Characterization of the Stability of Indium Tin Oxide and Functional Layers for Semitransparent Back‐Contact Applications on Cu(in,Ga)Se₂Solar Cells

Abstract

Herein, a detailed study of the stability of different ITO‐based back‐contact configurations (including bare ITO contacts and contacts functionalized with nanometric Mo, MoSe2, and MoS2 layers) under the coevaporation processes developed for the synthesis of high‐efficiency Cu(In,Ga)Se2 (CIGSe) solar cells is reported. The results show that bare ITO layers can be used as efficient back contacts for coevaporation process temperatures of 480 ºC. However, higher temperatures produce an amorphous In–Se phase at the ITO surface that reduces the contacts transparency in the visible region. This is accompanied by degradation of the solar cells’ efficiency. Inclusion of a Mo functional layer leads to the formation of a MoSe2 interfacial phase during the coevaporation process, which improves the cells’ efficiency, achieving device efficiencies similar to those obtained with reference solar cells fabricated with standard Mo back contacts. Optimization of the initial Mo layer thickness improves the contact transparency, achieving contacts with an optical transparency of 50% in the visible region. This is accompanied by a relevant decrease in back reflectivity in the CIGSe devices, confirming the potential of these contact configurations for the development of semitransparent CIGSe devices with improved optical aesthetic quality without compromising the device performance.

2‐step process for 5.4% CuGaSe2 solar cell using fluorine doped tin oxide transparent back contacts

“…The likeliest hypothesis is that temperatures of 600°C and above significantly degrade the FTO back contact/interface. A similar effect has been previously observed in the case of other TCO back contacts (ITO), which showed a detrimental influence on the solar cells' efficiency when the deposition temperatures increased above 600°C due to the appearance of the In–Se amorphous phase at the absorber/back contact interface 12 …”

“…It was found that the insertion of an ultrathin Mo layer at the absorber/TCO interface markedly improved the solar cells' performance, which can be ascribed to the formation of a MoSe 2 interlayer acting as hole‐selective material. Our group recently observed a similar trend working on lower band gap CIGS devices, 12 as well as with a wider bandgap (1.4 eV) CIGSe material with a 68% Ga content, 13 which seems to indicate that observations made on the low Ga content are to some extent transferrable to the higher Ga counterpart despite the modified band profile.…”

“…A similar effect has been previously observed in the case of other TCO back contacts (ITO), which showed a detrimental influence on the solar cells' efficiency when the deposition temperatures increased above 600 C due to the appearance of the In-Se amorphous phase at the absorber/back contact interface. 12 It should moreover be noted that the best results were obtained when annealing in a single step at 570 C low-pressure conditions (1.5 mbar) rather than at high-pressure conditions (500 to 800 mbar, rising with temperature), and multistep annealing processes (not reported here) combining both high-pressure and low-pressure steps did not lead to any gain in performance. In optimized conditions, a PV efficiency in the 3.5%-4.0% range can be consistently obtained, with a very low data spread for the current density and voltage.…”

Flexible Substrate‐Structured Sb2S3 Solar Cells with Back Interface Selenization

“…Furthermore, the Raman spectra and X‐ray diffraction (XRD) are applied to check the component and phase of MoSe 2 on the Mo foil. The Raman peaks 165 and 285 cm −1 corresponding to two weak vibration modes of E 1g and E 2g , and 242 cm −1 corresponding to the strong vibration mode of A 1g , [ 40 ] are typical peaks of MoSe 2 (Figure S2d, Supporting Information). As shown in the XRD spectra (see Figure 4e), the peaks of 26.1° and 58.7° match well with the standard PDF cards of MoSe 2 (pdf#20‐0757) and Mo (pdf#42‐1120) phases, respectively.…”

Wide‐Gap Chalcopyrite Solar Cells with Indium Oxide–Based Transparent Back Contacts

“…Fonoll-Rubio et al suggested the formation of an In-Se compound when CIGS is grown on ITO at T abs > 480 °C, deteriorating the electrical and optical properties of the TBC. [41] Since others measured decent solar cell characteristics using bare ITO back contacts for T abs up to 600 °C, [16] the evolution of secondary phases obviously depends on the absorber deposition process as well. For further details, Table S1 in Supporting Information lists the important contact properties of all identified works on CIGS-based solar cells with TBCs (not claiming completeness).…”