Absorber Layer In Solar Cells Research Articles

Effect of thiourea on structure, morphology and optical properties of spray deposited CZTS thin films for solar cell applications

<p>Cu2ZnSnS4 (CZTS) films of different Thiorea (SC (NH2)2) molarity were deposited by using simple chemical spray technique at substrate temperature 275°C. Analytical reagent Grade Copper chloride (CuCl2), Zinc chloride (ZnCl2), Tin chloride (SnCl4.5H2O) were used as Cu+ , Zn+ , and Sn+ ion sources respectively and thiourea (SC(NH2)2) (0.02, 0.04 0.06 0.08, and 0.1 M) was used as a Sion source. A set of five CZTS films was deposited using five different molarity of thiourea. The structure, Morphology, Elemental analysis and optical properties of these films were studied using X-ray diffratometer (XRD), Scanning Electron Microscopy (SEM) Energy Dispersive X-ray Analysis (EDX) and UV-Visible spectroscopy techniques respectively. The XRD spectra showed that all films are polycrystalline tetragonal structure with preferential orientation along (112) plane. The calculated crystallite size was increased with increase in thiourea concentration. Variations of optical band gap with thiourea molarity have been investigated using Tauc plots. SEM micrographs exhibits CZTS spherical granules regularly arranged with some void spaces. Purity of deposited films was investigated using EDX analysis. All the CZTS films exhibits higher absorption coefficient (𝛼 > 104) cm-1 and band gap in the reported range (1.2-1.53 eV) can be used as an absorber layer in solar cells.</p>

Stability, and electronic and optical properties of ternary nitride phases of MgSnN2: A first-principles study

We have studied the disordered rocksalt, orthorhombic, and disordered wurtzite phases of the ternary nitride semiconductor MgSnN2 by first-principles methods using density functional theory (DFT) and beyond. The results imply that MgSnN2 is mechanically and dynamically stable in all three phases. However, pCOHP analysis suggests that the disordered rocksalt structure has antibonding states below the Fermi level between −5 eV and −2 eV, as compared to the bonding states in the other two phases, indicative of its thermodynamic metastability. Computed lattice constant and electronic band-gap values of 4.56 Å and 2.69 eV for MgSnN2 in the disordered rocksalt structure compare well with experimentally reported values of 4.48 Å and 2.3 eV, respectively. Furthermore, band gaps were computed for MgSnN2-xOx (x = 0.5, 1.0, 1.5, 2.0) to elucidate the role of possible oxygen impurities. Band-gap bowing is suggested to occur upon alloying with oxygen. Of the three phases, the disordered rocksalt structure shows the lowest charge carrier effective masses. Moreover, the absorption coefficient and reflectivity of this phase make it promising for use as the absorber layer of tandem solar cells in the higher energy region of the visible portion of the solar spectrum. The other two phases, disordered wurtzite and orthorhombic, might be utilized as the window layer of solar cells owing to their larger band-gap values of 4.36 eV and 4.86 eV, respectively.

Present Status of Solution‐Processing Routes for Cu(In,Ga)(S,Se)2 Solar Cell Absorbers

AbstractPhotovoltaic technologies offer a sustainable solution to the challenge of meeting increasing energy demands. Chalcopyrite Cu(In,Ga)(S,Se)2, short CIGS—thin‐film solar cells—having intrinsically p‐type absorbers with a tunable direct bandgap—exhibits one of the highest stabilized power conversion efficiencies of 23.35%, utilizing absorbers typically fabricated via vacuum deposition methods. Research is increasingly devoted to absorbers deposited by solution processing techniques, which may inherently improve material usage, increase throughput, and lower financial barriers to commercialization. However, the performance of current devices with solution‐processed absorbers is still falling short of their vacuum‐processed counterparts with record power conversion efficiencies up to 18.7% reported to date. While hydrazine solvent‐based routes offer reduced residual impurities, their toxicity poses hindrances to widespread adoption. Alternatively, less toxic and environmentally friendly routes based on protic and aprotic solvents are being researched and are showing promising device efficiencies well above 14%. This review describes the current status of CIGS solar cell absorber layers fabricated by pure solution‐based deposition methods, provides a comparison of champion solution‐processed devices (with and without hydrazine), and offers an outlook for future improvements.

Routes to develop a [S]/([S]+[Se]) gradient in wide band-gap Cu2ZnGe(S,Se)4 thin-film solar cells

Wide band-gap kesterite-based solar cells are very attractive to be used for tandem devices as well as for semi-transparent photovoltaic cells. Here, Cu2ZnGe(S,Se)4 (CZGSSe) thin films have been grown by sulfurization of co-evaporated Cu2ZnGeSe4. The influence of a NaF precursor layer and of a Se capping film on CZGSSe absorbers and solar cells has been investigated. It has been found that the distribution of [S]/([S]+[Se]) through the CZGSSe absorber layer is strongly dependent on the Na content. Na promotes the diffusion of S towards the bulk of the absorber layer. Thicker NaF layers>6 nm lead to a higher S content in the bulk of the absorber layer, but to a decreased accumulation of sulphur at the surface, as detected by GIXRD, GD-OES, and Raman spectroscopy measurements. A relationship between Jsc, FF and Na-content supplied was found; higher Na content resulted in improved solar cell efficiencies. It has also been possible to modify the [S]/([S]+[Se])-gradient throughout the CZGSSe film by the absence of the Se capping layer, achieving devices with 2.7% performance and Eg = 2.0 eV. This work reveals two ways to control the [S]/([S]+[Se]) depth-profile to produce wide band gap CZGSSe absorber layers for efficient solar cells.

An investigation of physical properties and photovoltaic performance of methylammonium lead-tin iodide (CH3NH3Sn1-xPbxI3) solar cells

PurposeIn the past several years, CH3NH3PbI3 perovskite material has been extensively evaluated as an absorber layer of perovskite solar cells due to its excellent structural and optical properties, and greater than 22% conversion efficiency. However, improvement and future commercialization of solar cells based on CH3NH3PbI3 encountered restrictions due to toxicity and instability of the lead element. Recently, studies on properties of lead-free and mixture of lead with other cations perovskite thin films as light absorber materials have been reported. The purpose of this paper was the fabrication of CH3NH3Sn1-xPbxI3 thin films with different SnI2 concentrations in ambient condition, and study on the structural, morphological, optical, and photovoltaic performance of the studied solar cells. The X-ray diffraction studies revealed the formation of both CH3NH3PbI3 and CH3NH3SnI3 phases with increasing the Sn concentration, and improvement in crystallinity and morphology was also observed. All perovskite layers had a relatively high absorption coefficient >104 cm−1 in the visible wavelengths, and the bandgap values varied in the range from 1.46 to 1.63 eV. Perovskite solar cells based on these thin films have been fabricated, and device performance was investigated. Results showed that photo-conversion efficiency (PCE) for the pure CH3NH3PbI3sample was 1.20%. With adding SnI2, PCE was increased to 4.48%.Design/methodology/approachIn this work, the author mixed tin and lead with different percentages in the perovskite thin film. Also, the preparation of these layers and also other layers to fabricate solar cells based on them were conducted in an open and non-glove box environment. Finally, the effect of [Sn/Pb] ratio in the CH3NH3Sn1-xPbxI3 layers on the structural, morphological, optical, electrical and photovoltaic performance have been investigated.FindingsCH3NH3Sn1-xPbxI3 (x = 0.0, 0.25, 0.50, 0.75, 1.0) perovskite thin films have been grown by a spin-coating technique. It was found that as tin concentration increases, the X-ray diffraction and FESEM images studies revealed the formation of both CH3NH3PbI3 and CH3NH3SnI3 phases, and improvement in crystallinity, and morphology; all thin films had high absorption coefficient values close to 104 cm−1 in the visible region, and the direct optical bandgap in the layers decreases from 1.63 eV in pure CH3NH3SnI3 to 1.46 eV for CH3NH3Sn0.0.25Pb0.75I3 samples; all thin films had p-type conductivity, and mobility and carrier density increased; perovskite solar cells based on these thin films have been fabricated, and device performance was investigated. Results showed that photo-conversion efficiency (PCE) for the pure CH3NH3PbI3sample was 1.20%. With adding SnI2, PCE was increased to 4.48%.Originality/valueThe preparation method seems to be interesting as it is in an ambient environment without the protection of nitrogen or argon gas.

Highly crystalline methylammonium lead iodide films: Phase transition from tetragonal to cubic structure by thermal annealing

Herein, a very simple, solvent free, scalable, and single-step approach to prepare organometal halide perovskite powders via mechanochemical synthesis followed by the deposition of perovskite films by spin coating is reported. This work particularly deals with various parameters that influence the crystallization process and morphology (hyperbranched) of methylammonium lead iodide films. Moreover, the influence of growth temperature on the morphology and the transition from tetragonal to cubic structure are investigated. The mechanosynthesized perovskite provides hyperbranched morphology and crystalline films in a hexagonal shape and serves as a better precursor for the absorber layer in perovskite solar cells.

Direct synthesis of nanostructured silver antimony sulfide powders from metal xanthate precursors

Silver(I) ethylxanthate [AgS2COEt] (1) and antimony(III) ethylxanthate [Sb(S2COEt)3] (2) have been synthesised, characterised and used as precursors for the preparation of AgSbS2 powders and thin films using a solvent-free melt method and spin coating technique, respectively. The as-synthesized AgSbS2 powders were characterized by powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The crystalline AgSbS2 powder was investigated using XRD, which shows that AgSbS2 has cuboargyrite as the dominant phase, which was also confirmed by Raman spectroscopy. SEM was also used to study the morphology of the resulting material which is potentially nanostructured. EDX spectra gives a clear indication of the presence of silver (Ag), antimony (Sb) and sulfur (S) in material, suggesting that decomposition is clean and produces high quality AgSbS2 crystalline powder, which is consistent with the XRD and Raman data. Electronic properties of AgSbS2 thin films deposited by spin coating show a p-type conductivity with measured carrier mobility of 81 cm2 V−1 s−1 and carrier concentration of 1.9 × 1015 cm−3. The findings of this study reveal a new bottom-up route to these compounds, which have potential application as absorber layers in solar cells.

A Novel Ethanol-Based Non-Particulate Ink for Spin-Coating Cu2ZnSnS4Thin Film

The Cu2ZnSnS4(CZTS) thin films were fabricated by the direct solution coating method using a novel non-particulate ink. The ink was formulated using ethanol as the solvent and 1,2-diaminopropane as the complex-ing agent. The pure phase kesterite films with good crystallinity, large-sized crystals and excellent electrical properties were prepared by the spin-coating deposition technique using the homogeneous and air-stable ink. It was found that the subsequent pre-treatment temperature had an influence on the film crystallinity and electrical properties. The best film was obtained by pre-treating the spin-coated film at 250 °C, and then post-annealing at 560 °C. The film shows a narrow bandgap of 1.52 eV and excellent electrical properties, with a resistivity of 0.07 Ocm, carrier concentration of 3.0 x 1017cm-3, and mobility of 4.15 cm2V-1s-1. The novel non-particulate ink is promising for printing high quality CZTS thin films as absorber layers of thin film solar cells.

Copper concentration effect on physical properties of ultrasonically sprayed Cu2ZnSnS4 absorber thin films for solar cell applications

Cu2ZnSnS4 (CZTS) thin films have been successfully deposited on glass substrate using ultrasonic spray pyrolysis (USP) technique with different copper concentrations ranging from 0.005 to 0.020 M. The effect of copper concentration on the structural, morphological and optoelectrical properties of deposited films has been investigated. Choice of USP method and copper concentration was found to have a crucial role in stoichiometry, band gap and conductivity of sprayed CZTS thin films. X-ray diffraction indicated that all deposited films have CZTS Kesterite structure with a preferential orientation along (112) plane confirmed by Raman scattering measurements. Copper sulfide (CuS) is present as a secondary phase in all films. The presence of this secondary phase causes films optical band broadening. Good film with nearly stoichiometric CZTS film and desirable band gap of 1.51 eV was obtained at copper concentration of 0.015 M. This result indicates that this film is suitable for being applied as an absorber layer in photovoltaic solar cells.

Preparation and characterization of CuSbSe2 thin films deposited by pulsed laser deposition

CuSbSe2 has been a potential absorber layer for thin film solar cells by reason of its attractive photovoltaic properties and environmentally friendly constituent elements. In this work, high-quality CuSbSe2 thin film was prepared by pulsed laser deposition (PLD) and further in-situ annealing. Structural, morphological, compositional and optical properties of the unannealed and annealed films were characterized and compared separately. The results proved the existence of CuSbSe2, Sb2Se3 and Cu3SbSe3 in the unannealed film. With the rising of deposition temperature, the content of Sb2Se3 increased significantly. Moreover, in-situ annealing at 350 °C can effectively reduce the Sb2Se3 and Cu3SbSe3 in the film. The atomic ratio of Cu:Sb:Se in the annealed film is estimated to be 25.45:26.29:48.27, basically consistent with the stoichiometry ratio of CuSbSe2. Both annealed and unannealed CuSbSe2 films revealed a high absorption coefficient greater than 104 cm−1. The optical band gap of the annealed film was about 1.15eV, which meets the requirements for the absorber layer of solar cells.

Challenges in the deposition of (Ag,Cu)(In,Ga)Se2 absorber layers for thin-film solar cells

The partial replacement of Cu by Ag in Cu(In,Ga)Se2 thin-film solar cells is strategically interesting to achieve smooth devices with high conversion efficiencies. Yet, the industrial exploitation requires further understanding of the deposition process and control of the absorber layer properties. In this study, three-stage co-evaporation of (Ag,Cu)(Ga,In)Se2 films with [Ag]/([Ag] + [Cu]) contents up to 0.2 was investigated. Deep crevices and voids, sometimes extending down to the rear contact, were found. They mainly occur for high Ag contents and excessive group-I richness during the second stage of the deposition. The formation of cavities is attributed to the segregation of Ag–Se phases and slow Ag diffusion into the chalcopyrite during the deposition. Another identified challenge is the flattening of the desired bandgap grading which is correlated with the Ag content. Optimized process conditions allow fabrication of smooth (Ag,Cu)(Ga,In)Se2 films in a manufacturing-like inline deposition with cell efficiencies up to 20.5%.

Realizing Double Graded CIGSe Absorbers With the R2R Hybrid-CIGSe-Process

This article presents an approach for industrial roll-to-roll (R2R) deposition of Cu(In,Ga)Se2 (CIGSe) thin films on stainless steel foil for use as absorber layers in flexible thin-film solar cells. The hybrid-CIGSe-deposition-process combines magnetron sputtering with evaporation techniques and allows the realization of a double graded CIGSe layer, reaching a conversion efficiency of currently up to 14.0% (w/o antireflective coating). The influence of different power settings of the individual magnetrons on the gallium profile is described. For a deeper understanding of differences between a static and an R2R process, a calculated flux diagram is presented. In the second part of this contribution, the homogeneity of the industrial scale samples is discussed by comparing the change in CGI and GGI over the foil width of 30 cm. The hybrid-CIGSe-deposition-process excels in a straightforward scalability for industrial usage due to the application of planar magnetron sputtering sources as metal vapor supply.

Improving P3HT:PCBM absorber layers by blending TiO2/CdS nanocomposites for application in photovoltaic solar cells

In this work we propose the use of the semiconducting TiO2/CdS nanocomposite (NC) to improve the optical and electrical properties of the P3HT:PCBM polymeric system employed as the absorber layer in organic photovoltaic solar cells. Therefore, we report a methodology for obtaining the TiO2/CdS-NC, its incorporation into precursor solutions containing P3HT and PCBM polymers, and the fabrication of hybrid P3HT:PCBM:TiO2/CdS-NC absorber layers. The effect of the mass ratio between the TiO2/CdS-NC and the polymeric system was studied. XRD measurements conducted on the TiO2/CdS-NC showed that the TiO2 component has a tetragonal crystalline structure (anatase phase) with a nanocrystalline size of 15.8 nm, while the CdS component has the hexagonal close-packed crystalline structure with a nanocrystalline size of 14.9 nm. XPS analysis confirmed the formation of stoichiometric TiO2 and CdS compounds. The formation of any other compound comprising the elements Ti, O, Cd and S was not found, however, elements Cl, Br, N and C were observed at trace levels, which come from the capping agent that was used to obtain the nanocomposite. Measurements on the hybrid P3HT:PCBM:TiO2/CdS-NC absorber layers through UV-vis spectroscopy showed a shift on the absorption edge and an improved light absorption for wavelengths below 550 nm. The four-point probe measurements showed a decrease of the electrical resistivity from 8.5 × 108 Ω-cm in pristine P3HT:PCBM to a minimum of 4 × 105 Ω-cm in the hybrid absorber layers. Both the light absorption and the electrical resistivity are modulated by the mass ratio between the TiO2/CdS-NC and the P3HT:PCBM polymeric system.

Numerical Analysis of CsSnGeI<sub>3</sub> Perovskite Solar Cells Using SCAPS-1D

Recently, organic-inorganic perovskite-based solar cells have become promising devices due to their unique properties in the photovoltaic field. However, the factor of toxicity, stability, high production cost and complicated fabrication processes of these devices is a challenge to their progress in commercial production. Here a numerical modelling of Caesium Tin–Germanium Tri-Iodide (CsSnGeI₃) as an efficient perovskite light absorber material is carried out. In this paper, different inorganic Hole Transport Materials (HTMs) such as Cu₂O, CuI, CuSbS₂, CuSCN and NiO have been analyzed with C₆₀ as the Electron Transport Material (ETM). We intend to replace the conventional hole and electron transport materials such as TiO₂ and Spiro-OMeTAD which have been known to be susceptible to light induced degradation. Moreover, the influence of the Electron Transport Layer (ETL) and the perovskite layer properties, bandgap, doping concentration and working temperature for various Hole Transport Layers (HTL) on the overall cell performance have been rigorously investigated. The design of the proposed PSC is performed utilizing SCAPS- 1D simulator and for optimum device an efficiency greater than 30% was obtained. The results indicate that CsSnGeI₃ and C₆₀ are viable candidates for use as an absorber layer and electron transport layer in high-efficiency perovskite solar cells, with none of the drawbacks that other PSCs have.

Effects of mechanical deformations on P3HT:PCBM layers for flexible solar cells

The P3HT:PCBM mixture is the most promising system for absorber layers in flexible polymer-based photovoltaic solar cells. Nowadays, an increased number of studies have been devoted to improving electrical and optical properties of this absorber layer in order to rise the power conversion efficiency. However, studies dealing with its mechanical behavior are scarce, even though they are important on the fabrication processes for flexible devices and operation. Therefore, this work aims to elucidate the effect of tensile and bending deformations on P3HT:PCBM layers deposited on PET/ITO substrates. Mechanical and electrical characteristics were in-situ monitored while strains were applied. Tensile experiments showed that the layers are able to support a unitary strain up to 0.684%, when crack onset strain (COS) is reached, and crack propagation begins until the formation of a grid pattern observed in micrographs. Bending tests were performed with two span-lengths (50.8 mm and 25.4 mm) and two strain conditions on the P3HT:PCBM layers, i.e. tensile and compressive. For the tensile state, micrographs did not show any cracking on the layers regardless of the span-length, even when the bending radius was as low as 6.13 mm. On the other hand, the COS was observed when the layer was subjected to the compressive state with the lowest span and the same curvature radius.

Impurity Phases and Optoelectronic Properties of CuSbSe2 Thin Films Prepared by Cosputtering Process for Absorber Layer in Solar Cells

When there is a choice of materials for an application, particular emphasis should be given to the development of those that are low-cost, nontoxic, and Earth-abundant. Chalcostibite CuSbSe2 has gained attention as a potential absorber material for thin-film solar cells, since it exhibits a high absorption coefficient. In this study, CuSbSe2 thin films were deposited by radio frequency magnetron cosputtering with CuSe2 and Sb targets. A series of CuSbxSe2 thin films were prepared with different Sb contents adjusted by sputtering power, followed by rapid thermal annealing. Impurity phases and surface morphology of Cu–Sb–Se systems were directly affected by the Sb sputtering power, with the formation of volatile components. The crystallinity of the CuSbSe2 thin films was also enhanced in the near-stoichiometric system at an Sb sputtering power of 15 W, and considerable degradation in crystallinity occurred with a slight increase over 19 W. Resistivity, carrier mobility, and carrier concentration of the near-stoichiometric thin film were 14.4 Ω-cm, 3.27 cm2/V∙s, and 1.33 × 1017 cm−3, respectively. The optical band gap and absorption coefficient under the same conditions were 1.7 eV and 1.75 × 105 cm−1, which are acceptable for highly efficient thin-film solar cells.

A modeling study on utilizing ultra-thin inorganic HTLs in inverted p–n homojunction perovskite solar cells

Homojunction perovskite is used as a promising absorber layer in perovskite solar cells. In order to have p-n homojunction perovskite, we are able to induce n-type or p-type doping, thanks to controlling the stoichiometry of the perovskite precursors. In a p-n homojunction perovskite layer, the built-in electric field promotes oriented transport of the photo-induced carriers, thus reduces the carrier recombination losses. In this paper, by combining the modeling results of two basic structures, which are also compared with similar reported experimental data for their validation, an inverted structure with p-n homojunction MAPI perovskite and ultra-thin HTL film is suggested. Simulation results show that replacing the intrinsic MAPI perovskite with p-n homojunction MAPI perovskite leads to a 13% increase of efficiency in the inverted structure. Also, by investigating the various p-type inorganic materials, we propose CuAlO2 as a promising inorganic hole transport material, and its utilization for the inverted p-n homojunction perovskite solar cell delivers efficiency of 16.48%. We demonstrate that using the structure with homojunction perovskite is an effective approach, beyond existing planar structures, to achieve a highly efficient solar cell with reduced carrier recombination losses.

Climbing Jacob’s ladder: A density functional theory case study for Ag2ZnSnSe4 and Cu2ZnSnSe4

Recent advances in the development of exchange and correlation functionals to be employed in density functional theory calculations combined with the availability of ever more powerful high-performance computing facilities, let predictive computational materials science become reality. In order to assess the quality of calculated material properties, Jacob’s ladder provides an informal classification, where exchange and correlation functionals of similar capabilities are placed at the same rung of this ladder, while improved and more accurate ones are placed at higher rungs. Climbing Jacob’s ladder, i.e. employing more accurate exchange and correlation functionals, increases the quality of the results and the computational demands, and provides some guidance as to what accuracies and computational costs to expect from specific calculations. This is particular important for materials whose electronic ground state properties are incorrectly described, e.g. small band gap semiconductors, and materials, where system sizes for subsequent investigations, like defect properties or band offsets in heterostructures, become prohibitively large for more accurate exchange and correlation functionals.Here, we provide a systematic density functional theory study on the ground state properties of Ag2ZnSnSe4 and Cu2ZnSnSe4 for the lowest four rungs of Jacob’s ladder. Cu2ZnSnSe4, and in particular its alloys with Ag, is a promising candidate material for future thin-film solar cell absorber layers. In the present work, the obtained material properties are compared to available experimental data, allowing to benchmark the accuracy of the employed exchange and correlation functionals. We also provide a comparative study for subsequent quasiparticle calculations. Therein, the influence of differently obtained eigenvalues and orbitals as starting points are critically assessed with respect to available experimental data. Our results show that, structural properties based on the SCAN functional show overall best agreement with available experimental data, whereas additional hybrid functional calculations are necessary for satisfying results on electronic and optical properties.

Deducing transport properties of mobile vacancies from perovskite solar cell characteristics

The absorber layers in perovskite solar cells possess a high concentration of mobile ion vacancies. These vacancies undertake thermally activated hops between neighboring lattice sites. The mobile vacancy concentration N0 is much higher and the activation energy EA for ion hops is much lower than is seen in most other semiconductors due to the inherent softness of perovskite materials. The timescale at which the internal electric field changes due to ion motion is determined by the vacancy diffusion coefficient Dv and is similar to the timescale on which the external bias changes by a significant fraction of the open-circuit voltage at typical scan rates. Therefore, hysteresis is often observed in which the shape of the current–voltage, J–V, characteristic depends on the direction of the voltage sweep. There is also evidence that this defect migration plays a role in degradation. By employing a charge transport model of coupled ion-electron conduction in a perovskite solar cell, we show that EA for the ion species responsible for hysteresis can be obtained directly from measurements of the temperature variation of the scan-rate dependence of the short-circuit current and of the hysteresis factor H. This argument is validated by comparing EA deduced from measured J–V curves for four solar cell structures with density functional theory calculations. In two of these structures, the perovskite is MAPbI3, where MA is methylammonium, CH3NH3; the hole transport layer (HTL) is spiro (spiro-OMeTAD, 2,2′,7,7′- tetrakis[N,N-di(4-methoxyphenyl) amino]-9,9′-spirobifluorene) and the electron transport layer (ETL) is TiO2 or SnO2. For the third and fourth structures, the perovskite layer is FAPbI3, where FA is formamidinium, HC(NH2)2, or MAPbBr3, and in both cases, the HTL is spiro and the ETL is SnO2. For all four structures, the hole and electron extracting electrodes are Au and fluorine doped tin oxide, respectively. We also use our model to predict how the scan rate dependence of the power conversion efficiency varies with EA, N0, and parameters determining free charge recombination.

Benefits of direct electron detection and PCA for EELS investigation of organic photovoltaics materials

Electron energy-loss spectroscopy (EELS) is a powerful tool for imaging chemical variations at the nanoscale. Here, we investigate a polymer/organic small molecule-blend used as absorber layer in an organic solar cell and employ EELS for distinguishing polymer donor and small molecule acceptor domains in the nanostructured blend based on elemental maps of light elements, such as nitrogen, sulfur or fluorine. Especially for beam sensitive samples, the electron dose needs to be limited, therefore optimized acquisition and data processing strategies are required. We compare data acquired on a post-column energy filter with a direct electron detection camera to data from a conventional CCD camera on the same filter and we investigate the impact of statistical data processing methods (principal components analysis, PCA) on acquired spectra and elemental maps extracted from spectrum images. Our work shows, that the quality of spectra on a direct electron detection camera is far superior to conventional CCD imaging, and thereby allows clear identification of ionization edges and the fine structure of these edges. For the quality of the elemental maps, the application of PCA is essential to allow a clear separation between the donor and acceptor phase in the bulk heterojunction absorber layer of a non-fullerene organic solar cell.