Photocarrier Generation Mechanism Research Articles

Photocarrier generation mechanism of graphene-based field effect device

The carrier generation in a photodetecting device plays a critical role in the enhancement of photocurrent. Here we perform photocurrent measurements on graphene-based photodetecting device and unveil three carrier generation mechanisms, namely, (I) thermally assisted- and (II) optically induced- excitation of in-gap defect state electron, and (III) inter-band optical absorption of bulk Si. We further determine their carrier supply strength defined as P = dn/dt, which are P(I) = 2.09×1010 /cm⋅2 s, P(II) = 2.15×1010 /cm⋅2s, and P(III) = 3.55×1011 /cm⋅2s, respectively. In addition, we show that the characteristic IV behavior induced by the gate voltage cycling can be interpreted in terms of the three carrier generation mechanisms. This study provides a useful guide to designing practical graphene-based photodetecting devices.

Enhancing the graphene photocurrent using surface plasmons and a p-n junction

The recently proposed concept of graphene photodetectors offers remarkable properties such as unprecedented compactness, ultrabroadband detection, and an ultrafast response speed. However, owing to the low optical absorption of pristine monolayer graphene, the intrinsically low responsivity of graphene photodetectors significantly hinders the development of practical devices. To address this issue, numerous efforts have thus far been made to enhance the light–graphene interaction using plasmonic structures. These approaches, however, can be significantly advanced by leveraging the other critical aspect of graphene photoresponsivity enhancement—electrical junction control. It has been reported that the dominant photocarrier generation mechanism in graphene is the photothermoelectric (PTE) effect. Thus, the two energy conversion mechanisms involved in the graphene photodetection process are light-to-heat and heat-to-electricity conversions. In this work, we propose a meticulously designed device architecture to simultaneously enhance the two conversion efficiencies. Specifically, a gap plasmon structure is used to absorb a major portion of the incident light to induce localized heating, and a pair of split gates is used to produce a p-n junction in graphene to augment the PTE current generation. The gap plasmon structure and the split gates are designed to share common key components so that the proposed device architecture concurrently realizes both optical and electrical enhancements. We experimentally demonstrate the dominance of the PTE effect in graphene photocurrent generation and observe a 25-fold increase in the generated photocurrent compared to the un-enhanced cases. While further photocurrent enhancement can be achieved by applying a DC bias, the proposed device concept shows vast potential for practical applications.

Photocurrent Quantum Yield of Semiconducting Carbon Nanotubes: Dependence on Excitation Energy and Exciton Binding Energy

We address the dependence of the relative photocurrent quantum yield (QY) on the excitation energy and the exciton binding energy of semiconducting single-walled carbon nanotubes (s-SWNTs) having well-defined chiral indexes, by analyzing both the optical absorption and the photocurrent spectra. First, we examine the QY of a sample consisting of one sort of nanotube (such as (7,5)), which allows revealing that QY depends on the excitation energy and hence on the nature of the electronic transition. In particular, we demonstrate that the QY of the second excitonic transition (E22) is relatively higher than that of the first excitonic transition (E11). Then, we extend the analysis to a sample consisting of five kinds of nanotubes (namely, (7,5), (7,6), (8,6), (8,7), (9,7)), which permits demonstrating for the first time that QY increases with increasing the nanotube’s diameter and with decreasing the exciton binding energy, according to two categories known as type 1 and type 2 nanotubes. Finally, we discuss these results in the framework of the electric-field-assisted exciton dissociation model in order to gain further insight into the photocarrier generation mechanism in s-SWNTs.

Direct observation of ultrafast long-range charge separation at polymer-fullerene heterojunctions.

In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on ≲ 50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

Impact of pentacene film thickness on the photoresponse spectra: Determination of the photocarrier generation mechanism

Photocurrent response, optical absorption, and x-ray diffraction (XRD) measurements in pentacene films grown on glass substrates are performed in order to obtain an insight into the mobile photocarriers generation mechanism. For film thickness of the order of 50 nm and lower, the photocurrent response spectra are found to follow the optical absorption spectra demonstrating the so-called symbatic response. Upon increasing the film thickness, the photoresponse demonstrates a transition to the so-called antibatic response, which is characterized by a maximum and minimum photocurrent for photon energies of minimum and maximum optical absorption, respectively. The experimental results are not in accordance with the model of important surface recombination rate. By taking into account the XRD patterns, the experimental photoresponse spectra can be reproduced by model simulations assuming efficient exciton dissociation at a narrow layer of the order of 20 nm near the pentacene-substrate interface. The simulated spectra are found sensitive to the film thickness, the absolute optical absorption coefficient, and the diffusion exciton length. By comparing the experimental with the simulated spectra, it is deduced that the excitons, which are created by optical excitation in the spectral region of 1.7-2.2 eV, diffuse with a diffusion length of the order of 10-80 nm to the pentacene-substrate interface where efficiently dissociate into mobile charge carriers.

Photoluminescence Quenching and Photoconductivity in Devices Using 3,6-Diaryl-N-hexylcarbazole

Photocarrier generation mechanisms in 3,6-diaryl-N-hexylcarbazole (aryl = p-cyanophenyl and p-acetophenyl) were studied in single-layer, bilayer, and blend film devices in sandwich geometry between indium–tin oxide (ITO) and Al electrodes. Bilayer and blend film devices were made with N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD). Photoluminescence (PL) was strongly quenched by electric field. The photocurrent versus electric field plots and photoresponse action spectra were used to identify the source of photocarrier generation (interface versus bulk) in the three devices. In blend devices, the photocurrent density is directly proportional to PL quenching efficiency, indicating efficient carrier generation and collection in these devices. PL quenching and photocurrent results were fitted to theoretical equations to obtain material parameters such as exciton binding energy, exciton dissociation rate at zero field, PL quenching efficiency at zero field, and carrier mobility-lifetime product. The device properties of the two carbazole derivatives are suitable for visible-blind UV photodetectors.

Quantitative estimation of electronic quality of zinc phthalocyanine thin films

We determine the mobility-lifetime product (\ensuremath{\mu}\ensuremath{\tau}) of free charge carriers in pristine zinc phthalocyanine (ZnPc) films using photocurrent measurements. The photocurrent is proportional to the free charge carrier generation efficiency (\ensuremath{\eta}) and the \ensuremath{\mu}\ensuremath{\tau} product, and the carrier collection length is directly proportional to the latter. The \ensuremath{\mu}\ensuremath{\tau} product is thus an important parameter for the electronic quality of a material. We further determine the dominant photocarrier generation mechanisms in ZnPc. The free carrier generation efficiency is estimated from total carrier collection and electric field--induced photoluminescence quenching measurements. Using \ensuremath{\eta} and the electric field dependence of the photocurrent, we estimate the \ensuremath{\mu}\ensuremath{\tau} product of holes in ZnPc to be about 3\ifmmode\times\else\texttimes\fi{}10${}^{\ensuremath{-}11}$ cm${}^{2}$/V.

Structural and Electronic Properties of Extremely Long Perylene Bisimide Nanofibers Formed through a Stoichiometrically Mismatched, Hydrogen‐Bonded Complexation

Extremely long nanofibers, whose lengths reach the millimeter regime, are generated via co-aggregation of a melamine-appended perylene bisimide semiconductor and a substituted cyanurate, both of which are ditopic triple-hydrogen-bonding building blocks; they co-aggregate in an unexpected stoichiometrically mismatched 1:2 ratio. Various microscopic and X-ray diffraction studies suggest that hydrogen-bonded polymeric chains are formed along the long axis of the nanofibers by the 1:2 complexation of the two components, which further stack along the short axis of the nanofibers. The photocarrier generation mechanism in the nanofibers is investigated by time-of-flight (TOF) experiments under electric and magnetic fields, revealing the birth and efficient recombination of singlet geminate electron-hole pairs. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements revealed intrinsic 1D electron mobilities up to 0.6 cm(2) V(-1) s(-1) within nanofibers.

Polymer photovoltaic optimizations from exciton level

Future polymeric or ‘plastic’ solar cells are very attractive for solar energy conversions where large area, low cost, lightweight, and flexible shape are desired. In this brief review, fundamental mechanisms of inorganic and organic/polymeric photovoltaic materials will be briefly compared at exciton levels. This study shows that the frontier orbital couplings at atomic and molecular levels are crucial for the mechanisms of photo carrier generation and transport, either exciton or band models. Optimization approaches of polymer based solar cells are also addressed.

Photogeneration of Charge Carriers and Their Transport Properties in Poly[bis(p-n-butylphenyl)silane

The photocarrier generation mechanism and mobility in poly[bis(p-n-butylphenyl)silane] (PBPS) thin films doped with a variety of electron acceptors are studied by time-resolved microwave conductivity (TRMC) measurements. It was found that fullerene is a suitable electron acceptor for PBPS as it provides the highest product of photocarrier generation yield phi and mobility Sigmamu under excitation at 532 and 355 nm. The observed high phiSigmamu value of 4.5 x 10(-3) cm(2)/(V s) under excitation at 193 nm (6.39 eV) can be attributed to the direct ionization of PBPS molecules. The photoinduced electron transfer between C(60) and PBPS was investigated in a solution sample by laser flash photolysis under excitation at 532 nm. On the basis of the extinction coefficient of PBPS(*+), transient absorption of PBPS(*+) provides a maximum value of phi of 0.83% for the electron-transfer reaction from PBPS to (3)C(60). On the basis of this value of phi, the intrinsic intrachain mobility of holes on the PBPS backbone is estimated to be higher than 1.7 x 10(-2) cm(2)/(V s), suggesting the presence of a high conducting path along the Si backbone of PBPS.

Direct Observation of Photogenerated Carriers in Organic Photoreceptors by Time-Resolved Spectroscopic Techniques

Time-Resolved spectroscopy is a powerful tool for investigation of photogeneration and photoconduction mechanism in layered organic photoreceptors because it can directly detect transient species in each step. So far, we have measured the transient absorption spectra of layered organic photoreceptors using azo pigments as charge generation materials (CGMs). As an extension work of our previous study, we applied this technique to photoreceptors using titanyl phthalocyanines (TiOPcs) as CGMs. For photoreceptors containing TiOPcs and CTMs, we obtained almost the same transient absorption spectrum as we observed in azo-CGM/CTM layers. However, TiOPcs gave weak transient absorption even without CTMs, whereas azo CGMs did not. We also found that the response to external electric field largely depended on the crystalline phase of TiOPc. It is interesting that these results strongly correlate with the photocarrier generation mechanism and the sensitivity of photoreceptors. These experimental results are discussed in connection with photocarrier generation mechanism.

Investigation of the Schottky Junction at Poly(3-alkylthiophene) and Metal Interface

Poly(3-hexylthiophene) (head-tail coupled regioregular, HT-PHT)/Al contact shows a rectification as a result of formation of Schottky junction. The temperature dependence of J-V characteristics and action spectra of photocurrents are studied to investigate the mechanism of photocarrier generation and transport. A new method to estimate the thickness of depletion layer, which is found to be around 25 nm for the HT-PHT/Al contact, is proposed.

Study of the Photoconductivity Mechanism in Phthalocyanine Pigment Particles by Electric Field Modulated Time Resolved Fluorescence

Phthalocyanine pigment particles represent an important class of organic photoconductors, which are extensively used in organic xerographic photoreceptors. Despite numerous studies, a complete picture of the carrier generation process in this class of materials has yet to emerge. In this paper we will present a detailed description of the photocarrier generation mechanism in TiOPc(I), TiOPc(IV), HOGaPc and x-H2Pc. We have studied the influence of electric field on both the integrated and time resolved fluorescence in photoconductive particles dispersed in a polymer matrix. Time resolved fluorescence decays were analyzed by fitting the data to a sum of two exponentials representing the fast and slow fluorescence components. For HOGaPc, TiOPc(I) and TiOPc (IV) the fast fluorescence component exhibits both amplitude and lifetime quenching. These results indicate that carrier generation in HOGaPc, TiOPc(I) and TiOPc (IV) originates from both relaxed and nonrelaxed intrinsic excited singlet states, while the trapped excitons do not lead to significant carrier production. In contrast, for x-H2Pc significant amplitude quenching of the fast component is observed only at high field and the trapped excitons are an important source of photogenerated carrier. This indicates that x-H2Pc possesses at least some bulk sensitized photocarrier generation. All of the phthalocyanines studied exhibited a quadratic dependence of integrated fluorescence quenching on electric field indicating the existence of a neutral carrier precursor state.

Fabrication and characteristics of Schottky diodes based on regioregular poly(3-hexylthiophene)/Al junction

Abstract Schottky diodes based on Al/poly(3-hexylthiophene), PHT/Au (or indium tin oxide (ITO)) are fabricated with various preparation conditions. The current–voltage characteristics and photovoltaic effect have been studied to elucidate the mechanisms of rectification and photocarrier generation. The PHT/Al junction prepared by spin coating of PHT on fresh Al deposited in vacuum shows the larger rectification ratio up to 105 at ±2 V than that obtained by reversed way. The results indicate that the oxidation layer of Al plays the important role for the charge injection between Al and PHT. The thickness of depletion layer due to the Schottky junction is evaluated to be 70–150 nm from the capacitance vs. reversed bias plots. The excitation spectra and PHT thickness dependence of photocurrents upon illumination to Al or Au side are also measured. The results indicate that the carrier generation occurs predominantly within the depletion layer near the Al electrode with the help of strong built-in field.

Xerographic and electroabsorption study of primary photocarrier generation process in x-form metal-free phthalocyanine

Using a dual-layered photoconductor, we have investigated the primary photocarrier generation process in x-form metal-free phthalocyanine (x-H2Pc) over a wide range of illumination wavelengths. According to the results of quantum efficiency measurements, it has been established that the photocarrier generation mechanism in x-H2Pc occurs via two processes: (i) production of an intermediate that depends solely on the excitation energy, and (ii) subsequent free carrier production in the presence of an electric field. In addition, the spectral quantum efficiency and the electroabsorption spectrum were measured and compared. Based on that, the primary process efficiency was divided into four regions in terms of photon energy, which explained well the relationship between them. The excitation energy dependence of the primary efficiency was semiquantitatively validated based on the electron transfer theory.

Defect Auger exciton dissociation and impact ionization in conjugated polymers

We study theoretically the exciton dissociation and carrier generation in conjugated polymers via defect Auger process, in which the electron (hole) of the exciton drops into the deep defect level while the energy is released to the hole (electron) through Coulomb scattering. Contrary to the usual Auger process among free carriers at high densities, defect Auger process for excitons takes place independent of the exciton density, and is identified as the dominant mechanism of photocarrier generation for excitation below the band gap. The dissociation probability for each passage through the defect is found to be close to one for excitons with thermal velocity, consistent with the picture that exciton decay in oxidized polymers is controlled by diffusion on a chain with quenching centers. We also study the reverse process, i.e., defect impact ionization, in which excitons or free electron hole pairs are created via the impact of hot holes (electrons) on electrons (holes) in the defect level. Excitons are found to be produced predominantly for driving electric field in a window around ${10}^{5} \mathrm{V}/\mathrm{cm}$ along the chain. Light emission under unipolar carrier injection is predicted.

Mechanism of Photocarrier Generation and Transport in Conjugated Polymer

Mechanism of Photocarrier Generation and Transport in Conjugated Polymer

Generation and Transport of Photocarriers in Head-Tail Poly(3-Alkylthiophene) Films

ABSTRACTPhotovoltaic effects in Schottky Junction cells composed with Au/head-tail poly(3-hexylthiophene) (HT-P3HT)/Al have been studied as a function of excitation photon energy and temperature. It has been found that photocarriers are effectively generated at the interface of HT-P3HT/Al from the excitation spectra of photocurrents (photoaction spectra), which were obtained upon illumination to the Au or Al sides. The photoaction spectra also suggest that the photocarrier generation occurs at around 2.3 eV and 3.5 eV, which correspond to the peak of φ-φ* absorption and the photon energy with lower optical density region, respectively. The mechanisms of photocarrier generation are discussed taking the role of the Schottky junction into consideration.

Time-Resolved Fluorescence Study on Primary Photocarrier Generation Process in Layered Organic Photoreceptors

We have been studying the photocarrier generation mechanism in layered organic photoreceptors comprised of bisazo pigment based carrier generation materials (CGMs) and carrier transport materials (CTMs) using thermally stimulated current (TSC) technique. And it was concluded that the formation yield of initial ion-pair −(CGM-/CTM+) mainly determined the photosensitivity. It is known that the carrier generation yield is greatly enhanced by the coexistent CTMs in this type of photoreceptors. In the present study, we examine the primary process just after photoexcitation by means of time-resolved fluorescence spectroscopy. We observed that the CTMs considerably shortened the lifetime of CGM exciton. We attempt to correlate the quenching rate of CGM fluorescence by CTMs with the sensitivity of the photoreceptors.

Photocarrier Generation Mechanism of Layered Organic Photoreceptor Using Two-Coupler-Based Bisazo Pigment

We have investigated the photocarrier generation mechanism of a layered organic photoreceptor containing two-coupler-based fluorenone bisazo pigment in comparison with the conventional bisazo pigment which has the same coupler components. The magnitude of quantum efficiency of both layered photoreceptors in a higher electric field approaches that of the charge separation efficiency of a photogenerated exciton to form a geminate hole-electron pair. The charge separation efficiency of the layered photoreceptor containing the asymmetrically structured bisazo pigment is about 1.5 times larger than that containing the symmetrically structured pigment. The charge separation is considered to be attributed to the photoinduced electron transfer; the disparity of the reaction rate could not be explained based on the energy gap. The size of the primary particles of asymmetrical azo is found to be about half the size of those of the symmetrical azo. It is therefore concluded that the high photosensitivity is due to the size of the azo pigment particles which practically induce a change in the number of charge separation sites.