Issue
EPJ Photovolt.
Volume 11, 2020
Chalcogenide Materials for Photovoltaics 2020
Article Number 12
Number of page(s) 8
DOI https://doi.org/10.1051/epjpv/2020010
Published online 25 January 2021

© R. Hertwig et al., published by EDP Sciences, 2021

Licence Creative Commons
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

Prior to the recent record cells [1], the best performing thin film solar cells with chalcopyrite absorbers (Cu(In,Ga)(S,Se)2) have employed chemical bath deposited (CDB) CdS as the buffer layer [24] over alternative buffer materials or deposition methods [57]. CdS shows best performances when bath deposited, and is an inherently limiting material for a buffer layer due to its low band gap (Eg) of 2.4 eV [8] and the associated parasitic absorption. Therefore, industrial manufacturing is hindered by the added complexity of a highly uniform wet deposition step between vacuum deposition steps, the low material yield and the Cd-contaminated waste treatment. The ability to tailor the buffer layer to the absorber in terms of band alignment, transparency or lattice mismatch are increasingly necessary when absorber modifications such as alkali post deposition treatment (PDT) [2,9] or Ag incorporation affect the surface [1012]. High Eg materials like oxides, sulfides and selenides of In and Zn are recognized as the most promising material choices, as suggested in reviews discussing the difficulties of substituting CBD-CdS with alternative materials or deposition methods [1315]. The recent improvements on alternatives to CdS, namely CBD-Zn(O,S) [1] and notably the Zn1−xMgxO deposited by dry sputtering method [16], surpass the CBD-CdS buffer layer in terms of device efficiency, which was attributed to increased current, bespoke interface engineering and the consequential reduction of recombination [17].

Zn1−xMgxO is suited as buffer material due to its tuneable bandgap (3.2 eV for i-ZnO, 3.8 eV for Zn0.8Mg0.2O, 7.7 eV for MgO [18,19]), and the accompanied change of the conduction band minimum (CBM) position from below the CBM of CIS (i-ZnO) to clearly higher than CGS [20,21]. Zn1−xMgxO has been shown to be a viable buffer in case the conduction band offset (CBO) with the absorber is within 0–0.3 eV [22,23]. Zn1−xMgxO has been implemented as buffer layer in CIGS devices with efficiencies higher than 15% by sputtering [16,24] or atomic layer deposition (ALD) [2528]. Sputter deposition directly on the absorber is reported to cause sputter damage [29,30], leading to defective interfaces and V OC losses due to interface recombination [31,32]. This can be mitigated by introducing a thin intermediate layer, or using a soft deposition method, for example ALD or indirect sputtering [16].

Devices with alternative buffer layers are prone to metastabilities [3336], which can improve the performance after exposure to light, elevated temperatures or a combination of both [37,38]. The precise nature of metastable enhancement is still debated and may differ depending on the processing methods and materials of devices. The possible causes include presence of detrimental negative charges, for example associated with hydrogen or oxygen [34,37], which become inactive after hole capture holes upon light exposure. Another possibility is an interplay between vacancy complexes at the absorber and the buffer [35,39,40].

This work focuses on the substitution of CBD-CdS with a dry deposited high bandgap material and the effects of surface modifications before the growth of the buffer layer. The approach to prepare the absorber surface is inspired by the chemical environment in a typical CdS-CBD solution prior to CdS growth. First, the properties of the ALD deposited Zn1−xMgxO thin films are analysed in terms of thickness, composition, carrier density, mobility and sheet resistance. This high Eg material allows modification of the conduction band minimum [41,42] in the vicinity of the ones of CIS and CGS [21], which is used to match the conduction band of the absorber. The variation of the Mg content of the device performance will be discussed. We investigate the influence of different absorber surface treatments before Zn1−xMgxO deposition on the final device properties, notably KCN, ammonia and variants inspired by the successful CdS-CBD deposition process. Absorber dry treatments with trimethylaluminium (TMA) performed in the ALD chamber are also investigated. The effects of wet and dry absorber treatments before the buffer deposition are discussed and resulting cells are compared to CdS-CBD references.

2 Experimental

CIGS absorbers were grown on molybdenum-coated soda lime glass by a multistage co-evaporation process at about 450 °C and were subject to NaF and RbF PDT treatment. The growth process is described in [43]. The integrated ratio of Cu to In+Ga (CGI) and Ga to In+Ga (GGI) is 0.96 and 0.40, respectively, with the GGI near the surface being about 0.3. Typical gradings can be found in [43,44]. As standard treatment, the absorbers were etched in 10%w potassium cyanide (KCN) for 3 min, and in NH4OH (2 M [NH3]) aqueous solution for 1 min with H2O rinsing after each step. The transfer time between absorber surface treatments and ALD vacuum was less than 5 min. ZnMgO was deposited by ALD in a Fiji G2 system (Ultratech). Diethylzinc (DEZ), bis(cyclopentadienyl)magnesium (MgCp2), trimethylaluminium (TMA) and H2O (0.06 s) precursors were used at 120 °C substrate temperature, with Ar carrier gas at a base pressure of 13 Pa. Growth cycles are ZnO = H2O/N2/DEZ/N2 = 0.06/17/0.1/5 s, MgO = H2O/N2/MgCp2/N2 = 0.06/17/2/5 s, Al2O3 = H2O/N2/TMA/N2 = 0.06/17/0.06/10 s for ZnO, MgO and Al2O3, respectively. MgCp2 was heated to 90 °C, the other precursors were kept at room temperature. The stoichiometry of ZnMgO was varied by the relative numbers of DEZ to MgCp2 pulses, for example a ratio of 9 cycles DEZ/H2O followed by one cycle of MgCp2/H2O were used for Zn0.9Mg0.1O. Zn1−xMgxO thin films were simultaneously deposited on CIGS absorbers, Si (100) and soda-lime glass substrates. Targeted buffer thickness is 28 nm. A similar thickness is used for the CBD-CdS reference sample, which was performed using 185 ml H2O, 35 ml NH4OH, 15 ml Cd-acetate and 15 ml thiourea (TU) for 14 min at 70 °C. Where specified in the manuscript, solutions containing not all but the same ratio of ingredients as the CdS-CBD were used for surface treatments. The devices are finished with rf-magnetron sputtered ZnO (70 nm) and ZnO:Al (200 nm) layers in an Ar/O2 atmosphere at a pressure of 0.46 Pa and a power density of 1 and 2.5 W cm−2, respectively. Ni/Al grid contacts are e-beam deposited (50 nm, 4000 nm). The cell area is defined by mechanical scribing and determined from scans on a flatbed scanner. The cell area for the reference device is 0.512 cm2, for the Zn1−xMgxO buffer devices the area varies around 0.2 cm2.

The ALD buffer layer composition was analysed by X-ray photoelectron spectroscopy (XPS) using a Quantum 2000 photoelectron spectrometer from Physical Electronics with a monochromatic Al Kα source, operated at a base pressure below 10−9 mbar. The detailed high-resolution Mg 2p peak at 51 eV and Zn 2p peaks at 1022 eV with spin-orbit components splitting of 23 eV were acquired after 10 s of surface sputtering cycle. Peak spectra were recorded with an energy step size of 0.125 eV and a pass energy of 29.35 eV. An ion neutralizer using Ar+ of ≈ 1 eV was used to minimize the fluctuations of the binding energy values. The instrument work function was calibrated to give an Au 4f7/2 metallic gold binding energy of 84 eV. The acquired data were analyzed using PHI MultiPak software. Quantitative analysis was performed by measuring the Mg 2p and Zn 2p peak areas and by applying appropriate relative sensitivity factors (4.431 and 85.057).

The Zn1−xMgxO layer thickness was determined by ellipsometry on a M-2000 device using Si(100) substrates. Hall measurements were performed using a ECOPIA HMS3000 apparatus with 4 contacts in van der Pauw configuration. I-V characterisation was carried out with a Keithley 2400 source meter and four-terminal contacting under standard test conditions (1000 W m−2, 298 K) using an ABA type solar simulator on relaxed samples unless stated otherwise. Temperature dependent electrical measurements (TIV and J SC vs V OC) were performed in a cryostat cooled with liquid nitrogen with illumination provided by a halogen lamp with variable intensity filters. External quantum efficiency (EQE) characterisation was performed with a chopped halogen light source, a triple-grating monochromator and a lock-in amplifier at 298 K. A certified monocrystalline Si solar cell was used as reference device for both, EQE and I-V measurements.

3 Results and discussion

3.1 ZnMgO thin film

Zn1−xMgxO thin films were deposited on Si (100) and soda-lime glass substrates simultaneously with the deposition on CIGS absorbers. The nominal Mg content (ratio of pulses) is approximated from the ratio of Zn and Mg pulses, disregarding the intricacies of the ALD deposition process. The Mg content of deposited layers was varied between 5% and 16% by adjusting the ratio of Zn and Mg pulse numbers, and then analysed by XPS. As shown in Figure 1a, the Mg content determined using XPS does follow the nominal trend, similar to [25,28]. Zn1−xMgxO layers with Mg content of 16% deposited on Si substrates show linear growth of 1.65 Å per cycle between 50 and 1000 combined Zn and Mg cycles as determined by ellipsometry on Si(100) wafers, as visible in Figure 1b. Hall measurements could only be acquired with the thickest deposited layer (1000 cycles, 160 nm), resulting in a carrier density of 2.7 × 1019/cm3, a mobility of 1.8 cm2/Vs and a sheet resistance of 4.96 × 103 Ω/sq, measured on soda lime glass.

thumbnail Fig. 1

(a) Mg/(Zn+Mg) content in Zn1−xMgxO films as determined by XPS as function of the nominal value based on the number of pulses. (b) Zn1−xMgxO film thickness with number of deposition pulses as determined by ellipsometry. A linear fit (line) results in layer growth of 1.65 Å per pulse.

3.2 Influence of Mg content on device performance

A conduction band offset of 0–0.3 eV with respect to CIGS [23,45] is targeted by adjusting the Mg content in the Zn1−xMgxO buffer layer. The CBM of ZnO is reported equal to that of CIS, whereas the CBM of CuGaSe2 is 0.6–0.7 eV higher [21]. The addition of Mg in ZnO increases the CBM and lowers the VBM, which has been shown to result in a better alignment [22,25,46]. Considering the band gap variation from ZnO to Zn0.8Mg0.2O is reported to be 0.6 eV [19], a range of 10% nominal Mg content variations is expected to have little influence on device performance. The targeted buffer thickness is 28 nm, which is comparable to the thickness of the CBD-CdS buffer layer. The thickness of the Zn1−xMgxO buffer layer does affect the device performance, but since this can be attributed to an interplay of the buffer with the window layers, this effect is not discussed here [38].

The first absorber series was only rinsed in KCN before buffer deposition. This treatment cleans the absorber and removes alkali species from the surface as reported in [47]. As depicted in Figure 2a, all devices exhibit poor PV performance for Mg content between 10% and 20%, and the V OC is between 500 and 530 mV. There is no obvious correlation of the PV parameters with the content of Mg in the buffer layer, see Table 1. As CBD-CdS has been successfully applied to a variety of different absorbers, the hypothesis has been made that the absorber surface is beneficially modified during the initial stages of the deposition process [48,49]. This leads to the assumption, that a surface modification of the absorber which takes place during the initial stages of CdS-CBD is missing.

The following analysis investigates the effect of a treatment in a solution containing some, but not all ingredients of the CdS-CBD solution in the same ratio, for example, H2O, NH4OH and TU. After being cleaned in KCN and rinsed with H2O, the absorbers were immersed in NH4OH for 1 min. As shown in Figure 2b, a clear improvement in V OC and FF is observed for Zn1−xMgxO buffer layers with Mg content from 14% to 20%, with 16% Mg resulting in the best device performance, listed in Table 2. Devices with lower Mg content in the buffer layer exhibit lower V OC, while higher Mg content show a loss in FF, similarly as reported in [23,25,45,50]. This can be attributed to the formation of a cliff-like band alignment for low Mg content, which increases recombination and therefore reduces V OC. High Mg content raises the CB of the buffer layer to form a spike-like conduction band offset, which results in a blocking of photocurrent. It should be noted that variations in nominal Mg content of 2% already show drastic variations in device performance, narrower than the expected process window of about 10% nominal Mg content. In analogy to [51], a non-uniform composition over the very first monolayers could be speculated, which could lead to the formation of a blocking barrier.

The performance of the devices is not as good as the CdS reference, which is mostly due to the lower V OC and FF. The small deficit in J SC is attributed to reflection losses arising from non optimised layer thicknesses and comparatively larger grid shading. Analysis of the EQE curves reveals a potential for J SC gain arising from the wider buffer bandgap of about 1.1 mA/cm2 in the range of 360–550 nm (see Fig. 2c). The absorber bandgap extracted from EQE using Tauc fit procedure is 1.14 eV. The following sections will focus on the improvements of V OC and FF for buffer layers with 16% Mg content.

thumbnail Fig. 2

(a) IV curves of samples treated with KCN only before buffer deposition. (b) IV curves of samples subjected to KCN and NH4OH treatment. (c) EQE of samples with absorbers treated with KCN and NH4OH.

Table 1

PV parameters of devices with KCN only surface treatment, see Figure 2a. Percentages correspond to the Mg content of the buffer layer.

Table 2

PV parameters of devices with KCN and NH4OH surface treatment, see Figure 2b. Percentages correspond to the Mg content of the buffer layer.

3.3 Absorber surface treatments

As shown before, samples treated with KCN and NH4OH show an increase in V OC and FF compared to samples which were etched only with KCN. The improvement can be attributed to a beneficial surface modification during the NH4OH treatment. To investigate the analogies of KCN and NH4OH surface treatments with the surface modifications during CdS-CBD, wet treatments of 1 min have been conducted with solutions resembling parts of the CdS-CBD process. The solutions used are H2O + NH4OH, NH4OH, H2O + NH4OH + TU. Devices that have been treated with H2O + NH4OH show a reduction in V OC with 453 mV (FF 64%), see Figure 3a. The absorber treated with 28% NH4OH solution shows a significantly increased V OC of 597 mV (FF 73%). Solutions containing TU and NH4OH result in increases in V OC to 625 mV (FF 73%) over pure NH4OH, which is tentatively attributed to S incorporation and the consequential higher carrier density of the absorber surface and shorter depletion region as reported in [5254].

Even with TU and NH4OH treatments, the best achieved V OC and FF are significantly lower than the CdS reference sample, as reported in Table 3. To further treat the absorber surface, TMA was pulsed once in the ALD chamber before buffer deposition. The resulting V OC of 664 mV (FF 69%) yield a device efficiency of 16.5% without light and or heat soaking, as listed in Table 4. No further change in device performance was observed when repeating TMA pulses prior to Zn1−xMgxO deposition, similarly as reported in [55]. Hence, the improvement by a single pulse of TMA can be attributed to further etching of the absorber surface, formation of local Al2O3 islands with passivating effect, or a chemical reduction of surface species [56]. Another possibility which needs to be addressed is the potential doping of Zn1−xMgxO with Al and the corresponding change in carrier density [57]. The improvement in V OC is unlikely the result of a further increase in the CBM. ZnMgO:Al has a higher CB and lower VB than ZnMgO, hence it can be seen as a strong substitute for Mg. The introduction of Al will change the composition relative to the pulse ratio, which is one pulse of Al in 198 total pulses or roughly 0.5%. This would represent a ZnMgO:Al which is expected to behave like a ZnMgO with more than 16% Mg content, which are shown to have lower FF and not higher V OC, see Figure 3b. For comparison, TMA was also pulsed halfway through the Zn1−xMgxO deposition and directly compared to a device where TMA was pulsed at the very beginning, see Figure 3b. If Al distributes evenly in Zn1−xMgxO as reported in [58], the expected outcome would be the same. The device where TMA is pulsed in the middle of the buffer deposition exhibits a lower V OC and FF, which indicates absorber surface treatment instead of modification of the buffer layer. In case the single TMA pulse during the buffer deposition creates a uniform blocking layer, the charge carriers are able to tunnel through. The improvement of pulsing TMA at the beginning of the buffer deposition are primary in FF (61–69%) and secondary in V OC (647–664 eV), which support the hypothesis of a surface treatment further. In addition, the beneficial effect of TMA extends to a reduction in aging of the device. Devices without TMA display a loss in FF from 61% to 41% over 3 months storage in ambient conditions, as compared to 69–62% with TMA (Tab. 5).

thumbnail Fig. 3

(a) Effect of wet treatment before buffer deposition on JV curves. Absorber treatments with NH4OH or thiourea-ammonia solution show increased V OC. Additional (b) Influence of a single TMA pulse before and during buffer deposition, with unchanged total ZnMgO thickness. TMA before Zn1−xMgxO increases V OC and reduces ageing. TMA during Zn1−xMgxO reduces V OC.

Table 3

PV parameters of devices with KCN and CBD-CdS alike surface treatment, see Figures 3a and 3b. All devices have buffer layers of ALD deposited Zn0.84Mg0.16O, 28 nm.

Table 4

PV parameters of best devices with single TMA pulse during buffer deposition. “B” refers to a single cycle of TMA before buffer deposition, “M” refers to a single TMA pulse midway during buffer deposition.

Table 5

PV parameters of best performing device before and after light soaking at 1 sun, measured 90 days after finishing the cell.

3.4 Metastabilites and interface recombination

The presented devices with Zn1−xMgxO buffer show a reversible light soaking effect. The evolution of the IV curve for the best performing device with Zn1−xMgxO buffer device is presented in Figure 4a. The FF increases from 61% to 65 % already after 2 min (1 sun illumination). Light exposure of 45 min increases the FF to 71%, accompanied by an increase in V OC from 679 to 689 mV, resulting in 17.5% efficiency. Further light soaking up to 390 min improves FF to 72%, V OC 693 mV and 18% efficiency. Similar effects have been reported by others for Zn1−xMgxO and other materials [28,32,59] (Fig. 4).

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. 3.2).

In order to assess the interface quality, the best Zn1−xMgxO buffer device and the corresponding CdS reference device have been analysed with temperature dependant J SC − V OC measurements at different light intensities. The Zn1−xMgxO buffer device has been analysed before and after light soaking at 1 sun for 1 h. The respective diode quality factor A is presented in Figure 4b, the extrapolation of V OC at 0 K is presented in Figure 5. For the reference device, the ideality factor A is about 1.7 and almost insensitive to temperature. A pronounced difference before and after light soaking of the Zn1−xMgxO buffer device is visible in the respective diode ideality factors A. The relaxed Zn1−xMgxO buffer device shows A increase from 3.0 to 5.9 with decreasing temperature. After exposure to 1 sun, the ideality factor is still higher than 2, but the range has been limited to 2.1–2.9. The temperature dependence of the ideality factor of the Zn1−xMgxO buffer devices is probably linked to fluctuations in the activation energy of the dominating recombination process [62]. The fluctuation is reduced during light soaking, which is evident from the decreased values for A, but not fully eliminated.

The V OC at 0 K before and after light soaking is extrapolated for the temperatures from 200 to 300 K. The reference device shows an extrapolated V OC at 0 K between 1.14 and 1.15 eV, in good agreement with the bandgap of 1.14 eV extracted from the EQE curve using the Tauc fit method (Fig. 2). The device with Zn1−xMgxO buffer layer shows a spread in extrapolated V OC at 0 K from 1.18 eV down to 1.05 eV before light soaking, and ranges from 1.18 to 1.08 eV after light soaking (1 h at 1 sun). The decreased extrapolated V OC values suggest a dominant recombination path located at the absorber buffer interface [62] possibly mediated by a defect within the bandgap. Such an effect can also explain the reduced V OC. Improved FF and V OC due to the absence of shunts and a slight increase in device V OC are consistent with the IV analysis for different light soaking durations presented in Figure 4a.

thumbnail Fig. 4

(a) IV of Zn1−xMgxO buffer device before and after different light soaking duration at 1 sun. (b) Ideality factor extracted from J SC –V OC measurements for CdS reference (circle) and Zn1−xMgxO buffer device before (triangle) and after light soaking (square).

thumbnail Fig. 5

Extrapolation of V OC at 0 K for (a) CdS reference, (b) Zn1−xMgxO buffer (c) Zn1−xMgxO buffer after light soaking for 1 h at 1 sun.

4 Conclusion

CIGS solar devices with ALD ZnMgO buffer layers were fabricated with efficiencies reaching 18% after light soaking. The conduction band alignment was investigated by systematically varying the Zn1−xMgxO stoichiometry, revealing a best alignment with x = 0.16 composition. The compositional range for suitable band alignment with CIGS is found more narrow than expected. The treatments performed to investigate the effect on the absorber surface were performed immediately prior to buffer deposition. Wet treatments using NH4OH, thiourea or both resulted in improvements of V OC and FF. Additional improvements were achieved by a single TMA pulse in vacuum, before the Zn1−xMgxO buffer deposition, which we assign to reduction of surface species and subsequent chemical surface passivation. The best results were achieved by a combination of wet etching in KCN and NH4OH before exposure with TMA under vacuum. The Zn1−xMgxO devices exhibit a reversible metastable light soaking effect, improving the device efficiency. Temperature and illumination dependant electrical characterization revealed increased interface recombination in Zn1−xMgxO buffer devices, as compared to the CdS reference. We consider Zn1−xMgxO as a possible alternative buffer layer for CIGS solar cells, although further work is necessary to understand and overcome the unwanted recombination mechanisms and improve cell efficiencies.

Acknowledgments

This work was partially supported by the Swiss Federal Office of Energy (contract Nr SI/501614-01 ImproCIS) and from the Swiss State Secretary for Education, Research and Innovation (SERI) under contract number 17.00105 (EMPIR project HyMet). The EMPIR programme is co-financed by the participating states and by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement number 754364.

Author contribution statement

R.H. conceived the idea, which was co-developed with R.C., M.O. and A.N.T. S.N. deposited the absorbers. R.H. performed device fabrication after absorber deposition, characterization of thin films and devices. E.G. conducted the XPS measurements and analysis. All authors were involved in the interpretation of the data. R.H wrote the manuscript. All authors provided feedback and corrections to the manuscript.

References

  1. M. Nakamura, K. Yamaguchi, Y. Kimoto, Y. Yasaki, T. Kato, H. Sugimoto, IEEE J. Photovolt. 9, 1863 (2019) [CrossRef] [Google Scholar]
  2. A. Chirilă, P. Reinhard, F. Pianezzi, P. Bloesch, A.R. Uhl, C. Fella, L. Kranz, D. Keller, C. Gretener, H. Hagendorfer et al., Nat. Mater. 12, 1107 (2013) [CrossRef] [PubMed] [Google Scholar]
  3. P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte, M. Powalla, Phys. Status Solidi (RRL) − Rapid Res. Lett. 10, 583 (2016) [CrossRef] [Google Scholar]
  4. T. Kato, J.L. Wu, Y. Hirai, H. Sugimoto, V. Bermudez, IEEE J. Photovolt. 9, 325 (2019) [CrossRef] [Google Scholar]
  5. J. Lindahl, J. Keller, O. Donzel-Gargand, P. Szaniawski, M. Edoff, T. Törndahl, Sol. Energy Mater. Sol. Cells 144, 684 (2016) [CrossRef] [Google Scholar]
  6. F. Larsson, O. Donzel-Gargand, J. Keller, M. Edoff, T. Törndahl, Sol. Energy Mater. Sol. Cells 183, 8 (2018) [CrossRef] [Google Scholar]
  7. J. Löckinger, S. Nishiwaki, C. Andres, R. Erni, M.D. Rossell, Y.E. Romanyuk, S. Buecheler, A.N. Tiwari, ACS Appl. Mater. Interfaces 10, 43603 (2018) [CrossRef] [Google Scholar]
  8. S. Prabahar, M. Dhanam, J. Cryst. Growth 285, 41 (2005) [CrossRef] [Google Scholar]
  9. F. Pianezzi, P. Reinhard, A. Chirilă, B. Bissig, S. Nishiwaki, S. Buecheler, A.N. Tiwari, Phys. Chem. Chem. Phys. 16, 8843 (2014) [CrossRef] [PubMed] [Google Scholar]
  10. O. Donzel-Gargand, F. Larsson, T. Törndahl, L. Stolt, M. Edoff, Prog. Photovolt. Res. Appl. 27, 220 (2019) [CrossRef] [Google Scholar]
  11. H. Simchi, B.E. McCandless, K. Kim, J.H. Boyle, R.W. Birkmire, W.N. Shafarman, IEEE J. Photovolt. 2, 519 (2012) [CrossRef] [Google Scholar]
  12. N.H. Valdes, K.J. Jones, R.L. Opila, W.N. Shafarman, IEEE J. Photovolt. 9, 1846 (2019) [CrossRef] [Google Scholar]
  13. D. Hariskos, S. Spiering, M. Powalla, Thin Solid Films 480–481, 99 (2005) [CrossRef] [Google Scholar]
  14. S. Siebentritt, Solar Energy 77, 767 (2004) [CrossRef] [Google Scholar]
  15. N. Naghavi, D. Abou-Ras, N. Allsop, N. Barreau, S. Bücheler, A. Ennaoui, C.H. Fischer, C. Guillen, D. Hariskos, J. Herrero et al., Prog. Photovolt. Res. Appl. 18, 411 (2010) [CrossRef] [Google Scholar]
  16. J. Chantana, Y. Kawano, T. Nishimura, Y. Kimoto, T. Kato, H. Sugimoto, T. Minemoto, Prog. Photovolt. Res. Appl., pip. 28, 79 (2020) [CrossRef] [Google Scholar]
  17. J. Chantana, Y. Kawano, T. Nishimura, Y. Kimoto, T. Kato, H. Sugimoto, T. Minemoto, ACS Appl. Energy Mater. 3, 1292 (2020) [Google Scholar]
  18. O. Taurian, M. Springborg, N. Christensen, Solid State Commun. 55, 351 (1985) [CrossRef] [Google Scholar]
  19. Ü. Özgür, Y.I. Alivov, C. Liu, A. Teke, M. Reshchikov, S. Doğan, V. Avrutin, S.J. Cho, H. Morkoç, J. Appl. Phys. 98, 11 (2005) [Google Scholar]
  20. C. Kılıç, A. Zunger, Appl. Phys. Lett. 81, 73 (2002) [CrossRef] [Google Scholar]
  21. A. Klein, J. Phys.: Condens. Matter 27, 134201 (2015) [CrossRef] [Google Scholar]
  22. T. Minemoto, Y. Hashimoto, T. Satoh, T. Negami, H. Takakura, Y. Hamakawa, J. Appl. Phys. 89, 8327 (2001) [CrossRef] [Google Scholar]
  23. M. Murata, J. Chantana, N. Ashida, D. Hironiwa, T. Minemoto, Japanese J. Appl. Phys. 54, 032301 (2015) [CrossRef] [Google Scholar]
  24. Y. Kuwahata, T. Minemoto, Renew. Energy 65, 113 (2014) [CrossRef] [Google Scholar]
  25. T. Törndahl, C. Platzer-Björkman, J. Kessler, M. Edoff, Prog. Photovolt. Res. Appl. 15, 225 (2006) [CrossRef] [Google Scholar]
  26. C.-S. Lee, S. Kim, E.A. Al-Ammar, H. Kwon, B.T. Ahn, ECS J. Solid State Sci. Technol. 3, Q99 (2014) [CrossRef] [Google Scholar]
  27. J. Pettersson, C. Platzer-Björkman, M. Edoff, Prog. Photovolt. Res. Appl. 17, 460 (2009) [CrossRef] [Google Scholar]
  28. S. Kim, C.S. Lee, S. Kim, R. Chalapathy, E.A. Al-Ammar, B.T. Ahn, Phys. Chem. Chem. Phys. 17, 19222 (2015) [CrossRef] [PubMed] [Google Scholar]
  29. T. Minemoto, J. Julayhi, Curr. Appl. Phys. 13, 103 (2013) [CrossRef] [Google Scholar]
  30. M. Sugiyama, H. Sakakura, S.W. Chang, Electrochim. Acta 131, 236 (2014) [CrossRef] [Google Scholar]
  31. J. Chantana, T. Kato, H. Sugimoto, T. Minemoto, Prog. Photovolt. Res. Appl. 26, 127 (2018) [CrossRef] [Google Scholar]
  32. J. Löckinger, S. Nishiwaki, T.P. Weiss, B. Bissig, Y.E. Romanyuk, S. Buecheler, A.N. Tiwari, Sol. Energy Mater. Sol. Cells 174, 379 (2017) [Google Scholar]
  33. J. Serhan, Z. Djebbour, W. Favre, A. Migan-Dubois, A. Darga, D. Mencaraglia, N. Naghavi, G. Renou, J.F. Guillemoles, D. Lincot, Thin Solid Films 519, 7606 (2011) [CrossRef] [Google Scholar]
  34. N. Naghavi, S. Temgoua, T. Hildebrandt, J.F. Guillemoles, D. Lincot, Prog. Photovolt. Res. Appl. 23, 1820 (2015) [CrossRef] [Google Scholar]
  35. T. Lavrenko, T. Walter, B. Plesz, Phys. Status Solidi C 14, 1600197 (2017) [Google Scholar]
  36. I. Repins, S. Glynn, T.J. Silverman, R. Garris, K. Bowers, B. Stevens, L. Mansfield, Prog. Photovolt. Res. Appl. 27, 749 (2019) [Google Scholar]
  37. J. Serhan, Z. Djebbour, A. Darga, D. Mencaraglia, N. Naghavi, G. Renou, D. Lincot, J.F. Guillemeoles, Sol. Energy Mater. Sol. Cells 94, 1884 (2010) [CrossRef] [Google Scholar]
  38. N. Naghavi, G. Renou, V. Bockelee, F. Donsanti, P. Genevee, M. Jubault, J. Guillemoles, D. Lincot, Thin Solid Films 519, 7600 (2011) [CrossRef] [Google Scholar]
  39. S. Lany, A. Zunger, J. Appl. Phys. 100, 113725 (2006) [CrossRef] [Google Scholar]
  40. S. Lany, A. Zunger, Phys. Rev. Lett. 100, 016401 (2008) [CrossRef] [Google Scholar]
  41. W.S. Choi, J.G. Yoon, Solid State Commun. 152, 345 (2012) [CrossRef] [Google Scholar]
  42. F. Troni, G. Sozzi, R. Menozzi, A numerical study of the design of ZnMgO window layer for Cadmium-free thin-film CIGS solar cells, in 2011 7th Conference on Ph. D. Research in Microelectronics and Electronics (IEEE, 2011), pp. 193–196 [Google Scholar]
  43. R. Carron, S. Nishiwaki, T. Feurer, R. Hertwig, E. Avancini, J. Löckinger, S.C. Yang, S. Buecheler, A.N. Tiwari, Adv. Energy Mater. 9, 1900408 (2019) [CrossRef] [Google Scholar]
  44. B. Bissig, R. Carron, L. Greuter, S. Nishiwaki, E. Avancini, C. Andres, T. Feurer, S. Buecheler, A.N. Tiwari, Prog. Photovolt. Res. Appl. 26, 894 (2018) [Google Scholar]
  45. T. Minemoto, T. Matsui, H. Takakura, Y. Hamakawa, T. Negami, Y. Hashimoto, T. Uenoyama, M. Kitagawa, Sol. Energy Mater. Sol. Cells 67, 83 (2001) [CrossRef] [Google Scholar]
  46. C.S. Lee, Y.M. Shin, B.T. Ahn, ECS Trans. 41, 213 (2011) [CrossRef] [Google Scholar]
  47. E. Avancini, R. Carron, T.P. Weiss, C. Andres, M. Bürki, C. Schreiner, R. Figi, Y.E. Romanyuk, S. Buecheler, A.N. Tiwari, Chem. Mater. 29, 9695 (2017) [CrossRef] [Google Scholar]
  48. T. Nakada, A. Kunioka, Appl. Phys. Lett. 74, 2444 (1999) [CrossRef] [Google Scholar]
  49. T.M. Friedlmeier, P. Jackson, D. Kreikemeyer-Lorenzo, D. Hauschild, O. Kiowski, D. Hariskos, L. Weinhardt, C. Heske, M. Powalla, A closer look at initial CdS growth on high-efficiency Cu(In,Ga)Se2 absorbers using surface-sensitive methods, in 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC) , (IEEE, 2016), pp. 0457–0461 [Google Scholar]
  50. C. Platzer-Björkman, T. Törndahl, A. Hultqvist, J. Kessler, M. Edoff, Thin Solid Films 515, 6024 (2007) [CrossRef] [Google Scholar]
  51. F. Larsson, L. Stolt, A. Hultqvist, M. Edoff, J. Keller, T. Törndahl, ACS Appl. Energy Mater. 3, 7208 (2020) [Google Scholar]
  52. T. Nakada, H. Ohbo, T. Watanabe, H. Nakazawa, M. Matsui, A. Kunioka, Sol. Energy Mater. Sol. Cells 49, 285 (1997) [CrossRef] [Google Scholar]
  53. K. Nakada, M. Watanabe, T. Nishimura, N. Suyama, A. Yamada, Japanese J. Appl. Phys. 59, 031005 (2020) [CrossRef] [Google Scholar]
  54. W. Witte, D. Hariskos, M. Powalla, Thin Solid Films 519, 7549 (2011) [CrossRef] [Google Scholar]
  55. J. Keller, F. Gustavsson, L. Stolt, M. Edoff, T. Törndahl, Sol. Energy Mater. Sol. Cells 159, 189 (2017) [CrossRef] [Google Scholar]
  56. P. Alen, M. Juppo, M. Ritala, T. Sajavaara, J. Keinonen, M. Leskelä, J. Electrochem. Soc. 148, G566 (2001) [CrossRef] [Google Scholar]
  57. K. Koike, K. Hama, I. Nakashima, S. Sasa, M. Inoue, M. Yano, Japanese J. Appl. Phys. 44, 3822 (2005) [CrossRef] [Google Scholar]
  58. G. Luka, L. Wachnicki, B.S. Witkowski, T.A. Krajewski, R. Jakiela, E. Guziewicz, M. Godlewski, Mater. Sci. Eng. B 176, 237 (2011) [CrossRef] [Google Scholar]
  59. Y. Inoue, M. Hala, A. Steigert, R. Klenk, S. Siebentritt, Optimization of buffer layer/i-layer band alignment, in 2015 IEEE 42nd Photovoltaic Specialist Conference, PVSC 2015 Institute of Electrical and Electronics Engineers Inc. (2015) [Google Scholar]
  60. W. Metzger, I. Repins, M. Romero, P. Dippo, M. Contreras, R. Noufi, D. Levi, Thin Solid Films 517, 2360 (2009) [CrossRef] [Google Scholar]
  61. Y. Ando, S. Ishizuka, S. Wang, J. Chen, M.M. Islam, H. Shibata, K. Akimoto, T. Sakurai, Jpn. J. Appl. Phys. 57, 08RC08 (2018) [CrossRef] [Google Scholar]
  62. R. Scheer, H.W. Schock, Chalcogenide photovoltaics: physics, technologies, and thin film devices , (John Wiley & Sons, Weinheim, 2011) [CrossRef] [Google Scholar]

Cite this article as: Ramis Hertwig, Shiro Nishiwaki, Mario Ochoa, Shih-Chi Yang, Thomas Feurer, Evgeniia Gilshtein, Ayodhya N. Tiwari, Romain Carron, ALD-ZnMgO and absorber surface modifications to substitute CdS buffer layers in co-evaporated CIGSe solar cells, EPJ Photovoltaics 11, 12 (2020)

All Tables

Table 1

PV parameters of devices with KCN only surface treatment, see Figure 2a. Percentages correspond to the Mg content of the buffer layer.

Table 2

PV parameters of devices with KCN and NH4OH surface treatment, see Figure 2b. Percentages correspond to the Mg content of the buffer layer.

Table 3

PV parameters of devices with KCN and CBD-CdS alike surface treatment, see Figures 3a and 3b. All devices have buffer layers of ALD deposited Zn0.84Mg0.16O, 28 nm.

Table 4

PV parameters of best devices with single TMA pulse during buffer deposition. “B” refers to a single cycle of TMA before buffer deposition, “M” refers to a single TMA pulse midway during buffer deposition.

Table 5

PV parameters of best performing device before and after light soaking at 1 sun, measured 90 days after finishing the cell.

All Figures

thumbnail Fig. 1

(a) Mg/(Zn+Mg) content in Zn1−xMgxO films as determined by XPS as function of the nominal value based on the number of pulses. (b) Zn1−xMgxO film thickness with number of deposition pulses as determined by ellipsometry. A linear fit (line) results in layer growth of 1.65 Å per pulse.

In the text
thumbnail Fig. 2

(a) IV curves of samples treated with KCN only before buffer deposition. (b) IV curves of samples subjected to KCN and NH4OH treatment. (c) EQE of samples with absorbers treated with KCN and NH4OH.

In the text
thumbnail Fig. 3

(a) Effect of wet treatment before buffer deposition on JV curves. Absorber treatments with NH4OH or thiourea-ammonia solution show increased V OC. Additional (b) Influence of a single TMA pulse before and during buffer deposition, with unchanged total ZnMgO thickness. TMA before Zn1−xMgxO increases V OC and reduces ageing. TMA during Zn1−xMgxO reduces V OC.

In the text
thumbnail Fig. 4

(a) IV of Zn1−xMgxO buffer device before and after different light soaking duration at 1 sun. (b) Ideality factor extracted from J SC –V OC measurements for CdS reference (circle) and Zn1−xMgxO buffer device before (triangle) and after light soaking (square).

In the text
thumbnail Fig. 5

Extrapolation of V OC at 0 K for (a) CdS reference, (b) Zn1−xMgxO buffer (c) Zn1−xMgxO buffer after light soaking for 1 h at 1 sun.

In the text

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