# Performance evaluation of multi-junction solar cells by spatially resolved electroluminescence microscopy

- Lijing Kong
^{1}, - Zhiming Wu
^{1}Email author, - Shanshan Chen
^{1}, - Yiyan Cao
^{1}, - Yong Zhang
^{2}, - Heng Li
^{1}and - Junyong Kang
^{1}Email author

**10**:40

https://doi.org/10.1186/s11671-014-0719-9

© Kong et al.; licensee Springer. 2015

**Received: **28 November 2014

**Accepted: **29 December 2014

**Published: **5 February 2015

## Abstract

An electroluminescence microscopy combined with a spectroscopy was developed to visually analyze multi-junction solar cells. Triple-junction solar cells with different conversion efficiencies were characterized by using this system. The results showed that the mechanical damages and material defects in solar cells can be clearly distinguished, indicating a high-resolution imaging. The external quantum efficiency (EQE) measurements demonstrated that different types of defects or damages impacted cell performance in various degrees and the electric leakage mostly degraded the EQE. Meanwhile, we analyzed the relationship between electroluminescence intensity and short-circuit current density *J*
_{SC}. The results indicated that the gray value of the electroluminescence image corresponding to the intensity was almost proportional to *J*
_{SC}. This technology provides a potential way to evaluate the current matching status of multi-junction solar cells.

## Keywords

## Background

Multi-junction (MJ) solar cells have attracted broad interests, owing to their high conversion efficiency and wide future applications [1-8]. Generally, MJ solar cells consist of multiple thin semiconductor films, and the semiconductor in each junction has a characteristic bandgap, which only absorbs sunlight with the energy larger than its bandgap. The combination of several different semiconductor layers enables the solar cell to absorb sunlight efficiently and consequently improves the conversion efficiency. Up to now, the most popular MJ solar cells are based on III-V semiconductors (e.g., GaInP/GaInAs) epitaxially grown on single crystalline Ge substrate [5,6]. As is known to all, their efficiencies significantly depend on the crystal quality, electrode structure, and current matching status. However, due to their complex structures and manufacturing processes, the characterization of these devices as well as current matching remains extremely challenging, especially for an experimental access to the information of individual subcells in an MJ solar cell. Therefore, a fast, efficient and nondestructive detection technology used to derive the individual subcell electrical characteristics has a significant bright prospect.

In this work, we propose a method to characterize and evaluate the properties of each subcell in MJ solar cells by combining electroluminescence (EL) microscopy and spectroscopy. Four GaInP/GaInAs/Ge solar cells with different conversion efficiencies were systematically studied. The intrinsic defects and damages during the fabrication process could be conveniently recognized by comparison of the EL images of each junction. The external quantum efficiencies (EQEs) of these samples were measured, and the influence of defects on EQE was discussed as well. In addition, the theoretical relation between EL intensity and quantum efficiency was also deduced. It is believed that the EL imaging technique provides a pertinent and nondestructive means to characterize MJ solar cells.

## Methods

### Experimental equipment

### TJ solar cells

^{2}. To inspect instrument performance, four samples (labeled as A, B, C, and D) with different conversion efficiencies were characterized. Figure 2 shows their current-voltage (

*I*-

*V*) curves measured under AM1.5G illumination. The corresponding conversion efficiencies are marked in the figure. Cell A has the lowest conversion efficiency, and its open-circuit voltage is only 0.263 V, approximately equal to the open-circuit voltage of the Ge bottom cell. Cell B has a better performance than cell A, but its electrical parameters are still low. The steep

*I*-

*V*curves of cell A and cell B suggest the possibility of electric leakage. Among these cells, cells C and D are relatively normal, and the latter has the greatest conversion efficiency with the open-circuit voltage (

*V*

_{OC}) and the short-circuit current density (

*J*

_{SC}) of 2.60 V and 14.3 mA/cm

^{2}, respectively.

## Results and discussion

### Photographic diagnosis

*I*-

*V*parameters was mainly caused by a low shunt resistance and the increased current density in the upper left part. As for the top cell, the high current density spreading to the lower junction overcompensates the leakage defect, which leads to a dark EL image over the cell.

### Quantum efficiency measurement

### Reciprocity relation between electroluminescence and quantum efficiency

*X*= (

*x*,

*y*) on the surface of the solar cell, the EL intensity

*ϕ*

_{el}can be given by [19-27]

where *Q*
_{e}(*X*) and *ϕ*
_{bb} are the local external quantum efficiency and the black body photon flux with respect to the photon energy *E* of the EL peak of the subcell, respectively, *V*(*X*) is the local junction voltage, and *kT*/*q* is the thermal voltage.

*ϕ*

_{bb}as [22,26]

where *h* and *c* are Planck's constant and vacuum speed of light, respectively.

*C*

_{img}(

*λ*) is the imaging constant, which is related to multiple factors, such as vidicon quantum efficiency, vidicon conversion factor [counts/electron], maximum transmittance at the center of all optical elements, and geometry factor for the fraction of EL radiation that enters the optical elements. Since the vidicon signal loses spectral information, we assumed that the value is a constant one here.

*C*

_{off}represents the actual zero pixel value, which can be obtained by acquiring an absolutely dark image. Based on the above equations,

*ϕ*

_{ccd}of a raw EL image obtained directly from the vidicon can be expressed as

*V*(

*X*) is a constant [29]. It is clear that the intensity of the EL image is proportional to the EQE. Additionally, the short-circuit current density

*J*

_{SC}of the solar cell can be calculated from the EQE by

where *λ* is the wavelength, *h* and *c* have the same meaning as above, and *ϕ*
_{sun} (*λ*) is the solar radiation spectrum.

where *S*
_{aa} is the active surface area of the solar cell.

## Conclusions

In conclusion, the EL imaging technique for MJ solar cells was established by combining EL imaging with EL spectroscopy. By comparing the images taken from each subcell, different defects or damages can be definitely identified. The EL imaging system was proved to be a powerful diagnostic tool for investigating not only the material properties but also process-induced deficiencies in MJ solar cells. The EQE results confirmed different types of defects or damages impacting cell performance in various degrees, and the electric leakage mostly degraded the EQE. Moreover, the relationships between the gray value of the EL image and EQE or *J*
_{SC} were deduced and discussed. The results showed that the gray value was almost proportional to EQE or *J*
_{SC}. It is believed that this method will provide a simple and effective method with the calibrated parameters for evaluating the current matching status of MJ solar cells.

## Declarations

### Acknowledgements

This work was financially supported by the ‘973’ Program (Grant Nos. 2011CB925600 and 2012CB619301), the National Natural Science Foundation of China (Grant Nos. 61106008, 61106118, 91321102, U1405253, and 61227009), and the Natural Science Foundation of Fujian Province. This work was also supported partially by the Chinese Hungarian Intergovernmental S&T Cooperation Program (Project No: TÉT_12_CN-1-2012-0040, CH-6-26/2012).

## Authors’ Affiliations

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