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Charge Transport and Recombination in DyeSensitized Solar Cells on Plastic Substrates Alexander Robert Pascoe, Fuzhi Huang, Noel W. Duffy, and Yi-Bing Cheng J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Jun 2014 Downloaded from http://pubs.acs.org on June 27, 2014

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The Journal of Physical Chemistry

Charge Transport and Recombination in Dye-Sensitized Solar Cells on Plastic Substrates Alexander R. Pascoe1, Fuzhi Huang1, Noel W. Duffy2 and Yi-Bing Cheng1*. 1

Department of Materials Engineering, Monash University, Victoria 3800, Australia

2

CSIRO Energy Technology, Clayton, Victoria 3169, Australia

ABSTRACT: Dye sensitized solar cells (DSSCs) formed on plastic substrates boast immense potential for commercialization, however, plastic substrate DSSCs have only achieved efficiencies approximately half that of their glass substrate counterparts. Previous work has largely attributed these medial efficiencies to the inability to sinter the mesoporous TiO2 film. This study entailed the comparison of hightemperature and low-temperature fabricated DSSCs, moving beyond comparisons of device efficiencies, and quantitatively characterizing the physical mechanisms underpinning the gap between plastic and glass technologies. As shown through small perturbation techniques, the dominance of sintered working electrodes was reflected in their superior electron diffusion lengths, which were approximately three to four times greater than the non-sintered films. The improved performance gained after a TiO2 nanoglue treatment was also investigated, and was observed to be caused by a reduction in charge recombination dynamics and a modest improvement in electron diffusion rates. These findings present significant impacts for the benchmark methods currently used to fabricate plastic substrate DSSCs, illustrating the divide that needs to be bridged between low-temperature and high-temperature fabrication techniques. Keywords: Plastic, Dye Sensitised Solar Cells, IMVS, IMPS, Impedance Spectroscopy. 1

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1. Introduction Nanoporous TiO2 films printed on glass substrates have proven the mainstay of dye-sensitized solar cell (DSSC) research. It is this cell design that is responsible for the benchmark efficiencies reported in DSSC literature.1 However, the rigid nature of the glass substrate makes the continuous manufacture of DSSCs a complicated and costly procedure.2 The transfer of this technology to a flexible, low cost plastic substrate promises immense potential for the mass commercialization of DSSC technologies. Plastic substrates present the possibility of roll-to-roll manufacturing, which is an important step away from batch processing and towards mass production. The flexibility of plastics also offers a high degree of device portability and the ability to employ DSSC technologies in select niche applications. Also, the lightweight nature of plastic substrates present advantages for photovoltaic systems installation and applications where the weight of the array on the supporting structure is a limiting factor. Yet, the overriding impetus for the commercialization of plastic substrate DSSCs rests largely on the cell performance. Despite the immense potential of plastic substrate technologies, current cells have only been able to achieve efficiencies approximately 60% of their glass substrate counterparts.3 An obvious limitation of plastic substrate based DSSCs is the inability to sinter the film, due to the melting temperature of the plastic. This hinders the beneficial necking between nanoparticles which results in relatively low electron diffusion throughout the semiconductor film. To overcome the temperature limitations of the working electrode, fabrication techniques have been tailored to meet the low temperature requirements. TiO2 deposition techniques include doctor blade printing,4 electrophoretic deposition,5 chemical vapor deposition,6 spray deposition,7 and pulsed laser deposition,8 which can all be performed at low temperature. Post-deposition techniques have also been utilized to enhance the nanoparticle contact in substitution of the conventional sintering step. Three of these processes include a UV-ozone treatment9 and microwave heating10 and a TiO2 nanoglue treatment11 as a substitute for the sintering step. However, it is the me2


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chanical compression of the TiO2 film that has yielded the greatest results, and other techniques have not been able to replicate the same degree of success achieved by cold isostatic pressing (CIP)12, 13 and uniaxial compression processes.14, 15 The CIP technique used in this study, entails the compression of a TiO2 film while submerged in a pressurized fluid, and has been fully described in previous work.12 The high applied stress at particle contacts, attributed to the high pressures and small contact areas, has been shown to enhance the electrical contact between nanoparticles, mimicking the necking achieved through a sintering process. The low temperature limits of the plastic substrate also presents problems for the fabrication of the counter electrode.16 The deposition of a fluorine doped tin oxide (FTO) coating on glass substrates is typically performed at high temperature, which is not a feasible option for plastic substrate DSSCs. To overcome these barriers, plastic substrate based DSSCs have sought out low temperature equivalents to produce materials suitable for photovoltaic applications. However, to the best of our knowledge, the full effect of these alternate materials on the solar cell performance has not been explored. This study uses electrochemical impedance spectroscopy (EIS), intensity modulated photovoltage spectroscopy (IMVS), intensity modulated photocurrent spectroscopy (IMPS) and photocurrent transients to compare the charge transport and recombination dynamics of DSSCs on glass and plastic substrates. In this work we do not propose any new fabrication techniques, but provide a comprehensive examination of CIP and nanoglue treatments. Our results provide a novel analysis into the importance of the sintering step the quality of electrode materials, and hence we quantify the required improvements if plastic substrate DSSCs are to match their glass substrate counterparts. It is also important to note that the implications of this study also extend beyond DSSC technologies. With the recent development of CH3NH3PbI3 based perovskite solar cells on flexible plastic substrates,17, 18 there is added impetus to assess the efficacy of low-temperature produced mesoporous TiO2 films and blocking layers, which are paramount to 3



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these perovskite based devices. The characterization, presented in this study, of two benchmark lowtemperature TiO2 fabrication techniques will no doubt hold relevance for future photovoltaic technologies utilizing TiO2 nanoparticles on plastic substrates.

2. Experimental Fabrication of DSSCs Solar cell working electrodes were fabricated on both fluorine doped tin-oxide (FTO) coated glass substrates (TEC 8 Ω/□) and indium doped tin-oxide (ITO) polyethylene terephthalate (PET) substrates (Solutia 15 Ω/□). The FTO coated glass substrates were cleaned in three stages using Hellmanex solution, distilled water and ethanol. The ITO covered PET substrates were sonicated in ethanol for approximately 10 mins. A TiO2 slurry was formed using 25 %wt P25 in ethanol, and films were printed using the doctor blade technique on both glass and plastic substrates. The films were then compressed using the CIP method at pressures of 50, 100, 150 and 200 MPa, and one glass substrate film was sintered at 450 °C for 30 mins. Details of the CIP process are described in previous publications.12,

13

Films were

soaked in an N719 dye solution of 1:1 acetonitrile and propanol for an approximate duration of 18 h. Glass substrate cells were formed using a 25 µm Bynel gasket and a pyrolyzed 10 mM chloroplatinic acid (H2PtCl6) solution in isopropanol on FTO glass as a counter electrode. Plastic substrate cells were formed using a 25 µm Bynel gasket and a commercially sourced Pt-ITO-PET counter electrode (Solutia 20-30 nm Pt). Both glass and plastic substrate cells used an electrolyte of 0.4 M LiI, 0.4 M tetrabutylammonium iodide, 0.04 M I2, 0.3 M N-methylbenzimidazole in a mixture of acetonitrile and 3methoxypropionitrile (9:1 volume). Nanoglue solution and post-treatment The nanoglue solution was formed by mixing 2.86 g HNO3 (70 wt%, Aldrich) with 50 mL ethanol (99.7%, Merck), followed by the addition of 10.78 g TIB (titanium butoxide, 97% Aldrich) at room 4


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temperature. After stirring for 2 h, 4.56 g water was added to the solution and stirred for a further hour. The resulting solution was kept static overnight, and then ethanol and distilled water (1:2) was added to the solution. Doctor bladed films were dipped in the nanoglue solution for a duration of ten seconds, and were then heated at 150 °C for 30 mins. Characterization Techniques The film thickness of each working electrode was measured using a Veeco Dektak 150 profilometer prior to dye soaking. The current-voltage response of the completed cells was characterized using a solar simulator with an arc xenon lamp and AM1.5 spectral filter. The solar simulator intensity was calibrated using a silicon reference cell, and an anti-reflective mask was used to illuminate an area of 0.16 cm2 for each device tested. Current-voltage data for the tested devices are presented in Figure S1. Small perturbation measurements were made using a Zahner Zennium Electrochemical Workstation ECW IM6 as a frequency response analyser. Electrochemical impedance spectroscopy (EIS)19,

20

was per-

formed using a 10 mV applied perturbation in the 50 mHz to 500 kHz frequency range in the dark and at open-circuit under illumination. Intensity modulated photovoltage spectroscopy (IMVS)21, 22 and intensity modulated photocurrent spectroscopy (IMPS)23, 24 were performed using a < 5 % perturbation of the steady state illumination. Further information describing the small perturbation measurements is shown in Figures S2 and S3. Photocurrent transients and charge extraction measurements were performed in the same manner as previous reports.25, 26 Impedance, IMVS and IMPS data were analysed using Zview equivalent circuit modeling software (Scribner).

3. Results and Discussion CIP of plastic substrates The CIP technique has been previously used to increase the device performance, and it is widely assumed that the improvements in the efficiency are due to the enhanced contact between TiO2 nanoparti5



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cles.12, 27 However, to date this has not been formally shown. In this study EIS and IMPS small perturbation techniques were used to determine whether the source of the improvement after the CIP process is in fact due to a greater electrical contact between particles. The equivalent circuit used for the modelling of the impedance response is shown in the supplementary data, which has been derived from previous impedance studies.19, 20 The chemical capacitance (Cµ) of each film, as shown in Figure 1, provides an indication of the density of states as well as the charge accumulation.28 In this figure, the films deposited on glass substrates are represented in the filled markers and the films deposited on plastic substrates are presented in the hollow markers. The chemical capacitance density (F/cm3) for each film is presented as a function of the quasi Fermi level. The increase in the chemical capacitance at higher quasi Fermi levels reflects an increase in the electron population within the TiO2 film. The uniform slope of the chemical capacitance data for all films is attributed to the common distribution of trap states within the semiconductor band-gap.28, 29 The vertical shift between these log-linear capacitance responses can be ascribed to either a change in the conduction band edge,30-32 or a difference in the total density of trapping states within the band-gap.33,

34

As shown in previous analysis of the CIP method, the compression

causes a significant volume reduction in films,12 which we have considered when comparing the chemical capacitance density of different cells. Given that the electron density is the fundamental driver for charge transport and recombination, it is important to compare cells at an equivalent electron density for a meaningful analysis. This can be achieved by forming a corrected quasi Fermi level nEF,corr to align the chemical capacitance curves.35, 36 By plotting the measured data as a function of this corrected quasi Fermi level we are comparing cells at an equivalent charge density. This allows a more substantial comparison between cells as well as leading to more robust conclusions. Experimental data before and after the quasi Fermi level correction for all cells is presented in Figures S4 to S11. Using the above mentioned corrected quasi Fermi level, we compared the electron diffusion rates in a series of films deposited on plastic substrates pressed at different CIP pressures. The chemical diffusion 6


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coefficients Dn of the films pressed at 100, 150 and 200 MPa are shown in Figure 2. The diffusion coefficients calculated through impedance measurements are shown in the hollow triangles and the diffusion coefficients calculated through IMPS measurements are shown in the hollow circles. This figure shows an increase in the electron diffusion rate for all cells as a function of quasi Fermi level, which is in accordance with previous literature.37 The slope of this increase provides information concerning the distribution of trapping states within the TiO2 band-gap. Figure 2 most importantly illustrates that the electron diffusion rates increase with a higher applied CIP pressure, which reflects previous proposals that the source of the CIP improvement is from the enhanced electrical contact between nanoparticles. At higher applied pressures, there is not only an increase in the number of contacting surfaces for each individual nanoparticle, but the quality of these contacts is also enhanced given the highly concentrated energy at each point contact. In association with the absorption and charge separation characteristics, the electron diffusion rate plays a large role in determining the short circuit current. Our diffusion coefficient data also agrees with reports of higher short circuit currents observed after a compression process.3, 12 Figure 2 shows a discrepancy in the data measured through IMPS and EIS techniques. This discrepancy largely originates from the overlapping of impedance features measured when using EIS. At the lower potentials, the relative magnitude of the TiO2 transport resistance feature is obscured by the large response of the recombination resistance Rr. This complicates the fitting of the impedance features, and the IMPS measurements can be considered more reliable under these conditions. To provide a thorough and reliable analysis of the devices both EIS and IMPS data has been presented to support any trends.

Comparison of glass and plastic substrates In order to ascertain what the main factors limiting the efficiency of plastic substrate devices, films deposited on glass and plastic substrates were compared with a film deposited on a glass substrate and sin7



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tered at 450 °C for 30 minutes. A moderate CIP pressure of 150 MPa was used to compress both the non-sintered glass substrate and plastic substrate films. All cells were then characterized through EIS, IMPS and IMVS small perturbation techniques. Diffusion coefficients measured by impedance at open circuit and by IMPS at short circuit were used to calculate the shift in the quasi Fermi level between open circuit and short circuit measurements.38, 39 The calculation of the quasi Fermi level shift is presented in the supporting information. Chemical diffusion coefficients for the pressed films deposited on glass and plastic substrates as well as a sintered film are shown in Figure 3. Diffusion coefficients calculated through impedance measurements at open circuit are shown in the triangles and diffusion coefficients calculated through IMPS measurements at open circuit are shown in the circles. The two CIP prepared samples are presented in the red markers while the sintered sample is presented in the black markers. The sintered glass substrate film demonstrates electron diffusion rates approximately an order of magnitude better than the nonsintered film on a glass substrate. This increase in the electron diffusion rate after the sintering step is largely due to the increased nanoparticle necking achieved during the high temperature treatment. Despite the CIP method providing a good mechanical contact between nanoparticles, it does not provide the beneficial chemical necking produced by a high temperature sintering step. It has been suggested in other work that the sintering step reduces the concentration of transport limiting traps, thereby increasing diffusion rates.40 This would result in a significant decrease in the chemical capacitance of the film, as Cµ largely reflects the concentration of trapping states within the TiO2 band-gap. However, we see no evidence of a reduction in trapping states from our chemical capacitance data in Figure 1. The electron diffusion data also reveals a discrepancy between films deposited on glass and plastic substrates which were both pressed at 150 MPa. It is likely that the pressing procedure could damage the conductive oxide layer on the plastic substrate, as the flexible plastic offers little structural support to the brittle ITO layer. The damage of the ITO could in turn increase the transport resistance and reduce transport rates. 8


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If any damage is caused to the ITO layer during the CIP process then it would manifest as an increase in the TCO resistance Rs as measured through EIS. The damage of the ITO layer will be further discussed during analysis of the impedance data. The electron lifetime τn for the sintered and pressed films is presented in Figure 4. The pressed films are presented in the red markers and the sintered film is presented in the black markers. For all cells, the electron lifetime decreases as a function of the increasing quasi Fermi level, which has in accordance with previous experimentation and theory.39 The sintered glass film exhibits lifetimes that are comparable to that of the pressed glass film, yet we observed higher lifetimes in the pressed plastic substrate device. One likely factor that contributes to these higher lifetimes is the use of a different conductive oxide on the plastic substrate compared to that on a glass substrate. The recombination dynamics at the ITOelectrolyte interface could be slower than at the FTO-electrolyte interface, which would contribute to the longer lifetimes observed on a plastic substrate devices. In an optimized glass substrate DSSC, an FTO conductive layer is typically used due to its lower sheet resistance compared to an ITO layer. The low-temperature restrictions of the plastic substrate prevent the standard high-temperature deposition of an FTO material, which is why the less conductive ITO is used as a substitute. The low-temperature deposition process of the ITO also produces an inferior coating to what is possible under hightemperature. Given that the poor quality TCO layer on a plastic substrate is inherent to the temperature limitations of the material, it is not feasible to compare glass and plastic substrates with equivalent TCO materials. However, it is likely that one apparent benefit of this higher-resistive ITO layer is the slower recombination dynamics at the electrolyte/TCO interface. The electron lifetime may also be affected by the charge transport rates in each TiO2 film.40-42 Considering that we observed significantly faster diffusion rates on the glass substrate cells, electrons within these devices may spend less time impeded by trapping states, and are therefore more likely to encounter recombination sites such as I3- ions at the na9



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noparticle surface. The slower charge transport presented in the plastic substrate devices help to reduce recombination events and increase the effective lifetime. The combination of Dn and τn were used to calculate the electron diffusion length Ln, as shown in Figure 5. The Ln values calculated through impedance measurements are shown by the triangles and the values calculated through a combination of IMPS and IMVS measurements are shown by the lines. The measured electron diffusion lengths display a weak dependence on the illumination intensity as seen in previous studies,43-45 approaching the constant response implied by the quasi-static approximation.38 The dominance of the sintered film becomes apparent when observing the longer electron diffusion distances compared to the non-sintered films. The sintered film exhibits diffusion lengths of approximately 3 to 4 times larger than the non-sintered films. This results in a significant increase in the charge collection efficiency which ultimately elevates the total device efficiency. Another way of expressing this advantage is to state that the sintered film can be fabricated 3 to 4 times thicker than the CIP prepared films. This would allow for a greater light harvesting and increased current densities. Previous studies have revealed that nanoparticle P25 films sintered at 150 °C offer diffusion lengths one order of magnitude shorter than films sintered at 450 °C.41 However, we see only a 3 to 4 times decrease in Ln when comparing a 450 °C sintered film with a CIP treated film. Our findings indirectly illustrate the improvements to Ln achieved through the CIP process, which may boost diffusion lengths beyond what is achieved using a 150 °C sintering step. These results further support the CIP process as an effective low-temperature processing technique. Impedance measurements were also performed in the dark at an applied bias of -700 mV. The impedance spectra of the measured cells are shown in Figure 6. This figure reveals three significant features for each cell: the Zre intercept owing to the TCO and metal contact resistance Rs, a high-frequency feature owing to the counter electrode resistance RPt and capacitance CPt, and a mid-frequency feature owing to the TiO2/electrolyte interface recombination resistance Rr and chemical capacitance Cµ. The im10


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pedance spectra reveal that the glass substrate cells are characterized by relatively low Rs and RPt resistances which is in contrast to the high resistances shown by the plastic substrate cells. Once again, the temperature limitations of the plastic substrates restrict the quality of the TCO and Pt-TCO materials that can be deposited, which leads to these higher resistances. According to manufacturer’s data, the 15 Ω/□ sheet resistance of the plastic substrate ITO is approximately twice the value of the 8 Ω/□ FTO layer deposited on the glass substrate. However, the Rs values for the plastic substrates are approximately three times greater than those measured for the glass substrates. The disproportionate increase in the plastic substrate Rs after pressing provides evidence for the damage of the ITO material during the CIP compression of the plastic substrate films, as discussed previously. Further analysis of the Pt-TCO counter electrode resistances are presented in Figure S12. The comparably high resistances seen in the plastic substrate devices lead to significant ohmic losses which ultimately compromise the maximum efficiency of the device. The loss in efficiency may only be 5 to 10 % of the initial value, however this data outlines that it is not only the low-temperature deficiencies of TiO2 film that result in reduced performances, but also the quality of the materials used to fabricate plastic substrate devices.

Characterization of Nanoglue Working Electrode. Another promising low-temperature fabrication technique used to improve the performance of plastic substrate DSSCs is the use of a TiO2 nanoglue post-treatment. In this study, a nanoglue treated P25 film on an ITO/PET substrate was compared with an as-printed P25 film on an ITO/PET substrate as well as a sintered P25 film on an FTO/glass. The enhanced performance of a nanoglue treated TiO2 film has been previously attributed to the increased electron diffusion provided by the nanoglue particles.11, 46 Through the use of photocurrent transients,26 our results shown in Figure 7 indicate only a small increase in the electron diffusion rates. In this figure, the nanoglue film (shown in blue) only displays higher diffusion rates than the as-printed P25 film (shown in red) at charge densities exceeding 5 x 1017 11



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cm-3. The slight improvement in the electron diffusion rate as presented in our data cannot wholly explain the significant increase in the device performance as previously reported.11, 46 The more beneficial effect, which leads to the improved performance, appears to be the increase in the electron lifetime and chemical capacitance produced by the nanoglue treatment. Figure 8 displays the (a) chemical capacitance and (b) electron lifetime of the nanoglue treated film (blue), the untreated P25 film (red) and the sintered film (black). Part (a) of this figure illustrates a significant increase in the chemical capacitance of the nanoglue treated film compared to the as-printed and sintered P25 films, which implies that there is a greater build-up of electrons in the TiO2 layer. Part (b) of the figure reveals the source of this increased charge build-up, which is caused by the increase in the electron lifetime. The deposition of the amorphous TiO2 nanoparticles during the nanoglue treatment effectively form a core shell structure surrounding the existing P25 nanoparticle. This core shell structure suppresses recombination of conductive electrons to the adjacent electrolyte species. The significant decrease in the recombination rate (increase in the electron lifetime) bolsters the electron population within the film, which is reflected in our chemical capacitance data. In this regard, the application of the TiO2 nanoglue post-treatment is similar to a TiCl4 post-treatment which is popularly used to boost DSSC efficiencies.47-49 A TiCl4 posttreatment has been shown to reduce recombination rates and increase the charge injection efficiency due to a down-shift in the conduction band energy,50 however in this study we see no evidence to suggest a change in the conduction band energy level.

4. Conclusions The characterization of the electron diffusion lengths for both sintered and non-sintered films revealed the significant improvements achieved through the heat treatment of the TiO2 nanoparticle films. Sintered glass substrate devices exhibited electron diffusion lengths three to four times greater than the 12


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non-sintered working electrodes, permitting thicker TiO2 films and higher ensuing efficiencies. Impedance data also revealed that ohmic losses, due to differences in the material quality of glass and plastic substrates, partially accounted for the weaker performance of plastic substrate DSSCs. The low temperature restrictions of plastic substrate devices lie at the heart of their medial efficiencies. However, new fabrication techniques such as a TiO2 nanoglue post-treatment offer alternative approaches to the construction of plastic substrate DSSCs, and may be instrumental in bridging the divide between glass substrate and plastic substrate efficiencies.

Author Information Corresponding Author: *Email: [email protected] PH: +61 3 9905 4930 Department of Materials Engineering, Monash University, Victoria 3800, Australia

Acknowledgement This research was supported by the Victorian Organic Solar Cell Consortium, the Australian Renewable Energy Agency (ARENA) and the Australian Government Department of Education.

Supporting Information Available Additional data including the analysis of the small perturbation spectra as well as the calculation of the quasi Fermi level at open circuit and short circuit conditions is presented in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. 13



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Shifts, and Quantification of Recombination Losses at Short Circuit. J. Phys. Chem. C 2007, 111, 14001-14010.

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Figure Captions Figure 1. Chemical capacitance data measured through impedance spectroscopy. Data after the quasi Fermi level correction nEF,corr is presented in the inset graph. The filled markers represent the glass substrate cells and the empty markers represent the plastic substrate cells. Figure 2. Chemical diffusion coefficients in CIP treated plastic substrate cells as measured through IMPS (circles) and impedance (triangles) techniques. Figure 3. Chemical diffusion coefficients for a sintered film and films after a 150 MPa CIP treatment. The filled markers represent glass substrate DSSCs and the empty markers represent plastic substrate DSSCs as measured through IMPS (circles) and impedance (triangles) techniques. Figure 4. Electron lifetimes for a sintered film and films after a 150 MPa CIP treatment. The filled markers represent glass substrate DSSCs and the empty markers represent plastic substrate DSSCs as measured through IMVS (circles) and impedance (triangles) techniques. Figure 5. Electron diffusion lengths for a sintered film and films after a 150 MPa CIP treatment as measured through IMVS/IMPS (lines) and impedance (triangles). For the IMVS/IMPS data, the solid lines represent glass substrate DSSCs and the dashed lines represent plastic substrate DSSCs. As shown in the previous impedance data, filled markers represent glass substrate devices and empty markers represent plastic substrate devices. Figure 6. Impedance spectra measured in the dark at an applied potential of -700 mV. Experimental data are shown in the triangles and the equivalent circuit fits are shown in the lines. The filled markers and solid lines represent the glass substrate devices, while the empty markers and dashed lines represent the plastic substrate devices.

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Figure 7. Tracer diffusion coefficients as measured through photocurrent transients26

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D≅

4d 2 j(t ) π 2 Qt (t )

where

d is the film thickness, j(t) is the current density and Qt(t) is the charge remaining in the film. Figure 8. (a) Chemical capacitance data and (b) electron lifetime data for an as-printed P25 film, a sintered P25 film and a nanoglue treated P25 film as measured through impedance spectroscopy.

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Figures

Figure 1.

Figure 2.

Figure 3.

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Figure 4.

Figure 5.

Figure 6.

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Figure 7.

Figure 8.

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Table of Contents Image

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Charge Transport and Recombination in Dye ... - ACS Publications

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