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.
This article reviews key factors for the success of crystalline silicon photovoltaics and gives an update on promising emerging concepts for further efficiency improvement and cost reduction.
A technique to make poly-Si (p+)/SiOx contacts for crystalline silicon solar cells based on doping PECVD intrinsic amorphous silicon (a-Si) by means of a thermal BBr3 diffusion process is ...demonstrated. The thickness of the a-Si layer and the temperature of the boron diffusion are optimized in terms of suppressing carrier recombination and transport losses. Different interfacial layers are studied, including ultra-thin SiOx grown either chemically or thermally, and stacks of SiOx and SiNx. While the double SiOx/SiNx interlayers do not achieve the desired performance, both kinds of single SiOx layers produce satisfactory passivating contacts, with both a low recombination current and a low contact resistivity. By adjusting the boron diffusion temperature, recombination current parameter J0 values of ~16fA/cm2 to ~30fA/cm2 have been obtained for structures with initial a-Si thicknesses of 36–46nm, together with a contact resistivity of ~8mΩcm2.
•It describes a technique to make poly-Si contacts based on BBr3 diffusion and intrinsic a-Si.•The thickness of a-Si and temperature of boron diffusion are optimized.•We studied different interfacial layers, including single SiOx and stacks of SiOx and SiNx.•The degradation observed after FGA can be solved by doping poly-Si contacts.
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.
A high Schottky barrier (>0.65 eV) for electrons is typically found on lightly doped n‐type crystalline (c‐Si) wafers for a variety of contact metals. This behavior is commonly attributed to the ...Fermi‐level pinning effect and has hindered the development of n‐type c‐Si solar cells, while its p‐type counterparts have been commercialized for several decades, typically utilizing aluminium alloys in full‐area, and more recently, partial‐area rear contact configurations. Here the authors demonstrate a highly conductive and thermally stable electrode composed of a magnesium oxide/aluminium (MgOx/Al) contact, achieving moderately low resistivity Ohmic contacts on lightly doped n‐type c‐Si. The electrode, functionalized with nanoscale MgOx films, significantly enhances the performance of n‐type c‐Si solar cells to a power conversion efficiency of 20%, advancing n‐type c‐Si solar cells with full‐area dopant‐free rear contacts to a point of competitiveness with the standard p‐type architecture. The low thermal budget of the cathode formation, its dopant‐free nature, and the simplicity of the device structure enabled by the MgOx/Al contact open up new possibilities in designing and fabricating low‐cost optoelectronic devices, including solar cells, thin film transistors, or light emitting diodes.
A highly conductive and thermally stable electrode composed of a nanoscale magnesium oxide/aluminium (MgOx/Al) contact is demonstrated to achieve moderately low resistivity Ohmic contacts on lightly doped n‐type c‐Si. A power conversion efficiency of 20% is obtained at a proof‐of‐concept stage of n‐type c‐Si solar cells with full‐area MgOx/Al rear contacts.
Minimizing carrier recombination at contact regions by using carrier‐selective contact materials, instead of heavily doping the silicon, has attracted considerable attention for high‐efficiency, ...low‐cost crystalline silicon (c‐Si) solar cells. A novel electron‐selective, passivating contact for c‐Si solar cells is presented. Tantalum nitride (TaN
x
) thin films deposited by atomic layer deposition are demonstrated to provide excellent electron‐transporting and hole‐blocking properties to the silicon surface, due to their small conduction band offset and large valence band offset. Thin TaNx interlayers provide moderate passivation of the silicon surfaces while simultaneously allowing a low contact resistivity to n‐type silicon. A power conversion efficiency (PCE) of over 20% is demonstrated with c‐Si solar cells featuring a simple full‐area electron‐selective TaNx contact, which significantly improves the fill factor and the open circuit voltage (Voc) and hence provides the higher PCE. The work opens up the possibility of using metal nitrides, instead of metal oxides, as carrier‐selective contacts or electron transport layers for photovoltaic devices.
Tantalum nitride (TaNx) thin films are demonstrated to be an excellent hole‐blocking, electron‐selective passivating contact for crystalline silicon solar cells. An efficiency of over 20% is achieved on crystalline silicon solar cell featuring a full‐area TaNx heterocontact. This work opens up the possibility of using transition metal nitrides, instead of metal oxides, as electron transport layers for photovoltaic devices.
Borders and frontiers are often problematized in Agnieszka Dale’s Fox Season and Other Short Stories (2017), where mental borders seem to be more divisive than spatial boundaries. Many of these ...narratives feature Polish immigrants in Britain who struggle with their displaced condition in various ways. As some of the stories in the collection reveal, the scenario of post-Brexit Britain compromises conviviality amongst different groups, including the Polish community. Special attention is placed upon how several narratives in the volume underscore the prevalence in British society of Polish stereotypes as the crystallisation of the still widespread animosity against non-Europeans. Homi Bhabha’s notions regarding the formation and dynamics of stereotypes will be helpful in understanding the mechanisms beneath such constructions. Likewise, some of the major tenets of social theory, as well as Edward Said’s notion of ‘Orientalism’, will contribute to shedding light upon this resentment towards the Polish minority, occasionally adopted too by already established immigrants against their former compatriots. This article will ultimately intend to draw attention to the cautionary nature of Dale’s collection as a call for harmony and the appreciation of difference among nations, thus preventing the gloomy perspectives the dystopian futures of some of these stories forecast upon Europe.
Over the past five years, there has been a significant increase in both the intensity of research and the performance of crystalline silicon devices which utilize metal compounds to form ...carrier‐selective heterocontacts. Such heterocontacts are less fundamentally limited and have the potential for lower costs compared to the current industry dominating heavily doped, directly metalized contacts. A low temperature (≤230 °C), TiOx/LiFx/Al electron heterocontact is presented here, which achieves mΩcm2 scale contact resistivities ρc on lowly doped n‐type substrates. As an extreme demonstration of the potential of this heterocontact, it is trialed in a newly developed, high efficiency n‐type solar cell architecture as a partial rear contact (PRC). Despite only contacting ≈1% of the rear surface area, an efficiency of greater than 23% is achieved, setting a new benchmark for n‐type solar cells featuring undoped PRCs and confirming the unusually low ρc of the TiOx/LiFx/Al contact. Finally, in contrast to previous versions of the n‐type undoped PRC cell, the performance of this cell is maintained after annealing at 350–400 °C, suggesting its compatibility with conventional surface passivation activation and sintering steps.
An electron‐selective TiOx based heterocontact is developed and trialed as a dopant‐free partial rear contact in high efficiency silicon solar cells. This cell not only reaches an efficiency of above 23% but also maintains its performance after a short anneal at 400 °C—setting new benchmarks of performance and thermal stability for this cell architecture.
Recent advances in the efficiency of crystalline silicon (c‐Si) solar cells have come through the implementation of passivated contacts that simultaneously reduce recombination and resistive losses ...within the contact structure. In this contribution, low resistivity passivated contacts are demonstrated based on reduced titania (TiOx) contacted with the low work function metal, calcium (Ca). By using Ca as the overlying metal in the contact structure we are able to achieve a reduction in the contact resistivity of TiOx passivated contacts of up to two orders of magnitude compared to previously reported data on Al/TiOx contacts, allowing for the application of the Ca/TiOx contact to n‐type c‐Si solar cells with partial rear contacts. Implementing this contact structure on the cell level results in a power conversion efficiency of 21.8% where the Ca/TiOx contact comprises only ≈6% of the rear surface of the solar cell, an increase of 1.5% absolute compared to a similar device fabricated without the TiOx interlayer.
A novel passivated contact on crystalline silicon comprised of calcium and reduced titania is shown to result in a reduction in contact resistivity by up to two orders of magnitude compared to other titania‐based contacts to crystalline silicon. This enables the fabrication of a first‐of‐its‐kind passivated contact n‐type c‐Si PERC cell with an efficiency of 21.8%.
Low‐resistance contact to lightly doped n‐type crystalline silicon (c‐Si) has long been recognized as technologically challenging due to the pervasive Fermi‐level pinning effect. This has hindered ...the development of certain devices such as n‐type c‐Si solar cells made with partial rear contacts (PRC) directly to the lowly doped c‐Si wafer. Here, a simple and robust process is demonstrated for achieving mΩ cm2 scale contact resistivities on lightly doped n‐type c‐Si via a lithium fluoride/aluminum contact. The realization of this low‐resistance contact enables the fabrication of a first‐of‐its‐kind high‐efficiency n‐type PRC solar cell. The electron contact of this cell is made to less than 1% of the rear surface area, reducing the impact of contact recombination and optical losses, permitting a power conversion efficiency of greater than 20% in the initial proof‐of‐concept stage. The implementation of the LiFx/Al contact mitigates the need for the costly high‐temperature phosphorus diffusion, typically implemented in such a cell design to nullify the issue of Fermi level pinning at the electron contact. The timing of this demonstration is significant, given the ongoing transition from p‐type to n‐type c‐Si solar cell architectures, together with the increased adoption of advanced PRC device structures within the c‐Si photovoltaic industry.
Exceptionally low contact resistivity is demonstrated on lightly doped (5 × 1015 cm−3) n‐type silicon via a nanometer thin lithium fluoride (LiFx) interlayer. This advancement enables the fabrication of a first‐of‐its‐kind n‐type silicon solar cell with a LiFx/Al partial rear contact made to less than 1% of the cell's area. An efficiency of over 20% is achieved at the proof‐of‐concept stage.
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