Matevž Bokalič

Analysis of electroluminescence images in small-area circular CdTe solar cells

The electroluminescence (EL) imaging process of small area solar cells is investigated in detail to expose optical and electrical effects that influence image acquisition and corrupt the acquired image. An approach to correct the measured EL images and to extract the exact EL radiation as emitted from the photovoltaic device is presented. EL images of circular cadmium telluride (CdTe) solar cells are obtained under different conditions. The power-law relationship between forward injection current and EL emission and a negative temperature coefficient of EL radiation are observed. The distributed Simulation Program with Integrated Circuit Emphasis (SPICE®) model of the circular CdTe solar cell is used to simulate the dark J-V curve and current distribution under the conditions used during EL measurements. Simulation results are presented as circularly averaged EL intensity profiles, which clearly show that the ratio between resistive parameters determines the current distribution in thin-film solar cells. ...

Silicon Solar Cells on Glass with Power Conversion Efficiency above 13% at Thickness below 15 Micrometer

Liquid phase crystallized silicon on glass with a thickness of (10–40) μm has the potential to reduce material costs and the environmental impact of crystalline silicon solar cells. Recently, wafer quality open circuit voltages of over 650 mV and remarkable photocurrent densities of over 30 mA/cm2 have been demonstrated on this material, however, a low fill factor was limiting the performance. In this work we present our latest cell progress on 13 μm thin poly-crystalline silicon fabricated by the liquid phase crystallization directly on glass. The contact system uses passivated back-side silicon hetero-junctions, back-side KOH texture for light-trapping and interdigitated ITO/Ag contacts. The fill factors are up to 74% and efficiencies are 13.2% under AM1.5 g for two different doping densities of 1 · 1017/cm3 and 2 · 1016/cm3. The former is limited by bulk and interface recombination, leading to a reduced saturation current density, the latter by series resistance causing a lower fill factor. Both are additionally limited by electrical shading and losses at grain boundaries and dislocations. A small 1 × 0.1 cm2 test structure circumvents limitations of the contact design reaching an efficiency of 15.9% clearly showing the potential of the technology.

EL inspection of thin-film PV modules in between field operation

Homogeneity and stability of thin-film photovoltaic modules are of key importance for their long life-time and successful competition with other photovoltaic technologies. We have exposed and tracked three commercially available thin-film modules in outdoor conditions during the first months of operation. Before the start and periodically in-between the outdoor operation in maximum power point condition electroluminescence (EL) inspection and standard test condition (STC) performance data were measured. While the initial efficiency of thin-film silicon (both amorphous as well as micromorph tandem) PV modules decreased (-12% and -7%, respectively), the CIGS PV modules STC conversion efficiency improved (+13%) during the period of the first 144 kWh/m2 of solar irradiation. Much larger changes of EL intensity compared to the initial STC efficiency decrease have been observed in both thin-film silicon and CIGS PV modules indicating much higher sensitivity of EL to the temporal changes during operation.

Spatial characterization techniques for dye-sensitized solar cells

Spatial characterization techniques are applied to dye-sensitized solar cells (DSSCs). A comparison between transmittance imaging (TI), light-beam-induced-current (LBIC) scan and electroluminescence imaging is carried out. Detected types of inhomogeneities have different fingerprints by each applied technique. Electroluminescence (EL) is advantageous over TI because the electrical activity of the inhomogeneities influences the result. EL is also advantageous over the LBIC scan due to shorter acquisition time. Based on the above findings, EL has been used for characterization of DSSCs during outdoor short-term aging, showing that EL imaging is a proper method to follow the evolution of the inhomogeneities. Proof-of-concept EL inspection of a screen-printed dyesensitized solar module shows good uniformity.

Concluding Remarks and Outlook Trends

The last chapter summarises the book by pointing out key facts about spatial characterisation techniques and discussing possible future trends.

Spatially Resolved Characterisation Techniques

Techniques for spatial characterisation are presented: optical imaging, light beam induced current, electro- and photoluminescence, and thermography. An emphasis is placed on luminescence techniques, where image acquisition and processing are explained in detail with the aim of producing a true luminescence image. The steps required for absolute luminescence evaluation are described.

SPICE Model and Simulations

SPICE simulations are introduced by pointing out their benefits and describing the basic simulation principle. A 3-layer 2-dimensional SPICE model of a PV device is designed in three levels: microcell, cell and module level. The purpose and design of each level is explained and simplification guidelines are proposed. As the smallest homogeneous part of the model, different representations of the microcell are presented, and ways to determine the initial parameters are discussed. The simulation procedure is described separately for different measuring techniques, with emphasis on luminescence, current density map transformation to luminescence and iterative procedure for parameter extraction.

CdTe Solar Cells

In this chapter CdTe solar cells are presented as a typical thin-film photovoltaic technology. In the first part we describe their structure and fabrication together with their operation as required for understanding spatial inhomogeneities. A SPICE model of the cell is designed and described. The second part deals with current-voltage and electroluminescence characterisation of a cell. Electroluminescence image properties are explained. Characterisation includes back contact position dependence, forward bias current dependence, temperature dependence, and extracting the resistive parameters of the cell using SPICE simulations.

Module Level Electroluminescence Imaging

Electroluminescence imaging is extended to the photovoltaic (PV) module level. Qualitative characterisation of different types of commercially available PV devices is made based on electroluminescence measurements and photographic images. Examples include a conventional crystalline silicon PV module, a chalcopyrite PV module, a cadmium telluride PV module, an amorphous silicon single junction PV module, a tandem micromorph silicon PV module, and a flexible triple junction silicon PV module. For each type of module the discussion focuses on their topology and general electroluminescence intensity distribution, as well as a description of observed inhomogeneities.

Dye-Sensitised Solar Cells

Dye-sensitised solar cells are presented as an example of third generation photovoltaic devices. Their structure and fabrication are described as required for understanding spatial inhomogeneities. Because light interaction takes place via dye molecules, the operation of dye-sensitised solar cells is explained together with the operation under an electroluminescence regime. Conversion efficiency inhomogeneities of dye-sensitised solar cells are distinguished between manufacturing, other and unknown inhomogeneities. Each inhomogeneity type is described by a specific fingerprint obtained using transmittance imaging, light beam induced current and electroluminescence measurements. Ageing studies of dye-sensitised solar cells using transmittance imaging and electroluminescence measurements represents another use of spatial characterisation techniques and reveals the dynamics of the iodine present in the electrolyte.