Observation of transition metals at shunt locations in multicrystalline silicon solar cells

Abstract

By employing a combination of analytical tools including lock-in thermography and synchrotron-based x-ray fluorescence microscopy, transition metals have been identified at shunting locations in two types of low-cost multicrystalline silicon ͑mc-Si͒ solar cell materials: cast multicrystalline and ribbon growth on substrate ͑RGS͒. At a shunting location in the cast mc-Si cell, silver and titanium, both contact strip materials, have been identified at the shunting location, suggesting a process-induced error related to contact metallization. At a shunting location in the RGS cell, a material-specific shunting mechanism is described, involving channels of inverse conductivity type, where copper and iron are found. The possible roles of these metals in this shunting mechanism are discussed. These results illustrate the wide range of physical mechanisms involved with shunting in solar cells.

Analysis of copper-rich precipitates in silicon: Chemical state, gettering, and impact on multicrystalline silicon solar cell material

“…19 Multicrystalline silicon (mc-Si), the cost-effective material out of which an increasing 50% of solar cell modules are currently produced 20 , typically contains high transition metal concentrations combined with a high density and variety of structural defects. Not surprisingly, copper-rich particles have been observed at structural defects in poorly-performing regions of mc-Si solar cell material [21][22][23][24][25] , complementing neutron activation analysis (NAA) data reporting Cu concentrations in mc-Si as high as 10 13 cm -3 . 26 While Cu-rich clusters are undoubtedly not the only type of defect responsible for reducing the efficiencies of mc-Si solar cells, their known recombination activity and repeated observation in poorly-performing regions indicate they most certainly can be a contributing factor.…”

“…This is consistent with numerous µ-XRF observations of Curich clusters at structural defects in Al-gettered or fully processed mc-Si. 21,[23][24][25] Irrespective of the precise value of k, it follows from Eq. 2 that thicker Al layers should result in the removal of a larger fraction of the total Cu from the bulk during gettering.…”

Chemical natures and distributions of metal impurities in multicrystalline silicon materials

“…The incoming X-ray beam is used to generate electronhole pairs, which are then collected by a Schottky diode 41 or a pn junction. 9 XBIC can be used to determine the recombination activities of individual precipitates 16,40 and their impact on minority carrier diffusion length, 42 as well as to map larger sample areas by undersampling. 15 By combining these three techniques (XBIC, m-XRF, m-XAS) at the same synchrotron beamline, the recombination activity, spatial distribution, size, elemental composition and chemical state of metal-rich particles in mc-Si materials can be obtained with a micron or sub-micron spatial resolution.…”

“…[1][2][3] These impurities can decrease the efficiencies of siliconbased devices in a variety of ways, including bulk recombination, 4,5 increased leakage current, [6][7][8] and direct shunting. 9,10 The groundbreaking study of Davis et al 11 specified threshold concentrations for individual metal species in Czochralski silicon (CZ-Si) photovoltaic devices, correlating the total metal content in a CZ-Si wafer to a quantified decrease in solar cell efficiency.…”

A Next-Generation Hard X-Ray Nanoprobe Beamline for In Situ Studies of Energy Materials and Devices

“…[61] In summary, sustained synchrotron investigations into ''wild-type'' commercial multicrystalline silicon and ''model systems'' yielded such a high degree of microstructural insight, that predictive engineering of certain performance-limiting defects is now possible. Additionally, synchrotron investigations helped to reveal the natures of certain types of electrical shunts, [62] reverse-bias breakdown mechanisms, [63] crucible contamination sources, [64] and defect interactions, [65] helping improve module performance and reliability.…”