Nanostructured Si-alloys for silicon solar cells (2015)

In order to initiate further progress in silicon (Si) photovoltaics, a next-generation of Si solar cell concepts targeting both high performance and low-cost production needs to emerge. The objective of this thesis is to fabricate and characterize Si nanostructures embedded in a Si alloy matrix and implement these novel materials into Si based photovoltaic devices. In particular, silicon quantum dots (Si-QDs) in a silicon carbide (SiC)/Si-rich silicon oxide (SiOx) hetero-superlattice (HSL) structure as top cell absorber in all-Si tandem solar cells and nanocomposite microcrystalline silicon oxide (µc-SiOx:H) as contact layers in silicon heterojunction (SHJ) solar cells have been investigated. The main focus of the Si-QD absorber topic lies on the fundamental understanding of the material properties, whereas for the SHJ solar cell topic, developments on device level are the key aspect.Due to the competing nature of charge carrier confinement requiring high band offset and charge carrier transport demanding low band offset between Si-QDs and the embedding material, the choice of different materials for matrix and barrier is indicated. The novel Si-QD absorber approach based on HSL with near-stoichiometric SiC (low band offset) as vertical barrier layer and Si-rich SiOx (high band offset) as lateral matrix layer is motivated by pointing out the technical and theoretical difficulties of Si-QD formation in SiC matrix. After the successful development of laterally uniform, low rate and fully compatible plasma-enhanced chemical vapor depositions of high quality SiC and SiOx single layers, the processes were transfered into an automatic deposition sequence, which allows for the fabrication of SiC/SiOx HSL structures with excellent control over the sublayer composition and thickness. Even though the SiOx was found to fulfill the requirements for a matrix material upon high-temperature annealing (Si precipitation and crystallization, well passivated Si-QDs exhibiting clear signature of quantum confinement, remaining amorphous SiO2 phase after annealing), the SiC revealed several drawbacks that limit the usability of the SiC/SiOx HSL as a Si-QD absorber: (i) The Si-QDs extended into adjacent SiC layers especially with smaller SiOx sublayer thickness or higher silicon excess, which suggests that SiC is an insufficient Si diffusion barrier. (ii) The hydrogen release from SiC sublayers gave rise to the decrease of the optical band gap, the increase of the defect density and the increase of the sub-band gap absorption in HSL. (iii) The defects in the carbide layers seem to open non-radiative recombination paths for carriers generated in Si-QDs. (iv) The annealing-induced doping character of SiC is expected to increase recombination in these layers during carrier transport. The systematic analysis of HSL and single layers of SiC and SiOx annealed at various temperatures and passivated by various techniques using a number of characterization methods yielded a consistent picture of the fundamental differences between SiC and SiOx. The hydrogen effusion mechanisms during annealing and the hydrogen reincorporation mechanisms during passivation in HSL are well understood. The stronger increase of spin density upon annealing in SiC as compared to SiOx is ascribed to the effusion of a higher amount of hydrogen and the less efficient atomic diffusion. The much lower optical band gap and the significantly higher sub-band gap absorption of SiC in the annealed state as compared to SiOx can be traced back to the lower flexibility of its relatively dense 4-fold coordinated atomic structure. The less efficient stress relaxation in SiC increases the probability of creating strained bonds that additionally contributes to the sub-band gap absorption.Doped µc-SiOx:H, consisting of a phase mixture of Si crystallites and a-SiOx:H tissue, is an attractive contact layer material due to its high conductivity upon doping and at the same time high optical band gap and favorable refractive index. After finding out that the passivation quality of µc-SiOx:H is insufficient, an intrinsic amorphous silicon oxide (a-SiOx:H) buffer layer was inserted between the wafer and the doped layers yielding the first SHJ solar cells involving all buffer layers made from a-SiOx:H and all contact layers made from µc-SiOx:H materials. The advantages of a-SiOx:H, e.g., high optical band gap and high thermal stability were independently confirmed by analyzing the optical properties of a-SiOx:H layers and by showing good surface passivation on material and device level upon annealing, respectively. Implied open circuit voltages up to 742 mV achieved using a-SiOx:H passivation layers demonstrate a good reachability of high quality a-SiOx:H passivation within a low O content range. In addition, preconditioning by a-SiOx:H cover layer depositions after any change in the system was found to be crucial for the reproducibility of the passivation quality as well as the controlling of the buffer layer thickness. In fact, SHJ solar cells with front and rear a-SiOx layer thicknesses varying within a nanometer range showed largely deviating efficiency values. Structural, optical, electrical and passivation properties of the n- and p-type µc-SiOx:H layers deposited under various deposition conditions were consistently correlated to the cell results with the support of device simulations. The best SHJ solar cell using optimized a-SiOx:H and µc-SiOx:H layers on flat p-type wafer yielded an active area efficiency of 19 % with open circuit voltage Voc = 667 mV, short curcuit current density Jsc = 35.8 mA/cm² and fill factor FF = 79.6 %, which is up to now the highest efficiency shown for SHJ solar cells with µc-SiOx:H contact layers. In addition, the present work was able to provide several hints to explain the buffer layer thickness dependence of the surface passivation by a model, which assumes electrons and holes, generated in the crystalline silicon absorber, to be able to walk through the buffer layer and recombine at the next defect-rich interface.