NaF Precursor Research Articles

Precise Alkali Supply during and after Growth for High‐Performance Low Bandgap (Ag,Cu)InSe2 Solar Cells

Alkali treatments are crucial for low bandgap (Ag,Cu)InSe2 (ACIS) and Cu(In,Ga)Se2‐based solar cell performance. Traditionally, Ag‐alloying of CIS (ACIS) is grown on soda‐lime glass (SLG) at temperatures exceeding 500 °C, resulting in uncontrolled alkali diffusion from the substrate and variable photovoltaic properties. A substrate‐independent low‐bandgap ACIS growth process is introduced and the impact of controlled supplies of NaF and RbF alkali fluorides before and after absorber growth through precursor layers and post‐deposition treatments (PDT) are investigated. NaF and RbF precursor layers enhance carrier lifetimes and doping density, outperforming the previous SLG‐dependent strategy. Even small quantities of RbF significantly enhance device performance, while specific NaF amount during deposition are necessary to limit grain growth and achieve high doping densities and lifetimes. A certain density of grain boundaries appears crucial for high doping levels. Although subsequent NaF post‐deposition treatment (PDT) does not provide additional benefits with sufficient Na during growth, RbF‐PDT remains crucial. The best performance is achieved with a combination of NaF and RbF precursor layers along with RbF‐PDT, resulting in over 19% efficiency, 605 mV open‐circuit voltage (VOC), 73% fill factor (FF), a carrier density of 3 × 1016 cm−3, and a 700 ns lifetime. This approach supports high‐efficiency ACIS solar cell advancement, particularly for thin‐film tandem photovoltaic devices.

Wide‐Gap Chalcopyrite Solar Cells with Indium Oxide–Based Transparent Back Contacts

Herein, the performance of wide‐gap Cu(In,Ga)Se2 (CIGS) and (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells with In2O3:Sn (ITO) and In2O3:H (IOH) as transparent back contact (TBC) materials is evaluated. Since both TBCs restrict sodium in‐diffusion from the glass substrate, fine‐tuning of a NaF precursor layer is crucial. It is found that the optimum Na supply is lower for ACIGS than for CIGS samples. An excessive sodium amount deteriorates the solar cell performance, presumably by facilitating GaO x growth at the TBC/absorber interface. The efficiency (η) further depends on the absorber stoichiometry, with highest fill factors (and η) reached for close‐stoichiometric compositions. An ACIGS solar cell with η = 12% at a bandgap of 1.44 eV is processed, using IOH as a TBC. The best CIGS device reaches η = 11.2% on ITO. Due to its very high infrared transparency, IOH is judged superior to ITO for implementation in a top cell of a tandem device. However, while ITO layers maintain their conductivity, IOH films show an increased sheet resistance after absorber deposition. Chemical investigations indicate that incorporation of Se during the initial stage of absorber processing may be responsible for the deteriorated conductivity of the IOH back contact in the final device.

Defect passivation in Cu2ZnSnSe4 thin-film solar cells by novel sodium doping

Sodium (Na) alkali doping is one of the effective ways to improve Cu2ZnSnSe4 (CZTSe) device performance by passivating detrimental native defects caused by off-stoichiometric composition and Cu–Zn cationic disordering. Various methods to incorporate Na in CZTSe films have been considered to realize the ideal distribution of Na in CZTSe films. Na content, regardless of Na incorporation method, is significantly increased in bulk CZTSe. Interestingly, when a NaF precursor is supplied after CZTSe film growth, the surface Na content is slightly increased, resulting in improved device performance. For the first time, a sodium bifacial treatment was employed to obtain an optimal profile of Na in CZTSe films. Using this novel route for Na doping, the photovoltaic (PV) device exhibited a maximum efficiency of 6.02%, which was higher than that compared to CZTSe devices with single Na doping and CZTSe device with Na diffused from soda-lime glass. According to defect analyses, the improved device performance is associated with a reduction in the defect density from 1.1 × 1016 (without Na doing) to 5.7 × 1014 cm−3 (with optimal Na profile). These results indicate that the proposed bifacial treatment method is a desirable means to fabricate high-efficiency, flexible CZTSe solar cells.

Routes to develop a [S]/([S]+[Se]) gradient in wide band-gap Cu2ZnGe(S,Se)4 thin-film solar cells

Wide band-gap kesterite-based solar cells are very attractive to be used for tandem devices as well as for semi-transparent photovoltaic cells. Here, Cu2ZnGe(S,Se)4 (CZGSSe) thin films have been grown by sulfurization of co-evaporated Cu2ZnGeSe4. The influence of a NaF precursor layer and of a Se capping film on CZGSSe absorbers and solar cells has been investigated. It has been found that the distribution of [S]/([S]+[Se]) through the CZGSSe absorber layer is strongly dependent on the Na content. Na promotes the diffusion of S towards the bulk of the absorber layer. Thicker NaF layers>6 nm lead to a higher S content in the bulk of the absorber layer, but to a decreased accumulation of sulphur at the surface, as detected by GIXRD, GD-OES, and Raman spectroscopy measurements. A relationship between Jsc, FF and Na-content supplied was found; higher Na content resulted in improved solar cell efficiencies. It has also been possible to modify the [S]/([S]+[Se])-gradient throughout the CZGSSe film by the absence of the Se capping layer, achieving devices with 2.7% performance and Eg = 2.0 eV. This work reveals two ways to control the [S]/([S]+[Se]) depth-profile to produce wide band gap CZGSSe absorber layers for efficient solar cells.

Effect of Na and the back contact on Cu2Zn(Sn,Ge)Se4 thin-film solar cells: Towards semi-transparent solar cells

Cu2ZnSn1-xGexSe4 (CZTGSe) thin films have been grown onto Mo/SLG and Mo/V2O5/FTO/SLG substrates using thermal co-evaporation followed by a subsequent thermal annealing. A NaF precursor layer was evaporated prior to the deposition of the kesterite absorber layer. In the samples grown on Mo/SLG, it has been found that Na promotes Ge incorporation into the Cu2ZnSnSe4 lattice. The high concentration of incorporated Ge leads to the segregation of Sn-Se secondary phases as well as to an accumulation of Sn next to the Mo layer. The use of 12 and 16 nm NaF thick precursor layers prior to the CZTGSe deposition leads to absorber band gaps of 1.30 and 1.34 eV, and to device performances of 4.7 and 4.0%, respectively. A higher Na content, furthermore, caused the formation of bigger grains, a higher charge carrier concentration and a shorter depletion width. A 12 nm NaF precursor layer was used for the devices grown on FTO-based substrates, producing an optimal back contact that allows achieving efficiencies of 5.6% and transmittance of 30% in the near infrared range. This enhanced performance can be associated with the absence of secondary phases and Ge distribution through the absorber layer. The formation of a MoSe2 layer at the back interface in all the investigated devices seems to play a crucial role to improve the solar cell efficiency.

Effect of NaF pre-cursor on alumina and hafnia rear contact passivation layers in ultra-thin Cu(In,Ga)Se2 solar cells

In this work, we evaluate the effect of NaF layers on the properties of Al2O3 and HfO2 rear contact passivation layers in ultra-thin Cu(In,Ga)Se2 solar cells. The 6 nm thin passivation layers were deposited by atomic layer deposition and neither intentionally opened nor nano-patterned in any extra-fabrication step. NaF layers, 7.5 or 15 nm thin, were deposited as precursors prior to CIGS absorber co-evaporation. The 215 nm thick absorbers were co-evaporated with constant evaporation rates for all elements. Directly thereafter, a 70 nm thick cadmium sulfide layer was deposited. Photoluminescence measurements indicate a strongly reduced recombination at the rear contact for all passivated samples compared to an unpassivated reference. Although the sample with Al2O3 passivation and a 15 nm NaF precursor layer luminesces by far the least of the passivated samples, solar cells made from this sample show the highest efficiency (8.6% compared with 5.6% for the reference with no passivation). The current-voltage curves of the solar cells fabricated from the sample with 7.5 nm NaF on top of the Al2O3 layer and both samples with HfO2 exhibit blocking behavior to various degrees, but a high photoluminescence response. We conclude that NaF precursor layers increase conduction through the Al2O3 layer, but also reduce its effectiveness as a passivation layer. In contrast, conduction through the HfO2 passivation layers seem to not be influenced by NaF precursor layers, and thus requires nano-patterning or thinning for conduction.

Pre-incorporation of Na into flexible Cu(In,Ga)Se2 thin film solar cells

Incorporation of sodium into the Cu(In,Ga)Se2 (CIGS) absorber layer is a key step for the growth of high conversion efficiency CIGS solar cells. However, the doping of Na into CIGS layer on a flexible substrate, i.e. Titanium foil, is still a challenge. In this paper, we report our results on comparing two different Na pre-incorporation methods in constructing flexible CIGS solar cells on Ti substrates: i) by sputtering a soda-lime glass thin film (SLGTF) onto Ti substrates prior to the growth of CIGS and Mo back contact layers and ii) by evaporating a NaF precursor layer on the Mo layer before the CIGS growth. By carefully optimizing the growth conditions, the conversion efficiencies of 13.47% and 15.02% are achieved for the SLGTF sample and the NaF sample, respectively. The NaF precursor method shows a better effect. The diffusion of Na and the gradient of Ga play important roles in the performance of the solar cells.

Effect of different Na supply methods on thin Cu(In,Ga)Se2 solar cells with Al2O3 rear passivation layers

In this work, rear-contact passivated Cu(In,Ga)Se2 (CIGS) solar cells were fabricated without any intentional contact openings between the CIGS and Mo layers. The investigated samples were either Na free or one of two Na supply methods was used, i) a NaF precursor on top of the Al2O3 rear passivation layer or ii) an in situ post-deposition treatment with NaF after co-evaporation of the CIGS layer. The thickness of the ALD-Al2O3 passivation layer was also varied in order to find an optimal combination of Na supply and passivation layer thickness. Our results from electrical characterization show remarkably different solar cell behavior for different Na supplies. For up to 1 nm thick Al2O3 layers an electronically good contact could be confirmed independently of Na deposition method and content. When the Al2O3 thickness exceeded 1 nm, the current was blocked on all samples except on the samples with the NaF precursor. On these samples the current was not blocked up to an Al2O3 layer thickness of about 6 nm, the maximum thickness we could achieve without the CIGS peeling off the Al2O3 layer. Transmission electron microscopy reveals a porous passivation layer for the samples with a NaF precursor. An analysis of the dependence of the open circuit voltage on temperature (JVT) indicates that a thicker NaF precursor layer lowers the height of the hole barrier at the rear contact for the passivated cells. This energy barrier is also lower for the passivated sample, compared to an unpassivated sample, when both samples have been post-deposition treated.

Sulfurization of co-evaporated Cu2ZnSnSe4 thin film solar cells: The role of Na

The sulfurization of co-evaporated Cu2ZnSnSe4 (CZTSe) thin films is studied. In this work, a relationship between the Na concentration and the further S incorporation in the CZTSe absorber layer is found. Na is added by a NaF precursor layer before CZTSe co-evaporation and/or by diffusion from the soda-lime glass substrate. Higher Na concentrations result in bigger grain sizes, higher S/(S + Se) atomic ratios and photovoltaic devices with higher VOC, as investigated by SEM, GIXRD, Raman and I-V measurements. Results using an alternative substrate as ceramic verify the importance of the control of the Na content, which alters the carrier concentrations and elements distribution with much higher S and Na concentrations at the surface in this particular case. By using this methodology, it is possible to tune the kesterite band gap energy by the creation of S-gradient through the absorber thin film. This may provide a new approach for the control of Cu2ZnSn(S,Se)4 growth and development of high efficiency kesterite solar cells. Efficiencies of 5.5% and 6.4% are obtained using ceramic and SLG substrates respectively (without antireflection coatings and without grids).

Influence of Na in Cu2SnS3 film on its physical properties and photovoltaic performances

Cu2SnS3 (CTS) is the promising absorber of thin-film solar cell due to earth-abandant and non-toxic element. Influence of sodium (Na) on physical properties of CTS layers is investigated. In this work, CTS films with different Na concentrations are fabricated by sulfurizations of NaF/Cu/SnS2 and Cu/SnS2 stacked precursors on both soda-lime glass (SLG) and quartz substrates. The Na from NaF precursor layer and SLG substrate is introduced into CTS films. It is demonstrated that Na in forms of NaxCuSnS3 (x: 0.5–1.0) and Na2S is segregated in the large grain regions of CTS films. The crystal grains are significantly enlarged by Na addition into CTS films. As a result, the carrier lifetime, examined by time-resolved photoluminescence, is enhanced. From Raman scattering measurement, CTS films possess the peaks of monoclinic CTS structure. In addition, Cu2Sn3S7 and NaxCuSnS3 peaks are clearly observed in CTS films with high Na content. Finally, conversion efficiency of CTS thin-film solar cell is strongly enhanced from 0.61 (without Na addition) to 2.01% (with Na addition), mainly attributed to the increases in open-circuit voltage and fill factor through the decrease in carrier recombination.

Introduction of Na into Cu2SnS3 thin film for improvement of its photovoltaic performances

Impact of Na in Cu2SnS3 (CTS) films on their physical properties and photovoltaic performances is investigated. CTS films are fabricated by sulfurization of NaF/Cu/SnS2 stacked precursors. NaF precursor is used as primary Na source for Na introduction into the CTS films. It is demonstrated that Na content in the resulting CTS films is increased with enhancing thickness of NaF precursor, leading to the increase in Na/Cu atomic ratio but decrease in Cu/Sn atomic ratio. This suggests that the addition of NaF capping layer on the Cu/SnS2 stacked precursors suppresses evaporation of Sn-S compounds from the sample during sulfurization, thus forming Sn2S3 secondary. Therefore, with too thick NaF layer (greater than or equal to 75nm), Sn2S3 secondary phase in bulk and near surface of the resulting CTS films, having the detrimental effect on cell performances, is severely formed. However, the CTS grain sizes are notably enlarged as the thickness of NaF precursor increases. As a result, conversion efficiency of CTS thin-film solar cell with anti-reflective layer is increased up to 4.8% with the optimized NaF thickness of 60nm since the single-phase and large-grain CTS absorbing layer without Sn2S3 secondary phase near the surface is attained, thereby increasing open-circuit voltage and fill factor attributed to the decrease in the carrier recombination.

Synthesis and Characterization of Na3V2(PO4)2F3 Based Cathode Material for Sodium Ion Batteries

Na3V2(PO4)2F3 was synthesized via two-step carbothermal reduction from NaF and VPO4 intermediate precursor and tested as cathode material for aqueous and non-aqueous Na-ion batteries. The synthesized material was characterized using XRD, SEM-EDX, TOF-SIMS. The cyclic voltammetry and charge/discharge measurements for non-aqueous system indicated that the intercalation / deintercalation of Na+ ions is being carried out in two steps: the first one at 3.5/3.7 V and the second one at 4.1/4.3 V, indicative for a structural transformation of the cathode material. Discharge capacity of 55 mA g-1 was measured for the first cycle. However, the capacity decreased nearly 50 %, to 22 − 25 mA g-1 after the second galvanostatic cycle within 3.0 – 4.5 V. Discharge capacity decreases during the first 10 cycles but stabilizes after 50 cycles, maintaining ~ 20 mA g-1 after 3000 cycles. Cycling in aqueous media showed even faster capacity fade.

In situ process monitoring during multistage coevaporation of Cu2ZnSnS4 thin films

A multistage coevaporation process for the direct growth of Cu2ZnSnS4 (CZTS) thin films without additional atmospheric sulfurization was investigated. To obtain reproducible CZTS films, in situ process monitoring of the film growth was developed by measuring the apparent substrate temperature (Tpyro) using a pyrometer. After CZTS depositions terminated at various endpoints, ex situ characterization of the film properties was performed to clarify the growth mechanism of the films. The results provided clear evidence that CZTS phase formation was significantly delayed via re-evaporation of Sn–S-based compounds in the early part of the first stage, leading to the initial formation of a dominant (CuS + ZnS) structure that coexisted with a small amount of CZTS. CZTS phase formation was then facilitated by the (CuS + ZnS) precursor via a Cu-rich to Cu-poor sequence with an apparent variation in Tpyro during the second stage, and the slightly segregated CuS phase was nearly consumed under (Zn + Sn + S) fluxes. Consequently, CZTS thin films containing close-packed grains with a single kesterite structure were successfully grown under excess Sn and S fluxes, even at moderate Tsub below 500 °C. The best solar cell with a Glass/Mo/CZTS[Cu/(Zn + Sn) = 0.71, Zn/Sn = 1.6]/CdS/ZnO:Ga structure and a NaF precursor layer yielded an active area (0.170 cm2) efficiency of 3.84% (Voc = 567 mV, Jsc = 11.3 mA/cm2, and FF = 0.603). The diode properties under dark and light conditions were also evaluated.

Developments of High-Efficiency Flexible Cu(In,Ga)Se2 Thin Film Solar Cells on a Polyimide Sheet by Sodium Incorporation

We present the fabrication of flexible Cu(In,Ga)Se2 (CIGS) solar cells on a polyimide (PI) sheet with and without Na incorporation. A sodium element is incorporated into the CIGS absorber by using a NaF precursor after Mo back contact deposition. X-ray diffraction patterns show that the (112) preferred orientation of the as-grown CIGS films is decreased by Na incorporation. The secondary phase of (Inx,Ga1−x)2Se3 is observed for the CIGS films with Na. There is no significant difference in the grain size with and without Na incorporation from surface and cross-sectional SEM images. Additionally, the increase of carrier concentration and decrease of resistivity of CIGS absorber are induced by Na doping. Finally, the flexible CIGS solar cells on PI sheets with efficiency close to 11%, containing Na, are achieved. The improvement of cell efficiency can be attributed to the modified electrical properties of the CIGS film by Na incorporation.

Incorporation of Na in Cu(In,Ga)Se<inline-formula><tex-math>$_{\bf 2}$</tex-math></inline-formula> Thin-Film Solar Cells: A Statistical Comparison Between Na From Soda-Lime Glass and From a Precursor Layer of NaF

The presence of Na in Cu(In,Ga)Se 2 layers increases the electrical performance of this type of thin-film solar cell. A detailed comparison of incorporating Na in the CIGS layer by two different methods is performed by evaluating several hundred devices fabricated under similar conditions. The first method is based on the conventionally used Na diffusion from the soda-lime glass substrate, whereas the second method is based on a NaF precursor layer deposited on a Mo-coated alkali-free glass substrate. The sample where Na is introduced by using a NaF precursor layer shows an orientation weighted toward (2 0 4)/(2 2 0) and a net acceptor concentration of 3.4 × 10 16 cm -3 , while SLG shows a (1 1 2) orientation with a 2.9 × 10 16 cm -3 acceptor concentration. Both sample types show close identical elemental depth profiles, morphology, and electrical performance.

Na Doping of CIGS Solar Cells Using Low Sodium-Doped Mo Layer

Na plays an important role in the electrical performance of Cu(In,Ga)Se 2 (CIGS) thin-film solar cells. Traditionally, Na has been introduced during the growth of CIGS by thermal diffusion from the soda-lime glass (SLG) substrate; however, better control of the amount of Na is needed to have a more precise control of growth conditions. The introduction of Na into CIGS was studied in three different ways: from the SLG, from a NaF precursor, and from a Na-doped Mo (MoNa) back contact. The most successful approaches were obtained by using the conventional SLG and the NaF precursor. Different growth temperatures of CIGS were tested in an attempt to diffuse more Na from the MoNa layer.

Cu(In,Ga)Se$_{\bf 2}$ Solar Cells With Varying Na Content Prepared on Nominally Alkali-Free Glass Substrates

In this paper, Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) thin-film solar cells are prepared on nominally alkali-free glass substrates using an in-line CIGS growth process. As compared with, for example, borosilicate glass or quartz, the glass is engineered to have similar thermal expansion coefficient as soda-lime glass (SLG) but with alkali content close to zero. Na is incorporated in the CIGS material using an ex-situ deposited NaF precursor layer evaporated onto the Mo back contact. Several thicknesses of the NaF layer were tested. The results show that there is a process window, between 15 and 22.5 nm NaF, where the solar cell conversion efficiency is comparable with or exceeding that of SLG references. The effect of an NaF layer that is too thin on the solar cell parameters was mainly lowering the open-circuit voltage, which points to a lower effective dopant concentration in the CIGS layer and is also consistent with presented <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> measurements and modeling results. For excessively thick NaF layers, delamination of the CIGS layer occurred. Additional measurements, such as scanning electron microscopy (SEM), secondary ion mass spectrometry, capacitance-voltage analysis ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> ), time-resolved photoluminescence (TRPL), external quantum efficiency (EQE), current-voltage analysis (J-V), and modeling, are presented, and the results are discussed.

Characterization of Cu(In,Ga)Se2 Solar Cells Grown on Na-Free Glass with an NaF Layer on a Mo Film

Cu(In,Ga)Se2 (CIGS) solar cells fabricated on soda-lime glass (SLG) exhibited high efficiency due to the supply of Na from the SLG substrate. As a simple doping method, Na can be supplied into CIGS films using a Na compound deposited on Mo electrodes. In this study, the authors compared the properties of CIGS thin films grown on a NaF layer deposited on Mo-coated Na-free glass with those of CIGS films on Mo-coated SLG by a standard through high-resolution transmission electron microscopy and low-temperature photoluminescence analyzes. After NaF deposition on the Mo film, an amorphous interlayer was detected at the CIGS/MoSe2 interface, and the MoSe2 layer that formed on the Mo surface was thin. The photoluminescence study showed that NaF doping did not effectively prevent the formation of deep donors in the CIGS film, whereas Na supplied by SLG effectively prevents their formation. We concluded that the poor performance of CIGS solar cells incorporating a NaF precursor is due to the amorphous layer at the CIGS/Mo interface and different luminescence characteristic with standard CIGS films, resulting in a low minority carrier collection.

Analysis of NaF precursor layers during the different stages of the Cu(In,Ga)Se2 coevaporation process

ABSTRACTNaF precursor layers used for providing Na to Cu(In,Ga)Se2 (CIGS) grown on Na-free substrates have been studied. The NaF layers were deposited on top of the Mo back contact prior to the CIGS co-evaporation process. The co-evaporation process was interrupted after the preheating steps, and after part of the CIGS layer was grown. Completed samples were also studied. After the preheating, the NaF layers were analyzed with X-ray Photoelectron Spectroscopy and after growing part and all of the CIGS film, the Mo/NaF/CIGS stack was characterized using transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS). The NaF layers were found to be stable in thickness and composition during the pre-heating in selenium containing atmosphere before the CIGS process. The TEM analyses on the partly grown samples show a layer at the CIGS/Mo interface, which we interpret as a partly consumed NaF layer. This is corroborated by the SIMS analysis. In finalized samples the results are less clear, but TEM images show an increased porosity at the position of the NaF layer.

Effect of NaF precursor on preferential growth of Cu(In,Ga)Se2 thin films

We report the effect of NaF precursors on the preferential orientation of Cu(In,Ga)Se2 (CIGS) thin films. The CIGS films were grown on NaF/Mo/glass and Mo/glass substrates by using a multi-stage co-evaporation method. The 23-nm-thick NaF precursors were prepared by using an evaporation technique. The CIGS film with a NaF precursor shows a much higher I(112)/I(220/204) XRD peak intensity ratio than the film without it. The (112)- and (220/204)-oriented CIGS films were obtained from highly (0006)- and (30–30)-oriented (In,Ga)2Se3 (IGS) films, respectively. The IGS (0006) orientation was enhanced by the NaF precursor because the lattice misfit of IGS (0006) plane on NaF (−2.3%) was lower than that of IGS (0006) plane on Mo (5.1%) whereas the lattice misfit of IGS (30–30) plane on NaF (1.3%) was higher than that of IGS (30–30) plane on Mo (0.1%). The degree of lattice misfit is believed to play an important role in the preferential growth of CIGS films in the (112) orientation.