By combining ultraviolet and x-ray photoelectron spectroscopy with inverse photoemission spectroscopy, we find that the conduction-band alignment at the CdS/CuInSe2 thin-film solar- cell heterojunction is flat (0.0+/-0.2 eV). Furthermore, we observe a valence-band offset of 0.8+/-0.2 eV. The electronic level alignment is dominated by (1) an unusually large surface band gap of the CuInSe2 thin film (1.4 eV), (2) by a reduced surface band gap of the CdS overlayer (2.2 eV) due to intermixing effects, and (3) by a general influence of the intermixing on the chemical state near the interface

Room-temperature recombination dynamics has been investigated in a large set of different Cu(In,Ga)Se2 absorber films and compared to the electrical device characteristics of the respective solar cell modules. For a given cell preparation process, a characteristic relation between the low-injection minority-carrier lifetime of the absorber layers and the conversion efficiency of the solar cells is observed: Long lifetimes correlate with high open circuit voltages and conversion efficiencies, while no significant influence of the lifetime on the short circuit current is found.

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A combination of x-ray emission spectroscopy and x-ray photoelectron spectroscopy using high brightness synchrotron radiation has been employed to investigate the electronic and chemical structure of the buried CdS/Cu(In, Ga)Se2 interface, which is the active interface in highly efficient thin film solar cells. In contrast to the conventional model of an abrupt interface, intermixing processes involving the elements S, Se, and In have been identified. The results shed light on the electronic structure and interface formation processes of semiconductor heterojunctions and demonstrate a powerful tool for investigating buried interfaces in general.

The optical and vibrational properties of the quaternary chalcopyrite semiconductor alloy Ag x Cu 1x GaS 2

X-ray and UV photoelectron spectroscopy measurements of Cu(InGa)Se2 thin films grown on Mo coated soda-lime glass show segregation from the substrate and formation of two different Na species. One of these species is also identified after deliberate deposition of metallic Na. Moreover, the adsorption (or segregation) of this species reduces the native oxide SeO2, while the other, reacted Na species coexists with SeO2. Small amounts of Na (≤0.05 Å) induce a band bending and reduce the surface dipole. These findings reveal a positive influence of segregated Na on the morphology and electrical characteristics of Na-enriched films, improving the overall performance of the solar cells.

The laboratory performance of CIGS (Cu(In,Ga)Se 2) based solar cells (20.8% efficiency) makes them promising candidate photovoltaic devices. However, there remains little understanding of how defects at the CIGS/CdS interface affect the band offsets and interfacial energies, and hence the performance of manufactured devices. To determine these relationships, we use density functional theory with the B3PW91 hybrid functional that we validate to provide very accurate descriptions of the band gaps and band offsets. This confirms the weak dependence of band offsets on surface orientation observed experimentally. We predict that the conduction band offset (CBO) of perfect CuInSe 2 /CdS interface is large, 0.79 eV, which would dramatically degrade performance. Moreover we show that band gap widening induced by Ga adjusts only the valence band offset, and we find that Cd impurities do not significantly affect the CBO. Thus we show that Cu vacancies at the interface play the key role in enabling the tunability of CBO. We predict that Na further improves the CBO through electrostatically elevating the valence levels to decrease the CBO, explaining the observed essential role of Na for high performance. Moreover we find that K leads to a dramatic decrease in the CBO to 0.05 eV, much better than Na. We suggest that the efficiency of CIGS devices might be improved substantially by tuning the ratio of Na to K, with the improved phase stability of Na balancing phase instability from K. All these defects reduce interfacial stability slightly, but not significantly.

A characteristic, reversible metastability is observed for Cu(In,Ga)Se2 thin films and ZnO/CdS/Cu(In,Ga)Se2 heterojunctions. Annealing at 80 °C leads to a decrease of the dark conductivity of the thin films by up to a factor of 2 at room temperature and several orders of magnitude when measured at lower temperatures. By exposure to light, the initial state can be re-established. This reenhancement of the dark conductivity can be looked at as persistent photoconductivity. Admittance measurements at Cu(In,Ga)Se2 heterojunction solar cells display a reversible shift of the activation energy of a distinct dielectric loss peak ranging from 70 to about 160 meV upon illumination or annealing at 80 °C, respectively. We propose that both phenomena as well as commonly observed light-soaking effects of Cu(In,Ga)Se2 solar cells have a common origin.