A Polycrystalline Polymer Donor as Pre-Aggregate toward Ordered Molecular Aggregation for 19.3% Efficiency Binary Organic Solar Cells

The Role of Solution Aggregation Property toward High-Efficiency Non-Fullerene Organic Photovoltaic Cells

In organic photovoltaic cells, the solution-aggregation effect (SAE) is long considered a critical factor in achieving high power-conversion efficiencies for polymer donor (PD)/non-fullerene acceptor (NFA) blend systems. However, the underlying mechanism has yet to be fully understood. Herein, based on an extensive study of blends consisting of the representative 2D-benzodithiophene-based PDs and acceptor-donor-acceptor-type NFAs, it is demonstrated that SAE shows a strong correlation with the aggregation kinetics during solidification, and the aggregation competition between PD and NFA determines the phase separation of blend film and thus the photovoltaic performance. PDs with strong SAEs enable earlier aggregation evolutions than NFAs, resulting in well-known polymer-templated fibrillar network structures and superior PCEs. With the weakening of PDs' aggregation effects, NFAs, showing stronger tendencies to aggregate, tend to form oversized domains, leading to significantly reduced external quantum efficiencies and fill factors. These trends reveal the importance of matching SAE between PD and NFA. The aggregation abilities of various materials are further evaluated and the aggregation ability/photovoltaic parameter diagrams of 64 PD/NFA combinations are provided. This work proposes a guiding criteria and facile approach to match efficient PD/NFA systems.

Accomplishing High-Performance Organic Solar Sub-Modules (≈55 cm2) with >16% Efficiency by Controlling the Aggregation of an Engineered Non-Fullerene Acceptor

The fabrication of environmentally benign, solvent-processed, efficient, organic photovoltaic sub-modules remains challenging due to the rapid aggregation of the current high performance non-fullerene acceptors (NFAs). In this regard, design of new NFAs capable of achieving optimal aggregation in large-area organic photovoltaic modules has not been realized. Here, an NFA named BTA-HD-Rh is synthesized with longer (hexyl-decyl) side chains that exhibit good solubility and optimal aggregation. Interestingly, integrating a minute amount of new NFA (BTA-HD-Rh) into the PM6:L8-BO system enables the improved solubility in halogen-free solvents (o-xylene:carbon disulfide (O-XY:CS2)) with controlled aggregation is found. Then solar sub-modules are fabricated at ambient condition (temperature at 25 ± 3 °C and humidity: 30-45%). Ultimately, the champion 55 cm2 sub-modules achieve exciting efficiency of >16% in O-XY:CS2 solvents, which is the highest PCE reported for sub-modules. Notably, the highest efficiency of BTA-HD-Rh doped PM6:L8-BO is very well correlated with high miscibility with low Flory-Huggins parameter (0.372), well-defined nanoscale morphology, and high charge transport. This study demonstrates that a careful choice of side chain engineering for an NFA offers fascinating features that control the overall aggregation of active layer, which results in superior sub-module performance with environmental-friendly solvents.

Dilute Donor Organic Solar Cells Based on Non-fullerene Acceptors

The advent of small molecule non-fullerene acceptor (NFA) materials for organic photovoltaic (OPV) devices has led to a series of breakthroughs in performance and device lifetime. The most efficient OPV devices have a combination of electron donor and acceptor materials that constitute the light absorbing layer in a bulk heterojunction (BHJ) structure. For many BHJ-based devices reported to date, the weight ratio of donor to acceptor is near equal. However, the morphology of such films can be difficult to reproduce and manufacture at scale. There would be an advantage in developing a light harvesting layer for efficient OPV devices that contains only a small amount of either the donor or acceptor. In this work we explore low donor content OPV devices composed of the polymeric donor PM6 blended with high performance NFA materials, Y6 or ITIC-4F. We found that even when the donor:acceptor weight ratio was only 1:10, the OPV devices still have good photoconversion efficiencies of around 6% and 5% for Y6 and ITIC-4F, respectively. It was found that neither charge mobility nor recombination rates had a strong effect on the efficiency of the devices. Rather, the overall efficiency was strongly related to the film absorption coefficient and maintaining adequate interfacial surface area between donor and acceptor molecules/phases for efficient exciton dissociation.

Layer-by-Layer Organic Solar Cells Enabled by 1,3,4-Selenadiazole-Containing Crystalline Small Molecule with Double-Fibril Network Morphology

A double-fibril network of the photoactive layer morphology is recognized as an ideal structure facilitating exciton diffusion and charge carrier transport for high-performance organic solar cells (OSCs). However, in the layer-by-layer processed OSCs (LbL-OSCs), polymer donors and small molecule acceptors (SMAs) are separately deposited, and it is challenging to realize a fibril network of pure SMAs with the absence of tight interchain entanglement as polymers. In this work, crystalline small molecule donors (SMDs), named TDZ-3TR and SeDZ-3TR, were designed and introduced into the L8-BO acceptor solution, forcing the phase separation and molecular fibrilization. SeDZ-3TR showed higher crystallinity and lower miscibility with L8-BO acceptor than TDZ-3TR, enabling more driving force to favor the phase separation and better molecular fibrilization of L8-BO. On the other hand, two donor polymers of PM6 and D18 with different fibril widths and lengths were put together to optimize the fibril network of the donor layer. The simultaneously optimization of the acceptor and donor layers resulted in a more ideal double-fibril network of the photoactive layer and an impressive power conversion efficiency (PCE) of 19.38 % in LbL-OSCs.

π-Extended Nonfullerene Acceptor for Compressed Molecular Packing in Organic Solar Cells To Achieve over 20% Efficiency

Organic photovoltaics (OPVs) suffer from a trade-off between efficient charge transport and suppressed nonradiative recombination due to the aggregation-induced luminance quenching of organic semiconductors. To resolve this grand challenge, a π-extended nonfullerene acceptor (NFA) B6Cl with large voids among the honeycomb network is designed and introduced into photovoltaic systems. We find that the presence of a small amount of (i.e., 0.5 or 1 wt %) B6Cl can compress the molecular packing of the host acceptor L8-BO, leading to shortened π-π stacking distance from 3.59 to 3.50 Å (that will improve charge transport) together with ordered alkyl chain packing (that will inhibit nonradiative energy loss due to the suppressed C-C and C-H bonds vibrations), as validated by high-energy X-ray scattering measurements. This morphology transformation ultimately results in simultaneously improved JSC, FF, and VOC of OPVs. As a result, the maximum PCEs of PM6:L8-BO and D18:L8-BO are increased from 19.1 and 19.3% to 19.8 and 20.2%, respectively, which are among the highest values for single-junction OPVs. The university of B6Cl to increase the performance of OPVs is further evidenced in a range of polymer:NFA OPVs.

Electrostatic Potential Design of Solid Additives for Enhanced Molecular Order of Polymer Donor in Efficient Organic Solar Cells

Polymeric semiconducting materials struggle to achieve fast charge mobility due to low structural order. In this work, five 1H-indene-1,3(2H)dione-benzene structured halogenated solid additives namely INB-5F, INB-3F, INB-1F, INB-1Cl, and INB-1Br with gradually varied electrostatic potential are designed and utilized to regulate the structural order of polymer donor PM6. Molecular dynamics simulations demonstrate that although the dione unit of these additives tends to adsorb on the backbone of PM6, the reduced electrostatic potential of the halogen-substituted benzene can shift the benzene interacting site from alkyl side chains to the conjugated backbone of PM6, not only leading to enhanced π-π stacking in out-of-plane but also arising new π-π stacking in in-plane together with the appearance of multiple backbone stacking in out-of-plane, consequent to the co-existence of face-on and edge-on molecular orientations. This molecular packing transformation further translates to enhanced charge transport and suppressed carrier recombination in their photovoltaics, with a maximum power conversion efficiency of 19.4% received in PM6/L8-BO layer-by-layer deposited organic solar cells.

Impact of length of branched alkyl side chains on thiazolothiazole-based small molecular acceptors in non-fullerene polymer solar cells

It has been reported that the length of branched alkyl side chains on fused-ring electron acceptors confers different impacts on properties versus solubility of BJH blends. However, because this impact on a non-fused acceptor backbone has rarely been studied, we examined the impact of molecular optimization from alkyl chain tuning based on non-fused thiazolothiazole small-molecule acceptors. The length of the side chain on the thiophene bridge was modified from 2-butyloctyl to 2-ethylhexyl, which corresponds to small molecules TTz3(C4C6) and TTz3(C2C4), respectively. Compared with the reported TTz3(C6C8) with long alkyl side chains, TTz3(C4C6) and TTz3(C2C4) exhibited stronger molecular aggregation, higher absorption coefficients, and greater redshifted UV absorption. Unexpectedly, after the alkyl chain was slightly shortened in this type of acceptor system, devices were successfully fabricated, but it was necessary to reduce the blending concentration at low rotation speeds due to the sharp decrease in the solubility of corresponding acceptor materials. Thus, the obtained unfavorable thickness and morphology of the active layer caused a decrease in Jsc and FF. As a consequence, TTz3(C4C6)- and TTz3(C2C4)-based devices showed an unsatisfactory power conversion efficiency of 6.02% and 2.71%, respectively, when donors were paired with the wide bandgap donor J71, which is inferior to that of TTz3(C6C8)-based devices (8.76%). These results indicate that it is challenging to determine the limit of the adjustable range of side chains to modify non-fused thiazolothiazole small-molecule acceptors for high-performance non-fullerene solar cells.

Non-Covalent Interaction Enhancement on Active/Interfacial Layers via Two-Dimensional Vermiculite Doping for Efficient Organic Solar Cells

Interface modification plays an important role in improving the power conversion efficiency (PCE) of organic solar cells (OSCs). However, the low non-covalent interaction between the cathode interface layer (CIL) and nonfullerene acceptor (NFA) directly affects the charge collection of OSCs. Here, the non-covalent interaction between the CIL and NFA is enhanced by introducing the 2D vermiculite (VML) in the poly(9,9-bis(3'-(N,N-dimethyl)-Nethylammonium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) dibromide (PFN-Br) interface layer to form an efficient electron transport channel. As a result, the electron extraction efficiency from the active layer to the CIL is increased, and the PCE of OSCs based on PBDB-T:ITIC is boosted from 10.87% to 12.89%. In addition, the strategy of CIL doping VML is proven to be universal in different CIL materials, for which the PCE is boosted from 10.21% to 11.57% for OSCs based on PDINN and from 9.82% to 11.27% for OSCs based on PNDIT-F3N. The results provide a viable option for designing efficient CIL for high-performance non-fullerene OSCs, which may promote the commercialization of OSCs.

Alkoxythiophene-Directed Fibrillization of Polymer Donor for Efficient Organic Solar Cells

Realizing fibrillar molecular framework is highly encouraged in organic solar cells (OSCs) due to the merit of efficient charge carrier transport. This is however mainly achieved via the chemical structural design of photovoltaic semiconductors. In this work, through the utilization of three alkoxythiophene additives, T-2OMe, T-OEH, and T-2OEH, the intermolecular interactions among a series of BDT-type polymer donors, i.e., PM6, D18, PBDB-T, and PTB7-Th, are tuned to self-assemble into nanofibrils during solution casting. X-ray technique and molecular dynamics simulation reveal that the alkoxythiophene with (2-ethylhexyl)oxy (─OEH) chains can attach on the 2-ethylhexyl (EH) chains of these polymer donors and promote their self-assembly into 1D nanofibrils, in their neat films as well as photovoltaic blends with L8-BO. By adapting these fibrillar polymer donors to construct pseudo-bulk heterojunction (P-BHJ) OSCs via layer-by-layer deposition, generally improved device performance is seen, with power conversion efficiencies enhanced from 18.2% to 19.2% (certified 18.96%) and from 17.9% to 18.7% for the PM6/L8-BO and D18/L8-BO devices, respectively. This work provides a physical approach to promote the fibrillar charge transport channels for efficient photovoltaics.

Highly Efficient and Stable ITO-Free Organic Solar Cells Based on Squaraine N-Doped Quaternary Bulk Heterojunction

Simultaneously achieving high efficiency and robust device stability remains a significant challenge for organic solar cells (OSCs). Solving this challenge is highly dependent on the film morphology of the bulk heterojunction (BHJ) photoactive blends; however, there is a lack of rational control strategy. Herein, it is shown that the molecular crystallinity and nanomorphology of nonfullerene-based BHJ can be effectively controlled by a squaraine-based doping strategy, leading to an increase in device efficiency from 17.26% to 18.5% when doping 2 wt% squaraine into the PBDB-TF:BTP-eC9:PC71 BM ternary BHJ. The efficiency is further improved to 19.11% (certified 19.06%) using an indium-tin-oxide-free column-patterned microcavity (CPM) architecture. Combined with interfacial modification, CPM quaternary OSC excitingly shows an extrapolated lifetime of ≈23 years based on accelerated aging test, with the mechanism behind enhanced stability well studied. Furthermore, a flexible OSC module with a high and stable efficiency of 15.2% and an overall area of 5 cm2 is successfully fabricated, exhibiting a high average output power for wearable electronics. This work demonstrates that OSCs with new design of BHJ and device architecture are highly promising to be practical relevance with excellent performance and stability.

Optimized Crystal Framework by Asymmetric Core Isomerization in Selenium-Substituted Acceptor for Efficient Binary Organic Solar Cells

Both the regional isomerization and selenium-substitution of the small molecular acceptors (SMAs) play significant roles in developing efficient organic solar cells (OSCs), while their synergistic effects remain elusive. Herein, we developed three isomeric SMAs (S-CSeF, A-ISeF, and A-OSeF) via subtly manipulating the mono-selenium substituted position (central, inner, or outer) and type of heteroaromatic ring on the central core by synergistic strategies for efficient OSCs, respectively. Crystallography of asymmetric A-OSeF presents a closer intermolecular π-π stacking and more ordered 3-dimensional network packing and efficient charge-hopping pathways. With the successive out-shift of the mono-selenium substituted position, the neat films give a slightly wider band gap and gradually higher crystallinity and electron mobility. The PM1 : A-OSeF afford favourable fibrous phase separation morphology with more ordered molecular packing and efficient charge transportation compared to the other two counterparts. Consequently, the A-OSeF-based devices achieve a champion efficiency of 18.5 %, which represents the record value for the reported selenium-containing SMAs in binary OSCs. Our developed precise molecular engineering of the position and type of selenium-based heteroaromatic ring of SMAs provides a promising synergistic approach to optimizing crystal stacking and boosting top-ranked selenium-containing SMAs-based OSCs.