Impact of low 6,13-pentacenequinone concentration on pentacene thin film growth

Abstract

Thin films of pentacene ͑P͒ have been vacuum codeposited on SiO 2 with low concentrations of 6,13-pentacenequinone ͑PQ͒ in order to investigate the impact on the pentacene thin film structure. Within a range of 2%-20% PQ concentration no intercalation of the compounds can be observed by means of x-ray diffraction and infrared absorption spectroscopy. The crystalline quality of the P films stays unchanged by the presence of PQ, whereas P bulk phase contributions are being suppressed at PQ concentrations ജ5%, which could be confirmed by means of atomic force microscopy. From the results the authors suggest phase-separated PQ growth as well as PQ nucleation at P grain boundaries. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2767972͔ Thin films of conjugated organic molecules such as pentacene ͑P͒ ͑C 22 H 14 ͒ have been widely investigated during the last years due to the applicability in organic field effect transistors ͑OFETs͒.1-3 The charge carrier mobility in P thin films is governed by structural defects, the density of grain boundaries, 4,5 and the purity of the material. It was found that 6,13-pentacenequinone ͑C 22 H 12 O 2 ͒ is a main impurity in commercially available P and can reduce the charge carrier mobility in P single crystals. 6 In addition, it was suggested that PQ can form deep traps of 0.2-0.75 eV for electrons within a P matrix, depending on the mutual molecular orientation. 7,8 In a recent study it was shown that P and PQ exhibit pronounced phase separation if codeposited in a molar ratio of 1:1. 9 However, up to now no attention has been directed to possible structural changes of P thin films due to the presence of low concentrations of PQ during P thin film growth. We address this issue by controlled vacuum codeposition of P and PQ.Pentacene ͑Fluka, purum grade 99.9%͒ and 6,13-pentacenequinone ͑Sigma-Aldrich Co., purity 99%͒ were used without further purification. Films of nominally 30 nm thickness were obtained by vacuum codeposition ͑base pressure of 3 ϫ 10 −7 mbar, 1 nm/ min total deposition rate͒ from resistively heated ceramic crucibles. The nominal film thickness was measured in situ by a quartz microbalance. Substrates were ͑100͒ p-doped silicon wafers ͑Siegert Consulting, prime grade͒ with a thermally grown oxide layer of 50 nm, cut into 10ϫ 10 mm coupons. The root mean square roughness of the substrates was determined to 0.2 nm by atomic force microscopy ͑AFM͒ ͑all AFM investigations were done on Veeco Nanoscope III in TappingMode®͒; the substrates were used as received; the cleanliness was confirmed prior to organic film deposition by AFM investigations. X-ray diffraction ͑XRD͒ measurements were performed at the beamline W1.1 at the synchrotron radiation source HASYLAB ͑Hamburg, Germany͒; the wavelength was set to = 0.11808 nm. The upper limit of instrumental broadening of this setup was estimated by the 2⌰ width of the ͑111͒ reflection of a Ag͑111͒ single crystal ͑0.0382± 0.0003͒°. Fourier transform infrared absorption spectroscopy measurements ͑FTIR͒ ͑resolution of 2.0 cm...

Structural and electronic properties of pentacene-fullerene heterojunctions

“…Interestingly, this value is the same ͑within the error margin͒ as the value found for a pure PEN film of =30 nm ͑e = 0.0018Ϯ 0.0005͒ in a previous study. 16 An analogous analysis was done for the film of = 40 nm and yielded values of ͗L͘ V =87Ϯ 12 nm ͑e = 0.0038Ϯ 0.0008͒ and ͗L͘ V =45Ϯ 1 nm ͑e = 0.0015Ϯ 0.0003͒ on the bare and PEN precovered substrate, respectively.…”

“…46 The investigation of the microstructure was performed applying a Williamson-Hall analysis ͑WHA͒; 16,47,48 the results are shown in Fig. 4͑b͒.…”

“…[7][8][9] Alternatively, OPVCs based on bulk heterojunctions formed via codeposition have been reported to show enhanced device performance compared to layered heterojunctions, like in case of phthalocyanines combined with 3,4,9,10-perylene tetracarboxylic bis-benzimidazole 5 or C60, [10][11][12][13][14] which could be explained through improved charge transport as well as increased exciton dissociation efficiency at the bulk heterojunction. 6 However, no comprehensive study on the comparison of layered and bulk heterojunctions for PEN and C60 exists, although it is well established that PEN exhibits phase separation in case of codeposition 15,16 and that PEN based codeposited OPVCs show auspicious performance results. 17 The present work was motivated by our experimental finding that bulk-heterojunction OPVCs based on PEN and C60 codeposited films ͑PEN+ C60͒ show significantly lower efficiency compared to the respective layered structures ͑Fig.…”

Effect of impurities on pentacene island nucleation

“…Previous studies have shown that these tall islands are crystalline PnQ. 2,6,7,25 Sample AFM images of thick films, displayed in Fig. 2, show that increasing concentration of PnQ during growth causes an abrupt change for PnQ number fractions larger than 0.008 in the ultimate grain sizes and local structure of the bulk film.…”

“…Analysis of the distribution of capture zones areas of submonolayer islands as a function of impurity content shows that for large PnQ content the critical nucleus size for forming a Pn island is smaller than for low PnQ content. This result indicates a favorable energy for formation of Pn-PnQ complexes, which in turn suggests that the primary effect of PnQ on Pn mobility may arise from homogeneous distribution of PnQ defects.Pac Numbers: 68.65.-k, 05.10.Gg, 73.63.-b, 68.35.bm, 68.55.-a Published as PRB 77, 205328 (2008) The study of organic materials, particularly the various roles of morphology and impurity doping, remains an active subject for device physics, materials design, and applied statistical mechanics.1-3 Studies of the most promising organic electronic semiconductor, pentacene (Pn), have shown that its transport properties are sensitively dependent on crystalline quality 4, 5 and thin film preparation: for the work here, observations that low concentrations of impurities significantly affect film nucleation and growth, electronic transport, and electronic signal noise are of particular interest [6][7][8][9] . Extensive studies of the initial stages of pentacene film growth 3, 10-15 have shown that it follows the classical picture of nucleation, island growth, aggregation and coalescence that was developed for the growth of inorganic films.…”