Hydrogenated nanocrystalline silicon oxide (nc-SiO_x:H) layers exhibit promising optoelectrical properties for carrier-selective-contacts in silicon heterojunction (SHJ) solar cells. However, achieving high conductivity while preserving crystalline silicon (c-Si) passivation quality is technologically challenging for growing thin layers (less than 20 nm) on the intrinsic hydrogenated amorphous silicon ((i)a-Si:H) layer. Here, we present an evaluation of different strategies to improve optoelectrical parameters of SHJ contact stacks founded on highly transparent nc-SiO_x:H layers. Using plasma-enhanced chemical vapor deposition, we firstly investigate the evolution of optoelectrical parameters by varying the main deposition conditions to achieve layers with refractive index below 2.2 and dark conductivity above 1.00 S/cm. Afterwards, we assess the electrical properties with the application of different surface treatments before and after doped layer deposition. Noticeably, we drastically improve the dark conductivity from 0.79 to 2.03 S/cm and 0.02 to 0.07 S/cm for n- and p-contact, respectively. We observe that interface treatments after (i)a-Si:H deposition not only induce prompt nucleation of nanocrystals but also improve c-Si passivation quality. Accordingly, we demonstrate fill factor improvement of 13.5%_abs from 65.6% to 79.1% in front/back-contacted solar cells. We achieve conversion efficiency of 21.8% and 22.0% for front and rear junction configurations, respectively. The optical effectiveness of contact stacks based on nc-SiO_x:H is demonstrated by averagely 1.5 mA/cm² higher short-circuit current density thus nearly 1%_abs higher cell efficiency as compared with the (n)a-Si:H.

Solar cells based on black silicon (b-Si) are proven to be promising in photovoltaics (PVs) by exceeding 22% efficiency. To reach high efficiencies with b-Si surfaces, the most crucial step is the effective surface passivation. Up to now, the highest effective minority carrier lifetimes are achieved with atomic layer-deposited Al₂O₃ or thermal SiO₂. Plasma-enhanced chemical vapor deposition (PECVD)-grown hydrogenated amorphous silicon (a-Si:H) passivation of b-Si is seldom reported due to conformality problems. In this current study, b-Si surfaces superposed on standard pyramidal textures, also known as modulated surface textures (MSTs), are successfully passivated by PECVD-grown conformal layers of a-Si:H. It is shown that under proper plasma-processing conditions, the effective minority carrier lifetimes of samples endowed with front MST and rear standard pyramidal textures can reach up to 2.3 ms. A route to the conformal growth is described and developed by transmission electron microscopic (TEM) images. Passivated MST samples exhibit less than 4% reflection in a wide spectral range from 430 to 1020 nm.

This paper presents an analysis of physical mechanisms related to operation and optimization of interdigitated back contact (IBC) poly-silicon-based devices. Concepts of carrier selectivity and tunneling are used to identify the parameters that impact on the fill factor. Then, based on technology computer-aided design (TCAD) numerical simulations, we describe the device performance in terms of transport and passivation. A validation of the model is performed by matching measured and simulated R, T, and external quantum efficiency spectra and electrical parameters. As result of such process, the opto-electrical losses of the reference device are identified. Then, we execute a study of the impact of process parameters on the performance of the IBC device under analysis. Assuming a uniform SiO ₂ layer, simulation results reveal that both n-type and p-type poly-Si contacts can be theoretically perfect (i.e., approx. lossless), if assuming no interface recombination but considering tunneling of both carrier types. In other words, there exists an optimum oxide thickness (1 nm) for which majority carriers tunneling works already very well, and minority tunneling is still low enough to not result in significant recombination. Moreover, SiO ₂ thickness up to maximum 1.6 nm is crucial to achieve high efficiency. Regarding rear geometry analysis, the efficiency curve as a function of emitter width peaks at 70% of pitch coverage. Further, it is shown that diffused dopants inside crystalline silicon make the device resilient to passivation quality. Finally, the calibrated model is used to perform an optimization study aiming at calculating the performance limit. The estimated performance limit is 27.3% for a 100-μm-thick bulk, 20-nm-thick poly-silicon layers, silver as rear contact, and double ARC.

Broadband transparent conductive oxide layers with high electron mobility (μ_e) are essential to further enhance crystalline silicon (c-Si) solar cell performances. Although metallic cation-doped In₂O₃ thin films with high μ_e (>60 cm² V^-1 s^-1) have been extensively investigated, the research regarding anion doping is still under development. In particular, fluorine-doped indium oxide (IFO) shows promising optoelectrical properties; however, they have not been tested on c-Si solar cells with passivating contacts. Here, we investigate the properties of hydrogenated IFO (IFO:H) films processed at low substrate temperature and power density by varying the water vapor pressure during deposition. The optimized IFO:H shows a remarkably high μ_e of 87 cm² V^-1 s^-1, a carrier density of 1.2 × 10²⁰ cm^-3, and resistivity of 6.2 × 10^-4 ω cm. Then, we analyzed the compositional, structural, and optoelectrical properties of the optimal IFO:H film. The high quality of the layer was confirmed by the low Urbach energy of 197 meV, compared to 444 meV obtained on the reference indium tin oxide. We implemented IFO:H into different front/back-contacted solar cells with passivating contacts processed at high and low temperatures, obtaining a significant short-circuit current gain of 1.53 mA cm^-2. The best solar cell shows a conversion efficiency of 21.1%.

In this work we develop a rear emitter silicon solar cell integrating carrier-selective passivating contacts (CSPCs) with different thermal budget in the same device. The solar cell consists of a B-doped poly-Si/SiO_x hole collector and an i/n hydrogenated amorphous silicon (a-Si:H) stack acting as electron collector placed on the planar rear and textured front side, respectively. We investigate the passivation properties of both CSPCs on symmetric structures by optimizing the interdependency among annealing temperature, time and environment. The optimized B-doped poly-Si/SiO_x reaches a saturation current density of ~10 fA/cm² on n-type wafers and an implied open circuit voltage (iV_OC) of 716 mV. Furthermore, the i/n a-Si:H stack shows an effective carrier lifetime above 4 ms and iV_OC of ~705 mV for cell-relevant layers thickness. After a post-deposition annealing in H₂, lifetime is above 10 ms and iV_OC = 708 mV. Finally, we optimize the optoelectronic properties of indium-based transparent conductive oxide (Indium Tin Oxide ITO and hydrogenated indium oxide IO:H) to reduce parasitic absorption with a gain in short circuit current density of 0.23 mA/cm². In conclusion, the optimized layer stacks are implemented at device level obtaining a device with V_OC = 704 mV, fill factor of 73.8%, a short circuit current of 39.7 mA/cm² and 21.0% aperture-area conversion efficiency.

In this work we present a theoretical analysis of charge carriers transport mechanisms in IBC-SHJ solar cells. The concepts of contact and transport selectivity are correlated through the band bending at c-Si interface and are used to identify thin-film silicon parameters affecting fill factor (FF) and open-circuit voltage (V_OC). Additionally, the transport of carriers is associated to energy barriers at the conduction band for electrons and at the valence band for holes. In case of p-type contact, the transport of holes is mainly affected by activation energy and band gap of the p-type layer and work function of the TCO. In case of n-type contact, the activation energy and work function of the doped layer impact the most on transport of electrons. Selective transport is improved by maximizing the collection of majority carrier in each doped contact stack while blocking minority carriers. In particular, low activation energy values of doped layers are crucial to minimize energy barriers for majority carriers and increase the band bending at c-Si interface. Simulation results based on TCAD Sentaurus reveal that the FF increases as the activation energy of the doped layers is reduced. Also, for the p-type contact, the bandgap of p-type layer strongly affects the band bending at c-Si interface. Particularly, widening the bandgap of p-type layer enhances passivation and transport in terms of V_OC and FF but work function mismatch between the p-type layer and the related transparent conductive oxide (TCO) strongly increases as bandgap increases. This possibly makes the device less performant because it is more sensitive to activation energy of the p-layer in combination with the choice of the proper TCO. Considering realistic deposited layers, a wide bandgap p-type layer, in combination with low activation energy, potentially improves hole collection leading to maximal simulated FF = 86.8% and V_OC = 754 mV for a conversion efficiency η = 27.2%.

This paper shows the application of carrier-selective passivating contacts (CSPCs) in c-Si front-back contacted (FBC) and interdigitated back-contacted (IBC) solar cells with different thermal budgets. From lifetime analysis of our poly-Si and a-Si:H CSPCs, three FBC and one IBC architectures are devised to progressively increase efficiency (ç) and achieve record short-circuit current density (JSC): (i) a poly-poly cell (\eta = 19.6%); (ii) a selective emitter structure known as PeRFeCT (Passivated Emitter Rear and Front ConTacts, \eta = 20.0%); (iii) a so-called hybrid solar cell with poly-Si and a-Si:H CSPCs at rear and front, respectively (\eta = 21.0%); and (iv) an IBC solar cell with poly-Si CSPC (\eta = 23.0%, JSC = 42.2 mA/cm2).

This paper presents a detailed analysis on the impact of the emitter contact geometry on the performance of back contact-back junction (BC-BJ) c-Si solar cells by means of a TCAD-based electro-optical model. BC-BJ c-Si solar cells featuring a conventional linear-contact (LC) scheme and an advanced point-contact (PC) pattern are considered. In addition, the introduction of a selective emitter (SE) design in the PC structure and the effect of including multiple contacts in the emitter region of the considered BC-BJ solar cells are also investigated.

The poly-Si carrier-selective passivating contacts (CSPCs) parasitically absorb a substantial amount of light, especially in the form of free carrier absorption. To minimize these losses, we developed CSPCs based on oxygen-alloyed poly-Si (poly-SiO_x) and deployed them in c-Si solar cells. Transmission electron microscopy analysis indicates the presence of nanometer-scale silicon crystals within such poly-SiO_x layers. By varying the O content during material deposition, we can manipulate the crystallinity of the poly-SiO_x material and its absorption coefficient. Also, depending on the O content, the bandgap of the poly-SiO_x material can be widened, making it transparent for longer wavelength light. Thus, we optimized the O alloying, doping, annealing, and hydrogenation conditions. As a result, an extremely high passivation quality for both n-type poly-SiO_x (J₀ = 3.0 fA/cm² and iVoc = 740 mV) and p-type poly-SiO_x (J₀ = 17.0 fA/cm² and iVoc = 700 mV) is obtained. A fill factor of 83.5% is measured in front/back-contacted solar cells with both polarities made up of poly-SiO_x. This indicates that the carrier transport through the junction between poly-SiO_x and c-Si is sufficiently efficient. To demonstrate the merit of poly-SiOx layers' high transparency at long wavelengths, they are deployed at the back side of interdigitated back-contacted (IBC) solar cells. A preliminary cell efficiency of 19.7% is obtained with much room for further improvement. Compared to an IBC solar cell with poly-Si CSPCs, a higher internal quantum efficiency at long wavelengths is observed for the IBC solar cell with poly-SiO_x CSPCs, thus demonstrating the potential of poly-SiO_x in enabling higher J_SC.

In this work, we present the application of poly-Si carrier-selective passivating contacts (CSPCs) as both polarities in interdigitated back-contacted (IBC) solar cell architectures. We compared two approaches to form a gap between the back-surface field (BSF) and emitter fingers. It is proved that the gaps prepared by both approaches are efficient in preventing carriers’ recombination. To minimize the reflection losses, we developed a novel modulated surface texturing (MST) structure as anti-reflection coating (ARC). It is obtained by superposing a nano-textured SiO₂ layer on the conventional micro-textured pyramids, which are passivated with a-Si:H / SiN_x:H layers. This approach decouples the light harvesting from the Si surface passivation, which potentially results in the highest possible optical and electrical performances of the solar cells. The reflectance (R) of the MST-ARC is very close to that of the high-aspect ratio nano-structured silicon (black-Silicon), achieving R < 1% between 450 and 1000 nm. The J₀ of MST-ARC passivated Si surface (6.3 fA/cm²) is the same as that of standard a-Si:H/SiN_x:H layers passivated pyramidally-textured Si surface. By applying this novel MST-ARC in our IBC solar cell, the highest J_SC observed in a device is 42.2 mA/cm² with a V_OC as high as 701 mV. A spectral response enhancement in case of the MST-ARC cell is observed over the whole wavelength range with respect to the cell with standard SiN_x:H ARC. The highest efficiency achieved in this work is 23.0%, with the potential to reach 24.0% in short term by using more conductive metal fingers.

This article presents recent approaches for achieving high conversion efficiency of crystalline silicon solar cells at Delft University of Technology. The new approaches are based on heterojuction interfaces between the crystalline silicon (c-Si) absorber and carrier-selective passivating contact layers. We discuss silicon heterojunction solar cells with carrier-selective contact based on thin layers of hydrogenated amorphous silicon (a-Si: H) and on hydrogenated polycrystalline silicon (poly-Si) combined with a ultrathin silicon oxide layer. Both, the application of carrier-selective contacts in c-Si front-back contacted (FBC) and interdigitated back-contacted (IBC) solar cells with different thermal budgets are shown. The best performance was demonstrated with IBC c-Si solar cells with poly-Si carrier-selective passivating contacts with conversion efficiency η=23.0% and short-circuit current density JSC= 42.2 mAlcm².

Amorphous silicon carbide (a-SiC:H) is a promising material for photoelectrochemical water splitting owing to its relatively small band-gap energy and high chemical and optoelectrical stability. This work studies the interplay between charge-carrier separation and collection, and their injection into the electrolyte, when modifying the semiconductor/electrolyte interface. By introducing an n-doped nanocrystaline silicon oxide layer into a p-doped/intrinsic a-SiC:H photocathode, the photovoltage and photocurrent of the device can be significantly improved, reaching values higher than 0.8V. This results from enhancing the internal electric field of the photocathode, reducing the Shockley-Read-Hall recombination at the crucial interfaces because of better charge-carrier separation. In addition, the charge-carrier injection into the electrolyte is enhanced by introducing a TiO₂ protective layer owing to better band alignment at the interface. Finally, the photocurrent was further enhanced by tuning the absorber layer thickness, arriving at a thickness of 150nm, after which the current saturates to 10mAcm^-2 at 0V vs. the reversible hydrogen electrode in a 0.2m aqueous potassium hydrogen phthalate (KPH) electrolyte at pH4.

Highest conversion efficiency in crystalline silicon (c-Si) solar cells can be enabled by quenching minority carriers' recombination at c-Si/contact interface owing to carrier-selective passivating contacts. With the semi-insulating poly-crystalline silicon (SIPOS, poly-Si) a very good passivation of c-Si surfaces was obtained. We have explored these passivating structures on IBC solar cells and obtained over 22% efficiency with over 23% within reach on the short term. We present in detail the passivation quality of p-type and n-type ion-implanted LPCVD poly-crystalline silicon (poly-Si) and its relation to the doping profile. Optimized poly-Si layers in the role of emitter and BSF showed excellent passivation (J_0,emitter = 11.5 fA/cm² and J_0,BSF = 4.5 fA/cm²) and have been deployed in FSF-based IBC c-Si solar cells using a simple self-aligned patterning process. Applying an optimized passivation of FSF by PECVD a-Si:H/SiNx layer (J_0,FSF = 6.5 fA/cm²) leads to a cell with efficiency of 22.1% (V_OC = 709 mV, J_SC = 40.7 mA/cm², FF = 76.6%). Since over 83% FF has been reached with adjusted metallization technology on similar IBC structures, we believe 23% efficiency is within reach on the short term. Further improvement, especially at J_SC level, is expected by deploying less absorbing carrier-selective passivating contacts based on poly-Si or wide bandgap poly-SiO_x layers (J₀ ~ 10 fA/cm²).