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This study reports on the influence of air annealing and Na incorporation into the absorber on the performance of Cu2ZnSnS4 (CZTS) bifacial solar cells with FTO and W/FTO back contacts. Na was incorporated by depositing similar to 12 nm thick NaF on the CZTS precursors prior to the sulfurization process via thermal evaporation. After sulfurization, some of the samples were annealed in air at 300 degrees C for 90 s and subsequently at 200 degrees C for 600 s. Transmission electron microscopy confirmed sulfurization of the W interlayer to form WS2 which improves the FTO ohmicity. Na incorporation improved grain size of the absorber as revealed by scanning electron microscopy. Non-annealed samples had the unwanted SnS2 phase while the air annealed samples, particularly those with both W interlayer and Na incorporation, were exempt from SnS2 phase, as was confirmed through grazing incident X-ray diffraction and Raman spectroscopy. These results suggest that absorber air annealing and Na incorporation enhance absorber crystal growth which is advantageous in reducing bulk carrier recombination. As a result, the efficiency was significantly improved from 3.0% for solar cells fabricated directly on FTO to 5.2% for those whose absorbers were air annealed, incorporated with Na and made on W/FTO. The latter also exhibits the highest external quantum efficiency response and calculated short circuit current density for both sides illumination. This indicates that the air annealing, Na incorporation and W interlayer are enhancing the performance of bifacial CZTS solar cells with FTO back contact for both back side and front side illumination.

Recently, a record open-circuit voltage (1.1 V) is obtained for wide-bandgap Cu2ZnGeS4 (CZGS) thin-film solar cells with a Zn1-xSnxOy (ZTO) buffer grown by atomic layer deposition (ALD) (N. Saini et al. Sol. RRL 2021, 2100837). However, a low short-circuit current and a small variation in device performance with ZTO properties are observed, suggesting similar interface properties. Since the CZGS absorber cannot be etched due to peeling, it is suggested that the presence of an oxide interlayer can influence the device performance. Here, the chemical and electronic properties of the near-surface region of CZGS absorber and its interface with ZTO buffers with varying properties (bandgap and thickness) are studied nondestructively by employing excitation-dependent X-ray photoelectron spectroscopy. The formation of Ge oxide species is indicated at the absorber surface during device fabrication, which is preserved during ALD. Further, it is shown that the ZTO/CZGS interface properties are similar and not influenced by variations in ZTO properties, which can explain the small variation in device performance. Thus, attention shall be given to possible absorber surface cleaning treatment typically applied before the buffer deposition for Ge-containing CZGS, and an optimization of the ALD ZTO buffer layer growth shall be considered for a non-etched absorber.

This contribution studies the effect of ordered vacancy compounds (OVCs) on the minority carrier collection in wide-gap (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells. For this purpose, three samples with different ([Ag]+[Cu])/([In]+[Ga]) (I/III) values were processed: 1) a very off-stoichiometric absorber (I/III = 0.31), consisting of isolated chalcopyrite patches embedded in a major OVC bulk phase, 2) a moderately off-stoichiometric absorber (I/III = 0.77) with OVC patches at the front and back interfaces and 3) a close-stoichiometric absorber (I/III = 0.94) with only very few, isolated OVC patches. For each of these samples, synchrotron-based X-ray fluorescence (XRF) was measured on a nanoscale (55 nm resolution), while simultaneously recording the X-ray beam induced current (XBIC). The results, complemented by transmission electron microscopy and electron-beam induced current analyses, clearly indicate that the presence of the OVC phase reduces the carrier collection and thus the short-circuit current density of off-stoichiometric wide-gap ACIGS solar cells.

Kesterites are currently viewed as one of the most promising candidates for earth abundant and benign elements to substitute critical raw materials in photovoltaic technologies and may also be suitable for low-noise, room-temperature, self-powered photodetectors. However, while the impact of buffer layers on kesterite solar cell efficiency has been an active area of investigation, links between photodetector performance and intermediate layers are yet to be addressed. Herein, the impact of cadmium sulfide buffer layers on the performance of kesterite (Cu2ZnSnS4) photodetectors is probed. Specifically, the effect of buffer layer thickness on various photodetector performance metrices is clarified, including noise current, spectral responsivity, noise equivalent power, frequency response, and specific detectivity. Devices with a 100 nm cadmium sulfide layer perform the best, achieving a linear dynamic range of 180 dB and frequency responses in the range of tens of kHz. The key loss mechanisms are identified, and it is found that the photodetector performance to be primarily limited by shunt resistance-induced thermal noise and defect-induced nonradiative losses. Furthermore, we estimate the upper radiative limit of specific detectivity to be approximately 1019 Jones. Our results highlight the potential of kesterites to be used as an interesting earth abundant candidate for photodetection applications.

Alloying a Cu(In,Ga)Se-2 (CIGS) solar cell absorber with silver to form (Ag,Cu)(In,Ga)Se-2 (ACIGS) is an effective route for improving the performance of CIGS-based thin-film solar cells by increasing the optical band gap and open-circuit voltage. While the role of Ag on the solar cell's performance and crystal structure has been analyzed, important gaps in our understanding remain, especially regarding the atomistic (short-range) structure. Previous X-ray absorption spectroscopy (XAS) results have shown that local atomic arrangements in Ag-free CIGS deviate from the long-range crystallographic structure deduced from X-ray diffraction (XRD). However, it is unclear how these structural deviations evolve with Ag alloying, particularly in the presence of Ga depth gradient. In this work, we employ angle-resolved XAS to probe the local environment of Se atoms within different depths of ACIGS absorbers with varying Ag content and Ga depth gradient. By complementing XAS results with X-ray diffraction measurements for long-range structures, glow discharge optical emission spectroscopy for elemental profiles, and scanning transmission electron microscopy for morphologies, changes in element-specific bond lengths, cell parameters, and anion displacement depending on compositions of Group [I] (Cu, Ag) and Group [III] (In, Ga) elements were mapped. The results suggest that the local atomic arrangement of the investigated ACIGS thin-film solar cell samples is depth-dependent and deviates from the long-range crystallographic structure. Possible reasons include tetragonal distortion or the presence of other phases or off-stoichiometry compounds. For the sample with the highest Ag content, increased bond lengths of Se-Group [I] atoms and Se-Ga are observed from the absorber bulk toward the near-absorber/buffer interface, whereas, in Ag-free CIGS, no significant changes are found. Results further indicate nonlinear anion displacement with Ag addition in the absorber bulk or with dep

Thin film solar cells spanning a wide range of compositions within the (Ag,Cu)(In,Ga)Se2 system are fabricated and characterised using current-voltage, external quantum efficiency and capacitance-based measurements. The stability of the devices over time, after dry annealing and after lightsoaking is evaluated, and the role of Ag and Ga content is explored. Ag-free CuInSe2 and Cu(In,Ga)Se2 are observed to be stable, however high-Ag, high-Ga (Ag,Cu)(In,Ga)Se2 is observed to exhibit fluctuations in short-circuit current. High-Ag (Ag,Cu)InSe2 is instead observed to have stable current, but open-circuit voltage and fill-factor are strongly responsive. Thus, it is indicated that high fractions of Ag in the material lead to significant stability concerns, with high Ga fractions affecting the way in which degradation manifests. Discussion of probable causes and mechanisms follows.

The stability of thin-film solar cells spanning a wide range of compositions within the (Ag,Cu)(In,Ga)Se-2 material system is evaluated over time, after dry-heat annealing and after light soaking, and the role of Ag and Ga content is explored. Ag-free CuInSe2 is relatively stable to annealing and storage, while Cu(In,Ga)Se-2 suffers a degradation of fill factor and carrier collection. High-Ga (Ag,Cu)(In,Ga)Se-2 suffers degradation of carrier collection after prolonged annealing, reducing the short-circuit current by approximate to 12%. Ga-free (Ag,Cu)InSe2 loses up to a third of open-circuit voltage and a quarter of fill factor after all treatments are applied. All samples suffer voltage losses after light soaking, with the Ga-free devices losing up to 50 mV and those containing Ga losing up to 90 mV. Ag incorporation leads to a significant reduction in doping, and a significant increase in the response of doping to treatments, with the depletion width of (Ag,Cu)(In,Ga)Se-2 samples expanding from approximate to 0.1 mu m as-grown to beyond 1.0 mu m after all treatments, compared to the Cu(In,Ga)Se-2 sample variation of approximate to 0.1-0.3 mu m. Connections between Ag content, doping instability, and performance degradation are discussed.

Ag alloying and the introduction of alkali elements through a postdeposition treatment are two approaches to improve the performance of Cu(In,Ga)Se2 (CIGS) thin film solar cells. In particular, a postdeposition treatment of an alkali metal fluoride of the absorber has shown a beneficial effect on the solar cells performance due to an increase in the open circuit voltage (VOC) for both (Ag,Cu)(In,Ga)Se2 (ACIGS) and CIGS based solar cells. Several reasons have been suggested for the improved VOC in CIGS solar cells including absorber surface and interface effects. Less works investigated how the applied postdeposition treatment influences the ACIGS absorber surface and interface properties and the subsequent buffer layer growth. In this work we employed hard X-ray photoelectron spectroscopy to study the chemical and electronic properties at the real functional interface between a CdS buffer and ACIGS absorbers that have been exposed to different alkali metal fluoride treatments during preparation. All samples show an enhanced Ag content at the CdS/ACIGS interface as compared to ACIGS bulk-like composition, and it is also shown that this enhanced Ag content anticorrelates with Ga content. The results indicate that the absorber composition at the near-surface region changes depending on the applied alkali postdeposition treatment. The Cu and Ga decrease and the Ag increase are stronger for the RbF treatment as compared to the CsF treatment, which correlates with the observed device characteristics. This suggests that a selective alkali postdeposition treatment could change the ACIGS absorber surface composition, which can influence the solar cell behavior., Title in Web of Science: Surface/Interface Effects by Alkali Postdeposition Treatments of (Ag,Cu)(In,Ga)Se-2 Thin Film Solar Cells

Multijunction solar cells in a tandem configuration could further lower the costs of electricity if crystalline Si (c-Si) is used as the bottom cell. However, for direct monolithic integration on c-Si, only a restricted number of top and bottom cell architectures are compatible, due to either epitaxy or high-temperature constraints, where the interface between subcells is subject to a trade-off between transmittance, electrical interconnection, and bottom cell degradation. Using polySi/SiOx passivating contacts for Si, this degradation can be largely circumvented by tuning the polySi/SiOx stacks to promote gettering of contaminants admitted into the Si bottom cell during the top cell synthesis. Applying this concept to the low-cost top cell chalcogenides Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe), and AgInGaSe2 (AIGSe), fabricated under harsh S or Se atmospheres above 550 degrees C, we show that increasing the heavily doped polySi layer thickness from 40 to up to 400 nm prevents a reduction in Si carrier lifetime by 1 order of magnitude, with final lifetimes above 500 mu s uniformly across areas up to 20 cm(2). In all cases, the increased resilience was correlated with a 99.9% reduction in contaminant concentration in the c-Si bulk, provided by the thick polySi layer, which acts as a buried gettering layer in the tandem structure without compromising the Si passivation quality. The Si resilience decreased as AIGSe > CGSe > CZTS, in accordance with the measured Cu contamination profiles and higher annealing temperatures. An efficiency of up to 7% was achieved for a CZTS/Si tandem, where the Si bottom cell is no longer the limiting factor.

Herein, a new route of sulfur grading in CuInSe2 (CISe) thin-film solar absorbers by introducing an ultrathin (<50 nm) sacrificial sputtered CuInS2 (CIS) layer on top of the CISe. Different CIS top layer compositions (Cu-poor to Cu-rich) are analyzed, before and after a high-temperature treatment in selenium (Se)- or selenium+sulfur (SeS)-rich atmospheres. An [S]/([S] + [Se]) grading from the surface into the bulk of the Se- and SeS-treated samples is observed, and evidence of the formation of a mixed CuIn(S,Se)2 phase by Raman analysis and X-ray diffraction is provided. The optical bandgap from quantum efficiency measurements of solar cells is increased from 1.00 eV for the CISe reference to 1.14 and 1.30 eV for the Se- and SeS-treated bilayer samples, respectively. A ≈150 mV higher VOC is observed for the SeS-treated bilayer sample, but the cell exhibits blocking characteristics resulting in lower efficiency as compared with the CISe reference. This blocking is attributed to an internal electron barrier at the interface to the sulfur-rich surface layer. The difference in reaction routes and possible ways to improve the developed sulfurization process are discussed.

In 2017, Cu(In,Ga)Se2 (CIGS), Cu2(Zn,Sn)S4 (CZTS) and Cu2(Zn,Sn)(S,Se)4 (CZTSSe) thin film solar cells were irradiated by our group using 3 MeV protons to investigate the materials’ radiation hardness and subsequent recovery following dark storage. It was observed that the primary losses were in open-circuit voltage (VOC), with the CZTS and CZTSSe being more resistant than the CIGS, also recovering to ∼ 95% of initial performance, compared to ∼ 70% for CIGS after two months dark storage. In 2021 the cells were investigated by external quantum efficiency and current-voltage measurements once again, to investigate further recovery. The CIGS cells had continued to recover, whilst the CZTSSe devices appear to have fully recovered from radiation induced damage, but now suffer from aging-related degradation and exhibit slight bandgap widening over time. The CZTS cells were observed to recover fully from the radiation induced damage, whilst also showing gains in VOC., Title in WoS: Long term stability and recovery of 3 MeV proton irradiated Cu(In,Ga)Se-2 and Cu-2 (Zn,Sn)(S,Se)(4) thin film solar cells

The Cu2ZnGe X Sn1-X S4 (CZGTS) thin-film solar cells have a limited open-circuit voltage (V OC) due to bulk and interface recombination. Since the standard CdS buffer layer gives a significant cliff-like conduction band offset to CZGTS, alternative buffer layers are needed to reduce the interface recombination. This work compares the performance of wide bandgap Cu2ZnGeS4 (CZGS) solar cells fabricated with nontoxic Zn x Sn1–x O y (ZTO) buffer layers grown by atomic layer deposition under different conditions. The V OC of the CZGS solar cell improved significantly to over 1 V by substituting CdS with ZTO. However, V OC is relatively insensitive to ZTO bandgap variations. The short-circuit current is generally low but is improved with KCN etching of the CZGS absorber before deposition of the ZTO buffer layer. A possible explanation for the device behavior is the presence of an oxide interlayer for nonetched devices., Title in thesis list of papers: Record 1.1 V open circuit voltage for Cu2ZnGeS4 based thin-film solar cells using atomic layer deposition Zn1-xSnxOy buffer layers

Zn1-xSnxOy (ZTO) deposited by atomic layer deposition has shown promising results as a buffer layer material for kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Increased performance was observed when a ZTO buffer layer was used as compared to the traditional CdS buffer, and the performance was further increased after an air annealing treatment of the absorber. In this work, we study how CZTS absorber surface treatments may influence the chemical and electronic properties at the ZTO/CZTS interface and the reactions that may occur at the absorber surface prior to atomic layer deposition of the buffer layer. For this, we have used a combination of microscopy and synchrotron-based spectroscopies with variable information depths (X-ray photoelectron spectroscopy, high-energy X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy), allowing for an in-depth analysis of the CZTS near-surface regions and bulk material properties. No significant ZTO buffer thickness variation is observed for the differently treated CZTS absorbers, and no differences are observed when comparing the bulk properties of the samples. However, the formation of SnOx and compositional changes observed toward the CZTS surface upon an air annealing treatment may be linked to the modified buffer layer growth. Further, the results indicate that the initial N2 annealing step integrated in the buffer layer growth by atomic layer deposition, which removes Na-COx species from the CZTS surface, may be useful for the ZTO/CZTS device performance.

(Ag,Cu)(In,Ga)Se2 solar cells with bandgaps of ≈1.45 eV with a large spread in absorber stoichiometry are characterized with the intention of assessing the effect of composition on the stability of the devices. This material is observed to have a poor diffusion length, leading to very strong dependence upon the depletion region width for charge carrier collection. The depletion width is observed to depend strongly upon the stoichiometry value and shrinks significantly after an initial period of dark storage. It is also seen that the depletion width can be varied strongly through light-soaking and dry-heat treatments, with prolonged annealing leading to detrimental contraction and light soaking leading to expansion which increases current collection. The extent of depletion width variation in response to the treatments is also clearly linked to absorber stoichiometry. Consequently, the device performance, particularly the current output, exhibits a stoichiometry dependence and is considerably affected after each round of treatment. Possible causes of this behavior are discussed.

The chalcopyrite Cu(In,Ga)S-2 has gained renewed interest in recent years due to the potential application in tandem solar cells. In this contribution, a combined theoretical and experimental approach is applied to investigate stable and metastable phases forming in CuInS2 (CIS) thin films. Ab initio calculations are performed to obtain formation energies, X-ray diffraction (XRD) patterns, and Raman spectra of CIS polytypes and related compounds. Multiple CIS structures with zinc-blende and wurtzite-derived lattices are identified and their XRD/Raman patterns are shown to contain overlapping features, which could lead to misidentification. Thin films with compositions from Cu-rich to Cu-poor are synthesized via a two-step approach based on sputtering from binary targets followed by high-temperature sulfurization. It is discovered that several CIS polymorphs are formed when growing the material with this approach. In the Cu-poor material, wurtzite CIS is observed for the first time in sputtered thin films along with chalcopyrite CIS and CuAu-ordered CIS. Once the wurtzite CIS phase has formed, it is difficult to convert into the stable chalcopyrite polymorph. CuIn5S8 and NaInS2 accommodating In-excess are found alongside the CIS polymorphs. It is argued that the metastable polymorphs are stabilized by off-stoichiometry of the precursors, hence tight composition control is required.

Chalcopyrite Cu(In,Ga)Se-2 (CIGSe) solar absorbers are renowned for delivering high solar power conversion efficiency despite containing high concentration of lattice defects amounting to copper deficiencies of several atomic percent. The unique ability to incorporate this deficiency without triggering decomposition (i.e. "tolerance to off-stoichiometry") is viewed by many as the key feature of CIGSe. In principle, this property could benefit any solar absorber, but remarkably little attention has been paid to it so far. In this study, we assess the tolerance to off-stoichiometry of thin-film photovoltaic materials by carrying out ab initio analysis of group-I-poor ordered defect compounds (ODCs) in the extended family of I-III-VI systems (where I = Cu, Ag, III = Al, Ga, In, and VI = S, Se, Te). We analyze convex hulls and structural evolution with respect to group-I content, link them with experimental phase diagrams, and determine two empirical principles for the future identification of solar energy materials with high tolerance to off-stoichiometry. Practical implications for the deposition of I-III-VI absorbers are also discussed in light of our computational results and recent experimental findings.

Kesterite Cu2ZnSnS4 (CZTS), used for thin film solar cells, has a band gap energy around 1.5–1.6 eV with possibilities for further increase through alloying. In some applications for wide band gap solar cells, reduced absorber thickness can be beneficial, to allow partial light transmission. Reduced thickness can also be beneficial to reduce bulk recombination, and so called ultrathin solar cells (<700 nm thick) have been studied for several materials systems. Here, we report performance for CZTS devices down to 250 nm thickness and show that performance loss from thickness reduction is relatively small, partly due to short minority carrier diffusion length. Insertion of thin passivation layers (Al2O3, SiO2 or HfO2) at the Mo/CZTS interface gives improved performance of ultrathin devices, from 4.7% to 5.6% efficiency for best performing cells having 250 nm thick CZTS with Mo as compared to Mo/Al2O3 back contact. The approach of NaF post deposition for making isolating passivation layers conductive is tested for the first time for CZTS and is shown to work. For fabrication of CZTS devices on transparent ITO back contact, the insertion of passivation layers can reduce diffusion of indium into CZTS, but device performance is lower than on Mo back contacts.

The compound AgGaSe2 has received limited attention as a potential wide gap solar cell material for tandem applications, despite its suitable band gap. This study aims to investigate the potential of this material by deposition of thin films by co-evaporation and production of solar cell devices. Since AgGaSe2 has a very low tolerance to off-stoichiometry, reference materials of possible secondary phases in the Ag2Se–Ga2Se3 system were also produced. Based on these samples, it was concluded that X-ray diffraction is suited to distinguish the phases in this material system. An attempt to use Raman spectroscopy to identify secondary phases was less successful. Devices were produced using absorbers containing the secondary phases likely formed during co-evaporation. When grown under slightly Ag-rich conditions, the Ag9GaSe6 secondary phase was present along with AgGaSe2, which resulted in devices being shunted under illumination. When absorbers were grown under Ag-deficient conditions, the AgGa5Se8 secondary phase was observed, making the device behavior dependent on the processing route. Deposition with a three-stage evaporation (Ag-poor, Ag-rich, and Ag-poor) resulted in AgGa5Se8 layers at both front and back surfaces, leading to charge carrier blocking in devices. Deposition of the absorber with a one-stage process, on the other hand, caused the formation of AgGa5Se8 locally extended through the entire film, but no continuous layer was found. As a consequence, these devices were not blocking and achieved an efficiency of up to 5.8%, which is the highest reported to date for AgGaSe2 solar cells.

The Cu2ZnGe X Sn1-X S4 (CZGTS) thin-film solar cells have a limited open-circuit voltage (V OC) due to bulk and interface recombination. Since the standard CdS buffer layer gives a significant cliff-like conduction band offset to CZGTS, alternative buffer layers are needed to reduce the interface recombination. This work compares the performance of wide bandgap Cu2ZnGeS4 (CZGS) solar cells fabricated with nontoxic Zn x Sn1–x O y (ZTO) buffer layers grown by atomic layer deposition under different conditions. The V OC of the CZGS solar cell improved significantly to over 1 V by substituting CdS with ZTO. However, V OC is relatively insensitive to ZTO bandgap variations. The short-circuit current is generally low but is improved with KCN etching of the CZGS absorber before deposition of the ZTO buffer layer. A possible explanation for the device behavior is the presence of an oxide interlayer for nonetched devices., Title in thesis list of papers: Record 1.1 V open circuit voltage for Cu2ZnGeS4 based thin-film solar cells using atomic layer deposition Zn1-xSnxOy buffer layers

Kesterite-based solar cells are mainly restricted by their lower than expected open-circuit voltage (Voc) due to non-radiative recombination. Therefore, an approach to reduce bulk and interface recombination through band gap grading to induce a back surface field is attempted. This contribution presents the challenges in the formation of compositional grading of the wide bandgap material Cu2ZnGexSn1−xS4 (CZGTS) and successful fabrication of solar cells with an additional adhesive TiN interlayer. It is observed that the TiN interlayer improves adhesion between CZGTS and the back contact. The microstructure of the Cu2ZnSnS4 (CZTS) film is significantly affected by the concentration of Ge, and the existence of a Ge concentration gradient is strongly correlated to the formation of smaller Ge-rich and larger Sn-rich grains. The bandgap grading is exploited with a moderate Ge concentration of up to (Ge/(Ge+Sn) = 0.25) in CZTS. As the Ge profile stretched all the way to the front interface, the cliff-like band alignment at the front interface of the absorber could negate the beneficial effect of Ge inclusion in the bulk and back interface of the absorber. Ordering the absorber can introduce an additional downward shift in the valence band. In one of the samples, the increased ordering and high concentration of Ge in CZTS are suggested to enhance the hole barrier at the back interface. It is concluded that the effect of the bandgap grading with Ge can only be realized with optimization of interface band alignment and back contact formation.