Controlling the concentration of charge carriers near the surface is essential for solar cells. It permits to form regions with selective conductivity for either electrons or holes and it also helps ...to reduce the rate at which they recombine. Chemical passivation of the surfaces is equally important, and it can be combined with population control to implement carrier-selective, passivating contacts for solar cells. This paper discusses different approaches to suppress surface recombination and to manipulate the concentration of carriers by means of doping, work function and charge. It also describes some of the many surface-passivating contacts that are being developed for silicon solar cells, restricted to experiments performed by the authors.
•The principles for passivating the surfaces of crystalline silicon and developing carrier-selective contact systems are described.•The metal oxides Ga2O3, TiO2,Ta2O5, HfO2 and ZrO2 are shown to passivate silicon surfaces.•High work function materials, like MoOx, or low work function ones, like Ca, Mg, LiFx, MgFx and MgOx, selectively extract holes orelectrons.•Solar cells with such selective contacts in a partial or full-area configuration have conversion efficiencies above 20%.•Two polysilicon/SiO2 passivating contact technologies are demonstrated with 24.7% (n-type) and 23.0% (p-type) efficient solar cells.
We demonstrate effective passivation of a variety of crystalline silicon (c-Si) surfaces by a thin layer of thermal atomic layer deposited (ALD) titanium oxide (TiO2). Surface recombination ...velocities of 0.8cm/s, 2.5cm/s and 9.8cm/s have been obtained on n-type 10cm, 1Ωcm and p-type 1Ωcm undiffused wafers, respectively. Recombination current densities of 19 fA/cm2 and 780 fA/cm2 have been measured on 120Ω/□ boron diffused p+ and 100Ω/□ phosphorus diffused n+ regions. In addition to providing a superior passivation on p+ over n+ (c-Si) surfaces, the ALD TiO2 layers produces a strong injection-dependent effective lifetime on the n-type substrates, , both of which are consistent with the possible presence of negative charge in the passivating layer. The recombination at the interface between TiO2 and planar , planar and alkaline textured (c-Si) surfaces with upright pyramids is compared. We find that (i) a planar surface exhibits a 1.39 times higher recombination than a planar surface, and (ii) after area correction, the ratio of recombination on the textured and planar surfaces is 1.37. A thin film of TiO2 deposited by ALD has been applied to the front surface of a rear locally diffused p+nn+ front junction solar cell, performing the dual role of surface passivation and single-layer antireflection coating on the textured p+ diffusion. The best solar cell achieved Voc=655mV, FF=79.9% and efficiency=20.45%. The results presented in this work demonstrate that TiO2 re-emerging as a suitable optical and passivating material to produce high performance solar cells.
•The recombination decreases drastically with increasing TiO2 thickness on p, n and p+(c-Si) surfaces, and saturates at 15nm.•Effective passivation were achieved on both undiffused n-type (J0=6.4 fA/cm2) and p-type (J0=7.3 fA/cm2) (c-Si) wafers, as well as boron-diffused p+ surfaces (J0=19 fA/cm2), without any post deposition treatment.•Planar {111} Si exhibits a 1.39-fold higher recombination than planar {100} Si; and recombination at a textured surface is similar about 1.37 times to that at a planar {111} after surface area correction.•Application of a single layer TiO2 ARC on rear locally-diffused p+nn+solar cells resulted in average cell efficiency over 20% and the best efficiency of 20.45%.
Crystalline silicon (c-Si) solar cells have been dominating the photovoltaic (PV) market for decades, and c-Si based photoelectrochemical (PEC) cells are regarded as one of the most promising routes ...for water splitting and renewable production of hydrogen. In this work, we demonstrate a nanoscale tantalum oxide (TaO x , ∼6 nm) as an electron-selective heterocontact, simultaneously providing high-quality passivation to the silicon surface and effective transport of electrons to either an external circuit or a water-splitting catalyst. The PV application of TaO x is demonstrated by a proof-of-concept device having a conversion efficiency of 19.1%. In addition, the PEC application is demonstrated by a photon-to-current efficiency (with additional applied bias) of 7.7%. These results represent a 2% and 3.8% absolute enhancement over control devices without a TaO x interlayer, respectively. The methods presented in this Letter are not limited to c-Si based devices and can be viewed as a more general approach to the interface engineering of optoelectronic and photoelectrochemical applications.
There is tremendous interest in reducing losses caused by the metal contacts in silicon photovoltaics, particularly the optical and resistive losses of the front metal grid. One commonly sought-after ...goal is the creation of high aspect-ratio metal fingers which provide an optically narrow and low resistance pathway to the external circuit. Currently, the most widely used metal contact deposition techniques are limited to widths and aspect-ratios of ~40 μm and ~0.5, respectively. In this study, we introduce the use of a micropatterned polydimethylsiloxane encapsulation layer to form narrow (~20 μm) microchannels, with aspect-ratios up to 8, on the surface of solar cells. We demonstrate that low temperature metal pastes, electroless plating and atomic layer deposition can all be used within the microchannels. Further, we fabricate proof-of-concept structures including simple planar silicon heterojunction and homojunction solar cells. While preliminary in both design and efficiency, these results demonstrate the potential of this approach and its compatibility with current solar cell architectures.
This paper describes the optimization of a technique to make polysilicon/SiOx contacts for silicon solar cells based on doping PECVD intrinsic polysilicon by means of a thermal POCl3 diffusion ...process. Test structures are used to measure the recombination current density Joc and contact resistivity ρc of the metal/n+ polysilicon/SiOx/silicon structures. The phosphorus diffusion temperature and time are optimized for a range of thicknesses of the SiOx and polysilicon layers. The oxide thickness is found to be critical to obtain a low contact resistivity ρc, with an optimum of about 1.2nm for a thermal oxide and ∼1.4nm for a chemical oxide. A low Joc≤5fA/cm2 has been obtained for polysilicon thicknesses in the range of 32nm–60nm, while ρc increases from 0.016Ωcm2 to 0.070Ω-cm2 due to the bulk resistivity of polysilicon. These polysilicon/SiOx contacts have been applied to the rear of n-type silicon solar cells having a front boron diffusion, achieving Voc=674.6mV, FF=80.4% and efficiency=20.8%, which demonstrate the effectiveness of the techniques developed here to produce high performance solar cells.
•Optimization of poly-si/SiOx contacts for silicon solar cells.•A critically thin SiOx layer is essential to supress recombination.•Trade-offs in POCl3 diffusion processes and interfacial layer and poly-Si thickness.•Poly-Si/SiOx interface benefits from a hydrogenating anneal in forming gas.
While sputtering has been shown to be capable of depositing aluminum oxide suitable for surface passivation, the mechanisms for this are yet to be firmly established and its potential realized. In ...this paper, we investigate the relationships between the oxygen in the sputtering process to the resulting composition of the deposited film and the surface passivation obtained. We find that surface passivation is not strongly dependent on the bulk composition of the film. Instead the results indicate that the interfacial silicon oxide layer that forms after annealing between the aluminum oxide film and the silicon is a much more important factor; it is this combined structure of aluminum oxide, silicon oxide and silicon that is crucial for obtaining negative charges and excellent surface passivation.
Carrier-selective contacts based on silicon films deposited onto a thin SiOx layer combine high performance with a degree of compatibility with industrial solar cell metallization steps. This paper ...demonstrates an approach to form electron-selective passivating contacts that maximises the overlap with common industrial equipment; it is based on depositing an intrinsic amorphous silicon (a-Si) layer by PECVD and then doping and re-crystallizing it by means of a thermal phosphorus diffusion. By optimizing the intrinsic a-Si thickness and the phosphorus diffusion temperature, a low recombination current density Joc ≈ 3 fA/cm2 and a low contact resistivity of ρc ≈ 3 mΩ-cm2 have been achieved. Additionally, these electrical parameters have been found to be sensitive to the work function of the outer metal electrode. The application of these optimized electron-selective passivating contacts to n-type silicon solar cells has permitted to achieve a conversion efficiency of 24.7%. A loss analysis has been conducted through Quokka 2 simulations, which together with quantum efficiency measurements, indicate that further optimization should focus on the front boron-doped region of the device.
•An approach to form passivating contacts with industrial overlaps and compatibilities.•A-Si and POCl3 are furtherly optimized to reduce recombination and transport losses.•We studied the impact of different metals on the passivating contacts.•We demonstrate n-type silicon solar cells with a 24.7% conversion efficiency.
Defects and impurities in silicon limit carrier lifetimes and the performance of solar cells. This work explores the use of fluorine to passivate defects in silicon for solar cell applications. We ...present a simple method to incorporate fluorine atoms into the silicon bulk and interfaces by annealing samples coated with thin thermally evaporated fluoride overlayers. It is found that fluorine incorporation does not only improve interfaces but can also passivate bulk defects in silicon. The effect of fluorination is observed to be comparable to hydrogenation, in passivating grain boundaries in multicrystalline silicon, improving the surface passivation quality of phosphorus-doped poly-Si-based passivating contact structures, and recovering boron–oxygen-related light-induced degradation in boron-doped Czochralski-grown silicon. Our results highlight the possibility to passivate defects in silicon without using hydrogen and to combine fluorination and hydrogenation to further improve the overall passivation effect, providing new opportunities to improve solar cell performance.
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