Gallium gradients in Cu(In,Ga)Se₂thin-film solar cells

Abstract

The gallium gradient in Cu(In,Ga)Se2 (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co‐evaporation processes, plays a key role in the device performance of CIGS thin‐film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X‐ray measurements. In addition, the gallium grading of a CIGS layer grown with an in‐line co‐evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth‐dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co‐evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi‐Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross‐sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper‐vacancy‐mediated indium and gallium diffusion in CuInSe2 and CuGaSe2 were calculated using density functional theory. The migration barrier for the InCu antisite in CuGaSe2 is significantly lower compared with the GaCu antisite in CuInSe2, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co‐evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd.

Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers

“…However, alkali-metal doping also happens to hinder In/Ga interdiffusion in polycrystalline CIGS films. The effect was observed almost regardless of the alkali metal incorporation source 6,19–26 . This behaviour is ascribed to a reduction of the concentration of Cu vacancies (V Cu ) following Na addition.…”

“…In order to take advantage of a favourable band structure, CIGS devices are typically produced with a v-shaped Ga depth gradient that increases the collection of photogenerated electrons while reducing recombination at the CIGS/CdS interface 4,5 . Recent simulations have shown that the resulting trade-off between short circuit current and open circuit voltage depends on the position of the minimum (notch) in the Ga depth profile 6 . As a consequence, the efficiency could be further improved by shifting the notch position closer to the absorber surface, compared to recent record efficiency devices 7,8 .…”

Role of surface microstructure of Mo back contact on alkali atom diffusion and Ga grading in Cu(In,Ga)Se2 thin film solar cells

“…[24][25][26] A V-shaped Ga gradient which can increase the collection efficiency of photogenerated electrons and maintain a decent open circuit voltage of the solar cell has been widely accepted for the fabrication of high-efficiency CIGS solar cells. 34,35 The study of the above two topics has contributed to the improvement of the conversion efficiency of CIGS solar cells, however, the un-addressed mechanism of these two questions may have accounted for a large portion of the nonreproducibility of CIGS solar cells observed in many laboratories, rendering the industrialization process of CIGS far behind CdTe solar cells, even though the world record conversion efficiency of CIGS is higher. However, the formation mechanism of Ga gradient is still in debate.…”

Chemistry and Dynamics of Ge in Kesterite: Toward Band-Gap-Graded Absorbers

“…In CIGSe, this type of gradient leads to substantial improvements of open-circuit voltage (V oc ) and charge carrier collection. 4,5 Current efficiency levels of kesterites are below 13%, 6 because of short minority carrier lifetimes in the sub-nano-second regime. 7 Kesterite compounds like Cu 2 Zn(Sn,Ge)(S,Se) 4 as absorber for thin-film solar cells have the potential to contribute to the increase of solar energy in large volumes using earthabundant elements such as Sn and Zn.…”

“…4,5 Current efficiency levels of kesterites are below 13%, 6 because of short minority carrier lifetimes in the sub-nano-second regime. 7 Kesterite compounds like Cu 2 Zn(Sn,Ge)(S,Se) 4 as absorber for thin-film solar cells have the potential to contribute to the increase of solar energy in large volumes using earthabundant elements such as Sn and Zn. The capability of these compounds to host various elements in the same atomic position, such as Sn and Ge in Cu 2 Zn(Sn,Ge)Se 4 (CZTGeSe), allows tuning of the band gap of the compound by varying the composition similar to CIGSe.…”