Analysis of the Performance of Narrow-Bandgap Organic Solar Cells Based on a Diketopyrrolopyrrole Polymer and a Nonfullerene Acceptor

Abstract

The combination of narrow-bandgap diketopyrrolopyrrole (DPP) polymers and nonfullerene acceptors (NFAs) seems well-matched for solar cells that exclusively absorb in the near infrared but they rarely provide high efficiency. One reason is that processing of the active layer is complicated by the fact that DPP-based polymers are generally only sufficiently soluble in chloroform (CF), while NFAs are preferably processed from halogenated aromatic solvents. By using a ternary solvent system consisting of CF, 1,8-diiodooctane (DIO), and chlorobenzene (CB), the short-circuit current density is increased by 50% in solar cells based on a DPP polymer (PDPP5T) and a NFA (IEICO-4F) compared to the use of CF with DIO only. However, the open-circuit voltage and fill factor are reduced. As a result, the efficiency improves from 3.4 to 4.8% only. The use of CB results in stronger aggregation of IEICO-4F as inferred from two-dimensional grazing-incidence wide-angle X-ray diffraction. Photo- and electroluminescence and mobility measurements indicate that the changes in performance can be ascribed to a more aggregated blend film in which charge generation is increased but nonradiative recombination is enhanced because of reduced hole mobility. Hence, while CB is essential to obtain well-ordered domains of IEICO-4F in blends with PDPP5T, the morphology and resulting hole mobility of PDPP5T domains remain suboptimal. The results identify the challenges in processing organic solar cells based on DPP polymers and NFAs as near-infrared absorbing photoactive layers.

Revisiting the Classical Wide‐Bandgap HOMO and Random Copolymers for Indoor Artificial Light Photovoltaics

“…First, we measured the space charge limited current (SCLC) of hole‐only devices (HODs) and electron‐only devices (EODs) for pristine and blend films. The zero‐field mobilities of electrons ( μ e ) and holes ( μ h ) are extracted from the J – V curves of the devices, [ 49 ] and the values are summarized in Table 2. The J–V curves are shown in Figure S11 (Supporting Information).…”

“…The hole and electron mobilities were measured by the SCLC model using ITO/MoOx/polymer blends/MoOx/Ag and ITO/ZnO/polymer blend/Al devices, respectively. [ 49 ] Current–voltage measurements in the range of 0–10 V were obtained, and the results were fitted to a space‐charge limited function. The SCLC is described by $$\begin{eqnarray}{J_{{\rm{SCLC}}}} = \frac{9}{8}\varepsilon {\varepsilon _0}\mu \frac{{{V^2}}}{{{L^3}}} \cdot {\rm{exp}}\left( {0.89\gamma \sqrt {\frac{V}{L}} } \right)\end{eqnarray}$$where ε 0 , ε , µ , V , L , and γ represent the permittivity of free space (8.85 × 10 −14 F cm −1 ), relative dielectric constant of the active layer (=3), mobility of charge carriers, potential across the device ( V = V applied − V bi − V r ), active layer thickness, and field‐activation factor, respectively.…”

“…The hole and electron mobilities were measured by the SCLC model using ITO/MoOx/polymer blends/MoOx/Ag and ITO/ZnO/polymer blend/Al devices, respectively. [49] Current-voltage measurements in the range of 0-10 V were obtained, and the results were fitted to a space-charge limited function. The SCLC is described by…”

Impact of Self‐Absorption and Cavity Effects on the Electroluminescence Spectra of Thin‐Film Solar Cells

“…To disentangle the optical changes from possible differences in molecular packing and stress the impact of cavity effects, we fabricate semitransparent inverted PDPP5T:IEICO-4F We choose PDPP5T:IEICO-4F as active layer because of its broad EL spectrum extending far into the near-infrared. [32] Figure 1b shows the measured EL spectra for the cells with the MgF 2 | Ag optical spacer-mirror assembly and that of the corresponding semitransparent device. Figure 1b clearly demonstrates that cavity effects strongly affect the EL spectra.…”

Diketopyrrolopyrrole-Based Chlorinated Bithiophene Polymers for Organic Solar Cells: Effect of Thiophene or Pyridine Flank

“…Although the PCE of 5.15% obtained for DPP2ThCl:Y6 is moderate, this is the highest value among the non-benzodithiophene (BDT) and DPP-based Y6-OPVs (Figure S1). − …”