Mo Back Contact Research Articles

Simulation Study for Nb‐Doped MoS2${\rm MoS}_2$ Layer in CZTS‐Based Solar Cells: Assessment of Challenges

AbstractThe unintentional formation of a layer as a reaction product between the copper zinc tin sulfide (CZTS) thin film and the Mo back contact reduces cell efficiency due to high sheet resistance and carrier recombination. To limit the formation of , an intentionally grown p‐type Nb‐doped layer can serve as an effective hole transport layer. This study presents a detailed study and calculations for CZTS/Mo‐based solar cells, providing guidelines for calibration. Optimizing cell efficiency is influenced by various interconnected factors in Nb‐doped . While a high carrier concentration in Nb‐doped is assumed to enhance efficiency, other parameters such as band state, optical absorption, and carrier mobility also play crucial roles and can limit cell performance. This simulation study evaluated the effect of Nb‐doped layers with different carrier concentrations to determine the optimal conditions for this p‐type layer. This work reveals that a very thin layer (13 nm) of Nb‐doped p‐type can achieve a maximum efficiency of 11.34% (with = 1.5 ) and that for an Nb‐doped p‐type 62 nm is 15.82 % (with = 4.03 ).

Introducing Bi2S3 Interlayer to Synergistically Modify Mo Back Contact and Regulate Bulk Defects in Kesterite Solar Cells.

A suitable interlayer between the Mo back electrode and kesterite absorber layer has been proven to have a positive effect on limiting the bulk defects of the absorber by the constitute diffusion. Here, a thin Bi2S3 layer is used as the back-interface intermediate layer for the first time, this innovative approach allows for simultaneous modification of the back contact and reduction of bulk defects, resulting in improving the power conversion efficiency of the kesterite device from 9.66% to 11.8%. The evaporated Bi2O3 thin films turn into the Bi2S3 interlayers after sintering the Cu2ZnSnS4 precursor thin films. The Bi2S3 interlayer can inhibit the decomposition reaction of back contact and suppress the formation of the secondary phases. It can also optimize the Fermi level offset and promote the separation of the photoinduced carriers, resulting from its characteristic of high work function. Besides, a small part of the Bi element can diffuse into Cu2ZnSn(S, Se)4 film and induce the crystal growth and restrain Zn-related defects, which is attributed to forming the low melting-point liquid BiSex phase during the high-temperature selenization process. The conclusions highlight the bifunction of the thin Bi2S3 intermediate layer, which can provide a new approach to improve the photoelectric conversion efficiency of kesterite solar cells.

Multiple Impacts of the Aluminum Oxide Passivation Layer on the Properties OF Cu(In,Ga)Se2 Solar Cells

AbstractIn this study, the origins of efficiency gains in Cu(In,Ga)Se2 (CIGS) solar cells are investigated by introducing an Al2O3 passivation layer in terms of the oxidation condition of Mo back contact, alkali‐metal diffusion, minority carrier lifetimes (τ), and charge conditions. The study reveals that introduction of an Al2O3 back‐contact passivation layer into solar cells yields multiple impacts. Al2O3 deposition enhances the oxidation of the Mo back contacts, increasing Na solubility in Mo and Na diffusion from Mo into the CIGS layer, thereby modifying the metastable properties of CIGS. The charge condition at the CIGS/Al2O3 interface is not fixed negative charge but variable, dependent on whether electrons or holes are supplied. During solar cell operation, the interfacial charge condition is expected to be neutral or positive for Al2O3 grown using plasma or thermal atomic layer deposition techniques, respectively. Moreover, the mechanical peeling off of CIGS from Mo back contact enhanced τ in a similar way as with the insertion of Al2O3. Based on this study, the enhancement of alkali metal supply and the removal of direct contact of CIGS to the metal contact (Mo) can play crucial roles in improving the performance of CIGS solar cell.

Ultrasonic Al Bond on Mo Back-Contact Layer of CIGS Solar Panel Characterization Using Infinite Focus Microscope (IFM) and Micro-Ohmmeter

Ultrasonic Al Bond on Mo Back-Contact Layer of CIGS Solar Panel Characterization Using Infinite Focus Microscope (IFM) and Micro-Ohmmeter

Rear Interface Engineering in Solution‐Processed Submicron Cu(In,Ga)(S,Se)2 Solar Cells on Transparent Sn:In2O3 Back Contact

AbstractThe parasitic absorption in Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells emerging from the Mo back contact can be significantly reduced by replacing it with tin‐doped indium oxide (ITO). Commonly, an undesirable GaOx layer forms at the CIGSSe/ITO interface during the high‐temperature fabrication process, which has a detrimental effect on photo‐carrier extraction. Here, a Cu‐In‐TU‐DMF (TU: thiourea, DMF: N, N‐Dimethylformamide) intermediate layer for modification of the CIGSSe/ITO interface, which improves the efficiency of submicron CIGSSe solar cells significantly, is reported about. The reference submicron CIGSSe solar cells exhibit inferior performance (2.4% efficiency) and a large open circuit voltage deficit (Voc,def = 815.9 mV) due to a high barrier at the CIGSSe/ITO interface. At the modified rear interface, the recombination is reduced and hence carrier transport and collection are obviously improved. The efficiency of submicron CIGSSe solar cells on ITO with rear interface modification achieves 7.9% with an open circuit voltage of 565.8 mV, a short circuit current density of 23.4 mA cm−2, and a fill factor of 59.5%, as well as a Voc,def of 589.2 mV.

Influence of Processing Experimental Parameters of PEDOT: PSS on the Photovoltaic Performance on CdTe Solar Cells

Hybrid CdTe solar cells including the organic polymer poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) in the typical structure were fabricated. PEDOT:PSS was added as a p+ region in the structure of solar cells after CdTe deposition and before the processing of the Cu–Mo back contact and was compared with a CdTe reference solar cells without polymer. The PEDOT:PSS was deposited at different rotational speeds of 3000 to 6000 rpm for 30 s, by spin coating technique. The obtained thicknesses were around 20–40 nm. Adequate coverage of the CdTe surface and grain morphology was obtained, when the polymer was deposited at 5000 rpm of rotational speed this according to Scanning Electron Microscope measurements. The final structure of the solar cells was SnO2:F/ZnO+CdS-TT/CdTe+CdCl2-TT/PEDOT:PSS/Cu-Mo. In our results was clear that the solar cells with PEDOT:PSS processed at different rotational speeds had higher fill factor and thus higher photovoltaic efficiencies. A high photovoltaic efficiency of around 13% was obtained when PEDOT:PSS was deposited at 5000 rpm and the Cu–Mo back contact was processed with a substrate temperature of 200 °C. A particular increment in the photovoltaic efficiency of solar cells close to 8% was achieved, when the Cu–Mo back contact was processed at room temperature.

Enhanced Mechanical Stability of CIGS Solar Module with Glass/Polyimide/Indium Tin Oxide for Potentially Flexible Applications

Cu(In,Ga)Se2 (CIGS) is a promising candidate for flexible photovoltaics because of its outstanding efficiency and flexibility. Despite its advantages, achieving high-efficiency CIGS solar cells on a flexible polyimide (PI) substrate is challenging as it requires a low-temperature process and relaxation of the thermal expansion. This limitation is critical in CIGS modules, particularly for monolithic interconnection processes by laser scribing. Furthermore, Mo back-contact (BC)-based PI cells are sensitive to each laser processing step. Laser scribing is one of the important processes in thin-film module manufacturing. In this study, for the first time, we applied indium tin oxide (ITO) instead of Mo as a BC layer on the spin-coated PI on soda-lime glass to obtain mechanically durable CIGS modules. The ITO BC-based module not only provides a crack-free CIGS layer but also offers superior device performance owing to the excellent laser scribing quality. Additionally, electrical properties related to respective scribing steps are analyzed in correlation with observed morphologies to evaluate parasitic resistance and optimize the laser scribing conditions. Consequently, a CIGS monolithic-integrated module with 15.03% efficiency at 40.14 cm2 (16.3% at 0.480 cm2) is fabricated on a novel “soda-lime glass/coated-PI/ITO structure”. We propose ITO BC-based cells as promising candidates for achieving high-efficiency and flexible CIGS solar modules.

Probing the Interplay between Mo Back Contact Layer Deposition Condition and MoSe2 Layer Formation at the CIGSe/Mo Hetero-Interface.

The effect of Mo thin film deposition power in DC sputtering on the formation of a MoSe2 interfacial layer grown via the annealing of CIGSe/Mo precursors in an Se-free atmosphere was investigated. A Mo layer was deposited on glass substrates using the DC magnetron sputtering method. Its electrical resistivity, as well as its morphological, structural, and adhesion characteristics, were analyzed regarding the deposition power. In the case of thinner films of about 300 nm deposited at 80 W, smaller grains and a lower volume percentage of grain boundaries were found, compared to 510 nm thick film with larger agglomerates obtained at 140 W DC power. By increasing the deposition power, in contrast, the conductivity of the Mo film significantly improved with lowest sheet resistance of 0.353 Ω/square for the sample deposited at 140 W. Both structural and Raman spectroscopy outputs confirmed the pronounced formation of MoSe2, resulting from Mo films with predominant (110) orientated planes. Sputtered Mo films deposited at 140 W power improved Mo crystals and the growth of MoSe2 layers with a preferential (103) orientation upon the Se-free annealing. With a more porous Mo surface structure for the sample deposited at higher power, a larger contact area developed between the Mo films and the Se compound was found from the CIGSe film deposited on top of the Mo, favoring the formation of MoSe2. The CIGSe/Mo hetero-contact, including the MoSe2 layer with controlled thickness, is not Schottky-type, but a favourable ohmic-type, as evaluated by the dark I-V measurement at room temperature (RT). These findings support the significance of regulating the thickness of the unintentional MoSe2 layer growth, which is attainable by controlling the Mo deposition power. Furthermore, while the adhesion between the CIGSe absorber layer and the Mo remains intact, the resistance of final devices with the Ni/CIGSe/Mo structure was found to be directly linked to the MoSe2 thickness. Consequently, it addresses the importance of MoSe2 structural properties for improved CIGSe solar cell performance and stability.

Rear interface engineering via a facile oxidation process of Mo back contact for highly efficient CZTSSe thin film solar cells

The kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells (TFSCs) are promising absorber material for next-generation TFSCs technology because they exhibit earth-abundant, non-toxic elements and favorable optoelectronic properties. However, the progress and device performance development in CZTSSe TFSCs were limited because of unstable rear interface characteristics near the absorber/molybdenum (Mo) interface, which causes large open-circuit voltage loss. In this study, we report the rear interface engineering of CZTSSe TFSCs to improve device performance by introducing an amorphous thin MoOx intermediate layer by a facile oxidation process. We observed that (i) an amorphous thin MoOx intermediate layer can effectively control the rear interface properties (i.e., inhibiting the formation of Mo(S,Se)2 interfacial layers and voids at CZTSSe/Mo interface) and (ii) the improved device performance in CZTSSe TFSCs with oxidation (w-oxidation) is attributed primarily to the dramatically decreased shunt conductance, formation of large-sized grains within CZTSSe absorber layer, and improved rear interface characteristics. This study presents a simple and low-temperature oxidation process for rear interface engineering of highly efficient kesterite CZTSSe-based TFSCs.

(Invited, Digital Presentation) Application of Transition Metal Dichalcogenides for Chalcopyrite Solar Cells

IntroductionChalcopyrite compound materials, such as Cu(In,Ga)Se2 (CIGSe), Cu(In,Ga)S2 (CIGS), and Cu(In,Ga)(S,Se)2, are expected as promising photovoltaic materials since they can be applied for flexible light-weight solar cells, which realize a high-speed and low-cost production utilizing a roll-to-roll process. In this work, transition metal dichalcogenides, i.e., MoSe2 and MoS2 are applied to develop new functions in the chalcopyrite solar cells. First, a device peeling technique for CIGSe solar cells are developed by utilizing two-dimensional MoSe2 atomic layers. To boost an electric power generation via collecting of the ground albedo radiation, bifacial-type structures for the flexible CIGSe solar cells are constructed by depositing a TCO layer on CIGS rear side after applying device-peeling technique to traditional substrate-type structure. Second, an interfacial control technique to modify CIGS absorber / Mo back contact interfaces for the wide-gap CIGS solar cells are developed by introducing a p+-type MoS2 layer, where the carrier concentration is controlled by Nb-doping.Experimental methods1. Peeling techniques for flexible-bifacial CIGSe solar cells2-μm-thick CIGSe absorber was prepared on a Mo-covered glass substrate using a three-stage process, where the MoSe2 atomic layers were intentionally formed to control the adhesion at the Mo/CIGSe interface. CIGSe solar cells with a structure of glass/Mo/CIGSe/CdS/i-ZnO/Al:ZnO/Ni-Al grids were fabricated (Figure 1 (a)). For the peeling-off procedure, fluorinated ethylene propylene (FEP) films as alternative flexible substrates were attached to the front side with thermosetting epoxy glue under a temperature of 100 °C. While cooling down to room temperature, residual stress remained in the epoxy glue and FEP films due to their higher thermal expansion coefficients. Therefore, the CIGSe/CdS/i-ZnO/Al:ZnO/epoxy/FEP layers spontaneously detached from the Mo-covered glass. Finally, flexible-bifacial CIGSe solar cells were completed via deposition of 300-nm-thick ITO films on the rear side of the CIGSe by sputtering methods.2. Fabrication of Nb-doped MoS2 and wide-gap CIGS solar cells20-nm-thick Nb-Mo metal precursors were deposited on glass substrates via co-sputtering methods utilizing Mo and Nb targets. The sulfurization process for 30 min at 600 °C under H2S/Ar atmosphere was performed on the Nb-Mo metal precursors to form the p+-type Nb-doped MoS2 (Nb:MoS2) films. Hall effect measurement was performed for the Nb:MoS2 films to evaluate their electrical properties.The wide-gap CIGS solar cells with a structure of glass/Mo/Nb:MoS2/CIGS/CdS/i-ZnO/Al:ZnO/Ni-Al grids were fabricated (Figure 1 (b)). The Nb:MoS2 thin films with the [Nb] / ([Nb] + [Mo]) ratios of 0 and 0.02 were deposited on a Mo-covered glass substrate. Then, 2-μm-thick CIGS absorber were prepared. Stacked layers of Cu–Ga and Cu–In precursors were deposited using the evaporation method, and followed by a sulfurization process at the substrate temperature of 600 °C under H2S/Ar atmosphere.Results and discussions1. Conversion efficiency of flexible-bifacial CIGSe solar cellsThe CIGSe solar cells were successfully peeled from Mo back contacts, when the layered-grown MoSe2 (c-axis orientation) was formed at the Mo/CIGSe interface, suggesting the controllability of interfacial adhesion via cleavage by weak chemical bonding due to van der Waals force in the MoSe2 atomic layers. A high-performance ratio of 95.0% was achieved in the 11.5%-efficient lift-off cells (with alternative Au back contact) compared with 12.1%-efficient substrate cells on Mo back contact. Furthermore, the results demonstrated the device operation as a bifacial solar cell with conversion efficiency, V OC, J SC, and FF of 10.1% 0.487 V, 33.9 mA/cm2, and 0.609 under front illumination and 2.8%, 0.435 V, 9.8 mA/cm2, and 0.658 under rear illumination.2. Conductivity control of MoS2 interface layer for wide-gap CIGS solar cellsCarrier density of Nb:MoS2 thin films was monotonically increased from 9.9 × 1015 to 1.9 × 1020 cm-3 and the conductivity type was inverted from n- to p-type with increasing the [Nb] / ([Nb] + [Mo]) compositional ratio from 0 to 0.06. This result suggests that Nb element acts as an acceptor in MoS2.For the wide-gap CIGS solar cells the, the roll-over in the current density‒voltage curves was observed in the samples with the [Nb] / ([Nb] + [Mo]) ratio of 0 (without Nb-doping) in MoS2, whereas the roll-over was disappeared in the [Nb] / ([Nb] + [Mo]) ratio of 0.02. This demonstrated that a highly doped p-type Nb:MoS2 introduced in the CIGS/Mo back junction improved the performance of wide-gap CIGS solar cells.ConclusionsWe demonstrated a usefulness of the transition metal dichalcogenides, MoSe2 and MoS2, on the device peeling technique to fabricate the flexible-bifacial CIGSe solar cells and the interfacial modification technique for the wide-gap CIGS solar cells.AcknowledgementsWe gratefully acknowledge the support of JSPS KAKENHI (20K14780) and Kato Foundation for Promotion of Science (KJ–3020) in Japan. Figure 1

Influence of argon pressure on sputter-deposited molybdenum back contacts for flexible Cu(In,Ga)Se2 solar cells on polyimide films

In this study, polyimide films with various thicknesses and coefficients of thermal expansion were used as candidates of lightweight and flexible substrates for Cu(In, Ga)Se2 (CIGS) solar cells. The influence of the Ar background pressure (PAr) during deposition on the properties of the Mo back contact on the polyimide films was investigated. It was found that the stress condition of the Mo contact was the sum of thermal and intrinsic stresses. The thermal stress depended on the coefficient of thermal expansion (CTE) of polyimide films and induced compressive stresses. The intrinsic stress was largely influenced by the PAr. High PAr caused large tensile intrinsic stress and the subsequent formation of cracks in the Mo layer deposited on the polyimide films, whereas no apparent formation of cracks was observed on the reference rigid-glass substrates. The crack formation in Mo deposited on the polyimide films subsequently caused remarkable increase in sheet resistance, which deteriorated the fill factor of the CIGS solar cells. The decreasing PAr reduced the tensile intrinsic stress in the Mo layer, preventing the detrimental crack formation and improving the fill factor of the CIGS solar cells on the flexible polyimide substrates. The stress condition of the Mo was also found to influence thermal diffusion of alkali metal through the contact. The impact of the Mo stress on alkali metal diffusion was adjustable by varying the thickness of alkali silicate thin layers and/or alkali-metal postdeposition treatment conditions, improving conversion efficiency.

Characterization of the Stability of Indium Tin Oxide and Functional Layers for Semitransparent Back‐Contact Applications on Cu(in,Ga)Se2Solar Cells

Herein, a detailed study of the stability of different ITO‐based back‐contact configurations (including bare ITO contacts and contacts functionalized with nanometric Mo, MoSe2, and MoS2layers) under the coevaporation processes developed for the synthesis of high‐efficiency Cu(In,Ga)Se2(CIGSe) solar cells is reported. The results show that bare ITO layers can be used as efficient back contacts for coevaporation process temperatures of 480 ºC. However, higher temperatures produce an amorphous In–Se phase at the ITO surface that reduces the contacts transparency in the visible region. This is accompanied by degradation of the solar cells’ efficiency. Inclusion of a Mo functional layer leads to the formation of a MoSe2interfacial phase during the coevaporation process, which improves the cells’ efficiency, achieving device efficiencies similar to those obtained with reference solar cells fabricated with standard Mo back contacts. Optimization of the initial Mo layer thickness improves the contact transparency, achieving contacts with an optical transparency of 50% in the visible region. This is accompanied by a relevant decrease in back reflectivity in the CIGSe devices, confirming the potential of these contact configurations for the development of semitransparent CIGSe devices with improved optical aesthetic quality without compromising the device performance.

Mo Contact via High-Power Impulse Magnetron Sputtering on Polyimide Substrate

It has always been a huge challenge to prepare the Mo back contact of inorganic compound thin film solar cells (e.g., CIGS, CZTS, Sb2Se3) with good conductivity and adhesion at the same time. High-power impulse magnetron sputtering (HiPIMS) has been proposed as one solution to improve the properties of the thin film. In this study, the HiPIMS technology replaced the traditional DC power sputtering technology to deposit Mo back contact on polyimide (PI) substrates by adjusting the experimental parameters of HiPIMS, including working pressure and pulse DC bias. When the Mo back contact is prepared under a working pressure of 5 mTorr and bias voltage of −20 V, the conductivity of the Mo back contact is 9.9 × 10−6 Ω·cm, the residual stress of 720 MPa, and the film still has good adhesion. Under the minimum radius of curvature of 10 mm, the resistivity change rate of Mo back contact does not increase by more than 15% regardless of the 1680 h or 1500 bending cycle tests, and the Mo film still has good adhesion in appearance. Experimental results show that, compared with traditional DC sputtering, HiPIMS coating technology has better conductivity and adhesion at the same time, and is especially suitable for PI substrates.

The influence of atmospheric species on the degradation of the Mo/MoSe2 back contact in CIGS solar cells

The degradation mechanisms of the molybdenum (Mo) back contact for Cu(In,Ga)Se2 solar cells in the presence of atmospheric species have been investigated. Mo/MoSe2 films on glass were immersed in water purged with the gases oxygen (O2), nitrogen (N2), carbon dioxide (CO2), air, in non-purged water, and exposed to air. It was found that the degradation rate is the highest in the combined presence of liquid water and oxygen. Degradation in water was less in the absence of oxygen, while air exposure displayed the least degradation. The main degradation product was MoO2, which was found at places where metallic molybdenum was not fully covered by MoSe2. MoSe2 is thus a good protective measure against molybdenum oxidation and can play an important role to protect CIGS devices against environmental degradation.

Unraveling the Impact of Combined NaF/RbF Postdeposition Treatments on the Deeply Buried Cu(In,Ga)Se2/Mo Thin‐Film Solar Cell Interface

Cu(In,Ga)Se2 (CIGSe) is a promising absorber material for thin‐film photovoltaic devices. A key procedure to achieve high efficiencies is the application of alkali fluoride postdeposition treatments (PDT) of the CIGSe surface. While the effects of the PDT on the directly impacted CIGSe front surface have been subject to extensive studies, less is known about the impact on the deeply buried CIGSe/Mo interface. Exposing the CIGSe absorber backside by stripping it off the Mo back contact allows to use surface‐sensitive photoelectron spectroscopy to study the chemical and electronic structure of this interface in unprecedented detail. CIGSe /Mo stacks prepared using NaF only and combined NaF/RbF (with optimal and excess amount of RbF) PDT are studied. Rb is detected accumulating at the backside of the RbF‐treated CIGSe absorbers in conjunction with a depletion of Na. The exposed CIGSe backsides display a Cu‐deficient surface region, with increased presence of Rb correlating with a decreased Cu content. Rb appears to be incorporated into the Cu‐deficient absorber region as previously detected on the front surface. However, in contrast to the front surface, no distinct, secondary RbInSe‐type phase is found at the back surface.

A Numerical Investigation on the Combined Effects of MoSe2 Interface Layer and Graded Bandgap Absorber in CIGS Thin Film Solar Cells

The influence of Molybdenum diselenide (MoSe2) as an interfacial layer between Cu(In,Ga)Se2 (CIGS) absorber layer and Molybdenum (Mo) back contact in a conventional CIGS thin-film solar cell was investigated numerically using SCAPS-1D (a Solar Cell Capacitance Simulator). Using graded bandgap profile of the absorber layer that consist of both back grading (BG) and front grading (FG), which is defined as double grading (DG), attribution to the variation in Ga content was studied. The key focus of this study is to explore the combinatorial effects of MoSe2 contact layer and Ga grading of the absorber to suppress carrier losses due to back contact recombination and resistance that usually occur in case of standard Mo thin films. Thickness, bandgap energy, electron affinity and carrier concentration of the MoSe2 layer were all varied to determine the best configuration for incorporating into the CIGS solar cell structure. A bandgap grading profile that offers optimum functionality in the proposed configuration with additional MoSe2 layer has also been investigated. From the overall results, CIGS solar cells with thin MoSe2 layer and high acceptor doping concentration have been found to outperform the devices without MoSe2 layer, with an increase in efficiency from 20.19% to 23.30%. The introduction of bandgap grading in the front and back interfaces of the absorber layer further improves both open-circuit voltage (VOC) and short-circuit current density (JSC), most likely due to the additional quasi-electric field beneficial for carrier collection and reduced back surface and bulk recombination. A maximum power conversion efficiency (PCE) of 28.06%, fill factor (FF) of 81.89%, JSC of 39.45 mA/cm2, and VOC of 0.868 V were achieved by optimizing the properties of MoSe2 layer and bandgap grading configuration of the absorber layer. This study provides an insight into the different possibilities for designing higher efficiency CIGS solar cell structure through the manipulation of naturally formed MoSe2 layer and absorber bandgap engineering that can be experimentally replicated.

Detrimental Impact of Na Upon Rb Postdeposition Treatments of Cu(In,Ga)Se2 Absorber Layers

Passivation of the Cu(In,Ga)Se2 (CIGS)/Mo back contact using AlO x is studied to reduce the recombination at this interface. Herein, RbF postdeposition treatment (RbF‐PDT), a well‐established method to improve absorber and front interface properties is used on back‐passivated solar cells. It is found that this combination deteriorates the performance due to formation of an injection barrier at the front and reduced acceptor concentration. Photoluminescence yield and decay times show no indication of increased defect recombination, as both are improved. With time‐of‐flight secondary ion mass spectroscopy, in‐depth and lateral alkali profiles are measured. It is shown that the Na concentration is higher at the AlO x /Mo back contact and that Rb accumulates at the CdS/CIGS interface. It is hypothesized that Na at the back is released during the RbF‐PDT and inhibits Rb diffusion into the CIGS layer. Rb remains at the front and acceptor concentration is reduced. Modeling of dark and light current–voltage characteristics shows that the injection barrier and low doping are responsible for the reduced V oc and fill factor (FF). It is suggested that the commonly observed FF losses upon heavier alkali PDT can be eliminated by adapting the initial Na amount.

Efficiency boost of CZTS solar cells based on double-absorber architecture: Device modeling and analysis

In this study, we present a new alternative approach that utilizes Si as a second absorber along with the primary CZTS absorber to improve all photovoltaics performance parameters for CZTS based thin-film solar cell devices. The proposed ZnO:Al/CdS/CZTS/Si/Mo with a double-absorber hybrid structure has been extensively investigated and analyzed by employing the SCAPS-1D simulation package. Initial direct integration of a thin Si layer with thickness as low as 200 nm between the Mo back-contact and the primary CZTS active layer has resulted in a significant efficiency boost of 16.32%. Further optimization of several material parameters such as thickness, bandgap energy, and doping concentration for both absorbers have resulted in additional enhancement of many photovoltaic performance parameters such as the FF, Jsc, Voc, and PCE. The correlation study between the two absorbers suggests that maximum efficiency of 19.40%, FF of 88.54%, Voc of 0.84 V and JSC of 27.55 mA/cm2 can be attained at absorbers thicknesses of about 2 µm and high doping level up to 1018 cm−3. This can be attributed to the generation of more charge carriers with the increase of absorbers thicknesses. Additionally, the proposed model shows a stable performance with the rise of the operating temperature up to 350 K. Adding Si layer to the cell structure has resulted in pronounced quantum efficiency (QE) improvement due to the increase of solar spectrum absorption at longer wavelengths. A significant improvement in I-V characteristics was also detected with the decrease of the double-absorber structure's series resistance compared to the conventional single-absorber CZTS structure. The outcomes of this study represent a promising solution toward the fabrication of high-performance and cost-effective thin CZTS solar cell devices.

Influence of the Rear Interface on Composition and Photoluminescence Yield of CZTSSe Absorbers: A Case for an Al2O3 Intermediate Layer.

The rear interface of kesterite absorbers with Mo back contact represents one of the possible sources of nonradiative voltage losses (ΔVoc,nrad) because of the reported decomposition reactions, an uncontrolled growth of MoSe2, or a nonoptimal electrical contact with high recombination. Several intermediate layers (IL), such as MoO3, TiN, and ZnO, have been tested to mitigate these issues, and efficiency improvements have been reported. However, the introduction of IL also triggers other effects such as changes in alkali diffusion, altered morphology, and modifications in the absorber composition, all factors that can also influence ΔVoc,nrad. In this study, the different effects are decoupled by designing a special sample that directly compares four rear structures (SLG, SLG/Mo, SLG/Al2O3, and SLG/Mo/Al2O3) with a Na-doped kesterite absorber optimized for a device efficiency >10%. The IL of choice is Al2O3 because of its reported beneficial effect to reduce the surface recombination velocity at the rear interface of solar cell absorbers. Identical annealing conditions and alkali distribution in the kesterite absorber are preserved, as measured by time-of-flight secondary ion mass spectrometry and energy-dispersive X-ray spectroscopy. The lowest ΔVoc,nrad of 290 mV is measured for kesterite grown on Mo, whereas the kesterite absorber on Al2O3 exhibits higher nonradiative losses up to 350 mV. The anticipated field-effect passivation from Al2O3 at the rear interface could not be observed for the kesterite absorbers prepared by the two-step process, further confirmed by an additional experiment with air annealing. Our results suggest that Mo with an in situ formed MoSe2 remains a suitable back contact for high-efficiency kesterite devices.

Effect of Ultrasonic Bonding Parameters on the Contact Resistance and Bondability Performances of CIGS Thin Film Photovoltaic Solar Panel

This article aims to investigate the optimal ultrasonic bonding parameter namely bonding pressure, bonding energy, and bonding amplitude that can minimize the contact resistance (Rc), maximize the peel strength, and identify the possible failure modes of ultrasonic Al bond on Mo back contact layer of copper indium gallium (de)selenide (CIGS) thin-film photovoltaic (TFPV) solar panel. The transmission line method was used to measure Rc and a peel test was carried out to measure the peel strength and the possible failure modes of ultrasonic Al bond on the Mo layer. Design of experiment using D-optimal method was utilized to evaluate the effect of the ultrasonic bonding parameter toward the Rc and peel strength of the Al bond on the Mo layer. Individual and combination of the ultrasonic bonding parameter have a significant effect on the quality of Al bonds on Mo back contact layer of CIGS TFPV solar panel. Possible failure modes could be identified through the application of the higher value of individual ultrasonic bonding parameter while keeping constant other bonding parameters and through the qualitative result of the load-displacement profile obtained from the peel test. However, sample 2 with bonding pressure of 3 bar, bonding energy of 20 Ws, and bonding amplitude of 7.7 μm is the best-optimized bonding parameter window that can be applied to obtain ultrasonic Al bond with lower Rc and higher peel strength. It was noted that bonding pressure is the most sensitive bonding parameter followed by bonding amplitude and bonding energy.