Mg _x Zn_1−x O contact to CuGa₃Se₅ absorber for photovoltaic and photoelectrochemical devices

Abstract

CuGa3Se5 is a promising candidate material with wide band gap for top cells in tandem photovoltaic and photoelectrochemical (PEC) devices. However, traditional CdS contact layers used with other chalcopyrite absorbers are not suitable for CuGa3Se5 due to the higher position of its conduction band (CB) minimum. Mg x Zn1− x O (MZO) is a transparent oxide with adjustable band gap and CB position as a function of magnesium composition, but its direct application is hindered by CuGa3Se5 surface oxidation. Here, MZO is investigated as a contact (n-type ‘buffer’ or ‘window’) material to CuGa3Se5 absorbers pretreated in Cd2+ solution, and an onset potential close to 1 V vs reversible hydrogen electrode in 10 mM hexaammineruthenium (III) chloride electrolyte is demonstrated. The Cd2+ surface treatment changes the chemical composition and electronic structure of the CuGa3Se5 surface, as demonstrated by photoelectron spectroscopy measurements. The performance of CuGa3Se5 absorber with Cd2+ treated surface in the solid-state test structure depends on the Zn/Mg ratio in the MZO layer. The measured open circuit voltage of 925 mV is promising for tandem PEC water splitting with CuGa3Se5/MZO top cells.

Codesigning Alloy Compositions of CdSeyTe1−y Absorbers and MgxZn1−xO Contacts to Increase Solar Cell Efficiency

“…To codesign Mg x Zn 1− x O /CdSe y Te 1− y thin‐film solar cells, we fabricated PV device libraries that had orthogonal composition gradients in CdSe y Te 1− y absorbers and Mg x Zn 1− x O contacts using the combinatorial approach. For Mg x Zn 1− x O material characterization and growth optimization, thin‐film samples with compositional gradients were deposited by combinatorial radio frequency magnetron sputtering from ZnO and Mg targets [ 12 ] on 50 × 50 mm Eagle XG glass substrates. For CdSe y Te 1− y device fabrication, CdSe and CdTe were deposited by sequential thermal evaporation without breaking vacuum, on commercially available 3 × 3” TEC12D glass with bilayer SnO 2 and 120 nm of Mg x Zn 1− x O described earlier, with CdSe y Te 1− y gradient orthogonal to the Mg x Zn 1− x O gradient.…”

“…There are varying reports on the Mg x Zn 1Àx O band gap (4.0-4.2 eV) at maximum Mg composition (x ¼ 0.36-0.46) at which phase segregation occurs, as it depends on the deposition technique, deposition rate, and substrate temperature. [8,9] Mg x Zn 1Àx O has also been studied using different combinatorial methods, such as pulsed laser deposition, [10] chemical vapor deposition, [11] and sputtering, [12] where we demonstrated Mg x Zn 1Àx O conduction band tuning and integrated it with wide-bandgap CuGa 3 Se 5 absorber to increase V OC close to %1 V. The Mg x Zn 1Àx O/ CdSe y Te 1Ày device interface was also recently studied, demonstrating improvements in J SC and efficiency due to widened bandgap and better carrier collection at low wavelengths. [13] Finding the optimum composition in Mg x Zn 1Àx O and CdSe y Te 1Ày is complicated as Mg in Mg x Zn 1Àx O and Se in CdSe y Te 1Ày both change the conduction band alignment and interface defect density, which can have significant impacts on interface recombination, barrier heights, and V OC.…”

“…graded CdTe, [17] CIGS, [18] Cu 2 ZnSnCh 4 , [19] CuSbCh 2 [20] absorbers or Mg x Zn 1Àx O, [12] Zn-Ni-Co-O, [21] Cu x Zn 1Àx S, [22] and CdS [23] contacts in thin-film solar cells. However, the reports on codesign of absorbers and contacts are limited to early-stage development of all-oxide solar cells [24] or organic PV.…”

“…This coupled absorber/contact codesign problem can be addressed using high‐throughput experimental (combinatorial) methods, [ 16 ] by creating PV device libraries with orthogonal composition gradients in the CdSe y Te 1− y and Mg x Zn 1 −x O layers. In the past, combinatorial methods have been used to study compositionally graded CdTe, [ 17 ] CIGS, [ 18 ] Cu 2 ZnSnCh 4 , [ 19 ] CuSbCh 2 [ 20 ] absorbers or Mg x Zn 1− x O, [ 12 ] Zn–Ni–Co–O, [ 21 ] Cu x Zn 1− x S, [ 22 ] and CdS [ 23 ] contacts in thin‐film solar cells. However, the reports on codesign of absorbers and contacts are limited to early‐stage development of all‐oxide solar cells [ 24 ] or organic PV.…”

Electrical barriers and their elimination by tuning (Zn,Mg)O buffer composition in Cu(In,Ga)S2 solar cells: systematic approach to achieve over 14% power conversion efficiency

“…Higher power conversion efficiency (PCE) aspirations thus require the substitution of the CdS buffer layer with alternative buffer layers that possess a higher conduction band minimum (CBM) energy value than CdS. The zinc magnesium oxide ((Zn,Mg)O) and zinc oxy-sulfide (Zn(O,S)) buffers are particularly interesting, as it is possible to modify the bandgap and the CBM energy of the films by tuning the Mg content and O/S ratio in the films, respectively [13][14][15][16]. As a matter of fact, the record PCE of 15.5% has been achieved by replacing the CdS/i-ZnO buffer i-layer stack with a bilayer (Zn,Mg)O buffer stack [17].…”