Collection efficiency in amorphous silicon solar cells

A simple relation is presented for the field assisted collection efficiency for amorphous silicon solar cells, assuming zero geminate recombination. The ratio of the collection efficiency under forward biasing condition to short circuited condition is computed. It is found to agree reasonably well with the experimentally observed values.     

Decreased emitter sheet resistivity loss in high-eficiency silicon solar cells

State-of-the-art two-dimensional (2D) numerical semiconductor device simulation tools are applied to bifacially contacted silicon solar cells of practical dimensions in order to investigate the 2D effects arising from ohmic voltage drops in cell emitters due to finite front metal grid line spacings. the 2D simulations show that for typical front finger spacings of high-efficiency silicon solar cells the minority carrier flow in the base deviates strongly from the purely linear flow assumed by one-dimensional (1D) theory. Compared to conventional 1D theory, this 2D effect results in reduced emitter sheet resistivity losses, an increased optimum front finger spacing and a reduced impact of finger spacing on cell efficiency. the 2D effects are of particular importance for concentrator solar cells. The 2D simulations presented in this work considerably improve the general understanding of internal device physics of high-efficiency silicon solar cells and reveal the limits of 1D models for the simulation of these devices.     

17.8-percent efficiency polycrystalline silicon solar cells

A substantial increase to 17.8% in the efficiency of cast polycrystalline solar cells was achieved by incorporating phosphorus pretreatment and rear aluminium treatments into the passivated emitter solar cell (PESC) sequence. The deleterious effects of grain boundaries and defects were nullified to such an extent that the performance of cells produced on the less-expensive polycrystalline material of medium grain size matched the performance of those fabricated on expensive semiconductor-grade substrates. Surface texturing of polycrystalline solar cells by novel approaches appears feasible, with a corresponding 5% relative performance increase anticipated, as observed with crystalline cells     

A one-dimensional theory of high base resistivity tandem-junction solar cells in low injection

A one-dimensional theoretical model of the Tandem-Junction Solar Cell (TJC) with high base resistivity and under low-level injection is derived. The model provides a theoretical basis for a previously published conceptual model and allows for the calculation of the spectral response and other performance parameters, namely, Isc, Voc, Pm,FF, and η, under variation of one or more of the geometrical and material parameters and 1-MeV electron fluence. Sample calculation results of computer simulation of this model are presented.     

Diffusion‐free high efficiency silicon solar cells

Traditional POCl₃ diffusion is performed in large diffusion furnaces heated to ~850 C and takes an hour long. This may be replaced by an implant and subsequent 90‐s rapid thermal annealing step (in a firing furnace) for the fabrication of p‐type passivated emitter rear contacted silicon solar cells. Implantation has long been deemed a technology too expensive for fabrication of silicon solar cells, but if coupled with innovative process flows as that which is mentioned in this paper, implantation has a fighting chance. An SiOx/SiN_y rear side passivated p‐type wafer is implanted at the front with phosphorus. The implantation creates an inactive amorphous layer and a region of silicon full of interstitials and vacancies. The front side is then passivated using a plasma‐enhanced chemical vapor deposited SiN_xH_y. The wafer is placed in a firing furnace to achieve dopant activation. The hydrogen‐rich silicon nitride releases hydrogen that is diffused into the Si, the defect rich amorphous front side is immediately passivated by the readily available hydrogen; all the while, the amorphous silicon recrystallizes and dopants become electrically active. It is shown in this paper that the combination of this particular process flow leads to an efficient Si solar cell. Cell results on 160‐µm thick, 148.25‐cm² Cz Si wafers with the use of the proposed traditional diffusion‐free process flow are up to 18.8% with a V_oc of 638 mV, J_sc of 38.5 mA/cm², and a fill factor of 76.6%. Copyright © 2012 John Wiley & Sons, Ltd.     

Highest efficiency rapid thermal processed multicrystalline silicon solar cells

The formation of pn junctions and surface passivation by rapid thermal processing is being proved as a new and competitive method for silicon solar cell production. As the main process mechanisms are enhanced, the total process time at high temperature can be kept in the minute range, for the realization of emitter, back surface field (BSF) and surface passivation. In this work, we demonstrate for the first time that this knowledge, avoiding any in‐situ annealing step acquired on the sc‐Si, can also be applied on industrial mc‐Si (Polix_©) without bulk degradation, leading to a record conversion efficiency of 16·7%. Copyright © 2001 John Wiley & Sons, Ltd.     

Limiting efficiency of silicon solar cells under highly concentrated sunlight

Recent work has shown that the upper bound on the energy conversion efficiency of silicon cells under concentrated sunlight lies in the 36-37-percent range regardless of the concentration ratio. These bounds are reassessed at very high cencentration levels where loss of conductivity modulation and loss in carrier collection efficiency due to Auger effects become important. Previous work is shown to overestimate the efficiency bound at such levels as well as the cell thickness required to attain this bound.     

Excimer laser-processed oxide-passivated silicon solar cells of 19.5-percent efficiency

Single-crystal p+ -n-n+ silicon solar cells with AM1.5 efficiencies exceeding 19.5 percent have been fabricated using glow-discharge implantation and pulsed excimer laser annealing, together with techniques for reducing the recombination current. These techniques include extrinsic passivation by thermal oxide growth and fine-line photolithography for metallization in order to reduce the area of metal-silicon contact. These are the highest efficiency ion-implanted nonconcentrator cells reported to date and to our knowledge they are the highest efficiency p-on-n cells made by any technique.     

The limiting efficiency of silicon solar cells under concentrated sunlight

The intrinsic limits on the energy conversion efficiency of silicon solar cells when used under concentrated sunlight are calculated. It is shown that Auger recombination processes are even more important under concentrated sunlight than nonconcentrated sunlight. However, light trapping can be far more effective under concentrated light due to the better defined direction of incident light. As a result of these effects, the limiting efficiency lies in tile 36-37-percent range regardless of concentration ratio compared to the limiting value of 29.8 percent for a nonconcentrating cell with isotropic response.     

Fine line printed silicon solar cells exceeding 20% efficiency

Silicon solar cells with passivated rear side and laser‐fired contacts were produced on float zone material. The front side contacts are built up in two steps, seed and plate. The seed layer is printed using an aerosol jet printer and a silver ink. After firing this seed layer through the silicon nitride layer, the conductive layer is grown by light induced plating. The contact formation is studied on different emitter sheet resistances, 55 Ω/sq, 70 Ω/sq, and on 110 Ω/sq. These emitters are passivated with a PECVD silicon nitride layer which also acts as an anti‐reflection coating. Even on the 110 Ω/sq emitters it was possible to reach a fill factor of 80·1%. The electrical properties i.e., the contact resistance of the front side contacts are studied by transfer length model (TLM) measurements. On a cell area of 4 cm² and emitter sheet resistance of 110 Ω/sq, a record efficiency of 20·3% was achieved. Excellent open‐circuit voltage (V_oc) and short‐circuit current (j_sc) values of 661 mV and 38·4 mA/cm² were obtained due to the low recombination in the 110 Ω/sq emitter and at the passivated rear surface. These results show impressively that it is possible to contact emitter profiles with a very high efficiency potential using optimized printing technologies. Copyright © 2008 John Wiley & Sons, Ltd.     

High efficiency polycrystalline silicon solar cells using lowtemperature PECVD process

Conventionally directionally solidified (DS) and silicon film (SF) polycrystalline silicon solar cells are fabricated using gettering and low temperature plasma enhanced chemical vapor deposition (PECVD) passivation. Thin layer (~10 nm) of PECVD SiO₂ is used to passivate the emitter of the solar cell, while direct hydrogen rf plasma and PECVD silicon nitride (Si₃N₄) are implemented to provide emitter and bulk passivation. It is found in this work that hydrogen rf plasma can significantly improve the solar cell blue and long wavelength responses when it is performed through a thin layer of PECVD Si₃N₄. High efficiency DS and SF polycrystalline silicon solar cells have been achieved using a simple solar cell process with uniform emitter, Al/POCl₃ gettering, hydrogen rf plasma/PECVD Si₃N₄ and PECVD SiO₂ passivation. On the other hand, a comprehensive experimental study of the characteristics of the PECVD Si₃N₄ layer and its role in improving the efficiency of polycrystalline silicon solar cells is carried out in this paper. For the polycrystalline silicon used in this investigation, it is found that the PECVD Si₃N₄ layer doesn't provide a sufficient cap for the out diffusion of hydrogen at temperatures higher than 500°C. Low temperature (⩽400°C) annealing of the PECVD Si₃N ₄ provides efficient hydrogen bulk passivation, while higher temperature annealing relaxes the deposition induced stress and improves mainly the short wavelength (blue) response of the solar cells     

Effect of liquid dielectrics on the efficiency of silicon solar cells

The results of experimental studies of the change in the photoelectric characteristics of silicon solar cells produced as a result of depositing thin, liquid dielectric layers (glycerine, acetone, isopropyl alcohol, butanol, dioxane, deionized water) are presented. It is shown that the presence of these liquids reduces the forward and reverse currents, substantially raises the short-circuit currents and open-circuit voltage, and significantly increases the efficiency (by up to 40–60%). Possible physical models are proposed for this effect. Fiz. Tekh. Poluprovodn. 33, 1467–1468 (December 1999)     

The influence of diffusion-induced dislocations on high efficiency silicon solar cells

Heavy boron and phosphorus diffusions are used in many high efficiency, monocrystalline silicon solar cell designs to form localized contact diffusions and back surface fields. It is important to cell performance that these diffusion processes do not increase bulk recombination by the introduction of lattice defects. This paper investigates the effect of boron and phosphorus misfit dislocation networks on the bulk recombination parameters, and performance of high efficiency silicon solar cells. It demonstrates that the formation of either a boron or phosphorus misfit dislocation network generates bulk asymmetric Shockley-Read-Hall recombination centers, and that these adversely affect the current-voltage curve, local ideality factor, and ultimately the performance of p-type silicon solar cells.     

An experimental study of drift-field silicon solar cells

Silicon solar cells were constructed with drift fields of various widths and magnitudes. Both initial performance and performance after irradiation with up to 10¹⁶1 MeV electrons/cm²are compared with theory. Behavior is much as expected if the radiation damage is assumed to vary with doping level. This latter assumption leads to the conclusion that little change in cell performance occurs because of the field. Such a result was not anticipated at the start of the investigation, since the neglect of the capture cross-section variation gave the prediction of appreciably improved radiation tolerance.     

Improved efficiency silicon solar cell module

This paper describes a solar cell module efficiency of 22.3% independently measured at Sandia National Laboratories. This is the highest ever confirmed efficiency for a photovoltaic module of this size achieved by cells made on any material. This 787-cm² module used 40 large-area double-layer antireflection coated PERL (passivated emitter, rear locally-diffused) silicon cells of average efficiency of 23.1%. The double-layer coating, together with an improved cell structure and a shingled encapsulation technique, considerably contributed to this efficiency improvement. Also reported is an independently confirmed efficiency of 23.7% for a 21.60-cm² cell of the type used in the module, the highest efficiency ever reported for a silicon cell of this size     

p-Layer bandgap engineering for high efficiency thin film silicon solar cells

Spectroscopic ellipsometry (SE), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and optical transmittance measurements were used to study and establish a correlation between the open-circuit voltage (V_oc) of solar cells and the p-layer optical band gap (E_p). It is found that the ellipsometry measurement can be used as an inline non-destructive diagnostic tool for p-layer deposition in commercial operation. The analysis of ellipsometric spectra, together with the optical transmittance data, shows that the best p-layer appears to be very fine nanocrystallites with an E_p 1.95 eV. HRTEM measurements reveal that the best p-layer is composed of nanocrystallites ~9 nm in size. It is also found that the p-layer exhibits very good transmittance, as high as ~91.6% at ~650 nm. These results have guided us to achieve high V_oc value 1.03 V for thin film silicon based single junction solar cell.     

High efficiency screen printed bifacial solar cells on monocrystalline CZ silicon

We present industrialized bifacial solar cells on large area (149 cm²) 2 cm CZ monocrystalline silicon wafers processed with industrially relevant techniques such as liquid source BBr₃ and POCl₃ open‐tube furnace diffusions, plasma enhanced chemical vapor deposition (PECVD) SiN_x deposition, and screen printed contacts. The fundamental analysis of the paste using at boron‐diffused surface and the bifacial solar cell firing cycle has been investigated. The resulting solar cells have front and rear efficiencies of 16.6 and 12.8%, respectively. The ratio of the rear J_SC to front J_SC is 76.8%. It increases the bifacial power by 15.4% over a conventional solar cell at 20% of 1‐sun rear illumination, which equals to the power of a conventional solar cell with 19.2% efficiency. We also present a bifacial glass–glass photovoltaic (PV) module with 30 bifacial cells with the electrical characteristics. Copyright © 2010 John Wiley & Sons, Ltd.     

High efficiency inversion layer solar cells on polycrystalline silicon by the application of silicon nitride

Plasma enhanced CVD silicon nitride is introduced for the fabrication of inversion layer solar cells on p-type polycrystalline silicon. The same high interface quality as obtained for Si-nitride on monocrystalline silicon could also be achieved for polycrystalline silicon. This includes high interface charge densities up to 6.6 × 10¹²cm^-2and high UV sensitivity of the cells. For 4-cm²polycrystalline metal-insulator-semiconductor inversion layer (MIS/IL) solar cells active area efficiencies up to 13.4 percent (12.3-percent total area efficiency) under AM1 illumination could be reached, the highest values yet reported for polycrystalline silicon inversion layer solar cells on a total area basis. For the coprocessed MIS/IL cells on monocrystalline 0.7-ω. cm p-Si