High performance tandem organic solar cells via a strongly infrared-absorbing narrow bandgap acceptor

Loosely Bounded Exciton with Enhanced Delocalization Capability Boosting Efficiency of Organic Solar Cells

In organic solar cells (OSCs), electron acceptors have undergone multiple updates, from the initial fullerene derivatives, to the later acceptor-donor-acceptor type non-fullerene acceptors (NFAs), and now to Y-series NFAs, based on which efficiencies have reached over 19%. However, the key property responsible for further improved efficiency from molecular structure design is remained unclear. Herein, the material properties are comprehensively scanned by selecting PC71BM, IT-4F, and L8-BO as the representatives for different development stages of acceptors. For comparison, asymmetric acceptor of BTP-H5 with desired loosely bounded excitons is designed and synthesized. It's identified that the reduction of intrinsically exciton binding energy (Eb) and the enhancement of exciton delocalization capability act as the key roles in boosting the performance. Notably, 100 meV reduction in Eb has been observed from PC71BM to BTP-H5, correspondingly, electron-hole pair distance of BTP-H5 is almost two times over PC71BM. As a result, efficiency is improved from 40% of S-Q limit for PC71BM-based OSC to 60% for BTP-H5-based one, which achieves an efficiency of 19.07%, among the highest values for binary OSCs. This work reveals the confirmed function of exciton delocalization capability quantitatively in pushing the efficiency of OSCs, thus providing an enlightenment for future molecular design.

On the critical competition between singlet exciton decay and free charge generation in non-fullerene based organic solar cells with low energetic offsets

Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets - particularly the relationship between ionization energy offset (ΔIE) and free charge generation. Here, we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights, covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a ΔIE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small ΔIE blends, which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells, with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE-CT hybridization. In agreement with this conclusion, transient absorption spectroscopy consistently reveals that a smaller ΔIE slows the NFA exciton dissociation into free charges, albeit restorable by an electric field. Our experimental data align with Marcus theory calculations, supported by density functional theory simulations, for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE, but that this restriction is lifted in systems with a small reorganization energy for charge transfer.

What is the Limit Size of 2D Conjugated Extension on Central Units of Small Molecular Acceptors in Organic Solar Cells?

2D conjugated extension on central units of small molecular acceptors (SMAs) has gained great successes in reaching the state-of-the-art organic photovoltaics. Whereas the limit size of 2D central planes and their dominant role in constructing 3D intermolecular packing networks are still elusive. Thus, by exploring a series of SMAs with gradually enlarged central planes, it is demonstrated that, at both single molecular and aggerated levels, there is an unexpected blue-shift for their film absorption but preferable reorganization energies, exciton lifetimes and binding energies with central planes enlarging, especially when comparing to their Y6 counterpart. More importantly, the significance of well-balanced molecular packing modes involving both central and end units is first disclosed through a systematic single crystal analysis, indicating that when the ratio of central planes area/end terminals area is no more than 3 likely provides a preferred 3D intermolecular packing network of SMAs. By exploring the limit size of 2D central planes, This work indicates that the structural profiles of ideal SMAs may require suitable central unit size together with proper heteroatom replacement instead of directly overextending 2D central planes to the maximum. These results will likely provide some guidelines for future better molecular design.

Dual Function of the Third Component in Ternary Organic Solar Cells: Broaden the Spectrum and Optimize the Morphology

Ternary organic solar cells (T-OSCs) have attracted significant attention as high-performance devices. In recent years, T-OSCs have achieved remarkable progress with power conversion efficiency (PCE) exceeding 19%. However, the introduction of the third component complicates the intermolecular interaction compared to the binary blend, resulting in poor controllability of active layer and limiting performance improvement. To address these issues, dual-functional third components have been developed that not only broaden the spectral range but also optimize morphology. In this review, the effect of the third component on expanding the absorption range of T-OSCs is first discussed. Second, the extra functions of the third component are introduced, including adjusting the crystallinity and molecular stack in active layer, regulating phase separation and purity, altering molecular orientation of the donor or acceptor. Finally, a summary of the current research progress is provided, followed by a discussion of future research directions.

Highly-Efficient 2D Nonfullerene Acceptors Enabled by Subtle Molecular Tailoring Engineering

The conjugate expansion of nonfullerene acceptors is considered to be a promising approach for improving organic photovoltaic performance because of its function in tuning morphological structure and molecular stacking behavior. In this work, two nonfullerene acceptors are designed and synthesized using a 2D π-conjugate expansion strategy, thus enabling the construction of highly-efficient organic solar cells (OSCs). Compared with YB2B (incorporating dibromophenanthrene on the quinoxaline-fused core), YB2T (incorporating dibromobenzodithiophene on the quinoxaline-fused core) has red-shifted spectral absorption and better charge transport properties. Moreover, the more orderly and tightly intermolecular stacking of YB2T provides the possibility of forming a more suitable phase separation morphology in blend films. Through characterization and analysis, the YB2T-based blend film is found to have higher exciton dissociation efficiency and less charge recombination. Consequently, the power conversion efficiency (PCE) of 17.05% is achieved in YB2T-based binary OSCs, while YB2B-based devices only reached 10.94%. This study demonstrates the significance of the aromatic-ring substitution strategy for regulating the electronic structure and aggregation behavior of 2D nonfullerene acceptors, facilitating the development of devices with superior photovoltaic performance.

Organic Optoelectronic Materials: A Rising Star of Bioimaging and Phototherapy

Recently, many organic optoelectronic materials (OOMs), especially those used in organic light-emitting diodes (OLEDs), organic solar cells (OSCs), and organic field-effect transistors (OFETs), are explored for biomedical applications including imaging and photoexcited therapies. In this review, recently developed OOMs for fluorescence imaging, photoacoustic imaging, photothermal therapy, and photodynamic therapy, are summarized. Relationships between their molecular structures, nanoaggregation structures, photophysical mechanisms, and properties for various biomedical applications are discussed. Mainly four kinds of OOMs are covered: thermally activated delayed fluorescence materials in OLEDs, conjugated small molecules and polymers in OSCs, and charge-transfer complexes in OFETs. Based on the OOMs unique optical properties, including excitation light wavelength and exciton dynamics, they are respectively exploited for suitable biomedical applications. This review is intended to serve as a bridge between researchers in the area of organic optoelectronic devices and those in the area of biomedical applications. Moreover, it provides guidance for selecting or modifying OOMs for high-performance biomedical uses. Current challenges and future perspectives of OOMs are also discussed with the hope of inspiring further development of OOMs for efficient biomedical applications.

Interface Engineering for Highly Efficient Organic Solar Cells

Organic solar cells (OSCs) have made dramatic advancements during the past decades owing to the innovative material design and device structure optimization, with power conversion efficiencies surpassing 19% and 20% for single-junction and tandem devices, respectively. Interface engineering, by modifying interface properties between different layers for OSCs, has become a vital part to promote the device efficiency. It is essential to elucidate the intrinsic working mechanism of interface layers, as well as the related physical and chemical processes that manipulate device performance and long-term stability. In this article, the advances in interface engineering aimed to pursue high-performance OSCs are reviewed. The specific functions and corresponding design principles of interface layers are summarized first. Then, the anode interface layer, cathode interface layer in single-junction OSCs, and interconnecting layer of tandem devices are discussed in separate categories, and the interface engineering-related improvements on device efficiency and stability are analyzed. Finally, the challenges and prospects associated with application of interface engineering are discussed with the emphasis on large-area, high-performance, and low-cost device manufacturing.

Chemically Engineered Titanium Oxide Interconnecting Layer for Multijunction Polymer Solar Cells

We report chemically tunable n-type titanium oxides using ethanolamine as a nitrogen dopant source. As the amount of ethanolamine added to the titanium oxide precursor during synthesis increases, the Fermi level of the resulting titanium oxides (ethanolamine-incorporated titanium oxides) significantly changes from -4.9 eV to -4.3 eV, and their free charge carrier densities are enhanced by two orders of magnitudes, reaching up to 5 × 1018 cm-3. Unexpectedly, a basic ethanolamine reinforces not only the n-type properties of titanium oxides, but also their basicity, which facilitates acid-base ionic junctions in contact with acidic materials. The enhanced charge carrier density and basicity of the chemically tuned titanium oxides enable multi-junction solar cells to have interconnecting junctions consisting of basic n-type titanium oxides and acidic p-type PEDOT:PSS to gain high open-circuit voltages of 1.44 V and 2.25 V from tandem and triple architectures, respectively.

Toward ultrafast soft x-ray spectroscopy of organic photovoltaic devices

Novel ultrafast x-ray sources based on high harmonic generation and at x-ray free electron lasers are opening up new opportunities to resolve complex ultrafast processes in condensed phase systems with exceptional temporal resolution and atomic site specificity. In this perspective, we present techniques for resolving charge localization, transfer, and separation processes in organic semiconductors and organic photovoltaic devices with time-resolved soft x-ray spectroscopy. We review recent results in ultrafast soft x-ray spectroscopy of these systems and discuss routes to overcome the technical challenges in performing time-resolved x-ray experiments on photosensitive materials with poor thermal conductivity and low pump intensity thresholds for nonlinear effects.

Stereoisomeric Non-Fullerene Acceptors-Based Organic Solar Cells

Chiral alkyl chains are ubiquitously observed in organic semiconductor materials and can regulate solution processability and active layer morphology, but the effect of stereoisomers on photovoltaic performance has rarely been investigated. For the racemic Y-type acceptors widely used in organic solar cells, it remains unknown if the individual chiral molecules separate into the conglomerate phase or if racemic phase prevails. Here, the photovoltaic performance of enantiomerically pure Y6 derivatives, (S,S)/(R,R)-BTP-4F, and their chiral mixtures are compared. It is found that (S,S) and (R,R)-BTP-4F molecule in the racemic mixtures tends to interact with its enantiomer. The racemic mixtures enable efficient light harvesting, fast hole transfer, and long polaron lifetime, which is conducive to charge generation and suppresses the recombination losses. Moreover, abundant charge diffusion pathways provided by the racemate contribute to efficient charge transport. As a result, the racemate system maximizes the power output and minimizes losses, leading to a higher efficiency of 18.16% and a reduced energy loss of 0.549 eV, as compared to the enantiomerically pure molecules. This study demonstrates that the chirality of non-fullerene acceptors should receive more attention and be designed rationally to enhance the efficiency of organic solar cells.

Improved Short-Circuit Current and Fill Factor in PM6:Y6 Organic Solar Cells through D18-Cl Doping

Based on the PM6:Y6 binary system, a novel non-fullerene acceptor material, D18-Cl, was doped into the PM6:Y6 blend to fabricate the active layer. The effects of different doping ratios of D18-Cl on organic solar cells were investigated. The best-performing organic solar cell was achieved when the doping ratio of D18-Cl reached 20 wt%. It exhibited a short-circuit current of 28.13 mA/cm2, a fill factor of 70.25%, an open-circuit voltage (Voc) of 0.81 V, and a power conversion efficiency of 16.08%. The introduction of an appropriate amount of D18-Cl expanded the absorption spectrum of the active layer, improved the morphology of the active layer, reduced large molecular aggregation and defects, minimized bimolecular recombination, and optimized the collection efficiency of charge carriers. These results indicate the critical importance of selecting an appropriate third component in binary systems and optimizing the doping ratio to enhance the performance of ternary organic solar cells.

Complete Peripheral Fluorination of the Small-Molecule Acceptor in Organic Solar Cells Yields Efficiency over 19

Due to the intrinsically flexible molecular skeletons and loose aggregations, organic semiconductors, like small molecular acceptors (SMAs) in organic solar cells (OSCs), greatly suffer from larger structural/packing disorders and weaker intermolecular interactions comparing to their inorganic counterparts, further leading to hindered exciton diffusion/dissociation and charge carrier migration in resulting OSCs. To overcome this challenge, complete peripheral fluorination was performed on basis of a two-dimensional (2D) conjugation extended molecular platform of CH-series SMAs, rendering an acceptor of CH8F with eight fluorine atoms surrounding the molecular backbone. Benefitting from the broad 2D backbone, more importantly, strengthened fluorine-induced secondary interactions, CH8F and its D18 blends afford much enhanced and more ordered molecular packings accompanying with enlarged dielectric constants, reduced exciton binding energies and more obvious fibrillary networks comparing to CH6F controls. Consequently, D18:CH8F-based OSCs reached an excellent efficiency of 18.80 %, much better than that of 17.91 % for CH6F-based ones. More excitingly, by employing D18-Cl that possesses a highly similar structure to D18 as a third component, the highest efficiency of 19.28 % for CH-series SMAs-based OSCs has been achieved so far. Our work demonstrates the dramatical structural multiformity of CH-series SMAs, meanwhile, their high potential for constructing record-breaking OSCs through peripheral fine-tuning.

Proposal and Numerical Analysis of Organic/Sb2Se3 All-Thin-Film Tandem Solar Cell

The low bandgap antimony selenide (Sb₂Se₃) and wide bandgap organic solar cell (OSC) can be considered suitable bottom and top subcells for use in tandem solar cells. Some properties of these complementary candidates are their non-toxicity and cost-affordability. In this current simulation study, a two-terminal organic/Sb₂Se₃ thin-film tandem is proposed and designed through TCAD device simulations. To validate the device simulator platform, two solar cells were selected for tandem design, and their experimental data were chosen for calibrating the models and parameters utilized in the simulations. The initial OSC has an active blend layer, whose optical bandgap is 1.72 eV, while the initial Sb₂Se₃ cell has a bandgap energy of 1.23 eV. The structures of the initial standalone top and bottom cells are ITO/PEDOT:PSS/DR3TSBDT:PC₇₁BM/PFN/Al, and FTO/CdS/Sb₂Se₃/Spiro-OMeTAD/Au, while the recorded efficiencies of these individual cells are about 9.45% and 7.89%, respectively. The selected OSC employs polymer-based carrier transport layers, specifically PEDOT:PSS, an inherently conductive polymer, as an HTL, and PFN, a semiconducting polymer, as an ETL. The simulation is performed on the connected initial cells for two cases. The first case is for inverted (p-i-n)/(p-i-n) cells and the second is for the conventional (n-i-p)/(n-i-p) configuration. Both tandems are investigated in terms of the most important layer materials and parameters. After designing the current matching condition, the tandem PCEs are boosted to 21.52% and 19.14% for the inverted and conventional tandem cells, respectively. All TCAD device simulations are made by employing the Atlas device simulator given an illumination of AM1.5G (100 mW/cm²). This present study can offer design principles and valuable suggestions for eco-friendly solar cells made entirely of thin films, which can achieve flexibility for prospective use in wearable electronics.

Double-photoelectrode redox desalination of seawater

High energy consumption and low salt removal rate are key barriers to realizing practical electrochemical seawater desalination processes. Here, we demonstrate a novel solar-driven redox flow desalination device with double photoelectrodes to achieve efficient desalination without electrical energy consumption. The device consists of three parts: one photoanode unit, one photocathode unit, and one redox flow desalination unit sandwiched between the two photoelectrode units. The photoelectrode units include a TiO₂ photoanode and a NiO photocathode sensitized with N719 dye, triiodide/iodide redox electrolyte, and graphite paper integrated electrodes decorated with 3,4-ethylene-dioxythiophene. Two salt feeds are located between two ferro/ferricyanide redox flow chambers. Under light illumination, high-quality freshwater is obtained from brackish water containing different concentrations of NaCl from 1000 to 12,000 ppm with a high NaCl removal rate. The device can work in multiple desalination cycles without significant performance declines. Furthermore, natural seawater with an ionic conductivity of 53.45 mS cm^-1 is desalinated to freshwater. This new design opens opportunities to realize efficient and practical solar-driven desalination processes.

Characterization of various tandem solar cells: Protocols, issues, and precautions

In the search for a more efficient solar cell, various types of tandem solar cells (TSCs) have been actively developed worldwide as the performances of the single junction solar cells approach their theoretical limits. Meanwhile, various materials and structures are adopted in TSCs, which makes their characterizations and comparison difficult. Along with the classical monolithic TSC, which exhibits two electrical contacts, devices with three or four electrical contacts have been widely studied as a more performing alternative of commercialized solar cells. For a fair and accurate evaluation of the device performance of TSCs, understanding the effectiveness and limitations of the characterization of the different types of TSCs is crucial. In this paper, we summarize various types of TSCs and discuss their characterization methods.

Near-Infrared Acceptors with Imide-Containing End Groups for Organic Solar Cells

Near-infrared electron acceptors for organic solar cells (OSCs) mostly contain electron-withdrawing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) end groups, which can be modified by but limited to phenyl, thienyl, and naphthyl units with halogenated, methyl, and methyloxy substitution. In this work, we employed an imide-containing unit to construct a new IC end group, based on which a series of new electron acceptors were synthesized. The strong electron-deficient nature of imide units enables the new acceptors to show efficient intramolecular charge transfer and hence red-shifted absorption spectra compared to their IC counterparts. These new electron acceptors were applied to OSCs, providing efficiencies of over 17% with a low voltage loss of 0.52 eV. These results demonstrate that the new imide-containing end groups are promising fragments for the construction of near-infrared electron acceptors for high-performance OSCs.

Near-infrared absorbing acceptor with suppressed triplet exciton generation enabling high performance tandem organic solar cells

Reducing the energy loss of sub-cells is critical for high performance tandem organic solar cells, while it is limited by the severe non-radiative voltage loss via the formation of non-emissive triplet excitons. Herein, we develop an ultra-narrow bandgap acceptor BTPSeV-4F through replacement of terminal thiophene by selenophene in the central fused ring of BTPSV-4F, for constructing efficient tandem organic solar cells. The selenophene substitution further decrease the optical bandgap of BTPSV-4F to 1.17 eV and suppress the formation of triplet exciton in the BTPSV-4F-based devices. The organic solar cells with BTPSeV-4F as acceptor demonstrate a higher power conversion efficiency of 14.2% with a record high short-circuit current density of 30.1 mA cm^-2 and low energy loss of 0.55 eV benefitted from the low non-radiative energy loss due to the suppression of triplet exciton formation. We also develop a high-performance medium bandgap acceptor O1-Br for front cells. By integrating the PM6:O1-Br based front cells with the PTB7-Th:BTPSeV-4F based rear cells, the tandem organic solar cell demonstrates a power conversion efficiency of 19%. The results indicate that the suppression of triplet excitons formation in the near-infrared-absorbing acceptor by molecular design is an effective way to improve the photovoltaic performance of the tandem organic solar cells.

A Flexible, High-Voltage (>100 V) Generating Device Based on Zebra-Like Asymmetrical Photovoltaic Cascade

The mutual conversion between light and electricity lies at the heart of optoelectronic and photonic applications. Maximization of the photoelectric conversion is a long-term goal that can be pursued via the fabrication of devices with ad-hoc architectures. In this framework, it is of utter importance to harvest and transform light irradiation into high electric potential in specific area for driving functional dielectrics that respond to pure electric field. Here, a nano-fabrication technology has been devised featuring double self-alignment that is applied to construct zebra-like asymmetric heterojunction arrays. Such nanostructured composite, which covers a surface area of 5 × 4 mm² and contains 500 periodic repeating units, is capable of photo generating voltages as high as 140 V on a flexible substrate. This approach represents a leap over the traditional functionalization process based on simply embedding materials into devices by demonstrating the disruptive potential of integrating oriented nanoscale device components into meta-material.

Incorporation of a Boron-Nitrogen Covalent Bond Improves the Charge-Transport and Charge-Transfer Characteristics of Organoboron Small-Molecule Acceptors for Organic Solar Cells

An organoboron small-molecular acceptor (OSMA) M_B←N containing a boron-nitrogen coordination bond (B←N) exhibits good light absorption in organic solar cells (OSCs). In this work, based on M_B←N, OSMA M_B-N, with the incorporation of a boron-nitrogen covalent bond (B-N), was designed. We have systematically investigated the charge-transport properties and interfacial charge-transfer characteristics of M_B-N, along with M_B←N, using the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT). Theoretical calculations show that M_B-N can simultaneously boost the open-circuit voltage (from 0.78 V to 0.85 V) and the short-circuit current due to its high-lying lowest unoccupied molecular orbital and the reduced energy gap. Moreover, its large dipole shortens stacking and greatly enhances electron mobility by up to 5.91 × 10^-3 cm²·V^-1·s^-1. Notably, the excellent interfacial properties of PTB7-Th/M_B-N, owing to more charge transfer states generated through the direct excitation process and the intermolecular electric field mechanism, are expected to improve OSCs performance. Together with the excellent properties of M_B-N, we demonstrate a new OSMA and develop a new organoboron building block with B-N units. The computations also shed light on the structure-property relationships and provide in-depth theoretical guidance for the application of organoboron photovoltaic materials.

Synthesis and Ring-Opening Metathesis Polymerization of o-Dialkoxy Paracyclophanedienes

The highly strained ortho-diethylhexyloxy [2.2]paracyclophane-1,9-diene (M1) can be synthesized by ring contraction of a dithia[3.3]paracyclophane using a benzyne-induced Stevens rearrangement. This paracyclophanediene undergoes ring-opening metathesis polymerization to give well-defined 2,3-dialkoxyphenylenevinylene polymers with an alternating cis/trans alkene stereochemistry and controllable molecular weight. Fully conjugated block copolymers with electron-rich and electron-deficient phenylene vinylene polymer segments can be prepared by sequential monomer additions. These polymers can be readily isomerized to the all-trans stereochemistry polymer. The optical and electrochemical properties of these polymers were investigated by theory and experiment.

Picosecond Charge-Transfer-State Dynamics in Wide Band Gap Polymer-Non-Fullerene Small-Molecule Blend Films Investigated via Transient Infrared Spectroscopy

Organic solar cells based on wide band gap polymers and nonfullerene small-molecule acceptors have demonstrated remarkably good device performances. Nevertheless, a thorough understanding of the charge-transfer process in these materials has not been achieved yet. In this study, we use Fano resonance signals caused by the interaction of broad electronic charge carrier absorption and the molecular vibrations of the electron acceptor molecule to monitor the charge-transfer state dynamics. In our time-resolved infrared spectroscopy experiments, we find that in the small-molecule acceptor, they have additional dynamics on the order of a few picoseconds. A change in the solvent used in thin film deposition, leading to different morphologies, influences this time further. We interpret our findings as the dynamics of the charge-transfer state at the interface of the electron donor and the electron- acceptor. The additional mid-infrared transient signal is generated in this state, as both electron and hole polarons can interact with small-molecule acceptor vibrational modes.

Two-dimensional IV-VA3 monolayers with enhanced charge mobility for high-performance solar cells

High-performance photovoltaics (PVs) constitute a subject of extensive research efforts, in which silicon (Si)-based solar cells (SCs) have been widely commercialized. However, the low carrier mobility of Si-based SCs can limit the effective charge separation, thereby negatively impacting the device performance. Here, via calculating the physicochemical and PV performance based on density functional theory, we demonstrate SCs based on two-dimensional (2D) group IV and V compounds with an AX₃ configuration. Firstly, the cleavage energies of AX₃ (A = Si, Ge; X = P, As, and Sb) are calculated to be less than 1 J m^-2, providing an experimental feasibility to be exfoliated from the corresponding bulk. Secondly, electronic and optical properties have been systematically investigated. To be specific, the band gap of monolayer AX₃ falls in the range of 1.11-1.27 eV, which is comparable with that of Si. Significantly, the electron mobility of monolayer AX₃ can reach as high as ∼30 000 cm² V^-1 s^-1, which is one order of magnitude higher than that of Si. Furthermore, the optical absorbance of monolayer SiAs₃, SiP₃ and GeAs₃ exhibits high coefficients in visible light. Therefore, we believe that our designed AX₃-based PV systems with power conversion efficiency of 20% can offer great potential in the application of high-performance two-dimension-based PVs.

Renewed Prospects for Organic Photovoltaics

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (E_loss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller E_loss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

The Intrinsic Role of the Fusion Mode and Electron-Deficient Core in Fused-Ring Electron Acceptors for Organic Photovoltaics

The A-DA'D-A fused-ring electron acceptors with an angular fusion mode and electron-deficient core has significantly boosted organic photovoltaic efficiency. Here, the intrinsic role of the peculiar structure is revealed by comparing representative A-DA'D-A acceptor Y6 with its A-D-A counterparts having different fusion modes. Owing to the more delocalized HOMO and deeper LUMO level, Y6 exhibits stronger and red-shifted absorption relative to the linear and angular fused A-D-A acceptors, respectively. Moreover, the change from linear to angular fusion substantially reduces the electron-vibration couplings, which is responsible for the faster exciton diffusion, exciton dissociation, and electron transport for Y6 than the linear fused A-D-A acceptor. Notably, the electron-vibration coupling for exciton dissociation is further decreased by introducing the electron-deficient core, thus contributing to the efficient charge generation under low driving forces in the Y6-based devices.

Inorganic-Organic Hybrid Phototransistor Array with Enhanced Photogating Effect for Dynamic Near-Infrared Light Sensing and Image Preprocessing

Narrow-band-gap organic semiconductors have emerged as appealing near-infrared (NIR) sensing materials by virtue of their unique optoelectronic properties. However, their limited carrier mobility impedes the implementation of large-area, dynamic NIR sensor arrays. In this work, high-performance inorganic-organic hybrid phototransistor arrays are achieved for NIR sensing, by taking advantage of the high electron mobility of In₂O₃ and the strong NIR absorption of a BTPV-4F:PTB7-Th bulk heterojunction (BHJ) with an enhanced photogating effect. As a result, the hybrid phototransistors reach a high responsivity of 1393.0 A W^-1, a high specific detectivity of 4.8 × 10¹² jones, and a fast response of 0.72 ms to NIR light (900 nm). Meanwhile, an integrated 16 × 16 phototransistor array with a one-transistor-one-phototransistor (1T1PT) architecture is achieved. On the basis of the enhanced photogating effect, the phototransistor array can not only achieve real-time, dynamic NIR light mapping but also implement image preprocessing, which is promising for advanced NIR image sensors.

Efficient Optoelectronic Devices Enabled by Near-Infrared Organic Semiconductors with a Photoresponse beyond 1050 nm

Organic optoelectronic devices exhibit distinctive photoresponse to the near-infrared (NIR) light and show great potential in many fields. However, the optoelectronic properties of the existing devices hardly meet the technical requirements of new applications such as energy conversion and health sensing, thus raising the demand to develop high-performance NIR organic semiconductors. To address this issue, a new NIR material, namely, BFIC, is designed and synthesized by inserting fluorothieno[3,4-b]thiophene (FTT) as a π-bridge. Since the introduction of FTT can extend the conjugation, stabilize the quinoid resonant structure, and enhance the intramolecular charge transfer, BFIC displays a broad and intense absorption in the NIR region, ranging from 700 to 1050 nm. As a result, the organic solar cell based on BFIC and a polymer donor PTB7-Th realizes a power conversion efficiency of 10.38%. The semitransparent organic solar cell (OSC) shows a power conversion efficiency of 6.15%, accompanied by an average visible transmittance of 38.79% due to the selective photoresponse in the NIR range. The organic photodetector based on PTB7-Th:BFIC delivers a broad spectral response ranging from 330 to 1030 nm with a specific detectivity over 10¹³ Jones under the self-powered mode, which is one of the highest detectivities among the broad-band organic photodetectors.

High-Performance 1 cm2 Perovskite-Organic Tandem Solar Cells with a Solvent-Resistant and Thickness-Insensitive Interconnecting Layer

Organic solar cells (OSCs) and perovskite solar cells (PVSCs) are promising candidates for next-generation thin film photovoltaic technologies. The integration of OSCs with PVSCs in tandem devices is now attracting significant attention due to their similar fabrication procedures and the potential to afford a higher device performance. Here, a thickness-insensitive and solvent-resistant interconnecting layer is developed to efficiently connect perovskite and organic subcells with low contact resistance. The resultant perovskite-organic tandem devices maintain high efficiencies over a wide thickness range of the interconnecting layer, from ∼20 nm to ∼50 nm, providing an easily fabricated, solvent-resistant platform to integrate perovskite and organic active layers with low-temperature solution processing techniques. The tandem devices containing an ultrathin PVSC and a typical non-fullerene OSC give a maximum efficiency of 19.2%, which is much higher than those of the single-junction devices. Moreover, highly reproducible 1 cm² perovskite-organic tandem devices are achieved using the thickness-insensitive and solvent-resistant interconnecting layer, and an efficiency of 17.8% is realized. These 1 cm² tandem solar cells are used successfully in solar-to-hydrogen systems to afford a solar-to-fuel conversion efficiency of 11.2%. Overall, these advances represent significant progress in the design of ultrathin and facile solution processed perovskite-organic tandem solar cells.

A Review on the Materials Science and Device Physics of Semitransparent Organic Photovoltaics

In this review, the current state of materials science and the device physics of semitransparent organic solar cells is summarized. Relevant synthetic strategies to narrow the band gap of organic semiconducting molecules are outlined, and recent developments in the polymer donor and near-infrared absorbing acceptor materials are discussed. Next, an overview of transparent electrodes is given, including oxides, multi-stacks, thin metal, and solution processed electrodes, as well as considerations that are unique to ST-OPVs. The remainder of this review focuses on the device engineering of ST-OPVs. The figures of merit and the theoretical limitations of ST-OPVs are covered, as well as strategies to improve the light utilization efficiency. Lastly, the importance of creating an in-depth understanding of the device physics of ST-OPVs is emphasized and the existing works that answer fundamental questions about the inherent changes in the optoelectronic processes in transparent devices are presented in a condensed way. This last part outlines the changes that are unique for devices with increased transparency and the resulting implications, serving as a point of reference for the systematic development of next-generation ST-OPVs.

High-Efficiency Electron Transport Layer-Free Perovskite/GeTe Tandem Solar Cell: Numerical Simulation

The primary purpose of recent research has been to achieve a higher power conversion efficiency (PCE) with stable characteristics, either through experimental studies or through modeling and simulation. In this study, a theoretical analysis of an efficient perovskite solar cell (PSC) with cuprous oxide (Cu2O) as the hole transport material (HTM) and zinc oxysulfide (ZnOS) as the electron transport material (ETM) was proposed to replace the traditional HTMs or ETMs. In addition, the impact of doping the perovskite layer was investigated. The results show that the heterostructure of n-p PSC without an electron transport layer (ETL) could replace the traditional n-i-p structure with better performance metrics and more stability due to reducing the number of layers and interfaces. The impact of HTM doping and thickness was investigated. In addition, the influence of the energy gap of the absorber layer was studied. Furthermore, the proposed PSC without ETL was used as a top sub-cell with germanium-telluride (GeTe) as a bottom sub-cell to produce an efficient tandem cell and boost the PCE. An ETL-free PSC/GeTe tandem cell is proposed for the first time to provide an efficient and stable tandem solar cell with a PCE of 45.99%. Finally, a comparison between the performance metrics of the proposed tandem solar cell and those of other recent studies is provided. All the simulations performed in this study are accomplished by using SCAPS-1D.

Vinylene-Inserted Asymmetric Polymer Acceptor with Absorption Approaching 1000 nm for Versatile Applications in All-Polymer Solar Cells and Photomultiplication-Type Polymeric Photodetectors

The emerging polymerized small-molecule acceptors (PSMAs) with near-infrared (NIR) absorption have not only significantly boosted the power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) but have also exhibited great potential for sensitive NIR polymeric photodetectors (PPDs). However, there is no report regarding PSMAs with photo-response that can approach 1000 nm, which is an important criterion for applications in NIR-responsive all-PSCs and PPDs. Herein, by unidirectionally inserting vinylene segments into a selenophene-rich polymer backbone to improve the electron-donating strength and quinoidal character, an asymmetric PSMA, namely, PY3Se-1V, was developed, which showed an extensively red-shifted absorption approaching 1000 nm. The PBDB-T:PY3Se-1V-based binary all-PSCs achieve a decent PCE of 13.2% and a record-high photocurrent density of 25.9 mA cm-2 due to the significantly broadened photo-response and efficient photon-to-electron conversion. More encouragingly, narrowband photomultiplication (PM)-type PPDs based on poly(3-hexylthiophene-2,5-diyl) (P3HT):PY3Se-1V were developed, delivering an exceptionally high external quantum efficiency of 3680% and a responsivity of 28 A W-1 at an NIR peak of 960 nm under -50 V bias, which is reported for the first time in PM-type PPDs with a response approaching 1000 nm.

Theory-Guided Material Design Enabling High-Performance Multifunctional Semitransparent Organic Photovoltaics without Optical Modulations

Semitransparent organic photovoltaics (ST-OPVs) have drawn great attention for promising applications in building-integrated photovoltaics, providing additional power generation for daily use. A previously proposed strategy, "complementary NIR absorption," is widely applied for high-performance ST-OPVs. However, rational material design toward high performance has not been achieved. In this work, an external quantum efficiency (EQE) model describing this strategy is developed to explore the full potential of material design on ST-OPV performance. Guided by the model, a novel nonfullerene acceptor (NFA), ATT-9, is designed and synthesized, which possesses optimal bandgap for ST-OPVs, achieving a record short-circuit current density of 30 mA cm^-2 and a power conversion efficiency of 13.40%, the highest value among devices based on NFAs with bandgaps lower than 1.2 eV. It is notworthy that, at such a low bandgap, the energy loss of the device is only 0.58 eV, which is attributed to the low energetic disorder confirmed by an ultralow Urbach energy of 21.6 meV. Benefiting from the optimal bandgap and low energy loss, the ATT-9-based ST-OPV achieves a high light utilization efficiency of 3.33% without optical modulations, and meanwhile shows excellent thermal insulation, exceeding the commercial 3M heat-insulating window film, demonstrating the outstanding application prospects of multifunctional ST-OPVs.

Influence of Large Steric Hinderance Substituent Position on Conformation and Charge Transfer Process for Non-Fused Ring Acceptors

To obtain stable and planar molecular geometry in non-fused electron acceptors, A4T-25 and A4T-26 are designed and synthesized by introducing the bulk 2,4,6-triisopropylphenyl side groups onto different positions of the central two thiophene units. A4T-25 and A4T-26 both show a narrow-bandgap of 1.39 and 1.46 eV, with highest occupied molecular orbital/lowest unoccupied molecular orbital levels of -5.56/-3.81 and -5.65/-3.83 eV, respectively, and the electrostatic potential distributions imply that they have strong electron-accepting capability. The single crystal structure analysis and the transfer integral calculation demonstrate that the much more compact π-π stacking in A4T-26 can promote efficient charge transportation compared to that in A4T-25. Therefore, the electron mobility of A4T-26 is obviously higher and more balanced than that of A4T-25. When blending the two acceptors with the same polymer donor PBDB-TF, the photovoltaic cell based on PBDB-TF:A4T-25 has an inadequate power conversion efficiency (PCE) of 7.83%, while the PBDB-TF:A4T-26-based one yields an enhanced PCE of 12.1%. Overall, this study offers an insight into the relationship between the fine-tuning of the molecular structure of non-fused ring acceptors and the corresponding charge transfer process in organic solar cells.

Non-halogenated solvents processed efficient ITO-free flexible organic solar cells with up-scaled area

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Recent Progress in Organic Solar Cells: A Review on Materials from Acceptor to Donor

In the last few decades, organic solar cells (OSCs) have drawn broad interest owing to their advantages such as being low cost, flexible, semitransparent, non-toxic, and ideal for roll-to-roll large-scale processing. Significant advances have been made in the field of OSCs containing high-performance active layer materials, electrodes, and interlayers, as well as novel device structures. Particularly, the innovation of active layer materials, including novel acceptors and donors, has contributed significantly to the power conversion efficiency (PCE) improvement in OSCs. In this review, high-performance acceptors, containing fullerene derivatives, small molecular, and polymeric non-fullerene acceptors (NFAs), are discussed in detail. Meanwhile, highly efficient donor materials designed for fullerene- and NFA-based OSCs are also presented. Additionally, motivated by the incessant developments of donor and acceptor materials, recent advances in the field of ternary and tandem OSCs are reviewed as well.

Constructing Monolithic Perovskite/Organic Tandem Solar Cell with Efficiency of 22.0% via Reduced Open-Circuit Voltage Loss and Broadened Absorption Spectra

Combining the high stability under UV light of the wide bandgap (WBG) perovskite solar cells (pero-SCs) and the broad near-infrared absorption spectra of the narrow bandgap (NBG) organic solar cells (OSCs), the perovskite/organic tandem solar cells (TSCs) with the WBG pero-SC as front cell and the NBG OSC as rear cell have attracted attention . However, the photovoltaic performance of the perovskite/organic TSCs needs to be further improved. Herein, nonradiative charge recombination loss is reduced through bulk defect passivation in the WBG pero-SC front subcell and broadening the range of absorption spectra of the NBG OSC rear cell. For the WBG pero-SCs, an organic cation chloro-formamidinium is introduced into FA_0.6 MA_0.4 Pb(I_0.6 Br_0.4 )₃ to passivate the bulk defects in the perovskite film and the WBG pero-SC displays high open-circuit voltage of 1.25 V and high fill factor of 83.0%. For the NBG OSCs, a new infrared-absorbing organic small molecule acceptor BTPV-4Cl-eC9 is designed and synthesized. Then, a monolithic perovskite/organic TSC is fabricated with the WBG pero-SC as the front cell and the NBG OSC as the rear cell, and the TSC demonstrates high power conversion efficiency up to 22.0%. The results indicate that the perovskite/organic TSC is promising for future commercialization.

Near-Infrared Materials: The Turning Point of Organic Photovoltaics

Near-infrared (NIR)-absorbing organic semiconductors have opened up many exciting opportunities for organic photovoltaic (OPV) research. For example, new chemistries and synthetical methodologies have been developed; especially, the breakthrough Y-series acceptors, originally invented by our group, specifically Y1, Y3, and Y6, have contributed immensely to boosting single-junction solar cell efficiency to around 19%; novel device architectures such as tandem and transparent organic photovoltaics have been realized. The concept of NIR donors/acceptors thus becomes a turning point in the OPV field. Here, the development of NIR-absorbing materials for OPVs is reviewed. According to the low-energy absorption window, here, NIR photovoltaic materials (p-type (polymers) and n-type (fullerene and nonfullerene)) are classified into four categories: 700-800 nm, 800-900 nm, 900-1000 nm, and greater than 1000 nm. Each subsection covers the design, synthesis, and utilization of various types of donor (D) and acceptor (A) units. The structure-property relationship between various kinds of D, A units and absorption window are constructed to satisfy requirements for different applications. Subsequently, a variety of applications realized by NIR materials, including transparent OPVs, tandem OPVs, photodetectors, are presented. Finally, challenges and future development of novel NIR materials for the next-generation organic photovoltaics and beyond are discussed.

Two-dimensional InSb/GaAs- and InSb/InP-based tandem photovoltaic device with matched bandgap

The past several years have witnessed remarkable research efforts to develop high-performance photovoltaics (PVs), to curtail the energy crisis by avoiding dependence on traditional fossil fuels. In this regard, there is an urgent need to accelerate research progress on new low-dimensional semiconductors with superior electronic and optical properties. Herein, combining abundant related PV experimental data in the literature and our systematic theoretical calculations, we propose two-dimensional (2D) InSb/GaAs and InSb/InP-based tandem PVs with high solar-to-electric efficiency up to near 30.0%. Firstly, according to first-principles calculations, the stability, electronic and optical properties of single-layer group-III-V materials (XY, X = Ga and In, Y = N, P, As, Sb, and Bi) are systematically introduced. Next, due to the high bandgap (E_g) of GaAs and InP being a perfect match with the low E_g of InSb, InSb/GaAs- and InSb/InP-based tandem PVs are constructed. In addition, the complementary absorption spectra of these two subcells can facilitate the achievement of high tandem power conversion efficiency. Furthermore, we have analyzed in detail the influencing factors for PCE and the physical mechanism of the optimized match between the top and bottom subcells in the tandem configurations. Our designed 2D-semiconductor-based PVs can be expected to bring a new perspective for future commercialized high-efficiency energy devices.

Elucidating Charge Generation in Green-Solvent Processed Organic Solar Cells

Organic solar cells have the potential to become the cheapest form of electricity. Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene small-molecule acceptors. Next generation photovoltaics based upon environmentally benign “green solvent” processing of organic semiconductors promise a step-change in the adaptability and versatility of solar technologies and promote sustainable development. However, high-performing OSCs are still processed by halogenated (non-environmentally friendly) solvents, so hindering their large-scale manufacture. In this perspective, we discuss the recent progress in developing highly efficient OSCs processed from eco-compatible solvents, and highlight research challenges that should be addressed for the future development of high power conversion efficiencies devices.

Designs and understanding of small molecule-based non-fullerene acceptors for realizing commercially viable organic photovoltaics

Organic photovoltaics (OPVs) have emerged as a promising next-generation technology with great potential for portable, wearable, and transparent photovoltaic applications. Over the past few decades, remarkable advances have been made in non-fullerene acceptor (NFA)-based OPVs, with their power conversion efficiency exceeding 18%, which is close to the requirements for commercial realization. Novel molecular NFA designs have emerged and evolved in the progress of understanding the physical features of NFA-based OPVs in relation to their high performance, while there is room for further improvement. In this review, the molecular design of representative NFAs is described, and their blend characteristics are assessed via statistical comparisons. Meanwhile, the current understanding of photocurrent generation is reviewed along with the significant physical features observed in high-performance NFA-based OPVs, while the challenging issues and the strategic perspectives for the commercialization of OPV technology are also discussed.

Non-Fullerene Acceptors with an Optical Response over 1000 nm toward Efficient Organic Solar Cells

Non-fullerene acceptors (NFAs) with near-infrared (NIR) absorption show promising advantages in organic solar cells (OSCs). However, only a few NFAs can extend the absorption spectra over 1000 nm, and their photovoltaic performance has been unsatisfactory so far. To address this issue, three new NFAs, namely, 6-IFIC, 6-IF2F, and 6-IF4F, were synthesized by simultaneously introducing π-bridge units and different end groups. The π-bridge unit enlarges the conjugation and planarizes the molecular geometry, leading to intense absorption in the NIR range. The asymmetric configuration provides a large dipole moment, enhances the intermolecular interaction, and tunes the miscibility, consequently being beneficial for achieving a favorable morphology in OSCs. When blended with a donor polymer PTB7-Th, the 6-IF2F-based OSC yields the best power conversion efficiency (PCE) of 11.20%, which is among the highest PCEs based on NFAs with absorption over 1000 nm. More importantly, the absorption of the blend film provides a transparency window in the visible range from 400 and 650 nm. Therefore, the semitransparent OSCs based on these three NFAs can achieve over 28% average visible transmittance.

Y6 and its derivatives: molecular design and physical mechanism

Y6 and its derivatives have advanced the efficiency of organic solar cells to 15%-18%. This perspective reveals the device and photo physics features of Y-series based devices and proposed some guidelines for future molecular design.

Two-Dimensional Conjugated Benzo[1,2-b:4,5-b']diselenophene-Based Copolymer Donor Enables Large Open-Circuit Voltage and High Efficiency in Selenophene-based Organic Solar Cells

A two-dimensional electron-rich fused-ring moiety (ClBDSe) based on benzo[1,2-b:4,5-b']diselenophene is synthesized. Three copolymers (PBDT-Se, PBDSe-T, and PBDSe-Se) are obtained by manipulating the connection types and number of selenophene units on the conjugated main chains with two 2D fused-ring units and two different π-bridges, respectively. In comparison with PBDT-Se and PBDSe-Se, PBDSe-T with benzo[1,2-b:4,5-b']diselenophene unit and thiophene π-bridge exhibits the deepest HOMO energy level and the strongest crystallinity in neat films. The PBDSe-T:Y6 blend film exhibits the best absorption complementarity, the most distinctive face-on orientation with proper phase separation, the highest carrier mobilities, and the lowest charge recombination among three blend films. Finally, the PBDSe-T:Y6-based device delivers an impressive power conversion efficiency (PCE) of 14.50 %, which is higher than those of PBDT-Se:Y6 and PBDSe-Se:Y6. Moreover, a decent open-circuit voltage (V_oc ) of 0.89 V with a remarkably small energy loss of 0.44 eV is achieved for PBDSe-T:Y6. The efficiency of 14.50 % is the highest value for selenophene-containing copolymer-based binary organic solar cells (OSCs). This study provides evidence that introduction of 2D-benzo[1,2-b:4,5-b']diselenophene as a fused electron-rich unit with π-bridging into copolymeric donors is a valid strategy for providing high V_oc and excellent PCE simultaneously in selenophene-based OSCs.

A Tandem Organic Photovoltaic Cell with 19.6% Efficiency Enabled by Light Distribution Control

Despite more potential in realizing higher photovoltaic performance, the highest power conversion efficiency (PCE) of tandem organic photovoltaic (OPV) cells still lags behind that of state-of-the-art single-junction cells. In this work, highly efficient double-junction tandem OPV cells are fabricated by optimizing the photoactive layers with low voltage losses and developing an effective method to tune optical field distribution. The tandem OPV cells studied are structured as indium tin oxide (ITO)/ZnO/bottom photoactive layer/interconnecting layer (ICL)/top photoactive layer/MoO_x /Ag, where the bottom and top photoactive layers are based on blends of PBDB-TF:ITCC and PBDB-TF:BTP-eC11, respectively, and ICL refers to interconnecting layer structured as MoO_x /Ag/ZnO:PFN-Br. As these results indicate that there is not much room for optimizing the bottom photoactive layer, more effort is put into fine-tuning the top photoactive layer. By rationally modulating the composition and thickness of PBDB-TF:BTP-eC11 blend films, the 300 nm-thick PBDB-TF:BTP-eC11 film with 1:2 D/A ratio is found to be an ideal photoactive layer for the top sub-cell in terms of photovoltaic characteristics and light distribution control. For the optimized tandem cell, a PCE of 19.64% is realized, which is the highest result in the OPV field and certified as 19.50% by the National Institute of Metrology.

Tuning of the Interconnecting Layer for Monolithic Perovskite/Organic Tandem Solar Cells with Record Efficiency Exceeding 21

The photovoltaic performance of inorganic perovskite solar cells (PSCs) still lags behind the organic-inorganic hybrid PSCs due to limited light absorption of wide bandgap CsPbI_3-xBr_x under solar illumination. Constructing tandem devices with organic solar cells can effectively extend light absorption toward the long-wavelength region and reduce radiative photovoltage loss. Herein, we utilize wide-bandgap CsPbI₂Br semiconductor and narrow-bandgap PM6:Y6-BO blend to fabricate perovskite/organic tandem solar cells with an efficiency of 21.1% and a very small tandem open-circuit voltage loss of 0.06 V. We demonstrate that the hole transport material of the interconnecting layers plays a critical role in determining efficiency, with polyTPD being superior to PBDB-T-Si and D18 due to its low parasitic absorption, sufficient hole mobility and quasi-Ohmic contact to suppress charge accumulation and voltage loss within the tandem device. These perovskite/organic tandem devices also display superior storage, thermal and ultraviolet stabilities.