2, Laboratory of Thin Film Chemical Technologies, Tallinn University of Technology, Tallinn, , Estonia
ZnO nanowire (NW) arrays have emerged as promising building blocks for a wide variety of optoelectronic and photovoltaic devices, including the extremely thin absorber (ETA) solar cells. In this novel architecture, a direct p-type semiconductor absorber is typically deposited on a ZnO NW array to form core-shell p-n heterojunctions following the type-II band alignment. Increasing interest has been dedicated to this radial architecture owing to efficient light trapping and charge carrier management together with the use of a low amount of materials.1
Antimony trisulfide (Sb2S3) is a p-type semiconductor with a 1.7 eV band gap energy and a high absorption coefficient that has been integrated into mesoporous-TiO2-based dye-sensitized solar cells, showing a power conversion efficiency (PCE) as high as 7.5%.2 It is usually grown by low-cost, low-temperature chemical deposition techniques, which still make its combination with ZnO NWs difficult owing to their instability in acidic conditions.
In this work, the crystallization process of Sb2S3 thin films is investigated by in situ x-ray diffraction and in situ Raman spectroscopy, revealing the intermediate formation of a metallic antimony phase and showing the optimal annealing temperature of 270°C.3 Furthermore, an 8 nm-thick TiO2 protective layer is grown by atomic layer deposition onto ZnO NW arrays grown by chemical bath deposition. Sb2S3, as an absorbing shell, is subsequently deposited by chemical spray pyrolysis. The Sb2S3 10 nm-conformal shell with high crystalline quality covers the ZnO/TiO2 NW arrays from the bottom to the top. The photovoltaic performance of the ZnO/TiO2/Sb2S3 core shell NW heterostructures using P3HT as hole transporting material results in a promising PCE of 2.3% with a high Jsc of 7.5 mA/cm2, when considering that the Sb2S3 shell is 10 nm-thick, and a high Voc of 656 mV.4 The use of low-cost, surface scalable chemical deposition techniques for the fabrication of the whole ZnO/TiO2/Sb2S3 structure opens the way for improving the performances of ZnO NW-based ETA solar cells.
1E.C. Garnett et al. Annual Review of Materials Research 41 (2011), 269-295
2Y.C. Choi et al. Adv. Funct. Mater. 24 (2014), 3587
3R. Parize et al. Materials & Design 121 (2017), 1-10
4R. Parize et al. The Journal of Physical Chemistry C 121 (2017), 9672-9680