Influence of grain boundary modification on limited performance of wide bandgap Cu(In,Ga)Se2 solar cells

Raghuwanshi, Mohit; Cadel, E.; Pareige, P.; Duguay, S.; Couzinié-Devy, F.; Arzel, Ludovic; Barreau, Nicolas

doi:10.1063/1.4890001

Cited by 42 publications

(51 citation statements)

References 21 publications

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“…Also, the beneficial effects of Na at GBs must be similar for all the studied samples (same level), meaning that the reason for poor performance of wideband gap CIGS must arise mostly from differences in Cu concentration levels at GB. In previous works , our studies have shown that the modifications in GB compositions as a function of Ga ratio could explain the degradation of performances of Ga‐rich cells. Cu‐poor GBs (benign nature) were majorly found in Ga poor cells, and Cu‐rich GBs (detrimental effect) were mostly observed for Ga‐rich cells.…”

Section: Experiments and Resultsmentioning

confidence: 73%

Influence of Na on grain boundary and properties of Cu(In,Ga)Se₂ solar cells

Raghuwanshi

Cadel

Duguay

et al. 2017

Progress in Photovoltaics

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Whether added from external sources or supplied via substrate, incorporation of Na has always improved the conversion efficiency of polycrystalline thin‐film Cu(In,Ga)Se2 (CIGS) solar cells. Segregation of Na along grain boundary (GB) is thought to be one of the main reasons for better efficiency, and changing its concentration levels may tune the device properties accordingly. Our previous works have shown that changes in GB chemistry over various Ga/In concentrations are the main reason for poor performance of Ga‐rich (18%) CIGS, besides its optimum band gap (1.4 eV) for energy conversion. It was suspected that changes in GB chemistry or device performance or both could be driven by variation in Na concentration at GB. In this research, atomic level fluctuations of Na atoms at GB for various CuIn1−xGaxSe2 thin films were measured accurately with atom probe tomography technique. Fluctuations observed in Na concentration at GB are restricted to the range 0.5–6.5 atoms per nm2 and is irrespective of x value (GGI) and GB surface density. Results also show that changes in GB chemistry for similar CIGS compositions are not necessary due to changes in Na concentration levels at GB. However, an increased density of GBs per unit volume with GGI may indicate an overall rise in Na in thin films and could explain the drop in CIGS performance. Copyright © 2017 John Wiley & Sons, Ltd.

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Section: Experiments and Resultsmentioning

confidence: 73%

Influence of Na on grain boundary and properties of Cu(In,Ga)Se₂ solar cells

Raghuwanshi

Cadel

Duguay

et al. 2017

Progress in Photovoltaics

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“…A shorter diffusion length for higher gallium content could be caused by increased defect capture rates because of deeper trap states, which has been numerically predicted . Furthermore, recombination at grain boundaries might be enhanced, which has been identified as a possible limiting factor for wide band gap CIGS …”

Section: Resultsmentioning

confidence: 99%

“…The main problem is that the open‐circuit voltage ( V oc ) does generally not scale with the band gap energy as predicted, and it tends to saturate for absorber band gap energies above roughly 1.3 eV . This lack of performance in high gallium CIGS devices has been the subject of numerous studies …”

Section: Introductionmentioning

confidence: 99%

Record 1.0 V open‐circuit voltage in wide band gap chalcopyrite solar cells

Larsson

Nilsson

Keller

et al. 2017

Progress in Photovoltaics

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Tandem solar cell structures require a high-performance wide band gap absorber as top cell. A possible candidate is CuGaSe 2 , with a fundamental band gap of 1.7 eV. However, a significant open-circuit voltage deficit is often reported for wide band gap chalcopyrite solar cells like CuGaSe 2 . In this paper, we show that the open-circuit voltage can be drastically improved in wide band gap p-Cu(In,Ga)Se 2 and p-CuGaSe 2 devices by improving the conduction band alignment to the n-type buffer layer. This is accomplished by using Zn 1−x Sn x O y , grown by atomic layer deposition, as a buffer layer. In this case, the conduction band level can be adapted to an almost perfect fit to the wide band gap Cu(In,Ga) It has been proven difficult to maintain a good device quality when the gallium content is increased. The main problem is that the opencircuit voltage (V oc ) does generally not scale with the band gap energy as predicted, and it tends to saturate for absorber band gap energies above roughly 1.3 eV. 7,8 This lack of performance in high gallium CIGS devices has been the subject of numerous studies. 7-11The dominating recombination paths, in CuGaSe 2 devices, have been assigned to tunneling enhanced recombination either in the space-charge region or at the interface. 12,13 A possible explanation is trap states formed by cation anti-site (In Cu /Ga Cu ) or anion vacancy (V Se ) defects that become deeper positioned within the band gap when the gallium content increases, and thereby form more effective recombination centers. 11 Furthermore, the difference between the Fermi level and the valence band energy at the absorber surface seems to remain constant around 0.8 eV, independently of gallium content. 13 Consequently, the Fermi level position at the absorber/buffer interface is closer to the middle of the band gap at high gallium contents, and no beneficial type inversion can be expected. Thus, the influence of recombination close to or at the interface appears to become more prominent when the CIGS band gap is widened. The bulk recombination rate is also expected to increase with a large number of these defects, but it has not In the first section of this study, the growth and material characterization of nongraded CIGS absorbers with varying gallium contents (0.3 ≤ GGI ≤ 1) deposited in a single-stage co-evaporation process are described. These absorbers are applied in the second section, where we use the temperature dependence of ALD grown ZTO buffer layers to show that the V oc deficit in wide band gap CIGS can be reduced by improving the absorber/buffer conduction band alignment.In the last section, we further investigate the effect of an improved band alignment by using CuGaSe 2 absorbers of higher material quality, evaporated in a 3-stage type process. 21-24It can be observed in the SEM images that the grain size is reduced with increased GGI. This general observation is often reported in the literature. The grain size reduction has been found to be influenced by a number of factors, such as film t...

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“…In contrast, a high Ga-content indicated that the Cu had been enriched and the Ga depleted at the GBs relative to the IG regions. They suggested that Cu-enriched GBs, especially those with a high Ga-content, hinder V OC and act as recombination centers due their action as shunt paths [11]. In our results, a higher local current flows through the CIGS thin-film surfaces, which can act as shunt paths because the GBs of the high Ga-content are Cu enriched when compared to the IGs of CIGS thin-films.…”

Section: Resultsmentioning

confidence: 81%

“…However, it is desirable to obtain a wide band gap by controlling the Ga/(In þ Ga) ratio of CIGS. The Ga/(In þ Ga) ratio in CIGS is important because the theoretically optimal band gap (~1.4 eV) of a CIGS thin-film can achieve a conversion efficiency higher than 25% [11]. Many studies have discussed the impact that the Ga/(In þ Ga) ratio has on CIGS thin-film solar cells according to photo-capacitance spectroscopy and scanning probe microscopy (SPM) measurements.…”

Section: Introductionmentioning

confidence: 99%

Macroscopic and microscopic electrical properties of Cu(In,Ga)Se2 thin-film solar cells with various Ga/(In+Ga) contents

Kim¹,

Jo²,

et al. 2015

Current Applied Physics

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Influence of grain boundary modification on limited performance of wide bandgap Cu(In,Ga)Se2 solar cells

Abstract: Electron-beam-induced current at absorber back surfaces of Cu(In,Ga)Se2 thin-film solar cells

Cited by 42 publications

References 21 publications

Influence of Na on grain boundary and properties of Cu(In,Ga)Se₂ solar cells

Influence of Na on grain boundary and properties of Cu(In,Ga)Se₂ solar cells

Record 1.0 V open‐circuit voltage in wide band gap chalcopyrite solar cells

Macroscopic and microscopic electrical properties of Cu(In,Ga)Se2 thin-film solar cells with various Ga/(In+Ga) contents

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Influence of grain boundary modification on limited performance of wide bandgap Cu(In,Ga)Se2 solar cells

Abstract: Electron-beam-induced current at absorber back surfaces of Cu(In,Ga)Se2 thin-film solar cells

Cited by 42 publications

References 21 publications

Influence of Na on grain boundary and properties of Cu(In,Ga)Se2 solar cells

Influence of Na on grain boundary and properties of Cu(In,Ga)Se2 solar cells

Record 1.0 V open‐circuit voltage in wide band gap chalcopyrite solar cells

Macroscopic and microscopic electrical properties of Cu(In,Ga)Se2 thin-film solar cells with various Ga/(In+Ga) contents

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Influence of Na on grain boundary and properties of Cu(In,Ga)Se₂ solar cells

Influence of Na on grain boundary and properties of Cu(In,Ga)Se₂ solar cells