Beneficial effect of post-deposition treatment in high-efficiency Cu(In,Ga)Se2 solar cells through reduced potential fluctuations

Abstract

World-record power conversion efficiencies for Cu(In,Ga)Se2 (CIGS) solar cells have been achieved via a post-deposition treatment with alkaline metals, which increases the open-circuit voltage and fill factor. We explore the role of the potassium fluoride (KF) post-deposition treatment in CIGS by employing energy- and time-resolved photoluminescence spectroscopy and electrical characterization combined with numerical modeling. The bulk carrier lifetime is found to increase with post-deposition treatment from 255 ns to 388 ns, which is the longest charge carrier lifetime reported for CIGS, and within ∼40% of the radiative limit. We find evidence that the post-deposition treatment causes a decrease in the electronic potential fluctuations. These potential fluctuations have previously been shown to reduce the open-circuit voltage and the device efficiency in CIGS. Additionally, numerical simulations based on the measured carrier lifetimes and mobilities show a diffusion length of ∼10 μm, which is ∼4 times larger than the film thickness. Thus, carrier collection in the bulk is not a limiting factor for device efficiency. By considering differences in doping, bandgap, and potential fluctuations, we present a possible explanation for the voltage difference between KF-treated and untreated samples.

Optically induced metastability in Cu(In,Ga)Se2

“…We observed photoluminescence (PL) emission (at energies corresponding to band-to-band recombination 13 ) when the excitation photon energy was well below the bandgap ( E g ) of the CIGS absorbers. Therefore, the likely mechanism is carrier generation from the valence band (VB) into defect states near the conduction band (CB), as illustrated in the inset of Fig.…”

“…1(a)). The spectrum of the higher-energy Gaussian component (2) rises with a slope determined by the magnitude of the band-edge potential fluctuations 13 . In addition, we observe a broad spectrum up to 200 meV below the E g , labeled (1).…”

“…Pink curves indicate fits to the near-bandgap PL tails described in ref. 13 , corresponding to process (2) in the inset in ( a ), and orange curves show Gaussian fits to the sub-bandgap feature, process (1). Solid black lines show the sum of Gaussians (1) and (2).…”

“…Optical studies do not require contacts and can be carried out on polycrystalline films, which potentially simplifies data analysis and interpretation. We apply this analysis to CIGS films with band gaps of E g  = 1.10–1.12 eV 13 and E g  = 1.50 eV, respectively. For the low- E g samples we also investigated the effect of potassium fluoride (KF) post-deposition treatment, because such treatment is applied in the record-efficiency solar cells 1 .…”

Chalcopyrite compound semiconductors for thin film solar cells

“…An influence of the alkali treatment on the doping level has been observed, however a reduced [21] or an increased doping level [27] was found in each case compared to absorbers containing Na, but without potassium treatment. It was recognised that alkali treatment leads to an increased carrier lifetime [27,28], which goes along with an increased photoluminescence intensity [27,29], indicating reduced non-radiative recombination as the basis for the improved opencircuit voltage [27]. Temperature dependent photoluminescence measurements hint at a reduced amount of potential fluctuations in K treated samples [28].…”