The solution-processed layer-by-layer (LBL) method has potential to achieve high-performance polymer solar cells (PSCs) due to its advantage of enriching donors near the anode and acceptors near the cathode. However, power conversion efficiencies (PCEs) of the LBL-PSCs are still significantly lower than those of conventional one-step-processed PSCs (OS-PSCs). A method to solve the critical problems in LBL-PSCs is reported here. By employing a specific mixed solvent (o-dichlorobenzene [o-DCB]/tetrahydrofuran) to spin-coat the small-molecular acceptor IT-4F onto a layer of the newly designed polymer donor (PBDB-TFS1), appropriate interdiffusion between the PBDB-TFS1 and the IT-4F can critically be controlled, and then an ideal phase separation of the active layer and large donor/acceptor interface area can be realized with a certain amount of o-DCB. The PSCs based on the LBL method exhibit PCEs as high as 13.0%, higher than that of the counterpart (11.8%) made by the conventional OS solution method. This preliminary work reveals that the LBL method is a promising approach to the promotion of the photovoltaic performance of polymer solar cells.
current. The latter is the value of qV OC a solar cell with that bandgap would have at a temperature of 300 K illuminated by 1 sun unconcentrated illumination and assuming emission of luminescence into the 2π halfspace above the solar cell. [16][17][18][19] 2) qV OC SQ − qV OC rad (ΔE 2 ) is the loss due to the absorption edge being nonabrupt and therefore primarily due to radiative recombination below the bandgap. [20][21][22][23][24] 3) The third loss is due to nonradiative recombination (ΔE 3 ). As the first term ΔE 1 is unavoidable for any solar cell, the main opportunities for reducing E loss to improve the V OC lie in the other two terms.The second term, ΔE 2 , can be reduced by elevating the charge transfer (CT) state. For instance, it has been demonstrated that when the ionic potential (IP) and electronic affinity (EA) of a donor are nearly aligned with those of an acceptor, the energetic offset from E g to E CT can be very small, resulting in a negligible ΔE 2 . [20,21,25,26] In contrast, when the IP and EA of a donor are much larger than those of an acceptor, as observed in many OSCs based on polymer donors and fullerene acceptors, ΔE 2 can be as large as over 0.60 eV. [17,27,28] The third term, ΔE 3 , can be varied from 0.25 to 0.48 eV. [29][30][31][32][33][34][35] As is known, for an OSC under open-circuit condition, an electron-hole pair at the CT state transits to the ground state by either radiative recombination or nonradiative recombination, so the electroluminescence (EL) emission can be used as a tool to monitor the nonradiative recombination. [21,28,[31][32][33] There have been many studies to investigate ΔE 3 for OSCs based on various photoactive materials. [28,30,36] For instance, for an OSC based on the polymer donor P3TEA and acceptor SF-PDI2, the electroluminescence quantum efficiency (EQE EL ) was as high as 5 × 10 −5 , corresponding to a ΔE 3 of 0.26 eV, [20] while for OSCs based on polymer donors and fullerene acceptors, ΔE 3 can surpass 0.40 eV. [30,31] Up to date, the methodology for tuning ΔE 3 has not yet been established. Considering that ΔE 1 is unavoidable and ΔE 2 can be reduced to a negligible value, the modulation of ΔE 3 is critical in the field of OSCs.Here, we report a new method to reduce E loss in an OSC based on the polymer donor PBDB-T and nonfullerene acceptor IT-4F by incorporating a small molecular material named NRM-1 in the photoactive layer. We selected the PBDB-T:IT-4F system to conduct the study because the IP and EA of PBDB-T are much larger than those of IT-4F and the energetic offset between E g and E CT , referring to ΔE 2 , can be obviously Reducing energy loss (E loss ) is of critical importance to improving the photovoltaic performance of organic solar cells (OSCs). Although nonradiative recombination ( E loss nonrad ) is investigated in quite a few works, the method for modulating E loss nonrad is seldom reported. Here, a new method of depressing E loss is reported for nonfullerene OSCs. In addition to ternary-blend bulk heterojunction (BHJ) solar cells...
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