Guangye Zhang

A Wide-Bandgap Donor Polymer for Highly Efficient Non-fullerene Organic Solar Cells with a Small Voltage Loss

To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) small-molecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our non-fullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss.

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

An All-Solution Processed Recombination Layer with Mild Post-Treatment Enabling Efficient Homo-Tandem Non-fullerene Organic Solar Cells.

The first homo-tandem non-fullerene organic solar cell enabled by a novel recombination layer which only requires a very mild thermal annealing treatment is reported. The best efficiency achieved is 10.8% with a Voc over 2.1 V, which is the highest Voc for double-junction organic solar cells reported to date.

Design of Donor Polymers with Strong Temperature-Dependent Aggregation Property for Efficient Organic Photovoltaics

Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted intensive research attention over the past two decades owing to their unique advantages including mechanical flexibility, light weight, large area, and low-cost fabrications. To date, OSC devices have achieved power conversion efficiencies (PCEs) exceeding 12%. Much of the progress was enabled by the development of high-performance donor polymers with favorable morphological, electronic, and optical properties. A key problem in morphology control of OSCs is the trade-off between achieving small domain size and high polymer crystallinity, which is especially important for the realization of efficient thick-film devices with high fill factors. For example, the thickness of OSC blends containing state-of-the-art PTB7 family donor polymers are restricted to ∼100 nm due to their relatively low hole mobility and impure polymer domains. To further improve the device performance and promote commercialization of OSCs, there is a strong demand for the design of new donor polymers that can achieve an optimal blend morphology containing highly crystalline yet reasonably small domains. In this Account, we highlight recent progress on a new family of conjugated polymers with strong temperature-dependent aggregation (TDA) property. These polymers are mostly disaggregated and can be easily dissolved in solution at high temperatures, yet they can strongly aggregate when the solution is cooled to room temperature. This unique aggregation property allows us to control the disorder-order transition of the polymer during solution processing. By preheating the solution to high temperature (∼100 °C), the polymer chains are mostly disaggregated before spin coating; as the temperature of the solution drops during the spin coating process, the polymer can strongly aggregate and form crystalline domains yet that are not excessivelylarge. The overall blend morphology can be optimized by various processing conditions (e.g., temperature, spin-rates, concentration, etc.). This well-controlled and near-optimal BHJ morphology produced over a dozen cases of efficient OSCs with an active layer nearly 300 nm thick that can still achieve high FFs (70-77%) and efficiencies (10-11.7%). By studying the structure-property relationships of the donor polymers, we show that the second position branched alkyl chains and the fluorination on the polymer backbone are two key structural features that enable the strong TDA property. Our comparative studies also show that the TDA polymer family can be used to match with non-fullerene acceptors yielding OSCs with low voltage losses. The key difference between the empirical matching rules for fullerene and non-fullerene OSCs is that TDA polymers with slightly reduced crystallinity appear to match better with small molecular acceptors and yield higher OSC performances.

Optimal extent of fluorination enabling strong temperature-dependent aggregation, favorable blend morphology and high-efficiency polymer solar cells

Temperature-dependent aggregation is a key property for some donor polymers to realize favorable bulk-heterojunction (BHJ) morphologies and high-efficiency (>10%) polymer solar cells. Previous studies find that an important structural feature that enables such temperature-dependent aggregation property is the 2nd position branched alkyl chains sitting between two thiophene units. In this report, we demonstrate that an optimal extent of fluorination on the polymer backbone is a second essential structural feature that enables the strong temperature-dependent aggregation property. We compare the properties of three structurally similar polymers with 0, 2 or 4 fluorine substitutions in each repeating unit through an in-depth morphological study. We show that the non-fluorinated polymer does not aggregate in solution (0.02 mg mL−1 in chlorobenzene) at room temperature, which results in poor polymer crystallinity and extremely large polymer domains. On the other hand, the polymer with four fluorine atoms in each repeating unit exhibits an excessively strong tendency to aggregate, which makes it difficult to process and causes a large domain. Only the polymer with two fluorine atoms in each repeating unit exhibits a suitable extent of temperature-dependent aggregation property. As a result, its blend film achieves a favorable morphology and high power conversion efficiency. This provides another key design rationale for developing donor polymers with suitable temperature-dependent aggregation properties and thus high performance.

Side-chain engineering of perylenediimide-vinylene polymer acceptors for high-performance all-polymer solar cells

The side-chain structures of conjugated molecules are well recognized to sensitively influence the crystallinity, morphology and thus carrier transport properties of organic semiconductors. Here, by varying the alkyl side-chain length in the polymer acceptors, the effect of side-chain engineering on the photovoltaic performance is systematically studied in all-polymer solar cells. Clear trends of first an increase and then a decrease in the Jsc and FF values are observed as the branched alkyl groups are extended from 4 to 8 carbons. Correspondingly, the maximum average PCE (ca. 7.40%) is attained with an acceptor bearing a branched side-chain length of seven carbon atoms.

A wide bandgap conjugated polymer based on a vertically connected benzodithiophene unit enabling efficient non-fullerene polymer solar cells

We report a wide bandgap polymer PvBDTffBT based on a new building block: a vertical-benzodithiophene (vBDT) unit. Compared to traditional BDT based polymers, the vBDT unit in PvBDTffBT is connected via the phenyl group instead of the thiophene unit. Such modification leads to stronger torsion between the vBDT unit and the adjacent thiophene, which increases the bandgap of the polymer and introduces significant changes in the film morphology. When blended with a state-of-the-art narrow-bandgap small molecular acceptor (ITIC-Th), we find that this polymer modulation strategy significantly improves the photovoltaic performances from 3% to over 8%.

A perylene diimide-based electron transport layer enabling efficient inverted perovskite solar cells

A 3D type perylenediimide (PDI)-based molecule (TPE-PDI4) is successfully applied as an efficient electron transporting material in inverted perovskite solar cells (PSCs). TPE-PDI4 has been previously demonstrated as an excellent non-fullerene electron acceptor in high-performance bulk-heterojunction polymer solar cells. Considering its decent electron mobility and outstanding solution processability with favorable thin-film morphology, as well as compatible energy levels with perovskite materials, TPE-PDI4 serves as a promising candidate as the electron transport layer (ETL) material for perovskite solar cells. Herein, we report the fabrication of inverted perovskite solar cells using TPE-PDI4 as the electron transporting layer. A high PCE of 16.29% is obtained, which is higher than that obtained using a PCBM-based electron transporting layer under the same testing conditions. On the other hand, TPE-PDI4 also works well as an interfacial layer between perovskite and C60. A high efficiency of 18.78% is achieved in PSCs with TPE-PDI4 compared to a lower efficiency of 16.56% without this interlayer, indicating an enhanced charge transport/collection with the insertion of TPE-PDI4. Additionally, TPE-PDI4 shows a better water-resistibility than PCBM, which could more effectively protect the perovskite layer beneath. Therefore, devices with a TPE-PDI4-based ETL exhibit an enhanced stability. Our results demonstrate the great potential of TPE-PDI4 to replace expensive fullerene-based ETLs.

Use of two structurally similar small molecular acceptors enabling ternary organic solar cells with high efficiencies and fill factors

Ternary blends have shown great potential to increase the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we studied a ternary OSC system with a donor polymer (PM6) and two structurally similar non-fullerene acceptors (named ITCPTC and MeIC). Although these two small molecular acceptors (SMAs) exhibit similar absorption spectra, they introduce a surprising synergistic effect on tuning the domain size and crystallinity of the OSC blend. More specifically, MeIC is a SMA with strong crystallinity, which results in excessive phase segregation and large domain size for the PM6:MeIC binary blend. By adding a structurally similar and less crystalline SMA (ITCPTC) into the binary blend, the domain size and morphology of the blend are much improved without sacrificing the electron mobility of the blend. As a result, the optimal blend ratio of PM6 : ITCPTC : MeIC (1 : 0.4 : 0.6) led to an impressive FF of 78.2% and PCE of 14.13%, which are the highest values reported for ternary non-fullerene OSCs reported to date.

Efficient Nonfullerene Organic Solar Cells with Small Driving Forces for Both Hole and Electron Transfer

State-of-the-art organic solar cells (OSCs) typically suffer from large voltage loss (Vloss ) compared to their inorganic and perovskite counterparts. There are some successful attempts to reduce the Vloss by decreasing the energy offsets between the donor and acceptor materials, and the OSC community has demonstrated efficient systems with either small highest occupied molecular orbital (HOMO) offset or negligible lowest unoccupied molecular orbital (LUMO) offset between donors and acceptors. However, efficient OSCs based on a donor/acceptor system with both small HOMO and LUMO offsets have not been demonstrated simultaneously. In this work, an efficient nonfullerene OSC is reported based on a donor polymer named PffBT2T-TT and a small-molecular acceptor (O-IDTBR), which have identical bandgaps and close energy levels. The Fourier-transform photocurrent spectroscopy external quantum efficiency (FTPS-EQE) spectrum of the blend overlaps with those of neat PffBT2T-TT and O-IDTBR, indicating the small driving forces for both hole and electron transfer. Meanwhile, the OSCs exhibit a high electroluminescence quantum efficiency (EQEEL ) of ≈1 × 10-4 , which leads to a significantly minimized nonradiative Vloss of 0.24 V. Despite the small driving forces and a low Vloss , a maximum EQE of 67% and a high power conversion efficiency of 10.4% can still be achieved.