Engineering Silicon Interfaces with Transparent Al_yTiO_x/ZnO/TiO₂ Stack Exhibiting Exceptional Passivating Contact Performance

Abstract

To reduce the cost of solar photovoltaic electricity generation by overcoming current performance limitations in crystalline silicon (c-Si) solar cells, it is essential to switch from current silicon-based materials to more transparent materials as carrier-selective passivating contacts (CSPCs). Ideal CSPCs should perform three functions simultaneously: they should 1) passivate the silicon dangling bonds in order to provide low surface recombination current density prefactor J 0 (≤10 fA cm −2 ) which ensures high open-circuit voltages at a cell level; 2) operate as efficient contacts for either electrons or holes, with low contact resistivity (≤100 mΩ cm 2 ) to allow high fill factors; and 3) should be highly transparent in the wavelength range corresponding to the solar spectrum in order to reduce parasitic optical losses and hence enable high short-circuit current densities, particularly when applied to the front (illuminated) side of devices. In order to enable practical applications, they should also be thermally stable and prepared using earth-abundant materials. Multiple recent review articles have provided an overview of the development of passivating contact technology in silicon solar cells, testifying to the great interest in this topic. [1] Among passivating contact technologies, those based on hydrogenated amorphous Si (a-Si:H) and polycrystalline Si (poly-Si) layers represent the current state-ofthe-art, having enabled multiple silicon solar cells with worldrecord efficiencies in recent years. [2] However, these materials suffer from significant parasitic optical absorption, which imposes a limit on their ability to provide further improvements in device performance.Transition metal oxides (TMOs) are the leading candidate to replace Si-based materials as CSPCs in order to overcome these limitations. TMOs are very versatile since they can be utilized as both hole-selective contacts, such as MoO x , [3] VO x , [4] and WO x , [5] and electron-selective contacts, such as TiO 2 and ZnO. This is due to the fact that their work functions span a wide range from 3 to 7 eV. [6] Furthermore, TMOs are inexpensive, nontoxic, based on earth-abundant materials, compatible Passivating contact technologies are essential for fabricating high-efficiency crystalline silicon (c-Si) solar cells, and their application and incorporation into manufacturing lines has ranked as a hot topic of research. Generally, ideal passivating contacts should combine excellent electrical contact, outstanding surface passivation, and high optical transparency. However, addressing all these criteria concurrently is challenging since it is unlikely for any single material to exhibit both efficient carrier transport and surface-defect passivation while demonstrating negligible parasitic absorption. In this work, several earth-abundant, wide-bandgap materials are combined to engineer highquality transparent electron-selective passivating contact structures capable of overcoming these obstacles. A highly transparent Al y TiO x /ZnO/TiO 2 stack...

Dramatic Reduction of Silicon Surface Recombination by ALD TiOxCapping Layer from TiCl4and H2O: The Role of Chlorine

“…% as the deposition temperature increased from 75 to 200 °C. According to the literature, the concentration of Cl in ALD TiO x from TiCl 4 and H 2 O drops dramatically as deposition temperature increases . As a result, a lower deposition temperature is recommended to ensure sufficient chlorine content in the capping films and, hence, at the Si surface, which is beneficial to passivation quality, as shown in Figure S4 (Supporting Information).…”

“…It is important to note that the data pertaining to the ZnO/TiO x case was previously presented in the Supporting Information of our previous publication. 20 The pulse times of TiCl 4 and the following H 2 O pulse were also varied systematically. It is 22 Moreover, the first cycle results in the introduction of Cl into the film, and the remaining Cl is more accumulated near the Si surface.…”

“…8 We have recently further improved upon this stack by incorporating ZnO to form an Al y TiO x /ZnO/TiO x structure, with similarly exceptional surface passivation but improved contact characteristics. 20 These enhancements in surface passivation performance after capping Al 2 O 3, Al y TiO x , or Al y TiO x /ZnO with a TiO x layer (using TiCl 4 and H 2 O) were argued to be due to atomic Cl being liberated during the deposition process and diffusing across the stack, accumulating at the Si surface and passivating Si dangling bonds. In this context, it is worth noting that other halogens like iodine and bromine have previously been shown to be effective at passivating silicon surfaces.…”

“…However, from an examination of the literature, it is clear that passivation performance is not the same for all of these films, and the best results in passivation have been achieved using titanium tetrachloride (TiCl 4 ) as the Ti precursor, as shown in Figure . Figure depicts the surface recombination current density prefactor J 0 of TiO x -based passivating contacts deposited using three commonly used Ti precursors: tetrakis­(dimethyl amido)­titanium (TDMAT), , titanium­(IV) isopropoxide (TTIP), , and TiCl 4 . ,, Data are taken from the literature and include only thin layers (≤5 nm) relevant for contact applications. Interestingly, cells with a SiO x /TiO x stack fabricated using TiCl 4 also demonstrated the highest efficiency among solar cells with full-area contacts on the rear without an a-Si:H interlayer, as depicted in Figure .…”

“…Furthermore, we have recently observed that exceptional surface passivation can be accomplished by capping 2 nm of Al y TiO x with 2 nm of TiO x (using TiCl 4 and H 2 O) at 75 °C to form an Al y TiO x /TiO x stack, in contrast to a reduced passivation quality obtained for single layers of TiO x or Al y TiO x , or the mirrored stack (TiO x /Al y TiO x ) on planar Si surfaces . We have recently further improved upon this stack by incorporating ZnO to form an Al y TiO x /ZnO/TiO x structure, with similarly exceptional surface passivation but improved contact characteristics . These enhancements in surface passivation performance after capping Al 2 O 3, Al y TiO x , or Al y TiO x /ZnO with a TiO x layer (using TiCl 4 and H 2 O) were argued to be due to atomic Cl being liberated during the deposition process and diffusing across the stack, accumulating at the Si surface and passivating Si dangling bonds.…”