Dmitry Poplavskyy

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.

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.

Intermixing and Formation of Cu-Rich Secondary Phases at Sputtered CdS/CuInGaSe 2 Heterojunctions

The Cu migration behavior in PVD-CdS/PVD-Cu(In,Ga)Se2 (CIGS) heterojunctions is investigated by high-resolution electron microscopy (HREM) and energy dispersive X-ray spectroscopy (EDS). Incorporation of Cu into the CdS forms Cu-rich domains but has no effect on epitaxy of the CdS. Epitaxy is commonly observed in the CdS studied. Secondary ion mass spectroscopy depth profiles confirm the presence of Cu in the CdS. In some cases, Cd is completely replaced by Cu, resulting in a Cu-S binary compound epitaxially grown on the CIGS and fully coherent with the surrounding CdS. This is most likely to be cubic Cu2S, based on lattice spacing measurements from HREM images and EDS elemental quantification. In addition, we find that the buffer layer crystal structure influences the extent of Ga depletion at the CIGS surface, which is more pronounced adjacent to zinc-blende CdS than wurtzite CdS. Density functional theory calculations reveal that Cu clustering and different Ga depletion widths can be attributed to the inherent anisotropy of wurtzite CdS and differences in CIGS point-defect migration barriers. Understanding the influence of these effects on device properties is a critical step in developing more efficient CdS/CIGS-based photovoltaics.

Measurement of metal induced recombination in solar cells

Metal induced recombination is a significant loss mechanism in standard and especially in high efficiency solar cells. It is usually measured by making solar cells with different metal contact areas, and inferring the metal induced recombination rate from the change in open circuit voltage. There are many errors which can occur if the test structure is not designed correctly - a small finger spacing should be used, with variable finger width to change the contact area.

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.

Mechanisms for light-soaking induced carrier concentration changes in the absorber layer of Cu(In, Ga)Se 2 solar cells

We report evidence supporting two separate mechanisms that can change the device performance of Cu(In, Ga)Se2 (CIGS) solar cells upon light-soak through changes in the carrier concentration of the absorber layer. The first mechanism involves the Lany-Zunger (L-Z) copper-selenium divacancy complex (VSe-VCu), where this metastable defect is theoretically predicted to change from a shallow donor state to an acceptor state upon light-soak. We show that growing CIGS under selenium rich conditions can mitigate open-circuit voltage (Voc) change, consistent with the L-Z framework. Furthermore, we show that while carrier concentration increases with light-soak, Voc can either increase or decrease depending on the carrier concentration of the relaxed state. The second mechanism involves sodium redistribution in the CIGS layer, where sodium is known to affect absorber carrier concentration in CIGS solar cells. However, we argue that sodium redistribution by itself cannot account for the total light-soak induced carrier concentration changes observed in our devices.

Metastable defect measurement from capacitance-voltage and admittance measurements in Cu(In, Ga)Se 2 Solar Cells

Metastable defects in Cu(In, Ga)Se2 solar cells can account for performance changes during service. These defects and their response to light exposure are revealed in capacitance-voltage profiling and admittance spectroscopy. One-dimensional SCAPS modeling of the complete solar cell structure allows determination of the defect concentrations. The essential features of the absorber layer are found to be light-sensitive defects consistent with Lany-Zunger predictions and light-insensitive shallow and deep acceptors.