Efficient Inverted Perovskite Photovoltaics Through Surface State Manipulation.

Inverted perovskite solar cells (PSCs) are considered as the most promising avenue for the commercialization of PSCs due to their potential inherent stability. However, suboptimal interface contacts between electron transport layer (ETL) (such as C60 ) and the perovskite absorbing layer within inverted PSCs always result in reduced efficiency and poor stability. Herein, a surface state manipulation strategy has been developed by employing a highly electronegative 4-fluorophenethylamine hydrochloride (p-F-PEACl) to effectively address the issue of poor interface contacts in the inverted PSCs. The p-F-PEACl demonstrates a robust interaction with perovskite film through bonding of amino group and Cl- with I- and Pb2+ ions in the perovskite, respectively. As such, the surface defects of perovskite film can be significantly reduced, leading to suppressed non-radiative recombination. Moreover, p-F-PEACl also plays a dual role in enhancing the surface potential and improving energy-level alignment at the interfaces between the perovskite and C60 carrier transport layer, which directly contributes to efficient charge extraction. Finally, the open-circuit voltage (Voc ) of devices increases from 1.104 V to 1.157 V, leading to an overall efficiency improvement from 22.34% to 24.78%. Furthermore, the p-F-PEACl-treated PSCs also display excellent stability.

Giant Molecule Acceptor Enables Highly Efficient Organic Solar Cells Processed Using Non-halogenated Solvent.

High efficiency organic solar cells (OSCs) based on A-DA'D-A type small molecule acceptors (SMAs) were mostly fabricated by toxic halogenated solvent processing, and power conversion efficiency (PCE) of the non-halogenated solvent processed OSCs is mainly restricted by the excessive aggregation of the SMAs. To address this issue, we developed two vinyl π-spacer linking-site isomerized giant molecule acceptors (GMAs) with the π-spacer linking on the inner carbon (EV-i) or out carbon (EV-o) of benzene end group of the SMA with longer alkyl side chains (ECOD) for the capability of non-halogenated solvent-processing. Interestingly, EV-i possesses a twisted molecular structure but enhanced conjugation, while EV-o shows a better planar molecular structure but weakened conjugation. The OSC with EV-i as acceptor processed by the non-halogenated solvent o-xylene (o-XY) demonstrated a higher PCE of 18.27 % than that of the devices based on the acceptor of ECOD (16.40 %) or EV-o (2.50 %). 18.27 % is one of the highest PCEs among the OSCs fabricated from non-halogenated solvents so far, benefitted from the suitable twisted structure, stronger absorbance and high charge carrier mobility of EV-i. The results indicate that the GMAs with suitable linking site would be the excellent candidates for fabricating high performance OSCs processed by non-halogenated solvents.

High Efficiency Polymer Solar Cells with Efficient Hole Transfer at Zero Highest Occupied Molecular Orbital Offset between Methylated Polymer Donor and Brominated Acceptor.

Achieving efficient charge transfer at small frontier molecular orbital offsets between donor and acceptor is crucial for high performance polymer solar cells (PSCs). Here we synthesize a new wide band gap polymer donor, PTQ11, and a new low band gap acceptor, TPT10, and report a high power conversion efficiency (PCE) PSC (PCE = 16.32%) based on PTQ11-TPT10 with zero HOMO (the highest occupied molecular orbital) offset (ΔEHOMO(D-A)). TPT10 is a derivative of Y6 with monobromine instead of bifluorine substitution, and possesses upshifted lowest unoccupied molecular orbital energy level (ELUMO) of -3.99 eV and EHOMO of -5.52 eV than Y6. PTQ11 is a derivative of low cost polymer donor PTQ10 with methyl substituent on its quinoxaline unit and shows upshifted EHOMO of -5.52 eV, stronger molecular crystallization, and better hole transport capability in comparison with PTQ10. The PSC based on PTQ11-TPT10 shows highly efficient exciton dissociation and hole transfer, so that it demonstrates a high PCE of 16.32% with a higher Voc of 0.88 V, a large Jsc of 24.79 mA cm-2, and a high FF of 74.8%, despite the zero ΔEHOMO(D-A) value between donor PTQ11 and acceptor TPT10. The PCE of 16.32% is one of the highest efficiencies in the PSCs. The results prove the feasibility of efficient hole transfer and high efficiency for the PSCs with zero ΔEHOMO(D-A), which is highly valuable for understanding the charge transfer process and achieving high PCE of PSCs.

High-Performance All-Polymer Solar Cells: Synthesis of Polymer Acceptor by a Random Ternary Copolymerization Strategy.

Demonstrated in this work is a simple random ternary copolymerization strategy to synthesize a series of polymer acceptors, PTPBT-ETx , by polymerizing a small-molecule acceptor unit modified from Y6 with a thiophene connecting unit and a controlled amount of an 3-ethylesterthiophene (ET) unit. Compared to PTPBT of only Y6-like units and thiophene units, PTPBT-ETx (where x represents the molar ratio of the ET unit) with an incorporated ET unit in the ternary copolymers show up-shifted LUMO energy levels, increased electron mobilities, and improved blend morphologies in the blend film with the polymer donor PBDB-T. And the all-polymer solar cell (all-PSC) based on PBDB-T:PTPBT-ET0.3 achieved a high power conversion efficiency over 12.5 %. In addition, the PTPBT-ET0.3 -based all-PSC also exhibits long-term photostability over 300 hours.

Manipulating Polymer Backbone Configuration via Halogenated Asymmetric End-Groups Enables Over 18% Efficiency All-Polymer Solar Cells.

High-performance all-polymer solar cells (all-PSCs) deeply rely on the joint contributions of desirable optical absorption, adaptive energy levels, and appropriate morphology. Herein, two structural analogous polymerized small-molecule acceptors (PSMAs), PYFCl-T and PYF&PYCl-T, are synthesized, and then incorporated into the PM6:PY-IT binary blends to construct ternary all-PSCs. Due to the superior compatibility of PY-IT and PYFCl-T, the ternary all-PSC based on PM6:PY-IT:PYFCl-T with 10 wt% PYFCl-T, presents higher and more balanced charge mobility, suppressed charge recombination, and faster charge-transfer kinetics, resulting in an outstanding power conversion efficiency (PCE) of 18.12% with enhanced Jsc and FF, which is much higher than that (PCE of 16.09%) of the binary all-PSCs based on PM6:PY-IT. Besides, the ternary all-PSCs also exhibit improved photostability. The conspicuous performance enhancement principally should give the credit to the miscibility-driven phase optimization of the donor and acceptor. These findings highlight the significance of polymer-backbone configuration modulation of PSMAs in morphology optimization toward boosting the device properties of all-PSCs.

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.

n-Octyl substituted quinoxaline-based polymer donor enabling all-polymer solar cell with efficiency over 17.

Recently, the power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) have increased rapidly. To further increase the PCE of all-PSCs, it is necessary to create new donor polymers matching the polymer acceptors. In this paper, we synthesize a new quinoxaline-based polymer donor PBQ8 with n-octyl side chain on the quinoxaline unit, which possesses the same skeleton structure to the previously reported PBQ5 (with isooctyl side chain). The effects of alkyl side chains on the physicochemical properties of the polymer donor were investigated. In comparison with PBQ5, PBQ8 exhibits stronger intermolecular interactions and better molecular packing. When blending with polymer acceptor PY-IT, the PBQ8:PY-IT based devices demonstrated a higher PCE value of 17.04%, which is one of the highest PCEs occurred in the all-PSCs. And the PBQ5:PY-IT (PCE 15.56%, Voc 0.907 V, FF 69.72%, and Jsc 24.60 mA cm-2) is much lower. The PBQ8:PY-IT blend displayed more efficient exciton dissociation, better molecular stacking properties, preferable phase separation and higher mobility. These indicate that as an effective method, side chain engineering can improve the efficiency of the all-PSCs.

High performance polymerized small molecule acceptor by synergistic optimization on π-bridge linker and side chain.

The polymerized small-molecule acceptors have attracted great attention for application as polymer acceptor in all-polymer solar cells recently. The modification of small molecule acceptor building block and the π-bridge linker is an effective strategy to improve the photovoltaic performance of the polymer acceptors. In this work, we synthesized a new polymer acceptor PG-IT2F which is a modification of the representative polymer acceptor PY-IT by replacing its upper linear alkyl side chains on the small molecule building block with branched alkyl chains and attaching difluorene substituents on its thiophene π-bridge linker. Through this synergistic optimization, PG-IT2F possesses more suitable phase separation, increased charge transportation, better exciton dissociation, lower bimolecular recombination, and longer charge transfer state lifetime than PY-IT in their polymer solar cells with PM6 as polymer donor. Therefore, the devices based on PM6:PG-IT2F demonstrated a high power conversion efficiency of 17.24%, which is one of the highest efficiency reported for the binary all polymer solar cells to date. This work indicates that the synergistic regulation of small molecule acceptor building block and π-bridge linker plays a key role in designing and developing highly efficient polymer acceptors.

Terpolymer Donor with Inside Alkyl Substituents on Thiophene π-Bridges toward Thiazolothiazole A2 -Unit Enables 18.21% Efficiency of Polymer Solar Cells.

PM6 is a widely used D-A copolymer donor in the polymer solar cells (PSCs). Incorporating second electron-withdrawing (A2 ) units into PM6 backbone by ternary D-A1 -D-A2 random copolymerization strategy is an effective approach to further improve its photovoltaic performance. Here, the authors synthesize the PM6-based terpolymers by introducing thiazolothiazole as the A2 units connecting with thiophene π-bridges attaching alkyl substituent towards the A2 unit (PMT-CT) or towards D-unit (PMT-FT), and study the effect of the alkyl substituent position on the photovoltaic performance of them. Two terpolymers PMT-FT-10 and PMT-CT-10 are obtained by incorporating 10% A2 units in the terpolymers. The film of PMT-CT-10 shows slightly up-shifted highest occupied molecular orbital (HOMO) energy levels while better co-planar structure than that of PMT-FT-10. Meanwhile, the PMT-CT-10:Y6 blend film exhibits better molecular packing properties, more proper phase separation and more balanced hole and electron mobilities, which are beneficial to more efficient exciton dissociation, efficient charge transport and weaker bimolecular recombination. Consequently, the PMT-CT-10 based PSCs obtain the highest power conversion efficiency of 18.21%. The results indicate that side chain position on the thiophene π-bridges influence the device performance of the terpolymer donors, and PMT-CT-10 is a high efficiency polymer donor for the PSCs.

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 FA0.6 MA0.4 Pb(I0.6 Br0.4 )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.

16.52% Efficiency All-Polymer Solar Cells with High Tolerance of the Photoactive Layer Thickness.

All-polymer solar cells (all-PSCs) have drawn growing attention and achieved tremendous progress recently, but their power conversion efficiency (PCE) still lags behind small-molecule-acceptor (SMA)-based PSCs due to the relative difficulty on morphology control of polymer photoactive blends. Here, low-cost PTQ10 is introduced as a second polymer donor (a third component) into the PM6:PY-IT blend to finely tune the energy-level matching and microscopic morphology of the polymer blend photoactive layer. The addition of PTQ10 decreases the π-π stacking distance, and increases the π-π stacking coherence length and the ordered face-on molecular packing orientation, which improves the charge separation and transport in the photoactive layer. Moreover, the deeper highest occupied molecular orbital energy level of the PTQ10 polymer donor than PM6 leads to higher open-circuit voltage of the ternary all-PSCs. As a result, a PCE of 16.52% is achieved for ternary all-PSCs, which is one of the highest PCEs for all-PSCs. In addition, the ternary devices exhibit a high tolerance of the photoactive layer thickness with high PCEs of 15.27% and 13.91% at photoactive layer thickness of ≈205 and ≈306 nm, respectively, which are the highest PCEs so far for all-PSCs with a thick photoactive layer.

Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors.

Energy loss caused by exciton binding energy (Eb) has become a key factor that restricts further advancement of organic solar cells (OSCs). Herein, we used transient mid-IR spectroscopy to study direct photogeneration of free charge carriers in small-molecule acceptors (SMAs) Y6 and IDIC as well as polymerized SMAs (PSMAs) PYFT and PZ1. We found that free carrier concentration is higher in PSMAs than in their corresponding SMAs, indicating reduced exciton Eb, which is then confirmed by ultraviolet photoelectron spectroscopy, low-energy inverse photoemission spectroscopy, and film absorption spectra measurements. The measured Eb values of PYFT and PZ1 are 0.24 and 0.37 eV, respectively, smaller than those of Y6 (0.32 eV) and IDIC (0.47 eV). This work not only provides a method to directly monitor the photogenerated free carriers in OSC materials but also demonstrates that polymerization is an effective strategy to reduce the Eb, which is crucial to decrease the energy losses in high-performance OSCs.

Molecular Properties and Aggregation Behavior of Small-Molecule Acceptors Calculated by Molecular Simulation.

The power conversion efficiency of organic solar cells (OSCs) has increased rapidly to over 17% recently. The recent improvement in efficiency was mainly attributed to the development of small-molecule acceptors (SMAs) such as ITIC, Y6, and their derivatives. However, we still have little knowledge on how the molecular structures of the SMAs influence their photovoltaic properties. For the purpose of gaining more insight into the relationship between the molecular properties and photovoltaic performance of the SMAs, here, we carried out theoretical calculations on the most representative SMAs, such as ITIC, Y6, and their derivatives through molecular simulations, and tried to reveal their unique characteristic and aggregation behavior related to the general performance in OSCs, potentially helping to further improve the efficiency of OSCs.

A Cost-Effective Alpha-Fluorinated Bithienyl Benzodithiophene Unit for High-Performance Polymer Donor Material.

To reduce synthetic cost of the classic fluorinated bithienyl benzodithiophene (BDTT-F) unit, here, an alpha-fluorinated bithienyl benzodithiophene unit, namely, α-BDTT-F (F atom in the α position of the lateral thiophene unit), is developed by the isomerization strategy of exchanging the positions of the F atom and flexible alkyl chain on the lateral thiophene unit of the BDTT-F unit. The α-BDTT-F unit was synthesized with less synthetic steps, higher synthetic yield, and less purification times from the same raw materials as those of the BDTT-F unit, thus with low synthetic cost. Theoretical calculation indicates that the α-BDTT-F unit possesses a similar twisted conformation and electronic structures as those of the BDTT-F unit. The α-BDTT-F-based polymer α-PBQ10 exhibits similar light absorption and energy levels as those of the corresponding BDTT-F-based polymer PBQ10 but marginally increased molecular aggregation and stronger hole transport than PBQ10. In consequence, the α-PBQ10:Y6-based polymer solar cell demonstrates a slightly enhanced power conversion efficiency (PCE) of 16.26% compared with that of the PBQ10:Y6-based device (PCE = 16.23%). Also, the PCE is further improved to 16.77% through subtle microscopic morphology regulation of the photoactive layer with the fullerene derivative indene-C60 bisadduct as the third component. This work provides new ideas for the design of low-cost and high-efficiency photovoltaic molecules.

A Quinoxaline-Based D-A Copolymer Donor Achieving 17.62% Efficiency of Organic Solar Cells.

Side-chain engineering has been an effective strategy in tuning electronic energy levels, intermolecular interaction, and aggregation morphology of organic photovoltaic materials, which is very important for improving the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, two D-A copolymers, PBQ5 and PBQ6, are designed and synthesized based on bithienyl-benzodithiophene (BDTT) as the donor (D) unit, difluoroquinoxaline (DFQ) with different side chains as the acceptor (A) unit, and thiophene as the π-bridges. PBQ6 with two alkyl-substituted fluorothiophene side chains on the DFQ units possesses redshifted absorption, stronger intermolecular interaction, and higher hole mobility than PBQ5 with two alkyl side chains on the DFQ units. The blend film of the PBQ6 donor with the Y6 acceptor shows higher and balanced hole/electron mobilities, less charge carrier recombination, and more favorable aggregation morphology. Therefore, the OSC based on PBQ6:Y6 achieves a PCE as high as 17.62% with a high fill factor of 77.91%, which is significantly higher than the PCE (15.55%) of the PBQ5:Y6-based OSC. The PCE of 17.62% is by far one of the highest efficiencies for the binary OSCs with polymer donor and Y6 acceptor.

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

Tandem organic solar cells are based on the device structure monolithically connecting two solar cells to broaden overall absorption spectrum and utilize the photon energy more efficiently. Herein, we demonstrate a simple strategy of inserting a double bond between the central core and end groups of the small molecule acceptor Y6 to extend its conjugation length and absorption range. As a result, a new narrow bandgap acceptor BTPV-4F was synthesized with an optical bandgap of 1.21 eV. The single-junction devices based on BTPV-4F as acceptor achieved a power conversion efficiency of over 13.4% with a high short-circuit current density of 28.9 mA cm-2. With adopting BTPV-4F as the rear cell acceptor material, the resulting tandem devices reached a high power conversion efficiency of over 16.4% with good photostability. The results indicate that BTPV-4F is an efficient infrared-absorbing narrow bandgap acceptor and has great potential to be applied into tandem organic solar cells.

Polymerized small molecular acceptor based all-polymer solar cells with an efficiency of 16.16% via tuning polymer blend morphology by molecular design.

All-polymer solar cells (all-PSCs) based on polymerized small molecular acceptors (PSMAs) have made significant progress recently. Here, we synthesize two A-DA'D-A small molecule acceptor based PSMAs of PS-Se with benzo[c][1,2,5]thiadiazole A'-core and PN-Se with benzotriazole A'-core, for the studies of the effect of molecular structure on the photovoltaic performance of the PSMAs. The two PSMAs possess broad absorption with PN-Se showing more red-shifted absorption than PS-Se and suitable electronic energy levels for the application as polymer acceptors in the all-PSCs with PBDB-T as polymer donor. Cryogenic transmission electron microscopy visualizes the aggregation behavior of the PBDB-T donor and the PSMA in their solutions. In addition, a bicontinuous-interpenetrating network in the PBDB-T:PN-Se blend film with aggregation size of 10~20 nm is clearly observed by the photoinduced force microscopy. The desirable morphology of the PBDB-T:PN-Se active layer leads its all-PSC showing higher power conversion efficiency of 16.16%.

Highly Efficient All-Small-Molecule Organic Solar Cells with Appropriate Active Layer Morphology by Side Chain Engineering of Donor Molecules and Thermal Annealing.

It is very important to fine-tune the nanoscale morphology of donor:acceptor blend active layers for improving the photovoltaic performance of all-small-molecule organic solar cells (SM-OSCs). In this work, two new small molecule donor materials are synthesized with different substituents on their thiophene conjugated side chains, including SM1-S with alkylthio and SM1-F with fluorine and alkyl substituents, and the previously reported donor molecule SM1 with an alkyl substituent, for investigating the effect of different conjugated side chains on the molecular aggregation and the photophysical, and photovoltaic properties of the donor molecules. As a result, an SM1-F-based SM-OSC with Y6 as the acceptor, and with thermal annealing (TA) at 120 °C for 10 min, demonstrates the highest power conversion efficiency value of 14.07%, which is one of the best values for SM-OSCs reported so far. Besides, these results also reveal that different side chains of the small molecules can distinctly influence the crystallinity characteristics and aggregation features, and TA treatment can effectively fine-tune the phase separation to form suitable donor-acceptor interpenetrating networks, which is beneficial for exciton dissociation and charge transportation, leading to highly efficient photovoltaic performance.