Development of gallium gradients in three‐stage Cu(In,Ga)Se₂ co‐evaporation processes

Abstract

We use secondary-ion mass spectrometry (SIMS), X-ray diraction (XRD) and scanning electron microscopy (SEM) to investigate the development over time of compositional gradients in Cu(In, Ga)Se 2 thin lms grown in threestage coevaporation processes, and suggest a comprehensive model for the formation of the well-known`notch' structure. The model takes into account the need for compensating Cu diusion by movement of group-III ions in order to remain on the quasi-binary tie line and indicates that the mobilities of In and Ga ions dier. Cu diuses towards the back in the second stage and towards the front in the third, and this is the driving force for the movement of In and Ga. The [Ga]/[In + Ga] ratio then increases in the direction of the respective Cu movement because In has a higher mobility at process conditions than has Ga. Interdiusion of In and Ga can be considerable in the (In, Ga) 2 Se 3 lm of the rst stage, but seems largely to cease in CIGS and shows no signs of being boosted by the presence of a Cu 2 Se layer.

Voids and compositional inhomogeneities in Cu(In,Ga)Se2 thin films: evolution during growth and impact on solar cell performance

“…This could be a consequence of lateral Ga accumulation due to preferential In reactivity at facets of preferential growth. A similar mechanism has been used to explain the formation of Ga gradients in three-stage CIGS growth [12]. The appearance of small voids even before recrystallization could arise due to the formation of empty spaces between growing crystallites.…”

“…A good understanding has been achieved regarding the formation of beneficial GGI gradings across the depth of the CIGS absorbers [11,12], and also the existence of voids in the CIGS layer has previously been reported [13,14]. However, the formation mechanism of lateral inhomogeneities of the CIGS composition has not been investigated yet.…”

A comparison between thin film solar cells made from co‐evaporated CuIn1‐xGaxSe2 using a one‐stage process versus a three‐stage process

“…Finally, in the third step, In, Ga, and Se are evaporated, and the deposition is terminated when the desired composition and thickness are reached. More detailed information on this process can be found in previous studies [7,[11][12][13][14][15][16]. The success of this method has commonly been attributed to two properties: (1) the growth of CIGS with large grains [17]; and (2) the self-formation of an optimized Ga gradient [14].…”

Surface engineering in Cu(In,Ga)Se2 solar cells

“…The SIMS analyses suggest that the surface compositions correspond to Ga/(Ga + In) ratios of 0.44 and 0.32 for the reference and the 100 nm top layer samples, respectively. The formation of the notch‐like in‐depth variation of Ga/(Ga + In) is discussed more in detail in another study , where we observe that copper migration in one direction leads to preferential indium migration (as compared with gallium migration) in the opposite direction. As the final part in the three‐stage process is copper‐free, we believe that out‐diffusion of copper to the growth front may have enhanced indium–gallium intermixing between the bulk and the top layer.…”

“…Cu(In,Ga)Se 2 layers were prepared by co‐evaporation of the elements onto Mo‐coated soda‐lime glass substrates. The deposition followed a three‐stage process as described in an earlier publication , with metals being evaporated from open boat sources and their rates controlled by means of mass‐spectrometer feedback. The rates and temperatures are shown as functions of time in Figure .…”