Thin film solar cell of copper oxide

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Each student needs to discuss (a) Solar cell introduction and structure, (2) solar cell mechanism with band diagram, (3) future prospects and challenges for each solar cell you are working on.


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Solar Energy 80 (2006) 715–722 www.elsevier.com/locate/solener Thin film deposition of Cu2O and application for solar cells K. Akimoto *, S. Ishizuka, M. Yanagita, Y. Nawa, Goutam K. Paul, T. Sakurai Institute of Applied Physics, University of Tsukuba, CREST, Japan Science and Technology Corporation, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan Received 16 June 2005; received in revised form 9 September 2005; accepted 18 October 2005 Available online 5 January 2006 Communicated by: Associate Editor Arturo Morales-Acevedo Abstract Deposition conditions of cuprous oxide (Cu2O) thin films on glass substrates and nitrogen doping into Cu2O were studied by using reactive radio-frequency magnetron sputtering method. The effects of defect passivation by crown-ether cyanide treatment, which simply involves immersion in KCN solutions containing 18-crown-6 followed by rinse, were also studied. By the crown-ether cyanide treatment, the luminescence intensity due to the near-band-edge emission of Cu2O at around 680 nm was enhanced, and the hole density was increased from 1016 to 1017 cm3. Finally, polycrystalline pCu2O/n-ZnO heterojunctions were grown for use in solar cells. Two deposition sequences were studied, ZnO deposited on Cu2O and Cu2O deposited on ZnO. It was found that the crystallographic orientation and current–voltage characteristics of the heterojunction were significantly influenced by the deposition sequence, both being far superior for the heterojunction with structure Cu2O on ZnO than for the inverse structure. We successfully obtained a photoresponse for the first time in the deposited thin film of Cu2O/ZnO.  2005 Elsevier Ltd. All rights reserved. Keywords: Cuprous oxide; Zinc oxide; Solar cell; Cyanide treatment; Defect passivation; Photoresponse 1. Introduction Cuprous oxide (Cu2O), a direct-gap semiconductor with a band-gap energy of 2.0 eV, has been regarded as one of the most promising materials for application to photovoltaic cells (Pollack and Trivich, 1975). The attractiveness of Cu2O as a photovoltaic material lies in the fact that the constituent materials are nontoxic and abundantly available on the earth, and that the Cu2O has a high absorption * Corresponding author. Fax: +81 298 53 5205. E-mail address: akimoto@esys.tsukuba.ac.jp (K. Akimoto). coefficient in visible regions and low-cost producibility. There are several reports regarding the deposition method of Cu2O thin films (Siripala et al., 1996; Matsumura et al., 1996; Kawaguchi et al., 1994; Padiyath et al., 1994; Drobny and Pulfrey, 1979) and the dopant impurities (Musa et al., 1998). We have systematically investigated the deposition conditions and dopant impurities, and found that nitrogen acts as a p-type dopant in Cu2O with an activation energy of 140 meV (Ishizuka et al., 2000, 2001). Based on these results, we have been able to control the hole density in polycrystalline Cu2O thin films from 1015 to 1017 cm3. We have also 0038-092X/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2005.10.012 716 K. Akimoto et al. / Solar Energy 80 (2006) 715–722 Nomenclature I–V XRD PL P Na Nd Nv Ea current–voltage X-ray diffraction photoluminescence hole density acceptor density donor density effective density of state in the valence band acceptor activation energy reported that the hydrogen treatment, which is generally used for the passivation of defects in polycrystalline Si and Si/SiO2 interface states, is effective in improving electrical and optical properties of the polycrystalline Cu2O thin films (Ishizuka et al., 2002). The hydrogen treatment for polycrystalline Cu2O was, however, effective only in the absence of heat treatment above 150 C. Therefore, it is desired to develop another passivation technique whose effects are unchanged after relatively high-temperature heat treatment. Crown-ether cyanide treatment, which was recently developed for passivating interface states at Si/SiO2 interfaces and trap states in polycrystalline Si (Kobayashi et al., 1998, 2000) was applied to polycrystalline Cu2O, and its effects on electrical and optical properties were compared with those of the hydrogen treatment. Since Cu2O is a unipolar p-type semiconductor, probably due to Cu vacancies, a p-Cu2O/n-ZnO heterostructure was grown and the current–voltage (I–V) characteristics were studied. In this paper, successful control of hole density by nitrogen doping, defect passivation by cyanide treatment, and photoresponse of a Cu2O/ZnO/substrate thin film structure are reported. 2. Experimental Cu2O thin films were deposited by means of reactive radio-frequency (rf) magnetron sputtering on glass substrates (Corning 7059) at a substrate temperature of 400–500 C using a Cu target (99.99% purity) and Ar sputtering gas. Nitrogen and oxygen were introduced during the deposition through a nozzle whose end was placed near the substrate. The flow rate of oxygen used as the reactive gas was k T mh h Voc Jsc FF AFM Boltzmann constant temperature effective hole mass Planck’s constant open circuit voltage short circuit current fill factor atomic force microscope fixed at 200 ml/min, which was found to be the optimum condition to deposit a single phase of Cu2O (Ishizuka et al., 2000). The flow rate of nitrogen used as the source of a p-type dopant was varied from 0 to 20 ml/min. The film thickness was typically 2 lm, as determined by scanning electron microscopy. The structural properties were studied by X-ray diffraction (XRD) in the h–2h mode using Cu-Ka radiation. The resistivity, Hall mobility and carrier density were measured at temperatures ranging between 180 and 300 K by the Van der Pauw method using a BIO-RAD HL5500PC system. The specimen for the Hall measurement was square shaped with a size of 7 · 7 mm2, and the electrodes consisted of evaporated 1 mmB Au dots. Photoluminescence (PL) spectra of the Cu2O thin films were measured at 77 K using a He–Cd laser with an excitation wavelength of 325 nm. The crown-ether cyanide treatment was carried out by the following procedure: 0.2 mol of 18-crown-6 (C12H24O6) was dissolved in xylene and the solution was added to a 0.1 M KCN aqueous solution with the same volume. After keeping the solution still for 30 min, the part of the xylene solution which was well separated from the aqueous phase was taken out of the separatory funnel, and the Cu2O film was immersed in the KCN solution of xylene containing crown-ether for about 10 min, followed by successive rinses in acetone, ethanol, and de-ionized water. Hereafter, we call this treatment the ‘‘cyanide treatment’’. The 18-crown-6 molecule effectively captures a K+ ion, and consequently contamination by K+ ion is completely prevented. That is to say, K+ ions do not directly contact Cu2O, only CN ions do. Details of the cyanide treatment are described elsewhere (Kobayashi K. Akimoto et al. / Solar Energy 80 (2006) 715–722 717 et al., 1998, 2000). Hydrogen treatment was carried out by exposing Cu2O thin films to hydrogen plasma (Ishizuka et al., 2002). In order to study the thermal stability of the passivation effects, the Cu2O thin films with the hydrogen or cyanide treatment were annealed at temperatures between 150 and 350 C for 3 min in a rapid thermal annealing system under the vacuum of 1 · 104 Pa. A polycrystalline p-Cu2O/n-ZnO heterostructure was grown also by means of rf magnetron sputtering on Corning 7059 glass substrates using a Cu target of 99.99% purity, a ZnO target and Ar sputtering gas. At the interface between n-ZnO and p-Cu2O, an insulating ZnO (i-ZnO) layer was deposited. The iZnO and n-ZnO were deposited using undoped and 1 wt% Al2O3-doped ZnO targets, respectively. Two types of heterostructures, n-ZnO/i-ZnO/ p-Cu2O/Au/glass substrate (structure-1) and pCu2O/i-ZnO/n-ZnO/glass substrate (structure-2), were grown and the structural and electrical properties were studied. The deposition temperature of Cu2O was varied between 400 and 500 C to study the deposition temperature effect on the device performance. The carrier densities of n-ZnO and pCu2O were estimated to be about 5 · 1020 and 1 · 1015 cm3, respectively, by Hall-effect measurements on films of the individual materials. 3. Results and Discussion Fig. 1. XRD profiles of copper oxides thin films for various oxygen flow rates. Fig. 2. Hole density (a) and Hall mobility, (b) for Cu2O thin films deposited at various N2 flow rates. 3.1. Deposition of Cu2O and nitrogen doping Fig. 1 shows the results of XRD for film deposited at 400 C. Cu and Cu2O phases were observed for an oxygen flow rate of 180 ml/min. Also, Cu2O and CuO phases were observed for oxygen flow rates higher than 225 ml/min. Instead, for oxygen flow rates 210 ml/min and 225 ml/min, a single Cu2O phase was obtained, showing a major preferred crystal orientation on the (2 0 0) axis with a small (1 1 1) component. Fig. 2(a) and (b) shows the hole density and Hall mobility of Cu2O thin films as a function of N2 flow rate. As it can be seen, the hole density increases nearly linearly with N2 flow rate, with unitary slope, and varied from 1 · 1015 cm3 to approximately 1017 cm3 in controlling the electrical properties of this p-type semiconductor. The nitrogen was found K. Akimoto et al. / Solar Energy 80 (2006) 715–722 to have the effect of increasing hole density and to act as an acceptor. It is expected that the nitrogen in Cu2O can be incorporated in the oxygen lattice site, and this may be the reason for the nitrogen acting as an acceptor. The linear relationship between the hole density and nitrogen flow rate suggests that the incorporation rate of nitrogen is constant and no defects induced by nitrogen doping were created. The Hall mobility decreased with increasing nitrogen flow rate, as shown in Fig. 2(b). This result is reasonable since the carrier scattering is enhanced by ionized acceptors. These results indicate that the nitrogen is very effective in controlling the electrical properties in p-type conductivity. 3.2. Defect passivation in Cu2O by cyanide treatment Fig. 3 shows PL spectra of the nitrogen-doped, polycrystalline Cu2O thin films measured at 77 K before and after cyanide treatment. Although no luminescence peak was detected before the cyanide treatment, a peak due to near-band-edge emission was observed at 680 nm (1.82 eV) after the cyanide treatment. This peak is most likely attributable to acceptor-related luminescence, taking account of the band-gap energy of Cu2O (2.0 eV) and the nitrogen acceptor level (0.14 eV above the valence band maximum). These results suggest that CN ions passivate nonradiative recombination centers of Cu2O grains. As we reported previously (Ishizuka et al., 2002), the hydrogen treatment has an effect of increasing a luminescence intensity of polycrystalline Cu2O. The effect of the defect passivation from the cyanide treatment is similar to that of the hydrogen treatment since the luminescence intensities were almost the same after both treatments. Fig. 4(a) and (b) shows the hole density and Hall mobility of the Cu2O thin films, respectively, as a function of the nitrogen flow rate during the film deposition. After the cyanide treatment, the hole density increased and the Hall mobility decreased, irrespective of the nitrogen flow rate. The increase of the hole density and the decrease of the mobility may indicate that some kinds of hole traps are passivated by the cyanide treatment. It is generally believed that oxygen vacancies, that is, dangling bonds of Cu, act as donors that also act as hole traps (Philips, 1973). From these considerations, it 10 Hole density (cm-3) 718 17 10 16 10 15 0 (a) 10 20 N2 flow rate (ml/min) H- treatment He-Cd laser 77K 40 After CN- treatment As received Mobility (cm2/Vs) PL Intensity (a.u.) Before 20 0 0 (b) 600 700 10 20 N2 flow rate (ml/min) 800 Wavelength (nm) Fig. 3. PL spectra of the nitrogen-doped, polycrystalline Cu2O thin films before and after cyanide treatment. Fig. 4. Hole density (a) and Hall mobility (b) before (circle) and after hydrogen treatment (triangle) and after cyanide treatment (square), obtained from Hall-effect measurements for the Cu2O thin films deposited at various N2 flow rates. K. Akimoto et al. / Solar Energy 80 (2006) 715–722 P ¼ fðN a  N d Þ=2N d gN v expðEa =kT Þ; where Nv is the effective density of states in the valence band equal to 2ð2pmh kT =hÞ3=2 , and Na, Nd, Ea, k, T, mh , and h are the acceptor density, donor density, activation energy of the acceptors, Boltzmann constant, temperature, effective hole mass, and Planck’s constant, respectively. The value of the effective hole mass was taken to be 0.58m0 (where m0 is the mass of the free electron) (Hodby et al., 1976). From the slopes of the straight lines determined by the least squares method, as shown in Fig. 5, the values of Ea were estimated to be 0.14 and 0.12 eV for the nitrogen-doped Cu2O thin films before and after cyanide treatment, respectively. The change in Ea may be due to the formation of an impurity band. The compensation ratios Nd/Na were determined to be 0.56 and 0.18 for the samples before and after cyanide treatment, respectively. The significant reduction of the compensation ratio indicates the passivation of donor-like defects by the cyanide treatment. These results are consistent with the aforementioned passivation model that CN ions terminate the Cu dangling bonds, resulting in the formation of Cu–CN bonds. 10 19 10 18 Hole density (cm-3) is suggested that the effects of the cyanide treatment result from termination of Cu dangling bonds by CN ions. A similar hole density was also obtained from the hydrogen treatment. Fig. 5 shows the temperature dependence of the hole density for the nitrogen-doped Cu2O thin films before and after cyanide treatment. The hole density can be expressed as 719 10 17 10 16 10 15 0 100 200 300 400 Temperature (˚C) Fig. 6. Hole density of the Cu2O thin films with hydrogen treatment (circle), cyanide treatment (rectangular), and before treatment (triangle) measured as a function of the annealing temperature. The annealing effects on the hole density of three kinds of Cu2O films; cyanide treated films, hydrogen treated films and as received films, were examined, and the results are shown in Fig. 6. The data on the axis of 0 C means the samples without annealing. In the case of hydrogen treatment, the hole density began to decrease at around 200 C. In the case of cyanide treatment, on the other hand, the hole density remained almost constant up to 350 C. Therefore, it can be concluded that cyanide treatment has much better thermal stability than the hydrogen treatment. 3.3. Photoresponse from polycrystalline Cu2O/ZnO XRD profiles of the heterostructures are shown in Fig. 7. The curves (a) and (b) show the XRD profiles H-treatment As received Cu2O (110) Cu2O (220) Au (111) 10 Intensity (a.u.) Hole density (cm-3) CN-treatment 1018 17 10 1016 (a) Structure-I ZnO (0001) Cu2O (111) (b) 1015 3 4 1000/T Cu2O (200) 5 Structure-II (K-1) Fig. 5. Temperature dependence of the hole density for the nitrogen-doped Cu2O thin films before and after cyanide treatment. That of for hydrogen treatment was also plotted for reference. 20 30 40 50 2θ (degree) 60 70 Fig. 7. XRD profiles of structure-I (a) and structure-II (b). K. Akimoto et al. / Solar Energy 80 (2006) 715–722 from the n-ZnO/i-ZnO/p-Cu2O/Au/glass (structureI) and p-Cu2O/i-ZnO/n-ZnO/glass structures (structure-II), respectively. In the curve (a), three peaks corresponding to reflections from Cu2O(1 1 0), Cu2O(2 2 0) and Au(1 1 1) are observed. In the curve (b), two intense and sharp peaks corresponding to Cu2O(1 1 1) and ZnO(0 0 0 1) and a minor peak of Cu2O(2 0 0) are observed. These results show that in structure-I, Cu2O has a (1 1 0) orientation on Au(1 1 1), though the crystallographic orientation of ZnO is unknown. In structure-II on the other hand, ZnO is highly (0 0 0 1) oriented and the orientation of Cu2O is dominated by (1 1 1). Hence Cu2O is likely to have the (1 1 1) orientation selectively on ZnO(0 0 0 1). The differences in these structures may be due to the crystal structure; ZnO is hexagonal (wurtzite) and Cu2O is cubic (cuprite), and the hexagonal (0 0 0 1) and cubic (1 1 1) orientations have similar atomic arrangement. A schematic illustration of the atomic arrangements of Cu2O(1 1 1) and ZnO(0 0 0 1) is shown in Fig. 8. The lattice mismatch between Cu2O(1 1 1) and ZnO(0 0 0 1) is calculated to be 7.1%. The similarity in atomic arrangement would seem to be responsible for the crystallographic orientation in the heterostructure. The current–voltage (I–V) characteristics of samples with structure-I and structure-II were mea- sured. The n-ZnO is used as the window layer and also n-electrode since its resistivity was as low as 2 · 104 X cm for both structures. The electrode for p-side was Au. Some of the samples with structure-I showed electrical rectification, however, reproducibility was not good and the inverse current density was relatively high. On the other hand, all the samples fabricated with a type-II structure showed electrical rectification, and the inverse current density was clearly lower than that of struc0.5 Current density (mA/cm2) 720 0.0 -0.5 -1.0 -1.5 -2.0 dark dar k light li ght -2.5 -3.0 0.0 0.1 Voltage (V) 0.2 0.3 Fig. 9. I–V characteristics of a sample of structure-II. The dark current and the photocurrent are indicated by open squares and open circles, respectively. Fig. 8. Schematic illustration of atomic arrangements of Cu2O(1 1 1) and ZnO(0 0 0 1). K. Akimoto et al. / Solar Energy 80 (2006) 715–722 721 Fig. 10. AFM images of samples of p-Cu2O/i-ZnO/n-ZnO/glass structures. The deposition temperatures of Cu2O are 500 C (a) and 400 C (b). ture-I. Fig. 9 shows the I–V characteristics of Au/pCu2O (1.7 lm thickness)/i-ZnO (0.1 lm)/n-ZnO (0.8 lm)/glass structure (structure-II). The Cu2O layer of this sample was deposited at 400 C. As shown in the figure, we successfully obtained a photoresponse from the structure-II samples. This is the first reported demonstration of photoresponse from a deposited thin film of p-Cu2O/n-ZnO heterostructure. The open circuit voltage (Voc), short circuit current (Jsc) and fill factor (FF) were 0.26 V, 2.8 mA/cm2 and 0.55, respectively, under the condition of AM1.5. The conversion efficiency was calculated to be 0.4%. The value of Voc is smaller than expected, which may be due to interface defects and should be clarified in the future. A photoresponse was barely obtained from samples with structure-I. Such a clear difference may be due to the properties of the interface between ZnO and Cu2O. The defect density can be expected to be lower in structure-II due to the similarity of the atomic arrangement, as discussed above. Therefore, it can be said that the continuous atomic arrangement is important even in polycrystalline heterostructures. The effects of the deposition temperature of Cu2O on the I–V characteristics and the photoresponse were studied. The deposition temperature of Cu2O of the samples shown in Fig. 9 was 400 C. For samples with Cu2O grown at a deposition temperature of 500 C, the ratio of the electrical rectification, determined as the ratio of the current densities at bias voltages +1 V and 1 V, was 1.4 times greater than that for samples with Cu2O grown at 400 C, and the photocurrent was 1.2 times greater. The crystal quality may also be improved at higher deposition temperatures, and grain size seems to play an important role in improving device performance. Fig. 10 shows atomic force microscope (AFM) images of structure-II samples with Cu2O deposited at (a) 500 C and (b) 400 C. The average grain size increased from about 0.2 to 0.4 lm as the deposition temperature was increased. With increasing grain size, the density of surface dangling bonds decreased. Therefore, the improvement in device performance at higher deposition temperatures may be due to an increase in grain size. 4. Conclusions Cu2O thin films were deposited by the reactive rfmagnetron sputtering method on glass substrates. Single Cu2O phase films can be obtained by controlling the oxygen flow rate. Nitrogen was found to have the effect of increasing hole density by acting as an acceptor in Cu2O and to generate an acceptor level of about 0.14 eV. The effects of cyanide treatment on the electrical and optical properties of the nitrogen-doped, polycrystalline Cu2O thin films were studied. Luminescence properties were improved and the hole density increased from 1016 to 1017 cm3 after the cyanide treatment. These results indicate the passivation of nonradiative centers and hole traps, and it is attributed to the termination of Cu-dangling 722 K. Akimoto et al. / Solar Energy 80 (2006) 715–722 bonds by the formation of Cu–CN bonds. The magnitudes of the improvements in the optical and electrical properties caused by the cyanide treatment are comparable to those of the hydrogen treatment. However, the cyanide treatment has an advantage in that its passivation effect is more thermally stable than that of the hydrogen treatment. Polycrystalline p-Cu2O/n-ZnO heterojunctions were grown by rf-magnetron sputtering. The photoresponse of the deposited thin film structure was obtained for the first time. The I–V characteristics and photoresponse properties were studied with respect to the deposition sequence of ZnO and Cu2O and the deposition temperature of Cu2O. Crystallographic orientation, which relates to the continuous atomic arrangement and surface defect density (related to grain size), seem to be important in getting high performance in electrical rectification and photosensitivity. References Drobny, V.F., Pulfrey, D.L., 1979. Properties of reactivelysputtered copper oxide thin films. Thin Solid Films 61, 89–98. Hodby, J.W., Jenkins, T.E., Schwab, C., Tamura, H., Trivich, D., 1976. Cyclotron resonance of electrons and of holes in cuprous oxide, Cu2O. J. Phys. C 9, 1429–1439. Ishizuka, S., Maruyama, T., Akimoto, K., 2000. Thin-film deposition of Cu2O by reactive radio-frequency magnetron sputtering. Jpn. J. Appl. Phys. 39, L786–L788. Ishizuka, S., Kato, S., Maruyama, T., Akimoto, K., 2001. Nittrogen doping into Cu2O thin films deposited by reactive radio-frequency magnetron sputtering. Jpn. J. Appl. Phys. 40, 2765–2768. Ishizuka, S., Kato, S., Okamoto, Y., Akimoto, K., 2002. Hydrogen treatment for polycrystalline nitrogen-doped Cu2O thin film. J. Cryst. Growth 237, 616–620. Kawaguchi, K., Kita, R., Nishiyama, M., Morishita, T., 1994. Molecular beam epitaxy growth of CuO and Cu2O films with controlling the oxygen content by the flux ratio of Cu/O. J. Cryst. Growth 143, 221–226. Kobayashi, H., Asano, A., Kubota, T., Yoneda, K., Todokoro, Y., 1998. Studies on interface at ultrathin SiO2/Si(1 0 0) interface by means of X-ray photoelectron spectroscopy under biases and their passivation by cyanide treatment. J. Appl. Phys. 83, 2098–2103. Kobayashi, H., Asano, A., Takahashi, M., Yoneda, K., Todokoro, Y., 2000. Decrease in gap states at ultrathin SiO2/Si interfaces by crown-ether cyanide treatment. Appl. Phys. Lett. 77, 4392–4394. Matsumura, H., Fujii, A., Kitatani, T., 1996. Properties of highmobility Cu2O films prepared by thermal oxidation of Cu at low temperatures. Jpn. J. Appl. Phys. 35, 5631–5636. Musa, A.O., Akomolafe, T., Cartner, M.J., 1998. Production of cuprous oxide, a solar cell material, by thermal oxidation and a study of its physical and electrical properties. Sol. Energy Mater. Sol. Cells 51, 305–316. Padiyath, R., Seth, J., Babu, S.V., 1994. Deposition of copper oxide films by reactive laser ablation of copper formate in an r.f. oxygen plasma ambient. Thin Solid Films 239, 8–15. Philips, J.C., 1973. Bonds and Bands in Semiconductors. Academic, New York. Pollack, G.P., Trivich, D., 1975. Photoelectric properties of cuprous oxide. J. Appl. Phys. 46, 163–172. Siripala, W., Perera, L.D.R.D., De Silva, K.T.L., Jayanetti, J.K.D.S., Dharmadasa, I.M., 1996. Study of annealing effects of cuprous oxide grown by electrodeposition technique. Sol. Energy Mater. Sol. Cells 44, 251–260.
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Running head: Thin Film Solar Cell of CU2O

Thin Film Solar Cell of CU2O
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Thin Film Solar Cell of CU2O

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Thin Film Solar Cell of CU2O

(a) Solar cell introduction and structure.
A solar cell also known as photovoltaic cell is a device that trips a switch and opens the
circuit to transform light energy directly into electrical energy. According to (Akimoto, Ishizuka,
Nawa, Paul & Sakurai, 2006) the first solar cell was developed in 1839 by Edmund Becquerel.
When a solar cell is subjected to direct sunlight without being connected to any external source
of voltage, it produces and supports the flow of electric current. This phenomenon is known as
photovoltaic effect principle. Photovoltaic effect refers to the generation of voltage and electric
current in a matter when subjected to light. It is both a physical and chemical phenomenon and is
directly related to photoelectric effect as in both cases there is absorption of quantum energy
which causes excitation. There is a slight different between the two. In photoelectric effect there
is ejection of electron from a material when s...


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