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. 2024 Feb 29;9(10):12211–12218. doi: 10.1021/acsomega.4c00208

Reduced Surface Trap States of PbS Quantum Dots by Acetonitrile Treatment for Efficient SnO2-Based PbS Quantum Dot Solar Cells

Guannan Xiao †,‡,*, Taohua Liang †, Xiaoming Wang †, Chao Ying §, Kai Lv ∥, Chengwu Shi ∥,*
PMCID: PMC10938384  PMID: 38496937

Abstract

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The solution-phase ligand-exchange strategy offers a simple pathway to prepare PbS quantum dots (QDs) and their corresponding solar cells. However, the production of high-quality PbS QDs with reduced surface trap state density for efficient PbS QD solar cells (QDSCs) still faces challenges. As the hydroxyl group (−OH) has been demonstrated to be the primary source of the surface trap states on PbS QDs in the general oleic acid method, here, we present an effective and facile strategy for reducing the surface −OH content of PbS QDs by using acetonitrile (ACN) as precipitant to wash the surface of QDs, which significantly decreases the trap state density and enables the preparation of superior PbS QDs. The resulting solar cell with an ITO/SnO2/n-PbS/p-PbS/Au structure obtained an improved photoelectric conversion efficiency (PCE) from 8.53 to 10.49% with an enhanced air storage stability, realizing a high PCE for SnO2-based PbS QDSCs.

1. Introduction

As optoelectronic materials with great research potential and application prospect, PbS quantum dots (QDs), with their low synthesis cost, tunable band gap, multiple exciton generation, and large exciton Bohr radius of 18 nm, possess a broad range of applications in photovoltaics including photodetectors, light-emitting diodes, and photovoltaic devices.1−7 Their photoelectric properties can be widely adjusted through size control and surface chemical modification to meet the design requirements of various functions and properties.8,9 For QDSCs, with the improvement of QD synthesis, surface passivation, and solar cell structure, the photoelectric conversion efficiency (PCE) has improved rapidly,10−14 and the highest certified PCE of PbS QDSC has exceeded 13% so far.15 On the other hand, the large trap state density on the surface of PbS QD films seriously affects the photovoltaic performance of PbS QDSCs. Recent studies on PbS QDs have shown that for the oleate-capped PbS (o-PbS) QDs using PbO and oleic acid (OA) ligand, the nonpolar (001) surfaces can be passivated by the covalent bond of the carboxyl group (−COOH), whereas the polar (111) surfaces of Pb terminal are passivated by the deprotonated carboxyl group (COO–) and hydroxyl group (−OH), in which the −OH is regarded as the main cause of surface trap states on PbS QDs.16−18 There have been various reported strategies to reduce the −OH content, promising further improvement in the photovoltaic performance and stability of PbS QDSCs. Jang et al.17 prepared surface trap-passivated PbS QDs by using PDMII instead of TBAI as iodide source for ligand exchange, which effectively removed the −OH on the PbS QD surface. The resulting PbS QDSCs with the structure of ITO/ZnO/n-PbS/p-PbS/Au obtained a certified PCE of 10.89%. Konstantatos et al.18 successfully suppressed the −OH on the PbS QD surface through thermal annealing of QD layers at 80 °C and by using EMII instead of TBAI for ligand exchange, and the corresponding PbS QDSCs with the structure of ITO/ZnO/n-PbS/p-PbS/Au had a PCE of 9.6%. Therefore, developing a new strategy to reduce the −OH content on the surface of PbS QDs is an important approach to improve the photovoltaic performance and stability of PbS QDSCs. On the other hand, it is worth noting that most PbS QDSCs with high photovoltaic performance use ZnO or TiO2 thin film as the electron transporting layer (ETL), and there are currently limited reports on the application of SnO2 as the ETL in PbS QDSCs. Compared with ZnO and TiO2, SnO2 possesses high electron mobility, wide band gap, and good light transmittance.19−22 Song et al.23 assembled PbS QDSCs by using SnO2-Cl as the ETL, and the corresponding solar cells obtained a PCE of 9.37%. Ma et al.24 prepared a SnO2 thin film by magnetron sputtering and combined it with a 10 nm thick ZnO thin film to form a double-layer electron transporting layer. The corresponding PbS QDSC reached a PCE of 10.1%. Consequently, studies on the application of SnO2 in PbS QDSCs have great development potential and research significance.

Herein, we prepared a surface-passivated SnO2 thin film as the ETL for assembling PbS QDSCs. And for the first time, we present an effective and facile strategy for reducing the surface −OH content of PbS QDs by using ACN as precipitant to wash the surface of QDs, which is quite different from the previous work that usually applied different kinds of halide salts to reduce the surface −OH of PbS QDs. The resulting PbS QDs exhibited a significantly reduced surface −OH content, which effectively decreased the surface trap states and suppressed the charge recombination. The influence of different precipitants on surface trap states of PbS QDs and the photovoltaic performance of PbS QDSCs were systematically investigated. The corresponding PbS QDSC based on ACN-washed PbS QDs and SnO2 ETL achieved a PCE of 10.49% with high air storage stability, which was a high level of PCE for SnO2 ETL based PbS QDSCs.

2. Experimental Section

2.1. Synthesis of PbS QDs with Reduced Surface Trap States

PbS QDs were synthesized via a solution-phase ligand-exchange process with some modification.11,25,26 In this work, the hexamethyldisilathiane (TMS)/1-octadecene (ODE) mixture was obtained by adding 210 μL of TMS into 8 mL of the ODE that had been degassed at 80 °C for 12 h in advance. Pb-oleate was obtained by dissolving 0.45 g of PbO, 1.5 mL of OA, and 18 mL of ODE into a 50 mL three-neck flask and stirred at 95 °C for 2 h in a vacuum state. After that, a steady N2 flow was introduced into the flask to keep a N2 atmosphere. Then the prepared TMS/ODE mixture was injected into the Pb-oleate at 125 °C. The resulting mixture was centrifuged at 5000 rpm for 5 min after adding 50 mL of acetone and naturally cooling down to room temperature. After 8 mL of ACN was added, the precipitate of the mixture was dispersed in 4 mL of toluene followed by another centrifugation at 8500 rpm for 10 min. The obtained precipitates of PbS QDs were dispersed in 6 mL of octane and kept in reserve with a N2 atmosphere.

The PbI2 mixed solution was obtained by dissolving 0.031 g of CH3COONH4, 0.461 g of PbI2, and 0.073 g of PbBr2 into 10 mL of dimethylformamide (DMF) solution. The 4 mL of obtained PbS QD octane solution was diluted to 10 mL by octane and then mixed vigorously with the PbI2 mixed solution for 3 min. Triple washings with octane were then performed on the resulting PbS QDs in DMF phase, which were precipitated after mixing with 5 mL of toluene. After being centrifuged, the PbS QDs were dried by a constant N2 flow to obtain QD powder. The n-PbS QDs were prepared by dispersing the dried PbS QDs in 0.8 mL of butylamine, and the p-PbS QDs were obtained by filtering the remaining 2 mL of obtained PbS QD octane solution.

2.2. Device Fabrication

On the cleaned indium-doped tin oxide (ITO) substrate, the 1.5 wt % SnO2 solution was spin-coated at 3000 rpm for 30 s and then air-annealed at 150 °C for 30 min to obtain the SnO2 film. The passivated SnO2 electron transporting layer was prepared by spin-coating at 3000 rpm for 30 s with a 0.05 M SnCl2 ethanol solution on the SnO2 film followed by successive annealing at 100 °C for 30 min and 180 °C for 60 min, respectively.27 For the n-PbS film, 90 μL of n-PbS QDs was spin-coated on the passivated SnO2 electron transporting layer at 2500 rpm for 30 s. For the p-PbS film, 80 μL of p-PbS QDs was spin-coated on the n-PbS film at 2500 rpm for 30 s followed by the 1,2-ethanedithiol (EDT, 0.04 vol % in ACN) and ACN rinsing twice. The gold electrode was then deposited on the p-PbS film with a thickness of 80 nm. The structures of the PbS QDSCs were ITO/SnO2/n-PbS/p-PbS/Au.

2.3. Characterization and Measurement

The size of the PbS quantum dots was observed by transmission electron microscopy (FEI F20 FEG-STEM). The surface −OH content of PbS QDs was measured by X-ray photoelectron spectroscopy (XPS, Kratos Ultra). The space charge-limited current (SCLC) I–V measurements were applied on the devices with the structure of ITO/SnO2/n-PbS/BCP/C60/Ag for the electron-only device and the structure of ITO/PEDOT:PSS/p-PbS/spiro-OMeTAD/Au for the hole-only device. An Ivium-n-Stat was then used to measure the current under dark conditions with applied voltages, and the nonlinear response was analyzed based on the SCLC theory.28C–V characterization was performed by an Ivium-n-Stat under dark conditions with a voltage amplitude of 10 mV at a frequency of 1 kHz. Electrochemical impedance spectra (EIS) were collected by an electrochemical workstation under dark conditions, with frequencies from 1 to 1 × 106 Hz, and the Nyquist plot was obtained by fitting the measured data using the EC-lab software according to the corresponding equivalent circuit. The photocurrent–photovoltage characteristics of solar cells with an active area of 0.08 cm2 were characterized under a solar simulator (Newport, Oriel Class AAA, 94063A) at 100 mW·cm–2 illumination (AM 1.5 G) calibrated by an NREL-certificated silicon reference cell (Newport, 91150 V).

3. Results and Discussion

3.1. Influence of Different Precipitants on the Surface −OH of PbS QDs

As one of the main sources of surface trap states, the hydroxyl group on the (111) surface facets had a negative impact on the photovoltaic performance and stability of PbS QDSCs. Therefore, as shown in Figure 1, to decrease the surface trap states of PbS QDs and further improve the quality of n-type PbS (n-PbS) and p-type PbS (p-PbS), we developed a simple but effective strategy based on previous reports of PbS QDs synthesis using ACN instead of the traditional precipitant methanol (MeOH) to wash the surface of PbS QDs.11,25 The −OH on the PbS QDs surface was indicated by the O 1s peaks from XPS analysis, and the results were shown in Figure 2. According to previous reports, the peaks at 532.9, 530.3, and 531.5 eV can be corresponding to the characteristic peaks of −COO– or CO2, Pb–O, and Pb–OH, respectively.16,17,29,30 It can be clearly observed that the peak located at 531.5 eV showed less than one-third intensity in the ACN washed o-PbS compared to MeOH washed o-PbS, indicating that the content of −OH on the PbS QDs surface had a dramatic decrease by using ACN to wash PbS QDs. As shown in Figure 2c, Fourier transform infrared spectroscopy (FTIR) analysis has been added to further identify the surface chemistry of ACN-washed and MeOH-washed QDs. The peaks at around 1400 and 1550 cm–1 can be assigned to the symmetric and antisymmetric COO– stretching modes, and stronger intensities for the two peaks were observed from the ACN-washed QD surface compared with MeOH-washed QD surface, indicating that more oleate was bound to the QD surface washed by ACN. In addition, the stronger peaks of I 3d5/2 and I 3d3/2 in the I 3d XPS spectra were observed for the ACN-washed QD surface, which indicated that ACN washing could facilitate successful ligand exchange with PbI2 (see Figure S4 in the Supporting Information). Because it has been proven in the previous reports that Pb–OH is one of the main causes of the sub-band gap trap states in PbS QDs, the minimized −OH content after ACN washing illustrated that our strategy can effectively cure the surface trap states of PbS QDs and reduce the interface charge recombination.16,18

Figure 1.

Figure 1

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Schematic illustration of the PbS QD surface washed by ACN and MeOH.

Figure 2.

Figure 2

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O 1s XPS spectra of o-PbS QDs washed with (a) MeOH and (b) ACN. (c) FTIR spectrum of ACN-washed and MeOH-washed oleate-capped PbS QDs.

3.2. Photovoltaic Performance of PbS QDSCs

To investigate the influence of ACN washing and MeOH washing on the PbS QDs, we fabricated PbS QDSCs with the structure of ITO/SnO2/n-PbS/p-PbS/Au by using ACN-washed PbS QDs and MeOH-washed PbS QDs, respectively, where the SnO2 ETL was passivated by SnCl2. The passivation of the SnO2 ETL showed a suppressed charge combination in the interface of PbS/SnO2 (see Figure S1 and Table S1 in the Supporting Information), which improved the best PCE of the corresponding solar cells from 7.48 to 8.53% (see Figure S2 and Table S2 in the Supporting Information). More details and corresponding discussion can be obtained from the Supporting Information. The photocurrent–photovoltage characteristics of the champion PbS QDSCs are shown in Figure 3a, and the corresponding photovoltaic performance parameters are listed in Table 1. The results showed that the MeOH-washed PbS QDSC had a short-circuit photocurrent density (Jsc) of 24.19 mA·cm–2, an open-circuit voltage (Voc) of 0.56 V, and a fill factor (FF) of 0.63. In clear contrast, enhanced Jsc of 28.26 mA·cm–2, Voc of 0.58 V, and FF of 0.64 were achieved in the ACN-washed PbS QDSCs, which boosted the PCE from 8.53 to 10.49%. Although the PCE of 10.49% was lower than most ZnO ETL-based PbS QDSCs, it was a high efficiency for SnO2 ETL-based PbS QDSCs. The distinctly improved photovoltaic performance can be attributed to the minimization of −OH content on the surface of PbS QDs washed by ACN, which effectively decreased the trap states on the surface of PbS QDs, improved the quality of PbS QDs, and enhanced the charge transport in devices. To ensure the reproducibility of the above results, we assembled around 50 devices each for ACN-washed and MeOH-washed PbS QDSCs, and the corresponding PCE distribution is shown in Figure 3b. As shown in Figure 3b, the PCE distribution of MeOH-washed PbS QDSCs was from 6 to 8.5%, whereas an improved PCE distribution of 8 to 10.5% for ACN-washed PbS QDSCs was achieved, demonstrating the reliability of our strategy by using ACN to reduce surface trap states of PbS QDs, which can improve the photovoltaic performance of devices.

Figure 3.

Figure 3

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(a) Photocurrent–photovoltage characteristics of ACN-washed and MeOH-washed PbS QDSCs. (b) Statistics of the PCE distribution of ACN-washed and MeOH-washed PbS QDSCs.

Table 1. Photovoltaic Performance Parameters of PbS QDSCs with Different Precipitants.

precipitant Jsc (mA·cm–2) Voc (V) FF PCE (%)
ACN 28.26 0.58 0.64 10.49
MeOH 24.19 0.56 0.63 8.53
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Figure 4 summarizes the photovoltaic performance of ACN-washed PbS QDSCs and MeOH-washed PbS QDSCs with different thicknesses of light absorption layer. In agreement with previous reports, the Jsc, Voc, and FF of MeOH-washed PbS QDSCs increased with the absorber thickness increased from 250 to 350 nm, and then all decreased dramatically when the absorber thickness continued to increase from 350 to 550 nm, which can be attributed to insufficient carrier diffusion lengths.12,31−33 For ACN-washed PbS QDSCs, the Jsc, Voc, and FF reached the highest level when the absorber thickness reached 450 nm; therefore, the MeOH-washed PbS QDSCs and ACN-washed PbS QDSCs obtained the best PCE for the optimum absorber thickness of 350 and 450 nm, respectively. The result indicated that our strategy of using ACN to treat the surface of PbS QDs played an important role in extending the carrier diffusion lengths and enhancing the carrier transporting in PbS QDSCs, corresponding to the improved photovoltaic performance of ACN-washed PbS QDSCs.

Figure 4.

Figure 4

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(a) PCE, (b) Jsc, (c) Voc, and (d) FF of ACN-washed PbS QDSCs and MeOH-washed PbS QDSCs with different absorber thicknesses.

3.3. Effect of ACN-Washed PbS QDs on Surface Trap State Reduction

To confirm the surface trap state reduction of ACN-washed PbS QDs, SCLC measurements were applied on electron-only devices with the structure of ITO/SnO2/n-PbS/BCP/C60/Ag and hole-only devices with the structure of ITO/PEDOT:PSS/p-PbS/spiro-OMeTAD/Au. The n-PbS and p-PbS were obtained via ligand exchange of o-PbS and PbI2, and o-PbS and EDT, respectively. As shown in Figure 5, a linear (ohmic) increase appeared first at lower voltages, and a greater logarithmic slope can be observed with the rise of voltages, which represented the trap-filling. Normally, the trap-filling voltage (VTFL) is defined by the voltage corresponding to the intersection of the two lines with the resulting slopes.26 The VTFL's of the electron-only devices based on n-PbS films from ACN-washed o-PbS and MeOH-washed o-PbS were 0.197 and 0.499 V, respectively. The VTFL's of the hole-only devices based on p-PbS films from ACN-washed o-PbS and MeOH-washed o-PbS were 0.645 and 0.886 V, respectively. The trap state density (Ntrap) can be estimated by VTFL with the following equation33:

3.3. 1

where ε0 was the vacuum permittivity, ε was the relative dielectric constant of PbS (ε = 17.2),34e was the electron charge, and L was the film thickness. According to the equation, the electron trap densities of n-PbS films based on ACN-washed o-PbS and MeOH-washed o-PbS were estimated at 1.85 × 1015 and 4.69 × 1015 cm–3, whereas the hole trap densities of p-PbS films based on ACN-washed o-PbS and MeOH-washed o-PbS were estimated at 6.06 × 1015 and 8.33 × 1015 cm–3. The calculation results confirmed a lower trap density of ACN-washed devices, leading to the significant increase of Jsc, Voc, and FF of the corresponding PbS QDSCs, which demonstrated that our strategy of using ACN to minimize −OH content on PbS QDs surface was an effective way for reducing surface trap states of PbS QDs, improving the quality of PbS films, and achieving high photovoltaic performance of PbS QDSCs. To further understand the relationship of defects and the photovoltaic performance of PbS QDSCs, as shown in Figure 6, the defect density was determined by the Mott–Schottky equation using the capacitance–voltage (C–V) characterization35:

3.3. 2

where Na was the defect density, C was the capacitance, and A was the active area of PbS QDSCs. After calculation, the estimated defect density of ACN-washed PbS QDSCs was 8.74 × 1015 cm–3, which was only a quarter of that for MeOH-washed PbS QDSCs (3.34 × 1016 cm–3). Again, the lower defect density of ACN-washed PbS QDSCs indicated that the ACN-washed PbS QDs were essential for achieving significant trap state reduction and photovoltaic performance improvement.

Figure 5.

Figure 5

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SCLC curves of (a) electron-only devices and (b) hole-only devices based on ACN-washed o-PbS and MeOH-washed o-PbS.

Figure 6.

Figure 6

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C–V curves of ACN-washed and MeOH-washed PbS QDSCs.

The influence of the PbS QD trap states on the charge transporting and recombination of PbS QDSCs was investigated by analyzing EIS and light-intensity dependent Voc. First, Figure 7a shows the Nyquist plots of ACN-washed and MeOH-washed PbS QDSCs, and the corresponding fitted data by an equivalent circuit Rs (Rcr CPE) are listed in Table 2. Rs was the serial resistance describing the resistance of the ITO and Au electrode, whereas Rcr represented the charge recombination resistance on the interface of PbS/SnO2. The constant phase element (CPE) was a nonideal frequency-dependent capacitance, of which the admittance can be expressed by the following equation:

3.3. 3

where n was a constant ranging from 0 to 1 that described the difference from the ideal capacitance and Y0 represented the capacitance of the PbS/SnO2 interface.5,36−39 Therefore, the small difference of Rs between ACN-washed and MeOH-washed PbS QDSCs can be attributed to the same ITO substrate and electrode material of Au for fabricating PbS QDSCs. On the other hand, higher Rcr (7862 Ω) and lower Y0 (3.58 × 10–8·F·sn–1) of ACN-washed PbS QDSCs than those of MeOH-washed PbS QDSCs (3209 Ω and 10.67 × 10–8·F·sn–1, respectively) were obtained. The result indicated that the charge recombination at the interface of SnO2/PbS was suppressed by using ACN-washed PbS QDs, leading to improved interface charge separation and transport, which was consistent with the obviously enhanced photovoltaic performance of ACN-washed PbS QDSCs. As shown in Figure 7b, the light-intensity dependent Voc can be determined by the following equation29:

3.3. 4

where n was the diode ideality factor, k was the Boltzmann constant, T was the temperature, q was the elementary charge, α was the exponential factor, I was the light intensity, and C was the fitting parameter. The slopes of ACN-washed and MeOH-washed PbS QDSCs were 1.08kT/q and 1.62kT/q, respectively, demonstrating that the trap-assisted charge recombination was effectively suppressed by ACN washing. The result was in accordance with reduced trap density and improved Voc and FF values of ACN-washed PbS QDSCs.

Figure 7.

Figure 7

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(a) Nyquist plots and (b) light-intensity-dependent Voc for ACN-washed and MeOH-washed PbS QDSCs.

Table 2. Parameters Obtained by Fitting the Experimental Data with the Equivalent Circuit Rs (Rcr CPE).

solar cell Rs (Ω) Rcr (Ω) Y0 (10–8·F·sn–1) n
ACN washed 33.89 7862 3.58 0.987
MeOH washed 31.16 3209 10.67 0.906
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3.4. Stability of PbS QDSCs

The stability test of PbS QDSCs was also an important way to evaluate the surface condition of PbS QDs and the quality of the PbS QDSCs.29,30 Therefore, the PCE evolutions in a long storage time of ACN-washed and MeOH-washed PbS QDSCs without encapsulation were tested in a nitrogen atmosphere and in ambient air with a relative humidity of around 60%, respectively. As shown in Figure 8a, for the storage in nitrogen atmosphere, both the ACN-washed and MeOH-washed PbS QDSCs showed good stability with no significant decays of PCE for 45 days. However, from Figure 8b, for storage in ambient air, the MeOH-washed PbS QDSCs retained only about 90% of their original PCE after 45 days, wheresa the ACN-washed PbS QDSCs can still maintain over 105% of their original PCE after 45 days with a much slower trend of efficiency decay, possessing a better long-term air stability. The result demonstrated the improved surface condition of ACN-washed PbS QDs, which was in good agreement with the enhanced photovoltaic performance of ACN-washed PbS QDSCs.29

Figure 8.

Figure 8

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Long-term stability under (a) a nitrogen atmosphere and (b) ambient air of unencapsulated ACN-washed and MeOH-washed PbS QDSCs.

4. Conclusions

In summary, we developed an effective strategy to prepare high-quality PbS QDs with reduced surface trap states by using ACN as a precipitant to wash the surface of PbS QDs. The use of ACN can minimize the surface −OH content of PbS QDs, leading to reduced surface trap states of PbS QDs. The PbS QDSCs based on ACN-washed PbS QDs with the structure of ITO/SnO2/n-PbS/p-PbS/Au were successfully fabricated, which achieved a high PCE of 10.49% with enhanced air storage stability for SnO2 ETL-based PbS QDSCs. It may be anticipated that our strategy will expand the way that high-quality QDs and efficient photovoltaic devices are fabricated in future studies.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (51972091), the Sichuan Provincial Natural Science Foundation Youth Fund Project (2023NSFSC1178), the Opening Project of Material Corrosion and Protection Key Laboratory of Sichuan province (2023CL08), the Key Scientific Research Projects of Colleges and Universities in Anhui Province (2023AH051541), the Research and Development Centre of Photovoltaic Technology with Renewable Energy (23KYPT03), and the Scientific Research Projects of Chengdu Polytechnic (23CZYG046).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c00208.

  • The following files are available free of charge. Photovoltaic performance parameters of PbS QDSCs with SnO2 and passivited- SnO2 ETL, EIS analysis of PbS QDSCs with SnO2 ETL and passivated-SnO2 ETL, Photos of o-PbS standing for 20 s after adding MeOH and ACN, TEM image of PbS QDs after ACN washing, I 3d XPS spectra of ACN-washed and MeOH-washed PbS QDs after ligand exchange with PbI2 (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao4c00208_si_001.pdf (663.5KB, pdf)

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