An efficient approach is proposed to enhance the microstructure of the CIGS absorber, increasing the device efficiency from 4% to 11% by introducing a 5 nm thermally evaporated Ag interlayer at the CIGS/Mo interface.

This study evaluates In2O3:Ti as a transparent back contact (TBC) material in bifacial (Ag,Cu)(In,Ga)Se-2 (ACIGS) solar cells with a band gap of 1.1 eV and compares it to commonly used In2O3:Sn. Both TBC layers were processed with a sheet resistance <= 10 Omega/sq, as required in a monolithically series-connected ACIGS module. In contrast to several other high-mobility TBCs previously tested in ACIGS solar cells, In2O3:Ti retains its exceptionally high mobility (>100 cm(2)/Vs), low resistivity (2.4 & centerdot;10(-4) Omega cm), and minimal near-infrared absorption (<5% at ACIGS bandgap) after high-temperature absorber deposition. As a result, an up to 3 mA/cm(2) higher short-circuit current density is measured under rear illumination for cells with an In2O3:Ti back contact as compared with devices using highly doped In2O3:Sn. The best cell reaches an efficiency of 10.2% at rear illumination with a bifaciality factor of 68%. At front illumination, the cell performance is on the same level for both TBCs.

Incorporating ZnO as a buffer layer in thin film CdSeTe/CdTe solar cells leads to high conversion efficiencies. However, the sub-optimal band alignment at the ZnO/CdSeTe interface limits the V-oc. In this study, Ce is used to alloy the ZnO buffer layer to widen the band gap and improve band alignment, leading to an increase in J(sc) and V-oc. The 50 nm and 100 nm thick ZnO and CeZnO buffer layers are deposited on SnO2:F coated soda-lime glass using radio frequency sputtering. To study the effect of Ce alloying, the Ce atomic percent is varied from 3% to 9%. The buffer layers are fabricated into As-doped CdSeTe/CdTe devices using First Solar's process. The device incorporating the 3% CeZnO buffer layer leads to the highest efficiency and V-oc. However, the saturation current density and ideality factor are observed to increase as the Ce content increases, suggesting that Ce alloying degrades the quality of the front p-n junction. The interface defect density is estimated using C- V and DLCP profiling, the interface defect density is observed to increase significantly when incorporating more than 3% Ce. There is an apparent trade-off between front interface passivation and band alignment.

Lithium incorporation into Cu2ZnSn(S,Se)4 (CZTSSe) absorbers has led to improvements in power conversion efficiencies (PCEs), often linked to changes in doping density and improvement in grain growth. In this study, we shed new light onto the influence of Li on CZTSSe absorbers, specifically on a transparent support, by investigating the effect of adding lithium bis(trifluoromethane)sulfonimide (LiTFSI) from 0% to 5% Li/Cu ratios to molecular precursor solutions. Secondary ion mass spectrometry analysis confirmed both the uniform distribution of Li in annealed CZTSSe absorber, while there were also detectable changes to grain microstructure and surface electronic landscape. Upon the addition of 1% Li, the work function (WF) distribution, analysed by energy-filtered photoemission electron microscopy, broadens significantly from 85 to 200 meV, allowing for direct visualisation of the increase in surface electronic disorder of CZTSSe with Li incorporation for the first time. The Li additive was also found to induce the diffusion of Sn to the surface. A champion PCE of 5.07% was measured for F:SnO2 (FTO)-based CZTSSe with 1% Li, revealing the balance between microstructure, chemical disorder and Sn elemental distribution. This research demonstrates the importance of understanding alterations of nanoscale properties of CZTSSe when varying precursor solution conditions.

This study evaluates In 2 O 3 :Ti as a transparent back contact (TBC) material in bifacial (Ag,Cu)(In,Ga)Se 2 (ACIGS) solar cells with a band gap of 1.1 eV and compares it to commonly used In 2 O 3 :Sn. Both TBC layers were processed with a sheet resistance ≤10 Ω/sq, as required in a monolithically series‐connected ACIGS module. In contrast to several other high‐mobility TBCs previously tested in ACIGS solar cells, In 2 O 3 :Ti retains its exceptionally high mobility (>100 cm 2 /Vs), low resistivity (2.4·10 −4 Ωcm), and minimal near‐infrared absorption (<5% at ACIGS bandgap) after high‐temperature absorber deposition. As a result, an up to 3 mA/cm 2 higher short‐circuit current density is measured under rear illumination for cells with an In 2 O 3 :Ti back contact as compared with devices using highly doped In 2 O 3 :Sn. The best cell reaches an efficiency of 10.2% at rear illumination with a bifaciality factor of 68%. At front illumination, the cell performance is on the same level for both TBCs.

Thin film cadmium telluride is the most important second-generation solar cell technology. Although the photo-absorber is polycrystalline, high conversion efficiency has been achieved by effectively passivating the grain boundaries and other bulk defects. In this paper, we report on the use of a bilayer SnO2/ZnO n-type buffer to improve passivation at the p-n junction interface to achieve 21.7% efficiency. We have assessed the quality of the interface with highly sensitive electrical measurements and a combination of high-resolution electron microscopy and cathodoluminescence. High-resolution cathodoluminescence has enabled the measurement of the recombination velocity to quantify the improvement in interface defect passivation. We have combined this with direct observation of the coherence of the interface at the atomic scale. The use of these techniques will revolutionize our ability to assess future passivation strategies at the front p-n junction and also at the back contact of thin film photovoltaic devices.

The use of SnO2 buffer layers has led to highly efficient and stable thin film CdSeTe/CdTe solar cells. However, chlorine segregation was observed at the SnO2/CdSeTe interface, suggesting that the quality of the p-n junction could be improved. In this work, ZnO was incorporated between the SnO2 and CdSeTe to study its passivation effect. Including a similar to 10 nm thick ZnO layer improved conversion efficiency, V-oc, J(sc), and FF, suggesting an improvement in junction quality. The bilayer SnO2 (70 nm)/ZnO (30 nm) buffer layer achieved an efficiency of 21.7% without an anti-reflection coating. High-resolution TEM analysis showed atomically coherent boundaries at the SnO2/ZnO and ZnO/CdSeTe interfaces, indicating that ZnO creates a highquality interface. The discovery that ZnO can be used as a passivation layer opens up new pathways to achieve highly efficient solar cells.

Indium Tin-Oxide (ITO) transparent conducting films were prepared by RF magnetron sputtering on glass substrates. The effects of two different heat treatments - annealing and heating the substrate, both at 300 degrees C and 500 degrees C - were examined to determine how they affect the microstructure, optical and electrical properties of the films. Despite extensive work on both heat treatment methods in literature, the two have not been compared in a side-by-side study thus far, and this work aims to address this. Heating the substrate during deposition produced lower resistivity films with higher carrier concentrations and higher transmission over visible wavelengths. Microstructural analysis revealed that both heat treatments produced crystalline ITO films, but the preferred crystal orientations were different. Annealing resulted in a preferred cubic (222) orientation, and high substrate temperatures yielded a mixed phase crystal structure (cubic and rhombohedral) with a preferred cubic (211) orientation. Scanning Transmission Electron Microscopy (STEM) analysis showed that sputtering at room temperature with no added heat produced films with a mostly amorphous structure which were converted into equiaxed grains by annealing. High substrate temperatures resulted in a columnar grain structure. These results have useful implications for the deposition of ITO films for various opto-electronic applications.

Highly resistive ZnO thin films are effective n-type buffer layers for thin film CdSeTe/CdTe photovoltaic devices. However, high-temperature processing during device fabrication may alter their electrical properties. This study investigates the structural and electrical changes in ZnO films following different process treatments: as-deposited, annealed, and cadmium chloride (CdCl _2 ) treated. Carrier concentration, mobility, and electrical conductivity of the films were extracted using ultra-sensitive parallel dipole-line Hall effect measurements. Films deposited at higher substrate temperatures showed improved electrical conductivity, enabling reliable extraction of carrier concentration and mobility. Notably, 750 nm thick ZnO films deposited at 500 °C exhibited carrier concentrations around 10 ^13 cm ^−3 , which increased by four orders of magnitude after annealing and CdCl _2 treatment. The ZnO films also demonstrated strong environmental stability, showing minimal change in electrical properties after 1000 h of damp heat and ultraviolet exposure tests. Furthermore, analysis of CdSeTe/CdTe device cross sections using transmission electron microscopy and energy-dispersive x-ray spectroscopy has provided evidence of some diffusion of oxygen from the ZnO layer into the front CdSeTe absorber after the treatment. This work has simulated the influence of annealing and CdCl _2 treatment on ZnO thin films to explain how their electrical properties may change during CdSeTe/CdTe device fabrication.

The electrical properties of as-deposited sputtered ZnO films with varying oxygen content in the Ar\O2 working gas have been investigated using an ultra-sensitive parallel dipole line (PDL) Hall effect system. Carrier concentration and mobility of the films have been extracted from the Hall effect measurements. Films deposited with below 0.5% oxygen in the Ar\O2 working gas have resulted in high carrier concentration on the order of 1018 cm-3, whereas films with higher oxygen content were too resistive for measurement, suggesting a low carrier concentration below 1012 cm-3. ZnO films were used as a buffer layer in CdSeTe/CdTe devices, and the performance of complete devices was extracted from J-V measurements. Surprisingly, ZnO films with two extreme carrier concentrations did not lead to significant changes in device performance. The highest conversion efficiency was achieved with an as-deposited ZnO layer with low carrier concentration deposited with 0.5% oxygen in the Ar\O2 working gas.

Solution-based deposition of high-quality inorganic compound semiconductors onto a variety of substrates is a key challenge toward integrating photovoltaic technologies into a wide range of infrastructures. Cu2ZnSn-(S,Se)4 (CZTSSe) is one of the most promising solar absorbers processable by solution-based methods, however there is a substantial knowledge gap linking the chemistry of cation complexes and chalcogen precursors, e.g. thiourea (TU), to the microstructure and opto-electronic properties of the thin-films. In this study, we focus our attention on the complexation of zinc chloride (ZnCl2) and zinc acetate (ZnAc2) in dimethylformamide (DMF)-based CZTS precursor inks and how this ultimately affects the performance of CZTSSe thin-film devices on F/SnO2 (FTO) substrates. Acetate coordination not only improves the overall CZTSSe composition and structural uniformity but also lowers the zinc salt decomposition temperature, from 519 to 284 °C, which significantly affects the rate of grain growth during selenization and final microstructure. ZnAc2-based CZTSSe films show densely packed grain growth due to the fast rate of selenium incorporation during annealing, while ZnCl2-based CZTSSe displays slower rates due to the high decomposition temperature of ZnCl2. Upon the incorporation of 25 nm Mo at the CZTSSe/FTO interface, a champion power conversion efficiency of 6.02% was achieved with ZnAc2 precursor salt, over two times greater than the equivalent device architecture prepared with ZnCl2, at 2.62%. This investigation illustrates the significant role of the molecular complexes in tuning the grain growth kinetics and final microstructure and therefore improving device performance on semitransparent substrates.

The front buffer layer plays an important role in CdSeTe/CdTe solar cells and helps achieve high conversion efficiencies. Incorporating ZnO buffer layers in the CdSeTe/CdTe device structure has led to highly efficient and stable solar cells. In this study, the optimization of ZnO buffer layers for CdSeTe/CdTe solar cells is reported. The ZnO films are radio frequency sputter‐deposited on SnO 2 :F coated soda‐lime glass substrates. The substrate temperature for the ZnO deposition is varied from 22 to 500 °C. An efficiency of 20.74% is achieved using ZnO deposited at 100 °C. The ZnO thickness is varied between 40 nm and 75 nm. Following the ZnO depositions, devices were fabricated using First Solar's CdSeTe/CdTe absorber, CdCl 2 treatment, and back contact. The optimal ZnO deposition temperature and thickness is 100 °C and 65 nm, respectively. The STEM‐EDX analysis shows that within the detection limits, chlorine is not detected at the front interface of the devices using ZnO deposited at 22 °C and 100 °C. However, depositing ZnO at 500 °C results in chlorine segregation appearing at the ZnO/CdSeTe boundary. This suggests that chlorine is not needed to passivate the ZnO/CdSeTe interface during the lower temperature depositions. The nanocrystalline ZnO deposited at lower temperatures results in a high‐quality interface.

A range of microstructural changes occur during the deposition and activation of CdTe based thin film solar cells. In particular, the cadmium chloride (CdCl2) activation treatment results in wholesale recrystallisation which transforms the conversion efficiency of the solar cell. One of the noticeable effects is the change of preferred orientation of the CdTe absorber. Highly orientated [111] texture is observed in as deposited or under-treated CdTe based devices. Optimized activation results in a more randomized texture and the [111] preferred texture component is significantly weakened. In this paper we use Electron Backscatter Diffraction to characterise absorber cross-sections. The focus is on how randomization of the absorber texture reflects device performance. We have had access to a range of CdTe devices using a variety of deposition techniques. We have observed a clear pattern that shows that devices with a highly orientated [111] texture have poor efficiency. Devices with a randomized texture have much higher efficiency. Here we illustrate this empirical correlation using devices deposited by Metal Organic Chemical Vapour Deposition with a range of efficiencies from 13.1 % to 17 %. We have also included the analysis of an absorber from a 18.7% high efficiency CdSeTe/CdTe device to show that texture is similarly important in these advanced devices. We have been able to quantify the effect of texture by using multiples of uniform density or (MUD) values from the inverse pole figures. MUD figures close to 1 correlate with highest efficiency. Although the random texture of the absorber microstructure is only one of several important process factors, it appears to be a necessary feature for highest efficiency CdTe-based polycrystalline solar cells.

In this study, the use of intrinsic and highly insulating ZnO buffer layers to achieve high conversion efficiencies in CdSeTe/CdTe solar cells is reported. The buffer layers are deposited on commercial SnO2:F coated soda-lime glass substrates and then fabricated into arsenic-doped CdSeTe/CdTe devices using an absorber and back contact deposited by First Solar. The ZnO thickness is varied from 30 to 200 nm. The devices incorporating a 50 nm ZnO buffer layer achieved an efficiency of 21.23% without an anti-reflection coating. An improved efficiency of 21.44% is obtained on a substrate with a multilayer anti-reflection coating deposited prior to device fabrication. The highly efficient ZnO based devices are stable and do not develop anomalous J-V behavior following environmental tests. High resolution microstructural analysis reveals the formation of a high-quality ZnO/CdSeTe interface. Unusually, chlorine is not detected as a discrete layer at the interface, these observations point to a high-quality interface. The extrapolation of Voc to 0 K indicates that interface recombination dominates, suggesting that further improvement is possible. Using device modeling, an attempt is made to understand how this type of device performs so well. The incorporation of a ZnO buffer layer in the CdSeTe device structure has led to conversion efficiencies exceeding 21%. The ZnO carrier concentration is orders of magnitude lower than that of the conventional SnO2 buffer. ZnO creates a high-quality interface with CdSeTe, resulting in the unusual omission of Cl at the front interface.image

Thin film CdTe-based photovoltaic devices have achieved high efficiency above 22%. However, the device performance is limited by large open circuit voltage deficit. One of the primary reasons is non-ohmic back contacts. In this work, nickel oxide is used as a back buffer layer to form an ohmic back contact. We comprehensively investigate oxygen effects during sputtering on film properties and device performance. Increased oxygen in the deposition environment led to darker films, increased carrier concentration, decreased mobility and decreased resistivity. X-ray photoelectron spectroscopy showed peak shifts favouring Ni3+ over Ni2+, and X-ray diffraction demonstrated that crystallinity hit a peak at around 5% oxygen input. The NiO back buffer layer improves device performance by reducing barrier height at the gold back contact and improving valence band offset at the CdTe/NiO interface. The NiO layer deposited without oxygen improved the Voc to 710 mV, from a baseline of 585 mV. At 5% and 20% oxygen content during deposition, efficiency improved relative to the reference due to an increase in open circuit voltage (Voc) and short circuit current (Jsc). Voc increase is due to improved valence band offset between CdTe and NiO. The large conduction band offset also reflects minority carriers away from the CdTe/NiO interface and reduces interface recombination. SCAPS simulations demonstrated that an increase in valence band offset has shown pronounced effects of both s-kinks and rollover. Oxygen content during sputtering affects the characteristics of nickel oxide and was studied as a back buffer layer in cadmium telluride solar cells. Simulated valence band offsets outline ideal value range and are compared to measurements.

In this study, we compare the environmental stability of ZnO-based buffer layers for thin film CdSeTe/CdTe solar cells. The buffer layers were deposited on glass substrates using radio frequency sputtering and include zinc oxide (ZnO), magnesium-alloyed ZnO (MZO) and gallium-alloyed MZO. Accelerated lifetime tests (ALTs) were performed by exposing the samples to damp heat and ultraviolet radiation, complying with the conditions required for IEC standard 61215. The optical, electrical, chemical, and structural properties have been studied before and after ALTs. The optical degradation of the samples was evaluated by measuring the transmittance and reflectance spectra. The transmittance data was used to obtain Tauc plots which allowed the bandgaps to be compared. It was found that the electrical properties could not be measured due to the high resistivity of all the materials. The chemical composition of the buffer layers was analyzed using X-ray photoelectron spectroscopy. Scanning electron microscopy was used to observe the surface structure degradation. The results show that ZnO is the most environmentally stable buffer layer.

This paper presents the automation of an in-house build close space sublimation (CSS) system. This was mostly operated manually including temperature profile and pressure control, which added a lot of strain onto the user and made it susceptible to operating mistakes that can go unnoticed, resulting in greater variability of process parameters. The CSS has been automated to operate not only one, but two vacuum chambers, one for CdSe and Se containing alloy, and the second for CdTe deposition. Although they cannot run at the same time, this has reduced the overall cost of the system compared to two separate units, while eliminating the need to change source materials and potential cross contamination. The process pressure control is achieved via software controlling the upstream gas flow rate. This combined with a bypassable flow conduction restrictor at the downstream side, reduces the gas flow rate requirements and has eliminated the need for a costly butterfly valve type pressure controller. System automation has reduced manual interaction with the system to just loading and unloading the substrate. Greatly increasing the process control flexibility, control accuracy and repeatability with the added benefit of process parameter records throughout the entire deposition process. Since being completed to time of writing, the system has been used to fabricate over than 200 films.

Tin dioxide (SnO2) buffer layers with varying thicknesses were deposited onto commercial F: SnO2 coated soda lime glass substrates using RF sputtering. The buffer layers were then incorporated into arsenic doped CdSeTe/CdTe devices at First Solar. The devices and films were characterised to determine the effect of buffer layer thickness on device performance. The best performing device used a 70 nm thick SnO(2)buffer layer deposited in 25% oxygen at 500 degrees C. A strong trend was observed in which device efficiency increased with buffer layer thickness. The best device achieved a conversion efficiency of 19.79%, V-oc of 833 mV, J(sc) of 30.16 mA/cm(2) and Fill Factor of 78.74%.

Rationalizing the role of chemical interactions in the precursor solutions on the structure, morphology, and performance of thin-film Cu2ZnSn(S,Se)4 (CZTSSe) is key for the development of bifacial and other photovoltaic (PV) device architectures designed by scalable solution-based methods. In this study, we uncover the impact of dimethylformamide (DMF) and isopropanol (IPA) solvent mixtures on cation complexation and rheology of the precursor solution, as well as the corresponding morphology, composition, and PV performance of CZTSSe thin-film grown on fluorine-doped tin oxide (FTO). We find that increasing the proportion of IPA leads to a nonlinear increase in dynamic viscosity due to the strong repulsion between DMF and IPA, which is characterized by an interaction cohesion parameter of 3.06. The repulsive solvent interaction not only leads to complex dependence on absorber thickness and surface roughness but also on composition disorder in the annealed CZTSSe films. Systematic studies involving Raman, scanning electron microscopy, SIMS, XPS, and energy-filtered photoemission of electron microscopy show that adding 25% of IPA to DMF leads to thin films with a high degree of structure and composition homogeneity in comparison to pure DMF-based precursors. Further increasing the IPA content promotes Sn surface segregation and secondary phases, which have a clear impact on the surface electronic landscape of the absorber layer. This analysis allows for the rationalization of the device performance with the stack configuration glass/F:SnO2/CZTSSe/CdS (50 nm)/i-ZnO (50 nm)/Al:ZnO (500 nm)/Ag (500 nm).