Intermixing at the heterointerface between ZnS∕Zn(S,O) bilayer buffer and CuInS2 thin film solar cell absorber

Abstract

The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS 2 ͑CIS͒ based thin-film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. After identifying the deposited Zn compound, as ZnS / Zn͑S,O͒ bilayer buffer in former investigations ͓M. Bär et al., J. Appl. Phys. 99, 123503 ͑2006͔͒, this time the focus lies on potential diffusion/intermixing processes at the buffer/absorber interface possibly, clarifying the effect of the heat treatment, which drastically enhances the device performance of respective final solar cells. The interface formation was investigated by x-ray photoelectron and x-ray excited Auger electron spectroscopy. In addition, photoelectron spectroscopy ͑PES͒ measurements were also conducted using tunable monochromatized synchrotron radiation in order to gain depth-resolved information. The buffer side of the buffer/absorber heterointerface was investigated by means of the characterization of Zn͑S,O͒ / ZnS / CIS structures where the ZnS / Zn͑S,O͒ bilayer buffer was deposited successively by different deposition times. In order to make the ͑in terms of PES information depth͒ deeply buried absorber side of the buffer/absorber heterointerface accessible for characterization, in these cases the buffer layer was etched away by dilute HCl aq . We found indications that while ͑out-leached͒ Cu from the absorber layer forms together with the educts in the chemical bath a ͓Zn ͑1−Z͒ ,Cu 2Z ͔S-like interlayer between buffer and absorber, Zn is incorporated in the uppermost region of the absorber. Both effects are strongly enhanced by postannealing the Zn͑S,O͒ / ZnS / CIS samples. However, it was determined that the major fraction of the Cu and Zn can be found quite close to the heterointerface in the buffer and absorber layer, respectively. Due to this limited ͑in the range of one monolayer͒ spatial extent, these "diffusion" mechanisms were rather interpreted as a chemical bath deposition induced and heat-treatment promoted Cu-Zn ion exchange at the buffer/absorber interface. Possible impacts of this intermixing on the performance of the final solar cell devices will also be discussed.

Synchrotron-based spectroscopy for the characterization of surfaces and interfaces in chalcopyrite thin-film solar cells

“…This is corroborated by the fact that even hydrochloric acid (10% v/v) does not completely remove Zn from the surface even after several hours of etching, although bulk ZnS is readily dissolved in dilute mineral acids. A similar behavior was observed after removing wet-chemically deposited ZnS from a CuInS 2 substrate by etching with HCl as described above and in [60]. We analyzed the chemical composition of the Zn-containing residual by XAES, using Mg K a excitation.…”

“…7 After revealing the ZnS/Zn(S,O) bi-layer composition of the deposited buffer layer, the next aim is to investigate the chemical structure to the buffer/CIS interface. In order to do so the buffer layer was etched away by diluted HCl aq (for more details see [60]). Since the performance of the final solar cell devices with ZnS/ Zn(S,O) buffer layer increases significantly by annealing in air (5 min at 200 1C, [61]), the interface structure was also analyzed after such a heat treatment.…”

In situ monitoring the growth of thin-film ZnS/Zn(S,O) bilayer on Cu-chalcopyrite for high performance thin film solar cells

“…In the literature one can find various reports on the composition of CBD-ZnS layers ranging from ZnS [3] over Zn(S,OH) [4][5][6], ZnS(O,OH) [7,8], Zn(O,S, OH) x [9] to ZnS x O y H z [10]. Recently we have reported, on the composition of CBD-ZnS/Zn(S,O) bi-layer applied to low-gap CIGSSe (E g = 1.03 eV) and wide-gap CIS (E g =1.54 eV) [11][12][13][14][15][16]. In the present work this CBD process was monitored by measuring the transparency of the solution as a function of time, temperature, reactant concentration.…”

In Situ Analysis of the Zn(S,O) Buffer Layer Preparation for Chalcopyrite Solar Cells by Zn L‐edge X‐Ray Absorption Spectroscopy

“…[9] However, the conditions of chemical bath deposition (CBD) and post-annealing for the buffer layers of ZnO/ZnA C H T U N G T R E N N U N G (S,O)/ CuA C H T U N G T R E N N U N G (In,Ga)A C H T U N G T R E N N U N G (S,Se) 2 type solar cells have a strong impact on cell efficiency. [7,8,[10][11][12][13][14] Although the ZnA C H T U N G T R E N N U N G (S,O) buffer has proven to be an adequate replacement for the commonly used CdS, [15][16][17][18] the nature of its electronic structure is not yet fully known. The CBD process of ZnA C H T U N G T R E N N U N G (S,O) was shown to be sensitive to the solution chemistry.…”

“…Due to the predominant presence of Zn complexes rather than free Zn 2 + , formation of ZnO is partly suppressed even in alkaline solution, which is a prerequisite for ZnS formation. [9,15,16,20] The best ZnA C H T U N G T R E N N U N G (S,O) layers in terms of device efficiency were made by first adding thiourea to an aqueous solution of ZnSO 4 , heating this mixture up to 80 8C, and then adding ammonia [here referred to as CBD-T; Eqs. (1) and (2)].…”