Eco-friendly and stable silver bismuth disulphide quantum dot solar cells via methyl acetate purification and modified ligand exchange

Journal Pre-proof Eco-friendly and stable silver bismuth disulphide quantum dot solar cells via methyl acetate purification and modified ligand exchange Shuaiqiang Ming, Xiaohui Liu, Wenxiao Zhang, Qiaomu Xie, Yulei Wu, Lijun Chen, Hai-Qiao Wang PII:

S0959-6526(19)33836-3

DOI:

https://doi.org/10.1016/j.jclepro.2019.118966

Reference:

JCLP 118966

To appear in:

Journal of Cleaner Production

Received Date: 18 February 2019 Revised Date:

15 October 2019

Accepted Date: 19 October 2019

Please cite this article as: Ming S, Liu X, Zhang W, Xie Q, Wu Y, Chen L, Wang H-Q, Eco-friendly and stable silver bismuth disulphide quantum dot solar cells via methyl acetate purification and modified ligand exchange, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/ j.jclepro.2019.118966. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Eco-friendly and stable silver bismuth disulphide quantum dot solar cells via methyl acetate purification and modified ligand exchange Shuaiqiang Ming,a,b Xiaohui Liu,a Wenxiao Zhang,a,b Qiaomu Xie,a Yulei Wu,a,b Lijun Chena and Hai-Qiao Wang*a,b a

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences. No.1219 Zhong guan xi Road, Zhenhai, Ningbo 315201, China. E-mail: [email protected] b University of Chinese Academy of Sciences, Beijing 100049, China.

Keywords: Lead-free quantum dots; silver bismuth disulphide; low-toxic solvent; ligand exchange; performance; eco-friendly solar cell

Abstract From the point of view of environmentally-friendly, non- or less toxic photovoltaic material and fabrication method are desired by both the academia and industrial community. However, most efficient quantum dot solar cells have been fabricated based on heavy metal and toxic solvent consumption, which could cause serious ecological problems. Quantum dot solar cells were studied and explored based on lead-free active material silver bismuth disulphide, less toxic solvent purification of methyl acetate and lab-developed ligand exchange process. Twice ligand exchanges with tetramethylammonium iodide were applied for every deposited quantum dot layer kept in static. Methanol rinse was applied to the quantum dot film after each ligand exchange process to promote the ligand exchange and clean the organic residuals. Decent power conversion efficiency and good stability were demonstrated for the fabricated silver bismuth disulphide quantum dot solar cells. The best power conversion efficiency of 4.57% was achieved, with open-circuit voltage of 0.445 V, short-circuit current of 18.87 mA/cm2 and fill factor of 54.4%. Over 95% of the original power conversion efficiency was maintained after 150 days both in N2-filled glove box and ambient atmosphere with relative humidity of 30%. This work demonstrated fabrication of quantum dot solar cells based on non- or less-toxic quantum dots and solvent consumption. Which would benefit the development and commercialization of 1

quantum dot solar cell technology.

1. Introduction The energy crisis and environmental pollution are becoming increasingly serious for the world due to excessive exploitation of fossil fuels. Photovoltaic technologies are considered promising sustainable energy sources to solve these challenges (Şengül and Theis, 2011). Due to huge progresses achieved in material (Cai et al., 2017; Stroyuk et al., 2018) and fabrication techniques (Lu et al., 2015; Meredith and Armin, 2018), the typical emerging photovoltaics like organic solar cell (OSC) (Hou et al., 2018; Liu, X. et al., 2017), perovskite solar cell (PSC) (Fu et al., 2018; Rong et al., 2018) and quantum dots solar cell (QDSC) (Liu, Z. et al., 2017; Tang et al., 2011) have been promoted greatly in the past few years. Among which QDSC uses nanomaterials as the key part in device, which can be classified as nanotechnology. It possesses the advantages claimed for nanotechnologies, for instance high efficiency and elimination of waste and pollution (Reijnders, 2006). QDSC may possess the potential to overcome the barriers of photovoltaic technology: low efficiency, high fabrication cost and ecological impact. QDSC (Brabec et al., 2014; Liu, Z. et al., 2017) is catching up fast with its counterparts like dye sensitized solar cells (Mathew et al., 2014) and organic solar cells (Xue et al., 2018). The state-of-the-art power conversion efficiency (PCE) of 16.6% has been certified (NREL, 2019). Inorganic semiconducting quantum dots (QDs) present favorable properties like broad absorption from visible to near-infrared range, high absorption coefficient (Zhang et al., 2015), low exciton binding energy (Tang et al., 2010), size-dependent bandgap/tunable energy levels, facile tailoring of surface chemistry (Cao et al., 2016; Wang et al., 2018) and multiple excitons generation (Nozik et al., 2010). All of them could benefit the photovoltaic performance of QDSC. QDSC possesses the advantages of low temperature-solution 2

process, large scale and flexible fabrication compatibility and material/device stability as well in ambient conditions (Liu, Z. et al., 2017). They are attracting great attention from the academia and industrial community (Liu, Z. et al., 2017; Stroyuk et al., 2018). The aforementioned progresses and advantages of QDSC further convince us of its potential in practical application. However, the environmental impact issue of the technology must be concerned seriously and solved. The environmental impacts of a photovoltaic technology or product in its life cycle typically occur in the stages: raw material extraction, synthesis of starting products, fabrication, use and decommissioning (Babayigit et al., 2016). The most important hazards include toxicity of heavy metal salt precursors, heavy metal intoxication and acidification upon leakage of degradation products, increased risks from local accumulation, toxic fumes in case of fire, solvent-induced exacerbation of dermal and oral uptake (Babayigit et al., 2016). Candidate materials for QDSC applications consist primarily of compound semiconductors, most of them contain toxic elements like Pb and Cd. The toxic effects of these materials may be exacerbated by their nanoscale features (Şengül and Theis, 2011). Most efficient QDSCs have been fabricated with heavy metal element-based QDs, e.g. Pb and Cd etc. (Brabec et al., 2014; Zhou et al., 2011b). They are hazardous to human health and may also have ecotoxicological effects after discharge into environment (Reijnders, 2006). Renewable options are limited for these QDSCs. Solvent used in QD synthesis and device fabrication is another main source of toxic chemicals besides QD material itself (Choi and Jeong, 2018). Acetone or alcohol solvent with 5 to 10 times volume of that of the reaction solution is always needed (Liu et al., 2013) for instance to purify the synthesized PbS QDs in certain volume of reaction solution. Acetone and alcohols as volatile organic compounds (VOCs) can cause serious environmental impacts (Park et al., 2014). They are 3

potentially harmful to human health, other animals, plants and microorganisms present in the environment (Cheng and Hsieh, 2013; Ren et al., 2018). The consumption of these solvents in production is massive and cannot be neglected. They are limited to use in fields like coating and pharmacy etc. (Jr et al., 2007) and unfavorable for the photovoltaic application as well. Source reduction is considered effect and important to eliminate the harmful effect of nanomaterials (Reijnders, 2006). Non- or low toxic QD material and fabrication method are greatly demanded by the community to progress the development and real application of the QDSC technology. Although high efficient PCE (>16%) has been certified for QDSC (NREL, 2019). Progress achieved for non-toxic (Pb and Cd free) QDSC is limited (Brabec et al., 2014; Zhou et al., 2011a) due to unideal material properties and limited investigations. The highest PCE of non-toxic QDSC reported in literature is 6.3% based on silver bismuth disulphide (AgBiS2) QD (Bernechea et al., 2016). This semiconducting compound AgBiS2 comprises environmentally-friendly elements. The AgBiS2 QD exhibits photoconductivity (Pejova et al., 2011) and favorable thermoelectric properties (Pejova et al., 2008) and has been utilized as a sensitizer or counter-electrode in sensitized solar cells (Bernechea et al., 2016; Huang et al., 2013), showing promising potential in photovoltaic applications. However the study about AgBiS2 QDSCs is still quite limited (Hu et al., 2018; Pai et al., 2018). The VOC solvent acetone has always been used for QD purification (Bernechea et al., 2016) in device fabrication. Methyl acetate (MeOAc) is a carboxylate ester showing weakly polar and lipophilic properties, which is not considered as a VOC (Jr et al., 2007). It has been occasionally used to replace VOCs (acetone and butanone) in industrial manufacture since it can meet the requirement of the new environmental protection standard (Jr et al., 2007). It possesses potential and advantage as a low toxic solvent in QDSC applications. 4

In this work, efficient and stable QDSCs were fabricated based on lead-free QD active material, low toxic purification solvent and lad-developed ligand exchange procedure. Environmental impact of the technique was reduced by utilizing AgBiS2 QDs as active material and non-toxic purification solvent in material preparation. Decent PCE and good stability of device on the other hand were confirmed for AgBiS2 QDSCs kept both in N2-filled glove box and ambient atmosphere. This work demonstrated fabrication of QDSCs with decent PCE and promising stability, providing alternative material and solvent systems for efficient and environmentally-friendly quantum dot photovoltaic technology.

2. Experimental Section 2.1. Chemicals All reactants and solvents were of analytical grade and used without purification. Bi(OAc)3 (99.99%,

metals

basis),

Ag(OAc)

(99%),

Hexamethyldisilathiane

(HMS,

98%)

and

tetramethylammonium iodide (TMAI, 99%) were purchased from J&K Beijing. Oleic acid and 1-octadecene (ODE, 90%) was ordered from Sigma-Aldrich. Methyl acetate (MeOAc, AR), methanol (AR) and Toluene (AR) were obtained from Sinopharm, China. Abbreviations and full terms of the ligands and hole-transport materials (HTMs) used in the study are listed in Table 1. Table 1. Abbreviations and full terms of the ligands and hole-transport materials used in the study.

Ligands

Organic HTMs

Abbreviation

Full term

TMAI

Tetramethylammonium iodide

TBAI

Tetrabutylammonium iodide

BDT

Benzene-1,3-dithiol

ACR

Acrylic acid

Thiourea

Thiourea

Threonine

Threonine

PTB7

Poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-al t-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} 5

Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophen PCE10

e-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carbo xylate-2-6-diyl)]

PCE11

Poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3’’’-di(2-octyldod ecyl)-2,2’;5’,2’’;5’’,2’’’-quaterthiophen-5,5’’’-diyl)] Poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithi

PBDB-T

ophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4 ’,5’-c’]dithiophene-4,8-dione)]

P3HT

poly(3-hexylthiophene-2,5-diyl)

Spiro-OMeTAD 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene

2.2. Preparation of colloidal AgBiS2 QDs AgBiS2 QDs were synthesized following the work previously reported by Bernechea et al. (Bernechea et al., 2016) with modifications. 1 mmol Bi(OAc)3 and 0.8 mmol Ag(OAc) were added to 6 ml OA in flask. The mixture was heated to 100 °C, repeatedly degassed and purged with argon for about 6 h until reagents completely dissolved. The sulfur precursor was prepared by dissolving 1 mmol hexamethyldisilathiane (HMS) in 6 ml 1-octadecene, which was quickly injected into the flask. After 5 s, the reaction was quenched by ice-water bath. The as-synthesized quantum dots were precipitated by adding acetone or MeOAc, collected by centrifugation and re-dispersed in toluene. This purification process was repeated at least twice. The PbS QDs were dispersed in toluene in the end. 2.3. Device fabrication and ligand exchange The fabrication of the devices have been done in ambient atmosphere. The diagram of the whole fabrication process was shown in Figure 1. The patterned ITO substrates (ITO/glass: Rs ≤ 15 Ω per square) were cleaned by detergent, deionized water, acetone and isopropanol for 15 min in sequence. The substrates were treated with O2-plasma for 5 min. The ZnO solutions with different concentration were spin-coated onto the ITO substrates under 2000 rpm for 60 s and then annealed at 80 °C for 8 min in ambient. The AgBiS2 QD film was deposited on top of the ZnO 6

film from its nanocrystal solution (20 mg/ml in toluene), by a layer-by-layer (LBL) process. Every step of LBL includes deposition of AgBiS2 QD layer and exchange of the surface ligand. Then the samples were annealed at 100 °C for 10 min. The devices were stored in ambient atmosphere in the dark for overnight. Then a thin hole-transport layer (HTL) of polymer was deposited by spin-coating the polymer solution (5 mg/ml solution in chlorobenzene) at 2000 rpm. The device fabrication was completed by thermal evaporation of 4 nm MoO3 and 100 nm Ag as the anode.

Figure 1. Diagram of the QDSC device fabrication and the ligand exchange process.

In the ligand exchange process, 200 μl tetramethylammonium iodide (TMAI) solution in methanol (1 mg/ml) was added on AgBiS2 QD film to fully cover the QD film, wait for 20 s, then spin-cast at 2000 rpm for 20 s. Methanol was added for rinse. Then repeat the TMAI ligand exchange and methanol rinse once, finally rinsed by Toluene. This procedure has been applied for each AgBiS2 QD layer. In the dipping method, ligand exchange was carried out by dipping the AgBiS2 QD film into TMAI solution (1 mg/ml in methanol) for 30 s, followed by immersion in methanol for 10 s. The above two steps were repeated once. The film was immersed in toluene for 10 s and finally dried by spin at 2000 rpm for 20 s. 2.4. Characterizations

7

The transmission electron microscopy (TEM) image and the selected area electron diffraction pattern (SAED) are recorded using JEOL2100, Japan. TEM samples are prepared by dropping AgBiS2 QD solution onto TEM grids. The phase analysis of the as-prepared QD sample is performed by using a powder X-ray diffractometer (Bruker AXS D8 Advance, Germany) equipped with Cu Kα radiation ( λ = 0.154 nm). The UV–vis absorption spectra of QDs are recorded on a UV/Vis/NIR Spectrophotometer Lambda 950 (Perkin, Elmer). The Fourier transform infrared (FTIR) spectra are measured in transmittance mode using IR spectrometer instrument (Thermo, Nicolet 6700). The cross-section scanning electron microscope (SEM) images are recorded using field emission scanning electron microscope (Hitachi, S-4800) with an accelerating voltage of 6 kV.

The J–V curves are obtained by using Keithley 2400 source meter under the solar simulator (Newport Oriel Sol3A) with simulated AM 1.5 G illumination (100 mW/cm2), with light source of a 450 W xenon lamp calibrated by a standard Si reference solar cell (Newport, 91150 V). All the J–V curves are measured in glovebox at room temperature. The external quantum efficiency (EQE) is conducted to verify the measured current density of the solar cells in ambient atmosphere, using the Newport quantum efficiency measurement system (ORIEL IQE 200TM) combined with a lock-in amplifier and 150 W xenon lamp. The light intensity at each wavelength is calibrated by the standard Si/Ge solar cell. The device stability to resist moisture and oxygen was evaluated by monitoring the efficiency degradation both in both N2-filled glove box and ambient atmosphere with relative humidity (RH) of 30% and 80%. The efficiency was tested according to the J–V curves by using Keithley 2400 source meter under simulated AM 1.5 G illumination (100 mW/cm2).

8

3. Results and Discussions TEM images show that slightly bigger AgBiS2 QDs with an average particle size of ~5.3 nm (Figure 2 a) and a narrow size distribution (Figure 2 b) were obtained in experiment, compared with the results of the reference work (Bernechea et al., 2016). Cubic rock salt structure crystal phase was confirmed by the measured XRD pattern (Figure 2 c). All the observed diffraction peaks can be indexed to the cubic rock salt structure with widening and small deviations due to strain (Bernechea et al., 2016) . It is consistent with the obtained selected area electron diffraction (SAED) pattern. The high-resolution TEM image shows high crystallinity of the QDs (Figure 2 a, inset). The determined plane distance of 0.32 nm corresponds to (111) interplanar distances of the cubic. The UV-vis absorption spectrum of QDs in toluene is presented in Figure 2 d. It is worth to note that red shift is observed for the absorption of the QD film before and after ligand exchange.

Figure 2. a) TEM image of AgBiS2 QDs. Inset (top-right): SAED pattern of AgBiS2 QDs. Inset (bottom-right): HR-TEM image of AgBiS2 QD. b) Size distribution histogram of AgBiS2 QDs. Inset 9

(top-left): photo of AgBiS2 QD sample in toluene. c) XRD pattern of AgBiS2 QDs and reference standard pattern of cubic AgBiS2 (PDF#21-1178). d) UV–vis absorption spectra of AgBiS2 QDs in different situation.

The performance of QDSC is dependent on the quality of QD of different batches. AgBiS2-PTB7 QDSCs were firstly fabricated based on exactly the same device configuration to estimate the quality of the lab-made AgBiS2 QDs, with the same purification process and the same ligand exchange method as reported by the literature (Bernechea et al., 2016). QDs were precipitated by acetone in the purification process. TMAI solution (1 mg/ml in methanol) was dropped onto the AgBiS2 QD film spinning at the speed of 2000 rpm for 20 s for ligand exchange of the QD film. This step was repeated once. The TMAI-treated QD film was rinsed with methanol and toluene. Based on the lab-made AgBiS2 QDs and without systematic device optimization, lower device performance was obtained compared to the result reported in reference (6.3%) (Bernechea et al., 2016). The best performance with PCE of 1.44%, open circuit of voltage (VOC) of 0.416 V, short circuit current density (JSC) of 6.38 mA/cm2 and fill factor (FF) of 54.3% (Figure 3 a) was obtained when four-layer deposition of AgBiS2 QD was applied (Figure S1 a). Then the four-layer deposition was fixed for preparing the AgBiS2 QD film in the study. After systematical optimization for other layers in device including PTB7, ZnO and MoO3 (Figure S1b, c and d), the best PCE of 2.07% was obtained with VOC of 0.425 V, JSC of 8.76 mA/cm2 and FF of 55.6% (Figure 3 b) based on the lab-made QDs. This lower device performance compared to the result reported by Bernechea et al. is probably due to the inferior quality of our lab-made QDs. Then the optimized procedure was applied to prepare the solar cells in the following parts.

10

8

10

2

JSC (mA/cm )

b

2

JSC (mA/cm )

a

4

PCE= 1.44% VOC= 0.416 V 2

JSC= 6.38 mA/cm FF= 54.3% 0

PCE= 2.07% VOC= 0.425 V

5

2

JSC= 8.76 mA/cm FF= 55.6% 0

0.0

0.2

0.0

0.4

0.2

c

PCE (%)

PCE 2.07%

2

JSC (mA/cm )

d 3.0

PCE 2.61%

10

with spinning without spinning dip in solution

5

0.4

Voltage (V)

Voltage (V)

2.5

2.0 1 2 3 4 1+1

1.5

PCE 0.26% 0

1.0 0.0

0.2

0.4

1

Voltage (V)

2

3

4

1+1

Methanol rinse

Figure 3. a) J-V curve of the AgBiS2-PTB7 solar cell fabricated according to literature method. b) The best device performance obtained after preliminary optimization by tuning the thickness of different layers in device. c) J-V curves of the solar cells with ligand exchange conducted with/without substrate spinning or by dipping in solution. d) Boxplot statistics of device PCE based on different methanol rinse times for the AgBiS2 QD film after ligand exchange. The ‘1+1’ means one methanol rinse has been applied for each of the two ligand exchange steps.

It is considered that the spin of substrate in TMAI-treatment of AgBiS2 QD film could cause incomplete/insufficient ligand exchange due to less TMAI amount, low coverage and short ligand exchange time by spinning. In the present procedure, more TMAI solution (200 µl) was applied to fully cover the QD film when substrate was kept static. An extra 20 s of waiting was applied before the substrate spinning, to ensure enough time for ligand exchange and get TMAI penetrated deeper into the film in vertical direction. By which, improved performance with the best PCE of 2.61% was obtained (Figure 3 c). Since the organic ligands can hugely hinder the charge transport in QD film. The excess TMAI ligands and the removed OA molecules from QD surface must be removed after ligand exchange, especially for those films consist of multiple-deposition of QDs. Considering the fact that TMAI presents low solubility in methanol, 11

multiple rinses (1 to 4 times) by methanol were conducted in the study to improve the film conductivity. Twice-rinses delivered the best device performance (Figure 3 d). Further improvement of device performance was achieved (Figure 3 d 1+1) when one of the two methanol rinses was arranged between the two TMAI ligand exchange steps (noted as 1+1), probably due to promoted morphology and conductivity. The PCE was improved to 3.06% with much increased VOC of 0.47 V, JSC of 12.43 mA/cm2 and slightly reduced FF of 52.4%. Then this optimized rinse procedure was fixed for the device fabrication.

On the static substrates, different ligand molecules were tested to remove the oleic acid on QD surface in film. The obtained performance parameters of AgBiS2-PTB7 QDSs are shown in Table 2. The TMAI ligand exchange provided the best performance. Dipping strategy was tested as well for post-deposition ligand exchange. However extremely depressed device performance was obtained in the experiment (Figure 3 c, black curve). Table 2. Performance of different ligands treated AgBiS2-PTB7 solar cells, for which the synthesized QDs have been purified by acetone, ligand exchange has been carried out on static substrate, and with the ‘1+1’ rinse process. VOC

JSC

(V)

TMAI

PCE

(mA/cm )

(%)

(%)

0.472

12.43

52.4

3.06 (2.93±0.11)

TBAI

0.380

6.33

34.0

0.82 (0.71±0.14)

BDT

0.447

0.36

48.4

0.08 (0.06±0.02)

Ligands

a)

a)

FF 2

ACR

0.364

2.13

37.8

0.29 (0.16±0.11)

Thiourea

0.414

7.87

37.3

1.22 (1.10±0.08)

Threonine

0.214

0.58

39.3

0.05 (0.04±0.21)

The average values of PCEs were based on 24 individual devices.

It was observed in the experiment that the acetone purified AgBiS2 QDs tend to precipitate from solution in a few days to month (Figure 4 a, b). It is probably because of the chemical instability of surfactant and/or bigger particle size of the QDs. MeOAc was finally adopt to replace

12

the acetone as the purification solvent for AgBiS2 QDs. The obtained AgBiS2 QDs after MeOAc purification showed much better stability in toluene. There is no precipitation observed after ~6 months (Figure 4 a, b) kept in atmosphere at room temperature. More homogeneous and smoother QD film was obtained (Figure 4 c, d). Figure 5 a shows the FTIR spectra of AgBiS2 QD films on quartz treated by different method. The bands at 2852 and 2924 cm−1 were observed according to the stretch modes of –CH2– and –CH3 of OA. The bands at 1401 cm−1 and 1516 cm−1 were recognized and attributed to the asymmetric and symmetric stretching vibrations of –COO– functional group. Increased transmittances at the bands corresponding to carbon chain and carboxyl group were recorded for the films after ligand treatment due to the removal of organics after ligand exchange. The lowest corresponding bands were recorded for the QD film treated with MeOAc (Figure 5 a red), indicating promoted ligand exchange, which is consistent with the obtained device performances.

13

Figure 4. a) and b) Photos of the AgBiS2 QD solution in toluene purified by different solvents (left: acetone, right: MeOAc) after a period of time. c) and d) Photos of the QD films purified by different solvents. Untreated Reference method Our method

b

20

2

JSC (mA/cm )

80 10

60 -CH2-

1516

20 0

-COO-

-CH3

3200

2800

2400

2000

1600

PCE10 PTB7 PBDB-T PCE11 Spiro-OMeTAD P3HT

0

1401

40

2924 2852

Transmittance (%)

a 100

1200

0.0

-1

0.2

0.4

Voltage (V)

Wavenumber (cm )

Figure 5 a) FTIR spectra of AgBiS2 QD films by different ligand treatment. The vibrations at 2924.15 -1

-1

-1

-1

cm and 2852.94 cm are υasym (C-H) and υsym (C-H). The vibrations at 1516.28 cm and 1401.28 cm -

-

correspond to υasym (COO ) and υsym (COO ). b) The best J-V curves achieved for AgBiS2 QDSCs with different polymer hole-transport layer. Table 2. Performance parameters of AgBiS2 QDSCs with different polymer HTL. The synthesized QDs have been purified by MeOAc. Ligand exchange has been carried out on static substrate with the (1+1) rinse process.

a)

Polymer

VOC

JSC

(HTL)

(V)

PTB7

a)

FF

PCE

(mA/cm )

(%)

(%)

0.435

17.55

55.9

4.26 (3.81±0.31)

PCE10

0.445

18.87

54.4

4.57 (4.36±0.18)

PCE11

0.437

14.66

55.2

3.54 (3.19±0.22)

PBDB-T

0.443

13.03

47.5

2.74 (2.60±0.12)

P3HT

0.389

10.90

37.5

1.59 (1.29±0.18)

Spiro-OMeTAD

0.412

9.35

52.8

2.04 (1.98±0.06)

2

The average values of PCE were obtained based on 24 individual devices.

Improvement of JSC from 12.4 to 17.5 mA/cm2 was achieved for the AgBiS2-PTB7 QDSC due to the quantum dot purification by MeOAc, with maintained VOC and FF. Which improves further the PCE to 4.26% (Table 3). This strategy combining MeOAc purification and post-deposition ligand exchange works well for AgBiS2 QDSC with other polymer HTLs as well. Decent performances were obtained for all the AgBiS2-polymer QDSCs (Table 3), which indicates good applicability of the method based on QDs and solvent. The best PCE of 4.57% with VOC of 0.445 V, JSC of 18.87 mA/cm2 and FF of 54.4% (Figure 4b and Table 3) was achieved based on AgBiS2-PCE10. The EQE 14

curves of AgBiS2-PTB7 and AgBiS2-PCE10 based QDSCs and their coresponding absorption spectra are presented in Figure 6.

Figure 6. The EQE curves (red) of AgBiS2 QDSCs with PTB7 or PCE10 HTL and the corresponding absorption spectra (black) of AgBiS2-polymer films.

Figure 7. a) Cross-section SEM image of the optimal device. b) Normalized PCE decay curve of AgBiS2 QDSCs aged in N2-filled glove box or ambient atmosphere (RH: 30% or 80%) at room temperature.

The cross-section SEM image of the optimal device was measured and shown in Figure 7 a. Much thicker (90 nm vs 35 nm) AgBiS2 QD layer was recorded for the device compared to that of the device in literature (Bernechea et al., 2016), which would benefit the reproducibility in manufacture. All the fabrication steps of the solar cells have been carried out in ambient atmosphere, including the purification and storage of the QDs. The device degradation was monitored in different atmosphere at room temperature (Figure 7 b) to estimate the device stability to resist moisture and oxygen. Increase of PCE was observed for all the devices in different situation in the first 10 days, probably due to the further removal of the organics in film 15

by volatilization. After 150 days, slight increase of PCE was recorded for the device kept in ambient atmosphere with RH of ~30%. While the device degraded fast in ambient atmosphere with relative high humidity of 80%, with only ~53% of the original PCE retained after 20 days. Promising device stability was also confirmed for the device kept in N2-filled glove box, by retaining 95% of the original PCE in almost 200 days.

4. Conclusions Eco-friendly QDSCs were fabricated based on lead-free AgBiS2 QDs and low toxic purification solvent of MeOAc. Decent efficiency and good stability were demonstrated for the fabricated solar cells. Improvement of device performance was presented. PCE of 4.57% was obtained for AgBiS2 QDSC with a more environmentally-friendly solvent consumption way. Decent performances were verified for devices with different polymer HTL, which indicates good applicability of the active material and the solvent system in QDSCs. Over 95% of the original PCE was maintained after 150 days both in N2-filled glove box and ambient atmosphere with relative humidity of 30%. This work first deals with solvent system in device fabrication to reduce ecological impact of the QDSC technology. It provides specific strategy to fabricate decent QDSCs in

an

environmentally-friendly

way,

which

could

help

to

promote

the

development/commercialization of QDSC tecnology.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work was supported by Ningbo S&T Innovation 2025 Major Special Program (2018B10055)

16

and National Natural Science Foundation of China (No. 51672068). The authors thank Prof. Thomas Nann for proof reading of the manuscript.

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Highlights: •

Solar cell is obtained via AgBiS2, methyl acetate purification and ligand exchange.

•

Decent efficiency and stability are demonstrated for the quantum dot solar cell.

•

Low-toxic source consumption reduces the hazardous discharge of photovoltaics.

•

The work provides referential strategy for eco-friendly efficient solar cell.