Engineering topics
Publications and source records attributed to Lindholm, F. A..
Test circuit for silicon solar cells suppresses spurious effects. New circuit increases accuracy of measurements of recombination lifetime and effective surface recombination velocity in silicon solar cell. Fast electronic switch, circuit grounds forward-biased cell so rapidly transient voltage to be measured not affected significantly.
Method developed to raise value of open-circuit voltage in silicon solar cells by incorporating high/low junction in cell emitter. Power-conversion efficiency of low-resistivity silicon solar cell considerably less than maximum theoretical value mainly because open-circuit voltage is smaller than simple p/n junction theory predicts. With this method, air-mass-zero opencircuit voltage increased from 600 mV level to approximately 650 mV.
A determination is made of the temperature-dependence of emitter saturation current in bipolar devices which allows the derivation of a value for bandgap narrowing that is in better agreement with other determinations than previous results based on ohmic contact measurements of temperature dependence. The new values were obtained by varying the surface recombination velocity at the emitter surface. This improves accuracy by varying the minority carrier surface recombination velocity at the emitter contacts of otherwise indistinguishable emitters.
Several parameters of certain three-dimensional semiconductor devices including diodes, transistors, and solar cells can be determined without solving the actual boundary-value problem. The recombination current, transit time, and open-circuit voltage of planar diodes are emphasized here. The resulting analytical expressions enable determination of the surface recombination velocity of shallow planar diodes. The method involves introducing corresponding one-dimensional models having the same values of these parameters.
Air-mass-zero open-circuit voltage increased to 650 mV. Oxide-chargeinduced high/low emitter solar cell, electron accumulation layer induced by positive charge. High/low emitter suppresses dark emitter recombination current, resulting in power-conversion efficiencies significantly higher than previously achieved.
This work is a first detailed attempt to systematize the design of silicon solar cells. Design principles follow from three theorems. Although the results hold only under low injection conditions in base and emitter regions, they hold for arbitrary doping profiles and include the effects of drift fields, high/low junctions and heavy doping concentrations of donor or acceptor atoms. Several optimal designs are derived from the theorems, one of which involves a three-dimensional morphology in the emitter region. The theorems are derived from a nonlinear differential equation of the Riccati form, the dependent variable of which is a normalized recombination particle current.
High-low (HL) junctions form a part of many semiconductor devices, including back surface field solar cells. A first experimental determination and interpretation of the voltage across the HL junction under low- and high-injection conditions is presented as a function of the voltage across a nearby p/n junction. Theoretical analysis from first principles is shown to bear well on the experimental results. In addition, a test structure is proposed for measurement of the effective surface recombination velocity at the HL junctions.
The temperature dependence of the emitter saturation current for bipolar devices was studied by varying the surface recombination velocity at the emitter surface. From this dependence, the value was derived for bandgap narrowing that is in better agreement with other determinations that were obtained from the temperature dependence measure on devices with ohmic contacts. Results of the first direct measurement of the minority-carrier transit time in a transparent heavily doped emitter layer were reported. The value was obtained by a high-frequency conductance method recently developed and used for doped Si. Experimental evidence is presented for significantly greater charge storage in highly excited silicon near room temperature than conventional theory would predict. These data are compared with various data for delta E sub G in heavily doped silicon.
A reciprocity theorem is presented that relates the short-circuit current of a device, induced by a carrier generation source, to the minority-carrier Fermi level in the dark. The basic relation is general under low injection. It holds for three-dimensional devices with position dependent parameters (energy gap, electron affinity, mobility, etc.), and for transient or steady-state conditions. This theorem allows calculation of the internal quantum efficiency of a solar cell by using the analysis of the device in the dark. Other applications could involve measurements of various device parameters, interfacial surface recombination velocity at a polcrystalline silicon emitter contact, for rexample, by using steady-state or transient photon or mass-particle radiation.
The first use of a (silicon)/heavily doped polysilicon)/(metal) structure to replace the conventional high-low junction or back-surface-field (BSF) structure of silicon solar cells is reported. Compared with BSF and back-ohmic-contact (BOC) control samples, the polysilicon-back solar cells show improvements in red spectral response (RSR) and open-circuit voltage. Measurement reveals that a decrease in effective surface recombination velocity S is responsible for this improvement. Decreased S results for n-type (Si:As) polysilicon, consistent with past findings for bipolar transistors, and for p-type (Si:B) polysilicon, reported here for the first time. Though the present polysilicon-back solar cells are far from optimal, the results suggest a new class of designs for high efficiency silicon solar cells. Detailed technical reasons are advanced to support this view.
The activation behavior of the minority-carrier mobility and diffusivity in heavily doped (about 10 to the 20th per cu cm) Si(Si:As) was investigated in the temperature range, 20 - 350 K. Experimental results indicate that hole transitions between the valence band and localized shallow states give rise to the observed behavior. The activation energy is about 10 meV, which suggests that the localized states originate from band tails but does not rule out trapping at boron atoms in the compensated n(+) region.
The theoretical models now available that characterize heavily doped (highly conducting) regions in silicon are survyed. Analytical and numerical approaches that determine the influence of such regions on the conversion efficiency of solar cells are examined. Although dilutely doped silicon is well characterized except for some disagreement about optical absorption coefficients, what exists now for heavily doped silicon and its interplay with adjoining regions is an incomplete theory in which not all contributers to transport, recombination, generation, and trapping are defined. Further, the parameters relating to these mechanisms and their values as determined by experiment are subject to various interpretations. The characterization of heavily doped silicon is treated not as a theory but rather as an imperfectly articulated and incompletely formalized body of experience. This view is intended to help point the way toward the attainment of a more complete of heavily doped silicon and thereby toward more informed designs of solar cells. Because computer programs constitute tools both for design and for estimating performance limits, the review includes some remarks pertinent to existing and developing programs.
Recently it has been pointed out that the saturation current of a semiconductor filament which constitutes part of a p-n junction diverges when the surface recombination velocity at the faces become infinitely large. Here it is pointed out that this is to be expected on physical grounds since, whenever the carrier concentration is kept off equilibrium by an outside agent, and at the same time recombination lifetimes in the bulk or in surface layers tend to zero, concentration gradients tend to infinity. As also previously noted, the situation can be remedied by using realistic (finite) surface recombination velocities in model calculations. However, this procedure leads to mathematical complexities which have been circumvented recently by the introduction of a heuristic approach. It is the aim of this paper to assess the validity of the heuristic approach by means of detailed and exact calculations.
The use of a (silicon)/(heavily doped polysilicon)/(metal) structure to replace the conventional high-low junction (or back-surface-field, BSF) structure of silicon solar cells was examined. The results of an experimental study designed to explore both qualitatively and quantitatively the mechanism of the improved current gain in bipolar transistors with polysilicon emitter contact are presented. A reciprocity theorem is presented that relates the short circuit current of a device, induced by a carrier generation source, to the minority carrier Fermi level in the dark. A method for accurate measurement of minority-carrier diffusion coefficients in silicon is described.
The first use of a (silicon)/ (heavily doped polysilicon)/ (metal) structure to replace the conventional high-low junction (or back-surface-field, BSF) structure of silicon solar cells is reported. Compared with BSF and back-ohmic-contact control samples, the polysilicon-back solar cells show improvements in red spectral response and open-circuit voltage. Measurement reveals that a decrease in effective surface recombination velocity S is responsible for this improvement. Decreased S results for n-type (Si:As) polysilicon, consistent with past findings for bipolar transistors, and for p-type (Si:B) polysilicon, reported here for the first time. Though the present polysilicon-back solar cells are far from optimal, the results suggest a new class of designs for high efficiency silicon solar cells. Detailed technical reasons are advanced to support this view.
A model for bandgap shrinkage in semiconductors is developed and applied to silicon. A survey of earlier experiments, and of new ones, give an agreement between the model and experiments on n- and p-type silicon which is good as far as transport measurements in the 300 K range. The discrepancies between theory and experiment are no worse than the discrepancies between the experimental results of various authors. It also gives a good account of recent, optical determinations of band gap shrinkage at 5 K.
A study of the physics of heavily doped silicon and solar cell parameter measurement was undertaken. The parameters investigated were energy gap, lifetime, recombination velocity, diffusivity, mobility and if N or P is high.
Several of the key parameters describing the heavily doped regions of silicon solar cells are examined. The experimentally determined energy gap narrowing and minority carrier diffusivity and mobility are key factors in the investigation.