Improvement Of Open-circuit Voltage Research Articles

Lead-free tin-halide perovskite solar cells with 13% efficiency

Despite the similarities of the optoelectronic properties between tin halide and lead halide perovskites, the performance of tin perovskite solar cells (PSCs) is still far below that of lead PSCs with highest reported efficiency to date of around 10%. This is due to the chemical instability of tin halide perovskite crystals and the energy band levels mismatch between the perovskite and charge transport layers. In this work, tin halide PSCs with efficiency of more than 13% has been fabricated by regulating the A site cation to achieve a tolerance factor of nearly 1. The partial substitution of formamidinium cation with ethylammonium cation affords a more stable tin perovskite crystal and at the same time suppresses the trap density by as much as 1 order of magnitude. Furthermore, the more favourable energy levels of the EA-substituted tin perovskites enhanced the charge extraction process into the charge transport layers and thus reducing the charge carrier recombination. This work provides a proof that tin-halide PSCs has the potential to compete with more toxic lead-based PSCs, and hopefully will pave the future direction for fabricating more efficient and stable lead-free PSCs. Simultaneous band energy alignment and trap site passivation has been achieved by regulating the A-site cation of tin halide perovskites. As a result of efficient charge extraction due to low lattice disorder, a high efficiency of 13.24% with a V OC of 0.84 V has been achieved for tin-based perovskite solar cells. • Tin perovskite with small lattice strain and tolerance factor close to 1 achieved with ethylammonium substitution. • Matching energy levels of perovskite and charge transport layers with optimal substitution of small ethylammonium. • Significant improvement in electron lifetime and open-circuit voltage obtained via effective trap passivation. • Record efficiency of 13.24 % for lead-free perovskite solar cell demonstrated with mixed cation and surface passivation.

Hybrid Lead-Halide Polyelectrolytes as Interfacial Electron Extraction Layers in Inverted Organic Solar Cells.

A series of lead-halide based hybrid polyelectrolytes was prepared and used as interfacial layers in organic solar cells (OSCs) to explore their effect on the energy band structures and performance of OSCs. Nonconjugated polyelectrolytes based on ethoxylated polyethylenimine (PEIE) complexed with PbX2 (I, Br, and Cl) were prepared as polymeric analogs of the perovskite semiconductors CH3NH3PbX3. The organic/inorganic hybrid composites were deposited onto Indium tin oxide (ITO) substrates by solution processing, and ultraviolet photoelectron spectroscopy (UPS) measurements confirmed that the polyelectrolytes allowed the work function of the substrates to be controlled. In addition, X-ray photoelectron spectroscopy (XPS) results showed that Pb(II) halide complexes were present in the thin film and that the Pb halide species did not bond covalently with the cationic polymer and confirmed the absence of additional chemical bonds. The composite ratio of organic and inorganic materials was optimized to improve the performance of OSCs. When PbBr2 was complexed with the PEIE material, the efficiency increased up to 3.567% via improvements in open circuit voltage and fill factor from the control device (0.3%). These results demonstrate that lead-halide based polyelectrolytes constitute hybrid interfacial layers which provide a novel route to control device characteristics via variation of the lead halide composition.

Hetero-epitaxial growth of Cu2ZnSn(S1−x,Sex)4-Au nanocomposites: Microstructures and corresponding impacts on optoelectronic properties

Coupling plasmonic gold nanoparticles to sub-stoichiometric copper chalcogenide nanocrystals tends to generate promising nano-heterostructures. We report the research on plasmon-enhanced Cu2ZnSn(S1−x,Sex)4-Au nanocomposites (CZTSSe for 0 < x < 1, and CZTS if x = 0), emphasizing on exploring how hetero-epitaxial relationship at Cu2ZnSn(S1−x,Sex)4//Au interfaces affects their optoelectronic properties. Digital image processing tools, fast Fourier transform/Fourier inversion, are utilized to extract interfacial epitaxial orientations. We demonstrate that nonnegligible lattice mismatch in short-range order could be equivalently compensated by coincident lattice-matching at certain periodic intervals in a long-range order. By exploring polytypic CZTSSe, we further provide reasonable interpretation of abnormal symmetrical distribution of Au on CZTSSe, which induces lower gold coverage rate and accordingly moderates their localized surface plasmon resonance effect, compared to that of CZTS. It indicates that the shape and phase of CZTSSe should be elaborately controlled to realize optimal multi-site nucleation of heterostructured nanocomposites. As a result, by testing optoelectronic response of solar cells with Cu2ZnSn(S1−x,Sex)4-Au films as counter electrodes (CEs), we confirm that a superior hetero-epitaxial interconnection between Au and Cu2ZnSn(S1−x,Sex)4 can availably restrain interfacial defects of the junction and thus contribute efficient hole transfer/transportation in cathode region. The corresponding balanced electron and hole migration plays a quite significant role in the improvement of open-circuit voltage (Voc) and fill factor of photovoltaic devices.

Influence of WSe2 buffer layer at back electrode on performance of Cu2ZnSn(S,Se)4 solar cells

CZTSSe-based solar cells with a structure of Al/ITO/ZnO/CdS/CZTSSe/p-WSe2/Mo (denoted as WSe2-CZTSSe solar cell) were prepared on the Mo-coated soda-lime glass (SLG) with a pre-sputtering metal tungsten (W) layer. Comparing with the conventional CZTSSe solar cell with a structure of Al/ITO/ZnO/CdS/CZTSSe/n-MoSe2/Mo (denoted as CZTSSe solar cell), the WSe2-CZTSSe solar cell shows a significant improvement in open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF), and so its power conversion efficiency (PCE) enhances. The largest enhancement of the PCE is from 4.13% of the CZTSSe solar cell to 5.45% of the WSe2-CZTSSe solar cell. The increased VOC and JSC are mainly due to the enhancement of photogenerated current density (JL) and decrease of reversion saturation current density (J0), while the increased FF is ascribed to the increase of shunt resistance (RSh). The influence mechanism of the WSe2 buffer layer on JL, J0 and RSh is also investigated in detail. Our results present a route of increment of PCE by reducing and even removing back contact barrier between CZTSSe and MoSe2 layers.

High performance two-dimensional perovskite solar cells based on solvent induced morphology control of perovskite layers

In this work, two-dimensional inverted perovskite solar cells (2D PSCs) were fabricated with dimethylformamide (DMF) and γ-butyrolactone (GBL) as precursor solvent, respectively. The GBL processed 2D perovskite film showed better quality with fewer pinholes and higher crystallinity than the DMF processed case. The GBL processed 2D PSCs showed an average power conversion efficiency (PCE) of 11.17%, which obviously exceeded that (9.81%) of DMF processed devices, with simultaneous improvements in open-circuit voltage, short-circuit current density. The best-performed sample exhibited a PCE of 11.65% with stabilized current output and negligible hysteresis.

Passivating silicon-based hybrid solar cells through tuning PbI2 content of perovskite coatings

Silicon-nanowire (SiNW) based hybrid solar cells generally suffered from severe defect trapping initiated from high density of dangling bonds at nanowire surfaces and thus, passivation process was highly demanded for suppressing the surface recombination of photogenerated carriers. In this study, the strategy was proposed for minimizing the possible charge recombination by using perovskite layer as passivation design while tuning the excess PbI2 amount as well as employing the antisolvent treatment. The results indicated the uniform incorporation of perovskite coatings possessing 5% of excess PbI2 prepared under chlorobenzene treatment with aligned SiNW arrays demonstrated the improved cell efficiency of 14.1%, approximately 1.4 times beyond the un-passivated solar cells. This was supported by the increased carrier lifetime as well as reduction of charge-transfer resistance that greatly benefited the improvement of open-circuit voltage and fill factor; in addition, the superior light absorption of hybrid perovskite/SiNW nanostructures was evidenced through experimental and simulated examinations that introduced the additional gain of short-circuit current density. Such hybrid design shed light on the realization of all-solution processed photovoltaic devices, and might further offer an innovative approach for other advanced optoelectronic applications.

WITHDRAWN: Numerical analysis on the improvement of open circuit voltage of kesterite based Cu2ZnSnS4 thin films solar cell

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Improving CNT-Si solar cells by metal chloride-to-oxide transformation

Transitional metal oxides (TMOs) are important functional materials in silicon-based and thin-film optoelectronics. Here, TMOs are applied in carbon nanotube (CNT)-Si solar cells by spin-coating solutions of metal chlorides that undergo favorable transformation in ambient conditions. An unconventional change in solar cell behavior is observed after coating two particular chlorides (MoCl5 and WCl6, respectively), characterized by an initial severe degradation followed by gradual recovery and then well surpassing the original performance. Detailed analysis reveals that the formation of corresponding oxides (MoO3 and WO3) enables two primary functions on both CNTs (p-type doping) and Si (inducing inversion layer), leading to significant improvement in open-circuit voltage and fill factor, with power conversion efficiencies up to 13.0% (MoO3) and 13.4% (WO3). Further combining with other chlorides to increase the short-circuit current, ultimate cells efficiencies achieve >16% with over 90% retention after 24 h, which are among the highest stable efficiencies reported for CNT-Si solar cells. The transformation of functional layers as demonstrated here has profound influence on the device characteristics, and represents a potential strategy in low-cost manufacturing of next-generation high efficiency photovoltaics.

Improving the performance of lead-acetate-based perovskite solar cells using solvent controlled crystallization process

Lead accetate is an important lead source for preparation of organolead trihalide perovskites. In this work, we improved the performance of lead accetate based perovskite solar cells (PSCs) using a mix-solvent of γ-butyrolactone/dimethyl-sulfoxide (GBL/DMSO). Compared with the use of N,N-dimethylformamide (DMF)/DMSO, using GBL/DMSO efficiently retarded the crystallization of perovskite, which improved the crystallinity of the perovskite films. Consequently, the perovskite films processed from GBL/DMSO showed higher light harvesting, longer carrier lifetime and suppressed charge recombination properties in the PSCs. Power conversion efficiency (PCE) of the PSCs was significantly improved from 16.6 to 17.7% with simultaneous improvement in open-circuit voltage, short-circuit current density and fill factor. The best PSC showed a champion PCE of 18.1%, with a stable power output and negligible hysteresis. Moreover, the flexible PSCs (based on polyethylene naphthalate substrates) with perovskite processed from GBL/DMSO exhibited a high PCE of 13.2%. Our results indicate that using GBL/DMSO as a mix-solvent is a simple and effective way for performance improvement of lead-acetate-based PSCs.

The study on saturation current and ideality factor of CdTe solar cell based on CdS window layer deposited with hydrogen peroxide

By adding hydrogen peroxide (H2O2) to the solution, a more compact cadmium sulfide (CdS) film with larger optical band gap is fabricated by chemical bath deposition method. The oxidation of CdS is confirmed by the formation of cadmium sulfate (CdSO4) in the CdS window layer via x-ray photoelectron spectroscopy investigation. The analysis of quantum efficiency considering the alloy CdS1−yTey shows that the oxygenated CdS suppress the S-Te interdiffusion, which promotes the spectral response in short-wave range. Moreover, this oxygenated CdS layer promoted the conversion efficiency of the cadmium telluride (CdTe) based solar cell from11.2% to 14.0% due to the improvement of open-circuit voltage and the fill factor. The transport mechanisms of saturation current are evaluated by temperature dependent current–voltage curves. Results show that the oxygenated window layer reduces both the interfacial and bulk recombination, thus improves the ideality factor. The decrease of bulk recombination in depletion region is attributed to the fewer trap level induced from S-Te interdiffusion. The decrease of interfacial recombination is attributed to the larger optical band gap of oxygenated CdS, which results in a spike-like conduction band alignment at CdS/CdTe heterojunction.

Improvement of open-circuit voltage of poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate)/α-CsPbI3 based device by mixing different concentrations of hydrochloric acid and hydroiodic acid additives

Abstract The open-circuit voltage (Voc) of all-inorganic perovskite solar cells (PSCs) based on poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS)/α-CsPbI3 architectures doped with hydroiodic acid (HI) is usually unsatisfactory. Herein, a sophisticated concentration ratio of HI and HCl additives were investigated to improve the Voc of the all-inorganic PSCs devices. The SEM and XRD results showed that perovskite thin films with fewer pinholes and higher crystallinity are fabricated. Furthermore, we found that the best Voc of 0.92 V and power conversion efficiency (PCE) of 5.69% were obtained with formed using a HCl volume percentage of 6 vol%, which is higher than α-CsPbI3 solar cells with pristine HI additives (0.87 V of Voc). It is believed that take a minute quantity HCl doping at least for high-quality inorganic perovskite thin films growth and improves the Voc of PEDOT:PSS/α-CsPbI3 PSCs device.

Compositional Control in 2D Perovskites with Alternating Cations in the Interlayer Space for Photovoltaics with Efficiency over 18.

2D perovskites stabilized by alternating cations in the interlayer space (ACI) represent a very new entry as highly efficient semiconductors for solar cells approaching 15% power conversion efficiency (PCE). However, further improvements will require understanding of the nature of the films, e.g., the thickness distribution and charge-transfer characteristics of ACI quantum wells (QWs), which are currently unknown. Here, efficient control of the film quality of ACI 2D perovskite (GA)(MA)n Pbn I3 n +1 (〈n〉 = 3) QWs via incorporation of methylammonium chloride as an additive is demonstrated. The morphological and optoelectronic characterizations unambiguously demonstrate that the additive enables a larger grain size, a smoother surface, and a gradient distribution of QW thickness, which lead to enhanced photocurrent transport/extraction through efficient charge transfer between low-n and high-n QWs and suppressed nonradiative charge recombination. Therefore, the additive-treated ACI perovskite film delivers a champion PCE of 18.48%, far higher than the pristine one (15.79%) due to significant improvements in open-circuit voltage and fill factor. This PCE also stands as the highest value for all reported 2D perovskite solar cells based on the ACI, Ruddlesden-Popper, and Dion-Jacobson families. These findings establish the fundamental guidelines for the compositional control of 2D perovskites for efficient photovoltaics.

Interfacial Engineering at the 2D/3D Heterojunction for High-Performance Perovskite Solar Cells.

Perovskite solar cells based on two-dimensional/three-dimensional (2D/3D) hierarchical structure have attracted significant attention in recent years due to their promising photovoltaic performance and stability. However, obtaining a detailed understanding of interfacial mechanism at the 2D/3D heterojunction, for example, the ligand-chemistry-dependent nature of the 2D/3D heterojunction and its influence on charge collection and the final photovoltaic outcome, is not yet fully developed. Here we demonstrate the underlying 3D phase templates growth of quantum wells (QWs) within a 2D capping layer, which is further influenced by the fluorination of spacers and compositional engineering in terms of thickness distribution and orientation. Better QW alignment and faster dynamics of charge transfer at the 2D/3D heterojunction result in higher charge mobility and lower charge recombination loss, largely explaining the significant improvements in charge collection and open-circuit voltage (VOC) in complete solar cells. As a result, 2D/3D solar cells with a power-conversion efficiency of 21.15% were achieved, significantly higher than the 3D counterpart (19.02%). This work provides key missing information on how interfacial engineering influences the desirable electronic properties of the 2D/3D hierarchical films and device performance via ligand chemistry and compositional engineering in the QW layer.

(Invited) Application of ALD-Grown Metal Oxide Layers for Energy Applications: Photovoltaic and Photoelectrochemical Water Splitting

ALD is a versatile growth technique for depositing a thin conformal layer of various functional materials. In this presentation, I will discuss two applications of ALD in energy materials research—photovoltaics (PV) and photoelectrochemical (PEC) water splitting. In the PV part, I will present how ALD-growth thin dielectric layers can be used to passivate both the top and bottom interfaces of Cu2ZnSnSe4 solar absorber, which has been drawing attention in the PV community as an earth-abundant, non-toxic and cheap solar material. The passivation strategy with the thin ALD-grown layers led to significant improvement in open-circuit voltage by suppressing charge recombination.[1] Another example of the ALD application in thin film PV is the replacement of CdS—the most common buffer layer in CIGS solar cells but with a toxic Cd—with ALD-ZnTiO.[2] I will present the application of ALD-ZnTiO whose materials properties were tuned In the PEC part, various metal oxide layers prepared by ALD were applied to Cu(In,Ga)Se2 (CIGS) photocathode as protection layers to prevent photo-corrosion of the CIGS in a liquid electrolyte.[3],[4] Different degradation mechanisms were identified for PEC devices with different ALD protective overlayers, details of which will be discussed. [1] Kim et al., Progress in Photovoltaics, 25 (2017), 4. [2] Hwang et al., ACS Applied Materials & Interfaces, 10 (2018), 20920. [3] Koo et al., ACS Applied Materials & Interfaces, 9 (2017), 5279. [4] Koo et al., Advanced Functional Materials, 28 (2018), 16.

Polyaniline nanotube/reduced graphene oxide aerogel as efficient counter electrode for quasi solid state dye sensitized solar cell

Polyaniline nanotube/reduced graphene oxide aerogel (PAniNT/rGOA) is studied as a potential counter electrode material for a quasi-solid-state dye-sensitized solar cell (DSSC) fabricated with poly(methyl methacrylate) based polymer gel electrolyte. The aerogel, prepared via an organic sol–gel route, having a high surface area of 294.73 m2 g−1 exhibits excellent electro-catalytic activity towards the reduction of triiodide ions in the electrolyte. The excellent catalytic activity of the aerogel based electrodes is manifested with cyclic voltammetry based analyses. Electrochemical impedance analyses and current density vs voltage plots are employed to gauze the photovoltaic performance of the DSSCs fabricated with PAniNT/rGOA counter electrode. The thickness of the counter electrode is a crucial factor that affects the performance of the DSSC: thicker films offer more surface area for electro-catalytic reduction reaction whilst increasing the charge and mass transport resistances simultaneously. At an optimum PAniNT/rGOA electrode film thickness of 8.68 µm, a power conversion efficiency of 5.47% is achieved. In addition, an improvement of open circuit voltage is observed due to the capacitance involved at the electrolyte/electrode interface.

Understanding the effect of N2200 on performance of J71: ITIC bulk heterojunction in ternary non-fullerene solar cells

Abstract None-fullerene solar cells with ternary architecture have attracted much attention because it is an effective approach for boosting the device power conversion efficiency. Here, the crystalline polymer N2200 as the third component is integrated into J71:ITIC bulk heterojunction. A series of characterizations indicate that N2200 could increase photo-harvesting, balanced hole and electron mobilities, enhanced exciton dissociation, and suppressed charge recombination, which result in the comprehensive improvement of open circuit voltage, short circuit current and fill factor in the device. Moreover, after introduction of N2200, the morphology of the ternary active layer is optimized, and the film crystallinity is improved. This work demonstrates that adding a small quantity of high crystallization acceptor into non-fullerene donor: acceptor mixture is a promising strategy toward developing high-performance organic solar cells.

Study on the defect density of states in light soaking effect enhanced performance of perovskite solar cells

The light soaking phenomenon, in chlorine-based mixed halide perovskite solar cells (PSC) with p-i-n configuration, is studied. Due to light soaking, the solar cells have shown improved open circuit voltage, fill factor (FF), and power conversion efficiency without much change in short circuit current. The devices have returned to their original state upon extended exposure to dark conditions. Capacitance spectroscopy is used to understand the effect of light soaking on various photovoltaic parameters. The observed increase of capacitance value with light soaking in the low frequency region of the capacitance-frequency (C–F) plot is elucidated, as the coupled effect of electronic and ionic processes involving defects. The decrement in the mid-frequency capacitance is attributed to an increase in the width of the depletion region which eventually facilitated the improvement in open circuit voltage and FF. Defect densities, and its distribution in the energy space, are deduced before and after light soaking. The defects density of states of PSC studied are found to reduce from 1016 to 1015 eV−1 cm−3, with a Gaussian distribution, and having a peak energy shift from 0.27 to 0.26 eV.

Synthesis of phthalocyanines with donor–acceptor properties and their applications to dye-sensitized solar cell

In this study, photovoltaic cell devices based on phthalocyanine structures with metal-free and zinc metal-containing D–π–A substitutes consisting of biphenyl and cyano-containing substituents have been synthesized. The non-symmetrical phthalocyanine photo-sensitizer has three biphenyls and one cyano group, respectively, which act as electron release and withdrawal. The new light-sensitive dye cells were tested in dye-sensitive solar cells and the performance was compared to N719 dye. All compounds synthesized were characterized using FTIR, 1 H NMR, C NMR, UV–Vis and MS spectroscopic data. First, the relationship between symmetric and non-symmetric ZnPc and H2Pc morphology and photovoltaic properties was discussed. The effects of symmetric and asymmetric cyano groups on solar cell efficiency were investigated. Furthermore, according to the results from the non-symmetrical ZnPc device prepared at room temperature, it was determined that it exhibited relatively higher photovoltaic cell efficiency due to significant improvements in short-circuit photo-current (Jsc) and open-circuit voltage (Voc).

Improved coverage of rGO film on Si inverted pyramidal microstructures for enhancing the photovoltaic of rGO/Si heterojunction solar cell

Solar cell fabricated by reduced graphene oxide (rGO)/textured silicon (T-Si) structure is feasible due to the capability to form a continuous film of graphene oxide (GO) on Si surface by direct drop-casting or spin coating. In this article, we have fabricated a uniform array of Si inverted pyramidal microstructures by anisotropic etching using a mixture of tetramethyl-ammonium hydroxide and isopropyl alcohol. Low average reflectance of 10.4% is obtained by T-Si structure as compared to that of 53.5% obtained by flat Si (F-Si) structure in UV–visible range. As a result, the front surface area of solar cell has been increased up to 43.2%. The dispersion of GO on Si surface textured with inverted pyramidal microstructures is performed by direct drop-casting of GO droplet or a combination of spin-coating and drop-casting on. The reduction of GO to form rGO is achieved by annealing in N2 at 400 °C. Direct drop-casting of GO droplet on T-Si results to the formation of suspended GO film, covering only the top of Si inverted pyramidal array. Due to the minimum contact area of rGO and T-Si after the reduction, higher diode ideality factor, η = 4.02 is obtained as compared to the reference rGO/F-Si diode with η = 3.99. A combination of spin-coating and drop-casting of GO droplet on T-Si helps in filling the GO sheets into the pyramidal structures, thus creating a large contact area with sidewalls of Si inverted pyramidal structures. As a result, the η value has been significantly reduced to 3.50. The power conversion efficiency also shows an increment due to the improvement of both short circuit current (Jsc) and open circuit voltage (Voc). Jsc and Voc have been increased from 0.010 mA/cm2 up to 0.081 mA/cm2, and from 96 mV up to 304 mV, respectively. Hence, the power conversion efficiency has been improved in several orders due to the improvement in both Jsc and Voc, which is from 2.0 × 10−4% up to 5.2 × 10−3%.

Enhanced efficiency of CdS/P3HT hybrid solar cells via interfacial modification

The present paper examines the effects of surface modification of CdS with diverse dyes on fabricated CdSbased hybrid solar cells. The X-ray diffraction results showed that CdS thin films had a hexagonal phase with a preferred orientation along the (101) plane. Scanning electron microscopy indicated that the CdS specimen was composed of a granular structure while a P3HT layer was formed from tiny grains. Band gaps of the CdS thin films and the P3HT layer were 2.45 eV and 1.98 eV, respectively. The absorption spectra showed that different dye loading caused an increase in the absorbance of CdS thin films in the wavelength range of 400-650 nm. The photoluminescence of the CdS/P3HT structure including various dyes was lower than that of the pristine one, implying that efficient charge separation was achieved upon surface modification. Current density-voltage curves showed that the ITO/CdS/N719/Ag hybrid solar cell exhibited the best overall efficiency of 0.082%, which can be attributed to improvements in both short circuit current density (Jsc) and open circuit voltage (Voc) . These enhancements can be attributed to the creation of better interfacial contact between CdS and P3HT layers after dye loading.