Large-area Silicon Solar Cells Research Articles

Fundamental Aspects Concerning the Validity of the Standard Equivalent Circuit for Large‐Area Silicon Solar Cells

The standard equivalent circuit of a solar cell amounts to a lumped description by separate diode and resistor elements. As its application to a large‐area silicon solar cell effectively implies averaging the emitter resistance which, however, is closely coupled to the p–n junction, it is not self‐evident that it works more or less well. Using an analytically solvable distributed series resistance model and systematically treating the deviations from the ideal case of zero emitter resistance, the equivalent circuit is found in linear order in the sheet resistivity. In this linear order, the lumped voltage losses are fully compatible with the integrated Joule losses; this compatibility turns out to be a necessary and sufficient condition for modeling the local series resistance of a large‐area silicon solar cell. In higher orders of the sheet resistivity, however, the lumped voltage losses are not compatible with the integrated Joule losses, which means that the equivalent circuit cannot describe these higher orders. The equivalent circuit resulting from the linear‐order lumped series resistance accounts for the experimentally observed variation of the lumped series resistance along the current–voltage characteristic, which turns out to be fully described by a dependence on the dark diode current only.

Distributed series resistance in a one-dimensional two-diode model revisited

The lumped series resistance Rs of large-area silicon solar cells, obtained from current–voltage (I–U) data according to the two-light-level method, varies along the I–U characteristic. Such a variation can most simply be described by the linear-response series resistance model (LR-Rs), recently developed in connection with luminescence imaging. Here, independently obtained experimental data are used to test the applicability of the LR-Rs model to Rs data based on I–U characteristics. After subtracting a non-distributed part from the measured Rs data, the inverse of the remaining distributed part shows a scaling proportional to the inverse of the bias-dependent diode resistance; a slope value of 1 is used as predicted by the LR-Rs model applied to a laterally one-dimensional geometry. The same experimental data have previously been interpreted based on a mathematically rather complicated model published already many years ago; just recently it was found that in some cases this model may lead to unphysical results. The present LR-Rs model based proper interpretation of the variation of the lumped series resistance along the I–U characteristic leads to a roughly half-by-half splitting between the distributed and the non-distributed part of Rs. This share has been observed many years for “economically reasonable” solar cells investigated by the CELLO technique. The successful usage of the LR-Rs model for I–U based Rs data is a strong hint that its underlying physical concepts are of general validity.

Linear-response Description of the Series Resistance of Large-area Silicon Solar Cells: Resolving the Difference between Dark and Illuminated Behavior

Directly from luminescence images it can be shown that, for constant average injection (lumped dark current) and for not-too-large lateral voltage differences, besides the sign, the current flow direction doesn’t play any role for the voltages present, so the series resistances in the dark and under illumination are the same. This fits to the results of a linear-response based series resistance description, treating lateral voltage differences on large-area silicon solar cells in linear order in the series resistance as deviation from the case of zero resistance. In this approach it is found that for constant lumped dark current, emitter and grid of a large-area solar cell can be described as a passive network. Therefore, no difference occurs in the voltage distribution caused by inward and outward currents except for the sign. This contradicts several literature works reporting a smaller lumped series resistance of silicon solar cells in the dark than under illumination. However, we show that this contradiction is just a result of the series resistance definition applied in the respective works or that it can be the result of unsuitable measurement conditions. In a numerical modeling of a large-area silicon solar cell as a 1D distributed structure, using exactly the same parameters as Araújo et al. [IEEE-TED 33 (3), 391–401 (1986)] but calculating the lumped series resistance from the integrated Joule losses, we obtain completely different results than Araújo et al.: Under short-circuit condition, the series resistance stays constant, and there is no difference between the open-circuit and dark series resistance; the latter show the same dependence on the diode current density.

Statistical analysis and engineering fit models for two-diode model parameters of large area silicon solar cells

In this paper, an attempt has been made to find the correlation between various parameters of the two-diode equivalent circuit model of silicon solar cells. The statistical analysis has been done to find the engineering fit models between these parameters. The solar cell parameter data of 82 solar cell samples has been estimated using the Particle Swarm Optimization method from the measured illuminated I-V characteristics of the cells. This data on estimated parameters has been used to find the Pearson’s correlation coefficient between different parameters and the significant outcome of this work is that it revealed a high correlation between the first diode’s reverse saturation current and its ideality factor and a medium correlation between the second diode’s reverse saturation current and its ideality factor in the two-diode equivalent circuit model of a silicon solar cell. An engineering fit model has been suggested between the reverse saturation current and ideality factor of the first diode, based on the data of 82 large area (∼154.8cm2) silicon solar cells with AM1.5 conversion efficiency between 15% and 18.4%. The suggested engineering fit between the two would be a method to reduce the number of parameters needed for silicon solar cell modeling and to make it easier for predicting the module output.

Evaluation of advanced p‐PERL and n‐PERT large area silicon solar cells with 20.5% energy conversion efficiencies

AbstractIn this paper, we evaluate p‐type passivated emitter and rear locally diffused (p‐PERL) and n‐type passivated emitter and rear totally diffused (n‐PERT) large area silicon solar cells featuring nickel/copper/silver (Ni/Cu/Ag) plated front side contacts. By using front emitter p‐PERL and rear emitter n‐PERT, both cell structures can be produced with only a few adaptations in the entire process sequence because both feature the same front side design: homogeneous n+ diffused region with low surface concentration, SiO2/SiNx:H passivation, Ni/Cu/Ag plated contacts. Energy conversion efficiencies up to 20.5% (externally confirmed at FhG‐ISE Callab) are presented for both cell structures on large area cells together with power‐loss analysis and potential efficiency improvements based on PC1D simulations. We demonstrate that the use of a rear emitter n‐PERT cell design with Ni/Cu/Ag plated front side contacts enables to reach open‐circuit voltage values up to 676 mV on 1–2 Ω cm n‐type CZ Si. We show that rear emitter n‐PERT cells present the potential for energy conversion efficiencies above 21.5% together with a strong tolerance to wafer thickness and bulk resistivity. Copyright © 2014 John Wiley & Sons, Ltd.

Nanoimprint patterning for tunable light trapping in large-area silicon solar cells

We demonstrate the flexibility of UV nanoimprint lithography for effective light trapping in p –i –n a-Si:H/μc-Si:H tandem solar cells. A textured polymeric layer covered with pyramidal transparent conductive oxide structures is shown as an ideal system to promote front light scattering and thus enhanced photocurrent. The double structure incorporated into micromorph tandem thin film silicon solar cells is systematically investigated in order to find a relationship between interface morphology, optical properties and photovoltaic characteristics. To prevent the formation of defects during cell growth, a controllable smoothing of the imprinted texture is developed. Modules grown on polymer structures smoothed via multi-replication show excellent performance reaching a photocurrent of 12.6 mA/cm 2 and an efficiency of 12.8%.

Application of power loss calculation to estimate the specific contact resistance of the screen-printed silver ohmic contacts of the large area silicon solar cells

The establishment of a suitable contact formation methodology is a critical part of the technological development of any metal-to-semiconductor contact structure. Many test structures and methodologies have been proposed to estimate the specific contact resistance (ρc) of the planar ohmic contacts formed on the heavily doped semiconductor surface. These test structures are usually processed on the same wafer to monitor a particular process. In this study, new experimental procedure has been evolved to assess the value of ρc of the screen-printed front silver (Ag) thick-film metal contact to the silicon surface. The essential feature of this methodology is that it is an iteration technique based on the calculation of power loss associated with various resistive components of the solar cell normalized to the unit cell area. Therefore, this method avoids the complexity of making the design of any lay out of a standard contact resistance test structure like transmission line model (TLM) or Kelvin resistor, etc. It was shown that value of specific contact resistance of the order of 1.0 × 10−5 Ω−cm2 is measured for the Ag metal contacts formed on the n+ silicon surface. This value is much lower than the ρc data previously reported for the screen-printed Ag contacts. The sintering process of the front metal contact structure at different furnace setting is carried out to understand the possible wet interaction and metal contact formation as a function of the firing. Therefore, the study is further extended to study the peak firing temperature dependence of the ρc of screen-printed Ag metal contacts. It will help to assess the specific contact resistance of the ohmic contacts as a function of firing temperature of sintering process.

04/01406 Effect of spatial variation of incident radiation on spectral response of a large area silicon solar cell and the cell parameters determined from it: Singh, P. K. et al. Solar Energy Materials and Solar Cells, 2003, 80, (1), 21–31

04/01406 Effect of spatial variation of incident radiation on spectral response of a large area silicon solar cell and the cell parameters determined from it: Singh, P. K. et al. Solar Energy Materials and Solar Cells, 2003, 80, (1), 21–31

Effect of spatial variation of incident radiation on spectral response of a large area silicon solar cell and the cell parameters determined from it

Effect of spatial variation of incident monochromatic light on spectral response of an n+–p–p+ silicon solar cell and determination of diffusion length of minority carriers (Lb) in the base region and the thickness of the apparent dead layer (xd) in the n+ emitter from the spectral response have been investigated. Spectral response of a few 10 cm diameter and 10×10 cm2 pseudo-square silicon solar cells was measured with the help of a standard silicon solar cell of 2×2 cm2 area in 400–1100 nm wavelength range. Different areas (4, 9, 16, 25 and total area 78.6 or 96 cm2) were exposed. The effect of the radial variation of incident radiation was determined quantitatively by defining a parameter f1 as the ratio of the average intensity falling on the reference cell to that on the exposed area of the test cell. The value of f1 varied between 1 and 1.15 (1.25) as the exposed area of the cell varied from 4 cm2 to 78.6 (96) cm2 indicating that the spatial inhomogeneity of intensity increased with the increase in the exposed cell area. Short-circuit current densities, Jsc, computed from spectral response data for AM1.5 spectrum were less compared to the directly measured values by a factor which was nearly equal to f1. However, radial variation of intensity does not affect the determination of diffusion length of minority carriers in the base region (by the long wavelength spectral response, LWSR method using the measured spectral response data in 0.85<λ<1.05 μm range) and the thickness of the dead layer (by the method of Singh et al. using the data of 0.45<λ<0.65 μm range) significantly.

00/01459 Formation of porous silicon for large-area silicon solar cells: a new method

00/01459 Formation of porous silicon for large-area silicon solar cells: a new method

A novel method for the determination of the front contact resistance in large area screen printed silicon solar cells

The contact resistance at the interface of the semiconductor (Si)/metal (Ag) contact in a solar cell is generally small because it is made on a heavily doped silicon surface. In the case of large area terrestrial silicon solar cells contact is made by screen printing silver paste which has pores even after baking and sintering. Therefore the entire area is not active and the contact resistance is not entirely negligible. This paper describes the use of a simple method for the measurement of contact resistance of the front grid in a large area silicon solar cell based on the application of three-point probes using the existing fixed equidistant grid fingers of the solar cell. In addition, the value of the sheet resistivity is obtained from the measurement and is compared with that measured by the standard four-point probe method on different samples. The power losses due to current travelling through different resistive elements in the solar cell were obtained by using the standard formulae. The calculations show that power loss due to contact resistance is a small part of the total power loss. The total power loss agrees with that determined from the illuminated I-V characteristics of the solar cell.

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N +/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N +/P, 78.5 cm 2) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current ( I sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I– V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.

Substrate to substrate aluminized bonding of 10 cm round silicon substrates for application in solar photovoltaics

Large area silicon solar cells with screen printed contacts have been realized for the first time on 10 cm diameter, p-type, Cz silicon wafers which were bonded to silicon substrates by alloying of a suitably thick screen printed layer of Al on them. In cells made on 300 μm thick wafers without texturization, antireflection coating and passivation of the front surface, the values of the open-circuit voltage ( V oc), the short-circuit current density ( J sc), curve factor (CF) and the efficiency ( η) were found to be in the range 572–579 mV, 16–19.2 mA cm −2, 0.53–0.61 and 5.5–5.89%, respectively, under simulated tungsten halogen light of 100 mW cm −2 intensity. Using thinner wafers and having optical confinement, surface passivation and effective back surface field, the cell performance would be substantially improved. In fact, an efficiency close to 18% (AM1.5) would be realizable with this approach. Another attractive feature of this approach is that a low-cost silicon substrate could be used at the bottom that would act as support for the thin top surface without disadvantage to the cell performance. In this paper only the principle has been demonstrated experimentally. Possible improvements have been shown by computer simulation.

Fabrication of large area silicon solar cells by rapid thermal processing

Large area n+pp+ solar cells have been fabricated on 10 cm×10 cm pseudo-quasi-square CZ silicon wafers (1 Ω cm, p-type) predominantly used by the photovoltaic (PV) industry. All the high-temperature steps have been performed by rapid thermal processing (RTP). Emitter formation, back surface field (BSF) formation, and surface oxidation have been performed in just two RTP steps each lasting 50 s. Solar cells of 15% efficiency have been fabricated this way, demonstrating the applicability of this low thermal budget technology to large area, modulable size, industrial quality Si wafers. Furthermore, the rapid thermal oxidation (RTO) is shown to result in good quality thin oxides with Si/SiO2 interface trap densities (Dit)&amp;lt;1011 cm−3 eV−1 near-midgap.

Developmental experiments on large-area silicon solar cells

A comprehensive design/material/technological study was conducted with the aim of obtaining high-efficiency back-surface-field- (BSF-) type bifacial solar cells with reproducible parameters on large-area n- and p-silicon wafers. Practical ways of attenuating the severe limitations imposed by different kinds of areal inhomogeneities on the electro-optical performance of large-area BSF single-crystal silicon solar cells are described and tested experimentally. Various procedures leading to a substantial increase of both the emitter and the base contributions to the generated photocurrent are implemented and discussed in detail. The p+-n-n+ and n+-p-p+ cells were processed as bifacial devices and tested under both front and backside AM1 illumination. The trial devices were fabricated on 2-in and 3-in. commercially available lower-grade silicon and the minimization of the cell cost was one of the sought-after objectives. The combination of simple design/technological approaches described in this work has ultimately led to the development of low-cost, high-efficiency (17%–18%) large-area silicon solar cells with good overall electro-optical performance as bifacial devices. The results of this work show clearly that simple, but adequately designed/processed cells fabricated on an industrial scale on large-area silicon wafers could possess parameters similar to those of sophisticated laboratory samples elaborated on small-area silicon. With the design/technological approaches outlined in this work, the use of even cheaper, i.e., lower-grade single-crystal silicon, could still yield 2- and 3-in. cells with fairly acceptable conversion efficiencies.

Low cost molecular ion implantation equipment

A low cost, high current ion implantation system is described which includes fast annealing by laser induced surface melting. The implant machine is very simple, 20 cm in length and consists of an ion source with glow discharge in a gas atmosphere containing the dopant. No mass separation is used and the high density beam (1 mA/cm 2) consists of both atomic and molecular ions. Applications of this system are in the field of large area silicon solar cells.

A Proposed New Concept for a Solar-Energy Converter

The potential advantages of terrestrially utilizing nonpolluting solar energy are well-known. One of the more promising approaches to achieving the desired high solar-electrical conversion efficiency is the direct process, avoiding Carnot efficiency limitations of heat engines. The chief present-art example is the large area silicon solar cell based on utilization of quantum properties of light. In the United States it is now an established technology created from the space program. This paper presents the results of some preliminary new research exploring the possibilities of creating high efficiency solar-electricity converters utilizing wave-like properties of radiation interacting with absorber-converter elements. The concept is revealed for what is believed to be a new, unique, and potentially useful pyramidical solar radiation absorber-converter structure. It is based on the possibility of extending concepts of power absorbing antennas and converters to the visible light range. The resulting proposed converter structure would have a rough surface texture and yield a d-c output. It may have, if subsequently researched, significant efficiency, cost, and fabricating advantages, particularly for large-scale terrestrial utilization of solar energy. The concept is called an Electromagnetic Wave Energy Converter (EWEC). Concept validity evidences of a preliminary kind at both microwave and near light wavelengths are presented.