Ag heterojunction solar cell

Materials Research Bulletin 109 (2019) 1–9

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Photovoltaic properties of F:SnO2/CdS/CuO/Ag heterojunction solar cell 1

Sukdev Dolai, Rajkumar Dey, Shamima Hussain , Radhaballav Bhar, Arun Kumar Pal

⁎

T

Department of Instrumentation Science, Jadavpur University, Kolkata, 700032, India

A R T I C LE I N FO

A B S T R A C T

Keywords: CuO PV Heterojunction

Glass/F:SnO2/CdS/CuO/Ag hetero structure has been fabricated using direct current (d.c) magnetron sputtering and thermal evaporation technique. Cupric oxide (CuO) has been synthesized by direct current magnetron sputtering technique and Cadmium sulphide (CdS) has been deposited by thermal evaporation method. The micro-structural and optical studies were carried out by field emission scanning electron microscope, transmission electron microscope, atomic force microscope and UV–vis spectroscopy. The above heterojunction structure shows the efficiency ∼2.1%. Open-circuit voltage of the cells was ∼0.462 V. The efficiency values obtained here are the highest compared to those reported so far for this glass/F:SnO2/CdS/CuO/Ag structure. Sputtering acted as a viable and inexpensive technique for obtaining this glass/F:SnO2/CdS/CuO/Ag cell structure for PV application.

1. Introduction In the past few decades, modern technology demands the low-cost renewable sources of energy. Photovoltaic technology is the most promising field in which power generation can occurs from the natural sources (light sources). On the other hand, oxide semiconductor materials are getting wide attention for photovoltaic field. Among them cupric oxide are the most usable candidate for photovoltaic studies due to non-toxicity, inexpensive, earth-abundant which gives the more importance for large scale photovoltaic application. When copper is oxidized, it makes two most stable oxides (CuO and Cu2O). The band gap of CuO (1.2 eV) is less than that of Cu2O (2.1 eV) which is widely used as solar energy conversion, gas sensor and various types of energy storage devices. Performance status of thin film heterojunction solar cells based on copper oxide (Cu2O, CuO and Cu4O3) has been presented by Wong et al [1]. These non-toxic and sustainable photovoltaic materials were obtained by adopting different deposition techniques like: oxidation of copper, sputtering and electrochemical deposition. Guo et al. [2] adopted a facile one-step solution route to deposit flower-like CuO film on Cu foil substrate. Shabu et al. [3] studied spray deposited CuO absorber layers and indicated that the nanostructured CuO films prepared by them will be helpful for future developments of substrates for solar cells with CuO as absorber layers. However, defects at the copper oxide heterojunction and film quality are the major constraints for achieving higher performance in these cells [4]. Sustainability [5,6] in cells based

on CuO also depended critically on the inherent defects and the associated film quality. In this regard, report presented by Figueiredo et al. on the modulation of properties of CuO films by post deposition annealing [7] is note worthy. The films were obtained by annealing evaporated copper films on glass substrates. They observed that the electrical conductivity changed from p-type when annealed between 200 and 300 °C when the films contained Cu2O as dominant phase. The electrical conductivity changed to n-type when the annealing temperature id increased beyond 300 °C. Both elemental components (copper and oxygen) are plentiful in nature and would not cause any long term concern [8] for large scale PV power generation [9]. Additionally, the above copper oxides are non-toxic and can be deposited in thin film form by using simple low cost techniques. Monoclinic cupric oxide (CuO) and cubic cuprous oxide (Cu2O) [10] are the two stable copper oxide phases. These two semiconductor oxides show p-type conductivity which is suitable for photovoltaic applications. In the past, p-type copper oxide (CuO) was coupled with ntype ZnO or n-type Si to demonstrate the applicability of CuO as photovoltaic absorber material [10–15]. Reported highest efficiency obtained so far varied between 0.02–1.21 % even with the use of sufficiently high quality CuO films (Table 1) [11–17]. There are some reports on the formation of CuO/Cu2O heterojunction. As early as 1980 Herion et al. [18] reported the investigation on the CuO/Cu2O heterojuction. After the above report, Wijesundera et al. [19] prepared photoactive p-CuO/n-Cu2O heterojunction using electrodepostion technique. Oku et al. [20] fabricated Cu2O/C60 and CuO/

⁎

Corresponding author. E-mail address: [email protected] (A. Kumar Pal). 1 UGC-DAE CSR, Kalpakkam Node, Kokilamedu-603104, India. https://doi.org/10.1016/j.materresbull.2018.09.022 Received 28 July 2018; Received in revised form 13 September 2018; Accepted 13 September 2018 Available online 14 September 2018 0025-5408/ © 2018 Elsevier Ltd. All rights reserved.

Materials Research Bulletin 109 (2019) 1–9

S. Dolai et al.

Table 1 Review of CuO based solar cell. Device

Deposition Technique

VOC (V)

JSC(mA/cm2)

Fill Factor (FF)

Efficiency (η) (%)

Year

References

p-CuO/n-Si

Reactive magnetron sputtering Reactive magnetron Sputtering and Rapid thermal annealing RF-sputtering

0.33

6.27

0.2

0.41

2012

Gao et al. [8]

0.494

6.4

0.32

1.0

2015

S. Masudy-Panah et al. [9]

0.421

4.5

0.263

1.21

2014

CuO/ n-type tin-doped zinc oxide (TZO) CuO/ZnO FTO/ZnO/CuO/In

DC Sputtering

0.48

326 mA

0.63

0.232

2013

S. Masudy-Panah et al. [10] Omayio et al. [11]

Galvanostatic method Spin Coating

2.8 × 10−4 0.19

1.6 0.34

0.25 0.23

1.1 × 10−4 0.02

2012 2017

Ti/p-CuO/n-Cu2O/Au

Electro-deposition

0.22

6.8

–

0.64

2015

F:SnO2/CdS/CuO/Ag

Sputtering

0.462

13.4

0.59

2.1

CuO/n-Si Annealing (300 °C for 1 min(HP + N)) p-CuO/n-Si

C60 by electrospinning and spin coating method on fluorine doped and indium doped SnO2 substrate. Recently, Septina et al. [21] fabricated CuO/CdS heterojunction as a thin film photocathode for solar hydrogen production. We have recently demonstrated the possibility of synthesizing of high quality p-type Cu2O and CuO films by reactive d. c. sputtering technique [22,23]. Films deposited by us indicated one order of magnitude higher carrier concentration than those indicated by other workers. This report renewed the hope for utilizing this important material for photovoltaic application. At this juncture, it will be pertinent to cite the work on the fabrication of all-oxide TiO2/Cu2O heterojunction solar cells produced by spray pyrolysis technique by Pavan et al. [24]. The active cell area was ∼0.026 cm2 and the cells indicated photocurrent density ∼0.4 mA/cm2. They indicated that improvement on the cell performance could be obtained by increasing the grain size of Cu2O and reduction of band offset at the TiO2/Cu2O interface. In this communication, successfully fabrication of glass/F:SnO2/ CdS/CuO/Ag heterojunction solar cell structure utilizing sputtering on glass substrates is demonstrated. Among 25 cell structures fabricate here, we present the work based on fifteen beta cells having nearly identical I–V characteristics. For simplicity of presentation, data analysis for a representative cell is presented here. Performance of the solar cells thus obtained were studied by measuring I–V (dark and when illuminated), C–V and spectral response measurements. VOC decay characteristics were also documented for these cells. Values of cell parameters obtained here are the highest reported so far for a p-CuO/nZnO or p-CuO/n-Si p-n junctions operated in air (Table 1).

Kidowasky et al. [12] Hussain et al. [13] Jayathilaka et al. [14] This study

and oxygen (O2) (40%). A D.C source (∼0.6 kV; 1.2 mA/cm2) was used to deposit the CuO films. Substrate to target distance was ∼5 cm. Films were deposited at a substrate temperature (Ts) ∼ 623 K. It may be noted that the CuO films deposited here are very stable in air. We studied the change in sheet resistance of the CuO films over 10 days keeping the samples at room temperature. CuO films showed very good stability when exposed under ambient condition. 2.3. Device fabrication To fabricate p-n heterojunctions, we have taken three steps in which first two steps are already mentioned the section for deposition of CuO and CdS layer. Third step are mainly fabrication process. In this step, an F: SnO2 coated glass was used as back side contact and CdS films were deposited on F: SnO2 coated glass. After that, CuO layer was grown on CdS layer by sputtering technique. Finally, silver top contact was obtained by evaporating silver (99.99%) at a system pressure of 10 −6 Torr. Silver top contact was made using appropriate stainless steel mask containing circular holes (∼0.2 cm2) to define active cell area. Resulting glass/F:SnO2/CdS/CuO/Ag cell structure is shown in Fig. 3(a). 2.4. Characterizations of CdS, CuO thin films and their heterojunction structure

CdS thin films were deposited on flourine-doped SnO2 (F-SnO2) coated glass substrates using thermal evaporation method. Before deposition, F: SnO2 coated substrate was cleaned in acetone, isopropanol and DI water for 10 min followed by ultra sonication. CdS powder (Sigma-Aldridge, 99.99%) was evaporated on the glass and F:SnO2 substrate. During deposition, vacuum pressure was maintained ∼2 × 10−5 mbar and substrate temperature was kept∼423 K. Splattering of CdS powder was eliminated by using a quartz wool sheet on the top of the alumina crucible.

Structural and optical properties of the films were characterized by different measurements. A Carl Zeiss AURIGA field emission scanning electron microscopy (FESEM) is used to study microstructural morphology of the CdS and CuO films. Secondary emission mode with operating voltage of 10 kV in was utilized to record the image. X-ray diffraction (XRD) has performed to studies the micro-structural information using a Rigaku MiniFlex XRD (0.154 nm Cu Ka line). UV-VIS spectra were carried out by using a spectrophotometer (Hitachi-U3410) in the wavelength region 300–3300 nm. The resolution of spectrophotometer is λ ∼0.07 nm. The photometric accuracy was ± 0.3% for the above measurements. AFM pictures were recorded by using a Nanosurf Easy Scan2 in contact mode. J–V characteristics of p-n heterojunction were recorded using Keithley 2420 source-meter. Photocurrent response was recorded with the help of a spectrophotometer using conventional Xe-lamp illumination

2.2. CuO thin films deposition

3. Results and discussions

CuO thin films were deposited by D.C reactive magnetron sputtering technique details of which are given in reference [23]. In short, CuO thin films were synthesized using a metallic Cu (99.99%) target (3″ diameter). Deposition has been done in presence of Argon (Ar) (60%)

3.1. CdS window layer

2. Experimental details 2.1. CdS thin film depositon

Fig. 1a shows the FESEM micrograph of the CdS film deposited as above. Inset of Fig. 1a shows the EDAX spectrum which indicated Cd:S 2

Materials Research Bulletin 109 (2019) 1–9

S. Dolai et al.

Fig. 1. CdS layer : (a) SEM micrograph (inset shows EDAX spectra), (b) AFM picture of the same films and (c) Transmission spectra (inset shows the plot of (αhν)2 versus hν).

representative CuO film deposited here are shown in Fig. 2d and inset of Fig. 2d, respectively. The SAED pattern indicated diffraction rings for reflections from planes (-111), and (111) which are well matched with the XRD trace (inset of Fig. 2d). Appearance of similar diffraction planes in the SAED pattern was reported by Wang et al. [27]. They also observed an additional diffraction plane (−202) along with the above. The lattice image showed the fringes with spacing (d) = 0.244 nm (Fig. 2d) corresponding to the plane (−111) of monoclinic CuO phase. Transmission spectra are shown in Fig. 2e. Band gap of the CuO film was obtained using Eq. (1). (αhν) 2 versus hν (inset of Fig. 2e) plots which indicated the band gap of CuO layer used here to be around 1.54 eV [23].

ratio to be ∼1.4:1.0. The above sulphur deficiency imparted strong ntype conductivity in the films. Average surface roughness was determined from the AFM picture (Fig. 1b) and was ∼25 nm. This value is quite higher than that reported by others [25]. This increased surface roughness would facilitate light capturing capability of this cell structure. Transmission spectra of this CdS film are shown in Fig. 1c. Band gap of the CdS film was obtained using Tauc’s plot αhν =A (hν-Eg)

n

(1)

where, α is the absorbance, ‘A’ is a constant and for the direct band gap ‘n’ value is 1/2. Band gap of CdS layer was calculated from (αhν) 2 vs hν graph. Inset of Fig. 1c shows the optical band gap was around 2.45 eV which tallies well with the reported value of CdS band gap [26].

3.3. Cell characteristics

3.2. CuO absorber layer

3.3.1. Cell characteristics It may be mentioned that unlike Cu2O films, isolated CuO films deposited on glass substrates were quite stable to surface oxidation with occasional exposure to ambient air during cell fabrication. Additionally, when n-type ZnO is deposited by reactive sputtering technique on the above p-type CuO layer, properties of CuO get modulated by the presence of O+ ions and electrons in the plasma. It was evidenced when we tried to fabricate a simple Cu/CuO/ZnO/In cell structure which resulted in no success in fabricating a p-n junction. But cells with Cu/CuO/CdS/ In did show photovoltaic properties. Very poor cell performances were also observed for CuO/ZnO cells deposited by other deposition routes [11,12]. Thus, CuO layer would act as buffer layer for this cell

FESEM micrograph and AFM picture of the CuO layer used here are shown in Fig. 2a and Fig. 2b, respectively. The FESEM pictures indicate that the films are polycrystalline in nature. Corresponding histogram is shown in the inset of Fig. 2a. Average grain sizes in these films were ∼115 nm. It may be observed that the grains are nearly of the same size and films are more compact and faceted. AFM picture indicated a surface roughness ∼36 nm. This would facilitate light capturing capability of the cell structure. TEM micrograph (Fig. 2c) indicated a narrow grain distribution with average grain size ∼110 nm. Selected area electron diffraction (SAED) pattern and lattice image (HRTEM) of a 3

Materials Research Bulletin 109 (2019) 1–9

S. Dolai et al.

Fig. 2. CuO layer :(a) SEM micrograph and inset for histogram (b) AFM picture of the of the same films, (c) TEM micrograph, (d) HRTEM showing lattice image (inset showing selected area electron diffraction (SAED) pattern and XRD pattern of a representative CuO film), (e) Transmission spectra (inset shows the plot of (αhν)2 versus hν).

Electron-hole pairs created in the CuO absorber layer were separated at the CdS/CuO interface. Electrons are moved to the Ag electrode and holes are transported to the F:SnO2 electrode.

structure. A schematic of the PV cells structure used here is shown in Fig. 3a. The above CuO layers were deposited by reactive d.c. sputtering as discussed in details in the previous section. Tentative energy level diagram of the present structure (glass/F:SnO2/CdS/CuO/Ag) under this study is depicted in Fig. 3b. Previously reported and currently measured values of work function, electron affinities and band gaps were utilized to draw this energy diagram [28,29]. Radiation was incident from the F:SnO2 side and was absorbed in the CuO layer.

3.3.2. J–V Characteristics The J–V traces of a representative cell recorded here are shown in Fig. 4a. Values of series resistances (Rs) and shunt resistances (Rsh) were extracted from the dark I–V curve. Open circuit voltage (Voc) and short circuit current density (Jsc) were estimated from the illuminated I–V 4

Materials Research Bulletin 109 (2019) 1–9

S. Dolai et al.

dV RS = ⎛ ⎞ ⎝ dI ⎠V = VOC

(4)

and

dV RSh = ⎛ ⎞ ⎝ dI ⎠ I = ISC

(5)

Thus, using Eq. (4), Rsh could be evaluated from the dark characteristics in reverse direction for lower voltage values.Rs and Rsh obtained as above for the cell is shown in Table 2. The values of Rs are about ∼24.6 Ω-cm2 and Rsh ∼ 200.34 Ω-cm2. In this work, lower shunt resistance values in these cells might have caused the observed lower power conversion efficiency. Low shunt resistance is indicative of the presence of surface states and defects in the materials. Form Eq. (3), reverse saturation current (I0) can be estimated from the intercept of lnI–V graph at V = 0 (Fig. 4b) and apparent barrier height can be calculated using the equation

φb =

data in the usual manner. Different photovoltaic parameters like VOC, ISC, efficiency (η), RS, RSh etc. of the representative solar cells (with cell area ∼0.2 cm2) fabricated here are shown in tabular form in Table 2. Solar cell efficiency (η) was ∼2.1% while ISC was ∼6.65 mA, VOC ∼462 mV. The cells has poor fill factor ∼0.41 as we were fabricated with non-optimized Ag, CuO, CdS and F:SnO2 layers. It may be inferred that cells when fabricated with optimized layer thicknesses would yield more efficient solar cells. Recombination processes and corresponding losses in this cell may be understood by analyzing the I–V characteristics shown in Fig. 3b. For this one may opt for the basic diode model for a solar cell and a standard equivalent circuit diagram [30,31]. I–V characteristics could be written as:

⎜

n=

q ⎡ dV ⎤ ⎢ d (ln I ) ⎦ ⎥ kB T ⎣

(7)

where q, kB and T are the charge of electron, Boltzmann constant and temperature respectively. The slope of the linear portion of semi-log I–V characteristic would yield the value of ideality factor (n). The value of ideality factor (n) is found to be (Table 2) ∼3.5. All the values of ‘n’ computed as above are quite high compared to that for an ideal one. These higher values of n may be due to the recombination via mid-gap traps in the space-charge region [32]. It is clear that the ideality factor of this heterojunction is considerably larger than the unity. The deviation of the calculated n from the ideal may probably be due to the following factors: (i) inhomogeneities of the barrier height and (ii) the presence of surface states in the junction that provides multiple current pathways between the interfaces. The above would effectively control the current transport mechanism in this structure. The CuO films were polycrystalline in nature as had been observed from the FESEM measurements. This would mean that films that the surface of CuO would consist of alternate grain and grain boundaries. Whether the grains will be partially or fully depleted of charge carriers will depend on the relative amount of carrier concentration and density of grain boundary states present in the films. This would effectively modulate the barrier height. This in turn will effectively modulate the charge transport mechanism in the cells. Moreover, the window layer (CdS) is sulphur deficient in nature. Thus the Cd interstitials at the interface possibly may cause additional oxygen vacancy. The role of oxygen vacancy at the interface would be more complex which has been indicated by Deuermeier et al. [34]. and Nandy et al. [35] from their studies on grain boundaries and field emission properties in Cu2O films. The conduction mechanism which would control the hetero-junction behavior could be explained from the log I versus log V plot under

⎟

(2)

where IPh is the photocurrent. n, kB and T are the ideality factor, Boltzmann constant, and temperature, respectively. Io is the reverse saturation current and may be expressed as:

−qφb ⎞ Io = AA* T 2exp ⎛ ⎝ KT ⎠

(6)

Using the value of A, A*, T, k and q in Eq. (6), we can calculate the value of apparent barrier height which was found to be ∼ 0.73 eV. The ideality factor is a characteristic of the generation-recombination (G–R) processes in the photovoltaic device. As it is known, n = 1 would correspond to the radiation processes in the charge-neutral regions and n > 2 is associated with the recombination via mid-gap traps in the space-charge region (the Shockley–Read–Hall recombination). At high voltages, V > > kBT/e (kBT/e ∼ 25 mV at room temperature), the I–V curves are defined by the exponential term (Eq. (2)), from which the ideality factor, n, may be determined. The diode factor could be evaluated either by: (i) evaluating from the logarithmic plot of the dependence of Isc versus Voc which were measured for different illumination. The ideality factor is determined by the relation [33]:

Fig. 3. (a) Schematic of the solar cell structure and (b) Tentative energy level diagram.

q (V −IRS ) ⎞ ⎞ V −IRS I = I0 ⎛⎜exp ⎛ −1⎟ + −IPh RSh ⎝ nkB T ⎠ ⎠ ⎝

KT AA* T 2 ln ( ) q I0

(3)

where A, A* and φb are the rectifier contact area, effective Richardson constant and apparent barrier height, respectively. Utilizing Fig. 4a, the series resistances of the cell could be estimated. The series resistance (RS) in a solar cell would effectively modulate the short circuit current density. Inverse of the slope of I–V curve at the open circuit voltage may be utilized to evaluate Rs while Rsh could be obtained from the inverse of the slope of the I–V curve at the short circuit condition (V = 0). Thus, RS and RSh could be estimated using the following relations [32]: 5

Materials Research Bulletin 109 (2019) 1–9

S. Dolai et al.

Fig. 4. (a) I–V characteristics, (b) Semi logI–V characteristics and (c) log I vs log V characteristics for CdS/CuO heterojunction diode. Table 2 Cell parameters. Cell Structure

VOC (mV)

Isc (mA)

F.F.

η (%)

RS (Ω−cm2)

RSh (Ω− cm2)

Ideality Factor (n)

NA /cm3

τ (ms)

glass/F:SnO2/CdS/CuO/Ag

462

6.65

0.41

2.10

24.6

200.34

3.5

1.94 × 1014

15

3.3.3. C–V characteristics Capacitance (C)–voltage (V) characteristics may also reflect the quality of junction characteristics. For this, Mott-Schottky relation as indicated below may be used [37]:

forward bias conditions. It may be seen from the plot (Fig. 4c) that the current increases with increasing applied voltage and contains two distinct linear regions indicating a power law variation: J α Vm. The exponent ‘m’ obtained from the slope of each region would provide the corresponding current conduction mechanism. For a low bias voltage, i.e.V < 0.24 V (region-I), a nearly linear dependence of the current on the applied voltage is observed. The characteristic follows a relation: J α V0.96. The small deviation from the ohmic behavior indicates that the transport mechanism would probably depend on the nature of the contacts and the presence of surface states [36]. At voltages in the range of V < 0.24 (region II), an exponential dependence of the current with the bias voltage is evident with a variation: I α V2.2. This variation is indicative of a space charge limited current conduction for single carrier injection behavior [36]. Additionally, as indicated by Deuermeier et al [34]. and Nandy et al [35]. that problems associated with divacancies and also with grain boundaries would play a quite relevant role. This would also result in the observed poor solar cell efficiencies recorded. Effect of the above modulation of conduction mechanism in these cells will also be reflected in the measured minority carrier life time which is discussed in a later section of this report.

1 2(Vbi−V ) = C2 eε0 NA Ks A2

(8)

The terms, e, Ks, ε0, A, NA and Vbi have the usual meaning and can be had from reference [34]. Using Eq. (8), build-in potential can be obtained from the intercept of the extrapolated straight line plot of 1/C2 versus V (Fig. 5a). Negative slope of the Mott-Schottky plot of CuO in CuO/CdS junction would signify that CuO is a p-type semiconductor. The built-in potential of CuO/CdS junction is about ∼0.2 V. Again, NA could also be obtained from the slope of the above Mott-Schottky plot. Thus, the activated carrier density (NA) may be expressed as [37]:

NA =

2 × (qε0 Ks A2 )

1 d (1 / C2) dV

(9)

NA value computed as above ∼1.94 × 1014 cm−3. Straight line 1/C2-V plots (Fig. 5a) indicated a sharp interface. Width (w) of the depletion layer calculated from the expression [38]: 6

Materials Research Bulletin 109 (2019) 1–9

S. Dolai et al.

Fig. 5. (a) Mott-Schottky plot of a typical cell structure (b) VOC decay plot of a representative cell and (c) Photocurrent vs wavelength spectrum of glass/F:SnO2/CdS/ CuO/Ag cell structure.

w = εA/ C0

mean that τ would decay nonlinearly with n which would depend on the rate at which the open-circuit voltage would decay with time. This in turn, would depend on the electrons exchange between the conduction and valence bands. Voc decay with time can then be described as [45]:

(10) −7

where, Co is the device capacitance at V = 0 V. [Co = 1.33 × 10 F, ε = 18.1 × 8.85 × 10-12, A = 4 × 10-6 m2] Width of the depletion layer is obtained ∼4.82 nm. 3.3.4. Open circuit voltage decay measurement Minority carrier lifetime (τ) in a photovoltaic solar cell is an important parameter for designing cell structure. τ would be modulated by the nature of the contacts and the presence of surface states [31]. Presence of space charge limited current conduction for single carrier injection behavior in the cells would also affect the measured τ. One may determine the carrier life time by measuring the decay characteristics of open-circuit voltage (Voc). This method was originally presented by Gossick [39]. Subsequently, several researchers [40–42] utilized the above technique. In this technique, an external excitation is impinged on the cell momentarily. This would involve in creating excess minority carriers in the junction. Change in Voc is then monitored with time after the excitation is withdrawn abruptly. The above Voc decay curve could be used to determine the minority carrier lifetime. The process involved is that free electrons and holes are generated upon illumination. They are separated by the potential barrier of the p–n junction with the development of an open-circuit voltage. As soon as the illumination is turned off, electron and hole recombination causes a decreased in the excess carrier concentration. This decrease in carrier concentration with time (t) can be expressed as [43,44]:

dn/ dt = −R = −(n−n 0)/ τ

Voc (t ) = Voc (0) exp (−t / τ )

(12)

Eq. (12) indicates that the minority carrier life time can be calculated from the relation [46]:

τ=

2kB T ⎡ 1 ⎤ ⎥ ⎢ q ⎢ dVOC ⎥ ⎣ dT ⎦

(13)

Voc decay curve is shown in Fig. 5b. We have used a 1 s light pulse of 60 mW/cm2 intensity. Value of τ estimated as above was ∼15 ms. It may be noted here that the minority carrier life time for efficient cells is generally within ∼50 ms [47]. The observed lower τ value would suggest lower recombination losses in our cells. This would culminate in higher short circuit current density compared to copper oxide based cells fabricated by others. This result also suggests that the quality of CuO films deposited here is of better quality than those have been reported by other groups. Fig. 5c shows the photocurrent of glass/F:SnO2/CdS/CuO/Ag cell structure as a function of wavelength recorder under short circuit condition. In glass/F:SnO2/CdS/CuO/Ag, spectrum shows the maximum photocurrent obtained at wavelength 553 nm. The wavelength of the observed photocurrent in the ultraviolet range is related to that of the absorption edge of the CdS window layer. The cut-off wavelength around 800 nm is related to the absorption edge of CuO.

(11)

where, no and n are the equilibrium and non-equilibrium electron concentrations in the region of p-type absorber layer, respectively. τ is the carrier lifetime. The carrier recombination rate (R), being a nonlinear function of n, can be presented as R = (n − n0)/τ. This would 7

Materials Research Bulletin 109 (2019) 1–9

S. Dolai et al.

4. Conclusion

Photovolt. Res. Appl. 23 (2015) 637–645, https://doi.org/10.1002/pip.2483. [13] S. Masudy-Panah, G.K. Dalapati, K. Radhakrishnan, A. Kumar, H.R. Tan, Reduction of Cu-rich interfacial layer and improvement of bulk CuO property through twostep sputtering for p-CuO/n-Si heterojunction solar cell, J. Appl. Phys. 116 (2014), https://doi.org/10.1063/1.4893321. [14] E.O. Omayio, P.M. Karimi, W.K. Njoroge, F.K. Mugwanga, Current-voltage characteristics of p-CuO/n-ZnO:Sn solar cell, Int. J. Thin Film Sci. Technol. 2 (2013) 25–28, https://doi.org/10.12785/ijtfst/02005. [15] H. Kidowaki, T. Oku, T. Akiyama, Fabrication and characterization of CuO/ZnO solar cells, J. Phys. Conf. Ser. (2012), https://doi.org/10.1088/1742-6596/352/1/ 012022. [16] S. Hussain, C. Cao, G. Nabi, W.S. Khan, M. Tahir, M. Tanveer, I. Aslam, Optical and electrical characterization of ZnO/CuO heterojunction solar cells, Opt. – Int. J. Light Electron. Opt. 130 (2017) 372–377, https://doi.org/10.1016/j.ijleo.2016.10.099. [17] C. Jayathilaka, V. Kapaklis, W. Siripala, S. Jayanetti, Improved efficiency of electrodeposited p-CuO/n-Cu2O heterojunction solar cell, Appl. Phys. Express 8 (2015) 65503 http://stacks.iop.org/1882-0786/8/i=6/a=065503. [18] J. Herion, E.A. Niekisch, G. Scharl, Investigation of metal oxide/cuprous oxide heterojunction solar cells, Sol. Energy Mater. 4 (1980) 101–112, https://doi.org/ 10.1016/0165-1633(80)90022-2. [19] R.P. Wijesundera, Fabrication of the CuO/Cu2O heterojunction using an electrodeposition technique for solar cell applications, Semicond. Sci. Technol. 25 (2010) 45015, https://doi.org/10.1088/0268-1242/25/4/045015. [20] T. Oku, R. Motoyoshi, K. Fujimoto, T. Akiyama, B. Jeyadevan, J. Cuya, Structures and photovoltaic properties of copper oxides/fullerene solar cells, J. Phys. Chem. Solids 72 (2011) 1206–1211, https://doi.org/10.1016/j.jpcs.2011.06.014. [21] W. Septina, R.R. Prabhakar, R. Wick, T. Moehl, S.D. Tilley, Stabilized solar hydrogen production with CuO/CdS heterojunction thin film photocathodes, Chem. Mater. 29 (2017) 1735–1743, https://doi.org/10.1021/acs.chemmater.6b05248. [22] S. Dolai, S. Das, S. Hussain, R. Bhar, A.K. Pal, Cuprous oxide (Cu2O) thin films prepared by reactive d.c. sputtering technique, Vacuum 141 (2017) 296–306, https://doi.org/10.1016/j.vacuum.2017.04.033. [23] S. Dolai, R. Dey, S. Das, S. Hussain, R. Bhar, A.K. Pal, Cupric oxide (CuO) thin films prepared by reactive d.c. magnetron sputtering technique for photovoltaic application, J. Alloys Compd. 724 (2017) 456–464, https://doi.org/10.1016/j.jallcom. 2017.07.061. [24] M. Pavan, S. Rühle, A. Ginsburg, D.A. Keller, H.N. Barad, P.M. Sberna, D. Nunes, R. Martins, A.Y. Anderson, A. Zaban, E. Fortunato, TiO2/Cu2O all-oxide heterojunction solar cells produced by spray pyrolysis, Sol. Energy Mater. Sol. Cells. 132 (2015) 549–556, https://doi.org/10.1016/j.solmat.2014.10.005. [25] E. Yücel, S. Kahraman, H.S. Güder, Effects of different annealing atmospheres on the properties of cadmium sulfide thin films, Mater. Res. Bull. 68 (2015) 227–233, https://doi.org/10.1016/j.materresbull.2015.03.067. [26] Z.R. Khan, M. Zulfequar, M.S. Khan, Optical and structural properties of thermally evaporated cadmium sulphide thin films on silicon (1 0 0) wafers, Mater. Sci. Eng. B Solid-State Mater. Adv. Technol. (2010), pp. 145–149, https://doi.org/10.1016/j. mseb.2010.03.006. [27] L. Wang, W. Cheng, H. Gong, C. Wang, D. Wang, K. Tang, Y. Qian, Facile synthesis of nanocrystalline-assembled bundle-like CuO nanostructure with high rate capacities and enhanced cycling stability as an anode material for lithium-ion batteries, J. Mater. Chem. 22 (2012) 11297–11302, https://doi.org/10.1039/c2jm31023f. [28] S.O. Kasap, Principles of Electronic Materials and Devices, 2nd ed., McGraw-Hill, 2002. [29] T. Oku, T. Yamada, K. Fujimoto, T. Akiyama, Microstructures and photovoltaic properties of Zn(Al)O/Cu2O-based solar cells prepared by spin-coating and electrodeposition, Coatings 4 (2014) 203–213. [30] S.M. Sze, K.K. Ng, Physics of semiconductor devices, Phys. Semicond. Devices 164 John Wiley Sons, Inc., NJ, 2007, p. 682, https://doi.org/10.1002/9780470068328. fmatter. [31] K.A. Sablon, J.W. Little, A. Sergeev, N. Vagidov, V. Mitin, Solar cell with built-in charge: experimental studies of diode model parameters, J. Vac. Sci. Technol. A Vacuum Surf. Film 30 (2012), https://doi.org/10.1116/1.3703607 04D104. [32] B. Ghosh, M. Das, P. Banerjee, S. Das, Fabrication of the SnS/ZnO heterojunction for PV applications using electrodeposited ZnO films, Semicond. Sci. Technol. 24 (2009) 025024–025030, https://doi.org/10.1088/0268-1242/24/2/025024. [33] B.P. Modi, J.M. Dhimmar, The temperature dependent ideality factor effect on I-v characteristics of schottky diode, Proc. 2012 1st Int. Conf. Emerg. Technol. Trends Electron. Commun. Networking, ET2ECN 2012 (2012), https://doi.org/10.1109/ ET2ECN.2012.6470063. [34] J. Deuermeier, H.F. Wardenga, J. Morasch, S. Siol, S. Nandy, T. Calmeiro, R. Martins, A. Klein, E. Fortunato, Highly conductive grain boundaries in copper oxide thin films, J. Appl. Phys. 119 (2016), https://doi.org/10.1063/1.4954002. [35] S. Nandy, R. Thapa, M. Kumar, T. Som, N. Bundaleski, O.M.N.D. Teodoro, R. Martins, E. Fortunato, Efficient field emission from vertically aligned Cu2O1δ(111) nanostructure influenced by oxygen vacancy, Adv. Funct. Mater. 25 (2015) 947–956, https://doi.org/10.1002/adfm.201402910. [36] N. Zebbar, Y. Kheireddine, K. Mokeddem, A. Hafdallah, M. Kechouane, M.S. Aida, Structural, optical and electrical properties of n-ZnO/p-Si heterojunction prepared by ultrasonic spray, Mater. Sci. Semicond. Process. 14 (2011) 229–234, https://doi. org/10.1016/j.mssp.2011.03.001. [37] B. Ghosh, D. Ghosh, S. Hussain, G. Amarendra, B.R. Chakraborty, M.K. Dalai, G. Sehgal, R. Bhar, A.K. Pal, A novel CdCl2 treatment for glass/SnO2/CBD-CdS/ CdTe solar cell, Mater. Sci. Semicond. Process. 24 (2014) 74–82, https://doi.org/ 10.1016/j.mssp.2014.03.005. [38] T.D. Anthopoulos, T.S. Shafai, Influence of oxygen doping on the electrical and photovoltaic properties of schottky type solar cells based on α-nickel

Glass/F:SnO2/CdS/CuO/Ag heterojunction solar cell structure was successfully fabricated here. CuO acted as a good absorber layer. This layer has quite high absorption coefficient (α > 105 cm−1). It may be observed that the F:SnO2 films were quite transparent (Tr > 90%). The band gap was ∼3.45 eV. The CuO films are polycrystalline in nature with average grain sizes ∼115 nm. Cd interstitials at the interface possibly may cause additional oxygen vacancy. The role of oxygen vacancy at the interface would be more complex in modulating the charge transport in this PV structure. It Solar cells fabricated with this hetero-junction structure demonstrated a power conversion efficiency in between 2.0% and 2.1% with an open-circuit voltage ∼0.462 V and short circuit current ∼6.65 mA. Upon withdrawal of illumionation, the open circuit voltage decayed exponentially with time. The carrier life time (τ) determined from Voc decay measurement was ∼15 ms. The density of donors (Nd) was ∼1.94 × 1014 cm-3 and built in potential Vbi ∼0.20 V. These values were computed from the slope and the intercept, respectively, of the linear portion of Mott-Schottky plot. Acknowledgements The authors wish to thank the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India for the financial assistance to carry out this research program. R. Dey wishes to acknowledge with thanks the financial help for supporting his fellowship through DST-INSPIRE fellowship programme of the Department of Science and Technology, Government of India.Thanks are also due to Dr. P. V. Satyam and Mr. P. Guha, Institute of Physics, Bhubaneswar, India, for their assistance for recording TEM images. References [1] T. Wong, S. Zhuk, S. Masudy-Panah, G. Dalapati, Current Status and future prospects of copper oxide heterojunction solar cells, Materials (Basel) 9 (2016) 271, https://doi.org/10.3390/ma9040271. [2] F. Gao, L. Zhu, H. Li, H. Xie, Hierarchical flower-like CuO film: one-step room temperature synthesis, formation mechanism and excellent optoelectronic properties, Mater. Res. Bull. 93 (2017) 342–351, https://doi.org/10.1016/j.materresbull. 2017.05.033. [3] R. Shabu, A. Moses Ezhil Raj, C. Sanjeeviraja, C. Ravidhas, Assessment of CuO thin films for its suitablity as window absorbing layer in solar cell fabrications, Mater. Res. Bull. 68 (2015) 1–8, https://doi.org/10.1016/j.materresbull.2015.03.016. [4] A. Mittiga, E. Salza, F. Sarto, M. Tucci, R. Vasanthi, Heterojunction solar cell with 2% efficiency based on a Cu2O substrate, Appl. Phys. Lett. 88 (2006), https://doi. org/10.1063/1.2194315. [5] C. Wadia, A.P. Alivisatos, D.M. Kammen, Materials availability expands the opportunity for large-scale photovoltaics deployment, Environ. Sci. Technol. 43 (2009) 2072–2077, https://doi.org/10.1021/es8019534. [6] Y.S. Lee, M. Bertoni, M.K. Chan, G. Ceder, T. Buonassisi, Earth abundant materials for high efficiency heterojunction thin film solar cells, Conf. Rec. IEEE Photovolt. Spec. Conf. (2009) 002375–002377, https://doi.org/10.1109/PVSC.2009. 5411314. [7] V. Figueiredo, E. Elangovan, G. Gonçalves, N. Franco, E. Alves, S.H.K. Park, R. Martins, E. Fortunato, Electrical, structural and optical characterization of copper oxide thin films as a function of post annealing temperature, Phys. Status Solidi 206 (2009) 2143–2148, https://doi.org/10.1002/pssa.200881797. [8] N. Gupta, R. Singh, F. Wu, J. Narayan, C. McMillen, G.F. Alapatt, K.F. Poole, S.J. Hwu, D. Sulejmanovic, M. Young, G. Teeter, H.S. Ullal, Deposition and characterization of nanostructured Cu2O thin-film for potential photovoltaic applications, J. Mater. Res. 28 (2013), https://doi.org/10.1557/jmr.2013.150. [9] R. Singh, G.F. Alapatt, A. Lakhtakia, Making solar cells a reality in every home: opportunities and challenges for photovoltaic device design, IEEE J. Electron. Devices Soc. 1 (2013) 129–144, https://doi.org/10.1109/JEDS.2013.2280887. [10] Q. Zhang, K. Zhang, D. Xu, G. Yang, H. Huang, F. Nie, C. Liu, S. Yang, CuO nanostructures: synthesis, characterization, growth mechanisms, fundamental properties, and applications, Prog. Mater. Sci. 60 (2014) 208–237, https://doi.org/10. 1016/j.pmatsci.2013.09.003. [11] F. Gao, X.-J. Liu, J.-S. Zhang, M.-Z. Song, N. Li, Photovoltaic properties of the pCuO/n-Si heterojunction prepared through reactive magnetron sputtering, J. Appl. Phys. 111 (2012) 84507, https://doi.org/10.1063/1.4704382. [12] S. Masudy-Panah, G.K. Dalapati, K. Radhakrishnan, A. Kumar, H.R. Tan, E. Naveen Kumar, C. Vijila, C.C. Tan, D. Chi, P-CuO/n-Si heterojunction solar cells with high open circuit voltage and photocurrent through interfacial engineering, Prog.

8

Materials Research Bulletin 109 (2019) 1–9

S. Dolai et al.

[39] [40]

[41]

[42] [43]

[44] A. Zaban, M. Greenshtein, J. Bisquert, Determination of the electron lifetime in nanocrystalline dye solar cells by open-circuit voltage decay measurements, ChemPhysChem 4 (2003) 859–864, https://doi.org/10.1002/cphc.200200615. [45] A. Davletova, S.Z. Karazhanov, Open-circuit voltage decay transient in dislocationengineered Si p-n junction, J. Phys. D. Appl. Phys. 41 (2008), https://doi.org/10. 1088/0022-3727/41/16/165107. [46] J.E. Mahan, T.W. Ekstedt, R.I. Frank, R. Kaplow, Measurement of minority carrier lifetime in solar cells from photo-induced Open-circuit voltage decay, IEEE Trans. Electron. Devices 26 (1979) 733–739, https://doi.org/10.1109/T-ED.1979.19487. [47] B. Ghosh, D. Ghosh, A. Ghosh, S. Hussain, R. Bhar, A.K. Pal, Electrodeposited diamond-like carbon (DLC) films on n-Si(100) substrates for photovoltaic application, Mater. Sci. Semicond. Process. 25 (2014) 130–136, https://doi.org/10.1016/j. mssp.2013.10.005.

phthalocyanine, Thin Solid Films 441 (2003) 207–213, https://doi.org/10.1016/ S0040-6090(03)00887-3. B.R. Gossick, On the transient behavior of semiconductor rectifiers, J. Appl. Phys. 27 (1956) 905–911, https://doi.org/10.1063/1.1722512. W.H. Ko, The reverse transient behavior of semiconductor Junction diodes, IRE Trans. Electron. Devices. 8 (1961) 123–131, https://doi.org/10.1109/T-ED.1961. 14719. R.J. Bassett, Observations on a method of determining carrier lifetime in p+-v-n+ diodes, Solid-State Electron. 12 (1969) 385–391, https://doi.org/10.1016/00381101(69)90094-X. R.G. Mazur, H.R. Huffand, R.R. Burgess (Eds.), Semiconductor Silicon, 1973. A.B. Walker, L.M. Peter, K. Lobato, P.J. Cameron, Analysis of photovoltage decay transients in dye-sensitized solar cells, J. Phys. Chem. B 110 (2006) 25504–25507, https://doi.org/10.1021/jp064860z.

9