Back Electrode Research Articles

Fabrication of Organic Solar Cells with Branched Cauliflower-Like Nano Structures as a Back Electrode Replicated from a Natural Template of Cicada Wing Patterns

Fabrication of Organic Solar Cells with Branched Cauliflower-Like Nano Structures as a Back Electrode Replicated from a Natural Template of Cicada Wing Patterns

A novel MoS2-based hybrid film as the back electrode for high-performance thin film solar cells

A MoS2-based complex film was developed with a high work function and thickness-dependent conductivity, which greatly improved the CdTe cell efficiency.

Optimizing CdTe–metal interfaces for high performance solar cells

CdTe is widely applied in thin film solar cells as a p-type layer, which is usually in contact with a metal back electrode.

Critical Impact of Hole Transporting Layers and Back Electrode on the Stability of Flexible Organic Photovoltaic Module

Properties of hole transporting layers (HTLs) and back electrode are very critical to the stability of inverted bulk heterojunction organic photovoltaic (OPV) modules. Here, various deposition methods for back electrodes and materials of HTLs are examined by applying to inverted organic solar cells with a structure of indium tin oxide/ZnO/photoactive layer/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Ag. The experiment is performed on encapsulated modules with flexible barrier films under accelerated conditions. The OPV modules with screen‐printed Ag electrodes are shown to be electrically unstable with a reduction of the current density under damp heat condition at 85 °C/85% RH. Optical images for the active layer/PEDOT:PSS interface reveal that a reaction between the solvent from the Ag electrode and the underlying layers is the major cause for the degradation. In comparison with materials of the HTLs, the PEDOT:PSS layer shows low stability compared to the MoO3 layer under the accelerated conditions. Unusual chemical changes in the PEDOT:PSS film are observed through X‐ray photoelectron spectroscopy and this is further addressed by correlating the stability of the OPV devices.

Efficiency improvement of polymer solar cells with random micro-nanostructured back electrode formed by active layer self-aggregation

The formation of a micro-nanostructured back electrode provides an efficient route for enhancing light absorption in polymer solar cells by light scattering of the bumpy electrode. In this study, we incorporated propylene glycol mono-methyl ether acetate (PGMEA) into the poly (3-hexylthiophene) (P3HT) and [6:6]-phenyl-C61-butyric acid (PC61BM) solution, and the PGMEA induced P3HT aggregations give rise to a bumpy surface of the active layer. The sequential deposition of the Al electrode onto the active layer creates a polymer solar cell with interpenetrated micro-nanostructured morphology of the active layer/electrode interface. The higher crystallinity of P3HT induced by active layer self-aggregation improves carrier mobility. The bumpy active layer/electrode interface can not only facilitate charge carriers transfer and collection in the device, but also enhance optical scattering and leads to enhanced light absorption of the active layer. The resulting device shows improved photocurrent, corresponding to power conversion efficiency improvement of 17.9% as compared to the planar device. This work indicates that the active layer self-aggregation is a simple, cost-effective and mold-free methodology to manufacture high performance polymer solar cells with the micro-nanostructured back electrode.

Probing the electrical impedance of thin films on a quartz crystal microbalance (QCM), making use of frequency shifts and piezoelectric stiffening.

Using a temperature-responsive polymer film as an example, it was shown that a conventional quartz crystal microbalance (QCM) can probe a sample's electrical properties in addition to its thickness and softness. The film's electrical impedance was accessed by alternating between the driving voltage being applied to the front electrode and the back electrode. The opposing electrode was grounded in both cases. In the first configuration, the electrical properties of the sample do have an influence on the resonance frequency because of piezoelectric stiffening. In the second, they do not. Using this scheme, it was monitored how the electrical impedance of a film composed of a mixture of poly-N-isopropylacrylamide and polyvinylalcohol changes when the film swells and deswells.

The effects of electron and hole transport layer with the electrode work function on perovskite solar cells

The effects of electron and hole transport layer with the electrode work function on perovskite solar cells with the interface defects were simulated by using analysis of microelectronic and photonic structures-one-dimensional (AMPS-1D) software. The simulation results suggest that TiO2 electron transport layer provides best device performance with conversion efficiency of 25.9% compared with ZnO and CdS. The threshold value of back electrode work function for Spiro-OMeTAD, NiO, CuI and Cu2O hole transport layer are calculated to be 4.9, 4.8, 4.7 and 4.9 eV, respectively, to reach the highest conversion efficiency. The mechanisms of device physics with various electron and hole transport materials are discussed in details. The device performance deteriorates gradually as the increased density of interface defects located at ETM/absorber or absorber/HTM. This research results can provide helpful guidance for materials and metal electrode choice for perovskite solar cells.

Low Power FDSOI Technology and Devices for RF Applications

: We report RF Characteristics in a Low Power High Performance Ultra-thin Body and Box (UTBB) FDSOI technology. The technology features Si-Channel nFETs and SiGe-Channel pFETS, a gate first HiKMG stack with a gate length (LG) of 20nm, BOX thickness (TBOX) of 20nm and is similar to that described previously[1]. The use of the backgate gate is shown to dynamically change the operating range and expand the dynamic range for RF. Competitive f T s are reported for nFETs. The thin body architecture demonstrates high self gain. The technology is shown to be well optimized for applications like IoT where low power and RF performance are a premium. UTBB FDSOI technology offers a superior technology for IoT because of the low power performance of the technology and RF performance. A TEM micrograph of the nFET silicon channel and pFET SiGe channel is sown in Figure 1. The high K metal Gate, thin channel silicon and SiGe channel, and raised source drains architecture can be seen clearly seen in the tem Micrographs. The FDSOI transistor has a back gate electrode which provides a unique capability to shift the threshold voltage and operating point of the transistor. This capability is shown in Fig. 2 and Fig. 3 which is a linear plot of Id versus Vgs for Vbg = -2, 0, and 2 Volt bias for the nFET and pFET respectively. The Drain current is well behaved and is shown to shift as the back bias changes the threshold voltage. The digital low power performance of the technology is derived by the absence of drain to source leakage path and a steep subthreshold slope. The thin silicon channel and SiGe channel of 7nm prevents a leakage path and provides a fully depleted channel with superior electrostatics The technology Idsat current reaches 900μA/μm and 670μA/μm for NFET and PFET respectively, at off current (Ioff) of 100nA/μm and Vdd of 0.8V. The Log Id vs Vgs plots for the nFET and pFET are shown in Fig. 4 at a gate length of 20nm. The subthreshold slope of 85 mV/decade demonstrates the steep subthreshold that enables a low threshold voltage to be maintained with low leakage. The output characteristic for nFET Lg=30nm is shown in Fig. 5. The flat shape of the curve also indicates both nFET has low gDs. Out conductance curves (not shown here) show no roll-off at high Vd indicating little if any self heating. In this FDSOI technology the nFET tansconductance reaches gM ~ 1.8mS/um at Lg=20nm and the pFET transconductance reaches gMsat ~ 1.2.mS/um. The gMsat operating point may be adjusted with back gate bias as is shown in Fig. 6 for the nFET with a back gate bias with -2, 0, and 2 volts. The ratio of gMsat/gDS is the self gain wich is enhanced in fully depleted FDSOI technology. The self gain for the nFET is shown in Figure 7 with a back gate bias with -2, 0, and 2 volts. RF Performance is extremely important for IoT and other applications. The RF simulated RF performance of the nFET achieves a peak f T of over 300 GHz as is shown in Figure 8 which plots f T vs Vgs for Vbg = -2, 0, and 2 Volts. Note how the operating point can be moved around just as in the gMsat figure by applying the appropriate back gate bias. This expanded dynamic range is also true for the pFET operation. These are the first FDSOI RF characterization results published on a 20nm Lgate HiKMG process.

The Study of Optical and Electrical Properties of Short-Pitch Plasmonic Solar Cells

Metallic gratings have drawn great attention over the past few years due to effective light trapping and power conversion efficiency enhancement in organic solar cells (OSCs). Novel OSCs incorporated short-pitch metallic grating into the conventional OSCs as the back electrode is presented. The optical and electrical properties of OSCs are investigated by applying the finite-difference frequency-domain method. It is found that the light absorption at 400–650 nm in the active layer is strongly improved, compared with the standard structure without metallic grating. The underlying physical mechanism is due to the surface plasmon resonance and the interaction between two neighbor gratings, which undoubtedly enhances the electrical properties such as short-circuit current $(J_{\mathrm{SC}})$ , fill factor, and power conversion efficiency. Meanwhile, the optical and electrical properties of OSCs with different groove widths are also investigated. With an optimal groove width of 4 nm, the overall performance of OSCs is enhanced. This paper indicates that OSCs incorporated short-pitch metallic grating are promising candidates for high-performance OSCs.

Ohmic-like contact formation at the rear interface between Cu(In,Ga)Se2 and ZnO:Al in a lift-off Cu(In,Ga)Se2 solar cell

A lift-off Cu(In,Ga)Se2 (CIGS) solar cell, which could serve as the top solar cell in a tandem-type solar cell, was fabricated with a superstrate-type structure consisting of soda-lime glass (SLG)/epoxy/Al/n-type ZnO:Al (AZO)/ZnO/CdS/p-type CIGS/n-type AZO (back electrode)/Al. This study proposes a method of realizing the ohmic-like contact between p-type CIGS and n-type AZO in a lift-off CIGS solar cell to improve the conversion efficiency (η). The sputtering conditions under which the n-type AZO layer was deposited, specifically the sample position and radio frequency (RF) power density, were optimized to form ohmic-like contact through trap-assisted tunneling recombination. The sputtering damage near the CIGS surface that induced the trap-assisted tunneling recombination was evaluated by examining the photoluminescence peak intensity. Ultimately, ohmic-like characteristics were attained for the p-type CIGS/n-type AZO interface under optimal AZO deposition conditions, with the sample positioned 0cm from the AZO target center and an RF power density of 2.4W/cm2. The proposed method was successfully applied to the fabrication of a lift-off CIGS solar cell. Consequently, an η value of 9.2% was obtained for the lift-off CIGS solar cell, which is 70% that of a substrate-type CIGS solar cell.

Trap-assisted recombination for ohmic-like contact at p-type Cu(In,Ga)Se2/back n-type TCO interface in superstrate-type solar cell

Cu(In,Ga)Se2 (CIGS) solar cells with superstrate-type structure of soda-lime glass (SLG)/epoxy/Al/ZnO:Al (AZO)/ZnO/CdS/CIGS/back n-type transparent conductive oxide (TCO) electrode/Al are fabricated by lift-off process. AZO or In2O3:Sn (ITO) is used as the back n-type TCO electrode. Ohmic-like contact between p-type CIGS and n-type D-TCO (damage-TCO), namely, D-AZO or D-ITO, is formed through the trap-assisted recombination. The D-TCO, meaning TCO with high sputtering damage on the CIGS surface, is prepared under the optimization of its deposition condition, namely, the power density of 2.4 W/cm2 for D-AZO or 3.3 W/cm2 for D-ITO, for high defect density on the CIGS surface to promote the trap-assisted recombination. Ultimately, the superstrate-type CIGS solar cell with a bi-layer of D-AZO/AZO as back n-type TCO electrode with conversion efficiency (η) of 9.2% is achieved, which is 70% of η of the substrate-type CIGS solar cell before lift-off process. The bi-layer of D-AZO/AZO is utilized owing to high resistivity of D-AZO (about 0.1 Ω cm). On the other hand, the superstrate-type CIGS solar cell with D-ITO as the back n-type TCO electrode with η of 10.4% is attained, which is 93.7% of η of the substrate-type CIGS solar cell, where the resistivity of the D-ITO layer is low at about 5.0 × 10−3 Ω cm.

New advances in small molecule hole-transporting materials for perovskite solar cells

Organic π-functional molecules are the foundation and basic component of organic optoelectronic devices. For example, for ideal carrier transporting materials, extended π-conjugation and ordered π–π stacking are necessary to enhance the charge mobility and achieve desirable results. As a promising way to convert sunlight into electricity, organometal halide perovskite solar cells (PSCs) have captured a lot of attention due to its predominant merits especially in the aspect of remarkable photovoltaic performance and much potentially low production cost. For conventional planar PSC structure, hole-transporting layer which typically consists of organic π-functional materials plays a key role in suppressing hole–electron pair recombination, promoting charge transporting and ensuring ohmic contact of back electrode. Considering the key roles of HTMs and its soaring progress in recent years, here, we will summarize recent progress in small organic π-functional materials from its diverse functions in PSCs. Besides, aiming to further promote the development of organic π-functional molecules and HTMs, a promising direction toward highly efficient HTMs will also be discussed.

'Squeezing' near-field thermal emission for ultra-efficient high-power thermophotovoltaic conversion.

We numerically demonstrate near-field planar ThermoPhotoVoltaic systems with very high efficiency and output power, at large vacuum gaps. Example performances include: at 1200 °K emitter temperature, output power density 2 W/cm2 with ~47% efficiency at 300 nm vacuum gap; at 2100 °K, 24 W/cm2 with ~57% efficiency at 200 nm gap; and, at 3000 °K, 115 W/cm2 with ~61% efficiency at 140 nm gap. Key to this striking performance is a novel photonic design forcing the emitter and cell single modes to cros resonantly couple and impedance-match just above the semiconductor bandgap, creating there a ‘squeezed’ narrowband near-field emission spectrum. Specifically, we employ surface-plasmon-polariton thermal emitters and silver-backed semiconductor-thin-film photovoltaic cells. The emitter planar plasmonic nature allows for high-power and stable high-temperature operation. Our simulations include modeling of free-carrier absorption in both cell electrodes and temperature dependence of the emitter properties. At high temperatures, the efficiency enhancement via resonant mode cross-coupling and matching can be extended to even higher power, by appropriately patterning the silver back electrode to enforce also an absorber effective surface-plasmon-polariton mode. Our proposed designs can therefore lead the way for mass-producible and low-cost ThermoPhotoVoltaic micro-generators and solar cells.

Design and fabrication of capacitive nanosensor based on MOF nanoparticles as sensing layer for VOCs detection

Due to high porosity and increased surface area; metal organic frameworks (MOFs) have been widely used in gas storage applications as well as volatile organic compounds (VOCs) detection. Most of these sensors are electromechanical or electrochemical based; however there are few works in which capacitive sensors were developed using micrometer MOFs. In the present study, for the first time, MOF (Cu-BTC) nanoparticles were used to develop capacitive sensor device for detecting VOCs (e.g. methanol, ethanol, isopropanol, and acetone). The studied capacitive sensors were performed in a moderate environment (10% relative humidity and 25°C). Capacitive sensors were fabricated in a sandwich form or parallel plate by using copper plate as a back electrode, MOF nanoparticles layer as the dielectric, and interconnected silver spots as the upper electrode of the capacitor. Linearity of the response versus LCR meter frequency, reusability, reversibility, response time, and limit of detection (LOD) of the capacitive sensor were determined to evaluate the sensor performance. The results showed high ability of Cu-BTC nanoparticles synthesized as dielectric layer of a capacitive nanosensor to determine VOCs.

Cu2Sn1−xGexS3 solar cells fabricated with a graded bandgap structure

We fabricated Cu2Sn1−xGexS3 (CTGS) solar cells with a graded bandgap structure in order to improve their photovoltaic performance. Bandgap gradation was formed by changing the Ge/Sn ratio in the depth direction of the CTGS layers. The composition profile of each sample was measured by secondary ion mass spectrometry, and we confirmed that the Ge/Sn ratio near the buffer layer was lower than that near the back electrode. This means that the bandgap increases with depth from the surface. The performance of the cells was improved to over 6.7% in conversion efficiency.

Thermally activated hysteresis in high quality graphene/h-BN devices

We report on gate hysteresis of resistance in high quality graphene/hexagonal boron nitride (h-BN) devices. We observe a thermally activated hysteretic behavior in resistance as a function of the applied gate voltage at temperatures above 375 K. In order to investigate the origin of the hysteretic phenomenon, we compare graphene/h-BN heterostructure devices with SiO2/Si back gate electrodes to devices with graphite back gate electrodes. The gate hysteretic behavior of the resistance is present only in devices with an h-BN/SiO2 interface and is dependent on the orientation of the applied gate electric field and sweep rate. We describe a phenomenological model which captures all of our findings based on charges trapped at the h-BN/SiO2 interface. Such hysteretic behavior in graphene resistance must be considered in high temperature applications for graphene devices and may open new routes for applications in digital electronics and memory devices.

DSOI—a novel structure enabling adjust circuit dynamically

A double silicon on insulator (DSOI) structure was introduced based on fully depleted SOI (FDSOI) technology. The circuit performance could be adjusted dynamically through the separate back gate electrodes applied to N-channel and P-channel devices. Based on DSOI ring oscillator (OSC), this paper focused on the theoretical analysis and electrical test of how the OSC's frequency being influenced by the back gate electrodes (soi2n, soi2p). The testing results showed that the frequency and power consumption of OSC could change nearly linearly along with the back gate bias. According to the different requirements of the circuit designers, the circuit performance could be improved by positive soi2n and negative soi2p, and the power consumption could be reduced by negative soi2n and positive soi2p. The best compromise between performance and power consumption of the circuit could be achieved by appropriate back gate biasing.

Performance ratio study based on a device simulation of a 2D monolithic interconnected Cu(In,Ga)(Se,S)2 solar cell

We use a calibrated realistic 2D device simulation structure of a single cell from a monolithic interconnected Cu(In,Ga)(Se,S)2 solar module to investigate the impact of modifications of the patterning laser scribe P1 and the trench properties on the energy yield. This particularly includes examining the change of the low light behaviour and the performance ratio (PR). In order to study the effects of the P1 characteristics, we performed a variation of the P1 width, the absorber doping concentration within the P1 trench as well as a variation of the Mo(Se,S)2 band gap and the doping concentration. We simulated temperature-dependent current-voltage characteristics for different irradiances and apply the weather data of three different locations to calculate the PRs.An increased P1 width and decreased absorber doping concentration was found to increase the P1 (parallel) shunt resistance due to a reduced horizontal hole current component between the separated back electrodes. A higher shunt resistance enhances the low light behaviour of the solar cell and consequently leads to an improvement of the PR. A decreased Mo(Se,S)2 band gap and doping concentration increases the valence band barrier at the Cu(In,Ga)(Se,S)2/Mo(Se,S)2 interface and we obtained a moderate P1 shunt improvement accompanied by a strong PR enhancement. The increased barrier height enhanced the low light and temperature behaviour of the fill factor finally leading to the PR enhancement.

Characteristics of Sputtered ZnO Thin Films for an Inverted Organic Solar Cell.

Several research groups have claimed high energy conversion efficiency in organic solar cells. However, it still has low efficiency and is unstable, because organic materials are easily oxidized by atmospheric humidity and UV light. In this work, ZnO thin film as the blocking layer attributed to the interference of the injection of the hole from the P3HT and no charge carrier recombination. We obtained the maximum power conversion efficiency of 1.9% under AM 1.5 G spectral illumination of 100 MWcm(-2), when we used a ZnO film of 60 nm and the optimized P3HT:PCBM, and Au as the back electrode to solve the reaction problem of Al electrode and to control the work function between the HOMO level of P3HT and the energy level of the metal electrode. Power conversion efficiency of inverted organic solar cell (IOSC) is significantly dependent on the thickness of the ZnO thin film deposited by unbalanced magnetron sputtering method. Also, the stability of IOSC is measured under ambient conditions.

Graphene based plasmonic terahertz amplitude modulator operating above 100 MHz

The terahertz (THz) region of the electromagnetic spectrum holds great potential in many fields of study, from spectroscopy to biomedical imaging, remote gas sensing, and high speed communication. To fully exploit this potential, fast optoelectronic devices such as amplitude and phase modulators must be developed. In this work, we present a room temperature external THz amplitude modulator based on plasmonic bow-tie antenna arrays with graphene. By applying a modulating bias to a back gate electrode, the conductivity of graphene is changed, which modifies the reflection characteristics of the incoming THz radiation. The broadband response of the device was characterized by using THz time-domain spectroscopy, and the modulation characteristics such as the modulation depth and cut-off frequency were investigated with a 2.0 THz single frequency emission quantum cascade laser. An optical modulation cut-off frequency of 105 ± 15 MHz is reported. The results agree well with a lumped element circuit model developed to describe the device.