• Energy Research

Abstract Recent progress in the electronic quality of high-performance (HP) multicrystalline silicon material is reported with measurements and modeling performed at various institutions and research groups. It is shown that recent progress has been made in the fabrication at Trina Solar mainly by improving the high excess carrier lifetimes τ due to a considerable reduction of mid-gap states. However, the high lifetimes in the wafers are still reduced by interstitial iron by a factor of about 10 at maximum power point (mpp) conditions compared to mono-crystalline Cz wafers of equivalent resistivity. The low lifetime areas of the wafers seem to be limited by precipitates, most likely Cu. Through simulations, it appears that dislocations reduce cell efficiency by about 0.25% absolute. The best predictors for PERC cell efficiency from ingot metrology are a combination of mean lifetime and dislocation density because dislocations cannot be improved considerably by gettering during cell processing, while lifetime-limiting impurities are gettered well. In future, the material may limit cell efficiency above about 22.5% if the concentrations of Fe and Cu remain above 1010 and 1013 cm−3, respectively, and if dislocations are not reduced further.

An effective hydrogenation process for polycrystalline silicon based passivating contacts (TOPCon) is crucial to achieve a very high level of surface passivation. This work examines the hydrogenation characteristics of p-type TOPCon on textured surface morphology by applying dielectric layers such as AlOx, SiNx and stacks thereof followed by an activation in a furnace anneal or by fast-firing. In a direct comparison with n-type TOPCon, p-type TOPCon requires higher activation temperatures and a higher activation energy. For a successful integration of n-type and p-type TOPCon into bottom cell precursors with 726 mV implied Voc for tandem devices, stacks featuring AlOx are beneficial to increase the thermal stability especially for n-type TOPCon. With regards to fast-firing processes, the influence of an additional pre- or post-annealing step is investigated. The peak firing temperature can significantly be reduced when applying an annealing step beforehand and a post-firing anneal improves surface passivation to recombination current densities J0s as low as 7.9 fA/cm2 for p-type TOPCon on textured surface which is one of the lowest reported in literature.

This paper investigates and compares the impurity gettering effects of silicon nitride (SiN x ) films that are synthesized by plasma-enhanced chemical vapor deposition (PECVD) under various conditions. Both industrial- and laboratory-scale PECVD systems are employed to deposit SiN x films with a wide range of properties (with refractive indices from 1.93 to 2.45 at 632 nm), which covers the entire range of SiN x used for silicon solar cells. The gettering effects are quantified by monitoring the reduction kinetics of the interstitial iron concentration in the silicon wafer bulk as iron becomes gettered to the surface SiN x layers during cumulative annealing at 400 °C. The results show that the very different SiN x films generate similar gettering kinetics, indicating that the impurity gettering effect is likely present in most PECVD SiN x films for silicon solar cells. The gettering kinetics and the SiN x film properties of refractive index, Si–N, Si–H, N–H bond densities, and H content, are found to have no clear correlations.

AbstractThis study focuses on electron‐selective passivating contacts for crystalline silicon (c‐Si) solar cells where an interlayer is used to provide a low contact resistivity between the c‐Si substrate and the metal electrode. These electron contact interlayers are used in combination with other passivating interlayers (e.g., a‐Si:H, TiOx, and Nb2O5) to improve surface passivation whilst still permitting contact resistivities suitable for high‐efficiency solar cells. We show that a wide variety of thermally evaporated materials, most of which have ionic character, enable an Ohmic contact between n‐type c‐Si and Al. From this pool of compounds, we observed that CsBr has especially promising behavior because of its excellent performance and thermal stability when combined with thin passivating layers. With different test structures, we were able to demonstrate low contact resistance using TiOx/CsBr, Nb2O5/CsBr, and a‐Si:H/CsBr stacks on n‐type c‐Si. The quality of the provided surface passivation depended on the stack but we achieved the best overall passivation stability with TiOx/CsBr. Finally, we were able to demonstrate an efficiency >20% on a laboratory‐scale solar cell that implements the TiOx/CsBr/Al stack as full‐area rear‐side electron selective contact.

Metallic impurities in the silicon wafer bulk are one of the major efficiency-limiting factors in silicon solar cells. Gettering can be used to significantly lower the metal concentrations. Although gettering by silicon nitride films has been reported in literature, much remains unknown about its gettering behaviors and mechanisms. In this study, the gettering kinetics and mechanisms of silicon nitride films, from both plasma-enhanced chemical vapor deposition (PECVD) and low-pressure chemical vapor deposition (LPCVD), are investigated. By monitoring the kinetics of iron loss from the silicon wafer bulk, it is confirmed that silicon nitride gettering takes place mainly via segregation, even at a low annealing temperature of 400 °C. Simulation of the gettering kinetics suggests the presence of an interfacial diffusion barrier in some cases, which slows down the transport of iron impurities from the silicon wafer bulk to the silicon nitride gettering regions. The activation energy of the segregation gettering process is estimated to be 0.9 ± 0.1 eV for the investigated PECVD silicon nitride film at 400–900 °C and 1.6 ± 0.5 eV for the investigated LPCVD silicon nitride film at 400–700 °C.

Photoluminescence (PL) spectra from moderately doped, compensated silicon with boron and phosphorus concentrations on the order of ${\text{10}^{16}} - {\text{10}^{17}}\;{\text{c}}{{\text{m}}^{ - 3}}$ , which is representative of the low-cost upgraded metallurgical grade silicon potentially used for photovoltaics, are presented and explained. At 79 K, the captured PL spectra from the compensated silicon reveal the presence of the following radiative recombination channels in the material: band-to-band recombination, recombination through a single neutral dopant (phosphorus or boron), and recombination from the neutral donors (phosphorus) to the neutral acceptors (boron), i.e., the D°–A° pair recombination. The D°–A° pair luminescence peaks are found to appear as rather broad in the measured PL spectra from compensated silicon at 79 K. The relative PL intensity of the broad dopant features is shown to increase with increasing dopant concentrations. The dopant-related luminescence of the compensated silicon demonstrates strong excitation dependence, as a result of the D°–A° recombination channel in compensated silicon. The dopant features become increasingly suppressed at higher excitations due to the increasing dominance of the band-to-band recombination channel. With increasing temperature, the dopant-related luminescence features diminish and become undistinguishable at around 200 K, due to the increased ionization of dopants and the broadening of the band-to-band recombination peaks at higher temperatures.

AbstractThis paper presents findings on applying physical models in the literature to describe silicon luminescence spectra at 80 – 300K. Incorporation of exciton recombination models are shown to disagree with the measured luminescence spectra, whereas a free electron-hole recombination model is shown to match well with the luminescence spectra. However, the lack of consideration for excitons is not justified, as Bludau et al. [J. Appl. Phys., vol. 45, p. 1846, 1974] reported that excitons are present even at room temperature. The second part of the paper demonstrates the impact of shallow dopants on the silicon luminescence spectra at 79K. The ratio of the dopant-related peak to the band-to-band peak intensities correlates with the dopant concentration, indicating that luminescence spectroscopy has the potential for quantifying dopant concentrations in silicon in this temperature range.