Improved performance in Ag₂S/P3HT hybrid solar cells with a solution processed SnO₂ electron transport layer

Abstract

We successfully constructed a heterojunction structure composed of Ag₂S nanocrystals/P3HT conjugated polymer with a relatively high absorption coefficient and broader absorption from the ultraviolet to near-infrared region. The assembled P3HT:Ag₂S devices exhibited outstanding short-circuit current density around 19 mA cm⁻². Meanwhile, we demonstrated that a low-temperature solution-processed nanocrystalline SnO₂ thin film prepared by a facile synthesis method can be an excellent electron transport layer (ETL) material for hybrid solar cells. The SnO₂ based hybrid solar cells exhibit an open circuit voltage of 280 mV, which was 100 mV higher than those for devices without SnO₂. Based on the above work, we hypothesized that the higher power conversion efficiency is due to the excellent properties of nanocrystalline SnO₂ films, such as high electron mobility and excellent transparency at visible wavelength and enhanced exciton dissociation efficiency, and better energy level alignment between FTO and Ag₂S for greater photovoltage retention. The simple low temperature low energy consumption, and low-cost soft-chemical process is compatible with the roll-to-roll manufacturing of low-cost hybrid solar cells on flexible substrates.

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1. Introduction

Hybrid solar cells based upon an organic–inorganic semiconductor heterojunction are currently the theme of significant interest as they incorporate the attractive properties of both organic and inorganic materials. Hybrid solar cells utilize the advantages of the organic – enhanced light absorption in a wide range of wavelengths, adjustable molecular structures for energy band alignment, and use the advantages of inorganics, including facile solution process synthesis ability, high carrier mobility and compatible fabricating processes, and so on.^1–7

Monoclinic α-Ag₂S is an attractive material for use in photovoltaic devices due to its appropriate narrow band gap of 1.1 eV, a relatively high absorption coefficient from the ultraviolet (UV) to near-infrared (NIR) region, and excellent optical limiting properties. Therefore, it has also been used in a wide range of applications in optical and electronic devices such as photoconductive cells, infrared detectors, superionic conductors and solar-selective coatings. In addition, Ag₂S is reported to have negligible toxicity in organisms.^8–11

The development of Ag₂S photovoltaics (PVs) is limited by low device open-circuit voltages. A strong contributing factor to this underperformance is charge recombination can easily happen between Ag₂S and cathode. Therefore, it is highly desirable to develop an approach toward easy fabrication, low-cost and high-efficiency planar Ag₂S solar cells. Promising heterojunction candidates SnO₂, which demonstrate slightly positive conduction-band offsets and high V_OC potential. Tin oxide (SnO₂) is a promising wide band gap oxide material because of its higher electron mobility and large band gap (3.8 eV). Mobility reported in both single crystal SnO₂ (μ_e ∼ 250 cm² V⁻¹ s⁻¹) as well as nanostructures (μ_e ∼ 125 cm² V⁻¹ s⁻¹) are orders of magnitude higher than other possible candidates like ZnO and TiO₂. Furthermore, SnO₂ has a low sensitivity to UV degradation due to its larger band gap and hence has better long term stability.^12–15

In this study, we have fabricated hybrid cells with FTO/Ag₂S/P3HT/Au and FTO/SnO₂/Ag₂S/P3HT/Au configurations. Solution-processed SnO₂ was introduced between FTO and Ag₂S to explore the electron transport effects on the device performance. A remarkable enhancement of the device efficiency is found upon inserting a SnO₂ layer, dominantly explained by enhanced exciton dissociation efficiency or the reduction of the energetic barrier at the FTO/Ag₂S interface. Upon inserting a SnO₂ layer, the PCE increases from 1.1% to 2.1%, resembling an improvement of app. 90%.

2. Experimental details

2.1. Material

Silver target was purchased from Beijing InnoChem. Sulfur powder (99.99%) was bought from Aladdin Reagent. The polymer donor P3HT was bought from Rieke Metals. Tin chloride dehydrate (SnCl₂·2H₂O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All of the used reagents were analytical grade, without further purification. The purity of the gold top anode prepared by thermal evaporation was 99.99%. Fluorine-doped tin oxide (FTO) coated glass with a sheet resistance of 14 Ω sq.⁻¹ was purchased from Asahi Glass (Japan).

2.2. Preparation of light harvesting layer Ag₂S

The FTO substrate was cleaned by ultrasonic treatment in detergent, deionized (DI) water, acetone, and alcohol sequentially for 10 min each. Then, the FTO substrate was dried by N₂ gas. Finally the cleaned substrate was treated in an UV-ozone chamber for 15 min. Silver film was prepared by a radio frequency (RF) magnetron sputtering system using a metal silver (99.99% pure) target in Ar atmosphere. The base vacuum chamber was pumped down to a pressure below 10⁻⁴ Pa. The Ar gas flow rate was retained at 10 standard-state cubic centimeter per minute (sccm). The deposition progress was carried on under a pressure of 1 Pa at 60 °C and RF power of 60 W. By controlling the sputtering time, we could achieve copper films of various thicknesses.

In a typical procedure, the elemental silver coated FTO glass and 0.01 g sulfur powders were put into a 80 mL vessel, and then 20 mL of N,N-dimethylformamide (DMF) was added. The container was maintained at 80 °C for 6 h. After it cooled down to room temperature naturally, the FTO with the product was taken out of the autoclave and washed with ethanol for several times. Then the FTO/Ag₂S was dried in an oven at 60 °C for 30 min. Finally, a black thin film on the FTO substrate was obtained.

2.3. Solar cells fabrication

SnO₂ precursor solution was prepared by dissolving tin chloride dehydrate (SnCl₂·2H₂O) in ethylene glycol by stirring for several hours at a concentration of 5–40 mg per 2 mL to serve as a stock solution. The cleaned substrate was treated by oxygen plasma right before use. In SnO₂ devices, the SnO₂ precursor solution was spin-coated onto the FTO glass at 2000 rpm for 60 s. Afterwards, the samples were annealed at 200 °C in an air atmosphere for 2 h. The synthesized SnO₂ ETLs were treated by UV-ozone for 15 min before magnetron sputtering deposition of silver film. The Ag₂S was deposited on the substrates as above described. After the typical hydrothermal process, the substrates was dried at 60 °C in a vacuum oven overnight. The P3HT solutions (17 mg mL⁻¹ in chlorobenzene) were spin-coated onto Ag₂S/SnO₂/FTO film at 1500 rpm for 40 s and subsequently annealed in N₂ atmosphere at 100 °C for 10 minutes and the Au counter electrode was finally deposited by a thermal evaporator under a pressure of 1 × 10⁻³ Pa. The active area of the device was fixed at 0.03 cm².

2.4. Device characteristics

The current density–voltage (J–V) characteristics of the PSCs were measured on a CHI660D electrochemical workstation (ShangHai, China) with a scan rate of 100 mV s⁻¹ and the device test was carried out under illumination of AM 1.5G, 100 mW cm⁻² (The light intensity was calibrated using a Si photodiode) at room temperature using a solar simulator.

The crystal structure of the Ag₂S film was evaluated by X-ray diffraction (XRD) with a Bruker D8 Advance diffract meter using Cu Kα radiation at 40 kV and 40 mA. Line traces were collected over 2θ values ranging from 20° to 80°.

The morphology of hybrid solar cells were observed by a high resolution field emission scanning electron microscope (SEM, FEI XL-30). The thicknesses of the Ag₂S were also measured by SEM observation. The energy dispersive spectroscopy (EDS) measurement was made in a scanning electron microscope (FEI XL-30) equipped with an X-ray analyzer and an EDS spectrometer.

The square average roughness (RMS) of the SnO₂ films and bare FTO substrates were characterized by an atomic force microscopy (AFM, SPM-9500j3). The transmission and absorption spectra were measured by an ultraviolet-visible (UV-vis) spectrophotometer (CARY 5000, Varian) at room temperature.

X-ray photoelectron spectroscopy (XPS) analyses and ultraviolet photoemission spectroscopy (UPS) were performed on an ESCA Lab220i-XL electron spectrometer (VG Scientific) using a monochromatic Al Kα source (300 W).

The compositions and chemical states of the Ag₂S films and the SnO₂ ETLs were examined by XPS. Before been tested, samples were sputtering cleaned, to remove atmospheric contamination in the XPS chamber for approximately 30 s, by the lower energy of Ar⁺, and the Ar⁺ gun was operated at 0.5 kV, at a pressure of 1 × 10⁻⁷ Pa. The vacuum pressure of the analysis chamber was better than 1 × 10⁻⁸ Pa. The whole survey scan to identify the overall surface composition and chemical states were performed, using a monochromated Al Kα X-ray source (=1486.68 eV), detecting photoelectrons at a 150 eV pass energy and a channel width of 500 meV. The surface carbon signal at 284.6 eV was used as an internal standard.

The work function and band energy position were calculated by UPS measurement. UPS was carried out under a pressure of ∼2 × 10⁻¹⁰ Pa using helium Iα (21.22 eV) radiation from a discharge lamp operated at 90 W, a pass energy of 10 eV, and a channel width of 25 meV, and the energy resolution was 0.02 eV. A −9 V bias was applied to the samples, in order to separate the sample and analyze low-kinetic-energy cutoffs.

3. Results and discussion

3.1. Characterization of light absorber Ag₂S

Fig. 1(a) and (b) show the device structure of the hybrid solar cells and the typical XRD pattern of Ag₂S film on FTO substrates, respectively.

Fig. 1 The device structure and the XRD pattern of the Ag₂S films, which shows a monoclinic α-Ag₂S phase of the synthesized films from the hydrothermal progress.

The phase identification of as-synthesized Ag₂S films was performed using XRD measurement presented in Fig. 1(b). The peaks at 30.2°, 35.5°, and 51.6° 2θ correspond to the (111), (1̄21), and (2̄13) reflections of monoclinic α-phase structured Ag₂S, respectively (JCPDS, 14-72).^16–18

In order to study the morphology and confirm the end product of our hydrothermal synthesis, the scanning electron microscopy (SEM), EDS, XPS and UPS measurements were carried out. By varying the sputtering time of Ag target, we achieved Ag₂S films of different thicknesses.

The film morphology and chemical composition were determined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) in Fig. 2. Fig. 2(a) is a low magnification SEM image of the initial magnetron sputtered Ag. Well distributed particles of nanosized elemental silver grains with little size (about 60–80 nm in diameter) can be observed. The insert of Fig. 2(a) is an AFM image of the original magnetron sputtered Ag surface which was assembled by nanosized elemental silver grains. Fig. 2(b) shows typical SEM images of the as-prepare Ag₂S nanocrystal thin film obtained after 6 h of reaction with elemental sulfur at 80 °C. The SEM image of the Ag₂S thin film at low magnification reveals that the selected area of the FTO glass is fully covered with ordered Ag₂S nanocrystals. The nanocrystals have nonuniform diameters in the range of 60 to 150 nm. The insert of Fig. 2(b) is a SEM image of the vertical section of the Ag₂S film, showing that the mean thickness of the Ag₂S film is about 400 nm. Fig. 2(c) shows the SEM image of the fabricated P3HT/nanocrystals Ag₂S hybrid film. As we expected, the P3HT polymer is smooth and neatly infiltrate into the nanopores between adjacent Ag₂S nanocrystalline. The thickness of the fabricated P3HT film was also measured to be about 100 nm using cross-section SEM images of the film. The EDS spectrum in Fig. 2(d) of the Ag₂S films shows strong silver and sulfur signals with Ag : S ratios of 2.16 : 1.00, which proves that the atomic ratio of elemental silver and sulfur is approximately 2 : 1.

Fig. 2 (a) SEM images of Ag films made on FTO substrates. (b) SEM images of Ag₂S films after hydrothermal progress. (c) SEM images of P3HT films spin-coated on Ag₂S substrates. (d) The EDS spectrum of FTO/Ag₂S. The inset of Fig. 2(a) and (b) is the AFM images of Ag films made on FTO substrates and the cross-sectional SEM image of FTO/Ag₂S respectively.

The UV-vis-NIR absorption spectrum of a pristine P3HT film, Ag₂S film and P3HT/Ag₂S hybrid film are shown in Fig. 3(a). The pristine P3HT film exhibits the characteristic absorption spectrum ranging from 400 to 650 nm (Fig. 3(a), red curve). Meanwhile, the appearance of a broad absorption feature (320–750 nm) is observed, which is consistent with the presence of Ag₂S (Fig. 3(a), blue curve). Fig. 3(b) depicts the NIR absorption spectrum of pure Ag₂S film. A discernible absorption onset was detected in the NIR window at 1100 nm. As compared to bulk silver sulfide (E_g = 1 eV), the absorption spectrum of as-prepared films exhibits a large blue-shift, which suggests that the nanocrystalline behave within the quantum confined regime.^19,20 The another explanation is that different aggregate nanocrystalline sizes contribute different absorption sub-bands.^21,22

Fig. 3 The UV-vis-NIR spectra of Ag₂S films (a) (1) pristine P3HT thin film; (2) Ag₂S thin film; (3) P3HT:Ag₂S hybrid thin films. (b) The absorption curve of Ag₂S thin film between 600 and 1600 nm. The inserted curve reveals the diagram of (αhν)² against hν calculated from Fig. 3(b).

To investigate the basic properties of Ag₂S individually, we prepared Ag₂S films on silicon substrates for XPS characterization. The results are presented in Fig. 2. The binding energy of the spectra is calibrated by the C 1s peak of the aliphatic carbons at 284.6 eV as reference. Part of the oxygen in the spectra was attributed to adsorbed gaseous molecules. The XPS survey spectrum of Ag₂S is shown in Fig. 4. Fig. 4(a) shows a whole scale scan of the results of XPS test where we can find the peaks of silver and sulfur. The peaks at 367.74 and 373.68 eV in Fig. 4(b) could be assigned to the binding energies of Ag 3d_5/2 and Ag 3d_3/2, respectively. The doublet feature of the Ag 3d spectrum is due to the spin–orbit separation.^23,24 The spectrum of S 2p shown in Fig. 4(c) could be described as three peaks located at about 160.7 and 161.9 eV, respectively. The peaks at 160.7 and 161.9 eV could be assigned to the binding energies of S 2p_3/2 and S 2p_1/2, correspondingly, which were separated by a spin–orbit splitting of 1.2 eV.^25,26

Fig. 4 The XPS (a–c) and UPS (d) spectra of Ag₂S films. (a) A whole scale of X-ray photoelectron spectra of Ag₂S. (b) Ag 3d signals recorded for Ag₂S films. (c) Peak fitting of S 2p signals recorded for Ag₂S films. (d) The UPS spectrum of Ag₂S films.

In order to understand the band alignment of Ag₂S. UPS measurements were conducted to assist with determining the work function (WF) and ionization potential (IP) of the Ag₂S. UPS measurement was carried out with the same machine using helium Iα as the ultraviolet source. The wavelength of the ultraviolet is 58.13 nm and the energy is 21.22 eV. The work function of the Ag₂S is calculated by the following equation: E_K = hν − E_B, where E_K means the kinetic energy. We can conclude from Fig. 4(d) that the binding energy (E_B) is 16.3 eV. Thus, The WF of the Ag₂S film is determined to be 4.9 eV. The insert of Fig. 4(d) shows the energy difference between the top of valence band (E_V) and the Fermi level, which shows that E_V is 0.7 eV below the Fermi level. The IP is defined as the energy difference between valence band edge and vacuum level, which is 5.6 eV. The optical band gap is determined by a Tauc-plot analysis of the absorption spectra by the following equation: (αhν)² = A(hν − E_g), where E_g is the optical band gap of the samples and A is a constant.^27,28 The value of the bandgap of the Ag₂S film is estimated to be 1.1 eV. The electron affinity of the film was calculated to be 4.5 eV.

3.2. Study of simple planar Ag₂S solar cells

The effect of Ag₂S film thickness on device performance was systematically characterized. Typical J–V curves for the five conditions are shown in Fig. 5 and its parameters are given in Table 1, where the Ag₂S film were fabricated with different film thicknesses (300, 350, 400, 450 and 500 nm). Meanwhile, in our device, photogenerated carriers separate at P3HT/Ag₂S p–n junction interface. Electrons are injected into Ag₂S and are collected by FTO. Holes flow to the back side and are collected by Au contact. From the summarized device parameters chart in Fig. 5, Ag₂S films with 300 nm thick exhibited poor open circuit voltage (J_SC) and slightly lower fill factor (FF). When increasing film thickness to 350 nm, the J_SC was improved from 8.3 to 14.9 slightly. This illustrates that slight further increase the thickness of light harvesting layer Ag₂S benefits for the J_SC. However, excessive Ag₂S film thickness will reduce the device efficiency in the hybrid solar due to the separated exciton will be caught by defects in Ag₂S film and will induce larger series resistance which all contributed to the lower J_SC. The series resistance (R_s) raised from 13.5 Ω cm ² to 21.9 Ω cm ² which can also explain this phenomenon.^29,30 The highest PCE was achieved for the Ag₂S film when thickness is 400 nm. For film thickness less or greater than 400 nm, the device performances were degraded. The optimal devices show a V_OC of 0.18 V, a J_SC of 18.1 mA cm⁻², a FF of 33.5%, and a PCE of 1.1%.

Fig. 5 The performance of our devices: current density–voltage (J–V) plots for FTO/Ag₂S/P3HT/Au hybrid solar fabricated with different thickness of Ag₂S (Ag₂S film thicknesses = 300, 350, 400, 450 and 500 nm). Corresponding to the thin, barely sufficient and too thick stages.

Table 1 Summary of photovoltaic performance of the hybrid solar cell^a

Ag2S thickness (nm)	VOC (V)	JSC (mA cm^−2)	Rs (Ω cm^2)	FF (%)	PCE (%)
a There are 12 samples for each device in this study. The errors of PCEs are ±0.1%.
300	0.18	8.28	17.5	29.2	0.43
350	0.18	14.93	14.1	29.1	0.78
400	0.18	18.16	13.5	33.5	1.10
450	0.18	12.25	14.9	29.6	0.65
500	0.18	5.37	21.9	31.6	0.31

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3.3. Study and characterization of low-temperature solution-processed SnO₂

The SnO₂ solutions prepared as described were spin coated onto UV-ozone pre-treated FTO substrates and annealed at 200 °C for 2 hours in air.

X-ray photoemission spectroscopy (XPS) was performed to further investigate the surface characteristics of the SnO₂ films. Fig. 6 shows a full scan spectrum of the SnO₂ film made from the 0.1 M solution. O 1s and Sn 3d peaks are characteristic of SnO₂. The full XPS spectrum survey given in Fig. 6(a) shows the presence of O and Sn. The binding energies of 487.11 and 495.56 eV correspond to the Sn 3d_5/2 and Sn 3d_3/2 peaks, respectively (Fig. 6(b)). The main binding energy of 531.06 eV is attributed to the O 1s, which is the O²⁻ state in SnO₂ (Fig. 6(c)). The higher binding energy can be assigned to the chemisorbed oxygen atoms or hydroxyl groups.^15,31,32 Integrating the peaks and correcting for XPS sensitivity and emission efficiencies allows us to determine the quantity of remaining chloride in the sample to be approximately [Cl]/[O + Cl] ≈ 4.7 at%, indicating that more than 95% of the anions in the material has been successfully converted to oxygen. Moreover, the Sn 3d_5/2 peak is symmetric and no shoulder is observed, which indicates that the film consists mostly of SnO₂. The XPS spectrum for the SnO₂ film made from the 0.1 M solution was found to be identical to that from the 0.02 M solution. Therefore, all films fabricated from solutions of different SnO₂ concentrations are considered to have the same SnO₂ composition. According to the XPS spectra and the literature report, the energy level diagram of hybrid solar cells with SnO₂ ETL is shown in Fig. 6(d). The elevated conduction band minimum (CBM) contributes to the increase of the V_OC. The upshifted CBM of the blocking layer is more compatible with the FTO and the light harvesting layer, reducing the energy loss through the electron transportation. Meanwhile, the downshifted valence band maximum (VBM) also could be more effective to block holes, resulting in less recombination of electrons and holes at the film surface and thus, giving a better FF.³³

Fig. 6 XPS spectra of (a) survey, (b) Sn 3d, (c) O 1s peaks for a low-temperature solution-processed SnO₂ nanocrystalline film coated on a silicon substrate. (d) Energy band diagram of the hybrid solar cell, showing the separation and collection of photogenerated electrons and holes.

Fig. 7 presents the transmission spectra of SnO₂ samples on FTO used in our work and bare FTO substrates. As we know, SnO₂ is a wide band gap semiconductor, which has a strong absorption at short wavelengths. So we can see the SnO₂ sample is more transparent than the bare FTO between 300 and 600 nm. This is an encouraging result since high transparency of electrodes is vital for photovoltaic devices. The variation of (αhν)² versus hν are shown in Fig. 7 insert has been evaluated by extending the linear portion of the plots to the binding energy axis. The average intercept is 3.8 eV which is very close to those reported in literature.³⁴

Fig. 7 Transmission spectra of FTO substrates and with a 0.1 M SnO₂ nanocrystalline film. The inserted curve reveals the diagram of (αhν)² against hν calculated from Fig. 7.

For planar Ag₂S heterojunction solar cells, the morphologies of substrates also have an important influence on the device performance. A smooth interface is essential for achieving a higher FF for planar devices. We studied the interface between the absorber and the ETL. Fig. 8 shows atomic force microscopy (AFM) images of the SnO₂ films coated from solutions of different SnO₂ concentrations, 0.02, 0.1 and 0.5 M and bare FTO substrate. By increasing the concentration of SnCl₂·2H₂O in the solution, the films get thicker and smoother. The root-mean-square roughness (RMS) of the film made from a 0.1 M solution is only 0.769 nm which is as smooth as that of an ITO film. Films become rougher with increased concentration of SnO₂ in the solution and the surface roughness of the film from a 0.5 M solution is almost identical to that of an ITO substrate. Compared with 0.5 M, 0.02 M cause an increase of RMS from 0.853 nm to 0.945 nm. Thicknesses of the SnO₂ films from the 0.02 M and the 0.05 M solutions are too thin, it is not clear if those thin SnO₂ films from the 0.02 M solutions can cover the entire FTO surface. Considering about the slightly lower FF of devices, we may contribute this phenomenon to the imperfect smooth morphology of SnO₂ film.

Fig. 8 AFM images and the square average roughness (RMS) of (a) bare FTO, (b) 0.1 M SnO₂ films on FTO, (c) 0.02 M SnO₂ films on FTO and (d) 0.5 M SnO₂ films on FTO.

3.4. Study of novel SnO₂ ETLs based Ag₂S solar cells

J–V characteristics of photovoltaic devices using SnO₂ electron transport layers from the solutions of different SnO₂ concentrations are depicted in Fig. 9 and detailed parameters are summarized in Table 2. The device using a 0.1 M SnO₂ solution shows the best power conversion efficiency (PCE) of 2.1%, with open-circuit voltage (V_OC) of 0.28 V, short-circuit current (J_SC) of 19.3 mA cm⁻², and fill factor (FF) of 40.4%, in order to check the reproducibility of the device, more than 10 devices using the 0.1 M SnO₂ solution were examined with the same condition. The maximum and average PCE were 2.1% and 1.96%, respectively, which suggests the devices using the solution processed SnO₂ can constantly show the excellent performance. On the other hand, the device using a 0.5 M SnO₂ solution exhibits slightly lower PCE of 1.52% with J_SC of 12.6 mA cm⁻² and FF of 38.7%, which can be attributed to its high series resistance (R_s) of 14.8 Ω cm² compared to an 10.8 Ω cm² for the 0.1 M SnO₂ solution.

Fig. 9 J–V curves of the different SnO₂ concentrations hybrid solar cell using a 400 nm Ag₂S.

Table 2 Summary of photovoltaic performance of the hybrid solar cell^a

SnO2 concentration (M)	VOC (V)	JSC (mA cm^−2)	Rs (Ω cm^2)	FF (%)	PCE (%)
a There are 12 samples for each device in this study. The errors of PCEs are ±0.1%.
0.50	0.31	12.6	14.8	38.7	1.52
0.20	0.29	14.9	12.3	38.6	1.76
0.10	0.28	19.3	10.8	40.4	2.10
0.05	0.25	14.3	13.7	38.1	1.37
0.02	0.23	10.2	16.3	38.4	0.90

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Similar declined results were obtained with the device using a 0.02 M solution, PCE decreased compared to optimal SnO₂ concentration. As mentioned above, the morphologies of substrates have an important influence on the device performance. Compared to the interface RMS value of 0.94 nm using 0.02 M solution, the interface of 0.1 M solution is smoother. This smooth surface was very helpful to reduce the recombination of light induced excitations at the interface of the absorber and electron conductor, which would be beneficial for achieving a higher FF for planar devices.

By carefully controlling the interfaces of the devices and the film thickness, we achieved best-performing solar cells based on SnO₂ ETL efficiency of 2.10%, a V_OC of 0.28 V, a J_SC of 19.3 mA cm⁻² and a FF of 40.4%, whereas the best performing solar cells without ETL show an efficiency of 1.10% with a V_OC of 0.18 V, a J_SC of 18.1 mA cm⁻² and a FF of 33.5%. The corresponding J–V curves and parameters are presented in Fig. 10(a) and Table 3, we also make a comparison with the literature reported on metal-sulfide based solar cells in Table 3. We believe that our solution-processed SnO₂ provides a superior electron transport for highly-efficient, easily-fabricated and low-cost solar cells based on Ag₂S.

Fig. 10 (a) The J–V plots of the best-performing devices based on SnO₂ ETL and device without ETL. (b) IPCE spectrum of SnO₂/Ag₂S/P3HT hybrid solar cell. (c) Plots of dV/dJ vs. (J_SC − J)⁻¹ and the linear fitting curves calculated from the dark J–V curves of TiO₂ and SnO₂ devices. (d) Steady state PL spectra of the FTO/Ag₂S/P3HT and FTO/SnO₂/Ag₂S/P3HT samples.

Table 3 Photovoltaic performances of the devices based on FTO/No ETL or SnO₂/Ag₂S/P3HT/Au and metal-sulfide based thin film solar cells obtained in the literature

Cell structure	VOC (V)	JSC (mA cm^−2)	FF (%)	PCE (%)	References
FTO/Ag2S/P3HT/Au	0.18	18.16	33.5	1.10
FTO/SnO2/Ag2S/P3HT/Au	0.28	19.3	40.4	2.10
TCO/CdS/SnS/C/Ag	0.27	6	44	0.7	35
Mo/SnS/Zn(O,S)/ZnO/ITO	0.26	24.9	44.4	2.9	36
ITO/TiO2/CdS/Sb2S3:P3HT/PEDOT:PSS/Ag	0.73	3.85	46	1.29	37
Dense-TiO2/Sb2S3/P3HT/Au	0.55	12.3	69.9	5.06	38
FTO/Sb2Se3/CdS/ZnO/ZnO:Al/Au	0.35	17.8	33.5	2.1	39
FTO/SnO2/Sb2S3/P3HT/Au	0.58	10.5	45.2	2.8	40

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The prepared P3HT:Ag₂S devices exhibited outstanding short-circuit current density around 19.3 mA cm⁻². The basis for a high short circuit current of this type of solar cells is the photocurrent generation over a broad range of the solar spectrum, as is revealed by the IPCE spectrum in Fig. 10(b). Already at a wavelength of 800 nm, where Ag₂S is the only absorbing material, the IPCE shows significant photocurrent generation.

As mentioned above, a strong contributing factor of V_OC enhancement in SnO₂ ETL based devices is conduction-band offset between Ag₂S and its n-type ETLs material. As we know, the V_OC is determined by the hole and electron electrochemical potential difference, or quasi-Fermi level separation in the material. This separation may be limited by bulk recombination, which reduces the concentration of free photoexcited carriers, or limited by the electrochemical potential of electrons in the n-type ETL. A small positive conduction band offsets (spike-like) can moderately improve the V_OC by providing a barrier for electrons in the ETL from diffusing back into the low-lifetime bulk Ag₂S and recombining with holes. From a theoretical point of view, a small conduction band offsets in the range of 0 eV to 0.4 eV is the optimal band alignment.

We also noticed an obvious enhancement of photocurrent for SnO₂ devices. Except for the wider bandgap and a small positive conduction band offsets (spike-like), we also found other important characteristics for SnO₂-based devices. To further clarify the photovoltaic characteristics, a model based on the single heterojunction solar cell is put forward to analyze the I–V property of the device, from which we can get the series resistance (R_s) according to the linear fit of the data.

where A is the ideality factor, k_B is the Boltzmann constant, T is the absolute temperature and e is the elementary charge. Fig. 10(c) gives the plots of dV/dJ vs. (J_SC − J)⁻¹ with fit used to determine the R_s of the devices in the dark. As we can see, the R_s of the device without ETL is 13.5 Ω cm⁻² and it decreased to 10.8 Ω cm⁻² for SnO₂ devices. As we know, for solar cells, a smaller R_s is very important for achieving a higher current output. In order to investigate the interface transporting properties and charge carrier recombination losses of the devices, we carried out steady state photoluminescence (PL) measurements. Fig. 10(d) shows the PL spectra of FTO/Ag₂S/P3HT and FTO/SnO₂/Ag₂S/P3HT. The only difference in the samples is the electron conductor. The FTO/Ag₂S/P3HT sample has the higher PL intensity, indicating that serious recombination happens in the Ag₂S films or at the interface. When SnO₂ ETL were introduced into the samples, the PL intensities became weaker, suggesting that charge transfer occurred effectively before the carrier recombination in the Ag₂S films or at the interface. This conclusion can also explain the voltage and current enhancement of SnO₂ devices.

4. Conclusion

In conclusion, we present a facile solution-based route towards nanostructured, hybrid absorber layers based on Ag₂S. All of the material fabrication and device assembly processes were performed at low temperature and thus consumed negligible energy. The planar structure hybrid solar cell P3HT:Ag₂S exhibits outstanding short circuit current density around 19 mA cm⁻².

Moreover, using a low-temperature solution-processed SnO₂ as the ETL material, we most notably observed two characteristic improvements in the device performance: increased FF and V_OC. The largest of which occurs at the solution concentration from a 0.1 M solution. The best-performing hybrid solar cell, using solution-processed SnO₂ ETL, has achieved a PCE of 2.1% with a high V_OC of 0.28 V, a high J_SC of 19.3 mA cm⁻², and a FF of 40.4%. However, device performance was limited by the Ag₂S intrinsic bulk defects limited by the solvent thermal synthetic methods and suboptimal band gap. Further device configuration and processing optimization should boost the efficiency of Ag₂S solar cell to a new level.

Acknowledgements

This work was supported by the by the National High Technology Research and Development Program (2015AA050601), the National Natural Science Foundation of China (61376013), the Natural Science Foundation of Jiangsu Province (BK20131186), the program of Suzhou Science & Technology Bureau (SYG201449), the Fundamental Research Funds for the Central Universities (20152020203).

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