Showing papers in "IEEE Journal of Photovoltaics in 2011"

Local Reactive Power Control Methods for Overvoltage Prevention of Distributed Solar Inverters in Low-Voltage Grids

Abstract: The main objective of this study is to increase the penetration level of photovoltaic (PV) power production in low-voltage (LV) grids by means of solar inverters with reactive power control capability. This paper underlines weak points of standard reactive power strategies which are already imposed by certain grid codes, and then, the study introduces a new reactive power control method that is based on sensitivity analysis. The sensitivity analysis shows that the same amount of reactive power becomes more effective for grid voltage support if the solar inverter is located at the end of a feeder. Based on this fundamental knowledge, a location-dependent power factor set value can be assigned to each inverter, and the grid voltage support can be achieved with less total reactive power consumption. In order to prevent unnecessary reactive power absorption from the grid during admissible voltage range or to increase reactive power contribution from the inverters that are closest to the transformer during grid overvoltage condition, the proposed method combines two droop functions that are inherited from the standard cos φ(P) and Q(U) strategies. Its performance comparison in terms of grid losses and voltage variation with different reactive power strategies is provided by modeling and simulating a real suburban LV network.

Applicability of Metal/Insulator/Metal (MIM) Diodes to Solar Rectennas

Abstract: The current-voltage (I-V) characteristics of metal/insulator/metal (MIM) diodes illuminated at optical frequencies are modeled using a semiclassical approach that accounts for the photon energy of the radiation. Instead of classical small-signal rectification, in which a continuous span of the dc I-V curve is sampled during rectification, at optical frequencies, the radiation samples the dc I-V curve at discrete voltage steps separated by the photon energy (divided by the electronic charge). As a result, the diode resistance and responsivity differ from their classical values. At optical frequencies, a diode with even a moderate forward-to-reverse current asymmetry exhibits high quantum efficiency. An analysis is carried out to determine the requirements imposed by the operating frequency on the circuit parameters of antenna-coupled diode rectifiers, which are also called rectennas. Diodes with low resistance and capacitance are required for the RC time constant of the rectenna to be smaller than the reciprocal of the operating frequency and to couple energy efficiently from the antenna. Existing MIM diodes do not meet the requirements to operate efficiently at visible-to-near-infrared wavelengths.

Route Toward High-Efficiency Single-Phase Cu $_{\bf 2}$ ZnSn(S,Se) $_{\bf 4}$ Thin-Film Solar Cells: Model Experiments and Literature Review

Abstract: Thin-film chalcogenide kesterites Cu2ZnSnS4 and Cu2 ZnSnSe4 (CZTSSe) are promising candidates for the next-generation thin-film solar cells. They exhibit a high natural abundance of Cu, Zn, Sn and S2, a high absorption coefficient, and a tunable direct bandgap between 1.0 and 1.5 eV. A prerequisite for the use of CZTSSe as absorber layers in photovoltaic applications on large scales is a detailed knowledge of the formation reaction. Recently, we have shown that a decomposition/formation equilibrium governs the formation reaction. The presence of Sn(S,Se) during the high-temperature preparation steps is essential to prevent decomposition. This improves the solar cell efficiency from 0.02% to 6.1%. In this paper, we show that the decomposition is universal. Absorbers produced by high-temperature coevaporation and samples produced by low-temperature precursor fabrication followed by annealing in a tube furnace in S or Se atmosphere are compared in order to elucidate that in all cases, the loss of Sn(S,Se) forms a degraded surface region. We demonstrate that the degraded surface of CZTSe absorbers contains grains of ZnSe. These new insights can be used to explain why some of the synthesis routines described in the literature yield much better efficiencies than others.

Characterization of Grain Boundaries in Cu(In,Ga)Se $_{\bf 2}$ Films Using Atom-Probe Tomography

Abstract: This paper discusses the advantages of pulsed laser atom-probe tomography (APT) to analyze Cu(In,Ga)Se2-based solar cells. Electron backscatter diffraction (EBSD) was exploited for site-specific preparation of APT samples at selected Cu(In,Ga)Se2 grain boundaries. This approach is very helpful not only to determine the location of grain boundaries but also to classify them as well. We demonstrate that correlative transmission electron microscopy (TEM) analyses on atom-probe specimens enable the atom-probe datasets to be reconstructed with high accuracy. Moreover, EBSD and TEM can be very useful to obtain complementary information about the crystal structure in addition to the compositional analyses. The local chemical compositions at grain boundaries of a solar grade Cu(In,Ga)Se2 film are presented here. Na, K, and O impurities are found to be segregated at grain boundaries. These impurities most likely diffuse from the soda lime glass substrate into the absorber layer during cell fabrication and processing. Based on the experimental results, we propose that Na, K, and O play an important role in the electrical properties of grain boundaries in Cu(In,Ga)Se2 thin films for solar cells.

Silicon Surface Passivation by Thin Thermal Oxide/PECVD Layer Stack Systems

Abstract: For the passivation of p-type silicon surfaces, we investigate layer systems consisting of a thin layer of thermally grown SiO2 and different dielectric capping layers deposited by means of plasma-enhanced chemical vapor deposition (PECVD). We find that the thermal SiO2 layer thickness strongly impacts the passivation quality and interface parameters of the stacks. Capacitance-voltage measurements reveal that for Al2O3 and SiNx capping layers, an increased thermal SiO2 film thickness suppresses charge formation at the interface between SiO2 and the capping layer. Interface trap density and effective carrier lifetime data suggest that a certain thermal SiO2 thickness is required to achieve appropriate chemical passivation. The combination of a thin thermal SiO2 layer (~4 nm) and a PECVD-SiOx capping results in very low surface recombination velocities of a few centimeters per second, measured on p-type 1-Ω·cm float-zone silicon after contact firing and postmetallization annealing. The experimentally observed dependence of the surface recombination velocity on the fixed charge density, gate voltage, and injection density is reproduced very accurately by analytical calculations that use the measured interface trap density and total charge density at the Si/insulator interface. The model also includes additional recombination in the space charge region of inverted surfaces.

Can Luminescence Imaging Replace Lock-in Thermography on Solar Cells?

Abstract: The purpose of this paper is a detailed comparison of selected luminescence and lock-in thermography (LIT) results on one exemplary sample and the drawing of corresponding conclusions. Our focus is on solar cells, but some investigations on wafers will be discussed as well. The comparison will help to decide which characterization tools are needed to solve technological problems. It will be demonstrated that luminescence imaging may widely replace LIT with respect to the analysis of recombination-active bulk defects, cracks, series resistance, and junction breakdown sites. However, some important investigations can be done only by LIT. LIT allows for a quantitative analysis of different kinds of leakage currents both under forward and under reverse bias, enabling a reliable analysis of local I-V characteristics. It is shown that LIT and luminescence imaging are complementary to each other and should be used in combination.

Record Large-Area p-Type CZ Production Cell Efficiency of 19.3% Based on LDSE Technology

Abstract: A record independently confirmed production cell efficiency of 19.3% is presented for a large-area p-type Czochralski (CZ) silicon solar cell, based on the University of New South Wales (UNSW) laser-doped selective emitter technology. In this paper, the innovative and patented laser-doping technology is simply added to a standard Centrotherm turnkey line, operating with a modified process and the addition of the laser-doping and light-induced plating steps. Impressively, this record efficiency is achieved by using standard commercial grade p-type CZ-grown silicon wafers on standard production equipment and exceeds the previous independently confirmed record for any technology of 19.2% using a standard aluminum back-surface field with full rear coverage. The avoidance of laser-induced defects is discussed in this paper to overcome previous limitations of the laser-doping technology using conventional Q-switched lasers or the laser chemical processing method. It is demonstrated that the use of appropriate lasers can avoid defect formation through thermal cycling while still allowing for the sufficient mixing of dopants and allow laser doping to be performed through a standard SiN layer with contacts formed through a self-aligning metallization scheme.

Luminescent Ethylene Vinyl Acetate Encapsulation Layers for Enhancing the Short Wavelength Spectral Response and Efficiency of Silicon Photovoltaic Modules

Abstract: The inclusion of luminescent material in the ethylene vinyl acetate (EVA) encapsulation layer of multicrystalline silicon (mc-Si) photovoltaic modules offers a production-ready method for the improvement of their short-wavelength (λ) spectral response and overall conversion efficiency. Several luminescent materials and mixtures thereof were evaluated for this purpose in an EVA matrix, including perylene and violanthrone dyes and a novel ligand sensitised europium complex. The external quantum efficiency of mc-Si modules can be greatly improved in the region of 300 nm <; λ <; 400 nm via luminescent down-shifting (LDS) of the incident light. In the best-case scenario, an increase in efficiency of 0.3% absolute is reported for a 59-cm2 minimodule. The LDS technology cannot only be simply transferred to a standard production line with no added layers and/or manufacturing processes but can be used to color photovoltaic (PV) modules for architectural purposes as well.

The Contribution of Planes, Vertices, and Edges to Recombination at Pyramidally Textured Surfaces

Abstract: We present a methodology by which one may distinguish three key contributors to enhanced recombination at pyramidally textured silicon surfaces. First, the impact of increased surface area is trivial and equates to a √3-fold increase in Seff,UL. Second, the presence of {1 1 1}-oriented facets drives a fivefold increase in Seff,UL at SiO2-passivated surfaces but a small (1.5-fold) increase for SiNx passivation. A third factor, which is often proposed to relate to stress at convex and concave pyramids and edges, is shown to depend on pyramid period (and, hence, vertex/ridge density). This third factor impacts least on Seff,UL when the pyramid period is 10 μm. At this period, it results in a negligible increase in Seff,UL at SiO2 -passivated textured surfaces but causes at least a sevenfold increase at the Si/SiNx interface. Finally, we found that Seff,UL is 1.5-2.0 times higher at inverted pyramid texture than at surfaces featuring a random arrangement of upright pyramids. The results of this study, particularly for the Si/SiNx system, likely depend on process conditions, but the methodology is universally applicable. We believe this to be the first study to distinguish the impact of {1 1 1} facets from those of vertices and edges. Further, we find that {1 1 1} surfaces, rather than vertices and edges, are chiefly responsible for the poor-quality passivation achieved by thick oxides on textured surfaces.

Optimizing CdTe Solar Cell Performance: Impact of Variations in Minority-Carrier Lifetime and Carrier Density Profile

Abstract: Using numerical simulations, we study the combined effects of nonuniform minority-carrier lifetime τ and carrier density NA on device performance. In a uniformly doped device, maximum open-circuit voltage Voc is obtained for high τ and high NA. The fill-factor (FF) is mainly dependent on the lifetime. When the lifetime is low, and NA is high, the FF suffers losses due to voltage-dependant carrier collection. For a low carrier density and low lifetime, the electric field strength is low, recombination is a competitive process to drift, and the FF is reduced. Simulations predict that it might be possible to increase the device efficiency with lower carrier density, if the back of the absorber is highly doped. This configuration increases the built-in potential and the electric field close to the junction region, while keeping the space-charge region wide. In addition, a device with such a profile is very tolerant toward lifetime variations of the highly doped layer. With our simulation parameters, when the absorber properties are uniform, efficiencies >;18% require experimentally unrealistic doping and lifetime values. If the back of the absorber is doped significantly higher than the rest, such efficiencies can be achieved with realistic values of doping and lifetime.

Imaging of Metastable Defects in Silicon

Abstract: Photoluminescence imaging is able to provide quantitative information about carrier lifetime in silicon wafers. Recently, this technique has been applied to measure the distribution of iron and chromium point defects in p-type silicon. In this paper, we summarize the state of the art and extend the impurity analysis by photoluminescence imaging with the detection of the boron-oxygen defect. Solar cells from p-type Czochralski silicon material are mostly limited by this defect, but its impact may also be significant for multicrystalline silicon. For the presence of several metastable defect species, we demonstrate the preparation of a specific state of the metastable defects with appropriate conditions for temperature and illumination and show that the respective impurity concentrations can be determined in parallel. We complete the analysis by discussing the effects of lateral carrier diffusion on the measurement result.

High-Efficiency Large-Area Rear Passivated Silicon Solar Cells With Local Al-BSF and Screen-Printed Contacts

Abstract: This paper describes the cell design and technology on large-area (239 cm2) commercial grade Czochralski Si wafers using industrially feasible oxide/nitride rear passivation and screen-printed local back contacts. A combination of optimized front and back dielectrics, rear surface finish, oxide thickness, fixed oxide charge, and interface quality provided effective surface passivation without parasitic shunting. Increasing the rear oxide thickness from 40 to 90 A in conjunction with reducing the surface roughness from 1.3 to 0.2 μm increased the Voc from 640 mV to 656 mV. Compared with 18.6% full aluminum back surface field (Al-BSF) reference cell, local back-surface field (LBSF) improved the back surface reflectance (BSR) from 65% to 93% and lowered the back surface recombination velocity (BSRV) from 310 to 130 cm/s. Two-dimensional computer simulations were performed to optimize the size, shape, and spacing of LBSF regions to obtain good fill factor (FF). Model calculations show that 20% efficiency cells can be achieved with further optimization of local Al-BSF cell structure and improved screen-printed contacts.

N-Type, Ion-Implanted Silicon Solar Cells and Modules

Abstract: Ion-implanted, screen-printed, high-efficiency, stable, n-base silicon solar cells fabricated from readily available 156-mm pseudosquare Czochralski wafers are described, along with prototype modules assembled from such cells. Two approaches are presented. The first approach, which involves a single phosphorus implant, has been used to produce cells (239 cm2) having a tight distribution of Jsc, Voc, and fill factor over a wide range of wafer resistivity (factor of 10), with Fraunhofer-certified efficiencies up to 18.5%. In spite of the full screen-printed and alloyed Al back, a method has been developed to solder such cells in a module. The second approach, which involves implanting both phosphorus for back-surface field (BSF) and boron for front emitter, has been used to produce n-base cells having local back contacts and dielectric (SiNx/SiO2) surface passivation. Efficiencies up to 19.1%, certified by Fraunhofer, have been realized on 239-cm2 cells. A method is also presented to express recombination activity in the cell base as a component of total reverse saturation current density. This allows recombination activity in all three regions of the cell (n+ region and its surface, n-base, and p+ region and its surface) to be compared as components of the total cell J0 to aid in maximizing Voc.

Evaluation of Series Resistance Losses in Screen-Printed Solar Cells With Local Rear Contacts

Abstract: We demonstrate industrially feasible large-area solar cells with passivated homogenous emitter and rear achieving energy conversion efficiencies of up to 19.4% on 125 mm × 125 mm p-type 2-3 Ω·cm boron-doped Czochralski silicon wafers. Front and rear metal contacts are fabricated by screen printing of silver and aluminum paste and firing in a conventional belt furnace. However, these cells suffer from moderate fill factors below 76% due to an increased series resistance. In this paper, we analyze the main cause of this increase. We vary the rear contact geometry over a wide range. By subtracting the respective contribution of the base from the measured series resistance, we extract a value of (55 ± 10) mΩ·cm2 for the effective specific contact resistivity of our screen-printed local aluminum rear contacts. We verify this value by local series resistance mappings from photoluminescence measurements resulting in a contact resistivity of (40 ± 10) mΩ·cm 2. Our analysis reveals that the highest potential for a further energy conversion efficiency improvement is to decrease the rear contact resistivity.

Combined Effects of Shunt and Luminescence Coupling on External Quantum Efficiency Measurements of Multijunction Solar Cells

Abstract: The combined effects of shunt and luminescence coupling on the measurement artifact of external quantum efficiency (EQE) of multi-junction solar cells are studied. The EQE measurement artifact is modeled using DC and small-signal equivalent circuits under voltage and light bias conditions. The modeling results are verified with EQE measurements of a Ge bottom cell of a triple-junction solar cell. It is found that the optimal bias light intensity to minimize the EQE measurement artifact is the result of the tradeoff between the shunt and the luminescence coupling effects.

High Efficiency N-Type Emitter-Wrap-Through Silicon Solar Cells

Abstract: In the ALBA-II project, Q-Cells SE, Bitterfeld-Wolfen, Germany, and the Institute for Solar Energy Research Hamelin, Emmerthal, Germany, are developing high-efficiency emitter-wrap-through (EWT) solar cells on n-type silicon wafers. N-type silicon grown by the Czochralski (Cz) method forms the basis of this high-efficiency solar cell development as it offers high bulk carrier lifetimes. The EWT device structure allows us to employ a simplified process sequence compared with interdigitated back-contact back-junction solar cells. High open-circuit voltages of our solar cells are achieved by different passivation layers for base and emitter surfaces and picosecond laser ablated contact openings. An optimization of the resistances along the current paths in base and emitter leads to an improvement in fill factor (FF) over former EWT solar cells. Together with the inherently high current densities of EWT solar cells, we achieve on our small-area (4-cm2, designated area without busbars) cells a short-circuit current density JSC of 40.4 mA/cm2, an open-circuit voltage VOC of 661 mV, FFs well above 80%, and, thus, cell efficiencies of up to 21.6%.

Spatial Modeling of Thin-Film Solar Modules Using the Network Simulation Method and SPICE

Abstract: The use of the network simulation method (NSM) for the modeling of thin-film solar modules is discussed. It will be shown that for the accurate modeling of small defects using the NSM, a variable mesh is required to keep the computation time within reasonable limits. A simulation tool for thin-film solar modules is developed that implements the NSM with a variable, adaptive mesh. The results of this model will be compared with electroluminescence experiments on a Cu(In,Ga)Se2 solar module with a defect (shunt). Furthermore, it is demonstrated that the program can handle complex-shaped inhomogeneities such as local variations in solar cell properties, illumination, and contact layer resistance.

PC2D: A Circular-Reference Spreadsheet Solar Cell Device Simulator

Abstract: This paper introduces PC2D, which is a new silicon solar cell device simulator that models 2-D effects entirely within a Microsoft Excel spreadsheet. The semiconductor drift-diffusion equations for electrons and holes are implemented using an iterative solution of cell formulas containing circular references. The familiar spreadsheet user interface and open-source access make this 2-D simulator a natural companion to PC1D. The 2-D simulator can run “live,” responding immediately to every change in parameter, or in “drive” mode, using macros to guide the solver through common tasks such as generating a current-voltage or a quantum efficiency curve. Example device simulations are presented for a selective-emitter cell, an emitter wrap-through cell, and an interdigitated back-contact cell.

Analysis of Chromatic Aberration Effects in Triple-Junction Solar Cells Using Advanced Distributed Models

Abstract: The consideration of real operating conditions for the design and optimization of a multijunction solar cell receiver-concentrator assembly is indispensable. Such a requirement involves the need for suitable modeling and simulation tools in order to complement the experimental work and circumvent its well-known burdens and restrictions. Three-dimensional distributed models have been demonstrated in the past to be a powerful choice for the analysis of distributed phenomena in single- and dual-junction solar cells, as well as for the design of strategies to minimize the solar cell losses when operating under high concentrations. In this paper, we present the application of these models for the analysis of triple-junction solar cells under real operating conditions. The impact of different chromatic aberration profiles on the short-circuit current of triple-junction solar cells is analyzed in detail using the developed distributed model. Current spreading conditions the impact of a given chromatic aberration profile on the solar cell I-V curve. The focus is put on determining the role of current spreading in the connection between photocurrent profile, subcell voltage and current, and semiconductor layers sheet resistance.

20% Efficient Screen-Printed n-Type Solar Cells Using a Spin-On Source and Thermal Oxide/Silicon Nitride Passivation

Abstract: N-type Si cells offer a compelling alternative to p-type cells to achieve high, stabilized cell efficiencies because they do not suffer from light-induced degradation. However, the most common dielectric materials that are used to passivate the n+ emitters of p-type cells-thermal SiO2 and SiNX-have historically provided poor passivation of the p+ emitters required for n-type cells. In this paper, we demonstrate that a thin thermal-SiO2/SiNX stack can, when appropriately fired, provide similar passivation on both p+ and n+ surfaces. Passivation studies on textured, SiO2/SiNX passivated p+-Si surfaces indicate that a high-temperature firing cycle is the most important step to achieving high-quality passivation and that the positive charge in the dielectric stack may have little detrimental effect on industrial-type, high surface concentration emitters. In addition, the suitability of spin-on boric acid sources for forming uniform, well-passivated p+ emitters on textured surfaces was studied. This passivation scheme and spin-on boron source were used to achieve 4-cm2 screen-printed n-type cells with efficiencies over 20% and open-circuit voltages up to 650 mV.

Investigation of Aluminum-Alloyed Local Contacts for Rear Surface-Passivated Silicon Solar Cells

Abstract: We present a comprehensive study on the rear contact formation of rear surface-passivated silicon solar cells by full-area screen printing and alloying of aluminum pastes on the locally opened passivation layer. We show that the point contact distance has a significant influence on the local alloying process for the contact formation resulting in different structural and electrical contact properties when applying conventional Al pastes. Increasing the distance leads to 1) high contact depths resulting in an enlargement of the contact area and 2) severely reduced thicknesses of the Al-doped p+ regions in the contact points, leading to a strong increase in recombination within the contact points. This inadequate contact formation can be directly linked to the deficiently low percentage of silicon that dissolves into the Al-Si melt during alloying. We demonstrate that by intentionally adding Si to the Al paste, the contact point geometry can be significantly improved and particularly becomes independent of the contact distance. Further investigations on the internal reflectance and the specific resistivity of the rear contact suggest an upper limit for the Si content added to the Al paste. In summary, we present a simple way to significantly improve the rear contact formation of rear surface-passivated silicon solar cells.

Increasing Power and Energy in Amonix CPV Solar Power Plants

Abstract: Large-scale concentration photovoltaic (CPV) power plants deliver the high energy production and low electricity cost that will allow photovoltaics to become a substantial portion of the electrical grid. High concentration minimizes the semiconductor material costs, while tracking delivers higher capacity factors and provides a better match to demand. In order to prove the net cost benefits, however, annual deployments must exceed the tens of megawatt range. CPV companies, including Amonix, are reaching this level in 2011. Ongoing performance improvements combined with aggressive cost reductions are the means for Amonix to win the projects that are needed to gain more economies of scale. Energy modeling has been used to increase both the rated power and energy yield. Solar power generators deployed in 2011 exceed previous performance by more than 10%. Rated output now exceeds 60 kW AC-PTC, and ac system efficiency exceeds 27%. A similar increase in performance is expected in 2012.

Recombination Activity and Impact of the Boron–Oxygen-Related Defect in Compensated N-Type Silicon

Abstract: In this paper, we present experimental data regarding the recombination activity and concentration of the boron-oxygen complex in compensated n-type silicon, doped with phosphorus and boron, when subjected to illumination. Unlike the data of Bothe in n-type silicon compensated with thermal donors, our results suggest the dominant defect level in our doping range to be a shallow level (EC- ET = 0.15 eV), with a capture cross-section ratio σn /σp of around 0.006, suggesting a negatively charged center. We also confirm previous results showing an increasing defect density with bias light intensity. Due to the strong lifetime reduction observed, we suggest that this material might not be suited to make high-efficiency n-type solar cells, unless practical strategies to reduce the defect concentration can be developed.

Three-Dimensional Optoelectronic Model for Organic Bulk Heterojunction Solar Cells

Abstract: This paper describes a 3-D optoelectronic device model for organic bulk heterojunction solar cells. Three-dimensional full-wave optical simulation enables us to incorporate different modern light trapping techniques, such as subwavelength nanostructures, in a typical organic bulk heterojunction solar cell, while 3-D electrical simulation allows us to handle localized enhancement or reduction of polaron/charge generation, recombination, and transport induced by modern light trapping techniques in the device. We calibrate our model with an experimental poly (3-hexylthiophene) (P3HT):phenyl-C60-butyric acid methyl ester (PCBM) organic bulk heterojunction solar cell by tuning only one free parameter as compared with other device models, which have multiple fitting parameters. A 3-D example of a silver nanoparticle array in a typical P3HT:PCBM organic bulk heterojunction cell is also demonstrated, and the current density-voltage relation is predicted with our model.

High-Efficiency Cells From Layer Transfer: A First Step Toward Thin-Film/Wafer Hybrid Silicon Technologies

Abstract: Future low-cost Si photovoltaics shall combine the high-efficiency potential of ultrathin monocrystalline Si films with the low cost per area of the Si-thin-film photovoltaics. The literature describes various techniques for fabricating ultrathin monocrystalline Si films with no need for sawing wafers. Layer transfer using epitaxy on porous Si and subsequent layer separation is one option. We demonstrate an independently confirmed aperture efficiency of 19.1% for a 4-cm2-sized layer transfer cell with a thickness of 43 μm. This cell has a passivated emitter and rear contact structure with an Al2O3-surface passivation by atomic layer deposition and lasered contact openings. Highly efficient thin crystalline solar cells have to be integrated into modules. We also report on laser bonding of Si cells to a metalized carrier for module integration.

Comparative Study of Different Back-Contact Designs for High-Efficiency CIGS Solar Cells on Stainless Steel Foils

Abstract: In this paper, Cu(In,Ga)Se2 (CIGS) solar cells on stainless steel foils were developed using a three-stage evaporation processes at low and high substrate temperatures. Different CIGS back-contact designs were used: a thin Ti adhesion layer together with a Mo single layer, Mo bilayer, and Mo bilayer in combination with either an Si3N4 or TiN impurity diffusion barrier layer. Solar cells on contacts with an Si3N4 and a TiN barrier coating showed no improvement in performance compared with the cells on Ti/Mo/Mo triple-layer contact, regardless of the CIGS deposition temperature. The variation of the Mo back-contact designs with no impurity diffusion barrier showed a significant decrease in solar cell performance when thin Mo contacts were used. Furthermore, capacitance to voltage measurements showed a decrease in the carrier concentration in CIGS grown on thin Mo contacts compared with the layers for solar cells on Ti/Mo/Mo triple layer structures. Best cell efficiencies of 17.3% were obtained using a Mo back-contact with a thin Ti adhesion layer, in combination with a low-temperature CIGS deposition process with no additional oxide or nitride impurity diffusion barrier layer on stainless steel foils.

Ultrathin Flexible Crystalline Silicon: Microsystems-Enabled Photovoltaics

Abstract: We present an approach to create ultrathin ( ;15%) and highly flexible photovoltaic (PV) modules should be possible.

Defect Formation in Silicon During Laser Doping

Abstract: Laser doping in industrial crystalline solar cells creates a selective emitter and thereby enhances the efficiency. Nevertheless, if not done carefully, the irradiation of the semiconductor introduces defects. We have suppressed defect formation by using a laser beam that is focused to a line with a width of only several micrometers. The maximal line width for a defect free recrystallization, however, depends on the surface orientation of the silicon. Using a transmission electron microscope, we find a dislocation-free recrystallized layer on (100)-oriented silicon wafers that are irradiated with a line focus smaller than or equal to 15 μm. For (111)-oriented surfaces, this holds for the use of a 5.2-μm-wide line focus. Wafers that are irradiated with a circular laser focus of diameter D = 36 μm show the formation of microcracks, but no hints of dislocations are found using transmission electron microscopy (TEM).

Interdigitated Back-Contacted Silicon Heterojunction Solar Cells With Improved Fill Factor and Efficiency

Abstract: We compare recently reported results of efficient back-contacted amorphous/crystalline silicon heterojunction solar cells with fill factors up to 78.8% with calculated j-V characteristics that are derived from an area-weighted summation of recombination currents taken from lifetime measurements. We compare solar cell structures with and without an intrinsic buffer layer beneath the p-type amorphous silicon emitter. We find that the inclusion of the buffer layer reduces the fill-factor potential by changing the ideality of the recombination current. However, analyzing the series resistance by illumination-dependent j-V-measurements, we find that the major loss mechanism of the fill factor is the limitation of the charge-carrier transport.

Physics and Statistics of Non-Ohmic Shunt Conduction and Metastability in Amorphous Silicon p–i–n Solar Cells

Abstract: In this paper, we present a physical model of the non-Ohmic shunt current ISH in amorphous silicon (a-Si:H) p-i-n solar cells and validate it with detailed measurements. This model is based on space-charge-limited (SCL) transport through localized p-i-p shunt paths. These paths can arise from n-contact metal incorporation in the a-Si:H layer, causing the (n)a-Si:H to be counterdoped to p-type. The model not only explains all the electrical characteristics of preexisting shunts but also provides insight into the metastable switching that is observed in the shunt-dominated region of dark current as well. We first verify the SCL model using simulations and statistically robust measurements, and then use it to analyze our systematic observations of nonvolatile switching of the low-bias dark characteristics. This study interprets broad experimental observations regarding shunt behavior, and suggests possible techniques for alleviating shunt-induced performance and reliability issues.