Dicarbocyanine Dye-Based Organic Photodiodes

Abstract

We report on the utilization of 1′-1′-diethyl-4,4′-dicarbocyanine iodide (DDCI-4) as a photoactive material for organic photodiodes (OPDs). A device was fabricated using a ternary blended ratio in the conventional stack order of ITO/PEDOT:PSS/P3HT:DDCI-4:OXCBA/Al to improve stability and enhance light absorption. An investigation was carried out into the optical and morphological characteristics of the device along with its electrical performance using different concentrations of DDCI-4 in a blended ratio of P3HT:DDCI-4:OXCBA in the photoactive layer. The mechanism of the OPD device and its performance with a gradual increase in DDCI-4 concentration is explained throughout this work, in which the increase in DDCI-4 concentration caused the dislocation defect and a decrease in charge carriers. The appropriate concentration of DDCl-4 resulted in improved light broadening, especially in near-infrared (NIR) regions.

1. Introduction

The capability of conjugated organic dyes to aggregate in different modalities has drawn considerable attention. This interaction could lead to different structural arrangements of intimate cofacial π−π-stacking (H-aggregates) or slipped molecular packings (J-aggregates) and provide electronic couplings among monomeric dyes [1]. Consequently, the formation of H- and J- aggregates by the material could potentially lead to the fabrication of broadened or specific wavelength devices [2,3]. The advantage of having J-aggregates is the addition of a narrow and intense absorption characteristic that is highly desirable in photodetector applications where wavelength specificity may be essential. In addition, the variety of available J-aggregate materials permits organic photodiodes to be engineered to operate at narrow wavelength ranges of choice, spanning the visible and near-infrared spectrum [4]. Particularly relevant in this regard is the fabrication of detectors that operate at near-infrared wavelengths to enable applications such as biochemical sensing and night-vision imaging [5,6]. Liess et al. demonstrated that by controlling the donor substituent of the conjugated dye (merocyanine dyes), multiple strong H- or J- aggregates could be formed from its monomer. They managed to synthesize nine merocyanine dyes consisting of the same 2-aminothiophene donor (D) and 2-[4-(tert-butyl) thiazol-2(3H)-ylidene] malononitrile acceptor (A) moieties but with various donor substituents with each having its specific wavelength peak and full width at half maximum (FWHM) [1].

Cyanine dye, specifically, is the most common conjugated dye and it has been widely used as a fluorescent probe, a laser dye and in model systems for studying non-radiative decay processes [7]. Owing to their photodynamic characteristics, conjugated dyes have been the topic of numerous non-linear analysis spectroscopy experiments [8]. Scherer et al. characterized 1,1′,3,3,3′,3′-hexamethyl- 4,4′,5,5′-dibenzo-2,’-indo-carbocyanine (HDITC) by employing spectrally resolved transient absorption spectroscopy and discovered coherent oscillations associated with several modes and phase differences across the spectral bandwidth [9]. Furthermore, the cyanine-based conjugated dyes could mimic the light-harvesting antenna of green sulfur bacteria which leads to a strong light-collecting ability that can be utilized in light-harvesting sensors. [10]. Zhang et. al. managed to fabricate a visible, transparent organic photodiode using heptamethine cyanine dye, Cy7-T. The device obtained high responsivity of 165 mA/W and detectivity of 1 × 10¹² Jones with an EQE of 23% at −2 V [11].

Following the foregoing, this work has aimed to develop a NIR organic photodiode by utilizing a different concentration of 1,1′-diethyl-4,4′-dicarbocyanine iodide (DDCI-4) which derives from a cyanine dye derivative of 0 to 7 mg/mL. Although DDCI-4 has not yet been proven to be used as a photodiode, it has shown its capability in previous literature to work as a photorefractive material [12]. Ideally, for a cyanine dye-based NIR organic photodiode to work, the photoactive material needs to have a good response to NIR light. The presence of iminium cation and iodide anion in the DDCI-4 molecular structure shows a high tendency for it to operate in the NIR region [10]. To increase the stability and enhance the transportation of the charge carrier inside the donor-acceptor medium of the device, a mixture of poly[3-hexylthiophene] (P3HT) and oxylenyl-C60-bisadduct (OXCBA) is added in the photoactive layer. Herein, the optical, electrical and morphological properties of the device have also been studied and discussed. The overall interest is to establish the performance of a new conjugated cyanine dye, DDCI-4, that may contribute to the development of highly responsive NIR photodetectors.

2. Materials and Methods

Figure 1 illustrates (a) the molecular structure of P3HT, DDCI-4, and OXCBA; (b) the schematic diagram of photodiode architecture; and (c) the schematic energy level of active materials. 1,1′-diethyl-4,4′-dicarbocyanine iodide (DDCI-4), poly[3-hexylthiophene] (P3HT) and oxylenyl-C60-bisadduct (OXCBA) purchased from Sigma-Aldrich (St. Louis, MI, USA) were dissolved separately with organic solvent (chloroform). All the solutions were stirred for 24 h at 500 rpm. P3HT and OXCBA were dissolved with a concentration of 15 mg/mL each and then mixed with a 1.0:0.5 volumetric aspect ratio, respectively. Three different concentrations of DDCI-4 (3 mg/mL, 5 mg/mL, and 7 mg/mL) were added to the blend solution separately.

In the meantime, ITO pixelated glass substrates (from Ossila, UK) were cleaned with detergent, deionized water, acetone, and isopropyl alcohol using ultrasonic agitation for 15 min each and were finally dried by blowing nitrogen gas on them. PEDOT:PSS (PH1000 from H.C.Stack, Newton, MA, USA) was filtered using a 0.45 μm nylon syringe filter and was spin coated on top of ITO substrate at 3000 rpm for 60 s to act as a hole transport layer. The layer was then dried using an annealing process at 120 °C for 30 min so that the layer would be evaporated completely. After that, the blended photoactive layers were filtered using a 0.22 μm nylon syringe filter and spin coated onto the PEDOT:PSS surface at 2000 rpm for 30 s and then dried using the annealing process at 120 °C for 30 min to improve the crystallinity of the layer. After spin coating the photoactive materials, the sample was then thermally evaporated with aluminum (Al) under a base pressure of 2 × 10⁻⁷ torr through a shadow mask to form 100 nm top contact, which constituted a 0.045 cm² photo-active area for a single sample and finally the device was encapsulated using UV-cured epoxy and glass. Most of the operations including spin coating, thermal annealing, and thermal deposition were done in a glove box with an inert nitrogen gas atmosphere to avoid the influence of oxygen on the device.

The electrical properties of the device were characterized using a Keithley 236 Source Measure Unit and 940 nm LED pin (Tektronix, Beaverton, OR, USA) solar simulator with an optical power density that was fixed at 20 mW/cm² and the device efficiency was measured using the IPCE system. The thin films were characterized using ultraviolet–visible–near infrared light (UV–Vis–NIR) absorption spectroscopy (PerkinElmer LAMBDA 900, Waltham, MA, USA), a surface profilometer (model Bruker Dektak XT, Billerica, MA, USA), Hitachi AFM5100N using a SI-DF3 cantilever (Hitachi, Tokyo, Japan) with a scan range of 2 μm × 2 μm for each sample, and ultraviolet photoelectron spectroscopy (UPS) measurement of the BL3.2U conducted at Synchrotron Light Research Institute (SLRI, Nakhon Ratchasima, Thailand).

3. Results and Discussion

3.1. Strong Absorption in the NIR Region by Dicarbocyanine

In terms of light absorption capability, Figure 2 shows the absorption spectra of the active materials and the bulk-heterojunction blended ratio from the visible to the NIR region on a quartz substrate. Figure 2a shows that the influence of the NIR region absorption is solely due to DDCI-4, which exhibits two absorption peaks in the Q-band region (733 nm and 890 nm). As DDCI-4 has a long conjugated molecular structure, it tends to have strong absorption in the NIR region [13,14]. The presence of iodide anion and iminium cation in the DDCI-4 molecular structure also caused interactions between opposite charges together with π−π dispersive interaction between a highly polarizable group of atoms, which contributes to both peaks in the Q-band region [10]. Furthermore, DDCI-4 also shows a tendency to work in the visible region where DDCI-4 has a slight peak from 450 nm to the visible range with a strong valley in the 520 nm region. Furthermore, P3HT reveals a significant peak in the visible region at 520 nm and a vibrionic peak at 600 nm, which indicates the vibrational excitation of a highly ordered P3HT chain structure [15], whereas OXCBA does not contribute to any absorption from the visible to the NIR region. Therefore, the combination of all the materials could lead to a broadening of the absorption bandwidth so that the device could operate not only in NIR but also in visible conditions.

Upon the combination of a blended ratio (P3HT:DDCI-4:OXCBA), light absorption bandwidth was broadened in all visible to NIR ranges from 400 nm to 1000 nm and the valley at 520 nm diminished, as can be seen from Figure 2b. We also observed that the addition of P3HT:OXCBA to DDCI-4 led to a minimizing of the absorption of DDCI-4 in the NIR region by three times. This is possibly due to the higher concentration of P3HT:OXCBA blend that was used in the blended ratio. It is evident in Figure 2b that the peak in the Q-band region starts to decrease; the peak at 733 nm starts to diminish with the lower concentration of DDCI-4 used. On top of that, because absorption at NIR is only caused by DDCI-4, it is possible that DDCI-4 forms J-aggregates when more DDCI-4 is added to the blended ratio as its absorption region starts to experience a bathochromic shift where the peak at 890 nm is slightly shifted to 910 nm.

The energy gap (E_g) and alignment of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) play key and crucial roles in controlling the charge carrier transport for device performance [15]. The energy gap of a material could be defined as the difference between its HOMO and LUMO levels [16]. For this research, the energy gap of active materials is calculated from the Vis–NIR absorbance spectra using the Tauc plot interpretations technique and the following equation:

$${\left(\alpha hv\right)}^{\frac{1}{n}}=A\left(hv-E_g\right)$$

(1)

where Planck’s constant, photon frequency, absorption coefficient and energy gap are represented by h, v, α, and E_g, respectively, and $A$, on the other hand, is a proportionality constant. The value of the exponent, $n$, reveals the nature of transition electrons, including whether the transition is permitted or prohibited, and whether it is direct or indirect. Typically, the $n=\frac{1}{2}$ represents direct allowed transitions, $n=\frac{3}{2}$ is for direct forbidden transitions, $n=2$ is for indirect allowed transitions, and $n=3$ is for indirect forbidden transitions [17]. By plotting the (αhv)² against (hv), the resultant straight line from the linear region is used to extrapolate an x-axis intercept, which later gives the energy gap value. Figure 3c shows the Tauc plot interpretations technique for the active materials. The energy gaps for P3HT, DDCI-4, and OXCBA are 1.97 eV, 1.21 eV, and 2.08 eV, respectively. Determination of the HOMO and LUMO levels of the materials is characterized using ultraviolet photoelectron spectroscopy (UPS). Compared to other techniques such as cyclic voltammetry (CV) used to determine HOMO and LUMO, UPS can provide a precise assessment of the ionization potential of the thin film under vacuum conditions which results in a more accurate value [15,18,19]. The value obtained is then calculated via the following equations:

$$Work function=hv-\left(Cut-off energy\right)$$

(2)

$$HOMO=Work function+HOM{O}_{onset}$$

(3)

$$LUMO=HOMO-Energy band gap$$

(4)

Figure 3a,b show the UPS data obtained from the characterization. The data obtained were calculated and then summarized, as shown in

Table 1. Calculated HOMO and LUMO levels of P3HT, DDCI-4, OXCBA.

Materials	HOMOonset (eV)	Cut-Off Energy (eV)	Work Function (eV)	HOMO Level (eV)	LUMO Level (eV)	Eg (eV)
P3HT	0.93	35.56	3.94	4.87	2.90	1.97
DDCI-4	0.72	35.55	3.95	4.67	3.46	1.21
OXCBA	1.33	35.26	4.24	5.57	3.49	2.08

. HOMO and LUMO values for P3HT, DDCI-4 and OXCBA were calculated to be 4.87 eV and 2.90 eV, 4.67 eV and 3.46 eV, and 5.57 eV and 3.49 eV, respectively. All results for HOMO, LUMO and E_g values are comparable with previous works of literature and are summarized in

Table 2. Comparison of HOMO, LUMO and E_g values obtained from previous literature and this work.

Materials	HOMO (eV)	LUMO (eV)	Eg (eV)	Ref
P3HT	4.65	2.13	2.52	[20]
	4.94	3.08	1.86	[15]
	4.87	2.90	1.97	This work
DDCI-4	4.67	3.46	1.21	This work
OXCBA	5.66	3.47	2.19	[15]
	5.57	3.49	2.08	This work

. Slight differences may arise due to variations in the calibration process, experimental environments, sample types and data analysis techniques [18].

3.2. Photocurrent Performance Due to Dicarbocyanine

The current density–voltage (J-V) measurements were calculated to analyze the device performance under dark and illumination of light at 20 mW/cm² NIR optical power. Three devices with different concentrations of DDCI-4 (3, 5 and 7 mg/mL) were tested. The device was fabricated by using a ternary blend of P3HT:DDCI-4:OXCBA in the conventional stack order of ITO/PEDOT:PSS/P3HT:DDCI-4:OXCBA/Al under the same conditions. Use of an anode (PEDOT:PSS) and cathode interface layer have been proven by previous works of literature to help in reducing current leakage and preventing excitons from quenching, which could reduce the device’s performance [21]. However, in some cases, adding a cathode interface layer caused some problems such as poor contact between the active layer and the cathode that drove to grievous charge recombination [22]. In addition, the use of a ternary blend has also been proven as an alternative platform that results in the enhanced absorption of light as well as an increase in the performance of the device [15].

Table 3. Photo-sensing characteristics of P3HT:DDCI-4:OXCBA blended devices of different ratios at −1 V.

Devices	Jd (mA/cm2)	Jph (mA/cm2)	Jph/Jd	Rise Time, Tr (ms)	Decay Time, Td (ms)	R (mA/W)	D (×1010 Jones)
3 mg/mL	0.35	0.44	1.257	641	641	22	6.57
5 mg/mL	0.29	0.35	1.207	641	641	17.5	5.74
7 mg/mL	0.23	0.26	1.130	641	641	13	4.79

shows the summarized data for the performance of devices and their reaction behavior following the illumination of light at −1 V.

Low reverse bias dark current is often referred to as leakage current and it is required to achieve higher specific detectivity, D*, and large dynamic ranges which are the key figure of merit in any photodetector [23]. Based on Figure 4a,b, the dark current density (J_d) of the fabricated device decreases from 0.35 mA/cm² (3 mg/mL) to 0.23 mA/cm² with a higher concentration of DDCI-4 (7 mg/mL). This indicates the existence of an electron channel condition between the interfaces of the electrode and photoactive layer [15]. A lower J_d also shows that few current leakages occur between the interface that can minimize the noise in light detection that will eventually increase the photocurrent density (J_ph) of the device [24,25]. However, J_ph shows a downward trend where at 3 mg/mL of DDCI-4, the J_ph of the device is 0.44 mA/cm² and it decreases to 0.26 mA/cm² when 7 mg/mL of DDCI-4 is used. This is possibly due to the Frenkel defect or what is known as a dislocation defect that usually occurs in conjugated dye materials [10]. The effect causes the charges to be trapped in the geometrically distorted latter. This is owing to the symmetric vibrational coordinates of one or more monomers and this can be easily analogous to a ‘bowling ball on a mattress’ situation [26]. Because the absorption region at NIR experiences a bathochromic shift when more DDCI-4 is added, there is a tendency for DDCI-4 to form J-aggregates that deal with strongly bound excitons. These excitons that form may propagate through the aggregate, and at each instant of time the electron and hole occupy the same molecule. This causes the resonant excitation transfer interaction between constituting molecules to be much stronger than the interaction with the environment which directly lessens the number of electrons and holes that are being transferred to the electrodes [1,10].

To further understand this effect, the external quantum efficiency (EQE) of the fabricated devices was measured by using the incident-photon-to-current efficiency (IPCE) system. EQE is defined as the ratio between the number of charge carriers generated to the number of incident photons produced and is often conducted to describe the photo-conversion capability of photodiodes [16]. Figure 4c demonstrates the EQE value of each device in the NIR and visible regions and the values are summarized in

Table 4. External quantum efficiency (EQE) in the visible and NIR region of the devices.

Devices	Visible EQE (%)	NIR EQE (%)
0 mg/mL	15.3	0
3 mg/mL	23.7	0.29
5 mg/mL	5.0	0.21
7 mg/mL	2.6	0.05

. It is observed that in the visible region, the addition of DDCI-4 increases the EQE of the device from 15.3% to 23.7% showing that more electrons and holes are being transferred to the electrode. This is possible since DDCI-4 has been proven to assist in broadening the absorbance of the device in the visible region, as shown in the absorption spectra. Unfortunately, increasing DDCI-4 concentration leads to a sudden EQE drop in both the visible and NIR regions. EQE drops to as low as 2.6% for the visible region and 0.05% for the NIR region when 7 mg/mL of DDCI-4 is used. This occurs because of the possibility of DDCI-4 forming molecular aggregates that are often characterized as super-radiance or ultrafast radioactive decay [10,27]. The following equation can be used to derive linear J-aggregates’ radiative lifetime, τ_ο (J):

$${\tau }_o\left(J\right)=\frac{{\tau }_{o \left(M\right)}}{0.81 \left(N_c\right)}$$

(5)

where τ_ο (M) is the monomers’ radiative lifetime and N_c is the delocalization (coherence) length. The J-aggregates’ radiative lifetime is inversely proportional to the delocalization coherence length (N_c). N_c is determined as the number of monomers on which the exciton wave function is delocalized [28]. In this case, due to the transfer interaction between constituting molecules being stronger, electrons remain localized and the excitation is coherently delocalized over many monomers in the form of ‘excitation waves’ [28,29]. From the superpositions of these excitation waves, the ‘excitation packets’ can be generated, which describe the coherent motion of (localized) excitations, allowing the excitation to migrate over many monomers [30,31]. Based on this theory, the addition of more DDCI-4 leads to an increase in the number of monomers that will eventually lengthen the delocalization coherence length. This causes the radiative lifetime of the charges to be faster and decreases the number of holes and electrons formed as most of them will start to recombine back, which contributes to the lowering of the device’s EQE.

To assess the performance of the photodiodes, we calculated their responsivity (R) and specific detectivity (D*). R of the device indicates the amount of photocurrent produced when it is exposed to a specific intensity of light, which is usually measured in mA/W [15]. The following equation can be used:

$$R=\frac{J_{ph}}{P_{in}}$$

(6)

where J_ph is the photocurrent density and P_in is the power of incident light intensity. The value of J_ph and P_in are obtained from the J-V characteristics under illumination as depicted in Figure 4b. It reveals that the device with the lowest concentration of DDCI-4 (3 mg/mL) has the highest responsivity of 22 mA/W compared to the other two ratios (5 mg/mL = 17.5 mA/W and 7 mg/mL = 13 mA/W). On the other hand, specific detectivity (D*) is a metric that describes the signal-to-noise ratio of a photodetector and that is normalized to the detector area [2]. It is often measured in units of cmHz^1/2W⁻¹ or “Jones” and can be measured by using the equation:

$$D^{*}=\frac{R}{\sqrt{2q{J}_d}}$$

(7)

where R is the device responsivity, J_d is the dark current density, and q is the absolute value of the charge electron (1.60 × 10⁻¹⁹ C). As we can see in Table 3, higher responsivity contributes to higher detectivity of the device where the highest detectivity obtained is 6.57 × 10¹⁰ Jones for a 3 mg/mL device. In this case, we can suggest that a device with a lower concentration of DDCI-4 could lead to better performance of NIR photodiodes compared to the other devices in this research. In addition, the photo-response behavior of the devices when the lit light is controlled to the “ON” and “OFF” states has been investigated. This method involves repeatedly monitoring the rapid change in photocurrent as a function of time while a bias voltage of −1 V is applied. Figure 4d illustrates the photo-response behavior of the devices. Astonishingly, it reveals that the devices have a very stable and consistent response to light. The rise time, T_r (time for which device response rises from 10 to 90%) and decay time, T_d (time for which the device response falls from 90 to 10%) for all devices was consistent at 641 ms at each cycle for any concentration of DDCI-4. This shows the capability of DDCI-4 to hold and maintain charge stability because DDCI-4 can deal with strongly bound excitons [13].

3.3. Rougher Surface with Dicarbocyanine

The morphological and surface roughness of a dye film has clearly been shown to be a significant factor influencing the performance of a photodiode in previous works of literature [11,32]. The effect of the number of electron paths due to a smooth surface roughness also contributes to a decrease in the electron–hole recombination by the photodiode, which potentially controls the device’s performance [33,34]. Therefore, in this research, atomic force microscopy (AFM) analysis that operates in tapping mode using a cantilever with a scan size of 2 μm × 2 μm was utilized to examine the morphology and surface roughness of the thin film. Figure 5 depicts the surface morphology of the devices’ thin film with different concentrations of DDCI-4. It is shown that adding more DDCI-4 contributes to a rougher surface of the thin film. For a thin film without DDCI-4, surface roughness is observed to be 0.675 nm whereas adding 3 mg/mL, 5 mg/mL, and 7 mg/mL of DDCI-4 caused the surface roughness to increase to 0.923 nm, 1.06 nm, and 1.23 nm, respectively. This implies that when the concentrations of DDCI-4 were decreased, the number of electron paths increased, which in turn prevented electron–hole recombination, therefore resulting in a significant increase in quantum efficiency and in the efficiency of the photodetector [35,36,37,38].

4. Conclusions

The present work has been concerned with different concentrations of a newly utilized conjugated dye-type material of 1′-1′-diethyl-4,4′-dicarbocyanine iodide (DDCI-4). Examining the optical, morphological, and electrical performance of organic photodiodes showed that DDCI-4 has a good light sensing capability in the NIR region with a responsivity of 22 mA/W and detectivity of 6.57 × 10¹⁰ Jones at low concentrations. The effect of the Frenkel defect and the delocalization of charges that happened at high concentrations of DDCI-4 mostly contributed to controlling the charge flow and the performance of the device. Compared to another cyanine dye-based photodiodes, the DDCI-4 device showed a surprisingly very stable and consistent photoresponse to light at 641 ms even after several cycles.