Efficient Non-fullerene Solar Cells Research Articles

Efficient Non-Fullerene Solar Cells Enabled by a Temperature-Dependent Terpolymer with Controlled Aggregation and Orientation

Light harvesting, molecular aggregation, and orientation of photoactive materials are crucial for non-fullerene polymer solar cells. Random copolymerization is a feasible and effective way to modify the molecular structure of polymer donors. However, it is very challenging to transfer the edge-on orientation of oligothiophene-based polymers to face-on orientation. Herein, we employed random copolymerization strategy by incorporating a specific ratio of a benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD)-based monomer into the backbone of a temperature-dependent polymer PFBT4T to improve the device performance. The resulting terpolymer donor PFBT4T-T20 with a comparable synthetic complexity possesses a dominant face-on orientation and weakened aggregations in solution and film, which are different from the parent polymer PFBT4T. When blended with acceptor Y14, the PFBT4T-T20-based device exhibits a more balanced carrier transport and optimal morphology and then achieved a higher efficiency of 15.60%. Our results reveal that the terpolymer strategy involving an extra component of the C2v symmetric structure can revive the oligothiophene-based conjugated polymer toward efficient non-fullerene solar cells.

Design of simple-structure wide-bandgap conjugated polymers based on BDT for efficient non-fullerene solar cells

The recent evolution of non-fullerene acceptors yields brilliant progress for organic solar cells (OSCs). However, high-performance copolymer donors with simple chemical structures for new acceptors are a few. In this work, a series of donors based on BDT are developed with ester-substituted trithiophene as a repeating acceptor unit. Copolymerized polymers exhibit slightly red-shifted absorption and down-shifted the highest occupied molecular orbital (HOMO) as the side chains turning from alkyl (3TCO-R) and oxyalkyl (3TCO-OR) to thienyl (3TCO-Th) and chlorine-substituted thienyl (3TCO-Th-Cl), respectively. As a result, the corresponding power conversion efficiency (PCE) of 3TCO-Th-Cl based devices shows the most excellent performance (PCE = 7.70%) especially in open-circuit voltage (VOC = 1 V) when blending with ITIC and its PCE makes a breakthrough with Y6 (PCE = 10.30%). Moreover, the fabricated semitransparent OSCs exhibit a rational PCE of 8.16% with average visible transparency of 16.40%.

Small bandgap non-fullerene acceptor enables efficient PTB7-Th solar cell with near 0 eV HOMO offset

Three small bandgap non-fullerene (SBG NFAs) acceptors, BDTI, BDTI-2F and BDTI-4F, based on a carbon-oxygen bridged central core and thieno[3,4-b]thiophene linker, end-capped with varied electron-withdrawing terminal groups, were designed and synthesized. The acceptors exhibit strong absorption from 600 nm to 1000 nm. The optimal device incorporating designed NFA and PTB7-Th polymer donor achieves a power conversion efficiency of 9.11% with near 0 eV HOMO offset. The work presents a case study of efficient non-fullerene solar cells with small HOMO offsets, which is achieved by blending PTB7-Th with fine-tuned SBG acceptor.

Molecular design towards two-dimensional electron acceptors for efficient non-fullerene solar cells

Non-fullerene polymer solar cells (NF-PSCs) have gained wide attention recently. Molecular design of non-fullerene electron acceptors effectively promotes the photovoltaic performance of NF-PSCs. However, molecular electron acceptors with 2-dimensional (2D) configuration and conjugation are seldom reported. Herein, we designed and synthesized a series of novel 2D electron acceptors for efficient NF-PSCs. With rational optimization on the conjugated moieties in both vertical and horizontal direction, these 2D electron acceptors showed appealing properties, such as good planarity, full-spectrum absorption, high absorption extinction coefficient, and proper blend morphology with donor polymer. A high PCE of 9.76% was achieved for photovoltaic devices with PBDB-T as the donor and these 2D electron acceptors. It was also found the charge transfer between the conjugated moieties in two directions of these 2D molecules contributes to the utilization of absorbed photos, resulting in an exceptional EQE of 87% at 730 nm. This work presents rational design guidelines of 2D electron acceptors, which showed great promise to achieve high-performance non-fullerene polymer solar cells.

Design of New n-Type Porphyrin Acceptors with Subtle Side-Chain Engineering for Efficient Nonfullerene Solar Cells with Low Energy Loss and Optoelectronic Response Covering the Near-Infrared Region.

A series of tailor-made highly efficient and near-infrared (NIR) porphyrin-based acceptors is designed and synthesized for fullerene-free bulk-heterojunction (BHJ) organic solar cells. Constructing BHJ active layers using a PTB7-Th donor and porphyrin acceptors (P-x), which have complementary absorption, accomplishes panchromatic photon-to-current conversion from 300 to 950 nm. Our study shows that side chains of the porphyrin acceptors fairly influence the molecular ordering and nanomorphology of the BHJ active layers. Significantly, the porphyrin acceptor with four dodecoxyl side chains (P-2) achieves an open-circuit voltage (VOC) of 0.80 V, short-circuit current density (JSC) of 13.94 mA cm-2, fill factor of 64.8%, and overall power conversion efficiency of 7.23%. This great performance is attributable to the ascendant light-harvesting capability in the visible and near-infrared region, a high-lying LUMO energy level, a relatively high and more balanced carrier mobilities, and more ordered face-on molecular packing, which is beneficial for obtaining high VOC and JSC.

Sequential Deposition of Organic Films with Eco-Compatible Solvents Improves Performance and Enables Over 12%-Efficiency Nonfullerene Solar Cells.

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.

Sequential Deposition of Organic Films with Eco-Compatible Solvents Improves Performance and Enables Over 12%-Efficiency Nonfullerene Solar Cells

Sequential Deposition of Organic Films with Eco-Compatible Solvents Improves Performance and Enables Over 12%-Efficiency Nonfullerene Solar Cells

Polymer Semiconductors: Phthalimide‐Based High Mobility Polymer Semiconductors for Efficient Nonfullerene Solar Cells with Power Conversion Efficiencies over 13% (Adv. Sci. 2/2019)

In article number 1801743, Xugang Guo and co-workers develop two phthalimide-based polymers featuring a D-A1-D-A2 backbone motif. Eliminating benzodithiophene leads to polymers with substantial mobility. Nonfullerene polymer solar cells utilizing these high-mobility polymers achieve a remarkable power conversion efficiency >13%. The results demonstrate that phthalimides are excellent building blocks for enabling polymer semiconductors with outstanding solar cell performances and benzodithiophenes are not necessary for constructing such polymers.

Ladder-type dithienocyclopentadibenzothiophene-cored wide-bandgap polymers for efficient non-fullerene solar cells with large open-circuit voltages

Novel wide-bandgap copolymers based on ladder-type dithienocyclopentadibenzothiophene were developed for polymer solar cells with 9.46% efficiency and excellent stability.

Phthalimide-Based High Mobility Polymer Semiconductors for Efficient Nonfullerene Solar Cells with Power Conversion Efficiencies over 13.

Highly efficient nonfullerene polymer solar cells (PSCs) are developed based on two new phthalimide‐based polymers phthalimide‐difluorobenzothiadiazole (PhI‐ffBT) and fluorinated phthalimide‐ffBT (ffPhI‐ffBT). Compared to all high‐performance polymers reported, which are exclusively based on benzo[1,2‐b:4,5‐b′]dithiophene (BDT), both PhI‐ffBT and ffPhI‐ffBT are BDT‐free and feature a D‐A1‐D‐A2 type backbone. Incorporating a second acceptor unit difluorobenzothiadiazole leads to polymers with low‐lying highest occupied molecular orbital levels (≈−5.6 eV) and a complementary absorption with the narrow bandgap nonfullerene acceptor IT‐4F. Moreover, these BDT‐free polymers show substantially higher hole mobilities than BDT‐based polymers, which are beneficial to charge transport and extraction in solar cells. The PSCs containing difluorinated phthalimide‐based polymer ffPhI‐ffBT achieve a substantial PCE of 12.74% and a large V oc of 0.94 V, and the PSCs containing phthalimide‐based polymer PhI‐ffBT show a further increased PCE of 13.31% with a higher J sc of 19.41 mA cm−2 and a larger fill factor of 0.76. The 13.31% PCE is the highest value except the widely studied BDT‐based polymers and is also the highest among all benzothiadiazole‐based polymers. The results demonstrate that phthalimides are excellent building blocks for enabling donor polymers with the state‐of‐the‐art performance in nonfullerene PSCs and the BDT is not necessary for constructing such donor polymers.

Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination

Nonfullerene solar cells have increased their efficiencies up to 13%, yet quantum efficiencies are still limited to 80%. Here we report efficient nonfullerene solar cells with quantum efficiencies approaching unity. This is achieved with overlapping absorption bands of donor and acceptor that increases the photon absorption strength in the range from about 570 to 700 nm, thus, almost all incident photons are absorbed in the active layer. The charges generated are found to dissociate with negligible geminate recombination losses resulting in a short-circuit current density of 20 mA cm−2 along with open-circuit voltages >1 V, which is remarkable for a 1.6 eV bandgap system. Most importantly, the unique nano-morphology of the donor:acceptor blend results in a substantially improved stability under illumination. Understanding the efficient charge separation in nonfullerene acceptors can pave the way to robust and recombination-free organic solar cells.

Post-Treatment-Free Main Chain Donor and Side Chain Acceptor (D-s-A) Copolymer for Efficient Nonfullerene Solar Cells with a Small Voltage Loss.

Main chain donor and side chain acceptor (D-s-A) copolymers are an important branch of the D-A copolymer family. However, the development of D-s-A copolymers significantly falls behind the alternative D-A copolymers, especially for organic solar cells, because a breakthrough in device performance is not yet obtained with a reported power conversion efficiency (PCE) of 2%-4%. Herein, a newly developed D-s-A copolymer PDRCNBDT, bearing 2-(1, 1-dicyanomethylene) rhodanine pendant group as the donor material, delivers a high PCE of 5.3% for nonfullerene solar cells. To the best of our knowledge, this is the best value reported for D-s-A copolymers to date. The improved PCE is observed to be associated with a very small energy loss (Eloss ) of 0.57 eV, accompanied by a high open-circuit voltage (Voc ) of 1.015 eV. It is important to note that this efficient D-s-A copolymer is employed in organic solar cells (OSCs), free of additive and annealing treatments.

Terthieno[3,2‐b]Thiophene (6T) Based Low Bandgap Fused‐Ring Electron Acceptor for Highly Efficient Solar Cells with a High Short‐Circuit Current Density and Low Open‐Circuit Voltage Loss

AbstractA terthieno[3,2‐b]thiophene (6T) based fused‐ring low bandgap electron acceptor, 6TIC, is designed and synthesized for highly efficient nonfullerene solar cells. The chemical, optical, and physical properties, device characteristics, and film morphology of 6TIC are intensively studied. 6TIC shows a narrow bandgap with band edge reaching 905 nm due to the electron‐rich π‐conjugated 6T core and reduced resonance stabilization energy. The rigid, π‐conjugated 6T also offers lower reorganization energy to facilitate very low VOC loss in the 6TIC system. The analysis of film morphology shows that PTB7‐Th and 6TIC can form crystalline domains and a bicontinuous network. These domains are enlarged when thermal annealing is applied. Consequently, the device based on PTB7‐Th:6TIC exhibits a high power conversion efficiency (PCE) of 11.07% with a high JSC > 20 mA cm−2 and a high VOC of 0.83 V with a relatively low VOC loss (≈0.55 V). Moreover, a semitransparent solar cell based on PTB7‐Th:6TIC exhibits a relatively high PCE (7.62%). The device can have combined high PCE and high JSC is quite rare for organic solar cells.

Asymmetric Quinoxaline-Based Polymer for High Efficiency Non-Fullerene Solar Cells

Asymmetric Quinoxaline-Based Polymer for High Efficiency Non-Fullerene Solar Cells

A systematical investigation of non-fullerene solar cells based on diketopyrrolopyrrole polymers as electron donor

Diketopyrrolopyrrole (DPP)-based conjugated polymers have been successfully applied in high performance field-effect transistors and fullerene-based solar cells, but show limited application in non-fullerene solar cells. In this work, we use four DPP polymers as electron donor and a perylene bisimide dye as electron acceptor to construct non-fullerene solar cells. The donors and acceptor have complementary absorption spectra in visible and near-infrared region, resulting in broad photo-response from 300 nm to 1000 nm. The solar cells were found to provide relatively low power conversion efficiencies of 1.6–2.6%, which was mainly due to low photocurrent and fill factor. Further investigation reveals that the low performance is originated from the high charge recombination in photo-active layers. Our systematical studies will help better understand the non-fullerene solar cells based on DPP polymers and inspire new researches toward efficient non-fullerene solar cells with broad photo-response.