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The most common contaminants in silicon, its coexisting defect configurations, interstitial iron (FE(sub i)) and iron-boron pairs (FeB) are investigated on an intentionally iron-contaminated silicon sample by means of temperature-dependent lifetime spectroscopy (TDLS) and injection-dependent lifetime spectroscopy (IDLS). As the most sensitive technique to detect and determine an iron contamination in silicon heavily relies on the Fe(sub i) and Fe(sub B) defect parameters, the spectroscopic results are of special practical importance.

In this study, short-circuit current losses in high-efficiency n-type back-contacted back-junction silicon solar cells caused by the electrical shading effect have been investigated by two-dimensional simulations of the charge carrier collection probability. Based on the reciprocity theorem, the homogenous partial differential equation describing the probability of charge carriers being collected by the p-n junction on the rear side of the solar cell has been solved numerically using the finite element method implemented in the partial differential equation solver COMSOL Multiphysics. The method has been applied to study the impact of geometrical parameters of the solar cell, such as the pitch distance, as well as the emitter and back surface field width, on the local and global internal quantum efficiency and on the short-circuit current density. The influence of the rear surface recombination velocity of an undiffused gap and the effective rear surface recombination velocity of the back surface field region on the short-circuit current density is also presented. It has been found that the width and the surface recombination velocity of the undiffused gap on the rear side of the solar cell have a strong impact on the charge collection probability in the base. Thus, the surface recombination velocity of the undiffused gap has to be minimized or the undiffused gap has to be reduced or omitted completely in order to increase the short-circuit current density significantly. Furthermore, it has been found that low base doping concentrations are essential for minimizing the effective rear surface recombination velocity of the back surface field. It has also been shown that in the case of low base doping concentrations, the short-circuit current density reaches its maximum value and that in this case it is nearly independent of the back surface field doping concentration. [ABSTRACT FROM AUTHOR]

Upconversion (UC) of subband-gap photons is a promising possibility to enhance solar cell efficiency by making also the subband-gap photons useful. For this application, we investigate the material system of trivalent erbium doped sodium yttrium fluoride (NaYF4:20%Er3+), which shows efficient UC suitable for silicon solar cells. We determine the optical UC efficiency by calibrated photoluminescence measurements. Because these data are free from any influence of losses associated with the application of the upconverter to the solar cell, the obtained values constitute the upper limit that can be achieved with an optimized device. Subsequently, we compare the results of the optical measurements with the results obtained by using solar cells as detectors on which the upconverter material is applied. We find an optical UC quantum efficiency of 5.1% at a monochromatic irradiance of 1880 W m-2 (0.27 cm2 W-1) at 1523 nm. The device of silicon solar cell and applied upconverter showed an external quantum efficiency of 0.34% at an irradiance of 1090 W m-2 (0.03 cm2 W-1) at 1522 nm. The differences are explained by the optical losses occurring in the upconverter solar cell device, which are dominated by the transmission of the solar cell and the incomplete absorption of the upconverting layer, and the nonlinear behavior of the upconverter. [ABSTRACT FROM AUTHOR]

In this work, a one-dimensional analytical model to calculate the quantum efficiency in back-junction solar cells with and without a high-low junction on the front side is presented. The analytical model, based on the reciprocity theorem for charge collection, is compared with numerical device simulations taking into account the influence of high-injection effects. Using the analytical model, the influence of base doping concentration and surface recombination velocity on the internal quantum efficiency of a n-type back-junction solar cell is analyzed. [ABSTRACT FROM AUTHOR]

Boron-doped silicon wafers implanted with low doses of manganese have been analyzed by means of deep-level transient spectroscopy (DLTS), injection-dependent lifetime spectroscopy, and temperature-dependent lifetime spectroscopy. While DLTS measurements allow the defect levels and majority carrier capture cross sections to be determined, the lifetime spectroscopy techniques allow analysis of the dominant recombination levels and the corresponding ratios of the capture cross sections. Interstitial manganese and manganese-boron pairs were found to coexist, and their defect parameters have been investigated. In good agreement with the literature, this study identifies the defect level of manganese-boron pairs to be located in the lower half of the band gap at an energy level of Ev+0.55 eV with a majority carrier capture cross section of σp=3.5×10-13 cm2. The capture cross-section ratio was found to be k=σn/σp=6.0. This implies that the previously unknown minority carrier capture cross section is σn=2.1×1012 cm2. Concerning the defect related to interstitial manganese, this study identifies the most recombination-active level to be located in the upper half of the band gap at EC-0.45 eV with a corresponding ratio of the capture cross sections of k=9.4. In addition, the temperature-dependent association time constant of manganese-boron pairs is determined to be τassoc,Mn=8.3×105 K-1 cm-3 (T/Ndop)exp(0.67 eV/kBT) and found to differ from that for iron by a factor of 3 at room temperature, allowing this association time constant to be used as a fingerprint for a possible contamination with manganese. Also, the diffusion coefficient of interstitial manganese in silicon is determined from these experiments in a temperature range from 70 to 120 °C. It can be represented by the expression DMn=6.9×10-4 cm2 s-1 exp(-0.67 eV/kBT). [ABSTRACT FROM AUTHOR]

A method for spatially resolved measurement of the minority carrier diffusion length in silicon wafers and in silicon solar cells is introduced. The method, which is based on measuring the ratio of two luminescence images taken with two different spectral filters, is applicable, in principle, to both photoluminescence and electroluminescence measurements and is demonstrated experimentally by electroluminescence measurements on a multicrystalline silicon solar cell. Good agreement is observed with the diffusion length distribution obtained from a spectrally resolved light beam induced current map. In contrast to the determination of diffusion lengths from one single luminescence image, the method proposed here gives absolute values of the diffusion length and, in comparison, it is much less sensitive to lateral voltage variations across the cell area as caused by local variations of the series resistance. It is also shown that measuring the ratio of two luminescence images allows distinguishing shunts or surface defects from bulk defects. [ABSTRACT FROM AUTHOR]

Temperature- and injection-dependent lifetime spectroscopy (TIDLS) as a method to characterize point defects in silicon with several energy levels is demonstrated. An intentionally cobalt-contaminated p-type wafer was investigated by means of lifetime measurements performed at different temperatures up to 151 °C. Two defect energy levels were required to model the lifetime curves on basis of the Shockley-Read-Hall statistics. The detailed analysis is based on the determination of the recently introduced defect parameter solution surface (DPSS) in order to extract the underlying defect parameters. A unique solution has been found for a deep defect level located in the upper band gap half with an energy depth of EC-Et=0.38±0.01 eV, with a corresponding ratio of capture cross sections k=σn/σp=0.16 within the interval of uncertainty of 0.06–0.69. Additionally, a deep donor level in the lower band gap half known from the literature could be assigned to a second energy level within the DPSS analysis at Et-EV=0.41±0.02 eV with a corresponding ratio of capture cross sections k=σn/σp=16±3. An investigation of the temperature dependence of the capture cross section for electrons suggests that the underlying recombination process of the defect in the lower band gap half is driven by a two stage cascade capture with an activation energy of ΔE=52±2 meV. These results show that TIDLS in combination with DPSS analysis is a powerful method to characterize even multiple defect levels that are affecting carrier recombination lifetime in parallel. [ABSTRACT FROM AUTHOR]

21.4% efficient rear-contacted cells (RCC) with interdigitated contact grids processed at the Fraunhofer ISE on 1.25 Ωcm float-zone (FZ) silicon are analyzed in detail. The comprehensive description does not only include a two-dimensional numerical device simulation, but also a detailed analysis of the optical carrier generation using optical ray tracing and determination of the losses due to distributed metal resistance and perimeter currents employing circuit simulation. Bulk and surface recombination losses are separated, combining carrier lifetime and open-circuit voltage measurements with numerical device simulation. The interface surface recombination velocity of the thermally oxidized emitter covering the front surface is deduced to be 1500 cm/s and the bulk diffusion length within the 1.25 Ωcm FZ silicon base is 1200 µm. Despite this excellent bulk diffusion length, the simulations reveal that at a maximum power point 80% of the total recombination is due to Shockley-Read-Hall recombination in the base. It was determined that losses due to the distributed metal resistance within the contact grid (including nongeneration losses) caused an fill factor decrease of 1% absolute. Loss currents flowing out of the cell perimeter caused an additional fill factor loss of 1.5% absolute. It was predicted that changing the surface concentration of the front and rear emitter diffusion from 5 × 10[sup 18] to 1 × 10[sup 18] cm[sup -3], while keeping the sheet resistance constant, effected a relative improvement of 3% in the efficiency of the RCC structure. In fact, this modification has lead to an increase in the realized cell efficiency from 21.4% to 22.1% (V[sub OC] = 697.6 mV, J[sub SC] = 39.8 mA/cm², and FF = 79.4%), i.e., a relative improvement of 3.3%. This cell has an efficiency of 18.6% (V[sub OC] = 696.5 mV, J[sub SC] = 33.9 mA/cm², and FF = 78.8%) if illuminated from the rear side. [ABSTRACT FROM AUTHOR]

Carrier lifetime is very sensitive to electrically active defects. Apart from detecting the presence of recombination active defects, lifetime measurements allow for a direct identification of defects if the temperature and injection dependence of carrier lifetime is analyzed. In the present work, the Shockley–Read–Hall (SRH) equation is analyzed from a theoretical point of view in order to demonstrate the possibilities and basic restrictions of I njection and t emperature d ependent l ifetime s pectroscopy (IDLS and TDLS), respectively. To show the variety of attainable curve shapes and to evaluate their suitability for a spectroscopic determination of defect parameters, the general impact of the defect and material parameters on the injection and temperature dependence of SRH lifetime is investigated systematically. From this the basic requirements for an unambiguous defect characterization are derived. The experimental applicability of lifetime spectroscopy is demonstrated by TDLS and IDLS measurements performed on an intentionally molybdenum contaminated Si sample using the microwave-detected photoconductance decay technique and the quasi-steady-state photoconductance technique, respectively. The energy level of 0.335 eV below the conduction band determined from TDLS corresponds well with deep level transient spectroscopy-values reported in literature and could also be determined from the corresponding IDLS curve though not unambiguously. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]