The thermal decomposition behavior, the microstructural evolution and its influence on the mechanical properties of a supersaturated Cu-Co solid solution with ~100 nm average grain size prepared by severe plastic deformation is investigated under non-isothermal and isothermal annealing conditions. Pure fine-grained Cu and Co exhibit substantial grain growth upon annealing, whereas the Cu-Co alloy is thermally stable at the same annealing temperatures. The annealed microstructures are studied by independent characterization methods, including scanning electron microscopy, electron energy loss spectroscopy and atom probe tomography. The phase separation process in the Cu-Co alloy proceeds by the same mechanism, but on different length scales: a fine scaled spinodal-type decomposition is observed in the grain interior, simultaneously Co and Cu regions with a larger scale are formed near the grain boundary regions. Subsequent grain growth at higher annealing temperatures results in a microstructure consisting of the pure equilibrium phases. Such mechanisms can be used to tailor nano structures to optimize certain properties. Published in

To fully exploit the potential of a 2-junction photovoltaic (PV) tandem device, the top cell should exhibit an absorber bandgap energy (E g ) above 1.4 eV in a 4-terminal and E g % 1.6 eV in a 2terminal configuration. [1,2] Such high bandgaps can be reached for Cu(In,Ga)Se 2 (CIGS) films with [Ga]/([Ga]þ[In]) (GGI) values > 0.6. However, while efficiencies (η) of CIGS-based solar cells exceed 22% for GGI values ≤ 0.3, [3,4] a distinctly inferior performance is observed for wide-gap chalcopyrite solar cells with higher Ga contents (i.e., E g > 1.2 eV). [5] The most prominent bottleneck is the increasing open-circuit voltage (V OC ) deficit with respect to the bandgap energy. While some of the large V OC loss of wide-gap chalcopyrite solar cells can be mitigated by using alternative (i.e., not CdS) buffer layers with tunable electron affinity (χ), [6][7][8][9][10] a major part of the deficit seems to arise from a poorer bulk quality. [11,12] The lifetime deterioration with increasing GGI was suggested to originate from higher Shockley-Read-Hall (SRH) recombination via 1) energetically deeper Ga Cu antisite donor defects [13][14][15][16] or (V Se -V Cu ) complexes in acceptor state [17] ; 2) an increasing density of deep acceptors [18,19] ; 3) an increasing tetragonal lattice distortion [20] ; and/or 4) an increased fraction of Cu-enriched (supposedly detrimental [21] ) grain boundaries (GBs). [22] Instead of utilizing low-χ alternative buffers, interface recombination can also be reduced (or canceled out) for CdS-buffered wide-gap CIGS solar cells when the absorber is alloyed with silver, i.e., forming (Ag,Cu)(In,Ga)Se 2 (ACIGS). While a detrimental negative conduction band offset (CBO) at the CdS/CIGS interface is predicted for GGI > 0.5, [23][24][25] sufficient Ag-alloying allows to avoid a negative CBO in the entire compositional range of ACIGS (i.e., even for GGI ¼ 1). [7,26,27] Consequently, high V OC values close to 900 mV and beyond were achieved for wide-gap ACIGS solar cells with CdS buffer layers by several groups. [7,11,[28][29][30][31][32] However, it was reported that the chalcopyrite single-phase region of the ACIGS system is narrowing toward GGI and AAC ¼ 1 (i.e., AgGaSe 2 ). [33] As a result, a high volume share of ordered vacancy compound (OVC) patches is observed, which significantly increases for AAC and GGI > 0.5. [7,31,34,35] While OVCs at the back contact very likely cause a kink in current-voltage (I-V ) characteristics and thus a low fill factor (FF), it is currently not clear whether their occurrence at the heterojunction is beneficial, detrimental, or benign. [11,31] Further studies are ongoing to investigate the effect of the OVC patches on the electrical and optical properties of the ACIGS devices.Recently, we revealed a clear anticorrelation between V OC and the short-circuit current density ( J SC ) with varying group-I stoichiometry for wide-gap ACIGS solar cells. [11] Perfect carrier collection (high J SC ) is only achieved for very close-stoichiometric compositions, due to a complet...

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