Effects of bendable P3CT polymers layer on the photovoltaic performance of perovskite solar cells.

We report on the formation of bendable and edge-on poly[3-(4-carboxybutyl)thiophene-2,5-diyl] (P3CT) polymers thin layer used as a hole modification layer (HML) in the inverted perovskite solar cell. The aggregations of 2D layer-like P3CT polymers in dimethylformamide (DMF) solution can be formed via aromaticπ-πstacking interactions and/or hydrogen-bonding interactions with the different concentration from 0.01 to 0.02 wt%, which highly influences the photovoltaic performance of the inverted perovskite solar cells. The atomic-force microscopic images and water droplet contact angle images show that the P3CT polymers modify the surface properties of the transparent conductive substrate and thereby dominating the formation of perovskite crystalline thin films, which play important roles in the highly efficient and stable perovskite solar cells. It is noted that theVOC(JSC) of the encapsulated solar cells values are maintained to be higher than 1.115 V (22 mA cm-2) after 104 d when an optimizedπ-πstacked and hydrogen-bonded P3CT polymer is used as the HML. On the other hand, the solar cell showed a high long-term stability by maintaining 85% of the initial power conversion efficiency in the ambient air for 103 d.

Effects of drying time on the formation of merged and soft MAPbI3 grains and their photovoltaic responses.

The grain sizes of soft CH3NH3PbI3 (MAPbI3) thin films and the atomic contact strength at the MAPbI3/P3CT-Na interface are manipulated by varying the drying time of the saturated MAPbI3 precursor solutions, which influences the device performance and lifespan of the resultant inverted perovskite photovoltaic cells. The atomic-force microscopy images, cross-sectional scanning electron microscopy images, photoluminescence spectra and absorbance spectra show that the increased short-circuit current density (J SC) and increased fill factor (FF) are mainly due to the formation of merged MAPbI3 grains. Besides, the open-circuit voltage (V OC) of the encapsulated photovoltaic cells largely increases from 1.01 V to 1.15 V, thereby increasing the power conversion efficiency from 17.89% to 19.55% after 30 days, which can be explained as due to the increased carrier density of the MAPbI3 crystalline thin film. It is noted that the use of the optimized drying time during the spin coating process results in the formation of merged MAPbI3 grains while keeping the contact quality at the MAPbI3/P3CT-Na interface, which boosts the device performance and lifespan of the resultant perovskite photovoltaic cells.

Dislocation characterization inc-plane GaN epitaxial layers on 6 inch Si wafer with a fast second-harmonic generation intensity mapping technique.

Second harmonic generation (SHG) intensity, Raman scattering stress, photoluminescence and reflected interference pattern are used to determine the distributions of threading dislocations (TDs) and horizontal dislocations (HDs) in thec-plane GaN epitaxial layers on 6 inch Si wafer which is a structure of high electron mobility transistor (HEMT). The Raman scattering spectra show that the TD and HD result in the tensile stress and compressive stress in the GaN epitaxial layers, respectively. Besides, the SHG intensity is confirmed that to be proportional to the stress value of GaN epitaxial layers, which explains the spatial distribution of SHG intensity for the first time. It is noted that the dislocation-mediated SHG intensity mapping image of the GaN epitaxial layers on 6 inch Si wafer can be obtained within 2 h, which can be used in the optimization of high-performance GaN based HEMTs.

Stable and efficient soft perovskite crystalline film based solar cells prepared with a facile encapsulation method.

The quasi Fermi level for electrons in a soft perovskite crystalline thin film and the contact qualities at the PCBM/perovskite and perovskite/P3CT-Na interfaces can be increased using a facile encapsulation method, which improves the device performance and stability of the resultant perovskite solar cells. In the encapsulated perovskite solar cells, the averaged open-circuit voltage (VOC) largely increases from 0.981 V to 1.090 V after 9 days mainly due to the increased quasi Fermi levels. Besides, the reflectance and photoluminescence (PL) spectra show improved contact qualities at the PCBM/perovskite and perovskite/P3CT-Na interfaces, which can be used to explain the increase in the short-circuit current density (JSC) from 21.68 mA cm-2 to 23.48 mA cm-2 after the encapsulation process. Besides, nanosecond time-resolved PL and temperature-dependent PL spectra can be used to explain the increased VOC, which is mainly due to the increased shallow defect density and thereby increasing the exciton binding energy of the encapsulated perovskite sample. It is noted that the averaged power conversion efficiency (PCE) slowly decreases from 18.24% to 16.52% within 45 days.

Improving the photovoltaic performance of inverted perovskite solar cells via manipulating the molecular packing structure of PCBM.

In this study, the molecular packing structure of solution-processed phenyl-C61-butyric acid methyl ester (PCBM) thin film was manipulated by varying the volume ratio of chlorobenzene (CB) to bromobenzene (BrB) from 100:0 to 50:50, which largely influences the device performance of the PCBM/perovskite heterojunction solar cells. Absorbance spectra, photoluminescence spectra, atomic force microscopic images and contact angle images were used to investigate the molecular packing structure effects of the PCBM thin films on the device performance of the inverted perovskite solar cells. Our experimental results show that the formation of PCBM aggregates and the contact quality at the PCBM/perovksite interface significantly influence the open-circuit voltage, short-circuit current density and fill factor of the resultant solar cells simultaneously. It is noted that the PCE of the encapsulated inverted CH3NH3PbI3(MAPbI3) solar cells exhibited a stable and high power conversion efficiency of 18%.

Improving device performance of MAPbI3photovoltaic cells by manipulating the crystal orientation of tetragonal perovskites.

The properties of CH3NH3PbI3(MAPbI3) crystalline thin films and the device performance of highly efficient MAPbI3photovoltaic cells are investigated by varying the temperature of the antisolvent from 20 °C to 50 °C during the washing enhanced nucleation (WEN) process. The surface, structural, optoelectronic and defect properties of the perovskite thin films are characterized through atomic-force microscopy, X-ray diffractometry and photoluminescence spectrometry. The experimental results show that changing the temperature of the antisolvent during the WEN process can manipulate the MAPbI3crystalline thin films from the (110)-(002) complex phase to a (002) preferred phase. It is noted that the highest power conversion efficient of the inverted MAPbI3photovoltaic cells is 19.30%, mainly due to the increased carrier collection efficiency and reduced carrier recombination when the temperature of the antisolvent is 30 °C.

Two-Dimensional Self-Assembly of Boric Acid-Functionalized Graphene Quantum Dots: Tunable and Superior Optical Properties for Efficient Eco-Friendly Luminescent Solar Concentrators.

Carbon-based nanomaterials hold promise for eco-friendly alternatives to heavy-metal-containing quantum dots (QDs) in optoelectronic applications. Here, boric acid-functionalized graphene quantum dots (B-GQDs) were prepared using bottom-up molecular fusion based on nitrated pyrenes and boric acid. Such B-GQDs with crystalline graphitic structures and hydrogen-bonding functionalities would be suitable model systems for unraveling the photoluminescence (PL) mechanism, while serving as versatile building blocks for supramolecular self-assembly. Unlike conventional GQDs with multiple emissive states, the B-GQDs exhibited excitation-wavelength-independent, vibronic-coupled excitonic emission. Interestingly, their PL spectra can be tuned without largely sacrificing the quantum yield (QY) due to two-dimensional self-assembly. In addition, such B-GQDs in a polystyrene matrix possessed an ultrahigh QY (∼90%) and large exciton binding energy (∼300 meV). Benefiting from broadband absorption, ultrahigh QY, and long-wavelength emission, efficient laminated luminescent solar concentrators (100 × 100 × 6.3 mm3) were fabricated, yielding a high power conversion efficiency (1.4%).

Improvement of interfacial contact for efficient PCBM/MAPbI3planar heterojunction solar cells with a binary antisolvent mixture treatment.

Atomic-force microscopic images, x-ray diffraction patterns, Urbach energies and photoluminescence quenching experiments show that the interfacial contact quality between the hydrophobic [6,6]-phenyl-C61-buttric acid methyl ester (PCBM) thin film and hydrophilic CH3NH3PbI3(MAPbI3) thin film can be effectively improved by using a binary antisolvent mixture (toluene:dichloromethane or chlorobenzene:dichloromethane) in the anti-solvent mixture-mediated nucleation process, which increases the averaged power conversion efficiency of the resultant PEDOT:PSS (P3CT-Na) thin film based MAPbI3solar cells from 13.18% (18.52%) to 13.80% (19.55%). Beside, the use of 10% dichloromethane (DCM) in the binary antisolvent mixture results in a nano-textured MAPbI3thin film with multicrystalline micrometer-sized grains and thereby increasing the short-circuit current density and fill factor (FF) of the resultant solar cells. It is noted that a remarkable FF of 80.33% is achieved, which can be used to explain the stable photovoltaic performance without additional encapsulations.

Efficiency improvement of P3CT-Na based MAPbI3solar cells with a simple wetting process.

The averaged power conversion efficiency of polyelectrolytes (P3CT-Na) based MAPbI3solar cells can be increased from 14.94% to 17.46% with a wetting method before the spin-coating process of MAPbI3precursor solutions. The effects of the wetting process on the surface, structural, optical and excitonic properties of MAPbI3thin films are investigated by using the atomic-force microscopic images, x-ray diffraction patterns, transmittance spectra, photoluminescence spectra and Raman scattering spectra. The experimental results show that the wetting process of MAPbI3precursor solution on top of the P3CT-Na/ITO/glass substrate can be used to manipulate the molecular packing structure of the P3CT-Na thin film, which determines the formation of MAPbI3thin films.

19% Efficient P3CT-Na Based MAPbI3 Solar Cells with a Simple Double-Filtering Process.

A high-efficiency inverted-type CH3NH3PbI3 (MAPbI3) solar cell was fabricated by using a ultrathin poly[3-(4-carboxybutyl)thiophene-2,5-diyl]-Na (P3CT-Na) film as the hole transport layer. The averaged power conversion efficiency (PCE) can be largely increased from 11.72 to 18.92% with a double-filtering process of the P3CT-Na solution mainly due to the increase in short-circuit current density (JSC) from 19.43 to 23.88 mA/cm2, which means that the molecular packing structure of P3CT-Na thin film can influence the formation of the MAPbI3 thin film and the contact quality at the MAPbI3/P3CT-Na interface. Zeta potentials, atomic-force microscopic images, absorbance spectra, photoluminescence spectra, X-ray diffraction patterns, and Raman scattering spectra are used to understand the improvement in the JSC. Besides, the light intensity-dependent and wavelength-dependent photovoltaic performance of the MAPbI3 solar cells shows that the P3CT-Na thin film is not only used as the hole transport layer but also plays an important role during the formation of a high-quality MAPbI3 thin film. It is noted that the PCE values of the best P3CT-Na based MAPbI3 solar cell are higher than 30% in the yellow-to-near infrared wavelength range under low light intensities. On the other hand, it is predicted that the double-filtering method can be readily used to increase the PCE of polymer based solar cells.

Bright and fast-response perovskite light-emitting diodes with an ICBA:modified-C60 nanocomposite electrical confinement layer.

Bright and fast-response CH3NH3PbBr3 perovskite light-emitting diodes (PeLEDs) are realized by using ICBA:modified C60 (MC60) nanocomposites as the hole blocking layer (HBL) and electron transport layer (ETL). The photoluminescence spectrum shows that the use of hydrophilic MC60 in the ETL helps the surface passivation of the perovskite layer. In addition, the photoelectron spectra and water-droplet contact angle images show that the use of the ICBA:MC60 nanocomposite ETL can simultaneously confine the electrons and holes in the perovskite layer, which boosts the injected electron-hole radiative recombination efficiency and thereby increases the electroluminescence from 1 cd m-2 to 2080 cd m-2 at 6 V when the ICBA:3,5OEC60 nanocomposite ETL is used. In addition, the operational frequency of the optimal PeLED is up to 1.5 MHz.

Origins of the s-shape characteristic in J-V curve of inverted-type perovskite solar cells.

Fullerene derivative thin films have been widely used in inverted-type perovskite solar cells as the electron transport layer (ETL) and hole blocking layer. However, the smooth contact at the interface between the hydrophobic [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and hydrophilic CH3NH3PbI3 (MAPbI3) thin film has not yet been completely understood. The contact at the PCBM/MAPbI3 interface strongly influences the photovoltaic performance. The photovoltaic devices were characterized by measuring the light intensity-dependent current density-voltage (J-V) curves and impedance spectra, which show that the contact at the PCBM/MAPbI3 interface simultaneously influences the shunt resistance (carrier recombination) and series resistance (interfacial contact). In addition, x-ray diffraction patterns, atomic force microscopic images, absorbance spectra and photoluminescence spectra were used to explore the contact at the PCBM/MAPbI3 interface. The experimental results show that the flat MAPbI3 thin film cannot be completely covered by a PCBM thin film and thereby results in the s-shape characteristic in the J-V curve of the resultant solar cells. The s-shaped J-V curve can be suppressed by increasing the crystallinity and surface roughness of the MAPbI3 thin film. With the use of an interface modification layer in between the PCBM thin film and Ag cathode, the power conversion efficiency of MAPbI3 solar cells can be increased from 10.50% to 13.71%.

The Way to Pursue Truly High-Performance Perovskite Solar Cells.

The power conversion efficiency (PCE) of single-junction solar cells was theoretically predicted to be limited by the Shockley-Queisser limit due to the intrinsic potential loss of the photo-excited electrons in the light absorbing materials. Up to now, the optimized GaAs solar cell has the highest PCE of 29.1%, which is close to the theoretical limit of ~33%. To pursue the perfect photovoltaic performance, it is necessary to extend the lifetimes of the photo-excited carriers (hot electrons and hot holes) and to collect the hot carriers without potential loss. Thanks to the long-lived hot carriers in perovskite crystal materials, it is possible to completely convert the photon energy to electrical power when the hot electrons and hot holes can freely transport in the quantized energy levels of the electron transport layer and hole transport layer, respectively. In order to achieve the ideal PCE, the interactions between photo-excited carriers and phonons in perovskite solar cells has to be completely understood.