Radiation Tolerant Nanowire Array Solar CellsPilar Espinet-Gonzalez,† Enrique Barrigon,‡ Gaute Otnes,‡,○ Giuliano Vescovi,§ Colin Mann,∥

Ryan M. France,⊥ Alex J. Welch,† Matthew S. Hunt,∇ Don Walker,∥ Michael D. Kelzenberg,†

Ingvar Åberg,§ Magnus T. Borgstrom,‡ Lars Samuelson,‡,§ and Harry A. Atwater*,†

†Department of Applied Physics and Materials Science, California Institute of Technology, Pasadena, California 91125, UnitedStates‡Division of Solid State Physics, Lund University, Lund, SE-221 00, Sweden§Sol Voltaics AB, Lund, SE-223 63, Sweden∥The Aerospace Corporation, El Segundo, California 90245-4609, United States⊥National Renewable Energy Laboratory, Golden, Colorado 80401, United States∇The Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United States

*S Supporting Information

ABSTRACT: Space power systems require photovoltaicsthat are lightweight, efficient, reliable, and capable ofoperating for years or decades in space environment.Current solar panels use planar multijunction, III−V basedsolar cells with very high efficiency, but their specific power(power to weight ratio) is limited by the added mass ofradiation shielding (e.g., coverglass) required to protect thecells from the high-energy particle radiation that occurs inspace. Here, we demonstrate that III−V nanowire-arraysolar cells have dramatically superior radiation perform-ance relative to planar solar cell designs and show this formultiple cell geometries and materials, including GaAs andInP. Nanowire cells exhibit damage thresholds rangingfrom ∼10−40 times higher than planar control solar cells when subjected to irradiation by 100−350 keV protons and 1MeV electrons. Using Monte Carlo simulations, we show that this improvement is due in part to a reduction in thedisplacement density within the wires arising from their nanoscale dimensions. Radiation tolerance, combined with theefficient optical absorption and the improving performance of nanowire photovoltaics, indicates that nanowire arrayscould provide a pathway to realize high-specific-power, substrate-free, III−V space solar cells with substantially reducedshielding requirements. More broadly, the exceptional reduction in radiation damage suggests that nanowire architecturesmay be useful in improving the radiation tolerance of other electronic and optoelectronic devices.KEYWORDS: nanowire solar cells, radiation hard, space environment, space solar cells, high specific power,irradiation-induced defects, Monte Carlo simulations

Most spacecraft are powered by photovoltaic (PV)solar cells, which, because of the challenging spaceenvironment, are subject to stringent requirements,

including high-power conversion efficiency (PCE), highreliability, high tolerance to radiation, and high specific power,that is, power-to-weight ratio. The latter requirement is crucialbecause the energy-generating system is typically one of theheaviest components of a satellite,1 significantly impacting thecost of space launch. Thus, substantial efforts have been made toincrease the specific power of space solar panels, while ensuringthat the solar cells will produce adequate power throughout themission duration.In the space environment, solar panels are subject to severe

thermal cycles, high UV light exposure, atomic oxygen in low

earth orbit, space debris, meteoroid impact, electrostaticcharging, and high energy particle radiation. Standard spacesolar arrays composed of III−V solar cells degrade almostexclusively by high energy electron and proton irradiation. Theradiation flux, energy distribution, and fluence depend on theoperating orbit, orientation of the spacecraft, solar cycle, andmission duration. Two main approaches are typically used tomitigate the detrimental effects of radiation. First, a layer ofshielding (typically a cerium-doped cover glass) is used toreduce the amount of radiation reaching the solar cells but this

Received: July 2, 2019Accepted: October 18, 2019Published: October 18, 2019

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© XXXX American Chemical Society A DOI: 10.1021/acsnano.9b05213ACS Nano XXXX, XXX, XXX−XXX

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adds undesired weight.2 The second approach to mitigateradiation damage is to increase the intrinsic radiation toleranceof the solar cells through choice of semiconductor materialsand/or device architectures with enhanced radiation robustness,which experience less degradation for a given radiationexposure.3−7

State-of-the-art space solar panels composed of III−Vmultijunction solar cells8,9 are attractive because of high-power conversion efficiencies (>30% under extraterrestrialsolar spectrum (AM0)), enabling a reduction in the panel areafor a givenmission. The development of thinned-substrate10 andepitaxial lift-off (ELO)11 technologies have allowed extremelythin III−V solar cells to be produced, such that the mass of thepanels using these cells would be dominated largely by therequisite radiation-shielding. Yet, there is a growing demand forenergy conversion systems with higher specific power than canbe achieved with existing technologies (that is, exceeding ∼0.2W/g).12 This demand is particularly driven by systems that needto operate in environments dominated by high proton fluence,such as medium earth orbit, which require heavy shieldingbecause of the extensive exposure to radiation. Developing “rad-hard” thin-film solar cells would significantly reduce or eveneliminate the need for shielding, thus maximizing the specificpower.Many photovoltaic materials systems and cell geometries have

been investigated for their potentially enhanced radiationtolerance. Studies have reported radiation-tolerant thin-filmsolar cells based on copper indium gallium diselenide (CIGS),13

cadmium telluride (CdTe),14 and, more recently, perovskitecompounds.15−17 In particular, perovskite solar cells haveachieved impressive specific power values exceeding 29 W/g18

and have demonstrated outstanding radiation resistance. Whilepromising, these thin-film technologies are substantially lessefficient and less technologically mature than state-of-the-artmultijunction cells and have not found widespread use in space.Within the established III−V compound semiconductor ma-terial systems, numerous approaches for radiation hardeninghave also been explored. Ultrathin (nanometer thickness),planar GaAs19 has been demonstrated to reduce the short circuitcurrent loss due to radiation damage, but thus far such cells have

demonstrated a low beginning-of-life (BOL) efficiency of ∼3%.Also, GaAs solar cells with quantum dots and quantum wellshave been studied as potential candidates for radiation harddevices. Quantum dots and quantum wells have been reportedto have enhanced radiation tolerance20,21 owing to their reducedeffective cross section for particle-solid interactions and carrierconfinement. In solar cells, the current from the quantum dotsand quantum wells is significantly radiation tolerant. However,the overall radiation behavior reported in highly performingoptimized quantum dots/wells solar cells is similar to standarddevices.7,22,23

Here we propose the use of III−V nanowire (NW) arrays asspace solar cell architecture with dramatically enhancedradiation tolerance versus planar architectures. The NW solarcells consist of arrays of high-aspect-ratio semiconductorstructures (“wires”) that have appropriate dimensions toenhance light absorption24−27 and to reduce damage causedby high-energy particle radiation as we report here and it hasbeen observed in photodetectors.28−30 The NW solar cells arefabricated from the same III−V phosphide and arsenidesemiconductor compounds as conventional space solar cells.Although the NW technology is less mature, in principle thesecells should be capable of reaching similar high efficiencies andreliability. Reported efficiencies of NW solar cells have beenrapidly increasing over the past decade, yielding values of 15.3%and 15.0% (AM1.5G) for bottom up single-junction GaAs31 andInP32 NW solar cells, respectively, and 17.8% for top downtapered dry-etched InP NW solar cells.33 Furthermore, the NWgrowth processes can accommodate a far greater range of latticemismatch than planar epitaxy, suggesting that defect-freemultijunction cells could be grown from a diverse range ofmaterials, to further improve the efficiency.34−38 NWarrays havea low packing fraction (∼9−14%) and can be fabricated asdevices in a substrate-free,39−41 lightweight, flexible sheet formfactor,42−45 which is ideal for efficient packaging and deploy-ment in space.

RESULTS AND DISCUSSION

As high energy particles pass through a semiconductor crystal,they are slowed down by interactions with electrons and atomic

Figure 1. Schematics of all the devices tested (a,b,d,e), cross-sectional images of representative NW solar cells (c,f) and initial J−V curves ofrepresentative devices of each architecture (g). Panel c shows a completed GaAs NW solar cell, which was overcoated with metal and ion-milledto enable cross-sectional imaging. Panel f shows as-grown InP NWs with the Au growth catalyst particle still on top of the wire. In panel g, notethat the planar cells lack antireflective coatings.

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nuclei, producing ionization and atomic displacements(vacancies and interstitial defects) in the crystal lattice thatdegrade material performance primarily by generation ofnonradiative recombination centers. Particles with sufficientenergy to completely penetrate the active region of the solar cellproduce a roughly uniform damage concentration along theirpath.46 However, particles with lower energies that come to restinside the solar cell create an inhomogeneous defectdistribution. In the case of electrons, the displacement rate(displacement/cm) decreases gradually as they are slowed downin the semiconductor. Protons exhibit the opposite behavior; astheir energy decreases, the displacement rate increasesdramatically, producing a highly nonuniform defect profilewherein most of the damage is concentrated near the end of theproton trajectory.46 Shielding is primarily used to reduce thefluence of these highly damaging low-energy protons.46

Therefore, any solar cell technology which aims to eliminateor to reduce drastically the shielding without compromising theservice life of the solar array needs to be resistant not only to theradiation of high energy particles that penetrate the entire devicebut also to lower-energy particles (mainly protons) with shorterpenetration depth.To evaluate the radiation hardness of NW solar cells, we

irradiated GaAs and InP NW solar cells with protons at energiesof 100 and 350 keV and electrons at 1 MeV energy. The initialefficiency under terrestrial spectrum (AM1.5G) for the GaAsNW solar cells was ∼11.6% ± 0.9% and for the InP NW solarcells was∼6.5%± 0.9%. TheGaAsNWcells had a wire length of3−3.2 μm, whereas the InP NW cells had a wire length of 1.7−2μm. The GaAs NW cells had wire core radii of ∼80 nm and theInP NW cells of ∼90 nm. As experimental controls, we alsoincluded 4.4 μm thick ELOGaAs planar solar cells and∼1.3 μmthick InGaP cells. The cell geometries, representative crosssectional images, and J−V curves before the irradiation tests aredepicted in Figure 1. Although the InP NW solar cells have

moderate efficiencies, the performance of GaAs NW solar cells iscomparable to that of the planar ELOGaAs solar cells. However,it should be noted that the planar devices do not haveantireflective coatings; thus, a ∼30% increase in current densitycould be expected for both the GaInP and GaAs planar cells ifsuch a layer was used.Performance degradation was evaluated in terms of the short

circuit current density (Jsc) and open circuit voltage (Voc), asshown in Figure 2. A summary of the devices included in eachtest, their initial characteristics and their performance after theirradiation is presented in Table S.1 in the SupportingInformation (SI).First, we analyzed the results of irradiation with 100 keV

protons (left column panels in Figure 2), which have a shortpenetration depth (∼750 nm), such that the ions are implantedinside all of the tested devices. The degradation as a function offluence (degradation rate) for both Jsc and Voc for our GaAs NWsolar cells is lower than for the reference planar InGaP solar celleven though the InGaP is known to be a more radiation-tolerantmaterial than GaAs.3 Furthermore, the GaAs NW degradationresults are noticeably better than those previously reported forGaAs/Ge solar cells.47 The Jsc and Voc of the GaAs NW solarcells follow the degradation slope of the GaAs/Ge solar cellreported by Anspaugh but shifted to a fluence ∼13 times higher(see Figure S.1 in SI). Regarding the InP NW solar cells, theremaining factor of the Jsc > 0.95 after 1× 1012 p+/cm2 is notablein comparison with the degradation of standard space GaAs/Gesolar cells.47 For InP NW cells, the open circuit voltagedegradation with irradiation is similar to that for GaAs NW solarcells. However, due to the lower initial Voc in the InP NW cells,the radiation resistance of the Voc should be interpretedcautiously. Future improved InP NW structures with higherVoc could have a different radiation resistance.We also analyzed results of cell irradiation with 350 keV

protons (middle panels in Figure 2), which have a penetration

Figure 2. Solar cells performance after irradiation experiments. Degradation of the characteristic parameters (Jsc and Voc) of different solar cellsarchitectures tested under irradiation with 100 keV p+ (left panels), 350 keV p+ (center panels), and 1 MeV e− (right panels). The points showthe performance of the different solar cells included in each test. Data for the degradation of planar GaAs/Ge solar cells (red lines)47 and forplanar InP solar cells (green lines)4 have been included for comparison.

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depth of ∼2.4−2.9 μm. This is close to the full length (∼3−3.2μm) of the GaAs NWs in our solar cells and we therefore wouldexpect the protons to cause significant damage therein as well asin the thin film GaAs planar ELO devices (∼4.4 μm thick). Inthe InP NW solar cells (∼1.2−1.7 μm long), the protons crossthe semiconductor structure. We found that the GaAs NW solarcells can withstand a fluence ∼40 times higher than planardevices (see Figure S.1 in SI). In particular, the short circuitcurrent density degradation ratio (Jsc/Jsc0), exceeds 0.90 at thehighest fluence 1 × 1012 p+/cm2 in the GaAs NW solar cells,which is comparable to that of InP NW cells, and 9-fold timesbetter than the short circuit current density degradation ratio inthe planar GaAs solar cells (Jsc/Jsc0 ∼ 0.1), which are severelydamaged. Likewise the degradation of Voc in the GaAs NW solarcells (Voc/Voc0 > 0.80) is 1.8 times lower than the planar thin filmGaAs solar cells tested.Finally, we analyzed the results of irradiation with electrons of

1 MeV (right-hand panels in Figure 2), which penetrate around∼700 μm in the semiconductor, and consequently cross theentire solar cell in all devices. We found that the Jsc in the GaAsNW solar cells does not change when increasing the fluence upto 1 × 1015 e−/cm2 (equivalent to ∼15 years in geostationaryorbit) and only decreases slightly (Jsc/Jsc0∼ 0.90−0.96) after 5×1015 e−/cm2, which indicates exceptional tolerance to electronirradiation3 in comparison with previously reported data forvarious planar space GaAs solar cell architectures47,48 where ashort circuit current density degradation ratio (Jsc/Jsc0) between0.6 and 0.85 was observed at 5× 1015 e−/cm2. Also, theVoc of theGaAs NW solar cells is unchanged at low and medium fluences.However, we observe an abrupt drop at the highest fluence,showing a similar degradation Voc/Voc0 as seen in planar GaAsdevices. Unlike the protons, the 1 MeV electron degradationcurves of the planar and NW solar cells do not have the sameshape which might indicate a different nature of the defectsintroduced in both architectures.49 The InP NW solar cells alsohave lower degradation than previously reported values fordifferent designs of planar InP solar cells4 (green lines in theright-hand panels in Figure 2). A similar low short circuit currentdegradation rate has been reported for a planar InP solar cellwith a very low-doped base. However, the Voc degradation of theplanar devices with a low-doped base was significant, Voc/Voc0 =0.81, in contrast to the InP NW-based devices where thedegradation was almost negligible, Voc/Voc0 > 0.94. The initialVoc of the InP NW solar cells is low (620−660 mV) so theradiation resistance advantage should be corroborated in moreadvanced devices with higher initial Voc.These results suggest that the surfaces of the NWs,

particularly the interfaces between the InP NW core and theSiOx, as well as those for the GaAs core and Al0.9Ga0.1As shell, donot degrade significantly under irradiation by high energyparticles under conditions known to degrade the performance ofcells.27,32,50 NW solar cells made of different materials (InP,GaAs), with different architectures (e.g., junction geometry, wirelength), and with different initial efficiencies (∼6.5% in InP NWcells and ∼11.6% in GaAs NW cells) have all demonstrated thatboth theirVoc and Jsc are more resistant to radiation damage thantheir planar counterparts for both proton and electronirradiation at various energies having different penetrationdepths in the semiconductor. It has been reported that cellsmade from lower-quality materials exhibit increased radiationtolerance,51 thus because our NW cells have lower initialefficiencies than state-of-the-art planar cells, this may be partiallyresponsible for their improved radiation tolerance. However, at

least in the case of GaAs cells, our tests include both NW andplanar devices with comparable and relatively high initial Voc(>0.9 V), and in all conditions tested the NW cells show asignificantly higher radiation tolerance than planar devices.Therefore, collectively these results point to NW-based solarcells having superior radiation-tolerance compared to planardevices.To understand the origin of the observed high radiation-

tolerance of NW-based solar cells, we performed Monte Carlosimulations52 based on the binary collision approximation(BCA), which simulates the interaction of ions with matter bysolving the classical scattering equation between the incomingion and the target atom. We compared proton irradiationdamage in a GaAs planar cell and in a GaAs NW array, using asimplified geometry consisting of a solid GaAs cuboid 1.5 μm(width, X) × 1.5 μm (length, Y) × 3.0 μm (height, Z) for theplanar solar cells, and for the NW array, an array of 9 GaAs NWswith a radius of 80 nm, a pitch of 500 nm, and 3 μm longembedded in a cuboid with the same dimensions. Two sets ofNW-structures (see Figure S.2 in SI) have been simulated, thefirst one consists of free-standing GaAs wires in vacuum(hereafter NWvacuum), whereas in the second NW-structureGaAs NWs are radially covered by a Al0.9Ga0.1As shell 30 nmwide where the shell is enclosed with a 50 nm wide SiO2insulating layer and the NW array is in-filled with benzocyclo-butene (BCB) making the array planar (NWcore−shell). Thecore−shell planarized structure resembles the experimentalGaAs NW solar cells tested, whereas the freestanding NWs are anotional structure. In the simulations, we irradiated thestructures with a random distribution of protons (H+) at normalincidence to the top surface of each structure (perpendicular tothe planar devices, parallel to the NW orientation for the NW-based devices). Periodic boundary conditions were assumed inall structures to reproduce an infinite array. The distributions ofarsenic displacements (these are the primary defects reported onirradiated n- and p-type GaAs49) have been calculated. Furtherdetails on the simulation procedure are presented in Methods.The arsenic displacements produced per incident H+

(damage) as a function of the ion energy are shown in Figure3a) for the planar architecture. The damage (number ofdisplacements/ion) introduced in the planar device increaseswith proton energy up to ∼350 keV. At this energy, protons areimplanted at the bottom of the GaAs (see Figures 4 and 5) afterundergoing numerous collisions in the slowdown process. Athigher energies, protons cross the entire semiconductor andconsequently the interactions with the GaAs atoms aredrastically reduced. Figure 3b illustrates the damage ratio ofthe planar architecture to the NW array (left axis). The resultsindicate that the damage in the planar structure is noticeablyhigher, whereas only a small part of the ion energy is deposited inthe wires of the array. The benefit of the NW architecture isparticularly advantageous at the energies which produce themost detrimental effects in the planar geometry. In all cases, thefree-standing NW arrays (NWvacuum) exhibit the lowest damage.Three effects can happen concurrently in the interaction of

high energy protons with NW arrays: (1) some of the ions thatimpinge on the space between wires can leave the array withoutdepositing their energy in the wires (particularly in free-standingNWs), (2) some of the ions that initially impinge on a wire ortheir secondary recoils can be scattered out of the wire, and (3)some of the ions that initially enter the array through the infilledmaterial or their secondary recoils can be deflected toward awire. The array geometry, the density of the different materials

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used, and their stopping power are the main parameters whichdetermine the distribution of the irradiation induced defects inthe different layers of the NW array. On the other hand, from theviewpoint of optical absorption photons which impinge in thearea between wires are absorbed because NWs have opticalabsorption cross sections, which are greater than their physicalcross sections. Therefore, in addition to a comparison of damage(number of displacements/ion) integrated over the cuboidvolume (1.5 μm × 1.5 μm × 3 μm) for the differentarchitectures, the comparison of the damage introduced in thevolume covered by GaAs has been included in Figure 3b (rightaxis). At all the energies and for bothNW arrays (core−shell andvacuum), the ratio of As displacement/ion in planar versus NWarchitectures exceeds unity, confirming an effective drop in crosssection for defect production in the NWs.In order to have a better understanding of the damage features

observed in Figure 3, the As displacement distributions obtainedat 100 keV proton irradiation (H+ stop near the top surface), 350keV proton irradiation (H+ stop near the bottom surface), and500 keV proton irradiation (H+ cross the semiconductor) aregiven in Figure 4. The induced damage per angstrom has beenintegrated over the solar cell thickness (XY plane integrated overZ) in the top plots of Figure 4 and over the array width (YZ planeintegrated over X) in the bottom plots. The variation in thenumber of irradiation-induced defects in the differentarchitectures is exhibited in the top plots. As shown in Figure3b, the damage in the planar architecture is always higher than inthe NWs and particularly advantageous is the free-standing NWconfiguration (NWvacuum). The bottom plots in Figure 4 revealthat not only the number of irradiation-induced defects varies inthe different device architectures but also their depthdistribution within the cell.The penetration profiles of irradiation-induced damage per

angstrom averaged over the array area (1.5 μm × 1.5 μm, @

array) and over the NWs surface coverage (9× π× (80 nm)2, @NW) are represented in Figure 5 for a better visual comparison.In the NW solar cells, the particle tracks in the collision cascadeare truncated radially at the surfaces of the NWs, therebyreducing or eliminating the characteristic peak of induceddisplacements relative to that observed in bulk materials. Theions and recoils that radially leave the NWs continue theslowdown process in the surrounding material, as observed inFigures S.3. In particular, 350 keV protons are significantlyscattered in the planar structure and therefore the truncation ofthe collisions in the NW array is markedly pronounced. Theadvantage of the NW array is less significant at proton energiesthat create a moderate number of scattering events in a planarconfiguration. On the other hand, the scattering events in thesurrounding photovoltaically inactive material can deflect highenergy particles (ions or recoils) toward the NWs. In fact, forproton energies at which all the protons are implanted in theNW array (such as 100 keV) the damage originated in the NWsby scattered high energy particles coming from the surroundingmaterial is not negligible. In Figure 5a, two distinct regions in theirradiation-induced defect profile are observed: a first region(<750 nm) where the displacements predominantly originatevia ions or recoils impinging on the NWs and a second region(>850 nm) deeper inside the wire where the displacementsobserved originate via collisions from scattered high energyparticles that initially impacted on the surrounding material.However, as revealed in Figure 3b, the overall damageintroduced in the NWs is lower than in the planar configuration.Finally, the damage in the notional free-standing NW array

simulated is even lower than in the core−shell planarized NWarray. In the free-standing NWs, the ions can move undisturbedbetween NWs favoring larger angular and lateral spreading andultimately enabling ions to leave the array without depositingtheir energy. Also, the free movement of ions between wiresproduces a very different defect distribution with a peak-freedamage curve dispersed along the whole wire length (see Figure5).The energy requirements for space power systems demand

lightweight, efficient, and radiation hard PV materials anddevices designed for high specific power. In this study, we haveassessed III−V NW solar cells for space applications byirradiation using high-energy protons and electrons and bysimulating the implications of their reduced dimensions in thegeneration of particle irradiation-induced defects.On the basis of our experiments, we determined that the

degradation of III−V NW solar cells is significantly lower thantheir planar III−V counterparts for both high energy protonswith a short penetration depth (100 and 350 keV) and highenergy electrons (1 MeV) which cross the entire solar cells. Thehigh radiation tolerance exhibited by the NW solar cells impliesthat (1) NW solar cells require less (and potentially no)shielding against low energy protons and (2) NW solar cells canpotentially extend the lifetime of the mission because theunshielded high energy particles with high penetration depth arealso less harmful in NW solar cells.Because the planar and NW solar cells tested are compounds

of the same III−V semiconductor materials, the irradiationresults suggest that the radiation hardness of NW solar cellsbenefit from their array geometry and nanometric dimensions. AMonte Carlo BCA model has been used to compare protonirradiation-induced damage in a planar GaAs architecture and ina NW array resembling the device architecture of our irradiatedexperimental NW solar cells. The Monte Carlo BCA model

Figure 3. Comparison of the damage introduced by protons ofdifferent energies in planar and NW solar cells. (a) Arsenicdisplacements produced per incident ion in a planar architectureversus different proton energies. (b) Damage ratio of the planar tothe NW array (left axis) and damage ratio in GaAs of the planar tothe NWs (right axis).

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reveals a lower damage in the NWs than in the planar solar cellsacross the entire proton energy spectrum. The reduction in theirradiation-induced defect density is mainly due to thetruncation of collisions in the nanometer-scale wires. Monte

Carlo simulations reproduce the trend observed experimentallythat the NW array configuration is particularly advantageous atproton energies that induce a large number of scattering eventsin planar absorber structures. As a result, the benefit of the NW

Figure 4. Damage distribution per angstrom in planar and NW solar cells after the irradiation with protons of different energies. Front view ofthe As displacement distribution per ion and angstrom integrated over the solar cell thickness (Z) (top plots) and lateral view of the Asdisplacement distribution per ion and angstrom integrated over the solar cell width (X) (bottom plots) for different proton energies (100, 350,and 500 keV) and solar cell’s architectures.

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architecture is more pronounced for irradiation with 350 keVprotons than with 100 keV protons. However, the radiationhardness observed experimentally is higher than the MonteCarlo BCA simulations predict. In fact, the damage ratio of theplanar to the NW architecture estimated by the Monte Carlosimulations is approximately 10-fold lower than the radiationhardness observed experimentally in the irradiation tests. MonteCarlo BCA simulations model ballistic collisions of the incidention and the generated recoils on a time scale of ∼100 fs with thetarget atoms assuming a temperature of 0 K. The extra energyremaining after transferring the ion kinetic energy ballistically tothe target atoms is dissipated by heat conduction to thesurroundings.53 After the thermalization of the collisions on atime scale of 1−10 ps, thermally activated processes can causethe generated point defects (vacancy and interstitials) tomigrate, recombine, and create defect complexes on a longertime scale53 (nanoseconds to years). Therefore, whencomparing the performance of planar and NW solar cells uponirradiation-induced defects predicted by Monte Carlo BCAsimulations, the discrepancies could be due to (1) the intrinsiclimitations of the simulation code to describe the production ofpoint defects in planar and NWs architectures (n.b., theincapability of modeling the heat spike regime) or (2) otherthermally activated processes53,54 with a time scale >100 fswhich play a role in transforming and reducing the residualdamage in the NWs relative to that present immediately after theballistic collision cascade. Studies of bulk GaAs show that atroom temperature the intrinsic defects (vacancies andinterstitials) created during irradiation can interact withprevious generated defects in the crystal49 which influencesthe defect migration mechanisms as well as their annihilationand accumulation. Impurities can promote the creation of stabledefect complexes, whereas dislocations might act as sinks forpoint defects.55 Therefore, crystal growth conditions whichfavor incorporation of dislocation and other defects orimpurities might affect the resulting damage microstructure asit has been reported particularly in p-type GaAs.49 The planarand the NW solar cells tested are both grown by metallorganicvapor phase epitaxy reactors (MOVPE) but at very differentgrowth conditions and in different crystal orientations. Anexcellent crystal quality was observed in the transmissionelectron microscopy (TEM) images taken in the NW solar cellstested (Figures S.4).Finally, it has been widely reported in the literature that the

presence of surfaces/interfaces interact strongly with defectscreated by collision cascades.55,56 Interfaces can attract, absorb,and annihilate defects.57−60 Therefore, nanostructuredmaterials

with a high surface-to-volume ratio are expected to exhibit aradiation damage tolerance significantly different from bulkmaterials. Additionally, in the case of NWs, observation ofenhanced dynamic annealing and absence of extended defectsdue to the dimensional confinement has been reported in Ge61

and GaN NWs.62 Further computational work is required toextend the knowledge of the behavior of the NW arrays underirradiation and to correlate to the electronic properties observed.In order to achieve accurate predictions, the NW array needs tobe analyzed with tools which are able to model the evolution ofthe system from the ballistic collision until the irradiation-induced defects become immobile.60

However, Monte Carlo BCA simulations can only be used tomake qualitative predictions; they can be used to optimize theNW array to increase the radiation hardness. In fact, MonteCarlo BCA simulations revealed that the damage in free-standing NWs is even lower. An array of free-standing NWswould behave like a planar device for light absorption but like ahollow structure for the irradiation with high energy particles.

CONCLUSIONIn conclusion, III−V NW array solar cells have the potential tobecome efficient, lightweight, radiation-tolerant power-generat-ing devices for space applications. In addition, because III−VNW array solar cells are made of the same semiconductormaterials as standard space multijunction solar cells, they shouldbenefit from the long heritage in development of III−Vphotovoltaic device technologies for space applications. Ourresults also indicate that further increases in radiation-robust-ness and specific power can be anticipated for approaches thatfeature (1) processing of NW devices infilled with an ultralowdensity mechanically stable material (such as aerogel) and (2)developing efficient substrate-free devices; this could lead to abreakthrough in the development of electronic devices for spaceapplications.

METHODSSolar Cells Synthesis and Fabrication. NWs were grown in a

horizontal metal−organic vapor phase epitaxy (MOVPE) reactor withthe vapor−liquid−solid method on (111B) substrates with predefinedhexagonal or square array of Au nanodiscs with a pitch of ∼500 nmplaced by nanoimprint lithography.63 For GaAs NW growth, a p-typeGaAs substrate of 5 × 1017 cm−3 was employed, whereas for InP NWgrowth a p-type InP substrate of 5 × 1018 cm−3 was used. Typicalgrowth temperatures were around 450 °C at which the appropriatemetal−organics are added to the carrier gas (H2) to grow the linear n-pstructures. Total lengths are 1.7−2 and 3−3.2 μm for the InP and GaAscase, respectively. Given the high aspect ratio of the semiconductor

Figure 5. In-depth induced-damage per angstrom in planar and NW solar cells for different proton energies. Arsenic displacements averagedover the whole array area (@array dotted lines) and over the nanowires area (@NW solid lines) for the different device architectures andrepresentative proton energies.

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structures, special attention must be given to surface passivation. Inparticular, GaAs samples included an in situ grown AlGaAs passivationshell31 whereas passivation of InP relied on ex situ oxide deposition.32

Further details on InP and GaAs NWMOVPE growth can be found inreferences 32 and 31, respectively.Samples were processed into 1.049 mm2 solar cells in the case of the

GaAs nanowire solar cells and into 0.86mm2 solar cells in the case of theInP nanowire devices. Details about the processing can be foundelsewhere31 but basically it consist of (1) conformal SiOx depositionaround the wires, (2) planarization of the array by spin-coatingCYCLOTENE resin (Dow Chemical), (3) etch back the Au catalystparticles by selective wet etching to expose the tip of the wires, and (4)indium tin oxide (ITO) deposition top contact. Because of the smallcell area, no current spreading fingers were necessary in the InP solarcells and a comblike front grid was defined with a standardphotolithography process in the case of the slightly larger GaAs solarcells. In both cases, the sample back-side was glued with Ag paste to abrass coin, serving as back contact.The planar n on p GaAs solar cells included in this study were grown

inverted on top of a 5 nm sacrificial layer of Al0.9Ga0.1As grown on aGaAs substrate. The semiconductor structure was grown in a MOVPEreactor by Spectrolab and the samples were processed into 8.41 mm2

thin film solar cells at Caltech. Extended details about the solar cellprocessing can be found in ref 64 but in brief it comprises the followingsteps: (1) evaporation of a highly reflective back mirror which will alsoserve as the back contact, (2) electroplating a thick copper handlesubstrate (∼50 μm) on top of the rear mirror, (3) bonding the copperfilm plated to a silicon wafer with black wax to ease the handling of thesamples, (4) etch the Al0.9Ga0.1As sacrificial layer by immersing thesample into HF/C2H6O (1:1) to lift-off the thin film GaAs solar cellfrom the thick substrate, (5) standard photolithography techniques todefine the inverted square front grid contact, (6) evaporation of thefront contact which consists of Pd/Ge/Au, (7) metal lift-off, (8) frontcontact annealing at 200 °C, (9) contact layer removal, and (10) mesaisolation by using a second photolithography step and wet chemicaletching. It has to be noted that antireflecting coating (ARC) has notbeen deposited on the devices.The planar n on p InGaP solar cells tested were grown and fabricated

by the National Renewable Energy Laboratory (NREL). The solar cellswere grown inverted by atmospheric pressure MOVPE reactor on(001) GaAs substrates miscut 6° toward (111)A as describedpreviously.65 After growth, the consequent processing steps werefollowed:66 (1) electroplate a Au back contact, (2) bonding the samplesto Si handles, (3) substrate removal via chemical etching, (4) standardphotolithography to define the comblike front grid, (5) evaporation ofthe Ni/Au front contact, (5) contact layer removal by chemical etching,(6) second photolithography step to define the mesa etching areas, (7)wet chemical etching to isolate the devices by fully etching down thesemiconductor until the Au back contact is exposed. As in the case oftheGaAs ELO solar cells, ARCwas not deposited on the GaInP devices.Electron and Proton Radiation Testing. The proton testing was

carried out in The Aerospace Corporation facilities in vacuum (1 ×10−6 Torr), at room temperature, the proton beam had a normalincidence on the sample, and the proton flux was 6× 107 p+/cm2·s. Theelectron testing was performed in Boeing Radiation Effects Laboratoryalso in vacuum (3 × 10−7 Torr) at 23 °C and under normal radiationwith an electron flux of 2 × 1011 e−/cm2·s.Solar Cell Performance Characterization. The solar simulator

used in the characterization before and after the irradiation tests makesuse of an AM1.5G filter. In order to characterize and control thespectrum of the solar simulator, Spectrolab XTJ calibrated isotype solarcells for AM0 were used. The isotype top cell of InGaP (350−670 nm)measured∼0.5 suns (AM0) and the isotype middle cell of GaAs (500−950 nm) ∼ 0.9 suns (AM0). The solar cells were measured in air atroom temperature.The initial NW solar cells efficiency under AM1.5G of ∼11.6% ±

0.9% for GaAs NW solar cells and∼6.5%± 0.9% for InP NWs reportedin Results and Discussion has been obtained from the measurements inan Oriel Sol1A solar simulator with an AM1.5G filter. A Si calibrated

reference solar cell was used to adjust the power of the lamp, and themeasurements were carried out in air at room temperature.

Simulations Monte Carlo BCA. The evaluation of the damagedistribution in the different device architectures has been carried outwith the program iradina.52 The code simulates the ion transportthrough the matter by means of a Monte Carlo algorithm. Similar to thewell-known SRIM code,67 iradina uses the random phase approx-imation, the binary collision approximation, the central potentialapproximation, and treats the target as an amorphous structure withhomogeneous mass density. The main advantage of iradina is thepossibility of simulating 3D geometries. The target structure is definedby a box divided into equally sized elementary units which account forthe damage produced inside the unit, thus generating the 3D damagedistribution in the defined geometry. Further details about the physicsbehind the model and its limitations can be found in ref 52. Here, avolume of 1.5 μm × 1.5 μm × 3 μm subdivided into elementary units of10 nm × 10 nm × 20 nm has been simulated. Periodic boundaryconditions (PBC) have been assumed laterally to mimic an infinitearray. The front surface (1.5 μm × 1.5 μm) has been hit with 1 000 000normal H+ of different energies randomly distributed. The displace-ment threshold energy for Ga and As has been assumed in all the casesto be 10 eV.

ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.9b05213.

(1) Characteristic parameters of the different solar cellarchitectures before and after irradiation tests; (2)empirical correlation of the degradation of planar andNWGaAs solar cells under the irradiation with protons of100 and 350 keV; (3) cross-sectional sketches of the NWstructures simulated with iradina; (4) electronic energyloss distribution under the irradiation with normalincident 100 and 350 keV protons; (5) TEM images ofa NW solar cell irradiated with 100 keV protons at afluence of 1012 p+/cm2 (PDF)

AUTHOR INFORMATIONCorresponding Author*E-mail: haa@caltech.edu.ORCIDPilar Espinet-Gonzalez: 0000-0002-7656-0077Enrique Barrigon: 0000-0001-6755-1841Giuliano Vescovi: 0000-0001-9556-0710Alex J. Welch: 0000-0003-2132-9617Magnus T. Borgstrom: 0000-0001-8061-0746Lars Samuelson: 0000-0003-1971-9894Harry A. Atwater: 0000-0001-9435-0201Present Address○(G.O.) Institute for Energy Technology, Kjeller, NO-2007,Norway.NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSThe authors acknowledge financial support from the CaltechSpace Solar Power Project. The work performed withinNanoLund was supported by the Swedish Research Council(Vetenskapsrådet), the Swedish Foundation for StrategicResearch (SSF), and the Swedish Energy Agency. This researchhas been funded by Knut and Alice Wallenberg Foundation.This project has received funding from the European Union’s

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Horizon 2020 research and innovation programme under GrantAgreement 641023 (Nano-Tandem) and under the MarieSklodowska-Curie Grant Agreement 656208. This publicationreflects only the authors’ views and the funding agency is notresponsible for any use that may be made of the information itcontains. We acknowledge critical support and infrastructureprovided for this work by the Kavli Nanoscience Institute atCaltech. We acknowledge the helpful contributions of J. Lloydwith the solar cell processing of the ELO GaAs solar cells atCaltech. We acknowledge The Aerospace Corporation for theirradiation test with protons and Boeing Radiation EffectsLaboratory for the irradiation with electrons. This work wassupported by the U.S. Department of Energy under ContractNo. DE-AC36-08GO28308 with Alliance for SustainableEnergy, LLC, the operator of the National Renewable EnergyLaboratory.

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