The first-principles calculations based on density functional theory combined with cluster expansion techniques and Monte Carlo （MC） simulations were used to study the phase diagrams of both wurtzite （WZ） and zinc-blende （ZB） Cdl_xZnxS alloys. All formation energies are positive for WZ and ZB Cdl-xZnxS alloys, which means that the Cdl-xZnxS alloys are unstable and have a tendency to phase separation. For WZ and ZB Cdl_xZnxS alloys, the consolute temperatures are 655 K and 604 K, respectively, and they both have an asymmetric miscibility gap. We obtained the spatial distributions of Cd and Zn atoms in WZ and ZB Cd0.sZn0.sS alloys at different temperatures by MC simulations. We found that both WZ and ZB phases of Cdo.sZn0.sS alloy exhibit phase segregation of Cd and Zn atoms at low temperature, which is consistent with the phase diagrams.
We built an ideal perovskite solar cell model and investigated the effects of defect states on the so- lar cell＇s performance. The verities of defect states with a different energy level in the band gap and those in the absorption layer CH3NH3PbI3 （MAPbI3）, the interface between the buffer layer/MAPbI3, and the interface be- tween the hole transport material （HTM） and MAPbI3, were studied. We have quantitatively analyzed these effects on perovskite solar cells＇ performance parameters. They are open-circuit voltage, short-circuit current, fill factor, and photoelectric conversion efficiency. We found that the performances of perovskite solar cells change worse with defect state density increasing, but when defect state density is lower than 1016 cm^-3, the effects are small. Defect states in the absorption layer have much larger effects than those in the adjacent interface layers. The per-ovskite solar cells have better performance as its working temperature is reduced. When the thickness of MAPbI3 is about 0.3μm, perovskite solar cells show better comprehensive performance, while the thickness 0.05μm for Spiro-OMeTAD is enough.
Device modeling has been carried out to investigate the effects of defect states on the performance of ideal CulnGaSe2 （CIGS） thin film solar cells theoretically. The varieties of defect states （location in the band gap and densities） in absorption layer CIGS and in buffer layer CdS were examined. The performance parameters： open-circuit voltage, short-circuit current, fill factor, and photoelectric conversion efficiency for different defect states were quantitatively analyzed. We found that defect states always harm the performance of CIGS solar cells, but when defect state density is less than 10 14 cm-3 in CIGS or less than 10 18 cm-3 in CdS, defect states have little effect on the performances. When defect states are located in the middle of the band gap, they are more harmful. The effects of temperature and thickness are also considered. We found that CIGS solar cells have optimal performance at about 170 K and 2 μm of CIGS is enough for solar light absorption.
Using the first-principles plane-wave calculations within density functional theory, the perfect bi-layer and monolayer terminated WZ-CIS （100）/WZ-CdS （100） interfaces are investigated. After relaxation the atomic positions and the bond lengths change slightly on the two interfaces. The WZ-CIS/WZ-CdS interfaces can exist stably, when the interface bonding energies are -0.481 J/m2 （bi-layer terminated interface） and -0.677 J/m2 （monolayer terminated interface）. Via analysis of the density of states, difference charge density and Bader charges, no interface state is found near the Fermi level. The stronger adhesion of the monolayer terminated interface is attributed to more electron transformations and orbital hybridizations, promoting stable interfacial bonds between atoms than those on a bi-layer terminated interface.
