Sjoerd Smit

Influence of transparent conductive oxides on passivation of a-Si:H/c-Si heterojunctions as studied by atomic layer deposited Al-doped ZnO

In silicon heterojunction solar cells, the main opportunities for efficiency gain lie in improvements of the front-contact layers. Therefore, the effect of transparent conductive oxides (TCOs) on the a-Si:H passivation performance has been investigated for Al-doped zinc oxide (ZnO:Al) layers made by atomic layer deposition (ALD). It is shown that the ALD process, as opposed to sputtering, does not impair the chemical passivation. However, the field-effect passivation is reduced by the ZnO:Al. The resulting decrease in low injection-level lifetime can be tuned by changing the ZnO:Al doping level (carrier density = 7 × 1019–7 × 1020 cm−3), which is explained by a change in the TCO workfunction. Additionally, it is shown that a ~10–15 nm ALD ZnO:Al layer is sufficient to mitigate damage to the a-Si:H by subsequent sputtering, which is correlated to ALD film closure at this thickness.

Silicon passivation and tunneling contact formation by atomic layer deposited Al2O3/ZnO stacks

The passivation of Si by Al2O3/ZnO stacks, which can serve as passivated tunneling contacts or heterojunctions in silicon photovoltaics, was investigated. It was demonstrated that stacks with Al2O3 thicknesses >3 nm lead to lower surface recombination velocities (Seff,max < 4c m s −1 )o n n- and p-type Si than single-layer Al2O3 films for a wide range of ZnO thicknesses and irrespective of Al-doping of the ZnO. Stacks with an Al2O3 thickness of 1‐2 nm were found to combine reasonable surface passivation (Seff,max = 100‐700 cm s −1 ) with sufficiently high tunneling current densities (10‐300 mA cm −2 at 700 mV).

Optical modeling of plasma-deposited ZnO films: Electron scattering at different length scales

In this work, an optical modeling study on electron scattering mechanisms in plasma-deposited ZnO layers is presented. Because various applications of ZnO films pose a limit on the electron carrier density due to its effect on the film transmittance, higher electron mobility values are generally preferred instead. Hence, insights into the electron scattering contributions affecting the carrier mobility are required. In optical models, the Drude oscillator is adopted to represent the free-electron contribution and the obtained optical mobility can be then correlated with the macroscopic material properties. However, the influence of scattering phenomena on the optical mobility depends on the considered range of photon energy. For example, the grain-boundary scattering is generally not probed by means of optical measurements and the ionized-impurity scattering contribution decreases toward higher photon energies. To understand this frequency dependence and quantify contributions from different scattering ph...

Passivating Contacts for Crystalline Silicon Solar Cells: From Concepts and Materials to Prospects

To further increase the conversion efficiency of crystalline silicon (c-Si) solar cells, it is vital to reduce the recombination losses associated with the contacts. Therefore, a contact structure that simultaneously passivates the c-Si surface while selectively extracting only one type of charge carrier (i.e., either electrons or holes) is desired. Realizing such passivating contacts in c-Si solar cells has become an important research objective, and an overview and classification of work to date on this topic is presented here. Using this overview, we discuss the design guidelines for passivating contacts and outline their prospects.

Concepts and prospects of passivating contacts for crystalline silicon solar cells

To further increase the conversion efficiency of crystalline silicon solar cells it is vital to reduce the recombination losses between the photoactive part of the solar cell and the metal contacts. This is ideally achieved by fabricating contacts which passivate defects at the silicon surface while being simultaneously selective for only a single type of charge carrier, i.e. either electrons or holes. Despite the extensive research effort aimed at realizing such contacts, no clear overview of the fundamental physics of passivating contacts has appeared yet. Therefore, we present such an overview, introduce a clear classification of passivating contacts, and discuss their design guidelines and future prospects.

Variational method for the minimization of entropy generation in solar cells

In this work, a method is presented to extend traditional solar cell simulation tools to make it possible to calculate the most efficient design of practical solar cells. The method is based on the theory of nonequilibrium thermodynamics, which is used to derive an expression for the local entropy generation rate in the solar cell, making it possible to quantify all free energy losses on the same scale. The framework of non-equilibrium thermodynamics can therefore be combined with the calculus of variations and existing solar cell models to minimize the total entropy generation rate in the cell to find the most optimal design. The variational method is illustrated by applying it to a homojunction solar cell. The optimization results in a set of differential algebraic equations, which determine the optimal shape of the doping profile for given recombination and transport models.

Identifying parasitic current pathways in CIGS solar cells by modelling dark JV response

The presence of undetermined shunt pathways in CIGS solar cells can be severely limiting to the reproducibility of individual cell efficiency, both at lab-scale, and particularly in a roll-to-roll process. Here, a general model that describes the dark J-V characteristics of CIGS devices, accounting for three separate shunting pathways (Ohmic and non-Ohmic components, and a tunneling component), is presented. Excellent agreement between the model and experimental data is demonstrated throughout the temperature range 183 – 323K, whereas simpler models fail to accurate fit the data. To demonstrate the effectiveness of the model, a case study was carried out to investigate the cause of the large spread in efficiency in a single batch of CIGS cells. The model showed that the low efficiencies were entirely due to a higher prevalence of the three different shunt pathways, but not due to any degradation of the main junction. This methodology may therefore be used for rapid diagnosis of low (or inconsistent) efficiencies.

Status and prospects for atomic layer deposited metal oxide thin films in passivating contacts for c-Si photovoltaics

In the field of photovoltaics, atomic layer deposition (ALD) is mostly known for its success in preparing Al2O3-based surface passivation layers for c-Si homojunction cells. In the last years, many novel types of c-Si heterojunctions have appeared, referred to as passivating contacts. In these concepts, metal oxide thin films are used for surface passivation, carrier selectivity and as transparent conductive oxide. This leads to the question whether the success of ALD for homojunctions can be translated into this new field as well. Therefore, this work provides an overview of these new concepts, and highlights both the current role and prospects of ALD in this field.