Photoluminescence Quenching and Photoconductivity in Devices

ARTICLE pubs.acs.org/JPCC

Photoluminescence Quenching and Photoconductivity in Devices Using 3,6-Diaryl-N-hexylcarbazole Farman Ali,† N. Periasamy,† Meghan P. Patankar,‡ and K. L. Narasimhan*,‡ †

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

bS Supporting Information ABSTRACT: Photocarrier generation mechanisms in 3,6-diaryl-N-hexylcarbazole (aryl = p-cyanophenyl and p-acetophenyl) were studied in single-layer, bilayer, and blend film devices in sandwich geometry between indium
tin oxide (ITO) and Al electrodes. Bilayer and blend film devices were made with N,N0 -diphenyl-N,N0 -bis(3-methylphenyl)-1,10 -biphenyl-4,40 -diamine (TPD). Photoluminescence (PL) was strongly quenched by electric field. The photocurrent versus electric field plots and photoresponse action spectra were used to identify the source of photocarrier generation (interface versus bulk) in the three devices. In blend devices, the photocurrent density is directly proportional to PL quenching efficiency, indicating efficient carrier generation and collection in these devices. PL quenching and photocurrent results were fitted to theoretical equations to obtain material parameters such as exciton binding energy, exciton dissociation rate at zero field, PL quenching efficiency at zero field, and carrier mobility-lifetime product. The device properties of the two carbazole derivatives are suitable for visible-blind UV photodetectors.

1. INTRODUCTION Organic electronics has emerged as a distinct field over the past decade. Organic molecules in electronics are now used in a number of applications, such as organic light-emitting devices (OLEDs), thin-film transistors, photodetectors, and organic solar cells.1
3 In particular, OLED is the most successful technology, and some of the OLED-based products have already appeared in the market. Organic semiconductors are expected to fill an important niche for applications requiring large area displays, lighting, and solar cells. There is also a growing interest in organics-based visible-blind UV photodetectors. These UV photodetectors find applications ranging from astronomy to chemical sensors. The design of organic photodetectors and organic solar cells share functional similarities. In photodetectors it is possible to apply a large bias. This facilitates efficient exciton dissociation and carrier collection by the external applied electric field, which is not applicable for solar cells. In organic semiconductors, the primary optical excitation creates an exciton. The binding energy of the excitons is large and varies from 0.2 to 1 eV.4
6 Hence most of these materials exhibit bright photoluminescence. For efficient photodetectors and solar cells, free carrier generation by dissociation of these excitons is very important. Exciton dissociation can take place by different mechanisms. Dissociation can be assisted by electric field, occur spontaneously at organic
organic interfaces, etc., and consequently results in photoluminescence quenching. Complementary relation between photocurrent and photoluminescence is useful in the study of mechanisms of photocurrent in devices using organic materials.7
11 Carbazole derivatives are potential candidates for blue-emitting devices and visible-blind UV detectors. Addition of side groups to r 2011 American Chemical Society

carbazole influences the solubility, energy levels, and electronic properties of thin films of these molecules. N-Hexylcarbazole with several diaryl groups in 3- and 6-positions were investigated as candidates for blue emitters in OLEDs, and the results have been reported elsewhere.12 It was shown recently that a multilayer device using 3,6-dipyrenyl-N-hexylcarbazole (P2NHC) as the active layer is an efficient dual-function device, namely, blue OLED and UV photodetector.13 In this paper, we explore the electroluminescence, photoluminescence quenching, and photocurrent generation in two carbazole derivatives in single-layer, bilayer, and blend devices. We demonstrate efficient visible-blind UV photodetectors using these two molecules. The paper is organized as follows. We first describe the experimental results for one molecule, namely, CNHC. This is followed by discussion and interpretation of the results. We then present the results for the second molecule, ANHC, and discuss the results, comparing them with the results obtained for CNHC. We then present the conclusions.

2. EXPERIMENTAL SECTION 3,6-Di(p-cyanopheny)l-N-hexylcarbazole (CNHC) and 3,6di(p-acetophenyl)-N-hexylcarbazole (ANHC) were synthesized and purified by column chromatography.12 The other materials used in the device structures, N,N0 -diphenyl-N,N0 -bis(3methylphenyl)-1,10 -biphenyl-4,40 -diamine (TPD), 4,7-diphenyl-1, 10-phenanthroline (BCP), and lithium fluoride (LiF), were high-purity Received: October 5, 2011 Revised: December 14, 2011 Published: December 17, 2011 1298

dx.doi.org/10.1021/jp209586r | J. Phys. Chem. C 2012, 116, 1298–1306

The Journal of Physical Chemistry C or electronic-grade materials sourced from Sigma
Aldrich and used as obtained. 2,3,5,6-Tetrafluoro-7,70 ,8,80 -tetracyano-p-quinodimethane (F4TCNQ) was purchased from Lumtech (Taiwan). The device structures used are (1) single-layer devices, ITO/XX (90 nm)/Al (2) bilayer devices, ITO/TPD(20 nm)/XX (100 nm)/Al (3) blend devices, ITO/TPD
XX (100 nm)/Al where XX is CNHC or ANHC. Briefly, the devices were made by vacuum evaporation of the organic molecules, LiF, and Al cathode in a single pumpdown at a base pressure of 8  10
7 Torr. All devices were capped with a passivating layer (100 nm) of LiF. The device area was typically 2  1 mm2. After fabrication, the devices were transferred in air to a vacuum system for electrical measurements. The blend devices were made by either spin-coating or vacuum coevaporation of ANHC (CNHC) and TPD in a ratio of 1:1 (w/w). The spin-coated samples were made by dissolving the samples in chloroform with a concentration of 16 mg/cm3. The samples were spin-coated at 2000 rpm. No significant difference in the photocurrent was found between the evaporated and spin-cast devices. Electroluminescence (EL) intensity of OLED and its spectrum was measured as described earlier.12,13 Photocurrent measurements were made with 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 with a UV-020 calibrated silicon photodiode and corrected for the response of the photodiode. The intensity of illumination light at 400 nm was also determined by ferrioxalate actinometry.14 Photocurrent measurements were made with a SRS 530 lock-in amplifier and the light was chopped at 10 Hz. Photocurrent was measured in both reverse and forward bias. The photoresponse (A/W) is the measured photocurrent (A/cm2) per incident optical power (W/cm2). The lifetime of the samples was measured by measuring the luminescence decay via a time-correlated single-photon counting setup described earlier.15 The samples were excited at 305 nm. Luminescence of thin films and photoluminescence (PL) quenching measurements of devices were measured on a SPEX Fluorolog-3 fluorometer. The PL quenching efficiency was measured by monitoring the decrease in the PL intensity at the emission peak. We have also measured the spectral response of the PL quenching spectrum over the whole emission range to gain insight into the mechanisms of PL quenching. The PL quenching efficiency was also calculated from the integrated emission. The two results were within 5
10% of each other for all samples except the CNHC
TPD blend sample. For this sample, the PL quenching efficiency estimated from the peak position was 20% greater than that obtained from the integrated emission. The experimental data were fitted to theoretical equations by a nonlinear leastsquares method by use of ORIGIN software.

3. RESULTS AND DISCUSSION 3.1. Optical Spectra and Energy Levels. The absorption and emission spectra of ANHC and CNHC in solution and other properties are described elsewhere.12 Figure 1 shows the absorption spectra for thin films of CNHC, ANHC, and TPD deposited on quartz substrates by vacuum evaporation. The absorption spectra are similar for the three molecules, with two absorption peaks at ∼310 and 360 nm. The absorption coefficient is slightly smaller for TPD when compared with the other two molecules.

ARTICLE

Figure 1. Absorption coefficient (α, cm
1) as a function of wavelength for 100 nm thick films of (black) TPD, (green) CNHC, and (red) ANHC. The peak positions are at 317 and 358 nm (TPD), 312 and 355 nm (CNHC), and 313 and 361 nm (ANHC).

Figure 2. (a) PL spectra of (black) single-layer CNHC film, (red) TPD/CNHC bilayer film, and (green) TPD
CNHC blend deposited on ITO. (Inset) EL spectrum of blend device. (b) PL spectra of (black) single-layer ANHC film, (red) TPD/ANHC bilayer film, and (green) TPD
ANHC blend film deposited on ITO. (Inset) EL spectrum of blend device.

Figure 2a shows the PL spectrum for the single-layer (CNHC), bilayer (TPD/CNHC) and blend films of TPD
CNHC deposited on indium tin oxide (ITO). PL emission of the single-layer CNHC film has a peak emission at 424 nm, in agreement with earlier results.12 PL emission of the blend films is broad and dominated by the exciplex emission at 516 nm along with a contribution from the CNHC monomer (424 nm) and TPD (402 and 420 nm).16 The emission from the monomer is suppressed due to the exciplex emission. The concentration of the exciplex depends on the phase separation between the components and depends on the substrate and other preparation conditions. The relative contribution of CNHC and the exciplex to the PL determines the PL peak position. The inset in Figure 2a shows the electroluminescence (EL) spectrum for the blend film deposited on ITO. In agreement with earlier results, the EL spectrum is dominated by the exciplex emission and also has a small component due to monomer emission.16 The PL spectrum of the bilayer device is blue-shifted with respect to the single-layer CNHC device due to overlap of contributions from the PL of TPD (emission at 402 and 420 nm) and the PL of CNHC. There is no signature of exciplex formation at the TPD/CNHC interface. 1299

dx.doi.org/10.1021/jp209586r |J. Phys. Chem. C 2012, 116, 1298–1306

The Journal of Physical Chemistry C

Figure 3. J
V characterisics of (red circles) bilayer and (green triangles) blend devices of CNHC. (Inset) J
V characteristics of single-layer device.

Figure 2b shows the PL spectrum for ANHC, TPD/ANHC bilayer, and TPD
ANHC blend devices. The peak emission wavelength for the ANHC film is at 445 nm. The PL spectrum of the blend film is red-shifted due to the formation of an exciplex;16 the exact peak position depends, again, on the relative concentration of the exciplex and monomer concentration and is sensitive to preparation conditions. The inset in Figure 2b shows the EL for the blend sample. The PL spectrum of the TPD/ANHC bilayer film is redshifted to 454 nm. The emission peak of TPD is absent in this PL spectrum; the peak at 400 nm is an experimental artifact. If there is molecular diffusion in the film, then the TPD/ANHC interface is not a sharp interface but an extended interface and qualitatively accounts for the observed PL spectrum. 3.2. CNHC Devices. 3.2.1. Current
Voltage Characteristics and Electroluminescence. Three types of devices, single-layer, bilayer and blend, were prepared as described in the Experimental Section. The highest occupied molecular orbital (HOMO)
lowest unoccupied molecular orbital (LUMO) energy levels for the different molecules16 used here are shown in Supporting Information (Figure S1). There are substantial energy barriers (>0.6 eV) for both electron and hole injection from the cathode (Al) and anode (ITO) to CNHC (ANHC). The contacts used here are noninjecting contacts, which ensure that the dark current is small in both reverse bias and in forward bias (for low voltage). This enables the measurement of photocurrents in both forward and reverse bias with low dark current. Figure 3 shows the current density
voltage (J
V) characteristics for these three devices. The current density is very small (5  105 V/cm) is limited due to the

ARTICLE

Figure 4. Photoresponse (PR) vs electric field of (black squares) singlelayer, (red circles) bilayer, and (green triangles) blend devices of CNHC. The devices were illuminated with 370 nm light through ITO. (Inset) PR at low field to highlight the symmetry in forward and reverse bias in single-layer and blend devices and the asymmetry in bilayer device.

onset of current injection and electroluminescence. The inset in Figure 4 shows the low-field photoresponse for all three devices. There are three aspects to be noticed in the photoresponse plots shown in Figure 4. First, the photoresponse is symmetric with respect to field in forward and reverse bias for single-layer and blend devices and asymmetric for the bilayer device. Second, the photoresponse in reverse bias increases with electric field in all three devices but the trend is different in each case. In the single-layer device, the photoresponse increases slowly at low field and rapidly at high field. In the blend device, the increase of photoresponse with electric field is rapid even at low field. In the bilayer device, the photoresponse increases rapidly at low field (as in the blend device) with a tendency to saturate at intermediate field and then increases rapidly (as in the single-layer device). Third, the photoresponse of the blend device is the highest, showing a maximum of 60 mA/W at a field of 1.5  106 V/cm. The above variations in the plots of photoresponse versus field for the three devices may be qualitatively understood as follows. The increase of photoresponse with field symmetrically in forward and reverse bias in the single-layer and blend devices (inset, Figure 4) indicates that photocarrier generation occurs in the bulk. If photocarriers were to be generated at either electrode/organic interface, then the photoresponse will be identical in forward and reverse bias only if the mobility of electron and hole for the organic material is identical, which is unlikely. In the case of bilayer device, photocarriers are generated in the bulk of CNHC (as in the single-layer device) but there is also an additional contribution from photocarriers generated at low field in reverse bias to explain the observed asymmetry. We identify the TPD/CNHC interface to be the source of photocarrier (see below for confirmation), and we estimate quantitatively the bulk and interface contributions to the photocurrent in a latter section. Additional evidence that the origin of photocurrent generation in the above three devices is in bulk, interface, or both comes from the photoresponse action spectra. Panels a and b of Figure 5 show the photoresponse action spectra for single-layer and blend devices for illumination of light through ITO and Al, respectively. For these experiments, devices were prepared with thin Al cathode (15 nm) so that transmission of light through Al is 15
30% in the wavelength range shown in the action spectra. 1300

dx.doi.org/10.1021/jp209586r |J. Phys. Chem. C 2012, 116, 1298–1306

The Journal of Physical Chemistry C

Figure 5. PR action spectrum of CNHC devices for illumination at 370 nm light through (blue) Al and (red) ITO at an applied voltage of
10 V: (a) single-layer device and (b) blend device. Also shown in black are absorption spectra of CNHC (90 nm) and TPD
CNHC (100 nm) on quartz.

Figure 6. Photoresponse action spectra of bilayer device for different applied voltages: (a) illumination through Al and (b) illumination through ITO. (Brown diamonds) 0 V; (open circles)
5 V; (blue triangles)
10 V; (red circles)
20 V. Also shown (black solid line) is the absorption spectrum of the bilayer film.

The action spectra are normalized for easy comparison. The action spectra for the single-layer device are similar for illumination through ITO and Al contacts. In addition, the peak of the action spectrum (360 nm) of the single-layer device is similar to the corresponding peak in the absorption spectrum of CNHC. The absence of the absorption peak near 312 nm in the PR spectrum is presumably related to difficulties in carrier collection in CNHC film. The short-wavelength peak is seen in the PR spectra when measured on a thinner device (60 nm CNHC film; Figure S2, Supporting Information). In the case of the blend device, the action spectrum is redshifted by ∼20 nm with respect to the absorption spectrum. This suggests that low-lying CT states at the TPD/CNHC interface are important for photocarrier generation in the blend device. As noted earlier, the photoresponse is highest in the blend device. This explains the importance of the TPD/CNHC interface as a source of photocarrier even in the bilayer device (see below).

ARTICLE

Figure 7. PL quenching efficiency vs electric field for (black squares) single-layer, (red circles) bilayer, and (green triangles) blend devices of CNHC. Symbols represent the data points; solid lines are fits to eq 3.

The action spectrum for the bilayer device depends upon the illumination direction and also on the applied voltage. Figure 6a shows the action spectra when illuminated through Al at different applied voltages. The absorption spectrum of the sample with CNHC and TPD layers (prepared along with the device) is also shown in the figure. At low applied voltages, the action spectrum is antibatic with respect the absorption spectrum. (i.e., the action spectrum shows a minimum when absorption is maximum). When illuminated through ITO, the action spectra are symbatic at all applied voltages (Figure 6b). The antibatic relationship at low applied voltage between the action and absorption spectra, when illuminated through Al, is understood if photocarrier is generated solely at the TPD/CNHC interface, which is closer to the ITO end. The light illuminated through Al is attenuated by 100 nm thick CNHC before reaching the interface. At the peak wavelength of CNHC (355 nm), the absorbance is 0.85 and the intensity at the interface is small. This explains the antibatic relationship. On the other hand, when illuminated through ITO, the attenuation by thin TPD film is negligible, which explains the symbatic relationship. The antibatic relationship observed for illumination through Al is slightly blue-shifted at high voltage. This means that the photocarrier generation due to the bulk contributes significantly at high field, together with photocarriers generated at the interface. This is in agreement with the observation in the single-layer device, where photocurrent increases more rapidly at high electric field. Thus, we conclude that in the bilayer device photocarriers are generated at both the interface and in the bulk; the former is dominant at low electric field and the latter at high field. The photocurrent versus field results are explained below on the basis of quantitative modeling and by using the results of photoluminescence quenching by electric field. 3.2.3. Photoluminescence Quenching by Electric Field. Figure 7 shows the photoluminescence (PL) quenching efficiency as a function of electric field for single-layer, bilayer, and blend devices. The PL quenching efficiency η(F) is defined as ηðFÞ ¼ ½PLðFÞ
PLð0Þ=PLð0Þ

ð1Þ

where PL(F) and PL(0) are the photoluminescence at applied field of F and at zero field, respectively. In these experiments, the devices were illuminated through the ITO at an excitation wavelength of 365 nm. The onset of field quenching occurs at low field for the blend device, the quenching efficiency increasing to 0.88 at 2.7  106 V/cm with a tendency to saturation at high field. 1301

dx.doi.org/10.1021/jp209586r |J. Phys. Chem. C 2012, 116, 1298–1306

The Journal of Physical Chemistry C

ARTICLE

where ν is a pre-exponential frequency factor, ΔE is the binding energy, and kB is the Boltzmann constant. The parameter b is given by b = (q3F)/(8πεε0kB2T2), where q is the electronic charge, F is the electric field, ε is the relative dielectric constant, ε0 is the permittivity of free space, and T is the temperature. The PL quenching efficiency is then given by

The field quenching efficiency is lowest for the bilayer device and the onset field is highest. Electric-field-assisted dissociation of excitons can give rise to bulk free carrier generation, each exciton giving rise to a free electron and free hole. Hence, photoluminescence (PL) quenching by electric field provides a simple way to measure the quantum efficiency of free carrier generation due to electric field.7
11 Figure 8 shows the quenched PL spectra for single-layer, bilayer, and blend devices at different voltages. We see that the PL is quenched uniformly at all wavelengths for the single-layer and bilayer devices. In contrast, the quenched PL peak is slightly blue-shifted for the blend device, implying that the exciplex is more efficiently quenched than the exciton associated with the CNHC monomer. We hence use the value of η(F) measured at the peak wavelength in the quantitative analysis. The PL decays of CNHC (single-layer)16 and TPD
CNHC (blend)17 have also been reported earlier. The results for the PL lifetime for the various samples are summarized in Table 1. We now present a quantitative interpretation of the field-assisted PL quenching. The field dependence of PL quenching can be understood via the model of Braun.18 In this model, optical absorption gives rise to charge transfer (CT) state directly or indirectly by encounter of donor (D) and acceptor (A) molecules, and the CT state is the precursor for free carriers, as schematically shown in Figure 9. The rate constant kd(F) in Figure 9 is field-dependent and is given by18 kd ðFÞ ¼ ν expð
ΔE=kB TÞð1 þ b þ

b2 b3 þ þ :::Þ 3 18

ηðFÞ ¼ kd ðFÞ=½kd ðFÞ þ kr þ knr 

ð3Þ

where kr and knr are the radiative and nonradiative rates for CNHC in thin film. (kr + knr)
1 is the experimentally measured fluorescence lifetime of CNHC film. The value of the lifetime for a neat CNHC film has been determined to be 1 ns.16 η(F) is a nonlinear function of F and hence a nonlinear least-squares method was used to fit experimental data to eq 3, by use of eq 2 for kd(F). The free parameter to be optimized in the fit was kd(0) [=ν exp(
ΔE/kBT)]. The solid lines in Figure 7 show the fit. The fits for the bilayer and blend devices are good and only nominally so for the single-layer device. We need to estimate the prefactor ν in eq 2 to obtain ΔE. From detailed balance, the emission prefactor ν (also known as the attempt to escape frequency) is in general given by the product of the density of states and a volume trapping coefficient. The volume trapping coefficient has the dimensions of velocity times a cross section. The value of ν is difficult to estimate without a detailed knowledge of the mobility, density of states, effective mass, etc. A similar prefactor however, arises in the description of the emission of holes to the transport level in admittance spectroscopy. For carbazole derivatives,19 it was shown that a prefactor of 1010 s
1 satisfactorily describes the emission of holes to the transport level. We hence used the same for the prefactor to be valid for the molecules used in this study. We note in passing that Braun18 estimated ν to be 1015 s
1 for anthracene. The carrier mobility in anthracene is expected to be 4
5 orders larger than that in our materials, and we hence expect a suitably scaled-down value for the prefactor to be valid for the molecules used here.

ð2Þ

Figure 8. Electric-field-assisted PL quenching of devices based on (a) single-layer CNHC, (b) bilayer TPD/CNHC, and (c) blend TPD
CNHC devices at different applied voltages.

Figure 9. Kinetic scheme of luminescent, excited charge-transfer state dissociation.

Table 1. Parametersa Determined from PL and Photocurrent Studies of CNHC and ANHC Devices device

τ (ns)

kd(0) (s
1)

ΔE (meV)

η0

μτ (cm2/V)

G(calc)/G(expt)

CNHC 1

(2.4 ( 0.1)  104

331 ( 1

0.017 ( 0.005

(8.1 ( 0.6)  10
12

0.74

bilayer

1

(7.9 ( 0.1)  10

360 ( 0.3

0.017 ( 0.005

(5.0 ( 0.4)  10
11

0.81

blend

2.28

(6.4 ( 0.3)  104

306 ( 1

>10
10

0.37

single-layer

0.38

(1.3 ( 0.4)  105

288 ( 8

(3.12 ( 0.006)x10
12

0.6

bilayer

0.38

(1.6 ( 0.1)  10

5

283 ( 2

blend

0.5

(2.4 ( 0.2)  105

272 ( 2

>1.7  10
11

0.13

single-layer

3

ANHC 0.01 ( 0.005

Values of photoluminescence lifetime (τ), exciton dissociation rate at zero field [kd(0)], exciton binding energy (ΔE), PL quenching efficiency at zero field (η0), μτ product, and ratio of G(calc)/G(expt) are listed. a

1302

dx.doi.org/10.1021/jp209586r |J. Phys. Chem. C 2012, 116, 1298–1306

The Journal of Physical Chemistry C

ARTICLE

Clearly, a better understanding of the prefactor is necessary. Table 1 summarizes the results for kd(0) and ΔE derived from the fits to the data. 3.2.4. Quantitative Analysis of Photocurrent. Photocurrents arise due to exciton dissociation into free carriers. Exciton dissociation can take place at interfaces,20
23 in the bulk due to impurities,24 or by dissociation in an electric field.8
11,18,24 Following the dissociation of excitons, the charge carriers have to be transported and collected by the respective electrodes to give rise to a photocurrent in the external circuit. The nature of the contacts determines the gain.25 For noninjecting contacts, the maximum gain is unity and is obtained when all the carriers are collected at the contacts.25,26 For appropriate noninjecting contacts, the photocurrent density (Jpc) can be written as25,26 Jpc ¼ ηqGlc ½1
expð
d=lc Þ

ð4Þ

In eq 4, F is the electric field, η is the quantum efficiency for free carrier generation (exciton dissociation efficiency), q is the electronic charge, G is the exciton generation rate, d is the sample thickness, and lc is the collection length, given by lc = μτF, where τ is carrier lifetime and μ is carrier mobility. It is clear from eq 4 that for lc . d, all the carriers are collected and Jpc ¼ ηqGd

Figure 10. Photocurrent density vs field for single-layer device of CNHC for illumination of light at 370 nm through ITO. The intensity of light is 0.18 mW/cm2. Symbols represent data points, and the solid line is the fit to eq 8. (Inset) Plot of χ2 vs qG in the fit of eq 8 to the experimental data of J vs electric field for the single-layer CNHC device. χ2 was minimum for qG = 2.75 C 3 s
1 3 cm
3. Experimentally measured value for qG was 3.7 C 3 s
1 3 cm
3.

ð5Þ

If η is independent of the applied electric field, then for lc . d, Jpc will saturate and be given by eq 5. As shown in Figure 7, η is fielddependent for single-layer, bilayer, and blend devices. In such a case even for lc . d, Jpc will not saturate even for total carrier collection but will vary with η. In the other limit, lc , d, Jpc is given by Jpc ¼ ηqGlc ¼ ηqGμτF

ð6Þ

In this case, Jpc does not saturate and varies with field via η and F. Inspection of eqs 5 and 6 suggests that, for bulk free carrier generation, the photocurrent is expected to be symmetric with respect to field.25,26 However, if carrier generation is due to exciton dissociation at an interface, then the photocurrent will be asymmetric with respect to the electric field.21,27 To obtain a quantitative understanding of the photocurrent with electric field, we use a modified form of eq 4 to model the electric field dependence of photocurrent. We assume that the quantum efficiency for free carrier generation η in eq 4 can be written as η ¼ η0 þ ηðFÞ

ð7Þ

where η0 is the field-independent value. Substituting for η in eq 4 and lc = μτF, we obtain Jpc ¼ ½η0 þ ηðFÞqGμτF½1
expð
d=μτFÞ

ð8Þ

This equation describes the photocurrent for bulk generation of photocarriers. Figure 10 is a plot of photocurrent density versus electric field for the single-layer device. The experimentally measured Jpc versus F data were fitted to eq 8 by a nonlinear least-squares method to obtain values for η0 and μτ product, which are the only unknowns. qG was measured experimentally to an accuracy of (20%. Therefore, the best fit was obtained for different values of qG. The plot of χ2 versus qG is quasi-parabolic (see inset, Figure 10) and the minimum was taken to be the best fit for the experimental data. In this nonlinear fit, we use experimental values of η(F) (Figure 7), by interpolation wherever

Figure 11. Photocurrent density vs PL quenching efficiency for the blend device of TPD and CNHC (1:1) for illumination of light at 370 nm through ITO. The intensity of light is 0.2 mW/cm2. Symbols represent data points; the solid line is the linear fit to eq 5.

required. The solid line in Figure 10 shows the result of the best fit for the single-layer device; the best-fit values are qG = 2.75 C 3 s
1 3 cm
3, η0 = 0.017 ( 0.005, and μτ = (8.1 ( 0.6)  10
12 cm2/V. The experimentally measured value for qG was 3.7 C 3 s
1 3 cm
3, and the ratio of G(calc)/G(expt) = 0.74. The results are summarized in Table 1. Thus, the photocurrent data for the single-layer device fitted well to eq 8. However, the photocurrent data for the bilayer and blend devices did not fit well to eq 8. We hence consider alternative models to account for the photocurrent in these devices. Figure 11 is a plot of the photocurrent of the blend device versus η(F). We see that the photocurrent varies linearly with η(F). From eq 5, this implies that the photocurrent in the blend device is saturated even at the lowest field of 1  105 V/cm (estimated from the built-in voltage of 1 V) and gives a lower limit for μτ as >10
10 cm2/V for the blend film. The carrier mobility in the blend is expected to be smaller than in the singlelayer device due to increased disorder. Hence the larger value of μτ over the single-layer device suggests that the carrier lifetime is at least 1 order of magnitude larger than that in the single-layer device. This can happen when transport of electrons and holes is phase-separated in space, presumably due to nanoscale phase 1303

dx.doi.org/10.1021/jp209586r |J. Phys. Chem. C 2012, 116, 1298–1306

The Journal of Physical Chemistry C

ARTICLE

separation of the electron and hole transport layers. If we assume that all the carriers are collected even at zero applied bias (due to the built-in electric field), then in accordance with eq 5, the slope in Figure 11 should be qGd. qG was calculated to be 1.4 C 3 s
1 3 cm
3 from the slope of the plot, whereas the experimental value for qG = 3.8 C 3 s
1 3 cm
3, and the ratio of G(calc)/G(expt) is 0.37. We interpret the low value to be the fractional volume of the sample wherein the transport layers for the electrons and holes are phase-separated. We mention in passing that there is no physical significance for the value of the intercept in Figure 11. At low fields, the value of η(F) will drop to zero, ensuring that the photocurrent also goes to zero. This is clearly evident in the plot for the ANHC
TPD blend device, to be discussed in the next section. We now discuss the photocurrent in the bilayer device. We first explain the photocurrent qualitatively and then calculate it quantitatively. We have seen earlier that, for the bilayer device, photocarriers are generated at the (TPD/CNHC) interface and also in the bulk (due to exciton quenching by the electric field). We see in Figure 4 that the photocurrent tends to saturate at a field of 5  105 V/cm and then increases. This suggests that, at low field, photocurrent generated by exciton generation at the TPD/CNHC interface dominates. The tendency to saturate implies total collection of carriers. However, at higher electric field, bulk generation due to field dissociation of excitons becomes important and the photocurrent increases again. We now calculate the photocurrent quantitatively for the bilayer device. The photocurrent due to photocarriers generated at the interface is derived as follows. For carrier generation at the interface, we consider the interface layer thickness to be Ld, where Ld is the diffusion length of the exciton in the CNHC layer. Correspondingly, the thickness of the bulk region is reduced to (d
Ld). If G is the exciton generation rate for uniformly absorbed light in CNHC, then the generation rates for the interface and bulk region are G(Ld/d) and G(1
Ld/d), respectively. Following Ghosh and Feng,20 the photocurrent due to carriers generated at the bilayer for d . Ld and αd . 1 and exciton dissociation efficiency of unity, is given by J ¼ qGðLd =dÞdexpð
d=μτFÞ

ð9Þ

The term exp(
d/ μτF) in eq 9 is the transport term.21,27 Equation 9 can be qualitatively understood as follows: qGLd is the free carrier generation rate at the interface. During transit, the charges can be trapped at trap sites in the bulk, where τ is the bulk trapping time. When the transit time ttr (=d/μF) is smaller than the trapping time, the current will saturate as all the carriers are collected. The photoluminescence of CNHC is quenched by electric field. Therefore, the lifetime of the exciton and hence Ld is fielddependent, since Ld = (Dτf)1/2, where D is the exciton diffusion coefficient and τf is the exciton lifetime. It is easily derived that field-dependent Ld = Ld(0)[1
η(F)]1/2., where Ld(0) is the diffusion length at zero field. By substituting Ld in eq 9 and using eq 8 for photocurrent due to bulk with modified generation rate, Jpc for the bilayer device can be written as Jpc ¼ qGfLd ð0Þ½1
ηðFÞ1=2 =dgd expð
d=μτFÞ þ ½η0 þ ηðFÞqGf1
½Ld ð0Þð1
ηðFÞÞ1=2 =dgμτF½1
expð
d=μτFÞ ð10Þ

Figure 12. Photocurrent density vs electric field for bilayer device of TPD/CNHC for illumination of light at 370 nm through ITO. The intensity of light is 0.16 mW/cm2. Black squares represent data points, and the black solid line is the fit to eq 10. Red and blue curves are the contributions of interface and bulk, respectively. (Inset) Plot of χ2 vs qG in the fit of eq 10 to the experimental data of J vs electric field for the bilayer CNHC device. χ2 was minimum for qG = 2.2 C 3 s
1cm
3. Experimentally measured value for qG was 2.7 C 3 s
1 3 cm
3.

The first and second terms in eq 10 are the photocurrent contributions from the interface and bulk, respectively. In eq 10, η0 and η(F) are the values determined for the single-layer device. Ld(0) and μτ product are the only unknowns. The photocurrent versus F data for the bilayer device were fitted to eq 10 with Ld(0) and μτ as the free parameters in the nonlinear least-squares method. As explained before, qG was varied to generate a χ2 versus qG plot and the best-fit (lowest χ2) value for qG was determined as 2.2 C 3 s
1cm
3. The experimental value was 2.7 C 3 s
1 3 cm
3. The best-fit values of Ld(0) and μτ thus determined were ∼9 nm and ∼5  10
11 cm2/V, respectively. The interface and bulk contributions to the photocurrent are shown explicitly in Figure 12. This satisfactorily accounts for the photocurrent in the bilayer device. We summarize that for CNHC single-layer and blend devices, the photoresponse is due to electric-field-assisted exciton dissociation in the bulk of the sample. In the blend samples, the increase of photocurrent even at low applied fields is generationlimited and not transport-limited. This suggests phase separation of the electron and hole transport layers at a microscopic scale. For the bilayer device, exciton dissociation at low field takes place at the TPD/CNHC interface with additional contribution from the bulk region at high fields. We now extend the analysis of results for the ANHC devices. 3.3. ANHC Devices. The three devices, single-layer, bilayer, and blend, were studied in a manner similar to CNHC devices. Figures for the ANHC devices are shown in the Supporting Information. The current density
voltage plots for the three devices of ANHC are shown in Figure S3; they are similar to those of CNHC devices. Photoresponse versus electric field plots are shown in Figure S4. The photoresponse is symmetric in forward [to (5
7)  105 V/cm] and reverse bias for all three devices. This means that the carriers are generated in the bulk including the bilayer device. This conclusion is confirmed by the photoresponse action spectra for the three devices, which are symbatic with the absorption spectra irrespective of the illumination direction (Figure S5a,b). The photoluminescence is strongly quenched by the electric field (Figure S6). Figure S7 shows the quenched PL spectra for the single-layer, bilayer, and blend films. 1304

dx.doi.org/10.1021/jp209586r |J. Phys. Chem. C 2012, 116, 1298–1306

The Journal of Physical Chemistry C The spectral shape of the PL is blue-shifted at higher applied voltage for the blend. This again implies that the binding energy of the exciplex is smaller than the exciton binding energy of the monomer. Unlike the CHNC device, PL quenching by electric field is quite high even in single-layer and bilayer devices. The PL lifetime of ANHC
TPD blend was determined by time-resolved PL decay to be 0.5 ns. The PL quenching data were fitted to eq 3 and the values extracted for kd(0) and ΔE are given in Table 1. The photocurrent in all three ANHC devices arise due to photocarriers generated in the bulk region. The free carrier generation is primarily due to exciton dissociation by the electric field. Analysis of the results indicated that single-layer and blend ANHC devices were similar to the CNHC devices. In the case of single-layer device, the photocurrent density versus electric field data fitted well to eq 8 (Figure S8, Supporting Information). The values of η0, μτ product, and the ratio G(calc)/G(expt) extracted from the fits of above data for the three devices are shown in Table 1. In the case of the blend device, the photocurrent density was linear with η(F) at high electric field (Figure S9, Supporting Information), similar to the blend CNHC device. It follows from eq 5 that the increase of photocurrent with electric field is due to the free carrier generation by the electric field, similar to the CNHC
TPD blend sample. By use of the thickness of blend film (100 nm) and the lowest field from which linearity is observed (∼6  105 V/cm), the limiting value of μτ was obtained as >1.7  10
11 cm2/V. The ratio of G(calc)/G(expt) was calculated from the slope to be 0.13. The reason for the low value in the blend device was discussed with reference to the CHNC blend device, where a similar low value was obtained. The photocurrent versus field data for the bilayer device (Figure S10, Supporting Information) could not be fitted satisfactorily to eq 8 or eq 5. As discussed earlier, there was no indication of photocurrent due solely to carrier generation at the TPD/ANHC interface. Hence no attempt was made to fit the data to eq 10. The models discussed are inadequate to describe the photocurrent in the bilayer device. We now comment on the results obtained for CNHC and ANHC molecules in the three devices. We see from Table 1 that ΔE, the binding energy of the CT state that gives rise to free carriers for the TPD/CNHC blend, is ∼0.30 eV, while that for the single-layer device is ∼0.33 and 0.36 eV for the bilayer. Possible reasons for the smaller binding energy (by ∼0.03 eV) of the blend over the single-layer device could be as follows: The difference in the HOMO levels of TPD and CNHC or ANHC is about 0.2
0.4 eV. In the blend, the exciton dissociation may be facilitated by electric-field-assisted tunneling of holes from CNHC (ANHC) to TPD, analogous to the Poole
Frenkel effect.28 The most striking result is that the μτ product of the blend is very much larger than that of the single-layer device. This is surprising, as the carrier mobility is expected to decrease in the blend due to increased disorder. This implies that the lifetime has increased in the blend. This is possible only if there is very good phase separation at the nanoscale so that electrons and holes move in different parts of the sample so that they are not in close proximity. It is interesting that TPD forms an exciplex with both the compounds and the large μτ product is seen only in these systems. It is important to check this in other systems that form an exciplex to see if this is a general rule, as it has important implications for choice of donor
acceptor complexes for organic solar cells. We also see from Table 1that for the single-layer and bilayer CNHC devices the ratio G(calc)/G(expt) is marginally less than 1, within experimental error. The ratio is significantly less

ARTICLE

than 1 for the CNHC and ANHC blend devices, and this can have a simple interpretation. It is the fractional volume of the sample where the electron and holes are fully collected and hence is a measure of the microstructure. This volume tends to short out other smaller contributions to the photocurrent. However, for the single-layer device it implies recombination is also important and needs a modification of existing models for the photocurrent. We had shown recently13 that a multilayer organic device with 3,6-dipyrenyl-N-hexylcarbazole (P2NHC) as the active layer, a molecule similar to CNHC and ANHC, is an efficient dual-function device. In contrast to CNHC and ANHC, the PL quenching in the P2NHC device was very small (