Band-gap engineering in Cu(In,Ga) Se2 thin films grown from (In,Ga)2Se3 precursors

Gabor, Andrew M.; Tuttle, J. R.; Bode, M.; Franz, A.; Tennant, A.; Contreras, Miguel Á.; Noufi, R.; Jensen, D. G.; Hermann, A. M.

doi:10.1016/0927-0248(95)00122-0

Cited by 212 publications

(78 citation statements)

References 8 publications

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“…CuGaSe 2 (CGS) with a band gap of 1.7 eV is one of the ideal compounds to be used as the top cell for a tandem photovoltaic device. Different processes for preparing CGS films have been proposed, such as the bilayer and the three-stage process [2,3]. The three-stage process has produced the best device efficiencies to date [4].…”

Section: Introductionmentioning

confidence: 99%

Effect of Cu excess on three-stage CuGaSe2 thin films using in-situ process controls

Caballero¹,

Siebentritt²,

Sakurai³

et al. 2007

Thin Solid Films

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The objective of this work is to study the effect of Cu-excess and compare the growth mechanism of CuGaSe 2 (CGS) films coevaporated using a bilayer and a three-stage process and evaluate the consequences of the latter for CGS on transparent back contacts. CGS thin films are prepared by coevaporation in a three-stage process onto Mo/soda-lime substrates and onto FTO. In contrast to the bilayer process, Cu-Se phases are only observed on the surface at the end of the second stage, e 2 . This allows to work with a broader Cu-excess window. Atomic ratios (Cu/Ga)e 2 of around 1.3 at the end of deposition phase 2 in the three-stage process show the better device efficiencies due to a larger grain formation. Increasing the Cu-content leads to a slight decrease of the grain size and voids are observed in the film, reducing the FF of the device. The CGS morphology and the solar cells efficiency are dominated by the Cu excess more than by the T substrate between 535° C and 500° C. Similar results are obtained for CGS on FTO: (Cu/Ga)e 2 ∼1.3 as best composition at T substrate =500° C.

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Section: Introductionmentioning

confidence: 99%

Effect of Cu excess on three-stage CuGaSe2 thin films using in-situ process controls

Caballero¹,

Siebentritt²,

Sakurai³

et al. 2007

Thin Solid Films

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“…The transition from CuIn 3 Se 5 to CuInSe 2 can take place purely by cation exchange 26 within the fcc-type Se sublattice, accompanied by lattice expansion. 18 Thus, formation of compressive stress is expected due to the rigid substrate.…”

Section: B Correlation Between Strain Relaxation and Grain Growthmentioning

confidence: 99%

“…4042 This means that the diffusion of one In atom is directly coupled to diusion of 3 Cu atoms in the opposite direction. 26 Assuming further that the cation diusion takes place in a rigid Se sublattice, the three-component Cu-In diusion problem can be reduced to a one-component diusion problem. Since close to the CuInSe 2 stoichiometry the concentration of the defect complexes is dilute, Fickian diusion can be applied:…”

Section: Diusion and Phase Boundariesmentioning

confidence: 99%

Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation

et al. 2015

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In polycrystalline compound semiconductor thin lms, structural defects such as grain boundaries as well as lateral stress can form during lm growth, which may deteriorate their electronic performance and mechanical stability. In Cu-based chalcogenide semiconductors such as Cu(In, Ga)Se2 or Cu2ZnSn(S, Se)4, temporary Cu excess during lm growth leads to improved microstructure such as a reduced grain boundary density, a strategy that has been used for decades for high-eciency chalcopyrite thin lm solar cells. However, the mechanisms responsible for the benecial eect of Cu-excess are yet not fully claried. Here, we investigate the evolution of lateral stress, grain growth and Cu-Se segregation during Cu-Se deposition onto Cu-poor CuInSe2. Real-time x-ray diraction and uorescence analysis with a double-detector setup reveals that sudden stress relaxation occurs shortly prior to Cu-Se segregation at the surface and precisely coincides with domain growth and change of texture. Numerical reaction-diusion modeling provides an explanation for the observed delay of Cu-Se segregation. Our results show that partial recrystallization of the lm can be already reached without the necessity of an overall Cu-rich lm composition and thus suggest a new synthesis route for the fabrication of high-quality chalcopyrite absorber lms.Co-evaporation of Cu(In, Ga)Se 2 (CIGSe) lms -used as absorber layer in thin lm solar cells with world record energy conversion eciencies 1,2 -features a puzzling peculiarity: For the nal lm, a Cu-poor composi-is required to obtain highest eciencies; however, during deposition, the lm composition is changed from an initially Cu-poor composition to an intermediate Cu-rich composition and nally changed back to a Cu-poor composition. The composition modications during lm deposition are realized by varying the Cu and In+Ga evaporation uxes. Two crucial ndings point out the importance of the Cu-poor → Cu-rich transition: First, highest eciencies are only achieved if an intermediate Cu-rich lm composition was reached during lm deposition. 3 Second, a three-stage process with a Cu-poor → Cu-rich → Cu-poor sequence leads to higher eciencies than a two-stage process with only a Cu-rich → Cu-poor sequence. 46 Thus, it seems that the key challenge in understanding the success of the three-stage process over the two stage-process is -besides the adjustment of an ideal Ga gradient -the identication of the reactions and their driving forces acting during the Cu-poor → Cu-rich transition.While the eect of the Cu-poor → Cu-rich transition on structural and morphological changes in CIGSe lms such as grain growth 710 as well as on electronic properties of the material 3,11 has been thoroughly investigated in the past decade, the physical mechanisms and driving forces of these changes are not fully understood. Reduction of grain boundary (GB) energies or defect densities were proposed as possible driving forces for grain growth; 79 however, no attention has so far been paid to the potential role of stress energy for the mic...

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“…In addition, an increasing gallium content towards the molybdenum back contact is expected to enhance carrier collection and to decrease recombination via asymmetric diffusion, repelling electrons from the Cu(In,Ga)Se 2 /Mo interface. [5][6][7][8][9][10] The combination of a gallium grading towards the front and the back interfaces is commonly referred to as double grading. In general, a double grading within a Cu(In, Ga)Se 2 thin film can be achieved via the standard three-stage co-evaporation growth process.…”

Section: Introductionmentioning

confidence: 99%

Gallium gradients in chalcopyrite thin films: Depth profile analyses of films grown at different temperatures

Mönig

Kaufmann

Fischer

et al. 2011

Journal of Applied Physics

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Cu(In,Ga)Se 2 films are used as absorber layers in chalcopyrite thin film solar cells. As the gallium concentration in the absorber can be used to control the band gap, there have been many efforts to vary the gallium concentration in depth to gain an optimum balance of light absorption, carrier collection, and recombination at different depths of the absorber film, leading to improved quantum efficiency. In this study, we investigate the effect of the maximum substrate temperature during film growth on the depth dependent gallium concentration. For the in-depth gallium concentration analyses, we use two techniques, covering complementary depth ranges. Angle dependent soft x-ray emission spectroscopy provides access to information depths between 20 and 470 nm, which covers the depth range of the space charge region, where most of the photoexcited carriers are generated. Therefore, this depth range is of particular interest. To complement this investigation we use secondary neutral mass spectrometry, which destructively probes the whole thickness of the absorber (%2 lm). The two methods show increasingly pronounced gallium and indium gradients with decreasing maximum substrate temperature. The probing of the complementary depth ranges of the absorbers gives a consistent picture of the in-depth gallium distribution, which provides a solid basis for a comprehensive discussion about the effect of a reduced substrate temperature on the formation of gallium gradients in Cu(In,Ga)Se 2 and the device performance of the corresponding reference solar cells.

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Band-gap engineering in Cu(In,Ga) Se2 thin films grown from (In,Ga)2Se3 precursors

Cited by 212 publications

References 8 publications

Effect of Cu excess on three-stage CuGaSe2 thin films using in-situ process controls

Effect of Cu excess on three-stage CuGaSe2 thin films using in-situ process controls

Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation

Gallium gradients in chalcopyrite thin films: Depth profile analyses of films grown at different temperatures

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