A Low Resistance Calcium/Reduced Titania Passivated Contact for High Efficiency Crystalline Silicon Solar Cells

Abstract

Recent advances in the efficiency of crystalline silicon (c‐Si) solar cells have come through the implementation of passivated contacts that simultaneously reduce recombination and resistive losses within the contact structure. In this contribution, low resistivity passivated contacts are demonstrated based on reduced titania (TiOx) contacted with the low work function metal, calcium (Ca). By using Ca as the overlying metal in the contact structure we are able to achieve a reduction in the contact resistivity of TiOx passivated contacts of up to two orders of magnitude compared to previously reported data on Al/TiOx contacts, allowing for the application of the Ca/TiOx contact to n‐type c‐Si solar cells with partial rear contacts. Implementing this contact structure on the cell level results in a power conversion efficiency of 21.8% where the Ca/TiOx contact comprises only ≈6% of the rear surface of the solar cell, an increase of 1.5% absolute compared to a similar device fabricated without the TiOx interlayer.

Dopant‐Free Partial Rear Contacts Enabling 23% Silicon Solar Cells

“…For example, materials such as metal oxides, nitrides, and fluorides have been demonstrated to form electron and hole selective interfaces when applied to c-Si. [10,11] This specific architecture utilizes a near-ideal surface passivation layer, such as hydrogenated silicon nitride SiN x , [12] to cover the vast majority of the rear surface which can greatly reduce the average surface recombination factor J 0 and increase the rear reflection. [8,9] In addition, the unique interface properties of some metal compound/c-Si interfaces have even enabled novel solar cell architectures, for example, n-type c-Si cells with undoped partial rear contacts (PRCs).…”

“…[8,9] In addition, the unique interface properties of some metal compound/c-Si interfaces have even enabled novel solar cell architectures, for example, n-type c-Si cells with undoped partial rear contacts (PRCs). This came with the introduction of a TiO x /Ca/Al contact, [10] which was found to provide both reduced surface recombination and low contact resistivity, enabling an efficiency of 21.8%. Only a small percentage of the area is contacted, typically < 5%, where electrons flow to be collected.…”

TiO2 Films from the Low‐Temperature Oxidation of Ti as Passivating‐Contact Layers for Si Heterojunction Solar Cells

“…These issues have motivated studies to search for alternative new functional materials and simplified deposition technologies, whereby we can directly form high quality of heterojunctions with c‐Si substrates in a dopant‐free manner. During the past 5 years, dopant‐free HSCs with hole‐selective transport materials (e.g., molybdenum oxide (MoO x ), tungsten oxide (WO x ), vanadium oxide (V 2 O x ), and poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)) and electron‐selective transport materials (e.g., titanium dioxide (TiO 2 ), magnesium oxide (MgO x ), and magnesium fluoride (MgF x )) have already been implemented on c‐Si absorbers and promising progress has been achieved.…”

Tunnel Oxide – Magnesium as Electron‐Selective Passivated Contact for n‐type Silicon Solar Cell

“…Searching for the proper electron‐selective layer and the passivated tunnel layer is one of the most critical tasks in fabricating a high‐efficiency n‐type c‐Si passivated‐contact solar cell. First, it is proven that several kinds of the low‐work‐function metals display the robust property for collecting electrons. Unlike the metal oxides or the metal fluorides, using a low‐work‐function metal as the rear electron‐selective layer becomes less complication because of the avoidance of the strict thickness control during the fabrication.…”