When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to carrier collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells.
Through first‐principle density functional theory (DFT) calculations, the atomic structure and electronic properties of intrinsic and passivated Σ3 (114) grain boundaries (GBs) in Cu2ZnSnSe4 (CZTSe) are studied. Intrinsic GBs in CZTSe create localized deep states within the band gap and thus act as Shockley‐Read‐Hall recombination centers, which are detrimental to cell performance. Defects, such as ZnSn (Zn atoms on Sn sites), Na+i (interstitial Na ions), and OSe (O atoms on Se sites), prefer to segregate into GBs in CZTSe. The segregation of these defects at GBs exhibit two beneficial effects: 1) eliminating the deep gap states via wrong bonds breaking or weakening at GBs, making GBs electrically benign; and 2) creating hole barriers and electron sinkers, promoting effective charge separation at GBs. The results suggest a unique chemical approach for engineering GBs in CZTSe to achieve improved cell performance.
Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells.
Defect segregations at grain boundaries (GBs) of polycrystalline thin-film absorber are crucial to solar cell performance. The Σ3(114) GBs in CuInSe2 (CIS) have been studied through first-principle calculations. We reveal that the intrinsic CIS GBs produce deep gap states, which act as nonradiative recombination centers. However, the segregation of CuIn and OSe at GBs can clean the gap states and lead to electrically benign behavior. Our results suggest that the defect segregation at GBs could be an important feature for high efficiency CIS-based photovoltaic solar cells and it provides a general guidance for engineering GBs in other chalcogenide polycrystalline devices.
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