Solid-state electrolytes from polysulfide integrated polyvinylpyrrolidone for quantum dot-sensitized solar cells

Abstract

Solid-state electrolytes from S²⁻/S_n²⁻ integrated polyvinylpyrrolidone (PVP) are synthesized by a simple blending method. The ionic conductivity, charge-transfer ability, and therefore photovoltaic performances are optimized by adjusting the Na₂S/S stoichiometric ratio. The quantum dot-sensitized solar cell (QDSSC) is assembled by sandwiching the solid electrolyte between a CdS-sensitized TiO₂ anode and a CoSe alloy counter electrode. An optimal efficiency of 0.55% is measured for the QDSSC employing PVP/10Na₂S–S solid electrolyte. The present work demonstrates the feasibility of designing cost-effective solid-state electrolytes with PVP, and the photovoltaic performances of QDSSCs can be further elevated by optimizing the synthesis conditions.

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1 Introduction

Electricity generation by photovoltaic conversion has been a persistent objective for today's low-carbon economy.^1–3 Among the diverse range of photovoltaic cells, the quantum dot-sensitized solar cell (QDSSC) is displaying superiorities of high theoretical efficiency, cost-effectiveness, and scalable photosensitizers.^4–6 A typical QDSSC comprises a TiO₂ anode, CdS quantum dot, metal sulfide counter electrode, and liquid electrolyte containing S²⁻/S_n²⁻ redox couples. Under sunlight irradiation, the electrons in the valence band of CdS quantum dots absorb photons and jump to their conduction band, leaving holes to transfer in an opposite direction. The electrons subsequently flow to the conduction band of TiO₂ nanocrystallite and transfer to FTO glass along conducting channels formed by interconnected TiO₂.⁷ The oxide species such as holes react with S²⁻ species to generate S_n²⁻ ions, which subsequently transfer to electrolyte/counter electrode interface and are converted back to S²⁻ ions under the participation of reflux electrons (the electrons from external circuit to counter electrode). Therefore, the electrolyte is crucial in transferring S²⁻/S_n²⁻ redox couples, however, the leakage of traditional liquid electrolyte results in medium loss for charge transfer. A solution to this impasse is to develop all-solid-state electrolytes having high charge-transfer ability and cost-effectiveness.^8,9 Although the research on all-solid-state electrolytes for QDSSC applications is in a primary stage,^10–14 the promising peculiarity triggers our interest in them.

Inspired by the polyvinylpyrrolidone/iodide (PVP/I₂) complex electrolytes for dye-sensitized solar cell application,¹⁵ in which I₂ can be bonded onto PVP backbones to form stable complex electrolyte, we are impelled to develop cost-effective and robust S²⁻/S_n²⁻ integrated PVP electrolytes to the research on solid QDSSCs. As shown in Fig. 1, S²⁻/S_n²⁻ redox couples are expected to have bonding interaction with PVP surrounded by Na⁺ counter ions. The conformational transformation of PVP segments at relaxation motion precipitates the migration of S²⁻/S_n²⁻ couples. Here we communicate our preliminary results which support this concept. The electrical, electrochemical, and photovoltaic performances are optimized by adjusting Na₂S/S molar ratio, and an optimal efficiency of 0.55% is measured in its QDSSC device.

Fig. 1 The proposed schematic diagram for integration of PVP by polysulfide.

2 Experimental

2.1 Synthesis of solid-state electrolytes

The mixture was made by mixing Na₂S and S in a molar ratio of 1 : 1, 5 : 1, 10 : 1, or 20 : 1 in anhydrous methanol, and PVP with a mass percentage of 20% was added into the mixed solution at room temperature. After vigorous agitating at 50 °C for 24 h, the viscous reactant was casted onto CdS-sensitized TiO₂ anode for consolidation at room temperature. The resultant solid electrolytes were nominated as PVP/Na₂S–S, PVP/5Na₂S–S, PVP/10Na₂S–S, and PVP/20Na₂S–S, respectively.

2.2 Synthesis of CoSe alloy counter electrode

The feasibility of synthesizing CoSe alloy CEs was confirmed by the experimental procedures: a mixing aqueous solution consisting of Se powders and CoCl₂ was made by agitating 0.01 g of Se ultrafine powers and 0.0238 g of CoCl₂·6H₂O in 27.5 ml deionized water. 7.5 ml of hydrazine hydrate (85 wt%) was dropped into the above solution, after vigorous agitating for 10 min, the reactant was transferred into a 50 ml of Teflon-lined autoclave and cleaned FTO glass substrate (sheet resistance 12 Ω square⁻¹, purchased from Hartford Glass Co., USA) with FTO layer downward was immersed in. After the reaction at 120 °C for 12 h, the FTO substrate was rinsed by deionized water and vacuum dried at 50 °C.

2.3 Assembly of QDSSCs

A layer of TiO₂ nanocrystal anode film with a thickness of ∼10 μm was prepared by coating TiO₂ colloid synthesized with a sol-hydrothermal strategy.^16,17 Resultant anodes were alternatively soaked in 0.1 M Cd(NO₃)₂ ethanol solution for 1.5 min and 0.1 M Na₂S methanol solution for 1.5 min. By repeating these cycles for 12 times to obtain CdS sensitized TiO₂ anodes. The QDSSC was fabricated by sandwiching S²⁻/S_n²⁻ integrated PVP electrolyte between a CdS-sensitized TiO₂ anode and a CoSe counter electrode, as shown in Fig. 2.

Fig. 2 Schematic diagram for the QDSSC device.

2.4 Photovoltaic measurements

The photovoltaic test of the QDSSC with an active area of 0.25 cm² was carried out using a CHI660E Electrochemical Workstation under irradiation of a simulated AM1.5 solar light from a 100 W Xenon arc lamp (XQ-500 W) in ambient atmosphere. Each sample was repeatedly measured for at least five times, and a compromising J–V curve was employed to estimate its photovoltaic performance.

3 Results and discussion

Infrared spectrography is a useful technique in characterizing structures of materials.¹⁸ From the micro-transformations in FTIR spectra, we can detect the changes in structures and analyze the potential mechanism. Fig. 3 compares the FTIR spectra of pristine PVP and S²⁻/S_n²⁻ integrated PVP electrolytes. The peak centered at 1670 cm⁻¹ is assigned to the stretching vibration of C=O bond, whereas the band at 1658 cm⁻¹ refers to C–N stretching. After being integrated by S²⁻/S_n²⁻ redox couples, these two peaks shift to 1658 and 1293 cm⁻¹, respectively. It is due to a fact that the bonding of S²⁻/S_n²⁻ with C=O bond decreases the electron cloud density and therefore the vibration frequency. The detection of new band at 1174 cm⁻¹ for S–N stretching indicates the bonding of other end of S²⁻/S_n²⁻ species with –N< segments. Consequently, it is feasibility of utilizing schematic diagram in Fig. 1 to demonstrate the molecular structure of S²⁻/S_n²⁻ integrated PVP solid electrolytes.

Fig. 3 FTIR spectra of pristine PVP and solid electrolytes.

Fig. 4a shows the temperature dependence of ionic conductivities for the solid electrolytes. Solid electrolyte from PVP/10Na₂S–S display the highest conductivity, which is 1.3 mS cm⁻¹ at 25 °C and it is ∼6.3 mS cm⁻¹ at 75 °C. The high conductivity is attributed to the high conformational transformation of PVP segments at relaxation motion can accelerate the migration of S²⁻/S_n²⁻ redox couples. By plotting ln σ against 1000/T, as shown in Fig. 4b, we find the ionic conductivities of the electrolytes follow an Arrhenius relationship: σ = σ₀ exp(−E_a/RT), where σ is ionic conductivity, E_a represents activation energy. The straight line for ln σ–1/T plot suggests a typical ion-conducting behavior. The E_a,^19–21 which is the minimum energies required for ionic conduction through polysulfide integrated PVP solid-state electrolytes are obtained from the slopes in the linear fit, showing 36.5, 32.6, 30.2, and 33.9 kJ mol⁻¹, respectively. The lowest E_a for PVP/10Na₂S–S indicates that charge (S²⁻ and S_n²⁻) movement becomes easier in the solid electrolyte. As shown in Fig. 1, the diffusion of S²⁻/S_n²⁻ redox couples depends on the conformational transformation of PVP segments at relaxation motion. A low S²⁻/S_n²⁻ redox couple dosage within the solid-state electrolyte results in a low number of migrated ions in per unit time, therefore, the performances of the electrolyte are modest. However, at high S²⁻/S_n²⁻ redox couple dosages such as higher than 10 : 1, the conformational transformation of PVP segments will be restricted due to the bonding of networks from polysulfides. The PVP/10Na₂S–S electrolyte is believed to have highest performances after optimization.

Fig. 4 (a) Conductivity–temperature (σ–t) and (b) ln σ–1/T plots for the solid electrolytes at various stoichiometries.

Fig. 5a shows the characteristic J–V curves of the QDSSCs with various solid electrolytes and the photovoltaic parameters are summarized in Table 1. From the data, we can find a regular that the η increases by elevating Na₂S/S molar ratio from 1 : 1 to 10 : 1 and subsequently decreases beyond 10 : 1. The QDSSC with PVP/10Na₂S–S electrolyte yields an optimal η of 0.55% (J_sc = 2.84 mA cm⁻², V_oc = 0.67 V, FF = 28.9%), which is in the same level with previously reported values.^22,23 It is noteworthy to mention that J_sc has a peak value for the cell with PVP/10Na₂S–S. This may be attributed to a fact that the rapid interconversion between S_n²⁻ and S²⁻ can accelerate the generation of photoelectrons from CdS quantum dots and therefore elevate the electron flow from CdS to conduction band of TiO₂ and accumulative electron density on conduction band of TiO₂. From the dark J–V curves (Fig. 5b), one can see that the QDSSC with PVP/10Na₂S–S electrolyte has the smallest dark current density at the same voltage. The dark current in a QDSSC is attributed to the combination of S_n²⁻ species with electrons in the CB of TiO₂ at the TiO₂/electrolyte interface. The smaller dark current means that the reduction of S_n²⁻ at TiO₂/electrolyte interface is retarded. This is another factor for an elevated J_sc value for the cell with PVP/10Na₂S–S solid electrolyte.

Fig. 5 Characteristic J–V curves of the QDSSCs with solid electrolytes (a) under one sun irradiation and (b) in the dark.

Table 1 Photovoltaic and electrochemical parameters for the QDSSCs with solid electrolytes

Solid electrolytes	η (%)	Jsc (mA cm^−2)	Voc (V)	FF (%)	Rct1 (Ω cm^2)	Rct2 (Ω cm^2)	W (F cm^2)
PVP/Na2S–S	0.08	0.97	0.60	13.7	23.1	7511	10.8
PVP/5Na2S–S	0.37	1.87	0.62	31.9	3.3	2482	7.7
PVP/10Na2S–S	0.55	2.84	0.67	28.9	1.6	90.3	2.8
PVP/20Na2S–S	0.21	1.61	0.56	23.3	4.2	566.7	7.3

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Electrochemical impedance spectroscopy (EIS) was performed to evaluate the electrochemical performances of the QDSSCs. Fig. 6a–c show the Nyquist plots for the cell devices with various solid electrolytes, giving two semicircles which are assigned to the electrochemical reaction at electrolyte/CoSe counter electrode interface (a smaller one in high frequency region) and charge transfer at CdS–TiO₂/electrolyte (a larger one in low frequency region). An equivalent circuit (see the inset of Fig. 6a) is employed to fit the Nyquist plots for estimating the electrochemical parameters, such as series resistance (R_s), charge-transfer resistance at electrolyte/CoSe counter electrode interface (R_ct1), charge-transfer resistance at CdS–TiO₂/electrolyte interface (R_ct2), and Nernst diffusion impedance corresponding to diffusion resistance of S_n²⁻/S²⁻ redox couples (W). CPE1 and CPE2 are constant phase elements. Both R_ct1 and R_ct2 have peak valley for PVP/10Na₂S–S, indicating that this solid electrolyte has the highest charge-transfer ability. The charge-transfer kinetics of the electrolytes is cross-checked by electron lifetime (τ) at CdS–TiO₂/electrolyte interface.²⁴ In a real QDSSC, the electrons excited from CdS quantum dots will be injected to the conduction band of TiO₂ nanocrystallite, therefore, the τ is positively corrected to charge-transfer ability. From Fig. 6d, the τ values are calculated by equation:²⁵ τ = 1/2πf, where f is the peak frequency attributing to electrochemical reaction at CdS–TiO₂/electrolyte interface. The obtained τ follows an order of PVP/10Na₂S–S (2.83 ms) > PVP/5Na₂S–S (2.34 ms) > PVP/20Na₂S–S (1.63 ms) > PVP/Na₂S–S (0.91 ms), which is consistent with photovoltaic performances of QDSSCs. From Table 1, it is apparent that the W value for the QDSSC with PVP/10Na₂S–S is the lowest, indicating that the diffusion kinetics of S_n²⁻/S²⁻ redox couples within the solid electrolyte is more facile than other electrolytes. The facial diffusion of S_n²⁻/S²⁻ redox couples can accelerate the interconversion of S_n²⁻ ↔ S²⁻ species. Fig. S1† presents the photon-to-current conversion efficiency (IPCE) of QDSSC device with PVP/10Na₂S–S electrolyte. The broad IPCE curve, covering the spectrum from 400 to 800 nm, exhibits an IPCE value of ∼80% for the solar cell. The result demonstrates that the QDSSC can efficiently produce electrons for electricity generation.

Fig. 6 (a)–(c) Nyquist and (c) Bode EIS spectra for the QDSSCs with various solid electrolytes. The inset shows an equivalent circuit for the solid QDSSC.

The photocurrent dynamics of the solid-state QDSSC was probed in order to determine its start-up, charge diffusion, and photocurrent stability. A fast start-up and long-term stability in engine, vehicle and power source applications have been two challenges for nanoenergy devices.²⁶ Fig. 7a shows the start–stop switches of solar cell with solid electrolytes in a time slot of 0–1200 s. A sharp increase in photocurrent density suggests a quick start of cell operation under irradiation, indicating the rapid excitation of CdS quantum dots and high kinetics in electron transportation. For the quick start-up, fast kinetics at CdS-sensitized TiO₂ anode and CoSe alloy counter electrode are crucial to promote the cell launch. Moreover, a decrease in photocurrent density in each “on” state means a diffusion mechanism for charge transfer, which is in an agreement with EIS analysis. The photocurrent density of the cell with PVP/10Na₂S–S electrolyte versus time plots over 60 min display the photocurrent stability on prolonged exposure to light irradiation (100 mW cm⁻²). As shown in Fig. 7b, the photocurrent density over 60 min-irradiation remain 92.7% in comparison with initial value, referring to a relatively good stability.²⁷ Notably, the increase process in photocurrent means the temperature increment and therefore charge-tranfer acceleration under persistent irradiation.

Fig. 7 (a) Start–stop switches of the QDSSCs by alternatively irradiating and darkening the devices, (b) photocurrent stability of the QDSSC with PVP/10Na₂S–S electrolyte under persistent irradiation.

4 Conclusions

In summary, all-solid-state electrolytes from S_n²⁻/S²⁻ integrated PVP featured by high conductivity and charge-transfer ability are fabricated and employed in assembling efficient solid QDSSCs. Due to the superiorities of PVP in accelerating S_n²⁻/S²⁻ migration, the kinetics for S_n²⁻ ↔ S²⁻ interconvertion has been markedly enhanced. The QDSSC employing PVP/10Na₂S–S electrolyte exhibits an optimal efficiency of 0.55%. The research presented here is far from being optimized but these profound advantages along with cost-effectiveness, mild synthesis, and scalable materials promise the S_n²⁻/S²⁻ integrated PVP electrolytes to be strong candidates in full-solid-state QDSSCs.

Acknowledgements

The authors would like to acknowledge financial supports from Fundamental Research Funds for the Central Universities (201313001, 201312005), Shandong Province Outstanding Youth Scientist Foundation Plan (BS2013CL015), Shandong Provincial Natural Science Foundation (ZR2011BQ017), Research Project for the Application Foundation in Qingdao (13-4-198-jch), National Natural Science Foundation of China (51102219, 51342008), National Key and National Key Technology Support Program (2012BAB15B02).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10764k

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