Integration of thin n-type nc-Si:H layers in the window-multilayer stack of heterojunction solar cells

N-type nanocrystalline silicon (nc-Si:H(n)) layers are good candidates to improve current and transport properties in heterojunction solar cells. In this work, we perform thickness series alongside PH3 doping series to unravel the desirable characteristics of nc-Si:H(n) along its growth direction. While increasing the PH3 flow is necessary to improve the conductivity of the layer, we observe that too high flows lead to an amorphization of the first 10???15 nm of the layers. In consequence, either high crystallinity are reached at intermediate PH3 flow, resulting in high doping at the n/TCO interface but a poorly doped nucleation zone, or the material becomes more amorphous at very high PH3 flow, allowing a higher doping in the nucleation zone but a less efficient screening of the barrier with the TCO due to the lower crystallinity. To overcome this trade-off, we integrate a 2.5 nm a-Si:H(n) layer underneath the nc-Si:H(n) layer. We also report on the impact of higher ITO doping as well as on the beneficial effect of an additional SiOx capping not only to form a double anti-reflective coating (DARC), but also to improve contact properties. We observe that each of those three features can improve passivation and selectivity as well as reduce strongly the front contact resistivity (????????????????????????????????????). Our best results are achieved by using a thin a-Si:H(n)/nc-Si:H(n) stack together with an IZrO/SiOx DARC, enabling front ????????????????????????????????????lower than 15 m?? cm2 and an efficiency of 23.7% on screen-printed 2 ?? 2 cm2 solar cells. Superscript/Subscript Available

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

Bulk Defects and Hydrogenation Kinetics in Crystalline Silicon Solar Cells With Fired Passivating Contacts

Christophe Ballif, Franz-Josef Haug, Andrea Ingenito, Mario Joe Lehmann, Audrey Marie Isabelle Morisset, Père Roca i Cabarrocas, Philippe Wyss

In this article, the effect of the various processing steps during the fabrication of c-Si/SiOx/SiCx fired passivating contacts on the silicon bulk lifetime is studied, and the kinetics of defect deactivation by hydrogenation is investigated. It is found that the firing step at 800 degrees C induces shallow bulk defects in float-zone silicon wafers, which can subsequently be passivated with hydrogen provided by an a-SiNx:H/D reservoir layer upon annealing at 450 degrees C. Experimental results and numerical data treatment indicate a rapid passivation of the surface within less than 1 min, followed by a slower passivation of the shallow bulk defects. In situ lifetime measurements are consistent with a slow bulk lifetime improvement by showing similar lifetime evolutions for both p-type and n-type SiCx layers. The kinetics of the hydrogenation process seems to be limited by the available hydrogen supply at the c-Si/SiOx interface, rather than by its diffusion within the bulk of the wafer. Moreover, it is affected by the bulk doping as well as the SiCx layer thickness. Finally, it is shown that hydrogenation is also possible with an a-SiNx:H/D reservoir layer deposited on one side of the wafer only, although resulting in a lower passivation level (s similar to 700 mu s compared to similar to 1300 mu s for symmetrical samples), and slower kinetics (similar to 5 min compared to similar to 0.8 min).

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

Contact Design for Silicon Heterojunction Solar Cells

Luca Massimiliano Antognini

EPFL2022