Molecular Oligothiophene–Fullerene Dyad Reaching Over 5% Efficiency in Single‐Material Organic Solar Cells

Abstract

solar cells. [2][3][4][5][6][7][8][9] Typically, polymeric or oligomeric materials comprising covalently linked electron-rich donor (D) and electron-deficient acceptor (A) units are developed. In most examples D and A are connected by flexible insulating linkers of various lengths corresponding to a molecular bulk heterojunction model, whereas only few have rigid π-conjugated linkers or are directly connected. [1] Among the ambipolar D-A polymers, structurally challenging "double cable" polymers with high synthetic complexity [2][3][4][5] very recently showed significantly improved power conversion efficiencies (PCE) to over 8.4% in SMOSCs. In these materials, a lamellar phase-separation of D and A units is typically achieved at higher temperatures (up to 230 °C) resulting in solar cells with high thermal and light stability. [1c,3-5] Currently, these results have been outperformed by random D-A block copolymers [6][7][8] reaching a PCE of 8.6% [7] and even very promising 11.3% [8] catching the 10% technological barrier for industrial applications. [1c,10] It is well accepted that structurally defined semiconducting molecular materials have particular advantages such as monodispersity and reproducibility over polydisperse polymers, which contain chain length distributions and defects to a certain extent. [11] Various oligomeric D-A dyads and triads have been prepared and optimized for SMOSCs over time, [1] and general structural concepts include: 1) D and A are directly connected in an "in-chain" approach leading to fully π-conjugated alternating D-A systems. The best result in this category was achieved with oligo(fluorene-alt-bithiophene)perylene diimide dyads, which after post-treatment with solvent vapor annealing (SVA) gave device performances of up to 1.75% PCE; [12] 2) D and A are linearly linked via flexible linkers in the so-called "side-chain" approach in an 1:1 or 1:2 ratio, whereby an oligothiophene-PC 71 BM fullerene dyad recently described by Min et al. reached a PCE of 3.22% in SMOSCs; [13] 3) Similar to the subunits in "double cable" polymers, oligomeric donor backbones were substituted at the central unit with pending PC 61 BM or PC 71 BM fullerenes in a "T-shaped" fashion, but typically reached efficiencies of only below 2%. [1] An improved performance of up to 2.5% PCE was obtained by the attachment of non-fullerenic perylene diimide side chains to a conjugated cooligomer backbone comprising diketopyrrolopyrrole and benzodithiophene units. [14] We recently could further improve the performance of SMOSCs in this category by developing ambipolar "T-shaped" D-A dyads 1-3 consisting of a dithienopyrrole A novel donor-acceptor dyad, 4, in which the conjugated oligothiophene donor is covalently connected to fullerene PC 71 BM by a flexible alkyl ester linker, is synthesized and applied as photoactive layer in solution-processed single-material organic solar cells (SMOSCs). Excellent photovoltaic performance, including a high short-circuit current density (J SC ) of 13.56 mA cm −2 , is achieved,...

Unraveling the Charge‐Carrier Dynamics from the Femtosecond to the Microsecond Time Scale in Double‐Cable Polymer‐Based Single‐Component Organic Solar Cells

“…[15][16][17] Notable is the fact that the relative molecule-to-molecule arrangements are still complex to control and probably as well sensitive to processing conditions such as solvent vapor annealing, since the molecular electron donors and acceptors are restricted in their movement during thin-film formation by covalent linkers and steric hindrances. [18] Nevertheless, tedious developments resulted in molecular materials with up to around 5.34% power conversion efficiency [18,19] Further improvements are expected by understanding how to precisely adjust the desired microstructure morphology and exploiting high-efficiency alternatives as electron donor or acceptor moieties. [20] In-chain (or block co-) polymers are promising candidates for SCOSCs, which led to the most recent performance breakthroughs with particular efficiencies of 8.64% and 11.32%.…”

High‐Efficiency Organic Solar Cells with Reduced Nonradiative Voltage Loss Enabled by a Highly Emissive Narrow Bandgap Fused Ring Acceptor

“…To investigate the photovoltaic properties of these two acceptors, bulk heterojunction OSCs were fabricated with an invert device architecture of ITO/ZnO/Active layer/MoO 3 /Ag. [ 33 ] PBDB‐T, a well‐known wide bandgap polymer, was used as the donor. [ 34 ] As shown in Figure 2d, the smaller HOMO offset between PBDB‐T and SM16‐R than that between PBDB‐T and SM16 would have a negative effect on exciton dissociation in PBDB‐T: SM16‐R ‐based devices.…”

Achieving 12.6% Efficiency in Single‐Component Organic Solar Cells Processed from Nonhalogenated Solvents

“…There are three types of SCOSCs have been developed, [ 25 ] mainly including molecular dyads, [ 17,21 ] double‐cable conjugated polymers, [ 20,26 ] and conjugated block copolymers (CBCs). [ 27–29 ] Compared with the molecular dyads and double‐cable conjugated polymers, CBCs are capable of relatively simple synthesis (i.e., one‐pot polymerization), and various types of CBCs were developed with an impressive achievement of PCEs over 6%.…”

“…[14,15] In this respect, one of the prospective candidates to overcome the disadvantages of BHJ morphology is focused on the single-component OSCs (SCOSCs) containing only one photovoltaic material in the active layer, which integrate the electron-accepting moieties and the donating moieties in one molecule via the covalent linkage. [16,17] Apart from the stable active layer morphology, [18][19][20][21] SCOSC device also shows several attractive advantages. For instance, single-component devices possess considerably simplified device fabrication without complicated processing steps.…”

“…[24] As a consequence, the highest PCE of SCOSCs is still far lagging behind that of the reported BHJ OSCs. [5] There are three types of SCOSCs have been developed, [25] mainly including molecular dyads, [17,21] double-cable conjugated polymers, [20,26] and conjugated block copolymers (CBCs). [27][28][29]…”