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Graphite distributed electrodes for diamond-based photon-enhanced thermionic emission solar cells
Carbon 111 (2017) 48e53
Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Graphite distributed electrodes for diamond-based photon-enhanced thermionic emission solar cells Marco Girolami a, *, Luigino Criante b, Fabio Di Fonzo b, Sara Lo Turco b, Alessandro Mezzetti b, Andrea Notargiacomo c, Marialilia Pea c, Alessandro Bellucci a, Paolo Calvani a, Veronica Valentini a, Daniele M. Trucchi a a
CNR-ISM, Consiglio Nazionale delle Ricerche, Istituto di Struttura della Materia, Sede Secondaria di Montelibretti, Via Salaria km 29.300, 00015, Monterotondo Stazione, Roma, Italy CNST-IIT, Center for Nano-Science and Technology, Istituto Italiano di Tecnologia, Via Giovanni Pascoli 70/3, 20133, Milano, Italy c CNR-IFN, Consiglio Nazionale delle Ricerche, Istituto di Fotonica e Nanotecnologie, Sede Secondaria di Roma, Via Cineto Romano 42, 00156, Roma, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 July 2016 Received in revised form 23 September 2016 Accepted 25 September 2016 Available online 27 September 2016
Graphite conductive vertical microchannels, fabricated by femtosecond laser treatment, are proposed as distributed electrodes in defect-engineered (“black”) single-crystal diamond cathodes for innovative solar cells. Energy conversion is based on photon-enhanced thermionic emission, where the reduction of the cathode series resistance is crucial to preserve conversion efficiency. Process yield higher than 80% and resistivity as low as 0.75 ± 0.15 U cm were achieved by the optimization of laser parameters and the use of a multi-pass writing technique. A 100 100 array of graphite electrodes was integrated in a diamond-based cathode prototype, reducing the series resistance of more than 10 orders of magnitude with respect to bulk diamond. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction In the field of solar energy concentrating systems, conversion modules based on Photon-Enhanced Thermionic Emission (PETE) [1,2] have been proposed as a valid alternative to conventional solar cells. Basically, a PETE module consists of a semiconductor-based cathode, engineered for an optimal combination of thermionic and photovoltaic properties. Photon absorption in the solar spectrum range increases the population of conduction band, thus triggering the thermionic emission of electrons from the cathode surface at far lower temperatures than pure thermionic emitters. At the same time, PETE modules ensure a far higher conversion efficiency (up to 60%) [3] than conventional solar cells. Cathodes based on silicon [4] and III-V compounds [5] have been recently proposed: in particular, the bandgap of III-V structures is matched to the solar spectrum by adopting the so-called “bandgap engineering”. However, in order to make the electrons escape efficiently from these materials, it is mandatory to reduce electron affinity c by adding suitable coatings, usually made of alkali metal films [1,6,7], which
* Corresponding author. E-mail address: [email protected] (M. Girolami). http://dx.doi.org/10.1016/j.carbon.2016.09.061 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
suffer from instability at temperatures higher than 100e200 C: this strongly limits, at the moment, the use of III-V semiconductors as materials for PETE cathodes. An alternative way of developing a PETE device consists in the exploitation of diamond as active medium for the cathode. Diamond overcomes III-V compounds and silicon limitations thanks to the possibility of obtaining negative electron affinity (c ¼ 1.1 eV) by hydrogenating the emitting surface [8]: unlike alkali metal coatings, H-termination of diamond surface is indeed stable up to about 800 C [9]. On the other hand, diamond bandgap (5.47 eV) is too wide to be matched to solar spectrum, and conventional bandgap engineering is not applicable. For this purpose, a “defect engineering” strategy is being adopted by introducing in a controlled way a certain amount of defects within the bandgap able to interact more effectively with solar radiation. In previous works, we showed that surface nanotexturing, obtained by femtosecond laser treatment, is able to enhance solar absorptance (up to 95%) [10] and photogeneration [11] by inducing an intermediate band within the diamond bandgap, supporting an efficient photoelectronic conversion of solar photons. Nanotexturing of the lightreceiving surface and hydrogenation of the emitting surface can be both efficiently exploited in diamond-based PETE cathodes [12] (Fig. 1), in which photoelectrons generated under the illuminated
M. Girolami et al. / Carbon 111 (2017) 48e53
Fig. 1. Sketch of a PETE conversion module based on a defect-engineered black diamond cathode. A more accurate description of the cathode operating principle can be found in Ref. [12]. (A colour version of this figure can be viewed online.)
surface diffuse through the diamond bulk towards the emission surface, on the other side of the cathode: for this purpose, it is mandatory for the diamond bulk to have a high diffusion length, and that's the reason why single-crystal diamond plates are used in the present work. At the same time, absorption of the infrared part of the solar spectrum, as well as thermalization of photoelectrons, causes the overall increase of cathode temperature, enhancing thermionic emission from the emitting layer, that consists of a hydrogenated n-doped diamond film (few hundreds of nm thick) directly grown over the intrinsic diamond bulk. Hydrogenation of the emitting surface enables negative electron affinity: therefore, photoelectrons diffused from the light-receiving surface, as well as thermal electrons on the emitting surface (i.e. electrons on the tail of the Fermi-Dirac distribution overlapping the conduction band) are directly emitted into vacuum. In summary, the photonic process of light absorption and photogeneration is mostly attributed to the nanotextured lightreceiving surface, whereas thermionic emission is mainly triggered from the hydrogenated n-doped emitting layer. Ideally, illuminated and emitting surfaces should be as close as possible to each other, in order to make easier for photoelectrons to reach the emitting layer. Unfortunately, thickness of commercially available single-crystal diamond plates is in the range 300e1200 mm. The major drawback of such large values of thickness is that cathode series resistance, due to the extremely high electrical resistivity of bulk diamond (>1012 U cm), would be too high to sustain the emitted current. In other words, electrons emitted from the cathode (and collected by the anode) must be refilled by the external circuit at the cathode load contact: if the path from the load contact to the emitting layer is highly resistive, electrons injected into the cathode are hampered from reaching the hydrogenated surface and replace the emitted electrons, thus making impossible to sustain a steady-state current flow. Of course, this applies only to thermal electrons that follow Fermi-Dirac distribution, whereas transport of photoelectrons, directly promoted into the conduction band of bulk diamond, is only diffusion-limited. The technological innovative solution we introduce in the present work to drastically reduce the cathode series resistance by
49
several orders of magnitude, thus avoiding bottlenecks for the refilling of thermal electrons and sustaining efficiently the emitted current, is given by the laser writing of graphite microchannels within the diamond bulk, acting as distributed electrodes. In this way, electrically conductive graphite columns connect the lightreceiving surface, where a “frame-shaped” load contact is fabricated, to the emitting layer, forming a low-resistance path for the electrons injected by the external circuit. Electrical continuity between the array of graphite columns and the cathode load contact is ensured by a p-type highly conductive (0.1 U cm) layer, obtained by boron ion implantation just below the light-receiving surface [13]. Laser microstructuring of diamond is a well-established technology, usually employed for the fabrication of surface [14] and buried [15e17] graphitic contacts in 3D diamond detectors for ionizing radiation and charged particles. Ultrashort laser pulses cause a transition of diamond into graphite through an irreversible optical breakdown in the bulk material, induced locally by avalanche photoionization [18]. The structural, geometrical, and electrical properties of the graphite channels produced are strongly related to the laser processing parameters. The use of pulses in the fs range is preferred to longer pulses to avoid plastic strain due to thermal effects, which would cause lattice damage in the volume surrounding the channels, with detrimental effects on the charge transport properties of bulk diamond. The major drawback of fs pulses is the lower electrical conductivity of the channel if compared to structures fabricated with ns-pulsed laser sources, due to a lower concentration of the sp2 phase [19,20]. However, by a proper combination of energy density (laser power, repetition rate, objective used) and graphitization speed (i.e. the laser writing speed), it is possible to maximize the sp2/sp3 ratio, leading to channels with a higher degree of graphite crystallinity [21] and, as described later, with lower resistivity. In case of horizontal graphitic wires, i.e. fabricated parallel to the diamond surface, it is possible to further decrease resistivity by performing multiple laser scans [22]. Here we demonstrate that a similar result can also be obtained for graphitic vertical columns. 2. Experimental A standard grade, also known as “IIa grade”, single-crystal diamond sample (4.5 4.5 mm2, 500 mm thick, <110> oriented, polished on both sides, with boron concentration < 0.05 ppm and nitrogen concentration < 1 ppm), produced by Element Six Ltd. [23], was subjected to laser treatment for the fabrication of different groups of graphite channels, aimed at finding the best set of treatment parameters to be employed for the distributed electrodes of the PETE cathode prototype. A laser micromachining system equipped with a regeneratively amplified mode-locked femtosecond laser based on Yb:KGW active medium (Light Conversion, Pharos) was used, producing pulses with 515 nm wavelength, 280 fs duration, and 1 kHz repetition rate. Pulse energy E was varied in the range E ¼ 0.01e1 mJ. Columns were fabricated by focusing the laser beam through a 20 microscope objective (Mitutoyo, NA ¼ 0.4) onto the bottom surface of the sample, placed on a three-axis computer-controlled translation stage (ABL 1000, Aerotech) with a resolution of 2 nm, and then by moving the sample along the beam direction (z-axis) with speed v varying in the range v ¼ 2e1000 mm s1. Besides, a multi-pass writing process was employed for each (E, v) pair, similarly to what reported in Ref. [22] for horizontal wires, with laser focus scanning the sample vertically 1, 2 and 4 times, slightly shifting (1 mm) the focal position between each pass to avoid multiple treatments of the same spot. All the fabricated columns showed a diameter ranging from 2 to 10 mm, with larger diameters corresponding to higher values of laser pulse energy and to multiple scans.
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M. Girolami et al. / Carbon 111 (2017) 48e53
3. Results and discussion After the fabrication process, visible micro-Raman analysis was performed on all the different sets of graphite columns by using the emission wavelength (l ¼ 514.5 nm) of an Arþ laser. The (E, v) pair, as well as the number of laser passes, were selected on the basis of the best graphitic features shown in Raman spectra. As reported in Fig. 2(a) and (b), spectra obtained for intermediate values of pulse energy and graphitization speed show a better resolved G-band around 1580e1590 cm1 (typical of microcrystalline graphite), as well as a higher IG/ID ratio, where IG and ID are the G-band and D-band peak intensities, respectively, denoting a better graphitic quality and a lower structural disorder. These findings are in good agreement with data reported in the literature, showing that a pulse energy slightly above the graphitization threshold and an intermediate scanning speed [17,21,24] allow for the fabrication of conductive graphitic microchannels with a high sp2/sp3 ratio and a well-defined geometry. Moreover, the structural quality of the graphitic columns clearly takes advantage from a multi-pass writing process, as shown in Fig. 2(c). Indeed, several figures of merit improve significantly by increasing the number of laser passes: 1) ID/IG ratio decreases, 2) the full width at half maximum of G-band peak decreases, and 3) G-band peak position slightly downshifts, getting closer to 1582 cm1, which is the reference band for the highly oriented pyrolytic graphite. All the microchannels were characterized by atomic force microscopy (AFM) and conductive-AFM (C-AFM), to investigate the electrical properties of the surface layer and evaluate the conduction homogeneity within the columns area. Fig. 3 reports AFM and C-AFM images of the top surfaces of three sample columns fabricated with the same (E, v) pair, but varying the number of laser passes. The AFM images show the presence of pits with material pile up at the borders. The size of the pit (i.e. the graphitic channel size) increases with the number of laser passes, due to the treatment of a larger volume of material. The corresponding C-AFM images show a conductive background with superimposed current spots resembling AFM topographic features. These spots are mainly governed by the tip-sample contact configuration. By increasing the number of laser passes, the conductive background becomes more intense and homogeneous, as pointed out clearly in the 4pass case. This result suggests that charge transport properties
Fig. 3. AFM (left) and C-AFM (right) images of the top surfaces of three microcolumns, all fabricated with E ¼ 0.14 mJ and v ¼ 20 mm s1, but with different number of laser passes (1, 2, and 4). (A colour version of this figure can be viewed online.)
improve with multiple laser passes, most probably due to an improvement of the overall structural quality of the microcolumn, as pointed out from Raman analysis. The average resistivity of all the different sets of microchannels was evaluated by current-voltage measurements. For this purpose,
Fig. 2. Raman spectra of graphitic columns fabricated at: (a) different values of graphitization speed at fixed laser pulse energy (E ¼ 0.1 mJ) and 1 pass; (b) different values of pulse energy at fixed speed (v ¼ 20 mm s1) and 1 pass; (c) different number of laser passes at fixed pulse energy (E ¼ 0.14 mJ) and graphitization speed (v ¼ 20 mm s1). In each plot, blue curve represents the best spectrum obtained for that particular combination of fixed parameters. Among all the analyzed spectra, the best graphitic features were obtained for the set: E ¼ 0.14 mJ, v ¼ 20 mm s1, 4 passes. (A colour version of this figure can be viewed online.)
M. Girolami et al. / Carbon 111 (2017) 48e53
the bottom surface of the sample was covered by a ground electrode, contacting all the microcolumns, whereas on the top surface different metal pads, each one contacting a single column, were fabricated by optical lithography. Resistivity was then calculated as r ¼ R(S/d), where R is the column resistance measured between the metal top pad and the ground electrode, and S is the column section, assumed to be circular (diameter values were inferred by AFM images). Uncertainty in the evaluation of r arises mostly from the non-uniformity of the column diameter through the sample thickness. The benefits of a multi-pass laser treatment are clearly visible in Fig. 4 as an overall decrease of r at every set of experimental parameters used. As in the case of horizontal wires [22], the decrease of resistivity with multiple laser passes can be ascribed to an increase in the proportion of well-formed graphite, consistently with C-AFM and Raman measurements. Results pointed out indeed that graphitization is a dynamic process starting at the first laser scan with the transformation of a defined volume of bulk diamond into a composite of non-diamond phases, with a sp2/sp3 ratio depending strongly on the (E, v) pair used. Every subsequent laser pass further modifies the treated zones, increasing the volume of sp2-bonded conductive sites, and leading to a strong decrease of resistivity. However, the measured resistivity of columns, even in the best case (r ¼ 0.75 ± 0.15 U cm), is still far higher than the value reported for horizontal wires [22] (r ¼ 0.022 U cm): probably, the correction of optical aberrations in the material was crucial in that case for a better structural quality of the graphitized region. The lowest r we found is comparable to the lowest values reported in the literature for vertical fs-laser-induced graphitic columns [19,25] fabricated with a single laser pass, ranging from 0.29 to 0.9 U cm. It must be stressed here that multi-pass approach demonstrated indeed to decrease resistivity with respect to single-pass process (Fig. 4), but within the same experimental conditions, i.e. all other things being equal but the number of laser passes. The best combination of laser treatment parameters derived from Raman analysis (E ¼ 0.14 mJ, v ¼ 20 mm s1, 4 passes) was used for the fabrication of a 100 100 array of graphitic columns (Fig. 5), acting as distributed electrodes for the PETE cathode prototype, within a single-crystal diamond sample identical to the test one. Column pitch was set to 40 mm. Two 200 nm-thick Au metal contacts were deposited by sputtering on the top and bottom surface of the sample, thus connecting in parallel all the 10,000 columns. The
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Fig. 5. Optical microscope (100) standard image of a 100 100 array of graphitic columns. Its fabrication required about 22 h of continuous laser operation. Long scratches appearing throughout the surface are a consequence of the mechanical polishing of the sample. (A colour version of this figure can be viewed online.)
overall cathode series resistance was measured to be RTOT ¼ 36.2 U at room temperature. This result highlights the excellent yield of a multi-pass writing process for vertical columns, which is the real advantage of this approach over single-pass writing: being the average resistance measured for a single column about Ri ¼ 300 kU, we can roughly estimate that the process yield Y, defined in this case as the fraction of columns with resistance Ri, is at least Y ¼ Ri/(RTOT 10,000) ¼ 83%. In previous works [25] on 3D laser graphitization of diamond, success rate in fabricating conductive columns with a sub-GU resistance did not exceed 67%. Temperature-dependent measurements were also performed, showing that RTOT decreases linearly down to about 32 U at T ¼ 450 C, with a temperature coefficient of 2.7 103 C1. Therefore, at a nominal output voltage of a few V, a fully optimized diamond-based PETE cathode is expected to be able to sustain a photon-enhanced thermionic current of hundreds of mA, a range not achievable in case of bare single-crystal diamond
Fig. 4. Resistivity of graphite columns fabricated with different number of laser passes: (a) as a function of speed at a constant pulse energy of E ¼ 0.1 mJ (dots), and (b) as a function of pulse energy at a constant graphitization speed of v ¼ 20 mm s1 (squares). Red circle refers to the lowest value of resistivity obtained. (A colour version of this figure can be viewed online.)
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M. Girolami et al. / Carbon 111 (2017) 48e53
of electrical resistivity in vertical fs-laser-induced graphitic columns, with significant advantages in terms of process yield (>80%) with respect to single-pass technique. Multi-pass approach was used for the fabrication of an array of 10,000 graphite distributed electrodes within the bulk of a single-crystal diamond-based PETE cathode prototype, reducing the series resistance of the cathode of more than 10 orders of magnitude with respect to bulk diamond. This avoided the formation of current bottlenecks which may affect conversion efficiency of diamond-based PETE solar cells, as demonstrated by preliminary thermionic emission measurements.
Acknowledgements
Fig. 6. Thermionic current density as a function of temperature recorded for a surfacehydrogenated single-crystal diamond sample with (filled circles) and without (empty circles) graphite distributed electrodes. An electric field of 0.1 V mm1 was applied between the anode and the cathode load contact to ensure a complete collection of the emitted electrons.
devices, due to their extremely high bulk resistivity. Aimed at having a quick but solid feedback on the effectiveness of the fabricated array of graphite distributed electrodes in sustaining the thermally emitted current, the treated sample was surface-hydrogenated on one side, and thermionic emission measurements were performed and compared to those obtained with another surface-hydrogenated sample of the same quality with no graphite electrodes. The advantage of using graphite distributed electrodes is clearly highlighted in Fig. 6, showing the thermionic current density of the two tested samples (with and without graphitic columns) as a function of temperature: the sample with graphite electrodes was able to sustain a thermionic current up to 4 orders of magnitude higher than the untreated one, thanks to the formation of a low-resistance path from the load contact region to the emitting surface which avoids current bottlenecks. It's worth mentioning here that graphite distributed electrodes, even if occupying only 1.5% of the cathode volume, may capture some photoelectrons diffusing towards the surface, hampering their emission: for this reason, a trade-off must be found between decreasing the series resistance (by fabricating a large number of columns) and minimizing the loss of active volume (by increasing the column pitch). In the present work, as a result of such a tradeoff, column pitch was set to 40 mm, due to the constraints introduced by the limited lateral dimensions and the large thickness of the sample used. However, future diamond-based cathodes are expected to be larger and significantly thinner than the plates used in this preliminary work, allowing for a given series resistance to be obtained with a much lower number of columns and a wider pitch, and therefore minimizing the probability for a photoelectron of reaching a graphite microchannel before the emitting surface.
4. Conclusions In conclusion, the use of multiple laser scans demonstrated to bring benefits in terms of maximization of sp2 content and decrease
The activity was supported by the European Commission FP7Energy Project ProME3ThE2US2 “Production Method of Electrical Energy by Enhanced Thermal Electron Emission by the Use of Superior Semiconductors”, Grant Agreement n. 308975, website: www. prometheus-energy.eu. References [1] J. Schwede, I. Bargatin, D.C. Riley, B.E. Hardin, S.J. Rosenthal, Y. Sun, et al., Photon-enhanced thermionic emission for solar concentrator systems, Nat. Mater. 9 (2010) 762. [2] J. Schwede, T. Sarmiento, V.K. Narasimhan, S.J. Rosenthal, D.C. Riley, F. Schmitt, et al., Photon-enhanced thermionic emission from heterostructures with low interface recombination, Nat. Commun. 4 (2013) 1576. [3] K. Reck, O. Hansen, Thermodynamics of photon-enhanced thermionic emission solar cells, Appl. Phys. Lett. 104 (2014) 023902. [4] A. Varpula, M. Prunnila, Diffusion-emission theory of photon enhanced thermionic emission solar energy harvesters, J. Appl. Phys. 112 (2012) 044506. [5] G. Segev, Y. Rosenwaks, A. Kribus, Efficiency of photon enhanced thermionic emission solar converters, Sol. Energy Mater. Sol. Cells 107 (2012) 125. [6] A.G. Zhuravlev, A.S. Romanov, V.L. Alperovich, Photon-enhanced thermionic emission from p-GaAs with nonequilibrium Cs overlayers, Appl. Phys. Lett. 105 (2014) 251602. [7] A.G. Zhuravlev, V.L. Alperovich, Temperature dependence of photon-enhanced thermionic emission from GaAs surface with nonequilibrium Cs overlayers, Appl. Surf. Sci. (2016), http://dx.doi.org/10.1016/j.apsusc.2016.05.057 in press. [8] D. Takeuchi, H. Kato, G.S. Ri, T. Yamada, P.R. Vinod, D. Hwang, et al., Direct observation of negative electron affinity in hydrogen-terminated diamond surfaces, Appl. Phys. Lett. 86 (2005) 152103. [9] J.D. Rameau, J. Smedley, E.M. Muller, T.E. Kidd, P.D. Johnson, Properties of hydrogen terminated diamond as a photocathode, Phys. Rev. Lett. 106 (2011) 137602. [10] P. Calvani, A. Bellucci, M. Girolami, S. Orlando, V. Valentini, A. Lettino, D.M. Trucchi, Optical properties of femtosecond laser-treated diamond, Appl. Phys. A Mater. Sci. Proc. 117 (2014) 25. [11] P. Calvani, A. Bellucci, M. Girolami, S. Orlando, V. Valentini, R. Polini, D.M. Trucchi, Black diamond for solar energy conversion, Carbon 105 (2016) 401. [12] A. Bellucci, P. Calvani, M. Girolami, D.M. Trucchi, Defect engineering of diamond cathodes for high temperature solar cells, in: Proceedings of 2015 IEEE 15th International Conference on Environment and Electrical Engineering (EEEIC), Rome, Italy, 10e13 June 2015, IEEE, 2015, pp. 1616e1619. [13] A. Bellucci, M. Girolami, P. Calvani, S. Michaelson, A. Hoffman, V. Carcelen, D.M. Trucchi, Buried boron-doped layer for CVD diamond photo-thermionic cathodes, IEEE Trans. Nanotechnol. (2016), http://dx.doi.org/10.1109/ TNANO.2016.2566674 in press. [14] M. Pacilli, P. Allegrini, M. Girolami, G. Conte, E. Spiriti, V.G. Ralchenko, et al., Polycrystalline CVD diamond pixel array detector for nuclear particles monitoring, J. Instrum. 8 (2013) C02043. [15] S. Lagomarsino, M. Bellini, C. Corsi, F. Gorelli, G. Parrini, M. Santoro, S. Sciortino, Three-dimensional diamond detectors: charge collection efficiency of graphitic electrodes, Appl. Phys. Lett. 103 (2013) 233507. [16] T. Kononenko, V. Ralchenko, A. Bolshakov, V. Konov, P. Allegrini, M. Pacilli, et al., All-carbon detector with buried graphite pillars in CVD diamond, Appl. Phys. A Mater. Sci. Proc. 114 (2014) 297.
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Raman investigation of femtosecond laser-induced graphitic columns in single-crystal diamond, Appl. Phys. A Mater. Sci. Proc. 117 (2014) 143. B. Sun, P.S. Salter, M.J. Booth, High conductivity micro-wires in diamond following arbitrary path, Appl. Phys. Lett. 105 (2014) 231105. http://www.e6.com. M. Neff, T.V. Kononenko, S.M. Pimenov, V. Romano, W. Lüthy, V.I. Konov, Femtosecond laser writing of buried graphitic structures in bulk diamond, Appl. Phys. A Mater. Sci. Proc. 97 (2009) 543. €ni, P. Bergonzo, B. Caylar, G. Forcolin, I. Haughton, et al., A 3D F. Bachmair, L. Ba diamond detector for particle tracking, Nucl. Instr. Meth. Phys. Res. A 786 (2015) 97.
Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
Graphite distributed electrodes for diamond-based photon-enhanced thermionic emission solar cells Marco Girolami a, *, Luigino Criante b, Fabio Di Fonzo b, Sara Lo Turco b, Alessandro Mezzetti b, Andrea Notargiacomo c, Marialilia Pea c, Alessandro Bellucci a, Paolo Calvani a, Veronica Valentini a, Daniele M. Trucchi a a
CNR-ISM, Consiglio Nazionale delle Ricerche, Istituto di Struttura della Materia, Sede Secondaria di Montelibretti, Via Salaria km 29.300, 00015, Monterotondo Stazione, Roma, Italy CNST-IIT, Center for Nano-Science and Technology, Istituto Italiano di Tecnologia, Via Giovanni Pascoli 70/3, 20133, Milano, Italy c CNR-IFN, Consiglio Nazionale delle Ricerche, Istituto di Fotonica e Nanotecnologie, Sede Secondaria di Roma, Via Cineto Romano 42, 00156, Roma, Italy b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 July 2016 Received in revised form 23 September 2016 Accepted 25 September 2016 Available online 27 September 2016
Graphite conductive vertical microchannels, fabricated by femtosecond laser treatment, are proposed as distributed electrodes in defect-engineered (“black”) single-crystal diamond cathodes for innovative solar cells. Energy conversion is based on photon-enhanced thermionic emission, where the reduction of the cathode series resistance is crucial to preserve conversion efficiency. Process yield higher than 80% and resistivity as low as 0.75 ± 0.15 U cm were achieved by the optimization of laser parameters and the use of a multi-pass writing technique. A 100 100 array of graphite electrodes was integrated in a diamond-based cathode prototype, reducing the series resistance of more than 10 orders of magnitude with respect to bulk diamond. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction In the field of solar energy concentrating systems, conversion modules based on Photon-Enhanced Thermionic Emission (PETE) [1,2] have been proposed as a valid alternative to conventional solar cells. Basically, a PETE module consists of a semiconductor-based cathode, engineered for an optimal combination of thermionic and photovoltaic properties. Photon absorption in the solar spectrum range increases the population of conduction band, thus triggering the thermionic emission of electrons from the cathode surface at far lower temperatures than pure thermionic emitters. At the same time, PETE modules ensure a far higher conversion efficiency (up to 60%) [3] than conventional solar cells. Cathodes based on silicon [4] and III-V compounds [5] have been recently proposed: in particular, the bandgap of III-V structures is matched to the solar spectrum by adopting the so-called “bandgap engineering”. However, in order to make the electrons escape efficiently from these materials, it is mandatory to reduce electron affinity c by adding suitable coatings, usually made of alkali metal films [1,6,7], which
* Corresponding author. E-mail address: [email protected] (M. Girolami). http://dx.doi.org/10.1016/j.carbon.2016.09.061 0008-6223/© 2016 Elsevier Ltd. All rights reserved.
suffer from instability at temperatures higher than 100e200 C: this strongly limits, at the moment, the use of III-V semiconductors as materials for PETE cathodes. An alternative way of developing a PETE device consists in the exploitation of diamond as active medium for the cathode. Diamond overcomes III-V compounds and silicon limitations thanks to the possibility of obtaining negative electron affinity (c ¼ 1.1 eV) by hydrogenating the emitting surface [8]: unlike alkali metal coatings, H-termination of diamond surface is indeed stable up to about 800 C [9]. On the other hand, diamond bandgap (5.47 eV) is too wide to be matched to solar spectrum, and conventional bandgap engineering is not applicable. For this purpose, a “defect engineering” strategy is being adopted by introducing in a controlled way a certain amount of defects within the bandgap able to interact more effectively with solar radiation. In previous works, we showed that surface nanotexturing, obtained by femtosecond laser treatment, is able to enhance solar absorptance (up to 95%) [10] and photogeneration [11] by inducing an intermediate band within the diamond bandgap, supporting an efficient photoelectronic conversion of solar photons. Nanotexturing of the lightreceiving surface and hydrogenation of the emitting surface can be both efficiently exploited in diamond-based PETE cathodes [12] (Fig. 1), in which photoelectrons generated under the illuminated
M. Girolami et al. / Carbon 111 (2017) 48e53
Fig. 1. Sketch of a PETE conversion module based on a defect-engineered black diamond cathode. A more accurate description of the cathode operating principle can be found in Ref. [12]. (A colour version of this figure can be viewed online.)
surface diffuse through the diamond bulk towards the emission surface, on the other side of the cathode: for this purpose, it is mandatory for the diamond bulk to have a high diffusion length, and that's the reason why single-crystal diamond plates are used in the present work. At the same time, absorption of the infrared part of the solar spectrum, as well as thermalization of photoelectrons, causes the overall increase of cathode temperature, enhancing thermionic emission from the emitting layer, that consists of a hydrogenated n-doped diamond film (few hundreds of nm thick) directly grown over the intrinsic diamond bulk. Hydrogenation of the emitting surface enables negative electron affinity: therefore, photoelectrons diffused from the light-receiving surface, as well as thermal electrons on the emitting surface (i.e. electrons on the tail of the Fermi-Dirac distribution overlapping the conduction band) are directly emitted into vacuum. In summary, the photonic process of light absorption and photogeneration is mostly attributed to the nanotextured lightreceiving surface, whereas thermionic emission is mainly triggered from the hydrogenated n-doped emitting layer. Ideally, illuminated and emitting surfaces should be as close as possible to each other, in order to make easier for photoelectrons to reach the emitting layer. Unfortunately, thickness of commercially available single-crystal diamond plates is in the range 300e1200 mm. The major drawback of such large values of thickness is that cathode series resistance, due to the extremely high electrical resistivity of bulk diamond (>1012 U cm), would be too high to sustain the emitted current. In other words, electrons emitted from the cathode (and collected by the anode) must be refilled by the external circuit at the cathode load contact: if the path from the load contact to the emitting layer is highly resistive, electrons injected into the cathode are hampered from reaching the hydrogenated surface and replace the emitted electrons, thus making impossible to sustain a steady-state current flow. Of course, this applies only to thermal electrons that follow Fermi-Dirac distribution, whereas transport of photoelectrons, directly promoted into the conduction band of bulk diamond, is only diffusion-limited. The technological innovative solution we introduce in the present work to drastically reduce the cathode series resistance by
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several orders of magnitude, thus avoiding bottlenecks for the refilling of thermal electrons and sustaining efficiently the emitted current, is given by the laser writing of graphite microchannels within the diamond bulk, acting as distributed electrodes. In this way, electrically conductive graphite columns connect the lightreceiving surface, where a “frame-shaped” load contact is fabricated, to the emitting layer, forming a low-resistance path for the electrons injected by the external circuit. Electrical continuity between the array of graphite columns and the cathode load contact is ensured by a p-type highly conductive (0.1 U cm) layer, obtained by boron ion implantation just below the light-receiving surface [13]. Laser microstructuring of diamond is a well-established technology, usually employed for the fabrication of surface [14] and buried [15e17] graphitic contacts in 3D diamond detectors for ionizing radiation and charged particles. Ultrashort laser pulses cause a transition of diamond into graphite through an irreversible optical breakdown in the bulk material, induced locally by avalanche photoionization [18]. The structural, geometrical, and electrical properties of the graphite channels produced are strongly related to the laser processing parameters. The use of pulses in the fs range is preferred to longer pulses to avoid plastic strain due to thermal effects, which would cause lattice damage in the volume surrounding the channels, with detrimental effects on the charge transport properties of bulk diamond. The major drawback of fs pulses is the lower electrical conductivity of the channel if compared to structures fabricated with ns-pulsed laser sources, due to a lower concentration of the sp2 phase [19,20]. However, by a proper combination of energy density (laser power, repetition rate, objective used) and graphitization speed (i.e. the laser writing speed), it is possible to maximize the sp2/sp3 ratio, leading to channels with a higher degree of graphite crystallinity [21] and, as described later, with lower resistivity. In case of horizontal graphitic wires, i.e. fabricated parallel to the diamond surface, it is possible to further decrease resistivity by performing multiple laser scans [22]. Here we demonstrate that a similar result can also be obtained for graphitic vertical columns. 2. Experimental A standard grade, also known as “IIa grade”, single-crystal diamond sample (4.5 4.5 mm2, 500 mm thick, <110> oriented, polished on both sides, with boron concentration < 0.05 ppm and nitrogen concentration < 1 ppm), produced by Element Six Ltd. [23], was subjected to laser treatment for the fabrication of different groups of graphite channels, aimed at finding the best set of treatment parameters to be employed for the distributed electrodes of the PETE cathode prototype. A laser micromachining system equipped with a regeneratively amplified mode-locked femtosecond laser based on Yb:KGW active medium (Light Conversion, Pharos) was used, producing pulses with 515 nm wavelength, 280 fs duration, and 1 kHz repetition rate. Pulse energy E was varied in the range E ¼ 0.01e1 mJ. Columns were fabricated by focusing the laser beam through a 20 microscope objective (Mitutoyo, NA ¼ 0.4) onto the bottom surface of the sample, placed on a three-axis computer-controlled translation stage (ABL 1000, Aerotech) with a resolution of 2 nm, and then by moving the sample along the beam direction (z-axis) with speed v varying in the range v ¼ 2e1000 mm s1. Besides, a multi-pass writing process was employed for each (E, v) pair, similarly to what reported in Ref. [22] for horizontal wires, with laser focus scanning the sample vertically 1, 2 and 4 times, slightly shifting (1 mm) the focal position between each pass to avoid multiple treatments of the same spot. All the fabricated columns showed a diameter ranging from 2 to 10 mm, with larger diameters corresponding to higher values of laser pulse energy and to multiple scans.
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3. Results and discussion After the fabrication process, visible micro-Raman analysis was performed on all the different sets of graphite columns by using the emission wavelength (l ¼ 514.5 nm) of an Arþ laser. The (E, v) pair, as well as the number of laser passes, were selected on the basis of the best graphitic features shown in Raman spectra. As reported in Fig. 2(a) and (b), spectra obtained for intermediate values of pulse energy and graphitization speed show a better resolved G-band around 1580e1590 cm1 (typical of microcrystalline graphite), as well as a higher IG/ID ratio, where IG and ID are the G-band and D-band peak intensities, respectively, denoting a better graphitic quality and a lower structural disorder. These findings are in good agreement with data reported in the literature, showing that a pulse energy slightly above the graphitization threshold and an intermediate scanning speed [17,21,24] allow for the fabrication of conductive graphitic microchannels with a high sp2/sp3 ratio and a well-defined geometry. Moreover, the structural quality of the graphitic columns clearly takes advantage from a multi-pass writing process, as shown in Fig. 2(c). Indeed, several figures of merit improve significantly by increasing the number of laser passes: 1) ID/IG ratio decreases, 2) the full width at half maximum of G-band peak decreases, and 3) G-band peak position slightly downshifts, getting closer to 1582 cm1, which is the reference band for the highly oriented pyrolytic graphite. All the microchannels were characterized by atomic force microscopy (AFM) and conductive-AFM (C-AFM), to investigate the electrical properties of the surface layer and evaluate the conduction homogeneity within the columns area. Fig. 3 reports AFM and C-AFM images of the top surfaces of three sample columns fabricated with the same (E, v) pair, but varying the number of laser passes. The AFM images show the presence of pits with material pile up at the borders. The size of the pit (i.e. the graphitic channel size) increases with the number of laser passes, due to the treatment of a larger volume of material. The corresponding C-AFM images show a conductive background with superimposed current spots resembling AFM topographic features. These spots are mainly governed by the tip-sample contact configuration. By increasing the number of laser passes, the conductive background becomes more intense and homogeneous, as pointed out clearly in the 4pass case. This result suggests that charge transport properties
Fig. 3. AFM (left) and C-AFM (right) images of the top surfaces of three microcolumns, all fabricated with E ¼ 0.14 mJ and v ¼ 20 mm s1, but with different number of laser passes (1, 2, and 4). (A colour version of this figure can be viewed online.)
improve with multiple laser passes, most probably due to an improvement of the overall structural quality of the microcolumn, as pointed out from Raman analysis. The average resistivity of all the different sets of microchannels was evaluated by current-voltage measurements. For this purpose,
Fig. 2. Raman spectra of graphitic columns fabricated at: (a) different values of graphitization speed at fixed laser pulse energy (E ¼ 0.1 mJ) and 1 pass; (b) different values of pulse energy at fixed speed (v ¼ 20 mm s1) and 1 pass; (c) different number of laser passes at fixed pulse energy (E ¼ 0.14 mJ) and graphitization speed (v ¼ 20 mm s1). In each plot, blue curve represents the best spectrum obtained for that particular combination of fixed parameters. Among all the analyzed spectra, the best graphitic features were obtained for the set: E ¼ 0.14 mJ, v ¼ 20 mm s1, 4 passes. (A colour version of this figure can be viewed online.)
M. Girolami et al. / Carbon 111 (2017) 48e53
the bottom surface of the sample was covered by a ground electrode, contacting all the microcolumns, whereas on the top surface different metal pads, each one contacting a single column, were fabricated by optical lithography. Resistivity was then calculated as r ¼ R(S/d), where R is the column resistance measured between the metal top pad and the ground electrode, and S is the column section, assumed to be circular (diameter values were inferred by AFM images). Uncertainty in the evaluation of r arises mostly from the non-uniformity of the column diameter through the sample thickness. The benefits of a multi-pass laser treatment are clearly visible in Fig. 4 as an overall decrease of r at every set of experimental parameters used. As in the case of horizontal wires [22], the decrease of resistivity with multiple laser passes can be ascribed to an increase in the proportion of well-formed graphite, consistently with C-AFM and Raman measurements. Results pointed out indeed that graphitization is a dynamic process starting at the first laser scan with the transformation of a defined volume of bulk diamond into a composite of non-diamond phases, with a sp2/sp3 ratio depending strongly on the (E, v) pair used. Every subsequent laser pass further modifies the treated zones, increasing the volume of sp2-bonded conductive sites, and leading to a strong decrease of resistivity. However, the measured resistivity of columns, even in the best case (r ¼ 0.75 ± 0.15 U cm), is still far higher than the value reported for horizontal wires [22] (r ¼ 0.022 U cm): probably, the correction of optical aberrations in the material was crucial in that case for a better structural quality of the graphitized region. The lowest r we found is comparable to the lowest values reported in the literature for vertical fs-laser-induced graphitic columns [19,25] fabricated with a single laser pass, ranging from 0.29 to 0.9 U cm. It must be stressed here that multi-pass approach demonstrated indeed to decrease resistivity with respect to single-pass process (Fig. 4), but within the same experimental conditions, i.e. all other things being equal but the number of laser passes. The best combination of laser treatment parameters derived from Raman analysis (E ¼ 0.14 mJ, v ¼ 20 mm s1, 4 passes) was used for the fabrication of a 100 100 array of graphitic columns (Fig. 5), acting as distributed electrodes for the PETE cathode prototype, within a single-crystal diamond sample identical to the test one. Column pitch was set to 40 mm. Two 200 nm-thick Au metal contacts were deposited by sputtering on the top and bottom surface of the sample, thus connecting in parallel all the 10,000 columns. The
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Fig. 5. Optical microscope (100) standard image of a 100 100 array of graphitic columns. Its fabrication required about 22 h of continuous laser operation. Long scratches appearing throughout the surface are a consequence of the mechanical polishing of the sample. (A colour version of this figure can be viewed online.)
overall cathode series resistance was measured to be RTOT ¼ 36.2 U at room temperature. This result highlights the excellent yield of a multi-pass writing process for vertical columns, which is the real advantage of this approach over single-pass writing: being the average resistance measured for a single column about Ri ¼ 300 kU, we can roughly estimate that the process yield Y, defined in this case as the fraction of columns with resistance Ri, is at least Y ¼ Ri/(RTOT 10,000) ¼ 83%. In previous works [25] on 3D laser graphitization of diamond, success rate in fabricating conductive columns with a sub-GU resistance did not exceed 67%. Temperature-dependent measurements were also performed, showing that RTOT decreases linearly down to about 32 U at T ¼ 450 C, with a temperature coefficient of 2.7 103 C1. Therefore, at a nominal output voltage of a few V, a fully optimized diamond-based PETE cathode is expected to be able to sustain a photon-enhanced thermionic current of hundreds of mA, a range not achievable in case of bare single-crystal diamond
Fig. 4. Resistivity of graphite columns fabricated with different number of laser passes: (a) as a function of speed at a constant pulse energy of E ¼ 0.1 mJ (dots), and (b) as a function of pulse energy at a constant graphitization speed of v ¼ 20 mm s1 (squares). Red circle refers to the lowest value of resistivity obtained. (A colour version of this figure can be viewed online.)
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of electrical resistivity in vertical fs-laser-induced graphitic columns, with significant advantages in terms of process yield (>80%) with respect to single-pass technique. Multi-pass approach was used for the fabrication of an array of 10,000 graphite distributed electrodes within the bulk of a single-crystal diamond-based PETE cathode prototype, reducing the series resistance of the cathode of more than 10 orders of magnitude with respect to bulk diamond. This avoided the formation of current bottlenecks which may affect conversion efficiency of diamond-based PETE solar cells, as demonstrated by preliminary thermionic emission measurements.
Acknowledgements
Fig. 6. Thermionic current density as a function of temperature recorded for a surfacehydrogenated single-crystal diamond sample with (filled circles) and without (empty circles) graphite distributed electrodes. An electric field of 0.1 V mm1 was applied between the anode and the cathode load contact to ensure a complete collection of the emitted electrons.
devices, due to their extremely high bulk resistivity. Aimed at having a quick but solid feedback on the effectiveness of the fabricated array of graphite distributed electrodes in sustaining the thermally emitted current, the treated sample was surface-hydrogenated on one side, and thermionic emission measurements were performed and compared to those obtained with another surface-hydrogenated sample of the same quality with no graphite electrodes. The advantage of using graphite distributed electrodes is clearly highlighted in Fig. 6, showing the thermionic current density of the two tested samples (with and without graphitic columns) as a function of temperature: the sample with graphite electrodes was able to sustain a thermionic current up to 4 orders of magnitude higher than the untreated one, thanks to the formation of a low-resistance path from the load contact region to the emitting surface which avoids current bottlenecks. It's worth mentioning here that graphite distributed electrodes, even if occupying only 1.5% of the cathode volume, may capture some photoelectrons diffusing towards the surface, hampering their emission: for this reason, a trade-off must be found between decreasing the series resistance (by fabricating a large number of columns) and minimizing the loss of active volume (by increasing the column pitch). In the present work, as a result of such a tradeoff, column pitch was set to 40 mm, due to the constraints introduced by the limited lateral dimensions and the large thickness of the sample used. However, future diamond-based cathodes are expected to be larger and significantly thinner than the plates used in this preliminary work, allowing for a given series resistance to be obtained with a much lower number of columns and a wider pitch, and therefore minimizing the probability for a photoelectron of reaching a graphite microchannel before the emitting surface.
4. Conclusions In conclusion, the use of multiple laser scans demonstrated to bring benefits in terms of maximization of sp2 content and decrease
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