Film Photovoltaic Devices Research Articles

Adjustable photovoltaic performance driven by polarization in molecular ferroelectrics

The photovoltaic effect driven by ferroelectric polarization shows great application potential in photovoltaic devices. Two-dimensional (2D) Ruddlesden-Popper perovskite molecular ferroelectric materials, which combine polarization and low band gap properties, occupy a promising position in this field. However, literature reports mostly focus on single crystal blocks, and film devices are rarely reported. In this paper, 2D EA4Pb3Br10 (EA=ethylammonium) molecular ferroelectrics are used as the absorption layers to prepare photovoltaic devices, and the photovoltaic performance of the devices is enhanced by polarization. Dense EA4Pb3Br10 films are prepared by immediately adding antisolvent propylene glycol methyl ether acetate when the spin coating rate reaches the maximum. Surprisingly, the photovoltaic device exhibits significant photovoltaic effects under AM1.5 (100 mW/cm2) illumination with VOC ∼ 1.042 V and JSC ∼ 438.02 μA/cm2, which is much higher than that of ordinary inorganic ferroelectrics. In addition, the photovoltaic performance has been greatly improved by applying an external electric field, reaching to the maximum VOC ∼ 1.087 V and JSC ∼ 489.12 μA/cm2. This indicates the potential application prospects of 2D EA4Pb3Br10 molecular ferroelectric films in photovoltaic devices.

Laser-induced porous graphene from PEDOT:PSS films for dye-sensitized solar cells

• 3D porous graphene was firstly fabricated from PEDOT:PSS by a laser-scribing method. • Sheet resistance and morphology of LIG can be easily tuned by changing laser power. • LIG shows enhanced photovoltaic performance as a counter electrode for DSSCs. For the emerging laser-induced graphene (LIG) technologies, the exploration of laser irradiation interactions with various polymers and their new applications remains challenging. Herein, we report a novel strategy for the fabrication of 3D porous graphene electrodes from the aqueous PEDOT:PSS dispersion by a 445-nm diode laser in ambient air. By adjusting the laser power, LIG films exhibit a sheet resistance as low as 50 Ω/sq and their morphologies can change from a carbonized surface to a hierarchical porous structure. The dye-sensitized solar cell with a LIG70 counter electrode achieved a power conversion efficiency of 3.66%, which is over four times that of the pristine PEDOT:PSS electrode (0.80%). This work demonstrates the great potential for the application of PEDOT:PSS-based LIG films in photovoltaic devices.

Single‐Photon Emission from Single Microplate MAPbI3 Nanocrystals with Ultranarrow Photoluminescence Linewidths and Exciton Fine Structures

AbstractStimulated by the superior performance of organic–inorganic MAPbI3 perovskite films in photovoltaic devices, a lot of research interest is now being devoted to their low‐dimensional nanocrystals (NCs) to extend the optoelectronic functionalities as well as to promote the potential applications in quantum information technologies. Compared to other organic–inorganic and all‐inorganic counterparts that have been intensively studied, the MAPbI3 NCs suffer from the optical and chemical instabilities so that their single‐particle photophysical properties have been largely unexplored up to now. Here, the authors have synthesized single MAPbI3 microplates from a two‐step method and characterize their optical properties mainly at the cryogenic temperature, showing that localized optical emitters are universally present with the quantum feature of single‐photon emission. Photoluminescence (PL) linewidth measured for such a single quantum emitter can be as narrow as ≈200 μeV, as a direct consequence of its embedment inside the microplate with the spectral diffusion effect being greatly suppressed. This has allowed them to resolve the exciton fine structures from some of the studied single quantum emitters, each of which is manifested as a PL doublet with the energy separation of ≈600 μeV and the orthogonally linear polarizations.

Ionic Liquids Modulating CsPbI3 Colloidal Quantum Dots Enable Improved Mobility for High-Performance Solar Cells.

Colloidal all-inorganic CsPbI3 perovskite quantum dots (PQDs) have shown tremendous potential in photovoltaic applications in recent years due to their outstanding optoelectronic properties that general metal halide perovskites offer, along with the added advantages that originates from size reduction and the quantum confinement effect. However, the issue of low carrier mobility in PQD films caused by insulating organic ligands capped on the PQD surface still remains to be addressed while aiming for high-efficiency PQD solar cells. Herein, we propose a novel strategy that takes benefits of ionic liquids, which can offer the high polarity and the electron donating ability to boost the mobility of PQD films in photovoltaic devices. Specifically, 1-propyl-3-methylimidazolium iodide to modulate the colloidal CsPbI3 PQD surface and couple QDs is demonstrated for the first time. The lone pair electrons on the nitrogen of the imidazole ring within the ionic liquid binds to the empty nonbonding surface orbitals of CsPbI3 PQDs while the long-chain insulating ligands are replaced, which enables not only efficient charge transport but also reduced defect density in the assembled PQD solid films. The resulting CsPbI3 PQD solar cell shows a significant increase in efficiency with suppressed hysteresis, indicating the impressive potential of this strategy for developing highly efficient PQD solar cells.

Nd-Cr co-doped BiFeO3 thin films for photovoltaic devices with enhanced photovoltaic performance

BiFeO3 films and Nd-Cr co-doped BiFeO3 films were prepared by sol-gel method followed by spinning process on fluorine-doped tin oxide glass substrates. By testing the ultraviolet-visible absorption spectra, it was found that Nd-Cr co-doping will increase the light absorption rate of the film and reduce the optical band gap. The reduced bandgap can facilitate the transport of carriers. After Nd-Cr co-doping, the leakage current of the film is effectively reduced, which is near four orders of magnitude lower than the leakage current density of the pristine BiFeO3 film. The reduction of leakage current will enhance the ferroelectric polarization. The enhancement of ferroelectric polarization is more favorable for the separation of photogenerated carriers. Compared with the pristine BiFeO3 film, the short circuit photocurrent density, open circuit photovoltage and power conversion efficiency of Nd-Cr co-doped BiFeO3 film are all clearly improved. The Nd-Cr co-doped BiFeO3 films exhibited largely enhanced photovoltaic property, which favored the practical application of BiFeO3-based films in photovoltaic devices.

Effects of growth temperature on the photovoltaic properties of RF sputtered undoped NiO thin films

Abstract In this study, nickel oxide (NiO) thin films were deposited on soda lime glass using radio-frequency magnetron sputtering at different growth (substrate) temperatures ranging from room temperature (RT) to 400 °C. The effects of substrate temperature on the structural, morphological, electrical, and optical properties were investigated. The XRD pattern unveiled a dominant peak with (2 0 0) preferential orientations for the film grown at 100 °C. However, for samples grown at high temperatures, a gradual decrease of (2 0 0) peak intensity was observed, which may be the result of the decomposition of NiO as confirmed via EDX. Surface morphology from FESEM revealed that grains were randomly orientated on the surface with maximum grain size of 19.43 nm. Upon increasing the growth temperature, the crystal quality and grain size substantially deteriorated, which is consistent with the XRD results. Scanning probe microscopy (SPM) finds rough surface with the highest surface roughness obtained at RT with a value of 1.232 nm. Electrical resistivity was found to be highly dependent on the growth temperature that decreases from 2150 Ω cm to 72 Ω cm as the substrate temperature increases. For optical properties, the optical bandgap of the NiO films decreases from 3.8 eV to 3.2 eV as a function of substrate temperature as derived from the optical transmittance data. Results show the potential application of the NiO films in photovoltaic devices.

Two-Dimensional Growth of Large-Area Conjugated Polymers on Ice Surfaces: High Conductivity and Photoelectrochemical Applications.

Polymerizing monomers on an atomically flat solid surface and air/water, solid/liquid, or liquid/liquid interface is now a rapidly emerging frontier. Dimension-controlled synthesis of π-conjugated polymers is of particular interest, which can be achieved by precise control of monomer distribution during the polymerization. The surface of ice allows rapid polymerization of monomers in the plane direction along the air-water interface to yield large-area two-dimensional sheet-like poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (2D sheet-like PEDOT:PSS) films with a thickness of ca. 30 nm. The persuasive role of ice chemistry is reflected in the high degree of crystallinity and superior conductivity of resultant PEDOT:PSS films. Excellent photoelectrochemical features were further disclosed when the ice-templated PEDOT:PSS films were coupled to quantum dots. Utilization of these polymer films in photovoltaic devices also resulted in excellent current density and power conversion efficiency. This work presents an innovative material technology that goes beyond traditional and ubiquitous inorganic 2D materials such as graphene and MoS2 for integrated electronic applications.

Chemical Exfoliation of Black Phosphorus for Nanoelectronics Applications

ABSTRACTChemically exfoliated two-dimensional (2D) materials have shown promise in a variety of applications such as thin film transistors and photovoltaic devices. Here, we present a scalable method for preparing black phosphorus membranes via direct liquid-phase exfoliation of the bulk crystal in organic solvents. The black phosphorus sheets are characterized by Raman and Photoluminescence (PL) spectroscopy, and optical microscopy. The properties of these chemically exfoliated black phosphorus was compared to mechanically exfoliated nanomembranes. The liquid-phase exfoliation nonetheless yields the potential for large-area scalability. Our results highlight the important aspects of forming solution dispersions of 2D black phosphorus which can subsequently be the stepping stone for future work on the ink-jet printing of such dispersions over a wide range of substrates.

Properties of Electron-Beam Irradiated CuInSe2 Layers by Multi-Step Sputtering Method.

Typically, CuInSe2 (CIS) based thin films for photovoltaic devices are deposited by co-evaporation or by deposition of the metals, followed by treatment in a selenium environment. This article describes CIS films that are instead deposited by DC and RF magnetron sputtering from binary Cu2Se and In2Se3 targets without the supply of selenium. As a novel method, electron beam annealing was used for crystallization of Cu2Se/In2Se3 stacked precursors. The surface, cross-sectional morphology, and compositional ratio of CIS films were investigated to confirm the possibility in crystallization without any addition of selenium. Our work demonstrates that the e-beam annealing method can be a good candidate for the rapid crystallization of Cu-In-Se sputtered precursors.

Trimethylaluminum and Oxygen Atomic Layer Deposition on Hydroxyl-Free Cu(111).

Atomic layer deposition (ALD) of alumina using trimethylaluminum (TMA) has technological importance in microelectronics. This process has demonstrated a high potential in applications of protective coatings on Cu surfaces for control of diffusion of Cu in Cu2S films in photovoltaic devices and sintering of Cu-based nanoparticles in liquid phase hydrogenation reactions. With this motivation in mind, the reaction between TMA and oxygen was investigated on Cu(111) and Cu2O/Cu(111) surfaces. TMA did not adsorb on the Cu(111) surface, a result consistent with density functional theory (DFT) calculations predicting that TMA adsorption and decomposition are thermodynamically unfavorable on pure Cu(111). On the other hand, TMA readily adsorbed on the Cu2O/Cu(111) surface at 473 K resulting in the reduction of some surface Cu1+ to metallic copper (Cu0) and the formation of a copper aluminate, most likely CuAlO2. The reaction is limited by the amount of surface oxygen. After the first TMA half-cycle on Cu2O/Cu(111), two-dimensional (2D) islands of the aluminate were observed on the surface by scanning tunneling microscopy (STM). According to DFT calculations, TMA decomposed completely on Cu2O/Cu(111). High-resolution electron energy loss spectroscopy (HREELS) was used to distinguish between tetrahedrally (Altet) and octahedrally (Aloct) coordinated Al3+ in surface adlayers. TMA dosing produced an aluminum oxide film, which contained more octahedrally coordinated Al3+ (Altet/Aloct HREELS peak area ratio ≈ 0.3) than did dosing O2 (Altet/Aloct HREELS peak area ratio ≈ 0.5). After the first ALD cycle, TMA reacted with both Cu2O and aluminum oxide surfaces in the absence of hydroxyl groups until film closure by the fourth ALD cycle. Then, TMA continued to react with surface Al–O, forming stoichiometric Al2O3. O2 half-cycles at 623 K were more effective for carbon removal than O2 half-cycles at 473 K or water half-cycles at 623 K. The growth rate was approximately 3–4 Å/cycle for TMA+O2 ALD (O2 half-cycles at 623 K). No preferential growth of Al2O3 on the steps of Cu(111) was observed. According to STM, Al2O3 grows homogeneously on Cu(111) terraces.

Control of crystalline volume and nano crystal grain size in nanocrystalline silicon thin film deposited by PECVD

Application of the radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique was studied to fabricate amorphous and nanocrystalline silicon (a-Si and nc-Si) thin films for photovoltaic devices at substrate temperature of 200 °C. Amorphous-crystalline transition of silicon thin films in working conditions of PECVD system was shown as a function of deposition parameters, i.e., dilution ratio of silane (SiH 4 ) in hydrogen, total gas pressure during deposition and RF excitation power density. The crystalline volume as well as grain size of nanocrystalline silicon films could be successfully controlled by tuning those deposition parameters. Micro Raman scattering spectroscopy and spectroscopic ellipsometry (SE) methods were used to characterize the structure and crystallization of the deposited silicon thin films. We could make nc-Si thin films with various crystalline volumes. Nc-Si grain size was also controlled and was in the range of 3–5 nm.

Effect of Sodium Chloride (NaCl) as Crystallization Catalyst on Cu2ZnSnS4 (CZTS) Films Deposited by Wet-solution Coating Method

The CZTS ink was prepared by a sonochemical method and properties of the CZTS thin films deposited by a spraying method were investigated. We used CuCl2, ZnCl2, SnCl2 and thiourea as precursor materials, 2-methoxyethanol as solvent, monoethanolamine as stabilizer, and NaCl as catalyst. X-ray diffraction (XRD) patterns from the CZTS films mainly exhibited the (112), (200), (220), and (312) planes of the kesterite structure. The grain size of CZTS films increased with increasing NaCl concentration. The optical band-gap of CZTS films decreased from 1.27 eV to 1.08 eV, and the absorption coefficient was increased, when the concentration of NaCl was increased. The elemental ratio of Cu:Zn:Sn:S in CZTS films was approximately 2:1:1:3.6, and the ratio of the films seldom changed at different synthesis conditions. These results demonstrate that the major role of NaCl is the enhancement of crystallinity and light absorption in CZTS films. The CZTS ink developed in this study has a promising potential for the formation of high quality CZTS thin films for photovoltaic devices.

Synthesis of Cu2ZnSnS4nanoparticles and analysis of secondary phases in powder pellets

Sulfur-based kesterite Cu2ZnSnS4 (CZTS) nanoparticles (NPs) have been produced in a wet-chemical synthesis route which provides high yields of CZTS-NPs per synthesis cycle. These NPs can be used as raw material for electronic inks, which can be processed to light-harvesting absorber thin films in photovoltaic devices. In the corresponding phase diagram, the desired kesterite phase is found only in a very narrow, off-stoichiometric composition region. Accordingly, kesterite materials are likely to contain unwanted secondary phases such as ZnS, Cu2S, Cu2SnS3, which may limit the photovoltaic device performance in terms of current-blocking domains and formation of low-energy recombination channels. These phases can form already during the synthesis but may also evolve at high temperatures, for example, during annealing of the thin film. To limit the formation of Cu2S and Cu2SnS3 secondary phases Zn-rich and Cu-poor compositions were chosen corresponding to device-grade CZTS. In this paper we report on the phase and composition analysis of CZTS using X-ray diffraction, Raman spectroscopy with different excitation wavelengths, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The analysis was performed on annealed CZTS pellets prepared from condensed CZTS NP powders. To extract the phase constitution and the composition from the surfaces and from the sample interior the CZTS pellets were horizontally cleaved after the annealing step. The bulk of the pellet consists of the required CZTS phase with very small quantities of Cu2–xS and ZnS. The utmost amount of these secondary phases was found on the top of the sample whereby copper sulfide forms preferentially isolated domains. As these surface-located secondary phases can be removed chemically, CZTS absorber films in good crystalline quality are to be expected.

Improved photovoltaic performances by post-deposition acidic treatments on tetrapod shaped colloidal nanocrystal solids

The ligand exchange reaction with pyridine is the standard procedure for the integration of colloidal semiconductor nanocrystals (NCs) in photovoltaic devices; however, for large sized and irregularly shaped branched NCs, such as CdSe@CdTe tetrapods, this procedure can lead to a considerable waste of materials and the aggregation of NCs in the colloidal solution, therefore resulting in the formation of an inhomogeneous film and low device performances. Here, we report on alternative post-deposition treatments with carboxylic acids on films of CdSe@CdTe tetrapod shaped NCs. This approach guarantees the removal of the insulating surfactant, necessary to obtain good charge transport among NCs, while preserving the film integrity. We perform a complete characterization of the nanocrystalline films treated with different carboxylic acids and demonstrate the successful integration of such films in photovoltaic devices, showing a doubled efficiency with respect to the standard ligand exchange procedure. Our approach represents a general route towards the development of NC based devices with improved performances and minimized waste of material.

ZnO films grown using a novel procedure based on the reactive evaporation method

A novel procedure based on the reactive evaporation method was developed to deposit highly transparent thin films of i-ZnO and n+-ZnO in-situ. The opto-electrical properties of the ZnO films were optimized for using them as TCO layer in solar cells. The optimization of the preparation parameters was achieved through a figure of merit defined in terms of both, the transmittance and the resistivity. n+-ZnO films with resistivities around 8×10−4Ωcm and i-ZnO films with resistivities around 105Ωcm and transmittances greater than 80% (in the visible region) were obtained with this method.The applicability of the i-ZnO and n+-ZnO thin films in photovoltaic devices has been demonstrated by using them as interdiffusion barrier and TCO layer in CuInS2 based solar cells. Conversion efficiencies of 9.1% were achieved with CIS based solar cells using ZnO thin films deposited by reactive evaporation.

Propriedades óticas e microestruturais de filmes finos eletrodepositados de CdTe

Filmes finos de CdTe foram obtidos por eletrodeposição catódica, a partir de uma solução de CdSO4, H2SO4 e Te. Os filmes foram depositados à temperatura ambiente, sobre substratos de vidro cobertos com SnO2 ou In2O3:Sn. A influência do potencial de deposição nas propriedades microestruturais e óticas dos filmes foi investigada. Os valores de banda proibida dos filmes ficaram na faixa de 1,40 e 1,50 eV, próximos ao valor da banda do CdTe volumétrico. Um potencial de deposição ótimo foi determinado para cada substrato. O aquecimento do eletrólito a 50 ºC não foi suficiente para provocar grandes alterações na cristalinidade, mas promoveu a formação de filmes com menor quantidade de defeitos. Isto sugere que um tratamento térmico pós-deposição possa melhorar as propriedades destes filmes para aplicações fotovoltaicas.

Enhanced Intersystem Crossing via a High Energy Charge Transfer State in a Perylenediimide−Perylenemonoimide Dyad

The electronic relaxation processes of a photoexcited linear perylenediimide-perylenemonoimide (PDI-PMI) acceptor-donor dyad were studied. PDI-PMI serves as a model compound for donor-acceptor systems in photovoltaic devices and has been designed to have a high-energy PDI (-*)-PMI (+*) charge transfer (CT) state. Our study focuses on the minimal Gibbs free energy (Delta G ET) required to achieve quantitative CT and on establishing the role of charge recombination to a triplet state. We used time-resolved photoluminescence and picosecond photoinduced absorption (PIA) to investigate excited singlet (S 1) and CT states and complemented these experiments with singlet oxygen ( (1)Delta g) luminescence and PIA measurements on longer timescales to study the population of triplet excited states (T 1). In an apolar solvent like cyclohexene (CHX), photoinduced electron transfer does not occur, but in more polar solvents such as toluene (TOL) and chlorobenzene (CB), photoexcitation is followed by a fast electron transfer, populating the PDI (-*)-PMI (+*) CT state. We extract rate constants for electron transfer (ET; S 1-->CT), back electron transfer (BET; S 1<--CT), and charge recombination (CR) to lower-energy states (CT-->S 0 and CT-->T 1). Temperature-dependent measurements yield the barriers for the transfer reactions. For ET and BET, these correspond to predictions from Marcus-Jortner theory and show that efficient, near quantitative electron transfer ( k ET/ k BET >or= 100) can be obtained when Delta G ET approximately -120 meV. With respect to triplet state formation, we find a relatively low triplet quantum yield (Phi T < 25%) in CHX but much higher values (Phi T = 30-98%) in TOL and CB. We identify the PDI (-*)-PMI (+*) state as a precursor to the T 1 state. Recombination to T 1, rather than to the ground-state S 0, is required to rationalize the experimental barrier for CR. Finally, we discuss the relevance of these results for electron donor-acceptor films in photovoltaic devices.

Nano-structured GeySi1-y:H Films Deposited by Low Frequency Plasma for Photovoltaic Application

ABSTRACTIn this work we present the study of fabrication, Ge incorporation, structure and electronic properties of nano-structured GeySi1-y:H films with y&gt;0.5 prepared by low frequency (LF) PECVD. GeySi1-y:H films were deposited by LF PECVD at a frequency f= 110 kHz from SiH4+GeH4+H2 gas mixture. SiH4 and GeH4 flows were varied to fabricate the films in wide range of 0&lt;y&lt;l. Hydrogen dilution was varied in the range of RH =20 to 80. Structure of the films was studied by AFM and SEM with consequent image processing to extract statistical parameters such as grain distribution and mean values. Composition of the films was characterized by SIMS and EDX. Electronic properties were characterized by temperature dependence of conductivity, spectral dependence of optical absorption. Sub-gap absorption was characterized by Urbach energy, EU; and defect absorption, αD. We observed grain like nano-structure with Gauss distribution of grain diameters by both AFM and SEM measurements. The most interesting films had mean grain diameter&lt;D&gt; = 24.0±0.7 nm, dispersion D=11.0±0.2 nm and fill factor FF=0.313, Ge content y=0.96-0.97(by SIMS and EDS). These films showed also the lowest values of Urbach energy EU = 0.030 eV and low defect absorption αD = 5×102 cm −1 (at photon energy hv = 1.04 eV) indicating on low density of localized states in mobility gap. Doped films have been also fabricated and studied. Finally we shall discuss application of the above films in photovoltaic devices.

Deposition of CdS by evaporation through a gas discharge

Cadmium sulfide is an important material used in thin−film photovoltaic devices. The deposition of cadmium sulfide films by single indirect source evaporation through a gas discharge is investigated and the effects of the discharge on film properties and deposition rates are discussed. The discharge has a direct and advantageous effect on both the electrical and optical properties of the CdS thin films. These properties may be altered by suitable choices of evaporation and gas discharge parameters. Control of these properties is important in device applications and it is proposed that this technique is a practical method for controlling properties of CdS thin films.