Article pubs.acs.org/JPCC
Photoelectrochemical Investigations of Semiconductor Nanoparticles and Their Application to Solar Cells Jan Poppe, Stephen G. Hickey,* and Alexander Eychmüller Physikalische Chemie, Technische Universität Dresden, 01069 Dresden, Germany ABSTRACT: The objective of this review is to provide an overview concerning what the authors believe to be the most important photoelectrochemical techniques for the study of semiconductor nanoparticles. After a short historical background and a brief introduction to the area of photoelectrochemistry, the working principles and experimental setups of the various static and dynamic techniques are presented. Experimental details which are of crucial importance for their correct execution are emphasized, and applications of the techniques as found in the recent research literature as applied to semiconductor nanoparticles are illustrated.
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INTRODUCTION Due to their unique properties, semiconductor nanoparticles or quantum dots (QDs) are promising materials for use as optically active components for optoelectronic applications in a wide range of devices such as light-emitting diodes (LEDs), sensors, optical detectors, and telecommunication relays and in clean energy conversion cells.1−9 The ability to alter the optical and electronic properties of QDs by variation of their size in particular makes them a promising alternative as lightharvesting entities to the commonly used organic and metal organic dyes in injection type solar cells. Due to quantum confinement, the bandgap of a number of suitable narrow bandgap semiconductors (SCs) that have a direct allowed optical transition, such as PbS or PbSe, can be tuned to the optimal energy range for a maximum efficiency for AM 1.5 illumination at around 1.3−1.4 eV.10 An additional advantageous feature of these materials is that they have a steep absorption edge which quickly rises to reach very high values of the molar extinction coefficient (ε = 105−106 L mol−1 cm−1 for 4 nm PbS QDs in the visible range)11 and can therefore efficiently harvest the photon energies over the entire range of the spectrum. The interest in these materials has also grown in recent years because of the observation that such materials can demonstrate efficient multiple exciton generation (MEG) which is the ability to generate more than one charge carrier pair by the absorption of one high-energy photon.12−18 This provides the opportunity to enhance the efficiency of solar cells and overcome the Shockley/Queisser efficiency limit. Besides injection solar cells, QD-based photoelectrochemical systems can be used as sensors for biological and chemical detection.19−22 By comparison to the already common sensing systems in which QDs are employed as optical transducers, often based on their fluorescence, Förster resonant energy transfer (FRET), or the like, photoelectrochemical systems directly provide an electrical response to the measured property with the obvious advantage that an additional expensive optical detection system is not necessary.8 A wide range of photoelectrochemical techniques have been developed, many originating from earlier research undertaken © XXXX American Chemical Society
on bulk semiconductor/electrolyte interfaces, which can also be employed to characterize nanoparticles or related systems based on them. The objective of this review is to provide an overview concerning what the authors believe to be the most important of these methods. After a short historical background and a brief introduction to the area of photoelectrochemistry, the working principles and experimental setups of the various static and dynamic techniques are presented. Experimental details which are of crucial importance for their correct execution are emphasized, and applications of the techniques as found in the recent research literature as applied to semiconductor nanoparticles and in solar cell applications are illustrated. Historical Background. Research on photoelectrochemical phenomena began in the middle of the 19th century with the observations made by the French physicist Alexandre Edmond Becquerel while investigating the behavior of silver halides deposited at metal electrodes in contact with aqueous solutions under the influence of light. What may be considered to be the modern era of photoelectrochemistry was established in the mid-1950s with the search for the conditions under which semiconductor materials with defined purity could be produced and the means by which their electronic properties may be altered by specific doping. Experiments on n- and p-doped germanium electrodes in contact with aqueous solutions of HCl, KCl, and KOH demonstrated that the photocurrents generated depended on the surface concentrations of both the electrons and holes whose concentrations, in turn, depended on the applied electrode potential and the intensity of the illumination.23 Further important theories were established in the following decades; for example, by means of potentiometric measurements on a variety of different compound semiconductors Williams derived the relationship between the type of doping Special Issue: Michael Grätzel Festschrift Received: February 14, 2014 Revised: April 11, 2014
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Figure 1. Formation of the junction between a bulk n-type semiconductor and an electrolyte containing a redox couple in the dark. (a) Initial situation as the two phases come into contact. Electrons are transferred across the interface until (b) equilibration is reached and the Fermi levels in both phases become equal. (c) Junction under illumination. Adapted from ref 46.
present and the sign of the photovoltage measured.24 Dewald et al. established the concept of the flatband potential based on capacitance measurements of ZnO single crystals.25,26 Furthermore, theoretical concepts for the description of the kinetics of charge transfer27,28 and the phenomenon of photocorrosion were developed. Despite much scientific progress, photoelectrochemistry has tended to remain a niche research area in the field of physical chemistry. The announcement by Fujishima and Honda concerning the sustained photoelectrolysis of water at anodically biased nTiO2 electrodes29,30 and the oil crisis of the early 1970s caused a surge in research in the field of photoelectrochemistry with the goal being to develop a means by which clean and cheap energy could be produced. The introduction of nanocrystalline materials into photoelectrochemical systems has again fueled interest in solar energy conversion. The ability to tune the fundamental physical properties of matter through the synthesis of nanocrystalline materials, simply by varying the synthetic conditions, has opened up many possibilities for applications across a wide range of fields not only for solar energy harvesting but also for catalysis, sensing, light-emitting diodes (LEDs), and optoelectronics.3−6,31−33 Due to the fact that many of the fundamental physical properties are changing once one enters the nanometer size regime, there are a number of essential differences between bulk semiconductors and their nanoparticulate analogues. For semiconducting materials the most obvious manifestation of the size dependence of the physical quantities is the increase in the band gap as the particle diameter is decreased, an effect known as quantum confinement.34−37 As a consequence the spectral sensitivity of a photoelectrochemical cell consisting of nanoparticles is changed in comparison to the corresponding bulk semiconductor system. As the band gap energy of nanoparticles exhibiting the quantum confinement effect is higher than that of the bulk material, higher photon energies are necessary to promote an electron from the valence band (VB) to the conduction band (CB), leading to a blue shift in the spectral response. Consequently the absolute energy positions of the valence and conduction band edges are changed, leading to a different set of electrochemical properties.38−41 Whereas the conduction band is shifted to more negative potentials, the valence band is shifted to more positive ones. According to the Brus equation the extent to which each of the bands is shifted depends in general on the ratio of the effective masses of electron and hole for any given material.34,35
Another typical aspect of nanoparticulate matter is the large surface-to-volume ratio. For example, a 5 nm CdSe nanocrystal is composed of around 2500 atoms of which 20% are located at the surface. This immense interfacial surface area between the nanoparticle and the surrounding medium can have a fundamental influence on its physical properties. The surface can be regarded as a break in the crystal lattice. The atoms present at the surface of the inorganic nanoparticle are in general saturated by organic ligands which are present during the NP synthesis. Defects in the surface passivation, such as socalled dangling bonds, generate electronic levels within the bandgap which can act as trap states for the charge carriers. These surface states have the potential to become strongly involved in optical processes as shown by experimental and theoretical investigations and are also thought to play a major role in the electrochemical behavior of quantum dots.42−44 In bulk semiconductor electrochemistry the space charge layer plays a major role in the electronic processes that occur at the interface between the semiconductor and an adjacent phase. Depending on the doping density the thickness that the space charge layer occupies is in the range of a few tens of nanometers up to approximately 1 μm. Typically the dimensions of nanoparticles are much smaller than this, and therefore when considering these materials the concept of a space charge layer and its consequences are no longer valid. The doping density also has an important influence on the electronic properties of bulk semiconductor materials. If one imagines an ensemble of spherically shaped CdSe nanoparticles with an average diameter of 5 nm, then every particle consists of approximately 2500 atoms. Even at what are considered to be high doping densities of 1018 cm−3 only one in every 15 nanoparticles is calculated to contain a dopant atom. As a corollary to this, it can also be calculated that if every nanoparticle were to contain a dopant the doping density would be equal to ∼1019 cm−3. General Introduction to Photoelectrochemistry. In general, photoelectrochemistry can be thought to be concerned with the electrochemical behavior of interfaces consisting of a photoactive electrode, which is electronically excited through irradiation with light, in the presence of an appropriate electrolyte. The photoactive electrode can simply be a semiconductor or consist of other light-absorbing entities, such as organic dyes, colored metal complexes, or nanoparticles, which are in contact with either a semiconductor or metal electrode. The electrolyte can be a liquid such as an B
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aqueous or organic ion conductor or may be a solid ion conductor containing a suitable redox couple. Here, the basic concepts involved for the understanding of such an interface will be outlined for the more routine case of a semiconductor in contact with a liquid electrolyte. The physical properties of such a system are based on the formation of a semiconductor−liquid junction which occurs upon immersion of a semiconductor into an electrolyte. Upon contact the initial Fermi levels (which is the chemical potential of the electrons in a certain phase) of the two phases become thermodynamically equalized by transfer of the majority charge carriers (electrons in n-type material and holes in p-type materials) to the electrolyte phase.28,45,46 In Figure 1 this process is shown for the case of an n-type semiconductor. If the Fermi level EF of the n-type semiconductor lies above that of the redox potential of the electrolyte (i.e., the electrolyte Fermi level) then equilibration occurs via the transfer of electrons from the semiconductor conduction band ECB to the electrolyte. As a result the adjacent layer inside of the semiconductor becomes depleted in electrons and therefore overall positively charged. In contrast to metal electrodes the positive charge does not reside directly at the interface but rather is distributed across a certain width which stretches from the semiconductor/electrolyte interface to some predetermined depth and forms the so-called space charge layer (SCL). The electric field in this layer is manifested as a bending of the energies of the conduction and valence bands. Under illumination the absorption of a photon by the SC electrode can cause the promotion of an electron from the valence band to the conduction band. To enable this transition the photon energy must be equal to or greater than the energy difference between the bands. As a result, the excited state, which consists of an electron−hole pair, is formed. There are subsequently a number of different possibilities regarding the fate of the excited species. In the trivial case the electron−hole pair can recombine nonradiatively whereby the absorbed energy is either dissipated as heat or radiatively which is accompanied by the emission of a photon (photoluminescence). This happens primarily to electron−hole pairs generated deeper in the SC, i.e., beyond the space charge layer. If the photon is absorbed within the space charge layer, the band bending can lead to a sufficiently fast separation of the two charge carriers, where the electron (or generally: the majority carrier) is forced to move toward the inner regions of the bulk semiconductor and the hole (or generally: the minority carrier) toward the electrode/electrolyte interface. If a suitable redox couple is present in the electrolyte, the holes at the surface can cause the oxidation of the reduced species of the couple. The electrons can then be collected at the back-contact of the semiconductor and transferred through an external current to a counter electrode where some of the oxidized form of the redox couple can be reduced and the circuit completed as depicted in Figure 2. As previously mentioned, the concept of a space charge region layer hardly applies in the case of nanoparticles of semiconductor materials whose diameters are in the range of a few nanometers. Hence the photoelectrochemical properties of nanoparticulate systems often show a different behavior from that of the corresponding bulk semiconductor systems. Due to the absence of the electric field evoked by the space charge layer, a spatial separation of electron and hole is less likely, and therefore both charge carriers can be injected into the adjacent phases, as shown in Figure 3a. The direction of the
Figure 2. Scheme of a photoelectrochemical cell. Adapted from ref 47.
photocurrent flow or rather whether the system shows n- or p-type behavior depends to a greater degree on the electrolyte composition and/or surface effects.2,48,49 To describe the observed photoelectrochemical behavior and the electron/ hole separation, a model based on the kinetic differences in the charge injection into an electrolyte and the back contact (substrate electrode) is often proposed.50−53 The charge separation and therefore the overall cell performance in a nanoparticulate system can be significantly improved by introducing a combination of two or more semiconductor materials.2 Combinations with a staggered arrangement of the band edge energy positions as shown in Figure 3b lead to a rectification of the photocurrent flow. The benefit of such a system is a delay in the recombination of the photogenerated charge carriers due to their spatial separation. Additionally, an extended spectral response through the combination of a large and a small bandgap material further improves the cell efficiency. In the last decades this concept was realized for quantum dot sensitized solar cells (QDSSCs) for a wide range of material combinations such as TiO2−CdS,54−56 TiO2−CdSe,57−60 TiO2−CdTe,61−64 ZnO−CdSe,65−67 ZnO− CdTe,68 ZnO−PbS,69,70 and ZnO−PbSe,71,72 to name but a few. This is the general concept applied to convert solar energy into electrical energy, and much research has been done to find the correct circumstances by which the processes involved can be made more efficient since the aforementioned announcement of Fujishima and Honda in the early seventies.29 However, until now the efficiencies of QDSSCs cannot compete with those of conventional solid state devices based on silicon. Effects such as recombination, charge carrier trapping, and photocorrosion hamper the performance and long-term stability of such photoelectrochemical cells. To optimize the performance of such devices many experimental techniques have been developed to aid in the study of photoelectrochemical phenomena.
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EXPERIMENTAL TECHNIQUES IN PHOTOELECTROCHEMISTRY The experimental setup for many photoelectrochemical techniques is more or less similar to that used in advanced electrochemistry with the obvious addition of a light source by which the sample may be illuminated. The basic instrument present in every experimental setup employed for photoelectrochemical studies is the potentiostat which is used to control the voltage applied between the photoelectrode and a C
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Figure 3. (a) Schematic illustrating the interface of a quantum dot attached to a metallic substrate in the presence of an electrolyte. The constants ke1, ke2, kh1, and kh2 illustrate the different possible charge transfer rates. (b) A staggered arrangement of a large bandgap semiconductor (i.e., TiO2) and quantum dots leads to charge rectification and consequently enhances the efficiency of the photoelectrochemical system.
chosen reference electrode. Depending on the system under examination and on the experimental technique to be employed, the potentiostat has to meet a number of different requirements. Photoelectrochemical phenomena such as surface recombination can be very fast (microsecond time scale),73 and therefore the potentiostat must be capable of following the response to the stimuli signal at the appropriate speed. On the other hand, it should also be capable of measuring a very low current response without the signal becoming compromised by noise. The present state of the art devices have bandwidths of up to a few megahertz and are capable of measuring in the subnanoampere (nA) regime. For more sophisticated experiments the potentiostat should be capable of receiving and providing external inputs and outputs, respectively, so that connections to other instruments such as lock-in amplifiers, waveform generators, etc. may be facilitated. To suppress noise pickup and interference from light sources, e.g., room illumination (especially fluorescent lamps), the experiment should be carried out in a closed Faraday cage, i.e., under conditions of darkness and avoiding any stray light. Depending on the experiment, a wide range of light sources can be used in order to illuminate the interface of interest. Xenon short arc-lamps produce a bright white light that closely mimics natural sunlight and are therefore excellent for investigations under which conditions approaching those of real daylight are required to be simulated. Also, in combination with a monochromator, such light sources can provide all necessary wavelengths at sufficient intensities. For timeresolved experiments on the submicrosecond time scale, light sources that can respond in this time frame are necessary and diode lasers or LEDs can be used in such cases. Depending on the questions the experimenter wishes to have answers to, there are in general three variants on how a photoelectrochemical experiment can be performed: static, transient, or frequency-modulated techniques. It is usually observed that the photocurrent response of a particular system settles to a steady state value after a short period of time. Through the use of static or steady state methods, the magnitude of the resultant steady state photocurrent is measured against another quantity such as the applied potential or the wavelength (energy) of illumination. These experiments are usually performed under conditions of continuous illumination or, if it is necessary to increase the signal-to-noise ratio and reject background currents, under lowfrequency chopped illumination. These techniques are
especially interesting for testing the performances of photoelectrochemical solar cells as conversion efficiencies can be quite easily evaluated. However, other important parameters such as band gap magnitudes and band positions can be determined using these techniques. To gain a deeper understanding of the occurrences involved in photoelectrochemical systems the kinetic processes which take place and their corresponding rate constants need to be considered. Almost by definition data obtained using static techniques, e.g., photocurrent spectroscopy, possess little in the way of kinetic information as only the equilibrium behavior of the system is probed, and therefore to obtain information concerning the kinetics, time-resolved techniques are required. This can be achieved by sampling the photocurrent over time under square wave or pulsed illumination. By analysis of the photocurrent transients obtained for the on- and off-periods, i.e., the rise and decay portions of their response profile, information pertaining to the combined rate constants for the different processes can be evaluated. To obtain reliable data at short time scales, e.g., on the submillisecond regime, the iRdrop of the cell must also be carefully compensated for. Another means by which questions concerning the kinetics may be addressed is the investigation of photoelectrochemical systems by means of frequency-domain techniques. Although the treatment of a system in the frequency domain may often appear to be nonintuitive and require some degree of experience for its interpretation, the power of such techniques lies in the fact that they provide a number of distinct advantages over conventional transient methods. For example, attempts to deconvolute several time constants in the time domain from a transient profile can quickly lead to inaccuracies in the data interpretation. Furthermore, high intensities are often required for excitation during transient experiments to obtain acceptable signal-to-noise ratios. This has the potential to lead to nonlinear responses of the photoelectrochemical system and therefore potentially to an incorrect interpretation. Frequency-domain techniques use low ac intensity excitation signals of 2Eg can generate two carrier pairs in a semiconductor. In bulk semiconductors this effect of carrier multiplication is known as impact ionization, but practically this process is quite inefficient because the energy thresholds to obtain significant gains are much greater than 2Eg.90,91 However, studies exist that show that charge carrier multiplication is greatly enhanced in semiconductor quantum dots, and it has been suggested that this enhancement is caused by the discretization of the electronic structure associated with the strong three-dimensional carrier confinement. This so-called
(1)
where h is Planck’s constant, c the speed of light in a vacuum, iSC(λ) the wavelength-dependent photocurrent generated by the device, λ the wavelength of the incident light, and P(λ) the incident light power. To increase the signal-to-noise ratio and reject stray background currents, a lock-in amplifier (LIA) is usually employed in measuring the current response. The incident light is modulated, often by means of a mechanical chopper wheel, and the current response of the cell is subsequently routed from the potentiostat to the lock-in amplifier circuitry. In this way even very small photocurrents can be recovered from the raw signal. To obtain accurate IPCE measurements the chopping frequency should be chosen to be sufficiently low to ensure steady state behavior and any calibration done as soon as possible after the spectrum is recorded to rule out changes in the lamp intensity over time. If E
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The experimental procedure is very similar to a conventional chronoamperometric potential step experiment. The electrode is biased at the potential of interest, but instead of abruptly changing the applied potential, the electrode is abruptly excited by a beam of light. The induced response of the photocurrent is usually sampled until it settles to a steady state value. The experiment is then repeated at different electrode potentials, and depending on the difference between the applied potential and electronic states (i.e., band edges or trap states) the response will change characteristically. For example, a decay in the photocurrent after switching on the illumination and subsequent overshoot as the light is switched off can be indicative of recombination processes where surface trap states are involved.94−98 Altering the experimental conditions, i.e., by changing the concentrations of the redox species, pH, or the potential, can aid in establishing a kinetic model for the system under investigation.95 To provide rapid pulsed or step-function shaped illumination profiles, which are the preferred profiles in many of the optoelectrochemical techniques introduced here due to the ease of mathematical treatment of the response signal, fast light sources are essential. Diode lasers or LEDs which are triggered by a waveform generator can be operated with rise times which are below 100 ns and are therefore ideally suited as excitation sources in transient-based experiments. For sampling on short time scales a fast potentiostat response is also required. Since the bandwidth of the amplification stage decreases as the gain is raised, a compromise must be found between the available sampling speed and the sensitivity setting of the potentiostat. Under potentiostatic control, the speed of the overall system is usually limited by the RC constant of the cell, and therefore the electrolyte resistance should be minimized by the use of a Luggin capillary and/or by positive feedback compensation.53,94 Due to this limitation, charge injection from absorbing entities such as dyes or QDs usually cannot be sampled by means of an electrochemical interface as these processes have been determined by fast spectroscopic techniques to occur on the picosecond time scale.57,58,99,100 However, Bitterling et al. have shown measurements of photocurrent transients in the subnanosecond time regime by using a specialized experimental design. An impedance-matched, small-volume coaxial twoelectrode cell was mounted on a microstrip transmission line. In combination with high-frequency amplifiers and a mode locked dye laser providing pulses with less than 20 ps half width, the response time of the system could be reduced to 100 ps.101,102 The first suggestions for the theoretical description of the recombination-related transient response of the photocurrent were made during the early eighties.95,103−107 A generalized description of the kinetics of the interfacial charge transfer and recombination via surface states is given by Peter et al.73,94,108,109 The analytical expressions describing the photocurrent response to an arbitrary illumination profile are developed by combining the kinetic rate equations of the elementary steps occurring at the semiconductor/electrolyte interface with a proposed electric circuit model of the interface. In Figure 6 a simplified scheme illustrating the possible transfer routes for the charge carriers in the case of an n-type semiconductor under depletion conditions and with a set of surface states at a discrete energy level within the bandgap (SS) is shown. The photogenerated hole can either accept an electron from the reduced species of the redox couple in the electrolyte or accept an electron from the surface state
multiexciton generation (MEG) effect could be shown through the use of several spectroscopic techniques for the lead chalcogenides12,15,18,92 and also for CdSe,92 InAs,14,93 and Si.16 Recently this effect was also detected by means of photocurrent spectroscopy on PbS quantum dots as reported by Sambur et al.17 In this study PbS quantum dots were chemically linked to a TiO2 single crystal (anatase) by 3-mercaptopropionic acid, a short chained bifunctional molecule. This substrate was then brought into contact with an electrolyte containing the S/S2− couple as the redox system with a platinum wire acting as the counter electrode. The multiplication yields were obtained by normalization of the IPCE measured for this system by the fraction of the light being absorbed by the PbS quantum dots yielding the absorbed photon-to-current efficiency (APCE) which is a measure for the quantum yield of charge carrier extraction. Carrier multiplication thresholds of 2.5 ± 0.25Eg and multiplication yields of up to 50% at 3Eg were obtained for quantum dots of several sizes as shown in Figure 5.
Figure 5. (a) IPCE spectra of various bandgap PbS QDs linked to an anatase (001) surface. Photocurrents were acquired in an aqueous electrolyte (0.5 M Na2S, 0.01 M S in 0.1 M NaOH) at short circuit in a two-electrode configuration vs a Pt wire. (b) Magnified section of the IPCE spectra (solid dots) and the corresponding QD absorbance (dashed lines) in the NIR region. (c) APCE values as a function of the incident photon energy. (D) APCE values versus the incident photon energy normalized to the QD bandgap. Reprinted with permission from ref 17.
Transient Photocurrent Response to Square Wave Illumination. Using this technique the photoactive electrode interface is excited under square wave illumination, and the photocurrent response of the system is recorded as a function of time. The evaluation of the transient profiles provides insights concerning the rates of the different charge transfer processes involved, their competition, and the dominant direction of the charge carrier flux. As in control theory, the illumination profiles typically used for transient experiments are pulses which must be of sufficiently short duration to approximate the Dirac function (δ function) or square wave modulated illumination. F
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h+ + X → Y
(2c)
k2
e− + Y → X
(2d)
k3
(2e)
Y+R→X+O
The species X and Y in the scheme represent the surface concentration of occupied (in this case by an electron or in general by the majority carrier) and unoccupied surface states, respectively. By formulation of the corresponding kinetic differential equations the time-dependent photocurrent of the system can be derived. The photocurrent, in turn, is composed of a contribution from an electron and a hole jphoto (t ) = jh (t ) + je (t )
As the transit time for minority carriers (holes in this case) is short (
