Localized Electrical Characterization of Photovoltaic Cells Using Monochromatic Light

By AZoM.com Staff Writers

Table of Content

Introduction
Apparatus
Determination of Photovoltaic Characteristics
Analysis of Device Uniformity
Conclusion
About Imina Technologies

Introduction

Characterization is a key step in the analysis of innovative photovoltaic (PV) materials and solar cells. Characterizing photovoltaic cells and comparing devices locally is essential due to the possibility of influence on PV properties at small scales, such as differences between grains in polycrystalline PV materials or localized traps in the bandgap for semiconductor devices.

The characteristics of these devices can be evaluated by assessing parameters, such as short circuit current, fill factor, and conversion efficiency, utilizing voltage sweeps and current measurement by means of a semiconductor parameter analyzer. Laser illuminating and scanning the device can provide localized measurements. This article discusses the use of miBot micromanipulators from Imina Technologies to perform typical PV characterization of solar cells.

Apparatus

Figure 1. Schematic of the setup mounted on a standard optical breadboard (Thorlabs).

Apart from Imina Technologies components, the apparatus includes a 250µm diameter optical fiber equipped with a 5mW, 532nm, HeNe laser (light source) and standard commercial optomechanical modules, as shown in Figures 1 and 2.

Figure 2. Block diagram and photograph of the apparatus assembly, with the sample and micromanipulators (front-right), the syDrive piezoelectric controller and MultiBot (front-left) and the laser followed by optical components to couple it with the fiber (back).

Monocrystalline silicon solar cells that have top surface areas of 0.015 and 3.25 cm2 and open circuit voltage (VOC) of 0.45V are analyzed by placing them on a sample holder mounted on a manual X-Y stage with 25mm of travel. Two miBots equipped with 1µm tungsten electrical probes are positioned close to the sample on top of the X-Y stage. They are then travelled over individual stages (1-Bot) so that the miBot can translate over +/- 5mm in each direction and rotate over +/- 180°, while maintaining the configuration very compact.

Electrical contact is established between the positive electrode (a conductive coating on the reverse of the sample) and the negative electrode (the thin metalized strip on the PV surface) via the conductive metal sample holder. The optical fiber is placed with a third miBot whose 1-Bot stage is set above the X-Y stage. The fiber is kept at a distance of 5mm from the sample surface, yielding a roughly 2mm diameter laser spot.

Determination of Photovoltaic Characteristics

Figure 3. Forward bias I-V curve for the 0.015cm2 sample under a 5mW laser with corresponding the ISC and VOC.

Figure 4. Power curve for the 0.015cm2 sample under a 5mW laser with corresponding the Pmax, Imax, Vmax, and FF.

The first experiment uses a forward bias to determine the current-voltage characteristics of the samples by using an Agilent/HP 4156A semiconductor parameter analyzer under illuminated conditions.

At first, the photovoltaic characteristics of the environmental lights are acquired by testing the samples with no laser. Then, the 5mW laser is used to perform a voltage sweep of 0-500mV in 2.5mV steps. The I-V curve and the power curve are then obtained for the 0.015 cm2 sample, as shown in Figures 3 and 4, respectively.

Figure 5. Forward bias I-V curve in illuminated conditions showing PV cell parameters

The I-V curve provides data for the determination of the short circuit current (ISC), VOC, series resistance (Rs), and shunt resistance (Rsh), as shown in Figure 5. The power curve provides data for the determination of the maximum power (Pmax) and corresponding current (Imax) and voltage (Vmax). Furthermore, the field factor (FF) and the solar efficiency (η) can be calculated using the following equations:

The field factor (FF) and the solar efficiency (η) provide key insights into the behavior of PV cells.

Analysis of Device Uniformity

Figure 6. Line scan over the 3.25 cm2 with three passes.

In this experiment, the uniformity of the PV devices is analyzed by measuring the current, while the laser spot is moved in parallel along the sample. Although the use of more sophisticated methods such as LBIC helps in determining defective sites, a line scan provides an idea of the uniformity across the scan direction and enables observing changes in the current. A line scan over the 3.25cm2 sample moved linearly by the X-Y stage over 7mm in 10 seconds is illustrated in Figure 6.

To ensure reproducibility, the scan was repeated with three passes and changed to an average value to compromise for drift in the current values between passes. A region of apparently reduced current response noticed at the beginning of the scan is due to the presence of the laser spot near the sample edge.

Conclusion

The results clearly demonstrate the ability of miBot micromanipulators to help in determining the I-V characteristics of PV samples under illuminated conditions. It was quick and easy to place the electrical probes and optical fibers anywhere on the sample due to the mobility of the miBot micromanipulators over four degrees of freedom.

The piezoelectric actuators’ adjustable step resolution enabled localized illumination and measurements even on smaller samples. Furthermore, the individual miBot stages facilitated the manipulators to be integrated easily to the optical configuration, thus enabling users to leverage the compactness of the miBot to limit the size of the positioning devices on the overall footprint of the setup.

About Imina Technologies

Imina Technologies is a privately held company founded in 2009 to exploit more than ten years of research in high precision robotics at the Swiss Federal Institute of Technology in Lausanne, Switzerland (EPFL). With many years of experience in precision engineering, micro-robotics and nanomanipulation, Imina's interdisciplinary team is geared up for the needs of the most demanding users. Their unique combination of know-how enables them to propose complete solutions for even the most specific applications.

Imina Technologies introduces a new type of micro- and nanomanipulators. Based on a novel motion technology, the miBot is the world's smallest manipulator. It combines nanometer resolution of positioning, unprecedented ease-of-use and flexibility in an ultra compact design. Complete solutions of nanomanipulation are provided to quickly integrate the systems in electron microscopes (SEM/FIB) and light microscopes. The handling of samples like nanowires, MEMS or cells is effortless, speeding-up the production of experimental results.

This information has been sourced, reviewed and adapted from materials provided by Imina Technologies.

For more information on this source, please visit Imina Technologies.

Date Added: Aug 28, 2013 | Updated: Oct 21, 2013
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