CVD Boron Containing Glasses - An Attractive Alternative Diffusion Source for High Quality Emitters and Simplified Processing : A Review

This review presents the current state of the art and interesting questions with regard to CVD BSG layers. The advantages of CVD doping sources over the conventional POCl3 and BBr3 or BCl3 gaseous sources are the simple way to deposit a diffusion source on only a single side of the wafer and structuring the diffusion source to achieve dopant concentration profiles next to each other on the same side of the wafer. In addition, these CVD glasses are multifunctional. The same CVD BSG can serve e. g. as doping source, passivation layer, antireflective coating and as electrical insulator.Monofacial n-type solar cells achieve efficiencies up to 20.5% on a 156 x 156 mm2 n-type Cz-Si solar cell. Bifacial cells of that size show efficiencies up to 20.1%. Back junction back contact cells show an efficiency of 20.5% on 4 cm2 aperture area.publishe

Development and characterisation of novel high efficiency silicon solar cell concepts

High efficiency silicon solar cells generally require sophisticated and complex processing sequences. The key features of such solar cells are: a silicon dioxide for surface passivation, front side texture, local rear contacts, a low front contact coverage ratio and a large minority carrier lifetime. Within this work novel high efficiency silicon solar cell concepts were developed and characterised. These allow an efficiency improvement and/or a simplification of the solar cell processing compared to conventional methods. These high efficiency silicon solar cell concepts are based on single or double sided mechanical surface texturing.Lifetime measurements and the efficiencies of front side textured solar cells of up to 20.1% demonstrate that the mechanical texturing is suitable for high efficiency solar cells. The silicon solar cell processes developed within this work are mainly investigated with regard to the contacting schemes particularly designed for mechanically textured surfaces. The thorough characterisation of solar cells with either front or rear side texture as well as double side texture with parallel or perpendicularly aligned V-grooves was done using IV-, spectral response and reflection measurements, fit procedures and multidimensional optical and electrical simulations. The following maximum efficiencies for the different cell concepts were achieved: EASY&SIMPLE (Especially Advanced Structuring with high Yield & Selective emitter IMPLEmentation) solar cell 18.1%; BOSS (BOth Sides Structured with vertically aligned V-grooves) solar cell 19.3%; MECOR (MEchanically CORrugated) solar cell 18.5%

Towards a fast determination of the hydrogen concentration in thin passivating a-Si:H layers using GD-OES

In this contribution, the measurement of the hydrogen concentration in thin passivating amorphous silicon (a-Si:H) layers using Glow Discharge Optical Emission Spectroscopy (GD-OES) is suggested. Usually GD-OES hydrogen measurements suffer from additional signals from atmospheric contaminations, especially from H2O desorbed on the surfaces of such thin layers. This issue is addressed by the deposition of a hydrogen-free copper buffer layer on top, which provides a delay between sputtering the sample surface and the a-Si:H layer itself. Thus the signals from the atmospheric contaminations and from the a-Si:H layer itself are separated. The contamination-free hydrogen signals of the thin a-Si:H layers are eventually calibrated to average hydrogen concentrations.publishe

Passivation of Si wafers by Al2O3 films with different surface conditioning

We investigate the passivation quality of Al2O3 thin films grown by atomic laver deposition on differently etched surfaces of ∼500 μm thick, 2.13 Ωcm p-type float zone silicon wafers with an (100) crystal orientation. The applied CP- and KOH-etches lead to differently shaped surface morphologies but almost equal root-mean-squared (rms) roughnesses of ∼0.95 nm (CP) and ∼1.07 nm (KOH) measured within an area of 1 × 1 μm2 on the Si surface. The lowest surface recombination velocities after passivation resulting in effective carrier lifetimes up to 10 ms are achieved for samples treated with CP-etching. The lifetime is determined using a WCT-120 in the transient mode. It is shown that these lifetimes slightly exceed the theoretical limit given by a parameterization of the Auger recombination. Furthermore, scanning electron microscope images show that Al2O3 thin films with a thickness of ∼29 nm and ∼58 nm fully cover random pyramid textured Si surfaces of (111) orientation leading to high effective lifetimes up to 6 ms. The temperature of the wafer surface during the deposition is determined by in situ spectroscopic ellipsometry to be ∼180 °C

Contacting and recombination analysis of boron emitters via etch-back for advanced n-Type Si solar cells

In p-type c-Si solar cells, selective emitter structures generated by emitter etch-back (EEB) have been introduced in recent years in order to minimize electrical losses in the phosphorous emitter being one of the dominant factors limiting the performance of standard screen-printed p-type c-Si solar cells. In this work, a homogeneously or selectively etched-back boron emitter is demonstrated to provide additional benefits yielding an enhanced conversion efficiency in n-type Si solar cells. By means of subsequent B-EEB, contacting and recombination properties of B emitters dependent on their sheet resistance, surface concentration, and profile depth are studied indicating the latter to be the crucial parameter. Based on this, the characteristics of the optimal B emitter regarding low saturation current density and low specific contact resistivity are determined for the cases of homogeneous and selective etch-back. By employing the selectively etched-back B emitter in initial solar cells, a VOC gain of 5 mV and a significant shunting reduction compared with homogeneously doped devices is achieved

On the characteristics of the doping profile under local metal contacts

In many solar cell concepts, the recombination at local contacts is a bottleneck for the efficiency. Therefore, an optimized doping profile underneath the metal contact would improve the cell performance. We investigate the saturation current density (J0e,met) value of various doping profiles by TCAD simulation and showed that lowest J0e,met values are obtained for profiles with a surface concentration Ns > 5·1020 cm−3 as a consequence of the Pauli blocking and almost independently of the junction depth xj. For profiles with lower Ns we could show an approximate proportionality between J0e,met and the sheet resistance (Rsheet) making the recombination performance of these profiles quasi-independent of the profile shape. Therefore, profiles with even lower value of Rsheet as presently used, typically s > 1020 cm−3 could allow to reach even lower J0e,met, typically 2. In general Auger recombination is very low (2 for Rsheet > 5 Ω/sqr) and does not play a role in the optimal profile shape of the emitter.publishe

Multifunctional ICP-PECVD Silicon Nitride Layers for High-efficiency Silicon Solar Cell Applications

AbstractSilicon nitride layers can be deposited by a variety of methods leading to different physical characteristics, such as optical, electrical and mechanical properties. In photovoltaic applications highly hydrogenated (>20% H concentration) and transparent layers are necessary to passivate the surface (Seff°<20°cm/s) while allowing for optimal internal quantum efficiency. Secondary properties, such as a low pinhole density (<100/mm2) and various barrier-functions for even very thin films (<20°nm) are useful in more complex solar cell processes. This work investigates high deposition rate (2-7°nm/s) PECVD SiNx:H layers from inductively-coupled plasma (ICP) technology. The ICP-technology is capable of creating a variety of such layers, or even only one layer, with all these properties for cost reduction of high-efficiency silicon solar cell processes

Investigation of radiation damage to the Al₂O₃/Si wafer interface during electron beam evaporation by means of C-V and lifetime measurements

For the purpose of reducing recombination activity in crystalline silicon solar cells, atomic layer deposited aluminum oxide (Al2O3) has proven a promising candidate. Its excellent surface passivation is effected by an exceptionally high density of negative fixed charges Qf generating a strong field-effect along with a chemical passivation reducing the density of interface traps Dit. The dependence of these two measures on the temperature and the duration of the post-deposition anneal activating the passivation of Al2O3 is investigated by measuring the capacitance-voltage (C-V) characteristics. To directly correlate Qf and Dit with the effective minority carrier lifetime eff, a new kind of sample structure is developed, whereby both measurement types can be conducted on the same test sample. The interface properties of samples with thermal and electron beam evaporated metal contacts are compared and a correlation with the obtained passivation quality quantified by eff is identified in order to investigate the influence of the radiation damage. It is found that Qf, Dit and eff of Al2O3 passivated p-doped Si wafers exhibit a correlation when annealing parameters are varied and that an electron beam evaporation of Al damages the Al2O3/Si interface and significantly reduces eff. Finally, a method to restore the effective lifetime is developed and investigated which yields a recovery rate of 65% corresponding to a reduction of Dit and an increase of Qf

Injection in light beam induced current systems : An analytical model

In LBIC systems, the evaluation of the injection level is necessary when operating in low injection for defect recombination studies or in defined standard illumination conditions (one or several suns for concentrator applications) for quantum efficiency evaluation. We demonstrate in this contribution that evaluating the laser beam induced injection based on uniform illumination condition can lead to several decades of error because of the lateral carrier diffusion. Based on a parallel beam approximation, we propose here an analytical model to evaluate the maximum of injection of a laser with its parametrization valid for most LBIC system settings and material quality. State of the art high resolution LBIC (HR-LBIC) systems have so sharply focused laser beams that the beam divergence cannot be neglected anymore in the injection calculation. Although providing a quantitative criterion to determine whether the beam divergence can be neglected, we provide a more advanced model for describing the injection of the laser that includes beam divergence.publishe

Role of thermal SiO₂ on passivation of highly doped layer

In this paper we investigate the influence of thermal oxidation during and after POCl3 diffusion. The main focus here is on the question of how far the presence of a thermally grown oxide layer has an influence on the emitter passivation. For this purpose, characterization methods such as electrochemical capacitance-voltage measurements and QSSPC are used. The novel finding of this paper is that in the case of a stack of passivation layers, the presence of the thermal oxide does not make a significant contribution to improved emitter passivation.publishe