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In a constant effort to capture effectively more of the spectral range from the sun, multi-junction cells are being investigated. In this context, the marriage of thin film and dye-sensitized solar cells (DSC) PV technologies may be able to offer greater efficiency whilst maintaining the benefits of each individual technology. DSC devices offer advantages in the nature of both the metal oxide photo-electrode and dye absorption bands, which can be tuned to vary the optical performance of this part of a tandem device, while CdTe cells absorb the majority of light above their band-gap in only a few microns of thickness. The key challenge is to assess the optical losses with the goal of reaching a net gain in photocurrent and consequently increased conversion efficiency. This study reports on the influence of optical losses from various parts of the stacked tandem structure using UV-VIS spectrometry and EQE measurements. A net gain in photocurrent was achieved from a model developed for the DSC/CdTe mechanically stacked tandem structure.
In an effort to overcome the lack of a suitable metal as an ohmic back contact for CdTe solar cells, a study was carried out on the potential for using a highly arsenic (As) doped CdTe layer with metallization. The deposition of full CdTe/CdS devices, including the highly doped CdTe:As and the CdCl2 treatment, were carried out by metal organic chemical vapour deposition (MOCVD), in an all-in-one process with no etching being necessary. They were characterized and compared to control devices prepared using conventional bromine-methanol back contact etching. SIMS and C-V profiling results indicated that arsenic concentrations of up to 1.5 × 1019 at·cm-3 were incorporated in the CdTe. Current-voltage (J-V) characteristics showed strong improvements, particularly in the open-circuit voltage (Voc) and series resistance (Rs): With a 250 nm thick doped layer, the series resistance was reduced from 9.8 Ω·cm2 to 1.6 Ω·cm2 for a contact area of 0.25 cm2; the J-V curves displayed no rollover, while the Voc increased by up to 70 mV (~ 12 % rise). Preliminary XRD data show that there may be an As2Te3 layer at the CdTe surface which could be contributing to the low barrier height of this contact.
P-type CdTe can be produced via acceptor doping with As. However, as with other II/VI materials, the dopant behaviour is not simple, as there is the potential for compensating species to be formed from intrinsic defects and dopant-defect complexes. A further complication is introduced by the presence of grain boundaries in polycrystalline material. This study demonstrates that dopant concentration is a function of VI/II ratio in the growth ambient, and that resistivity is minimised for a dopant concentration of < 2 × 1018 at.cm-3. Grain size is also affected by the VI/II ratio, increasing slightly as the growth ambient becomes more Te-rich.
Laser-induced selected area epitaxy of CdTe thin films on GaAs substrates has been investigated and the role of vapour phase and surface reactions considered. Photo-enhanced growth rates of CdTe have been measured as a function of UV laser intensity and as a function of Cd to Te alkyl ratios. The growth rates are not simply determined by vapour phase photo-dissociation but also by a photolytic reaction on the surface. The latter enables good pattern definition where the growth rate is enhanced by the incident uv -radiation. The factors that determine the photo-enhancement are considered in the light of the Langmuir-Hinsheltwod model.
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