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Carrier-selective passivating contacts in crystalline silicon (c-Si) solar cells have expanded from doped silicon films to non-silicon wide-bandgap materials to reduce parasitic absorption and production costs. Titanium oxide (TiOx) has emerged as one of the most promising materials and has achieved high performance in c-Si solar cells. In this work, TiOx is explored as a passivation interlayer in hole-selective contacts rather than conventional electron-selective contacts. Theoretical calculations and experimental results demonstrate that negative charges and shallow states in TiOx, derived from oxygen vacancies (VO), enhance surface passivation and assist hole tunneling, respectively. As a strategy to modulate VO, forming gas annealing is performed to further improve hole selectivity. By incorporating the TiOx passivation interlayer into MoOx-based c-Si solar cells, we achieve an improved efficiency and stability of the device, with the highest efficiency of 21.28%. This work advances the understanding of TiOx as a promising material to enhance hole selectivity for c-Si solar cells. [ABSTRACT FROM AUTHOR]

The need to increase transparency in existing passivating contacts for crystalline silicon solar cells has motivated the development of transparent contacts based on transition metal oxides (TMOs). Among hole‐selective materials, molybdenum oxide (MoOx) has achieved the greatest success so far. However, despite providing low contact resistivity, MoOx relies on an intrinsic hydrogenated amorphous silicon (a‐Si:H(i)) interlayer to achieve high levels of surface passivation and thus high open‐circuit voltage at a device level, partially defeating the objective of improved transparency. Herein, we report unprecedented performance for a‐Si:H‐free MoOx‐based contacts by employing an alternative passivating interlayer based on a well‐engineered chlorine‐containing Al‐alloyed titanium oxide/titanium dioxide (AlyTiOx/TiO2)stack. The resulting AlyTiOx/TiO2/MoOx stack achieved record levels of passivation, reaching J0 values as low as 16 fA cm−2, closer to values reported for a‐Si:H‐based contacts, while maintaining lower contact resistivity, well below 100 mΩ cm−2. Additionally, the stack presents improved transparency compared to a‐Si:H‐based contacts, with gains in short‐circuit current density of at least 0.8 mA cm−2. The work pushes the performance of hole‐selective passivating contacts based on TMOs to new levels, enabling a record efficiency of 22.53% for cells with fully transparent hole‐selective passivating contacts. This work serves as an important stepping stone toward low‐thermal‐budget, simple manufacturing of high‐efficiency solar cells. [ABSTRACT FROM AUTHOR]

With the aid of the atomic layer deposition (ALD) technique, TiOx-based electron-selective contacts have been produced with high optical transparency and excellent passivation of silicon surfaces. These contacts are now considered to be on par with commercially available, optically absorbing Si-based contacts such as a-Si:H and poly-Si. On the other hand, these contacts have shown four stability challenges that prevent their commercialization: (1) degradation after metallization; (2) degradation following sputtering; (3) degradation over time; and (4) degradation at high temperatures. Although several approaches to overcoming some of these obstacles have been proposed in the literature, these impediments have remained unresolved over the past decade. Herein, the aforementioned problems have been successfully addressed by constructing an AlyTiOx/TiyZnOx/TiOx/ZnO/ITO multilayer structure, which includes an AlyTiOx/TiyZnOx/TiOx stack for surface passivation, an ALD ZnO buffer layer to minimize sputtering damage and hence enable recovery, and a sputtered indium tin oxide (ITO) capping layer to shield the passivation stack from moisture, act as a barrier for chlorine and hydrogen effusion, and eliminate TiOx-metal electrode interaction. Through systematic investigation, the key factors that degrade the performance of these layers are identified and resolved. This breakthrough in achieving highly stable and outstanding passivation performance using wide-bandgap transition metal oxides is a key step towards the development of highly efficient crystalline silicon (c-Si) solar cells that combine high open-circuit voltages and short-circuit current densities. [ABSTRACT FROM AUTHOR]

Passivating contacts based on transition metal oxides are of great interest for applications in crystalline silicon (c-Si) solar cells due to their improved optical transparency and potential cost reduction. In this work, the contact resistivity and passivation for thermally evaporated Cu2O are investigated and optimized, with and without an Al2O3 interlayer, as a hole-selective contact to c-Si. Additionally, we implement an AlyTiOx/TiO2 stack as a novel passivating tunnel interlayer for hole-selective contacts, achieving an implied open-circuit voltage iVoc of 630 mV and a record-low J0 of 212 fA cm-2 while maintaining a contact resistivity ρc of 62mΩ cm². A record-low ρc of 8mΩ cm² for Cu2O-based contacts is also demonstrated at the expense of passivation. The addition of the interlayer resulted in a 2% absolute improvement in the efficiency of proof-of-concept c-Si cells with full-area rear Cu2O contacts, reaching 19.1%.The demonstration of this novel interlayer stack provides new avenues to improve the performance also of other hole-selective passivating contacts. [ABSTRACT FROM AUTHOR]

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 high‐quality transparent electron‐selective passivating contact structures capable of overcoming these obstacles. A highly transparent AlyTiOx/ZnO/TiO2 stack with a total thickness of 3 nm, prepared by atomic layer deposition, is shown to provide a close‐to‐ideal passivating contact to Si surfaces by enabling dual functions of remarkable silicon surface passivation (with an effective minority carrier lifetime of 12.3 ms, an implied open‐circuit voltage of 730 mV, and a surface recombination current density prefactor of 2.6 fA cm−2), combined with efficient carrier transport with a very low contact resistivity of 3.4 mΩ cm2. These results demonstrate that low‐cost silicon interface‐engineering strategiesbased on transition metal oxides can push c‐Si solar cell performance to its theoretical limits. [ABSTRACT FROM AUTHOR]

Cu2O is an important p-type semiconductor material with applications in thin-film transistors, photovoltaics, and water splitting. For such applications, pinhole-free and uniform thin films are desirable, thus making atomic layer deposition (ALD) the ideal fabrication technique. However, existing ALD Cu precursors suffer from various problems, including limited thermal stability, fluorination, or narrow temperature windows. Additionally, some processes result in CuO films instead of Cu2O. Therefore, it is important to explore alternative precursors and processes for ALD of Cu2O thin films. In this work, we report the successful deposition of Cu2O using copper acetylacetonate as a precursor and a combination of water and oxygen as reactants at 200 °C. Saturation of the deposition rate with precursor and reactant dose time was observed, indicating self-limiting behavior, with a saturated growth-per-cycle of 0.07 Å. The Cu2O film was polycrystalline and uniform (RMS roughness ∼2 nm), with a direct forbidden bandgap of 2.07 eV and a direct allowed bandgap of 2.60 eV. [ABSTRACT FROM AUTHOR]

Passivating contacts based on transition metal oxides (TMOs) have the potential to overcome existing performance limitations in high‐efficiency crystalline silicon (c‐Si) solar cells, which is a significant driver for continuing cost/Watt reductions of photovoltaic electricity. Herein, innovative stacks of Al‐alloyed TiOx (AlyTiOx) and pure TiOx as transparent electron‐selective passivating contacts for n‐type c‐Si surfaces are explored. An optimized stack of 2 nm AlyTiOx and 2 nm TiOx is shown to provide both record‐quality surface passivation and excellent electrical contact, with a surface recombination current density prefactor J0 of 2.4 fA cm−2 and a specific contact resistivity ρc of 15.2 mΩ cm2. The performance of this innovative stack significantly exceeds previously reported values for pure or doped TiOx single layers, SiOx/TiOx stacks, a‐Si:H/TiOx stacks, and other transparent contact technologies. Furthermore, an excellent efficiency of 21.9% is attained by incorporating the optimized stack as a full‐area rear contact in an n‐type c‐Si solar cell. The findings set a new benchmark for the passivation performance of metal oxide‐based passivating contacts, bringing it to a level on par with state‐of‐the‐art SiOx/poly‐Si contacts while greatly improving optical transparency. [ABSTRACT FROM AUTHOR]

The capture cross sections of both electrons σn and holes σp were determined for interstitial molybdenum in crystalline silicon over the temperature range of -110 to 150 °C. Carrier lifetime measurements were performed on molybdenum-contaminated silicon using a temperature controlled photoconductance instrument. Injection dependent lifetime spectroscopy was applied at each temperature to calculate σp and σn. This analysis involved a novel approach that independently determined the capture cross sections at each temperature assuming a known defect density and thermal velocity. Since the energy state is in the lower half of the bandgap, the determination of σp is unaffected by the defect energy at all temperatures, and σp is found to decrease with temperature in a fashion consistent with excitonic Auger capture. At temperatures below 0 °C, the determination of σn is also unaffected by the defect energy due to the suppression of thermal emission, and σn decreases with temperature as well. It is shown that a projection of σn to higher temperature suggests the defect has an energy of 0.375 eV above the valance band edge of silicon. [ABSTRACT FROM AUTHOR]

We present a procedure to determine the dependence of photoconductance lifetime on the occupation of multiple defects. The procedure requires numerical iteration, making it more cumbersome than the analytical equations available for single-defect and simplified two-defect cases, but enabling the following features: (i) it accounts for the defect concentration when calculating the equilibrium carrier concentrations, (ii) it permits recombination through any number of defects, (iii) it calculates the occupation fraction of all defects at any injection, and (iv) it promotes a good understanding of the role of defect occupation in photoconductance measurements. The utility of the numerical procedure is demonstrated on an experimental sample containing multiple defects. The dependence of the sample’s photoconductance on carrier concentration and temperature can be qualitatively described by the generalized procedure but not by either analytical model. The example also demonstrates that the influence of defect occupation on photoconductance lifetime measurements is mitigated at elevated temperatures—a conclusion of particular worth to the study of multicrystalline silicon. [ABSTRACT FROM AUTHOR]

Amorphous thin-film TiO x prepared via atomic layer deposition (ALD) has been identified as one of the most promising materials for use in transparent passivating contacts in high-efficiency and low-cost crystalline silicon (c-Si) solar cells. As highlighted in this work, the passivation performance of ALD TiO x layers strongly depends on the metal precursor used, with films prepared using TiCl 4 recently showing the best results. However, a full understanding of how such films achieve their high level of surface passivation has not yet been demonstrated. This study provides a clear demonstration that a key part of this passivation mechanism is due to chlorine (Cl) accumulation at the Si surface. This mechanism is demonstrated to be quite general in nature by showing how 2 nm of ALD TiO x (TiCl 4 + H 2 O) can be applied as a capping layer for either ZnO or Al 2 O 3 interlayers to dramatically reduce silicon surface recombination. Cl depth profiles obtained using secondary ion mass spectrometry confirm the presence of Cl extending through the depth of the interlayers with a peak at the silicon interface. Remarkably, this diffusion of Cl is observed following low-temperature (75 °C) deposition of the TiO x capping layer, without any subsequent thermal treatment. Contrary to earlier studies that treated residual Cl in ALD films as a general contamination issue, these findings reveal unequivocally that chlorine plays a crucial role in Si surface passivation and can be classed as an effective passivation element, similar to hydrogen in its ability to passivate Si dangling bonds. The outcomes of this research emphasize the importance of residual chlorine in enhancing the passivation of buried interfaces and provide additional motivation for employing metal chloride precursors for silicon surface passivation applications.