Probing the Correlation of Twin Boundaries and Charge Transport of CdTe Solar Cells Using Electron Backscattering Diffraction and Conductive Atomic Force Microscopy

Abstract

The effect of grain boundaries (GBs) especially twin boundaries (TBs) on the performance of polycrystalline CdTe thin film (1−2 μm in thickness) solar cells remains largely controversial. Understanding the GB's microstructure and physical property and their impact on the photovoltaic efficiency is critically important to the optimization of the solar cell performance. In this work, we present a systematic study of GBs in CdTe thin film solar cells of different efficiencies in the range of 1.97−9.68%, aiming to elucidate a quantitative correlation of GBs especially TBs with the solar cell charge transport by integrating several advanced approaches including electron backscattering diffraction, transmission electron microscopy, conductive atomic force microscopy, and scanning Kelvin probe force microscopy. Importantly, we have confirmed {111} Σ3 TBs form in the PLD CdTe solar cells and are beneficial by providing an efficient channel for charge transport. The percentage of {111} Σ3 TBs was found to be correlated quantitatively with the solar cell power-conversion efficiency and a higher percentage led to a higher efficiency. In addition, a built-in field was observed between the random-angle GBs with respect to the grain interior, which may enhance exciton dissociation. This study provides a critical insight into the role of GBs in the polycrystalline CdTe thin film solar cells and is important to further improvement of their performance.

Perovskite Tandem Solar Cells: From Fundamentals to Commercial Deployment

“…CdTe solar cells are the most successful commercially available thin film solar cells due to the lowest cost for utility scale power production, low energy manufacturing (the LCOE from CdTe PV is ∼$0.04/kWh, lower than the national average LCOE from all sources of $0.11/kWh), and short payback time of ∼1 year. , The highest PCE of the CdTe solar cell is 22.1% . CdTe solar cells have consistently demonstrated the lowest cost for utility-scale power production .…”

Identification the Role of Grain Boundaries in Polycrystalline Photovoltaics via Advanced Atomic Force Microscope

“…Li et al used KPFM measurement (see Figure ) to reveal that the electric field is the largest near GBs, causing photogenerated electrons to follow near GBs. [ 107 ] The CPD between GIs and GBs can be observed according to the KPFM mapping and the corresponding cutting line (Figure 9b). CPD induces a local electric field, which contributes to the separation of electrons and holes during the optical absorption in the transverse direction of the CdTe film.…”

“…Magnified AFM image d) and KPFM image e) of the Cl-treated CdTe. f) Schematic description of the beneficial effect of GBs.Reproduced with permission [107]. Copyright 2004, American Chemical Society.…”

Characterizing Local Electronic States of Twin Boundaries in Copper

“…20 The direct interaction between the probe and the sample in AFM can provide feedback on the microelectrical properties of the materials. 21,22 The local electronic behaviors measured by several kinds of AFM methods may provide direct evidence of the electron transport properties of TBs and GBs in Cu.…”

“…30,31 Furthermore, due to the high chemical activity, the GB may attract more impurity atoms and introduce more electrons. 21,32 It is worth mentioning that the Fermi surface of Cu is a nonspherical structure, and k f in Cu is not monotonically related to the size of the Fermi surface. 33 The electron concentrations calculated by eqs 2 and 3 may not be able to realistically quantify free electron states at the Cu Fermi surface.…”