In this communication, we study the pure selenide phase CZTSe thin fi lm solar cells prepared by vacuum co-evaporation and demonstrate a power conversion effi ciency of 11.6%, which is the highest certifi ed effi ciency among CZTSe-based thin-fi lm solar cells reported to date. Device characterizations indicate that the higher effi ciencies appear to be related to signifi cantly enhanced transport properties for photogenerated carriers. The measured minority carrier diffusion length yields a record value of ≈2.1 µm, approaching that of state-of-the-art CIGS devices. The device also shows a record V OC -defi cit of 0.578 V, which can be attributed to reduced band tailing or the occurrence of shallower defect related states in the bandgap of the CZTSe layer. A comparative study of room-temperature photoluminescence (PL) of CZTSe and CZTS devices indicates that the PL emission peak from CZTSe is much closer in energy to the band edge than the case in CZTS. The improved device results are also consistent with the observation of a high-quality microstructure with large grains of ≈2.6 µm size.Polycrystalline CZTSe thin-fi lms with a thickness of ≈2.2 µm were deposited at 150 °C on 0.7-µm-thick Mo-coated soda-lime glass substrates using a vacuum co-evaporation technique, as described in earlier publications. [ 5,7 ] Knudsen-type elemental sources of Cu, Zn, and Sn, and a valved Se cracking source were used for the CZTSe co-evaporation. A ≈30-nm-thick NaF layer was deposited in a separate chamber prior to the CZTSe co-evaporation to promote grain coarsening of CZTSe thinfi lms. [ 16 ] Recrystallization and grain growth were carried out by annealing the co-evaporated CZTSe fi lm at ≈590 °C on a hot plate with excess selenium under N 2 atmosphere; similar to a process described elsewhere. [ 5,17,18 ] Inductively coupled plasma measurements showed a Cu-poor and Zn-rich elemental composition of the annealed fi lm: [Cu] Figure 1 a shows a θ -2 θ X-ray diffraction spectrum of the annealed CZTSe fi lm. The spectrum shows high crystallinity of the annealed fi lm with a major peak of (112)-orientation at ≈27.3°. The inset of Figure 1 a shows a top-view scanning electron microscopy (SEM) image of the annealed CZTSe fi lm, indicating dense and pinhole-free fi lm morphology. The average and standard deviation of grain size are determined to be 2.6 and 1.0 µm, respectively, estimated via digital image processing of SEM images.Using the co-evaporated CZTSe fi lms, solar cells with a total device area ( A ) of ≈0.43 cm 2 were fabricated with the following structure: MgF 2 /indium-tin-oxide (ITO)/ZnO/CdS/CZTSe/ Mo bottom electrode, as shown in Figure 1 b. The thicknesses Kesterite Cu 2 ZnSn( S x S e 1-x ) 4 (CZTSSe) has emerged as a promising candidate for scalable photovoltaic applications due to its earth-abundant elemental constituents and bandgap ( E g ) range between ≈1.0 eV ( x = 0) and ≈1.5 eV ( x = 1) with predicted theoretical maximum effi ciencies of over 30%. [ 1,2 ] To date, among thin-fi lm solar cells based on the kesteri...

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Weak H(2) physisorption energies present a significant challenge to even the best correlated theoretical many-body methods. We use the phaseless auxiliary-field quantum Monte Carlo method to accurately predict the binding energy of Ca(+)-4H(2). Attention has recently focused on this model chemistry to test the reliability of electronic structure methods for H(2) binding on dispersed alkaline earth metal centers. A modified Cholesky decomposition is implemented to realize the Hubbard-Stratonovich transformation efficiently with large Gaussian basis sets. We employ the largest correlation-consistent Gaussian type basis sets available, up to cc-pCV5Z for Ca, to accurately extrapolate to the complete basis limit. The calculated potential energy curve exhibits binding with a double-well structure.

Many recent calculations have been performed to study a Co atom adsorbed on graphene, with significantly varying results on the nature of the bonding. We use auxiliary-field quantum Monte Carlo (AFQMC) and a size-correction embedding scheme to accurately calculate the binding energy of Co on graphene. We find that as a function of the distance h between the Co atom and the six-fold hollow site, there are three distinct ground states corresponding to three electronic configurations of the Co atom. Two of these states provide binding and exhibit a double-well feature with nearly equal binding energy of 0.4 eV at h = 1.51 and h = 1.65 Å, corresponding to low-spin 2 Co (3d 9 4s 0 ) and high-spin 4 Co (3d 8 4s 1 ), respectively.