The double cascading charge transport and device performance
Fig. 1a-b show the chemical structure and absorption profiles of the materials used in this study. The Y6 acceptor absorbs from 700-950 nm, while PM6 and PM7 donors show complementary absorption from 400-700 nm. PC71BM, with a much lower absorption, can only be considered a transport medium. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital levels (LUMO) are -5.13 eV/-3.28 eV for PM6, -5.24 eV/-3.38 eV for PM7, -5.66 eV/-4.29 eV for Y6, and -6.10 eV/-4.10 eV for PC71BM, as measured by ultraviolet photoelectron spectroscopy (UPS)22 coupled with the optical band gaps (Fig. 1c-d). Solar cells were prepared with a forward structure (ITO/PEDOT:PSS/Active Layer/ PFNDI-Br /Ag). The donor to acceptor ratio was fixed at 1:1.2 (optimized conditions for PM6:Y6) to avoid light-absorption-induced performance change, and PC71BM was added separately. The concentrations of PM7 and PC71BM were varied to determine the optimal composition. Detailed performances of the devices are shown in Fig. S1, and Table S1-2. For the PM6:PM7:Y6 ternary blends, a mixing ratio of 0.8:0.2:1.2 (D1:D2:A1) was found to be optimal, while for the PM6:PM7:Y6:PC71BM quaternary blends, a mixing ratio of 0.8:0.2:1.2:0.25 (D1:D2:A1:A2) yielded optimum performance. The current density-voltage (J-V) curves are shown in Fig. 1e, with performance detailed in Fig. S2 and Table 1. PM6:Y6 binary devices showed a maximum PCE of 16.52%, with a VOC of 0.842 V, a JSC of 25.98 mA·cm-2, and an FF of 75.52%. Ternary devices showed a maximum PCE of 17.02%, a VOC of 0.848 V, a JSC of 26.17 mA·cm-2, and an FF of 76.70%. Quaternary devices showed a maximum PCE of 18.07%, with a VOC of 0.859 V, a JSC of 26.55 mA·cm-2, and an FF of 79.23%. The quaternary devices had a certified PCE of 17.35%, subject to the calibration procedures of the National Renewable Energy Laboratory (NREL), using a 0.032 cm2 photon mask (Fig. S3), which is the highest certified value reported for a single-layered BHJ device. In the ternary blends, the external quantum efficiency (EQE) improved slightly around 640 nm in comparison to the PM6:Y6 binary blends, as shown in Fig. S4. In the quaternary blends, the EQE from 450-600 nm and 650-800 nm improved slightly. Therefore, the PC71BM absorption and enhanced electron transport aided in improving the light extraction from the acceptor materials. The enhanced performance results from better charge collection channels for both electrons and holes, i.e. double cascading carrier transport pathways.
The stepwise-aligned energy levels lead to interesting device characteristics. The carrier recombination was determined from the dependence of the JSC and VOC on light intensity, as shown in Fig. S5, Fig. S7a and Table S3. A slope from VOC vs. Plight of 2 kT/q should be obtained if monomolecular or trap-assisted recombination dominate23, 24. The recombination parameter α, defined by JSC ∝ (Plight)α, is close to unity, suggesting minimal bimolecular recombination25. From the binary to ternary to quaternary blends, α increased from ~0.93 to ~0.94 and to ~0.96, and the slope of VOC vs. Plight decreased from 1.34 kT/q to 1.18 kT/q and 1.10 kT/q, respectively, consistent with the change in the FFs. These results indicate that the PM7 donor reduces trap-assisted recombination, due to a better HOMO level alignment. Adding PC71BM to the ternary blends leads to a further decrease from the electron transport side. Detailed hole and electron mobilities of the active layers were determined using space charge limited current (SCLC) (Fig. S6, Fig. S7b and Tab. 1), which are in good agreement with the JSC and recombination results26-28. Time-resolved microwave conductivity (TRMC) measurements were performed to characterize the free-charge generation characteristics29-31. The 𝜑∑𝜇 value at the lowest absorbed flux is used as an indicator of PV potential. As seen in Fig. S8a, PM6:Y6 blends peak at a value of 2.3×10-2 cm2·s-1·V-1. The addition of PM7 and PC71BM, slightly increases this value to 2.4×10-2 cm2·s-1·V-1 for the ternary and 2.9×10-2 cm2·s-1·V-1 for the quaternary blends, indicating an increased charge generation yield. Shown in Fig. S8b are the normalized photoconductivity transient spectra over 500 ns. PM7:Y6 shows the carrier lifetimes increase from 246 ns to 431 ns to 460 ns in going from the binary to ternary to quaternary blends, indicating increasingly efficient interfacial exciton dissociation and carrier extraction, which help to improve the long-term stability of the device32.
Stability tests were performed for 1000h under illumination equivalent to 1 sun, the performance of quaternary devices maintains an 81.0% PCE, with 5.2% VOC loss, 5% JSC loss and 10.1% FF loss (Fig. 1g), which is superior than binary devices. The storage stability for quaternary devices retain 97.2% PCE after 1000h aging in dark conditions (Fig. S9 and Fig. S10), underscoring the benefits of the double cascading quaternary blends in enhancing the morphology and operation stability for long-term use. Different cathode interlayers were also investigated during stability test, which yield large variations in performances, suggesting the necessity of cathode interlayer optimization in the future.
Ultrafast hole-transfer and efficient carrier transport along double cascading pathways
It is essential to understand the carrier transfer dynamics within the framework of the morphology. Femto-second transient absorption (TA) spectroscopy was used to probe the photo-induced hole transfer dynamics in the multi-component blends33-35. The results are shown in Fig. 2, with the corresponding hole transfer times summarized in Table S4. The static absorption peaks for the D and A are spectrally well separated, so both the spectral and temporal characteristics of hole transfer dynamics can be determined. An excitation wavelength of 750 nm was used to selectively excite Y6. The 2D color plot of TA spectra of PM6:Y6 blend film is shown in Fig. 2a, and a few representative TA spectra at the indicated delay times are shown in Fig. 2b. With the decay of the Y6 bleach peak at 770-860 nm, a few clear bleach peaks at 560-600 nm emerge in the TA spectra, matching well with the absorption features of PM6. The bleach decay process of the photoexcited Y6 agrees with the increase of the PM6 ground state bleach, confirming ultrafast hole transfer from Y6 to PM6, as shown in Fig. 2c. The increasing kinetics of the donor ground state bleach at wavelength where photoexcited acceptor has contribution provides a clean hole transfer process, we fit the donor kinetics with a bi-exponential function. The hole transfer process in the four blends (PM6:Y6, PM6:PM7:Y6, PM6:PM7:Y6:PC71BM and PM7:Y6) show a fast component with τ1 of ~ 0.25, ~0.39, ~0.37, ~0.47 ps, and a slow component τ2 of 8.18, 10.28, 13.22, 16.36 ps, respectively (Fig. 2d-e) and their relative contributions are shown in Table S4.The former fast component τ1 can be assigned to the ultrafast exciton dissociation of Y6 at the donor-acceptor interface and the latter to the diffusion of excitons in Y6 towards interface before dissociation. The interfacial exciton separation is one order of magnitude faster in comparison to the exciton diffusion. The trend in the hole transfer life-time and the hole transfer efficiency is consistent with the values of HOMO offsets or driving force (Fig. S11). The presence of PM7 in the BHJ blends reduces the driving force, aligning the cascading energy levels to ensure better transport and an increase in the JSC, a trade-off with the hole-transfer rate. PC71BM does not perturb the hole-transfer process, suggesting close interactions between Y6 and PM6/PM7. Such results also agree well with the observation that no PM6:PC71BM CT emission is seen in quaternary blends. While most studies focus on increasing absorption with the addition of more components, our findings indicate that a detailed balance between the driving force and hole-transfer rate is equally important to refine the carrier generation and extraction to generate high JSC’s in OSCs. We further compared the polaron decay dynamics of the quaternary blend films at different pump fluences to probe the charge recombination mechanism (Fig. S12). The fluence-dependence results suggest a dominant non-geminate charge recombination with small amount of CT excitons. From the above results, a picture of the mechanism emerges, as shown in Fig. 2f. The photon excitation first drives ultrafast and large amounts of free carriers accompanied with nongeminate recombination, biasing out the weak interfacially bound CT states (which are quite close in energies), along with a density dependent recombination process (step 4).
Energy loss optimization induced by CT energy management
In solar cell devices, the VOC decreases in going from quaternary to ternary to binary blends. This results from the energy level management, driving force, and energy loss (Eloss). Highly sensitive EQE (s-EQE), electroluminescence (EL) and electroluminescence quantum efficiency (EQE-EL) measurements were performed to investigate energy loss channels36-38. Two energy loss sectors, charge generation (∆E2 = Eg-ECT) and charge recombination losses (ECT-qVOC) were considered. The charge recombination loss could be further traced to radiative (∆E1) and non-radiative (∆E3) recombinations39. The ECT could be obtained by fitting the sub-gap absorption of the corresponding s-EQE curve (Fig. 3b) following the Marcus theory36, 40. The CT energy of the binary PM6:Y6 blend was 1.34 eV, yielding a ∆E2 = 0.066 eV. PM7:Y6 blend showed a CT energy of 1.37 eV, with ∆E2 = 0.061 eV. Ternary and quaternary blends showed the CT energies of 1.35 eV and 1.36 eV, respectively. The lowest ∆E2 of 0.048 eV was obtained for quaternary films, resulting in a higher VOC. The deeper HOMO of PM7 leads to a reduced hole transfer driving force but smaller energy loss, providing one avenue for device optimization, with an optimized ternary blend composition of 20% PM7. The PM6:PC71BM mixture has a CT energy of 1.49 eV, higher than the band gap of Y6, and thus would not introduce a low level CT state to trap electrons. Consequently, adding PC71BM to the ternary blends redefines the acceptor LUMO at a higher level, which further improves VOC.
Fig. 3a shows the normalized EL spectra of devices based on the pure materials and the BHJ films. PM6:Y6 blends show a single EL emission peak at 924 nm, similar to that of a Y6 film (920 nm). When the applied current was increased from 1 mA to 5 mA, no EL change was observed (Fig. S13). Thus, the CT states, if present, have a very low density. PM6:PC71BM blends have a CT emission at 980 nm, well below the emission in the binary mixtures. However, in the quaternary blends, the EL is dominated by Y6, eliminating the PM6: PC71BM CT states. Therefore, the BHJ blends can be viewed as simple OLED devices, with PM6/PM7 and PC71BM acting as hole and electron transporting layers, and emission happens at the Y6 acceptor or PM6:Y6 interfaces. Y6, then, functions as both a photovoltaic and electroluminescence material. The energy losses due to radiative recombination (∆E1) of charge carriers can be calculated using the fit parameters from the s-EQE spectra, and the losses due to the non-radiative recombination (∆E3) were quantified by measuring the EQE-EL41. As shown in Fig. 3c, the emission efficiency of PM7:Y6 blend was 1.33 × 10-2%, much higher than that of PM6:Y6 blend (5.05 × 10-3%). Thus, for ternary blends, the additional 20% PM7 actually reduced the energy loss caused by non-radiative recombination. The quaternary blends show a higher emission efficiency of 9.37×10-3% in comparison to the ternary film, representing a decrease in ∆E3 to 0.240 eV. Non-radiative recombination can be calculated from ∆E3 = -kT·ln(EQEEL). Consequently, it is important to maximize EQEEL to minimize ∆E3, and in the current case, the double cascading energy level alignment plays an important role. Different contributions to energy loss are shown in Fig. 3d and summarized in Table S5. The quaternary blends showed the lowest total energy loss of 0.548 eV, in comparison to the other blends (0.567 eV for PM6:Y6, 0.553 eV for PM7:Y6 and 0.568 eV for ternary blends). We attribute the elevated VOC of the quaternary devices to the smallest Eg-ECT energy offset and the suppressed non-radiative recombination losses, due to the addition of PM7 and PC71BM.
Thin film morphology of double cascading blends
The structure of the neat and BHJ thin films were determined using grazing incidence wide-angle X-ray diffraction (GIWAXS), the results of which are shown in Fig. 4 and Fig. S14. The PM6 donor assumed a dominant face-on orientation, with a broad (100) reflection in the in-plane (IP) direction at 0.28 Å-1 and a π–π stacking peak in the out-of-plane (OOP) direction at 1.69 Å-1. The crystal coherence lengths (CCLs) for (100) and (010) were 5.27 nm and 1.56 nm, respectively, as determined using the Scherrer analysis42. PM7 had a similar diffraction profile, with a (100) reflection in the IP direction at 0.29 Å-1 and (010) reflection in the OOP direction at 1.66 Å-1with CCLs of 4.52 nm and 1.31 nm, respectively. Y6 films showed a π−π stacking peak at 1.75 Å-1 in the OOP direction and a lamellar stacking peaks at 0.27 Å-1 in the IP direction. However, the banana-shaped Y6 molecules pack in a unique manner so that they can overlap by end group π–π stacking to form a polymer-like conjugated backbone, and the lamellae packing, determined from single crystal data, is assigned to the (110) lattice plane, which is shown in Fig. 4d and Fig. S15. Thus, Y6 assumes a tilted molecular orientation where the polymer-like backbone is tilted normal to surface which is more efficient for charge transport.
A summary of the 2D and line-cut GIWAXS profiles are shown in Fig. 4a and Fig. 4b, respectively. For the PM6:Y6 blends, Y6 showed well-defined in-plane (020) and (11-1) Y6 lattice reflections at 0.21 Å-1 and 0.42 Å-1. PM7:Y6 blends show weak crystalline order, with the Y6 (11-1) and (020) diffraction peaks absent. Even though PM6 and PM7 have similar chemical structures, PM7 interacts differently with Y6 and retards Y6 crystallization, such that only the Y6 (110) stacking can be seen. In ternary blends, the intensities of the reflections at 0.23 Å-1 and 0.42 Å-1 decreased significantly. In the quaternary blends, the Y6 reflections at 0.23 Å-1 and 0.42 Å-1 were quite weak, indicating that PC71BM also disrupts the packing of Y6. The polymer lamellar and Y6 (110) reflections could not be separated, and they were used in sum to estimate the lamellar ordering of the BHJ thin film. The polymer and Y6 π–π reflections are summarized in Tab. S6. Fig. 4c shows parameters derived from the lamellar and π–π stacking peaks of the different BHJ thin films. The quaternary blend showed the largest CCL and peak area for both the lamellar and π–π stacking peaks, indicating that the overall crystallinity and crystal quality are improved for the quaternary blends, which improves carrier transport pathways. The Y6 (020) peak showed a decrease in the peak area but an increase in the CCL in going from the binary to ternary to quaternary blends, as shown in Fig. S16 and Table S7. Consequently, the crystallization behavior of Y6 changed in the blends. The loss of primary axis coherence and intensity in Y6 but improvement in the (110) and π–π stacking reflect an extended polymer-like conjugated backbone by adopting a twisted or screw-like packing in the multi-component blends, providing a pathway for electron transport. A schematic of the molecular packing in the blend films is illustrated in Fig. 4e. The intimate mixing of PM6 and PM7 (as indicated by the linear dependence of the VOC on concentration) results in the formation of a homogeneous polymer-rich phase embedded in a fibrillar network. The difference in the interactions of PM6 and PM7 with Y6 optimizing the crystallization of Y6. PC71BM addition does not perturb the morphological framework of PM6:Y6 and is distributed uniformly, as evidenced by the absence of any feature characteristic of PC71BM aggregation. Thus, the plasticizing nature of PC71BM aids in the overall ordering, improving both electron and hole mobility, and a higher FF.
BHJ thin film phase separation was visualized using transmission electron microscopy (TEM). As shown in Fig. S17, all the BHJ thin films showed evidence of phase separation on the tens of nanometers length scale. Resonant soft x-ray scattering (Fig. S18) for PM6:Y6 and PM6:PM7:Y6 blends yielded an interference at a length scale of ~ 60 nm. The quaternary blends with different PC71BM loadings (Fig. S18b) show a shoulder gradually developing into a well-defined interference at higher PC71BM loadings, suggesting that uniform distribution of PC71BM in the mixed region enhances the scattering contrast. The uniform distribution of PC71BM in the mixture indicates that the close interactions with the other amorphous components produces a unique electronic structure with improved electron transport channels, where excited electrons in the donors can transfer onto the “LUMO” of mixed domain that are rapidly extracted. Though the HOMO of PC71BM is much deeper than that of Y6, a homogeneous mixing provides good contacts with the donor materials making exciton harvesting by PC71BM and Y6 efficient.