Robert E. Treharne

Combinatorial optimization of Al-doped ZnO films for thin-film photovoltaics

A rapid screening methodology for the development of transparent conducting oxides is presented. The methodology, based on a combination of spectrophotometry, ellipsometry and 4-point probe measurements, was used to map out the opto-electronic properties over a co-sputtered ZnO : Al2O3 film deposited from separate ceramic targets of ZnO and Al2O3. Clear distributions for the carrier density, ne, and mobility, μe, are determined as a function of wt%. Al2O3 content within the film. A minimum resistivity value of 7.6 × 10−4 Ω cm was achieved for a composition of 1.5 wt%. Al2O3.

Optical Design and Fabrication of Fully Sputtered CdTe/CdS Solar Cells

A model is presented for optimising the integrated intensity of light passing into the absorber layers of thin-film solar cells. Dispersion data for each of the transparent layers in the cells was obtained from variable angle spectroscopic ellipsometry. This data was then used in a transfer matrix model of multi-layer optics to determine the intensity spectrum passing over the interface from the transparent to the absorbing parts of the cell. Application of the method to the case of CdTe/CdS/ZnO/ITO/glass solar cells is described in detail, including the fitting of oscillator models to extract the complex dielectric function from the ellipsometry data. It was found that light coupling into the absorber was particularly sensitive to the CdS thickness but that tuning the ZnO (highly resistive transparent) layer was also influential. All-sputtered cells of this type were demonstrated as having efficiencies of 12.5% for 5 × 5 mm2 contacts.

The Effects of Zr Doping on the Optical, Electrical and Microstructural Properties of Thin ZnO Films Deposited by Atomic Layer Deposition

Transparent conducting oxides (TCOs), with high optical transparency (≥85%) and low electrical resistivity (10−4 Ω·cm) are used in a wide variety of commercial devices. There is growing interest in replacing conventional TCOs such as indium tin oxide with lower cost, earth abundant materials. In the current study, we dope Zr into thin ZnO films grown by atomic layer deposition (ALD) to target properties of an efficient TCO. The effects of doping (0–10 at.% Zr) were investigated for ~100 nm thick films and the effect of thickness on the properties was investigated for 50–250 nm thick films. The addition of Zr4+ ions acting as electron donors showed reduced resistivity (1.44 × 10−3 Ω·cm), increased carrier density (3.81 × 1020 cm−3), and increased optical gap (3.5 eV) with 4.8 at.% doping. The increase of film thickness to 250 nm reduced the electron carrier/photon scattering leading to a further reduction of resistivity to 7.5 × 10−4 Ω·cm and an average optical transparency in the visible/near infrared (IR) range up to 91%. The improved n-type properties of ZnO: Zr films are promising for TCO applications after reaching the targets for high carrier density (>1020 cm−3), low resistivity in the order of 10−4 Ω·cm and high optical transparency (≥85%).

In-depth analysis of chloride treatments for thin-film CdTe solar cells

CdTe thin-film solar cells are now the main industrially established alternative to silicon-based photovoltaics. These cells remain reliant on the so-called chloride activation step in order to achieve high conversion efficiencies. Here, by comparison of effective and ineffective chloride treatments, we show the main role of the chloride process to be the modification of grain boundaries through chlorine accumulation, which leads an increase in the carrier lifetime. It is also demonstrated that while improvements in fill factor and short circuit current may be achieved through use of the ineffective chlorides, or indeed simple air annealing, voltage improvement is linked directly to chlorine incorporation at the grain boundaries. This suggests that focus on improved or more controlled grain boundary treatments may provide a route to achieving higher cell voltages and thus efficiencies.

NH

The CdCl2 treatment is a key step in CdTe solar cell fabrication. However, despite its near ubiquitous use, the process is nonideal as CdCl2 is both expensive and potentially hazardous to utilize in processing. In this paper, we report on the development of a NH4Cl replacement to the CdCl2 process, which is a low-cost noncarcinogenic alternative. Comparative cells were fabricated and compared via C -V, J-V, scanning electron microscopy, and external quantum efficiency analysis. Further process optimization led to device efficiencies of up to 11.5%, achieved using this new process, with VOC values of up to 832 mV, which is relatively high.

_{\bf 4}

A combinatorial methodology, developed for the rapid optimisation of sputtered transparent conducting oxides, was applied to Si doped ZnO. A wide range of compositions have been explored over a single sample to determine an optimum composition, with respect to the minimisation of resistivity, of x = 0.65% wt. SiO2. A fundamental investigation of the conduction band non-parabolicity yields values of me0=0.35m0 and C = 0.3 eV−1 for the conduction band minimum effective mass and the non-parabolicity factor, respectively. The variation of extracted band gap values with respect to dopant concentration provided an estimate of the magnitude of re-normalization effects. A model is proposed to describe the carrier transport behaviour for a degenerate polycrystalline semiconductor by accounting for the tunnelling of carriers through grain boundaries.

Cl Alternative to the CdCl

CuSbS₂ is emerging as a novel absorber for sustainable photovoltaics. We fabricated CuSbS₂ thin films by the sulfurization of sputtered metallic layers. Characterization of the morphological, optical, electrical and surface properties is presented. Overpressure of inert gas during sulfurization reduced antimony loss. The largely single-phase films exhibit optimal characteristics for PV applications, including a band gap of about 1.5 eV and p-type conductivity. The carrier concentrations were ~10¹⁷ cm^-3 and the mobility was generally about 10 cm² V^-1 s^-1. Surface property measurements are dominated by Sb₂O₃ and a secondary phase may be responsible for PL emission above the optical gap.

_{\bf 2}

An all-sputtered fabrication route to CdTe/CdS solar cells has been developed. The reproducibility of device efficiency was investigated with respect to a post-deposition CdCl2 treatment. A maximum conversion efficiency, for optimized devices, of 12.4% and an average of 9.3±2.2% was achieved. Significant re-crystallization within the CdTe and CdS layers was observed, via cross-sectional SEM, following CdCl2 treatment.

Treatment Step for CdTe Thin-Film Solar Cells

Ellipsometry was used to measure the amplitude ratio and phase difference of light undergoing a phase shift as it interacts with a thin film of organic–inorganic hybrid perovskite CH3NH3PbI3 (MAPI) deposited onto a (100) silicon wafer. The refractive index and extinction coefficient was extracted from a multi-oscillator model fit to the ellipsometry data, as a function of wavelength, from 300 to 1500 nm.

Non-parabolicity and band gap re-normalisation in Si doped ZnO

The processes of the noncatalytic synthesis of structures with CdTe nanowires by magnetron sputtering deposition are studied. It is shown that the deposition of magnetron sputtered CdTe onto substrates covered by a porous SiO2 layer can result in CdTe nanowires formation. The porosity of SiO2 layers with thicknesses from 2 to 15 nm fabricated by magnetron sputtering deposition is estimated.