The synthesis of CdZnTe semiconductor thin films for tandem solar cells

A new approach is adopted to grow cadmium zinc telluride (CdZnTe) thin films using the close spaced sublimation (CSS) technique. The deposition parameters for the growth of cadmium telluride (CdTe) thin films onto the glass substrate were optimized. A zinc telluride (ZnTe) thin film layer was deposited onto already-deposited CdTe thin film to fabricate the CdZnTe (CZT) thin film sample as a ternary compound. Annealing was done after the successful deposition of CZT thin films before further characterization of the CZT thin film samples. The structures of the CZT thin film samples were studied using X-ray diffraction (XRD) and cubic phases were found. A spectrophotometer was used to study the optical parameters, and the energy band gap was found to be in the range of 1.45 eV to 1.75 eV after annealing. The nature of the direct band gap predicts that it might be an ideal component in second-generation solar cells. A Hall measurement system was used to find that the electrical conductivity was in the range of 4.6 × 10⁻⁶ to 8.2 × 10⁻¹¹ (ohm cm)⁻¹. XPS analysis confirmed the presence of Zn in the CdTe thin films. A significant change in electronic properties was observed. These results show that these CZT thin film samples can not only play a key role in the tandem structures of high-efficiency solar cells but they could also be used in the detection of X-rays and gamma rays.

A new approach is adopted to grow cadmium zinc telluride (CdZnTe) thin films using the close spaced sublimation (CSS) technique.     

Sample preparation induced phase transitions in solution deposited copper selenide thin films

Thin films of CuSe were deposited onto GaAs substrate. XRD showed that the as-deposited films were of the Klockmannite (CuSe – P6₃/mmc 194) phase with lattice parameters a₀ = b₀ = 0.3939 nm, c₀ = 1.7250 nm; however, electron diffraction in the TEM surprisingly indicated the β-Cu_2−xSe phase (Cu_1.95Se – R$\bar{3}$m 166) with lattice parameters a₀ = b₀ = 0.412 nm, c₀ = 2.045 nm. The discrepancy originated from the specimen preparation method, where the energy of the focused ion beam resulted in loss of selenium which drives a phase transition to β-Cu_2−xSe in this system. The same phase transition was observed also upon thermal treatment in vacuum, as well as when the 200 keV electron beam was focused on a powder sample in the TEM. The initial phase can be controlled to some extent by changing the composition of the reactants in solution, resulting in thin films of the cubic α-Cu_2−xSe (Cu_1.95Se – Fm$\bar{3}$m) phase co-existing together with the β-Cu_2−xSe phase.

Ion beam irradiation causes Klockmannite CuSe to lose Se and transform into β-Cu₂Se. Caution must be taken when using the dual beam FIB for preparing TEM specimen.     

Comparison of NiOx thin film deposited by spin-coating or thermal evaporation for application as a hole transport layer of perovskite solar cells

We compared nickel oxide (NiO_x) deposited by thermal evaporation and that deposited by the spin-coating process, for use in the hole transport layers of inverted planar perovskite solar cells (PSCs). Spin-coating deposition for NiO_x HTL has been widely used, owing to its simplicity, low cost, and high efficiency. However, the spin-coating process has a technical limit to depositing a large-area uniformly. In contrast, thermal evaporation fabrication has a low price and is able to produce uniform and reproducible thin film. Hence, the chemical states, energy band alignment, surface morphologies, and microstructures of NiO_x deposited by spin coating and thermal evaporation were analyzed. The PSC with NiO_x HTL deposited by thermal evaporation showed a higher power conversion efficiency of 16.64% with open circuit voltage 1.07 V, short circuit current density of 20.68 mA cm⁻², and a fill factor of 75.51% compared to that of PSC with spin-coated NiO_x. We confirmed that thermal evaporation can deposit NiO_x to give a better performance as a HTL with higher reproducibility than spin-coating.

We compared nickel oxide (NiO_x) deposited by thermal evaporation and that deposited by the spin-coating process, for use in the hole transport layers of inverted planar perovskite solar cells (PSCs).     

Pulsed laser deposited CoFe2O4 thin films as supercapacitor electrodes

The influence of the substrate temperature on pulsed laser deposited (PLD) CoFe₂O₄ thin films for supercapacitor electrodes was thoroughly investigated. X-ray diffractometry and Raman spectroscopic analyses confirmed the formation of CoFe₂O₄ phase for films deposited at a substrate temperature of 450 °C. Topography and surface smoothness was measured using atomic force microscopy. We observed that the films deposited at room temperature showed improved electrochemical performance and supercapacitive properties compared to those of films deposited at 450 °C. Specific capacitances of about 777.4 F g⁻¹ and 258.5 F g⁻¹ were obtained for electrodes deposited at RT and 450 °C, respectively, at 0.5 mA cm⁻² current density. The CoFe₂O₄ films deposited at room temperature exhibited an excellent power density (3277 W kg⁻¹) and energy density (17 W h kg⁻¹). Using electrochemical impedance spectroscopy, the series resistance and charge transfer resistance were found to be 1.1 Ω and 1.5 Ω, respectively. The cyclic stability was increased up to 125% after 1500 cycles due to the increasing electroactive surface of CoFe₂O₄ along with the fast electron and ion transport at the surface.

Cobalt ferrite thin films were grown by PLD at different temperatures as an electrode material for supercapacitors. The films deposited at room temperature exhibited the best power density (3277 W kg⁻¹) and energy density (17 W h kg⁻¹) values.     

The effect of solvent on Al-doped ZnO thin films deposited via aerosol assisted CVD

Aluminium-doped zinc oxide (AZO) thin films were deposited via aerosol assisted chemical vapour deposition (AACVD) from zinc acetylacetonate and aluminium chloride at 450 °C. The precursor solutions consisted of methanol in a mixture with one other secondary solvent, including toluene, tetrahydrofuran, n-hexane, cyclohexane, and ethyl acetate. The crystal structures, elemental compositions and surface morphologies of the resulting AZO films were determined, as well as the optoelectronic properties. It was found that the more polar solvents enhanced growth in the (002) plane of the wurtzite crystal structure, and that solutions with low viscosities resulted in superior grain growth. The film deposited from a solution consisting of methanol and ethyl acetate displayed the lowest visible transmittance, due to carbon contamination. However, it also exhibited 60% lower resistivity, in comparison to the film deposited using methanol only. This suggests that optoelectronic properties can be tuned for specific photovoltaic devices.

Aluminium-doped zinc oxide (AZO) thin films were deposited via aerosol assisted chemical vapour deposition (AACVD) from zinc acetylacetonate at 450 °C.     

All-vacuum deposited perovskite solar cells with glycine modified NiOx hole-transport layers

Organic–inorganic hybrid perovskite solar cells (PSCs) have attracted enormous research attention due to their high efficiency and low cost. However, most of the PSCs with high efficiencies still need expensive organic materials as their hole-transport layer (HTL). Obviously, the highly expensive materials go against the low-cost concept of advanced PSCs. In this regard, inorganic NiO_x was considered as an idea HTL due to its good transmittance in the visible region and outstanding chemical stability. But for most of the PSCs with a NiO_x HTL, the hole-extraction efficiency was limited by the unmatched valence band and too many surface defects of the NiO_x layer, especially for the vacuum-deposited NiO_x and perovskite. Herein, we developed a facile strategy to overcome this issue by using self-assembled glycine molecules to treat the NiO_x surface. With glycine on the surface, the NiO_x exhibited a deeper valence band maximum and a faster charge-extraction at the NiO_x/perovskite interface. What''s more, the vacuum-deposited perovskite showed a better crystallinity on the NiO_x + glycine substrate. As a result, the PSCs with a glycine interfacial layer achieved a champion PCE of 17.96% with negligible hysteresis. This facile approach is expected to be further developed for fabricating high-efficiency PSCs on textured silicon solar cells.

Self-assembled glycine molecules are used to modify E-beam evaporated NiO_x films. The glycine interlayer improved the crystallinity and band alignment of perovskite with NiO_x. The all vacuum-processed PSCs achieved a champion PCE of 17.96% with negligible hysteresis.     

Effect of thermal treatment on ZnO:Tb3+ nano-crystalline thin films and application for spectral conversion in inverted organic solar cells

Down conversion has been applied to minimize thermalization losses in photovoltaic devices. In this study, terbium-doped ZnO (ZnO:Tb³⁺) thin films were deposited on ITO-coated glass, quartz and silicon substrates using the RF magnetron sputtering technique fitted with a high-purity (99.99%) Tb³⁺-doped ZnO target (97% ZnO, 3% Tb) for use in organic solar cells as a bi-functional layer. A systematic study of the film crystallization dynamics was carried out through elevated temperature annealing in Ar ambient. The films were characterized using grazing incidence (XRD), Rutherford backscattering spectrometry (RBS), atomic force microscopy, and UV-visible transmittance and photoluminescence measurements at an excitation wavelength of 244 nm. The tunability of size and bandgap of ZnO:Tb³⁺ nanocrystals with annealing exhibited quantum confinement effects, which enabled the control of emission characteristics in ZnO:Tb³⁺. Energy transfer of ZnO → Tb³⁺ (⁵D₃–⁷F₅) was also observed from the photoluminescence (PL) spectra. At an inter-band resonance excitation of around 300–400 nm, a typical emission band from Tb³⁺ was obtained. The ZnO:Tb³⁺ materials grown on ITO-coated glass were then used as bi-functional layers in an organic solar cell based on P3HT:PCBM blend, serving as active layers in an inverted device structure. Energy transfer through down conversion between ZnO and Tb³⁺ led to enhanced absorption in P3HT:PCBM in the 300–400 nm range and subsequently augmented J_sc of a Tb³⁺-based device by 17%.

Thermal annealing of Tb doped ZnO thin films was undertaken and as proof of concept, pristine films were used as a bi-functional in inverted solar cell devices.     

Effect of annealing temperature on silicon-based MoSx thin film solar cells

A suitable annealing temperature was found by adopting the sol–gel method to prepare silicon-based molybdenum sulfide film heterojunction solar cells. As shown by the results, a change in the efficiency of the solar cells, which was attributed to the fact that as the annealing temperature rises, the degree of crystallization of the film increases continuously, the degree of order of the crystal particles goes up first and then goes down, and the temperature change affects the proportion of Mo in different valence states. By comparison, it was found that when the temperature reached 500 °C, the degree of order of the film was raised and the film was in the initial zone from the amorphous to the microcrystal phase change and the proportion of Mo 6+ was relatively large, increasing the conversion efficiency of the device power to 7.55% and laying a good basis for preparing high-performance solar batteries made in the two-dimensional materials. When the annealing temperature continues to rise, the intergranular defects increase, and the overall degree of order of the film decreases. Furthermore, the highly crystalline thin films and the improvement in the device efficiency can be controlled if we obtained the relationship between the annealing temperature and the layers of the two-dimensional materials.

A suitable annealing temperature was found by adopting the sol–gel method to prepare silicon-based molybdenum sulfide film heterojunction solar cells.     

Highly crystalline CsPbI2Br films for efficient perovskite solar cells via compositional engineering

All-inorganic CsPbI₂Br shows high thermal stability for promising application in perovskite solar cells (PSCs). The performance of PSCs is significantly affected by their morphology and crystallinity induced by compositional ratio, solvent/anti-solvent engineering and post thermal annealing. In this study, the compositional ratio effect of two precursors, PbI₂ and CsBr, on the power conversion efficiency (PCE) of a device with ITO/SnO₂/CsPbI₂Br/Spiro-MeOTAD/Au structure was investigated. With the assistance of anti-solvent chlorobenzene, perovskite with a PbI₂ : CsBr ratio of 1.05 : 1 showed a high quality thin film with higher crystallinity and larger grain size. In addition, the molar ratio of precursors PbI₂ and CsBr improved the PCE of the PSCs, and the PSCs fabricated using the perovskite with an optimal ratio of PbI₂ and CsBr exhibited a PCE of 13.34%.

All-inorganic CsPbI₂Br shows high thermal stability for promising application in perovskite solar cells (PSCs).     

Controlling the concentration gradient in sequentially deposited bilayer organic solar cells via rubbing and annealing

We elucidate the formation mechanism of adequate vertical concentration gradients in sequentially deposited poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM) bilayer solar cells. Using advanced analytical techniques, we clarify the origins of the enhanced photovoltaic performances of as-deposited and annealed bilayer P3HT/PCBM organic solar cells upon P3HT layer rubbing prior to PCBM deposition. Energy-dispersive X-ray spectroscopy reveals the individual effects of rubbing and annealing on the formation of adequate concentration gradients in the photoactive layers. Repetitive rubbing of P3HT strongly affects the active layer nanomorphology, forming an intermixed layer in the as-deposited devices which is retained after the annealing process. Infrared p-polarized multiple-angle incidence resolution spectrometry measurements indicate that rubbing induces a minor reorganization of the P3HT molecules in the polymer-only thin films towards face-on orientation. However, the deposition of the upper PCBM layer reverts the P3HT molecules back to their original orientation. These findings suggest that the formation of an adequate concentration gradient upon rubbing corresponds to the dominant contribution to the improved photovoltaic characteristics of rubbed bilayer organic solar cells. Using the reference low bandgap copolymer PCDTBT, we demonstrate that rubbing can be successfully applied to increase the photovoltaic performances of PCDTBT/PCBM organic solar cells. We also demonstrate that rubbing can be an efficient and versatile strategy to improve the power conversion efficiency of non-fullerene solar cells by using the reference materials in the field, PBDB-T and ITIC.

Rubbing the donor in bilayer organic solar cells promotes the formation of adequate concentration gradients in the active layers. The improved charge collection yields large enhancements in the performances of fullerene and non-fullerene solar cells.     

A simple synthesis of transparent and highly conducting p-type CuxAl1−xSy nanocomposite thin films as the hole transporting layer for organic solar cells

Inorganic p-type films with high mobility are very important for opto-electronic applications. It is very difficult to synthesize p-type films with a wider, tunable band gap energy and suitable band energy levels. In this research, p-type copper aluminum sulfide (Cu_xAl_1−xS_y) films with tunable optical band gap, carrier density, hole mobility and conductivity were first synthesized using a simple, low cost and low temperature chemical bath deposition method. These in situ fabricated Cu_xAl_1−xS_y films were deposited at 60 °C using an aqueous solution of copper(ii) chloride dihydrate (CuCl₂·2H₂O), aluminium nitrate nonohydrate [Al(NO₃)₃·9H₂O], thiourea [(NH₂)₂CS], and ammonium hydroxide, with citric acid as the complexing agent. Upon varying the ratio of the precursor, the band gap of the Cu_xAl_1−xS_y films can be tuned from 2.63 eV to 4.01 eV. The highest hole mobility obtained was 1.52 cm² V⁻¹ s⁻¹ and the best conductivity obtained was 546 S cm⁻¹. The Cu_xAl_1−xS_y films were used as a hole transporting layer (HTL) in organic solar cells (OSCs), and a good performance of the OSCs was demonstrated using the Cu_xAl_1−xS_y films as the HTL. These results demonstrate the remarkable potential of Cu_xAl_1−xS_y as hole transport material for opto-electronic devices.

Inorganic p-type films with high mobility are very important for opto-electronic applications.     

Correlation between surface scaling behavior and surface plasmon resonance properties of semitransparent nanostructured Cu thin films deposited via PLD

The surface scaling behavior of nanostructured Cu thin films, grown on glass substrates by the pulsed laser deposition technique, as a function of the deposition time has been studied using height–height correlation function analysis from atomic force microscopy (AFM) images. The scaling exponents α, β, 1/z and γ of the films were determined from AFM images. The local roughness exponent, α, was found to be ∼0.86 in the early stage of growth of Cu films deposited for 10 minutes while it increased to 0.95 with a longer time of deposition of 20 minutes and beyond this, it was nearly constant. Interface width w (rms roughness) scales with depositing time (t) as ∼ t^β, with the value of the growth exponent, β, of 1.07 ± 0.11 and lateral correlation length ξ following ξ = t^1/z and the value of 1/z = 0.70 ± 0.10. These exponent values convey that the growth dynamics of PLD Cu films can be best described by a combination of local and non-local models under a shadowing mechanism and under highly sticking substrate conditions. From the scaling exponents and power spectral density function, it is concluded that the films follow a mound like growth mechanism which becomes prominent at longer deposition times. All the Cu films exhibited SPR properties where the SPR peak shifts towards red with increasing correlation length (ξ) whereas bandwidth increases initially with ξ and thereafter decreases gradually with ξ.

The surface scaling behavior of nanostructured Cu thin films, grown on glass by the PLD technique, as a function of deposition time has been studied using height–height correlation function analysis from AFM images.     

Physicochemical conditions for ZnO films deposited by microwave chemical bath deposition

Physicochemical analysis was carried out to obtain the species distribution diagrams (SDDs) for the deposition of ZnO films as a function of OH⁻ ion concentration ([OH⁻]) in the reaction solution. The study of SDDs predicts nucleation and ZnO film growth by means of the dominant species at a given pH value. To confirm this, a series of experiments were made varying the [OH⁻] in the reaction solution and keeping the others parameters constant. Structured zinc oxide (ZnO) films were obtained on glass substrates by microwave chemical bath deposition (MWCBD). Structural, optical and morphological ZnO film properties were investigated as a function of [OH⁻]. X-Ray diffraction technique (XRD) measurements show multiple diffraction peaks, indicating the polycrystalline nature of ZnO films. Scanning Electron Microscopy (SEM) images of ZnO structures showed morphological changes with the variation of [OH⁻]. The stoichiometry of the structures changed as the [OH⁻] was varied in solution. From Raman spectra, it was observed that the [OH⁻] of the reaction mixture strongly affects the crystal quality of ZnO structures. A reaction pathway for the synthesis of ZnO structures based on our results is proposed. Experimental results are consistent with the physical–chemical analysis.

Physicochemical analysis was carried out to obtain the species distribution diagrams (SDDs) for the deposition of ZnO films as a function of OH⁻ ion concentration ([OH⁻]) in the reaction solution.     

Effect of BaTiO3 powder as an additive in perovskite films on solar cells

Perovskite solar cells (PSCs) are considered to be ideal energy devices, where perovskite-type organic metal halides act as light-absorbing materials. In PSCs, the photoexcitons are extracted and separated to afford high photoelectric conversion efficiency under the action of the built-in electric field (E_bi). However, the current challenge is that a low E_bi cannot provide a sufficient driving force to separate photonic excitons, which causes the captured charges to escape from the deep energy-level defect state. Here, the ferroelectric material barium titanate (BaTiO₃) was directly introduced into the perovskite precursor solution to reduce the defection density (to 8.58 × 10¹⁷ cm⁻³) in PSCs and promote the separation of photoexcitons. Furthermore, the addition of BaTiO₃ improved the quality of the perovskite film and significantly increased the photoelectric performance after the polarization treatment. This is mainly attributed to the residual polarization electric field generated by ferroelectric polarization, which increased the E_bi of the PSCs and the width of the depletion layer and inhibited the non-radiative recombination of carriers. This work provides a possibility to design and develop optoelectronic devices with high-efficiency optoelectronic response behavior.

During polarization treatment, the residual polarization electric field generated by BaTiO₃ increased E_bi of the cells and width of the depletion layer, promoted extraction and separation of carriers, and improved photoelectric performance of PSCs.     

Arrayed nanopore silver thin films for surface-enhanced Raman scattering

Active substrates are crucial for surface-enhanced Raman scattering (SERS). Among these substrates, large uniform area arrayed nanoporous silver thin films have been developed as active substrates. Arrayed nanoporous silver thin films with unique anisotropic morphologies and nanoporous structures can be fabricated onto the nanoporous anodic aluminum oxide (AAO) of controlled pore size and interspacing by precisely tuning the sputtering parameters. These thin films preserve locally enhanced electromagnetic fields by exciting the surface plasmon resonance, which is beneficial for SERS. In this study, nanoporous silver thin films were transferred into polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS) substrates using our recently invented template-assisted sol–gel phase inverse-imprinting process to form two different nanopore thin films. The as-formed Ag nanoporous thin films on PMMA and PDMS exhibited intensively enhanced SERS signals using Rhodamine 6G (R6G) as the model molecule. The two nanopore thin films exhibited opposite pore size-dependent SERS tendencies, which were elucidated by the different enhancement tendencies of the electric field around pores of different diameters. In particular, the Ag nanoporous thin film on PMMA exhibited an R6G detection limit of as low as 10⁻⁶ mol L⁻¹, and the SERS enhancement factor (EF) was more than 10⁶. The low detection limit and large EF demonstrated the high sensitivity of the as-prepared SERS substrates for label-free detection of biomolecules. Compared with conventional smooth films, this nanopore structure can facilitate future application in biomolecular sensors, which allows the detection of single molecules via an electronic readout without requirement for amplification or labels.

Typical active substrates are crucial for surface-enhanced Raman scattering (SERS).     

Effect of doping on the phase stability and photophysical properties of CsPbI2Br perovskite thin films

In this study, we demonstrate that the crystallization process of CsPbI₂Br films can be modulated when small amounts of additives are added to the precursor solution, leading to the formation of the bright brownish α-phase perovskite films with high orientation along the [100] crystallographic direction. Doped CsPbI₂Br films exhibit improved crystallinity, with high coverage, large grain size and pinhole-free surface morphology, suitable for making high performance optoelectronic devices. We also explored the role of Cl in the photophysical properties of CsPbI₂Br perovskite films using the temperature dependent photoluminescence technique. We found that the Cl ions enhance the photoluminescence emission by reducing the density of trap states, and also decrease the exciton binding energy from (22 ± 3) meV to (11 ± 2) meV. We believe this work contributes to understanding the effect of doping on the crystallization process with an in-depth insight into the photophysical properties of the cesium-based perovskite materials.

In this study, we demonstrate that in addition to improving the crystallization of CsPbI₂Br films, the incorporation of Cl⁻ and hydroiodic acid in the precursor solution leads to the formation of films with high coverage and large grains size.     

Physicochemical characterisation of reduced graphene oxide for conductive thin films

Graphene is a desirable material for next generation technology. However, producing high yields of single-layer flakes with industrially applicable methods is currently limited. We introduce a combined process for the reduction of graphene oxide (GO) via vitamin C (ascorbic acid) and thermal annealing at temperatures of <150 °C for times of <10 minutes, resulting in electrically conducting thin films with sheet resistances reducing by 8 orders of magnitude to as low as ∼1.3 kΩ □⁻¹, suitable for microelectronics, display technology and optoelectronic applications. The in-depth physicochemical characterisation of the products at different stages of GO preparation and reduction allows for further understanding of the process and demonstrates the suitability for industrial production methodologies due to an environmentally-friendly reducing agent, solution processability and no requirement for high temperatures. The presence of the vitamin C lowers the temperature required to thermally reduce the GO into an electrically conducting thin film, making the technique suitable for thermally sensitive substrates, such as low melting point polymers. Simultaneous spray coating and reduction of GO allows for large area deposition of conductive coatings without sacrificing solution processability, often lost through particle agglomeration, making it compatible with industrial processes, and applicable to, for example, the production of sensors, energy devices and flexible conductive electrodes for touchscreens.

We introduce a combined process for the reduction of graphene oxide (GO) via vitamin C (ascorbic acid) and thermal annealing.     

Wetting-induced formation of void-free metal halide perovskite films by green ultrasonic spray coating for large-area mesoscopic perovskite solar cells

A void-free metal halide perovskite (MHP) layer on a mesoscopic TiO₂ (m-TiO₂) film was formed via the wetting-induced infiltration of MHP solution in the m-TiO₂ film via a green ultrasonic spray coating process using a non-hazardous solvent. The systematic investigation of the behavior of ultrasonic-sprayed MHP micro-drops on the m-TiO₂ film disclosed that the void-free MHP layer on the m-TiO₂ film can be formed if the following conditions are satisfied: (1) the sprayed micro-drops are merged and wetted in the mesoscopic scaffold of the m-TiO₂ film, (2) the MHP solution infiltrated into the m-TiO₂ film by wetting is leveled to make a smooth wet MHP film, and (3) the smooth wet MHP film is promptly heat treated to eliminate dewetting and the coffee ring effect by convective flow in order to form a uniform void-free MHP layer. A void-free MHP layer on the m-TiO₂ film was formed under optimal ultrasonic spray coating conditions of substrate temperature of ∼30 °C, spray flow rate of ∼11 mL h⁻¹, nozzle to substrate distance of ∼8 cm, and MHP solution-concentration of ∼0.6 M under a fixed scan speed of 30 mm s⁻¹ and purged N₂ carrier gas pressure of 0.02 MPa. The mesoscopic MHP solar cells with an aperture area of 0.096, 1, 25, and 100 cm² exhibited 17.14%, 16.03%, 12.93%, and 10.67% power conversion efficiency at 1 sun condition, respectively.

A void-free metal halide perovskite (MHP) layer on a mesoscopic TiO₂ (m-TiO₂) film was formed via the wetting-induced infiltration of MHP solution in the m-TiO₂ film via a green ultrasonic spray coating process using a non-hazardous solvent.     

Flaky nano-crystalline SnSe2 thin films for photoelectrochemical current generation

We report chemical vapor deposition (CVD) and photoelectrochemical properties of large-area thin films of nano-crystalline SnSe₂ on conducting FTO glass. We show that densely packed, randomly oriented SnSe₂ nanoflakes can be grown by reacting vapors of tin monoselenide (SnSe) and selenium at 550 °C without compromising the electrical properties of FTO. We demonstrate that SnSe₂/FTO is photoelectrochemically active in visible to near-UV wavelengths (350–700 nm) and exhibits incident-photon-to-current-efficiency (IPCE) as high as 3.7% at 0 V versus the Ag/AgCl/1 M KCl reference electrode under 350 nm light illumination.

We report chemical vapor deposition (CVD) and photoelectrochemical properties of large-area thin films of nano-crystalline SnSe₂ on conducting FTO glass.     

Liquid phase assisted grain growth in Cu2ZnSnS4 nanoparticle thin films by alkali element incorporation

The effect of adding LiCl, NaCl, and KCl to Cu₂ZnSnS₄ (CZTS) nanoparticle thin-film samples annealed in a nitrogen and sulfur atmosphere is reported. We demonstrate that the organic ligand-free nanoparticles previously developed can be used to produce an absorber layer of high quality. The films were Zn-rich and Cu-poor, and no secondary phases except ZnS could be detected within the detection limit of the characterization tools used. Potassium was the most effective alkali metal to enhance grain growth, and resulted in films with a high photoluminescence signal and an optical band gap of 1.43 eV. The alkali metals were introduced in the form of chloride salts, and a significant amount of Cl was detected in the final films, but could be removed in a quick water rinse.

We present a route where organic ligand-free, KCl-functionalized Cu₂ZnSnS₄ nanoparticles grow into large, dense grains during annealing in nitrogen/sulfur atmosphere.

Thin-film photovoltaic materials with an absorber layer consisting of either CuInGaSe₂ (CIGS) or CdTe exhibit high power conversion efficiencies of 22.6% and 22.1%, respectively, and are already available on the solar panel market.¹ However, due to the relatively poor abundance of In, Ga, and partly Te, as well as the toxicity of Cd, it is important to look for substituting compounds, and here Cu₂ZnSnS₄ (CZTS) is a promising alternative. CZTS has reached a record efficiency of 9.5%,¹ and it has a high absorption coefficient of >10⁴ cm⁻¹ and a direct band gap of 1.45–1.51 eV, which furthermore also makes it an interesting material for a tandem solar cell with silicon.²For solution-processed CZTS, the current record efficiency for a nanoparticle solar cell is 4.8%,³ while devices made from molecular/precursor inks approach 6%.⁴ For the molecular precursor route, the synthesis step is circumvented, which should favor this method. It is, however, still important to develop the nanoparticle approach, as it allows a higher concentration of CZTS in the ink, and therefore facilitates different deposition techniques. As it is yet unknown which method will work best for future upscaling it is important to have different technologies.One challenge for the nanoparticle approach is to obtain uniform grain growth during the annealing step.³ CZTS nanoparticles are typically synthesized with long hydrocarbons as ligands/surfactants, e.g. oleylamine (OLA). After annealing thin films consisting of these particles, the formation of large grains on the surface can be observed, while a fine-grain layer is formed at the bottom interface.^3,5,6 This fine-grained material contains carbon, and its effect on the solar cell is not known. Even with ligands consisting of shorter hydrocarbon chains,⁶ the annealed CZTS nanoparticle film will remain porous. Therefore, it seems that solid state material diffusion which promotes grain growth is very slow under the specific annealing conditions.Grain growth in the form of sintering is desired as it can result in a dense film. In sintering particles are merged at a temperature below the melting point, and it occurs to minimize the surface area and thus the surface energy of a colloidal system.^7,8 The rate of sintering can be increased by hot pressing, impurity doping to enhance grain boundary diffusion, or incorporating an element that becomes a liquid phase at elevated temperature, i.e. liquid phase reactive sintering.^7,8For solution-processed CZTS, both hot-pressing⁹ and including a dopant have been attempted with promising film morphologies. Typically, the alkali metals Na and K are used, but promising results have also been achieved for Li and Sb. The incorporation of these elements has been done through a selection of different processes. For precursor CZTS films, the dopants can be incorporated directly in the ink, which has resulted in some of the highest efficiencies to date when using Sb(OAc)₃ and NaCl.⁴ A common way to include these elements in nanoparticle films is by soaking the film in a sodium salt solution,⁵ or depositing a thin NaF layer on top of the absorber before annealing, which adds an extra vacuum-deposition step;³ for both methods, the beneficial effects of the incorporated elements is typically restricted to the close proximity of the surface, and limited by its diffusion within the film.¹⁰ To obtain a more uniform distribution of incorporated dopants, the surface of the nanoparticles can be functionalized with e.g. SbCl₃,¹¹ or CF₃COONa.¹⁰We have previously reported a one-step synthesis of organic ligand-free nanoparticles, which minimizes the amount of organic material in the film, allows for using solvents like water and ethanol, and furthermore makes it possible to directly dissolve controllable amounts of various chloride salts in the ink.^12,13 In this paper we investigate the effect of LiCl, NaCl, and KCl on the structural evolution and opto-electronic properties of these thin films.