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Summary Recently, most of the highly efficient perovskite optoelectronic devices have been reported to employ effective surface passivation strategies, further confirming the significance of surface states in regulating their device performance. Therefore, an in-depth understanding and a systematic approach toward comprehensive investigations on perovskite surface states are urgently required. Here, we present methodical studies toward understanding the surface states in perovskite thin films utilizing a molecular “positively charged defect indicator” strategy. In formamidinium (FA)-methylammonium (MA) mixed-cation perovskite thin films, a nonuniform distribution of cations is uncovered with FA cations being close to the top and MA close to the bottom of the film, which leads to unique surface defect energetics. Antisite FAI was found to become a dominant deep trap on the surface of this system. As a result, a surface recombination velocity as low as 20 cm/s was achieved in such FA-based perovskite photovoltaic devices.

Intrinsic ion migration in the metal halide perovskite (MHP) absorber layer and its interfaces seriously limits the device stability of perovskite solar cells (PSCs). Despite considerable efforts to mitigate the ion migration issue, it remains a formidable challenge in the commercialization of PSCs. Here, we provide a short review of the device failure mechanisms induced by intrinsic ion migration and discuss the detrimental effects of ion migration on the different component layers of PSCs. We outline the corresponding strategies to mitigate ion migration in PSCs and provide an insight on materials engineering to attain long-term stabilized perovskite photovoltaics (PVs).

The effects of post-deposition CdCl2 annealing temperature on the electronic properties of CdTe solar cells were investigated. CdTe devices incorporate Mg doped ZnO as a buffer layer and selenization using a CdSe interlayer for reducing the buffer/absorber interface recombination and increasing solar energy absorption respectively. The post-deposition CdCl2 annealing treatments were done under separate, inert atmospheres of nitrogen and helium across the temperature range 380–430 °C. Electrical characterization of devices is carried out including temperature dependent current-voltage characteristics, admittance spectroscopy, and Shockley-Read-Hall recombination analysis. The best improvements in device efficiency are obtained upon annealing at temperature 410 °C. This anneal correlated with reduced back contact barrier in CdTe and reduced grain-boundary barrier height which is beneficial for enhanced charge transport.

Summary With rapid progress in perovskite light-emitting diodes (PeLEDs), the electroluminescence performance of large-area is of increasing interest. We investigated why large-area performance lags behind that achieved in laboratory-scale devices and found that defects in perovskite films—emerging from thermal convection during solvent evaporation, as well as electronic traps formed during perovskite crystallization—are chief causes. Here, we report a molecular modification strategy that simultaneously eliminates pinholes in perovskite layers by controlling the dynamics of film formation and that passivates defects in perovskites by incorporating Br species, thereby preventing shorts and non-radiative recombination. The molecular modifier 1,3,5-tris (bromomethyl) benzene (TBB) also modulates the electronic structure of injection or transport materials to achieve improved charge injection and balanced charge transport. As a result, we demonstrate 20 mm × 20 mm green perovskite nanocrystal LEDs that achieve an external quantum efficiency (EQE) of over 16%, a record for large-area PeLEDs.

Formamidinium lead triiodide (HC(NH2)2PbI3 or FAPbI3) is a promising light absorber for high-efficiency perovskite solar cells because of its superior light absorption range and thermal stability to CH3NH3PbI3 (MAPbI3). Unfortunately, it is difficult to fabricate high-quality FAPbI3 thin films to surpass the MAPbI3-based cells due to easily forming unwanted but more stable yellow δ-phase and thus requiring high annealing-temperature for wanted photovoltaic-active black α-phase. Herein, we reported a novel low-temperature fabrication of highly crystallized α-FAPbI3 film exhibiting uniaxial-oriented nature with large grain sizes up to 2 μm. First-principles energetic calculations predicted that this novel deposition should be ascribed to the formation of a high-energy metastable two-dimensional (2D) intermediate of MAFAPbI3Cl followed by a spontaneous conversion to α-FAPbI3. The ions exchange reaction during this MAFAPbI3Cl-FAPbI3 conversion account for the perovskite film uniaxial-oriented grown along the (1 1 1) direction. This large-grain and uniaxial-oriented grown α-FAPbI3 based solar cells exhibited an efficiency up to 20.4% accompanying with low density-voltage (J-V) hysteresis and high stability.

Electronic properties of a CdTe solar cell are reported using temperature-dependent capacitance spectroscopy and current-voltage characteristics, the latter in dark and illuminated conditions. The baseline solar cell material stack investigated is comprised of soda-lime-glass/SnO2:F/SnO2/CdS:O-buffer/CdTe-absorber/Cu/Au. Device properties are compared with CdTe solar cells in which the back surface was hydroiodic acid etched, before the back-contact formation, and a CdTe device in which Mg-doped ZnO (MZO) replaces buffer layers. Reduced back-contact barrier height and grain boundary barrier height are observed in the hydroiodic acid treated CdTe cell. Improved device performance in the MZO-based CdTe device is attributed to reduced emitter/absorber interface recombination when using the MZO window layer.

Band offsets at the back contact and front window layer in CdTe-based solar cells affect photovoltaic performance and challenge standard characterization methods. By analyzing the temperature and excitation dependence of both open circuit voltage and absolute photoluminescence intensity, we show that the effects band offsets can be separated from the effects of recombination and shunting. Solar cells were grown with MgZnO window layers and compared to cells with CdS window layers containing varying amounts of oxygen. It was demonstrated that band alignment rather than reduced recombination velocity is the reason for the success of MgZnO as a front interface contact. An assortment of thin back contact interlayers were also deposited, and a PbTe interlayer showed some promise as an Ohmic contact to the CdTe, though it appears to induce a photoconductive shunt. Finally, we show that the shunting resistance given by a standard current-voltage curve technique generally does not represent a physically meaningful quantity unless it is well below one kiloOhm square cm.

Wide-bandgap (∼1.7–1.8 eV) perovskite solar cells have attracted substantial research interest in recent years due to their great potential to fabricate efficient tandem solar cells via combining with a lower bandgap (1.1–1.3 eV) absorber (e.g., Si, copper indium gallium diselenide, or low-bandgap perovskite). However, wide-bandgap perovskite solar cells usually suffer from large open circuit voltage (Voc) deficits caused by small grain sizes and photoinduced phase segregation. Here, we demonstrate that in addition to large grain sizes and passivated grain boundaries, controlling interface properties is critical for achieving high Voc's in the inverted wide-bandgap perovskite solar cells. We adopt guanidinium bromide solution to tune the effective doping and electronic properties of the surface layer of perovskite thin films, leading to the formation of a graded perovskite homojunction. The enhanced electric field at the perovskite homojunction is revealed by Kelvin probe force microscopy measurements. This advance enables an increase in the Voc of the inverted perovskite solar cells from an initial 1.12 V to 1.24 V. With the optimization of the device fabrication process, the champion inverted wide-bandgap cell delivers a power conversion efficiency of 18.19% and sustains more than 72% of its initial efficiency after continuous illumination for 70 h without encapsulation. Additionally, a semitransparent device with an indium tin oxide back contact retains more than 88% of its initial efficiency after 100 h maximum power point tracking.

Summary Tandem solar cells coupling narrow- and wide-band-gap thin-film polycrystalline absorbers are attractive for achieving ultrahigh efficiency with low manufacturing cost. For established narrow-band-gap CIGS thin-film bottom cells, a challenge is to develop highly efficient polycrystalline wide-band-gap top cells. Here, we demonstrate a 1.68-eV (FA0.65MA0.20Cs0.15)Pb(I0.8Br0.2)3 wide-band-gap perovskite solar cell with an efficiency of ∼20% enabled by using PEAI and Pb(SCN)2 complementary additives in the perovskite precursor. The coupling of PEA+ and SCN− provides a synergistic effect that overcomes growth challenges with either additive individually and improves perovskite film quality with enhanced crystallinity, reduced formation of excess PbI2 (in comparison to using Pb(SCN)2 additive alone), lower defect density and energetic disorder, and an improved carrier mobility (∼47 cm2 V−1s−1) and lifetime (∼2.9 μs). When coupling a semi-transparent 1.68-eV perovskite top cell fabricated by this approach with a 1.12-eV CIGS bottom cell, we achieve 25.9%-efficient polycrystalline perovskite/CIGS 4-terminal thin-film tandem solar cells.

Optical characterization and simulations have been applied to study formamidinium + cesium lead triiodide (FA1-xCsxPbI3) perovskite based thin films and solar cells. Near infrared to ultraviolet complex dielectric function (e = e1 + ie2) spectra of varying cesium-to-formamidinium ratios in solution processed perovskite thin films have been extracted. Analysis of these e spectra track changes in the positions of critical point (CP) energies, including band gaps and above band gap transitions, with increasing cesium contents. Band gap values are identified as the lowest energy CP, with additional sub-gap features attributed to the presence of defects. Absorption onset values, for which absorption coefficient (α) = 4000 cm−1, are extracted, with x = 0.2 showing the sharpest absorption edge and the least contribution to absorption from defects below the band gap. External quantum efficiency (EQE) simulations of solar cells using e spectra as input are compared to experimental results and corroborate that the observed sub-gap absorption is due to defects as it does not contribute to EQE.

Incorporating multiple cations of the Ia alkali metal column of the periodic table (K+/Rb+/Cs+) to prepare m-AMCs perovskite film is promising for boosting the photovoltaic properties. However, contrary to K+, both Cs+ and Rb+ suffer from non-uniformity at the origin of performance and stability losses. In this paper, Ammonium chloride (NH4Cl) additive is shown to address this concern. We analyze step-by-step the effects of NH4Cl Additive and Alkali Metal Cations (K+/Rb+/Cs+) on the one-step film formation process of methylammonium-free, formamidinium-based, halide perovskites. First, we highlight the action of NH4Cl additive on the spin-coating and the anti-solvent dripping processes. We show that it improves the solubility of PbI2 in solution by forming an intermediate and then favor the perovskite phase formation. Then, we investigate the annealing process by introducing depth profile evolution monitoring by the glow discharge optical emission spectroscopy (GD-OES) technique. The real-time distribution changes of m-AMCs in the film upon annealing at 155°C is visualized. We show that K at low concentration is homogeneously distributed throughout the film. Cs is more concentrated at the surface and Rb in the film depth. With NH4Cl additive, these two alkali metals are more homogeneously distributed. We show that NH4Cl can slow down the movement of m-AMCs so that they are better evenly distributed into the perovskite layer. Moreover, it changes the growth direction of the perovskite film changes, making the overall crystallization quality improved and the distribution more uniform. It results in perovskite films with large monolithic grains, permitting a high stabilized power conversion efficiency (PCE) over 21%. Finally, by combining AC additive and film surface treatment with n-propylammonium iodide (PAI), the performance was further upgraded at a stabilized PCE of 22.04%.

Single crystal halide perovskite nanowires (NWs) have attracted much attention for use in high-performance photodetectors (PDs) due to their high photoluminescence quantum yields, large carrier mobilities, and long carrier diffusion lengths. So far, most high-performance NW PDs are fabricated based on individual or aligned NWs which require complicated preparation processing. Furthermore, it is challenging for these NW PDs to possess self-powered operation characteristics. Here, we report self-powered perovskite NW PDs with a vertical p-i-n structure in which the CsPbBr3 NWs are grown by a combination of solution-phase and halide exchange processes. Before depositing the hole selective layer, poly(methyl methacrylate) is coated on perovskite NW film to fill the internal voids, which passivates surfaces of perovskite NWs and reduces the dark current. Our optimized CsPbBr3 NW PDs show dark current density as low as 4.0 nA cm−2 and photocurrent density as high as 22.9 mA cm−2 under a 473 nm laser illumination with intensity of 641 mW cm−2, leading to a high linear dynamic range of 135 dB. In addition, our perovskite NW PDs display highly self-powered performance with ultrohigh on/off ratios of above 106, responsivity of up to 0.3 A W−1 and detectivity of up to 1 × 1013 Jones at 0 V.

To maximize the photovoltaic performance of perovskite solar cells (PVSCs）by developing new hole-transport layer (HTL) materials, the precise tuning of their energy levels especially the highest occupied molecular orbital (HOMO) is highly desirable. Here, a simple binary strategy for the first time is proposed to acquire ideal HOMO level by optimizing the composition of binary blend HTLs including CZ-TA (HOMO = −5.170 eV) and CZ-STA (HOMO = −5.333 eV). By adding 10 wt% CZ-STA, the binary HTM (HOMO = −5.199 eV) based perovskite solar cells achieve a maximum power conversion efficiency of 19.85% (18.32% for CZ-TA). The introducing of S atom in CZ-STA not only downshifts HOMO level but also forms stronger Pb-S interaction with perovskites than Pb-O in CZ-TA, leading to better device performance and reduced hysteresis. Importantly, the un-encapsulated PVSCs using CZ-TA:CZ-STA (90:10, w/w) binary HTL exhibit good environment stability in ambient air, maintaining over 82% of their initial efficiency after 60 days’ storage with a relative humidity around 50%. Therefore, this strategy provides new insights on HTL development to push forward the progress of the emerging PVSCs

Sb2Se3 is a promising candidate for low toxicity, low-cost and high efficiency solar cells. Thin film solar cells employing Sb2Se3 as the absorber have achieved power conversion efficiencies of up to 6.5%. However, these solar cells are typically fabricated using rapid thermal evaporation, a fast deposition technique but suffer from entangled substrate and source temperatures. Here, we report on the fabrication of stable and efficient CdS/Sb2Se3 thin film solar cells using close-space sublimation process, which allows separate control of the source and substrate temperatures. This enables better control of Sb2Se3 film deposition and reduction of interfacial diffusion, leading to higher quality absorber films and improved device heterojunctions. Additionally, we introduced a novel low-cost material 4,4',4'',4'''-(9-octylcarbazole-1,3,6,8-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)aniline) (CZ-TA) as the hole transport layer (HTL) to create n-i-p structured devices. This CZ-TA HTL reduced the back contact resistance and promoted photogenerated carrier collection, boosting device photo conversion efficiency to 6.84%.

A high-efficiency wide-bandgap (WBG) perovskite solar cell is critical for developing perovskite-related (e.g., all-perovskite, perovskite/Si, or perovskite/Cu(In,Ga)Se 2 ) tandem devices. Here, we demonstrate the use of non-stoichiometric precursor chemistry with excess methylammonium halides (MAX; X = I, Br, or Cl) for preparing high-quality ∼1.75-eV FA 0.83Cs0.17Pb(I0.6Br0.4)3 perovskite solar cells. Among various methylammonium halides, using excess MABr in the non-stoichiometric precursor exhibits the strongest effect on improving perovskite crystallographic properties and device characteristics without affecting the perovskite composition. In contrast, using excess MAI significantly reduces the bandgap of perovskite due to the replacement of Br with I. Using 40% excess MABr, we demonstrate a single-junction WBG perovskite solar cell with stabilized efficiency of 16.4%. We further demonstrate a 20.3%-efficient 4-terminal tandem device by using a 14.7%-efficient semi-transparent WBG perovskite top cell and an 18.6%-efficient unfiltered (5.6%-efficient filtered) Si bottom cell.

A carbazole-based hole-transporting material named 4,4′,4′′,4′′′-(9-octylcarbazole-1,3,6,8-tetrayl) tetrakis(N,N-bis(4-methoxyphenyl)aniline) (CZ-TA) has been developed through a one-step facile synthesis approach. The solution-processed planar perovskite solar cells (PVSCs) with 50 nm thick CZ-TA hole selective layer (HSL) contributes a power conversion efficiency of 18.32%, comparable to the most commonly used 200 nm spiro-OMeTAD (18.28%) HSLs. The improved hole extraction, transport and reduced recombination are found to endow CZ-TA-based devices with impressive fill factors over 81.0%. Importantly, the unit cost of HSL in PVSCs using CZ-TA can be as low as only 1/80 of that of spiro-OMeTAD, indicating that CZ-TA could be a promising candidate as HSLs for commercialization of the low-cost PVSC technology.

We report on compositional and morphological engineering of mixed methylammonium (MA) and formamidinium (FA) lead triiodide (MA1−xFAxPbI3) perovskite absorber layers to produce highly efficient planar and flexible perovskite solar cells (PVSCs) with reduced hysteresis. Incorporation of FA into the MAPbI3 extends the absorption edge of the perovskite to longer wavelengths, leading to enhanced photocurrent of the resultant PVSCs. Moreover, adding a small amount of lead thiocyanate (Pb(SCN)2) additive into mixed perovskite precursor solutions significantly enlarges the grain size and prolongs the carrier lifetime, leading to improved device performance. With optimal compositional and morphological engineering, the average power conversion efficiency (PCE) improves from 15.74±0.74% for pure MAPbI3 PVSCs to 19.40±0.32% for MA0.7FA0.3PbI3 PVSCs with 3% Pb(SCN)2 additive, exhibiting a high reproducibility and small hysteretic behavior. The best PVSC achieves a PCE of 20.10 (19.85)% measured under reverse (forward) voltage scan. Furthermore, the compositional and morphological engineering allowed the fabrication of efficient flexible PVSCs on indium-doped SnO2 (ITO)/polyethylene terephthalate (PET) substrates, with the best PCE of 17.96 (16.10)% with a VOC of 1.076 (1.020) V, a JSC of 22.23 (22.23) mA/cm2 and a FF of 75.10 (71.02)% when measured under reverse (forward) voltage scan. Our approach provides an effective pathway to fabricate highly efficient and reproducible planar PVSCs.

Bifacial solar cells have the potential to increase the energy yield per unit area over traditional monofacial devices without significant added cost, driving $/kWh costs lower and accelerating the adoption of solar photovoltaics. However, the performance of bifacial thin film solar cells significantly lags that achieved by crystalline silicon cells. Here we incorporate wide bandgap CuxAlOy as a back buffer layer for CdTe devices and achieve a backside illuminated device with high current density and high fill factor. Moreover, these values remain nearly constant even as the absorber layer thickness changes, indicating that a fully-depleted device is not required for efficient charge collection. We show that this response is indicative of a back surface field, albeit with a persistent high back surface recombination velocity. By managing electron reflection, we achieved a backside illumination conversion efficiency of 7.1% and bifaciality of 0.55 for a 3.3 µm CdTe device and 8.0% and 0.62 for a 2 µm device. Future improvements can be made by identifying and incorporating a passivation material that reduces the back surface recombination velocity.

Optically transparent p-type materials play a critical role in transparent electronics including photovoltaic (PV) devices. P-type sulfide materials offer an alternative to oxides for PV application due to improved hole transport properties. Here, we report the solution-based synthesis of earth-abundant p-type transparent conducting barium copper sulfide (α-BaCu4S3, BCS) thin films. These films were characterized using scanning electron microscopy, X-ray diffraction, UV–Vis–NIR spectrophotometry, Raman spectroscopy, and spectroscopic ellipsometry. BCS films of ~100 nm thickness transmit >70% of visible light. We report on tests of the hole transport properties of these BCS films for cadmium telluride (CdTe) photovoltaics, finding that the BCS deposition process forms a beneficial tellurium (Te) rich surface on CdTe by selectively removing Cd from the surface. Based on our study, the BCS interface layer plays dual functions for CdTe PV devices as a hole transport material and as an etchant, enhancing the resulting device performance. We observed a significant increase in open-circuit voltage of CdTe solar cells with the BCS buffer layer. Additionally, we discuss semitransparent CdTe solar cells with BCS as a hole transport layer and indium tin oxide as a finishing electrode. Semitransparent CdTe solar cells shows 13.3% conversion efficiency for the front side illumination and 1.2% efficiency for back side illumination, indicating high recombination of charge carriers generated close to the rear CdTe/BCS/ITO contact.

Perovskite photovoltaic cells (PVs) have attracted significant worldwide attention in the past few years. Although the stability of the power conversion is a concern, there is great potential for perovskites to enter the global PV market. To determine the future potential of perovskites, we performed a cradle-to-gate environmental life cycle (LCA) for two different perovskite device structures suitable for low cost manufacturing. Rather than examining current laboratory deposition processes like dipping and spinning, we considered spray and co-evaporation methods that are more amenable to manufacturing. A structure with an inorganic hole transport layer (HTL) was developed for both solution and vacuum based processes, and an HTL-free structure with printed with back contact was modeled for solution based deposition. The environmental impact of conventional Si PV technology was used as a reference point. The environmental impacts from manufacturing of perovskite solar cells were lower than that of mono-Si. However, environmental impacts from unit electricity generated were higher than all commercial PV technology mainly because of the shorter lifetime of perovskite solar cell. The HTL-free perovskite generally had the lowest environmental impacts among the three structures studied. Solution based methods used in perovskite deposition were observed to decrease the overall electricity consumption. Organic materials used for preparing the precursors for perovskite deposition were found to cause a high marine eutrophication impact. Surprisingly, the toxicity impacts of the lead used in the formation of the absorber layer were found to be negligible. Energy payback times were estimated as 1.0–1.5 years.

Long-term stability represents a major challenge for the commercial deployment of hybrid perovskite solar cells (PSCs). The stability of solar cells is commonly tested under continuous illumination over extended periods of time, for example, 1000 h. We have found that such a method does not adequately reflect the long-term performance of perovskite solar cells under the diurnal solar irradiation cycles experienced in real-world applications. We report a new characterization protocol of multiple 12-h cycles of darkness and illumination, uncovering a unique ‘fatigue’ behavior of PSCs. The PSC efficiency was found to decrease to 50% or less of its maximum value after storage in the dark for 12 h under open circuit conditions. The solar cell performance was capable of recovering to its maximum value in the subsequent 12-h illumination period, but the recovery rate slowed significantly with successive illumination/darkness cycles. This fatigue mechanism was strongly dependent on the cell temperature. The identification of this fatigue behavior renders our proposed characterization protocol an essential component of perovskite solar cell testing.

Segregated elements and their diffusion profiles within grain boundaries and interfaces resulting from post deposition heat treatments are revealed using atom probe tomography (APT), scanning transmission electron microscopy (STEM), and electron beam induced current (EBIC) techniques. The results demonstrate how these techniques complement each other to provide conclusive evidence for locations of space charge regions and mechanisms that create them at the nanoscale. Most importantly, a Cl dopant profile that extends ~5 nm into CdTe grains interfacing the CdS is shown using APT and STEM synergy, which has been shown to push the pn-junction into the CdTe layer indicative of a homojunction (revealed by STEM EBIC). In addition, Cu and Cl concentrations within grain boundaries within several nms and µms from the CdS/CdTe interface are compared, Na segregation of

We present efficient metal oxide-free and annealing-free planar perovskite solar cells with the regular cell structure using vacuum-deposited fullerenes C 60 and C 70 as the electron-selective layers and vacuum-processed perovskites as the light absorbers. The devices with an ultrathin C 60 layer (5.5 nm) yielded an average power conversion efficiency of 14.3% and a maximum efficiency of 15.7%. The best-performing cell produced a steady-state efficiency of 14.6%. The high performance is attributed to the efficient blocking of holes and extraction of electrons by C 60 due to a favorable energy level alignment between the C 60 and the fluorine-doped tin oxide electrodes. With the realization of efficient cells, the annealing-free vacuum deposition of perovskite absorbers and C 60 or C 70 electron-selective layers and intermediate layers demonstrates its power for fabricating all-perovskite tandem solar cells.

We focus on utilizing sputtered indium tin oxide (ITO) as a recombination layer, having low junction damage to an n-type silicon solar cell with a front-side tunnel oxide passivating electron contact, thereby enabling the development of a high efficiency monolithic perovskite/Si tandem device. High transparency and low resistivity ITO films are deposited via low power DC magnetron sputtering at room temperature onto a front-side thin SiOx/n+ poly-Si contact in a complete Cz n-Si cell with a back-side Al2O3/SiNx passivating boron-diffused p+-emitter on a random pyramid textured surface. We report the cell characteristics before and after ITO sputtering, and we find a cure at 250 °C in air is highly effective at mitigating any sputtering induced damage. Our ITO coated sample resulted in an implied open-circuit voltage (iVoc) of 684.7 ± 11.3 mV with the total saturation current density of 49.2 ± 14.8 fA/cm2, an implied fill factor (iFF) of 81.9 ± 0.8%, and a contact resistivity in the range of 60 mΩ-cm2 to 90 mΩ-cm2. After formation of a local Ag contact to the rear emitter and sputtered ITO film as the front-side contact without grid fingers, the pseudo-efficiency of 20.2 ± 0.5% was obtained with the Voc of 670.4 ± 7 mV and pseudo FF of 77.3 ± 1.3% under simulated one sun with the calculated short-circuit current density of 30.9 mA/cm2 from the measured external quantum efficiency. Our modelling result shows that efficiency exceeding 25% under one sun is practically achievable in perovskite/Si tandem configuration using the ITO recombination layer connecting a perovskite top cell and a poly-Si bottom cell.

The invention discloses nanocrystalline (NC) FeS 2 thin films as the back contact for CdTe solar cells. In one example, the FeS 2 NC layer is prepared from a solution directly on the CdTe surface using spin-casting and chemical treatment at ambient temperature and pressure, without a thermal treatment step. Solar cells prepared by applying the NC FeS 2 back contact onto CdTe yield efficiencies of about 95% to 100% that of standard Cu/Au back contact devices. In another example, FeS2 is interposed between Cu and Au to form a Cu/ FeS 2 NC/Au back contact configuration yielding an efficiency improvement of 5 to 9 percent higher than standard Cu/Au devices.

A nondestructive neutron scattering method to precisely measure the uptake of hydrogen and the distribution of hydride precipitates in light water reactor (LWR) fuel cladding was developed. Zircaloy-4 cladding used in commercial LWRs was used to produce hydrided specimens. The hydriding apparatus consists of a closed stainless-steel vessel that contains Zr alloy specimens and hydrogen gas. Following hydrogen charging, the hydrogen content of the hydrided specimens was measured using the vacuum hot extraction method, by which the samples with desired hydrogen concentrations were selected for the neutron study. Optical microscopy shows that our hydriding procedure results in uniform distribution of circumferential hydrides across the wall thickness. Small angle neutron incoherent scattering was performed in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Our study demonstrates that the hydrogen in commercial Zircaloy-4 cladding can be measured very accurately in minutes by this nondestructive method over a wide range of hydrogen concentrations from a very small amount (≈20 ppm) to over 1000 ppm. The hydrogen distribution in a tube sample was obtained by scaling the neutron scattering rate with a factor determined by a calibration process using standard, destructive direct chemical analysis methods on the specimens. This scale factor can be used in future tests with unknown hydrogen concentrations, thus providing a nondestructive method for determining absolute hydrogen concentrations.

We present a detailed assessment of the structural and electronic properties of CdTe calculated by density functional theory (DFT) with on-site Coulomb self-interaction potentials (LDA+U/GGA+U) on the Cd 4 d band. We systematically calculate the lattice constants, bulk moduli, elastic constants, band structure, and density of states as a function of the U value, and compare the results with those calculated by using standard LDA/GGA and the hybrid functional (HSE06). Our study gives a more accurate account of the strong localization effect of Cd 4 d electrons onto the overall electronic structure, in particular to the nature of localized Cd 4 d derived bands and delocalized Te 5 s derived bands and the coupling between them. We find that the s–d coupling is significant, which is underestimated within conventional DFT calculations (showing a single s-like peak, in disagreement with the experiments). LDA+U removes this discrepancy by shifting down the Cd- 4 d band closer to the Te- 5 s band, enhancing the s–d coupling, and leading to the appearance of two s-like peaks, which perfectly explains the so-called low intensity “shoulder” on the high-energy side of the Cd- 4 d peak in experimental spectra. Moreover, our results indicate LDA+U reveals an much more acceptable agreement with experiment at a adequate U than HSE06 does. A well balanced choice of U within LDA+U scheme is proposed to be at 7 eV.

Oxidation experiments were conducted at 1200 °C in flowing steam with tubing specimens of Zircaloy-4, 317, 347 stainless steels, and the commercial FeCrAl alloy APMT. The purpose was to determine the oxidation behavior and post-quench ductility under postulated and extended LOCA conditions. The parabolic rate constant for Zircaloy-4 tubing samples at 1200 °C was determined to be k = 2.173 × 10 7 g 2 /cm 4 /s, in excellent agreement with the Cathcart–Pawel correlation. The APMT alloy experienced the slowest oxidation rate among all materials examined in this work. The ductility of post-quenched samples was evaluated by ring compression tests at 135 °C. For Zircaloy-4, the ductile to brittle transition occurs at an equivalent cladding reacted (ECR) of 19.3%. SS-347 was still ductile after being oxidized for 2400 s (CP–ECR ≈ 50%), but the maximum load was reduced significantly owing to the metal layer thickness reduction. No ductility decrease was observed for the post-quenched APMT samples oxidized up to 4 h.

Thermochromic VO{sub 2} was directly synthesized using hydrothermal techniques. The effects of formation conditions on the structure and morphology of the final product were studied through X-ray diffraction (XRD), and scanning electron microscopy (SEM). Unique hollow sphere morphology was observed for the synthesized VO{sub 2} powders. Ex-situ XRD studies after heat treatment confirmed the thermal stability of the VO{sub 2} structure. Thermochromic properties, as a consequence of the reversible structural transformation between monoclinic VO{sub 2} and tetragonal phases, were observed by Fourier transform infrared spectroscopy (FTIR). - Graphical abstract: Thermochromic VO{sub 2} crystals with hollow spherical and asterisk shape were directly synthesized using hydrothermal techniques. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) studies confirmed the thermal stability and the reversible thermochromic properties of the VO{sub 2} structure. - Highlights: • One-step synthesis of thermochromic VO{sub 2} monoclinic phase, and VO{sub 2} (A and B phases) using hydrothermal technique. VO{sub 2} (A), VO{sub 2} (B). • Identification of the relationship between synthesis conditions and the morphology/structure of the final products. • Formation of VO{sub 2} monoclinic phase with an interesting hollow sphere shape. • Demonstration of superior thermal stability of the VO{sub 2} monoclinic phase. • Characterizing the thermochromicmore » properties of VO{sub 2} monoclinic phase.« less

In a recent paper published in Energy & Environmental Science, Avila et al. report a fully vacuum-processed dual-junction CH3NH3PbI3/CH3NH3PbI3 tandem solar cell featuring an unprecedentedly high open-circuit voltage of 2.30 V. This work demonstrates the promise of vacuum-based process for fabricating light-weight and flexible all-perovskite tandem solar cells with ultra-high power-conversion efficiencies.

Superstrate CdTe-based thin-film solar cells with conversion efficiencies higher than 15% have been synthesized on commercial SnO 2 :F/SnO 2 -coated soda-lime glass substrates without anti-reflection coating. The n -type CdS window layers were prepared by R.F. sputtering in an ambient containing a small amount of oxygen concentration. CdTe absorber layers were obtained by close-spaced sublimation (CSS). The post deposition CdCl 2 activation was conducted in a dry air ambient. With the application of elemental Cu/Au back contacts, the best small-area CdTe cells (0.08 cm 2 ) have shown an efficiency of 15.5% with an open-circuit voltage of 844 mV, a short-circuit current of 24.0 mA/cm 2 , and a fill-factor of 76.6% measured under an AM1.5 illumination. This efficiency was achieved when a 1% O 2 concentration was used in sputtering ambient during CdS deposition.

Aberration corrected scanning transmission electron microscopy (STEM) has been used to determine the structures of a variety of dislocation cores in CdTe, including 30° and 90° Shockley partial dislocations, positive and negative Frank sessile partial dislocations, and steps on twin boundaries. Structure models have been constructed from the images and electrical activity has been investigated with density functional calculations. An integrated electron energy loss spectroscopy, cathodoluminescence and electron beam induced current system has been designed and built to probe electrical and optical properties of individual defects. The first STEM-cathodoluminescence result shows strong impurity segregation between the CdTe and the glass. The correlation between the scanning electron microscopy-electron beam induced current and electron backscatter diffraction maps proves that the grain structures and boundaries dominate the electrical activity. After heat treatment in CdCl2, Cl is found to segregate to the grain boundaries, and they show higher efficiency than the bulk material.

To optimize materials and devices for solar photoelectrochemical hydrogen production, a detailed understanding of the chemical and electronic properties, in particular at the reactive surfaces and interfaces, is needed. In this review article we will show how electron and soft X-ray spectroscopies can provide such information. We will present exemplary studies using X-ray photoelectron spectroscopy, soft X-ray emission spectroscopy, UV photoelectron spectroscopy, and inverse photoemission. While the first two techniques mainly give insight into the chemical properties at and near the surface, the latter two methods allow us to derive the electronic levels relevant for photoelectrochemical water splitting at the surface of the investigated material. Ultimately, the ideal experiment would be performed in situ, in which the device is studied under working conditions, i.e., in a liquid environment and under illumination. We will give a short outlook on how this can be achieved experimentally under the strict requirements of the measurement environment.

The ZnO and mixed (ZnO:GaN) thin films are synthesized by (RF) magnetron sputtering in Ar and mixed O 2 and N 2 gas ambient at 100 °C, followed by post-annealing at 500 °C in ammonia for 4 h. The mixed (ZnO:GaN) thin films deposited under Ar gas ambient failed to reduce the bandgap, whereas (ZnO:GaN) thin films grown under mixed O 2 and N 2 gas ambient showed bandgap reduction. The (ZnO:GaN) films deposited under mixed O 2 and N 2 gas exhibited enhanced crystallinity, with shifting the optical absorption into the visible light regions. The bandgap reduction in mixed (ZnO:GaN) thin films is realized by varying the RF power. As a result, mixed (ZnO:GaN) films grown under mixed O 2 and N 2 showed higher photocurrents than the mixed (ZnO:GaN) thin films deposited under Ar gas ambient. Our results indicate that reduced bandgap with enhanced PEC response can be attained using the appropriate gas ambient and by varying the RF power using mixed (ZnO:GaN) films.

The performance of sputtered CdS/CdTe solar cells with MoO 3− x /Au contacts was examined. The cells were fabricated on Pilkington SnO 2 :F-coated soda-lime glass with a high resistivity transparent SnO 2 buffer layer. The MoO 3- x /Au back contact layers were deposited by thermal evaporation. The incorporation of the MoO 3− x layer was found to improve the open circuit voltage ( V OC ) but reduce the fill factor (FF) of the sputtered CdS/CdTe cells. While V OC is not sensitive to the thickness of the MoO 3 layer, FF is very sensitive. The optimal thickness for the MoO 3 layer is found to be 2–5 nm, which yields nearly 12% efficiency. The performance of these CdS/CdTe solar cells was further evaluated under thermal stress at 200 °C in controlled environments. We found that the cells degraded significantly under thermal stress in air, due to the increase of series resistance or decrease of FF.

Mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with significantly reduced bandgaps were synthesized by using zinc oxide and gallium nitride target at 100 °C followed by post-deposition annealing at 500 °C in ammonia for 4 h. All the films were synthesized by RF magnetron sputtering on Fluorine-doped tin oxide-coated glass. We found that mixed zinc oxide and gallium nitride (ZnO:GaN) thin films exhibited significantly reduced bandgap, as a result showed improved PEC response, compared to ZnO thin film. Furthermore, mixed zinc oxide and gallium nitride (ZnO:GaN) thin films with various bandgaps were realized by varying the O 2 mass flow rate in mixed O 2 and N 2 chamber ambient.

Using density functional theory calculation, we show that oxygen (O) exhibits an interesting effect in CdTe. The Te atoms with dangling bonds in a Te-rich rich Σ3 (112) grain boundary (GB) create deep gap states due to strong interaction between Te atoms. However, when such a Te atom is substituted by an O atom, the deep gap states can be shifted toward the valence band, making the site no longer a harmful non-radiative recombination center. We find that O atoms prefer energetically substituting these Te atoms and induce significant lattice relaxation due to their smaller atomic size and stronger electronegativity, which effectively reduces the anion–anion interaction. Consequently, the deep gap states are shifted to lower energy regions close to or even below the top of the valence band.

Thin-film photovoltaic modules hold great promise to produce sustainable, low-cost, and clean electricity from sunlight, because thin-film solar cells can potentially be fabricated by economical, high-volume manufacturing techniques. However, to achieve high sunlight-to-electricity conversion efficiency, thin-film solar cells require sophisticated control on interface formation and materials qualities. Transmission electron microscopy (TEM) provides unique methods to access this information at the nanometer scale. In this paper, we provide a brief review on TEM studies of the interfaces, microstructure, and lattice defects in chalcogenide thin-film photovoltaic materials. We analyze the potential effects of the observed interface formation and materials quality that could affect the performance of solar cells.

We report on our transmission electron microscopy study of dislocations and interfaces in CdTe solar cells. The atomic structure of dislocations formed inside CdTe grains have been determined by atomic-resolution transmission electron microscopy. We discuss the electronic properties of the dislocations and explore the effects of oxygen on the interdiffusion at CdS/CdTe interface. We find that the presence of oxygen in either CdS or CdTe suppresses the interdiffusion at the CdS/CdTe interface. We have further investigated interdiffusion at the CdS/Zn2SnO4 interface. We find that Zn diffuses into CdS from Zn2SnO4 and Cd diffuses into Zn2SnO4 from CdS. The possible effects of the interdiffusion are discussed. Finally, we have examined the distribution of intentionally introduced Cu at the CdTe/CdS junction, and we find that Cu is distributed uniformly in the CdS layer.

Combustion synthesized Li(Ni1/3Mn1/3Co1/3)O2 particles are coated with thin, conformal layers of Al2O3 by atomic layer deposition (ALD). XRD, Raman, and FTIR are used to confirm that no change to the bulk, local structure occurs after coating. Electrochemical impedance spectroscopy (EIS) results indicate that the surface of the Li(Ni1/3Mn1/3Co1/3)O2 are protected from dissolution and HF attack after only 4-layers, or ∼8.8 A of alumina. Electrochemical performance at an upper cutoff of 4.5 V is greatly enhanced after the growth of Al2O3 surface film. Capacity retention is increased from 65% to 91% after 100 cycles at a rate of C/2 with the addition of only two atomic layers. Due to the conformal coating, the effects on Li(Ni1/3Mn1/3Co1/3)O2 overpotential and capacity are negligible below six ALD-layers. We propose that the use of ALD for coating on Li(Ni1/3Mn1/3Co1/3)O2 particles makes the material a stronger replacement candidate for LiCoO2 as a positive electrode in lithium ion batteries.

Al and N co-doped ZnO thin films, ZnO:(Al,N), are synthesized by radio-frequency magnetron sputtering in mixed Ar and N2 and mixed O2 and N2 gas ambient at 100 °C. The ZnO:(Al,N) films deposited in mixed Ar and N2 gas ambient did not incorporate N, whereas ZnO:(Al,N) films grown in mixed O2 and N2 gas ambient showed enhanced N incorporation and crystallinity as compared to ZnO:N thin films grown in the same gas ambient. As a result, ZnO:(Al,N) films grown in mixed O2 and N2 gas ambient showed higher photocurrents than the ZnO:(Al,N) thin films deposited in mixed Ar and N2 gas ambient. Our results indicate that the gas ambient plays an important role in N incorporation and crystallinity control in Al and N co-doped ZnO thin films.

Upon analyzing an earlier experimental study by density-functional theory we have shown that graphene-like SiC layers can exist. We found that, for a particular stacking sequence, SiC double bond was responsible for the much larger interlayer distances observed in synthesized multi-walled SiC nanotubes. The Si/C ratios in SiC layers determine the extent of interlayer distances and bonding nature. It has been also shown that for some intermediate ratios of Si:C and/or with other stacking sequences, a collapse of SiC layers to tetrahedrally bonded system is possible. We have argued that these synthesized SiC double-bonded multi-wall silicon-carbide nanotubes may provide a pathway for future realization of SiC graphene-like materials.

We use the combination of high-resolution electron microscopy and density-functional theory to study the atomic structure and electronic effects of structural defects, such as lamellar twins, stacking faults, and double-positioning twin boundaries in polycrystalline photovoltaic materials such as Si, CdTe, and CuInSe2. We find that individual lamellar twins and stacking faults do not create deep levels in all these materials. However, areas with high density of these defects can form buried wurtzite layers that introduce a barrier to the majority carriers. Double-positioning twin boundaries, which contain dislocation cores, create deep levels in Si and CdTe. Surprisingly, however, they do not create deep levels in CuInSe2. These results may explain the fact that Si and CdTe solar cells usually require special passivation, whereas CuInSe2 solar cells do not. Our further study on the passivation effects indicates that grain boundaries in Si cannot be passivated completely by H alone. On the other hand, grain boundaries in CdTe can be passivated well by Cl and I.

We report on observations of structural stability of Sn-doped In 2 O 3 (ITO) thin films during thermal annealing at low temperature. The ITO thin films were deposited by radio-frequency magnetron sputtering at room temperature. Transmission electron microscopy analysis revealed that the as-deposited ITO thin films are nanocrystalline. After thermal annealing in a He atmosphere at 250 °C for 30 min, recrystallization, coalescence, and agglomeration of grains were observed. We further found that nanovoids formed in the annealed ITO thin films. The majority of the nanovoids are distributed along the locations of the original grain boundaries. These nanovoids divide the agglomerated larger grains into small coherent domains.

Ar ion beam and electron beam-induced damages in Cu(In,Ga)Se 2 thin films are investigated by transmission electron microscopy and X-ray energy-dispersive spectroscopy. We find that a high-energy Ar ion beam can cause severe damage in Cu(In,Ga)Se 2 surface regions by preferentially depleting Se and In. The depletion can occur with an Ar ion beam at energy as low as 0.5 keV. High-energy electron beams also cause damage in Cu(In,Ga)Se 2 thin films by preferentially depleting In and Ga. Our results imply that special care must be taken for measurements involving surface treatments using high-energy Ar ion beams or electron beams.

We have designed and synthesized a novel nanocrystalline manganese oxide with a nanofibrous morphology by employing an electrodeposition process in the presence of a non-ionic surfactant. This unique nanoporous/nanocrystalline material effectively accommodates the structural transformation during lithium insertion and avoids deleterious morphological changes as observed in battery materials composed of large particles. Consequently, the material exhibits outstanding cycling stability in addition to its high discharge capacity.

We report on our investigation of the microstructure and composition of the surface of CdTe films after mixed nitric and phosphoric (NP) acids etching, (HgTe, CuTe)-graphite pasting, and thermal annealing. We find that after this process, a thin layer of Cd x Hg 1− x Te forms between the CdTe and Te-rich layers, giving a structure of CdTe/Cd x Hg 1− x Te/Te. High-resolution electron microscopy reveals that the Cd x Hg 1− x Te layer has an epitaxial relationship with the CdTe. Bromine/methanol-etched samples or samples with intentionally deposited Te layers do not form the Cd x Hg 1− x Te layer after (HgTe, CuTe)-graphite pasting and thermal annealing, indicating that they cannot act as fully as the NP etching.

Thin-film polycrystalline photovoltaic devices based on Cu(In,Ga)Se2 have a demonstrated efficiency approaching 19%. The best performance was achieved when the Ga/In+Ga ratio was in the 25–30% range. The short-circuit current density exhibited for the device containing CdS was almost at its expected maximum. The open-circuit voltage was relatively low considering the optical bandgap (Eg) of the above absorber (∼1.15 eV); at best, it is 0.6×Eg. In this work, we examined the microstructural properties, e.g. defects due to misorientation, micro-twinning, stacking faults, and dislocations, for films prepared by our ‘ three-stage’ process, including the CIGS and Mo back-contact. We also attempted to make a correlation between the above observations and device performance.