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.

Solar cells based on polycrystalline Cu(In,Ga)Se(2) absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry.

Roll-to-roll manufacturing of CdTe solar cells on flexible metal foil substrates is one of the most attractive options for low-cost photovoltaic module production. However, various efforts to grow CdTe solar cells on metal foil have resulted in low efficiencies. This is caused by the fact that the conventional device structure must be inverted, which imposes severe restrictions on device processing and consequently limits the electronic quality of the CdTe layer. Here we introduce an innovative concept for the controlled doping of the CdTe layer in the inverted device structure by means of evaporation of sub-monolayer amounts of Cu and subsequent annealing, which enables breakthrough efficiencies up to 13.6%. For the first time, CdTe solar cells on metal foil exceed the 10% efficiency threshold for industrialization. The controlled doping of CdTe with Cu leads to increased hole density, enhanced carrier lifetime and improved carrier collection in the solar cell. Our results offer new research directions for solving persistent challenges of CdTe photovoltaics.

Concepts of localized contacts and junctions through surface passivation layers are already advantageously applied in Si wafer-based photovoltaic technologies. For Cu(In,Ga)Se2 thin film solar cells, such concepts are generally not applied, especially at the heterojunction, because of the lack of a simple method yielding features with the required size and distribution. Here, we show a novel, innovative surface nanopatterning approach to form homogeneously distributed nanostructures (<30 nm) on the faceted, rough surface of polycrystalline chalcogenide thin films. The method, based on selective dissolution of self-assembled and well-defined alkali condensates in water, opens up new research opportunities toward development of thin film solar cells with enhanced efficiency.

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...