Down-shifting by quantum dots for silicon solar cell applications

CHAPTER THIRTEEN

Down-shifting by quantum dots for silicon solar cell applications 1
Alvaro Flores-Pacheco1, 2 Mario Enrique Alvarez-Ramos ,
n2 Arturo Ayo 1

Posgrado en Nanotecnología, Departamento de Física, Universidad de Sonora, Hermosillo, Sonora, México MEMS Research Lab, Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, United States

2

13.1 Introduction Silicon is the second most abundant element available in the earth’s crust, it has relatively low production costs, and the processing techniques for electronic applications are well established. These are some of the reasons underlying the fact that worldwide, silicon-based solar cells are still the most widely employed type for electrical current generation and are still in continuous development [1,2]. The seminal work of William Shockley and Hans J. Queisser [3] theoretically detailed the limits of single pen junction solar cell that rendered the well-known upper efficiency limit of 30% in a semiconductor at room temperature with a 1.1 eV band gap. More recent calculations using the AM1.5G solar spectrum instead of the black body radiation approximation used in the Shockley’s and Queisser’s original work rendered an efficiency value of 32.23% with a 1.1 eV band gap, with maximum theoretical efficiency of 33.16% at 1.34 eV [4]. Crystalline silicon has an energy separation from the valence band to the conduction band of around 1.1 eV, value that corresponds to an approximate wavelength of 1100 nm. This band gap falls within the range suggested by the ShockleyeQueisser limit for reaching the upper efficiency limit and makes silicon suitable for photovoltaic applications. When studying the losses associated with the radiation spectrum, it is important to mention that in addition to the transparency to wavelengths longer to 1100 nm, there are other important losses above the band gap energy. The excess energy of incoming photons from the solar irradiation with energy above the band gap is lost by the emission of phonons by the lattice in a process known as thermalization [5]. Solar Cells and Light Management ISBN: 978-0-08-102762-2 https://doi.org/10.1016/B978-0-08-102762-2.00013-6

© 2020 Elsevier Ltd. All rights reserved.

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Another critical factor that influences solar cell performance is the external quantum efficiency (EQE), which is defined as the ratio of the photogenerated carriers to the number of photons of a given energy incident on the solar cell. It should be noted that the ShockleyeQueisser limit assumes a quantum efficiency of 100%. The maximum value of unity occurs when each incident photon promotes an electron to the conduction band which is collected at the respective electrode. The EQE of a solar cell is usually measured illuminating the solar cell with monochromatic light of wavelength l and measuring the photocurrent Ipc and is determined by the following relationship: EQEðlÞ ¼

Ipc ðlÞ qfph

(13.1)

where q is the elementary charge and fph is the photon flux incident on the solar cell. The photon flux is determined measuring the EQE in a calibrated detector under the same light source. The behavior of the EQE curve of a solar cell is determined by optical and electrical losses, like parasitic absorption and recombination loses. When an EQE spectrum is measured, the photocurrent Ipc can be determined using an amperemeter, but the photon flux must be determined indirectly. This can be achieved performing a measurement with a calibrated photodetector with a known EQE. The photon flux fph is determined by fph ¼

Ipcref ðlÞ

qEQEref ðlÞ Combining Eqs. (13.1) and (13.2): EQEðlÞ ¼ EQEref ðlÞ

Ipc ref Ipc ðlÞ

(13.2)

(13.3)

The EQE can be determined performing measurements on a welldefined reference Ipcref ðlÞ and on the actual sample Ipc ðlÞ. It is imperative to employ a stable light source during the measurement process because the photon flux is assumed to remain constant. If the EQE measurement is performed under short circuit conditions, the short circuit current density Jsc can be determined without the need of measuring the cell’s irradiated area. Specifically, the value of Jsc can be calculated combining the photon flux bs ðl) given by the AM 1.5 irradiance spectrum, in combination with the EQE values and integrating across all the relevant wavelengths, that is

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Figure 13.1 External quantum efficiency (EQE) and short current density (Jsc ) of a c-Si solar cell.

Zl2 Jsc ¼  q

bs ðlÞEQEðlÞdl

(13.4)

l1

Even though the solar spectrum extends from 200 to 2500 nm, the EQE is usually measured within the 300e1100 nm wavelength range as shown in Fig. 13.1. This range of evaluation is used due to the low energy density of the solar spectrum in wavelengths shorter than 300 nm and the transparency of c-Si to wavelengths longer than 1100 nm. Analyzing Fig. 13.1, an important optical loss mechanism can be identified in the EQE of a silicon solar cell. For short wavelengths, only a small fraction of the light is converted in electronehole pairs due to superficial absorption. This parasitic absorption is a consequence of the low penetration depth of c-Si within the 300e400 nm range. It is important to highlight that the proposed efficiency limits only take in account spectral losses. Photovoltaic devices have additional detrimental factors like parasitic resistances that affect the fill factor and crystalline defects in the semiconductor which create recombination centers and induce losses in the current density, among others. The study of those factors is beyond the scope of this work. One approach to enhance the energy harvesting in the UV region has been the use of spectral converter materials. For instance, rare earth co-doped systems are composed of sensitizer ions that absorb ultraviolet radiation and transfer this energy to activator ions that can have intense emissions in the near-infrared range (1000 nm), and such a wavelength is more

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suitable for the band gap of silicon [6,7]. However, in spite of their excellent optoelectronic properties, the extraction of these materials raises ecological concerns associated with the pollution from rare earth mines [8,9], and their limited supply make their long-term suitability questionable. Quantum dots (QDs) represent a viable option instead of rare earthe based spectral converters, due to their absorption and photoluminescent properties. These nanostructures are semiconductor nanocrystals that exhibit charge confinement in three dimensions, where the energetic states of electrons, holes, and excitons exhibit discrete values and are size dependent. Additionally, the surface/volume ratio is increased, enhancing their photoelectronic properties. The important length parameters involved in the classification of nanocrystals are the crystal lattice constant aL , the exciton Bohr radius a*B , and the wavelength, l, associated with the lowest optical transition of the semiconductor. If the size of the semiconductor nanocrystal R is within a range close to aL , an adequate description can only be provided in terms of quantum mechanics. When aL  R  l, a nanocrystal can be treated as the particle in a box model. The exciton Bohr radius value a*B divides this size range into two subranges, R[a*B and R  a*B , with different interpretations of the size-dependent properties in terms of either a hydrogen-like exciton confined motion or an electronehole, respectively. When R ¼ a*B , a nanocrystal is considered to be in the size range of a QD. A subrange can be identified within this range for smaller sizes, where a shell-like structure can be assigned to a nanocrystal on which every additional atomic shell discretely shifts optical spectra and electron energies. For a spherical potential box, analytical expressions can be derived for the weak and strong confinement limits proposed by Efros [10] and further explained by Gaponenko [11]. In “large” QDs, with a small radius R but still a few times larger than the exciton Bohr radius, a*B , the quantization of the exciton center of mass motion occurs. The kinetic energy of an exciton confined in a spherical box is expressed by Enml ¼ Eg 

Ry* Z2 c2ml þ ; n2 2MR2

n; m; l ¼ 1; 2; 3; .

(13.5)

Where cml is the root of the Bessel function, M ¼ m*e þ m*h is the exciton mass, me* and mh* are the effective masses of the electron and the hole, respectively, and Ry* is the exciton Rydberg energy that describes the

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ionization energy of the lowest exciton state. An exciton in a spherical QD is characterized by the quantum number n describing its internal states arising from the Coulomb electronehole interaction and by the additional numbers m and l describing the states connected with the center of mass motion in the presence of an external potential barrier with spherical symmetry. For the lowest state (n ¼ 1; m ¼ 1; l ¼ 0), with a value of c10 ¼ p, the energy can be expressed as p2 Z2 (13.6) 2MR2 * * Given that M ¼ me* þ mh* and the reduced mass of the electron and the * * hole is m ¼ me mh me þ mh , Eq. (13.6) can be written as "  * 2 # m paB (13.7) E110 ¼ Eg  Ry* 1  M R E110 ¼ Eg  Ry* þ

The first exciton resonance in a spherical QD has a size-dependent energy shift of "  # * 2 m pa B E110 ¼ Ry* (13.8) M R The value given by Eq. (13.8) is small compared to Ry* when R[ a*B . This is the quantitative justification of the term “weak confinement.” A weak confinement regime is feasible in wide band gap semiconductors of IeVII compounds featuring small exciton Bohr radius and large exciton Rydberg energy. The strong confinement limit corresponds to the condition R  a*B . In this regime, the confined electron and hole have no bound state corresponding to the hydrogen-like exciton. With this condition of no interaction between electron and hole, a good approximation could be proposed considering the quantization of an electron and hole motion separately. The “free” electron and hole in a spherical potential box have energy levels given by e Eml ¼ Eg þ

Z2 c2ml Z2 c2ml h ; E ¼  ml 2m*e R2 2m*h R2

(13.9)

The zero-point kinetic energy (valence band) of the electron and hole, relevant to the lowest state in a box, is considerably larger than the Ry* value. The energy and momentum conservation laws result in selection rules

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that allow optical transitions which couple electron and hole states with the same principal n and orbital l quantum numbers. The absorption spectrum will be reduced to a set of discrete bands with energies Z2 c2nl (13.10) 2mR2 QDs in the strong confinement limit are commonly referred as “artificial atoms,” due to the discrete optical spectrum controlled by the size (number of atoms), analog to an actual atom, where the discrete spectrum is controlled by the number of nucleons. However, due to the high confinement of the electronehole pairs, and the surrounding potential generated by the crystalline structure, a twoparticle Hamiltonian including the two kinetic energy terms, the Coulomb potential and the confinement potential, should be evaluated. These important considerations were outlined first by Brus [12]. This system is described by a hydrogen-like Hamiltonian: Enl ¼ Eg þ

b¼ H

Z2 2 Z2 2 e2 Ve  Vh  2Me 2Mh εjre  rh j

(13.11)

After solving Schr€ odinger’s equation under this condition, the energy of the lowest excited state of the QD EQD is written as Z2 p2 1:8e2  (13.12) 2R2 m 4pεR  The term e2 εR describes the effective Coulomb electronehole interaction in a medium with dielectric permittivity ε. In small QDs, the impact of the Coulomb interactions to the ground state energy is greater than that in bulk crystals. Nanocrystals can be synthetized from a variety of different materials, employing different techniques. CdTe, carbon, ZnO, CdSe/CdS, and silicon QDs reported in the present chapter were obtained by wet chemical routes, with the main advantage of being scalable to mass manufacture [13]. The growth process of colloidal nanocrystals can be explained in two steps: nucleation and growth. There are two kinds of nucleation processes, the particle cores originate from the precursor solution components (homogeneous nucleation) or are triggered by existing particles in the solution, for instance, by impurities (heterogeneous nucleation). In the homogeneous nucleation, the formation of nuclei of a substance under the liquidus temperature is a stochastic process which involves density EQD ¼ Eg þ

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fluctuations of the medium where several atoms assemble into small crystals that are thermodynamically stable. The stability condition is achieved when the composition fluctuation and temperature in a determined volume can generate stable ensembles. The formation of the crystalline phase involves a change in the Gibbs free energy DG of the system given by two terms, namely, a negative one that corresponds to the energy freed up in the volumetric crystallization process DGv and a positive component representing the energy required for the creation of a new surface s. In the case of spherical cores with a radius r, the free energy of the system will be 4 DG ¼  pr 3 DGv þ 4pr 2 s (13.13) 3 From the resulting balance of Eq. (13.13), the nucleation may or may not occur. If the formation of a new crystalline phase is thermodynamically favorable, but the nuclei radius is too small, the surface component will be predominant under the volumetric part, the total free energy of the system will not be negative, and the nuclei cannot be stable. On the other hand, when a nucleus radius is big enough to have a stable system, it is called a crystallization core. The critical nucleation radius rc corresponds to the maximum change in free energy, it can be obtained solving the optimization problem of Eq. (13.13) in terms of r: dDG ¼  4pr 2 DGv þ 8prs dr

(13.14)

from which after setting ðdDG=drÞ ¼ 0, we obtain rc : 2s (13.15) DGv The required activation energy to surpass the nucleation barrier can be obtained combining Eqs. (13.13) and (13.15) (Fig. 13.2): rc ¼

DGmax ¼

16ps3

(13.16) 3ðDGv Þ2 The surface component s is less affected thermodynamically than DGv , therefore, the variation of DG depends almost completely on DGv which is proportional to the subcooling degree of the saturated solution TL  T : DGv ¼

TL  T L TL

(13.17)

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Figure 13.2 Gibbs free energy dependence on nuclei radius.

where TL is the liquidus temperature, T is the nucleation temperature, and L is the latent heat of fusion of the crystalline phase per volume unit. With the lesser degree of subcooling, in other words, when the nucleation temperature gets closer to the liquidus temperature, the system will have the maximum free energy DGmax needed for nucleation. In the limit case TL ¼ T , the free energy term and the critical radius will be infinite. The kinetic properties of nucleation are described by the nucleation rate, yN , which is the number of cores formed by unit of time, which depends on the temperature, the probability of molecular grouping in stable cores, and by the diffusion activation energy ED of the chemical components needed for nucleation, that is DGmax þED

yN ¼ Ae kT (13.18) The pre-exponential factor A represents the probability of collisions in the current orientation. When there is a small subcooling, DGv will be low and DGmax will be high, giving a low nucleation rate. For the opposite case, at lower temperatures (bigger subcooling) DGmax will be reduced until it is equal to ED and the nucleation rate will be maximum. If the temperature is decreased even further, the process will be dominated by ED , and yN will be decreased. When a saturated solution has foreign particles that do not belong to the nucleating phase, the nucleation process experiments a significant

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modification. The existence of surfaces due to the presence of impurities, bubbles, or even the walls of the recipient when the reaction is occurring reduces the surface energy s and therefore the value of the Gibbs free energy against the homogeneous nucleation. The influence of the heterogeneous phase is given by the contact angle q that depends of the relationship between the superficial tensions of the different phases: sHS  sHC Cosq ¼ (13.19) sCS Where sHS represents the superficial tension between the heterogeneity and the solution, sHC is the superficial tension between the heterogeneity and the primary crystalline phase, and sCS is the superficial tension between the crystalline phase and the solution. The maximum Gibbs free energy for nucleation in a heterogeneous process can be described by the homogeneous process Gibbs free energy, times a proportionality factor fq : ðDGmax Þhet ¼ DGmax fq

(13.20)

ð2 þ cosqÞð1  cosqÞ2 (13.21) 4 If there is no affinity between the heterogeneity and the nucleating phase, the contact angle will have a value q ¼ 180 having fq ¼ 1. In this case, the nucleation work will be the same as homogeneous nucleation. For the contrary, in the limit q ¼ 0 and fq ¼ 0, there will not be nucleation work. The stable cores with critical radius formed by either of the previously mentioned processes start growing by successive material deposition. The growing process depends on nucleation and the supply of the chemical components to the growing crystal. The crystal growth rate, in the same way as nucleation, has a maximum that depends on temperature (Fig. 13.3). At high temperatures, the growth rate is small due to the difficulty to dissipate the generated crystallization heat. For low temperatures in which the aforementioned heat can be dissipated, the growth rate is reduced due to the increased viscosity of the medium. The crystal growth rate ygr is defined as the number of constituents deposited per unit of time and unit of area on a nucleus and is given by h ih i ED DG ygr ¼ aAeRT 1  eRT (13.22) fq ¼

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Figure 13.3 Temperature dependence of nucleation and growth rates.

where a represents the linear crystal growth, A is a frequency factor, R is the ideal gas constant, and DG ¼ TLTT L. L For the case of a sole component, the growth rate can be expressed in terms of the diffusion coefficient: ED

D ¼ a2 AeRT (13.23) Combining Eqs. (13.22) and (13.23):   ðT T Þ D  RTL T L (13.24) 1e L ygr ¼ a The maximum values of the nucleation and growth rates shown in Fig. 13.3 rarely coincide, being the nucleation curve typically centered in lower temperatures. The control of the kinetic of both processes is essential to obtain the desired results in the growth of nanoparticles like QDs. In the following paragraphs, we present and discuss examples of the application of down-shifting QDs in silicon solar cells that have been reported elsewhere [14e18].

13.2 Application of quantum dot layers on commercially available silicon solar cells 13.2.1 CdTe quantum dots CdTe is a IVeVI direct-band gap semiconductor with excellent optoelectronic characteristics, that are further enhanced when it is obtained

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in the form of QDs. Thiol-capped CdTe QDs have ideal characteristics in the UV-visible range that have been previously reported [19,20], including wide absorption spectra, size-dependent photoluminescence, and high quantum yield. Thioglycolic acid (TGA)-coated CdTe QDs were obtained employing an efficient aqueous refluxing method [20]. To this end, 50 mL of a 4 mM solution of Cd(CH3COO)2$2H2O in deionized (DI) water was prepared. Then 18 mL of TGA was added, and the pH was adjusted with a 1 M NaOH solution between 10.5 and 11. Separately, 50 mL of a 0.8 mM solution of K2TeO3 in DI water was prepared. The previously prepared solutions were mixed. Finally, 0.08 g of NaBH4 was added to the previous mixture for the reduction of Te4þ to Te2, making the nucleation of CdTe particles possible. The final solution was transferred to a single-neck, roundbottom flask that was attached to a Liebig condenser. During the refluxing time, the flask remained submerged in laboratory oil at 100 C and was stirred at 500 RPM while being refluxed. QD size and photoluminescent emission wavelength can be controlled by the refluxing time [20,21]. To incorporate the QDs to functional solar cells, polymethylmethacrylate (495 PMMA A2 from Microchem) was selected due its high transparency in the 300e1100 nm range, low fragility, weather and UV resistance, and excellent thermal insulation. 3 mL CdTe QDs solutions were mixed with acetone (volume ratio 1:1) and centrifugated at 10,000 RPM for 10 min to promote sedimentation. After the supernatant was removed, the precipitated product was mixed with 1 mL of PMMA, and the CdTe QDs were sonicated for proper redispersion for 5 min. Upon dispersion, PMMA þ QDs thin films were spin cast on the window side of 52  38 mm commercially available polysilicon solar cells (Eco-worthy Company). Spin casting was carried out starting at 300 RPM for 10 s, followed by a speed of 4000 RPM for 45 s. After completion, the samples were annealed at 180 C to evaporate the solvents. As shown in Fig. 13.4A for a refluxing time of 15 min, an emission peak at 510 nm is observed, which shifts to 585 nm for a refluxing time of 12 h. All measurements were performed with an excitation wavelength of 395 nm. UV-Vis characterization exhibits the wide absorption spectra of the CdTe QDs, starting around 450 nm for the 15 min sample, shifting up to 600 nm for the samples with the longest refluxing time of 12 h (Fig. 13.4B).

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Figure 13.4 (A) Evolution of photoluminescence (PL) emission of synthesized CdTe QDs with reflux time. The inset shows the observed color variation under UV excitation. (B) CdTe absorption spectra for different refluxing times. Reprinted from E. Pelayo, A. Zazueta, R. Lopez, E. Saucedo, R. Ruelas, A. Ayon, Silicon solar cell efficiency improvement employing the photoluminescent, down-shifting effects of carbon and CdTe quantum dots, Mater. Renew. Sustain. Energy. 5 (2016) 1e7. https://doi.org/10.1007/s40243-016-0070-4. Under Creative Commons CC BY license.

Solar cell performance was quantified with a solar simulator under standard testing conditions (AM1.5G irradiance). Measurements were collected before and after the deployment of the PMMA þ QDs layer with different refluxing times of 15, 30 min, 1, 3, 5, 6, 8, and 12 h, respectively. The EQE was also measured with a quantum efficiency kit using a spot size of approximately 2 mm2. Table 13.1 summarizes the measured power conversion efficiency (PCE) of solar cells before and after the application the CdTe þ PMMA layers, which in all cases exhibited improvements in PCE values after the QDs were added. The down-shifting effects of the CdTe QDs in the spectral response of the solar cells are noticeable in Fig. 13.5.

13.2.2 Carbon quantum dots Carbon QDs, as well as inorganic fluorescent semiconductor nanoparticles, exhibit tunable fluorescence emissions, which open the possibilities for applications from biological imaging [22] to electronic and photonic devices [23]. Carbon QDs have the important distinction of being made of an abundant and generally nontoxic element that can further help in the environmentally friendly development of photovoltaic technologies. The carbon nanostructures were synthesized employing an alkali-assisted electrochemical fabrication method utilizing graphite rods for both the

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Table 13.1 Average performance of solar cells with and without CdTe QDs. Poly-silicon solar cell Voc (mV) Jsc (mA/cm2) FF (%) PCE (%) D PCE (%)

Sample 1 Sample 1 þ QDs 15 min Sample 2 Sample 2 þ QDs 30 min Sample 3 Sample 3 þ QDs Sample 4 Sample 4 þ QDs Sample 5 Sample 5 þ QDs Sample 6 Sample 6 þ QDs Sample 7 Sample 7 þ QDs Sample 8 Sample 8 þ QDs

1h 3h 5h 6h 8h 12 h

620.76 623.4

36.38 36.81

58.82 58.88

13.30 13.48

1.35

620.51 624.12

37.09 37.72

56.59 59.06

13.03 13.91

6.75

621.91 625.18 618.73 620.23 612.92 615.51 607.31 610.71 615.35 623.45 618.41 625.26

37.19 37.16 37.09 37.03 36.51 36.66 36.32 36.14 37.13 37.26 37.53 37.56

57.00 59.27 56.72 57.75 58.55 59.46 59.02 61.66 57.65 59.06 58.53 59.20

13.17 13.77 13.01 13.28 13.11 13.42 13.02 13.62 13.18 13.73 13.58 13.90

4.56 2.07 2.36 4.61 4.17 2.36

FF, fill factor; PCE, power conversion efficiency; QDs, quantum dots. Reprinted from E. Pelayo, A. Zazueta, R. Lopez, E. Saucedo, R. Ruelas, A. Ayon, Silicon solar cell efficiency improvement employing the photoluminescent, down-shifting effects of carbon and CdTe quantum dots, Mater. Renew. Sustain. Energy. 5 (2016) 1e7. https://doi.org/10.1007/s40243-0160070-4. Under Creative Commons CC BY license.

Figure 13.5 Overlay of the AM1.5G spectrum and solar cell external quantum efficiency (EQE) before and after the addition of CdTe quantum dots (QDs). The absorption and emission spectra are also included as a reference. Reprinted from E. Pelayo, A. Zazueta, R. Lopez, E. Saucedo, R. Ruelas, A. Ayon, Silicon solar cell efficiency improvement employing the photoluminescent, down-shifting effects of carbon and CdTe quantum dots, Mater. Renew. Sustain. Energy. 5 (2016) 1e7. https://doi.org/10.1007/s40243-016-0070-4. Under Creative Commons CC BY license.

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anode and the cathode [24,25]. The size of the carbon QDs is controlled by varying the applied current during synthesis, for instance, between 10 and 60 mA in a fixed time period. Graphite rods of 5 mm diameter were employed, with a separation between anode and cathode of 25.4 mm that were submerged 30 mm in a 100 mL electrolyte solution with a 99.5/0.5 volume ratio of ethanol and DI water with 0.3 g of NaOH added. The synthesis current was applied for 1 h upon the submersion of the graphite rods within the specified current range. Subsequently, the samples were stored 48 h at room temperature for stabilization, and the produced solutions were evaporated until a 5 mL of concentrated carbon QDs solution was left from each original solution. After evaporation, the samples were filtered employing a silica-gel chromatography column with a 100 mL mixture of petroleum ether and diethyl ether with a volume ratio 30/70. Finally, the solvents were evaporated in each vial to increase the carbon QD concentration [26,27]. The carbon QDs were also dispersed in polymethylmethacrylate, adding 1 mL of PMMA solution to the dried C nanocrystals and sonicated for 5 min to disperse the QDs. The PMMA þ carbon QD solution was spin cast on the window side of commercially available polysilicon solar cells (Ecoworthy Company). Variations in the applied current determine the size of the synthetized carbon nanoparticles and subsequently the PL emission [24]. Under UV excitation with a wavelength of 360 nm, the emission of C QDs obtained with different currents (10e60 mA) can vary from violet (400 nm) to green (550 nm) with the maximum centered around 420 nm (Fig. 13.6A). Carbon QD solutions exhibit broad UV-Vis absorption spectra that were centered around 300 nm from the sample obtained with lowest current (10 mA) to approximately 370 nm in the latter sample (60 mA) (Fig. 13.6B). Solar cell performance and EQE were quantified under the same conditions of the CdTe QDs. In the same fashion as the CdTe QDs, the down-shifting effects of the carbon QDs exhibit a positive impact in the spectral response of the solar cells (Fig. 13.7). Table 13.2 summarizes the measured PCE of solar cells before and after the application the PMMA þ carbon QDs layers obtained with different currents of 10, 20, 30, 50 and 60 mA, respectively. Carbon QD layers exhibited modest but nonnegligible increases in PCE values after the QDs were added.

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Figure 13.6 (A) Photoluminescence (PL) emission spectra of synthesized carbon quantum dots (QDs) employing different currents. The inset shows the observed color variation under UV excitation. (B) Carbon QDs absorption spectra dependence of current. Reprinted from E. Pelayo, A. Zazueta, R. Lopez, E. Saucedo, R. Ruelas, A. Ayon, Silicon solar cell efficiency improvement employing the photoluminescent, down-shifting effects of carbon and CdTe quantum dots, Mater. Renew. Sustain. Energy. 5 (2016) 1e7. https://doi. org/10.1007/s40243-016-0070-4. Under Creative Commons CC BY license.

Figure 13.7 Overlay of the AM1.5G spectrum and solar cell external quantum efficiency (EQE) before and after the addition of carbon QDs. The absorption and emission spectra are also included as a reference. Reprinted from E. Pelayo, A. Zazueta, R. Lopez, E. Saucedo, R. Ruelas, A. Ayon, Silicon solar cell efficiency improvement employing the photoluminescent, down-shifting effects of carbon and CdTe quantum dots, Mater. Renew. Sustain. Energy. 5 (2016) 1e7. https://doi.org/10.1007/s40243-016-0070-4. Under Creative Commons CC BY license.

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Table 13.2 Average performance of solar cells with and without carbon QDs. Poly-silicon solar cell Voc (mV) Jsc (mA/cm2) FF (%) PCE (%) D PCE (%)

Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample Sample

1 1 þ QDs 2 2 þ QDs 3 3 þ QDs 4 4 þ QDs 5 5 þ QDs 6 6 þ QDs

10 mA 20 mA 30 mA 40 mA 50 mA 60 mA

616.62 619.91 613.91 616.99 618.76 621.93 619.81 622.48 620.85 625.23 617.15 622.96

36.92 37.27 36.13 36.53 36.93 37.32 36.77 37.12 36.9 37.12 36.9 36.49

57.92 58.73 56.14 57.67 58.49 59.38 60.25 60.59 59.48 60.57 57.72 59.44

13.21 13.56 12.45 13.03 13.37 13.71 13.72 14.00 13.62 14.06 13.13 13.51

2.64 4.65 2.54 2.04 3.23 2.89

FF, fill factor; PCE, power conversion efficiency; QDs, quantum dots. Reprinted from E. Pelayo, A. Zazueta, R. Lopez, E. Saucedo, R. Ruelas, A. Ayon, Silicon solar cell efficiency improvement employing the photoluminescent, down-shifting effects of carbon and CdTe quantum dots, Mater. Renew. Sustain. Energy. 5 (2016) 1e7. https://doi.org/10.1007/s40243-0160070-4. Under Creative Commons CC BY license.

13.2.3 ZnO quantum dots ZnO is a IIeVI wide direct-band gap semiconductor (3.3 eV) with multiple optoelectronic applications ranging from blue lasers to solar cells [28,29]. Similar to other nanostructured semiconductors like CdTe QDs, the optoelectronic properties of colloidal ZnO QDs are size dependent. Colloidal ZnO QDs were synthesized employing a controlled precipitation method [30], where the growing dynamics was controlled by the pH of the precursor solutions on fixed reaction times. Specifically, starting with a 0.02 M solution of zinc acetate, the pH was adjusted adding a 0.1 M LiOH solution dropwise under constant stirring, pH values of 10 and 12 were targeted. Upon reaching the desired pH value the solution was placed in an ultrasonic bath for 3 h. Once the reaction was finished, hexane was added to the ZnO QDs solution in a 3:1 volume ratio to precipitate the ZnO particles and remove undesired subproducts. After the supernatant was discarded, the precipitated ZnO QDs were washed in ethanol and dispersed for storage. Volumes from 0.5 to 2 mL of ZnO QD storage solutions were centrifugated in separate vials to promote the precipitation of the ZnO nanoparticles. Subsequently, the supernatant of each vial was removed, leaving the precipitated ZnO nanoparticles ready to be dispersed in 1 mL of PMMA solutions. The PMMA þ ZnO QDs solution was spin cast on the window side of commercially available polysilicon solar cells.

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Figure 13.8 Absorbance and photoluminescence (PL) emission spectra of synthesized ZnO QDs with different pH values. The PL spectrum was obtained using an excitation wavelength of 340 nm.

Fig. 13.8 shows the normalized photoluminescent spectra of the synthetized ZnO QDs with a UV excitation wavelength of 340 nm, which extends from around 420 to 650 nm. The PL emission peak was red-shifted from the 510 nm for the pH 12 sample to 540 nm for the ZnO QDs obtained with pH 10. Fig. 13.8 also shows the absorption spectra of the analyzed samples. ZnO QDs synthetized with a pH value of 12 exhibited a well-defined band edge around 320 nm, while those synthetized with a pH value of 10 showed a gradual absorption curve with a peak around 350 nm and extending until approximately 800 nm, this behavior could be associated with a broader QD size distribution. The absorbance spectrum of the pH 10 solution could also be affected by light scattering, given the fact that the pH 10 ZnO QD solution was noticeably more turbid [15]. The absorption spectra of Fig. 13.8 can be used for the calculation of the band gap of the semiconductor employing Tauc’s Plot method described below. While investigating the optical and electronic properties of amorphous germanium, Tauc et al. [31] proposed a method for band gap determination using the absorption data plotted in terms of energy. Based on Tauc’s work on amorphous semiconductors, Davis and Mott [32] described how the absorption strength depends on the difference between the photon energy and the band gap: ðahvÞ1=n ¼ Aðhv  Eg Þ

(13.25)

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where h is Planck’s constant, v is the photon’s frequency, a is the absorption coefficient, and A is a proportionality constant. The value of the exponent denotes the nature of the electronic transition. Typically, the absorption processes of the down-shifting QDs applied in solar cells have allowed transitions with either n ¼ 1=2 or n ¼ 2 for direct and indirect transitions, respectively. ZnO is a semiconductor with direct allowed transitions having n ¼ 1= 2. The plot of Eq. (13.25) for both pH values of ZnO QDs is shown in Fig. 13.9. Near the band gap value, the absorption becomes more pronounced and exhibits a region of linearity in the Tauc Plot. The extrapolation of the linear zones of both samples in Fig. 13.9 renders band gap values of 3.37 and 3.65 eV for the QDs obtained with pH 10 and pH 12, respectively. The energy dependence on particle size described by the Brus model can be used to extract the approximate radii value of the ZnO particles obtained with different pH values using Eq. (13.11) with a bulk band gap value (Eg ) for ZnO of 3.3 eV, effective electron and hole masses m*e ¼ 0:26 m0 and m*h ¼ 0:59 m0 , respectively, with permittivityε ¼ 8:5 ε0 [33]. The resulting estimated diameters of 5.83 and 4.08 nm, respectively, for the samples of pH 10 and pH 12 agree with the dimensions determined by TEM and DRX [15]. Solar cell performance and EQE were quantified under the same conditions of the previously reported QDs.

Figure 13.9 Band gap energy determination of the ZnO quantum dots synthetized with different pH values employing the Tauc plot.

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Table 13.3 Measured performance of solar cells before and after deploying different concentrations of pH 10 ZnO QDs dispersed in PMMA. Poly-silicon solar cell Voc (mV) Jsc (mA/cm2) FF (%) PCE (%) D PCE (%)

Sample 1 Sample 1 þ 0.5 mL Sample 2 Sample 2 þ 1.0 mL Sample 3 Sample 3 þ 1.5 mL Sample 4 Sample 4 þ 2.0 mL

630 QDs 620 630 QDs 630 630 QDs 620 630 QDs 630

35.54 35.42 36.38 36.00 36.27 36.30 36.38 36.26

66.75 66.89 66.44 63.89 67.75 67.93 67.48 67.71

14.87 14.78 15.22 14.40 15.39 15.41 15.41 15.43

0.61 5.39 0.13 0.13

FF, fill factor; PCE, power conversion efficiency; QDs, quantum dots. Reprinted with permission from A. Zazueta-Raynaud, R. Lopez-Delgado, J.E. Pelayo-Ceja, M.E. Alvarez-Ramos, A. Ayon, Utilization of down-shifting photoluminescent ZnO quantum dots on solar cells, Mater. Res. Express 4 (2017) 076203. https://doi.org/10.1088/2053-1591/aa7824.

The effects in the measured PCE of the solar cells before and after the application of the ZnO þ PMMA layers synthetized with pH 10 is summarized in Table 13.3. The prospective benefits of the photoluminescent effects of the ZnO is thought to be affected by the negative impact in transparency made evident by the dispersion observed in the absorption spectra of the pH 10 sample in Fig. 13.8. On the other hand, the effects of the samples obtained with pH 12 show promising results (Table 13.4), Table 13.4 Measured performance of solar cells before and after deploying different concentrations of pH 12 ZnO QDs dispersed in PMMA. Poly-silicon solar cell Voc (mV) Jsc (mA/cm2) FF (%) PCE (%) D PCE (%)

Sample 1 Sample 1 þ 0.5 mL QDs Sample 2 Sample 2 þ 1.0 mL QDs Sample 3 Sample 3 þ 1.5 mL QDs Sample 4 Sample 4 þ 2.0 mL QDs

620 620

34.42 34.50

65.44 66.83

13.97 14.33

2.58

620 620

35.02 35.35

65.90 66.87

14.25 14.71

3.22

620 620

34.26 34.40

66.00 68.33

14.00 14.67

4.78

620 620

34.60 34.76

64.40 65.58

13.76 14.19

3.13

FF, fill factor; PCE, power conversion efficiency; QDs, quantum dots. Reprinted with permission from A. Zazueta-Raynaud, R. Lopez-Delgado, J.E. Pelayo-Ceja, M.E. Alvarez-Ramos, A. Ayon, Utilization of down-shifting photoluminescent ZnO quantum dots on solar cells, Mater. Res. Express 4 (2017) 076203. https://doi.org/10.1088/2053-1591/aa7824.

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Figure 13.10 (A) JeV curve from solar simulator and (B) external quantum efficiency (EQE) performance of a representative sample (pH 12, 1.5 mL concentration).

with improved values of PCE in all cases and a maximum gain of around 4.8%. The ZnO QD layer should have a positive influence in the entire range of operation of the silicon solar cell. Nevertheless, the decrease of the quantum efficiency within the range of 450e750 nm observed in Fig. 13.10B can be attributed to a diffraction index mismatch between the antireflection layer of the commercial cells and the applied PMMA þ QD layer.

13.3 Fabrication of solar cells with quantum dot layers 13.3.1 CdTe Commercially available polysilicon solar cells comprise an optimized Si3N4 antireflection layer and surface texturization. The down-shifting effects of photoluminescent QDs on those solar cells are reduced by the refractive index mismatch between the antireflective coating and the PMMA þ QDs layers. Higher performance gains could be achieved with the fabrication of solar cells optimized for the use of PMMA þ QDs layers. CdTe QDs were obtained using the aqueous refluxing technique previously mentioned, with a pH value of 11 and reaction times of 0.5, 1, 6, 8, and 12 h. The photoluminescent down-shifting effects of the CdTe QDs were evaluated with an excitation wavelength of 400 nm. The obtained QDs exhibit strong absorption in wavelengths below 550 nm [16].

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Known concentrations of QDs were centrifugated with acetone (1:1) to promote precipitation; after the supernatant was removed, the precipitated QDs were dispersed in different QD to PMMA ratios. For solar cell fabrication, n-type <100> 3  3 cm Si samples, with thicknesses of 620, 300, and 150 mm were cleaned using Piranha solution for 10 min at 80 C. The samples were then rinsed with DI water and dried with a N2 gun. Afterward, a standard RCA cleaning process was performed. Finally, the samples were cleaned with a 50:1 solution of DI water and HF (49%) for 60 s to remove the native SiO2 layer, rinsed with DI water and dried with a N2 gun. The texturized samples were obtained by metal-assisted chemical etching (MacEtch) at room temperature [34,35]. The texturized samples exhibit random nanopillar patterns with an average height of 400 nm [36]. Silver nanoparticles were uniformly dispersed on the c-Si substrates immersing the samples for 60 s in a solution composed of 50 mL of DI water, 0.01 M concentration of AgNO3, 9.75 mL of HF. Then, the c-Si substrates coated with Ag nanoparticles were immersed in an etching solution composed of a 1:3:9 ratio of H2O2, HF, and DI water. After surface nanotexturization, the Ag nanoparticles were removed immersing the substrates in an aqueous solution of nitric acid (HNO3) for 10 min [37]. For the formation of the pen junction in planar and texturized samples, on the front surface as well as an ohmic contact on the back surface, boron (p-type) and phosphorous (n-type) spin on dopant (SOD) solutions, respectively, were prepared by the solegel method [38]. First, solutions with 10 mL of tetraethyl orthosilicate (TEOS), 20 mL of ethanol, and 10 mL of DI water were prepared for each dopant. Afterward, 5 g of boron oxide (B2O3) and 5 g of phosphorous pentoxide (P2O5) were added to each of the previously prepared solutions. 1 mL of the n-type solution was spin cast on the back side of a live n-type, <100>, Si sample at 300 rpm (10 s) ramping quickly to the final speed of 1000 rpm (1 min) and ending at 300 rpm (10 s). The p-type SOD solution was spin cast on the surface of a sacrificial 3  3 cm, c-Si sample with the same speed ramping sequence. Subsequently, the sacrificial and the live samples were baked at 120 C for 15 min to remove the organic solvents. The live c-Si sample was placed in a furnace with the pristine side facing the surface of the sacrificial sample with the p-type film. The two samples were separated by 620 mm with thin pieces of clean silicon wafers. The samples were then annealed at 1000 C for diffusion of the dopants. Upon the annealing step, the live samples were immersed in a 10:1 solution of DI water and HF (49%) for 2 min to remove

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the thick SiO2 layer formed during the doping process. Finally, the live samples were rinsed with DI water and dried with a N2 gun. Aluminum layers of 200 nm were deposited by thermal evaporation on each side of the live samples as contacts. The back side Al layer was also used as a back-surface reflector while the top surface was patterned with a comblike mask. After metallization, the samples were annealed in a furnace at 580 C for 10 min to obtain ohmic contacts on both sides. Finally, PMMA þ QDs 100 nm thin films were spin cast over the window surface of the solar cells. CdTe QDs refluxed for 8 h were selected due their emission around 560 nm and relatively good particle size distribution [16]. Solar cell performance was quantified with a solar simulator under standard testing conditions (AM1.5G irradiance). The tests were performed with and without down-shifting CdTe QDs employing different silicon substrate thicknesses (600, 300, and 150 mm) including planar and randomly texturized samples with an average pillar height of 400 nm. The EQE was also measured for all samples. The effects of the spin cast PMMA þ QDs layer in the measured PCE of planar and nanotexturized solar cells are summarized in Tables 13.5 and 13.6, respectively. The collected measurements indicate that the utilization of down-shifting nanostructures improved the Voc and Jsc values in each case, resulting in an improvement of PCE values in all samples and being more pronounced on thinner samples (150 mm). The best overall efficiency (15%) was achieved on a 150-mm c-Si texturized solar cell with a 100 nm QDs þ PMMA layer. The EQE values of planar and texturized devices (Fig. 13.11) exhibited higher values across the range of interest (300e1100 nm) due to the combination of the antireflective effect of the Table 13.5 Average performance of planar solar cells with different substrate thicknesses. c-Si planar solar cells Voc (mV) Jsc (mA/cm2) FF (%) PCE (%) D PCE (%)

620 mm 620 mm þ CdTe QDs 300 mm 300 mm þ CdTe QDs 150 mm 150 mm þ CdTe QDs

527.60 529.00 564.50 567.90 551.20 553.70

38.60 46.40 38.70 44.30 37.40 43.00

56.70 52.00 56.80 54.60 63.90 61.80

11.60 12.60 12.40 13.70 12.80 14.40

8.62 10.48 12.50

FF, fill factor; PCE, power conversion efficiency; QDs, quantum dots. Reprinted with permission from U. Tronco-Jurado, E. Saucedo-Flores, R. Ruelas, R. L
opez, M.E. Alvarez-Ramos, A.A. Ay
on, Synergistic effects of nanotexturization and down shifting CdTe quantum dots in solar cell performance, Microsyst. Technol. 23 (2015) 1e9. https://doi.org/10.1007/s00542015-2748-4.

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Table 13.6 Average performance of randomly texturized solar cells with different substrate thicknesses. c-Si texturized solar cells Voc (mV) Jsc (mA/cm2) FF (%) PCE (%) D PCE (%)

620 mm 620 mm þ CdTe QDs 300 mm 300 mm þ CdTe QDs 150 mm 150 mm þ CdTe QDs

535.3 537.5 537.5 540.7 514.4 521.2

41.20 47.30 44.70 49.20 45.10 49.60

55.00 52.20 54.70 56.60 60.80 60.90

12.10 13.20 13.10 13.90 13.50 15.00

9.10 6.11 11.11

FF, fill factor; PCE, power conversion efficiency; QDs, quantum dots. Reprinted with permission from U. Tronco-Jurado, E. Saucedo-Flores, R. Ruelas, R. L
opez, M.E. Alvarez-Ramos, A.A. Ay
on, Synergistic effects of nanotexturization and down shifting CdTe quantum dots in solar cell performance, Microsyst. Technol. 23 (2015) 1e9. https://doi.org/10.1007/s00542015-2748-4.

Figure 13.11 Measured external quantum efficiency (EQE) for (A) planar and (B) texturized c-Si solar cells of different thickness, with and without CdTe QDs down-shifting structures. Reprinted with permission from U. Tronco-Jurado, E.
pez, M.E. Alvarez-Ramos, A.A. Ayo
n, Synergistic effects of Saucedo-Flores, R. Ruelas, R. Lo nanotexturization and down shifting CdTe quantum dots in solar cell performance, Microsyst. Technol. 23 (2015) 1e9. https://doi.org/10.1007/s00542-015-2748-4.

PMMA layers and the down-shifting effects of the CdTe QDs. For the particular case of the texturized cells, the light-trapping due to surface texturization further improves general performance, achieving the best results in both PCE and EQE values.

13.3.2 CdSe/CdS core-shell quantum dots Photoluminescent down-shifting CdSe/CdS core-shell QDs absorb light in the UV range of the solar spectrum and emit photons with larger

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wavelengths centered around 625 nm, which is well suited for c-Si absorption and electronehole pair generation. CdSe QDs were synthesized using a hot injection approach [39]. The CdS shell on CdSe QDs was grown via successive ionic layer absorption and reaction (SILAR) [40]. In this synthesis process, 1 g of trioctylphosphine oxide (TOPO) and 1 mL of a 0.38 M solution of cadmium oleate were mixed in 8 mL of 1-octadecene (ODE). The resulting solution was purged by N2 at room temperature for 30 min. The reaction system was evacuated for 30 min at 100 C, then the temperature was increased to 300 C. A solution composed of a mixture of 4 mmol of pure selenium and 4 mL of trioctylphosphine (TOP, 97%), 3 mL of oleylamine (OLA, 70%), and 1 mL of ODE was quickly injected into the Cd-oleate suspension under vigorous stirring. To clean the core CdSe QDs, ethanol was added, the suspension was centrifuged at 6000 rpm for 5 min, and the supernatant was removed. The core CdSe QDs were then dispersed in toluene. After the core was synthetized, the CdS shell was grown via SILAR. A solution comprising 5 mL of OLA, 5 mL of ODE, and approximately 2  107 mol of CdSe cores in hexane was degassed at 110 C for 3 min. Then the temperature was raised to 240 C with stirring. 0.25 mL of 0.2 M of Cd-oleate dispersed in ODE solution was added dropwise, the mixture was allowed to react for 2.5 h, followed by dropwise addition of 0.25 mL of a 0.2 M sulfur solution and an additional 1 h reaction time. The resulting solution was further annealed at the same temperature (240 C) for 10 min. The reaction was cooled down to room temperature using ice water. Ethanol was added, the suspension was centrifuged at 6000 rpm, and the supernatant was removed. The core-shell QDs were finally dispersed in toluene for storage. In this evaluation, planar single crystal solar cells were fabricated employing 620 mm thick, n-type, <100> Si wafers using the procedure previously described in Section 14.3.1. The CdSe/CdS QDs were directly spin cast on the surface of the fabricated cells at 5000 rpm for 60 s. The viscosity of the CdSe/CdS colloidal solutions allows dispersion by spin casting without the need of a polymeric matrix like PMMA. Fig. 13.12 shows the photoluminescent spectra of the synthetized CdSe/ CdS QDs with an excitation wavelength of 420 nm and the absorption spectra within the 450e700 nm range. The photoluminescence maximum of the core-shell QDs is located around 625 nm with a narrow band. The CdSe/CdS QDs exhibit a strong absorption in the visible range of 400e500 nm (Fig. 13.12). The large Stokes shift (w120 nm) between

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Figure 13.12 Photon absorption and photoluminescence (PL) spectra (with an excitation wavelength of 420 nm) of the synthesized CdSe/CdS quantum dots (QDs). The left inset shows the QDs under visible light. The right inset shows the QDs under UV excitation. Reprinted from R. Lopez-Delgado, Y. Zhou, A. Zazueta-Raynaud, H. Zhao, J.E.
Pelayo, A. Vomiero, M.E. Alvarez-Ramos, F. Rosei, A. Ayon, Enhanced conversion efficiency in Si solar cells employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots, Sci. Rep. 7 (2017) 14104. https://doi.org/10.1038/s41598-017-14269-0. Under Creative Commons CC BY license.

absorption and emission of the core-shell nanostructures can largely decrease the self-absorption of QDs after excitation, making them good candidates as downconverters for Si solar cells. The extrapolation of the linear zone of the Tauc Plot of the CdSe/CdS QDs shown in Fig. 13.13 indicates a band gap value of 2.45 eV. The average diameter of the CdSe/CdS core-shell obtained by HRTEM was around 10.44 nm (Fig. 13.14). The tight size distribution with a s value of 1.76 nm (Fig. 13.14B) explains the narrow photoluminescent emission band observed in Fig. 13.14. Solar cell performance was quantified with a solar simulator under standard testing conditions (AM1.5G irradiance). The tests were performed with and without down-shifting core-shell CdSe/CdS QDs spin cast thin films. EQE measurements were employed to provide independent corroboration of the Jsc values collected with the solar simulator, the short circuit current density was extracted from the EQE spectra by using the relation given by Eq. (13.4). Since EQE is employed to provide an independent verification of the measured values of the short circuit current density, is an indicator of the consistency of the collected values.

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Figure 13.13 Band gap energy determination using Tauc plot. Reprinted from R. Lopez-Delgado, Y. Zhou, A. Zazueta-Raynaud, H. Zhao, J.E. Pelayo, A. Vomiero, M.E.
Alvarez-Ramos, F. Rosei, A. Ayon, Enhanced conversion efficiency in Si solar cells employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots, Sci. Rep. 7 (2017) 14104. https://doi.org/10.1038/s41598-017-14269-0. Under Creative Commons CC BY license.

Figure 13.14 (A) HRTEM and (B) size distribution of CdSe/CdS QDs. Reprinted from R. Lopez-Delgado, Y. Zhou, A. Zazueta-Raynaud, H. Zhao, J.E. Pelayo, A. Vomiero, M.E.
Alvarez-Ramos, F. Rosei, A. Ayon, Enhanced conversion efficiency in Si solar cells employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots, Sci. Rep. 7 (2017) 14104. https://doi.org/10.1038/s41598-017-14269-0. Under Creative Commons CC BY license.

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Figure 13.15 (A) Currentevoltage characteristics of fabricated solar cells without quantum dots (QDs) (solid lines); with QDs (dashed lines). (B) Measured external quantum efficiency (EQE) of the solar cells (solid lines) and calculated short circuit current density from EQE data (dashed lines) without QDs (black lines) and with QDs (red lines). Reprinted from R. Lopez-Delgado, Y. Zhou, A. Zazueta-Raynaud, H. Zhao, J.E. Pelayo, A.
Vomiero, M.E. Alvarez-Ramos, F. Rosei, A. Ayon, Enhanced conversion efficiency in Si solar cells employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots, Sci. Rep. 7 (2017) 14104. https://doi.org/10.1038/s41598-017-14269-0. Under Creative Commons CC BY license.

As shown by the JeV curves on Fig. 13.15A, the procedure for solar cell fabrication is highly reproducible, as evinced by the values of the functional parameters for the three samples. The application of the QDs in c-Si solar cells increases the short circuit current density, Jsc, and leads to an average improvement of the PCE from (12.0  0.2) to (13.5  0.2) for an overall PCE improvement of 12.7%. The EQE response was improved in the wavelength segment extending from the UV region to the visible, with the maximum increase between 350 and 630 nm that can be considered responsible for the increase in photocurrent density. The presence of the QDs decreases the reflectivity in the full range (300e1050 nm) and is rather uniform in the analyzed range (Fig. 13.16). The enhanced EQE values in the 350e630 nm region do not follow the reflectance trend, are more pronounced in the absorption region of the QDs. Reflectance spectroscopy considers only photons that are either absorbed or reflected (under the assumption of no transmission), but it does not take into consideration the probability of electronehole pair generation of the absorbed photons in a solar cell. The increases in EQE values in the 350e630 nm region shown in Fig. 13.15B can be explained by the absorption and down-shifted emission of photons more favorable for photocurrent generation, meanwhile the general, but not so pronounced, increase

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Figure 13.16 Reflectance from the surface of the c-Si solar cell before and after deposition of the quantum dots (QDs) film. Reprinted from R. Lopez-Delgado, Y. Zhou, A.
Zazueta-Raynaud, H. Zhao, J.E. Pelayo, A. Vomiero, M.E. Alvarez-Ramos, F. Rosei, A. Ayon, Enhanced conversion efficiency in Si solar cells employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots, Sci. Rep. 7 (2017) 14104. https://doi.org/10.1038/ s41598-017-14269-0. Under Creative Commons CC BY license.

in EQE in the 630e1000 spectral range can be attributed to the change in reflectance in the device due the presence of the CdSe/CdS QDs layer. The combination of the down-shifting effects and antireflective properties of the CdSe/CdS QDs helps to achieve a relative increase in PCE of approximately 13%.

13.3.3 Silicon quantum dots Down-shifting silicon QDs (Si QDs) are able to emit lower energy photons that fall within the range of absorption by the underlying solar cell [41]. Silicon-based QDs have the advantage of being a well-known material that is relatively abundant and nontoxic. Si QDs were prepared using a one-step synthesis method employing mild reagents [42]. First, 1.25, 1.5, 1.75, and 2.0 mL of (3-aminopropyl) triethoxysilane (APTES, 99%) were added to vials with 4 mL of DI water each and stirred for 10 min (three sets were made). After that, 1.25 mL of 0.1, 0.2, and 0.3 M of (þ)-sodium L-ascorbate (SA, 98%) were added to each of the previous solutions to make combinations of different APTES volumes and SA concentrations. The resulting solutions were stirred for an additional 20, 30, 40, and 50 min, respectively. APTES and SA were used as silicon source for the former and as reducing agent for the latter.

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A batch of preliminary tests including photoluminescence, absorption, and particle size were performed using samples with different ratios of APTES and SA as well as with various reaction times [18]. The samples with the best photoluminescent properties were synthesized using 2.0 mL of APTES with 0.1 M SA and 30 min reaction (QD1); 1.25 mL of APTES with 0.1 M SA and 30 min reaction (QD2); and 1.25 mL of APTES with 0.1 M SA and 40 min reaction time (QD3). The selected samples had their absorption peaks centered at w500 nm and photoluminescent emissions centered at w530 nm [18]. Three solar cell sets, containing three solar cells each, were fabricated on n-type <100> silicon wafers with the same methodology described in the previous section. Si QD films were spin cast on the window side of the solar cells at 4000 rpm for 60 s (Table 13.7). Fig. 13.17 shows the currentevoltage characteristics of the c-Si solar cells before and after application of down-shifting silicon QDs, quantified with a solar simulator under standard testing conditions (AM1.5G irradiance). The average performance parameters of the three cells that conform each solar cell set are summarized in Table 13.8. EQE measurements were employed to provide independent corroboration of the Jsc values collected with the solar simulator (Fig. 13.18). The performance parameters of the fabricated cells before and after the application of the silicon QDs shown in Table 13.8 reflect a small deterioration in the fill factor of the samples, as well as relatively small improvements in the Voc. The most significant influence in PCE was given by the improvement in Jsc values, consequence of an overall better EQE behavior for all samples (Fig. 13.18). A pronounced enhanced spectral Table 13.7 Average performance of planar solar cells with different substrate thicknesses. Jsc (mA/cm2) FF D PCE Jsc (mA/cm2) Voc (from EQE) (%) PCE (%) (%) (mV) (measured) Sample

Solar cell 543.4 32.50 Solar 545.9 37.00 cell þ QDs

32.00 36.00

68.00 12.00  0.2 67.00 13.50  0.2 12.70

FF, fill factor; PCE, power conversion efficiency; QDs, quantum dots. Reprinted from R. Lopez-Delgado, Y. Zhou, A. Zazueta-Raynaud, H. Zhao, J.E. Pelayo, A.
lvarez-Ramos, F. Rosei, A. Ayon, Enhanced conversion efficiency in Si solar cells Vomiero, M.E. A employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots, Sci. Rep. 7 (2017) 14104. https://doi.org/10.1038/s41598-017-14269-0. Under Creative Commons CC BY license.

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Figure 13.17 Currentevoltage characteristics for c-Si solar cells before (continuous lines) and after (dashed lines) the application of Si quantum dots (QDs) samples: QD1 (A), QD2 (B), and QD3 (C). Reprinted with permission from R.Lopez-Delgado, H.J. Higuera-Valenzuela, A. Zazueta-Raynaud, A. Ramos-Carrazco, J.E. Pelayo, D. BermanMendoza, M.E. Alvarez-Ramos, A. Ayon, Solar cell efficiency improvement employing downshifting silicon quantum dots, Microsyst. Technol. 24 (2018) 495-502. https://doi.org/10. 1007/s00542-017-3405-x. Table 13.8 c-Silicon solar cell performance parameters before and after application of Si-QDs. Sample Voc (mV) Jsc (mA/cm2) FF (%) PCE (%) D PCE (%)

c-Si solar cell c-Si solar cell 1 þ QD1 c-Si solar cell c-Si solar cell 2 þ QD2 c-Si solar cell c-Si solar cell 3 þ QD3

set 1 set

532.57 536.2

33.42 38.28

66.87 65.20

11.90 13.37

12.36

set 2 set

528.27 538.76

33.99 37.90

67.19 65.26

12.06 13.32

10.44

set 3 set

536.16 540.16

34.13 38.12

63.66 62.31

11.66 12.82

9.95

FF, fill factor; PCE, power conversion efficiency; QDs, quantum dots. Reprinted with permission from R. Lopez-Delgado, H.J. Higuera-Valenzuela, A. Zazueta-Raynaud,
A. Ramos-Carrazco, J.E. Pelayo, D. Berman-Mendoza, M.E. Alvarez-Ramos, A. Ayon, Solar cell efficiency improvement employing down-shifting silicon quantum dots, Microsyst. Technol. 24 (2018) 495e502. https://doi.org/10.1007/s00542-017-3405-x.

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Figure 13.18 Average external quantum efficiency (EQE) of c-Si solar cell sets before (continuous lines) and after (dashed lines) the application of Si quantum dots (QDs) samples: QD1 (A), QD2 (B) and QD3 (C).The inset graphs represent the difference of the EQE after the application of the Si-QDs. Reprinted with permission from R. Lopez-Delgado, H.J. Higuera-Valenzuela, A. Zazueta-Raynaud, A. Ramos-Carrazco, J.E.
Pelayo, D. Berman-Mendoza, M.E. Alvarez-Ramos, A. Ayon, Solar cell efficiency improvement employing down-shifting silicon quantum dots, Microsyst. Technol. 24 (2018) 495e502. https://doi.org/10.1007/s00542-017-3405-x.

response is evident starting at 530 nm, due to the photoluminescent properties of the Si QDs in all samples that easily surpasses the losses due to QD absorption of photons with wavelengths smaller than 500 nm.

13.4 Conclusions In addition to the improvements achieved on silicon solar cells, down-shifting QDs like ZnSe [43] and carbon [44] have been used in perovskite solar cells that doubled as UV protection layers as well as down-shifting materials.

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The synthesis and characterization of CdTe, carbon, and ZnO QDs exhibit the desired down-shifting effects, where relatively high energy photons (UV range) are absorbed and reemitted in wavelengths in the range of 480e620 nm, 350e550 nm, and 510e540 nm for CdTe, carbon, and ZnO, respectively. The characterization of commercially available polysilicon solar cells before and after the application of thin layers of the aforementioned nanocrystalline semiconductors on the surface of these photovoltaic devices indicate nonnegligible increases in the PCE with relative improvements up to 6.75%, 4.65%, and 4.78% for CdTe, carbon, and ZnO QDs, respectively. The efficiency gains achieved with down-shifting QDs were further improved in fabricated solar cells, thanks to the optimization of the QD thin films employed as antireflective layers. CdTe, CdSe/CdS, and silicon QDs were also synthetized and characterized. For each variety of QDs, the best results were achieved by selecting the samples with the best luminescent characteristics, with emissions centered around 560, 625, and 630 nm for CdTe, CdSe/CdS, and silicon, respectively. The characterization of the fabricated solar cells before and after the application of thin layers of QDs on the surface of these photovoltaic devices indicate considerable improvements in the PCE with relative improvements of 12.50%, 12.70%, and 12.36% for CdTe (in addition of surface nanotexturization), CdS/CdS, and silicon QDs, respectively. The best overall efficiency (15%) was achieved on a 150 mm c-Si texturized solar cell with a 100 nm QDs þ PMMA layer. The experimental results of the prototype systems described in the present work evince that down-shifting QDs represent a promising option to improve the efficiency of existing Si-based solar cells, which could promote the widespread adoption of photovoltaic energy harvesting.

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