Relationship between bandgap grading and carrier recombination for Cu(In,Ga)Se<sub>2</sub>-based solar cells

Ando, Yuta; Ishizuka, Shogo; Wang, Shenghao; Chen, Jingdong; Islam, Muhammad Monirul; Shibata, Hajime; Akimoto, Katsuhiro; Sakurai, Takayasu

doi:10.7567/jjap.57.08rc08

Cited by 17 publications

(14 citation statements)

References 33 publications

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“…[ 3,12,14 ] The gradient in the conduction band assists in driving electrons (minority carriers in p‐type CIGSe) towards the space charge region (SCR) and the heterojunction with the n‐type CdS layer. [ 15 ] The resulting decrease in electron density near the molybdenum (Mo) back contact has been shown to suppress recombination losses, [ 16–18 ] notably associated with interfacial recombination at the CIGSe/Mo junction, [ 14 ] thereby significantly increasing the device open‐circuit voltage ( V OC ). [ 19–21 ]…”

Section: Introductionmentioning

confidence: 99%

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se₂ Solar Cells

Chang

Carron

Ochoa

et al. 2021

Advanced Energy Materials

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CuInSe 2 (CISe) lattice widens the bandgap from 1.04 eV [5,6] up to 1.68 eV [7] in pure CuGaSe 2 , the change affecting the conduction band and leaving the valence band largely unaffected. [8-10] Since 1994, several studies have focused on optimising device efficiency by adjusting the Ga-concentration profile in the CIGSe layer [3,11,12] nowadays reaching record device efficiencies surpassing 23%. [13] A double Ga-gradient profile (Ga-rich/Ga-poor/Ga-rich) is commonly implemented in high-efficiency CIGSe solar cells. [3,12,14] The gradient in the conduction band assists in driving electrons (minority carriers in p-type CIGSe) towards the space charge region (SCR) and the heterojunction with the n-type CdS layer. [15] The resulting decrease in electron density near the molybdenum (Mo) back contact has been shown to suppress recombination losses, [16-18] notably associated with interfacial recombination at the CIGSe/Mo junction, [14] thereby significantly increasing the device open-circuit voltage (V OC). [19-21] Optimisation of the CIGSe film thickness, composition and Ga-gradient profile has largely been carried out by monitoring improvements in device efficiency, with only limited knowledge of the underlying dynamics and diffusion of the minority carriers to the n-contact. In particular, minority carrier mobility and driftdiffusion times in high-efficiency CIGSe solar cells are important parameters to quantify performance losses in state-of-the-art devices. A number of studies report carrier mobility in CISe and CIGSe measured with a variety of techniques as summarised in Table 1. However, the reported mobility values show variations of several orders of magnitude. Furthermore, only a few studies investigated device-relevant Ga-graded CIGSe layers, instead, focusing on simpler, ungraded absorbers with poorer performance. Combining time-resolved photoluminescence (TRPL) spectroscopy and numerical simulations, Weiss et al. extracted minority carrier mobilities between 32 and 45 cm 2 V −1 s −1 in Gafree CISe absorbers, however for back-graded CIGSe (GGI ratio increasing from 0 to 0.28 towards the back) only a lower limit of 8.3 cm 2 V −1 s −1 could be evidenced. [22] Kuciauskas et al. carried out TRPL studies on a typical Ga gradient device and estimated a minority carrier mobility of 55-230 cm 2 V −1 s −1 in the SCR near the CdS/CIGSe interface, [23] but did not address electron transport across the Ga-gradient towards the back contact region. Though transient absorption spectroscopy (TAS) has been utilized in the fields of organic and hybrid organic-inorganic Cu(In,Ga)Se 2 solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion efficiency of 23.3%. Key aspects to this efficiency progress are the engineered bandgap gradient profile across the absorber depth, along with controlled incorporation of alkali atoms via post-deposition treatments. Whereas the impact of these treatments on the carrier lifetime has been extensively studied in ungraded Cu(In,Ga...

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Section: Introductionmentioning

confidence: 99%

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se₂ Solar Cells

Chang

Carron

Ochoa

et al. 2021

Advanced Energy Materials

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“…The luminescence of the CIGS samples with a notch Ga-profile commonly presents two clear radiative transitions, ,,,− the origin of which is commonly related to the characteristic shape of the notch profile. In the case of a linear Ga-profile, a broad luminescence is observed. , Different Ga-profiles will lead to different electronic energy levels structures, and more importantly, to in-depth differences in the electronic energy levels structures that indicate the changes of the Ga content throughout the CIGS layer.…”

Section: Introductionmentioning

confidence: 99%

Recombination Channels in Cu(In,Ga)Se₂ Thin Films: Impact of the Ga-Profile

et al. 2020

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Depth bandgap profiles via a [Ga]/([Ga]+[In]) variation in the Cu(In,Ga)Se2 (CIGS) absorber layer have been implemented as a strategy to enhance the performance of CIGS solar cells. Since the [Ga]/([Ga]+[In]) determines to a large extent the position of the conduction band minimum, different Ga-profiles lead to different electronic energy levels structures throughout the CIGS layer. In this paper, from the investigation of the dependence of the photoluminescence (PL) on excitation power and temperature, we critically analyze the impact of a notch or a linear Ga-profile on the CIGS electronic energy levels structure and subsequent dominant recombination channels. Notwithstanding, two radiative transitions involving fluctuating potentials were observed for each sample, and significant differences in the luminescence resulting from the two Ga-profiles were identified. For the CIGS absorber with a notch Ga-profile, two tail-impurity radiative transitions involving equivalent donor clusters and the same deep acceptor level were ascribed to the CIGS/CdS interface region and to the notch region. The probability of radiative recombination in these two regions is discussed. For the CIGS absorber with a linear Ga-profile, two band-impurity radiative transitions involving an acceptor, with an ionization energy compatible with the V Cu defect were ascribed to the CIGS/CdS interface region. Our results show that the dominant acceptor defects are dependent on the Ga-profile, and they also highlight the complexity of the radiative and nonradiative recombination channels revealed by the tight control of the parameters in the experiment.

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“…Different mechanisms can decrease the V OC , for example increased interface recombination, reduction in carrier lifetimes at the interface [60,61], change in doping level of the absorber surface due to inter-diffusion [26,48], or photocurrent blocking barriers [32,59]. We can exclude inappropriate conduction band offsets as the Mg content in the buffer layer was systematically investigated (see Sect.…”

Section: Metastabilites and Interface Recombinationmentioning

confidence: 99%

ALD-ZnMgO and absorber surface modifications to substitute CdS buffer layers in co-evaporated CIGSe solar cells

Hertwig¹,

Nishiwaki²,

Ochoa³

et al. 2020

EPJ Photovolt.

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High efficiency chalcopyrite thin film solar cells generally use chemical bath deposited CdS as buffer layer. The transition to Cd-free buffer layers, ideally by dry deposition methods is required to decrease Cd waste, enable all vacuum processing and circumvent optical parasitic absorption losses. In this study, Zn1−xMgxO thin films were deposited by atomic layer deposition (ALD) as buffer layers on co-evaporated Cu(In,Ga)Se2 (CIGS) absorbers. A specific composition range was identified for a suitable conduction band alignment with the absorber surface. We elucidate the critical role of the CIGS surface preparation prior to the dry ALD process. Wet chemical surface treatments with potassium cyanide, ammonium hydroxide and thiourea prior to buffer layer deposition improved the device performances. Additional in-situ surface reducing treatments conducted immediately prior to Zn1−xMgxO deposition improved device performance and reproducibility. Devices were characterised by (temperature dependant) current-voltage and quantum efficiency measurements with and without light soaking treatment. The highest efficiency was measured to be 18%.

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Relationship between bandgap grading and carrier recombination for Cu(In,Ga)Se₂-based solar cells

Cited by 17 publications

References 33 publications

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se₂ Solar Cells

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se₂ Solar Cells

Recombination Channels in Cu(In,Ga)Se₂ Thin Films: Impact of the Ga-Profile

ALD-ZnMgO and absorber surface modifications to substitute CdS buffer layers in co-evaporated CIGSe solar cells

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Relationship between bandgap grading and carrier recombination for Cu(In,Ga)Se2-based solar cells

Cited by 17 publications

References 33 publications

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se2 Solar Cells

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se2 Solar Cells

Recombination Channels in Cu(In,Ga)Se2 Thin Films: Impact of the Ga-Profile

ALD-ZnMgO and absorber surface modifications to substitute CdS buffer layers in co-evaporated CIGSe solar cells

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Product

Resources

About

Relationship between bandgap grading and carrier recombination for Cu(In,Ga)Se₂-based solar cells

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se₂ Solar Cells

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se₂ Solar Cells

Recombination Channels in Cu(In,Ga)Se₂ Thin Films: Impact of the Ga-Profile