- Email: [email protected]
Al contact in organic optoelectronic devices
Organic Electronics 12 (2011) 1571–1575
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Poor photo-stability of the organic/LiF/Al contact in organic optoelectronic devices Qi Wang, Hany Aziz ⇑ Department of Electrical and Computer Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1
a r t i c l e
i n f o
Article history: Received 14 March 2011 Received in revised form 16 May 2011 Accepted 6 June 2011
Keywords: Photo-stability Organic/LiF/Al contact Photochemical reaction Diffusion
a b s t r a c t We study the photo-stability of the organic/LiF/Al contacts widely used in organic optoelectronic and photovoltaic devices. Exposing organic/LiF/Al to illumination is found to lead to a decrease in fluorescence (FL) yield of the organic layer. The decrease in FL is found to be due to the diffusion of certain positively charged species, possibly resulting from photochemical interactions with the LiF layer. Evidence of the correlation between the density of the excited states in the organic material and the rate of photo-degradation of the organic/LiF/Al contact is also reported. The results reveal that these widely used contacts suffer poor photo-stability. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Since the first demonstration of very efficient electron injection from a metal contact, especially Al, to organic layer in organic light emitting devices (OLEDs) by inserting an ultrathin LiF layer [1], LiF/Al contacts have been widely used in a variety of organic optoelectronic devices such as OLEDs [2–4] and organic photovoltaics (OPVs) [5–7]. In order to understand the mechanisms accounting for the improved performance of LiF/Al contacts, a large number of studies have been done on the electrical and chemical properties of tris-(8-hydroxyquinoline) aluminum (AlQ3)/LiF/Al contacts [8–13]. In these studies, it is concluded that a much better contact is formed when the three components AlQ3, LiF and Al co-exist [10,13], resulting in improved device performance such as a lower driving voltage (Vd) and a higher electroluminescence (EL) efficiency in case of OLEDs [3,4] or higher fill factor and power conversion efficiency in case of OPVs [6]. Despite the wide utilization and benefits of LiF/Al in organic optoelectronic devices, the stability, especially the photo-stability, of the organic/LiF/Al contacts has so ⇑ Corresponding author. E-mail address: [email protected] (H. Aziz). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.06.003
far been overlooked. So although an extensive number of studies addressed the chemistry and physics of the organic/LiF/Al contacts [8–13], the effect of exposure to light, as is the case in the typical operation of devices utilizing these contacts, has not been specifically addressed. In a recent study, we have demonstrated that the organic/metal interfaces in general are susceptible to photo-degradation, resulting in a deterioration in charge injection [14]. Thus, given its wide usage, it is worthwhile to specifically investigate the photo-stability of the organic/LiF/Al contacts, and its critical influence on the device performance. In this study, the photo-stability of the organic/LiF/Al contact is investigated. Exposing organic/LiF/Al to illumination is found to lead to a decrease in fluorescence (FL) yield of the organic layer. The decrease in FL is found to be due to the diffusion of certain positively charged species, possibly resulting from photochemical interactions with the LiF layer. Evidence of the correlation between the density of the excited states in the organic material and the rate of photo-degradation of the organic/LiF/Al contact is also reported. 2. Experimental section In this work, tris-(8-hydroxyquinoline) aluminum (AlQ3) and 9,10-bis (2-naphthyl)-2-t-butyl anthracene
1572
Q. Wang, H. Aziz / Organic Electronics 12 (2011) 1571–1575
(TBADN) are used as neat green and blue fluorescent materials, respectively. Indium-Tin-Oxide (ITO) and LiF/Al are used as the two electrodes between which the organic material is sandwiched. All samples are fabricated by the deposition of the organic materials and metals at a rate of 1.5–3 Å/s using thermal evaporation at a base pressure of 5 10 6 torr on UV ozone-cleaned ITO-coated glass substrates. For photo-degradation, monochrome illumination from a 200 W Hg–Xe lamp equipped with Oriel77200 monochromator is used. All tests are carried out in a nitrogen atmosphere. 3. Results and discussion A group of samples of structure ITO/AlQ3(50 nm)/LiF(x)/ Al(100 nm) are fabricated and tested. In theses samples, the thickness of the LiF, x, is varied from 0 to 20 nm. Fig. 1 shows the changes in FL yield (as evident from changes in FL intensity normalized to the initial value at the same excitation power density) of these samples versus time elapsed during which the samples are either left in the dark or exposed to 365 nm illumination of power density 0.5 mW/cm2. The temperature of the samples is kept at 22 ± 1 °C during the measurements to eliminate any thermal effects on FL yield. As the figure shows, the sample without LiF (solid triangle) shows no detectable decrease in FL yield after exposure to illumination for a period of 5 h, indicating that any photo-degradation in AlQ3 molecules under these irradiation conditions must be insignificant in the given time frame. On the other hand, the ones containing LiF show a significant decrease in FL yield under the same conditions. More notably, the decrease in FL becomes faster as the thickness of the LiF layer increases from 1 to 20 nm. Furthermore, the decrease in FL from a sample with 100 nm AlQ3 and 1 nm LiF (solid square) is found to be slower than that from a sample with 50 nm AlQ3 and 1 nm LiF (hollow triangle). These observations suggest that the decrease in FL of the samples incorporating LiF may be due to the diffusion of certain species
at the AlQ3/LiF interface to bulk AlQ3 which acts as FL quenchers, hence the less pronounced effect in samples with thicker AlQ3 and/or thinner LiF. It should be pointed out that keeping a sample of the structure ITO/ AlQ3(50 nm)/LiF(5 nm)/Al(100 nm) in the dark (solid diamond) leads to no detectable change in FL yield, indicating that the change in FL is induced by illumination. The observations therefore suggest that the diffusing species behind the decrease in FL must be the product of some photo-induced interactions between LiF and AlQ3. The inset of Fig. 1 displays the percentage change in FL yield observed in these samples after 1 h of illumination versus the thickness of the LiF layer. From the figure, the faster deterioration in FL with LiF thickness appears to proceed less rapidly as thickness increases, possibly reaching saturation at a thickness of about 20 nm. This can perhaps be explained in terms of the fact that the LiF/AlQ3 interface is not abrupt but rather diffuse possibly due to the disparate molecular sizes of AlQ3 and LiF [15], which would allow for significant interpenetration between molecules of the two materials at the interface. Therefore, the FL loss increases quickly with LiF thickness initially (when LiF is less than 5 nm) as a result of an increasing effective AlQ3–LiF interface, yet gradually saturates when LiF exceeds 10 nm when AlQ3 ‘‘valleys’’ have been filled with LiF and no more AlQ3–LiF interface is formed. To further investigate the nature of the processes behind the behavior observed in Fig. 1, we study the effect of applying electric fields on the photo-induced changes in the FL yield. Fig. 2a shows the changes in FL yield of a sample of structure ITO/AlQ3(50 nm)/LiF(1 nm)/Al(100 nm) versus time elapsed during which the samples are subjected to one of the following stress scenarios: (i) irradiation only (denoted by L), during which the sample is exposed to 405 nm illumination of power density 0.6 mW/cm2, (ii) reverse bias only (denoted by R), during which a reverse bias of 6 V is applied across the ITO and the Al electrodes of the sample (with ITO at a more negative potential relative to Al), and (iii) irradiation and reverse bias together
Fig. 1. The changes in FL yield versus time elapsed during which the samples are either left in the dark or exposed to 365 nm illumination of power density 0.5 mW/cm2. Inset: the percentage change in FL yield after 1 h of illumination versus the thickness of the LiF layer.
Q. Wang, H. Aziz / Organic Electronics 12 (2011) 1571–1575
1573
Fig. 2. (a) The changes in FL yield of samples of the structure ITO/AlQ3(50 nm)/LiF(1 nm)/Al(100 nm) versus time elapsed during which the samples are subjected to stress scenario L, R, L + R and L + F. Inset: the changes in FL yield of samples of the structure ITO/AlQ3(50 nm)/Al(100 nm) versus time elapsed during which the samples are subjected to stress scenario L, R and L + R. (b) The changes in Vd of the samples versus time elapsed during which the samples are subjected to scenario L’ (535 nm illumination, 1 mW/cm2), R, F, L’ + R and L’ + F, respectively.
(denoted by L + R), during which the sample is subjected to the conditions of (i) and (ii) simultaneously, and (iv) irradiation and forward bias together (denoted by L + F), during which the sample is subjected to the conditions of applying a forward bias of +4 V and (ii) simultaneously. A lower voltage is used in case of the forward bias in order to minimize electrical aging effects which could result from the higher current flow in this case [16]. Despite the voltage difference, the electric field across the AlQ3/LiF interface is expected to be similar in both forward and reverse bias. This is in view of the fact that carrier concentration in the AlQ3 will be much higher in case of forward bias (due to the good electron injection characteristics of the AlQ3/LiF/Al contact and the much higher mobility of electrons in AlQ3 versus that of holes [17]); hence an expectedly much lower field across the AlQ3 layer itself in this case. As Fig. 2a depicts, exposing the sample to illumination (scenario i) leads to a gradual decrease in its FL yield, which is consistent with the results in Fig. 1, and exposing it to illumination in the presence of reverse bias (scenario iii) significantly accelerates such change. In contrast, applying a reverse bias alone (i.e. without irradiation) (scenario ii) does not bring about any detectable change in FL yield, again verifying that the underlying mechanisms are photo-induced in nature. The fact that a reverse bias can accelerate the photo-induced deterioration in FL yield suggests that the underlying processes are also electrochemical in nature. Interestingly, applying a forward bias during irradiation (scenario iv) appears to have a little bearing on the changes in FL yield, evident in the fact that the decrease in FL yield in this case is very similar to that obtained by irradiation alone without bias (i.e. similar to that obtained from scenario i). This suggests that the diffusing species, produced by some photoinduced interactions between LiF and AlQ3 at the AlQ3/LiF interface, may be positively charged, hence its faster migration into the AlQ3 layer under reverse bias. In this regard, the slow decrease in FL yield observed in case of scenario i (i.e. L only) can be attributed to the diffusion of these positively charged species from the AlQ3/LiF interface
(where it is formed by photo-induced interactions) into the AlQ3 layer, driven by concentration gradients. In scenario iii (i.e. L + R), the application of a reverse bias during illumination significantly accelerates the migration of these positively charged species into the AlQ3 layer, hence the much faster deterioration in FL in this case. As the formation and/or release of these species from the AlQ3/LiF interface depends on photo-induced processes, the behavior is not observed with scenario ii (R only, no illumination), and the FL yield remains essentially unchanged in this case. According to this model, we would expect the application of an opposite (i.e. forward) bias to slow down the diffusion of these species. A close examination of the FL trends in the figure indeed supports this notion, where the application of a forward bias during illumination (scenario iv, L + F) can be seen to result in a slower decrease in FL yield versus that observed under scenario i (L only). Although the difference between the two trends on the figure is small, additional work shows that using a thicker LiF layer enhances the difference. It should be pointed out that the faster degradation in FL yield in case of scenario iii may, in principle, be alternatively attributed to an increased concentration of photogenerated carriers of AlQ3, due to electric field-induced dissociation of AlQ3 excitons [18], and hence to a subsequent increase in exciton–carrier interactions [19] or to a higher concentration of the unstable AlQ3 cations [16]. In order to investigate this alternative hypothesis, we tested other samples that do not contain the LiF layer. The results are shown in the inset of Fig. 2a. As shown in the figure, subjecting the samples of the structure ITO/AlQ3(50 nm)/ Al(100 nm) to scenarios i, ii and iii leads to no detectable change in FL yield, indicating that the changes observed above are associated with LiF and are not merely due to a higher concentration of photogenerated carriers in the AlQ3 layer. The results therefore rule out this alternative degradation mechanism. Besides the degradation in FL yield of the bulk organic layer, the AlQ3/LiF/Al contact also degrades as the samples
1574
Q. Wang, H. Aziz / Organic Electronics 12 (2011) 1571–1575
Fig. 3. Normalized FL yield of samples of structure ITO/AlQ3(50 nm)/LiF(3 nm)/Al(100 nm) as a function of time during which the samples are exposed to 405 nm illumination at three different power densities; 0.3, 0.7 and 1.2 mW/cm2. Inset: the FL loss after 1 h of illumination versus the illumination power density.
are subjected to irradiation. Fig. 2b shows the changes in Vd of the samples at current density 0.3 mA/cm2 versus time elapsed during which the samples are subjected to scenario L’ (535 nm illumination, 1 mW/cm2), R, F, L’+ R and L’ + F, respectively. As the figure depicts, exposing the sample to illumination (scenario L’, L’ + R and L’ + F) leads to a gradual increase in Vd, suggesting a deteriorated electron injection from the cathode and thus a degraded AlQ3/LiF/ Al contact. We have also investigated the dependence of this behavior on the illumination power density and on the optical absorbance of the organic layer. Fig. 3 shows normalized FL yield of samples of structure ITO/AlQ3(50 nm)/ LiF(3 nm)/Al(100 nm) as a function of time during which the samples are exposed to 405 nm illumination at three different power densities; 0.3, 0.7 and 1.2 mW/cm2. As
the figure shows, increasing the power density brings about a faster degradation in FL yield. The inset in Fig. 3 shows the FL loss after 1 h of illumination versus the illumination power density. Clearly, the figure shows a linear correlation, indicating that the effect depends linearly on the density of AlQ3 excited states. Fig. 4a shows normalized FL yield versus time for a sample of structure ITO/AlQ3(50 nm)/ LiF(5 nm)/Al(100 nm) subjected to monochromatic illumination of various wavelengths in the range 365–640 nm. We note that there are small variations in the illumination power density at the different wavelengths. We therefore also normalize the data to the same illumination power density, using the linear correlation as established from Fig. 3. Fig. 4b displays the same data after being normalized to the same illumination power density. Fig. 4c displays the FL loss after 1 h of illumination according to the results in
Fig. 4. (a) Normalized FL yield versus time for samples of structure ITO/AlQ3(50 nm)/LiF(5 nm)/Al(100 nm) subjected to monochromatic illumination of various wavelengths. (b) The same data after being normalized to the same illumination power density. (c) The FL loss after 1 h of illumination according to the results in Fig. 4b and the UV–Vis absorption spectrum of neat AlQ3.
Q. Wang, H. Aziz / Organic Electronics 12 (2011) 1571–1575
1575
Fig. 5. (a) Normalized FL yield versus time for samples of structure ITO/TBADN(50 nm)/LiF(5 nm)/Al(100 nm) subjected to monochromatic illumination of various wavelengths. (b) The same data after being normalized to the same illumination power density. (c) The FL loss after 1 h of illumination according to the results in Fig. 4b and the UV–Vis absorption spectrum of neat TBADN.
Fig. 4b. It also shows UV–Vis absorption spectrum of neat AlQ3. Clearly, the results show a strong correlation between the FL loss and the absorption spectrum of AlQ3, evident in the fact that illumination at wavelengths where AlQ3 has stronger absorption lead to a faster degradation in FL yield. The results indicate that absorption of light by the AlQ3 layer, rather than by other species at the contact, is behind the photo-induced processes. To further confirm this conclusion, a parallel study is carried out on another set of samples that utilize another organic material TBADN. Clearly, Fig. 5c also shows a strong correlation between the FL loss and the absorption spectrum of TBADN in this case. These results convincingly indicate that the photo-induced interactions at organic/LiF/Al contact involve interactions between LiF (or some LiF species) and adjacent organic molecules that are in an excited state; possibly due to some photo-chemical reactions, and which can lead to the release of some positively charged quenchers that can diffuse into the organic layers, and, subsequently reduce the FL yield. It should be pointed out that in case of OLEDs, the effect of the diffusion of the FL quenchers could be less serious due to the applied forward bias; however, the photo-degradation of the organic/LiF/Al contacts by the device own EL leads to a deteriorated electron injection and thus poor stability [14]. 4. Summary In summary, we study the photo-stability of the organic/LiF/Al contacts widely used in organic optoelectronic and photovoltaic devices. Exposing organic/LiF/Al to illumination is found to lead to a decrease in FL yield of the organic layer. The decrease in FL is found to be due to the diffusion of certain positively charged species, possibly resulting from photochemical interactions with the LiF layer. Evidence of the correlation between the density of
the excited states in the organic material and the rate of photo-degradation of the organic/LiF/Al contact is also reported. The results reveal that these widely used contacts suffer poor photo-stability. Acknowledgements Support by DALSA Corporation and the Natural Science and Engineering Research Council of Canada is acknowledged. References [1] L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 2. [2] L.S. Hung, C.W. Tang, M.G. Mason, P. Raychaudhuri, J. Madathil, Appl. Phys. Lett. 78 (2001) 4. [3] S.E. Shaheen, G.E. Jabbour, M.M. Morrell, Y. Kawabe, B. Kippelen, N. Peyghambarian, J. Appl. Phys. 84 (1998) 4. [4] G.E. Jabbour, B. Kippelen, N.R. Armstrong, N. Peyghambarian, Appl. Phys. Lett. 73 (1998) 9. [5] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, Appl. Phys. Lett. 78 (2001) 6. [6] C.J. Brabec, S.E. Shaheen, C. Winder, N.S. Sariciftci, Appl. Phys. Lett. 80 (2002) 7. [7] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Adv. Funct. Mater. 13 (2003) 1. [8] D. Grozea, A. Turak, X.D. Feng, Z.H. Lu, D. Johnson, R. Wood, Appl. Phys. Lett. 81 (2002) 17. [9] T. Mori, H. Fujikawa, S. Tokito, Y. Taga, Appl. Phys. Lett. 73 (1998) 19. [10] M.G. Mason, C.W. Tang, L.-S. Hung, P. Raychaudhuri, J. Madathil, D.J. Giesen, L. Yan, Q.T. Le, Y. Gao, S.-T. Lee, L.S. Liao, L.F. Cheng, W.R. Salaneck, D.A. dos Santos, J.L. Bre´das, J. Appl. Phys. 89 (2001) 5. [11] H. Heil, J. Steiger, S. Karg, M. Gastel, H. Ortner, H. von Seggern, J. Appl. Phys. 89 (2001) 1. [12] R. Schlaf, B.A. Parkinson, P.A. Lee, K.W. Nebesny, G. Jabbour, B. Kippelen, N. Peyghambarian, N.R. Armstrong, J. Appl. Phys. 84 (1998) 12. [13] P. He, S.D. Wang, S.T. Lee, L.S. Hung, Appl. Phys. Lett. 82 (2003) 19. [14] Q. Wang, Y. Luo, H. Aziz, Appl. Phys. Lett. 97 (2010) 063309. [15] C. Shen, A. Kahn, J. Schwartz, J. Appl. Phys. 89 (2001) 1. [16] H. Aziz, Z.D. Popovic, N. Hu, A. Hor, G. Xu, Science 283 (1999) 1900. [17] S. Berleb, W. Brutting, Phys. Rev. Lett. 89 (2002) 28. [18] Z.D. Popovic, H. Aziz, J. Appl. Phys. 98 (2005) 13510. [19] N.C. Giebink, B.W. D’Andrade, M.S. Weaver, J.J. Brown, S.R. Forrest, J. Appl. Phys. 105 (2009) 124514.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Poor photo-stability of the organic/LiF/Al contact in organic optoelectronic devices Qi Wang, Hany Aziz ⇑ Department of Electrical and Computer Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1
a r t i c l e
i n f o
Article history: Received 14 March 2011 Received in revised form 16 May 2011 Accepted 6 June 2011
Keywords: Photo-stability Organic/LiF/Al contact Photochemical reaction Diffusion
a b s t r a c t We study the photo-stability of the organic/LiF/Al contacts widely used in organic optoelectronic and photovoltaic devices. Exposing organic/LiF/Al to illumination is found to lead to a decrease in fluorescence (FL) yield of the organic layer. The decrease in FL is found to be due to the diffusion of certain positively charged species, possibly resulting from photochemical interactions with the LiF layer. Evidence of the correlation between the density of the excited states in the organic material and the rate of photo-degradation of the organic/LiF/Al contact is also reported. The results reveal that these widely used contacts suffer poor photo-stability. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Since the first demonstration of very efficient electron injection from a metal contact, especially Al, to organic layer in organic light emitting devices (OLEDs) by inserting an ultrathin LiF layer [1], LiF/Al contacts have been widely used in a variety of organic optoelectronic devices such as OLEDs [2–4] and organic photovoltaics (OPVs) [5–7]. In order to understand the mechanisms accounting for the improved performance of LiF/Al contacts, a large number of studies have been done on the electrical and chemical properties of tris-(8-hydroxyquinoline) aluminum (AlQ3)/LiF/Al contacts [8–13]. In these studies, it is concluded that a much better contact is formed when the three components AlQ3, LiF and Al co-exist [10,13], resulting in improved device performance such as a lower driving voltage (Vd) and a higher electroluminescence (EL) efficiency in case of OLEDs [3,4] or higher fill factor and power conversion efficiency in case of OPVs [6]. Despite the wide utilization and benefits of LiF/Al in organic optoelectronic devices, the stability, especially the photo-stability, of the organic/LiF/Al contacts has so ⇑ Corresponding author. E-mail address: [email protected] (H. Aziz). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.06.003
far been overlooked. So although an extensive number of studies addressed the chemistry and physics of the organic/LiF/Al contacts [8–13], the effect of exposure to light, as is the case in the typical operation of devices utilizing these contacts, has not been specifically addressed. In a recent study, we have demonstrated that the organic/metal interfaces in general are susceptible to photo-degradation, resulting in a deterioration in charge injection [14]. Thus, given its wide usage, it is worthwhile to specifically investigate the photo-stability of the organic/LiF/Al contacts, and its critical influence on the device performance. In this study, the photo-stability of the organic/LiF/Al contact is investigated. Exposing organic/LiF/Al to illumination is found to lead to a decrease in fluorescence (FL) yield of the organic layer. The decrease in FL is found to be due to the diffusion of certain positively charged species, possibly resulting from photochemical interactions with the LiF layer. Evidence of the correlation between the density of the excited states in the organic material and the rate of photo-degradation of the organic/LiF/Al contact is also reported. 2. Experimental section In this work, tris-(8-hydroxyquinoline) aluminum (AlQ3) and 9,10-bis (2-naphthyl)-2-t-butyl anthracene
1572
Q. Wang, H. Aziz / Organic Electronics 12 (2011) 1571–1575
(TBADN) are used as neat green and blue fluorescent materials, respectively. Indium-Tin-Oxide (ITO) and LiF/Al are used as the two electrodes between which the organic material is sandwiched. All samples are fabricated by the deposition of the organic materials and metals at a rate of 1.5–3 Å/s using thermal evaporation at a base pressure of 5 10 6 torr on UV ozone-cleaned ITO-coated glass substrates. For photo-degradation, monochrome illumination from a 200 W Hg–Xe lamp equipped with Oriel77200 monochromator is used. All tests are carried out in a nitrogen atmosphere. 3. Results and discussion A group of samples of structure ITO/AlQ3(50 nm)/LiF(x)/ Al(100 nm) are fabricated and tested. In theses samples, the thickness of the LiF, x, is varied from 0 to 20 nm. Fig. 1 shows the changes in FL yield (as evident from changes in FL intensity normalized to the initial value at the same excitation power density) of these samples versus time elapsed during which the samples are either left in the dark or exposed to 365 nm illumination of power density 0.5 mW/cm2. The temperature of the samples is kept at 22 ± 1 °C during the measurements to eliminate any thermal effects on FL yield. As the figure shows, the sample without LiF (solid triangle) shows no detectable decrease in FL yield after exposure to illumination for a period of 5 h, indicating that any photo-degradation in AlQ3 molecules under these irradiation conditions must be insignificant in the given time frame. On the other hand, the ones containing LiF show a significant decrease in FL yield under the same conditions. More notably, the decrease in FL becomes faster as the thickness of the LiF layer increases from 1 to 20 nm. Furthermore, the decrease in FL from a sample with 100 nm AlQ3 and 1 nm LiF (solid square) is found to be slower than that from a sample with 50 nm AlQ3 and 1 nm LiF (hollow triangle). These observations suggest that the decrease in FL of the samples incorporating LiF may be due to the diffusion of certain species
at the AlQ3/LiF interface to bulk AlQ3 which acts as FL quenchers, hence the less pronounced effect in samples with thicker AlQ3 and/or thinner LiF. It should be pointed out that keeping a sample of the structure ITO/ AlQ3(50 nm)/LiF(5 nm)/Al(100 nm) in the dark (solid diamond) leads to no detectable change in FL yield, indicating that the change in FL is induced by illumination. The observations therefore suggest that the diffusing species behind the decrease in FL must be the product of some photo-induced interactions between LiF and AlQ3. The inset of Fig. 1 displays the percentage change in FL yield observed in these samples after 1 h of illumination versus the thickness of the LiF layer. From the figure, the faster deterioration in FL with LiF thickness appears to proceed less rapidly as thickness increases, possibly reaching saturation at a thickness of about 20 nm. This can perhaps be explained in terms of the fact that the LiF/AlQ3 interface is not abrupt but rather diffuse possibly due to the disparate molecular sizes of AlQ3 and LiF [15], which would allow for significant interpenetration between molecules of the two materials at the interface. Therefore, the FL loss increases quickly with LiF thickness initially (when LiF is less than 5 nm) as a result of an increasing effective AlQ3–LiF interface, yet gradually saturates when LiF exceeds 10 nm when AlQ3 ‘‘valleys’’ have been filled with LiF and no more AlQ3–LiF interface is formed. To further investigate the nature of the processes behind the behavior observed in Fig. 1, we study the effect of applying electric fields on the photo-induced changes in the FL yield. Fig. 2a shows the changes in FL yield of a sample of structure ITO/AlQ3(50 nm)/LiF(1 nm)/Al(100 nm) versus time elapsed during which the samples are subjected to one of the following stress scenarios: (i) irradiation only (denoted by L), during which the sample is exposed to 405 nm illumination of power density 0.6 mW/cm2, (ii) reverse bias only (denoted by R), during which a reverse bias of 6 V is applied across the ITO and the Al electrodes of the sample (with ITO at a more negative potential relative to Al), and (iii) irradiation and reverse bias together
Fig. 1. The changes in FL yield versus time elapsed during which the samples are either left in the dark or exposed to 365 nm illumination of power density 0.5 mW/cm2. Inset: the percentage change in FL yield after 1 h of illumination versus the thickness of the LiF layer.
Q. Wang, H. Aziz / Organic Electronics 12 (2011) 1571–1575
1573
Fig. 2. (a) The changes in FL yield of samples of the structure ITO/AlQ3(50 nm)/LiF(1 nm)/Al(100 nm) versus time elapsed during which the samples are subjected to stress scenario L, R, L + R and L + F. Inset: the changes in FL yield of samples of the structure ITO/AlQ3(50 nm)/Al(100 nm) versus time elapsed during which the samples are subjected to stress scenario L, R and L + R. (b) The changes in Vd of the samples versus time elapsed during which the samples are subjected to scenario L’ (535 nm illumination, 1 mW/cm2), R, F, L’ + R and L’ + F, respectively.
(denoted by L + R), during which the sample is subjected to the conditions of (i) and (ii) simultaneously, and (iv) irradiation and forward bias together (denoted by L + F), during which the sample is subjected to the conditions of applying a forward bias of +4 V and (ii) simultaneously. A lower voltage is used in case of the forward bias in order to minimize electrical aging effects which could result from the higher current flow in this case [16]. Despite the voltage difference, the electric field across the AlQ3/LiF interface is expected to be similar in both forward and reverse bias. This is in view of the fact that carrier concentration in the AlQ3 will be much higher in case of forward bias (due to the good electron injection characteristics of the AlQ3/LiF/Al contact and the much higher mobility of electrons in AlQ3 versus that of holes [17]); hence an expectedly much lower field across the AlQ3 layer itself in this case. As Fig. 2a depicts, exposing the sample to illumination (scenario i) leads to a gradual decrease in its FL yield, which is consistent with the results in Fig. 1, and exposing it to illumination in the presence of reverse bias (scenario iii) significantly accelerates such change. In contrast, applying a reverse bias alone (i.e. without irradiation) (scenario ii) does not bring about any detectable change in FL yield, again verifying that the underlying mechanisms are photo-induced in nature. The fact that a reverse bias can accelerate the photo-induced deterioration in FL yield suggests that the underlying processes are also electrochemical in nature. Interestingly, applying a forward bias during irradiation (scenario iv) appears to have a little bearing on the changes in FL yield, evident in the fact that the decrease in FL yield in this case is very similar to that obtained by irradiation alone without bias (i.e. similar to that obtained from scenario i). This suggests that the diffusing species, produced by some photoinduced interactions between LiF and AlQ3 at the AlQ3/LiF interface, may be positively charged, hence its faster migration into the AlQ3 layer under reverse bias. In this regard, the slow decrease in FL yield observed in case of scenario i (i.e. L only) can be attributed to the diffusion of these positively charged species from the AlQ3/LiF interface
(where it is formed by photo-induced interactions) into the AlQ3 layer, driven by concentration gradients. In scenario iii (i.e. L + R), the application of a reverse bias during illumination significantly accelerates the migration of these positively charged species into the AlQ3 layer, hence the much faster deterioration in FL in this case. As the formation and/or release of these species from the AlQ3/LiF interface depends on photo-induced processes, the behavior is not observed with scenario ii (R only, no illumination), and the FL yield remains essentially unchanged in this case. According to this model, we would expect the application of an opposite (i.e. forward) bias to slow down the diffusion of these species. A close examination of the FL trends in the figure indeed supports this notion, where the application of a forward bias during illumination (scenario iv, L + F) can be seen to result in a slower decrease in FL yield versus that observed under scenario i (L only). Although the difference between the two trends on the figure is small, additional work shows that using a thicker LiF layer enhances the difference. It should be pointed out that the faster degradation in FL yield in case of scenario iii may, in principle, be alternatively attributed to an increased concentration of photogenerated carriers of AlQ3, due to electric field-induced dissociation of AlQ3 excitons [18], and hence to a subsequent increase in exciton–carrier interactions [19] or to a higher concentration of the unstable AlQ3 cations [16]. In order to investigate this alternative hypothesis, we tested other samples that do not contain the LiF layer. The results are shown in the inset of Fig. 2a. As shown in the figure, subjecting the samples of the structure ITO/AlQ3(50 nm)/ Al(100 nm) to scenarios i, ii and iii leads to no detectable change in FL yield, indicating that the changes observed above are associated with LiF and are not merely due to a higher concentration of photogenerated carriers in the AlQ3 layer. The results therefore rule out this alternative degradation mechanism. Besides the degradation in FL yield of the bulk organic layer, the AlQ3/LiF/Al contact also degrades as the samples
1574
Q. Wang, H. Aziz / Organic Electronics 12 (2011) 1571–1575
Fig. 3. Normalized FL yield of samples of structure ITO/AlQ3(50 nm)/LiF(3 nm)/Al(100 nm) as a function of time during which the samples are exposed to 405 nm illumination at three different power densities; 0.3, 0.7 and 1.2 mW/cm2. Inset: the FL loss after 1 h of illumination versus the illumination power density.
are subjected to irradiation. Fig. 2b shows the changes in Vd of the samples at current density 0.3 mA/cm2 versus time elapsed during which the samples are subjected to scenario L’ (535 nm illumination, 1 mW/cm2), R, F, L’+ R and L’ + F, respectively. As the figure depicts, exposing the sample to illumination (scenario L’, L’ + R and L’ + F) leads to a gradual increase in Vd, suggesting a deteriorated electron injection from the cathode and thus a degraded AlQ3/LiF/ Al contact. We have also investigated the dependence of this behavior on the illumination power density and on the optical absorbance of the organic layer. Fig. 3 shows normalized FL yield of samples of structure ITO/AlQ3(50 nm)/ LiF(3 nm)/Al(100 nm) as a function of time during which the samples are exposed to 405 nm illumination at three different power densities; 0.3, 0.7 and 1.2 mW/cm2. As
the figure shows, increasing the power density brings about a faster degradation in FL yield. The inset in Fig. 3 shows the FL loss after 1 h of illumination versus the illumination power density. Clearly, the figure shows a linear correlation, indicating that the effect depends linearly on the density of AlQ3 excited states. Fig. 4a shows normalized FL yield versus time for a sample of structure ITO/AlQ3(50 nm)/ LiF(5 nm)/Al(100 nm) subjected to monochromatic illumination of various wavelengths in the range 365–640 nm. We note that there are small variations in the illumination power density at the different wavelengths. We therefore also normalize the data to the same illumination power density, using the linear correlation as established from Fig. 3. Fig. 4b displays the same data after being normalized to the same illumination power density. Fig. 4c displays the FL loss after 1 h of illumination according to the results in
Fig. 4. (a) Normalized FL yield versus time for samples of structure ITO/AlQ3(50 nm)/LiF(5 nm)/Al(100 nm) subjected to monochromatic illumination of various wavelengths. (b) The same data after being normalized to the same illumination power density. (c) The FL loss after 1 h of illumination according to the results in Fig. 4b and the UV–Vis absorption spectrum of neat AlQ3.
Q. Wang, H. Aziz / Organic Electronics 12 (2011) 1571–1575
1575
Fig. 5. (a) Normalized FL yield versus time for samples of structure ITO/TBADN(50 nm)/LiF(5 nm)/Al(100 nm) subjected to monochromatic illumination of various wavelengths. (b) The same data after being normalized to the same illumination power density. (c) The FL loss after 1 h of illumination according to the results in Fig. 4b and the UV–Vis absorption spectrum of neat TBADN.
Fig. 4b. It also shows UV–Vis absorption spectrum of neat AlQ3. Clearly, the results show a strong correlation between the FL loss and the absorption spectrum of AlQ3, evident in the fact that illumination at wavelengths where AlQ3 has stronger absorption lead to a faster degradation in FL yield. The results indicate that absorption of light by the AlQ3 layer, rather than by other species at the contact, is behind the photo-induced processes. To further confirm this conclusion, a parallel study is carried out on another set of samples that utilize another organic material TBADN. Clearly, Fig. 5c also shows a strong correlation between the FL loss and the absorption spectrum of TBADN in this case. These results convincingly indicate that the photo-induced interactions at organic/LiF/Al contact involve interactions between LiF (or some LiF species) and adjacent organic molecules that are in an excited state; possibly due to some photo-chemical reactions, and which can lead to the release of some positively charged quenchers that can diffuse into the organic layers, and, subsequently reduce the FL yield. It should be pointed out that in case of OLEDs, the effect of the diffusion of the FL quenchers could be less serious due to the applied forward bias; however, the photo-degradation of the organic/LiF/Al contacts by the device own EL leads to a deteriorated electron injection and thus poor stability [14]. 4. Summary In summary, we study the photo-stability of the organic/LiF/Al contacts widely used in organic optoelectronic and photovoltaic devices. Exposing organic/LiF/Al to illumination is found to lead to a decrease in FL yield of the organic layer. The decrease in FL is found to be due to the diffusion of certain positively charged species, possibly resulting from photochemical interactions with the LiF layer. Evidence of the correlation between the density of
the excited states in the organic material and the rate of photo-degradation of the organic/LiF/Al contact is also reported. The results reveal that these widely used contacts suffer poor photo-stability. Acknowledgements Support by DALSA Corporation and the Natural Science and Engineering Research Council of Canada is acknowledged. References [1] L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 2. [2] L.S. Hung, C.W. Tang, M.G. Mason, P. Raychaudhuri, J. Madathil, Appl. Phys. Lett. 78 (2001) 4. [3] S.E. Shaheen, G.E. Jabbour, M.M. Morrell, Y. Kawabe, B. Kippelen, N. Peyghambarian, J. Appl. Phys. 84 (1998) 4. [4] G.E. Jabbour, B. Kippelen, N.R. Armstrong, N. Peyghambarian, Appl. Phys. Lett. 73 (1998) 9. [5] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, Appl. Phys. Lett. 78 (2001) 6. [6] C.J. Brabec, S.E. Shaheen, C. Winder, N.S. Sariciftci, Appl. Phys. Lett. 80 (2002) 7. [7] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Adv. Funct. Mater. 13 (2003) 1. [8] D. Grozea, A. Turak, X.D. Feng, Z.H. Lu, D. Johnson, R. Wood, Appl. Phys. Lett. 81 (2002) 17. [9] T. Mori, H. Fujikawa, S. Tokito, Y. Taga, Appl. Phys. Lett. 73 (1998) 19. [10] M.G. Mason, C.W. Tang, L.-S. Hung, P. Raychaudhuri, J. Madathil, D.J. Giesen, L. Yan, Q.T. Le, Y. Gao, S.-T. Lee, L.S. Liao, L.F. Cheng, W.R. Salaneck, D.A. dos Santos, J.L. Bre´das, J. Appl. Phys. 89 (2001) 5. [11] H. Heil, J. Steiger, S. Karg, M. Gastel, H. Ortner, H. von Seggern, J. Appl. Phys. 89 (2001) 1. [12] R. Schlaf, B.A. Parkinson, P.A. Lee, K.W. Nebesny, G. Jabbour, B. Kippelen, N. Peyghambarian, N.R. Armstrong, J. Appl. Phys. 84 (1998) 12. [13] P. He, S.D. Wang, S.T. Lee, L.S. Hung, Appl. Phys. Lett. 82 (2003) 19. [14] Q. Wang, Y. Luo, H. Aziz, Appl. Phys. Lett. 97 (2010) 063309. [15] C. Shen, A. Kahn, J. Schwartz, J. Appl. Phys. 89 (2001) 1. [16] H. Aziz, Z.D. Popovic, N. Hu, A. Hor, G. Xu, Science 283 (1999) 1900. [17] S. Berleb, W. Brutting, Phys. Rev. Lett. 89 (2002) 28. [18] Z.D. Popovic, H. Aziz, J. Appl. Phys. 98 (2005) 13510. [19] N.C. Giebink, B.W. D’Andrade, M.S. Weaver, J.J. Brown, S.R. Forrest, J. Appl. Phys. 105 (2009) 124514.