1) Collective Behavior of Molecular Dipoles in CH3NH3PbI3
C. Goehry, G. A. Nemnes and A. Manolescu; J. Phys. Chem. C 2015, 119, 19674−19680
ABSTRACT: Using ab initio molecular dynamics, we report a detailed exploration of the thermal motion occurring in perovskite crystals of the formula CH3NH3PbI3. We exploit the data generated to obtain estimates of the rotational relaxation time of the cation CH3NH3+. We examine the tetragonal and cubic phases, as both may be present under operational conditions. Influenced by each other, and by the tilting of PbI6 octahedra, cations undergo collective motion as their contribution to polarization does not vanish. We thereby qualitatively describe the modus operandi of formation of microscopic ferroelectric domains.
2) Optimization of halide perovskite solar cells based on nanocolumnar ZnO
Adela Nicolaev, T.L.Mitran, Sorin Iftimie, G.A.Nemnes; Solar Energy Materials and SolarCells (2015)
ABSTRACT: Halide perovskite solar cells based on nanocolumnar ZnO are considered alternative candidates to the more establishedTiO2 based devices. Replacing the scaffold oxide layer by ZnO has the benefit of a larger electron mobility and lower processing temperatures, still reaching power conversion efficiencies (PCEs) of over 15%. The perovskite layer stability and improving the PCE are currently two major issues under investigation. Regarding the latter, the presence of an interfacial layer of Al-doped ZnO (AZO) gives an enhanced power out put, as it was recently reported. Using density functional theory calculations we discuss the band alignment at the ZnO(AZO)-halide perovskite interface, indicating one possible source for the enhanced PCE.
3) Band alignment and charge transfer in rutile-TiO2/CH3NH3PbI3xClx interfaces
G. A. Nemnes, C. Goehry, T. L. Mitran, Adela Nicolaev, L. Ion, S. Antohe, N. Plugaru and A. Manolescu; Phys. Chem. Chem. Phys., 2015, 17, 30417–30423
ABSTRACT: Rutile-TiO2/hybrid halide perovskite CH3NH3PbI3xClx interfaces are investigated by ab initio density functional theory calculations. The role of chlorine in achieving enhanced solar cell power conversion efficiencies is in the focus of recent studies, which point to increased carrier mobilities, reduced recombination rates, a driven morphology evolution of the perovskite layer and improved carrier transport across the interface. As it was recently established that chlorine is preferentially localized in the vicinity of the interface and not in the bulk of the perovskite layer, we analyze the changes introduced in the electronic properties by varying the chlorine concentration near the interface. In particular, we discuss the effects introduced in the electronic band structure and show the role of chlorine in the enhanced electron injection into the rutile-TiO2 layer. Taking into account these implications, we discuss the conditions for optimizing the solar cell efficiency in terms of interfacial chlorine concentration.
4) Charge localization effects and transport in dendritic nanostructuresfor photovoltaic applications
G.A. Nemnes, Sorina Iftimie, Applied Surface Science, 2015 352,158–162
ABSTRACT: Charge localization effects and transport properties in dendritic interfaces are investigated in the frame-work of a coherent scattering formalism. Due to the large surface area, tree shaped interfaces enhancethe overall efficiency of bulk heterojunction photovoltaic applications. The charge localization effectsare analyzed for different tree shapes, band offset potentials and in the context of varying the thicknessof the tree branches. Two types of localization are pointed out – on the tree branches or inbetween.The transfer characteristics are influenced by the band offsets and the energy of the charged particles.A detailed statistical analysis shows the correlation between the average transmission function and thesurface area of the dendritic interface. The current study provides a framework for the characterizationand optimization of dendritic interfaces.
5) Transparent boundary conditions for time-dependent electron transport in the R-matrix method with applications to nanostructured interfaces
G.A. Nemnes, Alexandra Palici, A. Manolescu, Computer Physics Communications, 2016, 208, 109–116
ABSTRACT: Transparent boundary conditions for the time-dependent Schrödinger equation are implemented using the R-matrix method. The employed scattering formalism is suitable for describing open quantum systems and provides the framework for the time-dependent coherent transport. Transmission and reflection of wave functions at the edges of a finite quantum system are essential for an accurate and efficient description of the time-dependent processes on large time scales. We detail the computational method and point out the numerical advantages stemming from the open system approach based on the R-matrix formalism. The approach is used here to describe time-dependent transport across nanostructured interfaces relevant for photovoltaic applications.
6) Iodine Migration and Degradation of Perovskite Solar Cells Enhanced by Metallic Electrodes
C. Besleaga, L.E. Abramiuc,V. Stancu, A.G. Tomulescu, M. Sima,L. Trinca, N. Plugaru, L. Pintilie, G.A. Nemnes, M. Iliescu, H.G. Svavarsson, A. Manolescu, I. Pintilie, J. Phys. Chem. Lett. 2016, 7, 5168−5175
ABSTRACT: We monitored the evolution in time of pinhole-free structures based on TO/TiO2/CH3NH3PbI2.6Cl0.4 layers, with and without spiro-OMeTAD and counter electrodes (Ag, Mo/Ag, and Au), aged at 24 °C in a dark nitrogen atmosphere. In the absence of electrodes, no degradation occurs. While devices with Au show only a 10% drop in power conversion efficiency, remaining stable after a further overheating at 70 °C, >90% is lost when using Ag, with the process being slower for Mo/Ag. We demonstrate that iodine is dislocated by the electric field between the electrodes, and this is an intrinsic cause for electromigration of I− from the perovskite until it reaches the anode. The iodine exhaustion in the perovskite layer is produced when using Ag electrodes, and AgI is formed. We hypothesize that in the presence of Au the iodine migration is limited due to the buildup of I− negative space charge accumulated at the perovskite−OMeTAD interface.
7) Dynamic electrical behavior of halide perovskite based solar cells
G.A. Nemnes, C. Besleaga, A.G. Tomulescu, I. Pintilie, L. Pintilie, K. Torfason, A. Manolescu, Solar EnergyMaterials&SolarCells 2017, 159 ,197–203
ABSTRACT: A dynamic electrical model is introduced to investigate the hysteretic effects in the J-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured J-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current overshoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hys- teresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it be- comes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions.