Physics of grain boundaries in polycrystalline photovoltaic semiconductors

Abstract

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.

Methodologies toward Highly Efficient Perovskite Solar Cells

“…In photovoltaic devices, polycrystalline materials show typically poor performance compared to their single‐crystalline ones, because of GBs, which are usually effective recombination centers for photon‐generated electrons and holes, as well as scattering centers of free carriers. GBs in Si and CdTe were found to have ill‐effect on photovoltaic performance, whereas CIS (CuInSe 2 ) exhibited PCE as high as 20% without special GB passivation, significantly surpassing the single‐crystalline CIS‐based device (≈13%) . From the combined study of high‐resolution electron microscopy and first‐principles calculations, it was found that GBs in Si and CdTe create deep levels but GBs in CIS do not create deep levels and are electrically benign .…”

Sodium Passivation of the Grain Boundaries in CuInSe2 and Cu2ZnSnS4 for High‐Efficiency Solar Cells

“…Experiments observed that Na predominantly segregates at grain boundaries (GBs) or interfaces, [22,23] which typically acted as nonradiative recombination centers. [24][25][26][27][28] These indicate that wileyonlinelibrary.com than that of Cu doping. Therefore, we provide a mechanism for why Na treatment is crucial for achieving p-type samples under Cu poor conditions in multicompositional chalcogenide semiconductors.…”

“…In fact, Na substituting cations (Cu and In) can not affect the anion pairs' wrong bonds, so it would not be expected that the above doping approaches would improve the GB properties. [30] In this work, we study the electronic properties of CISe and CZTS anion-core prototype ∑3 (114) GB [28,32] and Na (Cu) doping. [30] These studies revealed that Na eliminating In Cu and In Ga defects can increase the p-type carriers, which are crucial for photocurrent in these materials.…”

Reducing Detrimental Defects for High‐Performance Metal Halide Perovskite Solar Cells

“…For example, grain boundaries lead to harmful deep levels in Si and CdTe, but they do not produce deep levels in CIGS, CZTSe, and CZTS materials. This is why passivation is critical in Si and CdTe solar cells, while passivation is less important in CIGS and CZTS solar cells . Organic semiconducting materials are more prone to the formation of trap states because of their disordered nature induced by the weak van der Waals interaction between molecules.…”

“…As a consequence, these defects play a similar important role in the performance of organic solar cells . Several Review articles provide insightful information about defects in Si, CdTe, CIGS, and organic based PV technologies …”