Cu(In,Ga)Se$_{\bf 2}$ Thin-Film Solar Cells and Modules—A Boost in Efficiency Due to Potassium

Abstract

Thin-film solar cells based on the chalcopyrite Cu(In,Ga)Se 2 (CIGS) absorber material show high potential for further cost reduction in photovoltaics. Compared with polycrystalline silicon (p-Si) wafer technology, thin-film technology has inherent advantages due to lower energy and material consumption during production but has typically shown lower conversion efficiency. However, in the past two years, new scientific insights have enabled the processing of CIGS solar cells with efficiencies up to 21%, surpassing the p-Si wafer value of 20.4% efficiency for the first time. Now several research groups report record cell efficiency values above 20% using different deposition processes and buffer layers. The presence of potassium was observed in many CIGS devices over the years, but it is only very recently that differences with Na have started being taken into full consideration for device processing and that K was added intentionally to the absorber. In this study, previous reports showing the presence of potassium are reviewed and discussed in more detail. Furthermore, on a scale-up perspective, additional progress has also taken place with CIGS minimodules achieving efficiency up to almost 19% and where further increase can be expected in the near future with the improvements induced by the use of potassium. This shows that the CIGS technology is continuously progressing not only on scientific level but on technological level as well.

Injection Current Barrier Formation for RbF Postdeposition‐Treated Cu(In,Ga)Se2‐Based Solar Cells

“…In order to verify that approach simulations are carried out using Sentaurus TCAD (see Section 6 for more details). The band diagram of the simulated structure is Among the thin-film solar cell technologies, Cu(In,Ga)Se 2 -based solar cells demonstrate the highest efficiencies, where the recent boost in efficiency is triggered by a KF postdeposition treatment (PDT). In this contribution, Cu(In,Ga)Se 2 -based solar cells are fabricated using RbF PDTs after absorber layer growth with varying substrate and RbF source temperature.…”

“…[1] A strong boost in efficiency was enabled due to the addition of a KF postdeposition treatment (PDT). [2] Recently, the addition of Rb and Cs has proven to be beneficial for device performance as well, and a 22.6% efficient CIGS device has been demonstrated using a RbF PDT. [3] The impact of the KF PDT has been investigated by several groups and mainly a modification of the CIGS surface was observed.…”

Effect of potassium fluoride post-deposition treatment on Cu(In,Ga)Se2 thin films and solar cells fabricated onto sodalime glass substrates

“…It has opened the experimental potential for higher CIGS solar cell efficiencies [9][10][11][12][13][14][15][16][17][18][19][20]. The EMPA group reported that KF-PDT reduces the Cu and Ga concentration at near-surface region of CIGS layer deposited onto alkali-free polyimide substrates, which leads to significant increase in the open circuit voltage (V oc ).…”

“…They interpreted the cause for increased V oc as a formation of a very dense shallow donor-type Cd Cu during the chemical bath deposition (CBD)-CdS process [9,12]. In addition, KF-PDT enables a significant reduction in the thickness of the CdS buffer layer without the commonly observed losses in device parameters [9][10][11][12][13][14][15]. Due to the Cu-deficient compounds at the very-near-surface region, a shift of the valence band to lower binding energies has been observed [18].…”

Photoelectrochemical water splitting using Cu(In,Al)Se2 photoelectrodes developed via selenization of sputtered Cu–In–Al metal precursors

“…Chalcopyrite p-type Cubased metal selenides (CuInSe 2 , CuGaSe 2 , and their solid solutions) are interesting photo-absorbers for solar-energy-related applications because of their suitable direct band gap, good absorption coefficient, and high flexibility for tuning the energy band gap without relying on toxic elements [19]. It has been reported that laboratory-scale thin-film solar cells with p-Cu(In,Ga)Se 2 (CIGS) and CdS heterogeneous junctions prepared using the thermal evaporation method can attain a high conversion efficiency of 21% [20]. Kumagai et al [21] reported PEC water splitting using p-type copper gallium selenide (CGS) films deposited onto Mo-and Ticoated glass substrates using the powder transfer method.…”