Spatial Confinement of the Optical Sensitizer to Realize Thin Film

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Spatial Confinement of the Optical Sensitizer to Realize Thin Film Organic Photodetector with High Detectivity and Thermal Stability Min Su Jang, Seongwon Yoon, Kyu Min Sim, Jangwhan Cho, and Dae Sung Chung J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02918 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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The Journal of Physical Chemistry Letters

Spatial Confinement of the Optical Sensitizer to Realize Thin Film Organic Photodetector with High Detectivity and Thermal Stability Min Su Jang, Seongwon Yoon, Kyu Min Sim, Jangwhan Cho, Dae Sung Chung* Department of Energy Science & Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Republic of Korea. AUTHOR INFORMATION Corresponding Author Dae Sung Chung, E-mail: [email protected]

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ABSTRACT

A thin film planar heterojunction organic photodetector (PHJ-OPD) is demonstrated. Different from a conventional sensitizer-doped photodetector, the limited spatial distribution of sensitizer in a PHJ-OPD enables significantly reduced thickness of the active layer without allowing the formation of unnecessary trap sites and electron percolation pathways. As a result, peak external quantum efficiency (EQE) of 120,700% and detectivity over 1013 Jones are demonstrated with thin active layer thickness of 150 nm, which can be a significant benefit for high resolution image sensor application. Furthermore, the operating voltage can be decreased to -5V while maintaining high detectivity over 1012 Jones. Remarkable thermal stability is also observed with minor change in detectivity for 2 hours of continuous operation at 60°C due to morphological robustness of PHJ. This work opens up a possibility of using a thin film PHJ-OPD as a key unit of high resolution image sensor.

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In modern society, photodetectors are widely used in private households, industries, and research fields.1 In particular, organic photodetectors (OPDs) have been actively studied for more than 10 years due to their suitability for applications requiring wide active areas, mechanical flexibility, low processing-cost, and wavelength selectivity.2,3 In most cases, the operating mechanism of OPD is following that of Si; pn-junction diode is introduced under a reverse saturation regime. In this case, OPD has an ultimate advantage of replacing Si diodes directly in CMOS image sensors, with much thinner thickness due to high absorption coefficient of organic semiconductor.4 In CMOS image sensor, thinner active layer thickness of photodiode is of great importance because it enables higher resolution. However, because too thin OPDs accompany high dark currents, currently reported high performance OPDs have marginal thicknesses of ~ 500 nm which is not very beneficial compared to conventional Si diode (3µm).5 Therefore, in order to realize thin film OPDs, a strategic approach is required to significantly enhance photocurrent so that high dark current can be compensated. The representative example of such approaches is realizing a photomultiplication mechanism in OPDs which can generate photoconductive gain higher than unity or external quantum efficiency higher than 100%.6 In such a photomultiplication OPD, the photoconductive gain is given by gain = τ × ttr-1, where τ and ttr are the carrier lifetime and transit time, respectively. This approach was developed by Zhang

et

al.

using

the

phenomenon

of

photo-induced

barrier-lowering

at

the

semiconductor/electrode interface.7-9 A high gain of up to 1,158 has been reported in such photomultiplication OPDs by using a high applied voltage of -19V.10 However, up to now, all the photomultiplication OPDs adapted bulkheterojunction (BHJ) when constructing the active layer, which possesses negative aspects in terms of thickness and stability. First of all, photomultiplication BHJ-OPDs have limitation in decreasing active layer thickness (~250nm)


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because they need to be prevented from the formation of percolation pathway of electron, which is a crucial factor for realizing photomultiplication.11 In addition, the operational stability of BHJ-OPDs at elevated temperature can be a weakness due to the possible enthalpy-driven morphological change which can induce the formation of electron percolation pathway. In this work, we introduce a photomultiplication planar heterojunction OPD (PHJ-OPD) to achieve high gain and detectivity while maintaining a thin active layer and thermal stability. Unlike the conventional photomultiplication BHJ-OPDs, we spatially confined phenyl-C71butyric acid methyl ester (PCBM), the optical sensitizer, only at the active layer / cathode interface so that a thinner active layer can be introduced without forming any electron percolating pathways, leading to not only low operating voltage but also higher gain. More importantly, the morphological robustness of a PHJ facilitates thermal stability during operation. A photomultiplication PHJ-OPD was constructed by sequentially depositing poly(3hexylthiophene) (P3HT) and PCBM onto the PEDOT:PSS/ITO surface by using orthogonal solvents of 1,2-dichlorobenzene and dichloromethane, respectively.12 Although layer-by-layer deposition was used, it was turned out that most of the PCBM diffused slightly into the P3HT layer, resulting in thin sensitizing layer (5nm) consisting of P3HT:PCBM mixture. (Figure 1a and b) As a result, the very outer surface mostly consists of more hydrophobic P3HT, as proved by surface energy analyses in Figure S1. Therefore, the estimated PCBM distribution in BHJOPD and PHJ-OPD can be speculated as the schemes in Figure 1c. The localized PCBM domains present only at the Al/active layer interface in PHJ-OPD introduces spatially confined trapping defects for electrons which work as a photoconductive sensitizer. Consequently, under reverse bias, the photogenerated electrons are trapped in the sensitizing layer near Al electrode and the corresponding holes drift towards the counter electrode (ITO) and is collected there.


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Because the electrons trapped at the sensitizing layer near the Al electrode form negatively charged layer, the Coulomb field induced by the negative charge lowers the potential at the injecting electrode, resulting in an additional injection of holes. The newly injected holes have a greater probability of crossing the semiconductor layer several times before recombining with the trapped electrons. Therefore, PHJ-OPD facilitates high photoconductive gain without the concern of forming electron percolation pathways, different to the case of BHJ-OPD.13 The optimal concentration of PCBM solution in dichloromethane was experimentally determined as 0.125 mg/ml which yielded the sensitizing layer thickness of ~ 5 nm. (Figure S2) The TEM cross-section image of the optimized PHJ-OPD with the active layer thickness of 150nm is shown in Figure 1b, together with the energy dispersive X-ray spectroscopy image in Figure 1c.


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Figure 1. (a) A Cross-sectional TEM image of the optimized PHJ-OPD structure: Anode (ITO) / Hole transport layer (PEDOT:PSS) / Active layer (P3HT) / Sensitizing layer (PCBM) / Cathode (Al); (b) The energy dispersive X-ray spectroscopy (EDX) line scan data of PHJ-OPD device with the y-axis as the distance from the ITO surface ; (c) Schematic image for the comparison of the operational mechanism of BHJ-OPD and PHJ-OPD under illumination

In order to study the thickness dependences of PHJ-OPDs in comparison to BHJ-OPDs, we prepared 250nm, 200nm, and 150 nm thick active layers for both cases. In the case of BHJOPDs, we followed a previously reported fabrication method. Figure 2 (a) and (b) shows the spectral gain characteristics of PHJ-OPDs and BHJ-OPDs, respectively. We can see that the 250 nm thick active layer yields similar gain values for both BHJ-OPD and PHJ-OPD (Figure 2a and b), which are also similar to gain of previously reported BHJ-OPD with similar thickness.7 However, with 200 nm and 150 nm, PHJ-OPD showed significantly enhanced photoconductive behavior with peak gain value up to ~1,200, while BHJ-OPD revealed only slight enhancement. In a photomultiplication PHJ-OPD, ttr of hole can be simply defined by ttr = L µh-1E-1, where µh is the hole mobility, E is the applied electric field, and L is the active layer thickness. Under the constant applied bias, ttr is simply proportional to L2, in other words, gain = µτV L-2.14 Therefore, roughly, a PHJ-OPD with a thickness of 150 nm would have a 2.8 times higher gain value than that with a thickness of 250 nm. Interestingly, very similar trend is observed when the gains of various PHJ-OPDs with different P3HT-layer thicknesses are compared at the same illumination wavelength, showing the relation ∝ (Figure 2a). In a strong contrast, BHJ-OPDs do not follow the relation as seen in Figure 2b. The inferior performance of BHJ-OPD compared to PHJ-OPD in decreased active layer thickness can be related with either the possibility of partial electron percolation pathway or the existence of undesired trap sites within the entire volume of


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the active layer. Although it is hard to say that perfect electron percolation pathway can be formed with only 1% of PCBM, we speculate that still some PCBM domains would form localized electron pathway which can limit the photomultiplication mechanism, especially in the case of very thin active layer thickness. Another possibility is related with the different aspects of PCBM distribution in BHJ-OPD and PHJ-OPD. In BHJ-OPD, all the PCBM domains other than the ones near the Al are undesired trap sites that can recombine with the injected holes, limiting the EQE of the device. Figure 2c shows the peak gain values of BHJ-OPDs and PHJ-OPDs with various active layer thicknesses. The peak gain value reached 1,207 for the 150 nm PHJ-OPD, which is the highest gain value reported ever with OPDs. In Figure 2d, we showed the voltagedependent gain for 150nm PHJ-OPD in comparison to conventional BHJ-OPD with previously used thickness of 250nm. Clearly one can see that the -5V of applied bias cannot guarantee photoconductive behavior (gain>1) in the case of thick BHJ-OPD, while gain higher than unity is easily achieved at the same voltage in the case of thin PHJ-OPD. Here we want to address that the achieved remarkably thin active layer thickness of ~150nm is very important for actual application of OPD for the pixel unit of high resolution image sensor. (see discussion in Section S2 in the Supporting Information (SI))


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Figure 2. The calculated gain spectra of (a) PHJ-OPDs and (b) BHJ-OPDs with different active layer thicknesses at -19 V. The inset of each figure shows the corresponding gain∝L-2 relation with the dashed line as theoretical fitting (c) The peak gain values of PHJ-OPD and BHJ-OPD as a function of the active layer thickness. (d) gain−V characteristics of PHJ-OPD (150nm) and BHJ-OPD (250nm) under 600 nm illumination with the light intensity of 33.5µW cm-2

A key figure-of-merit of OPD is specific detectivity (D*), which can be determined by ∗

√

(1)


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where λ is the illumination wavelength, G is the gain, A is the area of the active layer, h is the Planck’s constant, c is the speed of light, and in is the noise current.15 Therefore, in order to have high D*, not only high gain but also low in is essential. The measured noise current of the fabricated PHJ-OPD is shown in Figure S3. From the measured value of in, the resulting D* reached 1.3×1012 Jones with a gain value of six times unity at a low voltage of -5V (Figure 3a). Figure 3b shows the voltage-dependent D* spectra and gain of the optimized 150 nm PHJ-OPD. As the voltage increases, both D* and gain increase, which is a common phenomenon in photomultiplication-type photodetectors.8,10,11 Because the operating mechanism of PHJ-OPDs rely on repeated hole injection at the Al interface driven by the localized PCBM-trapped electrons, weak incident light may limit the gain generation. Therefore, it is very important to confirm that the PHJ-OPD can work at various intensities of incident light or have high linear dynamic range (LDR). The LDR of the PHJ-OPD was measured by recording the steady-state photocurrent under 445 nm light illumination with different light intensities from 2.31×10-6 W cm-2 up to 7.26×10-3 W cm-2, as shown in Figure 3c, which corresponds to the LDR of 89 dB. We tested the operational stability of the optimized PHJ-OPD by exposing it to a continuous light pulse (445 nm, 7.26 µW/cm2) in comparison to the case of BHJ-OPD. Figure 3d shows that the initial and final dark/photo-current difference of PHJ-OPD is much less pronounced compared to the case of BHJ-OPD, which is also reflected in transient gain measurement in Figure 3e. In commercial applications of OPDs such as pixels of image sensors, they are often exposed to high temperature environments. Therefore, we conducted a thermal stability test on the optimized PHJ-OPD by exposing the device to 60°C and measuring the transient detectivity. As shown in Figure 3f, PHJ-OPD nearly maintained its initial dark current characteristics even after 120 minutes of exposure to 60°C thermal stress, while BHJ-OPD showed fast increase of


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dark current. For the same condition, transient photocurrents were also measured for both cases and summarized in Figure S4. As a result, the measured transient detectivity of PHJ-OPD against thermal stress was much more robust compared to the case of BHJ-OPD as summarized in Figure 3g. This implies that the morphological robustness of the PHJ-OPD with a simple PHJ structure facilitates the generation of efficient photoconductive gain, even at elevated temperatures. This is the first demonstration of thermal stability among all OPDs.

Figure 3. a) The calculated gain spectrum of the PHJ-OPD (top) and the corresponding specific detectivity (D*) (bottom) as a function of wavelength both with -5V of applied bias. b) the gain (top) and D* (bottom) spectra of the optimized PHJ-OPD under different biases from -5 V to -19 V; c) LDR of the optimized PHJ-OPD at -10 V; d) the measured transient current for PHJ-OPD (left) and BHJ-OPD (right) against pulsed light (3Hz) exposure with 445 nm of wavelength and 7.26 mW cm-2 of the intensity e) the measured transient gain data for PHJ-OPD and BHJ-OPD;


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(f) dark J-V characteristics of PHJ-OPD (left) and BHJ-OPD (right) exposed to a 60°C for 2 hours. (g) the measured transient D* data for PHJ-OPD (left) and BHJ-OPD (right).

In this work, we have devised a thin film, low voltage, and high gain OPD by strategically positioning the PCBM within the interface of P3HT and the Al electrode. As a result, without the issue of forming electron percolation pathways, the thickness of the active layer could be reduced so that the carrier transit time of injected holes could be minimized. This ultimately enhanced the EQE or photoconductive gain of the device. The maximum gain achieved from this PHJ-OPD was as high as 1,207 at -19V, with a detectivity of over 1013 Jones. The PHJ-OPD also showed a fairly high detectivity of 1.3×1012 Jones at a low voltage of -5V with 89dB of LDR, which is comparable to recently reported OPDs, in terms of not only opto-electric performance but also operation voltage. Here it is noteworthy that all these high OPD performances were obtained from thin active layer of 150nm, much thinner than Si-PD or other OPDs. In addition, the PHJ-OPD exhibited an operational stability for as long as 120 minutes at 60°C, owing to the morphological simplicity of the PHJ junction. This work facilitates simple but effective enhancement of photoconductive gain and stability of OPDs by spatially confining sensitizer, which can be applied to a wide range of photovoltaic polymers.

AUTHOR INFORMATION Corresponding Author Dae Sung Chung, E-mail: [email protected] ACKNOWLEDGMENT


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This research was supported by the Space Core Technology Development Program and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant Numbers NRF-2014M1A3A3A02034707 and NRF015R1C1A1A02037219). ASSOCIATED CONTENT Supporting Information Experimental Section, additional information about the significance of thin layer OPD for application, surface energy analysis of pure P3HT film surface and P3HT:PCBM PHJ film surface, gain spectra and J-V characteristic of PHJ-OPD with different PCBM contents, noise measurement, J-V characteristic of OPDs with thermal stress.

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Spatial Confinement of the Optical Sensitizer to Realize Thin Film

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