Narrow-bandgap Acceptor Research Articles

Over 17.4% Efficiency of Layer‐by‐Layer All‐Polymer Solar Cells by Improving Exciton Utilization in Acceptor Layer

AbstractLayer‐by‐layer all‐polymer solar cells (LbL all‐PSCs) are prepared with PM6 and PY‐IT by using sequential spin coating method. The exciton dissociation efficiency in acceptor layer near electrode is rather low due to the limited exciton diffuse distance and impossible energy transfer from narrow bandgap acceptor to wide bandgap donor. In this study, less PM6 is incorporated into PY‐IT layer to enhance exciton dissociation in PY‐IT layer near electrode. A power conversion efficiency (PCE) of 17.45% is achieved in the LbL all‐PSCs incorporating 10 wt% PM6 into PY‐IT layer, which is much larger than 16.04% PCE of PM6/PY‐IT‐based LbL all‐PSCs. Over 8% PCE enhancement can be realized by incorporating 10 wt% PM6 into PY‐IT layer, which is attributed to the enhanced exciton utilization efficiency in PY‐IT layers near electrode. The enhanced exciton utilization efficiency in PY‐IT layer can be confirmed from the quenched photoluminescence (PL) emission in PY‐IT:PM6 films. Meanwhile, charge transport in acceptor layers can be optimized by incorporating less PM6, as confirmed from the optimized molecular arrangement. This study indicates that the strategy of incorporating less donor into acceptor layer has great potential in fabricating efficient LbL all‐PSCs by improving exciton utilization efficiency in acceptor layer near electrode.

A narrow band gap non-fullerene electron acceptor based on a dithieno-3,2-b:2',3'-dlpyrrole unit for high performance organic solar cells with minimal highest occupied molecular orbital offset.

Here, a new narrow band gap non-fullerene small molecular acceptor (NFSMA) based on a dithieno-3,2-b:2',3'-dlpyrrole(DTP) unit, namely SNIC-F, was designed and synthesized. Due to the strong electron-donating ability of the DTP-based fused-ring core, SNIC-F showed a strong intramolecular-charge transfer (ICT) effect and thus gave a narrow band gap of 1.32 eV. Benefiting from the low band gap and efficient charge separation, when pairing with a copolymer PBTIBDTT, the device optimized by 0.5% 1-CN gave a high short circuit current (Jsc) of 19.64 mA cm-2. In addition, a high open-circuit voltage (Voc) of 0.83 V was obtained due to the near 0 eV highest occupied molecular orbital (HOMO) offset between PBTIBDTT and SNIC-F. As a result, a high power conversion efficiency (PCE) of 11.25% was obtained, and the PCE was maintained above 9.2% as the active layer thickness increased from 100 nm to 250 nm. Our work indicated that designing a narrow band gap NFSMA-based DTP unit and blending it with a polymer donor with small HOMO offset is an efficient strategy for achieving high performance OSCs.

Photovoltaic properties and enhancement in near-infrared light absorption capabilities of acceptor materials for organic solar cell applications: A quantum chemical perspective via DFT

More than 22% power conversion efficiency (PCE) of organic solar cells (OSCs) has been reported with efficient narrow bandgap acceptor materials as the active layers. And end-capped modifications of such acceptor materials are an efficient approach for designing highly stable and efficient materials for organic solar cell applications. In this study, four new molecules (TOS1 to TOS4) are designed for the active layer of organic solar cells. Density functional theory and time-dependent functional theory have been employed for the computation of various geometric, photovoltaic, optoelectronic properties of newly designed molecules. Different analyses like frontier molecular orbitals, transition density matrix, open-circuit voltage, absorption maxima, excitation and binding energies have been performed and examined with great care. Further, reorganization energy of hole and electron of novel designed molecules has been calculated and results suggested that designed systems are effective contributors for future development of organic solar cells. Overall, the outcomes of this study urge the experimentalists for the future development of highly stable and near-infrared absorbing organic solar cells.

Novel Pyrrolo [3,4-b] Dithieno [3, 2-f:2″,3″-h] Quinoxaline-8,10 (9H)-Dione Based Wide Bandgap Conjugated Copolymers for Bulk Heterojunction Polymer Solar Cells.

Two D-A copolymers consisting of fused ring pyrrolo-dithieno-quinoxaline acceptors are synthesized with different donor units, i.e., benzodithiophene (BDT) with alkylthienyl (P134) and 2-ethylhexyloxy (P117) side chains. These copolymers are used as donors and a narrow bandgap acceptor Y6 to fabricate bulk heterojunction polymer solar cell devices. Owing to the strong electron-deficient fused ring pyrrolo-bithieno-quinoxaline and weak alkyl thienyl side chains in BDT, the polymer solar cells (PSCs) based on P134:Y6 attain the power conversion efficiency (PCE) of 15.42%, which is higher than the P117:Y6 counterpart (12.14%). The superior value of PCE for P134:Y6 can be associated with more well-adjusted charge transport, weak charge recombination, proficient exciton generation, and dissociation into free charge carriers and their subsequent charge collection owing to the dense π-π stacking distance and more considerable crystal coherence length for the P134:Y6 thin films. This investigation confirms the great potential of a strong acceptor-weak donor tactic for developing efficient D-A copolymers consists of quinoxaline acceptor for PSCs.

Effects of Heteroatom Substitution on the Photovoltaic Performance of Donor Materials in Organic Solar Cells

ConspectusOver the past few years, the innovation of narrow bandgap acceptor combined with wide bandgap donor materials significantly promotes the power conversion efficiencies (PCEs) of organic solar cells (OSCs) to exceed 18%. To build a state-of-the-art OSC, absorption spectra, frontier molecular orbital energy levels, molecular packing and crystallinity, and charge carrier mobilities of the photovoltaic materials should be considered in their molecular design. The donor and acceptor materials are the key components determining the photovoltaic performance of the OSCs. The side chain engineering on the conjugated backbone is a critical strategy to optimize the photovoltaic properties of the donor materials. In this Account, we focus on the topic of heteroatom substitution on the molecular backbone of the donor materials for improving their photovoltaic performance, aiming to provide in-depth understanding of the molecular structure optimization for the design of state-of-the-art photovoltaic materials.First, we highlight the halogen (fluorine and chlorine) atom substitution strategies applied on the conjugated molecular backbone of the organic photovoltaic materials. The strong electronegativity of halogen atoms can downshift the highest occupied molecular orbital (HOMO) energy levels of the donor materials, which could increase open-circuit voltages of the resulting OSCs. In addition, the hydrogen bonding aroused by the halogen atoms is beneficial to improve their charge transport property and crystallinity of the organic semiconductors. On the other hand, flexible side chains are critical components for improving the solubility of the photovoltaic materials. Using the flexible side chains of alkylthio and alkylsilyl is an easy and effective approach to tune the electronic energy levels and absorption spectra of the photovoltaic materials. Due to the formation of pπ(C)–dπ(S) orbital overlap, the empty 3d-orbitals of the sulfur atom in the alkylthio substituents can accept the π-electron of conjugated skeleton to modulate the optical and electrical properties of the photovoltaic materials. Similarly, the silicon atom in the alkylsilyl side chains can stabilize the lowest unoccupied molecular orbital (LUMO) level and downshift the HOMO level of the organic semiconductor, which leads to the improved photovoltaic performance of the organic donor materials. Finally, we briefly discussed the challenges for the photovoltaic materials toward performance optimization and practical application of the OSCs.

N-alkyl chain modification in dithienobenzotriazole unit enabled efficient polymer donor for high-performance non-fullerene solar cells

Molecular design of either polymer donors or acceptors is a promising strategy to tune the morphology of the active layer of organic solar cells, enabling a high-performance device. Thereinto, developing novel polymer donors is an alternative method to obtain high photovoltaic performance. Herein, we present a facile side-chain engineering on the dithiophenobenzotriazole (DTBTz) unit of newly-designed polymer donors (named pBDT-DTBTz-EH and pBDT-DTBTz-Me) to boost the performance of non-fullerene solar cells. Compared with pBDT-DTBTz-EH with long N-alkyl side chains, pBDT-DTBTz-Me with a short methyl exhibits stronger molecular aggregation, higher absorption coefficient, and preferred face-on orientation packing. As a consequence, pBDT-DTBTz-Me based devices achieve an optimal power conversion efficiency of 15.31% when donors are paired with the narrow bandgap acceptor Y6, which is superior to that of pBDT-DTBTz-EH based devices (9.17%). Additionally, the pBDT-DTBTz-Me based devices manifest more effective charge separation and transfer than pBDT-DTBTz-EH based devices. These results indicate that fine-tuning side chains of polymer donors provide new insights for the design of high-performance polymer donors in non-fullerene solar cells.

Semi-transparent organic solar cells with high visible transmission enabled by a transparent wide-bandgap donor

Abstract The reported semi-transparent organic solar cells (ST-OSCs) are commonly based on a middle bandgap donor and narrow bandgap acceptor blend, for example, the PTB7-Th:Y6 blend. However, the strong absorption of PTB7-Th in the visible region hinders the further improvement of average visible transmission (AVT) for the ST-OSCs. Here we report a new kind of all-small-molecule ST-OSCs based on TAPC:Y6 blend. The used donor TAPC has little absorption in the visible region, which is beneficial for attaining high transparency. The optimized ST-OSCs based on TAPC:Y6 blend realize a power conversion efficiency of 3.01%. Importantly, a high AVT of 47.99% is achieved for this kind of ST-OSCs, much higher than those of reported ST-OSCs. The proposed TAPC:Y6 ST-OSCs also have a good color perception, whose CIE 1931 coordinates are (0.292,0.313) and color rendering index is 94.5.

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.

Enabling High-Performance Tandem Organic Photovoltaic Cells by Balancing the Front and Rear Subcells.

In tandem organic photovoltaics, the front subcell is based on large-bandgap materials, whereas the case of the rear subcell is more complicated. The rear subcell is generally composed of a narrow-bandgap acceptor for infrared absorption but a large-bandgap donor to realize a high open-circuit voltage. Unfortunately, most of the ultraviolet-visible part of the photons are absorbed by the front subcell; as a result, in the rear subcell, the number of excitons generated on large-bandgap donors will be reduced significantly. This reduces the (photo) conductivity and finally limits the hole-transporting property of the rear subcell. In this work, a simple and effective way is proposed to resolve this critical issue. To ensure sufficient photogenerated holes in the rear subcell, a small amount of an infrared-absorbing polymer donor as a third component is introduced, which provides a second hole-generation and transporting mechanism to minimize the aforementioned detrimental effects. Finally, the short-circuit current density of the two-terminal tandem organic photovoltaic is significantly enhanced from 10.3 to 11.7mAcm-2 (while retaining the open-circuit voltage and fill factor) to result in an enhanced power conversion efficiency of 15.1%.

Dithienogermole-Based Nonfullerene Acceptors: Roles of the Side-Chains’ Direction and Development of Green-Tinted Efficient Semitransparent Organic Solar Cells

Inspired by the encouraging properties of Ge-fused heterocyclic dithienogermole (DTG) in optoelectronic applications, we here report two narrow-bandgap acceptor–donor′–donor–donor′–acceptor (A–D′–D...

Significantly Improved Morphology and Efficiency of Nonhalogenated Solvent-Processed Solar Cells Derived from a Conjugated Donor-Acceptor Block Copolymer.

A highly crystalline conjugated donor (D)–acceptor (A) block copolymer (PBDT2T‐b‐N2200) that has good solubility in nonhalogenated solvents is successfully synthesized. PBDT2T‐b‐N2200 shows a broad complementary absorption behavior owing to a wide‐band gap donor (PBDT2T) present as a D‐block and a narrow‐band gap acceptor (N2200) present as an A‐block. Polymer solar cells (PSCs) with conjugated block copolymer (CBCP) are fabricated using a toluene solution and PSC created with an annealed film showing the highest power conversion efficiency of 6.43%, which is 2.4 times higher than that made with an annealed blend film of PBDT2T and N2200. Compared to the blend film, the PBDT2T‐b‐N2200 film exhibits a highly improved surface and internal morphology, as well as a faster photoluminescence decay lifetime, indicating a more efficient photoinduced electron transfer. In addition, the PBDT2T‐b‐N2200 film shows high crystallinity through an effective self‐assembly of each block during thermal annealing and a predominant face‐on chain orientation favorable to a vertical‐type PSC. Moreover, the CBCP‐based PSCs exhibit an excellent shelf‐life time of over 1020 h owing to their morphological stability. From these results, a D–A block copolymer system is one of the efficient strategies to improve miscibility and morphological stability in all polymer blend systems.

A New Wide Bandgap Donor Polymer for Efficient Nonfullerene Organic Solar Cells with a Large Open-Circuit Voltage.

Significant progress has been made in nonfullerene small molecule acceptors (NF‐SMAs) that leads to a consistent increase of power conversion efficiency (PCE) of nonfullerene organic solar cells (NF‐OSCs). To achieve better compatibility with high‐performance NF‐SMAs, the direction of molecular design for donor polymers is toward wide bandgap (WBG), tailored properties, and preferentially ecofriendly processability for device fabrication. Here, a weak acceptor unit, methyl 2,5‐dibromo‐4‐fluorothiophene‐3‐carboxylate (FE‐T), is synthesized and copolymerized with benzo[1,2‐b:4,5‐b′]dithiophene (BDT) to afford a series of nonhalogenated solvent processable WBG polymers P1‐P3 with a distinct side chain on FE‐T. The incorporation of FE‐T leads to polymers with a deep highest occupied molecular orbital (HOMO) level of −5.60−5.70 eV, a complementary absorption to NF‐SMAs, and a planar molecular conformation. When combined with the narrow bandgap acceptor ITIC‐Th, the solar cell based on P1 with the shortest methyl chain on FE‐T achieves a PCE of 11.39% with a large V oc of 1.01 V and a J sc of 17.89 mA cm−2. Moreover, a PCE of 12.11% is attained for ternary cells based on WBG P1, narrow bandgap PTB7‐Th, and acceptor IEICO‐4F. These results demonstrate that the new FE‐T is a highly promising acceptor unit to construct WBG polymers for efficient NF‐OSCs.

High-Performance Polymer Solar Cell with Single Active Material of Fully Conjugated Block Copolymer Composed of Wide-Band gap Donor and Narrow-Band gap Acceptor Blocks.

We synthesized a novel fully conjugated block copolymer, P3, in which a wide-band gap donor block (P1) was connected to a narrow-band gap acceptor block (P2). As P3 contains P1 block with a wide bandgap and P2 block with a narrow bandgap, it exhibits a very wide complementary absorption. Transient photoluminescence measurement using P3 dilute solution demonstrated intramolecular charge transfer between the P1 block and the P2 block, which was not observed in a P1/P2 blend solution. A P3 thin film showed complete PL quenching because the photoinduced inter-/intramolecular charge transfer states were effectively formed. This phenomenon can play an important role in the photovoltaic properties of P3-based polymer solar cells. A single active material polymer solar cell (SAMPSC) fabricated from P3 alone exhibited a high power conversion efficiency (PCE) of 3.87% with a high open-circuit voltage of 0.93 V and a short-circuit current of 8.26 mA/cm2, demonstrating a much better performance than a binary P1-/P2-based polymer solar cell (PCE = 1.14%). This result facilitates the possible improvement of the photovoltaic performance of SAMPSCs by inducing favorable nanophase segregation between p- and n blocks. In addition, owing to the high morphological stability of the block copolymer, excellent shelf-life was observed in a P3-based SAMPSC compared with a P1/P2-based PSC.