Performance evaluation of multi-junction solar cells by spatially resolved electroluminescence microscopy
© Kong et al.; licensee Springer. 2015
Received: 28 November 2014
Accepted: 29 December 2014
Published: 5 February 2015
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.
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.
TJ solar cells
Results and discussion
Quantum efficiency measurement
Reciprocity relation between electroluminescence and quantum efficiency
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.
where h and c are Planck's constant and vacuum speed of light, respectively.
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.
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.
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).
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