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

Photovoltaic solar cells based on metal halide perovskites have gained considerable attention over the past decade because of their potentially low production cost, earth-abundant raw materials, ease of fabrication and ever-increasing power conversion efficiencies of up to 25.2%. This type of solar cells offers the promise of generating electricity at a more competitive unit price than traditional fossil fuels by 2035. Nevertheless, the best research cell efficiencies are still below the theoretical limit defined by the Shockley-Queissier theory owing to the presence of non-radiative recombination losses. In this Review, we analyse the predominant pathways that contribute to non-radiative recombination losses in perovskite solar cells, and evaluate their impact on device performance. We then discuss how non-radiative recombination losses can be estimated through reliable characterization techniques, and highlight some notable advances in mitigating these losses, which hint at pathways towards defect-free perovskite solar cells. Finally, we outline directions for future work that will push the efficiency of perovskite solar cells towards the radiative limit.

Minimizing non-radiative recombination losses in perovskite solar cells

Negative thermal expansion (NTE) is an intriguing physical property of solids, which is a consequence of a complex interplay among the lattice, phonons, and electrons. Interestingly, a large number of NTE materials have been found in various types of functional materials. In the last two decades good progress has been achieved to discover new phenomena and mechanisms of NTE. In the present review article, NTE is reviewed in functional materials of ferroelectrics, magnetics, multiferroics, superconductors, temperature-induced electron configuration change and so on. Zero thermal expansion (ZTE) of functional materials is emphasized due to the importance for practical applications. The NTE functional materials present a general physical picture to reveal a strong coupling role between physical properties and NTE. There is a general nature of NTE for both ferroelectrics and magnetics, in which NTE is determined by either ferroelectric order or magnetic one. In NTE functional materials, a multi-way to control thermal expansion can be established through the coupling roles of ferroelectricity-NTE, magnetism-NTE, change of electron configuration-NTE, open-framework-NTE, and so on. Chemical modification has been proved to be an effective method to control thermal expansion. Finally, challenges and questions are discussed for the development of NTE materials. There remains a challenge to discover a “perfect” NTE material for each specific application for chemists. The future studies on NTE functional materials will definitely promote the development of NTE materials.

Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

Recent Progresses on Defect Passivation toward Efficient Perovskite Solar Cells

Metal-halide perovskites are rapidly emerging as an important class of photovoltaic absorbers that may enable high-performance solar cells at affordable cost. Thanks to the appealing optoelectronic properties of these materials, tremendous progress has been reported in the last few years in terms of power conversion efficiencies (PCE) of perovskite solar cells (PSCs), now with record values in excess of 24%. Nevertheless, the crystalline lattice of perovskites often includes defects, such as interstitials, vacancies, and impurities; at the grain boundaries and surfaces, dangling bonds can also be present, which all contribute to nonradiative recombination of photo-carriers. On device level, such recombination undesirably inflates the open-circuit voltage deficit, acting thus as a significant roadblock toward the theoretical efficiency limit of 30%. Herein, the focus is on the origin of the various voltage-limiting mechanisms in PSCs, and possible mitigation strategies are discussed. Contact passivation schemes and the effect of such methods on the reduction of hysteresis are described. Furthermore, several strategies that demonstrate how passivating contacts can increase the stability of PSCs are elucidated. Finally, the remaining key challenges in contact design are prioritized and an outlook on how passivating contacts will contribute to further the progress toward market readiness of high-efficiency PSCs is presented.

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Defect and Contact Passivation for Perovskite Solar Cells.

We present solar cells fabricated with n-type Czochralski–silicon wafers grown with strongly compensated 100% upgraded metallurgical-grade feedstock, with efficiencies above 20%. The cells have a passivated boron-diffused front surface, and a rear locally phosphorus-diffused structure fabricated using an etch-back process. The local heavy phosphorus diffusion on the rear helps to maintain a high bulk lifetime in the substrates via phosphorus gettering, whilst also reducing recombination under the rear-side metal contacts. The independently measured results yield a peak efficiency of 20.9% for the best upgraded metallurgical-grade silicon cell and 21.9% for a control device made with electronic-grade float-zone silicon. The presence of boron-oxygen related defects in the cells is also investigated, and we confirm that these defects can be partially deactivated permanently by annealing under illumination.

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Upgraded metallurgical-grade silicon solar cells with efficiency above 20%

Si faceted dendrites grown from the same parallel twins under different undercooling (ΔT) were observed by in situ observation, and the dependence of faceted dendrite growth velocity (Vd) on undercooling was investigated. Vd increase linearly as the increasing ΔT. The faceted dendrite growth velocity-undercooling relationship was analyzed using a model developed on the basis of theoretical dendrite growth velocity. The results obtained using the proposed model fit the experimental results well and indicate that the twin spacing markedly affects the dendrite growth velocity-undercooling relationship.

Dependence of Si faceted dendrite growth velocity on undercooling

The formation mechanism of a cellular structure during the growth of Si-rich SiGe crystals was studied by in situ observation. We directly observed the morphological transformation of the crystal-melt interface during the unidirectional growth of Si-rich SiGe. It was found that the morphology of the interface transformed from a planar to a zigzag facets to a faceted cellular interface with increasing growth rate. It is clarified that Ge segregation at valleys of zigzag facets leads to the formation of a cellular structure in Si-rich SiGe crystals.

Formation mechanism of cellular structures during unidirectional growth of binary semiconductor Si-rich SiGe materials

In this work, carbon doped yttrium aluminum garnet (YAG) (YAG:C) crystal was grown by the temperature gradient technique in a reducing atmosphere. The optical and thermoluminescence (TL) properties of the as-grown YAG:C crystal were investigated. Four absorption bands at 235, 255, 298, and 370 nm and a main blue emission peak at 400 nm are observed in the YAG:C crystal. The absorption coefficients of the bands at 235 and 370 nm, which are associated with the F+-center, increase sharply in YAG crystal by the introduction of carbon. As-grown YAG:C crystal shows high TL sensitivity, and three main glow peaks at 390 K (P1), 465 K (P2), 524 K (P3) and two weak glow peaks at 580 and 625 K are found. The TL dose responses of the three main glow peaks show linear-supralinear characteristic, and wide linearity dose response are found in P1 and P2 glow peaks. The temperature position and TL intensity of three main glow peaks show a strong dependence of heating rate in YAG:C crystal.

Thermoluminescence properties of carbon doped Y3Al5O12 (YAG) crystal

The authors would like to thank Eric Schneller for assistance with analysis of the quantum efficiency and reflectance data. The authors acknowledge financial support from the Australian Renewable Energy Agency (ARENA) under the Postdoctoral Fellowship. The authors would also like to acknowledge support for this work by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, in the Solar Energy Technologies Program, under Award Number DE-EE0004947. Finally, the Materials Characterization Facility at University of Central Florida (UCF) is acknowledged for usage of its facilities.

Xinbo Yang

Papers

Upgraded metallurgical-grade silicon solar cells with efficiency above 20%

Dependence of Si faceted dendrite growth velocity on undercooling

Formation mechanism of cellular structures during unidirectional growth of binary semiconductor Si-rich SiGe materials

Thermoluminescence properties of carbon doped Y3Al5O12 (YAG) crystal

Transmission Electron Microscopy Studies of Electron-Selective Titanium Oxide Contacts in Silicon Solar Cells.