Suppressed Deep Traps and Bandgap Fluctuations in Cu₂CdSnS₄ Solar Cells with ≈8% Efficiency

Abstract

postdeposition alkali treatment to improve heterojunction diode quality in Cu(In,Ga) Se 2 (CIGS) solar cells and chloride treatment to passivate grain boundaries in CdTe solar cells. [1] These solar-cell technologies are already commercialized, with lab-scale photovoltaic efficiencies exceeding 22%. [2] However, kesterite-based solar cells, such as Cu 2 ZnSn(S,Se) 4 , which share many of the same characteristics of CIGS and CdTe, significantly lag behind, with a record power conversion efficiency (PCE) of 12.6%. [3] Although the dominant limiting factors for this low performance are a matter of considerable discussion, [4] the following observations are consistent among kesterite absorbers: i) a low photoluminescence quantum yield (PLQY) and a short chargecarrier lifetime, [5] ii) a high value of Urbach band tail energy (larger than 30 meV for S-rich kesterites) and lack of a steep absorption onset, [6,7] and iii) the presence of secondary phases. [4,6,8,9] The extent to which these factors individually affect the photovoltaic performance is debated, but their ubiquity among kesterite absorbers indicates the presence of a large density of point defects. [10][11][12] Specifically, i) the low PLQY arises from the presence of nonradiative mid-gap The identification of performance-limiting factors is a crucial step in the development of solar cell technologies. Cu 2 ZnSn(S,Se) 4 -based solar cells have shown promising power conversion efficiencies in recent years, but their performance remains inferior compared to other thin-film solar cells. Moreover, the fundamental material characteristics that contribute to this inferior performance are unclear. In this paper, the performance-limiting role of deep-trap-level-inducing 2Cu Zn +Sn Zn defect clusters is revealed by comparing the defect formation energies and optoelectronic characteristics of Cu 2 ZnSnS 4 and Cu 2 CdSnS 4 . It is shown that these deleterious defect clusters can be suppressed by substituting Zn withCd in a Cu-poor compositional region. The substitution of Zn with Cd also significantly reduces the bandgap fluctuations, despite the similarity in the formation energy of the Cu Zn +Zn Cu and Cu Cd +Cd Cu antisites. Detailed investigation of the Cu 2 CdSnS 4 series with varying Cu/[Cd+Sn] ratios highlights the importance of Cu-poor composition, presumably via the presence of V Cu , in improving the optoelectronic properties of the cation-substituted absorber. Finally, a 7.96% efficient Cu 2 CdSnS 4 solar cell is demonstrated, which shows the highest efficiency among fully cation-substituted absorbers based on Cu 2 ZnSnS 4 .

Defect Engineering in Earth‐Abundant Cu2ZnSn(S,Se)4 Photovoltaic Materials via Ga3+‐Doping for over 12% Efficient Solar Cells

“…Until now, many strategies have been applied for the purpose of improving the efficiency, and the most prevalent method is equivalent cation substitution, such as Ag or Li substitution for Cu, Cd, and Ba or Mn substitution for Zn. [ 21–28 ] These strategies demonstrated better device performance than the pure solar cells. Recently, Ma et al.…”

Assessing the defect tolerance of kesterite-inspired solar absorbers

“…13,14 However, the chemical trends in the defect tolerance of these kesterite-inspired compounds may be easier to discern by considering the fully substituted materials, since non-linear alloying effects are avoided and comparison between experiment and theory is more straightforward. Some fully-substituted, kesterite-inspired absorbers have received considerable attention and have achieved efficiencies above 5%: Ag 2 ZnSnSe 4 , 15 Cu 2 ZnGeSe 4 , 16 Cu 2 CdSnS 4 , 17 and Cu 2 BaSn(S,Se) 4 . 18 Interestingly, solar cells based on the pure sulfides Cu 2 SrSnS 4 (CSTS) and Cu 2 BaSnS 4 (CBTS) were also reported recently.…”

“…25 Despite this potential advantage, the record efficiencies of CBTS and CSTS solar cells (2.0% and 0.6% respectively) 19,20 are much lower than those of CZTS and CCTS solar cells (11.0% and 8.0% respectively). 17,26 In the first part of this work, we investigate chemical trends in the defect properties of the CXTS series by a combination of experimental techniques and first-principles calculations. Since defect spectroscopy measurements are not available in the literature for CBTS and CSTS, we start by performing temperature-and excitation dependent photoluminescence (PL) measurements on these materials and assign their shallow defect transitions to Cu vacancies and Cu interstitials by matching their measured ionization energies to the calculated charge transition levels of V Cu and Cu i .…”

Defect Control for 12.5% Efficiency Cu2ZnSnSe4 Kesterite Thin‐Film Solar Cells by Engineering of Local Chemical Environment

“…[3,13,20] Li and Na have been proven to reduce the nonradiative recombination centers as well as to improve the p-type doping density. [21,22] Isovalent cation substitution, such as substituting Ag for Cu, [23][24][25] Cd for Zn, [26][27][28] and Ge for Sn, [16] has demonstrated pronounced improvements in electronic properties and device performance compared to reference solar cells. Despite these efforts, the record efficiency for kesterite solar cells is still held by devices with nominally no doping nor cation substitution (but may, however, unintentionally incorporating Na from the glass substrates).…”