Ultra-wide Spectral Response of CIGS Solar Cells Integrated with

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Ultra-wide Spectral Response of CIGS Solar Cells Integrated with Luminescent Down Shifting Quantum Dots Ho-Jung Jeong, Ye-Chan Kim, Soo Kyung Lee, Yonkil Jeong, Jin-Won Song, Ju-Hyung Yun, and Jae-Hyung Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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ACS Applied Materials & Interfaces

Ultra-wide Spectral Response of CIGS Solar Cells Integrated with Luminescent Down Shifting Quantum Dots Ho-Jung Jeong, † Ye-Chan Kim, ‡ Soo Kyung Lee, ‡ Yonkil Jeong, § Jin-Won Song, ǁ Ju-Hyung Yun,*﬩ and Jae-Hyung Jang *†‡§ † Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea ‡ School of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea § Research Institute for Solar and Sustainable Energies, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea ǁ Materials Development Division, ECOFLUX Co., Ltd, Korea ﬩ Department of Electrical Engineering, Incheon National University, Yeonsu Incheon 406-772, Republic of Korea KEYWORDS : Solar cells, CIGS, Quantum dot, Luminescent down shifting, Light trapping, Light harvesting

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ABSTRACT

Conventional Cu(In1-x,Gax)Se2 (CIGS) solar cells exhibit poor spectral response due to parasitic light absorption in the window and buffer layers at short wavelength range between 300 nm to 520 nm. In this study, CdSe/CdZnS core/shell quantum dots (QDs) acting as a luminescent down-shifting (LDS) layer were inserted between the MgF2 anti-reflection coating and the window layer of the CIGS solar cell to improve the light harvesting in the short wavelength range. The LDS layer absorbs photons in the short wavelength range and re-emits photons in the 609 nm range, which are transmitted through the window and buffer layer and are absorbed in the CIGS layer. The average EQE in the parasitic light absorption region (300 – 520 nm) was enhanced by 51%. The resulting short circuit current density of 34.04 mA/cm2 and power conversion efficiency of 14.29% of the CIGS solar cell with CdSe/CdZnS QDs were improved by 4.35% and 3.85%, respectively, as compared to those of the conventional solar cell without QDs.

INTRODUCTION The Cu(In1-x,Gax)Se2 (CIGS) thin film solar cell is one of the most attractive candidates for replacing the crystalline silicon solar cell, which accounts for about 90% of the photovoltaic market today. The CIGS solar cell is currently the fastest growing photovoltaic technology, and its cell efficiency has increased annually by 0.7% during the last 3 years. Presently, the highest recorded CIGS cell efficiency is 22.6%1 that is close to the highest efficiency of 25% achieved by crystalline silicon solar cells. The typical structure of a CIGS device is a MgF2 anti-reflection coating (ARC)/Ni-Al grid/ZnO


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widow layer [Al-doped ZnO (ZnO:Al)/intrinsic-ZnO (i-ZnO)]/CdS buffer layer/CIGS absorber layer/Mo back contact/soda lime glass (SLG) configuration. This structure, however, has poor spectral response at short wavelengths, due to parasitic absorption losses in the window and buffer layers. The largest absorption loss takes place in the UV region, which accounts for 7% of the useful solar spectrum. Therefore, recycling the UV energy can provide a further margin of improvement to the CIGS solar cell efficiency. Photons with wavelengths shorter than 370 nm, which have an energy higher than the band gap energy of ZnO (3.4 eV), are absorbed in the top window layer. In addition, the CdS buffer layer with a bandgap energy of 2.4 eV absorbs photons with wavelengths shorter than 520 nm. The charge carriers generated in those layers are quickly recombined and cannot contribute to photocurrent. As a result, the quantum efficiency of the CIGS solar cell is significantly reduced in the range of wavelengths between 300 nm and 520 nm. Unfortunately, the record-high performing CIGS cell suffered from the same issue.1 For this reason, reducing the absorption losses in the both the window and buffer layers is the most urgent challenge to improve the performance of CIGS solar cells. In the last decade, research efforts aiming at harvesting more photons at shorter wavelengths have focused on finding wide bandgap window and buffer materials to substitute for ZnO and CdS. However, the performance of CIGS devices with such alternative materials have not been as high as that achieved with the combination of ZnO/CdS (window/buffer) layers,2 and a device without a CdS buffer layer also exhibited poor performance.3 One of the approaches to overcome this problem and achieve the theoretical CIGS solar cell maximum efficiency of 33%4 is to employ luminescent down-shifting (LDS), which was first proposed by Hovel et al.5 Organic dyes, rare-earth ions and semiconductor based quantum dots (QDs) are being widely studied as


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LDS materials. These materials have the ability to absorb the short wavelength photons and convert them into longer wavelength photons, which can provide a better spectral response by avoiding the lossy layers in the path of incident light. Among LDS materials, QDs have been intensively investigated in recent years due to their attractive optical properties. The QDs are nano-sized semiconductor materials with wide absorption and narrow emission bands, high quantum yield, and their band gap is tunable by changing their size.6-8 Various QDs, including CdS,9-11 CdSe,12 ZnSe,13 CuGaSe2,14 graphene,1517

CdSe/ZnS,18-21 CdZnS/ZnS22-23 and CuInS2/ZnS24 have been utilized as LDS materials in

several types of solar cells and their performance improvements have been reported, as shown in Table 1. Quantum dots with different PL peaks have been studied for LDS materials for silicon18 and GaAs solar cells20. CdSe/ZnS QDs having PL peak wavelengths of 542 nm and 530 nm have been utilized for silicon and GaAs solar cells, respectively. There have been several attempts to integrate CIGS solar cells with LDS materials, including QDs21 and others25-27. Previous studies have shown that the performance enhancement in the short wavelength range is very low, and most of the improvements resulted from light scattering effects caused by the LDS materials. In this study, we demonstrate a CIGS device integrated with CdSe/CdZnS core/shell QDs, and quantitatively analyzed its performance. Various quantities of QDs were dispersed between the MgF2 ARC and window layers. The CIGS solar cells with QDs exhibited superior spectral response over a broad solar spectrum. In particular, the cells’ quantum efficiency was significantly improved in the UV region, compared with the previous works.21, 25-27


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RESULTS AND DISCUSSION The schematic structure and the measured external quantum efficiency (EQE) of the CIGS solar cell that was used as a reference cell for this study are presented in Figures 1(a) and (b), respectively. The low EQE at wavelengths shorter than 520 nm is mainly due to the parasitic optical absorption loss in the ZnO layer and CdS buffer layer. The cut-off wavelengths of ZnO and CdS are 370 and 520 nm, respectively. (Supporting Information, Figure S2) The ZnO:Al and i-ZnO layer absorbs photons with wavelengths shorter than 370 nm (Region I) and the CdS buffer layer absorbs photons with wavelengths in the range between 370 nm and 520 nm (Region II). The parasitic absorption losses are 93.65% and 39.07% for regions I and II, respectively. The maximum achievable Jsc under an Air Mass 1.5 Global (AM1.5G) spectrum was calculated to be 0.72 mA/cm2 in region I and 7.03 mA/cm2 in region II. However, the Jsc extracted from the measured EQE of the reference CIGS solar cell was 0.07 mA/cm2 in region I and 4.54 mA/cm2 in region II, respectively, indicating that there is a further obtainable Jsc of 3.14 mA/cm2 from the regions I and II. To improve light harvesting at short wavelengths, CdSe/CdZnS QDs were employed in the LDS layer between the MgF2 ARC and ZnO window layer of the CIGS solar cell. Figure 2a displays the schematic structure of the CIGS solar cell with the LDS layer. For CIGS solar cells with a CdS buffer layer and ZnO based window layer, an LDS material with emissions at wavelengths longer than 570 nm is desired. Considering the finite spectral linewidth of the QD, the QDs were designed to have their PL peak position at 609 nm. The absorption and photoluminescence (PL) spectra of the QDs dispersed in toluene are shown in Figure 2b. The PL spectrum was obtained with an excitation wavelength of 325 nm. The


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absorption and PL spectra indicate that the CdSe/CdZnS QDs absorb light at wavelengths shorter than 630 nm and re-emit an orange-colored light with an emission peak at 609 nm. This downshifted light can pass through the window and buffer layers without energy loss and reach the CIGS absorber layer directly, generating charge carriers. Photographs of the CdSe/CdZnS QDs solution under room lighting and with a UV-lamp at 365 nm are shown in the inset of Figure 2b. The possible light paths and interactions between

neighboring QDs in the LDS layer are depicted in Figure 3. The emitted light ( = 609 nm) of ⑤, ⑧ can generate charge carriers within the CIGS absorber layer and produce photocurrent. However, other light paths cannot contribute to current generation. Specifically, the re-absorbed light of ⑥ is a major loss mechanism in the LDS layer. The light that is re-absorbed by neighboring QDs is not involved in the absorption-emission process, which results in a large spectral overlap between the absorption and emission spectra. A relatively small spectral overlap has been associated with the large Stokes shift observed in core/shell structured QDs, and is attributed to a small reabsorption loss compared to other luminecent materials, such as organic dyes and rare-earth ions.28 To investigate the optical properties of the LDS layer, a QDs layer was formed on a saphire substrate and capped with MgF2. The quality factor of the LDS, which is the ratio of the

absorption coefficient, , to the re-absorption coefficient, , was calculated to be 15.7~8.7 and 8.7~2.3 in loss regions I and II, respectively (Supporting Information, Figure S3). A low quality factor leads to a high re-absorption loss and directly limits the maximum available concentration of QDs in the LDS layer, which eventually limits the efficiency of the LDS layer in solar cells.29,30 The calculated quality factor in this study indicates that the current gain is maximized in region I (the UV-region) and is gradually reduced in region II, and re-absorption


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dominates near the emission band. In the structure of the Air/LDS/CIGS solar cell, the LDS process can be described by the

following. Once incident photons ( < 630 nm reach the LDS layer with a fraction of 1 −

R, the QDs will absorb the photons until the intensity is fully attenuated at a ratio of ,

and then emit down-shifted photons ( = 609 nm) in arbitrary directions. Diffused photons ( ) with beyond-critical angles (

!)

are guided laterally and transferred to the CIGS absorber

with multiple bounces on the LDS/ZnO interface. However, a quantity of photons with below-critical angles (

!)

escape to the air/LDS interface,

which reduces trapping efficiency. In this case, photon leakage through the four lateral sides of the LDS layer can be neglected due to the small escape area. The efficiency of the down-shifting layer (η#$% can be simply defined as:

η#$% = η × η'( × η)# 1 ,

where η is the fraction of the light absorbed into the LDS layer, η'( is the fraction of the

light trapped inside the LDS layer and η)# is the quantum yield of the QD itself. The η)# was

measured to be ≈ 0.9 in the PL spectra (Figure S4). η can be expressed as: η = 1 − R+1 − , 2 ,

where R is a reflection of the incident photon flux, and . is an average optical path length in the

LDS layer. Considering the high absorption coefficient of the QDs (on the order of 105 cm-1) and

the few µm-thick LDS layer (Figure S5), e is negligible and the equation η012 = 1 − R+1 − e , can be simply given by η = 1 − R.

The η'( is expressed by the escape cone loss, 3, which is the escape fraction of the total


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number of photons emitted from the LDS layer to air. Using Snell’s law, the difference in refractive index at the interface determines the critical angle to be Substituting

!

into the fraction of the escape cone, 3 = > 1 − cos 7

!

= 4567

! ,

89:

8; η'( = 1 − B1 − C1 − D E F 4 6#$% 2 To estimate the performance of the down-shifting layer, the refractive index of the LDS layer

(6#$% was obtained using spectroscopic ellipsometer. In the configuration with air/LDS layer

(6#$% ≈ 1.58 at ), η#$% was calculated to be ≈ 75%, where η = 0.94 and η'( = 0.89. The introduction of a quarter-wavelength-thick MgF2 ARC layer (refractive indexof MgF> ,

6VWXY ≈ 1.38 at ) on the LDS layer can improve η#$% to ≈ 78% by a dramatic suppression

of light reflection, where η = 0.97. It is important to note here that introducing the MgF2

layer does not affect the value of η'( , since the critical angle is identical to the air/LDS layer

for the same boundary conditions. To verify the effects of the LDS layer on top of the CIGS solar cell, we directly dropped CdSe/CdZnS QDs dispersed in toluene at a concentration of 40 µg/µl on the window layer. Different quantities of QDs, from 40 to 160 µg, were employed by using a micro-pipette, and then the QDs were capped with a 105-nm-thick MgF2 layer. The structural properties of the QDs on the window layer were analyzed by high resolution


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transmission electron microscopy (HRTEM). The HRTEM images of QDs prepared by dropcasting method on the window layer were taken using a JEOL JEM-2100F with the accelerating voltage of 200 kV, as shown in Figure 4. The cross sectional TEM image (Figure 4 a) of the interface between the QDs and window layer shows that the particles are well crystallized and well dispersed. In addition, since columnar grown ZnO:Al films have a pyramid shaped surface morphology, they provide highly efficient in-coupling between the ZnO:Al and CIGS absorber. The lattice structure of the QDs can be observed in the HRTEM image (Figure 4b). The lattice parameter was measured to be 0.36 nm, which corresponds to a wurtzite structure, and the average diameter of the QDs was found to be around 5.5 nm. The average diameter, D, of the CdSe core can also be calculated by the following Equation 518,31: D = 1.61 × 10\ λ^ − 2.66 × 10_ λ` + 1.62 × 10` λ> − 0.43λ + 41.57 5 , where λ is the first absorption peak. The calculated CdSe core diameter was determined to be about 4.6 nm with a 0.45-nm-thick CdZnS shell, where λ is 600 nm (Figure 2). To analyze the optical properties of the MgF2/LDS bilayer on the top surface of the CIGS solar cell, reflectance measurements were carried out in diffuse mode. The surface reflectance spectra of devices with just an LDS layer, and with a MgF2/LDS bilayer, are compared in Figures 5a and b, respectively. Figure 5a shows that the devices with only an LDS layer exhibit a reduction in surface reflection in the wavelength range from 400 to 1200 nm with increasing quantities of QDs. Although the introduction of QDs causes increased surface reflection in wavelengths below 400 nm, the increase in efficiency due to the energy down-shifting is much greater than the decrease in efficiency caused by the reflection loss. The huge mismatch of refractive index between the air and LDS causes a higher reflection of incoming light compared to the air/MgF2 interface. As can be seen in Figure 5b, the surface


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reflection spectra of the CIGS devices with the MgF2/LDS bilayer were reduced across the whole wavelength range from 300 to 1200 nm, as compared to that of the devices with the LDS single layer, via stepwise graded refractive index profile. As a result, the MgF2 layer can lead to superior light absorption in CIGS devices and prevent oxidation of the QDs as well. To investigate the photoelectric characteristics of CIGS solar cells with various quantities of QDs, EQE, current density-voltage (J-V) measurements were carried out under spectral irradiance (AM 1.5G). The spectral region highlighted in yellow can be effectively exploited by the QDs down-shifting mechanism, as shown in Figure 6. The conventional CIGS devices (with and without ARC) exhibited low EQEs at wavelengths shorter than 520 nm. However, the EQEs of the CIGS devices integrated with MgF2/LDS improved in the same wavelength range with increasing quantities of QDs. Furthermore, the devices exhibited enhanced performance in the visible wavelength region due to the reduced surface reflection. Figure 7 shows the photo-generated current gain, which was calculated using the following equation: Δde = fpq`rr st q × Φij7.kl λ × ∆EQEλdλ 6 , pq7>rr st

where λ is the wavelength, q is the charge of the electron, ΦAM1.5G(λ) is the photon flux of the AM1.5G solar spectrum. ∆EQE(λ) is EQE#$%/jvwY − EQExy or EQEjvwY −

EQExy , where EQExy is the EQE of the reference CIGS device without LDS, and MgF2 ARC layer, EQEjvwY and EQEjvwY /#$% are the EQEs of the devices with the MgF2 and MgF2/ LDS bilayer, respectively. The current gain calculated by Equation 6 are 0, 1.77, 1.99, 1.89 and 1.82 mA/cm2 for the solar cells having 0, 40, 80, 120 and 160 µg of QDs, respectively. As expected, all devices integrated


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with QDs had increased current gain over the entire wavelength region. In particular, the gains at short wavelengths are significantly improved by the effect of energy down-shifting. However, current losses (negative current gain) are observed in the wavelength range from 550 to 630 nm with the increasing quantities of QDs, caused by the re-absorption of light, as discussed above. The CIGS device integrated with 80 µg QDs had the maximum current gain of 1.99 mA/cm2. For an LDS layer consisting of 80 µg of QDs, the current gain and loss can be calculated by

integrating Δde with respect to Δλ in Figure 7. Down-shifted photons contribute to 37.7% of the total current gain, while 0.4% of current gain is lost due to re-absorption at wavelengths around 600 nm. The remaining 62.7% is the current gain obtained by suppressing the light reflection beyond the absorption spectrum of the QDs. This suggests that to fully exploit the LDS effect, light trapping schemes should also be considered to maximize the down-shift of both UV and blue-red light. The device parameters of the CIGS solar cells with the MgF2/LDS bilayer under AM 1.5G solar illumination are listed in Table 2.The Jsc and PCE are improved, whereas the open circuit voltages (Voc) are almost constant, and the fill factor (FF) is slightly reduced due to uncontrolled shunt paths caused by introducing a new layer. The EQE, internal quantum efficiency (IQE) and J-V curves of the CIGS device with 80 µg of QDs and the reference cell are shown in Figure 8. If all the photons absorbed in the QDs were converted without any loss, the IQE can be nearly 100%. However, the IQE of the device is saturated at a maximum value of 72% at 300 nm, due to the extra losses in the LDS layer resulting from re-absorption, the reflection of light and the non-unity quantum yields of the QDs. The device exhibited a significant average EQE enhancement of 51% in the wavelength range from 300 to 520 nm as compared with its


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corresponding reference device. The Jsc and PCE values of the device with 80 µg of QDs were relatively improved by 4.35% and 3.85%, respectively, as compared with the device without the QDs layer.

CONCLUSIONS This study demonstrated an enhancement in the photovoltaic characteristics of CIGS solar cells using CdSe/CdZnS QDs. By successfully integrating QDs as an LDS layer between the MgF2 ARC and ZnO window layers, the CIGS device performance was enhanced by extending its spectral response. The improved optical and electrical characteristics of the CIGS devices were achieved by optimizing the quantities of QDs. The optimized device exhibited relative enhancements in Jsc and PCE of 4.35% and 3.85%, respectively. This result can be attributed to the improvement in current collection due to down-shifting and light trapping effects in the MgF2/LDS layer. The greatly enhanced device performance proves that further current gain can be achieved by introducing CdSe/CdZnS QDs in conventional CIGS solar cells.

EXPERIMENTAL SECTION Device fabrication and characterization. Two-mm-thick SLG substrates (2.5×2.5 cm2), which are commonly used for high efficiency CIGS solar cells, were cleaned by acetone, methanol and deionized water. The 1-µm-thick Mo back contact was deposited on the SLG substrate by DC sputtering. The CIGS thin films were grown on the Mo/SLG substrate by three stage coevaporation process.32 Cu, In, Ga and Se elements were co-evaporated at high substrate temperature. In the first stage, the (In,Ga)2Se3 precursor layer was grown at a substrate


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temperature of 350°C. Then, the (In,Ga)2Se3 layer was exposed to Cu and Se to make a Cu-rich CIGS film. In the third stage, small amounts of In,Ga and Se were evaporated to make a slightly Cu-poor CIGS film. During the second and third stage processes, the substrate temperatures were maintained at 550°C. After the three-stage process, the thickness of the CIGS film was about 2 µm and the ratios of Cu/(In+Ga) and Ga/(In+Ga) film were ~0.9 and ~0.28, respectively. In order to form a p-n junction with the p-type CIGS, a 60-nm-thick CdS buffer layer was deposited on the CIGS layer by the chemical bath deposition method. Afterwards, transparent conductive ZnO window layers with 70-nm-thick i-ZnO and 350-nm-thick ZnO:Al were sequentially deposited by RF sputtering. Finally, 50-nm-thick Ni and 1-µm-thick Al metal grid were deposited by electron beam evaporation. To explore the effects of CdSe/CdZnS QDs on CIGS solar cells, different quantities of QDs, dissolved in toluene, were dropped on top of the devices using a micro-pipette. Afterwards, the cells were annealed at 80°C for 10 minutes to evaporate the solvent in the QDs solution. Finally, 105-nm-thick MgF2 was deposited to serve as ARC and capping of the LDS layer. The cell area of the device was 0.5cm2.The surface reflectance of the devices was investigated by UV-visible spectrophotometer (Cary-5000, Varian). The J-V characteristics were measured under AM 1.5G illumination calibrated by solar simulator (Oriel Sol3A Class AAA, Newport) with a light intensity of 100 mW/cm2. EQE spectra were obtained with an incident photon to current efficiency system (QEX7 series, PV Measurements, Inc.) under a 75 W xenon lamp (UXL-75XE, Ushio) as the light source.


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ASSOCIATED CONTENT Supporting Information. The following topics are described in the supporting information: Synthesis and characterization of CdSe/CdZnS QDs; photographs of CIGS solar cells under room light and UV-illumination (Figure S1); optical absorption characteristics of the ZnO and CdS layers (Figure S2); quality factor of LDS layer (Figure S3); photoluminescence quantum yield of QDs (Figure S4); thickness of the LDS layer (Figure S5); additional electrical characterization of CIGS devices (Figure S6 and Figure S7); the fractional improvements of Jsc and PCE with respect to the quantity of QDs (Figure S8); CIGS solar cells integrated with InP/ZnS QDs (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors Ju-Hyung Yun * E-mail : [email protected] Jae-Hyung Jang * E-mail : [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENTS This work was supported by the Core Technology Development Program for Next- Generation Solar Cells of Research Institute for Solar and Sustainable Energies (RISE), GIST, and by Korea Electric Power Corporation (Grant number:R17XA05-13), and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT and Future Planning) (No. 2017R1A2B3004049).

REFERENCES

(1) Jackson, P.; Wuerz, R.; Hariskos, D.; Lotter, E.; Witte, W.; Powalla, M. Effects of Heavy Alkali Elements in Cu(In,Ga)Se2 Solar Cells with Efficiencies up to 22.6%. Phys. Status Solidi RRL. 2016, 10, 583–586. (2) Naghavi, N.; Abou-Ras, D.; Allsop, N.; Barreau, N.; Bücheler, S.; Ennaoui, A.; Fischer, C. H.; Guillen, C.; Hariskos, D.; Herrero, J.; Klenk, R.; Kushiya, K.; Lincot, D.; Menner, R.; Nakada, T.; Platzer-Björkman, C.; Spiering, S.; Tiwari, A. N.; Törndahl, T. Buffer Layers and Transparent Conducting Oxides for Chalcopyrite Cu(In,Ga)(S,Se)2 Based Thin Film Photovoltaics: Present Status and Current Developments. Prog. Photovoltaics 2010, 18, 411– 433. (3) Orgassa, K.; Rau, U.; Nguyen, Q.; Schock, H. W.; Werner, J. H. Role of the CdS Buffer Layer as an Active Optical Element in Cu(In,Ga)Se2 Thin-Film Solar Cells. Prog. Photovoltaics 2002, 10, 457–463.


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(4) Siebentritt, S. What Limits the Efficiency of Chalcopyrite Solar Cells? Sol. Energy Mater. Sol. Cells 2011, 95, 1471–1476. (5) Hovel, H. J.; Hodgson, R. T.; Woodall, J. M. The Effect of Fluorescent Wavelength Shifting on Solar Cell Spectral Response. Sol. Energy Mater. 1979, 2, 19–29. (6) Reiss, P.; Protière, M.; Li, L. Core/shell Semiconductor Nanocrystals. Small 2009, 5, 154– 168. (7) Klampaftis, E.; Ross, D.; McIntosh, K. R.; Richards, B. S. Enhancing the Performance of Solar Cells via Luminescent Down-Shifting of the Incident Spectrum: A Review. Sol. Energy Mater. Sol. Cells 2009, 93, 1182–1194. (8) Huang, X.; Han, S.; Huang, W.; Liu, X. Enhancing Solar Cell Efficiency: The Search for Luminescent Materials as Spectral Converters. Chem. Soc. Rev. 2013, 42, 173–201. (9) Chen, H. C.; Lin, C. C.; Han, H. W.; Tsai, Y. L.; Chang, C. H.; Wang, H. W.; Tsai, M. A.; Kuo, H. C.; Yu, P. Enhanced Efficiency for c-Si Solar Cell with Nanopillar Array via Quantum Dots Layers. Opt. Express 2011, 19, A1141-A1147. (10) Chen, H. C.; Lin, C. C.; Han, H. V.; Chen, K. J.; Tsai, Y. L.; Chang, Y. A.; Shih, M. H.; Kuo, H. C.; Yu, P. Enhancement of Power Conversion Efficiency in GaAs Solar Cells with Dual-Layer Quantum Dots Using Flexible PDMS Film. Sol. Energy Mater. Sol. Cells 2012, 104, 92–96.


16

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Lin, C. C.; Chen, H. C.; Tsai, Y. L.; Han, H. V.; Shih, H. S.; Chang, Y. A.; Kuo, H. C.; Yu, P. Highly Efficient CdS-Quantum-Dot-Sensitized GaAs Solar Cells. Opt. Express 2012, 20, A319–A326. (12) Lee, Y. J.; Yao, Y. C.; Tsai, M. T.; Liu, A. F.; Yang, M. D.; Lai, J. T. Current Matching Using CdSe Quantum Dots to Enhance the Power Conversion Efficiency of InGaP/GaAs/Ge Tandem Solar Cells. Opt. Express 2013, 21, A953–A963. (13) Jung, J. Y.; Zhou, K.; Bang, J. H.; Lee, J. H. Improved Photovoltaic Performance of Si Nanowire Solar Cells Integrated with ZnSe Quantum Dots. J. Phys. Chem. C 2012, 116, 12409-12414. (14) Ho, C. R.; Tsai, M. L.; Jhuo, H. J.; Lien, D. H.; Lin, C. A.; Tsai, S. H.; Wei, T. C.; Huang, K. P.; Chen, S. A.; He, J. H. An Energy-Harvesting Scheme Employing CuGaSe2 Quantum Dot-Modified ZnO Buffer Layers for Drastic Conversion Efficiency Enhancement in Inorganic-Organic Hybrid Solar Cells. Nanoscale 2013, 5, 6350–6355. (15) Lee, K. D.; Park, M. J.; Kim, D. Y.; Kim, S. M.; Kang, B.; Kim, S.; Kim, H.; Lee, H. S.; Kang, Y.; Yoon, S. S.; Hong, B. H.; Kim, D. Graphene Quantum Dot Layers with EnergyDown-Shift Effect on Crystalline-Silicon Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 19043-19049. (16) Kim, J. K.; Park, M. J.; Kim, S. J.; Wang, D. H.; Cho, S. P.; Bae, S.; Park, J. H.; Hong, B. H. Balancing Light Absorptivity and Carrier Conductivity of Graphene Quantum Dots for HighEfficiency Bulk Heterojunction Solar Cells. ACS Nano 2013, 7, 7207–7212.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

(17) Tsai, M. L.; Wei, W. R.; Tang, L.; Chang, H. C.; Tai, S. H.; Yang, P.-K.; Lau, S. P.; Chen, L.-J.; He, J.-H. Si Hybrid Solar Cells with 13% Efficiency via Concurrent Improvement in Optical and Electrical Properties by Employing Graphene Quantum Dots. ACS Nano 2016, 10, 815–821. (18) Baek, S. W.; Shim, J. H.; Seung, H. M.; Lee, G. S.; Hong, J. P.; Lee, K. S.; Park, J. G. Effect of Core Quantum-Dot Size on Power-Conversion-Efficiency for Silicon Solar-Cells Implementing Energy-Down-Shift Using CdSe/ZnS Core/shell Quantum Dots. Nanoscale 2014, 6, 12524–12531. (19) Sadeghimakki, B.; Gao, Z.; Sivoththaman, S. Proof of Down-Conversion by CdSe/ZnS Quantum Dots on Silicon Solar Cells. IEEE Photovoltaic Spec. Conf., 40th 2014, 2262–2266. (20) Han, H. V.; Lin, C. C.; Tsai, Y. L.; Chen, H. C.; Chen, K. J.; Yeh, Y. L.; Lin, W. Y.; Kuo, H. C.; Yu, P. A. Highly Efficient Hybrid GaAs Solar Cell Based on Colloidal-Quantum-DotSensitization. Sci. Rep. 2014, 4, 5734. (21) Liao, Y. K.; Brossard, M.; Hsieh, D. H.; Lin, T. N.; Charlton, M. D. B.; Cheng, S. J.; Chen, C. H.; Shen, J. L.; Cheng, L. T.; Hsieh, T. P.; Lai, F. I.; Kuo, S. Y.; Kuo, H. C.; Savvidis, P. G.; Lagoudakis, P. G. Highly Efficient Flexible Hybrid Nanocrystal-Cu(In,Ga)Se2 (CIGS) Solar Cells. Adv. Energy Mater. 2015, 5, 1401280. (22) Baek, S. W.; Shim, J. H.; Park, J.-G. The Energy-down-Shift Effect of Cd0.5Zn0.5S-ZnS Core-Shell Quantum Dots on Power-Conversion-Efficiency Enhancement in Silicon Solar Cells. Phys. Chem. Chem. Phys. 2014, 16, 18205–18210.


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Page 19 of 31

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(23) Ghymn, Y. H.; Jung, K.; Shin, M.; Ko, H. A Luminescent Down-Shifting and Moth-Eyed Anti-Reflective Film for Highly Efficient Photovoltaic Devices. Nanoscale 2015, 7, 1864218650. (24) Gardelis, S.; Nassiopoulou, A. G. Evidence of Significant down-Conversion in a Si-Based Solar Cell Using CuInS2/ZnS Core Shell Quantum Dots. Appl. Phys. Lett. 2014, 104, 183902. (25) Klampaftis, E.; Ross, D.; Seyrling, S.; Tiwari, A. N.; Richards, B. S. Increase in ShortWavelength Response of Encapsulated CIGS Devices by Doping the Encapsulation Layer with Luminescent Material. Sol. Energy Mater. Sol. Cells 2012, 101, 62–67. (26) Muffler, H. J.; Bär, M.; Lauermann, I.; Rahne, K.; Schröder, M.; Lux-Steiner, M. C.; Fischer, C. H.; Niesen, T. P.; Karg, F. Colloid Attachment by ILGAR-Layers: Creating Fluorescing Layers to Increase Quantum Efficiency of Solar Cells. Sol. Energy Mater. Sol. Cells 2006, 90, 3143–3150. (27) Glaeser, G. C.; Rau, U. Improvement of Photon Collection in Cu(In,Ga)Se2 Solar Cells and Modules by Fluorescent Frequency Conversion. Thin Solid Films 2007, 515, 5964–5967. (28) Coropceanu, I.; Bawendi, M. G. Core/shell Quantum Dot Based Luminescent Solar Concentrators with Reduced Reabsorption and Enhanced Efficiency. Nano Lett. 2014, 14, 4097–4101. (29) Klimov, V. I.; Baker, T. A.; Lim, J.; Velizhanin, K. A.; McDaniel, H. Quality Factor of Luminescent Solar Concentrators and Practical Concentration Limits Attainable with Semiconductor Quantum Dots. ACS Photonics 2016, 3, 1138–1148.


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Page 20 of 31

(30) Banal, J. L.; White, J. M.; Ghiggino, K. P.; Wong, W. W. H. Concentrating AggregationInduced Fluorescence in Planar Waveguides: A Proof-of-Principle. Sci. Rep. 2014, 4, 4635. (31) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals. Chem. Mater. 2003, 15, 2854–2860. (32) Contreras, M. A.; Tuttle, J.; Gabor, A.; Tennant, A.; Ramanathan, K.; Asher, S.; Franz, A.; Keane, J.; Wang, L.; Scofield, J.; Noufi, R. High Efficiency Cu(In,Ga)Se2-Based Solar Cells: Processing of Novel Absorber Structures. Proc. IEEE 1st World Conf. Photovolt. Energy Convers. (WCPEC) 1994, 1, 68–75.


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Figures and captions

Figure 1. (a) Schematic structure of a typical CIGS thin film solar cell. (b) EQE of the reference CIGS device. The two main parasitic light absorption regions are the ZnO:Al/i-ZnO widow layer in region I (93.65%), and the CdS buffer layer in region II (39.07%). The total current loss in both regions I and II is 3.14 mA/cm2.


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Figure 2. (a) Schematic design of CIGS device incorporating CdSe/CdZnS QDs deposited onto the window layer. (b) The absorption and PL spectra of CdSe/CdZnS QDs. The PL spectrum was obtained using an excitation wavelength of 325 nm. The insets are photographs of QDs solution taken under room light, and with UV illumination of 365 nm.


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Figure 3. Schematic of light paths and interaction between neighboring particles after QDs absorb the incoming light. The light can be absorbed ① or reflected ② without emission. Light which is not down-shifted can pass through the QDs ③ or be absorbed by neighboring QDs ④. The light emitted by the QDs can be absorbed in the CIGS absorber layer without energy loss ⑤ and be re-absorbed by neighboring QDs without a new emission ⑥. The neighboring QDs can reflect ⑦ and emit ⑧ the light, after absorbing the light ④.


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Figure 4. (a) Cross section TEM images of the interface between the QD and the ZnO window layers. (b) High resolution image showing the particle sizes of the CdSe/CdZnS (core/shell) QDs.


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Figure 5. The surface reflectance spectra of the CIGS devices with a single LDS layer (a) and MgF2/ LDS bilayers (b).


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Figure 6. The EQE of CIGS devices with varying quantities of QDs, along with spectral irradiance (AM 1.5G). All the devices integrated with QDs exhibit improved EQE via the down shifting mechanism at short wavelengths (highlighted in yellow).


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Figure 7. The current gain and loss curves of CIGS devices for various quantities of QDs. The maximum current density gain of 1.99 mA/cm2 was achieved by introducing 80 µg of QDs in the LDS layer.


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Figure 8. (a) Comparison of EQE and IQE spectra for the CIGS solar cell incorporating 80 µg of QDs, and the EQE of the reference cell. (b) Current density and voltage characteristics of the device as compared to the reference cell.


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Tables and captions Table 1. Solar cells integrated with various types of QDs and their performance improvements.

Solar cell

Silicon

GaAs

Organic

CIGS

Quantum Dot

∆%Jsc

∆%PCE

Reference

CdS

2.76

33

[9]

ZnSe

23.75

12.88

[13]

Graphene

2.94

2.7

[15]

CdSe/ZnS

6.21

5.5

[18]

CdSe/ZnS

6.55

3.58

[19]

CdZnS/ZnS

6.45

6.4

[22]

CuInS2/ZnS

28

37.5

[24]

CdS

18.37

21.52

[10]

CdS

20.98

18.91

[11]

CdSe

9.78

10.4

[12]

CdSe/ZnS

21.9

24.65

[20]

CdZnS/ZnS

3.36

3.24

[23]

CuGaSe2

9.41

23.8

[14]

Graphene

5.92

13.43

[16]

Graphene

12.92

14.95

[17]

CdSe/ZnS

11.28

10.92

[21]


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Table 2. Performance parameters of the CIGS solar cells with various quantities of CdSe/CdZnS QDs.

CIGS solar cell

Jsc [mA/cm2]

Voc [V]

Efficiency [%]

Fill Factor [%]

Bare cell

32.71

0.62

14.04

68.5

MgF2/QD(40 µg)

34.11

0.62

14.58

68.2

Bare cell

32.62

0.61

13.76

68.4

MgF2/QD(80 µg)

34.04

0.61

14.29

68.1

Bare cell

32.22

0.61

13.63

68.6

MgF2/QD(120 µg)

33.56

0.61

14.09

67.8

Bare cell

32.25

0.62

13.7

67.8

MgF2/QD(160 µg)

33.49

0.62

14.2

67.7


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Abstract Graphic


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Ultra-wide Spectral Response of CIGS Solar Cells Integrated with

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