Single-layered organic photovoltaics with double cascading charge transport pathways: 18 % efficiencies

doi:10.21203/rs.3.rs-44585/v1

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Single-layered organic photovoltaics with double cascading charge transport pathways: 18 % efficiencies

https://doi.org/10.21203/rs.3.rs-44585/v1

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published 12 Jan, 2021

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The chemical structure of donors (Ds) and acceptors (As) limit the power conversion efficiencies (PCEs) achievable with bulk heterojunction (BHJ) active layers of binary D-A mixtures. However, using quaternary D-A blends, double cascading energy level alignment in BHJ organic photovoltaic active layers are realized, enabling efficient carrier splitting and transport. Numerous avenues to optimize light absorption, carrier transport, and charge transfer state energy levels are opened by the chemical constitution of the components. Record-breaking PCEs of 18.07% are achieved where, by electronic structure and morphology optimization, simultaneous improvements of the open-circuit voltage, short-circuit current and fill factor occur. The D and A chemical structures afford control over electronic structure and charge transfer state energy levels, enabling manipulation of hole-transfer rates, carrier transport, and non-radiative recombination losses.

Electronic Materials and Devices

Photonics/optics

Optical Materials and Devices

Materials Engineering

Polymer Science

quaternary solar cells

double cascading transport

thin film morphology

electronic structure

energy loss

A holistic strategy that can simultaneously improve open circuit voltage (V_OC)^1-3, short circuit current (J_SC)^{4, 5}, and fill factor (FF)^6-9 has been long sought in organic photovoltaics (OPVs) to maximize power conversion efficiency (PCE). Improving all characteristics simultaneously has been impossible to achieve, due to morphological and electronic structure constraints, leading to performance trade-offs. The ideal OPV device simultaneously maximizes light absorption, enhances exciton splitting, and facilitates carrier extraction. The carrier generation kinetics must shunt energy loss channels induced by low energy charge transfer (CT) states to improve the open circuit voltage (V_OC)^{10, 11}. Consequently, optimizing the electronic structure of photovoltaic blends along with optimizing morphology is essential^12-14.

We used quaternary blends, comprised of existing donor-acceptor pairs mixed with additional donor and acceptor components to mediate deficiencies in electronic performance or morphology, to address this challenge. Mixtures of the mid-bandgap donor PM6 (poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)]) and the low bandgap acceptor Y6 (2,2’-((2Z,2’Z) -((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2,’’3’’:4’,5’]thieno[2’,3’:4,5]pyrrolo[3,2-g]thieno[2’,3’:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1diylidene))dimalononitrile) have shown exceptional power conversion efficiency (PCE ~16%), high short-circuit current (J_SC, ~25 mA/cm²), and low energy loss (E_loss, 0.5-0.6 eV)^15-19. Enhancement of the photovoltaic characteristics requires more efficient exciton splitting and carrier transport pathways in the active layer. It is advantageous to establish a tiered energy level alignment, to form cascading charge hopping channels that mitigate the J_SC loss by fine-tuning the charge splitting and allow manipulation of multiple charge-transfer energies to ensure a high V_OC. This applies to both electron and hole transport pathways to maximize the energy gain. From the hole-transport side, PM7 (Poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-chloro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)]), which has a chemical structure similar to PM6 but a deeper HOMO energy level, can be used as an “added” donor^{20, 21}, since it is miscible with PM6, does not disrupt the morphology, and provides a tiered cascading energy level alignment. From the electron-transport side, PC₇₁BM, which is compatible with the base PM6:Y6 blends, is an acceptor with a slightly higher LUMO than Y6 that affords a cascading energy level alignment to improve electron transport and promote efficiency. Consequently, the quaternary blend strategy, where the composition can be fine-tuned to optimize device characteristics, represents a new strategy to improve PCEs.

In addition to reducing the charge splitting driving force, PM7 preferentially interacts with the Y6 acceptor, effectively regulating the crystallization of Y6 to better suit carrier transport. PM7 redirects the ultrafast hole-transfer from Y6 to the donor phase, forming tiered cascading energy levels that improve J_SC without further absorption gain. Adding PC₇₁BM reduces the light absorption, yet not the current output. The higher LUMO level of PC₇₁BM in the acceptor mixture improves the V_OC and electron transport, and offsets the absorption loss, leading to an enhancement in device performance. The synergy between the components in the PM6:PM7:Y6:PC₇₁BM quaternary devices result in a maximum PCE of 18.07%, the highest in single layered OPV devices, with an excellent stability (81% PCE after 1000h illumination) that is ~5% better than PM6:Y6 binary devices. The double cascading quaternary blend strategy is implemented in other material systems, which is reflected in similar behavior of device operation, demonstrating the novelty in the approach for OPV device fabrication. These results demonstrate the importance of manipulating the electronic structure in BHJ thin films while simultaneously manipulating the morphology, opening a new route to higher efficiency OPV devices.

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 V_OC of 0.842 V, a J_SC of 25.98 mA·cm^-2, and an FF of 75.52%. Ternary devices showed a maximum PCE of 17.02%, a V_OC of 0.848 V, a J_SC of 26.17 mA·cm^-2, and an FF of 76.70%. Quaternary devices showed a maximum PCE of 18.07%, with a V_OC of 0.859 V, a J_SC 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 J_SC and V_OCon light intensity, as shown in Fig. S5, Fig. S7a and Table S3. A slope from V_OCvs. P_light of 2 kT/q should be obtained if monomolecular or trap-assisted recombination dominate^{23, 24}. The recombination parameter α, defined by J_SC∝ (P_light)^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 V_OCvs. P_light 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 J_SC and recombination results^26-28. Time-resolved microwave conductivity (TRMC) measurements were performed to characterize the free-charge generation characteristics^29-31. 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^-1for 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% V_OC loss, 5% J_SC 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 blends^33-35. 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 J_SC, 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 J_SC’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 V_OC decreases in going from quaternary to ternary to binary blends. This results from the energy level management, driving force, and energy loss (E_loss). Highly sensitive EQE (s-EQE), electroluminescence (EL) and electroluminescence quantum efficiency (EQE-EL) measurements were performed to investigate energy loss channels^36-38. Two energy loss sectors, charge generation (∆E₂ = E_g-E_CT) and charge recombination losses (E_CT-qV_OC) were considered. The charge recombination loss could be further traced to radiative (∆E₁) and non-radiative (∆E₃) recombinations³⁹. The E_CT could be obtained by fitting the sub-gap absorption of the corresponding s-EQE curve (Fig. 3b) following the Marcus theory^{36, 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 ∆E₂of 0.048 eV was obtained for quaternary films, resulting in a higher V_OC. 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 V_OC.

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 ∆E₃to 0.240 eV. Non-radiative recombination can be calculated from ∆E₃ = -kT·ln(EQE_EL). Consequently, it is important to maximize EQE_EL 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 V_OC of the quaternary devices to the smallest E_g-E_CT 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 V_OC 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.

The results show the importance of achieving a detailed balance between morphology, energy loss reduction, and ultrafast charge transfer kinetics in double cascading quaternary blends. The double donor strategy yields a terraced HOMO energy level alignment that facilitates hole transport in the BHJ blends. Adding PC₇₁BM that is uniformly mixed with Y6 produces a terraced LUMO energy level alignment, and in the HOMO levels, the longer exciton diffusion times, in comparison to the splitting times, lead to a high probability that a suitable donor/acceptor interface can be found to split excitons. The close contact between Y6 and PM7 provides an important channel where Y6 trapped exciton or hole carriers can be extracted. Such a unique double cascading energy level simultaneously improves the V_OC, J_SC and FF. A more thorough correlation between the structure, photon-electron processes, and device performances needs to be quantified to establish a solid structure-property relationship. Shown in Fig. 5a-b are the correlations between the structural and device characteristics, Fig. 5c-e shows relationships between charge transfer, transport and performance, Fig. 5f, the correlation between energy loss and V_OC. A quantitative analysis was performed using the Pearson correlation method^{43, 44}, as shown in Fig. 5g-h in 3D and 2D. A correlation factor of 1, indicates a direct correlation, with decreasing values reflecting a loss in correlation. The Pearson correlation coefficient matrix is shown in Tab. S8. It is seen that crystallization is the major factor that accounts for J_SC and FF in photovoltaic devices, thus the quaternary blends reach the maximum value. Mobilities (µ_h and µ_e) are highly dependent on thin film crystallinity. 1/τ₁ shows a strong correlation with the driving force, indicating its strong influence on exciton separation, while the trend in driving force matches well with recombination parameters, suggesting an important mechanism correlating efficient charge transfer and extraction. V_OC should show an inverted correlation with energy loss, and ∆E₃, originating with non-radiative recombination, shows the most significant influence. Therefore, a detailed balance between morphology optimization, charge transfer efficiency, and energy loss channels controls the device output. These observations suggest a new strategy using double cascading quaternary blends for device optimization. The material properties and device performance parameters are highly interconnected, and optimizing only one parameter is not sufficient to optimize performance. Binary mixtures are effective for material screening, and ternary mixtures bring complementary absorption and energy level management into the device design, but quaternary mixtures enable a fine-tuning of both the morphology and electronic structure simultaneously. Fine-tuning such quaternary mixtures introduces a new strategy to achieve higher efficiency OPVs. We extended the double cascading quaternary mixture strategy to other well-studied NFA OPV systems (Fig. S19 and Table S9), with similar results been obtained as seen in PM6:PM7:Y6:PC₇₁BM mixture, underscoring the generality of this approach.

Here, we designed multi-components blends of polymer donors PM6 and PM7, a non-fullerene small molecule acceptor Y6, and a fullerene acceptor PC₇₁BM. An average PCE of 16.69% was obtained when 80% PM6 and 20% PM7 were blended with Y6. The addition of the fourth component PC₇₁BM significantly increased the average efficiency output to 17.71% (PCE_max of 18.07%), with a slightly improved J_SC of 26.55 mA/cm², which could be ascribed to a fine balance between light absorption and charge extraction. The elevated V_OC (0.859 V) of the quaternary device was attributed to the smallest E_g-E_CT energy offset and the suppressed non-radiative recombination losses after the addition of PM7 and, especially, PC₇₁BM. An FF of over 0.79 was obtained, since an optimized morphology formed with balanced crystallization features that ensured better charge transport, as shown in Fig. S20 that summarizes the holistic strategy in OPV efficiency optimization.

Acknowledgements

This work was financially supported by the grant from the National Natural Science Foundation of China (Grant No.51973110, 21734009, 21905102 and 61805138). Z.M. acknowledges the funding supported by the Natural Science Foundation of Shanghai (Grant No. 19ZR1401400). H.Z. acknowledges the funding supported by the National Key Research and Development Program of China (2017YFA0207700). T.P.R. and Q.H. were supported by the US Office of Naval Research under contract N00014-17-1-2244.

Author contributions

F.L. and T.P.R. conceived and directed the study. M.Z fabricated the BHJ devices and did morphology optimization. L.Z. characterized the BHJ devices and conducted the certification. TA results and corresponding analysis were provided by G.Z. and H.Z. Energy loss test was performed by T.H and assisted by Z.M and Z.T. J.X and C.Q. carried out recombination analysis. Q.H assisted in scattering measurement and device performance certification. B.L. carried out TRMC test and F.W provided TEM test. This manuscript was mainly prepared by F.L., M.Z. and L.Z., and all authors participated in the manuscript preparation and commented on the manuscript. We thank Cheng Wang and Chenhui Zhu from Advanced Light Source for providing x-ray scattering tests, which were carried out at beam line 7.3.3 and 11.0.1.2 at the Advanced Light Source, Molecular Foundry, Lawrence Berkeley National Laboratory, supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

Competing interests: The authors declare no competing interests.

Data and materials availability: All data are available and presented in the main text and the supplementary materials.

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Table 1

Photovoltaics of BHJ solar cells under illumination of AM 1.5G, 100 mW/cm². The average parameters were calculated from 40 devices, with the area of 0.032 cm². Values in parentheses denote the best optimal results.
Blend	V_OC (V)	J_SC (mA·cm^− 2)	FF (%)	PCE (%)	µ_e (cm²·s^− 1·V^− 1)	µ_h (cm²·s^− 1·V^− 1)
PM6:Y6	0.842 (0.842 ± 0.001)	25.98 (25.67 ± 0.19)	75.52 (74.91 ± 0.66)	16.52 (16.30 ± 0.11)	(4.42 ± 0.36) × 10^− 4	(1.41 ± 0.49) × 10^− 3
PM6:PM7:Y6	0.848 (0.847 ± 0.001)	26.17 (25.72 ± 0.31)	76.70 (75.91 ± 0.72)	17.02 (16.69 ± 0.27)	(4.69 ± 0.42) × 10^− 4	(1.47 ± 0.57) × 10^− 3
PM6:PM7:Y6:PC₇₁BM	0.859 (0.859 ± 0.001)	26.55 (26.24 ± 0.43)	79.23 (78.7 ± 0.32)	18.07 (17.71 ± 0.23)	(6.53 ± 0.64) × 10^− 4	(1.73 ± 0.16) × 10^− 3
PM7:Y6	0.879 (0.879 ± 0.001)	24.89 (24.79 ± 0.34)	69.10 (67.52 ± 1.25)	15.12 (14.79 ± 0.32)	(3.62 ± 0.56) × 10^− 4	(1.34 ± 0.22) × 10^− 3

Materials. All reagents and chemicals were purchased from commercial sources (Aldrich or Acros) and used without further purification. PM6 and Y6 were purchased from Solarmer Company. PM7 were purchased from Shanghai Vizuchemical Scientific Company. PC₇₁BM was purchased from American Dye Source, Inc. The silicon nitride membrane is from Clean -SiN Company.

Methods. UV-vis absorption spectra were recorded on a Shimadzu spectrometer model UV-1800 with films on the quartz plates at room temperature. The morphologies of the nanostructures were characterized by transmission electron microscopy (TEM, JEM-1400, JEOL, Japan). The GIWAXS characterization of the thin films was performed at the Advanced Light Source (Lawrence Berkeley National Laboratory) on beamline 7.3.3. The incidence angle was 0.16°, and the beam energy was 10 keV. Samples were prepared under device conditions on the Si/PEDOT:PSS substrates. RSoXS was performed at beamline 11.0.1.2 (ALS, LBNL) with a bean energy of 284.2eV. Samples were prepared under device conditions on the Si/PEDOT:PSS substrates, then placed in water and transferred to a silicon nitride window.

Device Fabrication. Organic solar cell devices with ITO/PEDOT:PSS/Active Layer/PFNDI-Br/Ag structures were fabricated according to the following procedure. Patterned ITO glass substrates were sequential cleaned by ultrasonicating in acetone, detergent, deionized water and isopropyl alcohol for 15 min each and then dried under dry oven. The precleaned substrates were treated in an ultraviolet-ozone chamber for 15 min, then a ~40 nm thick PEDOT:PSS (Clevious P VP AI 4083 H. C. Stark, Germany) thin film was deposited onto the ITO surface by spin-coating and baked at 150 °C for 15 min. The blend solution with different mixing ratio (14 mg mL^-1 in total) in CF (with 0.5% CN solvent additive) was stirred at 25 °C for 120 min in advance and then spin-coated on top of the PEDOT:PSS layer. The prepared films were treated with thermal annealing at 90^oC for 6 min. After cooling to room temperature, a ~5 nm thick of PFNDI-Br was spin-coated on the top of active layer. Then, those samples were brought into to an evaporate chamber and a 150 nm thick silver layer was thermally evaporated on the PFNDI-Br layer at a base pressure of 1 × 10^-6 mbar. The evaporation thickness was controlled by SQC-310C deposition controller (INFICON, Germany). Ten devices were fabricated on one substrate and the active area of each device was 0.032 cm² defined by a shadow mask.

Device Characterization. The current-voltage (J-V) curves were measured with Keithley 236 Source under the illumination of AM 1.5G irradiation (100 mW cm^-2) using a 150 W solar simulator (DM-40S3, SAN-EI ELECTRIC, Japan) in glove box at room temperature. The light intensity was determined by a 2 × 2cm² standardized monosilicon cell (Oriel PN 91150V, Newport, USA.) calibrated by the National Renewable Energy Laboratory (NREL). The external quantum efficiency (EQE) measurement was conducted on a TRACQ-BASIC System using a lock-in amplifier with a monochromator and 500 W xenon lamp. A calibrated silicon detector (PRL-12, Newport, USA) with known photo response was utilized as a reference. The J–V curves were measured along the forward scan direction from -0.2 to 1.0 V or the reverse scan direction from 1.0 to -0.2 V, yielding identical results. The scan speed and dwell times were fixed at 0.015 V s⁻¹ and 20 ms, respectively. White LEDs, and an ultraviolet LED (centred at 365 nm) were used as the light source for stability test, the intensity of which was controlled to consistently provide an output of 100 mW/cm². The LED intensity that produced a photocurrent equivalent to J_SC under AM1.5G illumination (J_SC, AM1.5G ) was defined as equivalent to 1-Sun intensity.

SCLC mobility measurements. The electron-only devices were fabricated with ITO/ZnO/Active Layer/ZnO/Ag structures and hole-only devices were fabricated with ITO/PEDOT:PSS/Active Layer/MoO_x/Al structures. The space charge limited current (SCLC) mobility was calculated according to the Mott-Gurney square law J = 9ε_rε₀μV²/8L³, where J is the current density, ε_r is the relative dielectric constant of the transport medium component, ε₀ is the vacuum permittivity, μ is the electron or hole mobility, V is the effective voltage, and L is the thickness of active layer.

Ultraviolet photoelectron spectroscopy measurement. Ultraviolet Photoelectron Spectroscopy analysis was conducted using an AXIS Ultra DLD spectrometer (Kratos Analytical Inc., Mancheser, UK) with a He discharge UV lamp with He I radiation (incident photo energy, 21.22eV), a hemispherical analyzer operating in the fixed analyzer transmission mode and the standard aperture (analysis area: 0.3 mm * 0.7 mm).

Transient photovoltage measurement. The lifetime of carriers can be measured by the transient photovoltage measurement. The background illumination was provided by a normal LED light source, and pulsed light was provided by arbitrary wave generator (AFG322C, Tektronix). The photovoltage traces were registered by the oscilloscope (AFG322C, Tektronix).

Electroluminescence measurement. Electroluminescence spectrum measurement was conducted by direct-current meter (PWS2326, Tectronix) to provide bias voltage for the test device, and the electroluminescence emissions were recorded by the fluorescence spectrometer (KYMERA-328I-B2, Andor technology LTD).

EQE_EL measurement. The EQE_EL was recorded with an in-house-built system comprising a standard silicon photodiode 1010B, Keithley 2400 source meter (for supplying voltages and recording injected currents), and Keithley 6482 picoammeter (for measuring the emitted light intensity).

Highly sensitive EQE (s-EQE). The halogen light source (LSH-75, Newport) passed through the monochromator(CS260-RG-3-MC-A, Newport) to form monochromatic light, which was focused on the device to generate electrical signals. Signals were finally collected by the front-end current amplifier (SR570, Stanford) and phase-locked amplifier (Newport). A corrected silicon solar cell (S1337-1010BR) was used as a standard detector.

Transient absorption spectroscopy (TA). For femtosecond transient absorption spectroscopy, the fundamental output from Yb:KGW laser (1030 nm, 220 fs Gaussian fit, 100 kHz, Light Conversion Ltd) was separated to two light beam. One was introduced to NOPA (ORPHEUS-N, Light Conversion Ltd) to produce a certain wavelength for pump beam (here we use 550 and 750 nm, 30 fs pulse duration), the other was focused onto a YAG plate to generate white light continuum as probe beam. The pump and probe overlapped on the sample at a small angle less than 10°. The transmitted probe light from sample was collected by a linear CCD array.

Time-resolved microwave conductivity (TRMC). TRMC measurements were performed using a pulsed Q-switched neodymium yttrium-aluminium-garnet (Nd:YAG) laser (3-5 ns pulses, 10 Hz repetition rate, Continuum Powerlite) and an optical parametric oscillator (Continuum Panther) pumped by the third harmonic (355 nm) of the Nd:YAG laser. Neutral density filters enabled the adjustment of the pump fluence by up to four orders of magnitude. A calibrated Schottky barrier diode (~1 ns response time) measured time-dependent changes in the microwave probe power, and an oscilloscope with subnanosecond resolution tracked the transient behaviour of changes in the microwave power. Although these types of measurements often employ a partially transparent (to microwaves) metal iris to create a resonant microwave cavity that enhances signal strength at the expense of time resolution, the signal here was sufficiently strong that we chose to omit the iris and maintain the full temporal resolution afforded by the ~4 ns optical pump.

There is NO Competing Interest.

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Single-layered organic photovoltaics with double cascading charge transport pathways: 18 % efficiencies

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