Pure sulfide kesterite solar cells with cation substituted absorber and back contact intermediate layer

Chalcogenide Cu(In,Ga)Se2 (CIGS) and CdTe thin film solar cells have the potential to reduce the cost of the photovoltaic (PV) technology. They showed high power conversion efficiency (PCE) of more than 20% shrinking the gap between them and champion crystalline Si solar cell. Despite of high efficiency and ease of fabrication, application of CdTe PV technology is limited on terawatt (TW) scale owing to scarcity of Te and because there is only one supplier. As for CIGS, a high demand for In from flat panel display industry along with its limited availability pushes the price of In up to several hundred USD/kg. Therefore, tremendous success of CIGS solar cells brought much attention to earth abundant, non-toxic and low cost kesterite Cu2ZnSnS4 (CZTS) where In and Ga are replaced with Zn and Sn. Similar electronic and optical properties of the materials enables CZTS solar cells to inherit the device structure of CIGS solar cells. CZTS is an intrinsic p-type semiconductor with a direct bandgap of about 1.5 eV and high absorption coefficient of >104 cm^(-1) for visible wavelengths. The bandgap of kesterite can be tuned from 1 eV to 1.5 eV by changing S/(S+Se) ratio from 0 to 1 which provides a degree of flexibility in device fabrication with the material. There is still large gap in PCE between the best chalcogenide and kesterite solar cells. Firstly, solid state reaction occurs at the back interface between CZTS absorber layer and Mo back contact during high temperature sulfurization. S diffuses into Mo to form resistive MoS2. In addition, as a result of the reaction, CZTS may decompose into secondary phases such as ZnS, Cu2S and volatile SnS causing void formation. Secondly, large open-circuit voltage deficit limits performance of the kesterite solar cells. Similar size of cations in CZTS facilitates cation disordering and formation of detrimental defects causing charge recombination. In this thesis, the effect of sputter of TaN, ZnS and CuO intermediate layers (IL) on the quality of the back interface, pure sulfide kesterite absorber layer and photovoltaic properties of the solar cells was studied in detail. The novel IL were used to slow down the back interface reaction and control crystal quality of kesterite absorber layer. It has been found that interfacial MoS2 thickness between sputtered Cu-poor CZTS and Mo can be effectively controlled using TaN IL. When 12 nm TaN IL is present, no continuous MoS2 is observed using scanning transmission electron microscopy after sulfurization at 600°C for 10 min. Incorporation of 10 nm ZnS IL at the interface between solution-processed Cd-substituted CZTS (CZCTS) and Mo enhances CZCTS grain growth and suppresses void formation. Moreover, deposition of the ZnS IL does not cause increase of MoS2 thickness. PCE of solution-processed CZCTS solar cells was enhanced from 9.5% to 10% as a result of 10 nm ZnS IL layer insertion. A higher PCE of 10.8% was achieved for solution processed CZCTS solar cells using CuO IL incorporation. Insertion of the optimized 4 nm CuO IL resulted in enhancement of short-circuit current density (Jsc) due to the increased width of depletion region while open-circuit voltage (Voc) and fill factor (FF) were almost constant. Moreover, further increase of CuO IL thickness enables to get as high Jsc as 28.5 mA/cm2. To the best of my knowledge, this is the highest reported Jsc for pure sulfide CZCTS solar cells. To address the problem of the absorber layer quality, impact of Mo on sputtered CZTS thin films was studied. Mo was found to be a dopant for CZTS enabling to tune CZTS resistivity in a wide range. In addition, it has been revealed that Mo incorporation enables to enhance absorbance of CZTS thin films while not affecting bandgap of CZTS based on measured external quantum efficiency spectra. Furthermore, Mo incorporation in CZTS demonstrates a substantial effect on the photovoltaic performance of the solar cells prepared by sputtering of quaternary compound CZTS target. When optimized Mo co-sputtering power is applied, PCE of CZTS solar cell is increased from 1.6% to 5.5%. In addition, effect of carbon (C) incorporation in sputtered CZTS and Cu doped ZnS (ZnS:Cu) thin films was investigated. It has been found that optimized content of C facilitates growth of CZTS crystallites and reduces void formation. Furthermore, the addition of C in sputtered CZTS and ZnS:Cu thin films enables reduction of tail states through defects passivation, decrease resistivity and tune bandgap. Moreover, formation of copper sulfide phase is suppressed while formation of ZnS is enhanced with increase of C content in p-type ZnS:Cu thin films resulting in increase of its bandgap from 3.2 eV to 3.6 eV.Doctor of Philosoph

Current Status and Future Prospects of Copper Oxide Heterojunction Solar Cells

The current state of thin film heterojunction solar cells based on cuprous oxide (Cu2O), cupric oxide (CuO) and copper (III) oxide (Cu4O3) is reviewed. These p-type semiconducting oxides prepared by Cu oxidation, sputtering or electrochemical deposition are non-toxic, sustainable photovoltaic materials with application potential for solar electricity. However, defects at the copper oxide heterojunction and film quality are still major constraining factors for achieving high power conversion efficiency, η. Amongst the Cu2O heterojunction devices, a maximum η of 6.1% has been obtained by using pulsed laser deposition (PLD) of AlxGa1−xO onto thermal Cu2O doped with Na. The performance of CuO/n-Si heterojunction solar cells formed by magnetron sputtering of CuO is presently limited by both native oxide and Cu rich copper oxide layers at the heterointerface. These interfacial layers can be reduced by using a two-step sputtering process. A high η of 2.88% for CuO heterojunction solar cells has been achieved by incorporation of mixed phase CuO/Cu2O nanopowder. CuO/Cu2O heterojunction solar cells fabricated by electrodeposition and electrochemical doping has a maximum efficiency of 0.64% after surface defect passivation and annealing. Finally, early stage study of Cu4O3/GaN deposited on sapphire substrate has shown a photovoltaic effect and an η of ~10−2%

Solution Processed Pure Sulfide CZCTS Solar Cells with Efficiency 10.8% using Ultra-Thin CuO Intermediate Layer

In this work, we demonstrate that incorporating an ultra‐thin p‐type cupric oxide (CuO) enhances performance and stability of the solution processed Cu2(Zn0.6Cd0.4)SnS4 (CZCTS)/CdS thin film solar cells. In sol‐gel CZCTS/CdS thin film solar cells, nanoscale CuO films (4 – 32 nm) were deposited on top of molybdenum (Mo) by magnetron sputtering and this was used as an intermediate layer (IL). The CuO IL thickness has a significant effect on the short‐circuit current density (JSC) in CZCTS/CdS solar cell devices. As a result, a maximum power conversion efficiency (PCE) of 10.77% has been measured for the optimized device with 4 nm CuO compared with 10.03% for the reference device without CuO layer. Furthermore, stability of the devices is enhanced significantly by incorporating CuO IL. The present work demonstrates that through proper design of the CuO intermediate layer thickness, both back interface quality and optical property of the CZCTS absorber can be tuned to enhance the device performance

Improving the crystallinity and texture of oblique-angle-deposited AlN thin films using reactive synchronized HiPIMS

Many technologies, such as surface-acoustic-wave (SAW) resonators, sensors, and piezoelectric MEMS require highly-oriented and textured functional thin films. The best results are typically achieved for on-axis sputter geometries, but in some scenarios, this is not feasible, such as during co-deposition from multiple magnetrons or when coating substrates with high aspect ratios. Ionized physical vapor deposition (PVD) techniques such as HiPIMS can be used to accelerate the film-forming species onto the growing film using substrate-bias potentials, thus increasing ad-atom mobility and film texture. However, gas-ion incorporation can limit the feasibility of such synthesis approaches for defect-sensitive functional thin films. This work reports on the oblique-angle deposition of highly textured, c-axis oriented AlN (0002) films using reactive metal-ion synchronized HiPIMS. AlN thin films deposited using direct current magnetron sputtering (DCMS) and HiPIMS are discussed for comparison and the effect of ion irradiation through substrate biasing is investigated. We find that combining HiPIMS with a moderate substrate bias of only −30 V improves the crystalline quality and texture of the films significantly, while the process-gas incorporation and point defects formation can be further reduced by synchronizing the negative substrate bias potential to the Al-rich fraction of each HiPIMS pulse. In addition to a pronounced out-of-plane texture, the films show uniform polarization of the grains making this synthesis route suitable for piezoelectric applications. While the compressive stress in the films is still comparatively high, the results already demonstrate, that metal-ion synchronized HiPIMS can yield promising results for the synthesis of functional thin films under oblique-angle deposition conditions - even with low substrate-bias potentials.ISSN:0257-8972ISSN:1879-334

Combinatorial Reactive Sputtering with Auger Parameter Analysis Enables Synthesis of Wurtzite Zn₂TaN₃

The discovery of new functional materials is one of the key challenges in materials science. Combinatorial high-throughput approaches using reactive sputtering are commonly employed to screen unexplored phase spaces. During reactive combinatorial deposition, the process conditions are rarely optimized, which can lead to poor crystallinity of thin films. In addition, sputtering at shallow deposition angles can lead to off-axis preferential orientation of the grains. This can make the results from a conventional structural phase screening ambiguous. Here, we perform a combinatorial screening of the Zn–Ta–N phase space with the aim to synthesize the novel semiconductor Zn2TaN3. While the results of the X-ray diffraction (XRD) phase screening are inconclusive, including Auger parameter analysis in our workflow allows us to see a very clear discontinuity in the evolution of the Ta binding environment. This is indicative of the formation of a new ternary phase. In additional experiments, we isolate the material and perform a detailed characterization confirming the formation of single-phase wurtzite Zn2TaN3. Besides the formation of the new ternary nitride, we map the functional properties of ZnxTa1–xN and report previously unreported clean chemical state analysis for Zn3N2, TaN, and Zn2TaN3. Overall, the results of this study showcase common challenges in high-throughput materials screening and highlight the merit of employing characterization techniques sensitive toward changes in the materials’ short-range order and chemical state

Cu₂O/CuO heterojunction catalysts through atmospheric pressure plasma induced defect passivation

A novel route to fabricate Cu2O/CuO heterojunction electrodes using an atmospheric pressure plasma jet (APPJ) is demonstrated. This process promotes favourable band alignment and produces nanoscale CuO surface features from Cu2O with low density of interfacial defects. This electrode can operate without any transparent current collector, showing remarkable currents and stability towards oxygen evolution reaction (OER) (6 mA cm−2 for 2 h at pH14) as well as photocatalytic hydrogen evolution reaction (HER) activity (−1.9 mA cm−2 for 800 s at pH7). When the electrocatalytic oxygen evolution (OER) activity was measured for Cu2O/CuO electrode deposited on FTO substrate the currents increased to ~40 mA cm−2 at 0.8 V vs SCE in 1 M KOH without compensating for the electrode electrolyte surface resistance (iR correction). The composite films also exhibited a high rate towards photo degradation of Methylene Blue (MB) and phenol in the visible spectra, indicating efficient charge separation. We modelled the electronic structure of this epitaxially grown Cu2O/CuO heterojunction using density functional theory. The calculations revealed the distinctive shifts towards Fermi level of the p-band centre of O atom in Cu2O and d-band centre of Cu atom in CuO at the interface contribute towards the increased catalytic activity of the heterostructure. Another factor influencing the activity stems from the high density of excited species in the plasma introducing polar radicals at the electrode surface increasing the electrolyte coverage. This work presents the potential of APPJ functionalization to tune the surface electronic properties of copper oxide based catalysts for enhanced efficiency in OER and HER water splitting

Impact of various dopant elements on the electronic structure of Cu₂ZnSnS₄ (CZTS) thin films:a DFT study

Abstract New structures made based on Cu₂ZnSnS₄ (CZTS) by substitutions with Cr, Ti, V, and Mo species were investigated via density functional theory. The total substitution of Zn by Cr and V leads to the vanishing of the bandgap, while n-type conductivity with a low bandgap of 0.19 eV was predicted in the case Ti. In addition, the conduction band minimum and valence band maximum overlapping were observed for the Mo/Sn ratio of 1/3. Therefore, our study suggests that even the low content of alternative cations in CZTS allows to control its band alignment. The obtained results can be helpful for designing CZTS-based intermediate layers to improve the quality of the back interface of the CZTS thin-film solar cells

Atmospheric pressure plasma engineered superhydrophilic CuO surfaces with enhanced catalytic activities

Cupric oxide (CuO) thin film has found widespread application as a low-cost, earth-abundant material for electro and photo catalytic applications. High surface wettability is a key factor to achieve enhanced efficiency in these catalytic applications. Here, we report a fast and environment friendly route to fabricate super hydrophilic CuO thin films using a low power (5 to 10 Watts) atmospheric pressure plasma jet (APPJ). With APPJ treatment for 5 minutes, the CuO surface transforms from hydrophobic to super-hydrophilic with threefold increase in catalytic activity. The electrodes were extensively characterized using various bulk and surface-sensitive techniques. APPJ introduces anisotropy in the crystal structure and creates unique three-dimensional surface morphology with distinct surface chemical and electronic features. Interestingly, presence of oxygen in the plasma was found to be critical for the enhanced activities and the activity decreased when the functionalised with nitrogen plasma. Oxygen plasma functionalisation of CuO electrodes resulted in a 130 mV reduction in the onset potential for oxygen evolution reaction along with enhanced current density ,10 mA cm-2 against 3 mA cm-2 at 1 V vs Saturated Calomel Electrode in 0.1M KOH without iR compensation. Importantly, without introducing any external dopants the work function could be decreased by 80mV. Moreover, the treated films exhibited a higher rate of photo degradation (0.0283 min-1 compared to 0.0139 min-1) of Methylene Blue and phenol indicating efficient charge separation. This work presents the potential of APPJ functionalisation of CuO surface to boost the activity of other thin film catalyst materials and solutions processed systems