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Fabrication of Regularly Arranged Chalcopyrite Micro Solar Cells via Femtosecond Laser-Induced Forward Transfer for Concentrator Application Berit Heidmann, Stefan Andree, Sergiu Levcenko, Thomas Unold, Daniel Abou-Ras, Norbert Schaefer, Joern Bonse, Jörg Krüger, and Martina Schmid ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017
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Fabrication of regularly arranged chalcopyrite micro solar cells via femtosecond laserinduced forward transfer for concentrator application Berit Heidmann1,2, Stefan Andree3, Sergiu Levcenko1, Thomas Unold1, Daniel Abou-Ras1, Norbert Schäfer1, Jörn Bonse3, Jörg Krüger3, and Martina Schmid1,2 1
Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany Universität Duisburg-Essen und CENIDE, Fakultät für Physik, Lotharstr. 1, 47057 Duisburg, Germany 3 Bundesanstalt für Materialforschung und –prüfung (BAM), Unter den Eichen 87, 12205 Berlin, Germany 2
Corresponding Author:
[email protected]
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Abstract A laser-based, bottom-up technique is presented to for the fabrication of Cu(In,Ga)Se2 (CIGSe) micro solar cells. We use femtosecond laser-induced forward transfer (LIFT) to transport a metallic precursor composed of copper, indium and gallium onto a molybdenum back contact layer on a glass substrate. A CIGSe absorber forms by subsequent selenization. An array of micro absorbers with defined spacing is fabricated to solar cells and characterized under concentrated light illumination. The solar cell array exhibited a conversion efficiency of 1.4% at one sun as well as a significant efficiency enhancement of 68 rel.% under 20-fold concentration. This work demonstrates the possibility of directly grown, micrometer-sized solar cells based on chalcogenide absorber layers, enabling an effective material usage. Keywords: micro solar cells, light concentration, LIFT, chalcopyrite, local growth, Cu(In,Ga)Se2 Table of Contents Image
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Cu(In,Ga)Se2 (CIGSe) semiconductors, are efficient absorber materials used in thin film photovoltaics. However, indium and gallium are rare elements1 limiting photovoltaic production on the GW scale. This together with the desire to cover specific application by novel solar cell designs motivates the development of material-saving processes and technologies in solar cell production. One approach is to reduce the absorber thickness.2, 3 This usually leads to a reduced cell efficiency, which can be compensated by absorption enhancing nano-structures.4, 5, 6 An alternative approach is to reduce the effective absorber area of the solar cells, while simultaneously concentrating the incident solar radiation to the desired cell dimensions, by using micro-lenses.7 In III-V multi-junction photovoltaics centimeter-sized concentrator solar cells have been realized.8 In such solar cells the photo current increases linearly with the concentration factor (C). This leads to a beneficial increase in open circuit voltage
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்
lnሺܥሻ where n is the ideality factor, k the Boltzmann constant,
T the solar cell temperature and q the elementary charge. For multi-junction solar cells an absolute efficiency enhancement of 5-9% at 300-500 suns has been obtained.7 Also CIGSe solar cells have been investigated under concentration.9,10,11 In a proof-of-principle experiment reference absorbers were scratched to micro-meter size and a efficiency enhancement of 4.8% absolute achieved in case of dot-shaped micro solar cells.9 Furthermore, dot-shaped absorbers were grown onto a SiO2 patterned template showing photovoltaic effect,10 and 105 and 1105 µm wide line-shaped absorbers were deposited by electrodeposition onto a back contact grid achieving efficiencies of 5.3% and 7.6%, respectively.11 In our previous studies, indium islands were formed in a spatially controlled manner on a femtosecond laser-patterned molybdenum-coated glass substrate12,13 and fabricated into CISe solar cells.14 In this letter, we report a site-controlled laser-based technique to transfer single dots of metallic Cu-In-Ga precursor on a Mo/glass substrate. By subsequent selenization, micro absorbers are produced, which are successfully completed to solar cells. The optoelectronic and structural properties as well as the cell characterization including concentrated light conditions are discussed. In order to gain insights to the microstructure and elemental distributions in microabsorbers on glass substrates and those embedded in a solar cell stack, SEM imaging and EDX measurements have been carried out. For obtaining ordered micro absorber arrays, precursor dots are transferred via the LIFT process on a molybdenum back contact, arranged in a rectangular grid with 500 µm spacing. The average diameter of each micro absorber dot is about 100 µm and the average thickness is 3 - 4 µm. This geometrical design was chosen to match the requirements of commercial micro-optics, with respect to available micro-lens spacings and the focusability of natural light.
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The reliability of the LIFT deposition technique followed by selenization is demonstrated by a successful fabrication of a 5x5 micro absorber array [Fig. 1(a)]. A top-view EDX analysis of one of these micro absorbers is shown in Fig. 1b. Here we can evaluate that all absorber components like, indium, gallium, copper and selenium are present in the micro absorber. Copper and indium are laterally homogeneously distributed, while gallium shows slight inhomogeneities. The selenium signal arises from the micro absorber area as well as from the Mo back contact, indicating the existence of molybdenum selenides. In Fig. 1(c) crosssectional SEM and EDX analysis are presented. It shows the presence of Zn and O in the front contact and Mo in the back contact, as expected. Furthermore, it is visible that the surface of the Mo back contact is selenized [Fig. 1(c)]. The Cu signal is very homogeneously distributed across the micro absorber, while indium is depleted in proximity to the back contact and gallium towards the front contact. This gallium gradient is often visible in sequential processes.15 It is well known that the control of stoichiometry of the chalcopyrite materials is challenging, as seen above. However, the advantage of the LIFT technique may lie in the flexibility to tailor the precursor composition by adequate choice of precursor stack and sequence of transfer. Hence, upon further optimization on horizontal homogeneity, the resulting micro absorbers are expected to be of high and controllable material quality.
Figure 1: (a) SEM image of a 5x5 array of micro solar cells, (b) Top-view SEM and EDX images of a single CIGSe micro absorber, and (c) SEM and EDX cross-sections of a CIGSe micro absorber. The optoelectronic properties are investigated by photoluminescence mapping. A map of the spectral PL peak emission position is shown in Fig. 2(a), where grey represents the 4 ACS Paragon Plus Environment
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background, red an emission energy of 1.10 eV and purple the energies above 1.23 eV. Fig. 2(b) shows the statistical analysis of the PL map, which indicates a broad distribution of band gap energies over the entire absorber area. Nevertheless, the statistical analysis and an averaged PL spectrum over all measured points [Fig. 2(c)], show an average peak energy of 1.15 eV. With the assumption that PL measurements at room temperature are dominated by band-to-band recombination an optical band gap of 1.15 eV can be estimated. In comparison, for planar absorbers a band gap energy of 1.16 eV is reported for absorbers with a Ga content of ~0.3,16 similar to that in our precursor material. According to the distribution of the PL peak positions we can estimate ±0.07 Ga content variation based on the band gap dependency on Ga in CIGSe,17 the full width at half maximum of the averaged spectrum is broad (> 100 meV).
Figure 2: (a) Top-view PL map of peak emission energies, (b) histogram of peak emission energies, and (c) averaged PL spectrum. 5 ACS Paragon Plus Environment
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Figure 3 shows the results of the electrical characterization performed by current-voltage (IV) measurements via a solar simulator (AM1.5 standard). The temperature is stabilized to 25°C at 1 sun. The illumination strength of the sun simulator is tunable from 1 sun (103 W/m2) to 100 suns (105 W/m2). In contrast to other measurements under concentrated light, we do not calculate the concentration factor by normalizing the current, but rather by direct measuring the lamp power. For I-V measurements an array consisting of 5x5 micro solar cells with a diameter of 100 µm of each cell is characterized. The sample area was homogeneously illuminated including the surface in between the micro absorbers. Under one sun illumination we could achieve an open circuit voltage (VOC) of 145 mV, a short circuit current (ISC) of 8 µA and a fill factor (FF) of 33%, associated with a solar cell efficiency of 1.4%. Furthermore, the performance of our 5x5 micro solar cell array is shown. I-V-characteristics from 1 sun to 100 suns are visible in Fig. 3(a). A decrease of series resistance as well as shunt resistance is observed. The series resistance drops from 2.7x10-3 Ωcm² at 1 sun to 2.4x104 Ωcm² at 100 suns, while the shunt resistance drops from 162 Ωcm² to 2.4 Ωcm², which is in both cases much lower than in flat high efficiency cells, where the shunt resistance is higher than 800 Ωcm² and the series resistance is around 0.3 Ωcm².18 Under concentrated light, the device performance is improving, visible by an increased Isc and Voc [Fig. 3(b)]. From theory a linear increase of Isc with increasing light concentration is expected, while the Voc will increase logarithmically. Contrary to theory, in our measurements the Voc increases up to 30 suns, followed by a decrease at higher intensities. The FF is increasing up to 10 suns (from 33.5% to 38%) and decreasing at higher light concentration (to 33.3%), where it stays constant (not shown). As a result the solar cell efficiency is raising up to 20 suns and decreasing afterwards. In total, an efficiency enhancement of 68% relative can be achieved by increasing the concentration from 1 sun to 20 suns.
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Figure 3: Electrical performance of a 5x5 micro solar cell array fabricated by fs-LIFT. (a) I-V characteristics for various light concentration factors and (b) Isc and Voc as a function of the light concentration factor C. The decrease in VOC for C > 20 suns can be explained by temperature effects. Due to homogeneous light illumination over an area of 2.5x2.5 cm² this whole area is heated to the same temperature. Therefore, no heat is dissipated from the cells to the surrounding area of the micro concentrator solar cell. Using micro optics to focus the light onto the micro absorbers will not lead to such heating effects due to the local illumination. The nonilluminated parts of the device can decrease the heating of the solar cell significantly.10 Some limitations of the device performance are still present. Reasons for the limitation could be the formation of MoSe2 [visible in Fig. 1(c)], micro absorbers with voids [visible in Fig. 1(b)], non-optimized CdS deposition and lateral band gap inhomogeneities [visible in Fig. 2(a)]. Selenization of the molybdenum back contact layer can easily take place due to the large uncovered surface between the micro absorbers. A possibility to protect the molybdenum layer is covering the surface with an isolation layer after precursor preparation but before selenization.10 Low parallel resistance can be attributed to the voids. These problems can be solved by optimizing the solar cell fabrication such as refinement of the deposition of the micro precursors and using an alternative isolation concept. Further studies are required to identify the most critical of the effects limiting the efficiency of the solar cells. Nevertheless, we demonstrated a side-controlled local growth of CIGSe, which leads to functional solar cells improving under concentrated light illumination.
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In summary, a bottom-up fabrication of working micro solar cell arrays was demonstrated using femtosecond laser-induced forward transfer to locally deposit precursors for chalcopyrite absorbers. This method is realizing full material saving and efficiency enhancement, when light is focused onto the micro cells. Material saving of around 90% is achieved by transferring precursor layers locally on the molybdenum back contact. The CIGSe array solar cell shows a photovoltaic effect with a short-circuit current of 8 µA and an open-circuit voltage of 145 mV, associated with a solar cell efficiency of 1.4%, at one sun. Under 20-fold light concentration, a relative efficiency enhancement of 68% relative is achieved. The characterization shows that improvements can be expected from optimization of the fabrication process, like a more homogeneous laser transfer as well as an improved solar cell isolation. Experimental Methods The manufacturing procedure of CIGSe micro absorbers via fs-LIFT19 is shown in Fig. 4(a)4(c). A 150 µm thick microscopic cover glass coated with copper, indium and gallium was used as metallic precursor (donor). Those precursors were made by physical vapor deposition. Prior to the coating the glass substrate was ultrasonically cleaned with acetone and rinsed with isopropanol, ethanol and water, followed by nitrogen drying. The metal layer deposition took place under vacuum at room temperature. A deposition rate of 1 Å/s for copper and 5 Å/s for indium was used. Gallium is deposited via net weight to achieve 100 nm thickness. The donor is builded up as following: glass/60 nm Cu/150 nm In/60 nm Cu/100 nm Ga/60 nm Cu/150 nm In. By focusing a single fs-laser pulse through the glass onto the glass-precursor interface [Fig. 4(a)], a transfer of precursor material to a molybdenum back contact on an acceptor takes place [Fig. 1(b)]. For LIFT, a Ti:sapphire fs laser (Femtolasers, Compact Pro) with a center wavelength of 800 nm and a pulse duration of 35 fs was used. The transferred Cu-In-Ga precursor was subsequently selenized by a rapid thermal process to produce a chalcopyrite absorber [Fig. 1(c)]. For this purpose, the sample is placed in a graphite box with 0.046 g elemental selenium, which was first heated up to 200°C (10 min) and afterwards to 550°C (6 min) under nitrogen atmosphere at 800 mbar pressure.
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Figure 4: Schematics of a LIFT process. Absorber processing: (a) LIFT of precursor layer stack, (b) Transfer of precursor material to the molybdenum back contact, (c) Selenization of transferred precursor. Solar cell processing: (d) CIGSe micro absorber, (e) Isolation of micro absorber, (f) Removal of capping isolation by plasma etching. Further solar cell processing is described in in the right part of Fig. 4(d)-4(f). To fabricate micro solar cells out of the micro absorbers [Fig. 4(d)] an electrical isolation between molybdenum back contact and ZnO front contact is necessary. Therefore, SU-8 photoresist was used as isolation layer [Fig. 4(e)]. It was spin coated on the sample surface and subsequently pre-baked at 100°C, exposed to UV-light with a wavelength of 385 nm and then hard-baked at 200°C. Due to the viscosity of the SU-8 and the spin coating process, the final isolation layer on top of the micro-absorbers was significantly thinner than in between them. An additional plasma etching process allowed the removal of the isolation layer from the micro absorbers [Fig. 4(f)]. Subsequently, the top surfaces of the micro absorbers were electrically contacted by a window layer consisting of a 50 nm thick CdS buffer and a 380 nm thick ZnO/ZnO:Al front contact. The CdS buffer layer was deposited by chemical bath deposition in a 1.1 mol ammonia, 0.14 mol thiourea and 0.002 mol cadmium acetate solution, heated to 60°C. The ZnO/ZnO:Al front contact was deposited by magnetron sputtering of a ZnO-bi-layer consisting of a 150 nm thick intrinsic ZnO and a 230 nm thick ZnO doped with 2 wt% Al2O3. For electrical characterization a regular array consisting of 25 micro cells (500 µm spacing) connected in parallel was measured. For an enhanced collection of carriers, Ni:Al grid fingers, located in between the absorber array rows, were deposited on the front contact. For structural analyses scanning electron microscopy (SEM) imaging and energy-dispersive Xray (EDX) analyses were performed using a LEO GEMINI 1530 and a Zeiss UltraPlus scanning 9 ACS Paragon Plus Environment
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electron microscopes, equipped with Thermo Noran and Oxford Instruments X-Max 80 X-ray detectors. For SEM imaging and EDX, electron energies of 3 and 10 keV were applied, respectively. Room temperature photoluminescence (PL) measurements using a pulsed laser diode (660 nm wavelength, 2.5 MHz pulse repetition frequency) as an optical excitation source were performed for optoelectrical analyses. The PL signals were detected by a 0.5 m Czerny-Turner grating monochromator (150 l/mm, spectral resolution ~ 0.52 nm) equipped with a liquid N2 cooled linear InGaAs diode array. Spatial maps of the PL signal were recorded with an acquisition time of 2 s, a 1.6 µm step size and an optical excitation spot diameter of about 1.5 µm. Electrical I-V-characterization of the micro solar cells was carried out with a class AAA solar simulator under AM1.5 standard illumination conditions. For light concentration measurements the power of the solar simulator lamp was tuned up to 100 suns.
Acknowledgements The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) through SCHM 2554/3-1 and KR 3638/3-1. B. Heidmann and M. Schmid are grateful to the Helmholtz Association for support from the Initiative and Networking Fund for the Young Investigator Group VH-NG-928. The authors would like to thank M. Kirsch for ZnO sputtering and grid deposition as well as O. Ernst (Leibniz Institute for Crystal Growth, Berlin) for preparation of the isolating layers. References 1. 2.
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