Most everyday surfaces are randomly rough and self-similar on sufficiently small scales. We investigated the tactile perception of randomly rough surfaces using 3D-printed samples, where the topographic structure and the statistical properties of scale-dependent roughness were varied independently. We found that the tactile perception of similarity between surfaces was dominated by the statistical micro-scale roughness rather than by their topographic resemblance. Participants were able to notice differences in the Hurst roughness exponent of 0.2, or a difference in surface curvature of 0.8 $$\hbox {mm}^{-1}$$
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for surfaces with curvatures between 1 and 3 $$\hbox {mm}^{-1}$$
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. In contrast, visual perception of similarity between color-coded images of the surface height was dominated by their topographic resemblance. We conclude that vibration cues from roughness at the length scale of the finger ridge distance distract the participants from including the topography into the judgement of similarity. The interaction between surface asperities and fingertip skin led to higher friction for higher micro-scale roughness. Individual friction data allowed us to construct a psychometric curve which relates similarity decisions to differences in friction. Participants noticed differences in the friction coefficient as small as 0.035 for samples with friction coefficients between 0.34 and 0.45.
The detrimental effect of tail states on the radiative and non‐radiative voltage loss has been demonstrated to be a limiting factor for the open circuit voltage (VOC) in Cu(In,Ga)Se2 solar cells. A strategy that has proven effective in reducing tail states is the addition of alkali metals, the effect of which has been associated with the passivation of charged defects at grain boundaries. Herein, tail states in Cu(In,Ga)Se2 are revisited by studying the effect of compositional variations and alkali incorporation into single‐crystal films. The results demonstrate that sodium and potassium decrease the density of tail states despite the absence of grain boundaries, suggesting that there is more to alkalis than just grain boundary effects. Moreover, an increase in doping as a result of sodium or potassium incorporation is shown to contribute to the reduced tail states, which are demonstrated to arise largely from electrostatic potential fluctuations and to be determined by grain interior properties. By analyzing the voltage loss in high‐efficiency polycrystalline and single crystalline devices, this work presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se2 based on the combined effect of doping on tail states and VOC.
Absolute photoluminescence measurements present a tool to predict the quality of photovolatic absorber materials before finishing the solar cells. Quasi-Fermi level splitting predicts the maximal open circuit voltage. However, various...
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