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Abstract
We provide a review of current understandings of mechanical properties and fracture behaviors of perovskites that are essential for flexible and stretchable solar cell (SC) applications. We first review the mechanical failure modes in perovskites. We further discuss the underlying mechanisms of mechanical failure and its impact on device degradation in flexible perovskite solar cells (PSCs). Then, we examine the strategies to mitigate these mechanical issues in flexible PSCs. Lastly, we assess the elevated challenges and present recommendations for future research directions to advance the technology towards a fully stretchable and wearable energy source.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
Letian Dou, Xiwen Gong, Sergey Makarov, Barry Rand, and Yongsheng Zhao, "Halide Perovskites for Photonics and Optoelectronics feature issue: publisher’s note," Opt. Mater. Express 12, 1009-1009 (2022)https://opg.optica.org/ome/abstract.cfm?uri=ome-12-3-1009
3 February 2022: A typographical correction was made to the article title.
1. Introduction
Metal halide perovskites (MHPs) have emerged as promising photovoltaic (PV) materials due to their excellent optical and electrical properties and the ease of manufacturing. Within the MHP community, improving power conversion efficiency (PCE), operational stability, and the replacement of lead (Pb) have been the main topics of interest. Recently, mechanical stability and flexibility of perovskites have attracted increasing attentions. Compared to rigid solar cells (SCs), flexible solar cells (fSCs) can minimize the damage caused by the vibration and turbulence during the handling and transportation process, which accounts for 6% of the device damage in the conventional rigid SCs [1]. The lightweight nature of perovskite solar cells (PSCs) enables their installation to remote areas by reducing the cost of transportation and installation compared to the conventional Si-based SCs. Moreover, enhancing mechanical conformability of SCs will generate new pathways to harvest solar energy beyond solar panels on the roof or in the dessert. For instance, fSCs can harvest solar energy on non-planar and dynamic structures by serving as power source for wearable electronics, vehicles, and small personal gadgets. In addition, fSCs also enable fast and large-area roll-to-roll manufacturing, which reduces the cost and allows the scaling-up on an industry relevant level.
There have been significant efforts in understanding the mechanical properties of perovskite thin films. In this article, we first review the mechanical properties of MHPs and present the most commonly observed mechanical failure modes in PSCs: crack and delamination. We further review solutions to address these issues: 1) reducing the grain boundaries, 2) grain boundary crosslinking 2) interfacial engineering, 4) pre-strained substrate, and 5) neutral plane strategy.
Moving towards the era of wearable technologies, there are increasing needs to further improve the mechanical deformability in electronics, to achieve versatility and compatibility with existing technology in energy sources [2,3]. Beyond flexibility, stretchability needs to be introduced in perovskites to develop next-generation wearable SCs.
2. Mechanical properties, common failures modes in MHPs and solutions to improve their flexibility
2.1 Fundamental mechanical properties of MHPs
MHPs feature low exciton binding energy (37 - 50 meV) [4,5], shallow trap states [6], high charge carrier mobility [7], and long charge diffusion length [8]. These superior optoelectronic properties make MHPs ideal candidates for PV applications. In addition, the solution processability of perovskites promises exciting applications in flexible and portable power sources for self-powered electronics.
To achieve flexibility in PSCs, significant efforts have been devoted to understanding their mechanical properties. Young’s modulus(E) is one of the most commonly reported indicators to evaluate the stiffness and mechanical compatibility of materials for flexible and wearable applications. Several researchers reported E value of MHPs based on experimental and theoretical studies. For instance, Rakita et al. reported an E of 14.3 GPa and 19.6 GPa for MAPbI3 and MAPbBr3, respectively, using the nano-indentation method [9]. Other studies on the mechanical properties of MAPbI3 also measured the E value with nano-indentation giving a range of values (Table 1) depending on dwell time and lattice plane [10,11,12,13]. Feng investigated the elastic properties and anisotropy of MABX3 (B = Pb, Sn; X = Br, I) using first-principles calculations based on density functional theory (DFT) [14]. Their research suggests that the elastic properties of perovskite depend on the chemical bonds between B and X. The calculated E of cubic phase MHPs ranges from 15 to 37 GPa (with E of MAPbI3 = 22.2 GPa). Faghihnasiri et al. also reported similar results (E of MAPbI3 = 22.8 GPa) [15] taking into account the interactions between MA and PBX3. The E of MHPs is in between that of rigid inorganic semiconductors (> 50 GPa) and flexible organic semiconductors (< 5 GPa) (Table 1).
Table 1. Mechanical Properties of Metal Halide Perovskite and other Photovoltaic Materials
Critical strain energy release rate (GC) is another important mechanical property which evaluates the fracture toughness of materials. Strain energy release rate (G) is defined as the amount of strain energy released by generating one unit of crack. Once the strain energy release rate reaches the critical value (GC), crack will propagate. Hence, the higher value of GC indicate that it is more energetically costly for a crack to propagate, i.e. more resistive to crack propagation. MHP thin films show comparable GC to organic semiconductors, which is significantly lower than that of silicon or CIGS thin films (Table 1) [16]. Therefore, it is more energetically favorable for MHP and organic semiconductors to form cracks than other inorganic semiconductors. In part 2.2, we will discuss in detail that using E or ${G_C}$ alone is insufficient to evaluate the flexibility of materials. Instead, the value of (E/GC)1/2 governs the critical (maximum possible) bending radius of materials, which better reflects the intrinsic flexibility of materials. Thus, the value of (E/GC)1/2 should be considered more often in the future research of flexible and stretchable electronics.
Overall, MHPs are regarded as promising candidates for fSCs applications in view of their reduced rigidity compared to inorganic semiconductors, excellent optoelectronic properties, solution processability, and low cost. However, in order to achieve the same level of flexibility such as in organic fSCs while maintaining the outstanding optoelectronic properties of perovskites, new material design and device engineering strategies are needed.
2.2 Effect of mechanical deformation in perovskite thin films
2.2.1 Strain-induced structural and optoelectronic property changes in perovskites
Flexible PSCs often operate under external strain, and thus the effect of strain needs to be considered carefully to develop high-performance and stable PSCs. Recent work has revealed that the strain in MHP can modulate their optoelectronic properties. For example, tensile strain increases the bandgap of MHP, while compressive strain decreases it [32]. Such change of the bandgap is ascribed to the different sensitivity of valence band and conduction band to the change of strength of Pb-X bond induced by the lattice expansion. In addition, strain also modulates the carrier mobility: compressive strain leads to the increased hole mobility due to the decrease of the hole effective mass [33].
Strain also negatively impacts the stability of MHP films. For example, tensile strain decreases the activation energy for ion migration, which accelerates the degradation of perovskite into PbI2. Conversely, compressive strain increases the activation energy thus suppressing such degradation [34]. Strain also affects the phase stability of perovskites. For example, residual tensile strain in FAPbI3 drives the photo-active α phase into photo-inactive δ phase at room temperature [33]. In the case of CsPbI3, tensile strain stabilizes the photoactive black γ-phase, preventing its transition to the photo-inactive yellow δ-phase [35].
Due to the importance of strain on MHPs, several groups have conducted thorough review on strain analysis and engineering in MHPs [36–38]. In the following discussion, we will instead focus on an important yet currently underexplored area: the fracture mechanics of PSCs and its impact on the photovoltaic performance.
2.2.2 Mechanical failure modes in perovskites
Due to the rigidity of MHP materials, mechanical failures are often present when rendering PSCs in flexible forms. Crack formation is one of the most commonly encountered mechanical failures in flexible PSCs (Fig. 1(b)) [39–42]. In general, cracks tend to initiate at 1) the grain boundaries of polycrystalline MHP, 2) within the grains, and 3) the interfaces between MHP/carrier transporting layers.
Fig. 1. Schematic illustration of mechanical failure modes in MHP thin films. (a) MHP thin film on PET without external stress(bending). (b) Crack formed on MHP thin film upon bending. (c) Crack sites make MHP film susceptible to moisture-induced degradation. (d) Diffused moisture degrades MHP into PbI2. (e) Hole transport layer materials penetrating through crack sites, leading to undesirable charge carrier recombination as indicated by the red dashed circle. The blue dots and white dots represent electrons and holes respectively. (f) Delamination results in the loss of electrical contact. Charge carriers fail to transport between the layers, leading to increased series resistance and device failure. Delamination also makes the film susceptible to degradation by generating pathways for moisture.
Dai et al. reported that grain boundaries are the primary locations of crack formation [41]. They found that MHPs with smaller grain size have grain boundaries parallel to the substrate. Using the double cantilever beam test on MHP film, they observed that cracks form along the grain boundaries, i.e. horizontally. In comparison, in MHP films with larger grain sizes, the horizontal grain boundaries are mostly absent. In this case, cracks tend to form at the next weakest sites: the MHP/electron transport layer interface. In addition, cracks can also form within the grain of MHP. Yadavalli et al. reported that upon e-beam exposure, α-FAPbI3 films experience crack formation primarily within each grain [39]. Lee et al. also observed cracks formation within the grain in MAPbI3 film during the bending test [40].
Interfacial delamination is another major cause of mechanical failures in flexible PSCs. Dong et al. observed interfacial delamination between MHP film and ITO/PET substrates post cyclic bend tests, in addition to the vertical crack formation [42]. They further confirmed that the gap caused by delamination and crack allows water and oxygen molecules to permeate and interact with perovskites, which accelerates the degradation of the MHP films (Fig. 1(c)).
2.2.3 Mechanistic studies of crack formation and interfacial delamination of perovskite thin films
Several in-depth studies have been carried out to unveil the mechanism of the mechanical failure in flexible PSCs. Dong et al. elucidated the effect of bending on crack formation in MHP films based on fracture mechanics studies [42]. Upon bending, thin film undergoes a combination of tensile stress at the outer layer and compressive stress at the inner layer of the film. When the film is under convex bending, the maximum tensile stress (σT) is given by Eq. (1) [39]:
where E is Young’s modulus of the MHP, h is the thickness of the substrate (when thickness of MHP is relatively small it is neglected), and R is the bending radius. The tensile stress drives the formation of fractures in MHP films. Using Eq. (2), one can predict the critical tensile stress when cracks start to form [43]: where K is the stress intensity factor, ψ is the geometrical constant (∼π0.5), c0 is the half length of the incipient crack (in the case of MHP films, grain-boundary grooves and facets are considered as incipient crack), KIC (MPa m0.5) is the critical stress intensity factor of MHP for mode I fracture, Gc (J/m2) is the critical strain energy release rate. When this criterion is met, i.e., σT ≥ (GCE)0.5/ψc00.5, an array of vertical channel cracks will form parallel to the bending line.In addition to the bending stress, residual stress(σR) that accumulated during the MHP crystallization also leads to the crack formation [44]. Specifically, the large difference of the thermal expansion coefficients between MHP and substrate generates significant tensile residual stress post annealing [10]. Furthermore, the lattice mismatch between the MHP film and substrate materials also results in the concentrated stress in the MHP film, which leads to crack formation [36]. Therefore, the net tensile stress should be considered in Eq. (2) and can be written as σnet = σT + σR (σR > 0 for tensile stress, σR < 0 for compressive stress).
Substituting ${\sigma _T}$ in Eq. (2) with Eq. (1), we can obtain the critical bending radius (Rc) at which the cracks start to propagate with Eq. (3). It should be noted that Rc is the most commonly reported parameter to quantify the flexibility of thin films. Smaller Rc corresponds to greater flexibility.
From Eq. (3), it is clear that the critical bending radius of thin film is governed by the term (E/GC)1/2: as the value of (E/GC)1/2 gets smaller, thin films can be bent to a smaller radius without forming cracks. It should be noted that we assumed certain thickness and initial crack size in Eq. (3), thus the value of (E/GC)1/2 reflects the intrinsic properties of material and is independent to material thickness or initial crack size.From Table 1, we can see that organic semiconductors exhibit lower values of (E/GC)1/2, while brittle inorganic materials such as silicon, CIGS, and ITO show higher values. MHP is comparable to those of brittle materials ((E/GC)1/2 value of 2.98 × 104.5 m-0.5). However, PSCs have demonstrated enhanced flexibility compared to the conventional SCs. The increased flexibility (smaller RC) of PSCs can be attributed to their thinner film thickness (lower h), due to their high absorption coefficient. I.e., PSCs can realize the comparable power conversion efficiencies with thinner absorber layers than Si SCs [45,46].
Since we assume the brittle fracture condition, the model described by Eq. (3) does not precisely predict the critical bending radius of materials that go through ductile fracture, such as elastomers, metals, and organic semiconductors. However, the value of (E/GC)1/2 reflects the intrinsic flexibility of a broad range of materials of interest. In Fig. 2, we presented the value of the inverse of (E/GC)1/2 of photovoltaic relevant materials: semiconductors, conductors, and substrate materials that are commonly employed in SCs. The increasing value of (GC/E)1/2 generally agrees with the higher flexibility and mechanical ductility of corresponding materials. MAPbI3 is at the higher end of the region of inorganic crystals, presenting both exciting opportunities and challenges for its applications in fSCs.
Fig. 2. Chart of (GC/E)1/2 value for photovoltaic materials (semiconductors, metals, and elastomers). Upper right direction indicates higher (Gc/E)1/2 and thus lower Rc and higher flexibility, which applies to elastomers and metals. The bottom left area features materials with lower (GC/E)1/2 and thus higher Rc and rigidity, which applies to inorganic crystals. See Supplementary Material 1 for numerical values.
In general, the value of (E/GC)1/2 can be considered as an empirical guide in search for new flexible and stretchable electronic materials. In the past, E has been the most commonly used indicator to predict the mechanical flexibility and stretchability in the wearable electronic community [47]. However, the tensile modulus (E) alone does not fully reflect the intrinsic flexibility of a material. For a given strain, materials with high E will have high stress building up. Should the material exhibit a high GC value meanwhile, it may as well sustain the high stress without fracture. Compared to E, the parameter of (GC/E)1/2 better reflects the resistance to deformation and to fracture of a material.
During the mechanical deformation of electronic device, interfacial slippage and delamination can also take place at the interface between different layers. While there are reports on interlayer slippage at the weak van der Waals interaction interfaces within the stack of 2D MHP layers [48,49], there has been yet study on the interfacial slippage between the MHP and adjacent layers of PSCs. The reports on the interfacial slippage have been so far scarce in PSCs, as it is not as noticeable as crack formation or delamination. However, interfacial slippage in inorganic flexible electronics has been investigated [50–53]. It has been shown that slippage always precedes delamination as the applied loading increases [51]. Clearly, the understanding of the interfacial slippage behavior is essential to analyze the failure behavior of flexible PSCs, more attentions should be paid from our community in the future.
The process of mechanical delamination of PSCs, on the other hand, has been well studied. The condition of delamination is described by Eq. (4) [10,42,54]:
where G is the driving strain energy release rate, v is the Poisson’s ratio (∼0.33 for MAPbI3), t is the thickness of the thin film, and GC is the critical strain energy release rate of the interface between photovoltaic active layer and its adjacent layers. Based on Eq. (4), there are three main directions to avoid delamination in PSCs: 1) decreasing the MHP film thickness, 2) reducing residual stress, and 3) increasing the interface adhesion between the MHP layer and the adjacent layer.2.2.4 Impact of mechanical failures on the photovoltaic performance of perovskites
It is widely accepted that the mechanical failures are detrimental to the functionality of PSCs, and the underlying mechanism of how mechanical deformation translates into decreased PV performance is being explored.
Hu et al. suggested that the cracks formed at the grain boundaries serve as the pathway for moisture to diffuse in and decompose MHPs (Fig. 2(d)) [55]. In addition to accelerated perovskite degradation, mechanical failures can negatively impact the PCE of PSCs. Figure 2(e) demonstrates that cracks lead to physical contact between the charge carrier transport layers, which leads to the undesirable recombination of charge carriers and the decrease in shunt resistance [56,57]. The shunting of PSCs leads to the leakage current and low fill factor (FF).
Furthermore, interfacial delamination leads to physical disconnection between the charge carrier transport layers and the MHP layer (Fig. 2(f)). This leads to the increased series resistance and decreased fill factor (FF) and PCE [58]. The effect of series resistance and shunting resistance on the fill factor (FFsh+s) can be described by Eq. (5) [59],
2.3 Material and device design strategies to improve mechanical flexibility of MHPs
To mitigate the mechanical failures in perovskites, several strategies have been developed to improve the mechanical stability of PSCs and performance in flexible devices (Fig. 3). From the previous discussion, it is established that grain boundaries not only facilitate mechanical failures but also accelerate the degradation of PSCs. Therefore, minimizing grain boundaries in MHP is one of the main focuses in the PSCs community.
Fig. 3. Mechanical Failure managing strategies. (a) Reducing grain boundaries leads to the improved electronic and mechanical stability of perovskites. (b) Crosslinking grain boundaries enhances the fracture resistance of MHPs. (c) Adhesive and low-modulus interfacial layer changes the strain distribution and reduces the overall strain in the MHP layer and the substrate. It also provides strong adhesion between the rigid layers, which increases the resistance to delamination. (d) Deposition of MHP film on the pre-strained substrate. After coating on the pre-stretched elastomer substrate, the thin film shows wrinkled structure that improves the mechanical flexibility and stretchability. (e) Reducing stress on MHP layer by placing it closer to stress-free neutral plane. The closed-up image shows the stress distribution in the layers, x-axis indicates the stress (tensile stress for positive direction) and y-axis indicates the distance from the neutral plane. Initially, the maximum tensile stress occurs at the surface of MHP film(left). However, by adjusting the neutral plane (red dashed line, right) in the PSC and placing it close to the MHP film, the stress in MHP film is reduced.
Deceleration of crystallization in MHP through additive engineering is one of the most effective approaches. Feng et al. reported that the addition of dimethyl sulfide led to the decreased Gibbs free energy and crystallization rate [60]. They attribute the prolonged crystallization process to the formation of a stable chelated intermediate $-{-}$ a product of strong interactions between PbI2 and dimethyl sulfide molecules. The resultant MHP film exhibited increased grain size and crystallinity, which led to an enhanced PCE value of 18.40%. In addition, the devices demonstrated an improved mechanical flexibility at 4 mm bending radius while maintaining 82% of the initial PCE after 5000 bending cycles. Other additives including DIO/H2O [61], NH4Cl [62], polycaprolactone [63] have also been explored to increase the grain size and reduce the grain boundaries of perovskite thin films.
Another approach is to crosslink the grains of perovskite to enhance its fracture resistance. Li et al. employed photo-crosslinked [6,6]-phenylC61-butyric oxetane dendron ester(C-PCBOD) to form the organic networks between grain boundaries of perovskites [64]. Not only the C-PCBOD network improves the mechanical stability of PSC, but it also reduces the compressive stress during thermal expansion in the film and leads to the reduced crack formation. The device maintained 62% of its initial PCE of 18.1% under 1% strain. Hu et al. used sulfonated graphene oxide(s-GO) to form cementitious grain boundaries in the film [55]. The sulfonic group of s-GO strongly interacts with [PbI6]4- at grain boundaries of perovskite film. The s-GO-[PbI6]4- complex enhances the bending resistance of perovskite film. The devices maintain 80% of their initial PCE after 10,000 bending cycles at bending radius of 3 mm. Polymers such as ethoxylated trimethylolpropane triacrylate (ETPTA) [65] or trimethyltrivinyl-cyclotrisiloxane (V3D3) [66] have also been introduced to form cross-linked network between grain boundaries and enhance the mechanical stability of PSCs.
The introduction of the adhesive and low-modulus interfacial layer is another important strategy to prevent the crack formation and delamination. It is known that continuous bending leads to the accumulation of highly localized stress in the MHP film. The concentrated stress often initiates mechanical failures. When a low-modulus interfacial layer is placed in between two rigid layers, shear deformation can be accommodated within the soft interfacial layer [67]. Thus, strain no longer continuously builds up linearly throughout the multilayer structure. This effect leads to the decoupling of strain in adjacent rigid layers and results in a decrease in overall strain in each rigid layer [68] as demonstrated in the right panel of Fig. 3(b).
Moreover, the strong adhesion provided by the interfacial layer between rigid films also increases the resistance to delamination. For example, Meng et al. introduced a polymer interfacial layer (Poly(3,4-ethylenedioxythiophene): poly(ethylene-co-vinyl acetate), PEDOT:EVA) in between the ITO and MHPs [69]. The soft polymer layer (E = 139 MPa) provides strong adhesion between the ITO and MHP films, and effectively reduces stress in the MHP layer and PET/ITO. The MHP films with PEDOT:EVA interfacial layer showed an enhanced bending resistance: no crack formed after 7000 cycles of bending with a bending radius of 3 mm. Xue et al. introduced a viscous PEDOT:graphene oxide(PEDOT:GO) gel (E = 347 MPa) as interfacial layer between the MHP and PET/ITO layers [70]. PEDOT:GO strongly adheres the adjacent layers and reduced the strain accordingly. In addition to improving the mechanical stability, PEDOT:EVA and PEDOT:GO interfacial layers also facilitate hole transport in the PSCs.
Depositing MHP films on pre-strained substrates can further enhance their flexibility and stretchability. Kaltenbrunner et al. fabricated PSC thin films (3 µm) and transferred them on pre-stretched elastomers to enhance the device flexibility [71]. When the strain of the substrate is released after the transfer, the device forms sinusoidal wave-like wrinkled geometry. This initial wrinkled structure allows the PSC to be repeatedly stretched and compressed without significant decrease in performance. Tavakoli et al. fabricated flexible PSC on substrate with inverted nanocone structures [72]. In the bending test, their device maintained 95% of the initial PCE after 200 cycles of bending with 6 mm bending radius, showing the improved flexibility over device on the flat substrate. The inverted cone structure makes the film foldable and leads to the effective stress release during the stretching and thermal expansion.
Adjusting the location of MHP layer to a specific plane in the PSC is another effective method to minimize stress on MHP. During the bending of a thin film, there exists a plane in which the net strain and stress is zero, and such location is defined as the neutral plane. By positioning the neutral plane closer to or within the MHP, researchers managed to reduce the strain on MHP. Specifically, the position of the neutral plane can be controlled by modifying the film thickness and/or modulus of the adjacent layers of MHPs [73], which is described by Eq. (7),
Lee at al. decreased the strain on the MHP layer by reducing the thickness of the flexible PET substrate from 100 µm to 2.5 µm [40]. More importantly, they added a protective parylene layer on top of their full device to adjust the position of neutral plane of PSC onto the MHP layer. By minimizing the strain applied on the MHP film, they achieved an improved cyclic durability from 20 cycles to 100 cycles of crumpling.
In the work by Lei et al., an innovative strategy that simultaneously reduces the grain boundaries and places MHP at the neutral plane was reported [74]. The authors fabricated PSCs using single crystal MHPs to eliminate the grain boundaries within MHP. The single crystal PSCs was placed in between PET(top) and SU-8/PDMS (bottom) layers. By precisely controlling the thickness of each layer, they managed to place the single crystal MHP film on the neutral plane. The thin film composite features great flexibility: no crack formation until the bending radius reaches 2.5 mm. They further demonstrated that single crystal MHP solar cells show improved mechanical and electronic stability over polycrystalline MHP solar cells, due to the absence of grain boundaries. By fabricating single crystal MHP SCs on the neutral plane, the authors achieved efficient flexible solar cells with the PCE of 20.04%. Their findings further validate that increasing the grain size of MPHs slows down the mechanical degradation of PSCs.
3. From flexible to stretchable solar cells
Looking forward, further advancing the mechanical deformability and stability of PSCs will be one of the key areas of interest. For example, introducing stretchability to PSCs can enable applications that are previously unimaginable, including wearable electronic textile, biomedical sensing devices, and solar harvesting on irregular areas (Fig. 4). However, the promise of stretchable PSCs entails the improved mechanical deformability, which are not yet attainable from the current flexible PV technology.
Fig. 4. Application of Stretchable Solar Cell. (a) textile solar cell. (b) power source for wearable electronics. (c) self-powered electronics for in-situ health monitoring. (d) light-weighted wearable power source for solar backpack.
For example, to achieve the desired stretchability for wearable applications, E of the electronic material needs to be reduced to the range of 200-500 MPa [3,75]. Few materials reported in today’s PSCs meet this requirement (Table 1). Crack on-set strain (CoS) is another commonly used parameter to quantify the stretchability of a material. CoS is defined as the strain at which crack occurs [76]. CoS of stretchable materials and structures is typically above 70%, while CoS of α-FAPbI3 is only 2.4% [36].
To improve the mechanical deformability of PSCs, innovative material design and device engineering strategies are urgently needed. Building heterostructures between perovskites and other low modulus materials is one potential pathway to achieve intrinsic stretchability [77]. Due to their ability to reconfigure the polymeric chain conformations, polymeric semiconductors feature superior mechanical deformability. Therefore, semiconducting polymers are considered as the promising matrix material to enable stretchable MHPs. Upon deformation, the entangled polymeric chains get straightened and aligned; once the stress is released, polymeric chains can resume their original state. The reversible process is driven by the change of entropy and the Gibbs free energy of the system. As a result, the CoS of semiconducting polymers have been reported beyond 120% [30]. Other innovative material syntheses have also reported semiconducting polymer stretchability over 100% [78,79]. In comparison, the covalent bonds in inorganic semiconducting materials can only withstand the deformation at a fraction of its length, leading to their poor stretchability (e.g., 1% for a-Si).
4. Challenges and outlook
The intrinsic mechanical and optoelectronic properties of perovskites allow ample opportunities in innovating high performance flexible and wearable photovoltaics. Considerable progress has been made to improve the mechanical stability of PSCs, as highlighted in this Review article. With the research on flexible and wearable PSCs continues to expand, multiple considerations, including lead toxicity, large-area scalability, and long-term stability, need to be advanced.
4.1 Lead toxicity
The best performing PSCs today still rely on Pb as the main element [80]. Lead toxicity gives rise to concerns in the commercialization of PSCs and will particularly restrict their applications in wearable technology [81]. Significant efforts have been made in overcoming the lead toxicity issue in PSCs: lead-free perovskite innovation, encapsulation, and recycling of PSCs have been the three main focuses [82,–85]. For wearable applications of PSCs, the devices often operate under unfavorable moist or straining conditions, which leads to the increased chance of lead leakage [81]. Advances of novel encapsulation techniques for wearable PSCs are urgently needed.
4.2 Device upscaling
PSCs with larger device area often suffer from lower PCEs than their smaller area counterparts [85]. The decreased performance is ascribed to the nonuniformity caused by pinholes and particulates and the difficulty in maintaining a thin yet robust film on a large scale. Significant efforts have been devoted to improving the deposition methods from spin coating (small-scale) to roll-to-roll techniques [86]. Yet, to successfully scale up PSCs, better control of the experimental conditions and further advances in the large area deposition of MHPs, electrodes, and carrier transporting layers are needed [87,88]. For instance, the deposition of brittle ITO onto flexible substrates often yields high surface roughness and low wettability, posing challenges in scaling up flexible PSCs.
4.3 Long-term stability
Improving the long-term stability of PSCs has been of paramount significance to realize their commercialization. MHPs are known to be sensitive to the external environment: humidity, temperature, and mechanical stimuli, etc. This should be of elevated concern for device with large area [88]. For flexible and wearable PSCs, the mechanical stimuli further accelerate the degradation of PSC through crack formation and delamination. In addition, the adhesion between MHPs and adjacent layers is weaker in flexible or stretchable PSCs due to the deformation, swelling, and poor surface roughness of the substrates, which will further limit the long-term stability of PSCs [89].
5. Summary
In this article, we reviewed the mechanical properties and fracture behaviors of MHPs and their impact on the photovoltaic performance of PSCs. We introduced a new term of (E/GC)1/2 that can be used to evaluate the intrinsic flexibility of electronic materials. We analyzed several strategies to address the crack formation and delamination in PSCs: 1) reducing grain boundaries, 2) crosslinking grain boundaries, 3) interfacial engineering, 4) pre-strained substrate, and 5) neutral plane strategy. Looking forward, we anticipate perovskite PVs to continuously advance, evolving from its current flexible form towards its stretchable and wearable form. With the strong growth in wearable technology, stretchable PSCs represent one of the most exciting opportunities for wearable energy sources. To accommodate unprecedented demand for the high mechanical deformability, we need to reimagine the material design and device engineering strategy. We recommend stretchable heterostructures between perovskites and polymer semiconductors as one possible pathway for future wearable PSCs design.
Acknowledgments
The authors acknowledge support from the Department of Chemical Engineering, and the College of Engineering at the University of Michigan. The authors thank Dr. Teng Cui and Dr. F Pelayo García de Arquer for helpful discussions. The authors also thank Hazel Mohamedali for the helpful comments on the manuscript.
Contributions. Sijun Seong and Yanmeng Liu have contributed equally to this work. Sijun Seong, Yanmeng Liu, and Xiwen Gong wrote the manuscript.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available in the Supplementary Material and Refs. [9––31,90,91].
Supplemental document
See Supplement 1 for supporting content.
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