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Comparison of the open-circuit voltage (external voltage Voc,ext) determined by Suns-Voc measurements with the implied voltage Voc,impl determined by transient photoconductance decay lifetime measurements can yield a quick and easy analysis of silicon heterojunction (SHJ) solar cells, especially in regard to finding the optimum doping concentration of the emitter layer [or back surface field (BSF)]. A sufficiently high doping concentration of the emitter and BSF is mandatory to extract the internal Fermi-level splitting and thus the internal voltage, at the solar cell contacts. However increasing the concentration of doping gases during the deposition of doped amorphous silicon layers results in a reduction of the interface passivation quality and Voc,impl. The best trade off is realized when the ratio of Voc,ext to Voc,impl (external/internal Voc-ratio ζ) reaches a saturation value near 1 upon increasing the doping concentration. AFORS-HET (Automat FOR Simulation of HETerostructures) simulations resulted in the conclusion that the characteristics of the external/internal Voc-ratio are mainly determined by the active doping concentration (doping minus defect concentration). [ABSTRACT FROM AUTHOR]

Fluorescent concentrators have gained new research interest recently. The development of new material systems for this type of solar concentrator requires the testing of a wide range of materials. The most important characteristic to be tested is the ability of the concentrators to guide light to their edges where the solar cells are mounted. This ability depends on a large set of parameters and cannot be determined with one simple measurement. We present a method to determine this light guiding efficiency spectrally resolved with transmission, reflection, and centermount measurements using a photospectrometer and an integrating sphere. This method represents considerable progress in comparison to external quantum efficiency (EQE) measurements on systems with attached solar cells. The method is fast and flexible. Because no optical coupling and no solar cells with varying properties are involved, accuracy and reproducibility are higher. The method gives an area average of the light guiding efficiency, rather than the value for the point of excitation as does the EQE measurement. Reasonably similar samples can be compared without requiring corrections. For fully quantitative results on an absolute scale, for samples with large Stokes shifts and/or very different properties, additional corrections must be applied. These corrections use data from samples without luminescent material incorporated, which are measured as references. The information obtained for this correction can be also used to derive additional relevant data, such as the absorption of the dyes used and the fraction of light lost into the escape cone. For the tested materials, the measured spectral collection efficiency reached up to 60% at certain wavelengths. [ABSTRACT FROM AUTHOR]

A valuable nondestructive measurement and analysis method for determining individual excess carrier lifetimes in multilayer systems is presented. This is particularly interesting for the characterization of crystalline silicon thin-film samples consisting of an electrically active epitaxial layer on top of a highly doped crystalline substrate. The analysis principle is based on a comparison between a measured photoluminescence intensity ratio and associated simulated radiative recombination ratios for samples with different epitaxial layer thicknesses. It benefits from the fact that for low excess carrier lifetimes within the epitaxial layer the carrier concentration in the said layer is limited by bulk recombination, while for high carrier lifetimes surface and interface recombination is dominating. In order to verify this measurement and analysis principle, results of a set of crystalline silicon thin-film samples with varying epitaxial layer thickness on a highly doped Czochralski substrate are presented. [ABSTRACT FROM AUTHOR]

Temperature-dependent lifetime spectroscopy allows for the determination of defect parameters (like ratio of the carrier capture cross sections and energy level) of pointlike defects in silicon. This necessitates reliable measurements of the low-level injection excess carrier lifetime. Photoluminescence-based measurement techniques have been shown to be ideal for this kind of measurements at room temperature, being immune to several measurement artifacts such as minority carrier trapping or depletion region modulation. In this article it will be shown how the effect of photon reabsorption influences the temperature-dependent photoluminescence measurements and how this can be accounted for using a theoretical model based on the generalized Planck equation. An intentionally titanium-contaminated silicon sample is investigated by means of temperature-dependent photoluminescence and injection-dependent photoconductance lifetime spectroscopy. Defect parameters of two independent recombination centers will be presented, which titanium introduces into the silicon band gap. One defect level at EC-0.24 eV agrees very well with a level found via deep-level transient spectroscopy measurements while another level at EC-0.49 eV could be measured for the first time using the advanced lifetime spectroscopic approach presented here. [ABSTRACT FROM AUTHOR]

Carrier density imaging, a further development of the infrared lifetime mapping technique [Bail et al., Proceedings 28th IEEE-PVSC (2000)], is presented as an extremely fast, spatially resolved lifetime measurement method. It is based on the principle of absorption of infrared radiation by free carriers. The physical principles and necessary calibration procedures are discussed, showing that the steady state carrier density and thus actual carrier lifetime at low-level injection is measured, whereas many standard lifetime measurement techniques measure differential lifetimes only. A charge coupled device camera operating in the infrared is applied for measuring the infrared transmissivity of the sample. This overcomes the necessity of scanning the sample in order to receive laterally resolved information. Thus measurement times can be cut from hours to minutes or even seconds. An additional advantage is the absence of any moving parts which may cause spatial errors or even failures in the measurement. As is shown in the article the use of lock in technique and an appropriate source of illumination is essential to receive good results on production type materials. The quantitative comparison to other lifetime measurement techniques and the application to silicon samples at different process stages is discussed. Finally methods for further reduction of measurement time are discussed, showing that this method may well be suitable for in-line process control. [ABSTRACT FROM AUTHOR]

We have established a simulation model for phosphorus-doped silicon emitters using Fermi-Dirac statistics. Our model is based on a set of independently measured material parameters and on quantum mechanical calculations. In contrast to commonly applied models, which use Boltzmann statistics and apparent band-gap narrowing data, we use Fermi-Dirac statistics and theoretically derived band shifts, and therefore we account for the degeneracy effects on a physically sounder basis. This leads to unprecedented consistency and precision even at very high dopant densities. We also derive the hole surface recombination velocity parameter S[sub po] by applying our model to a broad range of measurements of the emitter saturation current density. Despite small differences in oxide quality among various laboratories, S[sub po] generally increases for all of them in a very similar manner at high surface doping densities N[sub surf]. Pyramidal texturing generally increases S[sub po] by a factor of five. The frequently used forming gas anneal lowers S[sub po] mainly in low-doped emitters, while an aluminum anneal (A1 deposit followed by a heat cycle) lowers S[sub po] at all N[sub surf]. [ABSTRACT FROM AUTHOR]

Presents a study that demonstrated the modulated free-carrier absorption technique with high spatial resolution. Methodology; Results; Discussion.

Abstract: Tunnel oxide passivating contacts (TOPCon) consisting of an ultrathin tunnel oxide capped by a doped Si film exhibit excellent passivation and contact properties. The application of these contacts has so far resulted in efficiencies of up to 25.7% realized with an n‐type Si solar cell featuring a front‐side boron‐doped p+ emitter and n‐TOPCon as full‐area rear electron contact. In this work, we study the same cell structure on p‐type Si. In this case, the p+ diffusion on the front acts as a front surface field (FSF) and the n‐TOPCon layer as a full‐area rear emitter. One benefit of this rear‐junction cell design is that the whole base contributes to the hole transport towards the local contacts on the front, which means that the lateral current transport within the FSF is less important than in the case of the front emitter of the n‐type cell. To study this, we addressed the influence of the FSF lateral conductivity on the performance of these rear‐junction cells theoretically (based on a simulation study) as well as experimentally (with fabricated cells). Efficiencies up to 24.3% (independently confirmed) have been achieved with this structure applying a FSF and up to 23.9% without the full‐area FSF. As such, these results demonstrate a high device performance for these TOPCon rear emitter cells even without a lateral conductivity in the FSF. This bears the potential to simplify the process chain quite substantially as no full‐area boron diffusion is required. [ABSTRACT FROM AUTHOR]

Abstract: The experimental variation of wafer thickness and resistivity at device level combined with a comprehensive device simulation study allows the identification of dominating recombination‐induced power loss mechanisms in high‐efficiency n‐type silicon solar cells (A. Richter et al, Sol. Energy Mater. Sol. Cells 173, p. 96, 2017). Under the assumption of specific Shockley‐Read‐Hall (SRH) recombination parameters, impurity recombination within the silicon bulk was identified as one main source for efficiency losses particularly for solar cells made of high‐resistivity silicon. In this work, we extend that analysis approach focusing on the SRH recombination parameters in order to investigate whether certain properties of these recombination‐active impurities can be identified using this kind of simulation‐based analysis. Reported SRH recombination parameters of various common impurities (eg, Fe, Cr, or Ni) were considered. It was found that a dominating role of certain impurities as Cu, Au, Co, or Zn can be excluded. A general simulation study as a function of the fundamental SRH recombination parameters allowed us to reduce the possible SRH parameters significantly in particular of the capture cross‐section ratio of electrons and holes. These results demonstrate that our analysis approach, which combines an experimental variation of wafer thickness and resistivity with a comprehensive device simulation, can provide deep insights into the solar cells loss mechanisms. In particular, the conclusions regarding SRH recombination are of importance for solar cells made of n‐type silicon as there are no meta‐stable impurity states formed that would allow a direct identification of impurity recombination–related power loss as reported for p‐type Si. [ABSTRACT FROM AUTHOR]