Enhanced Electron Transport and Heat Transfer Boost Light Stability of Ternary Organic Photovoltaic Cells Incorporating Non‐Fullerene Small Molecule and Polymer Acceptors

Abstract

fabricating flexible, low-cost, low thermal budget, and lightweight solar panels. [1][2][3][4] Over the past decades, great efforts have been devoted for high-performance OPVs. These efforts include new material synthesis, fine tuning the donor:acceptor (D:A) compositions/morphology, and the optimization of fabrication process. [5][6][7][8][9][10][11][12] A key accomplishment of these efforts is the identification of small molecule acceptors (SMAs). In particular, a bulk heterojunction (BHJ) OPV cell using SMAs (e.g., ITIC-based acceptors) can attain simultaneously a low V oc loss and a high efficiency. With fused-ring electron acceptors (FREAs), the highest power conversion efficiencies (PCEs) for the single junction BHJ and tandem OPV devices have gone beyond 15% and 17%, respectively. [13,14] Besides the efficiency, the processibility and stability are also important attributes of BHJ cells. [15,16] However, at present, there is a general lack of knowledge in understanding the operation stability of the high efficiency SMA-based devices. To date, researchers have noticed the problem of photostability in SMA-based OPV devices. [17] Xiao et al. compared a series of PTB7-Th-based solar cells with different acceptors, and the results indicate that the poor miscibility between the donor polymers and SMAs is the primary cause of the morphology degradation, suppressed Operation stability remains the key hurdle for the best-performing nonfullerene small molecule acceptor (SMA)-based organic photovoltaic (OPV) devices. Among all SMAs, the ITIC-derivative is the most promising OPV cell using ITIC-derivative acceptors with a power conversion efficiency > 15%. However, the operation stability of SMA-based devices under illumination is relatively inferior when compared to bulk-heterojunction (BHJ) cells that employ polymeric acceptors. Here, a polymer acceptor N2200 is used as the ternary component to study the device performance of ITIC-derivative-based PBDB-T:ITIC-M and PBDB-T-2F:IT-4F BHJ solar cells, which currently are the representative state-of-the-art high-performance OPV devices. The ternary solar cells with low N2200 loading enjoy significantly improved operation stability, while maintaining a high power conversion efficiency. A comprehensive mechanism study is conducted on the ternary OPV systems in i) electronic and ii) thermal aspects. For i), the ternary BHJs show remarkably improved electron transport. For ii), the thermal diffusivity D of the ternary BHJ exhibits almost an order of magnitude improvement in D values, indicating that heat can be more effectively transferred out of such films than binary counterpart. The results show that N2200 ternary loading facilitates an improved network for both electron transport and heat dissipation, leading to improved photostability.

Reducing VOC loss via structure compatible and high lowest unoccupied molecular orbital nonfullerene acceptors for over 17%‐efficiency ternary organic photovoltaics

“…Comparing PM6: Y6 and PM6:Y6: ITIC‐Th blends, the increased acceptor domain size in the latter blend potentially favors electron transport, and can partially account for the increased electron mobilities from 3.96 × 10 −4 to 4.54 × 10 −4 cm 2 V −1 second −1 . Besides, it is understandable that PM6:Y6: IT‐4F exhibited the higher electron mobilities than the PM6: Y6 control, which is attributed to the increased CCL of oop π ‐ π peak and the decreased electron energy disorder 73‐75 . All the blend films exhibited reasonable phase segregation scales with acceptor domain size around 20 nm, except for the PM6: ITIC‐Th blend with an acceptor domain size of approximately 49.3 nm.…”

“…Besides, it is understandable that PM6:Y6: IT-4F exhibited the higher electron mobilities than the PM6: Y6 control, which is attributed to the increased CCL of oop π-π peak and the decreased electron energy disorder. [73][74][75] All the blend films exhibited reasonable phase segregation scales with acceptor domain size around 20 nm, except for the PM6: ITIC-Th blend with an acceptor domain size of approximately 49.3 nm. The excessively large phase separation in PM6: ITIC-Th devices is unfavorable for charge separation, F I G U R E 3 GIWAXS two-dimensional diffraction patterns of A, PM6: Y6 blend; B, PM6:Y6: IT-4F (1:1.12:0.08) blend; C, PM6: IT-4F blend; D, PM6:Y6: ITIC-Th (1:1.12:0.08) blend and E, PM6: ITIC-Th blend.…”

Deciphering the Role of Fluorination: Morphological Manipulation Prompts Charge Separation and Reduces Carrier Recombination in All‐Small‐Molecule Photovoltaics

“…The electron energy disorder, , was also evaluated by temperature‐dependent electron transport measurements using the Gauss disorder model (GDM)where k is the Boltzmann constant, T is the temperature, is the zero‐field mobility, β is the Poole–Frenkel slope, F is the applied electric field, and is the y ‐intercept from the plot of versus (1000/ T ) 2 . Figure f shows the of the films as a function of the ambient temperature and the linear fit to the experimental data, the slope of which can be used to derive .…”

A facile and robust approach to prepare fluorinated polymer dielectrics for probing the intrinsic transport behavior of organic semiconductors

“…38 In this model, charges hop in an energetic manifold with a Gaussian density of states. The width of the manifold is s. Under such a condition, the charge carrier mobility is temperature (T) and electric field (F) dependent and the mobility can be expressed as: [38][39][40][41]…”