Giuseppe Scardera

Silicon ink selective emitter process: Optimization of selectively diffused regions for short wavelength response

The Innovalight Cougar™ Platform is a portfolio of simple to implement technologies that, when combined with Innova-light Silicon Ink, enables the manufacture of a selective emitter solar cell with a non-masking single-step diffusion. Cell efficiencies of up to 19% have been achieved. Innova-light Silicon Ink is a highly engineered silicon nanoparticle colloidal dispersion, implemented for both high volume ink-jet and screen deposition, and further optimized to be produced and delivered in commercial volumes. A particular feature of the selective emitter structure is its enhanced quantum efficiency in the short wavelength region as result of reduced emitter recombination. In this report we demonstrate how the properties of the selectively doped regions on the front surface of the cell affect the short wavelength response of a Cougar cell. In particular, the importance of the emitter dopant profile for minimizing emitter recombination and the broad processing window of the Cougar cell structure with respect to emitter doping are illustrated. Further, the effect of the ink finger width on the short-wavelength response of the cell and the trade-off between the loss of short circuit current (Jsc) and metal-to-ink pattern alignment are demonstrated.

Localized doping using silicon ink technology for high efficiency solar cells

Controlled localized doping of selective emitter structures via Innovalight Silicon Ink technology is demonstrated. Both secondary ion mass spectrometry and scanning capacitance microscopy reveal abrupt lateral dopant profiles at ink-printed boundaries. Uniform doping of iso- and pyramidal surfaces is also verified using scanning electron microscopy dopant contrast imaging.

Simulation of emitter doping profiles formed by industrial POCl 3 processes

This paper quantifies the recombination losses associated with industrial POCl3 emitters. We examine three standard (STD) recipes and four of DuPonts lightly doped emitter (LDE) recipes. We find that an STD emitter has a higher effective surface recombination velocity than an LDE recipe of the same sheet resistance because it has a higher surface concentration. More significantly, we find that STD emitters have greater SRH recombination within the doped region, probably because they contain a greater concentration of inactive phosphorus atoms which are known to form silicon phosphide precipitates. These conclusions are drawn from simulations and experiments on the lifetime of test wafers and the quantum efficiency of solar cells. It is shown how this data can be used to distinguish the SRH recombination that occurs at the surface from the SRH that occurs inside an emitter.

Microstructural characterization of front-side Ag contact of crystalline Si solar cells with lightly doped emitter

Crystalline Si (c-Si) solar cell production has reached an annual scale of ~20 GW globally. Development of this leading technology has been boosted by continuous innovation in material science and reduced material and processing costs. An example of such innovation is the step-wise progression to more lightly doped emitters (LDE) that reduces recombination in the solar cell. Continuous improvement in front-side (FS) metallization pastes has enabled this progression to lower series resistance and higher cell efficiency. We report here homogeneous emitter LDE cells with efficiencies as high as 18.9%, printed with advanced FS Ag paste. A clear understanding of the microstructure of the interfacial region between Ag contact and Si emitter, and the associated electrical conduction mechanism of LDE cells can provide the guidance needed to drive overall efficiency higher and end-user cost lower. We report our latest investigation of the microstructure of the interface between FS Ag contact and lightly-doped emitter using scanning electron microscopy techniques. The microstructural features such as nano-Ag colloids, interfacial glass, and Ag crystallites are studied in detail. The relationship between microstructure, cell performance, and current conduction mechanism for LDE cells are discussed.

Highly tunable single step selective emitter diffusion process using Silicon Ink Technology

A key feature of the Innovalight™ Cougar™ Platform is the single step selective emitter diffusion process employing Silicon Ink Technology. Both the lightly doped emitter and the heavily doped contacting areas are formed simultaneously during one diffusion process. The tuning of doping strengths of both regions is decoupled and allows for a large separation in field and contact region sheet resistance. This paper will highlight the ability to achieve a wide range of dual doping strengths using the Cougar diffusion process and the corresponding impact of this tuning on cell performance.

Iron contamination in silicon solar cell production environments

The fundamental mechanisms of iron impurities in silicon have been thoroughly studied and are well explained in the literature. Of interest to solar cell manufacturers is to understand how these mechanisms manifest in a production environment and, more importantly, how to quickly diagnose and mitigate iron contamination as it occurs. This paper presents examples of iron contamination using p-type CZ wafers processed in production-style environments. The impact of iron on the IV performance of industrial screen printed solar cells is presented, including the time dependence of these effects and how they manifest in the various characterisation techniques that are typically used to diagnose solar cell performance. Examples are given of potential sources of iron contamination and the impact of subsequent processing on the redistribution of those contaminants. The paper demonstrates that iron contamination can occur in a variety of ways, can spread quickly and is severely detrimental to solar cell efficiency. Additionally, it is shown that the fundamental properties of iron in silicon can be used to quickly identify the root cause of contamination in a production environment.

Front metal and diffusion optimization for selective emitter

A complex optimization is required to reach the full potential of the selective emitter (SE) architecture. The focus of this report will be the front metal contact and phosphorous emitter diffusion strength. This optimization requires consideration of Rsheet (sheet resistance) of the field and contact regions as a function of diffusion recipe, metal Rcontact (contact resistance) as a function metal paste, and Remitter (emitter resistance) as a function of finger spacing. To cover the Rsheet,field range of interest, the field diffusion strength was varied from 60 - 100 Ω/□. To quantify the impact of Rcontact, an advanced formulation metal paste with reduced Rcontact was compared to an industry benchmark. The final parameter varied is the finger spacing, varied from 1.6 - 2.1 mm. Testing reveals there are marginal gains from increasing the diffusion Rsheet, field above 100 Ω/□ due to reduced front surface field below the metal contacts causing reduced Voc. More substantial gains were realized by from an advanced metal paste formulation which reduces Rcontact. Finally the concept that reduced finger spacing can be used to reduce Remitter component is pushed until it falls into printability limitations due to the need for progressively thinner fingers. The net result from diffusion strength, paste, and finger spacing optimization yield a mean efficiency of 19.2%, +0.2%abs gain over the control.

Screen-printed dopant paste interdigitated back contact solar cells

We report on progress made with an all-screen-printed interdigitated back contact (IBC) solar cell process. The rear side interdigitated doping pattern is defined using screen printed boron and phosphorus dopant pastes with thermal drive-in. High printing and doping fidelity as well as high wafer bulk lifetime are demonstrated. Screen printed fire-through metal pastes are also used for contacts. We present pilot line data for n-type CZ silicon, 156 mm pseudo-square cells achieving a 21.3% champion efficiency and 21% median efficiency. Improvements to the boron emitter metal contact are highlighted as a path to achieve 22% efficiency.

A review of manufacturing metrology for improved reliability of silicon photovoltaic modules

In this work, the use of manufacturing metrology across the supply chain to improve crystalline silicon (c-Si) photovoltaic (PV) module reliability and durability is addressed. Additionally, an overview and summary of a recent extensive literature survey of relevant measurement techniques aimed at reducing or eliminating the probability of field failures is presented. An assessment of potential gaps is also given, wherein the PV community could benefit from new research and demonstration efforts. This review is divided into three primary areas representing different parts of the c-Si PV supply chain: (1) feedstock production, crystallization and wafering; (2) cell manufacturing; and (3) module manufacturing.