Transient Photovoltage Decay Measurements Research Articles

The influence of different mask aperture on the open-circuit voltage measurement of perovskite solar cells

We studied the influence of different mask apertures on the open-circuit voltage measurement of perovskite solar cells. Aperture masks with different sizes were equipped to control the proportion of dark-state region. Statistical results of photovoltaic parameters, ideality factor, and reverse saturation current density revealed that different mask apertures would affect the J-V measurement results, especially the open-circuit voltage. A double diode model simulation together with the impedance spectroscopy and transient photovoltage decay measurement has been further employed to analyze the reasons and found that the mask with inappropriate shading, which is smaller than the cells active area, will introduce excess charge transfer and recombination pathways, thus lead to an underestimate of open-circuit voltage. This work provides a promising route towards a more accurate J-V measurement of perovskite solar cells.

Surface plasma resonance enhanced photocurrent generation in NiO photoanode based solar cells

Abstract Surface plasma resonance (SPR) effect has been demonstrated to improve solar cell performance. This work reports on the SPR effect from Au nanorod@SiO 2 on p-type dye-sensitized solar cells. Au nanorod@SiO 2 works as an antenna to transform photons with long wavelength into electric field followed by an enhanced excitation of dye. The devices using the NiO electrode containing Au nanorod@SiO 2 shows overall power conversion efficiencies of about 0.2% in combination with I − /I 3 − electrolyte, and 0.29% with T − /T 2 electrolyte, which are superior to those without adding Au nanorods. Detailed investigation including spectroscopy and transient photovoltage decay measurements reveals that plasma effect of Au nanorod@SiO 2 contribute to charge injection efficiency, and thus on the photocurrent. The effect of Au NRs can be further extended to the inverted planar perovskite solar cells, showing obviously improvement in photocurrent.

Alkyl-thiophene Functionalized D-π-A Porphyrins for Mesoscopic Solar Cells

An alkyl-thiophene functionalized D-π-A porphyrin (LW16) is designed and synthesized for dye-sensitized solar cells (DSSCs). Two hexyl-thiophene groups are attached to the meta-position of each meso-phenyl with a motivation to increase the light-harvesting ability as well as retarding the aggregation of porphyrins dyes. For comparison, none-alkyl substituted (LW14) and octyloxy substituted (LW15) porphyrin dyes are also synthesized to fully investigate the influence of porphyrin chromophore modification. These porphyrins present similar spectrum while the oxidation potentials vary as the functionalized group changes from the meta-position to ortho-position. The DSSCs based on the alkyl-thiophene functionalized (LW16), none-alkyl substituted (LW14), and octyloxy substituted (LW15) porphyrins can be achieved a power conversion efficiency of 8.5%, 6.9%, and 8.2% using I–/I3–redox electrolyte under full sunlight irradiation (AM 1.5G, 100mWcm−2), respectively. It is found that by tailoring the porphyrin chromophore with hexyl-thiophenes, the photocurrent of the corresponding devices could be increased without sacrifice the photovoltage. Detailed investigation, including spectroscopy, electrochemical and transient photovoltage decay measurement, provides general influence of π-conjugation extension at the meso-position onto the optoelectronic features of porphyrins dyes.

Integrated Design of Organic Hole Transport Materials for Efficient Solid‐State Dye‐Sensitized Solar Cells

A series of triphenylamine‐based small molecule organic hole transport materials (HTMs) with low crystallinity and high hole mobility are systematically investigated in solid‐state dye‐sensitized solar cells (ssDSCs). By using the organic dye LEG4 as a photosensitizer, devices with X3 and X35 as the HTMs exhibit desirable power conversion efficiencies (PCEs) of 5.8% and 5.5%, respectively. These values are slightly higher than the PCE of 5.4% obtained by using the state‐of‐the‐art HTM Spiro‐OMeTAD. Meanwhile, transient photovoltage decay measurement is used to gain insight into the complex influences of the HTMs on the performance of devices. The results demonstrate that smaller HTMs induce faster electron recombination in the devices and suggest that the size of a HTM plays a crucial role in device performance, which is reported for the first time.

Fine tuning of fluorene-based dye structures for high-efficiency p-type dye-sensitized solar cells.

We report on an experimental study of three organic push-pull dyes (coded as zzx-op1, zzx-op1-2, and zzx-op1-3) featuring one, two, and three fluorene units as spacers between donors and acceptors for p-type dye-sensitized solar cells (p-DSSC). The results show increasing the number of spacer units leads to obvious increases of the absorption intensity between 300 nm and 420 nm, a subtle increase in hole driving force, and almost the same hole injection rate from dyes to NiO nanoparticles. Under optimized conditions, the zzx-op1-2 dye with two fluorene spacer units outperforms other two dyes in p-DSSC. It exhibits an unprecedented photocurrent density of 7.57 mA cm(-2) under full sun illumination (simulated AM 1.5G light illumination, 100 mW cm(-2)) when the I(-)/I3(-) redox couple and commercial NiO nanoparticles were used as an electrolyte and a semiconductor, respectively. The cells exhibited excellent long-term stability. Theoretical calculations, impedance spectroscopy, and transient photovoltage decay measurements reveal that the zzx-op1-2 exhibits lower photocurrent losses, longer hole lifetime, and higher photogenerated hole density than zzx-op1 and zzx-op1-3. A dye packing model was proposed to reveal the impact of dye aggregation on the overall photovoltaic performance. Our results suggest that the structural engineering of organic dyes is important to enhance the photovoltaic performance of p-DSSC.

Effect of Extended π-Conjugation of the Donor Structure of Organic D–A−π–A Dyes on the Photovoltaic Performance of Dye-Sensitized Solar Cells

Two new D–A−π-spacer–A organic dyes, KM-10 and KM-11, containing a benzothiadiazole unit in a π-spacer and a cyanoacrylic acid as an acceptor have been synthesized and tested as sensitizers in dye-sensitized solar cells. Structural variations of the donor moiety, i.e., π-extension of the diphenylamine electron-donating groups, gave rise to different photovoltaic efficiencies—7.1% for KM-10 and 8% for KM-11—despite having comparable absorption properties. A detailed investigation, including transient photocurrent and photovoltage decay measurement, transient absorption spectroscopy, and quantum chemical methods, provided important conclusions about the nature of the substitution on the photovoltaic properties of dyes.

Modulated Charge Injection in p-Type Dye-Sensitized Solar Cells Using Fluorene-Based Light Absorbers

In this study, new pull-push arylamine-fluorene based organic dyes zzx-op1, zzx-op2, and zzx-op3 have been designed and synthesized for p-type dye-sensitized solar cells (p-DSCs). In zzx-op1, a di(p-carboxyphenyl)amine (DCPA) was used as an electron donor, a perylenemonoimide (PMID) as an electron acceptor, and a fluorene (FLU) unit with two aliphatic hexyl chains as a π-conjugated linker. In zzx-op2 and zzx-op3, a 3,4-ethylenedioxythiophene (EDOT) and a thiophene were inserted consecutively between PMID and FLU to tune the energy levels of the frontier molecular orbitals of the dyes. The structural modification broadened the spectral coverage from an onset of 700 nm for zzx-op1 to 750 nm for zzx-op3. The electron-rich EDOT and thiophene lifted up the HOMO (highest occupied molecular orbital) levels of zzx-op2 and zzx-op3, making their potential more negative than zzx-op1. When three dyes were employed in p-type DSCs with I(-)/I3(-) as a redox couple and NiO nanoparticles as hole materials, zzx-op1 exhibited impressive energy conversion efficiency of 0.184% with the open-circuit voltage (VOC) of 112 mV and the short-circuit current density (JSC) of 4.36 mA cm(-2) under AM 1.5G condition. Density functional theory calculations, transient photovoltage decay measurements, and electrochemical impedance spectroscopic studies revealed that zzx-op1 sensitized solar cell exhibited much higher charge injection efficiency (90.3%) than zzx-op2 (53.9%) and zzx-op3 (39.0%), indicating a trade-off between spectral broadening and electron injection driving force in p-type DSCs.

Adapting Ruthenium Sensitizers to Cobalt Electrolyte Systems.

In this work, we report the use of bulky substitutions in a new heteroleptic ruthenium(II) bipyridine complex, Ru(NCS)2LL', coded TT-230 to obtain high open-circuit potential in a dye-sensitized solar cell (where L is a bipyridine ligand appended with two cyclopenta(2,1-b;3,4-bA)dithiophene moieties, and L' = 4,4,'-dicarboxylic acid 2,2'-bipyridine). The electrolytes based on cobalt complexes have shown significant advantages in terms of attainable open-circuit potential compared to the standard iodide/tri-iodide redox mediators. These merits of the cobalt complexes were previously realized with a porphyrin sensitizer, achieving a VOC greater than 1 V in DSC. However, with conventional Ru(II)-polypyridyl complexes such as the C101 dye, similar increase in the VOC could not be attained due to the enhanced recombination. In this work, we have shown that the use of bulky substituents can prevent the back reaction of photogenerated electron and subsequently increase the open-circuit potential of the device. The recombination processes were investigated by transient photovoltage decay measurements.

A comparative study on properties of two phenoxazine-based dyes for dye-sensitized solar cells

Two phenoxazine (POZ)-based organic D–π–A sensitizers (POZ-1 and POZ-2) were synthesized. Then these two dyes were applied in dye-sensitized solar cells (DSSCs) to investigate the influence of different conjugated direction, extra phenyl ring and alkyl chain on the light-harvesting, energy level and photovoltaic properties through a joint spectral, electrochemical and photovoltaic study. The result shows that dye POZ-2 exhibits higher power conversion efficiency (η = 6.5%) than dye POZ-1 (η = 2.4%) under standard illumination (Global Air Mass 1.5). Besides, the geometries of the dyes were optimized to gain insight into the molecular structure and electron distribution. The charge extraction and transient photovoltage decay measurements were further performed to understand the alterative order of efficiency.

Wide bandgap Zn2GeO4 nanowires as photoanode for quantum dot sensitized solar cells

Zn2GeO4 nanowires were directly synthesized on fluorine-doped-tin-oxide substrates and were utilized as the photoanode in quantum dot sensitized solar cells (QDSSCs). CdSe QDs were deposited on Zn2GeO4 nanowires using successive ion layer adsorption and reaction (SILAR), and absorption measurement was performed to inspect their bandgaps and corresponding band alignment with the Zn2GeO4 nanowires. The performance of the QDSSCs was characterized, and a relatively high VOC of 0.64 V was observed. Transient photocurrent and photovoltage decay measurements were also conducted to study carrier transport and recombination processes in the cells, and the obtained transport and recombination time constants and charge collection efficiency exhibited dependence on the number of SILAR coating cycles.

Phenoxazine-based organic dyes with different chromophores for dye-sensitized solar cells

A series of organic dyes (POZ-2, POZ-3, POZ-4 and POZ-5) involving phenoxazine were synthesized as sensitizers for application in dye-sensitized solar cells (DSSCs). For comparison, three different electron donors namely 10-phenyl-10H-phe-nothiazine, 10-phenyl-10H-phenoxazine and triphenylamine were separately appended onto the 7-position of the model dye (POZ-2). The obtained four dyes exhibit considerably high values of conversion efficiencies of 6.6%, 7.8%, 7.1% and 6.4%, respectively, under the simulated AM1.5G conditions. The geometries of the dyes were optimized to gain insight into the molecular structure and electron distribution, and then the charge extraction and transient photovoltage decay measurements were further performed to understand the influence of electron donors on the photovoltaic behaviors.

Preventing Interfacial Recombination in Colloidal Quantum Dot Solar Cells by Doping the Metal Oxide

Recent research has pushed the efficiency of colloidal quantum dot solar cells toward a level that spurs commercial interest. Quantum dot/metal oxide bilayers form the most efficient colloidal quantum dot solar cells, and most studies have advanced the understanding of the quantum dot component. We study the interfacial recombination process in depleted heterojunction colloidal quantum dot (QD) solar cells formed with ZnO as the oxide by varying (i) the carrier concentration of the ZnO layer and (ii) the density of intragap recombination sites in the QD layer. We find that the open-circuit voltage and efficiency of PbS QD/ZnO devices can be improved by 50% upon doping of the ZnO with nitrogen to reduce its carrier concentration. In contrast, doping the ZnO did not change the performance of PbSe QD/ZnO solar cells. We use X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, transient photovoltage decay measurements, transient absorption spectroscopy, and intensity-dependent photocurrent measurements to investigate the origin of this effect. We find a significant density of intragap states within the band gap of the PbS quantum dots. These states facilitate recombination at the PbS/ZnO interface, which can be suppressed by reducing the density of occupied states in the ZnO. For the PbSe QD/ZnO solar cells, where fewer intragap states are observed in the quantum dots, the interfacial recombination channel does not limit device performance. Our study sheds light on the mechanisms of interfacial recombination in colloidal quantum dot solar cells and emphasizes the influence of quantum dot intragap states and metal oxide properties on this loss pathway.

Synthesis and characterization of conjugated polymers with main-chain donors and pendent acceptors for dye-sensitized solar cells

Herein, we report three conjugated polymers with main-chain donors and pendent π-acceptors as photosensitizers for dye-sensitized solar cells (DSSCs). The polymer PPTZF (which contains a phenothiazine (PTZ) chromophore and has a 9,9-dioctyl-fluorene (F) group as the donor unit in the main chain and a cyanoacrylic acid acceptor in the side chain, linked by a thiophene unit) was used as a basic structure. Then 9,9-dioctyl-carbazole (CZ) and triphenylamine (TPA) were selected to replace F and PTZ, respectively, as structural modifications to get another two polymers: PPTZCZ and PTPACZ. Finally, we fabricated devices and examined the effects of different donors with flexible alkyl chains on the photovoltaic performance of DSSCs. Our studies have revealed that the PTPACZ dye, which has a degree of polymerization (DP) of ∼4, exhibited excellent photovoltaic performance with a relatively high power conversion efficiency of 4.4% under standard (Global Air Mass 1.5) illumination. Charge extraction and transient photovoltage decay measurements have demonstrated that the charge recombination lifetime (τ) is a crucial influencing factor on the open-circuit voltage of these polymer dyes.

Coupled analysis of steady-state and dynamic characteristics of dye-sensitized solar cells for determination of conduction band movement and recombination parameters

A new research strategy for determining the conduction band movement of TiO(2) films and charge recombination between electrons in the TiO(2) film and electron acceptors in the electrolyte was proposed. Steady-state short-circuit current density versus open-circuit voltage was employed to attain the exchange current density and recombination reaction order. Transient photovoltage decay and open-circuit voltage decay measurements were carried out to obtain the energetic distribution of trapped electrons. Reduced voltage-dependent trapped electron concentration and trapped electron concentration-dependent recombination current density were used to analyze influence factors of open-circuit voltage, including contributions from conduction band movement and charge recombination. The simulated and measured electron concentration were in agreement and confirmed the validity of this method for extracting conduction band movement and recombination parameters. This new approach provides a physical insight which could help us to more conveniently and efficiently understand the operation of DSCs.

TiO2 Film Morphology, Electron Transport and Electron Lifetime in Ultra-fast Sintered Dye-sensitized Solar Cells

ABSTRACTWith the application of near-infrared radiation (NIR), TiO2 films for dye-sensitized solar cells (DSCs) on metallic substrates can be sintered in just 12.5 s. The photovoltaic performance of devices made with NIR sintered films match those devices made with conventionally sintered films prepared by heating for 1800 s. Here we characterise the electron transport, electron lifetime and phase-morphological properties of ultrafast NIR sintered films, using impedance spectroscopy, transient photovoltage decay and x-ray diffraction measurements. An important factor in NIR processing of TiO2 films is the peak metal temperature (PMT) and we show that during the 12.5 second heat treatment that a PMT of around 635 °C gives near identical electron transport, electron lifetime and morphological properties, as well comparable photovoltaic performance to a conventionally sintered (500 °C, 30 mins) film. What is perhaps most interesting is that the rapid heating of the TiO2 (to temperatures of up to 785°C) does not lead to a large scale rutile phase transition. As such photovoltaic performance of resultant DSC devices is maintained even though the TiO2 has been at temperatures which traditionally would have reduced cell photocurrents via anatase-to-rutile phase transition.

Ultra-fast sintered TiO2films in dye-sensitized solar cells: phase variation, electron transport and recombination

With the application of near-infrared radiation (NIR), TiO2 films for dye-sensitized solar cells (DSCs) on metallic substrates can be sintered in just 12.5 seconds. The photovoltaic performance of devices made with NIR sintered films match those devices made with conventionally sintered films prepared by heating for 1800 seconds. Here we characterise the electron transport, electron lifetime and phase-morphological properties of ultrafast NIR sintered films, using impedance spectroscopy, transient photovoltage decay and X-ray diffraction measurements. An important factor in NIR processing of TiO2 films is the peak metal temperature (PMT) and we show that during the 12.5 second heat treatment, a PMT of around 635 °C gives near identical electron transport, electron lifetime and morphological properties, as well comparable photovoltaic performance to a conventionally sintered (500 °C, 30 min) film. We demonstrate that the rapid heating of the TiO2 (to temperatures of up to 785 °C) does not lead to a large scale rutile phase transition. As such photovoltaic performance of resultant DSC devices is maintained since the heating period is insufficient to induce a significant transition from anatase to rutile or morphology changes which result in a loss of photocurrent.

Nondestructive Three-Dimensional Observation of the Electrolyte in a Dye-Sensitised Solar Cell Combined with the Local Photocurrent Dynamics

The three-dimensional (3D) visualisation of the photovoltaic active layer in a dye-sensitised solar cell (DSSC) was newly achieved by employing an optical interference measurement system. The technique was used to measure with high precision the active layer thickness of a complete cell to evaluate the diffusion resistance of the electrode layer made of titania and the electrolyte layer containing tetracyanoborate ionic liquid. Additionally, the visualised image was compared with the local photovoltaic characteristics in a region with diameter 100 µm studied using an AC impedance technique as well as by transient photocurrent and photovoltage decay measurements to elucidate the charge transport properties of the ionic-liquid-based electrolyte.

Nondestructive Three-Dimensional Observation of the Electrolyte in a Dye-Sensitised Solar Cell Combined with the Local Photocurrent Dynamics

The three-dimensional (3D) visualisation of the photovoltaic active layer in a dye-sensitised solar cell (DSSC) was newly achieved by employing an optical interference measurement system. The technique was used to measure with high precision the active layer thickness of a complete cell to evaluate the diffusion resistance of the electrode layer made of titania and the electrolyte layer containing tetracyanoborate ionic liquid. Additionally, the visualised image was compared with the local photovoltaic characteristics in a region with diameter 100 µm studied using an AC impedance technique as well as by transient photocurrent and photovoltage decay measurements to elucidate the charge transport properties of the ionic-liquid-based electrolyte.

TiO2 Nanocrystals Synthesized by Laser Pyrolysis for the Up‐Scaling of Efficient Solid‐State Dye‐Sensitized Solar Cells

AbstractA crucial issue regarding emerging nanotechnologies remains the up‐scaling of new functional nanostructured materials towards their implementation in high performance applications on a large scale. In this context, we demonstrate high efficiency solid‐state dye‐sensitized solar cells prepared from new porous TiO2 photoanodes based on laser pyrolysis nanocrystals. This strategy exploits a reduced number of processing steps as well as non‐toxic chemical compounds to demonstrate highly porous TiO2 films. The possibility to easily tune the TiO2 nanocrystal physical properties allows us to demonstrate all solid‐state dye‐sensitized devices based on a commercial benchmark materials (organic indoline dye and molecular hole transporter) presenting state‐of‐the‐art performance comparable with reference devices based on a commercial TiO2 paste. In particular, a drastic improvement in pore infiltration, which is found to balance a relatively lower surface area compared to the reference electrode, is evidenced using laser‐synthesized nanocrystals resulting in an improved short‐circuit current density under full sunlight. Transient photovoltage decay measurements suggest that charge recombination kinetics still limit device performance. However, the proposed strategy emphasizes the potentialities of the laser pyrolysis technique for up‐scaling nanoporous TiO2 electrodes for various applications, especially for solar energy conversion.

An Organic D-π-A Dye for Record Efficiency Solid-State Sensitized Heterojunction Solar Cells

The high molar absorption coefficient organic D-π-A dye C220 exhibits more than 6% certified electric power conversion efficiency at AM 1.5G solar irradiation (100 mW cm(-2)) in a solid-state dye-sensitized solar cell using 2,2',7,7'-tetrakis(N,N-dimethoxyphenylamine)-9,9'-spirobifluorene (spiro-MeOTAD) as the organic hole-transporting material. This contributes to a new record (6.08% by NREL) for this type of sensitized heterojunction photovoltaic device. Efficient charge generation is proved by incident photon-to-current conversion efficiency spectra. Transient photovoltage and photocurrent decay measurements showed that the enhanced performance achieved with C220 partially stems from the high charge collection efficiency over a wide potential range.