Dark Saturation Current Density Research Articles

A Comparative Study of the VOC in CuPc and SubPc Organic Solar Cells

The open-circuit voltage (VOC) with respect to the reverse dark saturation current density in planar CuPc/C60 and non-planar boron subphthalocyanine chloride (SubPc)/C60 organic solar cell (OSCs) devices is compared in the present study. The fabricated OSCs devices were: ITO/CuPc/C60/BCP(90Å)/Al(1000Å) and ITO/SubPc/ C60/BCP(90Å)/Al(1000Å) organic solar cells. We observed that the dark saturation current and the VOC values vary in the CuPc/C60 solar cells, whereas almost no variation of the dark saturation current and VOC values in SubPC/C60 solar cells occur. All SubPC devices show almost similar value of dark current. These differences may be attributed to the surface morphologies of planar CuPc and non-planar SubPC donor layers.

Highly efficient hybrid thin-film solar cells using a solution-processed hole-blocking layer

We report the origin of the improvement of the power conversion efficiency (PCE) of hybrid thin-film solar cells when a soluble C(60) derivative, [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM), is introduced as a hole-blocking layer. The PCBM layer could establish better interfacial contact by decreasing the reverse dark-saturation current density, resulting in a decrease in the probability of carrier recombination. The PCE of this optimized device reached a maximum value of 8.34% and is the highest yet reported for hybrid thin-film solar cells.

Spatially resolved photoluminescence imaging of essential silicon solar cell parameters and comparison with CELLO measurements

We present a method to image the following parameters spatially resolved of a solar cell: voltage Vxy, current density Jxy, power density Pxy, efficiency ηxy, series resistance Rs,xy, fill factor FFxy and dark saturation current densities of a two diode model J01,xy and J02,xy. The algorithm determines this set of self-consistent parameters by using a minimum of 5 electrical biased photoluminescence images. A comparison with CELLO measurements supports the presented method.

High resolution saturation current density imaging at grain boundaries by lock-in thermography

The electronic properties of multicrystalline solar silicon materials are dominated by low-lifetime defect regions containing recombination-active grain boundaries and dislocations. Besides reducing the carrier collection probability, these regions increase the dark saturation current density J01, which governs the open circuit voltage. By applying lock-in thermography with spatial deconvolution it is shown that the dominant contribution to J01 comes from recombination-active grain boundaries and to a lower degree from intra-grain defects like dislocations.

Modelling of Physically Relevant Features in Photoluminescence Images

Photoluminescence (PL) imaging is a promising measuring technique to rate the quality of multi-crystalline silicon wafers. Defect structures which form during crystallization can be observed in the PL-images. In recent approaches, image features are measured and correlated to global solar cell parameters. So far, an underlying physical effect has not been connected to the large variety of observed PL-image patterns. Next to global solar cell parameters, also spatially resolved solar cell parameters like the dark saturation current density can be measured to rate wafer quality. It is shown that these spatially resolved measurements strongly correlate to the global open circuit voltage. Based on the observation that special patterns within the as-cut PL-images correlate with regions of high and low dark saturation current, a method for the modelling of physically relevant image features in PL-images is introduced in this work. The modelling framework is based on the transfer of spatially resolved quality data of the dark saturation current onto prototypical patterns within the PL-images of as-cut wafers by means of pattern recognition techniques. We evaluate our approach by predicting the images of the local dark-saturation current of the finished cells based on PL-images of as-cut wafers.

Enhanced performance in dye-sensitized solar cells via carbon nanofibers–platinum composite counter electrodes

A composite counter electrode (CE) made of electrospun carbon nanofibers (ECNs) and platinum (Pt) nanoparticles has been demonstrated for the first time to improve the performance of dye-sensitized solar cells (DSCs). The new ECN-Pt composite CE exhibited a more efficient electro-catalytic performance with lower charge transfer resistance (R(ct)), larger surface area, and faster reaction rate than those of conventional Pt. It reduced the overall series resistance (R(se)), decreased dark saturation current density (J(0)) and increased shunt resistance (R(sh)) of the DSCs, thereby leading to a higher fill factor (FF) and larger open circuit voltage (V(oc)). The reduced electron transport resistance (R(s)) and faster charge transfer rate in the CE led to a smaller overall cell series resistance (R(se)) in the ECN-Pt composite based DSCs. The DSCs based on an ECN-Pt CE achieved a η of ∼8%, which was improved over those of pure Pt or ECN based cells.

Determination of the Intrinsic Diode Parameters of Polymer Solar Cells

Polymer solar cells offer a promising low cost alternative in photovoltaics if the expensive ITO electrode can be omitted. Recently an alternative based on highly conductive PEDOT:PSS in combination with current collecting grids was developed.1 Electrical modeling is carried out to optimize the grid pattern in these polymer solar cells. The basic inputs for this type of modeling are the resistivity of the materials, film thicknesses and the diode parameters of the solar cell. The diode parameters are often determined by fitting the experimental current-voltage measurements to a one-diode model. This gives the well-known dark saturation current density (J0), diode ideality factor (n), photocurrent density (JL), shunt resistance and series resistance. However, the fitted parameters do not always correspond with the intrinsic solar cell parameters, i.e. those that correspond to an infinitesimally small diode, but they are actually lumped parameters containing information of the heterogeneity of the system. For this reason, two one-diode fits corresponding to two different systems (in size and geometry) can yield different intrinsic diode parameters. The reason for this can be found in the heterogeneity of the system.We show an approach to determine the so-called intrinsic diode parameters, by fitting the experimental IV curve against a simulated IV curve that is obtained from a model in which the experimental solar cells are explicitly modeled in 3D.This model provides a simple basis to determine the intrinsic solar cell parameters that can be used for the optimization of grid patterns for polymer solar cells.

20.1% Efficient Silicon Solar Cell With Aluminum Back Surface Field

We present a standard p+pn+ solar cell device exhibiting a full-area aluminum back surface field (BSF) and a conversion efficiency of 20.1%. The front side features a shallow emitter which has been exposed to a short oxidation step and reduces the emitter dark saturation current density j0e to 160 fA/cm2 on a textured surface. The front contact is formed by light-induced nickel and silver plating. Also, devices featuring screen-printed front contacts have been realized that reach a conversion efficiency of 19.8%. PC1D simulations are presented in order to extract the electronic parameters of the BSF. Therefore, external quantum efficiency and reflectance have been determined for modeling the internal quantum efficiency by adapting surface recombination and lifetime of the PC1D-simulated silicon device. As a result, a recombination velocity of SBSF = 283 cm/s and a dark saturation current density of jBSF = 274 fA/cm2 in the Al BSF are determined. This results in an effective diffusion length Leff = 1150 μm .

Surface passivation of phosphorus-diffused emitters by inline thermal oxidation

Abstract We show that inline thermal oxidation is a very promising alternative to the use of conventional batch type quartz tube furnaces for the passivation of industrial phosphorus-diffused emitters. Our results reveal that both inline and batch oxidation allow similar results for oxide layer thickness, sheet resistance and, most important, emitter dark saturation current density J 0e even when compressed dry air is used as a cost effective oxygen source. Using a ~10 nm thin thermal oxide layer and a SiN x antireflection coating of adapted thickness, we achieve J 0e values as low as ~100 fA/cm² (68 Ω/sq., textured surface). Using a simple inline oxidation process in air ambient, solar cells with a conventional Al-BSF rear side and a thermal SiO 2 /SiN x passivated front yield average efficiencies of 18.1%, demonstrating 0.3% absolute efficiency gain compared to solely SiN x passivated emitter references. © 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV.

Metal pinning through rear passivation layers: characterization and effects on solar cells

We investigate local defects in rear passivation layers, in which the metal is forming a contact to silicon pinning through an insulating layer. At first, we studied these contacts by measuring the layer resistivity of different dielectrics sandwiched between Al and Si. Our study includes the influence of parameters like the surface roughness, the metallization techniques and the post-metallization annealing. In addition, we propose a characterization of these contacts on solar cell level, using photoluminescence-imaging performed before the finalization of the rear contacts. A good correlation between the contacts and the dark saturation current density suggests that these contacts can harm the rear surface passivation quality.

Recombination at Metal-Emitter Interfaces of Front Contact Technologies for Highly Efficient Silicon Solar Cells

Abstract We present an experimental approach to extract the dark saturation current density j0e-met at the emitter-metal interface of the front contact. For this purpose, 2×2 cm2 sized silicon solar cells have been realized featuring different metallization fractions FM. By simply applying the one-diode-model, the dark current density j01 is determined from the open circuit voltage Voc. From the slope of the j01 over FM plot, j0e-met is extracted. However, this is only valid if the dominant recombination mechanism at Voc features a diode character that is close to unity. Hence, the local ideality factor m is determined from the suns-Voc-curve indicating the required value close to one. Three main effects are observed. First, the metallization methods which are compared show different influences on j0e-met on the same emitter configuration. Second, an emitter drive-in due to an additional short thermal oxidation lowers j0e-met. Also, the field-effect passivation of the highly n-doped selective emitter decreases j0e-met effectively. By combining the field effect passivation with a short drive-in step the very low value of j0e-met = 549 fA/cm2 is reported.

New measurement method for the investigation of space charge region recombination losses induced by the metallization of silicon solar cells

Abstract Screen printed silver thick film contacts on the front side of industrial silicon solar cells induce parasitic impurities due to diffusion processes during the high-temperature contact formation process. This leads to space charge region recombination (SCR-recombination) corresponding to efficiency-limiting dark saturation current densities j02. The quantitative determination of j02 via fitting the two diode model to the dark IV curve of the solar cell is subject to errors as the result for j02 strongly depends on individual cell parameters like the series resistance as well as on fit parameters. For detailed studies on the dependency of SCR-recombination on different solar cell process parameters, mainly with focus on the realized front side metallization fraction, a special test structure (front side metallization test pattern) was developed and applied on Cz-Si H-patterned solar cells. With this new measurement method it is possible to vary the metallization fraction over different solar cell groups whereas the series resistance RS is kept nearly constant. This ensures a quantitative evaluation of SCR-recombination. We expect this new measurement method to allow for a more thorough optimization of metallization pastes, emitters and related processes by ensuring a quantitative determination of SCR-recombination. First results with respect to SCR-recombination related fill factor losses are presented.

Modelling carrier recombination in highly phosphorus-doped industrial emitters

Abstract One important parameter for modelling emitter recombination is the surface recombination velocity (SRV), which strongly depends on the surface doping concentration and the applied surface passivation. However, for highly phosphorus-doped surfaces with concentrations in excess of 2×1020 cm-3, where not all of the dopant is electrically activated, data is hardly available in the literature. Moreover, the bulk carrier lifetime in such supersaturated near surface regions is unknown. We use an analytical model to describe silicon-nitride-passivated phosphorus-diffused emitters. The model shows excellent agreement with a recently presented numerical solver, deviating less than 1%. In both cases we apply a Fermi-Dirac statistics correction and account for band gap narrowing to calculate the intrinsic carrier density. Our results from measurements of the emitter dark saturation current density indicate that either the SRV or the local carrier lifetime in the supersaturated region are strongly affected by the doping concentration, even if it exceeds the dopant activation limit by far. Assuming only Auger recombination in the supersaturated region, we derive an upper limit for the SRV that depends on the chemical phosphorus surface concentration.

Analysis of electroluminescence from silicon heterojunction solar cells

The density of defects at the interface between the amorphous and crystalline silicon layers limits the photovoltaic device performance of amorphous silicon / crystalline silicon heterojunction solar cells. We investigate the electroluminescence properties of these devices for a variation of interface defect densities. Electroluminescence efficiency, dark saturation current density and open-circuit voltage are related via Planck’s generalized law and the current-voltage relations. These analytical relations are reproduced by the full numerical simulation of the solar cell behavior, especially the exponential dependence of the electroluminescence efficiency on the open-circuit voltage. Experimentally we show that a planar silicon heterojunction solar cell with high open-circuit voltage achieves an electroluminescence efficiency around 0.13%.

A simple model of graded band-gap CuInGaSe2 solar cells

The effects of quasi-electric fields, caused by the band-gap variation of the active semiconductor, upon the illumination current density and open-circuit voltage of a solar cell will be discussed. In addition, it will be shown that open-circuit voltage improvement is possible when the band-gap is gradually increased within the space charge region from the neutral to the hetero-junction interface so that the dark saturation current density of the cell is reduced. Our estimation is that in the case of a solar cell where the band-gap increases from 1.15 eV to 1.4 eV, at the space charge region (of the order of 0.2 μm), an increase of the open-circuit voltage around 80 mV will be observed with respect to the single gap absorbing material case. A similar (increasing) band-gap variation within the bulk of the material will cause an increase of the drift-diffusion length of minority carriers by a factor of 5 with respect to a single band-gap material. Then, based on these physical concepts, two possible structures with variable band-gap CIGS layers are proposed in order to have higher efficiencies than cells without any band-gap grading. It will also be shown that very likely present record efficiency CIGS solar cells have achieved efficiencies as high as η=19.9% due to (intentional or non-intentional) band-gap grading of the CIGS layer. This band-gap grading can be optimized with the purpose of a further efficiency increase.

Variable band-gap semiconductors as the basis of new solar cells

Some basic concepts related to variable band-gap absorbing semiconductors in solar cell structures, such as the associated quasi-electric field, will be discussed. The effects of this quasi-electric field upon the minority carrier drift-diffusion length and the back surface recombination velocity may induce a larger generated carrier collection at the junction with the corresponding increase of the illumination current density. It will also be shown that an additional improvement of the open-circuit voltage is possible when the band-gap is reduced within the space charge region so that the dark saturation current density is reduced there. Our estimation is that in the case of a solar cell where the band-gap is changed about 0.5 eV within the space charge region, an increase of the open-circuit voltage around 115 mV will be observed with respect to the single minimum band-gap absorbing material case. A similar band-gap variation in the bulk of the material will cause an increase of the minority carrier drift-diffusion length by a factor of 10 with respect to the single band-gap material. Therefore, based on these physical concepts, two possible structures with variable band-gap layers are proposed in order to have higher efficiencies than for cells without any band-gap grading. It will be shown that these concepts can be applied to II–VI, III–V chalcopyrite and even amorphous semiconductor solar cells.

The Study of Silane-Free SiCxNy Film for Crystalline Silicon Solar Cells

We deposited plasma-enhanced chemical vapor deposition silicon carbon nitride antireflection coating and passivation layers using a silane-free process. We used a solid polymer source developed at SiXtron Advanced Materials to eliminate the storage and handling of dangerous pyrophoric silane gas. We used ammonia flow rate as a control for the chemical and optical properties in the silane-free process. As flow rate increases, the carbon content, refractive index, extinction coefficient, and surface charge density of the film decrease. At an ammonia flow rate of 3000 sccm, which is similar to the conventional , the extinction coefficients for the two films were similar. This led to an emitter dark saturation current density of for the two films on emitters. However, a stack passivation of on an emitter resulted in an emitter dark saturation current density of , which is enough to provide a good surface passivation for high efficiency solar cells. An energy conversion efficiency of 17.4% was obtained for a textured Czochralski screen-printed solar cell with this stack passivation. For a nontextured multicrystalline silicon, with only and a emitter, we obtained 14.9% efficiency.

Influence of SnO2 films with high resistance on the performance of CdTe solar cells

In this paper, we report the SnO2 thin films with high resistance prepared by magnetic reactive sputtering on the transparent conductive glass covered with SnO2:F layers, and study the structural, optical, and electrical properties of the films before and after annealing, as well as the influence of the annealing on the performance of CdTe solar cells. The results have shown that SnO2 thin films almost have the same structure as SnO2:F, and SnO2 high resistance transparent (HRT) insertion layer between transparent conductive oxide (TCO) and CdS affects little on light transmittance. After introducing the HRT layer, the tunneling leakage caused by the pinholes can be avoided, which effectively protects the p–n junction (Wu et al. Proceedings of NCPV program review meeting AIP, New York, 22–26, 2001). Meanwhile, higher parallel resistivity, fill factor (FF), short-wave response, carrier concentration, and lower dark saturation current density have been achieved. As a result, the conversion efficiency is improved by 14.5%, from 10.3% to 11.8%.

Textured silicon surface passivation by high‐rate expanding thermal plasma deposited SiN and thermal SiO2/SiN stacks for crystalline silicon solar cells

AbstractExpanding thermal plasma (ETP) deposited silicon nitride (SiN) with optical properties suited for the use as antireflection coating (ARC) on silicon solar cells has been used as passivation layer on textured monocrystalline silicon wafers. The surface passivation behavior of these high‐rate (>5 nm/s) deposited SiN films has been investigated for single layer passivation schemes and for thermal SiO2/SiN stack systems before and after a thermal treatment that is normally used for contact‐firing. It is shown that as‐deposited ETP SiN used as a single passivation layer almost matches the performance of a thermal oxide. Furthermore, the SiN passivation behavior improves after a contact‐firing step, while the thermal oxide passivation degrades which makes ETP SiN a better alternative for single passivation layer schemes in combination with a contact‐firing step. Moreover, using the ETP SiN as a part of a thermal SiO2/SiN stack proves to be the best alternative by realizing very low dark saturation current densities of <20 fA/cm2 on textured solar‐grade FZ silicon wafers and this is further improved to <10 fA/cm2 after the anneal step. Optical and electrical film characterizations have also been carried out on these SiN layers in order to study the behavior of the SiN before and after the thermal treatment. Copyright © 2008 John Wiley & Sons, Ltd.

Optimizing the diffused regions of interdigitated backside buried contact solar cells

AbstractRecombination and a number of other important factors must be considered in the optimization of the diffused regions of high‐efficiency silicon solar cells. In this paper, we examine issues related to the four types of diffusions used in rear‐junction, interdigitated backside buried contact solar cells made on n‐type silicon wafers: the phosphorus‐diffused front‐surface field (FSF), the boron‐diffused emitter, and the boron and the phosphorus diffused contact regions. Dark saturation current density, effective lifetime, implied open‐circuit voltage and sheet resistance are characterized for the optimization of the above‐mentioned diffused regions. Diffusion uniformity and the avoidance of the diffusion‐induced dislocations are also discussed for the heavily diffused, metal coated contact diffusions. It is found that the optimal sheet resistances of the FSF for planar and textured surfaces are 120 Ω/□ and 105 Ω/□ respectively, whereas the optimal post‐processing sheet resistance for the boron emitter is approximately 100 Ω/□. Moreover, sheet resistance as heavy as 10–20 Ω/□ for the boron groove diffusion and 5–10 Ω/□ for the phosphorus groove diffusion have been achieved without introducing the diffusion‐induced misfit dislocations. Careful consideration of the issues discussed here led to an absolute efficiency improvement on the planar n‐type IBBC solar cell of more than 0·6%. Copyright © 2006 John Wiley & Sons, Ltd.