Graduate College

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The doctoral dissertation and master's thesis reflects the scholarly research in a graduate program as required for the completion of the degree at the University of Delaware.

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Browsing Graduate College by Subject "(Ag,Cu)(In,Ga)Se2 (ACIGS)"

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    Advanced precursor reaction processing for Ag-alloyed Cu(In,Ga)Se2 solar cells
    (University of Delaware, 2017) Soltanmohammad, Sina
    Silver alloying of Cu(In,Ga)Se2 (CIGS) to form (Ag,Cu)(In,Ga)Se2 (ACIGS) has been investigated for thin film photovoltaics. Previous research has found that introducing silver into the CIGS lattice during co-evaporation resulted in improved VOC and diminished defect concentration, attributed to the lower melting point of the alloy. Also, Ag alloying gave significant improvement in the formation of ACIGS with the hydride gas reaction of metal precursors including altering the morphology of the metal precursors, enhancing adhesion to enable a wider process window and improved device performance. The main goal of this research is to understand the effects of Ag-alloying on the morphology and structure of Cu-In-Ga metal precursors and their reaction in H2Se to form ACIGS. This will lead to development of an advanced precursor reaction process with a controlled composition profile and provide the fundamental basis for scale-up of this technology. ☐ In this project, Ag-Cu-In-Ga precursor films were sputter deposited from Ag0.77Ga0.23, Cu0.77Ga0.23, and In targets using different layer sequences with Ag/(Cu+Ag) and (Ag+Cu)/(Ga+In) ratios fixed at 0.25 and 0.90, respectively. The most uniform morphology was achieved with a Ag-Ga layer followed by a layer with co-sputtered Cu-Ga and In. These precursor layers are shown to be unstable, with a phase evolution during storage at room temperature or low temperature (<150 ºC) annealing and formation of a previously unknown (Ag1-xCux)In2 phase with Cu content of ~30%. All precursor gave similar phase composition after annealing at 300ºC and contained intermetallic phases including In, (Ag,Cu)In2, Ag3In, γ-Cu9(In,Ga)4 and a new, previously unidentified Laves-type phase. This quaternary phase corresponds to (Cu1-xGax)2(AgyIn1-y) with x ≈ 0.2 and y ≈ 0.3. It has a Cu2Mg-type crystal structure with space group of Fd3 ̅m and lattice parameter of 7.090Å. ☐ Reaction pathways to formation of ACIGS were determined using ex-situ time-progressive experiments to help develop an advanced precursor reaction process with a controlled composition profile. Ag-Cu-In-Ga precursors with Ag/(Cu+Ag) = 0.0, 0.25, 0.75, and 1.0 and Ga/(Ga+In) = 0.25 were reacted at 450°C using rapid thermal processing (RTP) in a tubular reactor charged with 5% H2Se/Ar. The reaction time was varied from 2–45 min. ☐ XRD analysis of films with Ag/(Ag+Cu) = 0.0 indicated that the CuInSe2 phase initially formed after 3.5 min. But, slow reaction of a stable γ-Cu9(In,Ga)4 phase slowed down the complete reaction to more than 20 min. However, addition of 25% Ag into the CIGS film accelerated the reaction. EDS/XRF results showed that the complete Se uptake (Se/(Ag+Cu+Ga+In)) occurred in the first 10 min and XRD analysis indicated that the reaction completed within 10 min. EDS/XRF analyses of samples with Ag/(Ag+Cu) = 0.75 showed that Se uptake reached 0.9 within 10 minutes; however, SEM/EDS analyses measured from the Mo-side of the film after 20 min revealed unreacted Ag-In remnants. By increasing the Ag ratio to 1.0, Se uptake was delayed up to 20 minutes due to slow reaction of stable ζ-Ag3(In,Ga) phase. The reaction was completed with formation of Ag(In,Ga)Se2 and Ag(In,Ga)5Se8 phases. ☐ Finally, we show the Ga accumulated in AgGaSe2 and Ga-Se phases at the back-side of all films. However, the amount of Ga measured by EDS increased with increasing Ag concentration due to lower melting temperature of the Ag-alloyed compounds so Ag can assist in controlling the through film composition profile.