Decrease In Shunt Resistance Research Articles

Enhancement of power conversion efficiency of Ag-substituted Cu2ZnSn(S,Se)4 solar cells via tuning Cu2+/(Cu++Cu2+) percentage in precursor solution

Although trace silver (Ag) substitution for Cu in Cu2ZnSn(S, Se)4 (CZTSSe) offers benefits in improving open-circuit voltage (VOC) of CZTSSe solar cells, its further development is still hindered by the fairly low short-circuit current density (JSC) owing to the increase of bandgap (Eg) of the absorber layer. Herein, a strategy is demonstrated to mitigate the poor JSC arising from the Ag substitution by tuning the Cu2+/(Cu++Cu2+) percentage in the (Cu, Ag)2ZnSnS4 (CAZTS) precursor solution. Depth characterization indicates that the appropriate Cu2+/(Cu++Cu2+) percentage in the CAZTS precursor solution increases the photogenerated charges separation and carrier transport processes for Ag-substituted CZTSSe (CAZTSSe) solar cells. Benefiting from this strategy, the power conversion efficiency (PCE) of 9.57% is achieved when the Cu2+/(Cu++Cu2+) is 50 at.%. The main factor for the improvement of PCE is due to the increase in photogenerated current density (JL) and decrease in shunt resistance (Rsh). It is demonstrated that the increase in JL results from a widening in the depletion layer width (Wd) of the p-n junction and an increase in carrier lifetime, induced by optimization of the Cu2+/(Cu++Cu2+) percentage. While the decrease in Rsh comes from the change of reaction pathways due to the change in Cu2+/(Cu++Cu2+) percentage.

Long-term stability of graphene/c-Si Schottky-junction solar cells

A long operational lifetime is required for the use of solar cells in real-life photovoltaic applications. The optimization of operational lifetimes is achieved through understanding the inherent degradation phenomena in solar cells. In this study, graphene/Si Schottky-junction solar cells were produced, utilizing liquid-phase-exfoliated graphene as an active surface. The operational and interface stability of these solar cells over a period of 5 years in ambient conditions (following ISOS-D protocols: dark storage/shelf life) was examined, and the origin of their degradation was reported. It was found that the dominant degradation mechanism could be attributed to the degradation of silver contacts. This was indicated by a decrease in shunt resistance, an increase in the ideality factor (due to a higher carrier recombination), and a constant defect density in graphene films for up to 4 years. Measurements across the solar cell's active area during the 5-year period revealed neither significant spatial inhomogeneity, nor shunt channel defects.

Impact Analysis of Potential Induced Degradation on Crystalline Silicon Solar Cell Performance by Correlating Practical Diagnosis with MATLAB Simulation.

Extensive research on fault diagnosis is essential to detect various faults that occur to different photovoltaic (PV) panels to keep PV systems operating at peak performance. Here, we present an impact analysis of potential induced degradation (PID) on the current-voltage (I-V) characteristics of crystalline silicon (c-Si) solar cells. The impact of parasitic resistances on solar cell performance is highlighted and linked to fault and degradation. Furthermore, a Simulink model for a single solar cell is proposed and used to estimate the I-V characteristics of a PID-affected PV cell based on experimental results attributes. The measured data show that the fill factor (FF) drops by approximately 13.7% from its initial value due to a decrease in shunt resistance (Rsh). Similarly, the simulation results find that the fill factor degraded by approximately 12% from its initial value. The slight increase in measured data could be due to series resistance effects which were assumed to be zero in the simulated data. This study links simulation and experimental work to confirm the I-V curve behavior of PID-affected PV cells, which could help to improve fault diagnosis methods.

Thin film cadmium telluride solar cells on ultra‐thin glass in low earth orbit—3 years of performance data on the AlSat‐1N CubeSat mission

AbstractThis paper details 3 years of cadmium telluride (CdTe) photovoltaic performance onboard the AlSat‐1N CubeSat in low earth orbit. These are the first CdTe solar cells to yield I–V measurements from space and help to strengthen the argument for further development of this technology for space application. The data have been collected over some 17 000 orbits by the CubeSat with the cells showing no signs of delamination, no deterioration in short circuit current or series resistance. The latter indicating that the aluminium‐doped zinc oxide transparent front electrode performance remained stable over the duration. Effects of temperature on open circuit voltage (Voc) were observed with a calculated temperature coefficient for Voc of −0.19%/°C. Light soaking effects were also observed to increase the Voc. The fill factor decreased over the duration of the mission with a major contribution being a decrease in shunt resistance of all four of the cells. The decrease in shunt resistance is speculated to result from gold diffusion from the rear contacts into the absorber and through to the front interface. This has likely resulted in the formation of a deep trap state within the CdTe and micro shunts formed between the rear and front contact. Further development of this technology should therefore utilise more stable back contacting methodologies more commonly employed for terrestrial CdTe modules.

Chemical etching of Sb2Se3 solar cells: surface chemistry and back contact behaviour

The effect of (NH4)2S and CS2 chemical etches on surface chemistry and contacting in Sb2Se3 solar cells was investigated via a combination of x-ray photoemission spectroscopy (XPS) and photovoltaic device analysis. Thin film solar cells were produced in superstrate configuration with an absorber layer deposited by close space sublimation. Devices of up to 5.7% efficiency were compared via current–voltage measurements (J–V) and temperature-dependent current–voltage (J–V–T) analysis. XPS analysis demonstrated that both etching processes were successful in removing Sb2O3 contamination, while there was no decrease in free elemental selenium content by either etch, in contrast to prior work. Using J–V–T analysis the removal of Sb2O3 at the back surface in etched samples was found to improve contacting by reducing the potential barrier at the back contact from 0.43 eV to 0.26 eV and lowering the series resistance. However, J–V data showed that due to the decrease in shunt resistance and short-circuit current as a result of etching, the devices show a lower efficiency following both etches, despite a lowering of the series resistance. Further optimisation of the etching process yielded an improved efficiency of 6.6%. This work elucidates the role of surface treatments in Sb2Se3 devices and resolves inconsistencies in previously published works.

Thermally evaporated amorphous InZnO thin film applicable to transparent conducting oxide for solar cells

In recent years, amorphous InZnO (a-IZO) film gets recognition for the possibility as a transparent conducting oxide (TCO) layer with outstanding its optical and electrical properties in photovoltaic devices. Generally, the sputtering technique is normally used to deposit a-IZO film, which suffers from plasma damage exerting a negative influence on device performance. Here, we suggest an alternative to deposit a-IZO film with a thermal evaporation technique using a simply customized system to compensate defects by plasma damage. Thermally grown IZO films by controlling two variables of Zn rate and substrate temperature show the distinguishable characters in terms of transparency, conductivity and crystalline structure with a substantial change of Zn content in In1-xZnxO. Zn incorporation in IZO film is considered as a critical key to determining the crystal structure and IZO film quality. In the thermal evaporation system, phase transition occurs from crystalline to amorphous; the IZO film indicates the superior property in the amorphous phase than crystalline structure. We acquired the optimized IZO film with amorphous phase in the relative concentration of In0.73Zn0.27O grown at 200 °C. To demonstrate the feasibility of a-IZO film as a TCO layer, Cu(In,Ga)Se2 solar cell was practically fabricated, which shows excellent performance. It is attributed to the absence of plasma damage during deposition of the a-IZO film, proved by the decrease of shunt resistance in the solar cell.

Simulation of solar cells by delocalized recombination model and its applications

For lattice-mismatch, high-defect-density solar cells, the current-voltage curve is not perfects, and it always has high ideality factor and low shunt resistance. In traditional one-dimension circuit model of solar cells, there is a paralleled resistor Rp and recombination diodes with high ideality factor to simulate this imperfection. In this research, a delocalized recombination model is built based on the assumption that recombination could occur between trapped electrons from n type sides and trapped holes from p type side. The model with different parameters (i.e. defect volume w and recombination parameter Cp) is then added into current continuity equations and I-V curves of solar cells were calculated. With the increase of Cp, fill factor (FF) decreases obviously, followed by the decrease of short-circuit current and open-circuit voltage. Larger defect volume w would cause higher ideality factor. The effects of mobility on I-V curves were also calculated, low mobility leads to decrease of shunt resistance and fill factor. Two type of solar cells with different configurations were irradiated by 70 keV and the I-V curves show different degradation behaviors. Type A cell shows large decrease of shunt resistance indicating loss of mobility while type B cell shows increase of n(V) due to delocalized recombination. The difference is related to different thicknesses of GaInP.

RESISTANCE, TEMPERATURE AND IRRADIANCE PARAMETER ANALYSIS OF A SINGLE DIODE PHOTOVOLTAIC CELL MODEL

This paper presents an analysis of parameter variations of a single-diode solar cell model. The parameters analyzed are the series resistance, shunt resistance, temperature and radiation change. Model equations are derived and simulated. All simulations were performed in MatLab using looping iterative method. Results obtained show that an increase in series resistance causes a decrease in short-circuit current and output power. A decrease in shunt resistance also causes a decrease in short circuit current and output power. An increase in temperature above the nominal value of 25oC causes a significant decrease in the open circuit voltage. An increase in irradiance above a nominal value of 1000 W/m2 causes the short circuit current to increase from 8.21A at 1000 W/m2 to 10.67A at 1300W/m2. It can be seen that parameter variations have a net effect on the current-voltage (I-V) and power-voltage (P – V) characteristics.

Analysis of Potential-Induced Degradation in Soda-Lime Glass and Borosilicate-Glass Cu(In,Ga)Se2 Samples

Potential-induced degradation (PID) is investigated in small-size Cu(In,Ga)Se2 (CIGS) submodules of individual cells. Samples with conventional soda-lime glass (SLG) and low-alkaline glass (borosilicate glass, BSG) were analyzed applying positive and negative 1000 V bias at 85 °C and low relative humidity after a heat-soaking phase. During the heat-soaking phase, BSG samples showed degradation, with variations in their dark current–voltage ( I–V ) curve parameters compared with the stability of SLG samples. During the PID stress phase, back-grounded SLG samples with −1000 V applied to the cell experienced fast degradation, with complete failure in 25 h, whereas, BSG samples only showed changes because of the dark heat environment. When grounded on their front face, SLG samples remained stable during more than 150 h of PID testing, beginning degradation after longer applications of stress. The analysis of the dark I–V curves of SLG samples through the single-diode model showed that an increase in saturation current J 0 occurs first, attributed to Na+ ions migrating into the CdS/ZnO layer; later, a decrease in shunt resistance occurs. The application of the model in BSG samples showed an increase in series resistance evaluated at 25 °C. When the bias is reversed, SLG samples gradually recover their performance.

Degradation of up-grown metamorphic InGaP/InGaAs/Ge solar cells by low-energy proton irradiation

To gain higher efficiency than that of traditional lattice-matched solar cells, newly designed up-grown metamorphic (UMM) InGaP/InGaAs/Ge solar cells were grown on Ge substrates. The defects in UMM InGaP/InGaAs/Ge can be perfectly controlled, and thus the conversion efficiency exceeds 30%. Similarly, the radiation environment in space could also cause displacement damage in UMM solar cells, causing the degradation of electrical properties. Proton irradiation at 50 keV and 150 keV was tested on the UMM solar cells. Because the top InGaP layer is thick enough, the radiation defects are limited within the top subcell without affecting the underlying subcells. In this research, 50-keV protons cause more serious degradation of short-circuit current, whereas 150-keV protons cause more serious degradation of open-circuit voltage. The difference may be related to the distribution of defects, which is calculated by The Stopping and Range of Ions in Matter (SRIM). Defects in the junction region would cause the decrease in voltage, whereas defects in the surface region could cause degradation of current. As a comparison, lattice-matched GaInP/GaAs/Ge solar cells were also irradiated by low-energy protons. The degradation under 150-keV protons is more serious because the damage peak lies in the middle GaAs layer, which has poor radiation resistance. The degradation under 70-keV proton irradiation proves that a thinner GaInP layer improves the radiation resistance. It is notable that the degradation of short-circuit current and maximum power is influenced by the equivalent displacement damage dose (DDD), but this is not accurate for open-circuit voltage. The artifact signals at 700–750 nm after 50-keV proton irradiation indicate the decrease in shunt resistance of the InGaP subcell during proton irradiation.

Origin of Na causing potential-induced degradation for p-type crystalline Si photovoltaic modules

This paper presented whether Na ion in the front cover glass is absolute root cause of potential-induced degradation (PID) for p-type crystalline Si photovoltaic (PV) modules or not. P-type monocrystalline Si PV modules with and without the front cover glass, and with and without intentional Na contamination were subjected to PID test. Even without the front cover glass, a decrease in shunt resistance which is a characteristic feature of PID for p-type crystalline Si PV modules has been observed. Intentional Na incorporation on the cell or encapsulant also brings about remarkable PID. These results indicated that Na ion in the front cover glass is not a necessary condition for PID. Furthermore, PID occurs regardless of origin of Na ions. Relationship between PID and leakage current will be also discussed.

Collaborative R&D between multicrystalline silicon ingots and battery efficiency improvement—effect of shadow area in multicrystalline silicon ingots on cell efficiency* * Project supported by the National Natural Science Foundation of China (No. 51532007) and the Fundamental Research Funds for the Central Universities.

We characterized strip-like shadows in cast multicrystalline silicon (mc-Si) ingots. Blocks and wafers were analyzed using scanning infrared microscopy, photoluminescence spectroscopy, laser scanning confocal microscopy, field-emission scanning electron microscopy, X-ray energy-dispersive spectrometry, and microwave photoconductivity decay technique. The effect on solar cell performance is discussed. The results show that the non-microcrystalline shadow region in Si ingots consists of precipitates of Fe, O, and C. The size of these Fe–O–C precipitates found at the shadow region is ~25 μm. Fe–O–C impurities can slightly reduce the minority carrier lifetime of the wafers while severely decrease in shunt resistance, leading to the increase in reverse current of the solar cells and degradation in cell efficiency.

CSi Photovoltaic Modules, Main Degradation Factors Case Study: La Puna (Argentina)

This paper presents an analysis of the power loss of a set of photovoltaic modules exposed for more than fifteen years to solar radiation and other environmental factors. The study covered modules installed in 1989 and concluded with modules installed in 2004 (to 2009). Many of them were located in places high above the sea level and with high values of ultraviolet radiation. It is known that this power loss is caused by various factors: loss of optical properties of the protective glass, loss of transmittance of the encapsulation material (EVA), increase in series resistance, decrease in shunt resistance as well as other factors not analyzed in this study. The contribution of each of these factors to the power loss is measured and discussed in this analysis.

Influence of Niobium Doping in Hierarchically Organized Titania Nanostructure on Performance of Dye-Sensitized Solar Cells

Niobium doped hierarchically organized TiO2 nanostructures composed of 20 nm size anatase nanocrystals were synthesized using pulsed laser deposition (PLD). The Nb doping concentration could be facilely controlled by adjusting the concentration of Nb in target materials. We could investigate the influence of Nb doping in the TiO2 photoelectrode on the cell performance of dye-sensitized solar cells (DSSCs) by the exclusion of morphological effects using the prepared Nb-doped TiO2 anostructures. We found no significant change in short circuit current density (Jsc) as a function of Nb doping concentration. However, open circuit voltage (Voc) and fill factor (FF) monotonously decrease with increasing Nb concentration. Dark current characteristics of the DSSCs reveal that the decrease in Voc and FF is attributed to the decrease in shunt resistance due to the increase in conductivity TiO2 by Nb doping. However, electrochemical impedance spectra (EIS) analysis at open circuit condition under illumination showed that the resistance at the TiO2/dye/electrolyte interface increases with Nb concentration, revealing that Nb doping suppress the charge recombination at the interface. In addition, electron life time obtained using characteristic frequency in Bode plot increases from 14 msec to 56 msec with increasing Nb concentration from 0 to 1.2 at%. This implies that the improved light harvesting can be achieved by increasing diffusion length through Nb-doping in the conventional TiO2 photoelectrode.

MOCVD of Cd(1−x)Zn(x)S/CdTe PV cells using an ultra-thin absorber layer

Ultra-thin Cd(1−x)Zn(x)S/CdTe devices were produced by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) with varying CdTe absorber thicknesses ranging from 1.0 to 0.2μm and compared to baseline cells with total CdTe thickness of 2.25μm. The ultra-thin CdTe layers (≤1μm) were intentionally doped with As to induce p-type conductivity in the absorber. Cell performance reduced with CdTe thickness, with the magnitude of photo-current generation loss becoming more significant for the very thin CdTe layers. The decline in cell performance was lower than the optically limited performance relating to a decrease in shunt resistance, Rsh, especially for the thinnest cells due to areas of incomplete CdTe coverage and large presence of pin-holes leading to micro-shorts. Incorporation of Zn into the CdS window layer improved cell performance for all devices except when 0.2μm thick CdTe was used. This improvement was markedly in the blue region owing to enhanced optical transparency of the window layer. External quantum efficiency (EQE) measurements showed a red-shift of the window layer absorption edge due to leaching out of Zn during the CdCl2 treatment. Reduction of the CdCl2 deposition time was demonstrated to recover the blue response of the ultra-thin cells.

Analysis of the effect of parasitic resistances on the performance of photovoltaic modules

Parasitic resistances are detrimental to solar cell performance because they reduce the device power output. In this paper the effects that series- and shunt resistance have on photovoltaic module performance parameters are discussed. Techniques used to measure these resistances are presented and the results show how current–voltage characteristics and module performance are affected. An analysis of the performance degradation of an a-Si module revealed that, apart from the Staebler–Wronski effect, an increase in series resistance contributed to a power loss of 50% after outdoor exposure of 130 kWh/m 2. A 29% decrease in shunt resistance of a CuInSe 2 module, over the same exposure, translated to a power loss of 6%. This study shows that the effects of shunt and series resistances on module performance are significant and cannot be ignored. These effects must be understood and taken into consideration when employing photovoltaic modules in systems and when analysing performance degradation.