Photovoltaic Absorber Research Articles

Detailed Visualization of Phase Evolution during Rapid Formation of Cu(InGa)Se2 Photovoltaic Absorber from Mo/CuGa/In/Se Precursors

Amongst several processes which have been developed for the production of reliable chalcopyrite Cu(InGa)Se2 photovoltaic absorbers, the 2–step metallization–selenization process is widely accepted as being suitable for industrial–scale application. Here we visualize the detailed thermal behavior and reaction pathways of constituent elements during commercially attractive rapid thermal processing of glass/Mo/CuGa/In/Se precursors on the basis of the results of systematic characterization of samples obtained from a series of quenching experiments with set-temperatures between 25 and 550 °C. It was confirmed that the Se layer crystallized and then melted between 250 and 350 °C, completely disappearing at 500 °C. The formation of CuInSe2 and Cu(InGa)Se2 was initiated at around 450 °C and 550 °C, respectively. It is suggested that pre-heat treatment to control crystallization of Se layer should be designed at 250–350 °C and Cu(InGa)Se2 formation from CuGa/In/Se precursors can be completed within a timeframe of 6 min.

The effect of Na on Cu-K-In-Se thin film growth

Co-evaporation of Cu-KF-In-Se was performed on substrates with varied Na supply. Compositions of interest for photovoltaic absorbers were studied, with ratios of (K + Cu)/In ∼ 0.85 and K/(K + Cu) ∼ 0–0.57. Bare soda-lime glass (SLG) substrates had the highest Na supply as measured by secondary ion mass spectrometry, while SLG/Mo and SLG/SiO2/Mo substrates led to 3x and 3000x less Na in the growing film, respectively. Increased Na supply favored Cu1−xKxInSe2 (CKIS) alloy formation as proven by X-ray diffraction (XRD), while decreased Na supply favored the formation of CuInSe2 + KInSe2 mixed-phase films. Scanning electron microscopy and energy dispersive X-ray spectroscopy revealed the KInSe2 precipitates to be readily recognizable planar crystals. Extrinsic KF addition during film growth promoted diffusion of Na out from the various substrates and into the growing film, in agreement with previous reports. Time-resolved photoluminescence showed enhanced minority carrier lifetimes for films with moderate K compositions (0.04 < K/(K + Cu) < 0.14) grown on SLG/Mo. Due to the relatively high detection limit of KInSe2 by XRD and the low magnitude of chalcopyrite lattice shift for CKIS alloys with these compositions, it is unclear if the lifetime gains were associated with CKIS alloying, minor KInSe2 content, or both. The identified Na-K interdependency can be used to engineer alkali metal bonding in Cu(In,Ga)(Se,S)2 absorbers to optimize both initial and long-term photovoltaic power generation.

Highly phase-pure spray-pyrolysed Cu2SnS3 thin films prepared by hybrid thermal treatment for photovoltaic applications

The structure and physical properties of thin films of ternary photovoltaic absorber Cu2SnS3 on soda-lime glass substrate deposited by cost effective spray pyrolysis following a hybrid thermal annealing (HTA) have been investigated. The advantage of HTA-sulfurization over standard thermal annealing (STA) and rapid thermal annealing (RTA) is established for Cu2SnS3 (CTS). The X-Ray diffraction (XRD) and Raman spectroscopy reveals highly phase pure Cu2SnS3 is produced through effective relaxation of compressive stress and increased crystallinity by the HTA-sulfurization step. Additionally, scanning electron microscopy (FE-SEM) and energy dispersive x-ray spectroscopy (EDX) reveals that the evolution of secondary phases, CuxSy (x, y: 116, 64; 2, 1; 1.8, 1; 51, 27; 6, 6 etc) could be effectively prevented by the HTA process over the STA and RTA. The optimized HTA-CTS thin films obtained are of p-type conductivity with the carrier concentration, resistivity and mobility of 5.5 × 1020 cm−3, 9.8 × 10−3Ω-cm and 1.15 cm2V−1s−1, respectively. The optical band gap of about 1.65 eV with large absorption coefficient of 105 cm−1 demonstrates it as an ideal candidate for thin film solar cells. A solar cell configuration of glass/F:SnO2/CTS/CdS/ZnO:Al/Al shows an open circuit voltage of 225 mV.

Colloidal Nanocrystals of Lead-Free Double-Perovskite (Elpasolite) Semiconductors: Synthesis and Anion Exchange To Access New Materials.

Concerns about the toxicity and instability of lead-halide perovskites have driven a recent surge in research toward alternative lead-free perovskite materials, including lead-free double perovskites with the elpasolite structure and visible bandgaps. Synthetic approaches to this class of materials remain limited, however, and no examples of heterometallic elpasolites as nanomaterials have been reported. Here, we report the synthesis and characterization of colloidal nanocrystals of Cs2AgBiX6 (X = Cl, Br) elpasolites using a hot-injection approach. We further show that postsynthetic modification through anion exchange and cation extraction can be used to convert these nanocrystals to new materials including Cs2AgBiI6, which was previously unknown experimentally. Nanocrystals of Cs2AgBiI6, synthesized via a novel anion-exchange protocol using trimethylsilyl iodide, have strong absorption throughout the visible region, confirming theoretical predictions that this material could be a promising photovoltaic absorber. The synthetic methodologies presented here are expected to be broadly generalizable. This work demonstrates that nanocrystal ion-exchange reactivity can be used to discover and develop new lead-free halide perovskite materials that may be difficult or impossible to access through direct synthesis.

Intrinsic Point Defects in Inorganic Cesium Lead Iodide Perovskite CsPbI3

Cesium lead iodide (CsPbI3) has recently emerged as a promising solar photovoltaic absorber. However, the cubic perovskite (α-phase) remains stable only at high temperature and reverts to a photoinactive nonperovskite (δ-phase) CsPbI3 at room temperature. In this work, the formation energies and transition energy levels of intrinsic point defects in γ- (more stable than α-phase) and δ-phases have been studied systematically by first-principles calculations. It is found that CsPbI3 exhibits a unipolar self-doping behavior (p-type conductivity), which is in contrast to CH3NH3PbI3. Most of the intrinsic defects induce deeper transition energy levels in δ-phase than in γ-phase. This is due to the small Pb–I–Pb bond angles in δ-phase that results in the weak antibonding character of valence band maximum (VBM). However, the strong antibonding character of VBM plays a critical role in keeping defect tolerance in semiconductors. Therefore, these results indicate the importance of the large metal–halide–metal bond...

Infrared Solution-Processed Quantum Dot Solar Cells Reaching External Quantum Efficiency of 80% at 1.35 µm and Jsc in Excess of 34 mA cm-2.

Developing low-cost photovoltaic absorbers that can harvest the short-wave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Si-based and perovskite photovoltaic technologies, is a prerequisite for making high-efficiency, low-cost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganic-organic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 µm are reported, leading to a short-circuit current density of 34 mA cm-2 and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI3 perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm.

Solution Deposited Cu2BaSnS4–xSex from a Thiol–Amine Solvent Mixture

The discovery that cation antisite defects in Cu2ZnSn(S,Se)4 (CZTS) thin film photovoltaic absorbers are a major contributor to lower than expected device efficiencies has spurred recent interest in material design to develop similar materials that maintain the desirable optoelectronic properties of CZTS, while mitigating this source of loss. Recent reports have shown that this can be accomplished by replacing zinc with a larger cation of the same valency, such as barium, to give Cu2BaSn(S,Se)4. Until this point, Cu2BaSn(S,Se)4 has only been synthesized by solid-state and physical deposition methods. Herein, we use a thiol-amine solvent mixture to dissolve bulk Cu2S, BaS, and SnO to prepare a precursor ink that upon low temperature annealing yields phase-pure trigonal Cu2BaSnS4-xSex (nominal x = 0, 1, 2, 3) alloys with tunable direct band gaps from 1.56 – 1.86 eV.

Investigation of LaVO3 based compounds as a photovoltaic absorber

Bulk LaVO3 (LVO) is known to provide an optical bandgap of 1.1 eV which is interesting for photovoltaic applications. Here, we compare the properties of LVO thin films using different techniques, namely pulsed laser deposition, spin coating by solgel and sputtering. ZnO/LVO p-n junctions were fabricated in order to obtain solar cell architectures. The grown films were analyzed with Surface photovoltage and Scanning Kelvin Probe Microscopy. The analysis of the junctions showed that the photogenerated electron-hole pairs are poorly separated and therefore not efficiently collected. LVO has shown the particularity to provide very high work function (4.6 eV) and a great dependence of the electrical and optical properties on the growth conditions, including the oxygen stoichiometry and the La/V ratio of the films.

Reducing series resistance in Cu2ZnSn(S,Se)4 nanoparticle ink solar cells on flexible molybdenum foil substrates.

Earth abundant Cu2ZnSnS4 nanoparticle inks were deposited on molybdenum foil substrates and subsequently converted to high quality thin film Cu2ZnSn(S,Se)4 photovoltaic absorbers. Integration of these absorbers within a thin film solar cell device structure yields a solar energy conversion efficiency which is comparable to identical devices processed on rigid glass substrates. Importantly, this is only achieved when a thin layer of molybdenum is first applied directly to the foil. The layer limits the formation of a thick Mo(S,Se)x layer resulting in a substantially reduced series resistance.

Wurtzite CoO: a direct band gap oxide suitable for a photovoltaic absorber.

A direct band gap of 1.6 eV has been identified in wurtzite CoO thin films, which matches the required value to achieve a theoretically high conversion efficiency solar cell. Its p-type conduction has been determined and an intense sub-gap absorption between 0.7 and 1.1 eV has been observed.

Roles of Pseudo-Closed s2 Orbitals for Different Intrinsic Hole Generation between Tl-Bi and In-Bi Bromide Double Perovskites.

Although metal halide double perovskites A2B(I)B(III)X6 are expected as nontoxic alternatives for lead halide perovskites, recent studies have shown that only Tl(I)-Bi(III) and In(I)-Bi(III) bromides are thermodynamically stable and possess optoelectronic properties suitable for photovoltaic absorbers. Here, we show, through density functional theory calculations, that Tl-Bi and In-Bi bromide double perovskites exhibit significantly different semiconducting behaviors due to the different energy levels of the highest-occupied pseudoclosed s2 orbitals of Tl(I) and In(I). While Tl-Bi double perovskites can exhibit semiconducting p-type properties, In-Bi bromide double perovskites exhibit metallic p-type ones regardless of the synthesis condition due to the extremely low formation energy of In vacancy. Such difference makes Tl-Bi bromide double perovskites suitable for optoelectronic applications, but not In-Bi bromide double perovskites. Furthermore, there is a high probability for In to substitute a Bi site, forming a local In-In bromide double perovskite structure with a lower local conduction band minimum, detrimentally affecting the open circuit voltage of In-Bi bromide double perovskite-based thin film solar cells.

Mechanochemical bulk synthesis and e-beam growth of thin films of Sb2Se3 photovoltaic absorber

Antimonselite (Sb2Se3) bulk material has been prepared by mechanical alloying through ball milling of the elemental precursors antimony (Sb) and selenium (Se) in place of conventional solid state synthesis. X-ray diffraction (XRD), Raman spectroscopy studies and Energy dispersive spectroscopy (EDS) studies confirm the formation of polycrystalline Sb2Se3 along with elemental Sb. The as-milled bulk material was used as source to deposit Sb2Se3 thin films by e-beam deposition. Thin films have been grown at a substrate temperature of ∼373 K with an optimal electron beam current (Ib) of ∼30 mA at fixed voltage of ∼5 kV. The films were studied using XRD and Raman spectroscopy for the structural and phase analyses. Scanning electron microscopy (SEM) studies and energy dispersive spectroscopy (EDS) measurements revealed the formation of near stoichiometric thin films with smooth and uniform surface. Rutherford backscattered spectrometry (RBS) and proton induced X-ray emission spectroscopy (PIXE) tools were employed to confirm the chemical composition of as-grown Sb2Se3 thin films. Optical transmission spectrum showed a very sharp transition near the fundamental absorption edge ∼1.26 eV. The films exhibited an average absorption coefficient values exceeding > 104 cm−1 in the visible region making it a promising material for solar absorber in thin film solar cell.

Effect of the spraying temperatures and the sulfurization on the properties of the absorber Cu2FeSnS4 thin films in a solar cell

The quaternary Cu2FeSnS4 (CFTS) is regarded like the one of the semiconductors promoting as an effective photovoltaic absorber because of their earth-abundant and non-toxic elements. The thin films were elaborated by spray pyrolysis method deposited onto glass substrates at 370 °C for one hour with post-sulfurization treatment at 450 °C for 30 min. XRD spectra analysis showed that the CFTS thin films have tetragonal structure with a main peak corresponding to the direction (1 1 2). The Raman spectrum confirmed these results with an appearance of two main peaks corresponding to the CFTS situated at the positions 289 cm−1 and 318 cm−1. The CFTS films exhibit a homogeneous, rough and dense topography with a thickness of about 0.8 μm. The films of CFTS present a high absorption coefficient (≥105 cm−1) and the direct band gap energy is found to be 1.55 eV, which confirms their use as an absorber for solar cells.

Two‐step selenization using nozzle free Se shower for Cu(In,Ga)Se2 thin film solar cell

AbstractThe production of commercialized Cu(In,Ga)(S,Se)2 (CIGS) photovoltaic absorber layers uses expensive H2Se gas with a high utility cost. To reduce the manufacturing cost of CIGS photovoltaic modules, a process technology capable of supplying Se vapor uniformly over a large area is required to replace H2Se. In this study, a nozzle‐free Se shower was implemented using a porous material to pass Se vapor while confining liquid Se, and the highly effective selenization of the CuInGa precursor was performed. The nozzle‐free Se‐shower vehicle could be mounted in a commercial rapid thermal process chamber. The chamber pressure and the temperatures of the shower module and substrate, which were controlled independently by the upper and lower heaters, respectively, were varied to control the amount of Se supplied during the entire selenization reaction in real time. In particular, the precursor should be soaked with a sufficient amount of Se at a relatively low substrate temperature of 300°C or less to obtain a good quality absorber. In addition, at a chamber pressure of 100 Torr during the soaking stage, the Ga content in the surface region of the absorber increased considerably with a concomitant improvement in the open‐circuit voltage. The highest performance obtained using this method was an open‐circuit voltage of 0.638 V, short‐circuit current density of 34 mA/cm2, fill factor of 67.2%, and an active area efficiency of 14.57%. This performance is very high compared with other CIGS solar cells manufactured by a 2‐step process using Se vapor.

Solution processed CuSbS2 films for solar cell applications

CuSbS2 is a semiconductor with a band gap of 1.5eV and earth-abundant constituent elements, indicating potential promise as a photovoltaic absorber material. However, strategies to fabricate CuSbS2 films, especially using solution processing, have not been thoroughly developed. We report on two solution-based approaches to deposit CuSbS2 films: chemical bath deposition (CBD) and deposition of colloidal nanoplates. Conditions to directly deposit ternary CuSbS2 (chalcostibite) films were not found, but CuSbS2 films could be formed by annealing CBD-grown bilayers of CuS and Sb2S3. Simultaneous control over phase purity and film morphology proved elusive. To address this challenge, we synthesized colloidal nanoplates of phase-pure chalcostibite CuSbS2 capped with oleylamine ligands following a literature procedure. When colloids are condensed into thin films, these synthesis ligands are insulating and inhibit the inter-crystal charge transfer that is necessary for long-range charge transport. To solve this problem, two approaches were pursued: convective assembly followed by solid-state ligand exchange and a novel process involving solution-phase ligand exchange followed by electrophoretic deposition (EPD). Replacement of oleylamine with S2− increased the film conductivity by two orders of magnitude. S2− capping groups also increased the electrophoretic mobility and enabled EPD at bias voltages as low as 5V. Time-resolved terahertz spectroscopy indicated transient photoconductivity persisting beyond 1ns and carrier mobilities of ~1cm2V−1s−1. While many challenges remain, this work indicates the potential promise of solution-processed CuSbS2 nanoplates as building blocks for photovoltaic devices.

Core Levels, Band Alignments, and Valence-Band States in CuSbS2 for Solar Cell Applications

The earth-abundant material CuSbS2 (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core levels of the constituent elements, surface contaminants, ionization potential, and valence-band spectra are studied by X-ray photoemission spectroscopy. The ionization potential and electron affinity for this material (4.98 and 3.43 eV) are lower than those for other common absorbers, including CuInxGa(1-x)Se2 (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the valence band maximum is raised by the lone pair electrons from the antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies.

The green hydrothermal synthesis of nanostructured Cu2ZnSnSe4 as solar cell material and study of their structural, optical and morphological properties

Cu2ZnSnSe4 (CZTSe) has attracted intensive attention as an absorber material for the thin-film solar cells due to its high absorption coefficient, direct band gap, low toxicity, and abundance of its constituent elements. In this study nanostructured CZTSe nanoparticles are prepared via green hydrothermal synthesis without using toxic solvents, organic amines, catalysts or noxious chemicals. The structural, optical, and morphological properties of CZTSe nanostructured powder were studied using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV–vis absorption spectroscopy, and transmission electron microscope (TEM) techniques. Raman peaks at 170, 195, and 232 cm−1 confirm the formation of pure phase CZTSe nanostructured particles. In addition, the EDS and XPS results confirm the appropriate chemical purity of the annealed CZTSe nanoparticles. Meanwhile, the TEM analysis showed the presence of phase pure oval like CZTSe particle with size of about 80–140 nm. The UV–Vis-NIR absorption spectra analysis showed that the optical band gap of CZTSe nanostructured particles is about 1.14 eV. This band gap energy is close to the optimum value of a photovoltaic solar cell absorber material.

Conduction-band effective mass and bandgap of ZnSnN2 earth-abundant solar absorber

Pseudo III-V nitride ZnSnN2 is an earth-abundant semiconductor with a high optical absorption coefficient in the solar spectrum. Its bandgap can be tuned by controlling the cation sublattice disorder. Thus, it is a potential candidate for photovoltaic absorber materials. However, its important basic properties such as the intrinsic bandgap and effective mass have not yet been quantitatively determined. This paper presents a detailed optical absorption analysis of disordered ZnSnN2 degenerately doped with oxygen (ZnSnN2−xOx) in the ultraviolet to infrared region to determine the conduction-band effective mass (mc*) and intrinsic bandgap (Eg). ZnSnN2−xOx epilayers are n-type degenerate semiconductors, which exhibit clear free-electron absorption in the infrared region. By analysing the free-electron absorption using the Drude model, mc* was determined to be (0.37 ± 0.05)m0 (m0 denotes the free electron mass). The fundamental absorption edge in the visible to ultraviolet region shows a blue shift with increasing electron density. The analysis of the blue shift in the framework of the Burstein-Moss effect gives the Eg value of 0.94 ± 0.02 eV. We believe that the findings of this study will provide important information to establish this material as a photovoltaic absorber.

Double band gap gradients in sequentially processed photovoltaic absorbers from the Cu(In,Ga)Se2‐ZnSe pseudobinary system

AbstractWide band gap Cu(In,Ga)Se2‐ZnSe (CIGZSe) thin films have been synthesized using a sequential process, with the objective to demonstrate the possibility to tune the band gap and introduce double gradients playing with the content of Ga at the back and Zn at the front. In a first approach, we start by varying the 3‐valent cationic composition of the system, and then we modify the reactive annealing conditions in order to understand and control the elemental gradients. Structural, compositional, and morphological properties of the corresponding absorbers were analysed by X‐ray diffraction, Raman spectroscopy, energy dispersive spectrometer, Auger spectroscopy, and scanning electron microscopy. Solar cells were fabricated and characterized, focusing on the identification of the most promising cationic composition. The compounds from the complex Cu(In,Ga)Se2‐ZnSe system can adopt either the chalcopyrite or sphalerite phases depending on the [Zn]/[metals] and [Ga]/([Ga] + [In]) ratios. We demonstrate that Ga naturally diffuses towards the back region forming a Ga‐rich, wide band gap chalcopyrite phase at the rear contact, as is commonly observed for a Cu(In,Ga)Se2 synthesized via selenization process. On the contrary, Zn is preferably accumulated at the surface, forming wide band gap sphalerite Cu(In,Ga)ZnSe3 phases with high Zn and very low Ga contents at the surface. This opens an additional way to control the surface's band gap. With this approach, the formation of a doubly graded band gap profile with Ga‐rich layers at the back and Zn‐rich layers at the front is demonstrated in a single selenization step, showing promising efficiency and open circuit voltage values (up to 6.7% and 709 mV, respectively).

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites.

The role of organic molecular cations in the high-performance perovskite photovoltaic absorbers, methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3), has been an enigmatic subject of great interest. Beyond aiding in the ease of processing of thin films for photovoltaic devices, there have been suggestions that many of the remarkable properties of the halide perovskites can be attributed to the dipolar nature and the dynamic behavior of these cations. Here, we establish the dynamics of the molecular cations in FAPbI3 between 4 K and 340 K and the nature of their interaction with the surrounding inorganic cage using a combination of solid state nuclear magnetic resonance and dielectric spectroscopies, neutron scattering, calorimetry, and ab initio calculations. Detailed comparisons with the reported temperature dependence of the dynamics of MAPbI3 are then carried out which reveal the molecular ions in the two different compounds to exhibit very similar rotation rates (≈8 ps) at room temperature, despite differences in other temperature regimes. For FA, rotation about the N···N axis, which reorients the molecular dipole, is the dominant motion in all phases, with an activation barrier of ≈21 meV in the ambient phase, compared to ≈110 meV for the analogous dipole reorientation of MA. Geometrical frustration of the molecule-cage interaction in FAPbI3 produces a disordered γ-phase and subsequent glassy freezing at yet lower temperatures. Hydrogen bonds suggested by atom-atom distances from neutron total scattering experiments imply a substantial role for the molecules in directing structure and dictating properties. The temperature dependence of reorientation of the dipolar molecular cations systematically described here can clarify various hypotheses including those of large-polaron charge transport and fugitive electron spin polarization that have been invoked in the context of these unusual materials.