Optical Absorption and Photocurrent Spectra of CdSe Quantum Dots Adsorbed on Nanocrystalline TiO 2 Electrode Together with Photovoltaic Properties 481 Fig. 2. Photoacoustic spectra of nanostructured TiO 2 electrodes adsorbed with combined CdS/CdSe quantum dots for different adsorption times together with that adsorbed wth CdS quantum dots only (modulation frequency: 33 Hz). Fig. 3. Photoacoustic spectra of nanostructured TiO 2 electrodes adsorbed with CdSe quantum dots without a preadsorbed CdS quantum dot layer for different adsorption times (modulation frequency: 33 Hz). Solar Cells – New Aspects and Solutions 482 Fig. 4. Dependence of the average diameter on the adsorption time, both of combined CdS/CdSe (●) and CdSe without a preadsorbed CdS layer (○). Figure 5 shows the IPCE spectra of the nanostructured TiO 2 electrodes adsorbed with com- bined CdS/CdSe QDs for different adsorption times, together with that adsorbed with CdS QDs only. The pre-adsorption time of CdS QD layer is fixed at 40 min. Photoelectrochemical current in the visible light region due to the adsorbed CdSe QDs can be observed, indicating the photosensitization by combined CdS/CdSe QDs. With increasing adsorption time, the red- shift of photoelectrochemical current can be clearly observed, implying the growth of CdSe QDs. The IPCE peak value increases with the increase of adsorption time up to 8 hours (~ 75%), then decreases until 24 h adsorption owing to the increase of recombination centers or interface states, together with the decrease of energy difference between LUMO in CdSe QDs and the bottom of conduction band of TiO 2 . Also, the comparison between the adsorption of CdSe QDs on the TiO 2 electrodes with and without a pre-adsorbed CdS QD layer was carried out to evaluate the difference in IPCE spectra. For that, Figure 5 shows the IPCE spectra of the nanostructured TiO 2 electrodes adsorbed with CdSe QDs without a pre-adsorbed layer of CdS QD layer for different adsorption times. Photoelectrochemical current in the visible light region due to the adsorbed CdSe QDs can be observed in Fig. 6, also indicating the photosensitization by CdSe QDs. With increasing adsorption time, the red-shift of photoelectrochemical current can be clearly observed, implying the growth of CdSe QDs. However, the appearance of the spectrum in Fig. 6 is different from that of combined CdS/CdSe QDs, namely in the reduction of maximum IPCE value (~ 60%) and the adsorption time dependence of the spectrum shape. Also, the IPCE spectra below the CdSe QDs adsorption time of 8 h agree with that of pure nanostructured TiO 2 electrode within the experimental accuracy, indicating that the CdSe QDs adsorbed on the nanostructured TiO 2 electrode without a pre-adsorbed CdS layer show very slow growth or no growth similar to the results of PA characterization in Fig. 3. These results demonstrate that the spectral response of IPCE is enhanced upon combined CdS/CdSe sensitization rather than single CdSe QDs sensitization, indicating the possibility of the reduction in recombination centers and interface states owing to the possibilities of active CdSe QDs by the excess Cd remaining after CdS adsorption and passivation effect of CdS QDs on the nanostructured TiO 2 surface. Optical Absorption and Photocurrent Spectra of CdSe Quantum Dots Adsorbed on Nanocrystalline TiO 2 Electrode Together with Photovoltaic Properties 483 Fig. 5. IPCE spectra of nanostructured TiO 2 electrodes adsorbed with CdS/CdSe quantum dots for different adsorption times together with that adsorbed with CdS quantum dots only. Fig. 6. IPCE spectra of nanostructured TiO 2 electrodes adsorbed with CdSe quantum dots without a preadsorbed CdS QD layer for different adsorption times. The photocurrent-voltage cureves of (a) combined CdS/CdSe QD- and (b) CdSe QD- sensitized solar cells are shown in Fig. 7 (a) and (b), respectively, for different adsorption times, together with that obtained with cells adsorbed with CdS only. However, the Solar Cells – New Aspects and Solutions 484 appearance of the current-voltage curves of combined CdS/CdSe QD-sensitized solar cells is different from those of CdSe QD-sensitized solar cells. Figure 8 and 9 illustrates the photovoltaic parameters ((a) J sc ; (b) V oc ; (c) FF; (d) η) of combined CdS/CdSe QD-sensitized (●) and CdSe QD-sensitized (○) solar cells as a function of CdSe QDs adsorption times. Fig. 7. Photocurrent-voltage curves of (a) combined CdS/CdSe quantm dot- and (b) CdSe quantum dot-sensitized solar cells for different adsorption times together with that adsorbed with CdS quantum dots only. We observe that the parameter of J sc in combined CdS/CdSe QD-sensitized solar cells increases with the increase of CdSe QDs adsorption times up to 8 h. On the other hand, V oc and FF are independent of adsorption times. The performance of solar cells improved with Optical Absorption and Photocurrent Spectra of CdSe Quantum Dots Adsorbed on Nanocrystalline TiO 2 Electrode Together with Photovoltaic Properties 485 an increase in adsorption time up to 8 h due, mainly, not only to the increase of the amount of CdSe QDs but the improvement in crystal quality and decrease of interface states. However, the increase in adsorption times after more than 8 h leads to deterioration in J sc and V oc . High adsorption time of CdSe QDs might cause an increase in recombination centers, poor penetration of CdSe QDs, and the decrease of energy difference between LUMO in CdSe QDs and the bottom of conduction band of TiO 2 . Therefore, η of the combined CdS/CdSe QD- sensitized solar cell shows a maximum of 3.5% at 8 h adsorption times. On the other hand, J sc and η below the CdSe QDs adsorption time of 8 h without a pre-adsorbed CdS layer show very small values close to zero, indicating the very small amount of CdSe QDs adsorption similar to the results of PA and IPCE characterization. We can observe that J sc , V oc , FF, and η in Fig. 8. Dependence of the photovoltaic parameters ((a) J sc and (b) V oc ) on the adsorption time, both of combined CdS/CdSe (●) and CdSe without a preadsorbed CdS QD layer (○). Solar Cells – New Aspects and Solutions 486 Fig. 9. Dependence of the photovoltaic parameters( (c) FF and (d) η ) on the adsorption time, both of combined CdS/CdSe (●) and CdSe without a preadsorbed CdS QD layer (○). CdSe QD-sensitized solar cells without a pre-adsorbed CdS QD layer increase with the increase of adsorption times up to 24 h, indicating the difference of the crystal growth and the formation of recombination centers in combined CdS/CdSe and CdSe QDs. Figure 9 shows the preliminary ultrafast photoexcited carrier dynamics characterization of combined CdS/CdSe and CdSe without a pre-adsorbed CdS layer (average diameters of both CdSe QDs are ~ 6 nm) using a improved transient grating (TG) technique (Katayama et al., 2003; Yamaguchi et al., 2003; Shen et al., 2010). TG signal is proportional to the change in the refractive index of the sample due to photoexcited carriers (electrons and holes). TG method is a powerful time-resolved optical technique for the measurements of various kinds of dynamics, such as carrier population dynamics, excited carrier diffusion, thermal Optical Absorption and Photocurrent Spectra of CdSe Quantum Dots Adsorbed on Nanocrystalline TiO 2 Electrode Together with Photovoltaic Properties 487 diffusion, acoustic velocity and so on. Improved TG technique features very simple and compact optical setup, and is applicable for samples with rough surfaces. Comparing with transient absorption (TA) technique, improved TG method has higher sensitivity sue to its zero background in TG signals, which avoids the nonlinear effect and sample damage. Figure 9 shows that the hole and electron relaxation times of nanostructured TiO 2 electrodes adsorbed with combined CdS/CdSe QDs are faster about twice than those with CdSe QDs without a pre-adsorve CdS layer, indicating the decreases in recombination centers. Fig. 10. Ultrafast carrier dynamics of combined CdS/CdSe and CdS without preadsorbed CdS quantum dots layer with a transient grating (TG) technique. 4. Conclusion We have described the performance of quantum dot-sensitized solar cells (QDSCs) based on CdSe QD sensitizer on a pre-adsorbed CdS layer (combined CdS/CdSe QDs) together with the basic studies of optical absorption and photoelectrochemical current characteristics. It can be observed from optical absorption measurements using photoacoustic (PA) spectroscopy that the CdSe QDs on the nanostructured TiO 2 electrodes with a pre-adsorbed CdS layer grow more rapidly during the initial adsorption process than those without a pre-adsorbed CdS layer. Photoelectrochemical current in the visible light region due to the adsorbed CdSe QDs can be observed, indicating the photosensitization by combined CdS/CdSe QDs. The maximum IPCE value (~ 75%) of the CdSe QDs on the nanostructured TiO 2 electrodes with a pre-adsorbed CdS QD layer is 30% greater than that without a pre-adsorbed CdS layer. It indicates the possibilities of a decrease in recombination centers, interface states, and inverse transfer rate that is suggested by the preliminary ultrafast photoexcited carrier carrier dynamics characterization owing to the possibilities of active CdSe QDs by the excess Cd remaining after CdS adsorption and passivation effect of CdS QDs on the nanostructured TiO 2 surface. The short-circuit current (J sc ) in combined CdS/CdSe QD-sensitized solar cells shows Solar Cells – New Aspects and Solutions 488 maxima with the increase of CdSe QDs adsorption times between 2 h and 24 h, also indicating the decrease of recombination centers, interface states, and the increase in quasi Fermi level. The open-circuit voltage (V oc ) and fill factor (FF) are independent of adsorption times. The photovoltaic conversion efficiency (η) of the combined CdS/CdSe QD-sensitized solar cell shows a maximum value of 3.5%. 5. References Bawendi, M. G.; Kortan, A. R.; Steigerwals, M.; & Brus, L. E. (1989), J. Chem. Phys., Vol. 91, p. 7282. Chiba, Y.; Islam, A.; Watanabe, Y.; Koyama, R.; Koide, N.; & Han, L. (2006), Jpn. J. Appl. Phys.Vol. 43, p. L638. Diguna, L. J.; Shen, Q.; Kobayashi, J.; & Toyoda, T (2007), Appl. Phys. Lett., Vol. 91, p. 023116. Giménez, S; Mora-Seró, I; Macor, L.; Guijarro, N.; Lala-Villarreal, T.; Gómez, R.; Diguna, L.;Shen, Q.; Toyoda, T; & Bisquert, J (2009), Nanotechnology, Vol. 20, p. 295204. Gorer, S.; & Hodes, G. (1994), J. Phys. Chem., Vol. 98, p. 5338. Hines, M. A.; & Sionnet, P. G. (1996), J. Phys. Chem., Vol. 100, p. 468. Hodes, G; Manassen, J; & Cahen, D. (1980), J. Electrochem. Soc., Vol. 127, p. 544. Inoue, Y.; Toyoda, T; & Morimoto, J (2006), Jpn. J. Appl. Phys., Vol. 45, p. 4604. Jayakrishnan, R.; Nair, J. P.; Kuruvilla, B. A.; & Pandy, R. K. (1996) Semicond. Sci. Tech., Vol. 11, p. 116. Katayama, K.; Yamaguchi, M.; & Sawada, T. (2003), Appl. Phys. Lett., Vol. 82, p. 2775. Klimov, V. I. (2006), J. Phys. Chem. B, Vol. 110, p. 16827. Lee, Y-L.; & Lo, Y-S. (2009), Adv. Func. Mater., Vol. 19, p. 604. Mora-Seró, I; Giménez, S; Fabregat-Santiago, F.; Gómez, R.; Shen, Q.; Toyoda, T; & Bisquert, J. (2009), Acc. Chem. Res., Vol. 42, 1848. Niitsoo, O.; Sarkar, S. K.; Pejoux, P.; Rühle, S.; Cahen, D.; & Hodes, G., J. Photochem. Photobiol. A, Vol. 182, 306. Nozik, A. J. (2002), Physica E, Vol. 14, p. 16827. O’Regan, B.; & Grätzel (1991), Nature, Vol. 353, p. 737. Rosencwaig, A. & Gersho, A. (1977), J. Appl. Phys., Vol. 47, p. 64. Schaller, R. D.; Sykora, M.; Pietryga, J. M.; & Klimov, V. I. (2006), Nono Lett., Vol. 6. P. 424. Shen, Q.; & Toyoda, T (2004), Jpn. J. Appl. Phys., Vol. 43, p. 2946. Shen, Q.; Arae, D.; & Toyoda, T. (2004), J. Photochem. Photobiol. A, Vol. 164, p. 75 Shen, Q.; Kobayashi, J.; Doguna, L. J.; & Toyoda, T. (2008), J. Appl. Phys., Vol. 103, p. 084304. Shen, Q.; Ayuzawa, Y.; Katayama, K.; Sawada, T.; & Toyoda, T. (2010), Appl. Phys. Lett., Vol. 97, p. 263113. Shockley, W.; & Queisser, H. J. (1961), J. Appl. Phys., Vol. 32, p. 510. Sudhager, P.; Jung, J. H.; Park, S; Lee, Y-G.; Sathyamamoorthy, R.; Kang, Y. S.; & Ahn, H.(2009), Electrochem. Commun., Vol. 11, p. 2220. Tam, A. C. (1986), Rev. Mod. Phys., Vol. 58, p. 381. Toyoda, T.; Uehata, T.; Suganuma, R.; Tamura, S; Sato, A; Yamamoto, K.; Shen, Q. & Kobayashi, N. (2007), Jpn. J. Appl. Phys., Vol. 46, p. 4616. Toyoda, T; Tsugawa, S.; & Shen, Q. (2009), J. Appl. Phys., Vol. 105, p. 034314. Trinh, M. T.; Houtepen, A. J.; Schints, J. M; Hanrath, T.; Piris, J.; Knulst, W.; Goossens, P. L. M.; & Siebbeles, L. D. A. (2008), Nano Lett, Vol. 8, p. 1713 Yamaguchi, M.; Katayama, K.; & Sawada, T. (2003), Chem. Phys. Lett., Vol. 377, p. 589. Underwood, D. F.; Kippeny, T.; & Rosenthal, S. J. (2001), Eur. Phys, J. D, Vol. 16, p. 241. Yang, S. M.; Huang, C. H.; Zhai, J.; Wang, Z. S.; & Liang, L. (2002), J. Mater. Chem., Vol. 12,p. 1459. 23 Investigation of Lattice Defects in GaAsN Grown by Chemical Beam Epitaxy Using Deep Level Transient Spectroscopy Boussairi Bouzazi 1 , Hidetoshi Suzuki 2 , Nobuaki Kijima 1 , Yoshio Ohshita 1 and Masafumi Yamaguchi 1 1 Toyota Technological Institute 2 Myazaki University Japan 1. Introduction With only 3 % of N and 9 % of In, InGaAsN with a band gap of 1.04 eV was obtained and could be lattice matched to GaAs and Ge. This dilute nitride semiconductor has been selected as a promising candidate for high efficiency multijunction tandem solar cells (Geisz and Friedman, 2002). However, the diffusion length of minority carriers and the mobility are still lower than of that in GaAs or InGaAs and showed a considerable degradation with increasing the N concentration. These electrical properties are insufficient to insure the current matching in the multijunction solar cell structure AlInGaP/GaAs/InGaAsN/Ge (Friedman et al., 1998). An obvious reason of such degradation is the high density of N- related lattice defects that can be formed during growth to compensate for the tensile strain caused by the small atomic size of N compared with that of arsenic (As) and to the large miscibility of the gap between GaAs and GaN. These defect centers are expected to act as active recombination and/or scattering centers in the forbidden gap of the alloy (Zhang & Wei, 2001). However, no experimental evidence has yet been reported. On the other hand, the conductivity of undoped p-type InGaAsN or GaAsN and their high background doping (Friedman et al., 1998; Kurtz et al., 1999; Moto et al., 2000; Krispin et al., 2000) prevent the design of wide depletion region single junction solar cell and the fabrication of intrinsic layer to overcome the short minority carrier lifetime. This serious problem was expected in the first stage to the density of unintentional carbon in the film (Friedman et al., 1998; Kurtz et al., 1999; Moto et al., 2000). However, the carrier density in some InGaAsN semiconductors was found to be higher than that of carbon (Kurtz et al., 2002). Furthermore, the high density of hydrogen (up to 10 20 cm −3 ) and the strong interaction between N and H in InGaAsN to form N-H related complex were confirmed to be the main cause of high background doping in InGaAsN films (Li et al., 1999; Janotti et al., 2002, 2003; Kurtz et al., 2001, 2003; Nishimura et al., 2007). In addition, N-H complex was found theoretically to bind strongly to gallium vacancies (V Ga ) to form N-H-V Ga with a formation energy of 2 eV less than that of isolated V Ga (Janotti et al., 2003). These predictions were supported experimentally using positron annihilation spectroscopy results (Toivonen et al., 2003). Solar Cells – New Aspects and Solutions 490 On the other hand, similar electrical properties were obtained in InGaAsN grown by metal- organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) despite the large difference in the density of residual impurities, which excludes them as a main cause of low mobilities and short minority carrier lifetimes. For that, lattice defects, essentially related to the N atom, were expected to be the main reason of such degradation. Several theoretical and experimental studies have investigated carrier traps in InGaAsN films. Theoretically, using the first principles pseudo-potential method in local density approximation, four N-related defects were proposed: (As Ga -N As ) nn , (V Ga -N As ) nn , (N-N) As , and (N-As) As ( Zhang & Wei, 2001). While the two first structures were supposed to have lower formation probabilities, the two split interstitials (N-N) As , and (N-As) As were suggested to compensate the tensile strain in the film and to create two electron traps at around 0.42 and 0.66 eV below the conduction band minimum (CBM) of InGaAsN with a band gap of 1.04 eV, respectively (Zhang & Wei, 2001). Experimentally, the ion beam analysis provided a quantitative evidence of existence of N-related interstitial defects in GaAsN (Spruytte et al., 2001; Ahlgren et al., 2002; Jock, 2009). Furthermore, several carrier traps were observed in GaAsN and InGaAsN using deep level transient spectroscopy (DLTS). A deep level (E2/H1), acting as both an electron and a hole trap at 0.36 eV below the CBM, was observed (Krispin et al., 2001). Other electron traps in GaAsN grown by MBE were recorded: A2 at 0.29 eV and B1 at 0.27 eV below the CBM of the alloy (Krispin et al., 2003). In addition, a well known electron trap at 0.2  0.3 eV and 0.3  0.4 eV below the CBM of p-type and n-type GaAsN grown by MOCVD were observed, respectively (Johnston et al., 2006). Although the importance of these results as a basic knowledge about lattice defects in GaAsN and InGaAsN, no recombination center was yet experimentally proved and characterized. Furthermore, the main cause of high background doping in p-type films was not completely revealed. Chemical beam epitaxy (CBE) has been deployed (Yamaguchi et al., 1994; Lee et al., 2005) to grow (In)GaAsN in order to overcome the disadvantages of MOCVD and MBE. It combines the use of metal-organic gas sources and the beam nature of MBE. (In)GaAsN films were grown under low pressure and low temperature to reduce the density of residual impurities and to avoid the compositional fluctuation of N, respectively. Furthermore, a chemical N compound source was used to avoid the damage of N species from N 2 plasma source in MBE. Although we obtained high quality GaAsN films gown by CBE, the diffusion length of minority carriers is still short (Bouzazi et al., 2010). This indicates that the electrical properties of GaAsN and InGaAsN films are independent of growth method and the problem may be caused by the lattice defects caused by N. Therefore, it is necessary to investigate these defects and their impact on the electrical properties of the film. For that, this chapter summarizes our recent results concerning lattice defects in GaAsN grown by CBE. Three defect centers were newly obtained and characterized. The first one is an active non-radiative N-related recombination center which expected to be the main cause of short minority carrier lifetime. The second lattice defect is a N-related acceptor like-state which greatly contributes in the background doping of p-type films. The last one is a shallow radiative recombination center acceptor-like state. 2. Deep level transient spectroscopy To characterize lattice defects in a semiconductor, several techniques were used during the second half of the last century. Between these methods, we cite the thermally stimulated [...]... where A is the contact area, Vb is the built-in potential, 0 is the permittivity of the semiconductor material, and e is the elementary charge of an electron C0, NT, ND, and  492 Solar Cells – New Aspects and Solutions denote the junction capacitance at reverse bias, the density of filled traps under steady state conditions, the ionized donor concentration, and the time constant that gives the emission... NH2-Cj(cm-3) 2.23  1 015 2.88  1016 6.56  1016 7.55  1016 NH2-Cj/ NA 0.36 0.91 0.93 0.87 Table 3 Summary of EH2, H2, NH2-adj, Emax, NH2-est, and the ratio NH2-Cj/NA for CBE grown undoped p-type GaAsN Schottky junctions 508 Solar Cells – New Aspects and Solutions 5 Conclusion Three defect centers, related to the optoelectronic properties of GaAsN, were identified and characterized using DLTS and some related... of GaAsN based solar cells In conclusion, the results obtained in this study are very useful for scientific understanding of defects in III-V-N materials and to improve GaAsN and InGaAsN qualities for realizing high efficiency multi-junction solar cells 6 Acknowledgment Part of this work was supported by the New Energy Development Organization (NEDO) under the Ministry of Economy, Trade and Industry,... technology for photovoltaic solar energy applications Journal of Crystal Growth, Vol.136, pp.29-36 Zhang, S B & Wei, S H (2001) Nitrogen Solubility and Induced Defect Complexes in Epitaxial GaAs:N Physical Review Letters, Vol.86, pp 1789-1792 512 Solar Cells – New Aspects and Solutions Ziegler, J F.; Biersack J P & U Littmark (1985) The Stopping and Ion Range of Ions in Solids Pergamon, New York, Vol 1; SRIM... of Ncontaining n-type GaAsN The addition 56.7 E3 0.05 54.6 0.00 100 150 200 250 Tempertaure (K) 300 3.6 5.4 1000/T (1/K) Fig 2 DLTS spectra of (a) N free as grown and annealed GaAs, (b) as grown n-type GaAs0.998N0.002, (c) annealed n-type GaAs0.998N0.002, and (d) Arrhenius plots of DLTS spectra 7.2 496 Solar Cells – New Aspects and Solutions 1.4 (a) CBM 1.3 As grown Annealed 1.2 1.1 1.0 0.9 0.0 0.1... 1.5 1.0 0.6 0.5 0.0 0.00 0.05 0.10 0 .15 0.4 TEGa(sccm) Fig 13 N dependence of (a) NA and (b) TEGa flow rate dependence of growth rate and N concentration at a growth temperature of 420 C [N](%) 0.12 0.20 0.34 0.51 EH2 (eV) 0.210 0 .150 0.138 0.103 H2(cm2) 2.8  10-14 6.3  10-16 6.3  10-16 1.3  10-17 NH2-adj(cm-3) 2.64  1 015 3.08  1 015 5.20  1 015 9.12  1 015 Emax (V/cm) 6.2  104 1.4  105 2.1... by I d (T )  I  exp(  E ) kT (14) where I, E, k, and T denote the limit of the high-temperature current, the thermal activation energy of the reverse bias current, the Boltzmann constant, and the temperature, respectively The I-V characteristics deviate in the two samples from the thermionic emission This is 498 Solar Cells – New Aspects and Solutions Reverse Bias current |Id(A)| explained by the... E1 as function of lattice coordinate parameter 500 Solar Cells – New Aspects and Solutions 4.1.3 Possible origin of the N-related recombination center E1 It is worth remembering that the atomic structure of E1 may be free from impurities and doping atoms owing to the difference in the density of residual impurities in GaAsN grown with MOCVD, MBE, and CBE Furthermore, the uniform distribution of NE1... Thermometers Japanese Journal of Applied Physics, Vol.22, pp 878881 510 Solar Cells – New Aspects and Solutions Krispin, P.; Spruytte, S G.; Harris, J S & Ploog, K H (2000) Electrical depth profile of ptype GaAs/Ga(As, N)/GaAs hetero-structures determined by capacitance– voltage measurements Journal of Applied Physics, Vol.88, pp 4153 - 4158 Krispin P.; Gambin V.; Harris J S & Ploog K H (2003) Nitrogen-related... acceptor level obtained from theoretical prediction and identical to EA2 (Suzuki et al., 2008; Janotti et al., 2003) Furthermore, as given in Fig 12 (d), NH2-adj is in linear dependence with N concentration Therefore, H2 is proved to be the N-related hole trap acceptor-like state, which thermal ionization increased Cj and 506 Solar Cells – New Aspects and Solutions drops the depletion region width The contribution . Solar Cells – New Aspects and Solutions 484 appearance of the current-voltage curves of combined CdS/CdSe QD-sensitized solar cells is different from those of CdSe QD-sensitized solar cells. . built-in potential,  0 is the permittivity of the semiconductor material, and e is the elementary charge of an electron. C 0 , N T , N D , and  Solar Cells – New Aspects and Solutions. adsorption and passivation effect of CdS QDs on the nanostructured TiO 2 surface. The short-circuit current (J sc ) in combined CdS/CdSe QD-sensitized solar cells shows Solar Cells – New Aspects and