Photocurrent Decay Measurements Research Articles

Yellow luminescence band defect related photocurrent instability of GaN p-i-n ultraviolet photodetectors

In this work, a low leakage current and high responsivity GaN p-i-n ultraviolet (UV) photodetector (PD) is fabricated for a photocurrent instability study. When the illumination condition shifts from dark to constant UV illumination, the PD’s photocurrent is found first to increase sharply and then go through a slow rising process until reaching its saturation value in tens of seconds. The degree of photocurrent instability lessens as UV illumination intensity increases. Meanwhile, when the PD is illuminated by periodic square-wave UV light, its transient response time is measured to be ∼1–2 μs. The observed photocurrent instability behavior is likely due to photocarrier trapping by yellow luminescence band related defects in GaN, which is supported by multiwavelength light illumination and photocurrent decay measurements.

Silicon Surface Passivation for Silicon-Colloidal Quantum Dot Heterojunction Photodetectors.

Sensitizing crystalline silicon (c-Si) with an infrared-sensitive material, such as lead sulfide (PbS) colloidal quantum dots (CQDs), provides a straightforward strategy for enhancing the infrared-light sensitivity of a Si-based photodetector. However, it remains challenging to construct a high-efficiency photodetector based upon a Si:CQD heterojunction. Herein, we demonstrate that Si surface passivation is crucial for building a high-performance Si:CQD heterojunction photodetector. We have studied one-step methyl iodine (CH3I) and two-step chlorination/methylation processes for Si surface passivation. Transient photocurrent (TPC) and transient photovoltage (TPV) decay measurements reveal that the two-step passivated Si:CQD interface exhibits fewer trap states and decreased recombination rates. These passivated substrates were incorporated into prototype Si:CQD infrared photodiodes, and the best performance photodiode based upon the two-step passivation shows an external quantum efficiency (EQE) of 31% at 1280 nm, which represents a near 2-fold increase over the standard device based upon the one-step CH3I passivated Si.

Operando Analysis of Semiconductor Junctions in Multi‐Layered Photocathodes for Solar Water Splitting by Impedance Spectroscopy

AbstractAlthough electrochemical impedance spectroscopy (EIS) is a powerful technique for investigating optoelectronic devices, realistic equivalent circuit (EC) models suitable for multi‐layered water splitting electrodes have rarely been reported due to their complex nature. In the present study, the utility of the EIS method for investigating multi‐layered photocathodes for photoelectrochemical water splitting is demonstrated. By analyzing the EIS data of TiO2‐coated Sb2Se3 photocathodes, one is able to obtain information about the constituent semiconductors and interfaces such as recombination processes, carrier lifetimes, doping densities, and flat band potentials under operando conditions. The charge transfer time to the electrolyte is also extracted from the EIS data and confirmed by transient photocurrent decay measurements. In addition, the method is successfully applied to other photocathodes with different classes of light absorber, such as metal oxides (Cu2O) and crystalline Si, to compare the device characteristics under real operational conditions. It is shown that the lifetime of photo‐generated carriers in the Si photocathode is much higher than those of the Sb2Se3 and Cu2O photocathodes. It is believed that the EIS analysis method presented in this study will become a powerful routine characterization technique for discovering the limiting factors in a wide range of photo‐electrosynthetic as well as photovoltaic devices.

Porphyrin-sensitized quasi-solid solar cells with MOF composited titania aerogel photoanodes

ZnTPP-COOH porphyrin-based quasi-solid dye-sensitized solar cells were fabricated with the following three different photoanode materials: Ni-MOF@titania aerogel composite, Cu-MOF@titania aerogel composite, and pure titania aerogel. Their photovoltaic performances were investigated through photocurrent–photovoltage (J–V) curve, electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectroscopy, and transient measurements of photocurrent and photovoltage decay. Among the three cells, the enhancement of photocurrent is observed in cells containing Ni-MOF@titania aerogel photoanodes. As a result, the Ni-MOF@titania aerogel photoanode–based cell achieved the highest photocurrent density of 8.88 mA/cm2 and power conversion efficiency of 2.34%. This value is higher than that of the other two devices. Moreover, the EIS analysis is used to understand better the possible electron transfer behind the photocurrent boosting in Ni-MOF@titania aerogel photoanode–based cell. Low Rct value (44.1 Ω) and low τd value of the Ni-MOF@titania aerogel device are the key factors to improve the electron flow, the Warburg diffusion of electrolyte, as well as the regeneration process. These factors are attributed as the reason for their best performance among other devices. Based on the results obtained, the plausible mechanism for electron transfer is discussed using the energy level cascades of TiO2/MOF/porphyrin dye. The transient measurements indicated that the devices exhibited good reproducibility under multiple illuminations.

Understanding charge transfer dynamics in QDs-TiO2 nanorod array photoanodes for solar fuel generation

Harvesting light to drive water splitting for hydrogen generation is an attractive approach to satisfy the urgent energy demands. The design and fabrication of photoelectrode materials that are able to harvest sunlight is an important scientific undertaking. In this study, a two-quantum-dot (QD) layer is developed to decorate one-dimensional TiO2 nanorod arrays, which are subsequently utilized as photoanodes to harvest the wide-spectrum sunlight for water splitting. The QD-coated TiO2 nanorod arrays extend the light absorption range from the UV into the visible region yielding increased solar-to-hydrogen efficiencies. Transient photocurrent decay measurements demonstrate that the multi-layer CdSe-CdS QDs deposited onto the TiO2 nanorod arrays result in a stepwise band alignment that not only improves the hole extraction but also facilitates electron injection from the QDs to TiO2 rods. Moreover, the multi-heterojunction photoanode introduces interfacial states that act as recombination centers to trap the photogenerated electrons.

Silicon thin film photodetectors for multi‐channel fluorescence detection in a microfluidic point‐of‐care testing device

Fluorescence spectroscopy is a highly sensitive detection method widely used in analytical chemistry, bioanalytics, and medicine. The here presented amorphous silicon (a‐Si:H) based thin film photodetector arrays contribute to the miniaturization of this measurement method and thereby enable integration of a point‐of‐care testing (POCT) device. Revealed from photocurrent decay measurements, the limit of detection of the photodiodes with an active area A = 36.3 mm2 is in the 100 fA‐range. Dark current analysis at zero bias voltage yields a standard deviation σ = 4.4 fA for the thermally generated noise of the nip photodiodes. In the context of the desired application, a minimum detectable fluorescence radiation intensity Umin = 4.5 pW cm−2 with a dynamic range DNR = 78 dB is estimated. At the emission peak wavelength λem = 667 nm and with a bandwidth Δf = 0.25 Hz the photodiodes reach a noise equivalent power NEP = 54.6 fW Hz−1/2 and therewith a specific detectivity D* = 1.1 × 1013 Jones. Fluorescence measurements with the dye DY‐636 diluted in pure water with a molar concentration c = 1.31 μM demonstrate the suitability of the multi‐channel photodetectors for high‐sensitivity fluorescence spectroscopy.Low‐signal a‐Si:H photodetector multi‐channel chips in different geometrical layouts with an active area of up to 36.3 mm2 per diode.

Metal-free organic dyes for TiO2 and ZnO dye-sensitized solar cells

We report the synthesis and characterization of new metal-free organic dyes (namely B18, BTD-R, and CPTD-R) which designed with D-π-A concept to extending the light absorption region by strong conjugation group of π-linker part and applied as light harvester in dye sensitized solar cells (DSSCs). We compared the photovoltaic performance of these dyes in two different photoanodes: a standard TiO2 mesoporous photoanode and a ZnO photoanode composed of hierarchically assembled nanostructures. The results demonstrated that B18 dye has better photovoltaic properties compared to other two dyes (BTD-R and CPTD-R) and each dye has higher current density (Jsc) when applied to hierarchical ZnO nanocrystallites than the standard TiO2 mesoporous film. Transient photocurrent and photovoltage decay measurements (TCD/TVD) were applied to systematically study the charge transport and recombination kinetics in these devices, showing the electron life time (τR) of B18 dye in ZnO and TiO2 based DSSCs is higher than CPTD-R and BTD-R based DSSCs, which is consistent with the photovoltaic performances. The conversion efficiency in ZnO based DSSCs can be further boosted by 35%, when a compact ZnO blocking layer (BL) is applied to inhibit electron back reaction.

Contact‐Induced Mechanisms in Organic Photovoltaics: A Steady‐State and Transient Study

The role of the contacts in thin‐film, blended heterojunctions (<100 nm thick) organic photovoltaics is explored, specifically considering concepts of carrier selectivity, injection, and extraction efficiency, relative to recombination. Contact effects are investigated by comparing two hole‐collecting interlayers: a phosphonic acid monolayer on indium tin oxide (ITO) and a nickel oxide thin film. The interlayers have equivalent work functions (≈5.4 eV) but widely variant energy band offsets relative to the lowest unoccupied molecular orbital of the acceptor (electron blocking versus not), which are coupled to large differences in carrier density. Trends in open‐circuit voltages (VOC) as a function of light intensity and temperature are compared and it is concluded that the dominant mechanism limiting VOC for high density of states contacts is free carrier injection, not surface recombination or extraction barriers. Transient photocurrent decay measurements confirm excess reinjected carriers decrease the extraction efficiency via increased recombination and decrease free carrier lifetime, even at high internal electric fields, due to space charge accumulation. These results demonstrate that the energetics and injection dynamics of the interface between interlayers and high carrier density electrodes (typically ITO and metals) must be considered with fabrication and processing of interlayers, in addition to possible carrier selectivity and the interface with the active layer.

Interplay between efficiency and device architecture for small molecule organic solar cells.

Small molecule organic solar cells (OSCs) have experienced a resurgence of interest over their polymer solar cell counterparts, owing to their improved batch-to-batch (thus, cell-to-cell) reliability. In this systematic study on OSC device architecture, we investigate five different small molecule OSC structures, including the simple planar heterojunction (PHJ) and bulk heterojunction (BHJ), as well as several planar-mixed structures. The different OSC structures are studied over a wide range of donor:acceptor mixing concentrations to gain a comprehensive understanding of their charge transport behavior. Transient photocurrent decay measurements provide crucial information regarding the interplay between charge sweep-out and charge recombination, and ultimately hint toward space charge effects in planar-mixed structures. Results show that the BHJ/acceptor architecture, comprising a BHJ layer with high C60 acceptor content, generates OSCs with the highest performance by balancing charge generation with charge collection. The performance of other device architectures is largely limited by hole transport, with associated hole accumulation and space charge effects.

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.

Engineering of thiocyanate-free Ru(ii) sensitizers for high efficiency dye-sensitized solar cells

We synthesized a series of Ru(II) metal complexes TFRS-1, -2, -4, -21, -22 and -24 with a single 4,4′-dicarboxylic acid-2,2′-bipyridine together with two functionalized pyridyl azolate ancillary ligands consisting of pyrazolate or triazolate groups. Both photophysical measurements and DFT/TDDFT calculations were conducted to gain insight into their electronic and optical properties. The triazolate series of sensitizers TFRS-21, -22 and -24 showed an enlarged optical band gap with respect to their pyrazolate counterparts TFRS-1, -2 and -4. When employed in dye sensitized solar cells (DSCs), the triazolate sensitizers show slightly inferior JSC values due to the poor incident photon-to-current conversion efficiencies recorded compared to the pyrazolate series. Moreover, the endowed 5-(hexylthio)thiophen-2-yl substituents exert a notable hyperchromic effect and bathochromic shift in the absorption spectra, which then improves the short circuit current JSC to 18.7 and 15.5 mA cm−2 and the overall conversion efficiency to η = 10.2% and 8.25% for TFRS-4 and TFRS-24, respectively. For the evaluation of VOC, transient photocurrent and photovoltage decay measurements were carried out to compare the rates of interfacial recombination of electrons from the TiO2 conduction band to 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.

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.

Pore Filling of Spiro‐OMeTAD in Solid‐State Dye‐Sensitized Solar Cells Determined Via Optical Reflectometry

AbstractA simple strategy is presented to determine the pore‐filling fraction of the hole‐conductor 2,2‐7,7‐tetrakis‐N,N‐di‐pmethoxyphenylamine‐9,9‐spirobifluorene (spiro‐OMeTAD) into mesoporous photoanodes in solid‐state dye‐sensitized solar cells (ss‐DSCs). Based on refractive index determination by the film's reflectance spectra and using effective medium approximations the volume fractions of the constituent materials can be extracted, hence the pore‐filling fraction quantified. This non‐destructive method can be used with complete films and does not require detailed model assumptions. Pore‐filling fractions of up to 80% are estimated for optimized solid‐state DSC photoanodes, which is higher than that previously estimated by indirect methods. Additionally, transport and recombination lifetimes as a function of the pore‐filling fraction are determined using photovoltage and photocurrent decay measurements. While extended electron lifetimes are observed with increasing pore‐filling fractions, no trend is found in the transport kinetics. The data suggest that a pore‐filling fraction of greater than 60% is necessary to achieve optimized performance in ss‐DSCs. This degree of pore‐filling is even achieved in 5 μm thick mesoporous photoanodes. It is concluded that pore‐filling is not a limiting factor in the fabrication of “thick” ss‐DSCs with spiro‐OMeTAD as the hole‐conductor.

Characterization of Interfacial Modifiers for Hybrid Solar Cells

This study systematically investigates the influence of different TiO2 surface modifiers on the device properties of TiO2–poly(3-hexylthiophene) (P3HT) hybrid solar cells to infer design rules for interfaces in hybrid solar cells. Bare TiO2 is compared to TiO2 modified with a Ru(II) dye (Z907), phenyl-C61-butyric acid (PCBA), the carboxylated polymer poly[3-(5-carboxypentyl)thiophene-2,5-diyl] (P3HT-COOH), and Sb2S3, respectively. Self-assembled monolayers are investigated for the former three modifiers, whereas a thin coating of only a few nanometers is used in the case of Sb2S3. Photoluminescence quenching analysis is performed for the different TiO2–P3HT interfaces to gain insight into the mechanism of charge separation. For further analysis, the different modifiers are tested in solar cells. We focus on bilayered devices with a well-defined near-planar TiO2–P3HT interface for easier data analysis. Absorption of light by the modifiers can thus be neglected. In addition, detailed current density–voltage curve analysis, photovoltage, and photocurrent decay measurements reveal mechanisms of charge carrier recombination and extraction. Our study underlines the importance of recombination control and matching energy levels as well as proper alignment of P3HT at the interface.

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.

“Click-chemistry” approach in the design of 1,2,3-triazolyl-pyridine ligands and their Ru(ii)-complexes for dye-sensitized solar cells

The synthesis of new 1,2,3-triazolyl-pyridine ligands via “click-chemistry” and their corresponding Ru(L)(2,2′-bipyridyl-4,4′-dicarboxylic acid)(NCS)2 complexes (L = 1,2,3-triazolyl-pyridine) are presented. The complexes have been photophysically and electrochemically characterized and have been used as sensitizers in dye-sensitized solar cells (DSSC). In DSSCs with an acetonitrile-based electrolyte the cells comprising of Ru(2-(1-(4-hexylphenyl)-1H-1,2,3-triazol-4-yl)pyridine)(2,2′-bipyridyl-4,4′-dicarboxylic acid)(NCS)2 TBA salt 1 showed an overall power conversion efficiency of 7.8% under full sunlight intensity, and Ru(2-(4-(4-hexylphenyl)-1H-1,2,3-triazol-1-yl)pyridine)(2,2′-bipyridyl-4,4′-dicarboxylic acid)(NCS)2 TBA salt 2 an efficiency of 4.7%. Transient photovoltage and photocurrent decay measurements showed an enhanced performance for dye 1 due to faster electron transport into the TiO2 film and lower recombination rate in comparison to dye 2 sensitized devices. Additionally, solid-state devices were prepared with 2 μm thick TiO2 films using spiro-OMeTAD as a hole-transport material. The solid-state dye-sensitized solar cells showed power conversion efficiencies of 1.92% and 0.38% for sensitizer 1 and 2, respectively.

Charge collection and pore filling in solid-state dye-sensitized solar cells

The solar to electrical power conversion efficiency for dye-sensitized solar cells (DSCs)incorporating a solid-state organic hole-transporter can be over 5%. However, this is fordevices significantly thinner than the optical depth of the active composites and bycomparison to the liquid electrolyte based DSCs, which exhibit efficiencies inexcess of 10%, more than doubling of this efficiency is clearly attainable if allthe steps in the photovoltaic process can be optimized. Two issues are currentlybeing addressed by the field. The first aims at enhancing the electron diffusionlength by either reducing the charge recombination or enhancing the chargetransport rates. This should enable a larger fraction of photogenerated charges tobe collected. The second, though less actively investigated, aims to improve thephysical composite formation, which in this instance is the infiltration of mesoporousTiO2 with the organichole-transporter 2,2′,7,7′-tetrakis(N,N-di-p-methoxypheny-amine)-9,9′-spirobifluorene (spiro-MeOTAD). Here, we perform a broad experimental study to elucidatethe limiting factors to the solar cell performance. We first investigate the charge transport andrecombination in the solid-state dye-sensitized solar cell under realistic working conditionsvia small perturbation photovoltage and photocurrent decay measurements. From thesemeasurements we deduce that the electron diffusion length near short-circuit is as long as20 µm. However, at applied biases approaching open-circuit potential under realisticsolar conditions, the diffusion length becomes comparable with the film thickness,∼2 µm, illustrating that real losses to open-circuit voltage, fill factor and hence efficiency areoccurring due to ineffective charge collection. The long diffusion length near short-circuit,on the other hand, illustrates that another process, separate from ineffective chargecollection, is rendering the solar cell less than ideal. We investigate the process ofTiO2 mesopore infiltration with spiro-MeOTAD by examining the cross-sectional images of andperforming photo-induced absorption spectroscopy on devices with a range of thickness,infiltrated with spiro-MeOTAD with a range of concentrations. We present ourinterpretation of the mechanism for material infiltration, and by improving thecasting conditions demonstrate efficient charge collection through devices of over7 µm in thickness. This investigation represents an improvement in our understanding of thelimiting factors to the dye-sensitized solar cell. However, much work, focused on compositeformation and improved kinetic competition, is required to realize the true potential of thisconcept.

Electron Transport Analysis for Improvement of Solid-State Dye-Sensitized Solar Cells Using Poly(3,4-ethylenedioxythiophene) as Hole Conductors

Dye-sensitized solar cells (DSCs) using solid-state hole conductor, poly(3,4-ethylenedioxythiophene) (PEDOT), were fabricated using in-situ photoelectrochemical polymerization giving short-circuit photocurrent density of 3.20 mA cm-2, open-circuit voltage of 0.77 V, and fill factor of 0.50, and the resulting overall conversion efficiency of 1.25% on average under air mass 1.5 conditions. Furthermore, the electron transport properties of the DSCs based on PEDOT (PEDOT/DSCs) were analyzed using light intensity modulation induced photocurrent and photovoltage decay (SLIM-PCV) measurements and electrochemical impedance spectroscopy (EIS) measurements, and then compared to those of the DSCs based on organic liquid electrolyte containing I-/I3- as redox couple (liquid iodide/iodine electrolyte-DSCs, iodide/DSCs for short). The effective filling of PEDOT in the mesopores of dyed TiO2 layers is an important key to achieve the respectable conversion efficiency of PEDOT/DSCs that is comparable with iodide/DSCs.

Photogenerated carrier profile determination in polymeric light-emitting diodes by steady-state and transient photocurrent measurements

Photogenerated charge carrier profiles in poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) light-emitting diodes were determined from steady-state photocurrent spectra and transient photocurrent decay measurements. The observation that the photocurrent spectra behavior is strongly dependent on the bias polarity and amplitude suggests the existence of an intrinsic electric field, determined by the difference in the work function of the metallic electrodes, as well as a field dependence on the free charge carrier generation rates. The obtained results reveal built-in voltages of +0.2 and −0.6 V for Au and Al electrodes, respectively. The photocurrent spectra can be semi-quantitatively explained using a simple model which takes into account the internal built-in electric field of the device structure, the band gap energy and the migration/diffusion of the photogenerated charge carriers through the polymeric film. The drift mobility of carriers was investigated by the time-of-flight technique (TOF). The value for the mobility of holes was obtained from the change of slopes in the double logarithmic plot of the transient current and was independent of the applied field. The TOF transients for electrons showed no change of slopes in the double logarithmic plot and the drift mobility of electrons could not be calculated.