Integrated Organic Blue LED and VisibleâBlind UV Photodetector

ARTICLE pubs.acs.org/JPCC

Integrated Organic Blue LED and Visible-Blind UV Photodetector Farman Ali,† N. Periasamy,*,† Meghan P. Patankar,‡ and K. L. Narasimhan‡,* †

Department of Chemical Sciences and ‡Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India

bS Supporting Information ABSTRACT: In this paper, we report on an integrated OLEDphotodetector organic device using 3,6-dipyrenyl-N-hexylcarbazole (P2NHC) as the active layer. This device is an efficient blue emitter in forward bias and an efficient visible-blind UV photodetector in reverse bias. In forward bias, the device (as a blue emitter) has a low turn on voltage (2.7 V), a current efficiency of 2.0 cd/A for a brightness of 200-9000 cd/m2, and low droop at higher brightness. In reverse bias, the device has a saturated photoresponse of 77 mA/W at 390 nm, which corresponds to an internal quantum efficiency of 64%. The spectral response of a photodetector-only device indicated that carriers are generated in the interface of P2NHC/cathode. The singlet exciton diffusion length of P2NHC was determined to be 7.8 ( 1 nm, which is too small to account for the high photodetector efficiency. Alternate mechanisms are discussed to account for the high photodetector efficiency.

1. INTRODUCTION Organic electronics has emerged as an important technology over the past decade. The drivers for the technology are light emitting diodes, transistors, photodetectors, organic solar cells, photovoltaics, etc.1-3 Organic light emitting diodes (OLEDs) are important in the context of displays and lighting applications4,5 with many commercial displays already in the market. For color displays and lighting, the most critical devices are blue emitters and have been the subject of intense research.6,7 Simultaneously, there has been an interest in visible-blind UV detectors.8-11 These UV photodetectors are potentially low cost large area devices. The OLED device is active under forward bias and the organic photodetector is active under reverse bias. Interestingly, an integrated device which is an OLED in forward bias which also functions as a photodetector has received very little attention until recently.11 Such emitterdetector organic devices are important for all-organic integrated circuits. The architectures for the photodetector and the OLED are very different and have apparently conflicting requirements of material properties. OLEDs require that the carriers (electrons and holes) injected at the contacts (cathode and anode) form molecular excitons which recombine radiatively with high efficiency. Photodetectors, on the other hand, require efficient dissociation of molecular excitons (generated by optical absorption) into free carriers and collected by the contacts as a current. The exciton binding energy in organic semiconductors is typically r 2011 American Chemical Society

0.2-0.5 eV.12 Donor-acceptor blends in close proximity facilitate exciton dissociation13,14 and are normal architectures for detectors and solar cells. These molecules are by choice designed for efficient photoluminescence quenching and therefore poor candidates as emitters in OLED. A priori, it appears contradictory that an efficient OLED to be simultaneously an efficient photodetector, or vice versa. For this reason, the best performance of organic molecules is observed in LED-only or photodetector-only devices. The recent report15 of photoresponse 540 mA/W at 365 nm for an organic photodetector-only is probably the highest. It was noted that in polymer solar cells weak electroluminescence is often detected in forward bias which was identified with interfacial charge transfer states.16 Despite the apparent difficulty to make an integrated emitter-detector device, a multilayer organic device, Zhang et al11 showed that an integrated device gave a photoresponse of 135 mA/W at 365 nm and EL brightness of ∼4000 cd/m2. On the basis of comparison of PL and EL spectra, the EL emission in this device was concluded to be exciplex and not molecular. The physical and chemical mechanisms of photocurrent and EL in such integrated devices need to be fully understood. In this paper, we report on an integrated device that uses a new molecule, 3,6-dipyrenylN-hexylcarbazole (P2NHC) as the active layer. We show that Received: October 30, 2010 Revised: December 6, 2010 Published: January 7, 2011 2462

dx.doi.org/10.1021/jp110381h | J. Phys. Chem. C 2011, 115, 2462–2469

The Journal of Physical Chemistry C

Figure 1. Normalized absorption spectra of carbazole (black), pyrene (red), and P2NHC (green) recorded in chloroform and of P2NHC as a solid film (blue). Inset shows the molecular structure of P2NHC.

this device is an efficient blue LED in forward bias and simultaneously an efficient visible-blind UV detector in reverse bias. The mechanisms for high efficiency of EL and photoresponse are discussed.

2. EXPERIMENTAL SECTION 3,6-Dipyrenyl-N-hexylcarbazole (P2NHC) was synthesized and purified by column chromatography17 The other materials used in the integrated organic device, 2,3,5,6-tetrafluoro-7,70 ,8,80 -tetracyano-p-quinodimethane (F4TCNQ), N,N0 -diphenyl-N,N0 -(bis(3methylphenyl)-1,10 -biphenyl-4,40 -diamine (TPD), 4,7-diphenyl1,10-phenanthroline (BCP), and lithium fluoride (LiF), were sourced from Aldrich and used as obtained. The typical device structure fabricated is ITO/F4TCNQ(3 nm)/TPD(40 nm)/ P2NHC (40 nm)/BCP(6 nm)/LiF(1 nm)/Al. The details of the device preparation and electrical measurements have been described earlier.18 Briefly, the devices were made by vacuum evaporation of the organic molecules, LiF and Al cathode at a base pressure of 8 10-7 Torr in a single pumpdown. All devices were capped with 100 nm of LiF to act as a passivating layer. The device area was typically 2 mm2. After fabrication, the devices were transferred in air to a vacuum system for electrical measurements. EL intensity of OLED and its spectrum was measured as described in ref 18. Photocurrent measurements were made using a 75 W xenon lamp source. The light was coupled into a 0.15 m Acton monochromator for measuring the spectral response of the photodetector. The intensity of the illumination light was measured using a UV-020 calibrated silicon photodiode and corrected for the response of the photodiode. The intensity of illumination light was also determined by ferrioxalate actinometry.19 Photocurrent measurements were made using a SRS 530 lock-in amplifier, and the light was chopped at 10 Hz. Photoluminescence of molecules in thin films and solutions were measured using a SPEX Fluorolog-3 fluorimeter. An LED at 380 nm was used in measurement of electric field effect on PL quenching in devices. 3. RESULTS AND DISCUSSION 3.1. Optical Spectra and Energy Levels. Figure 1 shows the absorption spectra of P2NHC in chloroform and as a thin film.

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Figure 2. Normalized emission spectra of carbazole (black), pyrene (red), and P2NHC (green) recorded in chloroform and of P2NHC as a solid film (blue).

The inset in the figure shows the molecular structure of P2NHC. P2NHC is a molecule where two chromophores, pyrene and carbazole, are connected by a single bond. The figure also shows the absorption spectra of carbazole and pyrene in chloroform. Carbazole has multiple sharp absorption peaks at 291, 321, and 334 nm. The absorption spectrum of pyrene also has sharp multiple peaks at 275, 308, 322, and 337 nm. In contrast, the absorption spectrum of P2NHC in chloroform is broad and the peak is shifted to longer wavelength (347 nm). The absorption spectrum of solid thin film (vacuum evaporated) is slightly redshifted (354 nm) due to solid state effects. Figure 2 shows the photoluminescence (PL) spectrum of P2NHC recorded in chloroform and in thin film (vacuum evaporated), both excited at 360 nm. The figure also shows for comparison the PL of precursor molecules, carbazole and pyrene in chloroform. The emission spectra of carbazole and pyrene have multiple peaks below 400 nm. On the other hand, the PL of P2NHC in chloroform is characterized by a broad peak centered at 424 nm. The peak emission of P2NHC in solid thin film is at 448 nm-further red-shifted due to solid state effects. The absorption and emission spectra of P2NHC are very different from those of carbazole and pyrene which indicate that there is an overlap of the π electron wave functions of carbazole and pyrene. P2NHC is a more extensively conjugated molecule than its precursors. We mention in passing that there is a large Stokes shift (94 nm) between the absorption and emission peaks of P2NHC in solid state. The overlap of absorption and emission spectra is very small. The optical band gap of P2NHC was determined to be 2.94 eV using the wavelength of the intersection of the absorption and emission spectra as a measure of the band gap. Phosphorescence spectrum of P2NHC was recorded in chloroform glass at 77 K, which peaked at 612 nm (see Supporting Information). The triplet level of P2NHC was estimated to be 2.06 eV from the rising edge of the phosphorescence spectrum. However, phosphorescence spectrum of P2NHC solid could not be recorded at room temperature. The energy of the highest occupied molecular orbital (HOMO) was determined experimentally using cyclic voltammetry (CV).20 The HOMO- LUMO energy difference was taken to be the optical band gap. The HOMO and LUMO energies were 2463

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Figure 3. Schematic energy level diagram of HOMO and LUMO levels of the molecules used in the device.

found to be -5.44 and -2.50 eV, respectively, for P2NHC. This knowledge has guided the choice of molecules for charge transport and injection layers, electron and hole blocking layers, in the design using P2NHC as the active layer in a OLED architecture, ITO/F4TCNQ/TPD/P2NHC/BCP/LiF/Al. Figure 3 is a schematic of the HOMO and LUMO levels for the molecules used in the devices reported here. The energy levels for the other molecules (F4TCNQ, TPD, and BCP) were taken from the literature.21-23 Here, F4TCNQ facilitates hole injection, TPD is a hole transporting and electron blocking layer, and BCP is an electron transporting and hole blocking layer. The use of LiF facilitates electron injection presumably by doping the BCP layer with Li which is released with the deposition of Al.24 3.2. Blue OLED. We made a series of devices A, B, C, and D shown below. Of these, device D is the optimized OLED, the properties and performance of which will be described in detail. The properties of devices A, B, and C help to clarify carrier injection processes in device D. (A) ITO/TPD(40 nm)/ P2NHC(40 nm)/Al (B) ITO/F4TCNQ(3 nm)/TPD(40 nm)/ P2NHC(40 nm)/ Al (C) ITO/F4TCNQ(3 nm)/ P2NHC(40 nm)/BCP(6 nm)/ LiF(1 nm)/Al (D) ITO/F4TCNQ(3 nm)/TPD(40 nm)/ P2NHC(40 nm)/ BCP(6 nm)/LiF(1 nm)/Al Each of the above devices A-D has a different thickness. For a comparison of current, electroluminescence, and turn-on voltages in these devices, it is appropriate to use the nominal electric field F (F = V/d, where V is the applied voltage and d the total thickness), rather than the voltage across the device. Hence the current density and EL intensity for all the devices are plotted against the nominal electric field (not applied voltage) and are shown in parts a and b of Figure 4, respectively. In device A, current injection starts at a rather large field (6 105 V/cm) and increases slowly with the field. We see from Figure 3 that the barrier to hole injection from ITO into P2NHC is 0.44 eV. The magnitude of the barrier to electron injection from Al into P2NHC depends on the BCP/LiF interface and is larger than

Figure 4. (a) Current density as a function of electric field for the devices A-D. (b) EL intensity as a function of electric field for the devices A-D.

0.5 eV. Therefore, the current turn-on at high electric field is consistent with the existence of large barriers to both electron and hole injection. Since the barrier to hole injection is smaller than the barrier for electron injection and TPD being a good hole transporter, the current in device A is predominantly a hole current. As expected, the light emission in this device is very poor. In device B, the current is much larger than that in device A and also turns on at a lower field (3 105 V/cm). The F4TCNQ/ TPD layer acts as a good hole injection and transport layer. The current is still primarily a hole current. The electron injection is still very poor and the current efficiency which is the ratio of EL intensity to current density, is poorer than that in device A. In device C, an electron injection layer (BCP/LiF) is added and simultaneously the TPD layer is removed. This results in poor performance in current injection efficiency and EL. In fact, the performance of device C is worse than that of device A suggesting that the hole transport is more difficult across the ITO/F4TCNQ/P2NHC interface compared to the ITO/TPD/ P2NHC interface in device A. This shows that the F4TCNQ/ TPD layer in device B is critical to ensure good hole injection and transport. The multilayer architecture of device D is complete in all functional aspects. It is seen that in device D the current and electroluminescence intensity increase sharply with the electric field and the turn-on field remains low. Compared to device B which has a similar low current turn-on electric field, the sharp increase in current accompanied by sharp increase in EL by a factor of 1000 in device D over device B is due to efficient hole and electron injection accompanied by exciton formation 2464



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Figure 5. EL spectrum of the device D and PL spectrum of solid film of P2NHC (vacuum evaporated). Inset shows the photograph of device D.

and radiative recombination. We mention in passing that the efficiency in device D is also assisted by the electron and hole transport barrier due to TPD and BCP layers which serve to confine the electrons and holes in the P2NHC layer, as indicated by the energy levels shown in Figure 3. Besides, hole accumulation at the P2NHC/BCP interface presumably increases the electric field across the BCP facilitating electron injection in a manner similar to that reported for TPD/Alq3 devices.25,26 Figure 5 shows the EL spectrum of device D. The peak emission wavelength is 450 nm and this spectrum matches with the PL spectrum (peak at 448 nm) of the thin film of P2NHC. This shows that the EL spectrum is due to exciton formation and recombination in the P2NHC layer. The commission international de L’Eclairage (CIE) coordinates for this blue OLED are (0.16, 0.13). The inset shows the photograph of blue light emitting device D. Parts a and b of Figure 6 show the EL brightness (cd/m2)current efficiency (cd/A) characteristics of device D at low and high voltages, respectively. The turn-on voltage of this device is approximately 2.7 V. The light output increases sharply with the voltage and reaches a brightness of ∼500 cd/m2 at 4 V. The current efficiency is over 2 cd/A for a brightness in the range of 200-9000 cd/m2 with a low droop even when the device is driven to high levels of brightness (16000 cd/m2). The maximum current efficiency is 2.2 cd/A at a brightness of 2200 cd/m2. By appropriate use of doped contact layers, it should be possible to reduce the operating voltage at high levels of brightness. There was very little hysteresis when the device is repeatedly cycled suggesting that charge trapping was not important in P2NHC. The above results indicate that the performance characteristics of device D are very good and P2NHC is a suitable candidate molecule for a blue OLED. 3.3. Visible-Blind UV Detector. In the previous section, we have shown that P2NHC as the active layer in a multilayer device architecture (device D) results in efficient blue electroluminescence in forward bias. In this section, we show that in reverse bias, the same device functions as an efficient visible-blind UV detector. Device D was illuminated at 390 nm through ITO and photocurrent was measured for zero and various reverse bias voltages. Photoresponse (mA/W), the ratio of photocurrent to the optical power of illuminated light on ITO, was calculated. Figure 7 shows the photoresponse of device D as a function of reverse bias voltage. The photoresponse increases continuously

Figure 6. (a) Brightness (cd/m2) vs voltage and current efficiency (cd/A) vs voltage characteristics for device D at low voltage. (b) Brightness (cd/m2) vs voltage and current efficiency (cd/A) vs voltage characteristics for device D at high voltage.

Figure 7. Absolute magnitude of photoresponse of the device D as a function of applied voltage for illumination through ITO with 390 nm light. The plot includes results for voltage sweep from 0 to -16 and -16 to 0 V. Inset: Plot of photoresponse (PR) for device E for illumination at 370 nm through Al.

with the applied voltage and tends to saturate at 77 mA/W at -16 V. This corresponds to an external quantum efficiency (ratio of electron to photon) of 24%. Taking into account the reflection by Al electrode and absorption loss, the internal quantum efficiency of photocurrent generation due to P2NHC 2465



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is calculated to be 64%. The detector shows negligible hysteresis for repeated voltage cycling. Thus, the performance of the device D as a photodetector is very good. In the above discussion, we have assumed that the TPD layer does not contribute significantly to the photocurrent. Earlier work27 has shown that the internal quantum efficiency for free carrier generation in TPD is 1%. This is also consistent with the fact that the saturated photoresponse is 1 mA/W for TPD photodetector27 compared with 77 mA/W for device D. Even if we include this TPD contribution, then the free carrier generation efficiency for P2NHC would be reduced from 64% to 63.5%. Hence we conclude that TPD contributes negligibly to the photocurrent in device D. The photoresponse in device D can be understood as follows. Optical absorption in the P2NHC layer creates excitons and a fraction of these excitons dissociate to give free carriers. These carriers drift under the influence of the electric field and are collected by the respective electrodes. In reverse bias, there is no barrier for holes (electrons) to reach the ITO (Al) contacts. If the photocurrent is due to generation in the bulk, then at low electric field, the photocurrent is given by28 J ¼ GqημτF

ð5Þ

where J (A/cm2) is the current density, G (photon/(cm3 s)) the generation rate, q the electronic charge, η the efficiency of free carrier generation and collection, μ (cm2/(V s)) the mobility of the carrier, τ (s) the carrier lifetime, and F (V/cm) the electric field. In eq 5, μτ = μnτn þ μpτp, the suffix n and p stand for electron and hole, respectively. If I0 (photons/(cm2 s)) is the intensity of light on the sample then, G = (I0/L)(1 - exp(-RL)), where R (cm-1) is the absorption coefficient and L (cm) the sample thickness. We see from eq 5, the photocurrent density is linear with electric field (if η is independent of F) and is proportional to the collection length given by μτF. At sufficiently high electric field (μτF > L) when the collection length is greater than the sample thickness, the photocurrent will saturate because all the carriers generated in the sample are collected. The saturated photocurrent value, Jsat = GqLη and is independent of μ and τ. The plot of photoresponse versus applied voltage for device D (Figure 7) was replotted as photocurrent density (J) vs electric field (F) (plot not shown) which showed a linear variation between -0.5 and -1.2 106 V/cm. As noted earlier, the saturated photocurrent yields a value for η = 0.64. Using η = 0.64 calculated from the saturated photocurrent and the slope in the linear regime of J vs F plot, μτ product was calculated to be 2.8 10-12 cm2/V. The value of μτ product is 1 order less than that obtained for pure TPD27 which is 4.3 10-11 cm2/V but comparable to that of TPD/Alq3 blend9 which is 2.2 10-12 cm2/V. 3.4. Mechanism of Photocurrent Generation. To gain further insight into the working of the photodetector, we need to understand the mechanism of photocurrent generation. As noted earlier, optical absorption gives rise primarily to excitons with a binding energy between 0.2 and 0.5 eV.12 To generate photocurrent, the excitons need to be dissociated. This can happen at an interface, in the bulk due to impurities, or by an electric field. We first estimate the exciton dissociation efficiency due to an electric field using PL quenching efficiency as a measure of exciton dissociation.29 The PL quenching efficiency is defined as ηðFÞ ¼ ½ðPLð0Þ - PLðFÞÞ=PLð0Þ

ð1Þ

Figure 8. PL intensity as a function of applied field in device D illuminated through ITO using a LED at 380 nm.

where PL(0) and PL(F) are the PL intensity at the zero field and at an electric field F, respectively. Figure 8 shows the PL intensity as a function of electric field for device D illuminated through ITO. As the field increases the PL intensity of P2NHC decreases suggesting that the excitons are breaking up with the application of field. The value of η(F) at the highest field (-1 106 V/cm) is ∼4%. This is significantly less than the internal quantum efficiency of 64% determined from the saturated photoresponse for device D. In addition, the spectral response of the photocurrent (see below) indicated that carriers are generated in the vicinity of Al electrode and not in the bulk of the P2NHC layer. We hence conclude that electric field dissociation of the exciton does not account for the measured photocurrent in device D. The spontaneous exciton dissociation at the interface and/or in the bulk is the dominant contributor. It is possible to distinguish between generation of free carriers in bulk and at the interface by studying a simple (photodetectoronly) device with thick P2NHC layer and thin semitransparent Al cathode so that the device can be illuminated through the bottom (ITO) and top (Al) contact.27 For this, a device with 200 nm thick P2NHC and a thin Al (15 nm) was fabricated by vacuum evaporation. The device structure used in this measurement is ITO/F4TCNQ (3 nm)/P2NHC(200 nm)/Al(15 nm), denoted as device E. It may be noted that TPD and BCP/LiF layers were not used in this device and hence device E is not an OLED. We refer to this as photodetector-only device E. As will be shown later, the photoresponse of this photodetector-only device is similar to that measured for the OLED device D. Photoresponse of the device E was measured as a function of reverse bias voltage for illumination through ITO and Al. Parts a and b of Figure 9 show the spectral response of the device at zero bias for illumination through ITO and Al, respectively. The data are corrected for ITO and Al transmission losses respectively. The optical absorption of the 200 nm thick P2NHC film is also shown for comparison. For illumination through ITO, the spectral response is antibatic with respect to the absorption. That is, the photoresponse is minimum at wavelength of maximum absorption by P2NHC. In contrast for illumination through the top Al electrode, the photoresponse is symbatic with the absorption. These results imply that exciton dissociation takes place in the vicinity of the Al interface. For illumination through ITO, strongly absorbed light does not reach this interface and does not contribute to the photocurrent and hence the antibatic relationship. For illumination through Al, a symbatic 2466



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Figure 10. Photoresponse of the device E, ITO/F4TCNQ (3 nm)/ P2NHC (200 nm)/Al for illumination through Al recorded at -24 V. Absolute value of the photocurrent is used.

Figure 9. (a) Zero bias “antibatic” photoresponse (black) of device E, ITO/F4TCNQ (3 nm)/P2NHC (200 nm)/Al for illumination through ITO. The figure also shows the optical absorbance of 200 nm P2NHC film (blue). (b) Zero bias “symbatic” photoresponse (black) of device E, ITO/F4TCNQ (3 nm)/P2NHC (200 nm)/Al for illumination through Al. Absolute value of the photocurrent is used.

relationship is expected. We hence conclude that the observed photocurrent is due to exciton dissociation in the vicinity of the P2NHC/Al interface. Figure 10 is a plot of the saturated photoresponse for device E as a function of wavelength for illumination through Al at -24 V. To enable a comparison of device E with device D, we have corrected for the Al absorption in device E. The maximum photoresponse was measured as 115 mA/W in the UV at 360 nm, which is near the peak of P2NHC absorption. The photoresponse decreases to 60 mA/W at 300 nm. These values compare favorably with the UV photoresponse of inorganic UV detectors,30-32 GaN (150 mA/W) and SiC (120 mA/W). We now discuss the similarity of photoresponse (∼80 mA/W at 390 nm) for devices D and E despite the obvious differences in their structures and illumination direction. For example, device D is an efficient photodetector (77 mA/W at 390 nm) when illuminated through ITO, whereas device E showed similar performance when illuminated through a thin Al cathode. In device D illumination through thick Al was neither possible nor desirable. Similarly, in device E illumination through ITO did not yield a good result because of antibatic spectral response. The absence of antibatic effect in device D is attributed to two factors: (a) the thickness of P2NHC is 40 nm and the attenuation of light

before reaching the cathode is ∼50% at 390 nm as R = 1.03 105 cm-1 for P2NHC and 5 104 cm-1 for TPD at that wavelength, and (b) light is partially (∼50%) reflected at the Al cathode back into P2NHC layer, which compensates for the attenuation loss. Yet another difference is that P2NHC is in contact with BCP/LiF in device D whereas P2NHC is in contact with Al in device E. The efficient photoresponse in device D implies that BCP(6 nm)/LiF effectively functions as an extended cathode not only for electron injection in OLED in forward bias but also for exciton dissociation and carrier collection in reverse bias. Since the photocurrent is due to exciton dissociation at the P2NHC/Al interface in device E, the parameter that governs the detector efficiency is the exciton diffusion length (Ld). For high detector efficiency, Ld should be large but the value is unknown for P2NHC. Lunt et al.33 have proposed a method to determine Ld based on PL quenching. This method was used to determine Ld of P2NHC. The method required photoluminescence excitation spectra of spin-coated P2NHC film (57 nm) on a glass substrate and another identical sample of P2NHC on top of which a thin film of C60 (6 nm) was vacuum deposited. C60 is a PL quencher. The PL quenching ratio η(R) as a function of R is calculated as the ratio of the PL excitation intensity of the two samples. ð2Þ ηðRÞ ¼ PLP2NHC =PLP2NHC þ C60 where R (cm-1) is the absorption coefficient of P2NHC. It was shown that η(R) is related to the exciton diffusion length Ld and is given by33 η ðRÞ ¼ RðLd =cos φÞ þ 1

ð3Þ

where Ld is the exciton diffusion length and φ the angle of incidence of the excitation light. In our experiments φ is 45°. Figure 11 shows a plot of η(R) vs R and a straight line fitting the data. Using the slope, the diffusion length for P2NHC was obtained to be 7.8 ( 1 nm. Ghosh and Feng34 have derived the following equation which relates Ld to the photocurrent generated due to exciton dissociation at the electrode for thick samples (thickness . Ld). J ¼ NqψR=ðR þ 1=Ld Þ

ð4Þ

where J (electrons/s) is the photocurrent, N (photons/s) is the incident light intensity at a given wavelength λ, q is the electronic change, ψ is the product of efficiencies of exciton dissociation and carrier collection, and R (cm-1) is the wavelength-dependent 2467



Figure 11. Plot of the PL quenching efficiency η(R) vs extinction coefficient R(cm-1) of P2NHC film.

absorption coefficient of P2NHC film at λ. Device E has a film thickness of 200 nm and light in the wavelength range of 320380 nm was attenuated by 90% or more and the above equation may be used to calculate the photoresponse. The calculated value of photoresponse at 360 nm is 20 mA/W. This is about six times less than the experimental value, 115 mA/W. This discrepancy is indeed large and requires further analysis on the origin of the large photocurrent. Photoconductive gain is one such possibility. Gain or multiplication by several orders has been reported in organic polymer photodetectors.35-38 We now briefly discuss the role of gain for our system. To facilitate discussion, we assume that the photocurrent is carried primarily by holes, and deep electron traps exist in the material. Two conditions are necessary to have photoconductive gain: (1) The electron trapping time is very much greater than the hole transit time. (2) The top BCP/Al contact is an efficient hole injector. A large reverse dark current will guarantee the second condition.36 In our samples, since the reverse dark current is very small, we rule out the possibility of gain arising due to this mechanism. It is observed that in devices where photoconductive gain is observed the photocurrent or photoresponse does not saturate with applied voltage.35-37 In our device the photoresponse tends to saturate at high applied voltage thus ruling out photoconductive gain mechanism. An alternative possibility for gain is that under illumination trapped charges at the contact can lower the barrier height for minority carrier injection. Such a mechanism has been proposed by Yang et al.38 for gain in C60based devices. They attribute the gain to a lowering of the barrier for injection due to charge trapping in PEDOT:PSS. The existence of such traps should also be evident as large hysteresis in a dark I-V measurement, especially at high current drive. In our devices, there is almost no hysteresis even after the devices are driven to high current levels (100 mA/cm2 to 1 A/cm2) in forward bias for OLED operations. We hence discount photoconductive gain as being responsible in the devices reported here. We now discuss other possible reasons to account for the large photocurrent. Our experiments clearly show that exciton dissociation takes place near the cathode interface. However, the magnitude of the photocurrent is not consistent with singlet exciton dissociation at a sharp interface. One possibility is that metal diffusion takes place over a significant volume of the P2NHC layer and act as centers for exciton dissociation.

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Exciton dissociation will now mimic bulk photogeneration of carriers if the P2NHC layer thickness is comparable with the diffused metal region. Lunt et al.33 propose this mechanism as a possibility for the large photocurrent observed in some organic systems where the singlet exciton diffusion length is very small. Alternatively, if triplet exciton dissociation is responsible for the photocurrent39,40 and the triplet diffusion length is comparable with the sample thickness, this too will mimic bulk photogeneration of carriers. The photocurrent in either case will be larger than that calculated by eq 4 and thus qualitatively account for the large photocurrent and internal quantum efficiency. Finally, it may seem paradoxical for coexistence of an efficient photodetector and light emitter in the same device. This is possible due to the design of the device, and device D is an example. In forward bias, the injected carriers (both electrons and holes) are confined to the P2NHC layer (Figure 3) due to electron blocking TPD layer and hole blocking BCP layer increasing the possibility of exciton formation and radiative recombination. In contrast in reverse bias, the same device architecture facilitates carrier drift toward respective electrodes following exciton dissociation there by suppressing any recombination.

4. CONCLUSIONS We have demonstrated the fabrication of an integrated dual blue OLED and visible-blind UV photodetector device using a new molecule, P2NHC. This device shows efficient blue electroluminescence with a current efficiency 2.0 cd/A for a brightness of 200-9000 cd/m2 with a maximum current efficiency of 2.2 cd/A for a brightness of 2200 cd/m2. The current efficiency has a small droop at higher brightness and a low EL turn-on voltage of 2.7 V. The EL efficiency of the device is due to confinement of the carriers in the P2NHC layer in forward bias. The blue OLED structure also functions as an efficient visible-blind UV photodetector in reverse bias with an efficiency of 77 mA/W at 390 nm. The internal quantum efficiency is estimated to be 64%. The photocurrent in the detector arises from the exciton dissociation in the vicinity of the cathode. The singlet exciton diffusion length in P2NHC was determined to be 7.8 ( 1 nm, which is inadequate to explain the high photodetector efficiency. Alternate mechanisms which account for the high efficiency are discussed. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figures showing the phosphorescence spectrum and additional results on photodetector and LED. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], [email protected].

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