We show that the Cooperative Free Volume (CFV) rate model, successful at modeling pressure-dependent dynamics, can be employed to describe the temperature and thickness dependence of the segmental time of polymers confined in thin films (1D confinement). The CFV model is based on an activation free energy that increases with the number of cooperating segments, which is determined by the system's free volume. Here, we apply the CFV model to new experimental results on the segmental relaxation of 1D confined poly(4chlorostyrene), P4ClS, and find remarkable agreement over the whole temperature and thickness ranges investigated. This work further validates the robustness of the CFV model, which relates the effects of confinement on dynamics to pressure changes in the bulk, and supports the idea that confinement effects originate from local perturbations in density.

The power conversion efficiency boost of Cu(In,Ga)Se2 in the past years has been possible due to the incorporation of heavy alkali atoms. Their addition through postdeposition treatments results in an improvement of the open‐circuit voltage, the origin of which has been associated with grain boundaries. Herein, the effect of potassium fluoride postdeposition treatments on the optoelectronic properties of a series of sodium‐free Cu(In,Ga)Se2 single crystals with varying Cu and Ga content is discussed. Results suggest that improvement of the quasi‐Fermi level splitting can be achieved in the absence of grain boundaries, being greater in low‐gallium Cu‐poor absorbers. Secondary ion mass spectrometry reveals the presence of potassium inside the bulk of the films, suggesting that transport of potassium can occur through grain interiors. In addition, a type inversion from n to p in potassium fluoride‐treated low‐gallium Cu(In,Ga)Se2 is observed, which along with study of the carrier lifetime demonstrates that potassium can act as a dopant. The fact that potassium on its own can alter the optoelectronic properties of Cu(In,Ga)Se2 single crystals demonstrates that the effect of postdeposition treatments goes beyond grain boundary passivation.

Thin film semiconductors grown using chemical bath methods produce large amounts of waste solvent and chemicals that then require costly waste processing. We replace the toxic chemical bath deposited CdS buffer layer from our Cu­(In,Ga)­(S,Se)2 (CIGS)-based solar cells with a benign inkjet-printed and annealed Zn­(O,S) layer using 230 000 times less solvent and 64 000 times less chemicals. The wetting and final thickness of the Zn­(O,S) layer on the CIGS is controlled by a UV ozone treatment and the drop spacing, whereas the annealing temperature and atmosphere determine the final chemical composition and band gap. The best solar cell using a Zn­(O,S) air-annealed layer had an efficiency of 11%, which is similar to the best conventional CdS buffer layer device fabricated in the same batch. Improving the Zn­(O,S) wetting and annealing conditions resulted in the best device efficiency of 13.5%, showing the potential of this method.

We investigated the impact of sample preparation on the glassy dynamics of thin films of poly(4chlorostyrene), a polymer whose molecular mobility is particularly sensitive to changes in the specific volume. Samples were obtained by spincoating, the technique most commonly used to prepare thin organic layers, which consists of pouring dilute polymer solutions onto a plate rotating at a high rate. Our experimental results demonstrate that filtering the solutions before spincoating affects the value of the segmental relaxation time of the as-prepared films. Thin polymer layers obtained via filtered solutions show accelerated segmental dynamics upon confinement at the nanoscale level, once below 100 nm, while the samples obtained via unfiltered solutions exhibit bulk-like dynamics down to 15-20 nm. We analyzed these results by means of the cooperative free volume rate model, considering a larger free volume content in thin films obtained via filtered solutions. The validity of the model predictions was finally verified by measurements of irreversible adsorption, confirming a larger adsorbed amount, corresponding to a higher specific volume, in the case of samples obtained via unfiltered solutions. Our results prove that filtering is a crucial step in the preparation of thin films, and it could be used to switch on and off nanoconfinement effects.

In-depth understanding and subsequent optimization of the contact layers in thin film solar cells are of high importance in order to reduce the amount of nonradiative recombination and thereby improve device performance. In this work, the buried MoSe 2 /CuInSe 2 interface of stoichiometric absorbers is investigated with scanning tunneling spectroscopy and Kelvin probe force microscopy combined with compositional measurements acquired via photo-electron spectroscopy after a mechanical lift-off process. We find that the local density of states, as measured with scanning tunneling spectroscopy, is similar to the front-side of ultra-high vacuum annealed CISe absorbers. The grain boundaries exhibit a weak upward band bending, opposite to Cu-poor CuGaSe 2 , and we measure an increased Cu accumulation at the rear CISe surface compared to the bulk composition and a non-zero concentration of Cu on the Mo-side. Grazing incidence X-ray diffraction measurements corroborate that a small amount of a Cu x Se secondary phase is present at the MoSe 2 /CuInSe 2 interface in contrast to reports on Cu-poor material. Our findings shed new light into the complex interface formation of CuInSe 2 -based thin film solar cells grown under Cu-rich conditions. KEYWORDSCuInSe 2 , Kelvin probe force microscopy, scanning tunneling microscopy INTRODUCTIONCu(In,Ga)Se 2 (CIGSe) thin film solar cells offer high power conversion efficiencies (PCE), 1 low energy payback time, and long-term stability. 2Several major breakthroughs in the last decades allowed this material system to surpass the 20% efficiency benchmark on rigid and flexible substrates 1,3 and PCEs as high as 23.35% were reported. 1 One of the important steps that allowed for high PCE was the introduction of a Gallium back-gradient, which effectively reduced the recombination at the Mo/MoSe 2 back-contact. 4,5 The optimization of this interface [Correction added on 11 January 2021, after first online publication: surname of 'Harry Mönig' has been corrected in this version.]is still an area of intensive research due to the following reasons: The Molybdenum back-contact interface has a poor optical reflectivity and the recombination velocity is high, 6 which means that effective passivation strategies are indispensable. This is especially true for ultra-thin devices 7 where bandgap grading is not feasible.Pure CuInSe 2 (CISe) solar cells have several advantages compared to CIGSe such as an easier manufacturing process and a lower bandgap, which makes this material more attractive for tandem applications.However, the PCE was stuck for a long time at 15% 8 without post deposition treatment. Recently, KF treatment improved this number to 16%, 9 which is still far away from the record CIGSe absorbers. It was shown that the likely reason for the lower CISe solar cell performanceThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.