Optimizing Photovoltaic Response by Tuning Light-Harvesting

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Optimizing the photovoltaic response by tuning the light harvesting nanocrystal shape synthesized using a quick liquid-gas phase reaction Sayantan Mazumdar, Muthusamy Tamilselvan, and Aninda Jiban Bhattacharyya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08595 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 23, 2015

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ACS Applied Materials & Interfaces

Optimizing the photovoltaic response by tuning the light harvesting nanocrystal shape synthesized using a quick liquid-gas phase reaction Sayantan Mazumdar, Muthusamy Tamilselvan and Aninda J. Bhattacharyya* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India KEYWORDS: sensitized solar cell, photoanode, recombination resistance and electron lifetime, liquidgas phase reaction, nanocrystals with mixed crystal phases and shape, ac-impedance spectroscopy

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ABSTRACT The electron recombination lifetime in a sensitized semiconductor assembly is greatly influenced by the crystal structure and geometric form of the light harvesting semiconductor nanocrystal. When such light harvesters with varying structural characteristics are configured in a photoanode, its interface with the electrolyte becomes equally important and directly influences the photovoltaic efficiency. We have systematically probed here the influence of nanocrystal crystallographic structure and shape on the electron recombination lifetime and its eventual influence on the light to electricity conversion efficiency of a liquid junction semiconductor sensitized solar cell. The light harvesting cadmium sulfide (CdS) nanocrystals of distinctly different and controlled shapes are obtained using a novel and simple liquid-gas phase synthesis method performed at different temperatures involving very short reaction times. High resolution synchrotron X-ray diffraction and spectroscopic studies respectively exhibit different crystallographic phase content and optical properties. When assembled on a mesoscopic TiO2 film by a linker molecule, they exhibit remarkable variation in electron recombination life time by one order in magnitude, as determined by ac-impedance spectroscopy. This also drastically affects the photovoltaic efficiency of the differently shaped nanocrystals sensitized solar cells.


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1. Introduction The finite stocks of the unevenly distributed global fossil fuels are fast diminishing due to the excessive dependence of modern civilization on them. This has also led to detrimental impact on the global environment and sustainability. As a consequence, research focus has considerably shifted towards development of a wide variety of technologies aiming at efficient capture of solar energy and its conversion to electricity.1-10 In the context of photovoltaics, major emphasis is on the development of materials which can efficiently harvest solar photons and convert a significantly high proportion of the harvested photons in to stable electrical output.11-19 The sensitized solar cells, which were initiated with the advent of dye-sensitized solar cells (DSSCs), are attracting tremendous attention because of several reasons: easy processability using solution based techniques, low cost and high scalability synthesis processes, and satisfactory light to electricity conversion efficiencies.20-23 In DSSCs a mono-layer of light harvesting dye, mostly a ruthenium-based organic molecule is used to sensitize a mesoscopic metal oxide film. Several approaches have been suggested to replace the organic dye by alternative light harvesters, most important among them being the approach of using inorganic semiconductor nanocrystals as sensitizers. Replacement of the organic dyes by inorganic nanocrystals will expectedly lead to several advantages such as (a) higher chemical, mechanical and atmospheric stability, (b) very large extinction coefficient of visible light absorption, (c) tunability of the band gap and band edges via controlling the size and composition, (d) possibility of multiple exciton generation through impact ionization which may push the theoretical maximum conversion efficiency to 44%.12, 24-31 II-VI, IV-VI, III-V semiconductor nanocrystals have been demonstrated as efficient sensitizers in semiconductor sensitized solar cells including ternary and multinary materials.12,

28, 32-40

Cadmium chalcogenide nanocrystals particularly,

cadmium sulfide have been widely reported as light harvesters. Apart from existing in different sizes (and in rare cases shapes) in both ex situ and in situ assemblies, CdS based core-shell structures, metal-doped structures, and also ternary compounds involving Cd and S have been reported as efficient light harvesters.9,

18, 33, 37, 41-46

In spite of the availability of vast literature on various CdS nanostructures,



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precise clarification on the suitability of CdS structure or a class of CdS structures which will yield to high and efficient harvest of solar photons and eventual conversion to electrical energy is still lacking. In this light it would be useful to devise a synthesis strategy which would allow synthesis of nanocrystals with controlled morphological parameters and also systematic studies on their influence on the solar cell performance. In semiconductor sensitized assemblies, electron recombination resistance and lifetime are very important parameters which directly affect the light to electricity conversion efficiency.47 As a consequence, these parameters along with the various interfaces present in the solar cell constitute the figures of merit for performance of a sensitized solar cell.42, 47-49 Ac-impedance spectroscopy is a very powerful characterization tool to probe the various relaxation phenomena occurring at different time scales in bulk and various interfaces electrochemical devices.50-55 Contrary to other widely used techniques such as ultrafast spectroscopy, ac-impedance spectroscopy possesses a distinct advantage in the sense that it allows direct quantitative estimation of charge transfer processes at various interfaces of a semiconductor sensitized solar cell.42, 48-49, 51 In this study, we have employed ac-impedance spectroscopy to extensively probe the electronic properties of nanocrystals employed as light harvesters in semiconductor sensitized solar cells (SSSCs) and the various interfaces present in the SSSCs. Varied and well controlled shaped (and sized) nanocrystals are synthesized using a novel and simple liquid-gas phase synthesis method involving very short reaction times. The charge transfer characteristic parameters estimated using ac-impedance exhibit a direct correlation with the photovoltaic properties of the solar cell.

2. EXPERIMENTAL SECTION 2.1. Synthesis of CdS nanoparticles. Reactions are carried out under nitrogen atmosphere by using a suitable Schlenk line setup. 1 mM of cadmium nitrate is taken in a three neck flask, and is dissolved in 15 ml of oleylamine heated to 1400C to form Cd-oleylamine complex. The appearance of clear solution confirms the formation of the metal complex. Then, the temperature is raised to 1600C, which is taken


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here as the reaction temperature. As soon as the reaction temperature is achieved, H2S gas is introduced into the metal complex solution. H2S gas is prepared by treating FeS with HCl. With the introduction of H2S gas into the flask, the color of the solution starts to change from colorless to light yellow. To restrict the size of the particles, short nucleation burst is introduced through the super saturation of the precursor. After passing the H2S for 10 min, the reaction is stopped by removing the flask from the heating mantle. Following this, the reaction mixture is cooled to 1000C and requisite amount of chloroform is added to remove the unreacted oleylamine. Following the addition of ethanol, the yellow colored product is precipitated and is centrifuged at 6000 rpm for 5 times, each time for 5 min. The cleaned and centrifuged yellow colored powder is dried at 700C in vacuum. The same reaction is carried out at 2000C and 2400C respectively and the products are collected in the same way. The CdS synthesized samples are here after abbreviated as per their reaction temperature viz. CdS160, CdS200, CdS240. 2.2. Structural characterization. Transmission electron microscopy (including high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED)) is done using a JEOL-200kV FETEM. The TEM micrographs are studied by ‘Digital Micrograph’ software. Absorption and emission spectra are recorded using a Lambda 35 UV-Vis spectrophotometer and Perkin Elmer Luminescence Spectrometer (LS-55) respectively. Absorption and emission spectra are recorded for solid powder samples and photoanodes i.e. the CdS sensitized TiO2 films on a FTO substrate. Powder X-ray diffraction is carried out by a Philips X’Pert Pro diffractometer; Cu-Kα radiation, λ = 1.5418 Å, with step width and scan rate of 0.02 and 2.50 per min respectively. Synchrotron X-ray diffraction measurements are carried out with 14 KeV X-ray beam (λ = 1.0287 Å) at BL-18B (Indian beamline), Photon Factory, KEK, Tsukuba, Japan. All the measurements are carried out in Bragg−Brentano geometry with a divergence slit (200 µm), an anti-scattering slit (250 µm), and a receiving slit (200 µm). Well-grounded powder samples are loaded on quartz glass plate with groove. Dwell time for each measurement is adjusted such that a good signal-to-noise ratio can be obtained.



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2.3. Preparation of TiO2/CdS films as photoanodes of solar cell. A composite TiO2 structure i.e. multiple TiO2 layers on an optically transparent electrode has proven to be superior over that of a single layer of TiO2 in sensitized solar cells.56 Almost all the efficient SSSCs reported in recent years contain multiple TiO2 layers fabricated by different procedures and sometimes employ commercial titania pastes.45, 57-58 FTO glass plates are cleaned in a detergent solution using an ultrasonic bath for 30 min and then rinsed with water and ethanol. The FTO glass plates are immersed in 40 mM TiCl4 (aqueous) solution at 700C for 30 min in a Teflon container and washed with water and ethanol to obtain a compact and transparent layer of TiO2 on the FTO substrate. The compact layer prohibits the contact of the liquid electrolyte with the conducting substrate. Titanium butoxide is added in 6 N HCl solution drop wise and is stirred for 30 min in an ice-bath. Then 15 ml of this is transferred in a Teflon container along with a FTO-glass coated with the TiO2 compact layer. A hydrothermal reaction is carried out in a stainless steel autoclave at 1500C for 4 h to grow vertically aligned TiO2 rods on the FTO-glasses.59-60 Following the reaction, the FTO plate is again immersed in a 40 mM TiCl4 solution at 1000C for one hour. This is done to enhance the surface area and reduce the number of grain boundaries. After that the electrodes are annealed at 4500C for an hour resulting in a composite TiO2 film. The electrodes are dipped in a solution of thiolglycolic acid (TGA) in acetonitrile which acts as a bi-linker molecule, for 12 h to attach the various CdS nanocrystals with TiO2 under N2 atmosphere. Following this step, the bi-linker functionalized electrodes are immersed in dispersions of various CdS nano-powders in toluene for 48 h. Some of the CdS-TiO2 films on FTO substrates are additionally co-sensitized by cadmium selenide (CdSe) nanoparticles and by zinc sulfide (ZnS). Both CdSe and ZnS are coated using SILAR method. In brief, the electrodes are dipped in 0.03 M cadmium nitrate solution (Cd2+ source) in ethanol and 0.03 M sodium borohydride-selenium dioxide solution in ethanol with 0.06 M in cycle, 30 s cycles in each case and total 6 cycles. The dipping processes are carried out under N2 atmosphere. ZnS coating is also been done by SILAR where 0.2 M zinc acetate and 0.2 M sodium sulfide, both in aqueous solution serve as the Zn2+ and S2- sources respectively. The ZnS coating are performed in 2 cycles each of 1 minute duration.


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2.4. Solar Cell Fabrication. Scotch-tape (thickness ~ 50 µm) is stuck on the sides of the photoanode which acts as the spacer. A drop of sodium polysulfide electrolyte (1 M Na2S, 1 M S) is used as the electrolyte and Cu2S electrode obtained from a piece of brass serves as the dark cathode. A sandwich type cell is assembled, typically (5 × 5) mm being the active area for illumination and all spectroscopic characterizations. A schematic representation is portrayed in Scheme 1. 2.5. Electrochemical and photoelectrochemical measurements. PV performances of the as fabricated solar cells are measured under the illumination using an Oriel class 3A solar simulator (1 Sun, 1.5 G AM). I-V characteristics are measured by a Keithley sourcemeter (Model-2420). In case of OCVD measurements, solar cells are illuminated by the same light source and the decay of the open circuit voltage is measured as a function of time after switching off the light. AC-impedance spectroscopy is measured in the dark in a three electrode cell configuration. The photoanode serves as the working electrode, the brass electrode as the counter electrode, an Ag-wire as the reference electrode and an aqueous sodium polysulfide solution serves as the electrolyte. For each sample, ac-impedance spectroscopy (signal amplitude: 20 mV; frequency range: 1 MHz to 0.1 Hz; Solartron) is measured under constant applied bias voltages (0 - 0.8 V, interval= 0.05 V).

Scheme 1. Schematic of the semiconductor sensitized solar cell used in this study



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3.

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RESULTS AND DISCUSSIONS

3.1. Electron Microscopic and Synchrotron Powder X-ray Diffraction Analysis. The simple reaction carried out over short reaction times, as described in the experimental section, not only lead to excellent control over particle size and shape but also resulted in satisfactory yields. The shape, structure and size of the CdS samples are studied by transmission electron microscopy. In figure 1, representative TEM images for all the samples i.e. CdS160, CdS200, CdS240 are presented along with the corresponding HRTEM images. Additional TEM images, SAED patterns and size distributions are provided in the supporting information (c/f figures S1 and S2). It is evident that the particle size (average particle size≈ 6 nm) for the CdS160 sample is the smallest and is spherical in shape. The crystallinity of CdS160 is less than CdS200 and CdS240 as can be observed from the TEM images and ring structures in the SAED pattern (c/f figure S1). Additionally due to poor crystallinity, the planes are also not very clearly and consistently visible in the HRTEM. An approximate estimate of the d-spacing is measured using the regions where the planes are visible (d= 0.29 nm zinc blende (200) plane, d=0.335 nm zinc blende (111) plane). Only planes of zinc blende crystallographic phases are detected. This is also supported by the synchrotron powder XRD data (c/f figure 2) where lines corresponding to only zinc blende phases (JCPDS 80-0019) are detected (The laboratory XRD-data showed similar patterns but the peaks are not as well-resolved as the synchrotron data and suffer from low signal to noise ratios. In some cases few peaks are noisy (c/f figure S3). Due to lower crystallinity, the lines in the CdS160 sample are also broader than the other CdS samples. For CdS200, the nanocrystals are bigger in size and crystallinity is higher than the CdS160 nanocrystals and average diagonal length is ≈ 12 nm. The shapes of the nanocrystals are “rounded rectangle”. The XRD lines are narrower and the SAED patterns contain bright spots (c/f figure S1). Planes are clearly visible in the HRTEM


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images. Both wurtzite and zinc blende phases are detected. In the XRD pattern traces of wurtzite phase (JCPDS 41-1049) are found. Wurtzite lines corresponding to (100), (101), (103) planes are clearly visible along with a low intensity peak for (102) plane in CdS200. The zinc blende: wurtzite ratio is calculated to be 37: 63 from the peak intensity ratios (Table S1).

Figure 1. Transmission electron micrographs of CdS160, CdS200 and CdS240. Respective HRTEM images are shown on the right hand side. Various crystallographic planes with their d-spacing values are also mentioned. Planes for zinc blende and wurtzite phases are marked in green and red respectively. For the CdS240 sample, a particle’s FFT (marked in white) exhibiting the hexagonal pattern is shown in the inset.



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Sample CdS240 contains rectangular shaped samples with an average diagonal length of 12 nm. Crystallinity is even higher than both CdS200 and CdS160 as evidenced from the TEM, HRTEM images and synchrotron powder XRD patterns. Along with zinc blende phase, hexagonal wurtzite phase is clearly present and the fraction of wurtzite phase is more than that of CdS200.All the wurtzite planes including the (102) plane are clearly visible. The zinc blende: wurtzite ratio is calculated to be 19: 81 from the peak intensity ratios (Table S1). In spite of having the lowest zinc blende phase content compared to CdS160 and CdS200, due to higher crystallinity even a small peak (200) corresponding to the zinc blende plane is also visible. So, at low reaction temperature i.e. at 1600C only kinetically favorable zinc blende phase is formed. With increase in reaction temperature up to 2000C, the spherical shape tends to change to rectangular shape with increase in average particle size. The thermodynamically favorable wurtzite phase starts to form and crystallinity also increases. Further increment in reaction temperature to 2400C does not result in increment of the particle size, rather the particle shape changes slightly to rectangle. Also the crystallinity and the content of wurtzite phase increases significantly. We conclude that, following a certain reaction temperature the particle size does not change. It is rather the shape, crystallinity and content of the crystallographic phases which get altered.

Figure 2. Synchrotron powder X-ray diffraction data (λ = 1.0287 Å) of various CdS samples synthesized at different temperatures.


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3.2. Photoanodes for Liquid-junction Solar Cells. As discussed earlier, a composite multilayer photoanode (fabricated by TiCl4/TiO2/TiCl4 treatment) is used for the study. Unlike the previous study where a similar assembly of TiO2 nanoparticles were used (e.g. Zhong et al ref. [56]), here vertically aligned TiO2 rods are used as they facilitate unidirectional electron transfer and thus minimize recombination. This has been proven to be highly beneficial with respect to photoelectrochemical properties.61-62 Regarding assembly of light harvester nanocrystal with the oxide material, postsynthesis assemblies (direct absorption, linker assisted, electrophoretic deposition) have several advantages over direct growth deposition techniques (CBD, SILAR).63-66 Post-synthesis attachment of nanocrystals are advantageous because pre-synthesized highly crystalline semiconductor nanocrystals with controlled particle size and size distribution can be attached to the oxide film and uniform surface loadings can be achieved. Postsynthesis attachment of nanocrystals to the oxide film has proven to be a very efficient method for attachment of pre-synthesized semiconductor nanocrystals to the oxide film. This method involves the tethering of the sensitizer to the oxide film by a bi-functional linker molecule resulting in uniform and dense coverage of sensitizer over the oxide film.67 In this work, thiolglycolic acid is used to attach the pre-synthesized CdS nanocrystals with the mesoscopic composite TiO2 film. Additionally, the CdS sensitized TiO2 film is co-sensitized by cadmium selenide (CdSe) nanoparticles to shift absorption onset towards longer wavelengths and by zinc sulfide (ZnS) for passivation of the trap states.47 3.3. Steady State Optical Properties of Materials and Photoanodes. Figure 3 (a) shows the absorption spectra of CdS160, CdS200 and CdS240. All the samples exhibit an onset around (600 – 540) nm. The band gaps calculated from the Tauc plots show a decreasing trend with increasing synthesis reaction temperature (c/f figure S3): 2.31 eV (CdS160), 2.15 eV (CdS200), 2.05 eV (CdS240). So, the absorption onset slightly shifts towards longer wavelength with increasing synthesis temperature which also evidenced from sample’s color which turns to yellowish-orange from yellow with increasing synthesis temperature. The photoluminescence spectra (c/f figure 3 (c)) for all the three samples show broad profiles and all depict similar PL intensities. The optical properties of the photoanodes sensitized by the



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CdS samples are also characterized in order to ascertain the properties of nanocrystals following sensitization of the TiO2 film via a linker molecule. Figure 3 (b) depicts the absorption spectra of the photoanodes. Though all the samples show similar absorption onset, the photoanode sensitized by CdS160 shows the highest absorbance followed by the sample CdS200 having absorbance slightly lower than that of CdS160. The photoanode sensitized by CdS240 has much lower absorbance than that of other two photoanodes.

Figure 3. (a) and (c) Normalized absorbance spectra and photoluminescence spectra of various CdS powder samples, (b) and (d) Absorbance spectra and photoluminescence spectra of various CdS samples attached with a TiO2 film on a FTO substrate.


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Though the PL intensities are similar in the powder samples but when they are attached to the TiO2 film by a linker molecule in case of the photoanodes, the PL intensities for samples CdS200 and CdS240 are much less than that of the sample CdS160 (c/f figure 3 (d)). Higher quenching of photoluminescence in case of CdS200 and CdS240 indicates more favorable kinetics of electron transfer from the conduction band of CdS to the conduction band of TiO2 than that of the sample CdS160. 29 3.4. Characterization of the Semiconductor Electronic Properties and Semiconductor-Electrolyte Interfaces. AC-impedance spectroscopy measurements are carried out to gain direct insight on the recombination properties of the photoanodes used in the cell. As discussed earlier electron recombination properties directly affect the cell performance.47,

68

Impedance measurements are done on each

photoanodes sensitized by different CdS samples at different applied voltages. Representative Nyquistplots are shown in figure 4 for two particular voltages 0.1 V and 0.5 V respectively. The complex impedance spectra are fitted using a model (Figure 4 inset and the schematic in figure 4)47 proposed by the group of Bisquert. This model was first proposed to model highly efficient quantum-dot sensitized solar cells based on TiO2 films sensitized by CdS/CdSe nanocrystals grown by SILAR method. In this model, the recombination resistance of the photoanode at the electrolyte interface, indicated by the bigger semicircle in the Nyquist plot, is proposed to be a very important parameter to control the photovoltaic performance. It is inversely proportional to both the recombination rate and the density of electrons in TiO2.44, 47, 68-69 Moreover, electron lifetimes can also be calculated from this model which is a direct quantification of photovoltaic performance of a solar cell.47, 60 An array of semiconductor sensitized devices are modelled using this transmission line including high performance solar cells where the recombination resistance and electron lifetime are supposed to be a direct measure of performance of the device.37, 44, 47, 68-69 The potential of counter electrodes can also be measured using the same model however, this is beyond the scope of this paper as same counter electrode is used in all the cases.70



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Figure 4. Nyquist plots of different photoanodes (TiO2 film on a FTO substrate sensitized by different CdS samples) at different applied voltages 0.1 V and 0.5 V. The simplified transmission line (inset) is used to fit the impedance data. The scheme below is depicting the equivalent circuit model for a semiconductor sensitized solar cell.


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In the schematic in figure 4, Rs is the series resistance between the TiO2 film and the OTE, Rtr (= rtrL) the electron transport resistance in the TiO2 film, Rr (= rrL) is the recombination resistance i.e. the charge transfer resistance at the semiconductor-electrolyte interface and Cµ (=cµL) is the chemical capacitance which is a resultant of the change of electron density. In the schematic in figure 4, the three circuit elements corresponding to Rtr, Rr and Cµ are depicted in lower case as they are the element per unit length for a film of thickness L and distributed in a repetitive arrangement of a transmission line. RCE and CCE are respectively the charge transfer resistance and the interfacial capacitance between the electrolyte and the counter electrode. As the model represents the impedance of diffusion and recombination, this directly leads to an easy correlation between the impedance spectra and the ongoing processes in the SSSC.47 Figure 5 (a) shows the plot of recombination resistance (Rr) plotted against the applied voltages. The photoanode sensitized by the CdS200 sample shows the highest recombination resistance over all applied voltages. The recombination resistances of the photoanodes sensitized by the CdS240 and CdS160 particles are approximately one order in magnitude lower than that of the CdS200 sensitized photoanode. The CdS240 has higher recombination resistances at lower applied voltages but it steadily decreases and the Rr values are lower than that of the CdS160 at applied voltage > 0.4 V. The chemical capacitance versus applied voltage is shown in figure 5 (b). The plots are in general, complex showing a slow increase with increasing applied voltages. Cµ is the parameter which is directly proportional to the energy difference between the Fermi level and the conduction band edge of the TiO2 film. The changes in Cµ values may be attributed to the varying degrees of CdS loading which prevents direct contact of the electrolyte with the TiO2 film and accordingly move the TiO2 conduction band. The lifetime τ is calculated by the following equation, τ = RrCµ.51 The obtained lifetimes from ac-impedance spectroscopy measurements are plotted against the applied voltages in figure 5 (c). Again, the lifetime is almost the highest for the photoanode sensitized by the CdS200 sample over all the applied voltages. The lifetime for CdS240 is lesser than CdS200 followed by CdS160. The lifetime values for CdS240 tend to gain almost the same values as that of CdS200 at around 0.2 V. Then, it steadily decreases and attains lower lifetime



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values than that of CdS160 at higher applied voltages. The lifetime values for the photoanode sensitized by CdS160 remain almost the same over the applied voltage range. It is worth mentioning here that the recombination resistance and lifetime values for the photoanode sensitized by the CdS200 sample is comparable with values achieved in photoanodes sensitized by CdS/CdSe nanocrystals grown by SILAR method or doped CdS/CdSe nanocrystals.47, 56, 68 The low electron lifetimes in case of CdS160 may be attributed to unfavorable electron transfer kinetics from CdS to TiO2 which is also evidenced from the PL spectra of the electrodes. However, the lifetime remains almost unchanged over applied voltage range due to higher coverage and loading on TiO2 film as evidenced from the absorption spectrum.

Figure 5. (a) Recombination resistance, (b) chemical capacitance of photoanodes (c) lifetime of different CdS sensitized samples as a function of applied voltage. (a), (b) and (c) are obtained from ac-impedance spectroscopy of the photoanodes sensitized by different CdS samples under dark conditions. (d) lifetime values as a function of open circuit voltage obtained from open circuit voltage decay (OCVD) experiment.


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The CdS240 sensitized photoanode showed higher lifetime than CdS160 at lower applied voltages. This may be due to a favorable kinetics of electron transfer (as evidenced from the PL spectrum) but gradually decreases and becomes lesser than that of CdS160 at higher applied voltage. The latter observation can be accounted on the basis of lower CdS loading on the TiO2 film as observed from the absorption spectrum (figure 3c). The open circuit voltage decay (OCVD) measurements are carried out to justify the lifetime estimates by ac-impedance spectroscopy. Sandwich type solar cells as discussed in the experimental section are used for both the OCVD measurements and photovoltaic performances (vide infra). The lifetimes from the OCVD (estimated using τ= -kT/e (dVoc/dt)-1) are plotted against the open circuit voltage (VOC) in figure 3 (d).37, 47 Here as well, the lifetime for the solar cell sensitized by the CdS200 sample is highest among the three samples discussed over the full range of open circuit voltage. The lifetime for the CdS240 is lower than CdS200 but higher than CdS160 at lower Voc however, it decreases steadily and gains lower values than CdS160 at Voc > 0.3 V. On the other hand CdS160 retains almost the same lifetime values at lower Voc (0 V - 0.3 V) but decreases steadily at higher open circuit voltages. Thus, the lifetimes measured from the ac-impedance spectroscopy and OCVD experiment are in good agreement with each other and agree well with reported literature. 3.5. Photovoltaic Characterization. Figure 6 depicts the J-V plots (under 1 sun illumination) of solar cells where different CdS samples are used as the light sensitizer without and with CdSe co-sensitizer. For each sample, measurements are carried out on at least five different cells. The various cell parameters are shown in Table 1.


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Figure 6. Current density-Voltage (J-V) characteristics of photovoltaic cells assembled with photoanodes sensitized by different CdS samples under 1 sun illumination from a class 3A solar simulator (1.5 G AM). Between the devices sensitized only by CdS, the CdS200 sensitized solar cell has the highest short circuit current (Jsc = 3.9 mA/cm2) as well as highest open circuit voltage (Voc = 0.55 V), followed by the CdS240 (Jsc = 2.13 mA/cm2, Voc = 0.54 V) and CdS160 (Jsc = 1.97 mA/cm2, Voc = 0.51 V). The light to electricity conversion efficiency also follows the similar trend. It is highest for the CdS200 (η = 1.29 %), followed by CdS240 (η = 0.6 %) and CdS160 (η = 0.5 %). The same trend continues when the photoanodes are co-sensitized by CdSe. The best efficiency of 3.01 % is obtained for CdS200/CdSe whereas CdS240/CdSe and CdS160/CdSe respectively exhibits efficiencies of 1.62 % and 1.23 %. Co-sensitization by CdSe causes absorbance of


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longer wavelengths and results in higher photocurrent than that of where only CdS is used. CdS200/CdSe sensitized solar cell shows photovoltaic efficiency almost double than those of the other two samples. Table 1. Photovoltaic performance of various solar cells sensitized by CdS samples Jsc (mAcm-2)

Voc (V)

fill factor

η (%)

CdS160

1.97

0.51

0.5

0.5

CdS200

3.9

0.55

0.6

1.29

CdS240

2.13

0.54

0.52

0.6

CdS160/CdSe

4.93

0.49

0.51

1.23

CdS200/CdSe

9.81

0.52

0.59

3.01

CdS240/CdSe

6.01

0.51

0.53

1.62

From the observations on CdS samples synthesized at different reaction temperature it is felt that the CdS200 sample has the highest potential as a light harvester in a sensitized solar cell. The CdS200 sample is highly crystalline and biphasic which helps in decreasing recombination via charge separation due to differences in the conduction band energies. The bi-phasicity is also observed in case of the CdS240 sample. CdS160 suffers from poor crystallinity and is monophasic. Photoanodes sensitized by CdS160 and CdS200 have much higher absorbance than that sensitized by CdS240. The photoluminescence of the photoanodes sensitized by CdS200 and CdS240 are quenched in comparison to CdS160 in spite of all three showing similar PL values when not attached with TiO2 films. This indicates favorable electron transfer from CdS200 and CdS240 to TiO2 than from CdS160. The recombination resistance values, obtained from the ac-


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impedance spectroscopy, are highest in case of CdS200. The electron lifetime values which are proposed to be a key parameter to determine the photovoltaic performance of a solar cell, are highest for the CdS200 sensitized cell obtained from both the ac-impedance measurements and OCVD measurements. All of these result in the best light to electricity conversion efficiency in case of the solar cell sensitized by CdS200. The similar kind of approach is observed in the work of Jara et al where optimization of size of pyramidal shaped CuInS2 nanocrystals is reported. The photovoltaic efficiency increases with increasing size (i.e. the reaction time at constant reaction temperature for all cases) of the nanocrystals but, after a certain size again η starts to decrease. Thus, they find the optimum size of the CuInS2 nanocrystal to obtain the best efficiency.57

Conclusion We have convincingly shown here the growth of CdS nanocrystals of various shapes using a simple and novel liquid-gas phase synthesis method. The liquid-gas phase method provides excellent control over the formation of distinct shapes and properties at very less reaction times. The variation in the recombination resistances and electron recombination, time estimated using ac-impedance spectroscopy and OCVD vary significantly between the CdS samples and shown to be directly correlated to the light to electricity conversion efficiency. The sample having the highest recombination resistance and highest electron lifetime exhibits the highest photovoltaic efficiency, nearly double of the other two samples. The need for optimization of the synthesis parameters such as temperature for a nanocrystal shape is strongly felt to play an important role in not only generating high performance solar cells but will also trigger manifold interests in other contemporary photoresponsive applications such as photocatalysis, optoelectronics and optical image sensing.


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ASSOCIATED CONTENT Supporting Information. More TEM images, SAED patterns, Powder XRD plots, Tauc plots, absorbance spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Tel.: +91 80 2293 2616, E-mail - [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT SM acknowledges CENSE for fellowship, AJB acknowledges DST Nano Mission, New Delhi, CSIR, New Delhi and India-Taiwan Science and Technology Cooperation (GITA/DST/TWN/P46/2013) for financial support. Authors also acknowledge Professors N. Munichandraiah (IPCIISC, Bangalore), S. Natarajan (SSCU-IISc, Bangalore), Chemistry Division-IISC (TEM), AFMM (TEM) and CENSE-IISc for instrumental support.

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Table of content (TOC) Graphics


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Optimizing Photovoltaic Response by Tuning Light-Harvesting

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