Letter pubs.acs.org/NanoLett
Mapping of Charge Distribution in Organic Field-Effect Transistors by Confocal Photoluminescence Electromodulation Microscopy Wouter W. A. Koopman,† Stefano Toffanin,*,† Marco Natali,† Stefano Troisi,† Raffaella Capelli,† Viviana Biondo,‡ Andrea Stefani,‡ and Michele Muccini†,‡ †
CNR-ISMN, Bologna Via P. Gobetti 101, 40129 Bologna (BO), Italy ETC s.r.l., Via P. Gobetti 101, 40129 Bologna (BO), Italy
‡
S Supporting Information *
ABSTRACT: A novel method for mapping the charge density spatial distribution in organic field-effect transistors based on the electromodulation of the photoluminescence is demonstrated. In field-effect transistors exciton quenching is dominated by exciton−charge carrier interaction so that it can be used to map the charge distribution in different operating conditions. From a quantitative analysis of the photoluminescence quenching, the thickness of the charge carrier accumulation layer is derived. The injection of minority charge carriers in unipolar conditions is unexpectedly evidenced, which is not displayed by the electrical characteristics.
KEYWORDS: Organic transistors, charge density, confocal microscopy, electromodulation, exciton quenching This capability is particularly relevant for field-effect devices, since charge accumulation and transport processes are related to the (buried) semiconductor−insulator (S/I) interface. Indeed, scanning Kelvin-probe microscopy although endowed with a lateral resolution better than 50 nm is allowed to access only the top semiconductor surface (precluding the analysis of top-gated devices). Among far-field optical techniques, EFSHG spectroscopy7 has raised interest in acquiring local information regarding device operation. However, the demonstrated resolution was limited to few micrometers, and the collected signal intensity is generally low, given that the technique is based on nonlinear optical process. Another very interesting optical approach relies on charge-modulation spectroscopy,9,10 a technique in which the characteristic polaronic absorption and bleaching features induced by accumulated charge carriers are measured. In the case of the technique version which implements confocal microscopy hundreds-of-nanometers lateral resolution is achievable.10 However, a deep knowledge of the spectral absorptive features of the material is mandatory, especially in discriminating the causes of the intensity and spectral broadening of the collected signal across the area of interest. Moreover, in addition to requiring a fully transparent sample, transmittance-based techniques can only be used to probe the
M
any fundamental issues regarding the mechanism of charge transport in organic field-effect transistors (OFETs) are still under debate. Indeed, charge carrier ambipolarity is theoretically expected for all organic semiconductors, while experimental evidence mainly reports on either hole- (p-type) or electron- (n-type) transporting materials.1−5 Moreover, charge carrier injection, diffusion, and trapping are fundamental processes that preside over different device operational modes, such as charge accumulation and depletion. Many investigations derive these physical parameters indirectly from the device electrical characteristics, employing models based on inorganic semiconductor theory. As these models assume a homogeneous semiconductor, their validity for OFETs based on disordered thin films is however questionable. In this respect, probing techniques capable of providing morphology-correlated distribution of electric field, potentials, and charge carriers in field-effect devices are of great importance. In recent years, several techniques have been presented that allow direct observation with microscopic resolution of the electric-field and potential distributions in operating transistors. These include on the one hand scanning probe techniques like Kelvin-probe microscopy6 and on the other hand several optical methods which are based on electric-field second-harmonic generation (EFSHG),7 electro-reflectance microscopy,8 and charge-modulated microscopy.9,10 Even though far-field optical techniques have a lower lateral resolution, they allow for nonintrusive probing changes in material and device parameters even in devices with stacked architecture and buried interfaces. © 2014 American Chemical Society
Received: July 15, 2013 Revised: February 13, 2014 Published: March 10, 2014 1695
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channel active area, while the under-contact regions remain inaccessible. In this Letter we demonstrate a different approach for mapping the charge distribution in operating OFETs employing photoluminescence (PL) electromodulation (EM). EM has been broadly implemented in vertically stacked devices.11−15 In particular, PL quenching phenomena in biased vertical organic optoelectronics devices, such as organic light-emitting diodes (OLEDs), are usually investigated for better understanding the quenching mechanisms affecting the overall quantum efficiency in working devices.12 Indeed, due to the high electric fields typically present in diode-like architectures, potentially exceeding 108 V/cm, the dominating source of EM is found to be the field-enhanced exciton dissociation.13,15 The scenario is fundamentally different in the case of organic field-effect transistors, given the planar intralayer charge transport. Indeed, the electric field intensity in the channel of a typical OFET presenting a 500-nm-thick dielectric layer remains below 106 V/cm for a gate bias up to 100 V. Such a value is usually considered too low to generate sizable exciton dissociation in a single-layer and single-material organic device.16 For this reason, photosensitive transistors usually employ a blend of active materials, as it is normally implemented in organic photovoltaic cells. Another well-known source for EM is the interaction of excitons with charges. In this process the excitation energy is transferred via dipole−dipole interaction (Förster transfer) from the excited neutral molecule to a charged molecule and subsequently dissipates nonradiatively. In vertically stacked devices the charge density is typically about 1015 cm−3, which can be correlated to an average charge−charge distance of about 100 nm. Considering that the exciton diffusion length is in the range of tens of nanometers, exciton−charge interaction is negligible in devices with vertical stack geometry.17,18 On the contrary, the sheet charge density can reach values up to 1014 cm−2 (ref 19) in OFETs, which corresponds to a distance of 1 nm among charges, thus well within the diffusion length of excitons. Based on these considerations, we implement photoluminescence electromodulation (PL-EM) as a novel tool for direct probing the charge density at the semiconductor− insulator interface (S/I) in an OFET. The EM signal η(x,y) is observed as a variation of the laser-induced PL intensity upon biasing the device. The use of a confocal laser scanningmicroscope (Nikon EZ-C1) allowed us to map the spatial variation of the PL-EM, by scanning an excitation laser beam across the device active regions while collecting the PL (Figure 1a). For every measurement the images of the biased and of the unbiased transistor were collected for a pixelwise evaluation of the PL intensity variation, according to: η(x , y ) =
PL0(x , y) − PL V(x , y) ΔPL = PL(x , y) PL0
Figure 1. (a) Confocal PL-EM setup together with schematics of the investigated OFET (DM: dichroic mirror, PMT: photomultiplier tube). (b) OFET n-type transfer curve characteristic. (c) OFET n-type multiple output curve characteristcs at different applied Vg values from 0 to 100 V.
given the PL spectrum peaked at around 680 nm (Figure S1) and the use of a 60× objective with 0.7 numerical aperture. The laser power (10−100 μW) impinging on the device active layer within the expected confocal laser beam spot area is optimized for guaranteeing linear correlation with collected PL intensity. It is important to note that PLV collection was delayed by 100 ms with respect to the application of the gate bias to guarantee the complete polarization of the gate dielectric. This allowed us to correlate the PL-EM measurements with the standard transistor electrical characteristics. Finally, no source− drain photocurrent was detected during the PL-EM measurements so that we can exclude the formation of photogenerated charges due to exciton dissociation. The samples used in the experiments were single-layer topcontact bottom-gate OFETs based on a thermally evaporated 15-nm-thick N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) layer. This perylene derivative is one of the best performing n-type field-effect materials21,22 and exhibits high fluorescence intensity. Moreover, the sufficiently low attenuation coefficient of P13 thin film at the excitation wavelength (Figure S1) avoids pronounced quenching contributions by bimolecular recombination. To probe the material underneath the injecting electrodes, we used a glass substrate and a transparent indium tin oxide (ITO) gate electrode. As injecting electrodes we evaporated two 30-nm-thick gold contacts with a source−drain channel length of L = 70 μm. The gate dielectric was formed by a spincoated 450 nm-thick PMMA layer. To protect the organic layer from oxygen and water during the measurements, all devices were encapsulated in a nitrogen glovebox. The devices were electrically characterized using an Agilent B1500 semiconductor analyzer. Typical saturation transfer and multiple output characteristics are shown in Figure 1b and c, respectively. From the slope of the locus curve, we extracted the average saturation mobility μ and threshold voltage Vth, using the approximation for space-charge saturated currents: Id = 1/ 2(μCiW/L)(Vg − Vth)2 (refs 1, 23). With an aerial capacitance of Ci = 7.58 nF/cm2 for 450 nm PMMA, we calculated μ = 0.26 cm2/(V s) and Vth = 12.6 V.
(1)
Here, η(x,y) denotes the PL-EM signal at the image coordinates (x,y), PL0 the unaltered PL intensity and PLV the PL intensity from the biased device. The PL emission of the device active material was excited by the second-harmonic of a Ti:sapphire femtosecond laser (Spectra-Physics, Tsunami) tuned at the material thin-film absorption maximum wavelength (∼480 nm, Figure S1 of the Supporting Information). Given our experimental conditions20 we can estimate the lateral resolution of PL-EM confocal microscope of around 300 nm, 1696
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Biasing only the gate electrode (that is, without applying a potential difference between source and drain electrodes), the transistor can be considered as a metal−insulator−semiconductor (MIS) diode. In this configuration charge carriers are accumulated in a small layer at the semiconductor− insulator (S/I) interface, until the generated space-charge fully screens the external gate bias. We report in Figure 2 the PL-EM
Figure 2. Electromodulation signal (ΔPL/PL) across the OFET active region at different applied Vg values. The channel region is white, and the electrode regions are gray.
Figure 3. (a) Dependence of the PL quenching on the applied gate voltage for different semiconductor thickness values. (b) The reduced maximum quenching f as a function of semiconductor thicknesses dsemi. (c) Charge density (unless a K multiplicative value) dependence from the applied gate voltage within the channel of a 15-nm-thick P13based OTFT. The threshold gate voltage value is located at the vertical line.
signal in this bias configuration as a function of the position across the transistor channel. Positive PL-EM values correspond to an effective quenching of the luminescence in the operating device compared to the unbiased one. For small gate bias the quenching is uniform throughout the device, while the luminescence is more strongly quenched underneath the electrodes for higher Vg. In the proximity of the electrode the quenching increases nonlinearly. The maximum attainable quenching underneath the electrode region is up to 20% higher than in the channel. Figure 3a illustrates the dependence of the PL-EM on the applied gate bias within the channel area. The relation can roughly be divided into two regimes. For Vg < Vth the signal increases rapidly and almost linearly with the applied Vg, while at higher voltages it reaches a plateau (saturation). Given that in organic field-effect devices it is usually assumed that charge accumulation takes place at the S/I interface, we assume that only the luminescence in the space charge region is modulated. To corroborate experimentally this assumption, we varied the thin-film interface-to-bulk ratio by increasing the active layer thickness dsemi of the OFETs. Figure 3a shows the PL-EM signal for devices with dsemi varying from 7 to 50 nm. While the signal slope is almost unaffected by the semiconductor thickness, the saturation signal is inversely proportional to dsemi. This evidence confirms that the PL quenching is mainly an interfacial phenomenon and excludes any effects from the bulk of the film. We describe the exciton−charge interaction as a type of collision quenching, so that the magnitude of the PL-EM signal can be correlated to the average probability for an exciton to encounter a charge carrier during its lifetime, given that the probability of energy transfer once the interaction takes place is equal to 1.17 To model the dependence of the PL-EM signal on the charge density, we calculated the fraction of photoexcited species I that decay radiatively with respect to the total population photoexcited species I0. The ratio I/I0 can be
expressed as the ratio of the decay rate in the absence of quenchers kr,0 with respect to the decay rate in the presence of quenchers kr,0 + kqρ (Stern−Volmer relation):24 k r,0 I = I0 k r,0 + kqρ(x , Vg)
(2)
Here kq is the quenching rate due to Förster energy transfer and ρ the charge density as a function of the position and the electric field. To account for the interface limitation we divide the photoexcited species into two populations, one of which is located at the S/I interface and thus accessible to chargeinduced quenching, and the other one located in the bulk of the thin film, being inaccessible. Thus, the total fluorescence intensity is given by I = Iinterface + Ibulk. The interface contribution to the luminescence is subject to quenching according to the Stern−Volmer relation, while the bulk contribution remains unaffected. Since the PL intensity is proportional to the population of photoexcited species I, the PL-EM signal can be derived as: η=
Kρ(x , Vg) ΔPL ΔI = =f× PL0 I0 1 + Kρ(x , Vg)
(3)
where ΔI = (I0 − I) is the intensity difference of PL between the biased and unbiased conditions, while f = Iint/(Iint + Ibulk) is the fraction of photoexcited species accessible to quenching and K = kq/kr,0 the ratio of the rate constant of exciton−charge quenching with respect to the radiative rate constant in the absence of quenchers. In the equation the prefactor f constitutes the upper limit for luminescence quenching, while 1697
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the product Kρ determines the slope of the quenching curve as the charge density increases. As shown in Figure 3a, the experimental data are well-fitted by eq 3 for all devices with different active layer thickness. The two parameters f and Kρ can be extracted from the interpolated curves. The parameter f is correlated to the fraction of the semiconductor layer that corresponds to the effective charge accumulation layer. Figure 3b shows the reciprocal dependence of f on the semiconductor thickness dsemi. By assuming a sharp transit from accumulation layer to bulk material when the device is in accumulation mode, we could approximate the charge accumulation layer thickness h by f = h/dsemi. For the PTCDI-P13-based OFET with a PMMA dielectric we extracted h = 4.5 nm. As Tatemichi et al. determined a height of 2.5 nm for a PTCDI-P13 monolayer,25 the accumulation layer corresponds roughly to two monolayers. This value agrees surprisingly well with typical accumulation layer thicknesses of 2−3 monolayers reported in literature for other typical organic semiconductors.26,27 Differently from earlier investigations, however, PL-EM confocal microscopy offers the possibility to map the accumulation layer thickness throughout the entire device, even underneath the injecting electrodes. The parameters K and ρ cannot be determined independently by quenching measurements alone. Indeed, K is a material constant given that the Förster-transfer rate depends on the spectral overlap between the emission of the donor (neutral excitation) and the absorption of the acceptor (charged molecule), but not on the applied electric field. For a quantitative evaluation of the charge density distribution, the quenching rate constant has to be determined independently. Though, by suitable rearrangement of the relation reported in eq 3 it is possible to plot the quantity Kρ as a function of Vg as reported in Figure 3c. It can be observed the linear increase of the charge density as the applied gate voltage increases up to the gate threshold voltage (vertical line in the graph). Indeed, for higher gate voltage values, the functional dependence of the charge density on the applied gate voltage is still almost linear but with less pronounced slope. The linear behavior is in well accordance with the wellknown linear relation between ρ and Vg in a transistor operating in accumulation mode within the linear-channel approximation theory. Although, no charge density is typically expected for gate voltage values lower than Vth. So far, we have dealt with the charge accumulation induced by the gate bias. To investigate the influence of the lateral electric field on the charge density distribution in a working transistor, we measured the variation of the PL-EM signal across the channel versus the source−drain voltage, while keeping the gate voltage constant (thereby miming the electrical output characteristic). The results for Vg = 20 and 5 V are displayed in Figures 4 and 5 together with 2D surface plots. In these graphs, the ordinate axis indicates the applied Vds and the abscise axis the position along the channel, while the colors represent the PL-EM signal, from blue (low signal) to red (high signal). For a gate bias of Vg = 20 V, the transistor is in its normal operation conditions (Vg > Vth). If the lateral voltage is small compared to the effective gate voltage (Vds ≪ Vg − Vth), the transistor operates in the linear regime.1,28 As long as the local voltage exceeded the threshold along the entire channel, an accumulation zone is formed from source to drain. The Vdsdependent PL-EM mapping is almost identical with respect to the accumulation mode, apart from a linear gradient in the PL-
Figure 4. (a) 2D surface PL quenching plot at fixed Vg = 20 V. The ordinate axis indicates the applied Vds and the abscise axis the position along the channel, while the colors represent the PL-EM signal, from blue (low signal) to red (high signal). (b) Linear gradient in the PL quenching along the channel for the specific Vds value indicated by the line in a.
Figure 5. (a) 2D surface PL quenching plot at fixed Vg = 5 V. (b) PL quenching behavior along the channel for the specific Vds value indicated by the line in a.
EM signal, extending from the source to the drain electrode (Figure 4b). However, when the source−drain voltage increases above the effective gate voltage (Vds ≥ Vg − Vth), a depletion zone is formed in the proximity of the drain electrode,1 which in turn is clearly correlated to the intensity decrease of the PLEM signal (blue and green regions). Further increasing Vds, the depletion zone extended into the channel. This phenomenological behavior offers an experimental validation of the macroscopic model implemented for describing OFETs, which is based upon the linear-channel approximation. Interestingly, PL-EM signal has a minimum for Vds = Vg, while increasing again for higher values of Vds. This observation 1698
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confirms our assumption that the PL-EM signal is sensitive to both the mobile and the fixed charges, since below Vth no mobile charges are present in the channel. Furthermore, it shows that the thickness of the accumulation layer is determined by the fixed space charge. Finally, the increase of the PL-EM signal in the proximity of the drain electrode for higher Vds voltages is not expected in macroscopic OFET models. Indeed, given that the electric field resulting from Vg − Vth is still small, we can rule out exciton dissociation as the origin of this unexpected effect. Indeed it is even more striking the observation of a negative PL-EM signal crossing the channel (dark blue zone in Figure 4), which corresponds to luminescence enhancement during device operation. This behavior is studied in more detail for a gate bias of Vg = 5 V (Figure 5a). Given that the transistor is operated in subthreshold conditions, all charges are expected to fill trap states once injected into the semiconductor layer without contributing therefore to the collected drain current. A depletion zone is immediately formed upon application of a small Vds, while for Vds = Vg the PL-EM signal dropped below zero at the drain. Again the depletion zone extended into the channel, and the PL-EM signal increases in the proximity of the collecting electrode for increasing Vg. A plausible explanation for both these observations is the injection of holes (positive charge carriers) into the semiconductor for high Vds. Since the photoluminescence signal can be modified by both negative and positive charge carriers, we cannot distinguish the types of charge carriers from the PL-EM signal. The negative PL-EM signal could then be the result of charge recombination. This explanation implies the injection and accumulation of positive charges into a semiconductor transport material known as n-type. On the other hand, theoretical calculations on a similar PTCDI molecule by Chesterfield et al. predicted a hole mobility equal to the electron mobility.29 They explained the lack of hole transport by the presence of trapping sites. We therefore interpret our observations as a signature of the injection of both types of charge carriers into the organic thin-film. Clearly, all positive charge carriers are preferentially trapped without contributing to IDS current. Hence, they cannot be simply observed by standard electrical characterization of the transistor. In this Letter we investigated the charge-density spatial distribution in an organic field-effect transistor using confocal photoluminescence electro-modulation microscopy. Implementing a model based on the Stern−Volmer relation, we established a direct correlation between the PL-EM signal and the charge density. We could extract the local charge accumulation layer thickness in the transistor, which in general is very difficult to be evaluated with other experimental methods. We furthermore showed that the average accumulation layer does not increase above two monolayers independently from the applied gate voltage. In the operating transistor, we detected an unexpected behavior which can most likely be attributed to the injection of nonmobile holes into the n-type semiconductor. Our results show the potential of confocal photoluminescence electro-modulation microscopy as a fast, highthroughput, and high in-plane resolution optical tool for the investigation of opto-electronic performances in organic field-effect transistors.
Letter
ASSOCIATED CONTENT
S Supporting Information *
Absorption and photoluminescence spectra of 100-nm-thick thin film of P13 deposited onto quartz (Figure S1). This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail address: s.toff
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from Consorzio MIST E-R through Programma Operativo FESR 2007-2013 della Regione EmiliaRomagna - Attività I.1.1. and from EU FP7 Marie Curie ITN316832 project OLIMPIA and by Fabbrica del Futuro: Silk-IT is kindly acknowledged.
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NOTE ADDED AFTER ASAP PUBLICATION This Letter was published ASAP on March 20, 2014. Since that time, an Acknowledgment has been added to the manuscript. The correct version was published on March 26, 2014.
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