shuhui

The device with binary additive of octance‐1,8‐dithiol:1,8‐diiodooctane (ODT:DIO) (0.375%:0.125%) based on FTAZ:ITIC‐Th blends exhibits a higher power conversion efficiency of 10.93% than the devices processed with only 0.5% ODT, 0.5% DIO, or excess binary additive of ODT:DIO (0.5%:0.5%). The reason is that additives with different boiling point work in different stages during the whole filming process as in situ grazing incidence wide‐angle X‐ray scattering characterization indicates.

Abstract

The power conversion efficiency of polymer solar cells (PSCs) is strongly affected by active layer morphology. Here, two solvent additives (ODT: octance‐1,8‐dithiol; DIO: 1,8‐diiodooctane) are used to optimize the bulk heterojunction morphology of FTAZ:ITIC‐Th based PSCs and ≈11% efficiency is obtained, which is 10% higher than the untreated device. Based on the morphological characterizations, the influence of binary solvent additives on manipulating molecular packing and phase separation of blend films is successfully revealed. More importantly, in situ grazing incidence wide‐angle X‐ray scattering characterization is adopted to explore the crucial role played by these two solvent additives at different stages of the film‐forming process, that is, ODT influences the initial stage of the film‐forming process, while DIO later establishes the ultimate photoactive film formation. Due to the impacts of two additives at different film processing stages, an optimal ratio of ODT:DIO (0.375%:0.125%) is obtained, which helps in realizing the optimized morphology.

Two solid additives are proven to improve the molecular packing of acceptors, while devices processed with different additives exhibit different photovoltaic performances due to the different volatilities. The working mechanism and basic design rules of solid additives are revealed, and a feasible method for achieving high‐efficiency polymer solar cells is established.

Abstract

Fine‐tuning of the nanoscale morphologies of the active layers in polymer solar cells (PSCs) through various techniques plays a vital role in improving the photovoltaic performance. However, for emerging nonfullerene (NF) PSCs, the morphology optimization of the active‐layer films empirically follows the methods originally developed in fullerene‐based blends and lacks systematic studies. In this work, two solid additives with different volatilities, SA‐4 and SA‐7, are applied to investigate their influence on the morphologies and photovoltaic performances of NF‐PSCs. Although both solid additives effectively promote the molecular packing of the NF acceptors, due to the higher volatility of SA‐4, the devices processed with SA‐4 exhibit a power conversion efficiency of 13.5%, higher than that of the control devices, and the devices processed with SA‐7 exhibit poor performances. Through a series of detailed morphological analyses, it is found that the volatilization of SA‐4 after thermal annealing is beneficial for the self‐assembly packing of acceptors, while the residuals due to the incomplete volatilization of SA‐7 have a negative effect on the film morphology. The results delineate the feasibility of applying volatilizable solid additives and provide deeper insights into the working mechanism, establishing guidelines for further material design of solid additives.

Suppression of the interfacial recombination are achieved by facile alkali chloride modification of the nickel oxide in inverted perovskite solar cells. It is demonstrated that the interface modification induces the ordering of the perovskite crystal at the interfaces, which in turn reduces defect/trap density, causing reduced interfacial recombination. This results in dramatically improvement of the open circuit voltage and power conversion efficiency.

Abstract

In this work, significant suppression of the interfacial recombination by facile alkali chloride interface modification of the NiOx hole transport layer in inverted planar perovskite solar cells is achieved. Experimental and theoretical results reveal that the alkali chloride interface modification results in improved ordering of the perovskite films, which in turn reduces defect/trap density, causing reduced interfacial recombination. This leads to a significant improvement in the open‐circuit voltage from 1.07 eV for pristine NiOx to 1.15 eV for KCl‐treated NiOx, resulting in a power conversion efficiency approaching 21%. Furthermore, the suppression of the ion diffusion in the devices is observed, as evidenced by stable photoluminescence (PL) under illumination and high PL quantum efficiency with alkali chloride treatment, as opposed to the luminescence enhancement and low PL quantum efficiency observed for perovskite on pristine NiOx. The suppressed ion diffusion is also consistent with improved stability of the devices with KCl‐treated NiOx. Thus, it is demonstrated that a simple interfacial modification is an effective method to not only suppress interfacial recombination but also to suppress ion migration in the layers deposited on the modified interface due to improved interface ordering and reduced defect density.

The efficiency limits in non‐fullerene organic solar cells are examined using a numerical simulator. Power conversion efficiency (PCE) of over 18% using recently reported carrier mobility values and voltage losses, are predicted. Increasing the mobility to >10⁻³ cm² V⁻¹ s⁻¹ and decreasing the recombination constant to <10⁻¹² cm³ s⁻¹ is shown to yield a single‐junction and 2T‐tandem cell with PCEs of >20% and >25%, respectively.

Abstract

The reported power conversion efficiencies (PCEs) of nonfullerene acceptor (NFA) based organic photovoltaics (OPVs) now exceed 14% and 17% for single‐junction and two‐terminal tandem cells, respectively. However, increasing the PCE further requires an improved understanding of the factors limiting the device efficiency. Here, the efficiency limits of single‐junction and two‐terminal tandem NFA‐based OPV cells are examined with the aid of a numerical device simulator that takes into account the optical properties of the active material(s), charge recombination effects, and the hole and electron mobilities in the active layer of the device. The simulations reveal that single‐junction NFA OPVs can potentially reach PCE values in excess of 18% with mobility values readily achievable in existing material systems. Furthermore, it is found that balanced electron and hole mobilities of >10⁻³ cm² V⁻¹ s⁻¹ in combination with low nongeminate recombination rate constants of 10⁻¹² cm³ s⁻¹ could lead to PCE values in excess of 20% and 25% for single‐junction and two‐terminal tandem OPV cells, respectively. This analysis provides the first tangible description of the practical performance targets and useful design rules for single‐junction and tandem OPVs based on NFA materials, emphasizing the need for developing new material systems that combine these desired characteristics.

By using the new electron‐rich heptacyclic anthracene(cyclopentadithiophene) (AT) core, together with energy level modulations by end‐group optimizations enabling the match with polymer donors, two new nonfullerene small molecule acceptors AT‐NC and AT‐4Cl are synthesized. With both halogenated donor and acceptor, the organic photovoltaics device based on AT‐4Cl achieves a high power conversion efficiency of 13.27% with simultaneously improved J _sc and fill factor.

Abstract

Two new nonfullerene small molecule acceptors (NF‐SMAs) AT‐NC and AT‐4Cl based on heptacyclic anthracene(cyclopentadithiophene) (AT) core and different electron‐withdrawing end groups are designed and synthesized. Although the two new acceptor molecules use two different end groups, naphthyl‐fused indanone (NINCN) and chlorinated INCN (INCN‐2Cl) demonstrate similar light absorption. AT‐4Cl with chlorinated INCN as end groups are shifted significantly due to the strong electron‐withdrawing ability of chlorine atoms. Thus, desirable V _oc and photovoltaic performance are expected to be achieved when polymer PBDB‐T is used as the electron donor with AT‐NC as the acceptor, and fluorinated analog PBDB‐TF with down‐shifted energy levels is selected to blend with AT‐4Cl. Consequently, the device based on PBDB‐TF:AT‐4Cl yields a high power conversion efficiency of 13.27% with a slightly lower V _oc of 0.901 V, significantly enhanced J _sc of 19.52 mA cm⁻² and fill factor of 75.5% relative to the values based on PBDB‐T:AT‐NC. These results demonstrate that the use of a new electron‐rich AT core, together with energy levels modulations by end‐group optimizations enabling the match with polymer donors, is a successful strategy to construct high‐performance NF‐SMAs.

N₂ proximity to MAPbI₃ cations acts in mitigating the formation of defects and concurrently assists a lattice reconstruction during heating/operation. This is beneficial for the optical response of the material in absorption (spectroscopic ellipsometry) and photoemission (photoluminescence). For the same reasons, the resistance to charge extraction is reduced.

Abstract

On the basis of experiment and theory, a general paradigm is drawn that reconsiders N₂ not simply being an inert species but rather an effective healing gas molecule if entering a methylammonium lead iodide (MAPbI₃) layer. Nitrogen is soaked into polycrystalline MAPbI₃ via a postdeposition mild thermal treatment under slightly overpressure conditions to promote its diffusion across the whole layer. A significant reduction of radiative recombination and a concurrent increase of light absorption, with a maximum benefit at 80 °C, are observed. Concomitantly, the current of holes drawn from the surfaces with nanometer resolution through a biased tip is raised by a factor of 3 under N₂. This is framed by a reduction of the barrier for carrier extraction. The achieved improvements are linked to a nitrogen‐assisted recovery of intrinsic lattice disorder at the grain shells along with a simultaneous stabilization of undercoordinated Pb²⁺ and MA⁺ cations through weak electrostatic interactions. Defect mitigation under N₂ is reinforced in comparison to the benchmark behavior under argon. It is additionally unveiled that surface stabilization through N₂ is morphology‐independent and thus can be applied after any preparation procedure. Such simple and low‐cost strategy can complement other stabilizing solutions for perovskite solar cells or light‐emitting diode engineering.

A bifunctional saddle‐shaped small molecule α, β‐COTh‐Ph‐OMeTAD is synthesized and systemically charactered as a dopant‐free hole transporting material and interfacial layer in perovskite solar cells (PSCs). A higher power conversion efficiency (PCE) (17.22%) and stable‐enhanced PSCs devices are achieved when compared with that based on spiro‐OMeTAD (16.66%). Noteworthily, after storing nearly for 800 h, 86% of the maximum PCE is retained without any encapsulation.

Herein, a new bifunctional saddle‐shaped organic small molecule named 2,2′,7,7′‐tetrakis(N, N‐di‐p‐methoxyphenyl‐aniline)‐α, β‐cycloocta[1,2‐b:4,3‐b′:5,6‐b′:8,7‐b″′]tetrathiophenyl (α, β‐COTh‐Ph‐OMeTAD) is synthesized. When compared with spiro‐OMeTAD, a star hole transporting material (HTM) for highly efficient perovskite solar cells, the new material has a deeper highest occupied molecular orbital (HOMO) energy level of −5.30 eV, and a higher hole mobility of 2.88 × 10⁻⁴ cm² V⁻¹ s⁻¹. With dopant‐free α, β‐COTh‐Ph‐OMeTAD as a HTM and an interfacial layer combinined with chlorobenzene (CB) as the anti‐solvent, mesoporous perovskite solar cells (PSCs) are fabricated, which exhibit a power conversion efficiency (PCE) of 17.22% under AM 1.5 conditions, which is a little higher than that of devices based‐on doped spiro‐OMeTAD under the same conditions, which is 16.83%. Notably, the PSCs devices with dopant‐free α, β‐COTh‐Ph‐OMeTAD as both the HTM and interfacial layer show better stability, and after being stored in dark and dry air without encapsulation for nearly 800 h, the PCE can still be maintained at 86% of the maximum. This opens a new avenue for efficient and stable PSCs by exploring new dopant‐free materials as alternatives to spiro‐OMeTAD.

A low‐temperature solution‐processed TFB is demonstrated as an ideal hole‐transporting layer to push the PCE of the inverted perovskite solar cells (PVSCs) up to 20.2%. Moreover, this TFB‐based inverted PVSC exhibits good stability, retaining 90% of its original efficiency after storage for 30 days in ambient air.

Abstract

Low‐temperature‐processed inverted perovskite solar cells (PVSCs) attract increasing attention because they can be fabricated on both rigid and flexible substrates. For these devices, hole‐transporting layers (HTLs) play an important role in achieving efficient and stable inverted PVSCs by adjusting the anodic work function, hole extraction, and interfacial charge recombination. Here, the use of a low‐temperature (≤150 °C) solution‐processed ultrathin film of poly[(9,9‐dioctyl‐fluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐secbutylphenyl) diphenylamine)] (TFB) is reported as an HTL in one‐step‐processed CH₃NH₃PbI₃ (MAPbI₃)‐based inverted PVSCs. The fabricated device exhibits power conversion efficiency (PCE) as high as 20.2% when measured under AM 1.5 G illumination. This PCE makes them one of the MAPbI₃‐based inverted PVSCs that have the highest efficiency reported to date. Moreover, this inverted PVSC also shows good stability, which can retain 90% of its original efficiency after 30 days of storage in ambient air.

Nonfullerene‐based small‐molecule organic solar cells with a new record efficiency of 12.08% are achieved by first incorporation of near‐infrared absorbing molecules and by tuning the sequentially evolved crystalline morphology. The improved crystallinity of both donor and acceptor materials facilitates the formation of multilength scale morphologies, which further enhance charge mobility and extraction, and reduce the nongeminate recombination.

Abstract

In this paper, two near‐infrared absorbing molecules are successfully incorporated into nonfullerene‐based small‐molecule organic solar cells (NFSM‐OSCs) to achieve a very high power conversion efficiency (PCE) of 12.08%. This is achieved by tuning the sequentially evolved crystalline morphology through combined solvent additive and solvent vapor annealing, which mainly work on ZnP‐TBO and 6TIC, respectively. It not only helps improve the crystallinity of the ZnP‐TBO and 6TIC blend, but also forms multilength scale morphology to enhance charge mobility and charge extraction. Moreover, it simultaneously reduces the nongeminate recombination by effective charge delocalization. The resultant device performance shows remarkably enhanced fill factor and J _sc. These result in a very respectable PCE, which is the highest among all NFSM‐OSCs and all small‐molecule binary solar cells reported so far.

An effective but simple approach to rationally tune the crystallinity and miscibility of small‐molecular acceptors is reported. With a phenyl introduced at the tail of alkyl side chains, the morphology and molecular orientations of heterojunction are greatly improved. Outstanding efficiencies of 13.23% and 14.04% are detected from the as‐cast and annealed devices, promoted by the fairly high fill factors.

Abstract

Research on fused‐ring small‐molecular‐acceptors (SMAs) has deeply advanced the development of organic solar cells (OSCs). Compared to fruitful studies of ladder‐type cores and end‐caps of SMAs, the exploration of side chains is monotonous. The widely utilized alkyl and aryl side chains usually produce a conflicting association between SMAs' crystallinity and miscibility. Herein, a fresh idea about the modification of side chains is reported to explore the subtle balance between the crystallinity and miscibility. Specifically, a phenyl is introduced to the tail of the alkyl side chain whereby a new acceptor IDIC‐C4Ph is reported. Moderately weakened crystallinity is observed, while maintaining preferred absorption profiles and face‐on orientations. Concurrently, remarkably improved heterojunction morphologies and stacking orientations are detected. PBDB‐T:IDIC‐C4Ph devices exhibit greater efficiency of 11.50% than devices from alky and aryl modified acceptors. Notably, the as‐cast OSCs of PBDB‐TF:IDIC‐C4Ph reveal outstanding FF over 76% with the best efficiency up to 13.23%. The annealed devices reveal further increased efficiency exceeding 14% with the state of the art FF of 78.32%. Overall, an effective but easily navigable approach is demonstrated to modulate the crystallinity of SMAs toward synergistically improved morphologies and molecular orientations of bulk heterojunction enabling highly efficient OSCs.

A novel wide‐bandgap nonfullerene acceptor TfIF‐4FIC is synthesized. PBDB‐T‐2F:TfIF‐4FIC‐based organic solar cell acquires a power conversion efficiency (PCE) of 13.1%, a high open‐circuit voltage of 0.98 V, which is the best performed device with bandgap larger than 1.60 eV. When using PBDB‐T‐2F:TfIF‐4FIC as front cell and PTB7‐Th:PCDTBT:IEICO‐4F as back cell to construct tandem device, PCE of 15% is achieved.

Abstract

A tandem organic solar cell (OSC) is a valid structure to widen the photon response range and suppress the transmission loss and thermalization loss. In the past few years, the development of low‐bandgap materials with broad absorption in long‐wavelength region for back subcells has attracted considerable attention. However, wide‐bandgap materials for front cells that have both high short‐circuit current density (J _SC) and open‐circuit voltage (V _OC) are scarce. In this work, a new fluorine‐substituted wide‐bandgap small molecule nonfullerene acceptor TfIF‐4FIC is reported, which has an optical bandgap of 1.61 eV. When PBDB‐T‐2F is selected as the donor, the device offers an extremely high V _OC of 0.98 V, a high J _SC of 17.6 mA cm⁻², and a power conversion efficiency of 13.1%. This is the best performing acceptor with such a wide bandgap. More importantly, the energy loss in this combination is 0.63 eV. These properties ensure that PBDB‐T‐2F:TfIF‐4FIC is an ideal candidate for the fabrication of tandem OSCs. When PBDB‐T‐2F:TfIF‐4FIC and PTB7‐Th:PCDTBT:IEICO‐4F are used as the front cell and the back cell to construct tandem solar cells, a PCE of 15% is obtained, which is one of best results reported to date in the field of organic solar cells.

Fluorinated perylenediimide (F‐PDI) is first introduced to optimize photovoltaic performance and stability of perovskite solar cells. Conductive F‐PDI effectively passivates defects and promotes charge transfer. The hydrophobicity of F‐PDI preventing moisture penetration as well as the strong hydrogen bonding immobilizing methylamine ions, thereby, endow excellent moisture and thermal stability with nearly 70% efficiency retention after thermal treatment at 100 °C.

Abstract

The notoriously poor stability of perovskite solar cells is a crucial issue restricting commercial applications. Here, a fluorinated perylenediimide (F‐PDI) is first introduced into perovskite film to enhance the device's photovoltaic performance, as well as thermal and moisture stability simultaneously. The conductive F‐PDI molecules filling at grain boundaries (GBs) and surface of perovskite film can passivate defects and promote charge transport through GBs due to the chelation between carbonyl of F‐PDI and noncoordinating lead. Furthermore, an effective multiple hydrophobic structure is formed to protect perovskite film from moisture erosion. As a result, the F‐PDI‐incorporated devices based on MAPbI₃ and Cs_0.05 (FA_0.83MA_0.17)_0.95 Pb (Br_0.17I_0.83)₃ absorber achieve champion efficiencies of 18.28% and 19.26%, respectively. Over 80% of the initial efficiency is maintained after exposure in air for 30 days with a relative humidity (RH) of 50%. In addition, the strong hydrogen bonding of F···H‐N can immobilize methylamine ion (MA⁺) and thus enhances the thermal stability of device, remaining nearly 70% of the initial value after thermal treatment (100 °C) for 24 h at 50% RH condition.

The ITC‐2Cl‐based device yields an excellent power conversion efficiency of 13.6% with a low E _loss of 0.67 eV, which is superior to those of the devices based on ITCPTC, IT‐4F, and IT‐4Cl.

Abstract

Generally, highly efficient organic solar cells require both a high open‐circuit voltage (V _OC) and a high short‐circuit current density (J _SC). Reducing the energy loss (E _loss) is an effective way to achieve a high V _OC without compromising the photocurrent, which is ideal for enhancing the power conversion efficiencies (PCEs). Herein, a new chlorinated nonfullerene acceptor (ITC‐2Cl) with chlorinated thiophene‐fused end groups is developed. In comparison with the unchlorinated counterpart (ITCPTC), the introduction of Cl improves not only the electronic properties by redshifting the absorption spectra and deepening the lowest unoccupied molecular orbital energy levels, but also the molecular packing and thus thin‐film morphology. The PM6:ITC‐2Cl‐based device yields a significantly higher PCE (13.6%) with a lower E _loss (0.67 eV) than the ITCPTC‐based device (PCE of 12.3% with E _loss of 0.70 eV). More importantly, compared to the archetypal nonfullerene acceptors such as IT‐4F (PCE of 12.9% with E _loss of 0.73 eV) and IT‐4Cl (PCE of 12.7% with E _loss of 0.76 eV), the ITC‐2Cl‐based device shows a higher PCE and a lower E _loss. These results demonstrate that the chlorinated thiophene‐fused end group is a promising candidate for a high‐performance nonfullerene acceptors with low energy loss.

The C₆₀‐containing polystyrene (PS‐C₆₀) is used to obtain thick‐layer efficient polymer solar cells. The PS‐C₆₀‐processed device exhibits a power conversion efficiency (PCE) of 10.34% and, remarkably, a power conversion efficiency (PCE) of 7.30% for a 450‐nm thick layer. Furthermore, the addition of PS‐C₆₀ improves the film stability, resulting in ≈90% retentivity of its initial PCE after 8 h of annealing at 150 °C.

In this study, a C₆₀‐containing polystyrene (PS‐C₆₀) is used as an effective polymer additive for thick‐film high‐performance polymer solar cells, which are necessary for performing roll‐to‐roll mass production in the future. The PS‐C₆₀‐processed device exhibits a power conversion efficiency (PCE) of 10.34 ± 0.10%; the PCE is still high (7.30 ± 0.10%) for an active layer thickness of 450 nm, which is more than twice that observed in non‐additive devices (3.11 ± 0.15%). The usage of PS‐C₆₀ results in an efficient exciton dissociation and charge extraction, less bimolecular recombination, and superior charge transport. These effects lead to an improved device performance, even with a thick active layer. Surprisingly, PS‐C₆₀ also helps the film to retain its morphology at high temperatures, thereby improving its thermal stability. The PS‐C₆₀ device retains ≈90% of the initial PCE after conducting a high‐temperature treatment, whereas a remarkable decrease (≈55%) is observed in case of the non‐additive one. The versatility and applicability of the strategy that is presented in this study can considerably help the development of stable thick‐layer devices in terms of satisfying the requirements of the roll‐to‐roll production of solar cells.

A new methodology is presented for preparing a dopant‐free Spiro‐OMeTAD film by dynamic spin‐coating the pristine Spiro‐OMeTAD solution from a halogen‐free green solvent THF, which yields a record efficiency of 17% as along with negligible hysteresis in planar PSCs. Importantly, the strategy brings the field a step closer toward cost‐effective and environmental friendly production of PSCs with enhanced longevity.

In this paper, highly efficient (17%) perovskite solar cells (PSCs) based on a hole‐transporting layer (HTL) made of dopant‐free Spiro‐OMeTAD processed from a non‐halogenated solvent (THF) are reported for the first time. In addition to the high efficiency, a negligible hysteresis effect is observed for the devices with dopant‐free Spiro‐OMeTAD hole‐transporting material (HTM), which is often a problem for planar n‐i‐p type PSCs. By eliminating the hydroscopic dopants, the ambient stability of the completed PSC devices are much improved. Another advantage of using THF as a solvent is that much less of the Spiro‐OMeTAD material is required (5 mg ml⁻¹) to coat the HTL compared to that used in a conventional chlorobenzene solvent (70 mg ml⁻¹). Our result provides a simple yet effective method to fabricate dopant‐free PSCs toward cost‐effective and environmental friendly production of PSCs with enhanced stability.

Publication date: May 2019

Source: Nano Energy, Volume 59

Author(s): Biao Liu, Mengqiu Long, Mengqiu Cai, Liming Ding, Junliang Yang

Graphical abstract

The charge recombination center is at the 2D BA₂PbI₄ interface in 2D/I interface heterostructure.

Publication date: 17 April 2019

Source: Joule, Volume 3, Issue 4

Author(s): Anita Ho-Baillie, Meng Zhang, Cho Fai Jonathan Lau, Fa-Jun Ma, Shujuan Huang

Context & Scale

Summary

Graphical Abstract

3D chiral hybrid organic–inorganic perovskites are both kinetically and thermodynamically stable based on theoretical calculation, and chirality is transferred from chiral cations to the perovskite framework, which is of great interest in the fields of piezoelectricity, pyroelectricity, ferroelectricity, topological quantum engineering, circularly polarized optoelectronics, and spintronics.

Abstract

Hybrid organic–inorganic perovskites (HOIPs), in particular 3D HOIPs, have demonstrated remarkable properties, including ultralong charge‐carrier diffusion lengths, high dielectric constants, low trap densities, tunable absorption and emission wavelengths, strong spin–orbit coupling, and large Rashba splitting. These superior properties have generated intensive research interest in HOIPs for high‐performance optoelectronics and spintronics. Here, 3D hybrid organic–inorganic perovskites that implant chirality through introducing the chiral methylammonium cation are demonstrated. Based on structural optimization, phonon spectra, formation energy, and ab initio molecular dynamics simulations, it is found that the chirality of the chiral cations can be successfully transferred to the framework of 3D HOIPs, and the resulting 3D chiral HOIPs are both kinetically and thermodynamically stable. Combining chirality with the impressive optical, electrical, and spintronic properties of 3D perovskites, 3D chiral perovskites is of great interest in the fields of piezoelectricity, pyroelectricity, ferroelectricity, topological quantum engineering, circularly polarized optoelectronics, and spintronics.

A facile and eco‐friendly approach is introduced to greatly promote the molecular order of nominally amorphous polymers and thus realize high‐efficiency in sequentially deposited (SD) nonfullerene solar cells. Applying a green solvent, (R)‐(+)‐limonene, enhances the polymer order and yields the best efficiency. Additionally, strong relationships between solvent, interaction parameter, and long period are observed for these new SD devices.

Abstract

Casting of a donor:acceptor bulk‐heterojunction structure from a single ink has been the predominant fabrication method of organic photovoltaics (OPVs). Despite the success of such bulk heterojunctions, the task ofcontrolling the microstructure in a single casting process has been arduous and alternative approaches are desired. To achieve OPVs with a desirable microstructure, a facile and eco‐compatible sequential deposition approach is demonstrated for polymer/small‐molecule pairs. Using a nominally amorphous polymer as the model material, the profound influence of casting solvent is shown on the molecular ordering of the film, and thus the device performance and mesoscale morphology of sequentially deposited OPVs can be tuned. Static and in situ X‐ray scattering indicate that applying (R)‐(+)‐limonene is able to greatly promote the molecular order of weakly crystalline polymers and form the largest domain spacing exclusively, which correlates well with the best efficiency of 12.5% in sequentially deposited devices. The sequentially cast device generally outperforms its control device based on traditional single‐ink bulk‐heterojunction structure. More crucially, a simple polymer:solvent interaction parameter χ is positively correlated with domain spacing in these sequentially deposited devices. These findings shed light on innovative approaches to rationally create environmentally friendly and highly efficient electronics.