Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20%. This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance. Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters. Ion exchange processes, well known in other research areas, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.

Transparent conductive oxides (TCO) are a unique class of materials exhibiting optical transparency combined with metallike electrical conductivity and thus are of utmost importance for the rapidly expanding fi elds of transparent electronics and sustainable energy generation. [ 1a-e ] For most applications the standard compound is ITO exhibiting best optoelectronic performance amongst all TCO materials but to ensure a sustainable supply of such materials earth abundant and inexpensive alternatives such as aluminum doped zinc oxide (AZO) are crucial. [ 2 ] In fact this is also refl ected in predicted markets of almost $1 Billion in 2016 for alternative TCOs. [ 3 ] Nowadays, plasma based (magnetron-sputtering), [ 4a ] pulsed laser deposition (PLD), [ 4b ] atomic layer deposition(ALD) [ 4c,d ] or chemical vapor deposition (CVD) [ 4e ] methods are employed on industrial scale to obtain high quality AZO thin fi lms with resistivity of 10 −3 -10 −4 Ω cm and visible transparency > 90%, although instrumental complexity poses high investment costs as well as limits scalability. In this respect low cost non-vacuum methods for AZO thin fi lms are of immense interest.A variety of solution based approaches have been described but to obtain good optoelectronic properties comparable to vacuum deposition techniques, high temperature annealing (300-600 ° C) preferably in vacuum (10 −1 -10 −4 mbar), and for long time (60-90 min) are necessary. Some approaches employ fl ammable or toxic organic solvents (e.g. 2-methoxyethanol), [ 5a-c ] and in the case of electrodeposition conductive substrates are inevitable. [ 5d ] The most straightforward yet challenging approach for deposition of ZnO thin fi lms is aqueous solution growth [ 6 ] (e.g., chemical bath deposition or hydrothermal synthesis) on seeded substrates as illustrated in Figure 1 a. The solution chemistry comprises a water soluble zinc salt and a complexant (usually ammonium salts, ethanolamine, or NH 4 OH) which is also used for adjusting the pH to a basic regime. The crystal growth is associated with the decreasing thermodynamic stability of the zinc complex leading to controlled supersaturation and the retrograde solubility of ZnO upon increased temperature. [ 7 ] Thus, phase pure ZnO thin fi lms can be obtained already at temperatures of 60 ° C. The use of water as solvent makes the method environmental friendly and due to the use of inexpensive chemicals -as mostly water soluble metal salts are employed -it can also be designated as low cost. A number of approaches have been reported to obtain intrinsic, undoped ZnO in the form of nanorod, nanoneedle, or nanopillar thin fi lms [ 8a ] using aqueous solution deposition, but only a few studies attempt to obtain conductive AZO thin fi lms. [ 8b,c ] However, achieving a compact, dense, and highly conductive (<100 Ω /sq) AZO thin fi lm at low process temperatures has not been successful yet, although for most of the applications high conductivity is essential.To overcome this bottleneck, we present a new concept to...

The performance characteristics of AlGaN-based deep ultraviolet light emitting diodes (UV-LEDs) grown by metalorganic vapor phase epitaxy on sputtered and high temperature annealed AlN/sapphire templates are investigated and compared with LEDs grown on epitaxially laterally overgrown (ELO) AlN/sapphire. The structural and electro-optical properties of the devices on 350 nm sputtered and high temperature annealed AlN/sapphire show similar defect densities and output power levels as LEDs grown on low defect density ELO AlN/sapphire templates. After high temperature annealing of the 350 nm sputtered AlN, the full widths at half maximum of the (0002) and (101¯2) reflections of the high resolution x-ray diffraction rocking curves decrease by one order of magnitude to 65 arc sec and 240 arc sec, respectively. The curvature of the sputtered and HTA AlN/sapphire templates after regrowth with 400 nm MOVPE AlN is with −80 km−1 much lower than the curvature of the ELO AlN/sapphire template of −160 km−1. The on-wafer measured output powers of 268 nm LEDs grown on 350 nm sputtered and high temperature annealed AlN/sapphire templates and ELO AlN/sapphire templates were 0.70 mW and 0.72 mW at 20 mA, respectively (corresponding to an external quantum efficiency of 0.75% and 0.78%). These results show that sputtered and high temperature annealed AlN/sapphire provide a viable approach for the fabrication of efficient UVC-LEDs with reduced complexity and thus reduced costs.

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