Hamed Simchi

Backwall superstrate configuration for ultrathin Cu (In, Ga) Se2 solar cells

A backwall superstrate device structure that outperforms conventional substrate Cu(In,Ga)Se2 devices for thin absorbers is demonstrated. The backwall structure (glass/In2O3-SnO2/MoO3-x/Cu(In,Ga)Se2/CdS/i-ZnO/Ag) utilizes a MoO3−x transparent back contact to allow illumination of the device from the back. In combination with a silver front reflector this cell structure is tailored to enhance performance of devices with submicron thick absorbers. It was found that devices with the backwall configuration outperform substrate devices in the absorber thickness range dCIGS = 0.1-0.5 μm. The advantage of the backwall configuration is mainly through superior JSC, achieved by application of a front reflector and elimination of parasitic absorption in CdS.

Improved Performance of Ultrathin Cu(InGa)Se

A backwall superstrate device structure that outperforms conventional substrate Cu(In,Ga)Se₂ devices for thin absorbers is described. The backwall structure of glass/ITO/MoO₃/Cu(In,Ga)Se₂ /CdS/i-ZnO/Ag utilizes a MoO₃ transparent back contact to allow illumination of the device from the back. The device performance has been improved by modifying the Cu(In,Ga)Se₂, including alloying with Ag to form (AgCu)(InGa)Se₂ absorber layers. In addition, sulfized back contacts including ITO-S and MoS₂ are compared. Interface properties are discussed based on the XPS analysis and thermodynamics of reactions.

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(Ag,Cu)(In,Ga)Se2 alloy absorber layers with various Ga/(Ga+In) and Ag/(Ag+Cu) ratios were deposited using multisource elemental evaporation and analyzed by glancing incidence X-ray diffraction and energy dispersive X-ray spectroscopy. All films exhibit chalcopyrite reflections in the X-ray diffraction pattern and films with 0.5 ≤ Ga/(Ga+In) <; 1 and Ag/(Ag+Cu) >; 0.5 have additional reflections consistent with an ordered defect phase which is limited to the near-surface region of the film. X-ray photoelectron spectroscopy measurements show that all films studied have low (Ag+Cu)/Se and (Ag+Cu)/(Ga+In) ratios near the surface relative to the bulk composition, consistent with an ordered defect compound identified as (Ag,Cu)(In,Ga)5Se8. Additionally, the near-surface region of (Ag,Cu)(In,Ga)Se2 films contains a higher Ag/(Ag+Cu) ratio than the bulk and the Ag(In,Ga)Se2 film contains excess Ag near the surface.

Solar Cells With a Backwall Superstrate Configuration

The addition of Ag to Cu-Ga-In precursors for reaction to form (AgCu)(InGa)Se2 has shown benefits including improved adhesion, greater process tolerance and potential for improved device performance. In this study, sequential layer sputtering of Ag-Ga, Cu-Ga, and In targets with different layer sequences is characterized. Ag/(Cu+Ag) and (Ag+Cu)/(Ga+In) ratios were fixed at 0.25 and 0.9, respectively. The most uniform morphology is achieved in Ag-Ga/Cu-In-Ga co-sputtered precursors. No metallic In phase was found in this case, and only the Ag(In,Ga)2 phase was detected. Varying the sputtering sequence for stacked layers results in dissimilar morphologies and structural phases. X-ray diffraction (XRD) analyses reveal that the Ag-Ga and In layers intermix when they are in contact, forming a Ag(In,Ga)2 phase. Incorporating a Cu-Ga layer between the Ag-Ga and In layers prevents the formation of such a phase. Finally, solar cells fabricated from the Cu-Ga/In/Ag-Ga metal precursor sequence showed the highest overall performance.

An Investigation of the Surface Properties of (Ag,Cu)(In,Ga)Se

Molybdenum oxide (MoO₃) and tungsten oxide (WO₃) are considered as transparent back contacts for (Ag,Cu)(In,Ga)Se₂ thin film solar cells. MoO₃ and WO₃ films were deposited by reactive RF sputtering at room temperature in an Ar/O₂ ambient on (Ag,Cu)(In,Ga)Se₂ absorber layers with various Ga/(Ga + In) and Ag/(Ag + Cu) ratios. Determination of the valence band offsets by XPS showed that Ag-alloying of absorber layer changes the energy band alignment at the absorber-back contact interface with MoO₃ and WO₃ contacts. This produces a primary contact with lower valence band offset compared with Cu(In,Ga)Se₂ counterparts. The effect is less significant in films with Ga > 0.5 and Ag > 0.5 (corresponding to E_g > 1.4 eV) probably due to the different nature of ordered vacancy compounds forming near the surface phases.

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AgCu(InGa)Se<inf>2</inf> alloy absorber layers with various Ga/(Ga+In) and Ag/(Ag+Cu) ratios were deposited using multi-source elemental evaporation and analyzed by glancing incidence x-ray diffraction and energy dispersive x-ray spectroscopy. All films exhibit satellite chalcopyrite reflections in the x-ray diffraction pattern and films with 0.5 ≤ Ga < 1 and Ag > 0.5 have additional reflections consistent with an ordered defect phase which is limited to the near-surface region of the film. X-ray photoelectron spectroscopy results show that all films have low (Ag+Cu)/Se ratios near the surface, consistent with an ordered defect compound. Films with 0 < w < 1 have (Ag+Cu)/Se and (Ag+Cu)/(Ga+In) ratios at the surface close to the (AgCu)(InGa)<inf>5</inf>Se<inf>8</inf> ordered defect phases. Additionally the near-surface region of (AgCu)(InGa)Se<inf>2</inf> films contains a higher Ag/(Ag+Cu) ratio than the bulk and the Ag(InGa)Se<inf>2</inf> film contains excess Ag near the surface.