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.

Cu2ZnSn(S,Se)4 thin layers processed from solution-deposited earth-abundant precursors emerge as absorber materials for low-cost thin film solar cells. A frequently observed drawback of the chemical solution processingpoor crystallinity of the chalcogenide absorbercan be overcome by employing a sodium-containing reactive agent. We demonstrate a massive improvement in grain growth in the presence of sodium. It enhances the surface chemisorption of selenium molecules and can promote the formation of liquid Na2Se x phases during reactive annealing of the precursor. The sodium is also incorporated into the semiconductor absorber and significantly modifies its electronic properties. By adjusting the sodium precursor quantity, it is possible to tune doping levels and gradients to maximize the collection of photogenerated carriers in thin film Cu2ZnSn(S,Se)4 solar cells. The presented approach can be extended to other solution-processed metal chalcogenides to enhance their structural and electronic properties, which are critical for applications such as thin film solar cells and transistors.

and has yielded 10.3% effi cient solar cells with a V oc defi cit of 0.60 V [ 10 ] or recently, even up to 11.8% measured on active area. [ 11 ] Some commonly reported problems of the DMSO-processed kesterite layers are their high porosity, nonuniformity, and numerous grain boundaries that can lead to undesirable recombination. [ 12 ] Here, we employ a three-stage annealing process under controlled selenium atmosphere in an SiO x coated graphite box to drastically improve the grain size and morphology of the absorber layer. Importantly, the V oc defi cit can be reduced to 0.57 V, which appears to be one of the lowest values reported for kesterite devices. Systematic electrical characterization of absorbers and fi nished solar cells with time-resolved photoluminescence (TRPL), temperature-dependent currentvoltage measurements ( JV-T ), and admittance spectroscopy (AS) are used to identify the reasons of the improved voltage. Figure 1 shows the scanning electron microscopy (SEM) cross sections of four different Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorbers A-D yielding effi ciencies from 6.6% to 10.1% (total cell area of 0.3 cm 2 including metal grid lines). The annealing conditions are varied from uncoated graphite box (sample A,B) to SiO x -coated graphite box (sample C,D) and two-stage temperature gradient (sample A,C) to three-stage temperature gradient (sample B,D); temperature gradients are presented in Figure S1 (Supporting Information). The selenization of sample A was conducted in an uncoated graphite box employing a two-stage temperature gradient, and the absorber layer exhibits a distinct bilayer structure with a thick small-grain bottom layer. [ 13 ] Sample B was selenized in an uncoated graphite box similar to A but employing a three-stage temperature gradient. The SEM cross section shows an improved crystallization and grain size in both upper crust and bottom layer. However, the distinct bilayer structure of the absorber layer remains. The selenization of sample C was conducted in an SiO x -coated graphite box using the two-stage process, and the morphology of the fi lm exhibits a comparably thin upper layer with small grains, but an improved crystallization in the bottom layer in contrast to sample A. Finally, sample D was selenized in the SiO x -coated graphite box with the three-stage temperature gradient and shows an overall improved crystallization with large grains and a signifi cant reduction of the small-grain bottom layer. X-ray diffraction (XRD) pattern in Figure 2 b shows a double kesterite refl ex at 53.4° for all samples, indicating two regions with different S/(S + Se) ratio in the absorber layer. Grazing incidence XRD with varying incident angles confi rms that the region with lower S/(S + Se) ratio coincides with the upper crust and the region with higher S/(S + Se) ratio belongs to the small-grain bottom layer. The refl exes corresponding to the higher S/(S + Se) ratio extenuate with the shift from uncoated On the way towards a marketable and industrially-relevant photo voltaic technology, ke...

Attempts to improve the efficiency of kesterite solar cells by changing the intrinsic stoichiometry have not helped to boost the device efficiency beyond the current record of 12.6%. In this light, the addition of extrinsic elements to the Cu 2 ZnSn(S,Se) 4 matrix in various quantities has emerged as a popular topic aiming to ameliorate electronic properties of the solar cell absorbers. This article reviews extrinsic doping and alloying concepts for kesterite absorbers with the focus on those that do not alter the parent zinc-blende derived kesterite structure. The latest state-of-the-art of possible extrinsic elements is presented in the order of groups of the periodic table. The highest reported solar cell efficiencies for each extrinsic dopant are tabulated at the end. Several dopants like alkali elements and substitutional alloying with Ag, Cd or Ge have been shown to improve the device performance of kesterite solar cells as compared to the nominally undoped references, although it is often difficult to differentiate between pure electronic effects and other possible influences such as changes in the crystallization path, deviations in matrix composition and presence of alkali dopants coming from the substrates. The review is concluded with a suggestion to intensify efforts for identifying intrinsic defects that negatively affect electronic properties of the kesterite absorbers, and, if identified, to test extrinsic strategies that may compensate these defects. Characterization techniques must be developed and widely used to reliably access semiconductor absorber metrics such as the quasi-Fermi level splitting, defect concentration and their energetic position, and carrier lifetime in order to assist in search for effective doping/alloying strategies.

Core/shell CdSe/ZnS quantum dot fluorescence-blinking statistics depend strongly on excitation wavelength. Excitation on the band gap (575 nm) results in inverse-power law "on" time distributions. However, distributions resulting from excitation above the band gap (525 nm) require a truncated power law and are 100 times less likely to display 10-s fluorescence. "Off" time statistics are insensitive to the excitation wavelength. The results may be explained by nonemissive trap states accessed with the higher-photon excitation energies.