High Vapor Transport Deposition: A Novel Process to Develop Cu₂ZnSn(S_xSe_1–x)₄ Thin Film Solar Cells

Abstract

Materials such as cadmium telluride (CdTe), copper indium gallium selenide (CIGSe), and copper zinc tin selenide (CZTSe) with higher absorption coefficient are trying to make headway in the competitive photovoltaic market covered mainly by silicon materials. [1] The most important key factors of those emerging semiconductors are the cost-effective mass production, together with high photovoltaic conversion factor (PCE) and long-term stability. Despite CdTe and CIGSe are already industrialized, and their PCE is reaching the commercial silicon ones, the kesterite (CZTSe) is still an emerging technology with high absorption and low raw material cost. But one of its main bottlenecks is its hidden capability to be manufactured at large scale with reasonably good quality. [2] The CZTSe has the properties desired for photovoltaic materials to be selected as potential p-type semiconductors: direct bandgap, high absorption coefficient (10 4 cm À1 in the visible light range), and optical bandgap energy of the range 1.4-1.5 eV, close to the optimum singlejunction value predicted by Shockley-Queisser model. Today, CZTSe is considered a good competitor for CIGSbased solar cells. CZTSe is described by the structural model of two natural minerals: stannite (space group I-42 m) and kesterite (space group I-4). [3][4][5] These structures are very similar: in both structures the cations are located on tetrahedral sites, but their distributions on planes perpendicular to the x-axis are not the same. Additionally, the position of the chalcogen atom is slightly different in both structures. Design and manufacturing of high efficiency CZTSe solar cells need high accuracy in the composition of the absorber material: slight modifications in composition, structural electronic, and defect properties of the alloys have a high impact in the bandgap energy. [6] CZTSe solar cells have achieved the highest efficiency of 12.6%, [7] which is still far from the offered by CIGS-based solar cells: 23.35%. [8] It has known that this limitation is mainly related with the short minority carrier lifetime and the high series resistance imposed by the contact barrier due to the formation of the MoSe 2 at the CZTSe/Mo interface. The presence of secondary phases and defect states in the absorber also increases the recombination rate. [9,10] The influence of the deposition method on the structure, morphology, optical, and electrical properties of CZTSe has been investigated by other researchers: sputtering, thermal evaporation, spray pyrolysis, electrodeposition, dip coating, SILAR method, spin coating, sol-gel, solvothermal method, and chemical bath deposition (CBD). [1] In all of these techniques, the control of the presence of secondary phases is the major critical issue, so the final performance of the device is dependent on the manufacturing route selected: the highest conversion efficiency of CZTS achieved using vacuum techniques such as

Zn(O,S) Buffer Layer Deposited by High Vapor Transport Deposition for Controlling Band Alignment of Cu2ZnSn(SxSe1−x)4 Thin‐Film Solar Cell Heterojunction

“…Then, a 2 μm‐thick CZTSSe absorber was deposited using the HVTD method. [ 21 ] The 50 nm‐thick Zn(O,S) layer was deposited by HVTD, too. No atmospheric contamination was expected because the absorber and buffer layers were kept under vacuum conditions, and both depositions were sequential in the same process chamber.…”

“…For the Cd-free buffer layer Zn(O,S)-based CZTSSe solar cells, the Zn(O,S) thin films with a thickness of about 50 nm were manufactured under different oxygen partial pressures and applied between the Al:ZnO and the CZTSSe absorber layers Ericson et al [39] Photochemical deposition CZTS 516 16.8 35 3.0 Zhang et al [40] Sulfurization CZTS 610 21.18 40 5.1 Yang et al [41] CBD CZTS 708 19.2 54 7.3 Huang et al [42] CBD CZTS 641 15.9 54 5.5 Correa et al [43] Sputtering CZTS 311 12.2 56 2.1 Kim et al [44] Modeling CZTS 640 23.9 65 10.0 Cherouana et al [18] CBD CZTSe 379 30.7 60 6.5 Neuschitzer et al [45] CBD CZTSe 336 25.0 51 5.0 Steirer et al [40] CBD CZTSe 358 33.5 60 7.2 Li et al [21] Sputtering CZGSSe 730 13.0 48 4.6 Schnabel et al [41] ALD CZTSSe 327 26.6 51 4.7 Hong et al [29] ALD CZTSSe 313 29.4 33 3.1 Hong et al [30] ALD CZTSSe 496 35.6 56 9.8 Jeong et al [46] ALD CZTSSe Barkhouse et al [47] CBD CZTSSe 389 29.0 52 5.8 Grenet et al [48] CBD CZTSSe 220 25.2 46 3.8 Park et al [49] CBD CZTSSe 400 21.8 51 4.5 Gao et al [50] CBD CZTSSe 388 22.8 42 3.7 Grenet et al [51] Modeling CZTSSe 640 24.1 41 6.3 Cherouana et al [18] CBD CZTSSe 596 15.4 49 4.5 Nguyen et al [52] HVTD CZTSSe 574 20.3 59 6.9 This work of the solar cells. To ensure statistical analysis, ten samples (0.25 cm 2 area) were processed for each process condition.…”

“…Moreover, the Zn(O,S) layer is deposited using vacuum conditions in the same process chamber where the kesterite absorber layer is grown, using the high vapor transport deposition (HVTD) technique. [ 21 ] The consequences of depositing both semiconductor layers inline are studied, at the time that the manufacturing process represents a simple in‐line, which might reduce the production cost, shorten the process time, and provide a better control of layer thickness.…”

Inorganic Thin-Film Solar Cells: Challenges at the Terawatt-Scale

“…Kesterite CZTS (Table 4, entry 6) is still an emerging technology with high absorption and low raw material cost. Today, CZTSe is considered a good competitor for less sustainable yet more efficient counterpart CIGS solar cells [10,[58][59][60][61][62]. However, one of the main bottlenecks of CZTSSe is its hidden capability to be manufactured on a large scale with reasonably good quality [63].…”