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ZnS overlayer on in situ chemical bath deposited CdS quantum dot-assembled TiO2 films for quantum dot-sensitized solar cells
Current Applied Physics 12 (2012) 1459e1464
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ZnS overlayer on in situ chemical bath deposited CdS quantum dot-assembled TiO2 films for quantum dot-sensitized solar cells Sung Woo Jung a, Jae-Hong Kim a, Hyunsoo Kim b, *,1, Chel-Jong Choi b, Kwang-Soon Ahn a, **,1 a b
School of Chemical Engineering, Yeungnam University, Dae-dong, Gyeongsan 712-749, Republic of Korea School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2011 Received in revised form 12 March 2012 Accepted 7 April 2012 Available online 19 April 2012
ZnS overlayers were deposited on the CdS quantum dot (QD)-assembled TiO2 films, where the CdS QDs were grown on the TiO2 by repeated cycles of the in situ chemical bath deposition (CBD). With increasing the CdS CBD cycles, the CdS QD-assembled TiO2 films were transformed from the TiO2 film partially covered by small CdS QDs (Type I) to that fully covered by large CdS QDs (Type II). The ZnS overlayers significantly improved the overall energy conversion efficiency of both Types I and II. The ZnS overlayers can act as the intermediate layer and energy barrier at the interfaces. However, the dominant effects of the ZnS overlayers were different for the Types I and II. For Type I, ZnS overlayer dominantly acted as the intermediate layer between the exposed TiO2 surface and the electrolyte, leading to the suppressed recombination rate for the TiO2/electrolyte and the significantly enhanced charge-collection efficiency. On the contrary, for Type II, it dominantly acted as the efficient energy barrier at the interface between the CdS QDs and the electrolyte, leading to the hindered recombination rate from the large CdS QDs to the electrolyte and thus enhanced electron injection efficiency. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Quantum dot-sensitized solar cell Recombination Charge-collection efficiency Electron injection efficiency Intermediate layer Energy barrier
1. Introduction Dye-sensitized solar cells (DSSCs) based on mesoporous TiO2 films are potential low-cost alternatives to commercial Si-based solar cells [1e4]. They are composed of a dye-sensitized mesoporous TiO2 film and a Pt catalytic counter electrode, with an electrolyte containing a redox couple (I/I 3 ) between the two. Dye molecules adsorbed on TiO2 can be excited by light absorption. Electrons photo-excited in the dye molecules can then be injected into the conduction band of TiO2 and transferred to transparent conducting oxide (TCO) for use in an external circuit. The use of a ruthenium complex photo-sensitizer has reached an energy conversion efficiency of about 11% [5]. Semiconducting quantum dots (QDs) [CdS, CdSe, PbS, InP, etc.] have been tested as photo-sensitizers for DSSCs [6e16] because (1) their optical band gaps can be readily tuned through quantum confinement effects by varying their sizes [6,7,12e14], (2) they have much higher extinction coefficients than conventional dyes due to
* Corresponding author. ** Corresponding author. Tel.: þ82 53 810 2524; fax: þ82 53 810 4631. E-mail addresses: [email protected] (H. Kim), [email protected] (K.-S. Ahn). 1 Both of Prof. K.-S. Ahn and H. Kim contributed equally to this work as the corresponding authors. 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.04.012
the multiple energy levels [6], and (3) they are robust, inorganic materials [6e9]. However, despite these advantages, quantum dotsensitized solar cells (QD-SSCs) have low energy conversion efficiencies (reaching 2.5% for CdS QD-SSCs) [6,9], due to several possible recombination routes at the interfaces of QD/TiO2/electrolyte [6,9,17,18]. Fig. 1 shows the schematic energy diagrams of the redox couple in the electrolyte, QD and TiO2. The solid arrows represent desirable, forward charge transfer and the dotted arrows show several recombination routes; where pathways 1, 2, and 3 are the back electron transfers from QDs to the redox couple, from TiO2 to the redox couple, and from TiO2 to QDs, respectively, and pathway 4 is the inner recombination of photo-excited electrons in QDs. Furthermore, these recombination pathways are more facilitated by the defect traps existing in QDs and TiO2, which led to much quicker electronehole recombination in QD-SSCs (1011 to 106 s) than in DSSCs (107 to 104 s) [6]. The ZnS overlayer on the QD-assembled TiO2 films has been demonstrated to improve the overall energy conversion efficiency of the QD-SSCs, because the ZnS has the conduction band minimum higher than the CdS, leading to the effective energy barrier and passivation effects [6,9,17,19e22]. However, more detailed studies on the ZnS overlayer remain poorly understood [6,17]. Direct deposition of QDs on the TiO2 surface using an in situ chemical bath deposition (CBD) has resulted in superior electron injection and better coverage than indirect contact of the
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Polysulfide electrolyte containing 0.5 M Na2S, 2 M S, and 0.2 M KCl was prepared in water/methanol (3:7 by volume) solution [24e26]. The resulting cells’ photovoltaic currentevoltage characteristics were measured under 1 Sun illumination (100 mW cm2, AM 1.5) verified by an AIST-calibrated Si-solar cell. For open circuit voltage decay (OCVD) measurement, cells were illuminated to a steady voltage and the decay of the open circuit voltage was then measured after the illumination was cut off by a shutter. Incident photon-to-current conversion efficiency (IPCE) was measured with an action spectrum measurement setup (PEC-S20, Peccell Ltd.). The QDs’ sizes and coverage on the TiO2 surface were estimated from UVevis absorption spectra (Cary5000, Agilent Tech. Co.). Fig. 1. Schematic energy diagram of the redox couple, quantum dot, and TiO2; (a) desired charge transfer (solid arrows) and recombination pathways (dotted arrows).
pre-synthesized QDs linked by functional molecular linkers on TiO2 [7e10,23]. However, it has the disadvantage of being difficult to separately control the QDs’ coverage and size [23] which are simultaneously increased with the increased CBD cycles. Therefore, as the CBD cycles increased, the CdS QD-assembled TiO2 films were transformed from TiO2 films assembled by small QDs with low coverage to films assembled by large QDs with high coverage, where the former and latter are denoted as the Types I and II, respectively [7,10,24]. Consequently Type I features the TiO2 surface partially covered by small QDs, leaving the exposed TiO2 surface. In contrast, Type II consists of TiO2 fully covered by large-sized QDs without exposure of the TiO2 surface. In this paper, the CdS QDs were in situ grown on the mesoporous TiO2 films by repeated CdS CBD procedure (3, 5, 8, and 12 cycles), whose surfaces were then coated with the ZnS overlayer. We report here that the dominant effects of the ZnS overlayers were different for the Types I and II films and hence resulted in enhanced overall energy conversion efficiency. Their properties, including electron lifetime, driving force for the electron injection, recombination pathways, energy barrier, charge-collection efficiency, and electron injection efficiency, were systematically studied.
3. Results and discussion The QDs were deposited by in situ CBD process, which has the disadvantage of being difficult to separately control the QD’s size and coverage [23]. It indicates that, as the cycle number increases, the QD’s size increases together with the increased coverage of the QDs on the TiO2 surface. Fig. 2 shows (aec) the SEM top-viewed images of bare TiO2, TiO2/CdS(3), and TiO2/CdS(8), respectively. It clearly shows that the CdS QD’s size and coverage were simultaneously increased with increasing the cycle number. The TiO2/CdS films with and without the ZnS overlayer had no apparent change in the SEM surface morphology (not shown here), because the ZnS layer was deposited on the TiO2/CdS(X) films only by one CBD cycle, leading to the ZnS monolayer. The ZnS overlayer was investigated by the energy-dispersive X-ray spectroscopy (EDS). Fig. 2(d) shows the EDS spectrum of the TiO2/CdS(8)/ZnS, which exhibited the ZnS overlayer deposited in the 1.56 at.%. Fig. 3(a) shows optical absorbance curves of TiO2/CdS(X) and TiO2/CdS(8)/ZnS, where X is the CBD cycle number. TiO2 absorbs only UV light with wavelengths less than 370 nm due to its wide band gap (3.4 eV) [1e4]. The absorption edge of TiO2/CdS(X) redshifted with the increased cycles of the CdS CBD, indicating the increased CdS QD’s size. The Brus equation was used to estimate the average diameters of the CdS QDs: [8,27].
" # h2 1 1 1:8e2 ¼ Eg þ 2 þ 4p3 3 0 r 8r m*e mh
2. Experimental details
EQD
Mesoporous TiO2 films were prepared by doctor-blading TiO2 paste (Ti-Nanoxide T/SP, Solaronix SA) onto fluorine-doped tin oxide (FTO) substrates, followed by sintering at 450 C for 30 min. The films were deposited to be 10 mm thick, as measured by stylus profilometry. The cells’ active areas were 0.16 cm2. Ethanol and methanol were used to dissolve Cd(NO3)2 and Na2S, respectively, for the synthesis of the CdS QDs. In situ chemical bath deposition (CBD) involved immersing the mesoporous TiO2 films in 0.5 M Cd(NO3)2 solution for 5 min, rinsing with ethanol, and subsequent 5 min immersion in 0.5 M Na2S solution followed by further rinsing with methanol [24]. Each series of two immersions was considered as one CBD cycle and the experiments were performed for3, 5, 8, and 12 cycles to vary the size of the CdS QDs assembled on the TiO2 surface which is denoted by TiO2/CdS(X) with X being the CBD cycle number. Finally, the ZnS overlayer was then coated on the CdS QD-assembled TiO2 films by immersing alternately into 0.5 M Zn(CH3COO)2 and 0.5 M Na2S solution each for 5 min, which is then referred to as TiO2/CdS(X)/ZnS [9,19,20]. Semitransparent Pt counter-electrodes were prepared by doctor-blading Pt nanocluster-containing Pt paste (PT-1, Dyesol. Ltd.) onto FTO transparent conducting substrates followed by calcination at 450 C for 30 min in air. The TiO2/CdS(X) photoanodes both with and without the ZnS overlayer and the Pt-coated counter-electrodes were sandwiched by 60 mm-thick seals.
where EQD and Eg are the lowest excitation energy of the QDs and the band gap of bulk CdS, respectively. r is the radius of the QDs. me and mh are the effective masses of electrons and holes in CdS, respectively, 3 0 is the vacuum permittivity, and 3 is the relative permittivity of CdS. [24,27,28]. The optical band gaps of QDs were estimated by intersecting the base line and shoulder line, as shown in Fig. 3(a). Average QD sizes were calculated to be 4.4, 4.9, 5.4, and 6.2 nm in diameter, respectively, for the TiO2/CdS(3), TiO2/CdS(5), TiO2/CdS(8), and TiO2/CdS(12). As the CBD cycles increased, the coverage of QDs on the TiO2 surface is simultaneously improved with the increased QDs’ size [7,10,23,24]. Our previous work [24] showed that, with increasing the CdS CBD cycles, the surface morphologies of the CdS QD-assembled TiO2 films were transformed from Type I to the Type II, where Type I is composed of TiO2 partially covered by small QDs, whilst Type II consists of TiO2 fully covered by large-sized QDs without any exposed TiO2 surface. The ZnS overlayers were then coated on all of the TiO2/CdS(X) films, and their optical absorption curves were also measured. The absorbance of the TiO2/CdS(3) was gradually increased from the absorbance edge to the shorter wavelengths, whereas the TiO2/CdS(8) exhibited sharply increased, saturated absorbance curve. The SEM images and absorbance data of the Figs. 2 and 3 indicate that the surface area of the TiO2 in TiO2/CdS(3) was not fully covered by the QDs and the surface area of the TiO2 in TiO2/CdS(8) was almost fully
(1)
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Fig. 2. (aec) the SEM top-viewed images of bare TiO2, TiO2/CdS(3), and TiO2/CdS(8), respectively. (d) The energy-dispersive X-ray spectroscopy (EDS) spectrum of the TiO2/CdS(8)/ ZnS.
covered so that the TiO2/CdS(3) and TiO2/CdS(8) were classified as the Types I and II, respectively. Fig. 3(b) illustrates the schematics of the ZnS overlayers coated on (upper) Type I and (lower) Type II, respectively. The dominant effects of the ZnS overlayer may be significantly influenced by the difference in the morphology of the CdS QD-assembled TiO2 films, which will be discussed later. The optical absorbance of the TiO2/CdS(8)/ZnS was the same as that of the TiO2/CdS(8), as seen in Fig. 3(a), due to wide band gap (3.6 eV) of ZnS.19 The other ZnS-coated TiO2/CdS(X) films had the same absorption curves as the corresponding ZnS-uncoated TiO2/CdS(X) (not shown here). Electron lifetimes on the TiO2/CdS(X) films with and without the ZnS overlayer were estimated through open circuit voltage decay (OCVD) measurements [29,30]. Fig. 4(a) exhibits open circuit voltage (Voc) decay curves of TiO2/CdS(3) and TiO2/CdS(8) with and without the ZnS overlayer recorded during the relaxation from an illuminated quasi-equilibrium state to darkness. Fig. 4(b) plots the
electron lifetimes calculated from the Voc decays according to: [17,18,29,30]:
kB T dVoc 1 e dt
se ¼
(2)
where kBT, e, and dVoc/dt are the thermal energy, the positive elementary charge, and the derivative of the open circuit voltage transient, respectively. The open circuit voltage is determined by the difference in potential between the Fermi level of TiO2 and the redox couple. The photovoltage decay rate is directly related to electron lifetime, because excess electrons in the conduction band of TiO2 are removed by recombining with the redox couple in the electrolyte when the illumination at open circuit is interrupted [18,30]. The electrons in the CdS QDs may be recombined by the electrolyte. In this experiment, the electron lifetimes were estimated by the open circuit voltage (Voc) decay curves. The Voc is the potential difference
Fig. 3. (a) Absorbance curves of TiO2/CdS(X) with and without the ZnS overlayer, where X represents the cycle number of the CdS CBD. (b) Schematics of the ZnS overlayers coated on Type I (upper) and Type II (lower), respectively.
a
0.5
TiO2/CdS(3) TiO2/CdS(3)/ZnS TiO2/CdS(8) TiO2/CdS(8)/ZnS
Light off
(V)
0.4
V
oc
0.3 0.2 0.1 2
3
4
5
Time (s)
b
7
100
10 9
TiO2 /CdS(3) TiO2 /CdS(3)/ZnS
8
TiO2 /CdS(5)
7
TiO2 /CdS(5)/ZnS
6
TiO2 /CdS(8) TiO2 /CdS(8)/ZnS
5
TiO2 /CdS(12)
4
TiO2 /CdS(12)/ZnS
3 2 1 0 0.0
8
0.1
0.2
0.3
0.4
0.5
0.6
Potential (V) TiO2/CdS(3) TiO2/CdS(3)/ZnS TiO2/CdS(8) TiO2/CdS(8)/ZnS
101
Lifetime (s)
6
a
Current density (mAcm-2)
S.W. Jung et al. / Current Applied Physics 12 (2012) 1459e1464
-1
10
b
60 TiO2/CdS(3) TiO2/CdS(3)/ZnS
50
TiO2/CdS(5)
IPCE (%)
1462
TiO2/CdS(5)/ZnS
40
TiO2/CdS(8) TiO2/CdS(8)/ZnS
30
TiO2/CdS(12) TiO2/CdS(12)/ZnS
20 10
-2
10
0.0
0.1
0.2
0.3 V
oc
0.4
0.5
(V)
Fig. 4. (a) Voc decay curves of the TiO2/CdS(3) and TiO2/CdS(8) films with and without the ZnS overlayer recorded during the relaxation from illuminated quasi-equilibrium to the dark. (b) Electron lifetimes estimated from (a).
between the quasi-Fermi level of the TiO2 and the redox couple in the electrolyte, indicating that the Voc decay is related to the decreased number of the electrons in the conduction band of the TiO2 which lower the quasi-Fermi level. M. Miyauchi’s group [18] reported that the charge recombination from the TiO2 to the electrolyte is mainly responsible for the Voc decay of the QD-SSCs. They measured the voltage transients under the dark condition by switching the applied bias to zero and compared these results with the Voc decay curves under the light condition. They found that the Voc decay and electron lifetimes of the QD-SSCs do not differ from the dark condition to the light condition, indicating that the electron lifetimes estimated by the Voc decay curves were mainly caused by the charge recombination from the TiO2 to the electrolyte. The TiO2/ CdS(3) film exhibited significantly enhanced electron lifetime after the ZnS overlayer was coated, compared to the uncoated-TiO2/ CdS(3) film. The absorbance spectrum of the TiO2/CdS(3) as seen in Fig. 3(a) exhibited low light absorption as well as small QD’s sizes (with an averaged diameter of 4.4 nm), indicating the presence of Type I where the TiO2 surface is only partially covered and it also partly contacts with the electrolyte, leading to the interfacial contact between the two. Furthermore, mesoporous TiO2 films composed of nanoparticles smaller than 25 nm do not develop a depletion layer at the interface between TiO2 and electrolyte, indicating that the electrons in the conduction band of TiO2 move according to the diffusion process rather than the faster drift mechanism [1e4,31,32]. Therefore, the exposed TiO2 surface in Type I facilitates the recombination of TiO2 and the electrolyte (pathway 2 in Fig. 1), which is
0 300
400
500
600
700
Wavelength (nm) Fig. 5. (a) Photovoltaic currentevoltage curves of QD-SSCs with the TiO2/CdS(X) and TiO2/CdS(X)/ZnS measured under 1 Sun illumination. (b) The QD-SSCs’ incident photon-to-current conversion efficiency (IPCE).
responsible for its poor electron lifetime. The ZnS overlayer coated on the TiO2/CdS(3) Type I film exhibited significantly enhanced electron lifetime, indicating that the ZnS overlayer covered the exposed TiO2 surface and acted as the intermediate layer which hindered the back electron transfer (or recombination) from the conduction band of TiO2 to the redox couple (pathway 2 in Fig. 1). In contrast, the electron lifetime of the TiO2/CdS(8) film was much higher than that of the TiO2/CdS(3) film. This is becauseTiO2/CdS(8) is much closer to Type II than Type I, owing to the increased QD’s size and better coverage on the TiO2 surface at the higher CdS CBD cycle. That is, the CdS QDs in the TiO2/CdS(8) almost fully cover the TiO2 surface and hence there is only a smaller interfacial contact area between TiO2 and the electrolyte, resulting in significantly improved electron lifetime, compared to TiO2/CdS(3). The ZnS overlayer coated on TiO2/CdS(8) exhibited only slightly increased electron lifetime, compared to the TiO2/CdS(8) without the ZnS overlayer, indicating that the ZnS overlayer had little effect as the intermediate layer for the Type II, unlike the ZnS overlayer’s effect for the Type I. It is because the CdS QDs of the Type II fully covered the TiO2 surface and acted as the intermediate layer between the TiO2 and the electrolyte interface regardless of the ZnS overlayer. The ZnS overlayers act as the energy barrier as well as the intermediate layer in the Types I and II. However, the dominant effect may differ for the Types I and II. The Types I and II had two different features relating to the QD’s size and coverage. The QD’s size is much smaller in the Type I, leading to the enhanced size quantization. V. Kamat’s group [33] reported that the electron injection rate from the small QDs to the TiO2 was much faster than
S.W. Jung et al. / Current Applied Physics 12 (2012) 1459e1464 Table 1 The photovoltaic properties of QD-SSCs based on the CdS QDs with and without the ZnS overlayer. Samples
Jsc / mA cm2
Voc / V
FF / %
h/%
TiO2/CdS(3) TiO2/CdS(3)/ZnS TiO2/CdS(5) TiO2/CdS(5)/ZnS TiO2/CdS(8) TiO2/CdS(8)/ZnS TiO2/CdS(12) TiO2/CdS(12)/ZnS
3.519 4.038 4.519 5.738 6.694 7.813 5.494 6.288
0.41 0.44 0.44 0.48 0.48 0.49 0.51 0.51
45.96 46.55 42.69 43.54 40.04 44.83 37.48 40.69
0.66 0.83 0.85 1.20 1.29 1.72 1.05 1.30
that from the large-sized QDs to the TiO2, due to higher driving force for the electron injection from the conduction band of the QDs to the TiO2. The TiO2 surface in the Type I was not fully covered by the CdS QDs, indicating that the dominant role of the ZnS layer in the Type I may be the intermediate layer rather than the energy barrier effect. In contrast, the TiO2 surface in the Type II was almost fully covered by the CdS QDs. The QD’s size in the Type II was much larger, leading to the reduced size quantization. The lowered conduction band minimum (CBM) of the large-sized QDs reduces the driving force for the electron injection from the CdS QDs to the TiO2. Therefore, the dominant effect of the ZnS overlayer in the Type II may be the energy barrier effect among the two effects. Fig. 5(a) presents the photovoltaic currentevoltage (IeV) performances of the QD-SSCs under 1 Sun illumination, which are summarized in Table 1. The overall energy conversion efficiency (h) of QD-SSCs with the ZnS-uncoated TiO2/CdS(X) increased as the CdS CBD cycles increased up to 8 times, due to the increased amount of electrons injected from the increased amount of assembled QDs and enhanced light absorption in the long wavelengths. However, further increased CdS CBD cycle reduced the cell performance. It can be attributed to the poor charge-injection efficiency caused by (1) the reduced size quantization of the large QDs leading to the reduced driving force for carrier (electrons and holes) injection, (2) the increased number of recombination traps in the large QDs themselves, and (3) hindered ion transport of the redox couple caused by blocked pores [24]. The ZnS overlayer improved all of the parameters (short-circuit current (Jsc), open circuit voltage (Voc), and fill factor (FF)) for all the TiO2/CdS(X) films, resulting in the significantly enhanced overall energy conversion efficiency. Fig. 4 showed that the ZnS overlayer for Type I TiO2/CdS(3) acted as the efficient intermediate layer between the exposed TiO2 surface and the electrolyte, giving rise to the significantly enhanced electron lifetime. On the contrary, the ZnS overlayer influenced the electron lifetime of Type II TiO2/CdS(8) to a much smaller extent, due to the full coverage of the TiO2 surface. Nevertheless, Fig. 5(a) shows that QD-SSC with ZnS-coated Type II TiO2/CdS(8) exhibited dramatically improved cell efficiency of 1.72%, which was an improvement of approximately 30% over that with uncoated Type II TiO2/CdS(8). It therefore indicates that the ZnS overlayer had a different dominant role in Type II TiO2/CdS(8), which will be discussed later. The efficiency gain by the ZnS overlayer was mainly due to the increased Jsc, which was studied in more detail using the incident photon-to-current conversion efficiency (IPCE) curves [Fig. 5(b)]. IPCE is the product of lightharvesting efficiency, charge-injection efficiency, and chargecollection efficiency [24,34,35]:
IPCE ¼ LHE$hinj $hcc
(3)
where LHE is the light-harvesting efficiency, hinj is the chargeinjection efficiency, and hcc is the charge-collection efficiency. LHE is determined from the amount of attached photo-sensitizer, light scattering, and the concentration of the redox species; hcc is by the competition between recombination rate and charge
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transport; and hinj is by the driving forces related to the energy levels of TiO2, QDs, and the redox couple and trap-mediated electronehole recombination through QDs. [6,33]. The IPCE values of the ZnS-uncoated TiO2/CdS(X) films increased as the CdS CBD cycles increased up to 8 times, whereas further increased CBD cycles (12 times) reduced the IPCE value. These results are in a good agreement with the short-circuit currents as shown in Fig. 5(a), and the details of which were described elsewhere [24]. The onset wavelength of the IPCEs in the TiO2/CdS(X) films without the ZnS coating red-shifted with the increased CBD cycles (or increased CdS QD’s size). The TiO2/CdS(X) with the ZnS overlayer had the same onset wavelength as the corresponding TiO2/CdS(X) without the ZnS, due to the wide band gap of ZnS. [19]. The ZnS coating significantly enhanced the IPCE values for all the TiO2/CdS(X) films. The light absorption of the TiO2/CdS(X) was unchanged even in the presence of the ZnS overlayer [Fig. 3(a)], implying the same LHE values regardless of the ZnS coating. The electron lifetime of Type I TiO2/CdS(3) film was significantly enhanced by the ZnS overlayer (Fig. 4), because the ZnS overlayer dominantly acted as the effective intermediate layer between the exposed TiO2 surface and the electrolyte, leading to the hindered back electron transfer from TiO2 to the electrolyte (pathway 2 in the Fig. 1). Therefore, the improved IPCE value of Type I TiO2/CdS(3) by the ZnS overlayer can be attributed to the increased charge-collection efficiency (hcc) caused by the significantly enhanced electron lifetime. The ZnS overlayer on Type II film (e.g. TiO2/CdS(8)) had little effect on the electron lifetime, as seen in Fig. 4, because the CdS QDs covering on the whole TiO2 surface acted as the intermediate layer instead. It indicates that the significantly improved IPCE value of Type II TiO2/ CdS(8) by the ZnS overlayer is no longer caused by the hcc value. The large-sized QDs in Type II reduce the size quantization, leading to the lowered conduction band minimum (CBM) and a raised valence band maximum (VBM) of the CdS QDs. [6,24,33]. The CBM of the large CdS QDs in Type II is located closer to the CBM of the TiO2 than that of the small QDs in Type I. It indicates the reduced driving force for the electron injection through the large-sized CdS QDs to TiO2 so that the recombination from the QDs to the electrolyte (pathway 1 in Fig. 1) may be much more facilitated, compared to the recombination from the small QDs. [33]. In this case, the ZnS overlayer formed at the interface between the largesized CdS QDs in Type II and the electrolyte can act as the much more effective energy barrier against the recombination pathway 1, owing to its much lower electron affinity (or higher CBM position) than that of the CdS. [6,9,21]. The energy barrier effect of the ZnS thus reduces the recombination rate from the large QDs to the electrolyte (pathway 1 in the Fig. 1), leading to enhanced electron injection efficiency (hinj) and the significantly improved IPCE of TiO2/CdS(8)/ZnS. Therefore, as the CdS CBD cycles increased, the morphologies of the TiO2/CdS(X) films changed from Type I to Type II. The ZnS overlayers coated on the TiO2/CdS(X) films may act as the energy barrier and the intermediate layer. However, the dominant roles of the ZnS overlayer were different from the Types I and II; the ZnS overlayers dominantly acted as the effective intermediate layer in the Type I and the energy barrier in the Type II. 4. Conclusions The ZnS overlayers were coated on the CdS QD-assembled TiO2 films, where the CdS QDs were grown on the TiO2 by varying the cycle number of the CdS chemical bath deposition (CBD). With increasing CdS CBD cycles from 3 to 12 times, the TiO2/CdS films transformed from Type I to the Type II, where Type I consists of the TiO2 surface partially covered by small CdS QDs and Type II features the TiO2 surface fully covered by large CdS QDs. The ZnS overlayers significantly improved the overall energy conversion efficiencies
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for all of the TiO2/CdS(X) films. Although the ZnS overlayers act as the intermediate layer and energy barrier at the interfaces, the dominant effects of the ZnS overlayers for the Types I and II were different. For Type I the ZnS overlayer covered the exposed TiO2 surface and dominantly acted as the effective intermediate layer between TiO2 and the electrolyte, leading to the reduced recombination rate from TiO2 to the electrolyte (pathway 2) and thus significantly enhanced charge-collection efficiency. On the other hand, the ZnS overlayer for Type II dominantly acted as the much more effective energy barrier between the CdS QDs and the electrolyte, because the large CdS QDs fully covering the TiO2 surface acted as the intermediate layer instead. The energy barrier effect of the ZnS overlayer on Type II suppressed the recombination rate through the CdS QDs to the electrolyte (pathway 1), leading to the enhanced electron injection efficiency. These results can potentially provide an insight to the QD-sensitized nanostructures coated by the semiconductor overlayers to advance their applications such as solar cells, catalysts, and photoelectrochemical cells.
Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant number 2010e0003968) and the Human Resources Development Program of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20104010100580) funded by the Korean Ministry of Knowledge Economy. References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] S. Ito, T. Kitamura, Y. Wada, S. Yanagida, Sol. Energy Mater. Sol. Cells 76 (2003) 3. [3] S.H. Kang, S.-H. Choi, M.-S. Kang, J.-Y. Kim, H.-S. Kim, T. Hyeon, Y.-E. Sung, Adv. Mater. 20 (2008) 54. [4] K.-S. Ahn, M.S. Kang, J.K. Lee, B.C. Shin, J.W. Lee, Appl. Phys. Lett. 89 (2006) 013103. [5] K.N. Mohammad, D.A. Filippo, F. Simona, S. Annabella, V. Guido, L. Paul, I. Seigo, T. Bessho, M. Grätzel, J. Am. Chem. Soc. 127 (2005) 16853. [6] G. Hodes, J. Phys. Chem. C 112 (2008) 17778.
[7] C.-H. Chang, Y.-L. Lee, Appl. Phys. Lett. 91 (2007) 053503. [8] W.J. Lee, S.-H. Kang, S.K. Min, Y.-E. Sung, S.-H. Han, Electrochem. Commun. 10 (2008) 1579. [9] Q. Shen, J. Kobayashi, L.J. Diguna, T. Toyoda, J. Appl, Phys 103 (2008) 084304. [10] S.-C. Lin, Y.-L. Lee, C.-H. Chang, Y.-J. Shen, Y.-M. Yang, Appl. Phys. Lett. 90 (2007) 143517. [11] X. Song, Y.-S. Fu, Y. Xie, J.-G. Song, H.-L. Wang, J. Sun, X.-W. Du, Semicond. Sci. Technol. 25 (2010) 045031. [12] W.-T. Sun, Y. Yu, H.-Y. Pan, X.-F. Gao, Q. Chen, L.-M. Peng, J. Am, Chem. Soc. 130 (2008) 1124. [13] X.-F. Gao, W.-T. Sun, Z.-D. Hu, G. Ai, Y.-L. Zhang, S. Feng, F. Li, L.-M. Peng, J. Phys. Chem. C 113 (2009) 20481. [14] X.-F. Gao, H.-B. Li, W.-T. Sun, Q. Chen, F.-Q. Tang, L.-M. Peng, J. Phys. Chem. C 113 (2009) 7531. [15] S.-Q. Fan, B. Fang, J.H. Kim, B. Jeong, C. Kim, J.-S. Yu, J.J. Ko, Langmuir 26 (2010) 13644. [16] S.-Q. Fan, B. Fang, J.H. Kim, J.-J. Kim, J.-S. Yu, J.J. Ko, Appl. Phys. Lett. 96 (2010) 063501. [17] I. Mora-Seró, S. Giménez, F. Fabregat-Santiago, R. Gómez, Q. Shen, T. Toyoda, J. Bisquert, Acc. Chem. Res. 42 (2009) 1848. [18] Z. Liu, M. Miyauchi, Y. Uemura, Y. Cui, K. Hara, Z. Zhao, K. Sunahara, A. Furube, Appl. Phys. Lett. 96 (2010) 233107. [19] S.-M. Yang, C.-H. Huang, J. Zhai, Z.-S. Wang, L. Jiang, J. Mater, Chem 12 (2002) 1459. [20] S. Giménez, I. Mora-Seró, L. Macor, N. Guijarro, T. Lana-Villarreal, R. Gómez, L.J. Diguna, Q. Shen, T. Toyoda, J. Bisquert, Nanotechnol 20 (2009) 295204. [21] L.J. Diguna, Q. Shen, J. Kobayashi, T. Toyoda, Appl. Phys. Lett. 91 (2007) 023116. [22] Y.-L. Lee, C.-F. Chi, S.-Y. Liau, Chem. Mater. 22 (2010) 922. [23] N. Guijarro, T. Lana-Villarreal, I. Mora-Seró, J. Bisquert, R. Gómez, J. Phys. Chem. C 113 (2009) 4208. [24] S.W. Jung, J.-H. Kim, H. Kim, C.-J. Choi, K.-S. Ahn, J. Appl. Phys. 110 (2011) 044313. [25] Y.-L. Lee, C.-H. Chang, J. Power Sources 185 (2008) 584. [26] G. Zhu, Z. Cheng, T. Lv, L. Pan, Q. Zhao, Z. Sun, Nanoscale 2 (2010) 1229. [27] S. Huang, Q. Zhang, X. Huang, X. Guo, M. Deng, D. Li, Y. Luo, Q. Shen, T. Toyoda, Q. Meng, Nanotechnol 21 (2010) 3752021. [28] M.G. Bawendi, A.R. Kortan, M.L. Steigerwald, L.E. Brus, J. Chem. Phys. 91 (1989) 7282. [29] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 6 (2006) 215. [30] A. Zaban, M. Greenshtein, J. Bisquert, ChemPhysChem 4 (2003) 859. [31] M.D. Archer, A.J. Nozik, Nanostructured and Photoelectrochemical Systems For Solar Photon Conversion, in: R.J.D. Miller, R. Memming (Eds.), Chapter 2, Imperial College Press, Singapore, 2008, p. 130. [32] J.-H. Park, J.-Y. Kim, J.-H. Kim, C.-J. Choi, H.S. Kim, Y.-E. Sung, K.-S. Ahn, J. Power Sources 196 (2011) 8904. [33] I. Robel, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 129 (2007) 4136. [34] B.C. O’Regan, J.R. Durrant, P.M. Sommeling, N.J. Bakker, J. Phys. Chem. C 111 (2007) 14001. [35] Z.-S. Wang, T. Yamaguchi, H. Sugihara, H. Arakawa, Langmuir 21 (2005) 4272.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
ZnS overlayer on in situ chemical bath deposited CdS quantum dot-assembled TiO2 films for quantum dot-sensitized solar cells Sung Woo Jung a, Jae-Hong Kim a, Hyunsoo Kim b, *,1, Chel-Jong Choi b, Kwang-Soon Ahn a, **,1 a b
School of Chemical Engineering, Yeungnam University, Dae-dong, Gyeongsan 712-749, Republic of Korea School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2011 Received in revised form 12 March 2012 Accepted 7 April 2012 Available online 19 April 2012
ZnS overlayers were deposited on the CdS quantum dot (QD)-assembled TiO2 films, where the CdS QDs were grown on the TiO2 by repeated cycles of the in situ chemical bath deposition (CBD). With increasing the CdS CBD cycles, the CdS QD-assembled TiO2 films were transformed from the TiO2 film partially covered by small CdS QDs (Type I) to that fully covered by large CdS QDs (Type II). The ZnS overlayers significantly improved the overall energy conversion efficiency of both Types I and II. The ZnS overlayers can act as the intermediate layer and energy barrier at the interfaces. However, the dominant effects of the ZnS overlayers were different for the Types I and II. For Type I, ZnS overlayer dominantly acted as the intermediate layer between the exposed TiO2 surface and the electrolyte, leading to the suppressed recombination rate for the TiO2/electrolyte and the significantly enhanced charge-collection efficiency. On the contrary, for Type II, it dominantly acted as the efficient energy barrier at the interface between the CdS QDs and the electrolyte, leading to the hindered recombination rate from the large CdS QDs to the electrolyte and thus enhanced electron injection efficiency. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Quantum dot-sensitized solar cell Recombination Charge-collection efficiency Electron injection efficiency Intermediate layer Energy barrier
1. Introduction Dye-sensitized solar cells (DSSCs) based on mesoporous TiO2 films are potential low-cost alternatives to commercial Si-based solar cells [1e4]. They are composed of a dye-sensitized mesoporous TiO2 film and a Pt catalytic counter electrode, with an electrolyte containing a redox couple (I/I 3 ) between the two. Dye molecules adsorbed on TiO2 can be excited by light absorption. Electrons photo-excited in the dye molecules can then be injected into the conduction band of TiO2 and transferred to transparent conducting oxide (TCO) for use in an external circuit. The use of a ruthenium complex photo-sensitizer has reached an energy conversion efficiency of about 11% [5]. Semiconducting quantum dots (QDs) [CdS, CdSe, PbS, InP, etc.] have been tested as photo-sensitizers for DSSCs [6e16] because (1) their optical band gaps can be readily tuned through quantum confinement effects by varying their sizes [6,7,12e14], (2) they have much higher extinction coefficients than conventional dyes due to
* Corresponding author. ** Corresponding author. Tel.: þ82 53 810 2524; fax: þ82 53 810 4631. E-mail addresses: [email protected] (H. Kim), [email protected] (K.-S. Ahn). 1 Both of Prof. K.-S. Ahn and H. Kim contributed equally to this work as the corresponding authors. 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2012.04.012
the multiple energy levels [6], and (3) they are robust, inorganic materials [6e9]. However, despite these advantages, quantum dotsensitized solar cells (QD-SSCs) have low energy conversion efficiencies (reaching 2.5% for CdS QD-SSCs) [6,9], due to several possible recombination routes at the interfaces of QD/TiO2/electrolyte [6,9,17,18]. Fig. 1 shows the schematic energy diagrams of the redox couple in the electrolyte, QD and TiO2. The solid arrows represent desirable, forward charge transfer and the dotted arrows show several recombination routes; where pathways 1, 2, and 3 are the back electron transfers from QDs to the redox couple, from TiO2 to the redox couple, and from TiO2 to QDs, respectively, and pathway 4 is the inner recombination of photo-excited electrons in QDs. Furthermore, these recombination pathways are more facilitated by the defect traps existing in QDs and TiO2, which led to much quicker electronehole recombination in QD-SSCs (1011 to 106 s) than in DSSCs (107 to 104 s) [6]. The ZnS overlayer on the QD-assembled TiO2 films has been demonstrated to improve the overall energy conversion efficiency of the QD-SSCs, because the ZnS has the conduction band minimum higher than the CdS, leading to the effective energy barrier and passivation effects [6,9,17,19e22]. However, more detailed studies on the ZnS overlayer remain poorly understood [6,17]. Direct deposition of QDs on the TiO2 surface using an in situ chemical bath deposition (CBD) has resulted in superior electron injection and better coverage than indirect contact of the
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Polysulfide electrolyte containing 0.5 M Na2S, 2 M S, and 0.2 M KCl was prepared in water/methanol (3:7 by volume) solution [24e26]. The resulting cells’ photovoltaic currentevoltage characteristics were measured under 1 Sun illumination (100 mW cm2, AM 1.5) verified by an AIST-calibrated Si-solar cell. For open circuit voltage decay (OCVD) measurement, cells were illuminated to a steady voltage and the decay of the open circuit voltage was then measured after the illumination was cut off by a shutter. Incident photon-to-current conversion efficiency (IPCE) was measured with an action spectrum measurement setup (PEC-S20, Peccell Ltd.). The QDs’ sizes and coverage on the TiO2 surface were estimated from UVevis absorption spectra (Cary5000, Agilent Tech. Co.). Fig. 1. Schematic energy diagram of the redox couple, quantum dot, and TiO2; (a) desired charge transfer (solid arrows) and recombination pathways (dotted arrows).
pre-synthesized QDs linked by functional molecular linkers on TiO2 [7e10,23]. However, it has the disadvantage of being difficult to separately control the QDs’ coverage and size [23] which are simultaneously increased with the increased CBD cycles. Therefore, as the CBD cycles increased, the CdS QD-assembled TiO2 films were transformed from TiO2 films assembled by small QDs with low coverage to films assembled by large QDs with high coverage, where the former and latter are denoted as the Types I and II, respectively [7,10,24]. Consequently Type I features the TiO2 surface partially covered by small QDs, leaving the exposed TiO2 surface. In contrast, Type II consists of TiO2 fully covered by large-sized QDs without exposure of the TiO2 surface. In this paper, the CdS QDs were in situ grown on the mesoporous TiO2 films by repeated CdS CBD procedure (3, 5, 8, and 12 cycles), whose surfaces were then coated with the ZnS overlayer. We report here that the dominant effects of the ZnS overlayers were different for the Types I and II films and hence resulted in enhanced overall energy conversion efficiency. Their properties, including electron lifetime, driving force for the electron injection, recombination pathways, energy barrier, charge-collection efficiency, and electron injection efficiency, were systematically studied.
3. Results and discussion The QDs were deposited by in situ CBD process, which has the disadvantage of being difficult to separately control the QD’s size and coverage [23]. It indicates that, as the cycle number increases, the QD’s size increases together with the increased coverage of the QDs on the TiO2 surface. Fig. 2 shows (aec) the SEM top-viewed images of bare TiO2, TiO2/CdS(3), and TiO2/CdS(8), respectively. It clearly shows that the CdS QD’s size and coverage were simultaneously increased with increasing the cycle number. The TiO2/CdS films with and without the ZnS overlayer had no apparent change in the SEM surface morphology (not shown here), because the ZnS layer was deposited on the TiO2/CdS(X) films only by one CBD cycle, leading to the ZnS monolayer. The ZnS overlayer was investigated by the energy-dispersive X-ray spectroscopy (EDS). Fig. 2(d) shows the EDS spectrum of the TiO2/CdS(8)/ZnS, which exhibited the ZnS overlayer deposited in the 1.56 at.%. Fig. 3(a) shows optical absorbance curves of TiO2/CdS(X) and TiO2/CdS(8)/ZnS, where X is the CBD cycle number. TiO2 absorbs only UV light with wavelengths less than 370 nm due to its wide band gap (3.4 eV) [1e4]. The absorption edge of TiO2/CdS(X) redshifted with the increased cycles of the CdS CBD, indicating the increased CdS QD’s size. The Brus equation was used to estimate the average diameters of the CdS QDs: [8,27].
" # h2 1 1 1:8e2 ¼ Eg þ 2 þ 4p3 3 0 r 8r m*e mh
2. Experimental details
EQD
Mesoporous TiO2 films were prepared by doctor-blading TiO2 paste (Ti-Nanoxide T/SP, Solaronix SA) onto fluorine-doped tin oxide (FTO) substrates, followed by sintering at 450 C for 30 min. The films were deposited to be 10 mm thick, as measured by stylus profilometry. The cells’ active areas were 0.16 cm2. Ethanol and methanol were used to dissolve Cd(NO3)2 and Na2S, respectively, for the synthesis of the CdS QDs. In situ chemical bath deposition (CBD) involved immersing the mesoporous TiO2 films in 0.5 M Cd(NO3)2 solution for 5 min, rinsing with ethanol, and subsequent 5 min immersion in 0.5 M Na2S solution followed by further rinsing with methanol [24]. Each series of two immersions was considered as one CBD cycle and the experiments were performed for3, 5, 8, and 12 cycles to vary the size of the CdS QDs assembled on the TiO2 surface which is denoted by TiO2/CdS(X) with X being the CBD cycle number. Finally, the ZnS overlayer was then coated on the CdS QD-assembled TiO2 films by immersing alternately into 0.5 M Zn(CH3COO)2 and 0.5 M Na2S solution each for 5 min, which is then referred to as TiO2/CdS(X)/ZnS [9,19,20]. Semitransparent Pt counter-electrodes were prepared by doctor-blading Pt nanocluster-containing Pt paste (PT-1, Dyesol. Ltd.) onto FTO transparent conducting substrates followed by calcination at 450 C for 30 min in air. The TiO2/CdS(X) photoanodes both with and without the ZnS overlayer and the Pt-coated counter-electrodes were sandwiched by 60 mm-thick seals.
where EQD and Eg are the lowest excitation energy of the QDs and the band gap of bulk CdS, respectively. r is the radius of the QDs. me and mh are the effective masses of electrons and holes in CdS, respectively, 3 0 is the vacuum permittivity, and 3 is the relative permittivity of CdS. [24,27,28]. The optical band gaps of QDs were estimated by intersecting the base line and shoulder line, as shown in Fig. 3(a). Average QD sizes were calculated to be 4.4, 4.9, 5.4, and 6.2 nm in diameter, respectively, for the TiO2/CdS(3), TiO2/CdS(5), TiO2/CdS(8), and TiO2/CdS(12). As the CBD cycles increased, the coverage of QDs on the TiO2 surface is simultaneously improved with the increased QDs’ size [7,10,23,24]. Our previous work [24] showed that, with increasing the CdS CBD cycles, the surface morphologies of the CdS QD-assembled TiO2 films were transformed from Type I to the Type II, where Type I is composed of TiO2 partially covered by small QDs, whilst Type II consists of TiO2 fully covered by large-sized QDs without any exposed TiO2 surface. The ZnS overlayers were then coated on all of the TiO2/CdS(X) films, and their optical absorption curves were also measured. The absorbance of the TiO2/CdS(3) was gradually increased from the absorbance edge to the shorter wavelengths, whereas the TiO2/CdS(8) exhibited sharply increased, saturated absorbance curve. The SEM images and absorbance data of the Figs. 2 and 3 indicate that the surface area of the TiO2 in TiO2/CdS(3) was not fully covered by the QDs and the surface area of the TiO2 in TiO2/CdS(8) was almost fully
(1)
S.W. Jung et al. / Current Applied Physics 12 (2012) 1459e1464
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Fig. 2. (aec) the SEM top-viewed images of bare TiO2, TiO2/CdS(3), and TiO2/CdS(8), respectively. (d) The energy-dispersive X-ray spectroscopy (EDS) spectrum of the TiO2/CdS(8)/ ZnS.
covered so that the TiO2/CdS(3) and TiO2/CdS(8) were classified as the Types I and II, respectively. Fig. 3(b) illustrates the schematics of the ZnS overlayers coated on (upper) Type I and (lower) Type II, respectively. The dominant effects of the ZnS overlayer may be significantly influenced by the difference in the morphology of the CdS QD-assembled TiO2 films, which will be discussed later. The optical absorbance of the TiO2/CdS(8)/ZnS was the same as that of the TiO2/CdS(8), as seen in Fig. 3(a), due to wide band gap (3.6 eV) of ZnS.19 The other ZnS-coated TiO2/CdS(X) films had the same absorption curves as the corresponding ZnS-uncoated TiO2/CdS(X) (not shown here). Electron lifetimes on the TiO2/CdS(X) films with and without the ZnS overlayer were estimated through open circuit voltage decay (OCVD) measurements [29,30]. Fig. 4(a) exhibits open circuit voltage (Voc) decay curves of TiO2/CdS(3) and TiO2/CdS(8) with and without the ZnS overlayer recorded during the relaxation from an illuminated quasi-equilibrium state to darkness. Fig. 4(b) plots the
electron lifetimes calculated from the Voc decays according to: [17,18,29,30]:
kB T dVoc 1 e dt
se ¼
(2)
where kBT, e, and dVoc/dt are the thermal energy, the positive elementary charge, and the derivative of the open circuit voltage transient, respectively. The open circuit voltage is determined by the difference in potential between the Fermi level of TiO2 and the redox couple. The photovoltage decay rate is directly related to electron lifetime, because excess electrons in the conduction band of TiO2 are removed by recombining with the redox couple in the electrolyte when the illumination at open circuit is interrupted [18,30]. The electrons in the CdS QDs may be recombined by the electrolyte. In this experiment, the electron lifetimes were estimated by the open circuit voltage (Voc) decay curves. The Voc is the potential difference
Fig. 3. (a) Absorbance curves of TiO2/CdS(X) with and without the ZnS overlayer, where X represents the cycle number of the CdS CBD. (b) Schematics of the ZnS overlayers coated on Type I (upper) and Type II (lower), respectively.
a
0.5
TiO2/CdS(3) TiO2/CdS(3)/ZnS TiO2/CdS(8) TiO2/CdS(8)/ZnS
Light off
(V)
0.4
V
oc
0.3 0.2 0.1 2
3
4
5
Time (s)
b
7
100
10 9
TiO2 /CdS(3) TiO2 /CdS(3)/ZnS
8
TiO2 /CdS(5)
7
TiO2 /CdS(5)/ZnS
6
TiO2 /CdS(8) TiO2 /CdS(8)/ZnS
5
TiO2 /CdS(12)
4
TiO2 /CdS(12)/ZnS
3 2 1 0 0.0
8
0.1
0.2
0.3
0.4
0.5
0.6
Potential (V) TiO2/CdS(3) TiO2/CdS(3)/ZnS TiO2/CdS(8) TiO2/CdS(8)/ZnS
101
Lifetime (s)
6
a
Current density (mAcm-2)
S.W. Jung et al. / Current Applied Physics 12 (2012) 1459e1464
-1
10
b
60 TiO2/CdS(3) TiO2/CdS(3)/ZnS
50
TiO2/CdS(5)
IPCE (%)
1462
TiO2/CdS(5)/ZnS
40
TiO2/CdS(8) TiO2/CdS(8)/ZnS
30
TiO2/CdS(12) TiO2/CdS(12)/ZnS
20 10
-2
10
0.0
0.1
0.2
0.3 V
oc
0.4
0.5
(V)
Fig. 4. (a) Voc decay curves of the TiO2/CdS(3) and TiO2/CdS(8) films with and without the ZnS overlayer recorded during the relaxation from illuminated quasi-equilibrium to the dark. (b) Electron lifetimes estimated from (a).
between the quasi-Fermi level of the TiO2 and the redox couple in the electrolyte, indicating that the Voc decay is related to the decreased number of the electrons in the conduction band of the TiO2 which lower the quasi-Fermi level. M. Miyauchi’s group [18] reported that the charge recombination from the TiO2 to the electrolyte is mainly responsible for the Voc decay of the QD-SSCs. They measured the voltage transients under the dark condition by switching the applied bias to zero and compared these results with the Voc decay curves under the light condition. They found that the Voc decay and electron lifetimes of the QD-SSCs do not differ from the dark condition to the light condition, indicating that the electron lifetimes estimated by the Voc decay curves were mainly caused by the charge recombination from the TiO2 to the electrolyte. The TiO2/ CdS(3) film exhibited significantly enhanced electron lifetime after the ZnS overlayer was coated, compared to the uncoated-TiO2/ CdS(3) film. The absorbance spectrum of the TiO2/CdS(3) as seen in Fig. 3(a) exhibited low light absorption as well as small QD’s sizes (with an averaged diameter of 4.4 nm), indicating the presence of Type I where the TiO2 surface is only partially covered and it also partly contacts with the electrolyte, leading to the interfacial contact between the two. Furthermore, mesoporous TiO2 films composed of nanoparticles smaller than 25 nm do not develop a depletion layer at the interface between TiO2 and electrolyte, indicating that the electrons in the conduction band of TiO2 move according to the diffusion process rather than the faster drift mechanism [1e4,31,32]. Therefore, the exposed TiO2 surface in Type I facilitates the recombination of TiO2 and the electrolyte (pathway 2 in Fig. 1), which is
0 300
400
500
600
700
Wavelength (nm) Fig. 5. (a) Photovoltaic currentevoltage curves of QD-SSCs with the TiO2/CdS(X) and TiO2/CdS(X)/ZnS measured under 1 Sun illumination. (b) The QD-SSCs’ incident photon-to-current conversion efficiency (IPCE).
responsible for its poor electron lifetime. The ZnS overlayer coated on the TiO2/CdS(3) Type I film exhibited significantly enhanced electron lifetime, indicating that the ZnS overlayer covered the exposed TiO2 surface and acted as the intermediate layer which hindered the back electron transfer (or recombination) from the conduction band of TiO2 to the redox couple (pathway 2 in Fig. 1). In contrast, the electron lifetime of the TiO2/CdS(8) film was much higher than that of the TiO2/CdS(3) film. This is becauseTiO2/CdS(8) is much closer to Type II than Type I, owing to the increased QD’s size and better coverage on the TiO2 surface at the higher CdS CBD cycle. That is, the CdS QDs in the TiO2/CdS(8) almost fully cover the TiO2 surface and hence there is only a smaller interfacial contact area between TiO2 and the electrolyte, resulting in significantly improved electron lifetime, compared to TiO2/CdS(3). The ZnS overlayer coated on TiO2/CdS(8) exhibited only slightly increased electron lifetime, compared to the TiO2/CdS(8) without the ZnS overlayer, indicating that the ZnS overlayer had little effect as the intermediate layer for the Type II, unlike the ZnS overlayer’s effect for the Type I. It is because the CdS QDs of the Type II fully covered the TiO2 surface and acted as the intermediate layer between the TiO2 and the electrolyte interface regardless of the ZnS overlayer. The ZnS overlayers act as the energy barrier as well as the intermediate layer in the Types I and II. However, the dominant effect may differ for the Types I and II. The Types I and II had two different features relating to the QD’s size and coverage. The QD’s size is much smaller in the Type I, leading to the enhanced size quantization. V. Kamat’s group [33] reported that the electron injection rate from the small QDs to the TiO2 was much faster than
S.W. Jung et al. / Current Applied Physics 12 (2012) 1459e1464 Table 1 The photovoltaic properties of QD-SSCs based on the CdS QDs with and without the ZnS overlayer. Samples
Jsc / mA cm2
Voc / V
FF / %
h/%
TiO2/CdS(3) TiO2/CdS(3)/ZnS TiO2/CdS(5) TiO2/CdS(5)/ZnS TiO2/CdS(8) TiO2/CdS(8)/ZnS TiO2/CdS(12) TiO2/CdS(12)/ZnS
3.519 4.038 4.519 5.738 6.694 7.813 5.494 6.288
0.41 0.44 0.44 0.48 0.48 0.49 0.51 0.51
45.96 46.55 42.69 43.54 40.04 44.83 37.48 40.69
0.66 0.83 0.85 1.20 1.29 1.72 1.05 1.30
that from the large-sized QDs to the TiO2, due to higher driving force for the electron injection from the conduction band of the QDs to the TiO2. The TiO2 surface in the Type I was not fully covered by the CdS QDs, indicating that the dominant role of the ZnS layer in the Type I may be the intermediate layer rather than the energy barrier effect. In contrast, the TiO2 surface in the Type II was almost fully covered by the CdS QDs. The QD’s size in the Type II was much larger, leading to the reduced size quantization. The lowered conduction band minimum (CBM) of the large-sized QDs reduces the driving force for the electron injection from the CdS QDs to the TiO2. Therefore, the dominant effect of the ZnS overlayer in the Type II may be the energy barrier effect among the two effects. Fig. 5(a) presents the photovoltaic currentevoltage (IeV) performances of the QD-SSCs under 1 Sun illumination, which are summarized in Table 1. The overall energy conversion efficiency (h) of QD-SSCs with the ZnS-uncoated TiO2/CdS(X) increased as the CdS CBD cycles increased up to 8 times, due to the increased amount of electrons injected from the increased amount of assembled QDs and enhanced light absorption in the long wavelengths. However, further increased CdS CBD cycle reduced the cell performance. It can be attributed to the poor charge-injection efficiency caused by (1) the reduced size quantization of the large QDs leading to the reduced driving force for carrier (electrons and holes) injection, (2) the increased number of recombination traps in the large QDs themselves, and (3) hindered ion transport of the redox couple caused by blocked pores [24]. The ZnS overlayer improved all of the parameters (short-circuit current (Jsc), open circuit voltage (Voc), and fill factor (FF)) for all the TiO2/CdS(X) films, resulting in the significantly enhanced overall energy conversion efficiency. Fig. 4 showed that the ZnS overlayer for Type I TiO2/CdS(3) acted as the efficient intermediate layer between the exposed TiO2 surface and the electrolyte, giving rise to the significantly enhanced electron lifetime. On the contrary, the ZnS overlayer influenced the electron lifetime of Type II TiO2/CdS(8) to a much smaller extent, due to the full coverage of the TiO2 surface. Nevertheless, Fig. 5(a) shows that QD-SSC with ZnS-coated Type II TiO2/CdS(8) exhibited dramatically improved cell efficiency of 1.72%, which was an improvement of approximately 30% over that with uncoated Type II TiO2/CdS(8). It therefore indicates that the ZnS overlayer had a different dominant role in Type II TiO2/CdS(8), which will be discussed later. The efficiency gain by the ZnS overlayer was mainly due to the increased Jsc, which was studied in more detail using the incident photon-to-current conversion efficiency (IPCE) curves [Fig. 5(b)]. IPCE is the product of lightharvesting efficiency, charge-injection efficiency, and chargecollection efficiency [24,34,35]:
IPCE ¼ LHE$hinj $hcc
(3)
where LHE is the light-harvesting efficiency, hinj is the chargeinjection efficiency, and hcc is the charge-collection efficiency. LHE is determined from the amount of attached photo-sensitizer, light scattering, and the concentration of the redox species; hcc is by the competition between recombination rate and charge
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transport; and hinj is by the driving forces related to the energy levels of TiO2, QDs, and the redox couple and trap-mediated electronehole recombination through QDs. [6,33]. The IPCE values of the ZnS-uncoated TiO2/CdS(X) films increased as the CdS CBD cycles increased up to 8 times, whereas further increased CBD cycles (12 times) reduced the IPCE value. These results are in a good agreement with the short-circuit currents as shown in Fig. 5(a), and the details of which were described elsewhere [24]. The onset wavelength of the IPCEs in the TiO2/CdS(X) films without the ZnS coating red-shifted with the increased CBD cycles (or increased CdS QD’s size). The TiO2/CdS(X) with the ZnS overlayer had the same onset wavelength as the corresponding TiO2/CdS(X) without the ZnS, due to the wide band gap of ZnS. [19]. The ZnS coating significantly enhanced the IPCE values for all the TiO2/CdS(X) films. The light absorption of the TiO2/CdS(X) was unchanged even in the presence of the ZnS overlayer [Fig. 3(a)], implying the same LHE values regardless of the ZnS coating. The electron lifetime of Type I TiO2/CdS(3) film was significantly enhanced by the ZnS overlayer (Fig. 4), because the ZnS overlayer dominantly acted as the effective intermediate layer between the exposed TiO2 surface and the electrolyte, leading to the hindered back electron transfer from TiO2 to the electrolyte (pathway 2 in the Fig. 1). Therefore, the improved IPCE value of Type I TiO2/CdS(3) by the ZnS overlayer can be attributed to the increased charge-collection efficiency (hcc) caused by the significantly enhanced electron lifetime. The ZnS overlayer on Type II film (e.g. TiO2/CdS(8)) had little effect on the electron lifetime, as seen in Fig. 4, because the CdS QDs covering on the whole TiO2 surface acted as the intermediate layer instead. It indicates that the significantly improved IPCE value of Type II TiO2/ CdS(8) by the ZnS overlayer is no longer caused by the hcc value. The large-sized QDs in Type II reduce the size quantization, leading to the lowered conduction band minimum (CBM) and a raised valence band maximum (VBM) of the CdS QDs. [6,24,33]. The CBM of the large CdS QDs in Type II is located closer to the CBM of the TiO2 than that of the small QDs in Type I. It indicates the reduced driving force for the electron injection through the large-sized CdS QDs to TiO2 so that the recombination from the QDs to the electrolyte (pathway 1 in Fig. 1) may be much more facilitated, compared to the recombination from the small QDs. [33]. In this case, the ZnS overlayer formed at the interface between the largesized CdS QDs in Type II and the electrolyte can act as the much more effective energy barrier against the recombination pathway 1, owing to its much lower electron affinity (or higher CBM position) than that of the CdS. [6,9,21]. The energy barrier effect of the ZnS thus reduces the recombination rate from the large QDs to the electrolyte (pathway 1 in the Fig. 1), leading to enhanced electron injection efficiency (hinj) and the significantly improved IPCE of TiO2/CdS(8)/ZnS. Therefore, as the CdS CBD cycles increased, the morphologies of the TiO2/CdS(X) films changed from Type I to Type II. The ZnS overlayers coated on the TiO2/CdS(X) films may act as the energy barrier and the intermediate layer. However, the dominant roles of the ZnS overlayer were different from the Types I and II; the ZnS overlayers dominantly acted as the effective intermediate layer in the Type I and the energy barrier in the Type II. 4. Conclusions The ZnS overlayers were coated on the CdS QD-assembled TiO2 films, where the CdS QDs were grown on the TiO2 by varying the cycle number of the CdS chemical bath deposition (CBD). With increasing CdS CBD cycles from 3 to 12 times, the TiO2/CdS films transformed from Type I to the Type II, where Type I consists of the TiO2 surface partially covered by small CdS QDs and Type II features the TiO2 surface fully covered by large CdS QDs. The ZnS overlayers significantly improved the overall energy conversion efficiencies
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for all of the TiO2/CdS(X) films. Although the ZnS overlayers act as the intermediate layer and energy barrier at the interfaces, the dominant effects of the ZnS overlayers for the Types I and II were different. For Type I the ZnS overlayer covered the exposed TiO2 surface and dominantly acted as the effective intermediate layer between TiO2 and the electrolyte, leading to the reduced recombination rate from TiO2 to the electrolyte (pathway 2) and thus significantly enhanced charge-collection efficiency. On the other hand, the ZnS overlayer for Type II dominantly acted as the much more effective energy barrier between the CdS QDs and the electrolyte, because the large CdS QDs fully covering the TiO2 surface acted as the intermediate layer instead. The energy barrier effect of the ZnS overlayer on Type II suppressed the recombination rate through the CdS QDs to the electrolyte (pathway 1), leading to the enhanced electron injection efficiency. These results can potentially provide an insight to the QD-sensitized nanostructures coated by the semiconductor overlayers to advance their applications such as solar cells, catalysts, and photoelectrochemical cells.
Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant number 2010e0003968) and the Human Resources Development Program of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20104010100580) funded by the Korean Ministry of Knowledge Economy. References [1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737. [2] S. Ito, T. Kitamura, Y. Wada, S. Yanagida, Sol. Energy Mater. Sol. Cells 76 (2003) 3. [3] S.H. Kang, S.-H. Choi, M.-S. Kang, J.-Y. Kim, H.-S. Kim, T. Hyeon, Y.-E. Sung, Adv. Mater. 20 (2008) 54. [4] K.-S. Ahn, M.S. Kang, J.K. Lee, B.C. Shin, J.W. Lee, Appl. Phys. Lett. 89 (2006) 013103. [5] K.N. Mohammad, D.A. Filippo, F. Simona, S. Annabella, V. Guido, L. Paul, I. Seigo, T. Bessho, M. Grätzel, J. Am. Chem. Soc. 127 (2005) 16853. [6] G. Hodes, J. Phys. Chem. C 112 (2008) 17778.
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