Alberto Mittiga

Heterojunction solar cell with 2% efficiency based on a Cu2O substrate

We report on the fabrication of heterojunction solar cells made by deposition of transparent conducting oxide (TCO) films on Cu2O substrates. The TCO films have been grown by ion beam sputtering on good quality Cu2O sheets prepared by oxidizing copper at a high temperature. The best solar cell has reached an open-circuit voltage of 0.595V, a short-circuit current density of 6.78mA∕cm2, a fill factor of 50%, and a conversion efficiency of 2% under simulated AM1.5G illumination, which is the highest efficiency value reported for this kind of heterojunction devices. These devices represent a good starting point for the development of very low cost solar cells.

Valence band offset at the CdS/Cu2ZnSnS4 interface probed by x-ray photoelectron spectroscopy

The valence band offset (VBO) at the interface CdS/Cu2ZnSnS4 was investigated by x-ray photoelectron spectroscopy (XPS). The VBO was measured by two different procedures: an indirect method involving the measurements of the core levels together with the XPS bulk valence band (VB) spectra and a direct method involving the analysis of XPS VB spectra at the interface. The indirect method resulted in a VBO value of (?1.20???0.14)?eV while the direct method returned a similar value of (?1.24???0.06)?eV but affected by a lower uncertainty. The conduction band offset (CBO) was calculated from the measured VBO values. These two measured values of the VBO allowed us to calculate the CBO, giving (?0.30???0.14)?eV and (?0.34???0.06)?eV, respectively. These values show that the CBO has a cliff-like behaviour which could be one of the reasons for the Voc limitation in the CdS/CZTS solar cells.

Effect of the order-disorder transition on the optical properties of Cu2ZnSnS4

The effect of the order-disorder transition on the band gap of kesterite Cu2ZnSnS4, an interesting material for solar cells, has been investigated by optical spectroscopy. The band gap energy (Eg) decreases continuously with increasing annealing temperature, Ta, and reaches its minimum at Ta ∼ 273 °C. Eg is about 200 meV higher in the most ordered state, than in the fully disordered state. Its value and the transition kinetic depend on the sample stoichiometry. A simplified model able to explain the order degree and stoichiometry effects on the Eg value is developed. Ordering results in narrower Raman peaks without affecting the shape of the photoluminescence spectrum—except for the change in Eg—or the characteristic energy of the exponential tail below the fundamental absorption edge. Although a prolonged annealing increases the order degree, the material properties are still influenced by residual disorder as well as by defects related to the off-stoichiometry composition.

Electrical properties of heavily doped μc‐Si:H

We have investigated the conduction in heavily doped μc‐Si:H samples grown by glow discharge by measuring the conductivity and the Hall coefficient in a wide temperature range. We show that the conductivity of these materials cannot be satisfactorily interpreted using the models already present in the literature. We put forward a model in which the material is considered composed of small crystalline grains embedded in an amorphous tissue and in which potential barriers exist between adjacent grains due to the band discontinuities. A comparison between detailed calculations and transport data shows that the tunneling (probably aided by the presence of localized states) is the fundamental conduction mechanism.

Stoichiometry effect on Cu2ZnSnS4 thin films morphological and optical properties

Thin films of Cu 2ZnSnS4 (CZTS) were prepared by sulfurization of multilayered precursors of ZnS, Cu, and Sn, changing the relative amounts to obtain CZTS layers with different compositions. X-Ray Diffraction (XRD), Energy Dispersive X-Ray spectroscopy, and SEM were used for structural, compositional, and morphological analyses, respectively. XRD quantitative phase analysis provides the amount of spurious phases and information on Sn-site occupancy. The optical properties were investigated by spectrophotometric measurements and Photothermal Deflection Spectroscopy. These films show a clear dependence of the optical and microstructural properties on the tin content. As the tin content increases we found: (i) an increase in both crystalline domain and grain size, (ii) an abrupt increase of the energy gap of about 150 meV, from 1.48 to 1.63 eV, and (iii) a decrease of sub-gap absorption up to two orders of magnitude. The results are interpreted assuming the formation of additional defects as the tin content is reduced.

Numerical modeling of laser induced phase transitions in silicon

A new one-dimensional numerical model of laser induced phase transitions in silicon is presented. In addition to the heat flow phenomena, it includes a first order description of the nucleation and growth of the new grains. The simulations are used to assess both the relevance of different nucleation mechanisms and the numerical values of some fundamental parameters.

Advances in screen printing metallization for a-Si:H/c-Si heterojunction solar cells

Amorphous / crystalline silicon heterojunction is the most attractive technique to obtain high efficiency solar cells. Usually such a kind of cells is produced starting from n-type silicon wafers, because of several advantages, like the high bulk lifetime and the possibility to easily contact the n-type base with i-n amorphous layers. The emitter is usually covered by Transparent Conductive Oxides (TCO) which works as high conductive layer and Anti Reflection Coating (ARC). The device is completed by a metal grid, made by screen printed silver, sintered at low temperature. Both the TCO and the grid strongly influence the final cell series resistance, and consequently the cell efficiency. When p-type wafer is adopted as substrate for heterojunction cell, the base contact is more difficult to obtain because of the energy bands alignment between the c-Si and the p-type a-Si:H layer. Recently n-type doped SiOx layer has attracted interest as emitter layer in heterojunction device, therefore in this work we show the results obtained on the metallization of n-type SiOx/ptype c-Si heterojunction solar cells by means of low temperature screen printing technique. In particular a new kind of low temperature sintering (<; 200°C) screen printable silver paste has been developed able to ensure high linear conductivity, low specific contact resistivity and strong adhesion to TCOs. We present electrical characterization using Transfer Length Method (TLM) technique. Since the base contact of SiOx/c-Si heterojunction is ensured by laser doping technique starting from p-type a-Si:H layer, we also show how the screen printed Ag paste can enhance the base contact of this solar cell.

Density of states and photoconductivity light degradation in a-Si:H at different temperatures

We have studied the light degradation process of a-Si:H at different temperatures monitoring the defect density and the photoconductivity. Our data are not in complete agreement with the predictions of the two most widely accepted models (the recombination induced bond breaking model and the stretched exponential one). We show that our experiments can be well interpreted by a model in which the creation rate is proportional to the square of the photoconductivity and in which the annealing rate is computed considering a distribution of energy barriers and it is enhanced by the degradation light.

Chloride-based route for monodisperse Cu2ZnSnS4 nanoparticles preparation

A new approach based on hot injection method is proposed to gram-scale Cu2ZnSnS4 nanoparticles production minimizing the use of organic solvents. Nanocrystal synthesis was performed starting from metal chlorides and pure sulphur powder and using Oleylamine as capping agent. As a result, core-shell nanoparticles with a narrow size distribution were obtained.

Evaluation of Hydrogen plasma effect in a-Si:H/c-Si interface by means of Surface Photovoltage measurement and FTIR spectroscopy

The amorphous/crystalline silicon technology has demonstrated its potentiality leading to high efficiency solar cells. To enhance the interface quality we investigate the effect of hydrogen plasma and thermal annealing treatments performed on thin amorphous silicon layer deposited over crystalline silicon surface. To this aim we use surface photovoltage technique, as a contact-less tool for the evaluation of the energetic distribution of the state density at amorphous/crystalline silicon interface, and FTIR spectroscopy of the same samples to evaluate the evolution of Si-H and Si-H2 bonds. The surface photovoltage technique results to be very sensitive to the different experimental treatments, and therefore it can be considered a precious tool to monitor and improve the interface electronic quality. We found that thermal annealing produces a metastable state which goes back to the initial state after just 48 hours, while the effect of hydrogen plasma post-treatment results more stable. In particular the latter reduces the defect density of one order of magnitude and keeps constant also after one month. The hydrogen plasma is able to reduce the defect density but at the same time increases the surface charge within the a-Si:H film due to the H+ ions accumulated during the plasma exposure, leading to a more stable configuration.