A Generally Applicable Approach Using Sequential Deposition to Enable Highly Efficient Organic Solar Cells

Abstract

Organic solar cells (OSCs) have attracted enormous research interest due to their promise for the realization of lightweight, mechanically flexible, and portable power generators using low-cost printing techniques. [1-3] Solution-processable OSCs are typically based on a bulk heterojunction (BHJ) architecture comprising electron-donating (donor) and electronaccepting (acceptor) semiconductors. In recent years, the continual development of innovative photovoltaic materials, [4-14] device and interface engineering [15-19] in conjunction with better understanding of fundamental mechanisms [20-23] have contributed to the rapid progress of BHJ OSCs. Their power conversion efficiencies (PCEs) have now reached more than 17% for both single-junction [24-28] and tandem [29] devices, providing great prospects for practical applications. The achievable performance of OSCs is strongly dependent on the BHJ blend morphology, wherein the donor and acceptor are expected to form nanoscale phase separation and bicontinuous pathways for maximizing charge generation and transport. [30] At present, BHJ photoactive layers are fabricated normally via spin-coating of the mixture of donor and acceptor materials, which usually leads to complicated dynamics and kinetics during the morphological evolution. [31,32] One of the long-standing challenges in realizing optimum BHJ configuration through such a method is to strike a delicate balance between the miscibility and self-organization of two active components during processing, thus imposing stringent requirements on the treatment conditions. Specifically, in the state-of-the-art nonfullerene systems, the donor and acceptor typically possess similar planar molecular structures and good miscibility, which enhances the difficulty in controlling the BHJ morphology. [33] An alternative way is to deposit neat donor and acceptor sequentially trying to reduce the complexity of interaction between two components. Previous studies have indicated that this sequential deposition method is feasible to create a p-in like structure due to the interdiffusion of top layer into bottom one and higher gradient concentration of acceptors near the cathode and donors near the anode. [34-36] Additionally, the two-step procedure allows each layer to be regulated and optimized independently, affording new opportunities for Bulk-heterojunction (BHJ) organic solar cells (OSCs) are prepared by a common one-step solution casting of donor-acceptor blends often encounter dynamic morphological evolution which is hard to control to achieve optimal performance. To overcome this hurdle, a generally applicable, sequential processing approach has been developed to construct high-performance OSCs without involving tedious processes. The morphology of photoactive layers comprising a polymer donor (PM6) and a nonfullerene acceptor (denoted as Y6-BO) can be precisely manipulated by tuning Y6-BO layer with a small amount of 1-chloronaphthalene additive to induce the structural order of Y6-BO molecules to impact the blend phase. Th...

Sequential Deposition of Donor and Acceptor Provides High‐Performance Semitransparent Organic Photovoltaics Having a Pseudo p–i–n Active Layer Structure

“…Thus, we conclude that a substantial fraction of Y6 became intercalated within the PBDB-T-2F layer during the solution-deposition of Y6, forming a continuous pathway for charge transport in the vertical direction throughout the SD bilayer. [7] To examine the origin of the suppressed charge recombination in the SD film, we undertook a mechanistic study of the differences in the morphologies and degrees of spontaneous mutual molecular diffusion of the films. [14] Figure 4a and Figure S3a, Supporting Information, present 2D grazing-incidence wideangle X-ray scattering (2D-GIWAXS) patterns of films fabricated using our SD method and the conventional BHJ method, respectively.…”

“…This approach is not entirely possible in a conventional BHJ structure, in which the concentrations of the donor and acceptor must be fixed at an optimal ratio for the optimum PCE, because a delicate balance between the two active components during processing imposes stringent requirements on the treatment conditions and limits the degree of tunability. [7] Consequently, decreasing the donor concentration or layer thickness for increasing the transmittance in a conventional ST-OPVs having a BHJ active layer generally sacrifices the device performance, owing to a less-than-ideal active layer morphology. Here, we describe a two-step processing method involving SD of the donor and acceptor layers, to control their initial thicknesses, by simply changing the concentrations of their individual solutions.…”

Asymmetric Acceptors Enabling Organic Solar Cells to Achieve an over 17% Efficiency: Conformation Effects on Regulating Molecular Properties and Suppressing Nonradiative Energy Loss

“…Organic solar cells (OSCs) as a green photon-to-current conversion technology have shown great potential as a clean energy source because it can be fabricated into large-area, light-weight, flexible and/ or semitransparent devices through costeffective solution processing method. [1][2][3] In the past few years, multiple contributions stemming from material design [4][5][6][7][8][9][10][11] and device engineering [12][13][14][15][16][17] have greatly improved the performance of OSCs to achieve over 16% certified power conversion efficiencies (PCEs) for single-junction devices. [18][19][20][21][22] However, the prospect of commercialization of OSCs is still lagging behind that inorganic silicon and hybrid perovskite solar cells due to their large energy loss (E loss ) in the charge generation processes.…”

Layer‐by‐Layer Solution Processing Method for Organic Solar Cells

“…A total of 17.2% is by far the highest efficiency in the field of bilayer OSCs. [73] Dong et al used a mixed solution of CB: ODCB as the solvent to deposit the donor layer and acceptor layer and prepared bilayer devices based on the PffBT4T-2OD: PC 71 BM system. After optimizing the film thickness, the best achieved PCE was 9.4%, equivalent to that of BHJ devices (9.3%).…”