Pohl, Johan (2013)
Structure and properties of defects in photovoltaics absorber material: Atomic scale computer simulations of Si and Cu(In,Ga)Se2.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Solar cell technology is becoming a viable alternative to fossil fuels. The main challenge remains to deliver electricity at grid parity. To achieve this goal increasing the efficiency of solar cells remains the top priority. Most of the solar cells on the market are still based on silicon wafers. Cu(In,Ga)Se2 thin-film technology, however, is becoming one of the main competitors with substantial advantage through reduced material and energy consumption in the production process. Knowledge-based improvement of the silicon as well as the Cu(In,Ga)Se2 absorber material requires a better understanding of the material at the atomic scale. For this purpose, atomistic simulations are a useful approach to gain an understanding of properties and processes in the absorber material, which are hard or even impossible to access experimentally.

The scope of this thesis is to investigate the properties of intrinsic point defects in Cu(In,Ga)Se2 and interface-related defect formation processes in silicon grown from the melt. For this purpose, various atomic-scale simulations methods are employed, ranging from ab-initio methods such as screened-exchange hybrid density functional theory to molecular dynamics simulation employing classical interatomic potentials and Lattice Monte Carlo techniques. This choice of methods allows to access the relevant system sizes and time scales relevant to the chosen problems. The detailed characterization of the intrinsic point defects in CuInSe2 and CuGaSe2 based on screened-exchange hybrid density functional theory yields a complete and consistent picture of the defect thermodynamics and the electronic properties of all relevant defects. Most importantly, copper self-diffusion is found to be mediated by both copper vacancy as well as by the interstitial and interstitialcy mechanisms. The interstitial mechanism has a particularly low migration barrier and is a likely source of fast electric field-enhanced diffusion as measured in some experiments. CuIn and CuGa antisites are found to act as a hole traps in both CuInSe2 and CuGaSe2 and are assigned to the N2 level, a prominent signal in admittance spectroscopy measurements in many samples. It is most likely also the source of a second deeper hole trap level which is consistent with measurements using photocapacitance spectroscopy. GaCu antisites are found to exhibit a deep electron trap level in Cu(In,Ga)Se2 only when the gallium content is sufficiently high, whereas InCu antisites are always shallow. The deep GaCu trap level was confirmed by photoluminescence measurements in ternary CuGaSe2 devices. The full picture of the intrinsic point defects including several proposed metastable defects is analyzed in detail and conclusions for device optimization are drawn. In addition, the results are put into perspective to former results from local density theory in the literature and possible sources of deviations are discussed.

With respect to defect formation processes at the solid-liquid interface in silicon crystal growth, the formation mechanism of twin boundaries at the interface is revealed by molecular dynamics simulations. In contrast to former models, it is shown that twin boundaries can form at the triple line between two grains and the melt. In contrast, the spontaneous formation of twin boundaries at the interface is generally not possible in the absence of grain boundaries since this would inevitably lead to the formation of coherency and anti-coherency dislocations. The excess formation energy of these dislocations inhibits spontaneous twin boundary formation without grain boundaries. At elevated undercooling, however, faulted dislocation loops can be grown into the crystal at the interface. Molecular dynamics simulations show that subsequent shrinkage of these loops is responsible for an interface-related mechanism by which nanoscale vacancy clusters can be directly grown into the crystal. These clusters are large enough to be sustained at typical growth temperatures and may act as nucleation seeds for larger voids. Finally, a new lattice Hamiltonian model for the simulation of the solid-liquid growth interface is presented. This model extends the commonly used lattice models to include stacking faults and twin boundaries for the diamond cubic lattice. The model is applied to study the growth dynamics at the solid-liquid interface and accurately takes into account the formation energy of stacking faults. The simulations show that an undercooling of 50K is sufficient for the spontaneous formation of faulted islands to occur at the interface.

Frage zum Eintrag

Redaktionelle Details anzeigen