Enhanced Power Conversion Efficiency Research Articles

Ion-migration and carrier-recombination inhibition by the cation-π interaction in planar perovskite solar cells

Abstract In organic-inorganic hybrid perovskite solar cells, migration of intrinsic ions (e.g., MA+, Pb2+, I−) have a significant impact on the current-voltage hysteresis and stability of devices. Here, N, N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine (NPB) was introduced into MAPbI3 perovskite layer to facilitate the stability of perovskite film and improved the efficiency of planar perovskite solar cells (PSCs). The results suggest that migration of intrinsic ions are inhibited effectively by cation-π interaction between NPB and MA+, and lead to reduce intrinsic defects in perovskite films, which is benefit for the stability of devices. Lewis basicity of NPB enhances the crystallization, passivates the perovskites films and addresses the issue of low electron extraction efficiency. Consequently, solar cells made using NPB modified MAPbI3 resulted hysteresis-free, enhanced power conversion efficiency of 19.22% with improved stability.

Additive Engineering to Grow Micron-Sized Grains for Stable High Efficiency Perovskite Solar Cells.

A high‐quality perovskite photoactive layer plays a crucial role in determining the device performance. An additive engineering strategy is introduced by utilizing different concentrations of N,1‐diiodoformamidine (DIFA) in the perovskite precursor solution to essentially achieve high‐quality monolayer‐like perovskite films with enhanced crystallinity, hydrophobic property, smooth surface, and grain size up to nearly 3 µm, leading to significantly reduced grain boundaries, trap densities, and thus diminished hysteresis in the resultant perovskite solar cells (PSCs). The optimized devices with 2% DIFA additive show the best device performance with a significantly enhanced power conversion efficiency (PCE) of 21.22%, as compared to the control devices with the highest PCE of 19.07%. 2% DIFA modified devices show better stability than the control ones. Overall, the introduction of DIFA additive is demonstrated to be a facile approach to obtain high‐efficiency, hysteresis‐less, and simultaneously stable PSCs.

Microwave-assisted Fe-doped ZnO nanoparticles for enhancement of silicon solar cell efficiency

We have investigated the performance of a silicon solar cell coated with ZnO nanoparticle layers doped with iron varying from (0–20%). We synthesized nanoparticles using a microwave method and fabricated thin film layers on silicon solar cells through a spin coating technique. The structure and morphology of nanoparticles were analyzed through transmission electron microscopy (TEM) and X-ray diffraction spectroscopy (XRD). The size of the particles decreased as iron concentration increased from 0 to 15%. Optical properties were analyzed through photoluminescence (PL) and absorption spectroscopy. PL spectra show a blueshift in the near band edge emission peaks for 0–15% doped samples and a redshift for the 20% doped sample. The band gap of Fe–ZnO nanoparticles (0–15%) was found to increase from 3.20 eV to 3.24 eV, and the bandgap decreased to 3.22 eV for 20% Fe–ZnO nanoparticles. Current-Voltage measurements confirm the enhancement in power conversion efficiency by 30% with iron doping in ZnO up to 15%. Efficiency increased with nanoparticle layers up to 180.29 nm in every sample and decreased with further increase in thickness. Enhancement in external quantum efficiency (EQE) was also observed with the increase in dopant concentration up to 15%. The optimum thickness of the ZnO nanoparticle layer was found to be approximately 180 nm for maximum enhancement in conversion efficiency.

Dual-functional cathode buffer layer for power conversion efficiency enhancement of bulk-heterojunction solar cells

Abstract Here, we report a luminescent cathode buffer layer (CBL) for power conversion efficiency (PCE) enhancement of organic solar cells (OSCs). ZnO doped with Aluminum (Al) and Europium (Eu) was cast as CBL by a solution-processed method. CBL serves a dual purpose by acting both as a spectral conversion and an electron transporting layer. The luminescent ZnO:Al,Eu CBL layer has broad absorption spanning the ultraviolet (300–400 nm) spectrum, contributing to absorption enhancement. Moreover, the emission of CBL layer overlaps with the absorption of poly [N -9′-heptadecanyl-2,7-carbazole- alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) polymer thus elevating the overall absorption of the active layer and improving the photocurrent. With this ZnO:Al,Eu CBL in the inverted device configuration (ITO/CBL/active layer/MoOx/Al), an enhanced power conversion efficiency (PCE) of 6.9% was obtained while the device with pristine ZnO as CBL showed PCE of 5.9%. A blend of PCDTBT donor and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) acceptor was used as an active layer in both the cases. In ZnO:Al,Eu CBL layer, Al doping improves the conductivity, while Eu doping significantly enhances the emission in the visible region by down-shifting the incoming solar UV light to the visible range which overlaps with the absorption of PCDTBT polymer resulting in energy transfer and improved overall device efficiency. The findings of the study show the significance of luminescent ZnO:Al,Eu nanoparticle CBL in enhancing the performance of organic solar cells.

Enhancing the power conversion efficiency of SrTiO3/CdS/Bi2S3quantum dot based solar cell using phosphor

The role of down-conversion phosphorescence in the enhancement of power conversion efficiency (PCE) of TiO2: SrTiO3/CdS/Bi2S3/ZnS: Cu heterostructure solar cell is reported here. ZnS:Cu is used as phosphor which converts a fraction of the ultra violet energy to visible range, increasing the light harvesting capacity. Also this acts as a passivation layer, to reduce the charge recombination. SrTiO3 nanoparticles with spherical morphology were synthesized by co-precipitation technique and CdS, Bi2S3 and ZnS:Cu were grown on the SrTiO3 by successive ionic layer adsorption and reaction (SILAR) method after optimizing the Cu concentration. The structural, morphological and composition were studied by XRD, SEM, TEM and XPS measurements. J-V measurements of TiO2:SrTiO3/CdS/Bi2S3/ZnS:Cu3 structure based solar cell are done by solar simulator under 1.5 AM and found to exhibit an enhancement in PCE to the tune of 55% than SrTiO3/CdS/Bi2S3 (pristine) based solar cell. Incident Photon Current Efficiency (IPCE) spectrum covered a spectral range of 400 to 900 nm for TiO2:SrTiO3/CdS/Bi2S3/ZnS:Cu3 cell with the maximum IPCE ~51%. Electrochemical impedance analysis is also carried out in the form of Nyquist and Bode phase plots for the fabricated solar cells which possess low values of Rs, Rct and longer life time for the charge carriers which is favorable for higher PCE.

Enhancing the JSC of P3HT-Based OSCs via a Thiophene-Fused Aromatic Heterocycle as a "π-Bridge" for A-π-D-π-A-Type Acceptors.

Poly(3-hexylthiophene) (P3HT)-based organic solar cells (OSCs) have been extensively studied due to unique advantages of P3HT such as roll-to-roll and large-area printing fabrication, but a poor short-circuit current density greatly limits the enhancement of power conversion efficiency (PCE). Herein, via the thiophene-fused aromatic heterocycle as a "π-bridge", two "A-π-D-π-A"-type acceptors have been designed and synthesized for P3HT-based OSCs. The aromaticity of the fused thiophene ring has effectively stabilized the quinoid population, thus strengthening the intramolecular charge transfer and further improving the current density. Owing to the weaker electron-withdrawing ability of the thiophene-fused benzotriazole unit in JC2 than the thiophene-fused benzothiadiazole unit in JC1, a blue-shifted absorption occurs for JC2 to show a better complementarity with P3HT to improve the light-harvesting efficiency and current density of the derived OSCs, and an uplifted lowest unoccupied molecular orbital energy level is also achieved for JC2 to obtain higher voltages. Thus, the P3HT:JC2-based device exhibits a PCE of 6.24% with a high JSC of 13.96 mA cm-2 and a VOC of 0.71 V, significantly exceeding those of the P3HT:JC1 device with a PCE of 2.80%, a JSC of 10.66 mA cm-2, and a VOC of 0.48 V. This indicates that the fusion of a thiophene ring onto a benzotriazole unit is an effective strategy to balance the VOC and JSC of P3HT-based OSCs to achieve excellent photovoltaic performances.

Reprint of “Oxygen-plasma processed Spiro-OMeTAD toward high performance for perovskite solar cell”

Efficient perovskite solar cell was achieved using a small hole transport material, Spiro-OMeTAD, after oxygen plasma treatment. It was demonstrated that the plasma treatment of hole transport layer for the solids state perovskite solar cell was enhanced power conversion efficiency. Reason for enhancing the cell performance of solar cell was good match of homo energy level between perovskite and Spiro-OMeTAD. It leads to transfer well hole carrier at Active/HTL interface. This study obtained highly improved performance by a current density of 25.4 mA/cm2, an open-circuit voltage of 1.02 V, and a fill factor of 60.2%, which resulted showed above 15% of cell efficiency.

Novel 2D material from AMQS-based defect engineering for efficient and stable organic solar cells

In this work, soluble two-dimensional (2D) material of antimonene quantum sheets (AMQSs) is used to form a bilayer hole extraction layer (HEL) with CuSCN. It proves that the application of AMQSs helps to passivate surface defects of CuSCN, resulting in diminished recombination loss and depressed exciton quenching effect, and thereby achieving improved photovoltaic performance. In OPVs based on poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]: [6,6]-phenyl C71-butyric acid methyl ester (PTB7-Th:PC71BM), over 12% enhancement of power conversion efficiency (PCE) is observed compared to that of the reference cell fabricated with pure CuSCN as an HEL. The advantage of the bilayer CuSCN/AMQS HEL is also confirmed in non-fullerene systems of PBDB-T-2F:IT-4F and PTB7-Th:ITIC. In a cell based on PBDB-T-2F:IT-4F, a PCE of 10.14% was obtained after application of AMQSs, which improved by about 10% compared to that of the reference cell using pure CuSCN as an HEL. Furthermore, cells based on CuSCN and CuSCN/AMQS HEL exhibit superior air stability. The use of a bilayer CuSCN/AMQS HEL provides a promising approach to obtain efficient and stable organic solar cells.

Facile non-pressurized synthesis of nanowire-constructed hierarchical TiO2 nanomaterials with improved dye sensitized solar cell performance

Nanowire-constructed hierarchical TiO2 nanomaterials (HTN) are prepared by a facile non-hydrothermal method at atmospheric pressure. The product is characterized in detail using powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV–Vis spectrophotometer. The synthesized TiO2 hierarchical nanomaterials consist of nanowires of 15 nm in diameter, it possesses a high surface area of 144.65 m2/g, and demonstrate an excellent dye absorption ability (155.55 nmol/cm2), which is 2.7 times more than that of the Degussa P25. Corresponding photovoltaic performance of dye-sensitized solar cells (DSSCs) fabricated from HTN is also investigated. It exhibits an overall light-to-electricity conversion efficiency of 8.9% accompanying a short-circuit current density of 20.43 mA/cm2, an open-circuit voltage of 720 mV and fill factor of 0.61, which is much higher than that of P25 (5.5%). The significant enhancement of short-circuit current density and power conversion efficiency of HTN can be attributed to its larger dye loading amount, better charge transportation ability and excellent light scattering ability.

Oxygen Plasma-Induced p-Type Doping Improves Performance and Stability of PbS Quantum Dot Solar Cells.

PbS quantum dots (QDs) have been extensively studied for photovoltaic applications, thanks to their facile and low-cost fabrication processing and interesting physical properties such as size dependent and tunable band gap. However, the performance of PbS QD-based solar cells is highly sensitive to the humidity level in the ambient air, which is a serious obstacle toward its practical applications. Although it has been previously revealed that oxygen doping of the hole transporting layer can mitigate the cause of this issue, the suggested methods to recover the device performance are time-consuming and relatively costly. Here, we report a low-power oxygen plasma treatment as a rapid and low-cost method to effectively recover the device performance and stability. Our optimization results show that a 10 min treatment is the best condition, resulting in an enhanced power conversion efficiency from 6.9% for the as-prepared device to 9% for the plasma treated one. Moreover, our modified device shows long-term shelf-life stability.

High Power UV-Light Irradiation as a New Method for Defect Passivation in Degraded Perovskite Solar Cells to Recover and Enhance the Performance

Although the power conversion efficiency (PCE) of perovskite solar cells (PSCs) reached up to 23%, their short lifetime and fast degradation still remain as the main challenges. In this work, a new facile optical method based on the high power UV-irradiation is presented for the recovery of the degraded PSCs. Addition to the full recovery of the performance, about 20% PCE enhancement and hystersis reduction are also achieved by UV-irradiation. UV-treatment causes modifications in both the bulk properties of the perovskite layer and the energy equilibrium at the interfaces. It is shown that UV-treatment effectively passivates the surface and grain boundaries defects in different types of the devices comprising normal and inverted configurations that is confirmed by the reduction of the density of defect states (DOS). It is proposed that UV-light passivates the shallow and deep defects by dissociation of adsorbed hydroxyl groups and water molecules during the device storage.

High-Efficiency Perovskite Solar Cell Based on Sequential Doping of PTAA

Perovskite solar cells have been extensively studied and have recently demonstrated a power conversion efficiency above 23%. In our present approach, we select poly(triaryl amine) (PTAA) as the hole-transport material and use an inverted solar cell planar structure. We propose a sequential method to dope the PTAA thin film in order to reduce the solar cell's series resistance. A modified inter-diffusion method is adopted to improve the perovskite film. We optimize the solvent-assisted annealing condition to maximize the perovskite grain size and reduce the pin-hole density. The impact of PTAA's doping concentration on the solar cell's PV performance is investigated. Compared to conventional PEDOT:PSS-based perovskite solar cell, we observe an improvement in both the open-circuit voltage and the short-circuit current, resulting in a power conversion efficiency enhancement from 12.7% to 15.3%.

Power Conversion Efficiency Enhancement of Low-Bandgap Mixed Pb–Sn Perovskite Solar Cells by Improved Interfacial Charge Transfer

The power conversion efficiency (PCE) of low-bandgap mixed Pb–Sn perovskite solar cells (PSCs) has been significantly hindered by large open-circuit voltage (Voc) loss and poor fill factor (FF). He...

A series of V-shaped small molecule non-fullerene electron acceptors for efficient bulk-heterojunction devices

Two simple semiconducting acceptor-acceptor1-donor-acceptor (A-A1-D-A) modular, small molecule non-fullerene electron acceptors, 2-(4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)-3-(6-((4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)ethynyl)-9-octyl-9H-carbazol-3-yl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (NDICz-5) and 2-(4-(3,3-dicyano-2-(4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)-1-(6-((4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)ethynyl)-9-octyl-9H-carbazol-3-yl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (NDICz-6), were designed, synthesized and characterized for application in solution-processable bulk-heterojunction solar cells. The optoelectronic and photovoltaic properties of NDICz-5 and NDICz-6 were directly compared with those of a structural analogue, 7,7'-(((9-octyl-9H-carbazole-3,6-diyl)bis(ethyne-2,1-diyl))bis(4,1-phenylene))bis(2-hexadecylbenzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone) (NDICz-4), which was designed based on an A-D-A format. All of these new materials were designed to be V-shaped and comprised an electron rich carbazole donor core (D) together with electron deficient naphthalene diimide terminal core (A). In the simple A-D-A system, naphthalene diimide was the terminal functionality, whereas in the A-A1-D-A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) derived functionalities were incorporated as A1 units by keeping D/A units constant. The inclusion of A1 was mainly done to induce cross-conjugation within the molecular backbone and hence to improve light-harvesting. The physical and optoelectronic properties were characterized by ultraviolet–visible, thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry. These new materials exhibited energy levels complementing those of the conventional donor polymer poly(3-hexyl thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with NDICz-4, NDICz-5 and NDICz-6 as non-fullerene electron acceptors. Studies on the photovoltaic properties revealed that the best P3HT: NDI-Cz6-based device showed an impressive enhanced power conversion efficiency of 7.58%, an increase of around two-fold with respect to as-cast blend, after solvent vapor annealing using carbon disulfide. Not only are the results among the best in the current literature using the conventional donor polymer P3HT, but clearly demonstrate that A-A1-D-A type, V-shaped small molecules are promising non-fullerene acceptors in the research field of organic solar cells.

Optimization of Light Management Layers for Light Harvest of Perovskite Solar Cells

Recently, with rapid development of perovskite solar cells, the record efficiency has exceeded 24.2%. However, the research of efficiency improvement never stops. This work introduces an effective approach to remarkably reduce the device reflection loss by using light management (LM) cover layer. A specific model combined with ray and wave optics was built to numerically optimize the texture of the LM layers for light harvesting in photovoltaics cells and explore the optical principles in the light trapping. The LM layer texture is made of 2D periodic triangle structures. Both symmetric and asymmetric triangle structures were investigated. The results indicate that asymmetric structures possess the close performance compared with symmetric ones, and notably, on contrary to the symmetric structures, the light trapping abilities are much more insensitive to the bottom thicknesses of asymmetric structures. It will benefit the practical fabrication, since the precise bottom thickness control can be avoided. From the optimization results, 3 optimal structures are found. With the optimal LM layers, a thin active layer (165nm) device is able to produce the short circuit current intensities (Jsc) of around 21mA/cm2, which is approximately equivalent to the Jsc of a thick active layer (800nm) device without LM layers. Meanwhile, about 23% power conversion efficiency (PCE) enhancement is achieved. On the other hand, active layer thickness also makes influence on the light trapping abilities. As the active layer thickness is increased from 165nm to 800nm, the Jsc only improves by about 2.4mA/cm2, while the PCE enhancement is dropped from ~23% to ~11%. In consideration of the compromise between total device energy output and PCE enhancement, as well as material cost reduction, the active layer with around 165nm should be an optimal thickness.

Solution-processed bathocuproine cathode buffer layer towards efficient planar heterojunction perovskite solar cells

Solution process boosts organic and hybrid organic–inorganic electronic devices towards low-cost, scalable and high-throughput production. Facilely solution-processed bathocuproine (sBCP) is innovatively introduced as an effective cathode buffer layer (CBL) sandwiched between the electron transport layer of (6, 6)-phenyl-C61-butyric acid methyl ester (PCBM) and Ag electrode in perovskite solar cells (PSCs) with structure of ‘indium-tin-oxide/poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate)/CH3NH3PbI3/PCBM/sBCP/Ag’. The effect of sBCP concentration on PSC performance is intensively investigated. Experiments show that sBCP with optimum concentration extremely favors filling voids at the perovskite/PCBM film, which improves surface morphology and ameliorates interface characteristics of PCBM/Ag. It also contributes to the enhancement of power conversion efficiency, owing to the promotion in short circuit current density and fill factor. Impressive power conversion efficiencies of 12.47% average value and 13.24% maximum value with about 60% increment in relative to the reference device without CBL are achieved. Moreover, the incorporation of sBCP effectively blocks diffusion of Ag atoms inside the PSCs, which reasonably accounts for long-term stability of the PSCs. Our results open up an alternative approach for constructing high-performance PSCs with solution process and advancing their applications.

Picosecond timescale tracking of pentacene triplet excitons with chemical sensitivity

Singlet fission is a photophysical process in which an optically excited singlet exciton is converted into two triplet excitons. Singlet fission sensitized solar cells are expected to display a greatly enhanced power conversion efficiency compared to conventional single-junction cells, but the efficient design of such devices relies on the selection of materials capable of harvesting triplets generated in the fission chromophore. To this aim, the possibility of measuring triplet exciton dynamics with chemical selectivity paves the way for the rational design of complex heterojunctions, with optimized triplet conversion. Here we exploit the chemical sensitivity of X-ray absorption spectroscopy to track triplet exciton dynamics at the picosecond timescale in multilayer films of pentacene, the archetypal singlet fission material. We experimentally identify the signature of the triplet exciton in the Carbon K-edge absorption spectrum and measure its lifetime of about 300 ps. Our results are supported by state-of-the-art ab initio calculations.

Bilayer polymer solar cells prepared with transfer printing of active layers from controlled swelling/de-swelling of PDMS

Herein, we demonstrate a facile method for transferring thin films to achieve polymer solar cells having stacked structures. By controlling the swelling/de-swelling properties of Polydimethylsiloxane (PDMS) via solvent treatment, we formed uniform organic films upon the PDMS surface and then transferred them to target substrates. We prepared bilayer and graded bilayer structures after transferring indene-C60 bis-adduct (ICBA) and ratio-controlled poly(3-hexylthiophene) (P3HT:ICBA) blends, respectively, onto the P3HT layer. The optimal graded bilayer solar cell exhibited a power conversion efficiency (PCE) of 5.13% an impressive value compared with that obtained for the corresponding bilayer cell (3.67%). We attribute this enhancement in PCE to the greater number of junction interfaces and the balanced carrier transfer properties. This residue-free and place-lift-off transferring method appears to have great promise in the solution processing of multilayer stacked thin film optoelectronics.

Enhanced power conversion efficiency and preferential orientation of the MAPbI3 perovskite solar cells by introduction of urea as additive

Abstract In this investigation, urea (NH2CONH2) was added into the perovskite precursor solution with Pb(CH3COO)3 as precursor. The effects of the urea content in the solution on the crystallization dynamics and the orientation of the perovskite films, and their photovoltaic properties were investigated. The results showed that the optimal annealing duration for the perovskite film increased with the urea content in the solution. An interaction between the urea and the source materials was observed. As a result, urea addition in the Pb(CH3COO)3 based solution has an effect on the crystallization rate and the grain size of the perovskite phase. We found that the suitable amount of urea in the solution was beneficial for the formation of highly (110)-oriented perovskite films and this resulted in the corresponding solar cells with enhanced power conversion efficiency of nearly 17%.

Defect passivation of CsPbI2Br perovskites through Zn(II) doping: toward efficient and stable solar cells

Defect passivation is an important strategy to achieve perovskite solar cells (PVSCs) with enhanced power conversion efficiencies (PCEs) and improved stability because the trap states induced by defects in the interfaces and grain boundaries of perovskites are harmful to both large open circuit voltage and high photocurrent of devices. Here, zinc cations (Zn2+) were used as a dopant to passivate defects of the CsPbI2Br perovskite leading to Zn2+-doped CsPbI2Br film with fewer trap states, improved charge transportation, and enhanced light-harvesting ability. Thus, the best-performance PVSC based on CsPbI2Br with the optimal Zn2+ doping shows a higher PCE of 12.16% with a larger open-circuit voltage ( V OC) of 1.236 V, an improved short-circuit current ( J SC) of 15.61 mA cm−2 in comparison with the control device based on the pure CsPbI2Br which exhibits a PCE of 10.21% with a V OC of 1.123 V, a J SC of 13.27 mA cm−2. Time-resolved photoluminescence results show that the Zn2+ doping leads to perovskite film with extended photoluminescence lifetime which means a longer diffusion length and subsequently enhanced photocurrent and open circuit voltage. This work provides a simple strategy to boost the performance of PVSCs through Zn2+ doping.