Dopant-Free Bifacial Silicon Solar Cells

Herein, challenges in the fabrication of full dopant-free bifacial silicon solar cells are discussed and efficient devices utilizing a MoO3/ indium tin oxide (ITO)/Ag hole-selective contact and ZnO/LiFx/Al electron-selective contacts with up to 79% short-circuit current bifaciality are demonstrated. The ZnO/LiFx/Al rear electron contact features a full-area ZnO antireflective coating and a LiFx/Al finger contact, allowing sunlight absorption from the back side, thus producing more overall power. The ZnO/LiFx/Al electron contacts with a thinner ZnO layer and a larger contact fraction display a better selectivity and a lower resistance loss. When considering rear-side irradiance of 0.15 sun, the dopant-free bifacial solar cell with 60 nm ZnO and 50% LiFx/Al metal contact fraction achieves a 3% estimated output power density improvement compared with its monofacial counterpart (21.0 mW cm(2) compared to 20.3 mW cm(2)) using the full-area back contact. Both the efficiency and bifaciality factor of this dopant-free device are still significantly lower than those of state-of-the-art devices relying on doped-silicon-based layers. The required improvement for this technology to become industry-relevant is discussed.

Silicon Solar Cells on Glass with Power Conversion Efficiency above 13% at Thickness below 15 Micrometer

Jan Haschke, Natalie Preissler

Liquid phase crystallized silicon on glass with a thickness of (10-40) mu m has the potential to reduce material costs and the environmental impact of crystalline silicon solar cells. Recently, wafer quality open circuit voltages of over 650 mV and remarkable photocurrent densities of over 30 mA/cm(2) have been demonstrated on this material, however, a low fill factor was limiting the performance. In this work we present our latest cell progress on 13 mu m thin poly-crystalline silicon fabricated by the liquid phase crystallization directly on glass. The contact system uses passivated back-side silicon hetero-junctions, back-side KOH texture for light-trapping and interdigitated ITO/Ag contacts. The fill factors are up to 74% and efficiencies are 13.2% under AM1.5 g for two different doping densities of 1.10(17)/cm(3) and 2.10(16)/cm(3). The former is limited by bulk and interface recombination, leading to a reduced saturation current density, the latter by series resistance causing a lower fill factor. Both are additionally limited by electrical shading and losses at grain boundaries and dislocations. A small 1 x 0.1 cm(2) test structure circumvents limitations of the contact design reaching an efficiency of 15.9% clearly showing the potential of the technology.

Nature Publishing Group2017

Transparent Passivating Contacts for Front Side Application in Crystalline Silicon Solar Cells

Josua Andreas Stückelberger

In this thesis, we present the development and characterization of novel approaches to carrier-selective passivating contacts for crystalline silicon (c-Si) solar cells. In our first approach, we benefit from a wide-bandgap material, namely mixed-phase silicon oxide (SiOx). High hydrogen dilution during the plasma-enhanced chemical vapor deposition of a SiOx layer leads to the growth of vertically oriented silicon filaments embedded in a silicon oxide matrix. The resulting mixed-phase material combines transparency and vertical conduction through the layer. We propose a layered contact structure starting with an ultra-thin chemically grown SiOx (~1.2 nm) at the c-Si wafer surface, followed by a phosphorus-doped bilayer of the mixed-phase SiOx, finishing with a nanocrystalline silicon layer that ensures contact with the metallization. With such a stack, an electron-selective passivating contact with a saturation current density J0 of 5.3 fA/cm2 is achieved on n-type wafers with hydrogenation, whereas on p-type wafers the contact reached 5.8 fA/cm2 even without hydrogenation. The contact is analyzed in detail with respect to its structure and its change upon thermal treatment. Two different growth regions are detected: one with the vertically oriented silicon inclusions and a second one in which the silicon phases have more lateral growth. Crystallization and atomic redistribution are analyzed during in-situ annealing in an electron microscope, leading to a better understanding of the change in distribution of phosphorus and oxygen as well as the nucleation sites. The influence of the annealing temperatures and dwell times as well as the initial doping concentration on the resulting in-diffused region and the passivation quality is reported. We use simulations to relate the influence of these parameters on passivation to the different recombination mechanisms. From the analyzed phosphorus doping profiles, an interesting observation is reported. The chemical oxide varies in thickness and density depending on the wafer polarity as well as the base resistivity. Proof-of-concept cells including this novel structure demonstrate the suitability for electron-selective passivating contacts at the device level with conversion efficiencies of 19.0% on planar wafers and 20.1% on textured wafers. Promisingly high short-circuit current densities (Jsc) of 35 mA/cm2 (40 mA/cm2) on planar (textured) with low parasitic absorption indicate its potential as a front contact in next-generation solar cells. Additionally, the beneficial feature of the smooth refractive index change within the layers that leads to a low reflectance over a broad wavelength range, similar to a built-in anti-reflection coating, is presented. A second approach to reach highly transparent electron-selective passivating contacts is demonstrated by the use of a plasma process based on the precursor gases phosphine (PH3), silicon tetrafluoride (SiF4), hydrogen (H2), and argon (Ar). The beneficial effect of fluorine in terms of passivation without the need for additional hydrogenation processes is shown, reaching implied open-circuit voltages of 730 mV on planar surfaces and 707 mV on textured surfaces. These layers are implemented in first proof-of-concept working devices with efficiencies of 19.5% on single-side-textured wafers and Jsc values of up to 40 mA/cm2.

EPFL2018

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.