Influence of the Dopant Gas Precursor in P-Type Nanocrystalline Silicon Layers on the Performance of Front Junction Heterojunction Solar Cells

Silicon heterojunction solar cells can employ p-type hydrogenated nanocrystalline silicon nc-Si:H(p) on their front side, since these can provide better transparency and contact resistance compared to hydrogenated p-type amorphous silicon layers. We investigate here the influence of trimethyl boron (TMB) and BF3 as dopant source on the layer properties and its performance in solar cells. Both gases enable high efficiencies but yield a different crystallinity and effective doping. A high BF3 flow lowers the series resistance through a low activation energy of dark lateral conductivity and maintains a high crystallinity. This allows fill factors up to 83%, however with the apparition of a parasitic absorption in the UV. A low TMB flow enables simultaneously a high crystallinity and a low activation energy. As an illustration of this layer potential, a 23.9%-certified efficiency is achieved with a 2 x 2 cm(2) screen-printed device. We finally suggest that similar transport versus transparency trade-offs can be reached for both dopant types for front junction application, while high BF3 flow allowing lower series resistance might be of interest when placed on the rear side.

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

Amorphous silicon-germanium for triple and quadruple junction thin-film silicon based solar cells

Christophe Ballif, Mathieu Gérard Boccard, Maximilien Joseph Marie Bonnet-Eymard, Grégory Bugnon, Matthieu Despeisse, Franz-Josef Haug, Björn Niesen, Michael Elias Stückelberger

We study amorphous silicon-germanium (a-SiGe:H) as intrinsic absorber material for thin-film silicon-based triple and quadruple junction solar cells. First, we present the development of a-SiGe:H single junction devices, in particular the Ge-content grading in the absorber layer, the influence of the Ge-content on electrical properties and (infra)red-response, and the influence of using different types of players. We subsequently show the incorporation of optimized single-junction devices in triple junction cells and discuss the interplay between Ge-content and intermediate reflector thickness. For triple junction devices with amorphous silicon (a-Si:H) top cells, a-SiGe:H middle cells and microcrystalline silicion (mu c-Si:H) bottom cells, we obtained an initial efficiency of 13.6% and an efficiency of 11.3% after light-soaking. We also present a quadruple junction device with an a-Si:H top cell, a low Ge-content a-SiGe:H second cell, and mu c-Si:H third and bottom cells. In this device configuration, we obtained an open-circuit voltage as high as 2.57 V. The performance of these cells was limited by not yet optimized current matching, leading nevertheless to an initial efficiency of 10.1%. A brief roadmap towards quadruple-junction devices with stabilized efficiencies of 14% is also outlined. (C) 2014 Elsevier B.V. All rights reserved.

Elsevier2015

Explaining Morphological and Electrical Features of Boron-doped Zinc Oxide to Tailor New Electrodes for Photovoltaics

Lorenzo Fanni

TCOs are a class of metal oxides that combine transparency to visible light with electrical conductivity. Each TCO is characterized by the trade-off of these two properties which can be tuned for a particular application. This thesis is dedicated to the investigation of one TCO material: boron doped zinc oxide (ZnO:B) deposited by low-pressure chemical vapor deposition (LP-MOCVD). Its main distinction is low absorptance which makes ZnO films deposited by LP-MOCVD ideal as transparent electrodes in thin-film solar cells. Although ZnO:B properties have previously been optimized for this application, the exact processes behind the film formation have not been fully described. This thesis substantially clarifies the processes of the film nucleation and growth evolution, and the mechanisms of incorporation of the B atoms into the ZnO. First, for non-intentionally doped ZnO we establish that the main deposition parameters influencing the film properties are the deposition temperature, the gas precursor ratio and the total gas flow. We experimented with these three parameters across a wide range of parameter values. The films deposited were characterized at different stages of their growth using atomic force microscopy, X-ray diffraction and automated crystallographic orientation mapping. These data reveal the dependence of the film preferential orientation on the deposition conditions. We propose a model based on the adsorbed atom mean free path to explain this dependence. Using deposition parameters learnt from this model, we control the preferential orientation during film growth to increase the grain sizes. Second, we analyze the conductivity to B-doped ZnO films. Boron atoms act as electron donors in ZnO, increasing the electrical conductivity and decreasing the transparency. This work quantifies how the B concentration and spatial distribution affect the film conductivity. Quantification of the B atoms incorporated in the film was performed using nuclear reaction analysis. We found that a high level of O precursor gas favors B incorporation in the film. Combining nano secondary ion mass spectroscopy and Kelvin probe force microscopy we demonstrate that the dopant atoms incorporate in only one of the two sides of each grain. This is a new observation because dopant atoms are commonly assumed to be uniformly distributed in the film. The transparency and electrical conductivity of ZnO depend also on the carrier mobility. The sources of carrier scattering are investigated for intrinsic (i.e. O vacancies and Zn interstitials) and extrinsic (B atoms) impurity concentrations. We observed that for the same B concentration the source of electron scattering depends on the concentration of intrinsic defects: for films with a lower concentration of intrinsic defects, the main source of scattering is the grain boundaries; for films with a higher concentration of intrinsic defects, the main sources are the mechanisms happening in the crystalline region. Finally, LP-MOCVD ZnO:B films were optimized using the previous results and successfully applied in four different types of solar cells: amorphous/microcrystalline silicon, amorphous silicon, copper indium gallium selenide and silicon heterojunction. The advantages of the developed films to these four cells include the simplification of the fabrication process, the reduction of reflectance and parasitic absorption, and a better understanding of carrier transport mechanisms through the device.