A NANOWIRE HARD X-RAY DETECTOR AT ITS LIMITS An axial p-n junction GaAs nanowire X-ray detector that enables ultrahigh spatial resolution (~200 nm) was examined via X-ray analytical techniques based on a focused synchrotron X-ray nanobeam. By probing the internal electrical field, hot electron effects at the nanoscale were observed, inducing a selective oxidisation in the n-doped segment of the p-n junction.

X-RAY NANOPROBE

88 ESRF

Revealing the origin of the beneficial effect of cesium in highly efficient Cu(In,Ga)Se2 solar cells; P. Schöppe (a), S. Schönherr (a), M. Chugh (b), H. Mirhosseini (b), P. Jackson (c), R. Wuerz (c), M. Ritzer (a), A. Johannes (d), G. Martínez-Criado (d,e), W. Wisniewski (f), T. Schwarz (g), C.T. Plass (a), M. Hafermann (a), T.D. Kühne (b), C.S. Schnohr (a,h) and C. Ronning (a), Nano Energy 71, 104622

(2020); https://doi.org/10.1016/j. nanoen.2020.104622. (a) Institut für Festkörperphysik, Friedrich- Schiller-Universität Jena (Germany) (b) Dynamics of Condensed Matter and Center for Sustainable Systems Design, University of Paderborn, (Germany) (c) Zentrum für Sonnenenergie und Wasserstoff-Forschung Baden- Württemberg, Stuttgart (Germany)

(d) ESRF (e) Instituto de Ciencia de Materiales de Madrid (CSIC) (Spain) (f) Otto-Schott-Institut für Materialforschung, Friedrich-Schiller- Universität Jena (Germany) (g) Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf (Germany) (h) Felix-Bloch-Institut für Festkörperphysik, Universität Leipzig (Germany)

PRINCIPAL PUBLICATION AND AUTHORS

The distribution of Cs was measured via nano- XRF using a highly focused hard X-ray nanobeam, and the obtained and corresponding Cs intensity map is depicted in Figure 71c. Clearly, the Cs distribution is not homogeneous throughout the absorber layer and Cs accumulates at random grain boundaries, where the adjacent grains also show a reduced Cu concentration and increased In and Se concentrations. On the other hand, no accumulation of caesium was found at S3 twin boundaries or the grain interior. Furthermore, an accumulation of Cs was clearly detected at the p-n junction of the solar cell along with variations in the local CIGS composition, indicating the formation of a beneficial secondary phase with a laterally inhomogeneous distribution.

These experimental findings are in excellent agreement with complementary ab-initio calculations, where the formation energies of Cs point defects were calculated for a variety of different grain boundaries. The inset of Figure 71d shows one example, where a Cs atom forms an interstitial at a grain boundary. The respective calculated density of states, shown in Figure 71d, demonstrates that these grain boundaries are clearly passivated by the presence of Cs and no states occur in the band gap. Finally, it is unlikely that Cs with its large ionic radius is incorporated into the CIGS grains, where it would cause detrimental defects.

Nanowire chip-based electrical and optical devices such as biochemical sensors, physical detectors or light emitters combine outstanding functionality with a small footprint, reducing expensive material and energy consumption. The core functionality of many nanowire-based devices such as LEDs, solar cells and detectors is given by their p-n junctions. To fully unleash their potential, such nanowire-based devices require besides a high performance stability

and reliability. However, strong device-to- device variations, huge surface-to-volume ratios, and reduced thermal conductivity limit device stability and make nanowires extremely sensitive to environmental changes.

Using a synchrotron X-ray nanobeam at ID16B, a single GaAs nanowire axial p-n junction working as a hard X-ray detector was thoroughly analysed with ultrahigh spatial resolution.

Fig. 72: a) X-ray beam-induced current (XBIC) signal at 0 V bias voltage measured with an incident X-ray

energy of 11.9 keV (i.e., above the Ga and As K-edges). The GaAs nanowire (as located in the Ga XRF map) is

indicated by the dashed, white lines. b) Edge region of the XANES spectra measured around the Ga K-edge

at the p-doped (blue) and the n-doped (orange) part of the GaAs nanowire at 0 V (light colours) and -5 V

(dark colours), respectively. References for GaAs and Ga in octahedral coordination (Ga(AcAc)3 are added as dashed and dotted lines [1,2]. A clear shift in the

absorption edge can be observed in the n-doped region for the measurement at -5 V.