Thin Solid Films 447 – 448 (2004) 80–84

Photovoltaic effect in nanocrystalline Pb1yxFexS–single crystal silicon heterojunctions Rakesh K. Joshia,*, Satish Mohanb, S.K. Agarwalb, H.K. Sehgala a

Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India b Solid State Physics Laboratory, Timarpur, New Delhi 110054, India

Abstract Photovoltaic effect is observed in self-assembled heterojunctions obtained in p-type nanocrystalline Pb1yx Fex S (xs0.25, 0.33, 0.50) films on n-type single crystal silicon substrates. The p-type ternary Pb1yx Fex S films grown on these substrates by a solution growth technique were observed to be homogeneous and grow with basic lead sulfide structure. The lattice parameter of the nanocrystalline ternary films was observed to decrease with increase in x. The films were observed to have direct optical band gap from 1.90 to 1.60 eV as the iron concentration (x) increased from 0.25 to 0.50. The decrease in band gap with increase in iron concentration suggests the alloying of FeS (Eg s0.04 eV) with the nanocrystalline PbS. The current–voltage (I–V) and capacitance–voltage (C–V) characteristics confirmed the formation of heterojunctions in all the samples. Increase in the value of forward current and capacitance with increase in iron concentration (x) in the junctions can be attributed to the increase in surface area contact caused by decrease in the grain size of the films with x. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Chemical bath deposition; Nanocrystalline films; Heterojunction

1. Introduction Growth of semiconductor nanoparticle films for device applications is an emerging field of research w1x. PbS in bulk form is a material of potential application for infrared detection w2x. Ternary Pb1yxFexS if prepared in the bulk form could be an emerging material for higher wavelength IR detection due to the possibility of varying the band gap from 0.41 eV (PbS) to 0.04 eV (FeS) w3x by varying the composition (x). On increasing the optical band gap of PbS from 0.41 (bulk) to 5.4 eV w4x by reducing the grain size in nanometer range provides the possibility to exploit the material for photovoltaic applications. Therefore, an attempt has been made to prepare ternary Pb1yxFexS alloys in the nanocrystalline form so that the required band gap engineering for different applications could be achieved. Single-phase nanocrystalline films of Pb1yxFexS were grown by chemical bath deposition. Hall effect study shows that the films are grown as p-type for xs0.25, 0.33, 0.50 w5x. Therefore, it was expected that p*Corresponding author. Tel.: q91-11-6591349; fax: q91-116581114. E-mail address: [email protected] (R.K. Joshi).

Pb1yxFexS nanocrystalline films grown on n-Si would form a heterojunction and an increased optical band gap value of the present nanoparticle films indicates the possibility to use the heterojunction for the study of photovoltaic effect. In this paper, we report the study of heterojunctions between p-Pb1yxFexSyn-silicon and the photovoltaic effect observed in these self-assembled heterojunctions. 2. Experimental Nanocrystalline films of Pb1yxFexS (xs0.25, 0.33, 0.50) were grown on the n-single crystal silicon by chemical bath deposition. (My30) lead acetate, (My25) FeCl2 and (My25) thiourea aqueous solutions were found to be most appropriate for growth of good quality ternary films with the desired compositions. Initially, cleaned silicon (1 0 0) substrates were dipped in the lead acetate and thiourea solutions mixed in equal proportions. The chemical bath was maintained at a temperature of 35 8C ("1 8C) and the solution was stirred continuously. Liquor ammonia was gradually added to bring pH of the solution to 9.75. Appropriate quantity of aqueous FeCl2 (My25) solution was added

0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.09.026

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Fig. 2. hn vs. (ahn)2 plot for the Pb0.66Fe0.33S nanocrystalline films. Fig. 1. TEM micrograph of Pb0.5Fe0.5 S nanocrystalline film grown on n-Si substrate.

at this stage to obtain a particular value of x in the films. pH of the solution was then driven to 10.75 ("0.1). Films were allowed to grow on the silicon substrate for an optimized time to achieve a thickness of 200 nm. Compositions of the films are determined with the help of X-ray fluorescence technique. Films were thoroughly cleaned by deionized water and then dried and chemically dissolved from the edges and back side of the silicon crystal. Indium dots (1 mm in diameter) were used to take the electrical contacts from the p-type film and n-type silicon. X-ray diffraction and transmission electron microscopy were used to study the structural properties of the films. Grain size in the film was estimated by TEM in plane-view (PVTEM) mode, using Philips CM20 instrument operating at 200 keV. The ternary films were also grown in quartz substrates to study the optical properties. The current–voltage (I– V) characteristics of the p–n junctions were studied with the help of a programmable curve tracer (Tektronix 370). I–V characteristic was studied in dark and in illumination of light (AM 1.5 sunlight). Junction capacitance (C) as a function of the applied forward bias voltage (V) was measured at 1 MHz by using MDC,

RM-1600 C–VyG–V analysis system and Keithley Quasi static system. 3. Results and discussion Heterojunctions were fabricated between p-type Pb1yxFexS (xs0.25, 0.33, 0.50) films and n-type single crystal silicon substrates. Pb1yx Fex S films were observed to grow with structure identical to PbS with a small decrease in lattice parameter with increase in iron concentrations in the films w5x. Typical bright-field image of the PVTEM taken with the electron beam direction close to the (1 1 0)-zone axis under a strongly focused condition for xs0.50 is shown in Fig. 1. The average grain size in the films was observed to decrease from 29 to 20 nm with increase in iron concentration x (Table 1). The ternary films were observed to have a direct optical band gap, which is decreases from 1.90 to 1.60 eV when iron concentration (x) was increased from 0.25 to 0.50 as shown in Table 1. It is observed that the absorption edge shifts towards higher wavelengths with increase in iron concentration (x) in the present nanocrystalline films. Absorption coefficient a (cmy1) was computed from the experimentally observed reflectance and transmittance data obtained for the films grown in quartz substrates. (ahn)n vs. hn plots for ns2, 1y2 and

Table 1 Variation of average grain and optical band gap with composition in Pb1yx Fex S nanocrystalline films and diode parameters in p-Pb1yxFexSyn-Si heterojunctions for different values of x Pb1yxFexS

Grain size (nm)

Band gap (eV)

Built in potential (eV)

Ideality factor (n)

xs0.25 xs0.33 xs0.50

29 26 20

1.90 1.72 1.60

1.14 1.12 1.08

6.35 6.85 7.50

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Fig. 3. Schematic diagram of the p-Pb1yxFexSyn-Si heterojunction.

1y3 show a linear behaviour for ns2, indicating the presence of direct optical band gap in these films for all x. Fig. 2 shows hn vs. (ahn)2 plots from a typical Pb0.66Fe0.33S film. On extrapolating the linear plot of hn vs. (ahn)2 to zero value of (ahn)2, the optical band gap is obtained. The decrease in band gap with increase in iron concentration is attributed to the alloying of FeS (Egs0.04 eV, bulk value) with the nanocrystalline PbS. The self-assembled heterojunctions were characterised by the I–V and C–V characteristics at 300 K. Schematic diagram of the p-Pb1yxFexSyn-Si heterojunction is shown in Fig. 3. The I–V characteristic of p-Pb1yxFexSy n-Si is shown in Fig. 4a for different values of x. Current

Fig. 4. (a) I–V characteristic of p-Pb1yxFexSyn-Si heterojunction; (h) xs0.25, (j) xs0.33 and (s) xs0.50. (b) The ln(I) vs. forward bias voltage for p-Pb1yxFexSyn-Si heterojunctions; (h) xs0.25, (j) xs0.33 and (s) xs0.50.

Fig. 5. Junction capacitance vs. forward bias voltage for the p-Pb1yxFexSyn-Si heterojunctions; (a) xs0.25; (b) xs0.50.

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Fig. 6. Energy band diagram for p-Pb0.5Fe0.5Syn-Si heterojunction.

in the forward bias regime increases almost exponentially and is higher compared to reverse bias regime. The forward current is observed to increase with increase in iron concentration x in the films, which is possibly attributed to increased interface area due to increase in surface to volume ratio with decrease in particle size w6x. Diode ideality factor (n) for the heterojunctions from films with different x is estimated from the plots between ln(I) vs. forward bias voltage (Fig. 4b). The values, summarised in Table 1, are similar to those reported by Nanda and Sahu w6x for PbSyITO heterojunction. Junction capacitance measured as function of forward bias voltage for the heterojunctions shown in Fig. 5 confirms the formation of junctions. Liner fit of the data points (solid line) indicates the formation of abrupt junctions w7x. The value of normalised capacitance was observed to increase with iron concentration in the films, which suggests an increase in interface area with decrease in particle size with x. Intercept of the straight

Fig. 7. Effect of illumination on the I–V characteristic of (a) pPb0.75Fe0.25Syn-Si; (b) p-Pb0.66Fe0.33 Syn-Si and (c) p-Pb0.5Fe0.5Syn-Si heterojunctions.

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line on x-axis gives the value of built-in potential (Vd) for the heterojunctions (Table 1). On the basis of the I–V and C–V characteristics of the heterojunctions, energy diagram is proposed in Fig. 6, which helps understand formation of junction between the Pb1yxFexS nanocrystalline films and n-Si. For the energy band profile we need the values of electron affinity (x) for Si and Pb1yxFexS. For Si, xs4.01 and the band gap (Eg)s1.12 eV. In n-Si with donor concentration of ;2=1016 the position of fermi level is 0.21 eV below the conduction band. Value of x is not known for Pb1yxFexS, therefore we assume the PbS value, which is 4.6 eV w8,9x. As Pb1yxFexS films are p-type, the fermi level is expected to lie above the valence band. Photovoltaic effect was observed in these heterojunctions. I–V characteristic for the junctions, illuminated from the film side by AM 1.5 sunlight, is shown in Fig. 7. These diodes were observed to show a large open circuit voltage (Voc) and short circuit current (Isc). The values of Isc and Voc were found to increase in diodes of increasing iron concentrations. Effect of film thickness on Isc was investigated by the study of I–V characteristics (under illumination) for three heterojunctions with different film thicknesses. Results obtained from this study indicate that Isc increases up to a certain thickness (200 nm) of the nanoparticle films after which it shows a decrease which is depicted for the pPb0.5Fe0.5Syn-Si heterojunction in Fig. 8. 4. Conclusions Electroless solution growth technique has been used to fabricate the heterojunctions between p-type nanocrystalline Pb1yxFexS films and n-type single crystal silicon substrates. Increase in the value of forward current and capacitance with increase in iron concentration is observed in the junctions. Increased value of Isc and Voc in the diodes of higher iron concentration with optimized film thickness suggests that the material has potential for solar cell application.

Fig. 8. Variation of Isc with thickness of the film in p-Pb0.5Fe0.5SynSi heterojunction.

Acknowledgments The authors are grateful to Dr P. Srivastav and Mr B.S. Sahu of Semiconductors Engineering Laboratory, Department of Physics, at IIT Delhi for providing the C–V measurement facility. References w1x C.D. Grant, A.M. Schwartzberg, G.P. Smestad, J. Kowalik, L.M. Tolbert, J.Z. Zhang, J. Electroanal. Chem. 522 (2002) 40. w2x G.W. Carmichael, Infrared Phys. 4 (1964) 9. w3x J.R. Gosselin, M.G. Townsend, R.J. Trembly, Solid State Commun. 19 (1976) 799. w4x R. Thielsch, T. Bohme, ¨ ¨ R. Reiche, D. Schlafer, H.D. Bauer, H. ¨ Bottcher, Nanostruct. Mater. 10 (1998) 131. w5x R.K. Joshi, H.K. Sehgal, J. Cryst. Growth 247 (2003) 425. w6x K.K. Nanda, S.N. Sahu, Appl. Phys. Lett. 79 (2001) 2743. w7x R.L. Anderson, Solid State Electron. 5 (1962) 341. w8x R.M. Oman, J. Appl. Phys. 36 (1965) 2091. w9x R.A. Knapp, Phys. Rev. 132 (1963) 1891.