Shunt Leakage Research Articles

Cu2ZnSnSe4 Thin Film Solar Cell with Depth Gradient Composition Prepared by Selenization of Sputtered Novel Precursors

In this study, we proposed a new method for the synthesis of the target material used in a two stage process for preparation of a high quality CZTSe thin film. The target material consisting of a mixture of CuxSe and ZnxSn1-x alloy was synthesized, providing a quality CZTSe precursor layer for highly efficient CZTSe thin film solar cells. The CZTSe thin film can be obtained by annealing the precursor layers through a 30 min selenization process under a selenium atmosphere at 550 °C. The CZTSe thin films prepared by using the new precursor thin film were investigated and characterized using X-ray diffraction, Raman scattering, and photoluminescence spectroscopy. It was found that diffusion of Sn occurred and formed the CTSe phase and CuxSe phase in the resultant CZTSe thin film. By selective area electron diffraction transmission electron microscopy images, the crystallinity of the CZTSe thin film was verified to be single crystal. By secondary ion mass spectroscopy measurements, it was confirmed that a double-gradient band gap profile across the CZTSe absorber layer was successfully achieved. The CZTSe solar cell with the CZTSe absorber layer consisting of the precursor stack exhibited a high efficiency of 5.46%, high short circuit current (JSC) of 37.47 mA/cm2, open circuit voltage (VOC) of 0.31 V, and fill factor (F.F.) of 47%, at a device area of 0.28 cm2. No crossover of the light and dark current-voltage (I-V) curves of the CZTSe solar cell was observed, and also, no red kink was observed under red light illumination, indicating a low defect concentration in the CZTSe absorber layer. Shunt leakage current with a characteristic metal/CZTSe/metal leakage current model was observed by temperature-dependent I-V curves, which led to the discovery of metal incursion through the CdS buffer layer on the CZTSe absorber layer. This leakage current, also known as space charge-limited current, grew larger as the measurement temperature increased and completely overwhelmed the diode current at a measurement temperature of 200 °C. This is due to interlayer diffusion of metal that increases the shunt leakage current and decreases the efficiency of the CZTSe thin film solar cells.

Crystalline Engineering Toward Large-Scale High-Efficiency Printable Cu(In,Ga)Se2 Thin Film Solar Cells on Flexible Substrate by Femtosecond Laser Annealing Process

Ink-printing method emerges as a viable way for manufacturing large-scale flexible Cu(In,Ga)Se2 (CIGS) thin film photovoltaic (TFPV) devices owing to its potential for the rapid process, mass production, and low-cost nonvacuum device fabrication. Here, we brought the femtosecond laser annealing (fs-LA) process into the ink-printing CIGS thin film preparation. The effects of fs-LA treatment on the structural and optoelectronic properties of the ink-printing CIGS thin films were systematically investigated. It was observed that, while the film surface morphology remained essentially unchanged under superheating, the quality of crystallinity was significantly enhanced after the fs-LA treatment. Moreover, a better stoichiometric composition was achieved with an optimized laser scanning rate of the laser beam, presumably due to the much reduced indium segregation phenomena, which is believed to be beneficial in decreasing the defect states of InSe, VSe, and InCu. Consequently, the shunt leakage current and recombination centers were both greatly decreased, resulting in a near 20% enhancement in photovoltaic conversion efficiency.

Breakthrough to Non-Vacuum Deposition of Single-Crystal, Ultra-Thin, Homogeneous Nanoparticle Layers: A Better Alternative to Chemical Bath Deposition and Atomic Layer Deposition.

Most thin-film techniques require a multiple vacuum process, and cannot produce high-coverage continuous thin films with the thickness of a few nanometers on rough surfaces. We present a new ”paradigm shift” non-vacuum process to deposit high-quality, ultra-thin, single-crystal layers of coalesced sulfide nanoparticles (NPs) with controllable thickness down to a few nanometers, based on thermal decomposition. This provides high-coverage, homogeneous thickness, and large-area deposition over a rough surface, with little material loss or liquid chemical waste, and deposition rates of 10 nm/min. This technique can potentially replace conventional thin-film deposition methods, such as atomic layer deposition (ALD) and chemical bath deposition (CBD) as used by the Cu(In,Ga)Se2 (CIGS) thin-film solar cell industry for decades. We demonstrate 32% improvement of CIGS thin-film solar cell efficiency in comparison to reference devices prepared by conventional CBD deposition method by depositing the ZnS NPs buffer layer using the new process. The new ZnS NPs layer allows reduction of an intrinsic ZnO layer, which can lead to severe shunt leakage in case of a CBD buffer layer. This leads to a 65% relative efficiency increase.

Eliminating the excess Cu x Se phase in Cu-rich Cu(In,Ga)Se 2 by In 2 Se 3 treatment

Abstract The residual Cu x Se phase results in shunt leakage paths in Cu-rich Cu(In,Ga)Se 2 (CIGS) which can cause serious degradation in the conversion efficiency. The conductive atom force microscopy(C-AFM) was employed to obtain a direct evidence of shunt leakage paths in the Cu-rich CIGS thin film. The results reveal that Cu x Se phase exists not only near the surface but also inside the Cu-rich CIGS thin films. In order to eliminate the residual Cu x Se, it is recommended to deposit a thin layer of In 2 Se 3 on Cu-rich CIGS with subsequent annealing treatment (In 2 Se 3 treatment for short). The In 2 Se 3 treatment can effectively remove Cu x Se phase as well as the shunt leakage paths in Cu-rich CIGS thin films and improve the efficiency of Cu-rich CIGS based solar cells from 1.2% to 9.5%.

High Short-Circuit Current Density in CIS Solar Cells by a Simple Two-Step Selenization Process With a KF Postdeposition Treatment

In this work, we present a CuInSe2 (CIS) thin-film solar cell with 14.7% efficiency and 40.2-mA/cm 2 short-circuit current. The samples were fabricated by the selenization of stacked elemental layers under selenium vapor in atmospheric ambient within a simple graphite box. Some samples were treated with a postdeposition treatment (PDT) with potassium fluoride (KF), whereas some were not, thus allowing for useful comparison. The use of the KF PDT led to two interesting improvements on our samples: formation of a hole-blocking layer between CdS/CIS interface, which reduces the tunneling recombination and significantly increases the open-circuit voltage. In addition, the KF PDT clearly reduces the symmetry of the nonohmic shunt leakage at low forward bias and reverse bias, which can be explained by a space-charge-limited current model, indicative of metal–semiconductor–metal (MSM) shunts. Discussion is presented to understand possible defects introduced during our processing steps and the physical origin of the shunt paths and why they might be of the MSM type. New directions to improve the performance of CIS solar cells are presented for future work using this very simple fabrication technique.

Small-area Si Photovoltaics for Low-Flux Infrared Energy Harvesting.

Silicon photovoltaics are prospective candidates to power mm-scale implantable devices. These applications require excellent performance for small-area cells under low-flux illumination condition, which is not commonly achieved for silicon cells due to shunt leakage and recombination losses. Small area (1-10 mm2) silicon photovoltaic cells are studied in this work, where performance improvements are demonstrated using a surface n-type doped emitter and Si3N4 passivation. A power conversion efficiency of more than 17% is achieved for 660 nW/mm2 illumination at 850 nm. The silicon cells demonstrate improved power conversion efficiency and reduced degradation under low illumination conditions in comparison to conventional crystalline silicon photovoltaic cells available commercially.

Energy Harvesting for GaAs Photovoltaics Under Low-Flux Indoor Lighting Conditions.

GaAs photovoltaics are promising candidates for indoor energy harvesting to power small-scale (≈1 mm2) electronics. This application has stringent requirements on dark current, recombination, and shunt leakage paths due to low-light conditions and small device dimensions. The power conversion efficiency and the limiting mechanisms in GaAs photovoltaic cells under indoor lighting conditions are studied experimentally. Voltage is limited by generation-recombination dark current attributed to perimeter sidewall surface recombination based on the measurements of variable cell area. Bulk and perimeter recombination coefficients of 1.464 pA/mm2 and 0.2816 pA/mm, respectively, were extracted from dark current measurements. Resulting power conversion efficiency is strongly dependent on cell area, where current GaAs of 1-mm2 indoor photovoltaic cells demonstrates power conversion efficiency of approximately 19% at 580 lx of white LED illumination. Reductions in both bulk and perimeter sidewall recombination are required to increase maximum efficiency (while maintaining small cell area near 1 mm2) to approach the theoretical power conversion efficiency of 40% for GaAs cells under typical indoor lighting conditions.

Amorphous Silicon Photovoltaic Modules on Flexible Plastic Substrates

ABSTRACTThis paper reports on a monolithic 10 cm × 10 cm area PV module integrating an array of 72 a-Si:H n-i-p cells on a 100 μm thick polyethylene-naphtalate substrate. The n-i-p stack is deposited using a PECVD system at 150 °C substrate temperature. The design optimization and device performance analysis are performed using a two-dimensional distributed circuit model of the photovoltaic cell. The circuit simulator SPICE is used to calculate current and potential distributions in a network of sub-cell circuits, and also to map Joule losses in the front TCO electrode and the metal grid. Experimental results show that the shunt leakage is one of the factors reducing the device performance. Current-voltage characteristics of individual a-Si:H p-i-n cells were analyzed to estimate a variation of shunt resistances. Using the LBIC technique, the presence of multiple shunts in the n-i-p cell was detected. To understand the nature of electrical shunts, the change in the surface roughness of all device layers was analyzed throughout fabrication process. It is found that surface defects in plastic foils, which are thermally induced during the device fabrication, form microscopic pinholes filled with highly conductive top electrode material.

Development of a new thin film composition for SnS solar cell

The work is devoted to the development of a new thin film composition for SnS solar cell based on semiconductor heterojunction n-Zn(O,S)/p-SnS by the combination of two low-cost liquid methods, namely electrodeposition and Successive Ionic Layer Adsorption and Reaction (SILAR). Structure and optical properties of the obtained layers have been investigated. The p-SnS absorber is fabricated by combining the two methods. The creation of the main SnS film was carried out by sulfurization of the electrodeposited Sn thin film. The elimination of the shunt leakage was made by SILAR deposition of nanocrystalline SnS into pores of the main SnS. All SnS films are single-phase and consisted of tin monosulfide with orthorhombic structure of Herzenbergite. Wide band gap and transparent buffer layer n-Zn(O,S) is created by air annealing of ZnS thin film prepared by SILAR. Series and shunt resistances as well as a height of the rectifying barrier of this n-Zn(O,S)/p-SnS heterostructure were investigated due dark current–voltage characteristics. The diode and electronic parameters of this new thin film composition is superior ones for the obtained by using tin sulfide absorber produced only by SILAR.

Conduction Mechanism in n-CdSe/p-ZnTe Heterojunction

This work reports on fabrication using vacuum evaporation and characterization of n-CdSe/p-ZnTe heterojunctions. Before forming the junction, CdSe and ZnTe layers were characterized for crystal structure and chemical composition to account for observed electrical properties. The heterojunction was characterized by current–voltage (I–V) measurements, temperature dependence of reverse saturation current, admittance, and capacitance–voltage (C–V) measurements. I–V characteristics of the heterojunction exhibited clear diode nature with rectification ratio of 9.05 at ±0.5 V and ideality factor n = 3.34. From the temperature dependence of the I–V characteristic, a barrier height ϕ b of 0.36 eV was determined for the CdSe–ZnTe junction. Conduction mechanism analysis revealed contributions from both thermionic and space-charge-limited conduction. Furthermore, the shunt leakage current was found to be space-charge limited, showing symmetry in current near V = 0 V. The dependence of capacitance on frequency and bias voltage has been analyzed to identify the bulk and interface defects. These measurements indicate the presence of bulk defects and high series resistance, severely affecting current transport.

Characterization of a-Si:H Solar Cell Modules on Plastic Substrates by High Resolution LBIC Technique

This article reports on characterization of hydrogenated amorphous silicon (a-Si:H) photovoltaic modules fabricated on 100μm thick PEN plastic films. Experimental results show that the shunt leakage is one of the factors reducing the device performance. Current-voltage characteristics of individual a-Si:H p-i-n cells were analysed to estimate a variation of shunt resistances. A SPICE model of the a-Si:H p-i-n cell with local shunt leakage was also developed to analyse the impact of leakage currents on the device performance. Using the LBIC technique, the presence of multiple shunts in the cell was detected. They are attributed to surface defects in plastic foils, which are thermally induced during the device fabrication.

Novel modification of voice prosthesis.

The undesired dilatation of the tracheooesophageal shunt after surgical implantation of voice prosthesis is a typical complication of this procedure. Temporary removal of the prosthesis and reinsertion after a short period of time is a first-line therapeutical option aiming shrinkage of the shunt. Failure of this measure generally is an indication of revision surgery. We present first experiences treating leakage problems with novel modified voice prosthesis without surgical intervention in specified cases. 11 patients (1 female, 10 male) aging between 51 and 71 years were presented with shunt leakage between 11/2008 and 11/2012 in the ENT-Department of the University Hospital of Homburg/Saar after a custom built voice prosthesis had been used initially successfully. A "Provox 2"(®) voice prosthesis was modified with two discs made of silicone each on the tracheal and oesophageal side and additionally reinforcing the diameter of the prosthesis by a silicone tube. The modified prosthesis was inserted in a retrograde way under general anesthesia, analogical to the approach used with the "Provox 1"(®)-prosthesis. The period of observation ranged between 12 and 48 months. As a measure of control swallowing of methylene blue was used. In all cases leakage suspended. Durability of the modified prosthesis ranged between 2 and 6 months. Neither the patients' complained about, nor did the physicians notice subjectively an impairment of the voice quality. Modifications of "Provox 2"(®)-prosthesis should be regarded in individual cases and constitute a reasonable alternative to revision surgery. A surgical approach is more intricate and costly, more taxing for the patient and susceptible to failure. We regard the necessity of general anesthesia for the insertion of the modified prosthesis as a disadvantage.

<inline-formula> <tex-math notation="TeX">\(640\times 512\) </tex-math></inline-formula> Extended Short Wavelength Infrared In<sub>0.83</sub>Ga<sub>0.17</sub>As Focal Plane Array

We present the detailed characteristics of solid source molecular beam epitaxy (MBE) grown large format ( \(640\times 512\) ) extended short wavelength infrared In0.83Ga0.17As sensor with desirable performance at both pixel and focal plane array (FPA) levels. The FPA pixels in the mesa structure grown on a graded AlInAs buffer layer with 2.65- \(\mu \) m, 300-K cutoff wavelength exhibited 300- and 200-K peak detectivities as high as \(\sim \!\! 2.5\times 10^{10 }\) and \(\sim \!\!1\times 10^{12}\) cmHz \(^{1/2}\) /W, which are both equivalent to the theoretical limits set by the Johnson noise of the detector. Dark current analysis of the pixels displayed no considerable tunneling component with the dark current being dominated by generation-recombination and shunt leakage mechanisms >200 K up to a reverse bias voltage of 3 V. Moreover, the noise measurements displayed no 1/ \(f\) noise in the FPA pixels. In spite of the large lattice mismatch, the FPA yielded very good response linearity, as well as impressively good responsivity nonuniformity and pixel operability of 5.5% and 99.8%, respectively. The results showing the feasibility of both solid source MBE and graded AlInAs buffer for the growth of extended InGaAs photodetectors are very encouraging for the large format FPA implementation of these detectors with desirable imaging performance and an extended cutoff wavelength as high as \(\sim 2.7\) \(\mu \) m.

Extended wavelength SWIR InGaAs focal plane array: Characteristics and limitations

We present the characteristics of large format (640×512) short wavelength infrared (SWIR) InGaAs photodetector focal plane array (FPA) with ∼2.65μm room temperature cut-off wavelength. The detector epilayers were grown with solid source molecular beam epitaxy on InP substrates using a linearly graded InAlAs buffer layer. In spite of the large lattice mismatch, the FPA yields reasonably good responsivity nonuniformity (9%) with a pixel detectivity above 2×1010cmHz1/2/W at room temperature. The dark current of the pixels are dominated by generation–recombination (G–R) and shunt leakage mechanisms with negligible tunneling above 200K up to a reverse bias voltage of 3V. The results are very encouraging for further study toward the optimization of the epilayer structure and growth conditions in order to provide a lower cost technology alternative to HgCdTe in the SWIR band.

Impact of series and shunt resistances in amorphous silicon thin film solar cells

In this paper hydrogenated amorphous silicon (a-Si:H) thin film solar cells have been investigated using advanced simulation tools and the obtained electrical values were compared with experimental results. The analysis considers the effects of series resistance and shunt leakage. The simulation tool is parameterized with standard theoretical models for the density of states in the mobility gap, generation/recombination statistics, and experimental optical data of a-Si:H thin films. Experimental measurements were used to implement a textured TCO. Calculation of the illuminated and dark current voltage characteristics for the initial and stabilized states of the ideal solar cell was done. The ideal device was adjusted to the experimental curves using a non-ohmic model for the shunt leakage (simulated as microscopic pip structures, arisen due to aluminum diffusion) and an ohmic series resistance, finding good correspondence. The simulation shows that these mechanisms reduce the cell’s efficiency in around an absolute 1%.

Interdigitated front contact crystalline silicon solar cell

Interdigitated front-contact front-junction (IFC) silicon solar cell using advanced semiconductor technologies was studied. Narrow width, high aspect-ratio finger was made to reduce optical shadow effects and electrical resistive losses. Precise junction control and surface field passivation by ion implantation were employed. Self-aligned front local base contact was developed using spacer technology to prevent shunt leakage from emitter contact while maximizing emitter area. Total point-contact areal fraction was kept as low as 0.4% to reduce contact recombination and contact resistance was lowered by metal-silicide formation. To reduce optical normal reflection, angle shape metal grid was developed. By using parallel wires between solar cells, 20.1% maximum module efficiency was achieved.

Low-boiling-point solvent additives can also enable morphological control in polymer solar cells

Processing organic photovoltaic (OPV) blend solutions with high-boiling-point solvent additives has recently been used for morphological control in bulk-heterojunction OPV cells. Here we show that even low-boiling-point solvents can be effective additives. When P3HT:PCBM OPV cells were processed with a low-boiling-point solvent tetrahydrafuran as an additive in parent solvent o-dichlorobenzene, charge extraction increased leading to fill factors as high as 69.5%, without low work-function cathodes, electrode buffer layers or thermal treatment. This was attributed to PCBM demixing from P3HT domains and better vertical phase separation, as indicated by photoluminescence lifetimes, hole mobilities, and shunt leakage currents. Dependence on solvent parameters and applicability beyond P3HT system was also investigated.

A look into the origin of shunt leakage current of Cu(In,Ga)Se2 solar cells via experimental and simulation methods

This study investigates how to apply space-charge-limited (SCL) current to describe shunt leakage current in a CIGS solar cell. Possible factors inducing SCL current have been observed through conductive atomic force microscopy (C-AFM), which supports the SCL current theory, describing the shunt current of a CIGS solar cell. In simulations derived from experimental data, deviation of dark IV curves is due to flaws in the real device. These flaws are absent in simulation, but investigation verifies the characteristics of SCL current component's experimental IV curves within shunt leakage current. A device with a metal/CIGS/metal structure could simulate SCL current and confirm its characteristics. Such a simulated structure, representing flaws inserted into a CIGS solar cell, generates the same dark-current behavior revealed in experimental dark IV curves. This study investigates the response of dark current to varying sizes of the flaw within the CIGS solar cell.

Influence of shunt conduction on determining the dominant recombination processes in CIGS thin-film solar cells

We investigated the transport and photovoltaic properties of Cu(In1-xGax)Se2 (CIGS) thin-film solar cells. The shunt-current-eliminated diode current could be obtained from the current–voltage characteristics by subtracting the parasitic shunt leakage current from the total current. The temperature dependence of the open-circuit voltage, extracted from the shunt-eliminated (total) current, suggested that the recombination activation energy is comparable to (much less than) the CIGS bandgap. The low-temperature characteristics of the diode ideality factor supported bulk-dominated recombination in the same cell. This suggests that shunt-current subtraction can provide the proper diode parameters of CIGS solar cells.

Enhancements in device efficiency of poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester based solar cells with incorporation of bathocuproine

The overall enhancement of poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester based organic solar cell with a thin layer of bathocuproine (BCP) inserted between active layers and aluminum (Al) cathodes was investigated. X-ray and ultraviolet photoemission spectroscopy show that no reaction occurs at the active layer/BCP interface and 2nm of BCP could effectively suppress the chemical reactions between Al and active layers. Atomic force microscope images also indicate that BCP layers can provide smoother contact surfaces with Al cathodes and suppress the generation of shunt leakage, resulting in larger Voc and better power conversion efficiency of devices.