Heterovalent doping and energy level tuning in Ag2S

Solar Energy Materials and Solar Cells 182 (2018) 339–347

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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat

Heterovalent doping and energy level tuning in Ag2S thin-films through solution approach: pn-Junction solar cells Goutam Paul, Soumyo Chatterjee, Amlan J. Pal

T

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Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Doped Ag2S Scanning tunneling spectroscopy Fermi energy upon of heterovalent doping pn-Junction solar cells

In this work, we have fabricated heterojunction solar cells between layers of p-type CuS and n-type Ag2S deposited through successive ionic layer adsorption and reaction (SILAR) method. We have introduced Sn2+ and Al3+ as heterovalent-dopants in the cationic sites of Ag2S and studied their effect on the photovoltaic performance of the heterojunctions. As evidenced from the band diagrams drawn from scanning tunneling spectroscopy (STS) of the components, such doping shifted the Fermi energy of pristine Ag2S towards the conduction band-edge influencing the solar cell activities of the heterojunction in turn. While the trivalent Al-dopants introduced defects states in Ag2S and hence deteriorated the cell efficiency, doping with 5 at% of Sn at the silver site resulted in band-engineered pn-junction solar cells with a power conversion efficiency of 2.85%.

1. Introduction Elemental doping in semiconductors is a fundamental approach to tune their optical and electrical properties in accordance with device functionality [1]. In a range of doped semiconductors with intentional dopants, the foreign elements play critical roles leading to stimulated research on their properties and subsequent device applications [2–5]. In contrast to isovalent substitutions, heterovalent dopants can modulate the type of conductivity and other electrical properties of pristine semiconductors, leaving the band gap mostly unaltered [6–8]. A heterovalent dopant with one or more valence electrons than the host atom can be ionized by thermal energy in donating the extra electron(s) to the semiconductor (n-type doping). Similarly, a dopant-element with one or more less electrons can provide extra hole(s) or p-type doping. These electrons or holes are not only available as carriers for electrical conduction but also decides the type of majority carriers of dopedsemiconductors. Silver sulfide (Ag2S) is an n-type binary chalcogenide compound which has been studied for numerous applications in photocatalysis [9,10] and electronic devices [11–13]. Its potential as a light-absorbing material for solar cells is primarily due to its non-toxicity, high absorption coefficient in the near-infrared (NIR) region [14], process-dependent conductivity [15], stability against moisture [12], and more importantly multiple exciton generation (MEG) capability [16]. In spite of these advantages, application aspects of Ag2S have so far remained limited to quantum dot sensitized solar cells (QDSSCs) with the material's lower-dimensional forms [11,14,17]. In such devices, power ⁎

conversion efficiency (η) was low (1.7%) due to unfavorable energy levels of Ag2S sensitizer and TiO2 for electron-transfer from the chalcogenide to the oxide [11]. Instead, a hybrid bulk-heterojunction (BHJ) with a conventional polymer (P3HT) yielded an efficiency of 2.04% having a high short-circuit current (ISC); nanocrystals of Ag2S acted as electron-acceptors in the energy-level-matched BHJ having a type-II band-alignment at the P3HT:Ag2S interface [12]. However, the efficiency still remained far below the Shockley–Queisser limit of ~ 30% at the energy corresponding to the material's optical gap. For completeness, P3HT refers to poly(3-hexylthiophene-2,5-diyl), which is a commonly used donor polymer. Though reports on photovoltaic performance of Ag2S in its thin-film form are limited, a number of fabrication techniques have been considered in this direction. Solution-based approaches like chemical bath deposition (CBD) [18], spray pyrolysis deposition (SPD) [19], sol-gel method [15], successive ionic layer adsorption and reaction (SILAR) technique [17,20], and vacuum-based schemes like molecular beam epitaxy (MBE) [21], thermal evaporation [22], sulfurization method [23], gamma irradiation [24], and so forth have been reported for formation of Ag2S thin-films. Amongst these approaches, solution-based deposition methods are economically effective, versatile, and industrially viable for large-area film-formation and thereby device fabrication. SILAR is a well-known solution-based sequential dip-coating approach for depositing large-area, uniform, and single-phase semiconductor thin-films with a precise control over thickness at ambient condition [25]. Thin-films of a range of oxides [26–28] and chalcogenides [29–31] have been formed on virtually any kind of substrates

Corresponding author. E-mail address: [email protected] (A.J. Pal).

https://doi.org/10.1016/j.solmat.2018.03.022 Received 8 September 2017; Received in revised form 5 February 2018; Accepted 11 March 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.


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prepared by dissolving 0.1 M CuCl2·2H2O in methanol (20 mL). The solution was then complexed with 25% NH4OH at pH = 11.8. In doing so, the light-green colored homogeneous solution of the copper salt turned deep-blue due to formation of copper:amine complex. Following adsorption of the cationic precursor and rinsing in methanol, the substrates were dipped in methanol solution of 0.1 M Na2S·xH2O for sulfurization. Rinsing of the substrate in methanol completed one layer of the SILAR film. The above-mentioned steps were sequenced to obtain thin-films of CuS with a desired thickness.

through this method. Suitable homo- or heterovalent dopants can be introduced in the ionic sites to dope these semiconductors during the film-formation process [7,32]. Though a few reports on SILAR deposited Ag2S thin-films are available in literature [13,17,20,33], application prospects of such films as an active material in solar cells have remained an unexplored domain. In this work, we hence report thin-film formation of pristine and heterovalent-doped Ag2S by SILAR method to fabricate pn-junction solar cells. Due to monovalent nature of the cation in the binary semiconductor, heterovalent substitution at the metal site led to n-type doping in the semiconductor. Here we have considered bivalent tin (Sn2+) and trivalent aluminum (Al3+) to replace Ag+ leading to a gradual shift of Fermi energy towards the conduction band (CB). While CB and valence band (VB) edges were measured with scanning tunneling spectroscopy (STS) and density of states (DOS) thereof, pnjunctions were formed between a layer of CuS, which is a p-type semiconductor, and the pristine and doped-Ag2S layer as an n-type one. Solar cell characteristics of the junctions have been correlated with the band-diagram of the junctions.

2.3. Fabrication of devices Devices were formed on patterned indium tin oxide (ITO) glass substrates having a sheet resistance of 15 Ω/square. The substrates were cleaned by following a usual protocol and treated with ozone so that an intimate contact between ITO with its immediate layer could be achieved. A 40 nm thick hole-transport layer (HTL) of Cu@NiO (5 at% Cu) was first formed on the electrode following a well-documented method [34]. The active layers of pn-junctions, namely p-type CuS and n-type Ag2S were formed in sequence following the as-stated SILAR method. After formation of each layer, the films were annealed at 150 °C for 15 min in nitrogen environment to reduce the possibility of oxidation. As an electron-transport layer (ETL), a 20 nm thick film of PCBM was formed by spin-coating the fullerene from a 20 mg/mL solution in chlorobenzene at 2500 rpm for 30 s. Here PCBM represented the common abbreviation for the fullerene derivative [6,6]-phenyl-C61butyric acid methyl ester. The multilayered films were transferred to a glovebox and annealed at 120 °C for 15 min in nitrogen environment. Finally, aluminum was thermally evaporated in vacuum to form top electrodes (100 nm) orthogonal to the ITO strips. This completed fabrication of 4–5 cells in a single substrate each having an effective area of 10 mm2. The thermal evaporator was connected to the same glovebox in which devices were characterized.

2. Experimental 2.1. Formation of pristine and doped Ag2S thin-films by SILAR method Thin-films of Ag2S were formed through SILAR method following a reported route [20]. Briefly, substrates were first dipped in a bath containing a cationic precursor, namely 0.2 M AgNO3 in methanol (20 mL). After allowing 30 s for adsorption of cations, the substrates were rinsed in methanol and dried in air. They were then immersed in an anionic precursor composed of 0.1 M Na2S·xH2O in methanol (20 mL) for 45 s followed by the same rinsing and drying protocol. This completed formation of one layer of Ag2S; the dipping sequence was repeated to obtain a desired film-thickness of the semiconductor. During the process, the silver ions combine with the sulfides to form silver sulfide. Since Ag2S is one of the most insoluble compound with a low solubility product, KSP = 1.0 × 10−51, the ionic product (QSP) in the solutions employed during this work was always higher than this value at a moderate concentration ensuring formation of Ag2S thinfilms. In order to introduce cationic dopants in Ag2S thin-films, a calculated atomic percentage of a dopant was added to the cationic precursor. In order to introduce Sn2+ or Al3+ separately, SnCl2·2H2O and AlCl3·6H2O were used, respectively, as the source of dopants. After addition of the dopant-salt, the clear AgNO3 solution immediately turned turbid evidencing formation of a white insoluble AgCl precipitate. To dissolve this precipitate, an excess amount of NH4OH was added to the precursor solution until its complete dissolution. The entire process can be represented by the following double replacement reactions in ionic form:

2.4. Characterization of materials The materials were characterized with optical absorption spectroscopy, X-ray diffraction (XRD) patterns, and energy dispersive X-ray analysis (EDXA). These measurements were carried out with a Shimadzu UV-2550 Spectrophotometer, Bruker D8 Advanced X-ray Powder Diffractometer (Cu Kα radiation, λ = 1.54 Å), and Jeol JEM2100F transmission electron microscope, respectively. Thickness of the films was measured with a Nanosurf Easyscan2 atomic force microscope (AFM). In addition, ultrathin films of the semiconductors were characterized with a Nanosurf Easyscan2 scanning tunneling microscope (STM). While the films were deposited on arsenic-doped silicon having a resistivity of 3–10 mΩ cm, the STM was operated in ambient condition to record tunneling current through the films; voltage was applied to the Pt/Ir tip with respect to the substrate. From first derivative of the tunneling current having a correspondence to the DOS spectrum, conduction and valence band-edges (CB and VB, respectively) of the semiconductors with respect to the Fermi energy were estimated through a conventional approach.

Sn2+ + 2Cl− + 2Ag+ + 2NO3− = Sn2+ + 2NO3− + 2AgCl↓ Al3+ + 3Cl− + 3Ag+ + 3NO3− = Al3+ + 3NO3− + 3AgCl↓ AgCl + 2NH4OH = [Ag(NH3)2]+ + Cl− + 2H2O

2.5. Device characterization

The diamine:silver complex formed in presence of excess NH4OH underwent decomposition releasing Ag+ ions in the cationic precursor solution and thus the turbidity disappeared allowing formation of SILAR films of Ag2S with heterovalent cationic-doping.

The sandwiched structures were characterized under a dark and 1 sun illumination conditions. Current-voltage (I–V) characteristics of the devices were recorded with Keithley 2636 Electrometer using LabTracer software. For the measurements, devices were retained in the glovebox after thermal evaporation of the top-electrode in order to avoid oxidation of aluminum. 3-axes micro-positioners having pressureloaded spring-based probe-contacts were used to connect to the electrometer. A 300 W solar simulator (Newport-Stratfort model 76500) attached with an AM1.5 filter placed below the glovebox acted as a source for illumination. Intensity of the simulated solar light on the

2.2. Fabrication of CuS thin-films by SILAR technique CuS thin-films were formed by using highly pure copper(II) chloride dihydrate (CuCl2·2H2O), ammonium hydroxide (NH4OH), and sodium sulfide hydrate (Na2S·xH2O) as copper precursor, complexing agent, and anionic precursor, respectively. The cationic precursor was 340


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Fig. 1. (a) Optical absorption spectrum, (b) absorbance at 400 nm versus number of layers, (c) Tauc plots to determine optical gap (as shown by dotted lines), and (d) XRD patterns of Ag2S, Sn@Ag2S, and Al@Ag2S thin-films. The vertical sticks in the top-abscissa of (d) represent the respective positions of the planes according to JCPDS card no. #14-0072.

devices was 100 mW/cm2. While recording I–V characteristics under the illumination, area outside the cell was covered to avoid any contribution from neighboring regions or cells.

obtain optical gap of the semiconductors. From the slope of the plots, optical gap of the materials could be estimated to be around 1.05 eV, which was independent of film-thickness. The value is in consistent to those obtained in films formed through other deposition methods [12,35]. Furthermore, all the three materials showed an identical gap and threshold for photon absorption ensuring an invariance of optical properties under elemental doping. Since presence of secondary phases in these compounds cannot be completely ruled out from optical spectroscopy, we recorded XRD patterns of the undoped and doped Ag2S thin-films. The patterns, as presented in Fig. 1d, show well-defined crystallographic planes, which matched perfectly with the JCPDS card no. #14-0072. Absence of any additional peaks even in doped-materials indicated phase-purity of the compounds and a possible substitutional nature of doping. For further confirmation on the nature of doping, we have carried out EDX measurements of the thin-films. Since EDX is a localized mode of measurement, we recorded EDX spectrum on several locations of each thinfilm. Typical EDX spectra of the doped and undoped semiconductors along with measured atomic-percentage of elements are presented in Fig. S1 and Table S1 in the Supplementary section. The results evidenced the presence 4.4–4.6 at% of dopants in the compound as a replacement of silver when 5 at% was aimed during the SILAR film-formation.

3. Results and discussion 3.1. Characterization of Ag2S thin-films To ensure formation of phase-pure Ag2S thin-films and also to understand the nature of doping in the doped-semiconductors, the pristine and Sn2+- or Al3+-doped thin-films were characterized through usual manner. To begin with, we have presented optical absorption spectra of the materials in their thin-film form (Fig. 1a). The spectra showed a strong absorption over a broad spectral region extending to the NIR region. Identical nature of the spectra and comparable absorbance of films having an equal thickness evidenced that optical properties of Ag2S films were unaffected due to the cationic-doping. Since formation of doped-Ag2S thin-films by SILAR technique is of prime importance, we have monitored growth of the films with the number of SILAR cycles and compared with the growth-process of undoped-Ag2S thin-films (Fig. 1b). As can be seen in the figure, absorbance grew linearly with film thickness in all three cases demonstrating success of the film-formation process even in presence of heterovalent dopant-ions. In Fig. 1c, we show plots of (αhυ)2 versus energy (Tauc plot) for 6-layer films to 341


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Fig. 2. (a)–(f) Typical DOS spectrum and histogram of CB and VB energies for pristine Ag2S, Sn@Ag2S, and Al@Ag2S. Each of the histograms was fitted to Gaussian distribution. (g) Schematic diagram showing band-edges of the pristine and doped-semiconductors. The dashed-line represents the semiconductors’ Fermi energy, which was aligned to 0 V.

tunnel conductance (dI/dV) spectra and thereby DOS versus energy characteristics, we located CB and VB-edges of the semiconductors with respect to the Fermi energy. Here we aimed to study any shift of Fermi energy upon heterovalent doping which was expected to introduce free carriers (electrons in the present cases) in the binary semiconductor.

3.2. Characterization of the semiconductors through scanning tunneling spectroscopy To obtain a further evidence of doping, we have characterized ultrathin-films of the pristine and doped-Ag2S in a STM. From differential 342


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each doping-content, we have summed up the results in Fig. 3, which contains band-diagrams of Sn- and Al-doped Ag2S semiconductors at different doping-concentrations. In both the cases, the concentration was varied up to 7%. The respective dI/dV plots and associated histograms of band-edges have been placed in Figs. S2 and S3 in the Supplementary section. Due to a substitutional nature of doping in both the cases that wouldn’t as such affect the nature of conduction process in these semiconductors [37], band-gap of the pristine semiconductor has also been found to be unaffected under doping. A shift of Fermi energy towards the CB-edge was prominent in all the cases. As expected, with an increase in doping-concentration, the Fermi energy shifted closer to the CB-edge ensuring more n-type conductivity in the semiconductor. Due to a higher valency of Al3+ as compared to Sn2+, the degree of shift was more in Al-doped Ag2S than in bivalent-doped semiconductors at the same dopant-content. From the histograms, we can measure transport gaps of the updoped and the doped-semiconductors. As stated, the gap did not change upon the low-level of doping. It may be stated that the optical gap as estimated from Tauc plots was around 1.05 eV and did not evidence any shift. The transport gap being 1.07–1.10 eV was a little higher than the optical gap due to involvement of exciton binding energy in the latter mode of measurement. We may add here that the precision of measurement in STS has a higher degree of accuracy to determine the transport gap as compared to that of the optical gap measured through optical spectroscopy.

Band-edges and more importantly a shift in Fermi energy in fact would determine the energy-diagram of a heterojunction solar cell that we would envisage in this work. A typical dI/dV spectrum of Ag2S ultrathin-films is shown in Fig. 2a. Since bias was applied to the tip, a positive voltage implied withdrawal of electrons from the VB of the semiconductor; the first peak in the positive voltage of the spectrum hence implied location of its VB-edge. Similarly, the first peak at a negative voltage appearing due to injection of electrons inferred the CB-edge of the semiconductor. Here, the bandedge energies were located with respect to the Fermi energy, which was aligned at 0 V. Since STS is a localized mode of measurement, it is necessary to record tunneling current at many different points on a film. From each measurement and thereby DOS spectrum, we located VBand CB-edges to finally plot histogram of their energies (Fig. 2b). The histogram shows that the Fermi energy was closer to the CB-edge and hence a mild n-type nature of the pristine semiconductor. The n-type behavior in chalcogenides generally appears due to sulfur vacancies in the compounds. We continued with the STS studies in Sn- and Al-doped Ag2S to infer on the shift of Fermi energy upon the heterovalent doping. In Fig. 2c and d, we have presented a typical DOS spectrum and a histogram of CB and VB energies for Sn@Ag2S. For Al-doped Ag2S, such plots have similarly been presented in Fig. 2e and f. While the content of the dopants in the semiconductor was varied (and will be discussed in the following section), the plots in Fig. 2c–f represent 5 at% concentration for the two dopant-ions. DOS spectrum and histogram plot of energies for Sn@Ag2S and Al@Ag2S semiconductors were compared with those of pristine Ag2S; such a comparison has been summed up as a plot of band-edges of the pristine and doped-semiconductors (Fig. 2g). The plot clearly shows that the Fermi energy shifted towards the CB-edge upon heterovalent doping sans any change in the band gap. The shift was more in case of trivalent doping as compared to the bivalent one. Since each Sn- and Aldopant introduced one and two additional free-electrons in the system, respectively, such a trend in the shift of Fermi energy is well-accounted. DOS and thereof the histogram of band-edges provide more information than the VB and CB energies. The histograms can be seen to be distributed over the energy scale due to defect or disordered states. The energy distributions were as such symmetric and Gaussian in nature. The full width at half maximum (FWHM) of the distribution that can be considered to be a measure of defect or disordered states in a semiconductor can be seen to increase with introduction of dopants. Interestingly, the FWHM of only the CB-edge could be seen to be affected due to introduction of dopants, which were cationic in nature, leaving FWHM of the VB-edge unaffected (Table 1). Reports on band structure calculations of Ag2S established that top of the VB had primarily S 3p character, whereas bottom of the CB had Ag s and S s-p mixed character [36]. Dopants in the metal site hence would affect distribution of only the CB-edge. The results presented in the histograms and Table 1 are hence experimental evidence of broadening in CB energy distribution in cationic-doped Ag2S with Al-dopants yielding wider broadening due to a large mismatch between ionic radii of silver and aluminum. Since density of additional free-carriers introduced in a system is expected to depend on the concentration of heterovalent-dopants, we varied the concentration to determine the degree of shift in Fermi energy. While the measurements comprised of STS at many different points followed by determination of band-edges and their histograms at

3.3. Band-diagram from scanning tunneling spectroscopy With band-energies of pristine and doped-Ag2S, we proceeded to form band-diagram of CuS/Ag2S pn-heterojunctions. As such, the sulfides of copper (CuxS) are well studied p-type semiconductors where the p-type conduction originates from free holes of acceptor levels of copper vacancies. The optical gap of CuxS in its thin-film form depends on the stoichiometry and increases with a decrease in the copper content in the film. For example, the copper-rich phase Cu2S (x = 2) or chalcocite has a direct band gap ~ 1.2 eV which gets blue-shifted to ~ 2.0 eV for the maximum copper deficient phase CuS (x = 1) or covelite. Such a shift in the band gap energy is also accompanied by an increment in electrical conductivity [38]. The selection of CuS as a p-type layer in our devices hence would not affect the absorption process in the subsequent Ag2S layer markedly but of course would take part in exciton generation process. We have characterized the SILAR-grown CuxS thin-films through usual experimental methods. The results as presented in Fig. S4 in the Supplementary section matched well with reported results of the covelite phase and infer absence of other copper-rich phases including Cu2S. We then proceeded to form band-diagram of pn-junction solar cell structures that contained hole- and electron-transport materials as well. dI/dV plots and associated histograms of band-edges of p-type CuS and Cu@NiO are shown in Fig. S5 in the Supplementary section. The figure also contained dI/dV plots along with histogram of HOMO and LUMO levels of PCBM. Here, HOMO and LUMO signify highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. Band-edges of the materials with respect to their Fermi energy inferred p-type nature of the compound semiconductors and electron-transport ability of the organics. When we drew band-diagram of Cu@NiO/CuS/ Ag2S/PCBM in “after-contact” configuration (Fig. 4), we found that the CuS/Ag2S heterojunction would form a type-II band-alignment at the interface. Energies of the hole- and electron-transport layers could be seen to aid carrier transport to the respective electrodes. The figure moreover infers that the desired type-II band-alignment at the pnjunction continued to stand upon doping in Ag2S. The energy level diagrams hence infer that the Cu@NiO/CuS/Ag2S/PCBM heterojunctions may act as solar cells.

Table 1 FWHM of VB and CB energy-histograms. Material

FWHM of CB (eV)

FWHM of VB (eV)

Ag2S Sn@Ag2S Al@Ag2S

0.22 0.25 0.27

0.27 0.26 0.26

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Fig. 3. Schematic diagram showing band-edges of (a) Sn-doped and (b) Al-doped Ag2S having a dopant content of 0–7 at% with respect to silver. The dashed-lines represent the semiconductors’ Fermi energy, which was aligned to 0 V.

such a mismatch, Al3+ ions, when introduced at the silver site, yielded a large number of recombination pathways (trap states) affecting the device performance in an adverse manner. The results hence infer that ionic radius needs to be matched while introducing heterovalent dopants having an intention to introduce free carriers and thereby shift in Fermi energy, so that recombination pathways in solar cells remain to a minimum. I–V plots of dopant-content-optimized heterojunction solar cells have been summed in Fig. 7, in which characteristics of CuS/Ag2S device (without doping) have also been added. The figure clearly brings out the importance of doping in semiconductors while forming and characterizing solar cells. Parameters of the solar cells have been enlisted in Table 4 for comparison. It may be stated that Ag2S thin-films are known for poor intergranular conduction through their small-sized grains [33]. In our SILAR grown Ag2S thin-films, size of the grains as calculated from the Williamson-Hall plot appeared also to be small (D ~ 20 nm). The efficiency of the solar cells hence did not exceed 3%.

3.4. Solar cell characteristics With the ensured type-II alignment at the interface between CuS and all the Ag2S layers, we characterized the pn-heterojunction solar cells. With an efficiency-optimized thickness of CuS (20 nm), we first varied the layer number of Ag2S. I-V characteristics of the devices under dark and 1 sun illumination conditions are shown in Fig. 5. In the dark characteristics, current could be seen to decrease with increasing film thickness. The characteristics were rectifying in nature implying that the device might exhibit photovoltaic properties upon illumination. Under light, the devices indeed acted as solar cells. Open-circuit voltage (VOC), short-circuit current (JSC), fill-factor (FF), and power conversion efficiency (η) of the devices have been enlisted in Table 2. The parameters could be seen to have optimized at 3 layers of Ag2S, which is equivalent to 70 nm, yielding an η of 1.98%. As such, in pn-junction solar cells, thickness of each component should be the sum of the width of the depletion region extending to that semiconductor and diffusion length of the minority carriers. This will ensure that the minority carriers generated anywhere in the device out of photoexcitation can migrate through the depletion region and finally contribute to photocurrent. Due to its high absorption coefficient, the 70 nm Ag2S layer, in conjunction with the CuS layer, absorbed sufficient amount of incident light. Thicker p or n layers generally increases series resistance and also facilitate carrier recombination, both of which are detrimental to solar cell activities. We then proceeded to study the effect of dopant-concentration in Ag2S on the performance of thickness-optimized devices. Since introduction of dopants does not alter optical absorption spectrum, the devices having identical thickness absorbed the same solar illumination. Here we considered both the dopants, which are known to shift the Fermi energy of the n-type semiconductor and hence the band-diagram of pn-junction solar cells. While the I–Vs under light are shown in Fig. 6, the parameters have been enlisted in Table 3. The results infer that the solar cell parameters expectedly depended on the doping content. With increasing concentration of the dopant and also their valency, the VOC of the devices could be seen to decrease. Since introduction of dopants in the Ag2S structure pushes the Fermi energy towards the CB-edge resulting in more n-type conductivity, the offset between VB of CuS and CB of Ag2S decreases which in turn would reduce the VOC of the devices. The additional carriers introduced in the system, on the other hand, increase conductivity and hence yield a higher JSC; the efficiency hence optimized to 2.85% at an intermediate doping-content. A comparison between results with Sn- and Al-doped Ag2S reveals that in spite of higher valency of Al3+ than Sn2+ and correspondingly a better conductivity of Al@Ag2S, the JSC and FF were lower in all such devices. This is due to the fact that Al3+ and Ag+ have a large mismatch in ionic radius that in turn introduces lattice strain in the Ag2S lattice. Due to

3.5. Junction properties In addition to the solar cell parameters, such as VOC, ISC, fill-factor, and η, we have analyzed the I-V characteristics further to get an insight on the performance governing factors namely, series resistance (RSeries), shunt resistance (RShunt), and ideality factor (n) of the devices. Typically, in a junction solar cell, a low RSeries is desired to improve charge-extraction possibilities; a high RShunt similarly implies a lower recombination loss. Ideality factor, on the other hand, represents the

Fig. 4. Schematic band-diagram of CuS, Ag2S and doped-Ag2S thin-films along with bandedges of Cu@NiO and energy levels of PCBM. The dashed line represents each semiconductor's Fermi energy, which was aligned to 0 V.

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Fig. 5. Current-voltage characteristics of CuS/Ag2S heterojunction solar cells under (a) dark and (b) 1 sun illumination conditions with different number of Ag2S layers as stated in the legends.

I = ISC − I0exp ⎡ ⎢ ⎣

Table 2 Photovoltaic parameters of CuS/Ag2S heterojunction solar cells with different number of Ag2S layers. Number of Ag2S layers

JSC (mA/cm2)

VOC (V)

FF (%)

η (%)

1 2 3 4

3.9 4.8 6.4 6.0

0.63 0.66 0.69 0.67

38 43 45 45

0.94 1.36 1.98 1.81

q (V + IRSeries ) ⎤ ⎛ V + IRSeries ⎞ − ⎥ nkB T ⎦ ⎝ RShunt ⎠ ⎜

⎟

(1)

where, I0 the dark saturation current density. Forward bias I–V characteristics can hence be expressed in terms of RSeries as:

I = ISC − I0exp ⎡ ⎢ ⎣

q (V + IRSeries ) ⎤ ⎥ nkB T ⎦

(2)

Logarithm on the both sides followed by first derivative of the expression for V would result RSeries as:

− junction quality and hence the carrier recombination mechanism of a solar cell. When the diode current of a pn-junction is dominated by carrier diffusion in the neutral region of the semiconductors, the value of n approaches unity; on the other hand, in the event of carrier recombination in the depleted region dominating the diode current, the ideality factor approaches 2. In most devices, both the phenomena take place simultaneously yielding a value of n within 1 and 2. In a junction diode, the current flowing through an external load could be expressed as:

dV nk T = B + IRSeries d (lnI ) q

(3)

dV d (lnI )

Thus the plot of versus I can provide the values of RSeries as the slope; the ideality factor, on the other hand, would appear as the intercept with the ordinate (Fig. 8a). RShunt of the devices could be evaluated from first derivative of Eq. (1) as:

−

qV ⎞ I q 1 dI = + 0 exp ⎛ nk dV RShunt nkB T ⎝ BT ⎠ ⎜

⎟

(4)

Here, the contribution of RSeries has been neglected. From the plot of dI/dV versus V, RShunt could be derived from the reciprocal of the

Fig. 6. I–V characteristics under 1 sun of (a) CuS/Sn@Ag2S and (b) CuS/Al@Ag2S heterojunction solar cells having different concentration of dopants in Ag2S as stated in the legends.

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Table 3 Photovoltaic parameters of CuS/Ag2S heterojunction solar cells with different dopingconcentration in Ag2S. Dopant content

0% 3% 5% 7%

CuS/Sn@Ag2S heterojunction

Table 4 Photovoltaic parameters of CuS/Ag2S heterojunction solar cells with different dopants in Ag2S layer.

CuS/Al@Ag2S heterojunction

JSC (mA/ cm2)

VOC (V)

FF (%)

η (%)

JSC (mA/ cm2)

VOC (V)

FF (%)

η (%)

6.4 7.2 8.3 8.5

0.69 0.68 0.66 0.63

45 47 52 51

1.98 2.30 2.85 2.73

6.4 6.1 5.6 4.8

0.69 0.67 0.65 0.60

45 42 40 35

1.98 1.72 1.46 1.01

Ag2S layer

JSC (mA/cm2)

VOC (V)

FF (%)

η (%)

Undoped 5 at% Sn2+ 5 at% Al3+

6.4 8.3 5.6

0.69 0.66 0.65

45 52 40

1.98 2.85 1.46

Table 5 Diode parameters of CuS/Ag2S heterojunction devices having different Ag2S layers. Ag2S film

RSeries (Ω)

Ideality factor (n)

RShunt (kΩ)

Undoped Sn@Ag2S Al@Ag2S

140.3 126.6 194.1

3.5 2.6 6.1

6.3 9.1 4.7

higher than 2 in all the devices leading to the conclusion that recombination of trapped charges was prevalent in those devices. 4. Conclusions To summarize, we have introduced Sn2+ and Al3+ as heterovalent dopants in Ag2S so that Fermi energy of the semiconductor can be tuned that in turn have altered the band-diagram of pn-junctions. The junctions were formed between p-type CuS and doped-Ag2S (n-type) through a solution-approach. Band-edges of the semiconductors were determined from dI/dV spectrum which has correspondence to their DOS. STS studies of Ag2S inferred a shift in Fermi energy towards the CB-edge upon heterovalent cation-substitution and thereby evidenced n-type nature of the doped-semiconductors. Due to the shift, the dopants played a large role in the type-II band-alignment at the interface between the two semiconductors. From solar cell characteristics of the pn-junctions, we could correlate photovoltaic parameters with the band-diagram. We have observed that the junction between CuS and Sn2+-doped Ag2S resulted better solar cell parameters with an efficiency of 2.85% as compared to that based on Al3+-doped Ag2S, which contained a large number of defect states due to mismatch in ionic radii of the dopant-ions and the cation. The variation in solar cells parameters with dopant content/concentration were found to be in agreement with series- and shunt-resistances of the devices.

Fig. 7. Current-voltage characteristics of pn-junction solar cells under 1 sun illumination condition. While CuS was the p-layer, pristine and doped Ag2S acted as the n-type layer.

intercept at ordinate (Fig. 8b). The values of RSeries, RShunt, and n, as derived from the two plots, for the CuS/Ag2S heterojunction solar cells having different dopants in the Ag2S layer have been collated in Table 5. It can be observed that RSeries was the least and RShunt was the highest in the CuS/Sn@Ag2S heterojunction that actually yielded an optimum efficiency. RSeries of the heterojunctions appeared to be on the higher side and could hence be the performance limiting factor in these solar cells. Ideality factor was

Fig. 8. Plots of (a)

dV d (lnI )

versus I and (b) dI/dV versus V of pn-junction solar cells under 1 sun. While CuS was the p-layer, pristine and doped Ag2S acted as the n-type layer.

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Acknowledgements [17]

A.J.P. acknowledges JC Bose Fellowship (SB/S2/JCB-001/2016) of SERB. G.P. and S.C. acknowledge CSIR Junior Research Fellowship Number 09/080(1042)/2017-EMR-I (Roll no. 523509) and DST INSPIRE Fellowship [IF 140158], respectively. The authors acknowledge financial assistance from SERIIUS project bearing Grant no. IUSSTF/JCERDC-SERIIUS/2012.

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Appendix A. Supplementary material [21]

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2018.03.022.

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