Thickness-dependent Performance Research Articles

Thermal evaporated CuI film thickness-dependent performance of perovskite solar cells

CuI, as a promising hole transport material, possesses high hole mobility and is compatible with perovskite materials about band structure. Herein, CuI hole transport films are first fabricated by vacuum thermal evaporation procedure for n-i-p perovskite solar cell. The results show that vacuum thermal evaporated CuI films have no obvious influence on perovskite underlayer and 200 nm thick CuI film has the best carrier extraction ability. To further improve the stability of the device, reduced graphene oxides are introduced to avoid the accelerated attenuation caused by the reaction of CuI with Au. Finally, the power conversion efficiency of the dual-modified device improves to 8.69%, with the short circuit current density of 18.86 mA cm−2, the open circuit voltage of 0.832 V and the filling factor of 0.550. The work demonstrates a potential application of vacuum thermal evaporated CuI films in organic-inorganic hybrid perovskite solar cells.

Metal–2D multilayered semiconductor junctions: layer-number dependent Fermi-level pinning

Thickness-dependent performance of metal–two-dimensional semiconductor junctions in electronics/optoelectronics have attracted increasing attention but, currently, little knowledge about the micro-mechanism of this thickness dependence is available.

A Heterogeneous Kinetics Model for Triplet Exciton Transfer in Solid-State Upconversion.

High internal quantum efficiency semiconductor nanocrystal (NC)-based photon upconversion devices are currently based on a single monolayer of active NCs. Devices are therefore limited in their external quantum efficiency based on the low number of photons absorbed. Increasing the number of photons absorbed is expected to increase the upconversion efficiency, yet experimentally increasing the number of layers does not appreciably increase the upconverted light output. We unravel this mystery by combining kinetic modeling and transient photoluminescence spectroscopy. The inherent energetic disorder stemming from the polydispersity of the NCs means that the kinetics are governed by a stochastic transfer matrix. By drawing the rates from a probabilistic distribution and constructing a reaction network with realistic connectivity, we are able to fit complex photoluminescence traces with a very simple model. We use this model to explain the thickness-dependent performance of the upconversion devices and can attribute the reduced efficiencies to the low excitonic diffusivity of the exciton within the NC layers and increased back transfer of the created singlets from the organic annihilator rubrene. We suggest some avenues for overcoming these limitations in future devices.

Increase in Interfacial Adhesion and Electrochemical Charge Storage Capacity of Polypyrrole on Au Electrodes Using Polyethyleneimine

High-performance devices based on conducting polymers (CPs) require the fabrication of a thick CP-coated electrode with high stability. Herein, we propose a method for enhancing the interfacial adhesion strength between a gold electrode and an electropolymerized polypyrrole (pPy) layer by introducing a polyethyleneimine (PEI) layer. Although this insulating layer hinders the initial growth of the pPy layer on the Au surface, it improves the adhesion by up to 250%. Therefore, a thick layer of pPy can be fabricated without delamination during drying. X-ray photoelectron spectroscopy shows that the PEI layer interacts with the Au surface via polar/ionic groups and van der Waals interactions. Scanning electron microscopy reveals that the cohesion of the pPy layer is stronger than the interfacial adhesion between the PEI layer and the pPy layer. Importantly, the electroactivities of pPy and its dopant are unaffected by the PEI layer, and the electrochemical storage capacity of the pPy layers on the PEI-coated Au electrodes increases with thickness, reaching ~530 mC/cm2. Negative potential sweep is detrimental to pPy layer adhesion: pPy layers on a bare Au electrode peel off instantly as the potential is swept from 0.2 to −0.7 V, and most of the charge stored in the layer becomes inaccessible. In contrast, pPy layers deposited on PEI coated Au electrode are mechanically stable and majority of the charge can be accessed, demonstrating that this method is also effective for enhancing electrochemical stability. Our simple approach can find utility in various applications involving CP-coated electrodes, where thickness-dependent performance must be improved without loss of stability.

High Efficiency Inverted Organic Solar Cells with a Neutral Fulleropyrrolidine Electron-Collecting Interlayer.

A novel fulleropyrrolidine derivative, named FPNOH, was designed, synthesized, and utilized as an efficient electron-collecting (EC) layer for inverted organic solar cells (i-OSCs). The grafted diethanolamino-polar moieties can not only trigger its function as an EC interlayer, but also induce orthogonal solubility that guarantees subsequent multilayer processing without interfacial mixing. A higher power conversion efficiency (PCE) value of 8.34% was achieved for i-OSC devices with ITO/FPNOH EC electrode, compared to that of the sol-gel ZnO based reference devices with an optimized PCE value of 7.92%. High efficiency exceeding 7.7% was still achieved even for the devices with a relatively thick FPNOH film (16.9 nm). It is worthwhile to mention that this kind of material exhibits less thickness dependent performance, in contrast to widely utilized p-type conjugated polyelectrolytes (CPEs) as well as the nonconjugated polyelectrolytes (NCPEs). Further investigation on illuminating intensity dependent parameters revealed the role of FPNOH in reducing interfacial trap-induced recombination at the ITO/active layer interface.

Characterization of Ambient Light-Induced Inversion Current in MOS(n) Tunneling Diode With Enhanced Oxide Thickness-Dependent Performance

The dark and light current mechanisms of n-type substrate MOS(n) tunneling photodiode for visible light sensing were investigated. By measuring current-voltage and capacitance-voltage curves for devices with different oxide thicknesses, we found that the light current at $V_{G} = -2.5$ V first decreases but then increases with oxide thickness. The turning point of oxide thickness is $\sim 2.3$ nm. We attributed this phenomenon to the oxide voltage tuning induced by the lateral diffusion current. Therefore, the photosensitivity enlarges remarkably when an oxide is thicker than 2.45 nm, and the photosensitivity and responsivity of the device with an oxide thickness of 2.5 nm reach up to 669 and 463 mA/W under the visible ambient light irradiance of 30 mW/cm2, respectively. MOS(n) tunneling photodiode exhibits the increasing photosensitivity with the oxide thickness, which is opposite to the behavior of MOS(p). This difference may be properly utilized as the ambient light image sensor integration in modern CMOS technology. In addition to the optoelectrical properties, the light current flow path was examined to elucidate the current mechanism and gate structural dependence.

Room-Temperature Solution-Processed n-Doped Zirconium Oxide Cathode Buffer Layer for Efficient and Stable Organic and Hybrid Perovskite Solar Cells

In this study, we present a simple and effective method to improve the performance and stability of organic and hybrid perovskite solar cells by the incorporation of solution-processed cetyltrimethylammonium bromide (CTAB)-doped zirconium oxide (ZrOx) as cathode buffer layer (CBL). This novel n-doped ZrOx CBL possesses several remarkable features, including ease of fabrication without the need for thermal annealing or any other post-treatment, reasonable electrical conductivity (2.9 × 10–5 S cm–1), good ambient stability, effective work function modulation of Ag electrode, relative weak thickness-dependent performance property, and wide applicability in a variety of active layers. Compared with ZrOx CBL without CTAB dopant, CTAB-doped ZrOx can significantly improve the power conversion efficiency (PCE) from 0.57% to 2.48% in organic solar cells based on diketopyrrolopyrrole-thiophene-bezothiadazole low-bandgap polymer (PDPP-TBT):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend. With this n-doped ...

Thickness-Dependent Performance of a Free-Standing Membrane LED

We propose a free-standing membrane light-emitting diode (LED) on a GaN-on-silicon platform. Finite-difference time-domain (FDTD) simulation is used to investigate thickness-dependent performance of a free-standing membrane LED. The simulation results show that the light extraction efficiency is increased, and a directed emission pattern is obtained in free-standing membrane LEDs, which are experimentally implemented by removing the silicon substrate and back wafer thinning of the epitaxial layer. The light extraction efficiency and current-voltage (I-V) performance of membrane LEDs is increased in comparison with LEDs with silicon substrate.

High-Performance Flexible Tandem Polymer Solar Cell Employing a Novel Cross-Linked Conductive Fullerene as an Electron Transport Layer

A novel thermally cross-linkable, n-doped conductive fullerene material is developed by incorporating tetrabutylammonium iodide (TBAI) as the dopant into an azidofullerene derivative PCBN3. The application of TBAI-doped cross-linked PCBN3 film as the electron transport layer (ETL) in polymer solar cells (PSCs) delivers several remarkable features, including easy solution-processability, reasonable electrical conductivity (2.8 × 10–3 S cm–1), good ambient and chemical stability, fine-tunability of the work function of the electrode, wide applicability in a variety of efficient polymers, relative weak thickness-dependent performance property, and moderate cross-linking temperature (∼140 °C). With this ETL, a single-junction solar cell based on the blend of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) delivers a power conversion efficiency (PCE) up to 8.8...

Well-defined star-shaped conjugated macroelectrolytes as efficient electron-collecting interlayer for inverted polymer solar cells.

A star-shaped monodisperse conjugated macroelectrolyte grafted with cationic side chains, TrNBr, was designed, synthesized, and utilized as efficient electron-collecting cathode interlayers for inverted polymer solar cells. A neutral one composed of identical star-shaped conjugated backbone, TrOH, was also investigated for comparison. The surface properties and the function as interfacial layers on modulating the work function of bottom electrode (indium tin oxide) were systematically studied. Both interfacial electron-selective materials show strongly thickness-dependent performance for inverted polymer solar cells, and the best performance could be achieved via optimizing the thickness with 2.4 nm of TrNBr and 8.7 nm of TrOH. Parallel investigations of optimized TrNBr and TrOH interlayer in inverted architecture with active blend layer of poly(3-hexylthiophene):indene-C60 bisadduct (P3HT:ICBA) demonstrated a remarkable power conversion efficiency (PCE) enhancement (PCE of 4.88% for TrNBr and 4.74% for TrOH) in comparison with those of conventional noninverted devices using Ca/Al cathodes (3.94%) and inverted devices with sol-gel ZnO buffer layer (4.21%). In addition, the inverted devices using the TrNBr and TrOH interlayer exhibited improved device stability in contrast to conventional noninverted devices using Ca/Al cathodes.

Study of the characteristics of UVA LEDs grown by HVPE: active region thickness-dependent performance

We report on results of the performance study of UVA LEDs depending on the thickness of the active region. UVA LEDs are based on GaN/AlGaN heterostructures grown on Al2O3 (0001) substrates by hydride vapor phase epitaxy (HVPE). It is shown that the use of thick (-100 nm) single layer as the active region of UVA LED is a promising concept to achieve enhanced efficiency.

Field- and thickness-dependent performance of tandem organic light-emitting diodes with buffer-modified fullerene/copper phthalocyanine heterojunction as interconnector

Tandem organic light-emitting diodes (OLEDs) with the buffer-modified fullerene/copper phthalocyanine (LiF/C60/CuPc/MoO3) as interconnector have been investigated. The properties of tandem OLEDs are dependent on electric field, which is induced by electric-field-dependence of charge generation (by electric filed and photons) and separation in such an interconnector structure. At low electric field, because of less generated-charges and light-absorption of C60/CuPc film, current efficiency and brightness of tandem OLEDs with two emissive units exhibit less than twice higher than those of conventional single-unit device. But more than twofold enhancement of current efficiency and brightness is observed at higher electric field due to a large number of charges generated in the interconnector. Meanwhile, the performance of tandem devices is affected by the thickness of C60 and CuPc. The optimal thickness is related to their exciton diffusion length.

Engineering the interfaces of ITO@Cu2S nanowire arrays toward efficient and stable counter electrodes for quantum-dot-sensitized solar cells.

Among the issues that restrict the power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSSCs), insufficient catalytic activity and stability of counter electrodes (CEs) are critical but challenging ones. The state-of-the-art Cu/Cu2S CEs still suffer from mechanical instability and uncertainty due to the reaction of copper and electrolyte. Herein, ITO@Cu2S core-shell nanowire arrays were developed to fabricate CEs for QDSSCs, which have no such issues in Cu/Cu2S CEs. These nanowire arrays exhibited small charge transfer resistance and sheet resistance, and provided more active catalytic sites and easy accessibility for electrolyte due to the three-dimensional structure upon use as CEs. More interestingly, it was found that the interface of ITO/Cu2S significantly affected the performance of ITO@Cu2S nanowire array CEs. By varying synthetic methods, a series of ITO@Cu2S nanowire arrays were prepared to investigate the influence of ITO/Cu2S interface on their performance. The results showed that ITO@Cu2S nanowire array CEs with a continuous Cu2S nanocrystal shell fabricated via an improved cation exchange route exhibited excellent and thickness-dependent performance. The PCE of corresponding QDSSCs increased by 11.6 and 16.5% compared to that with the discrete Cu2S nanocrystal and the classic Cu/Cu2S CE, respectively, indicating its promising potential as a new type of CE for QDSSCs.

Understanding the Thickness-Dependent Performance of Organic Bulk Heterojunction Solar Cells: The Influence of Mobility, Lifetime, and Space Charge

We investigate the reasons for the dependence of photovoltaic performance on the absorber thickness of organic solar cells using experiments and drift-diffusion simulations. The main trend in photocurrent and fill factor versus thickness is determined by mobility and lifetime of the charge carriers. In addition, space charge becomes more and more important the thicker the device is because it creates field free regions with low collection efficiency. The two main sources of space-charge effects are doping and asymmetric mobilities. We show that for our experimental results on Si-PCPDTBT:PC71BM (poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,50-diyl]:[6,6]-phenyl C71-butyric acid methyl ester) solar cells, the influence of doping is most likely the dominant influence on the space charge and has an important effect on the thickness dependence of performance.

Debye Length and Active Layer Thickness-Dependent Performance Variations of Amorphous Oxide-Based TFTs

We analyzed the active layer thickness-dependent performance variations of amorphous oxide-based semiconductor thin-film transistors (AOS TFTs), which are typically operated in depletion mode by using an ATLAS 2-D device simulator. The negative shift of threshold voltage was originated from increasing the amount of intrinsic carrier as active layer thickness is increased. On the contrary, off-current level was a function of Debye length, which is in inverse proportion to the square root of carrier density and the amount of valence band deep states, as well as active layer thickness. Therefore, the relation between Debye length and active layer thickness determines the off-current level, which also enables to explain the off-current behavior of AOS TFTs under light illumination.

Zinc cadmium oxide thin film transistors fabricated at room temperature

Zinc cadmium oxide (ZnCdO) transparent thin film transistors (TFTs) have been fabricated with a back-gate structure using highly p-type Si (001) substrate. For the active channel, 30 nm, 50 nm, and 100 nm thick ZnCdO thin films were grown by pulsed laser deposition. The ZnCdO thin films were wurtzite hexagonal structure with preferred growth along the (002) direction. All the samples were found to be highly transparent with an average transmission of about 80%~ in the visible range. We have investigated the change of the performance of ZnCdO TFTs as the thickness of the active layer is increased. The carrier concentration of ZnCdO thin films has been confirmed to be increased from 10 16 to 10 19 cm −3 as the film thickness increased from 30 to 100 nm. Base on this result, the ZnCdO TFTs show a thickness-dependent performance which is ascribed to the carrier concentration in the active layer. The ZnCdO TFT with 30 nm active layer showed good off-current characteristic of below ~ 10 11, threshold voltage of 4.69 V, a subthreshold swing of 4.2 V/decade, mobility of 0.17 cm 2/V s, and on-to-off current ratios of 3.37 × 10 4.

Cathode thickness-dependent tolerance to Cr-poisoning in solid oxide fuel cells

This work aimed to set new guidelines for the quantification of Cr accumulation in solid oxide fuel cell cathodes after operation, and enabled to pinpoint a diffusion-controlled tolerance to Cr-poisoning for increased cathode thickness; the additional cathode material decreases the deposition rate from Cr vapor species in the active layer. These experimentally based findings were obtained by direct comparison of cathode performances measured on a segmented test arrangement enabling the independent control of four cathodes, with different thicknesses, on an anode-support. The cathode thickness-dependent performance degradation was correlated to deliberate poisoning by volatile Cr species stemming from the test arrangement.

Indium Oxide Thin-Film Transistors Fabricated by RF Sputtering at Room Temperature

Thin-film transistors (TFTs) were fabricated using an indium oxide (In2O3) thin film as the n-channel active layer by RF sputtering at room temperature. The TFTs showed a thickness-dependent performance in the range of 48-8 nm, which is ascribed to the total carrier number in the active layer. Optimum device performance at 8-nm-thick In2O3 TFTs had a field-effect mobility of 15.3 cm2 · V-1 · s-1, a threshold voltage of 3.1 V, an ON-OFF current ratio of 2.2 × 108, a subthreshold gate voltage swing of 0.25 V · decade-1, and, most importantly, a normally OFF characteristic. These results suggest that sputter-deposited In2O3 is a promising candidate for high-performance TFTs for transparent and flexible electronics.

Thermochromic VO2 Thin Films: Solution-Based Processing, Improved Optical Properties, and Lowered Phase Transformation Temperature

This paper describes a solution-phase synthesis of high-quality vanadium dioxide thermochromic thin films. The films obtained showed excellent visible transparency and a large change in transmittance at near-infrared (NIR) wavelengths before and after the metal-insulator phase transition (MIPT). For a 59 nm thick single-layer VO(2) thin film, the integral values of visible transmittance (T(int)) for metallic (M) and semiconductive (S) states were 54.1% and 49.1%, respectively, while the NIR switching efficiencies (DeltaT) were as high as 50% at 2000 nm. Thinner films can provide much higher transmittance of visible light, but they suffer from an attenuation of the switching efficiency in the near-infrared region. By varying the film thickness, ultrahigh T(int) values of 75.2% and 75.7% for the M and S states, respectively, were obtained, while the DeltaT at 2000 nm remained high. These results represent the best data for VO(2) to date. Thicker films in an optimized range can give enhanced NIR switching efficiencies and excellent NIR blocking abilities; in a particularly impressive experiment, one film provided near-zero NIR transmittance in the switched state. The thickness-dependent performance suggests that VO(2) will be of great use in the objective-specific applications. The reflectance and emissivity at the wavelength range of 2.5-25 microm before and after the MIPT were dependent on the film thickness; large contrasts were observed for relatively thick films. This work also showed that the MIPT temperature can be reduced simply by selecting the annealing temperature that induces local nonstoichiometry; a MIPT temperature as low as 42.7 degrees C was obtained by annealing the film at 440 degrees C. These properties (the high visible transmittance, the large change in infrared transmittance, and the near room-temperature MIPT) suggest that the current method is a landmark in the development of this interesting material toward applications in energy-saving smart windows.

Thickness dependent performance of Na0.5K0.5NbO3/sapphire thin film varactors

Perfect c-axis oriented Na0.5K0.5NbO3 (NKN) films have been pulsed laser deposited on Al2O3(0112) single crystals (r-cut sapphire) for voltage tunable microwave device applications. Thickness dependence of dielectric performance of the NKN/sapphire interdigital capacitors (IDCs) has been studied. 40 V bias tunability and dielectric loss tan δ of 4 μm slot IDCs have been found to be 24.6 % and 2.86 % for 1.2 μm thick NKN film, and 6.1 % and 0.83 % for 0.14 μm thick NKN film, respectively. Low leakage currents and high breakdown voltages are observed in these structures.