPc. A direct comparison of the photocurrent measured at the ZnPc sensitized film with the same Si film prior to ZnPc deposition is presented in Fig. 7b for selected wavelengths. A wavelength of 500 nm was chosen at which ZnPc showed only minor absorption and Si still absorbed at reasonable intensity (Fig. 1). Here the deposition of ZnPc resulted in a subtle decrease of the observed photocurrent, probably caused by additional scattering of light. At the wavelengths of 600, 620 or 700 nm, however, at which the ZnPc shows intense optical absorption (Fig. 1), the observed photocurrent was increased significantly. Since the dark current was observed at a constant level of

0.1 nA throughout the experiments the observations clearly speak for a sensitization of the Si film by the deposited ZnPc. The effects were accumulated to an increase of the photoconduction of about 15% under white light illumination, a remarkable value considering the small interfacial contact area of ZnPc with the smooth Si films. With the preparation conditions for microcrystalline silicon at low substrate temperature as given in the literature [4], Si grew in the amorphous phase using the remote HW source. New process parameters as silane concentration and process gas pressure had to be found in order to deposit mc-Si [24]. The transition from a-Si to mc-Si is strongly dependent on parameters like the SiH4 to H2 ratio, the deposition pressure [30,31], the substrate temperature [32,33], and others. As measure of crystallinity the crystalline content calculated from Raman spectra is taken. A broad Raman line with a maximum at 480 cm1 indicates amorphous silicon, while mc Si is indicated by a sharp line at 520 cm1. Fig. 8 shows the Raman spectra of silicon deposited at substrate temperature TS ¼ 235 1C with silane concentrations and deposition pressures of 1% and 6 Pa, 5% and 1.5 Pa, and 5% and 0.7 Pa, respectively. Increasing mc-to amorphous Raman signals with increasing pressure and decreasing SiH4 content was found. 3.2. Composites of Si and ZnPc 3.2.1. Codeposited Si and ZnPc composites Composite films of Si and ZnPc were prepared by simultaneous deposition of ZnPc during CVD of Si at the parameters found for mc-Si deposition with the remote HW source. A Raman spectrum of a composite prepared under these conditions is depicted in Fig. 9 and compared to spectra of a layer of ZnPc in the b crystalline structure and of ZnPc powder [29].

Fig. 7. (a) Spectral dependence of the photoconductivity observed at a 120 nm thick film of a-Si following room-temperature deposition of a-ZnPc (equivalent amount of 10 nm thickness) measured at 2000 V cm1; (b) Comparison of the photocurrent at 2000 V cm1 observed under white light and at selected wavelengths (indicated by arrows in part a) for a pure 120 nm a-Si film (hatched columns) and the same covered with an equivalent amount of 10 nm a-ZnPc (solid columns).

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Fig. 8. Raman spectra of silicon deposited by remote HW CVD under different silane concentrations in H2 and process gas pressures. Microcrystalline silicon is indicated by a characteristic Raman line at 520 cm1, amorphous Si by a broad band with its maximum at 480 cm1. At 5% SiH4 and deposition pressure of 1.5 Pa contributions of both indicate a transitional state.

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composite exhibits all the Raman lines of pure ZnPc and the features of the composite are very similar to the spectrum of ZnPc powder. In addition to the ZnPc lines we find a broad emission around 500 cm1 clearly indicating amorphous silicon. As for the bilayer, the fluorescence background is drastically decreased in the composite, which may be taken as an indication of energy transfer to the a-Si matrix. An extremely sensitive method to changes in the molecular structure is UPS, which gives information on the occupied electronic states. In Fig. 10 spectra with increasing ZnPc content in a-Si matrix are displayed together with a spectrum of a pure ZnPc film deposited onto Si(1 1 1):H [26]. Inelastic secondary electron background has been removed. ZnPc exhibits characteristic structures in the valence band region. These structures persist in the composites. While the HOMO state in the composite is found 170 meV below the HOMO of the a-ZnPc film, the HOMO-1, HOMO-2 and HOMO-3 are shifted by approximately 300 meV to higher binding energies. The different shifts may be due to varied initial or final state effects in the different surroundings indicating charge redistribution within the molecule or variations in (photo) hole screening, respectively. An optical absorption spectrum of a codeposited composite film is shown in Fig. 11a. The contribution of both constituents was clearly detected when compared to Fig. 1. By the absorption band at 760 nm, it was shown that ZnPc clusters of the b-structure were formed under these preparation conditions, also expected from ZnPc deposition on heated substrates (see above) [27]. But samples showing absorbance similar to amorphous ZnPc

Fig. 9. Raman spectra of a codeposited Si–ZnPc composite film, a pure b-ZnPc film and ZnPc powder [29]. All vibrational features of crystalline ZnPc are conserved in the composite, but broadened to similar line shapes as in the powder. Amorphous-Si is indicated by a broad additional line around 500 cm1.

Using a 633 nm laser Raman source resonance excitation of the ZnPc Q-band causes a strong background in the spectra of pure ZnPc due to a broad fluorescence line with a maximum at 780 nm. The resonance also affects the intensity ratios of the Raman lines enhancing the lines in the lower wave number region [29]. The spectrum of the

Fig. 10. UP spectra of simultaneously deposited composites with increasing ZnPc content and of an a-ZnPc film. The electronic valence states of the ZnPc film are resembled with minor differences in relative line position, which are attributed to initial or final state variations caused by the different molecular surroundings.

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shutters installed in front of the silicon and the ZnPc source, respectively. The SiH4/H2 gas flow was switched off during the ZnPc deposition period but the HW temperature was kept constant. The organic source temperature varied with the SiH4/H2 pressure due to pressure dependent convection. Six silane sequences of 30 min alternated five dye sequences of 30 min. Raman spectra of a series of samples deposited at different substrate temperatures between 210 and 300 1C together with a spectrum of a bZnPc film are displayed in Fig. 12. Up to 235 1C the spectra basically resemble the spectrum of b-ZnPc. Changes in the intensity ratio are attributed to different arrangements of the molecules, as mentioned above. At 260 1C quenching of the fluorescence sets in, while all the Raman lines of a ZnPc film are present and a clear signal of mc-Si at 520 cm1 is observed. At 300 1C substrate temperature this signal is further increased while only scarce features of ZnPc are detected indicating a low sticking coefficient of ZnPc at this temperature. The respective absorbance spectra of the Q-band region of ZnPc are displayed in Fig. 13. Two main maxima are observed mirroring the line shape of pure ZnPc. Up to 235 1C the positions and relative intensities are similar to a

Fig. 11. Optical absorption spectrum of a 170 nm thick composite Si–ZnPc film prepared at 250 1C substrate temperature consisting of a-Si and b-ZnPc (see text); (b) Photoconductivity spectrum measured for a film prepared under the same conditions as in (a) measured at 2000 V cm1.

have also been grown when Si and ZnPc were simultaneously deposited [24]. From the optical data, an approximate content of 90% Si and 10% ZnPc can be estimated. Also in the photoconduction data at such composite films, the sensitization by ZnPc was observed by a contribution in the ZnPc absorption region around 700 nm (Fig. 11b). This contribution within the overall photocurrent spectrum approximately equals the contribution of the ZnPc absorption to the absorption spectrum, indicating similar quantum efficiency for both, photons absorbed in Si and in ZnPc. This can be considered a great success since the photoconductivity quantum efficiency for pure Si thin films often approaches unity. We are presently working on a further quantification of this efficiency. 3.2.2. Sequentially deposited Si and ZnPc composites Since co-depositing organic dye and Si leads to an amorphous Si matrix, we changed the deposition mode to sequential deposition [24]. Parameters for HW temperature, deposition pressure and silane concentration have been set in the sequential mode as mentioned above for mcSi production. Sequential deposition was controlled by two

Fig. 12. Raman spectra of sequentially deposited Si–ZnPc composites at increasing substrate temperatures. Growth of mc-Si is indicated by the Raman line at 520 cm1 marked by an arrow.

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b-ZnPc film, while for 260 1C the positions of the maxima are shifted to low photon energy. A structure at 570 nm in the spectrum of the 210 1C sample and to a smaller extent in the 225 1C sample might be an indication of additional

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ZnPc in the low temperature a phase. The silicon contribution is detectable as increasing background for increasing substrate temperature. Since Raman spectra show the mc-Si feature at 260 1C substrate temperature, but the absorbance spectra show somewhat disturbed features, we will produce sequential composites of ZnPc and mc-Si at 250 1C in the future. The electronic quality of the mc-Si in the composite was tested by contactless transient photoconductivity measurements in the microwave frequency range by the time resolved microwave conductivity (TRMC) method [34]. As figure of merit the effective mobility meff is used which correlates with the height of the TRMC signal [35]. This parameter refers to the majority carrier (in the present case electrons). A high value of meff indicates good transport properties. It must be noted that for these samples the effective mobility refers to a trap limited mobility as charge carrier trapping is already active during the excitation. TRMC signals of pure mc-Si deposited by the remote HWCVD process, mc-Si in a sequential composite with ZnPc, and for comparison a low temperature mc-Si film with a high density of defect states are displayed in Fig. 14 using 10 ns pulse excitation at 532 nm. For pure mc-Si a value for meff of 0.2 cm2V1s1 was found. This is comparable to values obtained elsewhere for HW material. The electron band mobility is certainly larger and considerable trapping leads to the lower value observed. This can also be inferred from the strong decay after the excitation (Fig. 14). So this material can be considered as state of the art mc-Si. The material produced by HW deposition from silane and sequential ZnPc deposition is characterized by a lower (about 30%) value of meff. Also here a strong electron trapping is already active during the excitation. The lifetime of the excess electrons in both systems does not exceed a few nanoseconds due to the electron trapping process. For comparison a TRMC signal of a low temperature mc-Si film with a high density of defect states is displayed in Fig. 14. Due to extensive

Fig. 13. Absorbance spectra of sequentially deposited composites. The spectra largely resemble the b-ZnPc wave structure. At 260 1C a shift to lower photon energy is observed which is taken as an indication of an upper temperature limit.

0.14 Excitation 10ns (FWHM) pulses 532nm

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Fig. 14. TRMC signal of pure remote HWCVD mc-Si, mc-Si/ZnPc sequential composite and mc-Si of inferior quality. The microwave conductivity was induced by 10 ns light pulses at 532 nm.

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R4

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1.78 VB

1.8 1.85

Fig. 15. (a) Zinc phthalocyanine and different fluorinated derivatives prepared by cyclotetramerization and purified by zone sublimation. (b) Calculated HOMO and LUMO levels of the FxZnPc in comparison to band edges of Si.

trapping meff observed does not exceed a value of 0.01 cm2V1s1. This shows that the sequential deposition described above still allowed preparation of high quality mc-Si. TRMC signals induced with 730 nm light pulses suggest a slightly higher absorption in the sensitized film than in the pure film. As 730 nm light excites ZnPc, this may point to an injection of electrons from ZnPc into the mc-Si. However, this has to be considered with care, because the sensitized film may contain a somewhat higher fraction of a-Si:H (as suggested by preliminary measurements) and this may also lead to a higher absorption at 730 nm. 3.3. Tuning of orbital line up As injection of both photo-generated charges, electron and hole, from the excited state of the dye into the semiconductor matrix is mandatory to run a continuous sensitisation process, it is crucial to provide for driving

forces for both charges, which may be reached by tuning the energetic position of the dye HOMO state below the silicon valence band and the LUMO above the conduction band. An effective way of shifting molecular ionisation potentials is substitution of H ligands by more or less electronegative species as e.g. fluorine or CH3, respectively. Calculations on the semi empirical level for ZnPc with 0, 4, 8 and 16 fluorine atoms attached predicted a stepwise frontier orbital shift of approx. 400 meV to higher binding energy increasing the number of F atoms by 4 placed at positions (4), (4, 5) and (3–6) (Fig. 15a), while the width of the HOMO LUMO gap is kept almost unchanged (Fig. 15b). We performed detailed photoelectron spectroscopy measurements at the synchrotron BESSY to investigate whether the increased ionization energy also leads to a similar change of the orbital line up at dye–Si interfaces. Details of determining the line up and results for ZnPc, F16ZnPc and ZnTPP on H terminated Si(1 1 1) have been

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Fig. 16. Photoemission spectra of the C1s core level of FxZnPc on Si(1 1 1):H with x ¼ 0, 4, 8, 16 as indicated. The spectra have been normalized in energy and intensity to the emission attributed to C–N. With increasing F number the emission at higher binding energy labelled C–F grows in, while the emission addressed to C–C, which is not discernible from C–H is diminished.

published before [26]. The results for the complete Fluorine substituted series F0, F4, F8, and F16ZnPc are depicted in Figs. 16–18 compiling C1s and valence band spectra and a schematic of the orbital line up, respectively. None of the investigated organic dyes led to the formation of a chemically shifted Si 2p component in addition to the detected emissions attributed to the Si bulk and to Si–H bonds. Also the dye core levels give no indication for a chemical reaction. Increasing the number of fluorine atoms is indicated by decreasing C–C/C–H and increasing C–F addressed emissions in the C1s core level spectra displayed in Fig. 16. The spectra have been normalized in energy and intensity to the component addressed to C–N bonds. The HOMO positions as measured relative to the Fermi level position (as determined on metallic reference substrates) are indicated in the valence band spectra displayed in Fig. 17. The spectral shapes of the HeI valence band spectra of ZnPc and F16ZnPc agree well with spectra measured in the gas phase and on Au [36]. As at higher coverage the Si substrate valence band maximum can no more be identified, its energy has to be deduced from the position of the Si 2p core level using a binding energy difference to the valence band maximum of 98.8 eV for EVBM–ESi 2p 3/2 [37]. Using a literature value of EUPS/IPES ¼ 3.0 eV for the HOMO LUMO gap as derived from ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) on gold [38], the frontier orbital line up can be given as sketched in Fig. 18. In addition to the HOMO LUMO gap as derived from UPS and IPES two more HOMO LUMO gaps are considered in each case [26], indicated as Eopt related to optical absorption measurements and Et the transport gap. The differences have been addressed to exciton binding energy (Eopt), bulk polariza-

Fig. 17. Valence band photoemission spectra of FxZnPc on Si(1 1 1):H with x ¼ 0, 4, 8, 16 from bottom to top. The position of the HOMO maximum referenced to the Fermi level is indicated.

tion (Et) and surface polarization (EUPS/IPES) [39]. As a first estimate we place Eopt and Et symmetrically to the experimentally determined UPS HOMO and IPES LUMO positions assuming similar values of relaxation and coulomb binding energies for the electron as for the hole. While the Q-band absorption of dissolved FxZnPc molecules is narrow (fwhm ¼ 20 meV) with maxima at almost the same energy of 1.81 eV, the absorption of deposited films is broad (fwhm ¼ 450 to 670 meV) with two maxima at a distance of several hundred meV [40]. Because due to instrumental broadening and averaging over different molecular surroundings the line width of UPS and especially IPES are broad as compared to optical spectra, we use in Fig. 18 a mean value for the optical absorption gap of 1.8 eV for all four phthalocyanine films, ignoring any details of the absorption spectra. Using an exciton binding energy of 0.6 eV (as determined for CuPc [38], the position of the transport gap can be given. While the increase in frontier orbital shift from ZnPc to F16ZnPc at the hydrogen terminated Si(1 1 1):H wafer surface (1.1 eV) is similar to the calculated shift (1.07 eV), for F4ZnPc(4) and F8ZnPc(4,5) the same experimental value of 550 meV was found in contrast to the calculation (Fig. 15b). Specific interactions at the interface as the development of interface dipoles cause this deviation from the calculated ionization potentials. As a result of the model experiments, we find the HOMO LUMO gap of F4ZnPc(4) and F8ZnPc(4,5) symmetrically lined up versus

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Fig. 18. HOMO position of FxZnPc on Si(1 1 1):H with x ¼ 0, 4, 8, 16 as determined experimentally (hatched HOMO peak). In addition, the ZnPc LUMO is sketched, using the HOMO-LUMO gap as measured with photoemission and inverse photoemission on gold [39]. The bulk transport gap and the optical (excitonic) gap are indicated, where 0.3 eV both for electron and for hole relaxation and a 0.6 eV exciton binding energy have been used as determined for CuPc [38]. Using the optical gap of F16ZnPc [40], the HOMO-LUMO gaps EUPS/IPES, Et, and Eopt have been sketched also. While EUPS/IPES and Et are given by states of positive and negative ions, Eopt is given by states of the neutral molecule.

the Si(1 1 1):H bands leading to similar driving forces for electron as for hole injection, which we consider optimum for the sensitisation process. The HOMO and LUMO positions of the exciton are decisive for exciton injection in a Dexter like process and also for the injection of the first charge carrier in a sequential electron hole injection reaction. The energy positions of ionic states as indicated by the transport gap are decisive for the injection of the

second charge carrier in a sequential reaction or for the injection of both carriers following exciton dissociation in an organic cluster. 4. Summary and conclusions The feasibility of depositing organic inorganic composites for optoelectronic applications has been studied for

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the prototypical dye ZnPc and the standard semiconductor material silicon. The materials have been chosen with respect to possible application as photovoltaic thin film material. Characterization has been carried out by Raman, absorbance and photoelectron spectroscopy and photocurrent and TRMC measurements. Bilayers of mc-Si on ZnPc and ZnPc on a-Si gave first indication that the dye molecules sustain the deposition conditions of mc-Si, indicated by similarity of vibrational and absorption features and that the dye molecules and Si interact optoelectronically, indirectly indicated by quenching of dye fluorescence and directly measured in increased photocurrent in the spectral range of dye absorptivity. Composites of ZnPc guest molecules in a-Si and mc-Si host matrices have been grown for the first time using a remote HW CVD source for Si and a Knudsen type source for ZnPc. In the codeposition mode composites of amorphous silicon and ZnPc are formed. Raman and UPS measurements again prove that ZnPc molecules survive the Si deposition conditions. In the sequential mode, composites of microcrystalline Silicon and ZnPc have been grown. A temperature series shows that below 235 1C, Si is deposited amorphously and at 300 1C almost no dye molecules are found in the film. At 260 1C, mc-Si was deposited with good electronic quality (high TRMC maximum) and ZnPc still shows all Raman features of a pure film. The ZnPc fluorescence is quenched giving an indirect indication of the attempted energy transfer from the dye to the mc-Si matrix. Tailoring of the orbital line up has been reached by fluorine substitution of ZnPc hydrogen atoms. The series ZnPc, F4ZnPc(4), F8ZnPc(4,5), and F16ZnPc has been synthesized, purified and deposited on Si(1 1 1):H surfaces in order to measure the HOMO line up versus the Si valence band maximum using photoelectron spectroscopy. Relaxation at the surface and in the bulk and coulomb interaction in the exciton have been considered for the HOMO LUMO line up in different situations. From the derived energetic HOMO and LUMO position F4ZnPc(4) and F8ZnPc(4,5) are identified as best candidates for the attempted sensitization process since similar driving forces for the injection of the electron and of the hole are expected in the bulk composite. In next future a device structure of composite mc-Si and F8ZnPc films deposited on transparent conductive substrates on glass and a back conduct will be used for detailed photocurrent characterization. We have shown that hybrid films composed of inorganic semiconductor matrices and organic dye molecules can be produced keeping the favourable properties of the inorganic and of the organic material, even in the rough case of HWCVD of mc-Si. First indication of the challenging optoelectronic interaction for photovoltaic applications have been shown. Also a rout to optimize the transfer of generated electron hole pairs from the dye molecules to the mc-Si matrix exploiting the flexibility of organic synthesis, e.g. in tailoring ionization energies by substitution. Of course optimization is needed in order to lessen undesired side effects as a increased number of recombination centres

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in the mc-Si that annihilate the possibly increased generation rate of electron hole pairs. Other combinations of organic molecules in inorganic semiconductor matrices may allow to design hybrids for different optoelectronic applications. Acknowledgements Funding by the Volkswagen Foundation under the Initiative Complex Materials, Co-operative Projects of the Natural, Engineering and Biosciences, is gratefully acknowledged. References [1] J. Yang, A. Banerjee, S. Guha, Solar Energy Mater. Solar Cells 78 (2003) 597. [2] H.W. Schock, Appl. Surf. Sci. 92 (1996) 606. [3] B. Rech, T. Roschek, T. Repmann, J. Muller, R. Schmitz, W. Appenzeller, Thin Solid Films 427 (2003) 157. [4] R.E.I. Schropp, Thin Solid Films 451–52 (2004) 455. [5] K.F. Brennan, The Physics of Semiconductors, Cambridge University Press, Cambridge, 1999. [6] L.S. Hung, C.H. Chen, Mat. Sci. Eng. R 39 (2002) 143. [7] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [8] D. Wohrle, D. Meissner, Adv. Mater. 3 (1991) 129. [9] D. Wohrle, L. Kreienhoop, G. Schnurpfeil, J. Elbe, B. Tennigkeit, S. Hiller, D. Schlettwein, J. Mater. Chem. 5 (1995) 1819. [10] N. Trombach, H. Tada, S. Hiller, D. Schlettwein, D. Wohrle, Thin Solid Films 396 (2001) 109. [11] B. Maennig, J. Drechsel, D. Gebeyehu, P. Simon, F. Kozlowski, A. Werner, F. Li, S. Grundmann, S. Sonntag, M. Koch, K. Leo, M. Pfeiffer, H. Hoppe, D. Meissner, N.S. Sariciftci, I. Riedel, V. Dyakonov, J. Parisi, Appl. Phys. A—Mater. Sci. Process. 79 (2004) 1. [12] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [13] L. Schmidt-Mende, A. Fechtenkotter, K. Mullen, R.H. Friend, J.D. MacKenzie, Physica E—Low Dimensional Systems Nanostruct. 14 (2002) 263. [14] P. Peumans, S. Uchida, S.R. Forrest, Nature 425 (2003) 158. [15] B. O’Rregan, M. Gratzel, Nature 353 (1991) 737. [16] R.W. Fessenden, P.V. Kamat, J. Phys. Chem. 99 (1995) 12902. [17] T. Oekermann, T. Yoshida, D. Schlettwein, T. Sugiura, H. Minoura, Phys. Chem. Chem. Phys. 3 (2001) 3387. [18] T. Yoshida, M. Iwaya, H. Ando, T. Oekermann, K. Nonomura, D. Schlettwein, D. Wohrle, H. Minoura, Chem. Commun. 400 (2004). [19] C. Sanchez, B. Lebeau, F. Chaput, J.P. Boilot, Adv. Mater. 15 (2003) 1969. [20] J.R. Darwent, P. Douglas, A. Harriman, G. Porter, M.-C. Richoux, Coord. Chem. Rev. 44 (1982) 83. [21] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [22] T. Fo¨rster, Ann. Phys. 437 (1948) 55. [23] U. Weiler, T. Mayer, E. Mankel, W. Jaegermann, C. Kelting, D. Schlettwein, D. Wo¨hrle, O. Abdhallah, M. Kunst, in: Proceedings of WCPEC IV Hawaii 2006. [24] U. Weiler, K. Schwanitz, C. Kelting, D. Schlettwein, D. Wohrle, T. Mayer, W. Jaegermann, Thin Solid Films 511 (2006) 172. [25] C. Kelting, U. Weiler, T. Mayer, W. Jaegermann, S. Makarov, D. Wo¨hrle, O. Abdallah, M. Kunst, D. Schlettwein, Org. Electron. 2006. [26] U. Weiler, T. Mayer, W. Jaegermann, C. Kelting, D. Schlettwein, S. Makarov, D. Wohrle, J. Phys. Chem. B 108 (2004) 19398. [27] J. Simon, J.-J. Andre, Molecular Semiconductors, Springer, Berlin, 1985. [28] J.H. Parker, D.W. Feldman, M. Ashkin, Phys. Rev. 155 (1967) 712.

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