Methylammonium (MA)-free perovskite solar cells have the potential for better thermal stability than their MA-containing counterparts. However, the efficiency of MA-free perovskite solar cells lags behind due to inferior bulk quality. In this work, 4-methylphenethylammonium chloride (4M-PEACl) is added into a MA-free perovskite precursor, which results in greatly enhanced bulk quality. The perovskite crystal grains are significantly enlarged, and defects are suppressed by a factor of four upon the incorporation of an optimal concentration of 4M-PEACl. Quasi-2D perovskites are formed and passivate defects at the grain boundaries of the perovskite crystals. Furthermore, the perovskite surface chemistry is modified, resulting in surface energies more favorable for hole extraction. This facile approach leads to a steady state efficiency of 23.7% (24.2% in reverse scan, 23.0% in forward scan) for MA-free perovskite solar cells. The devices also show excellent light stability, retaining more than 93% of the initial efficiency after 1000 h of constant illumination in a nitrogen environment. In addition, a four-terminal mechanically stacked perovskite-silicon tandem solar cell with champion efficiency of 30.3% is obtained using this MA-free composition. The encapsulated tandem devices show excellent operational stability, retaining more than 98% of the initial performance after 42 day/night cycles in an ambient atmosphere.

The year 2014 marks the point when silicon solar cells surpassed the 25% efficiency mark. Since then, all devices exceeding this mark, both small and large area, with contacts on both sides of the silicon wafer or just at the back, have utilized at least one passivating contact. Here, a passivating contact is defined as a group of layers that simultaneously provide selective conduction of charge carriers and effective passivation of the silicon surface. The widespread success of passivating contacts has prompted increased research into ways in which carrier‐selective junctions can be formed, yielding a diverse range of approaches. This paper seeks to classify passivating contact solar cells into three families, according to the material used for charge‐carrier selection: doped amorphous silicon, doped polycrystalline silicon, and metal compounds/organic materials. The paper tabulates their current efficiency values, discusses distinctive features, advantages, and limitations, and highlights promising opportunities going forth towards even higher conversion efficiencies.

This article presents a comprehensive study regarding the impact of the Al electrode on the surface passivation of three TiO x ‐based passivating selective contacts: TiO x :Al/LiF x /Al,TiO x /LiF x /Al, and a‐Si/TiO x :Al/LiF x /Al. A deterioration in passivation is recorded after the deposition of the Al electrode at close to room temperature, where the deterioration correlated to the Al thickness. A thin Al (10 nm) electrode resulted in the most severe passivation decline, while samples with a 100 nm Al electrode showed much less passivation deterioration. Furthermore, it is found that a low‐temperature annealing step led to a partial recovery of the passivation, particularly in the case of TiO x :Al/LiF x /Al and a‐Si/TiO x :Al/LiF x /Al contacts. The presented discovery in this article provides crucial insight into the importance of characterization and evaluation of passivating contacts, which is demonstrated here to be highly sensitive to the deposited metal thickness and the interfacial layers, as well as to the post‐deposition annealing.

Magnetic imaging with nitrogen-vacancy centers in diamond, also known as quantum diamond microscopy, has emerged as a useful technique for the spatial mapping of charge currents in solid-state devices. In this work, we investigate an application to photovoltaic (PV) devices, where the currents are induced by light. We develop a widefield nitrogen-vacancy microscope that allows independent stimulus and measurement of the PV device, and test our system on a range of prototype crystalline silicon PV devices. We first demonstrate micrometer-scale vector magnetic field imaging of custom PV devices illuminated by a focused laser spot, revealing the internal current paths in both short-circuit and open-circuit conditions. We then demonstrate time-resolved imaging of photocurrents in an interdigitated back-contact solar cell, detecting current build-up and subsequent decay near the illumination point with microsecond resolution. This work presents a versatile and accessible analysis platform that may find distinct application in research on emerging PV technologies.

Thin SiOx interlayers are often formed naturally during the deposition of transition metal oxides on silicon surfaces due to interfacial reaction. The SiOx layer, often only several atomic layers thick, becomes the interface between the Si and deposited metal oxide and can therefore influence the electrical properties and thermal stability of the deposited stack. This work explores the potential benefits of controlling the properties of the SiOx interlayer by the introduction of pregrown high-quality SiOx which also inhibits the formation of low-quality SiOx from the metal-oxide deposition process. This work demonstrates that a high-quality pregrown SiOx can reduce the interfacial reaction and results in a more stoichiometric MoOx with improved surface passivation and thermal stability linked to its lower Dit. Detailed experimental data on carrier selectivity, carrier transport efficiency, annealing stability up to 250 °C, and in-depth material analysis are presented.

Crystallographic structures, optoelectronic properties, and nanoscale surface morphologies of ex situ phosphorus‐doped polycrystalline silicon (poly‐Si)/SiO x passivating contacts, formed by different deposition methods (sputtering, plasma‐enhanced chemical vapour deposition [PECVD], and low‐pressure chemical vapour deposition [LPCVD]), are investigated and compared. Across all these deposition technologies, we noted the same trend: higher diffusion temperatures yield films that are more crystalline but that have rougher surface morphologies due to bigger surface crystal grains. Also, the recrystallization process of the as‐deposited Si films starts from the SiO x interface, rather than from the film surface and bulk. However, there are some distinct differences among these technologies. First, the LPCVD method yields the lowest deposition rate, roughest surfaces, and smallest degree of crystallinity on finished poly‐Si films. In contrast, the PECVD method has the highest deposition rate and smoothest surfaces for both as‐deposited Si and annealed poly‐Si films. Second, as‐deposited sputtered and PECVD Si films contain only an amorphous phase, whereas as‐deposited LPCVD films already has some crystalline phase. Third, the LPCVD phosphorus in‐diffusion into the substrate depends strongly on the initial film thickness, whereas for the other two methods, it is weakly dependent on thickness. Finally, the passivation quality of every poly‐Si film type has different responses to the film thickness and diffusion temperature, suggesting that the ex situ doping optimization should be performed independently.

Surface texturing of a silicon solar cell is critical to provide surface antireflection and light trapping. The common texturing method based on KOH as an etchant with isopropyl alcohol (IPA) as a wetting agent suffers two disadvantages: introducing metal contamination and low repeatability. To circumvent the limitation of the KOH-IPA method, we develop a new texturing regime by substituting TMAH for KOH, and a commercial surfactant RENA monoTEX for IPA. This TMAH-monoTEX method shows advantages of non-metal contamination, high reproducibility, short process time and small random pyramids. IBC solar cells fabricated with the TMAH-monoTEX texture achieved an efficiency of 25% with J(sc) of 42.9 mA/cm(2), V-oc of 719 mV and FF of 81.1%.

This paper reports that current widely used metal electrode aluminum results in dramatic passivation deterioration of some dopant-free passivated contacts (DFPC) (e.g. TiOx/LiFx， a-Si/LiFx and a-Si/TiOx/LiFx) after the Al electrode is deposited on the DFPC, which significantly lowers the device performance since excellent surface passivation is one of the key requirements for the successful integration of a full area contact in a solar cell. More interestingly, we find that the Al electrode thickness has a significant impact on the passivation deterioration. A thin Al (10nm) electrode results in the most severe passivation degradation for TiOx/LiFx. The thin TiOx behaves as a sacrifice layer in an a-Si/TiOx/LiFx contact improving the passivation stability upon thermal treatment. The presented result in this study implies that the passivation performance of a passivated contact assessed based on a the widely usage of nanometer-scale metal film on DFPC to mimic a metalized device structure is not fully reliable for evaluating the contact surface passivation quality, and instead a metal layer replicating the final intended device should be implemented, in combination to PL analysis for the extraction of the passivation quality.

Adv. Energy Mater. 2020, 10, 1903553 In the original manuscript, the spelling “Anita Ho-Ballie” is incorrect. The correct spelling is, “Anita Ho-Baillie” The authors apologize for any inconvenience caused.

This work presents results of a laboratory‐scale interdigitated back contact (IBC) solar cell with an independently measured efficiency of 25.0%, featuring open‐circuit voltage of 716 mV, short‐circuit current of 43.0 mA.cm−2 and fill factor of 81.0%. Notably, the high efficiency was achieved based on significant improvements resulting from the optimised cell structure, excellent SiO2‐SiNx‐SiOx (ONO) surface passivation, detailed bulk lifetime management strategy and improved random pyramid texturing. Experimental details and analysis of the individual improvements over prior work are presented in‐depth.

Titania (TiOx) is re-emerging to be a passivating material for the surfaces of high-efficiency crystalline silicon solar cells. Numerous sources in the literature suggest that the surface passivation and thermal stability of TiOx deteriorates with increasing film thickness when the TiOx film is thicker than a sufficient thickness. To circumvent this limitation, this study presents a novel process of Al-doped TiOx (TiOx:Al) film, which demonstrates the potential for improved thermal stability and surface passivation. Based on grazing incident X-ray diffraction and UV-Raman measurements, the incorporation of Al impurity in TiOx effectively restrains the crystal phase transformation of the amorphous TiOx layer during deposition. Furthermore, the TiOx:Al films provide better thermal robustness up to 350 °C, which makes it highly compatible with Si solar cell fabrication processes.

Mixed-dimensional perovskite solar cells combining 3D and 2D perovskites have recently attracted wide interest owing to improved device efficiency and stability. Yet, it remains unclear which method of combining 3D and 2D perovskites works best to obtain a mixed-dimensional system with the advantages of both types. To address this, different strategies of combining 2D perovskites with a 3D perovskite are investigated, namely surface coating and bulk incorporation. It is found that through surface coating with different aliphatic alkylammonium bulky cations, a Ruddlesden-Popper "quasi-2D" perovskite phase is formed on the surface of the 3D perovskite that passivates the surface defects and significantly improves the device performance. In contrast, incorporating those bulky cations into the bulk induces the formation of the pure 2D perovskite phase throughout the bulk of the 3D perovskite, which negatively affects the crystallinity and electronic structure of the 3D perovskite framework and reduces the device performance. Using the surface-coating strategy with n-butylammonium bromide to fabricate semitransparent perovskite cells and combining with silicon cells in four-terminal tandem configuration, 27.7% tandem efficiency with interdigitated back contact silicon bottom cells (size-unmatched) and 26.2% with passivated emitter with rear locally diffused silicon bottom cells is achieved in a 1 cm(2) size-matched tandem.

Immeasurably low surface recombination of crystalline-silicon wafers is demonstrated with an oxide-nitride-oxide (ONO) corona charged dielectric stack. We detail experimental variations to each layer of the dielectric stack to establish a procedure which provides outstanding passivation properties on textured and planar silicon wafers. We demonstrate surface recombination velocities of < 1 cm/s and surface recombination prefactors of < 1 fA/cm2, and we show that passivation remains stable over a 2-year period when stored in ambient conditions. The effective carrier lifetimes of n-type silicon are found to exceed the commonly accepted intrinsic lifetime limit, and in one case, a lifetime of 170 ms is attained. These high lifetimes indicate that ONO passivation is amongst the best dielectric passivation, and as such, might find applications in high-efficiency silicon solar cells.

We investigate the versatility of anodically grown silicon dioxide (SiO2) films in the context of process durability and exceptional surface passivation for high efficiency (> 23%) silicon solar cell architectures. We show that a room temperature anodic oxidation can achieve a thickness of similar to 70 nm within similar to 30 min, comparable to the growth rate of a thermal oxide at 1000 degrees C. We demonstrate that anodic SiO2 films can mask against wet chemical silicon etching and high temperature phosphorus diffusions, thereby permitting a low thermal budget method to form patterned structures. We investigate the saturation current density J(0) of anodic SiO2/silicon nitride stacks on phosphorus diffused and undiffused silicon and show that a J(0) of < 10 fA cm(-2) can be achieved in both cases. Finally, to showcase the anodic SiO2 films on a device level, we employed the anodic SiO2/silicon nitride stack to passivate the rear surface of an interdigitated back contact solar cell, achieving an efficiency of 23.8%.

This work presents the investigation of low pressure in-situ thermal oxidation as the interfacial oxide for n+ polysilicon-oxide passivated contact structure, achieving excellent surface passivation below 1 fA·cm-2 and contact resistivity below 1 mΩ·cm2. The results from the process optimisation are presented in detail, showing the importance of accurate control of oxidation conditions, and presenting the correlation to the electrical properties. Additionally, a method of fabricating contact resistivity structures from symmetrical photoconductance decay lifetime samples, and the extraction of the specific contact resistivity using 3D numerical simulation is presented.

As silicon photovoltaic technology advances, charge carrier losses at the contacted interfaces of the silicon absorber are coming to dominate power conversion efficiency. The so-called passivated contact, which provides selective charge-carrier extraction while simultaneously reducing interface recombination, is thus of significant interest for next-generation silicon solar cells. However, achieving both low recombination and low resistance to charge carrier extraction has proven challenging. Here, we present a passivated contact technology based on polysilicon deposited using low pressure chemical vapour deposition (LPCVD) over an ultra-thin silicon dioxide layer, which achieves an excellent surface passivation with implied open-circuit voltage of 735 mV, a recombination prefactor below 1 fA cm(-2) and contact resistivity below 1 m Omega cm(2). Key to this technology is the deposition of an ultra-thin silicon dioxide interlayer under high temperature and low pressure condition, performed in-situ within a single process with the polysilicon deposition. Additionally, the passivating contact structure maintains its electronic properties at temperatures of up to 900 degrees C and is compatible with existing industrial processes. The presented work therefore represents a significant advancement in industrially-applicable passivated contact technology.

This work presents the investigation of low pressure in-situ thermal oxidation as the interfacial oxide for n+ polysilicon-oxide passivated contact structure, achieving excellent surface passivation below 1 fA∙cm and contact resistivity below 1 mΩ∙cm. The results from the process optimisation are presented in detail, showing the importance of accurate control of oxidation conditions, and presenting the correlation to the electrical properties. Additionally, a method of fabricating contact resistivity structures from symmetrical photoconductance decay lifetime samples, and the extraction of the specific contact resistivity using 3D numerical simulation is presented.

Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping over cells with front-side only texture. However, increased surface area, roughness and exposed <111> crystal planes of textured surfaces not only causes increased recombination, but also makes cells susceptible to shunting through pinholes in the dielectric at the sharp peaks and valleys of the textured pyramids. A polyimide film as an insulating interlayer film is investigated to circumvent the tradeoff between improved light trapping, increased recombination and increased shunt paths. When applied at the rear of the interdigitated back contact silicon solar cell structure, the polyimide film provides an excellent electrical insulation (> 1000 MΩ of insulation resistance) and increases photocurrent (~ 1.1 mA/cm2) owing to an increased rear internal reflectance. The polyimide is also compatible with metal annealing of passivating dielectrics such as silicon nitride. Optical simulation and experimental results are combined in a 3D semiconductor simulation (Quokka) to quantify the possible gain of implementing the double-sided texture in high efficiency silicon solar cells.