On of the most fascinating properties of tetrahedrally coordinated solids is their negative thermal expansion. For Si, Ge and a number of binary compounds (cubic materials crystallizing in the diamond-type and sphalerite- or wurtzite-type crystal structure) it was shown that their thermal expansion coefficient becomes negative at temperatures between 50 to 100 K [1-3]. In ternary A^IB^IIIX₂^VI chalcogenides, crystallizing in the tetragonal chalcopyrite-type structure, the linear thermal expansion behaviour is described by the both independent linear thermal expansion coefficients α_a and α_c, which are anisotropic. It was shown that CuBX₂ chalcopyrites (B=In,Ga) exhibit negative linear thermal expansion at temperatures below 30 K, for both a and c. The lattice parameter in these compounds first decrease with decreasing temperature going through a minimum and increase at low temperatures [4-8]. In the exceptional case of  AgBX₂ semiconductors the lattice parameter c increases with decreasing temperature in the whole temperature range [9].

Because of negative linear thermal expansion coefficients α_a and α_c, negative Grueneisen parameters may be expected in the low-temperature region and as a consequence the existence of low-energy lattice vibrational modes [10]. Moreover high-pressure induced structural phase transitions are caused by low-energy lattice vibrational modes with negative Grueneisen parameters. It is known from literature that the binary compounds ZnS and ZnSe as well as ternary CuInSe₂ show a pressure induced structural phase transition to the rocksalt-type structure [11-12].

Not much is known about the low temperature behaviour of quaternary chalcogenide compound semiconductors, like A₂^IB^IIC^IVX₄^VI. According to the tetrahedrally coordinated crystal structure, a negative thermal expansion can be expected. This assumption is strenghtened by the observation of a pressure induced structural phase transition in Cu₂ZnSnS₄ [13]. This compound shows a transition from the tetragonal kesterite-type structure to ta distorted rocksalt-type structure at ~ 15 GPa [13].

The compound semiconductors discussed above are used as absorber layers in thin film solar cells. Photovoltaic (PV) devices with chalcopyrite-type Cu(In,Ga)Se₂ absorbers show very high efficiencies [14]. The quaternary semiconductors Cu₂ZnSn(S,Se)₄, crystallizing in the tetragonal kesterite-type structure, are the absorbers in the only critical raw material free PV technology. Record efficiencies have been reached with an (Ag,Cu)₂ZnSnSe₄ absorber layer [15].

Thin film solar cells are the most ubiquitous and reliable energy generation systems for aerospace applications, because of their appealing properties such as lightweightness, flexibility, cost-effective manufacturing, and exceptional radiation resistance [16]. For  longer  missions,  PV devices  in  conjunction  with  rechargeable  batteries  are  the  only  available  option  to  provide  uninterrupted, sustainable and stable  electrical  power. Especially satellites  on  inner  planets  missions  employ  solar  cells, because at  these  distances  the  power  density  of  sunlight  is  sufficient  for  the  production  of  electricity [16]. For these applications the harsh conditions in space, like radiation and low temperatures, have to be taken into account. The temperature in outer space far away from earth is just 3 K [17], and extreme temperature swings occur. Thus the low temperature behaviour of absorber materials in solar cells potentially used in space applications, are of extreme importance. 

The presentation will give an overview of our detailed in situ neutron diffraction based structural investigation comparing the thermal expansion behaviour of Cu₂ZnSnSe₄, Ag₂ZnSnSe₄ and (Ag,Cu)₂ZnSnSe₄ mixed crystals from room temperature to 3 K. These materials crystallize in the tetragonal kesterite-type structure in the whole temperature range studied. The linear thermal expansion coefficients α_a and α_c are highly anisotropic. The end member Cu₂ZnSnSe₄ shows negative thermal expansion coefficients below 50K. In case of Ag₂ZnSnSe₄ the lattice parameter c increases with decreasing temperature thus showing a negative thermal expansion coefficient over the whole temperature range studied, indicating the special behaviour of Ag-containing tetrahedrally coordinated semiconductors.