Deliberate and Accidental Gas-Phase Alkali Doping of Chalcogenide Semiconductors: Cu(In,Ga)Se2

in Scientific Reports (2017), 7

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of ... [more ▼]

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of gas phase alkali transport in the kesterite sulfide (Cu2ZnSnS4) system (re)open the way to a novel gas-phase doping strategy. However, the current understanding of gas-phase alkali transport is very limited. This work (i) shows that CIGSe device efficiency can be improved from 2% to 8% by gas-phase sodium incorporation alone, (ii) identifies the most likely routes for gas-phase alkali transport based on mass spectrometric studies, (iii) provides thermochemical computations to rationalize the observations and (iv) critically discusses the subject literature with the aim to better understand the chemical basis of the phenomenon. These results suggest that accidental alkali metal doping occurs all the time, that a controlled vapor pressure of alkali metal could be applied during growth to dope the semiconductor, and that it may have to be accounted for during the currently used solid state doping routes. It is concluded that alkali gas-phase transport occurs through a plurality of routes and cannot be attributed to one single source. [less ▲]

Vapour phase alkali species for Cu(In,Ga)Se2 solar cells

Scientific Conference (2015, September)

Alkalis are essential in Cu(In,Ga)Se2 absorber layers for efficient solar cells. Current doping methods rely on solid state diffusion of an alkali through to the absorber layer, e.g. a thin NaF layer on ... [more ▼]

Alkalis are essential in Cu(In,Ga)Se2 absorber layers for efficient solar cells. Current doping methods rely on solid state diffusion of an alkali through to the absorber layer, e.g. a thin NaF layer on Mo or NaCl dissolved in a metal precursor ink[1]. The apparent concentration of alkali in the final absorber is determined by the initial alkali dosing and the use of an interfacial barrier to stop alkali diffusion from the substrate. Until now the vapor–absorber interface as a source or sink of alkali doping has been largely ignored. We show that device efficiency improves from 2 to 8% by gas phase Na adsorption alone. Conversely initial results show that Na can also be desorbed to the gas phase. Although these efficiencies are lower than those obtained by including Na directly in the precursor (device efficiency 13.3% [1]), the findings are relevant to all chalcogenide growers as they show that exact doping, and thus control of device efficiency, is only possible when gas phase adsorption/desorption processes are controlled. [less ▲]

Laser Annealing of CuInSe2 Electrodeposited Precursors as the Absorber Layer in Thin Film Solar Cells

Doctoral thesis (2014)

This thesis demonstrates a method by which a non-vacuum deposited CuInSe2 precursor is transformed into a semiconductor absorber layer, suitable for completion into a photovoltaic device, using 1 s laser ... [more ▼]

This thesis demonstrates a method by which a non-vacuum deposited CuInSe2 precursor is transformed into a semiconductor absorber layer, suitable for completion into a photovoltaic device, using 1 s laser annealing. A final device produced using this absorber fabrication method gave a 1.6 % best conversion efficiency. This represents the first reported working device originating from electrodeposition - laser annealing. In this thesis, the steps taken to achieve this result are analysed in detail. This work is split into four parts. Firstly, the precursor structure and the optical properties of its component materials were investigated to ensure optimal interaction between the laser beam and sample. Using the selected codeposited CuInSe2 precursor it was shown that it is possible to stimulate grain growth and atomic diffusion by using a 1064 nm Nd:YAG laser with only a 1 s annealing time. Secondly, it is shown that even on these rapid annealing time-scales the thermodynamic equilibrium reactions between the ternary CuInSe2 and its constituent materials must be considered. This is proven by the formation of the first device from a laser annealed absorber only when a sufficiently elevated partial pressure of Se is supplied during the annealing process. Absorbers formed under lower Se activities led to devices which were shunted or showed negligible efficiency. A correlation was identified between the Se partial pressure and the absorber properties including its Se content, grain size and PL yield. Thirdly, a finite element model capable of predicting the film temperature during laser annealing is demonstrated. By considering the Gaussian nature of the irradiating beam it is seen that the peak temperature of the CuInSe2 film, which formed the 1.6 % device, fluctuated spatially by > 300 oC during processing. Variations in temperature led to different rates of atomic diffusion and grain growth and resulted in lateral inhomogeneities in the absorber. The low device efficiency is believed to be partially caused by these variations. Finally, the chemical composition of the film and its effect on absorber properties is established. Increasing the Cu/In ratio of CuInSe2 precursors and incorporating Na into CuInSe2 absorbers is shown to increase their crystal coherence length. However, the positive effects related to these elements must be balanced against their impact on the optoelectronic properties of the absorber when present in high concentrations. Therefore it is hoped that this initial ‘first device’ efficiency validates this fabrication route and motivates future research on this interesting topic. [less ▲]

Single Second Laser Annealed CuInSe2 Semiconductors from Electrodeposited Precursors as Absorber Layers for Solar Cells

in Journal of Physical Chemistry. C, Nanomaterials and interfaces (2014), 118 (3)

Three ways to grow faster and better CIGSe

Scientific Conference (2013)

Effects of annealing in sulfur vapor on electrodeposited CuInSe2 films

Scientific Conference (2011)