Cold‐Aging and Solvent Vapor Mediated Aggregation Control toward 18% Efficiency Binary Organic Solar Cells

Abstract

Besides the intrinsic optoelectronic properties of photovoltaic materials and the device architectures, the nanoscale morphology within the photoactive layer, including molecular packing in molecule level and molecular aggregation in nanoscale, [12][13][14][15][16] represents a vital factor in optimizing the device performance and can be manipulated via various approaches. [17][18][19][20][21][22][23][24] It is known that the pre-aggregates of organic semiconductors in solution have a profound influence toward their morphology in solid thin films, and various physical and chemical approaches, including the modulation of solvent, [25] solvent additive, [26] temperature [27] and molecular structure, [28,29] have been demonstrated in the literature to manipulate the pre-aggregation behavior. [13,30] For example, additives can induce polymer aggregates with short range order in solution, which can then act as the nuclei for polymer crystallization during the solution casting process, leading to many small polymer domains with jagged interfaces, resulting in enhanced light harvesting and charge separation. [31] Co-solvents have been used to induce polymer aggregation and prevent the formation of large domains of fullerene as well as restraining the liquidliquid phase separation in PDPP5T:PC 70 BM system in order to receive high performance. [32] Ma et al. controlled the solution and substrate temperatures during the casting of PM6:Y6 from hydrocarbon solvents to enable similar aggregation states in solutions and solid films compared to those cast from halogenated solution, and therefore maintained the device PCE cast from hydrocarbon solvents. [33] Zhang et al. tuned the molecular weight of PM6 to optimized OSCs with suitable size and purity of the domains to achieve well-balanced charge transport, and found that PM6 with higher molecular weight possessed a stronger aggregation degree in solutions. [34] The temperature-dependent aggregation property of conjugated polymers offers the approach to manipulate the pre-aggregation through temperature control, which is also a facile and low-cost approach. [35,36] Different from previous work on tuning the aggregation in a temperature range from room-temperature to over 100 °C, weThe molecular ordering and pre-aggregation of photovoltaic materials in solution can significantly affect the nanoscale morphology in solid photoactive layers, and play a vital role in determining the power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, a cold-aging strategy is reported to mediate the pre-aggregation of PM6 polymer in solution through a disorder-order transition, which leads to dense and fine PM6 aggregates with enhanced π−π stacking in its blend thin films with either fused-ring and non-fused-ring non-fullerene acceptors (NFAs) including Y6-BO, N3, IT-4F, and PTIC. The fine aggregates of PM6 and slightly enlarged NFA domains improve the continuous networks with enhanced and balanced charge mobility. The resulting OSCs all demonstrate enhanced PCEs compared to the...

Alkyl Chain Tuning of Non-fullerene Electron Acceptors toward 18.2% Efficiency Binary Organic Solar Cells

“…While a number of literature studies have reported PCEs over 18%, the majority of them are achieved via the ternary strategy, − where a third component has been incorporated in the photoactive layer to either increase light absorption or regulate the morphology to enhance the power conversion process. A literature survey shows that only L8-BO and Y6-BO have been reported very recently to achieve a high PCE over 18% in a single-junction, binary OSC, ,, and systematic investigations are still lacking, which urges the development of new NFAs that can deliver high PCEs, for example, via fine-tuning of the alkyl side chains.…”

Non‐Volatile Perfluorophenyl‐Based Additive for Enhanced Efficiency and Thermal Stability of Nonfullerene Organic Solar Cells via Supramolecular Fluorinated Interactions

“…Compared to the GIWAXS patterns and 1D profiles measured respectively for the neat Y6 and PM6 films (Figure 7d,e; Figure S6, Supporting Information), these two highly perpendicularly oriented peaks are attributed mainly to face‐on bilayer structure of PM6 (as crystallites of Y6 tend not to have a preferred orientation), as also suggested in previous reports. [ 21 ] We note that no obvious GIWAXS peaks of Y6 crystals (other than the characteristic peak at q ≈ 0.3 Å −1 , overlapping with that of PM6) could be observed in the PM6:Y6 film (Figure 7d,e), suggesting that Y6 are dispersed relatively well by PM6 in the blend film. These two characteristic peaks at q z = 1.756 Å −1 and q r = 0.30 Å −1 (Figure 7e) with orthogonal orientations could be enhanced markedly after optimum 12 min of 100 °C annealing, suggesting particularly enhanced and oriented crystallizations of PM6 nanodomains; after cooled down to room temperature, these two prominent peaks, however, reduced significantly by ≈50%, with the π–π peak position slightly shifted to a higher q z = 1.775 Å −1 (Figure 7d), toward characteristic Y6 crystalline peaks (Figure S6e, Supporting Information).…”

“…We notice that a recent report suggested a cold‐aging process to control the phase separation of a similar system, which favors specifically the PM6 crystallization at a low temperature at 273 K. Such a process successfully improved PM6 crystallization with suppressed Y6‐derivative crystallization; as a result, the charge mobility ratio can be better balanced to μ h / µ e = 1.1, at the cost of substantially reduced hole and electron mobility (7.1 × 10 −4 and 6.3 × 10 −4 cm 2 V −1 s −1 ). [ 21 ] In our case, the BF7 additive is used to selectively interact with Y6 for an earlier phase separation of Y6 from the PM6 polymer matrix during spin‐coating process, resulting in the much more prominent PM6 crystallization observed before the annealing (Figure 8a), compared to that without BF7. The annealing process, however, is needed to enhance the second stage of film morphology tuning to improve Y6 crystallization (Figure 8f) and the film network structures (Figure 8i).…”

Realizing 19.05% Efficiency Polymer Solar Cells by Progressively Improving Charge Extraction and Suppressing Charge Recombination

“…[1,2] Over the past decades, the utmost endeavors have been devoted to modulating the structural and optoelectronic properties by designing new organic semiconductors, [3][4][5] controlling the morphology of photoactive layer via additives and various post-treatments, [6][7][8] maximizing the harvest of light by using ternary or tandem devices, [9][10][11][12][13][14][15][16][17] improving the extraction of electrons and holes by interfacial modification. [18][19][20] With these efforts, PSCs have witnessed rapid progress, the cutting-edge PSCs have delivered over 18% power conversion efficiencies (PCEs) (Table S1, Supporting Information), [7,8,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] and there still has much space to further improvement. As is well-known, simultaneously improving the open-circuit voltage (V oc ), short-circuit current density (J sc ), and fill factor (FF) is essential to maximize the PCE of PSCs.…”