A Player Often Neglected: Electrochemical Comprehensive Analysis of

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A player often neglected: electrochemical comprehensive analysis of counter electrodes for quantum dot solar cells Riccardo Milan, Mehwish Hassan, Gurpreet Singh Selopal, Laura Borgese , Marta Maria Natile, Laura E. Depero, Giorgio Sberveglieri, and Isabella Concina ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11508 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 13, 2016

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ACS Applied Materials & Interfaces

A player often neglected: electrochemical comprehensive analysis of counter electrodes for quantum dot solar cells Riccardo Milan1,2, Mehwish Hassan3, Gurpreet Singh Selopal1,2, Laura Borgese3, Marta Maria Natile4, Laura E. Depero3, Giorgio Sberveglieri1,2, Isabella Concina1,2,5* 1

Department of Information Engineering, University of Brescia, Via Valotti, 9 - 25131 Brescia, Italy 2

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CNR-INO, SENSOR Laboratory, Via Branze 45, 25123 Brescia, Italy

INSTM & Chemistry for Technologies Laboratory Dipartimento di Ingegneria Meccanica e Industriale Via Branze 38, University of Brescia, 25123 Brescia, Italy

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CNR- Istituto per l'Energetica e le Interfasi, Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova (Italy) 5

Luleå University of Technology, 971 98, Luleå, Sweden

KEYWORDS: Quantum dot sensitized solar cells, Counter electrodes, Solar energy conversion, Charge transfer resistance, Copper sulfide, Treated brass

ACS Paragon Plus Environment

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ABSTRACT: The role played by the counter electrode (CE) in quantum dot sensitized solar cells (QDSSCs) is crucial: it is indeed responsible for catalyzing the regeneration of the redox electrolyte after its action to take back the oxidized light harvesters to the ground state, thus keeping the device active and stable. The activity of CE is moreover directly related to the fill factor and short circuit current through the resistance of the interface electrode-electrolyte that affects the series resistance of the cell. Despite that, too few efforts have been devoted to a comprehensive analysis of this important device component. In this work we combine an extensive electrochemical characterization of the most common materials exploited as CEs in QDSSCs (namely Pt, Au, Cu2S obtained by brass treatment and Cu2S deposited on conducting glass via spray) with a detailed characterization of their surface composition and morphology, aimed at systematically defining the relationship between their nature and electrocatalytic activity.

INTRODUCTION Counter electrodes (CEs) in excitonic solar cells, both dye- and quantum dotsensitized solar cells (DSSCs and QDSSCs, respectively), play a critical role in sustaining the device functional performances, being in charge of the regeneration of oxidized electrolyte.1 This process is critical from both thermodynamic and kinetic viewpoint: on one hand, standard redox potential alignment is mandatory for favoring charge transport between the two partners (electrolyte and cathode) and, on the other hand, speeding the charge transfer kinetics up at this interface is


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critical for reducing the back electron transfer to the photoanode, which eventually heavily affects the capability of device of converting solar energy into electricity. Low charge transfer rate at the CE implies high overpotential for the reduction of the oxidized electrolyte, which results in hurdling the overall electron flow, thus promoting more favored processes, such for instance the mentioned back reaction at the photoanode.2 Beside electrochemical considerations, strategies devoted to material engineering need as well to be considered, related in particular to fabrication of systems featuring high surface area, mechanical and chemical stability, with respect to both the QDs involved as light harvesters and the electrolyte. Polysulfide in water (or sometimes in a solvent mixture considering the addition of a sacrificial hole conductor like methanol) has been demonstrated to stabilize semiconductor QDs against photocorrosion phenomena and is thus the most widely exploited hole transport material (HTM).3 However, most intensive research in the field of QDSSCs has been devoted to the photoanodes, focusing on either the semiconductor metal oxide (nano) structuring or light harvester assembly,4 aimed at enhancing final device functional performances. The tale of cathodes for QDSSCs is a peculiar one: it is indeed a long story as for the knowledge of good electrocatalysts suitable for this application, while at the same time it looks rather short and poor as for the efforts devoted to enhance material properties, especially from a stability viewpoint. Here, stability refers to chemical stability in the medium used as an electrolyte, but considers as well mechanical stability of the involved active species (especially Cu2S, as we are going to discuss below), electrocatalytic activity over the time and reproducibility of CE fabrication approaches.


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More than three decades ago, Hodes and coworkers4 pointed out that metallic sulfides (Cu, Co and Pb) possess good characteristics for being excellent candidates as cathodes in polysulfide electrolyte. Despite this important point being ascertained, relatively low efforts have been carried out over the years devoted to setting up reliable fabrication procedures of CEs based on these materials. However, more recently, a renaissance of interest for this topic seems taking place, which is good news.1, 5-20 We recently investigate the possibility to apply spray deposition for the fabrication of Cu2S-based CEs for QDSSCs,9 and the reader is referred to this study for a broader discussion of the issues of reproducibility. What is surprising in the field is that a comprehensive study shining a definitive light on the relationship between material composition and surface and electrocatalytic activity of the different materials applicable as CEs in QDSSCs has been not yet carried out. Several studies report indeed on the application of different CEs, together with the experimental determination of main corresponding electrochemical features,14-20 but often comparison with both treated brass and gold, whose use is still extremely diffuse, is omitted, thus not definitely clarify the electrocatalyst performances of new cathode materials. Moreover, a reference anode, i.e. an electron transport material sensitized with a semiconductor chosen as a system model, is not employed. Although mentioned studies are valuable and helpful to the scientific community working in the field, definitive clarity on choices needed for counter electrodes in QDSSCs is still to be drawn. Most commonly applied materials as CEs in QDSSCs are platinum, gold and brass, this latter after undergoing a specific treatment (with HCl and the polysulfide electrolyte aimed at generating Cu2S species on the surface). The use of Pt has been loaned from the more studied DSSCs, and gold is often used as a Pt substitute, but noble metals, which present the advantage


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of being easily deposited on conductive glass as thin films, present limited electrocatalytic activity as well-known drawbacks in polysulfide electrolyte,21-26 especially related to corrosion and poisoning effects associated to sulfur species in the hole conductor, which eventually heavily affects the fill factor of the solar cell. The electrocatalytic activity of Pt, indeed, significantly diminishes because of irreversible surface adsorption of sulfur species, which decreases the catalyst active surface, causing an increase of charge transfer resistance at the electrolyte/CE interface and promoting charge carrier recombination at the photoanode.1,27 Furthermore, for practical considerations, high cost and low elemental abundance of platinum preclude cost effective and scalable QDSSC fabrication. Gold is used as alternative to platinum due to its lower interactions with sulfide ions and better electrocatalytic nature, with respect to Pt.27 In particular, Au electrocatalytic activity can be modulated by varying the size and crystal orientation of Au nanoparticles (NPs), as previously observed.29 The catalytic performance of Au NPs based counter electrodes was further improved by using reduced graphene oxide substrate by Zhu et al.30 However, gold is prone to chemisorption of sulfur ions in polysulfide solution and lower electrocatalytic activity is concomitant to surface passivation by sulfur ions in polysulfide electrolyte.24,31 The use of treated brass has gained fame over the years, being probably the most commonly exploited CE, thanks to the good solar to energy conversion performances assured by this material, despite issues related to stability are known also for this approach.32 This cathode is fabricated by treating in polysulfide electrolyte a piece of brass after acidic activation to expose copper by partial removal of zinc (treatment in HCl at about 70ºC for variable times according to the procedure adopted in the specific laboratory). However, mechanical instability has been reported for brass, due to the adverse corrosion effects of polysulfide electrolyte on brass


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substrate, leading to lack of long-term stability,32,33 not to mention the fact the reproducibility is impaired by the differences in brass composition (relative amount of Zn and Cu). Fortunately, as briefly mentioned above, in the last three years several attempts have been done to set up a procedure of Cu2S-based CEs intended to answer the call for reproducibility, stability and, of course, enhanced electrocatalytic activity.1,5-19 Despite drawbacks are known from wellestablished scientific literature as for noble metals (and partially for brass, too), still many studies appear on QDSSCs applying these CEs, which, if allowing result reproducibility (with the application of Pt and Au), possibly underestimate the potential of fabricated devices for solar to energy conversion that would be better highlighted by the use of proper cathode. The aim of the present study is then to go back to basics: we performed a comprehensive electrochemical study of the most commonly materials applied as CEs in QDSSCs, namely platinum, gold, brass and Cu2S. Main electrochemical parameters, such for instance charge transfer resistance, are determined and compared, allowing to quantify the catalytic activity and the voltage drop on the counter electrode/electrolyte interface directly related to the loss of performance in terms of fill factor. The electrochemical characterization is integrated by a morphological and compositional surface characterization of the CEs before and after the contact with polysulfide electrolyte, devoted to investigate the possible changes imparted by the redox processes occurring at the cathode surface.

EXPERIMENTAL DETAILS Materials. Copper nitrate trihydrate (≥ 98%), ethanol (≥ 99.8%), methanol (≥ 99.9%), hydrochloric acid (37%), cadmium nitrate tetrahydrate (≥ 99.0%), zinc nitrate hexahydrate


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(98%), sulfur (≥ 99.5%), sodium sulfide nonahydrate (≥ 98%), sodium hydroxide (99.99%) were purchased from Sigma Aldrich. Thiourea (99.0%) was purchased from Fluka. Bidistilled water was purchased from Carlo Erba. All chemicals were used as received without any further purification. Counter electrode preparation. Pt and Au counter electrodes. 5 nm thick Pt and Au films on FTO glass (sheet resistance 10 Ω/ )) were deposited by sputtering (area of 25 mm2). Brass counter electrode. A piece of brass was treated in HCl (13 M) at 70 °C for the time needed (5-10 min) to activate the surface of the alloy (a pink homogeneous color is detected). Cu2S active species was then generated by treating the CE with polysulfide electrolyte (10 min). Spray deposition of Cu2S. Spray deposition technique was applied to prepare counter electrodes. Detailed preparation procedure can be found in ref. 9. Briefly, a mixture of aqueous solutions of 0.1M Cu(NO3)2 x 3 H2O and 0.05M CS(NH2)2, was sprayed on ultrasonically cleaned fluorinedoped tin oxide (FTO) glass substrate (sheet resistance 10 Ω/ ) maintained at 300°C; the nozzleto-substrate distance was set at 30 cm; pressure of gas carrier (N2) was 4 psi. Spray time: 10 sec. The area of sprayed material was 0.25 cm2 obtained with a square mask (5 mm x 5 mm) above FTO. Polysulfide in bidistilled water (1M S2-, 1M S and 0.1 M NaOH) was used as electrolyte. Prior to use, all the electrodes were immersed in polysulfide electrolyte for 10 min. Symmetric cell assembly. For electrochemical impedance spectroscopy (EIS) in two-electrode configuration, a symmetric cell was fabricated by sandwiching two identical counter electrodes using 25 µm thick plastic spacer (Surlin, Solaronix) and filling with polysulfide electrolyte. Characterization


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Scanning electron microscopy (SEM). SEM analysis was carried out by a field-emission LEO 1525 microscope, equipped with an In-Lens detector for secondary electron imaging. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded by using a Perkin-Elmer PHI 5600 ci spectrometer with a standard Al Kα source (1486.6 eV) working at 250 W. The working pressure was less than 7 × 10−7 Pa. The spectrometer was calibrated by assuming the binding energy (BE) of the Au 4f7/2 line to be 84.0 eV with respect to the Fermi level. Extended spectra (survey) were collected in the range 0−1350 eV (187.85 eV pass energy, 0.5 eV step, 0.025 s/step). Detailed spectra were recorded for the following regions: Pt 4f, Au 4f, Cu 2p, O 1s, Na 1s, and C 1s (23.5 eV pass energy, 0.1 eV step, 0.2 s/step). The reported binding energies (standard deviation ±0.1 eV) were corrected for the charging effects by considering the adventitious C 1s line at 285.0 eV. The atomic percentage, after a Shirley type background subtraction,34 was evaluated by using the PHI sensitivity factors.35 Electrochemical characterization. Electrochemical measurements were made in three and two electrodes configuration. In the first case the cell was made using saturated calomel electrode (SCE) as reference electrode, Pt foil (4 cm2 geometrical area) as counter electrode and the active materials (Pt, Au, brass and Cu2S) as working electrodes (geometrical area 0.25 cm2). The two electrodes configuration was realized by fabricating a symmetric cells, composed of two identical electrodes sandwiched together, with a thin plastic film (25 m) as a spacer, filled with electrolyte (Figure 1). A Frequency analyzer equipped PGZ402 Potentiostat from Voltalab (Radiometer Analytical) was used to carry out the measurements in the three electrodes cell configuration. A SOLARTRON 1260A Impedance/Gain-Phase Analyzer, in a frequency range of 100 mHz to 300


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kHz with an AC amplitude of 10 mV and the DC bias voltage fixed on 0 V was used for the symmetric cell measurements. Electrochemical Impedance Spectroscopy (EIS) measurements were collected in potentiostatic mode at zero potential bias by superposing a 10 mV sinusoidal perturbation and the scan was performed in the frequency range of 100 mHz to 50 kHz. Impedance data were analyzed using commercially available Z-View software and fitted in terms of equivalent electric circuits. EIS measurement to test electrodes stability were scheduled at 1 h, 2 h and 24 h after immersion in polysulfide electrolyte solution. All measurements were done at room conditions (25°C and 1 atm) under dark. Potentiostatic current voltage measurements were carried out in the same three electrodes cell configuration as described above. An overpotential of 100 mV was applied in both cathodic and anodic direction and the respective current densities were measured. The charge transfer resistance is proportional to the reciprocal slope of the current overpotential curve.4,10

Figure 1. Scheme of the symmetric cell used for EIS analysis in two electrodes configuration, showing a couple of equal electrodes sandwiched together, separated by a plastic spacer and in contact through the redox electrolyte.


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Solar cells assembly and functional test. Double layer mesoporous TiO2 photoanodes were prepared by tape casting a transparent layer of 20 nm-sized anatase TiO2 nanoparticles (18 NR-T from Dyesol) on FTO glass substrates (sheet resistance 10 Ω/□), followed by a scattering layer of anatase TiO2 nanoparticles (150–250 nm-sized, WER2-O from Dyesol). After drying 15 min at ambient conditions and then 6 min on hot plate at 120 ºC, the photoanodes were annealed at 500 ºC for 30 min under ambient atmosphere. Thickness was measured by profilometry (average thickness: 12 µm). TiO2 photoanodes were sensitized with CdS QDs by using the successive ionic layer adsorption and reaction (SILAR) technique. A 0.05 M ethanolic solution of Cd(NO3) x 4H2O and a 0.05 M solution of Na2S x 9H2O in methanol/water (50/50 V/V ) were used as sources of Cd2+ and S2-, respectively. For each SILAR cycle 1 min dipping the TiO2 photoanode in metallic precursor was applied, then after washing for unadsorbed chemicals removal and drying under N2, the same process was applied for sulfide precursor. Finally, a passivating ZnS capping layer was deposited by SILAR (1 cycle) using as precursors 0.1M methanolic solution of [Zn(CH3COO)2 x 2H2O] and 0.05M Na2S x 9H2O. Polysulfide in bidistilled water was used as electrolyte. QDSSCs cells were fabricated by sandwiching the CdS sensitized TiO2 photoanodes (4 SILAR cycles) and the counter electrode using 25 µm-thick plastic spacer (Surlin, Solaronix). The current-voltage (I-V) measurements of QDSSCs were carried out using an ABET 2000 solar simulator under one sun simulated sunlight at AM 1.5G (100 mW/cm2), calibrated with silicon reference cell.

RESULTS AND DISCUSSION


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Morphological and chemical surface characterizations. Scanning electron microscopy (SEM) analysis of CEs before and after treatment with polysulfide electrolyte is shown in Figure 2. SEM analysis allows to evidence relevant differences in terms of material morphology, which are reflected in their electrocatalytic activity. As expected, before polysulfide treatment, sputtered noble metals do not present any relevant morphological feature and the deposited thin films are rather homogeneous (Figure 2 a and b). After the contact with polysulfide solution (Figure 2 e and f), both CEs show the formation of discontinuous structures, due to poisoning by sulfur species (as it will be evidenced by both XPS and electrochemical analyses). Moreover, cracks are evidenced on platinum surface (Figure 2 e), more sensitive than gold to poisoning, also ascribable to sulfur scavenging, which could impair the charge transport processes needed for electrolyte regeneration in working devices.

Figure 2. Scanning electron images of Pt (a-e), Au (b-f), regenerative brass (c-g) and spray deposited Cu2S (d-h) counter electrodes showing the changes imparted by the action of the polysulfide electrolyte on material morphology. (a)-(d): CEs before and (e)-(h) CEs after integration in solar cells. Scale bars: 1 µm (insets: 200 nm).


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As for brass, before the treatment with polysulfide electrolyte, the material presents a rather smooth surface (Figure 2 c), characterized by several scratches, without any particular morphological characteristics. After polysulfide treatment, hierarchical quasi-spherical structures are visible (average diameter 4.5 µm), which are composed of self-assembled sheets featuring lateral size of about 100 nm (Figure 2 g). These structures could be extremely interesting from an electrocatalytic viewpoint, presenting a suitable surface area for contact with electrolyte, thus being able, in principle, to speed redox processes up. Similarly, spray deposited CEs undergo relevant morphological changes due to polysulfide action (Figure 2 d and h). As deposited CE features elongated structures, presenting sizes in the nanometric scale as for width and in micrometric scale as for length, which are converted in hierarchical super-structures after the action of polysulfide.9 As evidenced by SEM analysis, Cu2S structures are generated by the reaction of polysulfide with the surface.24 Both HCl-treated brass and spray deposited Cu2S, present a certain similarity in shape, but the relevant difference in size is particularly worth noting. This feature, as it will be confirmed by the electrochemical analysis, will reflect in different electrocatalytic behavior, possibly ascribable to the different surface area available for electrolyte absorption. An extensive XPS analysis (reported in Figure 3, Table 1 and Table 2) has been as well carried out in order to investigate the chemical nature of cathode surface before and after the immersion in polysulfide solution. Surface features, in fact, can be critical for redox interactions within a working device. The BEs observed for Pt 4f (71.1 and 74.4 eV for Pt 4f7/2 and 4f5/2, respectively) on the as deposited Pt CE (Figure 3 a and Table 1) are in agreement with those reported in literature for


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Pt(0).35 The small amount of oxygen revealed on the surface can be ascribed to the presence of hydroxyl species, which is not unusual for a metal.

Figure 3. XPS analysis of counter electrode surface before and after reaction with polysulfide electrolyte. (a) Pt, Au CEs before (continuous line) and after polysulfide treatment (dotted line); (b) brass CE: as received (yellow line); after HCl treatment (pink line) and after polysulfide treatment (black line); (c) spray deposited Cu2S CE before (black line) and after polysulfide treatment (red line). Figure 3 c reprinted in part with permission from ref. 10. Copyright 2014 Elsevier.

After treatment with polysulfide, no significant changes in Pt 4f peak positions and shape are visible, but contributions characteristic of sodium (1071.5 eV, not shown) and sulfur (Figure 3 a) become evident. Analysis of S 2p core level evidences the presence of sulfur in different


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environments (Figure 3 a and Table 1): sulfide (161.9 eV), sulfite (165.5 eV) and sulfate (168.2 eV).35,36 Measurements recorded in two different surface zones of the sample reveal a good homogeneity of the polysulfide treatment.

Table 1. Binding energies (in eV) of different chemical species as revealed by XPS on the surface of materials applied as CEs (before and after polysulfide treatment). Counter electrodes

As deposited Pt

Pt 4f

O 1s

S 2p

Na 1s

71.1,

531.8

----

----

531.5

161.9

1071.5

74.4 Polysulfide treated 71.1 Pt

74.3

165.5 168.2

As prepared Au

Au 4f

O 1s

S 2p

Na 1s

84.0

531.5

----

----

531.5

161.7

1071.6

87.7 Polysulfide treated 84.0 Au

87.6

165.6 168.2

Cu 2p3/2

Zn 2p

S 2p

Cl 2p

O 1s


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Brass

932.6

1021.8

----

----

531.3

1021.9

----

198.7

530.4

952.4 HCl-treated brass

932.5 952.3

HCl-

and 932.3

polysulfide-treated

934.5(shoulder)

brass

942.4(shakeup)

532.1 ---

161.5

----

166.8

531.1 534.7

952.1 954.8(shoulder) 962.8(shakeup)

Cu 2p3/2

Cu

S 2p

O 1s

915.8

169.0

532.0

915.1

162.6

531.4

918.0

168.7

535.8

LMM Cu2S on TCO glass

932.7

as prepared

(shoulder) 935.1 942.9 (shakeup) 955.1 963.0 (shakeup)

Cu2S on TCO glass after

932.3

treatment ---


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with polysulfide

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952.2 ---

Similar considerations can be done for the Au CE. In this case, too, two different regions of the sample were analyzed to confirm the homogeneity of polysulfide treatment. The Au 4f spectral region does not significantly change after polysulfide treatment: peak positions (84.0 and 87.7 eV for Au 4f7/2 and 4f5/2, respectively) and shape are consistent with Au(0).35 Also in this case the analysis of S 2p core level indicates the presence of sulfur in different environments (sulfide at 161.7 eV, sulfite at 165.6 eV and sulfate at 168.2 eV).35,36 The Cu2p/Zn2p3/2 XPS atomic ratio in native brass is 2.0. After treatment with HCl, the amount of copper on the surface drastically increases and it is found four times (8.0) higher than in untreated material (Table 1). As expected, chloride (at 198.7 eV, peak not shown) is as well present on the surface after HCl treatment. Concerning Zn 2p3/2 the BE is characteristic of Zn(0).35 The peak shape and positions observed for Cu 2p in native brass (932.6 and 952.4 eV for Cu 2p3/2 and 2p1/2, respectively) and after treatment with HCl (932.5 and 952.3 eV for Cu 2p3/2 and 2p1/2, respectively) are consistent with Cu(0) and Cu(I) species.37 The presence of Cu(I) on the surface is not surprising and is also confirmed from O 1s signal.37 After treatment with polysulfide significant change of Cu 2p signal occurs. Besides the contribution at 932.3 and 952.1 eV ascribable to Cu(I), other contributions around 934.5 and 954.8 eV are evident (Figure 3b), suggesting the presence of Cu(II) in CuSO4. The presence of Cu(II) is also confirmed by shake-up contributions at 942.4 and 962.8 eV.35,36 The low signal to noise ratio of S 2p core level suggests a poor presence of sulfur on the surface. It is however evident that sulfur is present in two different environments: the main peak at 161.5 eV is due to


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sulfide in Cu2S, while the signal at higher BE (166.8 eV) is characteristic of sulfur in higher oxidation state, such as sulfate (Figure 3 b). In Figure 3 c XPS analysis of spray deposited Cu2S CE has been reported. As reported in detail by Selopal et al.,10 after the treatment with polysulfide of the as prepared CE the desired Cu2S species forms on the surface. The polysulfide treatment, in fact, promotes the reduction of the deposited Cu(II) to Cu(I). This reduction is confirmed by changes in shape and position of both Cu 2p and Cu LMM peaks (Figure 3 c). The formation of Cu2S is also confirmed by a new contribution at 162.6 eV in S 2p spectral region, although the contribution at high BEs characteristic of sulfite/sulfate is still evident. The atomic surface compositions obtained by XPS analyses are summarized in Table 2. It is interesting to remark how the polysulfide treatment heavily affects by physisorption the Pt and Au surface composition, which results dramatically depleted in metals (representing a less than 3%), i.e. of the species electrocatalytically active. This will represent a critical point as for potentially deliverable electrochemical performances and constitute one of the reasons behind the scarce capability of electrolyte regeneration shown by noble metals (especially reflecting in FF of the solar energy converting devices). Higher amount of sulfur is detected on sprayed Cu2S (compared with treated brass), which is possibly due to the higher surface area of smaller hierarchical structures, indicated by SEM. On the contrary, a very reduced amount of sulfur is detected on the surface of brass after the treatment with polysulfide electrolyte, which suggests a reduced amount of Cu2S on electrode surface.

Table 2. Elemental atomic percentages of CE surface, as determined by XPS analysis.


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Counter electrodes Pt

O

S

Na

As deposited Pt

71.9

28.1

----

----

Polysulfide treated Pt

2.7

45.0

26.9

25.4

Au

O

S

Na

As prepared Au

83.1

16.9

----

----

Polysulfide treated Au

0.3

50.8

24.5

24.4

Cu

Zn

S

O

Cl

Brass

22.3

11.0

----

66.7

--

HCl treated brass

45.6

5.7

----

41.2

7.5

--

1.5

72.5

--

Cu

S

O 1s

21.0

9.5

69.5

12.1

16.0

71.9

HCl and Polysulfide 26.0 treated brass

Cu2S on TCO glass as prepared Cu2S on TCO glass after treatment with Polysulfide


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Electrochemical analyses. Both photoanode and counter electrode contribute to the overall cell resistance in excitonic solar cells, which is directly correlated to the obtainable photocurrent in short circuit conditions, according to the relation reported in Equation 1.38 = − [ ( /) − ] − /

Equation 1

The contribution of counter electrode (RCE) to the overall series resistance depends on three main parameters: the charge transfer resistance (RCT) between the electrode and the electrolyte, the resistance due to transparent conducting oxide substrates (RTCO), and the diffusion resistance (RDIFF) of the active species through the electrolyte towards the electrode. The total contribution to RCE is expressed in Equation 2: = + +

Equation 2

One obvious although not trivial strategy to enhance solar device performances is thus lowering the contribution of resistances. Unlike RTCO (which is an intrinsic parameter of transparent conductive oxide), RCT and RDIFF can be in principle modulated by appropriate choice of materials for electrode and electrolyte, towards an optimization of the overall device performances. In particular, a high value of RCT implies a high value of voltage drop in the solar device with a decrease of overall functional performances of the cell (mainly in terms of fill factor).12 It should be remarked that strategies devoted to improve charge transport processes at the interface anode/electrolyte have been indeed implemented over the years,38 especially related to engineering both QDs/TiO2 and QDs/electrolyte interfaces, while detailed analysis of counter electrode electrochemical features, on which subsequent material design should rely, are seldom reported in literature.


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The electrocatalytic activity of the counter electrode, i.e. the rate at which the cathode is able to regenerate the oxidized electrolyte, is indeed a critical parameter, often underestimated, of a working QDSSC. A more effective charge transportation at the counter electrode would decrement its contribution to the overall resistance, eventually resulting in higher device functional performances. Indeed, speeding up this charge transfer would mitigate what is currently considered the bottleneck of electron transport in QDSSCs, i.e. charge accumulation at this interface, which slows down the overall charge transport in the circuit.1 In this work, we analyzed RCT for four different CEs applied in QDSSCs, evaluating it by means of EIS and current-voltage measurements in the three electrodes and the symmetric cell configurations. In particular, the three electrodes configuration allows evaluating the values of charge transfer resistance at the interface between the working electrode and the electrolyte, while all the other contributions are kept constant. In the symmetric cell configuration, no reference electrode is applied and the amount of electrolyte is quantitatively limited by the volume imposed by the plastic spacer, as it is in the working devices. This approach is not widely used to measure RCT, although commonly applied in studies focusing on counter electrodes.12-19,40-46 Indeed, the similarity of the structure of these cells with the configuration of a working QDSSC enables the characterization of CE under working conditions without the presence of interfaces that determine the electrochemical processes pertaining to the photoanode. Since the third electrode (reference electrode) is not present, in this configuration the bias must be fixed to 0 mV, to avoid overpotentials on the electrodes.47 We compared the electrocatalytic behavior of Au, Pt, brass and Cu2S with respect to the reduction reaction of polysulfide electrolyte, whose simplest process is given in Equation 3. + → Equation 3


20

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The RCT of this semireaction corresponds to that of the counter electrode. Figure 4 shows the Nyquist plot of Au, Pt, brass and sprayed Cu2S obtained by EIS measurements, respectively in three (Figure 4 a, b and c) and two electrodes (Figure 4 d) configuration. Two different equivalent circuits (Figure 4 e and f) have to be claimed for the description of physical phenomena occurring in noble metals and semiconductors.

Figure 4. Nyquist plots of different counter electrodes measured with electrochemical impedance spectroscopy on (a, b) symmetric cell configuration and (c, d) in three electrodes configuration at zero potential bias for Pt (triangle), Au (rhombus), Cu2S (reversed triangle) and brass (square). Electrical equivalent circuits for fitting of electrochemical impedance data are as well reported for (e) Pt and Au deposited on FTO glass and (f) brass and Cu2S deposited on FTO glass


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electrodes. RCT is the charge transfer resistance at the electrode/electrolyte interface, Rs is the electrolyte resistance, Cdl is the double-layer capacitance, and Ws is the Warburg element representing diffusion phenomena. The capacitance was replaced by a constant phase element (due to non-ideality).

One Randles circuit (Figure 4 e) was used to fit data pertaining to Pt and Au, while the addition of the Warburg element (accounting for diffusion phenomena), was necessary (Figure 4 f) for the analysis of Cu2S and brass CEs. The first impressive difference between the counter electrodes based on noble metals and semiconductors is indeed the complete absence of the diffusion pattern. This indicates sluggish kinetics of the polysulfide reaction on Pt and Au electrodes, which is coherent with both SEM observation (limited surface area) and XPS analysis (depleted amount of Pt and Au after reaction with polysulfide). On the contrary, the contribution of diffusion is clearly visible at low frequencies for the other two electrode materials, indicating a faster kinetics. Comparison of the curves clearly shows that the diffusion resistance is lower for brass with respect to the sprayed Cu2S. A further dramatic difference between Cu2S-based CEs and noble metals based CEs is moreover observed from a quantitative viewpoint comparing the values of the charge transfer resistances (Table 3). Differences of about four orders of magnitude were indeed revealed between the two kinds of CEs by both considered EIS approaches. The RCT value obtained for Pt is comparable with those reported in literature,1 while the value for Au is higher than the reported one for sputtered Au on FTO.41 As for sprayed Cu2S CE, comparison with literature results shows that these electrodes are extremely good with respect to similar materials prepared by different


22

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approaches. Cu2S analyzed herein shows, indeed, a higher electrocatalytic activity than those deposited on reduced graphene oxide1 and electroplated on FTO,32 which reported respectively values of 1.61 Ωcm2 and 2.72 Ωcm2. Similar values found here have been found in case of Cu2S deposited on FTO glass recently investigated by Zeng and coworkers, while use of a graphite film as a conductive support for Cu2S resulted in almost doubling the charge transfer resistance.19

Table 3. Comparison of RCT values of counter electrodes under investigation (reported values are normalized by the geometrical area of the electrode) with previously reported data. Technique

EIS

(symmetric

RCT Pt

RCT

RCT

RCT Cu2S

RCT other materials

(Ωcm2)

Au

brass

(Ωcm2)

(Ωcm2)

(Ωcm2)

(Ωcm2)

2112.3

0.053

---

---

cell) 1528.8

0.20

(this work) EIS (symmetric cell)12

8000

13.2 (CuS) 11.3 (CuS/CoS)

EIS (symmetric cell)13

49.86

---

---

7.02 (Hollow

core

mesoporous

shell

carbon)


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8.25 (CMK) EIS (symmetric cell)14

38.5

---

---

1.89 (Cu2ZnSn(S1-xSex)4 ; x=0.5)


---

---

---

75.45 (CuInS2) 18.79 (CuInS2:carbon, 1:1)


1510

---

---

0.56 (bare Cu2S) 1.02 (graphite powder

film

supported Cu2S) EIS (symmetric cell)19

19700

---

---

130 (PbS)


47.3

---

13.2

2600 EIS2

998

5.7 (bare Cu2S)

---

---

1.61 (Cu2S

on

reduced


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graphene oxide) Relevant differences in Pt RCT are ascrivable to the presence of absence of methanol as sacrificial hole scavenger in the electrolyte (which drastically reduces the value of charge transfer resistance).

Other metal sulfide have been investigated and RCT were found at least two orders of magnitude higher than bare Cu2S (Table 3), while very good RCT value has been found for Cu2ZnSn(S1xSex)4.

Among the metal sulfides potentially useful as cathodes in QDSSCs, CoS, PbS and NiS

are particularly worth mentioning. CoS and PbS present similar electrocatalytic activity as Cu2S (as estimated from polarization curves),5 while NiS shows reduced activity, and rather high RCT (1057.9 Ω/cm2), even when investigated in relatively high concentrated polysulfide solution.42 PbS CE, although presenting a charge transfer resistance in the order of hundreds Ω/cm2, can deliver quite high fill factor when inserted in a working device (close to 60%),20 but, apart from obvious environmental concerns related to the use of lead, has been shown to reduce its activity (about 10%) after some time, possibly due to a formation of a porous PbO2 layer on the surface.4 CoS seems to be among the most promising materials to find actual applications in QDSSCs: it delivers rather high currents in polysulfide electrolyte and shows acceptable RCT (see Table 3). However, a significant decrease (15-20%) in output power is observed for CoS, possibly due to deactivation of the photoelectrode, which require for electrode restoring after use and impairs the application of this material for short term studies of QDSSCs.5 However, treated brass still outperforms the other materials: this is possibly associated to the presence of metallic zinc, whose sulfidization is rather limited.4 The trend in also confirmed by


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the polarization curves, reported in Figure 5 a. Polarization curves are strictly correlated to electrocatalytic activity of the electrodes: the higher the current under forward bias the higher the electrocatalytic activity.4,41 Both gold and platinum show reduced current values (1.6 mA/cm2 and 2.4 mA/cm2, respectively at 100 mV), while Cu2S CE features a slightly higher current (4.5 mA/cm2 at 100 mV). Highly enhanced current is instead featured by treated brass (27 mA/cm2 at 100 mV).

Figure 5. (a) Polarization curves (normalized to the electrode geometrical area) for the counter electrodes under investigation. (b) J-V characteristics under dark of corresponding quantum dot sensitized solar cells. Brass: black line, Cu2S: red line, Pt: blue line and Au: green line.

It has to be moreover remarked that electrolyte concentration strongly influences the shape of polarization curves, as remarked by previous studies: with particular reference to counter electrodes for excitonic solar cells, Kamat and coworkers2 used the effect of polysulfide concentration to highlight how the irreversibility characteristics featured by platinum, as well as the needed overpotential, are more pronounced in ten times diluted polysulfide, while Cu2Sbased CE still displays very rapid kinetics, although, as obvious, reduced currents. The critical


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role played by electrolyte composition, and in particular by the [S]/[S2-] ratio, has been elucidated by Hodes,5 who found no obvious dependence of polarization curves of MS electrodes (M = Cu, Co, Pb) from the relative amount of sulfur species (while the role of OH- ions resulted much less important). In the present study, we explored the behavior of CEs in a polysulfide electrolyte whose composition is the most commonly used in the literature pertaining to QDSSCs, with no sacrificial hole scavenger (such for instance methanol) added in the solution. Although, the time applied to treat the electrodes with the polysulfide solution prior to use can also potentially affect the subsequent performances, this point is almost neglected in literature. In the present work, all the CEs have been treated in polysulfide electrolyte for 10 min at room temperature: this time ensured a complete physisorption/reaction of the sulfur species with the material constituting the electrode, as evidenced by SEM and XPS analyses previously discussed. These findings strongly suggest a complex relationship between electrode morphology/chemical composition and electrocatalytic activity. Noble metals are subjected to both chemical and morphological changes under polysulfide contact, as evidenced by SEM and XPS analyses. As remarked, sulfur species are present on electrode surface, poisoning both Pt and Au and limiting a direct contact between the electroactive species and the electrolyte. Moreover, the surface of Au CE shows the formation of complex and disordered structures after the interaction with polysulfide (Figure 2 f), while the electrolyte is able to scavenge the Pt electrode causing even more serious structural damages (cracks in Figure 2 e). Cu2S-based electrodes are more resistant in polysulfide electrolyte and deliver higher currents together with highly reduced charge transfer resistances. Moreover, reaction with sulfur species results in the formation of complex hierarchical structures, featuring higher surface areas than noble metals, increased number of


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reactive sites for electrolyte regeneration and an overall porous network extremely suitable for proper electrolyte infiltration, with subsequent benefit for electrocatalytic activity. Polarization curves at very low overpotentials (within 100 mV with respect to the open circuit potential) can also be used to estimate RCT, which is proportional to the reciprocal of the curve slope. The response of Pt and Au CEs overlap in almost all the explored bias range, thus confirming a very similar behavior of the noble metals in polysulfide electrolyte. The slope of the I-V curve of brass is very close to the unity and currents much higher than those recorded for the other materials are observed in all the explored bias range. These findings are probably also partially ascribable to the compact nature of brass electrode: Cu2S deposited on FTO glass indeed experiences also a certain electrical inertia due to the FTO, which is absent in case of Cu2S generated on brass. Previous investigations have indeed highlighted that compactness of electrode strongly contributes to RCT.12 These features enhance the effect of the amount of electro-active material on the current values. Obtained results thus indicate a similar kinetic mechanism of brass and Cu2S materials, as it should be expected, with main differences due to the order of magnitude of the involved currents. Overall, these results would suggest that brass would a better choice than Cu2S due to the highest current associated to lower RCT. However, J-V characteristics of solar cells under dark (reported in Figure 5 b) indicate a different behavior of the CEs under investigation after integration in a working device. Under forward bias in the dark, electrons enter the device through the metal oxide layer (electron transport material) and reduce the electrolyte. Oxidation of resultant species in the electrolyte occurs then at the counter electrode. Slow kinetics of oxidation process at counter electrode||electrolyte interface eventually results in depletion of the electrolyte of species to be reduced at the photoanode (Sn2-), resulting in smaller currents at the same forward bias. At


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relative high forward bias (> 200 mV) higher dark currents are recorded for Cu2S compared with all the other materials under investigation (noble metals are still before their onset point), thus indicating greater catalytic activity (even compared with treated brass) and hence a faster reduction of polysulfide electrolyte.

Functional performances. In order to verify the performances of considered counter electrodes in an actual device, a series of CdS sensitized solar cells were assembled. CdS deposited by SILAR, although not resulting in high functional performances, has been chosen as system model, due to its simplicity of deposition and to its widespread use in literature.9 Figure 6 shows the J-V characteristics of four QDSSCs working with different counter electrodes. Devices exploiting noble metals as cathodes present reduced open circuit photovoltages (VOC = 0.34 V for Pt and 0.32 V for Au) and moderate short circuit photocurrent values (JSC = 5.42 mA cm-2 in case of Pt and 8.04 mA cm-2 in case of Au), with Au outperforming Pt, as expected. Overall curve shape indicates high resistances inside the devices, associated to degradation of counter electrodes during cell operation, heavily affecting the FF of the devices, which are extremely low (17% for Pt and 16% for Au) as often reported in literature.38


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Figure 6. J-V characteristics of CdS sensitized solar cells assembled with different counter electrodes Pt: blue line; Au: green line; brass: black line; Cu2S: red line. VOC values are comparable when brass is exploited as cathode (0.34 V) and significantly higher when Cu2S is applied (0.45 V). Photocurrent density is lower than Au in case of brass (6.72 mA cm-2), as previously remarked,9 while a remarkable enhancement (JSC = 9.58 mA cm-2) is observed when working with sprayed Cu2S. As expected, FF values with Cu2S-based CEs are dramatically increased (brass: 45%; sprayed Cu2S: 40%) as compared with those recorded for noble metals (below 20%) and overall photoconversion efficiency (PCE) is drastically improved (Pt: 0.32%; Au: 0.41%; brass: 1.05%; sprayed Cu2S: 1.70%). Device functional performances highlight that, despite featuring the best RCT among the studied batch, brass is not the CE delivering the best PV parameters in a real device. As expected for the aforementioned reasons, Pt CE is the worst CE of the analyzed series, while Au CE is able to slightly improve the overall performances thanks to a better photocurrent density. Between the two CEs based on Cu2S, sprayed copper (I) sulfide is able to assure the best device performances, although the RCT is almost one order of magnitude higher than that of brass. Surface chemistry seems thus dominating the skill of these electrodes in electrolyte regeneration, and Cu2S guaranteeing the best short circuit photocurrent and consequently the best PCE. Devices sensitized with CdS QDs through SILAR and fabricated using innovative CEs show very variable functional performances (Table 4), depending on both the number of SILAR cycles used to sensitize the photoanodes and on the composition of electrolyte (mixture of water/methanol as solvent results in increased performances). In particular, application of a quasi solid electrolyte results in a relevant increase of open circuit voltage from which a quite high PCE is derived,16 but the simultaneous use of a CoS CE does not guarantee a likewise gain in


30

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short circuit photocurrent. In all cases, platinum is the disadvantaged choice, while treated brass considerably increase the device capability of converting solar light, with particular reference to useful photocurrent and fill factor. Cu2S delivers in all cases the best short circuit photocurrent, suggesting an extremely high catalytic activity, even irrespective of the number of CdS SILAR cycles, i.e. of the amount of exciton couples generated under irradiation, although improvements in terms of fill factor should be still pursued.

Sensitizer

Counter Electrode

FF

PCE

VOC

JSC

(V)

(mA/cm2)

CdS SILAR 4 Pt

0.34

5.42

0.17

0.32

cycles

Au

0.32

8.03

0.16

0.41

Treated brass

0.35

6.72

0.44

1.05

Cu2S

0.44

9.58

0.40

1.70

CdS SILAR 12 Pt

0.61

3.47

0.49

1.05

cycles13

0.54

4.31

0.47

1.08

0.55

3.10

0.48

0.81

0.62

8.43

0.61

3.19

CdS SILAR 9 Pt

0.51

6.23

0.35

1.10

cycles20

Treated brass

0.52

8.26

0.43

1.85

Cu2S

0.53

10.6

0.47

2.60

HCMSC-1

CdS SILAR 4 Treated brass

(%)

cycles47 CdS SILAR 4 CoS cycles16


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Table 4. Functional parameters of CdS sensitized solar cells fabricated in this work and compared with similar devices reported in literature.

Stability. Another key feature for good electrodes is the overall stability (electrocatalytic and mechanical) over the time under working conditions. Stability of all the studied CEs has been evaluated over 24 h by EIS measurements in the three electrodes configuration, Figure 7 shows the related Nyquist plots and fitting data are reported in Table 5.

Electrode

Resistance

Immersion time (h)

material 1

2

24

Pt

RCT (Ω)

1.05 x 105

2.02 x 105

7.43 x 104

Au

RCT (Ω)

6.03 x 104

6.13 x 104

8.40 x 105

Brass

RCT (Ω)

7.10 x 10-4

1.43 x10-2

1.67 x 10-1

RDIFF

1.78 x 10-1

2.25 x 10-1

6.13 x 10-1

RCT (Ω)

1.83 x 101

2.60 x 101

7.95 x 103

RDIFF

5.33 x 101

4.60 x 102

---

Cu2S

Table 5. RCT and Rw obtained by fitting data of stability tests (reported values are normalized by the geometrical area of the electrode). Au and Pt CEs show similar behavior. RCT of Au shows a sharp increase with time, covering the diffusional part of the curve. This behavior is ascribable to both electrode material poisoning by the hole transporter and partial detachment from the substrate, which is suggested by SEM,


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although after 24 h Au is still visible on the FTO glass. The same conclusion can be drawn for Pt, whose RCT has similar values and trend. Very low values of RCT are observed in case of brass, from the first hour of immersion and diffusion resistance slightly increases with time. However, the values are much lower than those reported for all the other studied CEs. Sprayed Cu2S features RCT values three orders of magnitudes lower than those recorded for Pt and Au, but its behavior clearly changes with time: curves show the increase of charge transfer resistance, together with the increase of diffusion resistance (Figure 7 a-b) with time. After 24 hours of immersion in polysulfide, RCT value is so high that the diffusional part of the curve is no longer visible. At this time the fitting of data requires to neglect the contribution of the Warburg element and RCT has increased more than two orders of magnitude. Moreover, Cu2S is no more visible on the substrate. Observed trend may thus be interpreted as due to the progressive detachment of the electrode material from the substrate, which could be also confirmed by the increase of diffusion resistance (Rw) over the time.


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Figure 7. Nyquist plots related to stability of investigated counter electrodes over the time. (a) Gold; (b) platinum; (c) brass and (d) Cu2S. Immersion time: Square (black) 1h; triangles (red line) 2h; circles (blue line) 24h.

CONCLUSIONS

The choice of suitable materials as counter electrodes in quantum dot sensitized solar cells is critical. On one hand, reproducibility of results strongly ask for both materials and preparations that have to be as reproducible as possible, and in this frame Au (being the classic Pt excluded for reasons of chemical, and hence mechanical, stability) is able to satisfy this relevant need. On the other hand, enhancing the electrocatalytic activity of the electrodes is mandatory to try and boost the overall device functional performances and this leads towards materials other than noble metals, if polysulfide is to be the HTM in QDSSCs. This study, meant to go to basics, i.e. to the relationship between surface morphology and composition and electrocatalytic features of most commonly applied materials as CEs, highlights that the use of noble metals should be definitely quit, since they present very high charge transfer resistance in polysulfide electrolyte and do not allow to evaluate the actual functional performances of devices. On the other hand, brass shows a very reduced charge transfer resistance, however increasing over the time (probably due to the consumption of surface Cu2S), which should guarantee optimal performances once integrated in a solar cell. Nevertheless, best functional PV parameters are obtained by applying as CE the spray deposited Cu2S, which can be ascribed to better surface composition and morphology.


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Thus, Cu2S appears as the best choice, taking also into account the issue of reproducibility of electrode preparation (impossible with commercial brass), although more efforts should be devoted to improve the mechanical stability of the material on the conductive substrate. The purpose of this study was to shine definitive light on the issue of counter electrodes for quantum dot sensitized solar cells, motivating the choice on solid basis of material science and electrochemistry, thus clarifying the reasons why a standard cell fabrication is definitely needed to understand what the actual functional performances of these solar energy converting devices could be.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources R. M, G. S. S. and I. C. thank Regione Lombardia and National Research Council Project (“Tecnologie e Materiali per l’utilizzo efficiente dell’energia solare”) for partial funding. ACKNOWLEDGMENT


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I.C. acknowledges VINNOVA, under the Marie Curie Incoming Mobility for Growth Programme (project “Light Energy”.).

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(14)

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