Numerical simulations of hole carrier selective contacts in p-type c-Si solar cells

This work presents a systematic analysis of the transport mechanism and surface passivation of tunneling oxide (SiO2)/p-type poly-silicon (poly-Si(p)) junctions applied to p-type crystalline silicon (c-Si) solar cells by means of TCAD numerical simulations. We report on the impact of the buried doped region (BDR) in the c-Si wafer on the transport and passivation of SiO2/poly-Si(p) junctions. We show that a BDR is not necessary for carrier selective contacts (CSCs) with a tunnel oxide thinner than 1.2 nm and for surface recombination velocity at SiO2/c-Si interface below 1.10(3) cm/s. Then, we explore alternative semiconductors to poly-Si for tunnel oxide passivating contacts. We rind that 3C-SiC(p) is a promising candidate thanks to its valence band offset with respect to silicon, driving the wafer surface into a condition of strong accumulation. We show that excellent SiO2/3C-SiC(p) junctions are obtained for doping density of the 3C-SiC(p) larger than 5.10(19) cm(-3) and for SiO2 thinner than < 1.2 nm. Finally, with the aim of deriving guidelines for material selection, we present an investigation on the influence of the electron affinity and bandgap of the semiconductor layer forming the passivating contact, demonstrating that conversion efficiency is maximized for built-in voltages between 0.4 and 2.6 eV.

Mitigating Plasmonic Absorption Losses at Rear Electrodes in High‐Efficiency Silicon Solar Cells Using Dopant‐Free Contact Stacks

Christophe Ballif, Mathieu Gérard Boccard, Stefaan De Wolf, Julie Amandine Dreon, Fan Fu, Quentin Thomas Jeangros, Sihua Zhong

Although charge‐carrier selectivity in conventional crystalline silicon (c‐Si) solar cells is usually realized by doping Si, the presence of dopants imposes inherent performance limitations due to parasitic absorption and carrier recombination. The development of alternative carrier‐selective contacts, using non‐Si electron and hole transport layers, has the potential to overcome such drawbacks and simultaneously reduce the cost and/or simplify the fabrication process of c‐Si solar cells. Nevertheless, devices relying on such non‐Si contacts with power conversion efficiencies (PCEs) that rival their classical counterparts are yet to be demonstrated. In this study, one key element is brought forward toward this demonstration by incorporating low‐pressure chemical vapor deposited ZnO as the electron transport layer in c‐Si solar cells. Placed at the rear of the device, it is found that rather thick (75 nm) ZnO film capped with LiFx/Al simultaneously enables efficient electron selectivity and suppression of parasitic infrared absorption. Next, these electron‐selective contacts are integrated in c‐Si solar cells with MoOx‐based hole‐collecting contacts at the device front to realize full‐area dopant‐free‐contact solar cells. In the proof‐of‐concept device, a PCE as high as 21.4% is demonstrated, which is a record for this novel device class and is at the level of conventional industrial solar cells.

2019

Contact Design for Silicon Heterojunction Solar Cells

Luca Massimiliano Antognini

Today more than ever the world needs clean energy sources and thus a fast deployment and scaling up of the photovoltaic industry. In this context improving solar cell efficiency plays a major role. In order to achieve the maximum single junction efficiency for this material, technologies featuring carrier selective passivating contacts are foreseen to be the next successors of actual industrial cells. Among them, the silicon heterojunction (SHJ) solar cell is a promising technology which demonstrated efficiencies up to 26.7%. Its main specificity consists in the use of intrinsic and doped amorphous silicon (a-Si:H) to provide surface passivation and carrier selectivity to the c-Si, which allows to reach very high operating voltages in comparison to other c-Si based technology. The major drawbacks of SHJ solar cell is the large parasitic absorption occurring in those layers. To address this challenge, a strong focus on alternative non-silicon based thin films with wider band gap has built up in the literature. A first part of this thesis focuses on reviewing the fundamental process behind carrier selectivity and discuss the ability of different models from the literature to draw informations directly from the peculiar shapes of IV curves commonly observed in the solar cells featuring newly developed materials. We first discuss the simple four ideal diodes model of Roe et al to explain the occurrence of S-shapes depending on the contact quality. We discuss from a theoretical point of view how to improve the model by modifying the ideal diodes by other circuit elements as well as adding bulk recombinations. We test the theory experimentally and identified modifications that should be incorporated in the model.In a second part of the thesis, we explore the development of doped nano-crystalline silicon (nc-Si) layers to replace classical amorphous silicon layers. We investigate the influence of TMB and BF3 as dopant sources on the transparency and contact properties of nc-Si:H(p) layers as well as their integration in solar cells. We expose the roles of both gas to modify the crystallinity of the layer and find different optimum for both of them to lead high efficiencies, illustrated by a certified 23.9%-efficient solar cell. We report also on further optimization steps of this front junction architecture resulting in certified efficiency of 24.44%.We explore next the n-type nc-Si layers for application as window layer. In particular, we perform thickness and doping series to unravel the layer properties along its growth direction. In order to improve the low doping of the nucleation zone of nc-Si:H(n) at the i/n interface, a thin a-Si:H(n) buffer layer is introduced and shown to improve the passivation, selectivity and contact resistivity. Finally, we also report on the beneficial effect of an additional SiOx capping layer which improves both the reflection properties as well as all contact properties, broadening the optimal parameter window. The absorption loss in the front silicon layers remains however the highest share of losses, and therefore we explore theoretically the opportunity of using thinner front side layers or using thin TCO / silicon nitride bilayers to reduce the absorption losses. We finish the discussion by presenting an optimization roadmap, showing a possibility for double-side contacted SHJ cells to reach efficiencies above 26% without requiring any patterning or localization step.

EPFL2022

Phosphorous-Doped Silicon Carbide as Front-Side Full-Area Passivating Contact for Double-Side Contacted c-Si Solar Cells

Christophe Ballif, Matthieu Despeisse, Luca Gnocchi, Franz-Josef Haug, Andrea Ingenito, Philipp Friedrich Hermann Löper, Gizem Nogay, Josua Andreas Stückelberger, Philippe Wyss

We present an electron selective passivating contact based on a tunneling SiOx capped with a phosphorous doped siliconcarbideandpreparedwithahigh-temperaturethermalanneal. We investigate in detail the effects of the preparation conditions of theSiCx(n)(i.e.,gasﬂowprecursorandannealingtemperature)on the interface recombination rate, dopant in-diffusion, and optical properties using test structures and solar cells. On test structures, our investigation reveals that the samples annealed at temperatures of 800–850 °C exhibit an increased surface passivation toward higher gas ﬂow ratio (r = CH4/(SiH4 + CH4)). On textured and planar samples, we obtained best implied open-circuit voltages (i-VOC) of 737 and 746 mV, respectively, with corresponding dark saturation current densities (J0) of∼8 and∼4 fA/cm2. The SiCx(n)layerswithdifferentrvalueswereappliedonthetextured front side of p-type c-Si solar cells in combination with a borondoped SiCx(p) as rear hole selective passivating contact. Our cell results show a tradeoff between VOC and short-circuit current density (JSC) dictated by the C-content in the front-side SiCx(n). On p-type wafers, best VOC = 706 mV, FF = 80.2%, and JSC = 38.0 mA/cm2 with a ﬁnal conversion efﬁciency of 21.5% are demonstrated for 2 × 2 cm 2 screen-printed cells, with a simple and patterning-free process based on plasma depositions and one annealing step 800 °C < T < 850 °C for the formation of both passivating contacts.

2018

Contact Design for Silicon Heterojunction Solar Cells

Luca Massimiliano Antognini

EPFL2022