Daniel Biro

Field-effect passivation of the SiO2Si interface

The field-effect passivation of the interface of thermal oxides on silicon is experimentally investigated by depositing corona charges on the oxide of solar cells and of lifetime test structures. The open circuit voltage of solar cells with interdigitated rear contacts can be increased by +12 mV or decreased by −34 mV, respectively, by depositing positive or negative corona charges on top of the front oxide. The resulting effective surface recombination velocity, Seff, is determined on carrier lifetime test structures for different injection levels and charge densities using microwave-detected photoconductance decay and a new expression for the Auger-limited bulk lifetime. Seff can be varied between 24 cm/s and 538 cm/s on a 1 Ω cm p-type wafer with a thermal oxide of 105 nm thickness. The measurements are compared with theoretical predictions of an analytical model for the calculation of the surface recombination. Measured values for the capture cross sections and interface trap densities are used for th...

Comprehensive analytical model for locally contacted rear surface passivated solar cells

For optimum performance of solar cells featuring a locally contacted rear surface, the metallization fraction as well as the size and distribution of the local contacts are crucial, since Ohmic and recombination losses have to be balanced. In this work we present a set of equations which enable to calculate this trade off without the need of numerical simulations. Our model combines established analytical and empirical equations to predict the energy conversion efficiency of a locally contacted device. For experimental verification, we fabricate devices from float zone silicon wafers of different resistivity using the laser fired contact technology for forming the local rear contacts. The detailed characterization of test structures enables the determination of important physical parameters, such as the surface recombination velocity at the contacted area and the spreading resistance of the contacts. Our analytical model reproduces the experimental results very well and correctly predicts the optimum cont...

Solar cells with efficiencies above 21% processed from Czochralski grown silicon

Czochralski-Si (Cz-Si) of several manufacturers and with resistivities ranging from 1 to 13 /spl Omega/cm were processed into solar cells with efficiencies higher than 20% (AM1.5) using the LBSF/PERL processing sequence. The highest efficiency was 21.7%. The investigation of high efficiency Cz-Si solar cells was augmented by computer simulation and a study of the carrier lifetime before and after processing. A small degradation of solar cell performance in the lower resistivity material is discussed. Furthermore, a much simpler processing sequence is presented revealing efficiencies well above 19% on Cz-silicon and 21% on float zone-silicon.

Silicon Surface Passivation by Thin Thermal Oxide/PECVD Layer Stack Systems

For the passivation of p-type silicon surfaces, we investigate layer systems consisting of a thin layer of thermally grown SiO2 and different dielectric capping layers deposited by means of plasma-enhanced chemical vapor deposition (PECVD). We find that the thermal SiO2 layer thickness strongly impacts the passivation quality and interface parameters of the stacks. Capacitance-voltage measurements reveal that for Al2O3 and SiNx capping layers, an increased thermal SiO2 film thickness suppresses charge formation at the interface between SiO2 and the capping layer. Interface trap density and effective carrier lifetime data suggest that a certain thermal SiO2 thickness is required to achieve appropriate chemical passivation. The combination of a thin thermal SiO2 layer (~4 nm) and a PECVD-SiOx capping results in very low surface recombination velocities of a few centimeters per second, measured on p-type 1-Ω·cm float-zone silicon after contact firing and postmetallization annealing. The experimentally observed dependence of the surface recombination velocity on the fixed charge density, gate voltage, and injection density is reproduced very accurately by analytical calculations that use the measured interface trap density and total charge density at the Si/insulator interface. The model also includes additional recombination in the space charge region of inverted surfaces.

19.7% Efficient All-Screen-Printed Back-Contact Back-Junction Silicon Solar Cell With Aluminum-Alloyed Emitter

A back-contact back-junction solar cell on n-type silicon with an aluminum-alloyed emitter is introduced, where both structuring and metallization are realized by screen-printing. The process sequence for realizing the cell with a pitch of 2 mm is displayed and described. The cell parameters are shown for three different emitter coverages on the rear side (45%, 58%, and 72%). An analysis of the saturation current density Jο of the different areas was carried out on lifetime samples, and the calculated Voc is compared to the measured one. The potentially critical edge of the Al finger is analyzed in a scanning electron microscopy cross section. No damaging of the aluminum paste to the passivation layer and a homogenous p+-layer over the whole finger width is observed. A conversion efficiency of 19.7% is presented for a cell with an aperture area of 16.65 cm2 that was exclusively processed in Fraunhofer ISE PV-TEC, which is an industrial-like fabrication environment.

n-type silicon - enabling efficiencies > 20% in industrial production

In the first part of this paper we estimate the efficiency potential of crystalline silicon solar cells on conventionally pulled p-type boron-doped Czochralski-grown silicon with typical oxygen concentrations. Taking into account an industrial high-efficiency cell structure featuring fine-line metallization, shallow and well-passivated emitter and a rear surface structure with dielectric passivation and local laser-fired point contacts, the maximum achievable efficiency is around 20%. The main limitation of such a cell is due to the rather low bulk lifetime after light-induced degradation. Even when avoiding the metastable boronoxygen defect by using Gallium-doped or magnetic Cz-silicon, it has to be kept in mind that the detrimental impact of metal contaminations on p-type silicon is greater than on n-type silicon. A potential strategy to reduce this loss is the use of n-type silicon. Therefore, the second part of the paper discusses different architectures for solar cells on n-type silicon substrates and shows the latest results achieved at Fraunhofer ISE in this field.

Towards 19% efficient industrial PERC devices using simultaneous front emitter and rear surface passivation by thermal oxidation

Higher solar cell efficiencies enable a reduction of the cost per watt ratio, if production effort is maintained at an acceptable level. A proven high-efficiency concept is the passivated emitter and rear cell (PERC) [1]. However, the transfer of this solar cell structure from demonstrator level to industrial application is challenging. We present a simple approach for the industrial fabrication of PERC solar cells which utilizes the simultaneous passivation of the front emitter and the rear surface by a thin layer of thermally grown oxide. This Thermal Oxide Passivated All Sides (TOPAS) structure represents an industrially feasible implementation of the PERC concept.

Aluminum Alloying in Local Contact Areas on Dielectrically Passivated Rear Surfaces of Silicon Solar Cells

We present a detailed study on the rear contact formation of rear-surface-passivated silicon solar cells by full-area screen printing and alloying of aluminum pastes on the locally opened passivation layer. We demonstrate that applying conventional Al pastes exhibits two main problems: (1) high contact depths leading to an enlargement of the contact area and (2) low thicknesses of the Al-doped p+ Si regions in the contact points resulting in poor electron shielding. We show that this inadequate contact formation can be directly linked to the deficiently low percentage of silicon that dissolves into the Al-Si melt during alloying. Thus, by intentionally adding silicon to the Al paste, we could significantly improve the contact geometry by reducing the contact depth and enlarging the Al-p+ thickness in the contact points, enabling a simple industrially feasible way for the rear contact formation of silicon solar cells.

Co-Diffused Back-Contact Back-Junction Silicon Solar Cells without Gap Regions

In this paper, first generation back-contact back-junction (BC-BJ) silicon solar cells with cell efficiencies well above η = 20% were fabricated. The process sequence is industrially feasible, requires only one high-temperature step (codiffusion), and relies only on industrially available pattering technologies. The silicon-doping is performed from pre-patterned solid diffusion sources, which allow for the precise adjustment of phosphorusand boron-doping levels. Based on the investigated process technologies, BC-BJ solar cells with gap and without gap between adjacent n+ - and p+ -doped areas were processed. On the one hand, a strong reduction of the process effort is possible by omitting the gap regions. On the other hand, parasitic tunneling currents through the narrow space charge region may occur. Hence, deep doped areas were realized to avoid tunneling currents in gap-free BC-BJ cells. This paper finishes with a detailed characterization of the manufactured cells including important cell measurements like I-V, SunsVOC, quantum efficiency, and an analysis of the cell specific fill factor losses.

Inkjet Technology for Crystalline Silicon Photovoltaics

The worlds ever increasing demand for energy necessitates technologies that generate electricity from inexhaustible and easily accessible energy sources. Silicon photovoltaics is a technology that can harvest the energy of sunlight. Its great characteristics have fueled research and development activities in this exciting field for many years now. One of the most important activities in the solar cell community is the investigation of alternative fabrication and structuring technologies, ideally serving both of the two main goals: device optimization and reduction of fabrication costs. Inkjet technology is practically evaluated along the whole process chain. Research activities cover many processes, such as surface texturing, emitter formation, or metallization. Furthermore, the inkjet technology itself is manifold as well. It can be used to apply inks that serve as a functional structure, present in the final device, as mask for subsequent structuring steps, or even serve as a reactant source to activate chemical etch reactions. This article reviews investigations of inkjet-printing in the field of silicon photovoltaics. The focus is on the different inkjet processes for individual fabrication steps of a solar cell. A technological overview and suggestions about where future work will be focused on are also provided. The great variety of the investigated processes highlights the ability of the inkjet technology to find its way into many other areas of functional printing and printed electronics.