Photovoltage Decay Research Articles

Nitrogen doping of TiO2 and annealing treatment of photoanode for enhanced solar cell performance

Mild doping of nitrogen (N) in TiO2 followed by an optimized aerial annealing treatment of CdS quantum dot (QD) sensitized photoanode resulted in efficient electron transfer and low recombination rates for the corresponding quantum dot solar cell (QDSC). Nitrogen doping passivates the surface defects in TiO2, reduces the density of recombination centers, and promotes electron injection into the current collector. N-doping also modifies the electronic band structure of TiO2 and reduces the band gap from 3.17 to 2.91 eV. Therefore in the QDSC with the N-TiO2/CdS photoanode, both N-TiO2 and CdS, undergo charge separation upon illumination thereby producing a higher photocurrent compared to the undoped-TiO2/CdS based QDSC. N-doping also increased the redox activity of TiO2, allowing facile ion and electron transport across its cross-section which is advantageous for solar cell performance. Optimal annealing temperature of 150 °C for the N-TiO2/CdS or TiO2/CdS photoanode restricted the back electron movement effectively, and imparted a significantly enhanced power conversion efficiency (PCE) to the N-TiO2/[email protected] °C/polysulfide gel/C-fabric- solar cell, greater by 55% compared to its unannealed counterpart. Lowered average excited electron lifetime, increased-incident photon-to-current conversion efficiency, recombination resistance and photovoltage decay response time, confirm the ability of the annealed photoanodes to undergo enhanced charge separation contrasting with the unannealed photoanode. This first time study relies on two simple approaches of nitrogen doping and annealing treatment to achieve considerably improved photovoltaic parameters, and opens up realistic possibilities for commercialization of QDSCs.

Overcoming the Limitations of Transient Photovoltage Measurements for Studying Recombination in Organic Solar Cells

Transient photovoltage (TPV) measurements are frequently used to study recombination processes in thin‐film solar cells by probing the decay of a small optically induced voltage perturbation to infer the charge carrier dynamics of devices at open circuit. However, the validity of this method to probe organic semiconductors has recently come into doubt due to large discrepancies in the reported carrier lifetime values for the same systems and the reporting of unrealistic reaction order values. Herein, the validity of TPV to extract reliable charge carrier lifetimes in thin‐film solar cells is explored through the use of time‐dependent drift‐diffusion simulations and measurements. It is found that in low‐mobility materials, TPV serves primarily as a probe of charge carrier redistribution in the bulk rather than bulk recombination dynamics and that the extracted time constant is highly mobility dependent. To address this shortcoming, transient photocharge, a new technique to measure the charge carrier density during photovoltage decay, is introduced and applied to study the recombination dynamics in a series of (fullerene and nonfullerene) organic solar cell systems. It is shown that using this technique the charge carrier recombination lifetime in the active layer is more accurately determined.

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Summary Capacitance-based techniques have been used to measure the electrical properties of halide perovskite solar cells (PSCs) such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. Here, we show that capacitance-based techniques cannot be used to reliably analyze the properties of defects in the perovskite layer or at its interface, because the high-frequency capacitance signature is due to the response of charge carriers in the hole-transport layer (HTL). For HTL-free PSCs, high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer because there is no trapping and de-trapping of charge carriers in the perovskite layer. We further find that the low-frequency capacitance signature can be used to calculate the activation energy of the ionic conductivity of the perovskite layer, but the overlapping effects with charge transport materials must be avoided.

Double-Mesoscopic Hole-Transport-Material-Free Perovskite Solar Cells: Overcoming Charge-Transport Limitation by Sputtered Ultrathin Al2O3 Isolating Layer.

The electrically insulating space layer takes a fundamental role in monolithic carbon-graphite based perovskite solar cells (PSCs) and it has been established to prevent the charge recombination of electrons at the mp-TiO2/carbon-graphite (CG) interface. Thick 1 μm printed layers are commonly used for this purpose in the established triple-mesoscopic structures to avoid ohmic shunts and to achieve a high open circuit voltage. In this work, we have developed a reproducible large-area procedure to replace this thick space layer with an ultra-thin dense 40 nm sputtered Al2O3 which acts as a highly electrically insulating layer preventing ohmic shunts. Herewith, transport limitations related so far to the hole diffusion path length inside the thick mesoporous space layer have been omitted by concept. This will pave the way toward the development of next generation double-mesoscopic carbon-graphite-based PSCs with highest efficiencies. Scanning electron microscope, energy dispersive X-ray analysis, and atomic force microscopy measurements show the presence of a fully oxidized sputtered Al2O3 layer forming a pseudo-porous covering of the underlying mesoporous layer. The thickness has been finely tuned to achieve both electrical isolation and optimal infiltration of the perovskite solution allowing full percolation and crystallization. Photo voltage decay, light-dependent, and time-dependent photoluminescence measurements showed that the optimal 40 nm thick Al2O3 not only prevents ohmic shunts but also efficiently reduces the charge recombination at the mp-TiO2/CG interface and, at the same time, allows efficient hole diffusion through the perovskite crystals embedded in its pseudo-pores. Thus, a stable VOC of 1 V using CH3NH3PbI3 perovskite has been achieved under full sun AM 1.5 G with a stabilized device performance of 12.1%.

Insight on Shallow Trap States-Introduced Photocathodic Performance in n-Type Polymer Photocatalysts.

Graphitic carbon nitride (g-C3N4) is a robust organic semiconductor photocatalyst with proven H2 evolution ability. However, its application in a photoelectrochemical system as a photocathode for H2 production is extremely challenging with the majority of reports representing it as a photoanode. Despite research into constructing g-C3N4 photocathodes in recent years, factors affecting an n-type semiconductor’s properties as a photocathode are still not well-understood. The current work demonstrates an effective strategy to transform an n-type g-C3N4 photoanode material into an efficient photocathode through introducing electron trap states associated with both N-defects and C–OH terminal groups. As compared to the g-C3N4 photoelectrode, this strategy develops 2 orders of magnitude higher conductivity and 3 orders of magnitude longer-lived shallow-trapped charges. Furthermore, the average OCVD lifetime observed for def-g-C3N4 is 5 times longer than that observed for g-C3N4. Thus, clear photocathode behavior has been observed with negative photocurrent densities of around −10 μA/cm2 at 0 V vs RHE. Open circuit photovoltage decay (OCVD), Mott–Schottky (MS) plot, and transient absorption spectroscopy (TAS) provide consistent evidence that long-lived shallow-trapped electrons that exist at about the microsecond time scale after photoexcitation are key to the photocathode behavior observed for defect-rich g-C3N4, thus further demonstrating g-C3N4 can be both a photoanode and a photocathode candidate.

Honeycomb-shaped charge collecting electrodes for dipole-assisted back-contact perovskite solar cells

Dipole-field-assisted charge-transporting-material-free lead halide perovskite solar cells (PSCs) using a back-contact configuration feature intrinsic advantages, such as no parasitic light absorption and high architectural defect tolerance. Herein, a newly designed, highly defect tolerant honeycomb-shaped back-contact (HBC) electrode was incorporated into dipole-field-assisted-back-contact PSCs, aiming to optimize the charge transport distance before being collected by the electrodes. HBC-PSCs with three feature sizes were fabricated in order to understand the effect of charge transport distance on device performance. The photovoltaic performance of HBC-PSCs correlates inversely proportional to the feature sizes. The mechanism behind the performance difference is elucidated via a detailed analysis through device current voltage characterization, transient photovoltage decay measurement and photocurrent mapping. A comprehensive comparison between honeycomb-shaped and interdigitated finger back-contact electrodes for PSCs is also conducted.

Coupled Ionic-Electronic Equivalent Circuit to Describe Asymmetric Rise and Decay of Photovoltage Profile in Perovskite Solar Cells

In this research, we employed transient photo-voltage rise and decay measurements to investigate the origin of slow unsymmetrical rise and decay profiles in single and triple cation perovskite solar cells. Drastic changes in photo-voltage decay profile were observed upon insertion of Br−, Cs+ and FA+ ions into perovskite structures. In order to explain our observations, the activation energy for ionic defects was measured and an equivalent circuit model was proposed containing both electrical and ionic components. The electrical branch consists of a diode, the bulk capacitance and resistances for charge transport and recombination. In parallel we introduced an ionic branch describing the ionic response by a resistance for ionic charge transport and a capacitance describing ion accumulation at the interface to the charge transport layer. To reproduce the asymmetry of photo-voltage rise and decay, a diode with a parallel resistor is introduced leading to a belayed backflow of the accumulated ions. The results revealed that the activation energy of ionic defects became larger upon insertion of either halides or cations. There is larger amount of ionic defects in the case of MAPbI3 while the de-accumulation process of ions happens in much larger time scale in triple cation perovskite. The presence of ions at the interfaces results in band bending generating a potential barrier restraining electrons and holes from recombination; so the loss of built-in potential is delayed until de-accumulation of ionic double layer happens. Our model proposes that the loss of built-in potential depends on electrostatic potential drop, suggesting coupled electronic-ionic phenomenon in perovskite solar cells.

Sonochemical Modification of SiGe Layers for Photovoltaic Applications

To improve the photovoltaic response of SiGe and amorphous silicon (a‐Si)/SiGe surfaces, a sonochemical treatment in chloroform (CHCl3) is used. The use of the sonochemical reaction slows down the observed surface photovoltage (SPV) decay and enhances its magnitude in SiGe and a‐Si/SiGe thin layers grown on Si. The average surface‐integrated photovoltage and decay time can increase up to 50%. This effect is not observed in distilled water, indicative of the fact that CH‐containing radicals can lead to the observed improvements. It is suggested that the effect can be explained as follows: CHCl3 is decomposed and produces hydrocarbon chains, which are then decomposed further away into hydrogen and carbon. The reactive Si dangling bonds revealed on the surface of SiGe alloys are saturated by the hydrocarbon species to passivate the surface. It is, therefore, advantageous to use sonochemical surface modification of silicon–germanium‐based photovoltaic materials in chloroform solutions.

Pyridyl/hydroxyphenyl versus carboxyphenyl anchoring moieties in Zn - Thienyl porphyrins for dye sensitized solar cells.

For developing efficient DSCs, anchoring moieties need to be understood in detail with respect to a particular dye. Herein, we have synthesized new Zn-thienyl porphyrin dyes based on pyridyl and hydroxyphenyl anchors and compared with traditional carboxyphenyl anchoring group. The effect of such variation in anchoring groups was investigated with UV-vis and fluorescence spectroscopy as well as electrochemical studies. The hydroxyphenyl appended dye showed higher molar extinction coefficient and red shifted emission bands. Both pyridyl and hydroxyphenyl dyes showed adsorption over TiO2 semiconductor. The dye loading amount was found to be greater for carboxyphenyl dye due to stronger bidendate coordination of the carboxy group, as revealed by FT-IR studies. The electrochemical potentials were found to be strongly influenced by the anchoring functionality and all the dyes showed positive driving force for electron injection to the conduction band of TiO2 and regeneration of oxidized dye by the redox electrolyte. DSC devices were fabricated for each of the dyes and characterized well. Hydroxyphenyl dye showed higher Voc as a result of lower back reactions. Transient photovoltage decay measurments indicated lower recombination rate in hydroxyphenyl and pyridyl than the carboxyphenyl anchored thienyl porphyrins offering potential for further dye optimization.

Poly(3,4-ethylenedioxypyrrole) coating and poly(4-styrenesulfonate) polyanions enhance solar cell performance

Co-sensitized photoanode with nitrogen doped graphene particles (NGPs) and CdS capable of harvesting light efficiently in the visible region is combined with a poly(3,4-ethylenedioxypyrrole) (PEDOP)@carbon(C)-cloth based counter electrode (CE) for the first time. The conduction band of NGPs is wedged between that of CdS and TiO2, and charge separation is improved in TiO2/NGPs/CdS compared to TiO2/CdS, thus resulting in a power conversion efficiency (PCE), greater by 37% and 305% in comparison to TiO2/CdS and TiO2/NGPs cells. The electrolyte is a polysulfide/silica gel modified with a polyanionic additive of sodium poly(4-styrenesulfonate) (NaPSS). NaPSS polymer chains serve as a passivating layer at the photoanode/electrolyte interface and reduce back electron transfer, while simultaneously allowing efficient hole transfer to the electrolyte. An optimal NaPSS content is ascertained from the highest PCE, conductivity and longest photovoltage decay time achieved for the electrolyte with 5 wt% NaPSS, compared to electrolytes with lower and higher NaPSS contents. The efficacy of PEDOP@C-cloth as a CE for the solar cell is confirmed from a lower charge transfer resistance and a higher diffusion current density for polysulfide reduction at the CE/electrolyte interface compared to sole C-cloth. This amplified solar cell performance is attained owing to the synergy of all the three components: NGPs at the photoanode, NaPSS in the electrolyte and PEDOP at the CE thus manifesting in a PCE of 7.7% for the TiO2/NGPs/CdS/polysulfide/silica/NaPSS gel/PEDOP@C-cloth cell relative to 3.5% attained for the TiO2/CdS/polysulfide/silica gel/C-cloth cell.

Flash Surface Treatment of CH3NH3PbI3 Films Using 248 nm KrF Excimer Laser Enhances the Performance of Perovskite Solar Cells

For perovskite solar cells (PSCs), the surface traps of perovskite films have great influence on the charge carrier behavior at the interface of perovskite and charge transport layers. In this investigation, a 248 nm KrF excimer laser with high photon energy and shallow penetration depth is introduced to perform surface modification on the CH3NH3PbI3 film through irradiation for reducing its surface trap density for the first time. A whole excimer laser surface modification (ELSM) process can be completed in few seconds, and the actual interaction time of the excimer laser and perovskite film is only several hundred nanoseconds. After ELSM, the trap density of the CH3NH3PbI3 film decreases from 1.61 × 1016 cm−3 to 5.81 × 1015 cm−3, and the nonradiative recombination is suppressed effectively. As a result, the open‐circuit voltage and the power conversion efficiency (PCE) of CH3NH3PbI3‐based PSCs increase from 1082 ± 27 to 1117 ± 16 mV and from 16.69% ± 0.77% to 18.50% ± 0.65%, respectively, and the PCE of the champion device reaches 19.38%. In addition, the measured larger charge recombination resistance, slower open‐circuit photovoltage decay, and longer charge recombination lifetime confirm the effective suppression effect of ELSM on the charge carrier recombination in PSCs.

Origin of Exceptionally Slow Light Soaking Effect in Mesoporous Carbon Perovskite Solar Cells with AVA Additive

A range of slow dynamic processes occurring in perovskite solar cells have been linked to ionic migration, including J–V hysteresis and long photovoltage rise and decay times. This work demonstrates the remarkably slow response time of triple mesoporous carbon-based cells, containing the additive 5-aminovaleric acid iodide (AVA). The photovoltage rise under illumination is 1–2 orders of magnitude longer than has previously been observed for planar and mesoporous TiO2 based devices. Transient photovoltage measurements during this slow rise in voltage show a strong negative transient feature which demonstrates the presence of fast recombination. By analyzing the rate of Voc rise and the decay of this negative transient, we show a clear link between this recombination process and the limiting of the Voc. The reduction of recombination over time and the resultant rise in Voc are influenced by the movement of ions in the perovskite. From temperature-dependent measurements, an activation energy consistent with ...

Photovoltage Decay Measurements by Photo-Assisted Kelvin Probe Force Microscopy on Cu(In,Ga)Se2 Solar Cells

Photovoltage measurements are performed on Cu(In,Ga)Se2 (CIGS) solar cells with various Ga composition ratios [Ga/(Ga + In)] using photo-assisted Kelvin probe force microscopy (P-KFM). From the temporally-averaged photovoltages acquired by P-KFM under intermittent light illumination at various frequencies, photovoltage decay phenomena resulting from different carrier recombination processes are investigated. The sample with high Ga composition in CIGS exhibits an elongated time constant for the slow recombination process and an increased contribution ratio of the fast recombination process in the entire photovoltage decay processes. The slow process is attributed to the recombination of spatially separated electrons and holes in the surface n -type layer and CIGS bulk layer, respectively, or to the existence of the electron trap near the CdS/CIGS interface. On the other hand, the fast process is attributed to direct recombination of electrons and holes in the CIGS bulk. The influence of the carrier separation effect on photo-carrier recombination around grain boundaries in the CIGS solar cells is also discussed.

Role of carbon nanodots in defect passivation and photo-sensitization of mesoscopic-TiO2 for perovskite solar cells

In this work, a recently developed carbon nanodots (CnDs) were introduced in the mesoscopic TiO2 to form a TiO2: CnDs composite, to be used as an electrode in CH3NH3PbI3 based perovskite devices. It resulted the improvement in morphology of perovskite film and induced a carbon-doping passivating the oxygen defects. In addition, the presence of carbon dots enhanced the charge carrier mobility of TiO2 and provided extra photo-generation ability to the perovskite device due to its absorbing nature. The improved morphology, carbon-doping, photogeneration ability and the improved charge transfer characteristics with the introduction of carbon dots was verified by the scanning electron microscopy (SEM), atomic force microscopy (AFM), X-Ray photoelectron spectroscopy (XPS), UV-visible absorption, X-ray diffraction, space charge limited current (SCLC) measurement, photoluminescence (PL), electrochemical impedance spectroscopy (EIS), current leakage and open circuit photovoltage decay (OCPD) measurements. An optimized concentration of CnDs (3 wt%) within the TiO2 resulted in a maximum power conversion efficiency of 19.45% with negligible hysteresis, which is 16% higher than the device with pristine TiO2.

Synthesis and photovoltaic performance of novel ullazine-based organic dyes for dye-sensitized solar cells

Two new ullazine-based organic dyes have been synthesized for applications in nanocrystalline TiO2-based dye-sensitized solar cells (DSSCs). Overall conversion efficiencies up to 3.63% were obtained for DSSCs based on these dyes. The dye with a tert-butyl antenna exhibited a higher and redshifted absorption pattern resulting in increased light-harvesting property. Compared to the unsubstituted dye, the photocurrent density, open-circuit photovoltage and overall power conversion efficiency (PCE) of solar cells based on the tert-butyl substituted dye are improved. The decreased PCE of the solar cell based on unsubstituted dye results mainly from the increased recombination rate and decreased electron lifetime, as evidenced from impedance and photovoltage decay measurements. Density functional theory (DFT) and time-dependent DFT calculations were performed to predict the geometries, electronic structure, and the optical and electrochemical properties of the dyes.

Reduced Defects of MAPbI3 Thin Films Treated by FAI for High‐Performance Planar Perovskite Solar Cells

AbstractOrganolead trihalide perovskite films with a large grain size and excellent surface morphology are favored to good‐performance solar cells. However, interstitial and antisite defects related trap‐states are originated unavoidably on the surfaces of the perovskite films prepared by the solution deposition procedures. The development of post‐growth treatment of defective films is an attractive method to reduce the defects to form good‐quality perovskite layers. Herein, a post‐treatment tactic is developed to optimize the perovskite crystallization by treating the surface of the one‐step deposited CH3NH3PbI3 (MAPbI3) using formamidinium iodide (FAI). Charge carrier kinetics investigated via time‐resolved photoluminescent, open‐circuit photovoltage decay, and time‐resolved charge extraction indicate that FAI post‐treatment will boost the perovskite crystalline quality, and further result in the reduction of the defects or trap‐states in the perovskite films. The photovoltaic devices by FAI treatment show much improved performance in comparison to the controlled solar cell. As a result, a champion solar cell with the best power conversion efficiency of 20.25% is obtained due to a noticeable improvement in fill factor. This finding exhibits a simple procedure to passivate the perovskite layer via regulating the crystallization and decreasing defect density.

A Phosphonic Acid Anchoring Analogue of the Sensitizer P1 for p-Type Dye-Sensitized Solar Cells

We report the synthesis and characterization of the first example of an organic dye, PP1, for p-type dye-sensitized solar cells (DSCs) bearing a phosphonic acid anchoring group. PP1 is structurally related to the benchmarking dye, P1, which possesses a carboxylic acid anchor. The solution absorption spectra of PP1 and P1 are similar (PP1 has λmax = 478 nm and εmax = 62,800 dm3 mol−1 cm−1), as are the solid-state absorption spectra of the dyes adsorbed on FTO/NiO electrodes. p-Type DSCs with NiO as semiconductor and sensitized with P1 or PP1 perform comparably. For PP1, short-circuit current densities (JSC) and open-circuit voltages (VOC) for five DSCs lie between 1.11 and 1.45 mA cm−2, and 119 and 143 mV, respectively, compared to ranges of 1.55–1.80 mA cm−2 and 117–130 mV for P1. Photoconversion efficiencies with PP1 are in the range 0.054–0.069%, compared to 0.065–0.079% for P1. Electrochemical impedance spectroscopy, open-circuit photovoltage decay and intensity-modulated photocurrent spectroscopy have been used to compare DSCs with P1 and PP1 in detail.

Revealing Solar-Cell Photovoltage Dynamics at the Picosecond Time Scale with Time-Resolved Photoemission Spectroscopy

Understanding carrier transport and recombination is the key to improving the efficiency of photovoltaic energy conversion, but it is difficult to directly monitor these processes on their nano- to picosecond time scales in complete devices. This study applies time-resolved photoemission spectroscopy to measure transient photovoltage in an actual solar cell. With temporal resolution as fine as tens of picoseconds, charge separation and recombination are observed respectively as the rise and decay of photovoltage. This approach affords a unique opportunity to characterize the microscopic properties of complete photovoltaic devices.

Effect of Grain Cluster Size on Back‐Contact Perovskite Solar Cells

AbstractIncorporating interdigitated back‐contact electrodes into organic–inorganic halide perovskite solar cells overcomes the optical losses and low architectural defect tolerance present in conventional “sandwich” cell configurations. However, other factors limit device performance in back‐contact architectures, such as the short charge‐carrier diffusion length within the perovskite film relative to the electrode spacing. As charge‐carrier diffusion length is crystal‐size related, in order to understand the effect of perovskite morphology on the performance of back‐contact perovskite solar cells (bc‐PSCs), perovskite films with four different grain cluster sizes, i.e., large, medium, small, and extra small, are fabricated via a solvent annealing approach. Crystallization of the perovskite is found to be closely related to the surface chemistry and topography of the substrate. The bc‐PSC photovoltaic performance correlates positively with the perovskite grain cluster size. Through a detailed analysis of transient photovoltage decay measurements, time‐resolved photoluminescence, and space charge‐limited current measurements, the effect of defect densities associated with grain cluster boundaries is elucidated.

Hot‐Injection Synthesized Ag2S Quantum Dots with Broad Light Absorption and High Stability for Solar Cell Applications

AbstractA hot‐injection synthesis method was used to synthesize low‐toxicity Ag2S colloidal quantum dots (CQDs) with strong and broad light absorption as an ultra‐thin photo‐absorber in CQD heterojunction solar cells. By using iodide and sulfur linkers it was possible to accomplish efficient charge carrier extraction, resulting in a high photocurrent due to the broad absorption spectrum. Transient photovoltage decay measurements were used to obtain information about trap states in the CQDs and the effect on the lifetime of the photoinduced carriers. The devices show very promising stability under constant long‐term illumination and they are stable under ambient storage conditions with low losses to the performance over a period of over two months. These results show that Ag2S CQDs have high potential within solar cell applications, and point the direction for further improvements.