With a global market share of about 90%, crystalline silicon is by far the most important photovoltaic technology today. This article reviews the dynamic field of crystalline silicon photovoltaics from a device-engineering perspective. First, it discusses key factors responsible for the success of the classic dopant-diffused silicon homojunction solar cell. Next it analyzes two archetypal high-efficiency device architectures – the interdigitated back-contact silicon cell and the silicon heterojunction cell – both of which have demonstrated power conversion efficiencies greater than 25%. Last, it gives an up-to-date summary of promising recent pathways for further efficiency improvements and cost reduction employing novel carrier-selective passivating contact schemes, as well as tandem multi-junction architectures, in particular those that combine silicon absorbers with organic–inorganic perovskite materials.

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High-efficiency crystalline silicon solar cells: status and perspectives

The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al2O3 films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al2O3 thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al2O3 uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can be used to synthesize Al2O3, this review focuses on ALD—a new technology in the field of c-Si photovoltaics. The authors discuss how the unique features of ALD can be exploited for interface engineering and tailoring the properties of nanolayer surface passivation schemes while also addressing its compatibility with high-throughput manufacturing. The recent progress achieved in the field of surface passivation allows for higher efficiencies of industrial solar cells, which is critical for realizing lower-cost solar electricity in the near future.

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Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells

Al2O3 is a versatile high-κ dielectric that has excellent surface passivation properties on crystalline Si (c-Si), which are of vital importance for devices such as light emitting diodes and high-efficiency solar cells. We demonstrate both experimentally and by simulations that the surface passivation can be related to a satisfactory low interface defect density in combination with a strong field-effect passivation induced by a negative fixed charge density Qf of up to 1013 cm−2 present in the Al2O3 film at the interface with the underlying Si substrate. The negative polarity of Qf in Al2O3 is especially beneficial for the passivation of p-type c-Si as the bulk minority carriers are shielded from the c-Si surface. As the level of field-effect passivation is shown to scale with Qf2, the high Qf in Al2O3 tolerates a higher interface defect density on c-Si compared to alternative surface passivation schemes.

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On the c-Si surface passivation mechanism by the negative-charge-dielectric Al2O3

The properties and applications of molybdenum oxides are reviewed in depth. Molybdenum is found in various oxide stoichiometries, which have been employed for different high-value research and commercial applications. The great chemical and physical characteristics of molybdenum oxides make them versatile and highly tunable for incorporation in optical, electronic, catalytic, bio, and energy systems. Variations in the oxidation states allow manipulation of the crystal structure, morphology, oxygen vacancies, and dopants, to control and engineer electronic states. Despite this overwhelming functionality and potential, a definitive resource on molybdenum oxide is still unavailable. The aim here is to provide such a resource, while presenting an insightful outlook into future prospective applications for molybdenum oxides.

Molybdenum Oxides – From Fundamentals to Functionality

Thin Al2O3 films with a thickness of 7–30 nm synthesized by plasma-assisted atomic layer deposition (ALD) were used for surface passivation of crystalline silicon (c-Si) of different doping concentrations. The level of surface passivation in this study was determined by techniques based on photoconductance, photoluminescence, and infrared emission. Effective surface recombination velocities of 2 and 6 cm/s were obtained on 1.9 Ω cm n-type and 2.0 Ω cm p-type c-Si, respectively. An effective surface recombination velocity below 1 cm/s was unambiguously obtained for nearly intrinsic c-Si passivated by Al2O3. A high density of negative fixed charges was detected in the Al2O3 films and its impact on the level of surface passivation was demonstrated experimentally. The negative fixed charge density results in a flat injection level dependence of the effective lifetime on p-type c-Si and explains the excellent passivation of highly B-doped c-Si by Al2O3. Furthermore, a brief comparison is presented between the ...

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Silicon surface passivation by atomic layer deposited Al2O3

The performance of many silicon devices is limited by electronic recombination losses at the crystalline silicon c-Si surface. A proper surface passivation scheme is needed to allow minimizing these losses. The surface passivation properties of amorphous hydrogenated silicon a-Si:H on monocrystalline Si wafers are investigated here. We introduce a simple model for the description of the surface recombination mechanism based on recombination through amphoteric defects, i.e. dangling bonds, already established for bulk a-Si:H. In this model, the injection-dependent recombination at the a-Si:H/c-Si interface is governed by the density and the average state of charge of the amphoteric recombination centers. We show that with our surface recombination model, we can discriminate between the respective contribution of the two main mechanisms leading to improved surface passivation, which is achieved by a the minimization of the density of recombination centers and b the strong reduction of the density of one carrier type near the interface by field effect. We can thereafter reproduce the behaviors experimentally observed for the dependence of the surface recombination on the injection level on different wafers, i.e., of both p and n doping type as well as intrinsic. Finally, we are able to exploit the good surface passivation properties of our a-Si:H layers by fabricating flat heterojunction solar cells with open-circuit voltages exceeding 700 mV.

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Model for a-Si: H/c-Si interface recombination based on the amphoteric nature of silicon dangling bonds

The electronic properties of hydrogenated amorphous silicon (a-Si:H) relax following stretched exponentials. This phenomenon was explained in the past by dispersive hydrogen diffusion, or by retrapping included hydrogen motion. In this letter, the authors report that the electronic passivation properties of intrinsic a-Si:H/crystalline silicon (c-Si) interfaces relax following a similar law. Carrier injection dependent a-Si:H∕c-Si interface recombination calculations suggest this originates from amphoteric interface state (or Si dangling bond) reduction, rather than from a field effect. These findings underline the similarity between a-Si:H∕c-Si interface recombination and the electronic properties of a-Si:H bulk material.

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Stretched-exponential a-Si:H∕c-Si interface recombination decay

To study recombination at the amorphous/crystalline Si (a- Si:H/c-Si) heterointerface, the amphoteric nature of silicon (Si) dangling bonds is taken into account. Modeling interface recombination measured on various test structures provides insight into the microscopic passivation mechanisms, yielding an excellent interface defect density reduction by intrinsic a-Si:H and tunable field-effect passivation by doped layers. The potential of this model's applicability to recombination at other Si heterointerfaces is demonstrated. Solar cell properties of a-Si:H/c-Si heterojunctions are in good accordance with the microscopic interface properties revealed by modeling, that are, e.g., slight asymmetries in the neutral capture cross-sections and band offsets. The importance of atomically abrupt interfaces and the difficulties to obtain them on pyramidally textured c-Si is studied in combination with transmission electron microscopy.

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Properties of interfaces in amorphous/crystalline silicon heterojunctions

Crystalline silicon wafer (c-Si) can be extremely well passivated by plasma enhanced chemical vapor deposited (PECVD) amorphous silicon (a-Si:H) films. As a result, on flat substrates, solar cells with very high open circuit voltage are readily obtained. On textured substrates however the passivation is more cumbersome, likely due to the presence of localized recombinative paths situated at the pyramid valleys. Here, we show that this issue may be resolved by selecting a silicon substrate morphology featuring large pyramids. Chemical post-texturization treatments can further reduce the surface recombination velocity. This sequence has allowed us to fabricate solar cells with open circuit voltage over 700 mV, demonstrating also on device level the effect of pyramid density and surface micro-roughness on the surface passivation quality.

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Modification of textured silicon wafer surface morphology for fabrication of heterojunction solar cell with open circuit voltage over 700 mV

In this article, we report on the use of transmission electron microscopy (TEM) for the fabrication of high-performance textured amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction (HJ) solar cells. Whereas classical thin-film characterization techniques allowed us to optimize the a-Si:H layer properties for flat HJ solar cells (open-circuit voltages (VOC) up to 710 mV and energy conversion efficiencies up to 19.1%), these techniques can not always be fully exploited on textured c-Si surfaces. Nevertheless, in this situation, TEM micrographs permit us to identify device performance limiting factors, such as, e.g., local epitaxy in pyramid valleys as the main source of our VOC-loss. Minimizing this local epitaxy by adjusting the amorphous Si based layers growth conditions and improving the c-Si surface morphology yields Si HJ solar cells with VOCs over 700 mV.

S. Olibet

Papers

Model for a-Si: H/c-Si interface recombination based on the amphoteric nature of silicon dangling bonds

Stretched-exponential a-Si:H∕c-Si interface recombination decay

Properties of interfaces in amorphous/crystalline silicon heterojunctions

Modification of textured silicon wafer surface morphology for fabrication of heterojunction solar cell with open circuit voltage over 700 mV

Textured silicon heterojunction solar cells with over 700 mv open-circuit voltage studied by transmission electron microscopy