Tesis

Figure 3.4 Encapsulation tools, (a) clear glass on the left and encapsulation glass on the right, and (b) UV curable adhesive. Figure 4.15 Summary of Dark Current Density, EQE, Responsiveness, and Specific Detectivity after Thermal Shock with Different Applied Bias of -0.5, -1, and -2 V.

Overview

The dilemma between low dark current density and high thermal stability prohibits the potential of TPD towards commercial applications. Here, an optimization of the TPD device structure is studied to maximize the trade-off between dark current density and thermal stability.

Figure 1.1 The Sun’s illustration of transparent mobile phone [9] based on LG’s US patent [10]

Objective

Outline

TPD Device Structure

Photoactive Layer

The process of photocurrent conversion in the photoactive layer begins with the absorption of a photon by the donor or acceptor (assume that the light is coming from the donor). For efficient dissociation of photon into electron and hole, parameters such as thickness, energy band and doping ratio (for bulk heterojunction) are essential in the design of D-A heterojunction.

Blocking Layer

Blue sine waves indicate photons, dashed line indicates Coulomb coupling between electron and hole (purple colors indicate excitons and red color indicates dissociated exciton), black circles indicate electrons, and white circles indicate holes. In this structure, the high LUMO level of EBL inhibits electron transfer from the active layer to the anode, and the deep HOMO level of HBL inhibits hole transfer to the cathode.

Transparent Electrode and Capping Layer

Cover layer is an extra thin layer on the outside of the structure of the device (see figure 2.1). The main purpose of the cover layer is to change the optical properties of the device structure; it is not electrically connected to the internal layers.

Characterization of TPD

Dark Current Density
Photocurrent Density
EQE and Responsivity
Noise Current
Specific Detectivity
Transmittance and AVT
Photoresponse and f-3 dB
LDR

Despite its superiority in transparency and sheet resistance, ITO is one of the most durable materials compared to others. From the photocurrent density curve as shown in Figure 2.6, parameters such as open circuit voltage (Voc), short circuit current (Jsc), field factor (FF) and PCE (η) can be determined. EQE is the ratio of the ejected electron to the photon radiation incident on the device.

Parameters that largely determine the EQE are the absorption of the donor-acceptor junction, charge transport loss from the transition to the electrode, and the refractive index of the device structure. The first and second parameters are determined by the inherent properties of the material, while the third parameter can be optimized by adjusting the thickness of the layers in the unit structure. The correct design of the thicknesses will induce the optical cavity effect that can be used to improve the EQE at certain wavelengths.

Furthermore, responsivity is defined as the response signal from the photodetector corresponds to an incident irradiation on the photodetector, the illustration of the responsivity curve can. LDR is the ratio of the highest to the lowest illumination intensity, where the linearity of photocurrent vs.

Figure 2.5 The illustration of typical J dark curve. Reverse dark current density is J dark in negative applied voltage bias and forward dark current density is J dark in positive applied bias

Previous Work in NIR TPD and Outlook

Previous Work in NIR TPD

Compared to other previous results in TPD or semi-transparent OPD, our TPD shows the best transmission below 515 nm and dark current density under -2 V bias, as shown in Figure 2.13 [17]. Dark current density of reference OPD and TPD with different ClAlPc:C60 ratios in photoactive layer is shown in figure 2.14. The low dark current density of NIR TPD is attributed to the smooth surface morphology of Cu:Ag/WO3 (see Figure 2.15).

For comparison, the reference OPD with the same device structure and dark silver electrode shows a dark current density of 4.13 nA cm-2. The higher dark current density of the reference OPD is due to a rougher surface morphology of silver (RMS = 3.17 nm) compared to Cu:Ag (1.48 nm). The dark current density of TPD increases by three orders of magnitude after 30 min of thermal shock of 100.

To replace BPhen, an alternative blocking layer material with high thermal stability, such as TmPyPb, successfully improved thermal stability up to 4 hours of thermal shock without any significant change in dark current density. However, the dark current density of TmPyPb-based TPDs is on the order of 10-7 A cm-2, which is three orders of magnitude higher than BPhen-based TPDs.

Figure 2.13 Comparison of dark current density under -2 V applied bias and transmittance at 515 nm with the other previous works on TPD and semitransparent OPD [17]

Outlook

As shown in the previous work, EQE is relatively stable even after 4 hours of thermal shock at 100 °C, but the dark current density increases by three orders of magnitude after 30 minutes of thermal shock at 100 °C for Bphen-based TPDs. The alternatives, TmPyPB based TPDs, offer high thermal stability with high dark current density. In the future, low dark current density and high thermal stability are needed to bring TPD closer to LIDAR or IoT applications.

The values are measured with an applied bias voltage of -2 V; b), c) Spectral response values were measured at -2 V and a wavelength of 730 nm; d) Values are calculated from Eq.

Table 2.2 Performances summary of the previous TPDs [17]

Materials and Device Fabrication

Materials
Substrate Preparation
Device Fabrication
Encapsulation
Thin Film Preparation

The device is manufactured by thermal evaporation method inside the vacuum chamber with a vacuum level of 10-6 torr or less. Organic materials are deposited sequentially with some masking, as can be seen in Figure 3.3. Unit structures deposited in this report are shown in Table 3.1. The first part of the device structure is proposed to compare the performance of TPD with different HBL of C60, CN-T2T and C60:CN-T2T, while the second part is the optimization of HBL thickness.

There are two types of encapsulations used in the experiment, the encapsulation glass for opaque OPD and the bare glass for TPD as can be seen in Figure 3.4. The bare glass and encapsulation glass are cleaned by the same procedure as the substrates before the encapsulation process. Device encapsulation is processed in the glove box with nitrogen gas (N2) with oxygen (O2) and moisture level < 0.1 ppm.

Thin films are fabricated for characterization purposes such as sheet resistance, transmittance, reflectance, absorbance and surface morphology. Thin film is stored in the N2 glove compartment before the measurement to prevent the degradation process due to the surrounding environment.

Figure 3.2 Steps of substrate preparation processes. (a) the substrate is rinsed by the soap, (b) the substrate is soaked in the liquid, and (c) vibrated with the ultrasonic cleaner for ten minutes

Device Characterization

Dark Current Density
Photocurrent Density
EQE and Responsivity
Transmittance and AVT
LDR
Transient Photoresponse
Thermal Shock

The photocurrent density is measured using a single solar system consisting of a solar simulator light source and a Keithley 2401 SourceMeter for J-V characterization (see Figure 3.6). After preheating, the solar simulator is calibrated with a standard silicon solar cell (1 sunlight equals a photocurrent of 75.6 mA at 0 V). Parameters that can be extracted from a single solar measurement are short-circuit photocurrent (Isc), open-circuit photovoltage (Voc), maximum solar cell power (Pmax), efficiency, duty factor, short-circuit photocurrent density (Jsc), series resistance (Rs) , and shunt resistance (Rsh).

EQE is performed by QE-R (Enli Technology Co., Ltd., Taiwan) as shown in Figure 3.7. Before using this system, the light source must be preheated for 30 minutes and then calibrated by the standard reference sensor of silicon or germanium; EQE is measured with AC mode. LDR is measured by the home-made setup, 780 nm light source system (Thorlabs, M780L3) is aimed at filter wheel (Thorlabs, FW102CNEB) to control the light intensity, as can be seen in Figure 3.9.

The light intensity is swept from 1 mW cm-2 to 1 nW cm-2, and the resulting photocurrent is measured by a Keithley 2636A SYSTEM SourceMeter. The signal from the photodetector is amplified by the preamplifier (Ametek, model 5182) with a gain factor of 105 and displayed with a 2.5 GHz oscilloscope (Teledyne.

Figure 3.5 (a) Keithley 2636A SYSTEM SourceMeter and (b) Labview software of dark current measurement system

Selection of HBL

The EQE and responsivity spectra have two peaks at 350 nm and 730 nm as can be seen in Figure 4.3, the first peak (UV peak) corresponds to the absorption of C60, while the second peak (NIR peak) is due to the absorption of ClAlPc. Another parameter associated with dark current density and responsivity data is specific detectivity, which can be obtained by replacing the noise current with the shot noise as mentioned in Eq. Specific detectivity, EQE, responsivity and dark current density data indicate that CN-T2T is the optimized HBL in this group of materials.

T2T exhibits a dark current density of A cm-2 under an applied bias of -2 V, which can potentially be further reduced by tuning the CN-T2T thickness.

Table 4.1 Summary of dark current density at different point of voltage with HBL of C 60 , CN-T2T, and C 60 :CN-T2T (1:1)

CN-T2T Thickness Optimization

Regarding the reciprocal relationship between specific detection and dark current density, thicker CN-T2T monotonically increases TPD performance, however, other parameters such as EQE and responsivity may decrease more significantly due to CN thickness -T2T resulting lower. As shown in Figure 4.6 and Table 4.5, EQE and responsivity spectrum are highest with CN-T2T thickness of 10 nm, then the spectrum decreases with increasing thickness, moreover, the spectrum values are extremely low with 40 nm CN -T2T. To observe the accumulative result of those opposite effects, specific detection can more accurately determine the dependence of CN-T2T thickness on TPD performance.

Therefore, the 20 nm CN-T2T is the most balanced trade-off between the hole trapping ability to reduce the dark current density and the electron extraction ability to extract the photogenerated carrier. This result is probably due to the electron and hole transit time imbalance when the thickness of CN-T2T increases, which results in a lower EQE and responsivity spectra [36,37]. To comprehensively examine the performance of TPDs with different thicknesses of CN-T2T, the photocurrent density is measured under a solar illumination, as shown in Figure 4.8, the photocurrent density decreases as the thickness of CN-T2T increases, e which can be attributed to transit time. imbalance [36,37].

On the other hand, the dependence of CN-T2T thickness on rise time and fall time is more obvious than f-3 dB. LDR value for 40 nm thick CN-T2T is omitted due to low light response.

Figure 4.5 Dark current density of TPDs with with CN-T2T thickness of 10, 20, 30, and 40 nm

Thermal Shock and Voltage Adjustment

As shown in Figure 4.15, dark current density performance improves with lower applied bias, conversely, EQE and responsiveness improve with higher applied bias. This can be attributed to the lowest dark current density point in Figure 4.12, a higher detection can be achieved by choosing the applied bias closer to the voltage at which the lowest dark current density is set (approx. to ≈-0.5 V). This lower dark current density can be attributed to the lower HOMO level of CN-T2T compared to C60, which leads to a more efficient hole blocking layer than C60.

The dark current density can be further reduced by tuning the thickness of CN-T2T as HBL. The optimized TPD shows excellent thermal stability under thermal shock of 100 oC for up to one hour of exposure, with no significant performance degradation after this thermal test interval with residual dark current density of A cm-2, EQE of 28, 91%, response of 0.17 A W-1, and specific Jones detection. Liu, Transparent photodetectors with ultra-low dark current and high photoresponse for near-infrared detection,.

Lugli, Origin of Dark Current and detailed description of the operation of organic photodiodes under different illumination intensities, IEEE Trans. Wong, Vacuum-processed small molecule organic photodetectors with low dark current density and strong response to near-infrared wavelength, Adv.