Progression of metalorganic chemical vapour‐deposited CdTe thin‐film PV devices towards modules

Abstract

This paper reports important developments achieved with CdTe thin-film photovoltaic devices produced using metalorganic chemical vapour deposition at atmospheric pressure. In particular, attention was paid to understand the enhancements in solar cell conversion efficiency, to develop the cell design, and assess scalability towards modules. Improvements in the device performance were achieved by optimising the high-transparency window layer (Cd 0.3 Zn 0.7 S) and a device-activation anneal. These increased the fill factor and open-circuit voltage to 77 ± 1% and 785 ± 7 mV, respectively, compared with 69 ± 3% and 710 ± 10 mV for previous baseline devices with no anneal and thicker Cd 0.3 Zn 0.7 S. The enhancement in these parameters is associated with the two fold to three fold increase in the net acceptor density of CdTe upon air annealing and a decrease in the back contact barrier height from 0.24 ± 0.01 to 0.16 ± 0.02 eV. The optimum thickness of the window layer for maximum photocurrent was 150 nm. The cell size was scaled from 0.25 to 2 cm 2 in order to assess its impact on the device series resistance and fill factor. Finally, micro-module devices utilising series-connected 2-cm 2 sub-cells were fabricated using a combination of laser and mechanical scribing techniques. An initial module-to-cell efficiency ratio of 0.9 was demonstrated for a six-cell module with the use of the improved device structure and processing. Prospects for CdTe photovoltaic modules grown by metalorganic chemical vapour deposition are commented on.

A combined Na and Cl treatment to promote grain growth in MOCVD grown CdTe thin films

“…The As doping of the CdTe absorber layer during the MOCVD growth is essential in order to achieve high p-type conductivity and high efficiency for superstrate configuration devices [14]. The organic precursors used were dimethylcadmium (DMCd), diisopropyltelluride (DiPTe) and trisdimethylaminoarsine (tDMAAs) for Cd, Te and As, respectively.…”

Domain decomposition approach for parallel improvement of tetrahedral meshes

“…It was deduced by the authors that the decreasing slope in the EQE curves towards the CdTe band edge may show a reduced minority carrier lifetime for air annealed ultrathin cells. The thermal anneal in air increases carrier concentration [20] but consequently depletes the collection depth with increased recombination for carriers generated deeper into the absorber further from the junction.…”

“…It has been demonstrated [19] that for solar cell devices with a CdTe thickness of 2.25 µm (baseline) produced by MOCVD in a hydrogen atmosphere, annealing the device in air after the CdCl 2 treatment also leads to an increase in V oc . Capacitance measurements [20] of as-grown and air annealed devices have shown that the carrier concentration in bulk CdTe increases for the air annealed devices. Both devices PZN07 and PZN14 were subjected to the same low temperature air anneal (170 C for 30 minutes), prior to gold metallisation for back contact formation.…”

“…In addition to the improvements in using a Cd 1-x Zn x S alloy window layer, a post-CdCl 2 treatment air anneal at low temperature has been found [19,20] to be beneficial to MOCVD-produced CdTe solar cells. X-ray photoelectron spectroscopy (XPS) has shown [19] that there is oxidation of the CdTe after the air anneal and capacitance-voltage (C-V) measurements have confirmed [20] an increase in acceptor carrier concentration, as well as a reduction in the back contact barrier.…”

Thin CdTe Layers Deposited by a Chamberless Inline Process using MOCVD, Simulation and Experiment

“…The growth rate of thin-film Cadmium Telluride (CdTe) using metalorganic chemical vapour deposition (MOCVD) has been found to be sensitive to both substrate temperature and reactant partial pressures [1], presenting a complex transport and kinetic process. Among the various deposition techniques used for CdTe growth, MOCVD is a favourable method, allowing control of the microstructure and stoichiometry of the films [2][3][4]. MOCVD is an attractive method for depositing CdTe and another group II-VI compound semiconductor thin films [5,6].…”