The Physics of Twin Boundary Termination in Cu(In, Ga)Se₂ Absorbers

Abstract

Thin-film solar cells of polycrystalline Cu(In 1Àx ,Ga x )(S,Se) 2 (based on Cu(In,Ga) Se 2 also known as CIGS) now demonstrate efficiency as high as 23.35%. [1] However, the Shockley-Queisser [2] limit predicts the efficiency can be as high as 33.7% for single-junction CIGS devices. Recombination of charge carriers at interfaces, grain boundaries (GBs), and the bulk are the main reasons behind this efficiency loss of CIGS-based cells. [3,4] That is, the suppression of such recombination promises an improvement in efficiency. Contrary to Si solar cells, the efficiency of polycrystalline CIGS cells is observed to be superior to their monocrystalline counterparts (%20% in ref. [3]). The reasons for high efficiency of polycrystalline absorbers are associated with passivation of defects arising from heavy and light alkali elements, [5][6][7] particularly at the GBs.GBs in CIGS can be roughly divided into two prevalent types: random GBs and Σ3 twin boundaries (TBs). [8,9] For chalcopyrite solar cells like CIGS (and CdTe), multiple groups have reported variations in the nature of the random GBs, which are summarized in the review in ref. [10]. Since the amount of TBs is exceptionally high (>40%) in CIGS, [9,11] the role of TBs and their corresponding defects are very important. However, regardless of the solar cell material and system, almost all existing reports on TBs claim their bulk-like behavior. [8,12,13] To specifically select the TBs of interest and to characterize them, correlative measurements are required. Several correlative measurements on CIGS have been performed in the past. Among these are correlative cathodoluminescence (CL)-electron backscatter diffraction (EBSD) measurements, [11,14] which have indicated neither spectral shift nor significant reduction in the CL signal at the Σ3 TBs, thus suggesting that the radiative recombination of charge carriers is unaffected at these TBs. Another technique used was correlative Kelvin probe force microscopy (KPFM)-EBSD, [15,16] which has shown that these Σ3 TBs exhibit no fluctuations in potential and no band bending, due to their high degree of symmetry, [17] whereas the random GBs exhibit either positive or negative potential barriers. However, only in our recent study it was found that some TBs have peculiar electronic properties. This study consists of correlative electron beam-induced current (EBIC)-EBSD measurements and reports

Sodium in Cu(In, Ga)Se2 Solar Cells: To Be or Not to Be Beneficial

“…The absence of Na at TBs could also be related to the disability of the current techniques in measuring the Na content at very low levels. [42] To investigate the effect of Na doping on the texture of CIGSe, the pole figures of the three samples are plotted (see Figure S1, Supporting Information). Figure S1 (Supporting Information) 4 and denotes the space charge capacitance.…”

“…The absence of Na at TBs could also be related to the disability of the current techniques in measuring the Na content at very low levels. [ 42 ]…”

Precise Alkali Supply during and after Growth for High‐Performance Low Bandgap (Ag,Cu)InSe2 Solar Cells

“…This effect was first reported by Rudmann et al [ 32 ] and was assigned to Na atoms that do not dissolve in (A)CI(G)S grains but rather saturate at GBs and interfaces and form a Na‐In‐Se compound. [ 40 ] A recent study by Karami et al [ 26 ] reports that the number of random high‐angle GBs increased with increasing Na amount present during recrystallization, while the number of twin boundaries decreased. They claim GBs decorated by Na are less mobile during growth resulting in smaller grain sizes.…”

“…[ 21–24 ] Numerous investigations have aimed to understand the mechanisms involving different alkali metals (Na, K, Rb, Cs) and their influence on electronic device properties. [ 18,20,25–28 ] Despite diverse and sometimes conflicting explanations regarding the effect on doping density, carrier lifetime, conductivity, and formation of alkali‐rich compounds at interfaces, these studies universally highlight the significance of a precisely controlled alkali supply for achieving high device performance.…”