Yiyi Zhu , Qianpeng Zhang , Lei Shu , Daquan Zhang , and Zhiyong Fan ,

1Department of Electronic& Computer Engineering, The HongKong University of Science and Technology,Hong Kong999077, China2HKUST-Shenzhen Research Institute, Shenzhen 518057, China3Guangdong-HongKong-Macao Joint Laboratory for Intelligent Micro-Nano Optoelectronic Technology, HKUST, HongKong 999077, China

Abstract: Flexible solar cells are important photovoltaics (PV) technologies due to the reduced processing temperature, less material consumption and mechanical flexibility, thus they have promising applications for portable devices and building-integrated applications. However, the efficient harvesting of photons is the core hindrance towards efficient, flexible PV. Light management by nanostructures and nanomaterials has opened new pathways for sufficient solar energy harvesting. Nanostructures on top surfaces provide an efficient pathway for the propagation of light. Aside from suppressing incident light reflection,micro-structured back-reflectors reduce transmission via multiple reflections. Nanostructures themselves can be the absorber layer. Photovoltaics based on high-crystallinity nanostructured light absorbers demonstrate enhanced power conversion efficiency (PCE) and excellent mechanical flexibility. To acquire a deep understanding of the impacts of nanostructures, herein, a concise overview of the recent development in the design and application of nanostructures and nanomaterials for photovoltaics is summarized.

Key words: solar cells; nanotechnology; antireflection; bendability; PCE

1.Introduction

Sunlightprovides the most abundant sustainable energy to our world. Flexible thin-film photovoltaics (TF-PV) are important technologies in the PV community due to the reduced pay-backtime[1]and materialconsumption[2].Meanwhile, their lightweight and excellent mechanical flexibility are particularly suitable for portable and wearable power supply and building-integrated applications[2,3]. Reducing solar radiation loss becomes the foremostconcerntoreach the requirement of efficient flexible solar cells[4,5]. Whereas, the transparent dielectrics or metal oxide layer with a high refractive index produce an unfavored reflection. Researchers are searchingfor anti-reflectionschemesto address thisissue. Currently, theconventional quarter-wavelength(λ/4)coatings can be only effective for the photons at a typical wavelength under normal incidence[6−9]. To achieve enhanced broadbandabsorption, advanced light trapping techniqueshave been utilized. In industry, the pyramid structure or randomtextures are the most widely used light-trapping techniques for textured crystalline silicon solar cells[10,11]. However, this micro-sizedtexturing is not an optionfor TF-PV with onlyseveral hundred nanometers thick active layers.

The utilization of nanostructures for advanced light management is a realistic path to minimize optical losses of TFPV. Implementingthesenano/microstructures, forinstance,nano/micro-pyramid[5], nanowire (NW)[10,12−14], nanopillar[15,16],nanocone (NC)[17], nanodome[18], nanosphere[19,20],nanobowl(NB)[21], and nanosheets (NSs)[22]onsolarcellscontributeto several times enhancementof short-circuitcurrent(Jsc)[23]. In the meantime, effectively improved light-harvesting makes the utilization of ever-thinner cells possible.Thereby, a larger proportion of carriers can reach the boundary andbe separatedinto freecarriers before recombination.The reduced collectionpath potentially resultsinenhanced open-circuit voltage (Voc)[4].

It is highly desirable to have a nanostructured film with broadband anti-reflection and self-cleaning capacity on the topsurface ofsolar panels forrealapplication. As theincidentangle of sunlightvaries duringtheday, theangular dependence performance determines a solar cell's daily electrical energy output[24]. Considerable study has already confirmed that the utilization of nanostructures effectively improvesoutputpower overabroad range ofincident angles.Besides, there isaneed for manpower tomaintain thesurface cleaning of solar panels inoutdoorconditions,as the dust stuck on the solar panels will block the solar radiation and lead to performance degradation. The nanostructured film with the function of self-cleaning capacity becomes a practical solution to thisissue[25−27].

The activelayeritself can also be nanostructured. Taking one-dimensional (1D) NW, for example, photovoltaics based on high crystallinity (even monocrystalline) NW provide remarkable improvement in PCE. One contributing factor is the sufficient photogenerated carrier generationandcollection in optoelectronic nanodevice[28]. Meanwhile, the single crystallinity of NW is beneficial to form a direct charge transport pathway, which makes carrier mobility several orders of magnitudes outperform polycrystallinity TF counterpart[29]. Additionally, nanostructured devices are considered ideal structures for studying geometry's effect on optoelectronic and mechanical properties. Massive work has already demonstrated their merits in flexibility in an assortment of highly flexible optoelectronic device[7].

This review provides a comprehensive review of the recent developments in various light management nanostructures for photovoltaics. In particular, we focus on nanostructures on top as a broadband anti-reflection layer with selfcleaning capacity and survey micro-structured back-reflectors to reduce transmission via multiple reflections. We also summarize the recent progress in developing flexible photovoltaics based on NW with improved bendability, longevity, and PCE. The distinct merits and challenges of these strategies are discussed.

2.Nanostructures at the front surface

It is known that there are two categories of absorption losses: reflection and transmission. The transparent dielectrics or metal oxide layer with a high refractive index produce an unfavored reflection. One practical solution to minimize reflection is the nanostructures implemented on the top surface of solar panels. Nanostructures offer an efficient pathway for the photons flux and reduce solar radiation power loss due to the antireflection effect from the geometry and gradual refractive index gradient provided by nanostructures[4]. Careful modification of the shape and size of nanostructures provides new degrees of light manipulation. Theoretical calculations have been performed to maximize absorption in cells. Fig. 1(a1) is the typical prototype of PV consisting of nanostructures on top for light-trapping and a back-reflector at the bottom[30]. The simulated light absorption of the optimum structure is close to the Yablonovitch limit (Fig. 1(a2)). By comparison, shown in Fig. 1(b1), only the high-aspect-ratio,dense nanostructure arrays are utilized in the front, working as an anti-reflection layer. Most photons at short wavelengths, from 400 to 500 nm, are fully absorbed in the single path of semiconductors. Thismakestheantireflection on the top surfacemorecritical forthese wavelengths(Fig. 1(b2))[31].Fig. 1(c1) showsthelow-aspect-ratio,low-density microstructure arrays used in the back to reflect light into the light active layer. As shown in Fig. 1(d), for the neat TF, the light-harvesting of photons decreases with increasing wavelength.Thus, multiple reflections from the back reflector in Fig. 1(c1)improveoptical path lengthfor thephotonsat the wavelengthnear the bandgap. The theoretical calculation,Fig. 1(c2), shows that the microstructure arrays significantly improve light absorption at the wavelength of 0.8–1.1μm compared with the planar counterpart.

Fig. 1.(Coloronline) Three-dimensional(3D)nanostructured silicon solar cellsandtheir corresponding absorption spectra. (a1,a2)Double-sided nanostructure.(b1,b2) Top-only nanostructure. (c1,c2) Bottom-onlynanostructure.(d1, d2)Flatfilm.Red curves standfor the Yablonovitch limit, green curves are the single-pass absorption spectra, andblack curves represent spectrafor correspondingstructures. Reproducedwith permission[31]. Copyright 2014, Wiley-VCH.

Apart from the light-trapping effect, wavelength-scale nanospheres can diffractively couple photons and assist confined resonant modes.Moreover, owning to whispering galleryresonances withinthespheres, the light coupling betweenthe spheres iswitnessed inthe highlyperiodic array of dielectric nanospheres[32−36]. These will significantly enhance the optical path length inside the light absorber[4].

Tsuiet al. reported a cost-effective method for flexible plastic with three-dimensional (3D) light-trapping nanocone(NC) arrays[25]. Figs.2(a)and2(b)show the scanningelectron microscopy (SEM)images ofinverseNC template andNC arrays, respectively. Tsuiet al. illustrate the modification of NC arrays morphology, such as pitch and height, via changing the template's geometry. Notably, the NC arrays films are adhesive-free and easily attached to substrates, including glass and silicon. A noticeable light-trapping enhancedperformanceis observed in the opticalmeasurements andtheoretical simulations. ThisisfurtherconfirmedinFig.2(c) external quantum efficiency (EQE) measurements of CdTe solar cells.In addition, their nanostructures also possess broadband light trapping capability[37]. This is confirmed in the daily electrical energy output measurement. After implementing anti-reflection films, theoutput ofCdTesolar cells reachesashigh as1kW·h/m2,showing 7% enhancement compared with the control group[25]. Similarly, Tanget al. proposed a large-scale manufacturing approach of adhesive-free, 3D NC structured flexible, and anti-reflection films[26]. As expected, the antireflection NC arrays effectively boost light-harvesting, verified in finite-differencetime-domain(FDTD) simulations.

Fig. 2. The scanning electron microscope (SEM) of inverse nanocone (NC) template (a) and NC arrays (b). (c) The external quantum efficiency(EQE) spectra of CdTe solar cells with and without NC film. The inset of (c) is the schematic structure of the device. (a–c) Reproduced with permission[25]. Copyright 2014, Wiley-VCH. (d) SEM of NC arrays. The inset is a drop of water on NC arrays, illustrating a contact angle of 155°. (e) The current density–voltage (J–V) characteristics of perovskite solar cells with and without NC arrays (inset is a photo of the flexible device). (f) Under different incident angles, the short-circuit current density (Jsc) and the power conversion efficiency (PCE) with and without NC arrays. (d–f) Reproduced with permission[38]. Copyright 2015, American Chemical Society. (g) Schematic procedure of 3D nanostructured a-Si:H solar cells. (1) Spin coating ZnO film on polyimide film. (2) Patterned ZnO film. (3) A a-Si:H solar cell constructed on the as-fabricated substrate. (4) Fabrication of nanoindentation on aluminum foils. (5) The anodic aluminum oxide (AAO) template with inverse NC arrays. (6) The NC arrays film peeled off from the template. (7) The a-Si:H solar cell with nanostructured back-reflector and top anti-reflection NC arrays. (h) Normalized PCE under different bending angles. (i) Normalized PCE as a function of bending cycles. The insets (h) and (i) demonstrate bending angles and a bent solar cell mounted on the set-up. Reproduced with permission[39]. Copyright 2017, Wiley-VCH.

The energy outputof solar cells canbe interfered withor even cutdownby the duston thetopof solar moduleswhen it comes to outdoor conditions, especially in the solar farm locatedinthe desert. The duststuckon solar panels willblock the solarradiationand leadto performancedegradation.Therefore,thereisa need formanpower or machine to maintainthe surface cleaning ofsolarpanels.Apart fromthe constantmaintenancecost,an abundantamountof water isneeded to cleanthe solar systemthat ispreciousin thedesert area. The nanostructured film with the function of self-cleaning capacity becomes a practical solution to this issue[25−27].

Tavakoliet al. fabricated flexible perovskites solar cells on ultrathin willow glass substrates with polydimethylsiloxane (PDMS) NC array films on top as a light-trapping and selfcleaning layer[38]. Illustrated in Fig. 2(d), the NC array demonstrates a depth and opening width of 1μm. Besides, as shown in the inset of Fig. 2(d), the NC structure demonstratesa water-repellentcapacity, with ahigh watercontact angleof 155°,suggestinga self-cleaningfunction. To further confirmtheself-cleaning property,the experiment is carried outby spreadingthesand onthetop surfaceof solar cells.The dustis easy toremove byrolling a waterdroplet across the surface, comparedwith the devicewithout nanostructures. Fig.2(e)is thecurrent density–voltage (J–V)characteristicsofflexiblesolar cells withand without theNCarray under simulated AM1.5Gillumination. The insetisaphotograph of a perovskite solar cellbased on flexiblewillowglass. Anoticeable enhancedJscfrom 17.7 to 19.3 mA/cm2is observed after applying the NC structure. This leads to PCE improvement from 12.06% to 13.14%, corresponding to ~9% increment.The improved performance is also witnessed under oblique light, which is critical for the solar cell's daily operation, as the sunlight varies during the day. Fig. 2(f) is the angular-dependentperformanceofJscand PCEof thedevicewith and without the NC array. To have a fair comparison with conditions where the device is not inclined under the collimated irradiation, bothJscandPCE measuredunderdifferent incident angles are normalized with the horizontal light projection area. Clearly, the improvement ofJscand PCE of NC-based devicesis around1.5–2.5 mA/cm2and1%–1.75%,respectively. Notably, bothJscand PCE drop more notably with the increase of incident angle for the control group than the case with NC structure.

Similarly, through nanoimprinting lithography, Zhanget al. realized highly ordered metal oxide nanotextures on polyimide(PI)substratefor the highlyflexible amorphoussilicon(a-Si:H) solar cells[39]. Fig. 2(g) shows the fabrication process of a-Si:H solar cells on patterned ZnO/PI substrates covered with NC structures.Briefly, the backreflectorZnOfilmswith highly orderednanoholesarrays arefabricated through nanoimprinting lithography. Then, a-Si:H solar cells are constructed on the patterned PI flexible substrates. The anodic aluminum oxide (AAO) film is used as atemplate to pattern NC arrays film that is attached on the top of a-Si:H solar cell. Benefitting from the sufficient light absorption, the PCE of the nanostructureddevice increased upto 8.17%, whichisnearly 48.5% improvement over the planar control group. To evaluate the mechanical robustness, shown inFig. 2(h), the PCE of the device under bending anglesfrom 0° to 180°arecharacterized[40]. The PCE is normalized by the projection area. Notably, the NC-based devices only encounter 17% PCE drop under the bending angleof 180°. As shown inFig. 2(i), theperiodic nanopatterns contribute to better excellent flexibility. NCbased devices only experience a negligible drop in PCE after 100000 bendingcycles. Zhanget al.attributedthe excellent mechanicalflexibility to the nanostructure, effectively minimizing the strain and stress generated during bending. Because thestrainandstressraise thepossibilityofcracknucleation and delamination attheinterface, leading toperformance degradation.

3.Nanostructures at the back surface

As mentioned above, antireflection coatings affect the optical loss caused by reflection. In contrast, light-trapping schemes address thelossbytransmission.Especially,photons at the longer wavelength are less absorbed in the single path because of decreased absorption with increasing the wavelength towards thebandgap[31].

The loss of light absorption in the red region is the cause of undesired reddish-brown color for conventional semi-transparentperovskitesolarcells (ST-PSCs) based on thecontinuous TF[41−48]. The reddish-brown hue is unfavored for the application of power-generated windows for building-integratedphotovoltaics[49]. The periodic arrays ofmicrostructure implemented at back work as reflectors to increase the optical path length through multiple reflections. Besides, the reflection andangular distributions of the scattered light can be controlled by geometry modification, forinstance, shape,diameters, and periodicities. With this regard, inFig. 3(a), Zhuet al. proposeda moth-eye-inspiredstructure (MEIS) for STPSCs[50]. Fig.3(b) is the reflection spectraofMEIS. Thebiomimetic structure is a perfect back reflector that only reflects photons in the wavelength range where the human eye is less discerning. This is further confirmed in the inset of Fig. 3(b) that the patterned area only reflects blue and red light. In Fig. 3(c), the unique optical property contributes to the improvements in ST-PSCs performance without compromising the average visible transmittance (AVT). MEIS device raises PCE to 10.53% at AVT of 32.50%, which is a significant improvement compared to the planar counterpart (PCE =8.78%, AVT = 35.00%). Thus, a record high figure-of-merit for ST-PSCs, defined as the productofPCEandAVT,is achieved.Besides,confirmedby theinset of Fig. 3(c), theimproved light-harvesting in the longer wavelengthhelps convert visual appearance from thereddish-brown (planarcontrolgroup)to adesired near-neutralcolor (MEIS).

Similarly,to addressthe insufficient light-harvesting,Zhenget al. proposed a strategy of TiO2nanobowl (NB) array with controlled morphology and fabricated carbon cathodebased perovskite solar cells[21]. Fig. 3(d) shows the top-view and cross-section view SEM of the TiO2NB array. TiO2NB array is a light-trapping layer to quench photons and thereby reduce transmission and boost absorption. Fig. 3(f) reveals theJ–Vcurves of TiO2NB-based devices and planar counterparts.As expected, enhanced performance is achieved in the TiO2NB-based devices comparedwith theplanarcounterpart. The contributing factor for theimproved performanceis enhancedlight absorption arisingfromthe NB back reflector.Tostudy the light management,shown inFig.3(f), the crosssectionalelectric fieldintensity (|E|) distributions of the electromagnetic (EM) wave at 600 nm are calculated.Interestingly,the TiO2NB-based devices reveal stronger electric field intensity than the planar control group, resulting from a larger proportion of photons coupled into the TiO2NB array.

Xiaoet al. systematically investigated the performance of a-Si:H solar cells based on the different thicknesses of oxide spacer layers[40]. The nanopatterned Al is used as the substrate, and the device structure is shown in Fig. 3(g). The corresponding SEM image of the substrates coated with 100 nm Ag/100 nmconductiveAl-dopedZnO (AZO) isrepresented in Fig.3(h).Interestingly, the increasedthickness ofthe spacer layer AZOreduces the absorption in theAg layer andinduces enhanced light-harvestingin thesilicon layerin return.As shownin Fig.3(i),the highestcalculatedcurrentdensity in the silicon layeriswitnessed inthe device basedon the 100 nm spacer layer (ND100 device). This suggests that the light-harvesting of the nanopatterned device can be rationally controlled by modification of device geometry. More importantly, aluminum foils hold large-scale manufacturing possibilities and excellent mechanical flexibility[51,52]. Shown in Fig. 3(j), the normalized PCE of the ND100 device under different bending angles is measured. The PCE only encounter a negligible 8.8% drop even under the bending angle of 120°.

Fig. 3. (Color online) (a)Complete compound motheyes and amoth-eye-inspired structure(MEIS) device structurediagram. (b) Reflectancespectraof MEISandhuman luminosity curve, inset is thephoto of MEIS (scale bars, 3 cm). (c) J–V curves of the MEIS ST-PSCs anda planar reference under simulated AM1.5G illumination, theinsetis thephotographs of (c)(scale bars,2 cm). Reproducedwith permission[50].Copyright 2021,Wiley-VCH.(d) SEMimages of TiO2nanobowlwith adiameterof 180 nm(NB-180). (e) J–V curvesof thedevicebasedon differentdiameters.(f)Simulatedcross-sectional |E| distributionof theelectromagnetic(EM) waves at 600nmwavelength inthe perovskite deposited on(f1) TiO2NB-180,(f2) TiO2 NB-220, (f3) TiO2 NB-500,and (f4)planar TiO2,Reproducedwith permission[21].Copyright2021, Wiley-VCH.(g)Schematic viewof nanostructured a-Si:Hthin-film. (h)SEM image of the 100 nmAg-coated substrates deposited with100nmconductive Al-doped ZnO (AZO). (i) The calculated Jsc of thedevice based ondifferent diameters TiO2.(j)NormalizedPCEunderdifferent bending angles. Theinset (j1)representsaphoto of the measurement set-up and (j2)a schematic of bending angles. Reproducedwithpermission[40]. Copyright 2021, Wiley-VCH.

4.3D nanostructured device

4.1.3D nanostructured for light management &boosted carrier collection

The most crucial part of photovoltaicsis the adequate harvesting of photons, exciting electrons to theconductive band, and leaving holes behind. Nanomaterials provide the opportunityto minimizelossofeach step, for instance,absorption, carrier generation, separation, and collection. Besides,the nano-scale geometry offers unique advantages, includingsuppressed reflection, lighttrapping,facilestrainrelaxation, new charge separation mechanisms, better defect tolerance, etc.[53]. Theseadvantagesare notexpectedto improve the PCE above the standard limits. Instead, nanotechnology decreasesthequantity and quality of material requirements to obtain a highly efficient device[54]. The solar cells based on nanopillar-arraywithradial p–n junctions arean excellent example of this point.

In Figs.4(a)and 4(b),Fanet al. pioneeredananopillar-array CdTe/CdS photovoltaics with a 3D geometric configuration[28]. The 3D device structure affords effective radialcharge collection and light absorption[10,13,55]. The experimental result Fig. 4(c) and simulation calculations Fig.4(d) show thatabsorption is improved in 3D nanodevice. One contributing factoristhat 3D geometric configurationreduces reflection and thereby enhances optical absorption. In addition, the 3D nanopillar arrayoffers excellent lightabsorption along the length of the wire[15,38].

The carrier separationand collectionadvantagesof the radial geometry are more noticeable. As the single crystallinity ofnanopillar (Figs.4(e)and 4(f)) isbeneficialto formguiding channels for carriers, the direct charge transport pathway makeselectronmobilityinNWs several orders ofmagnitude higher than in the polycrystallinity TF counterpart[29,56−58].More importantly, the orthogonalcarriercollection inthe radial built-in electric field (Fig. 4(a)) benefits photogenerated carriercollection[28]. Thephotogeneratedcarrier only needs to travel a short path and reach the boundary of p–n junctions.This isespecially beneficial for photovoltaic materials with a short diffusion length[59].

To achieve the optimumperformance cell, the detailed optimization of the optical and electronic properties is required,which are strongly determinedby the geometry of the nanopillar[60]. To investigate the dependency of the performance on the geometricconfiguration,the theoreticalsimulations of Shockley–Read–Hall (SRH) recombination as a function of height(H)are carriedout. The visualization of SRH recombination forH= 0 nm (Fig. 4(g1)) andH= 900 nm (Fig. 4(g2)) is plotted.Notably, as shown in Fig.4(g1),the spacecharge and carrier collection region are quite low forH= 0 nm. This leads to alarge proportionof photogenerated carriers lost in the upper portion of the film via recombination, where there is ahigh level ofphotogenerated carrier generation.However, as shown in Fig. 4(g2), the space charge and carriercollectionregionaredramaticallyimprovedforH=900 nm. This decreased the total volumetric recombination in return. However, thenanostructuremay lead to a drop inPCE in contrast to planar counterparts when surface recombination isthe limitingfactor.

Fig.4. (Color online)(a)Cross-sectional schematicdiagram of a3Dsolarnanopillar cell, demonstrating improvedcarrier separation andcollection. (b) SEM images of daCdS nanopillar array. The experimental(c) andsimulated(d)absorptionspectraof the nanowire (NW)plottedas a function of diameter and pitch. (c, d) Reproduced withpermission[15]. Copyright 2012, American Chemical Society.(e)SEMimages of InSbNW. (f) The electron mobilityof InSb NW.(e,f) Reproduced withpermission[58]. Copyright 2019, AmericanChemicalSociety. (g) Visualizationof the Shockley–Read–Hall(SRH) recombinationinthe3D nanopillar cellsplottedwithafunctionof heigh(H): H = 0 nm (g1) and H =900 nm (g2).(a,b,g) Reproducedwithpermission[28].Copyright2009, NatureResearch. (h) Schematicrepresentation of BiI3structure. (h)Reproducedwith permission[75]. Copyright 2017, Wiley-VCH.(i) Cross-sectional schematicdiagram of 3DBiI3 nanosheets (NSs) cell. (j)SEM of BiI3 NSs.(k) J–V curvesof BiI3 NSs solar cells fromdifferent precursor Bithicknesses. (i–k)Reproducedwithpermission [22]. Copyright 2020, Wiley-VCH.

Multilayered photovoltaic absorbers, such as BiI3, two-dimensional (2D)perovskites, and transition metal dichalcogenides have gained enormous attention because of their unique properties. Shown in Fig. 4(h), BiI3is a layered 2D material constructed by the repeating unit of the I-Bi-I layer. Carriersaremobilein the layer andimmobileacrossplanes[22].The carrier transportandcollectioninthe randomly oriented polycrystalline TF is insufficient,which is the core hindrance for high-performance photovoltaics based on 2Dmaterials.To tackle this issue, Zhuet al. fabricated 3D BiI3nanosheets(NSs)basedphotovoltaics that embeds vertically aligned monocrystallineBiI3NSs into2,2’,7,7’-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9’-spirobifluorene (spiro-OMeTAD)[22].It is noteworthy that verticallyaligned monocrystallineBiI3NSs aredirectlyfabricatedon the substratewithacontrolled geometricconfiguration throughthe vapor-solid-solidreaction.The directgrowth of nanomaterial onthe substrate reduces productioncosts. Fig. 4(i)showsthecross-sectional schematicdiagram ofthe BiI3NSscell.BiI3NSs areembedded into spiro-OMeTAD to form the 3D heterojunction. Thetop-view SEM image ofvertically aligned monocrystallineBiI3NSs is shownin Fig.4(j). Light is prevented frombouncingoffthe top surface and couplinglight in the nanostructured active layer.BiI3NSsbased deviceshows onemagnitude lowerinthe light reflection over a planer reference. This is further confirmed in theJ–Vcurve measurement. The experimental result Fig. 4(k)shows thatJscis significantly improved compared witha planar reference due to less trap-assisted recombination in themonocrystalline BiI3and largep–n junction areas of3D heterojunctionstructure[10,61,62].Asaresult, a record-high PCEof1.45% is achieved (Fig. 4(k)).Notably, the BiI3NSs-based devicedemonstrates robust stability against moisture and oxygen, resulting from the self-passivated surface of monocrystalline BiI3NSs. The non-packaged device retained 96% of the original PCE after 24 h of continuous AM 1.5 illumination at ~70% humidity and 82% of the initial PCE after one-month storage at ~30% humidity.

Fig. 5. (Color online) (a) Schematic diagram of the 3D nanospike. (b) Angular and wavelength-dependent absorption of a nanospike solar cell and a planar reference. (c) Normalized PCE of the nanospike device under different bending angles, inset is the schematic of a flexible nanospike solar cell. (a–c) Reproduced with permission[52]. Copyright 2014, The Royal Society of Chemistry. (d) SEM image of a-Si:H solar cells on 0.5 aspect ratio nanocone. The aspect ratio is the ratio between height and pitch. Simulated cross-sectional stress distribution of flat (e) and nanocone devices (f), with their photos after bending with a radius of 4 mm shown. (d–f) Reproduced with permission[73]. Copyright 2016, The Royal Society of Chemistry.

4.2.3D nanostructured devices for better flexibility

High-performance flexible electronics increasingly gained attention during recent decades, owing to the promising potential in building-integrated photovoltaics, portable and wearable power supplies, etc.[63−65]. Various flexible active materials, such as amorphous silicon and organic semiconductors,have been studied for flexible electronics[66−68]. However, owingto thetraps in amorphous silicon andthelong-term stabilityof organicmaterial, searchingforanewcandidate for flexible electronics is still theneed of the hour[69,70].

For flexible photovoltaics,thebending and stretching of thedevice should not have anotable impacton PCE[71]. Currently, the dominant TF flexible photovoltaic suffers from rapid performancedegradation during continuous bending. Nanodevice caneffectivelyrelax tensile andcompressive stresses and avoid the formation of cracks during bending and stretching. They have demonstrated their merits in flexibility in an assortment of highly flexible optoelectronic devices[7].

InFig. 5, Leunget al. fabricated flexible, nanospike arrays of Al substrate for single-junction a-Si:H solar cells[52].Thefabrication process is shown inFig. 5(a). By systematic analysis ofthe geometry, Leunget al. resolve the dilemma between light-harvesting andsurface recombination.After the geometry optimization, a PCE of 7.92% is achieved. In addition, as illustrated in Fig. 5(b), the nanostructured device exhibits superior angular-dependent performance. The daily integrated power is 32% outperformed the planar counterpart.Fig. 5(c) is the mechanical stability test. The device based on nanospike arrays maintains 82% of original PCE remains after 1000 cycles. This type of nanostructurescombines versatile merits in terms of excellentflexibility,lightweight, andcost-effectiveness.

Tavakoliet al. reported efficient, flexible, and mechanically robust organometallic perovskite solar cells on plastic substrates with inverted NC structures[72]. PCE of 11.29% is achieved in the NC-based device, a 37% improvement from the planar control group. The mechanical simulation demonstrates that the NC structures contribute to relaxing stress and strainduring continuousbending and suppressing cracks formation due to the sub-micrometer diameters. The NC-baseddevice retained 90%of the original PCEafter 200 mechanical bending cycles. In comparison, the planar group dropped to 60% of the initial PCE under the same condition.

In Fig. 5(d), Linet al. present acost-effective approach towards periodical NC arrays of polyimide (PI), which possesses excellent mechanical flexibility andunique optical management[73]. The flexible a-Si:H solar cells are made on the nanopatterned PI substrate.Fig. 5(d)shows the top-view and cross-sectionalviewSEMimagesofthedevice. The a-Si:H devices demonstrate magnificent conformality and uniformity. Their result further confirmed that nanostructures are beneficial to release stress, which is verified in both the mechanical simulation and experimental observations of Figs. 5(e) and 5(f). Similarly, Linet al.reportedflexible a-Si:H solar cells with a 3D nanostructure light-trapping scheme[74]. Apart from improvedJsc, excellent flexibilityisachieved. The devicemaintained97.6% of the initial efficiencyeven after 10000 bending cycles.

5.Summary and outlook

Nanostructuresand nanomaterialspossesspromising potentialtoimprove thelight-harvestingcapability ofsolar cells.Nanostructureson thetop surfaceofferbroadband antireflectionand self-cleaningcapacities forsolarcells. More importantly,the device's overall performance has beenremarkably upgradedviaimplementing these nanostructures. Nevertheless,it is still farfrom the terminalobjectives of the entire solar spectrumcoverage for solar cells. Future workstill requires optimizing geometry design and fabricationprocess to draw out the fullpotency of these light-trapping strategies.

Thenanostructured light absorbersoffer photovoltaics unique optoelectronic and mechanical properties. The 3D geometric configuration leadstosufficient orthogonalization light-harvesting and carrier collectionachieved in nanodevice. Also,nanodevicespossessexcellent mechanical highly flexibility.Despite thepromising potential, surfacerecombinationisthecore hindrance forahigh-performance nanodevice. Thus, the investigation of the surface property of materialreallymatters.In this perspective, effortsare requiredtounderstand better the carrier dynamics atthesurface, such as charge transfer,surface recombination, minority carrier diffusion, dopant density, surface state, and conductivity measurements. It isnoteworthy that the PCE relieson efficient photogenerated carrier collection. Thenanodevices' performance canbe significantly improvedthrough interface engineering forbettersurface quality and interfacial band alignment. Therefore, toexplore thepotentiality of nanodevice,efforts should bedevoted to nanodevice geometry design, material choice,surface passivation/treatments, andenergy-level alignment engineering.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Project No. 51672231), the Science and Technology Plan of Shenzhen (Project Nos.JCYJ20170818114107730, JCYJ20180306174923335), the General Research Fund (Project Nos. 16309018, 16214619) from the Hong Kong Research Grant Council. Guangdong-Hong Kong-Macao Intelligent Micro-Nano Optoelectronic Technology Joint Laboratory (Project No. 2020B1212030010), HKUST Fund of Nanhai (Grant No. FSNH-18FYTRI01). The authors also acknowledge the support from the Center for 1D/2D Quantum Materials and the State Key Laboratory of Advanced Displays and Optoelectronics Technologies at HKUST and Foshan Innovative and Entrepreneurial Research Team Program (2018IT100031).