Reduced Energy Offsets and Low Energy Losses Lead to Efficient

Sep 13, 2018 - Two organic semiconductor molecules, CS01 and MPU3 – an electron donor and electron acceptor, respectively – are described for use ...
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Reduced Energy Offsets and Low Energy Losses Lead to Efficient (∼10% at 1 sun) Ternary Organic Solar Cells ACS Energy Lett. Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/08/18. For personal use only.

Maria Privado,†,# Cristina Rodríguez Seco,‡,§,# Rahul Singhal,∥ Pilar de la Cruz,† Fernando Langa,*,† Ganesh D. Sharma,*,⊥ and Emilio Palomares*,‡,∇ †

Universidad de Castilla La Mancha, Institute of Nanoscience, Nanotechnology and Molecular Materials (INAMOL), Campus de la Fábrica de Armas, E-45071 Toledo, Spain ‡ Institute of Chemical Research of Catalonia, The Barcelona Institute of Science and Technology (ICIQ-BIST), Avda. Països Catalans, 16, E-43007 Tarragona, Spain § Departament d’Enginyeria Electrònica, Elèctrica I Automàtica, URV, Avda. Països Catalans, 26, E-43007 Tarragona, Spain ∥ Department of Physics, Malviya National Institute of Technology, JLN Marg, Jaipur 302017, Rajasthan, India ⊥ Department of Physics, The LMN Institute of Information Technology, Jamdoli, 302031 Jaipur, Rajasthan, India ∇ ICREA, Passeig Lluis Companys, 23, E-08010 Barcelona, Spain S Supporting Information *

ABSTRACT: Two organic semiconductor molecules, CS01 and MPU3, an electron donor and electron acceptor, respectively, are described for use in organic bulk heterojunction solar cells. The highest occupied molecular orbital (HOMO) energy offset (ΔΕHOMO) between these two molecules is as low as 0.29 eV. Moreover, the lowest unoccupied molecular orbital (LUMO) energy offset (ΔΕLUMO) is as low as 0.14 eV. Nonetheless, the interfacial charge-transfer process upon light excitation is extremely efficient, and solar cells with efficiencies, under normal conditions, as high as 7.81% have been fabricated. Furthermore, the incorporation of PC71BM (another electron acceptor molecule) to fabricate a triple organic heterojunction led to efficiencies close to 10% at 1 sun irradiation conditions. The improvement is due to a better panchromatic harvesting of the sunlight and a better charge balance, which allows the rapid extraction of carriers before they can recombine.

T

Interestingly, ternary OSCs based on small-molecule donor and acceptor (all SMs) have hardly been studied, and their overall PCE values still lag behind those of their polymer counterparts. Nevertheless, in binary OSCs, small molecules display excellent efficiencies due to the known advantages over polymers.24−27 In recent years, the development of nonfullerene small-molecule acceptors has progressed rapidly,28−33 and overall PCE values in the range of 13−14% have been reported, i.e., values higher than those of the fullerene counterparts. OSCs based on SM donors and nonfullerene small-molecule acceptors have shown PCE values over 11%.27,34−39 The achievement of low energy loss values is a target of great interest.40−42 Recently, we reported CS01 (Scheme 1), and we used it as a donor in conjunction with PC71BM to give an overall PCE of 4.80%.43 The PCE of this device is mainly due

he low-cost manufacturing benefits together with the wide abundance of building block materials and the ability to be applied to flexible substrates make organic solar cells (OSCs) one promising technology to address the increasing green energy demand.1−6 Power conversion efficiency (PCE) of binary bulk heterojunction (BHJ) OSCs has already exceeded around 11%7−9 and 13−14%10−13 when using fullerene and nonfullerene acceptors, respectively. Despite these impressive PCE values, some unsolved issues include the increase of the light absorption and better optimization of the donor/acceptor energy levels.14,15 One of the most promising strategies to overcome these issues is to employ a ternary active layer in OSCs.13,16−20 The OSCs based on a ternary active layer harvest more photons and provide better PCE than OSCs based on binary active layers in some cases.21,22 Recently, Xiao et al. reported an excellent PCE of more than 14% for an OSC based on a ternary active layer consisting of a conjugated polymer donor and two acceptors (fullerene and nonfullerene).23 © XXXX American Chemical Society

Received: August 1, 2018 Accepted: September 13, 2018

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DOI: 10.1021/acsenergylett.8b01400 ACS Energy Lett. 2018, 3, 2418−2424

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Cite This: ACS Energy Lett. 2018, 3, 2418−2424

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ACS Energy Letters

The optical absorption spectra for the materials are shown in Figure 1. It can be seen from this figure that the donor CS01

Scheme 1. Chemical Structures of CS01, MPU3, and PC71BM

Figure 1. Normalized optical absorption spectra of CS01, MPU3, and PC71BM in thin film.

and acceptor MPU3 have complementary absorption spectra with different absorption bands centered at 574 and 702 nm, respectively. The absorption spectrum of CS01, MPU3, and PC71BM in the chloroform solution is shown in Figure S1 (Supporting Information). The highest occupied molecular orbital (HOMO)/LUMO energy levels of CS01 and MPU3, as determined from the electrochemical data, are −5.32/−3.60 eV and −5.61/−3.74 eV,43,44 respectively (Figure 2).

to the moderate Jsc value, which is related to the limited photoresponse range. Moreover, we have designed a nonfullerene small-molecule acceptor, MPU3 (Scheme 1). This system gave an overall PCE of 9.14% with a low energy loss of 0.54 eV.44 In the work reported here, we used MPU3 as an acceptor and CS01 as a donor for the fabrication of binary BHJ OSCs and achieved a PCE of 7.81%, with an energy loss of 0.48 eV, for the optimized CS01:MPU3 active layer. These results are believed to be related to the low lowest unoccupied molecular orbital (LUMO) offset between the CS01 and MPU3. The values obtained are higher than those of the fullerene-based counterpart. The CS01:MPU3 films showed absorptions in the range from 500 to 900 nm, but the absorption below 500 nm was very weak. As a consequence, it was realistic to incorporate PC71BM as a second acceptor, with an absorption in the range from 350 to 500 nm, in order to assist the photon harvesting and further enhance the PCE of OSCs. Although the LUMO offset between CS01 and MPU3 is very low (0.14 eV), i.e., below that threshold value for charge transfer from donor to acceptor (∼0.3 eV), the photoluminescence measurements showed that electron transfer from CS01 to MPU3 was efficient and led to a high Voc and low voltage loss for the CS01:MPU3-based OSC. After optimization of the ternary active layer, i.e., the weight ratio of electroactive molecules and solvent vapor annealing time, the ternary OSC gave a PCE of 9.94% with an energy loss of 0.56 eV, which is the lowest value obtained for a ternary OSC to the best of our knowledge. Based on the energy levels of the CS01, MPU3, and PC71BM in the ternary active layer, a cascade energy level alignment is formed, and it is believed that this can reduce the LUMO energy offsets between CS01 and PC71BM, which in turn can facilitate the germinate recombination through the chargetransfer state45 and result in an improvement in the FF of ternary OSC.

Figure 2. Energy level diagram of CS01, MPU3, and PC71BM.

The HOMO offset (ΔEHOMO) between the CS01 and MPU3 is 0.29 eV (very close to the empirical threshold value of ∼0.3 eV), which is sufficient for efficient hole transfer from MPU3 to CS01. On the other hand, the LUMO offset (ΔELUMO) between CS01 and MPU3 is only 0.14 eV, i.e., below the threshold value of 0.3 eV. In order to achieve high Jsc and Voc values for OSCs, it is desirable to minimize the ΔELUMO between the donor and acceptor while still allowing efficient electron transfer from donor to acceptor. In our OSCs based on a CS01:MPU3 binary BHJ active layer, the electron transfer from CS01 to MPU3 is efficient, despite the small ΔELUMO value (0.14 eV), according to the photoluminescence (PL). Therefore, we expect that an OSC based on a CS01:MPU3 binary BHJ active layer may provide values for 2419

DOI: 10.1021/acsenergylett.8b01400 ACS Energy Lett. 2018, 3, 2418−2424

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ACS Energy Letters Voc and Jsc that are higher than those of a CS01:PC71BM counterpart. The results indicate that a ΔELUMO larger than 0.3 eV might not be an essential requirement for efficient dissociation and charge transfer in nonfullerene electron acceptors used in OSCs. The PL spectra were recorded in order to investigate the charge-transfer yield in the CS01:MPU3 thin film. The PL spectra of CS01, MPU3, and their blended films are shown in Figure 3. The maximum absorption peaks of CS01 (520 nm)

Figure 4. Current−voltage (J−V) characteristics under illumination of the OSCs based on optimized CS01:PC71BM (black), CS01:MPU3 (red), and CS01:PC71BM:MPU3 (blue).

The Voc values for OSCs based on the MPU3 acceptor are higher than that obtained using PC71BM, which was expected because of the higher LUMO energy level of MPU3. The MPU3 solar cells displayed greater Jsc than the fullerene-based devices. One parameter that is directly related to the greater capability to convert sunlight into electrical current is the active layer thickness. Thus, it is of interest to record the ultraviolet (UV)−visible spectrum of the films as used in active devices (see Figure 5). The absorption spectra of the optimized binary and ternary in solution are shown in Figure S2 for their molar extinction coefficient. As can be seen, CS01:MPU3 shows panchromatic absorbance from the UV to the infrared (IR) region of the sun spectra. In contrast, the lack of MPU3 limits the light absorption to the visible region of the sun spectra. The broader absorption of the CS01:PC71BM system may be related to the improvement in the Jsc. It has been shown that the organic solar cell voltage is related to the quasi Fermi level splitting between the electron donor and the electron acceptor. In our particular case, the solar cells having MPU3 displays higher open-circuit voltage due to the LUMO energy level value, which is higher than the fullerene derivative.46−50 Moreover the HOMO energy values for the CS01:MPU3 blend is deeper than for CS01:PC71BM, and furthermore, the small energy offset between the HOMO and LUMO values for the electron acceptor and the electron donor, respectively, also leads to minor energy losses in the Voc. The IPCE spectra of the devices based on the optimized CS01:MPU3 and CS01:PC71BM are shown in Figure 6. The CS01:MPU3 device showed a photoresponse from 410 to 900 nm, whereas the CS01:PC71BM device showed a response only up to 700 nm. The IPCE responses of the OSCs closely resemble the optical absorption spectra of the corresponding active layers (Figure 5), which indicates that both the donor (CS01) and acceptor (PC71BM or MPU3) contribute to the photocurrent generation. Although the ΔE LUMO for CS01:MPU3 is very small (0.14 eV), the IPCE spectra indicate that there is sufficient driving force for the electron transfer to take place from the CS01 to MPU3. The Jsc values estimated from the integration of the IPCE spectra of the

Figure 3. Photoluminescence spectra of pristine CS01 and MPU3 and their binary and ternary blends with PC71BM in thin films.

and MPU3 (710 nm) were chosen to excite the CS01 and MPU3, respectively. It can be seen that excitation at 520 nm led to significant quenching of the PL intensity of CS01 (by up to 84%) for the CS01:MPU3 blend, thus suggesting efficient electron transfer from CS01 to MPU3. Similarly, excitation at 710 nm also led to quenching of the PL intensity of MPU3 (by 79%) for the CS01:MPU3 blend film. In order to evaluate the photovoltaic properties of the OSCs based on CS01:MPU3 binary BHJ active layers, we initially screened the performance of OSCs by varying the weight ratio of CS01:MPU3 using chloroform as solvent. The OSCs were fabricated with a conventional device arrangement, i.e., ITO/ PEDOT:PSS/active layer/PFN/Al. We have varied the donor (CS01) to acceptor (MPU3 or PC71BM) weight ratio to optimize the as-cast active layer, and the photovoltaic data are summarized in Tables S1 and S2 for CS01:MPU3 and CS01:PC71BM, respectively. The OSCs based on a 1:2 weight ratio showed the best performance, with an overall PCE of 3.34(±0.08)% (Jsc = 9.48 mA/cm2, Voc = 1.10 V, and FF = 0.32). After optimization of the CS01:MPU3 active layer by SVA treatment, the OSC behavior was enhanced to give a PCE of 7.81(±0.06)% (Jsc = 13.04 mA/cm2, Voc = 1.07 V, and FF = 0.56). This PCE is higher than that of the PC71BM counterpart processed under identical conditions (CS01:PC71BM; PCE = 4.80 ± 0.07%). The current−voltage characteristics of the OSCs based on optimized CS01:MPU3 and CS01:PC 71BM are shown in Figure 4, and the photovoltaic parameters are collected in Table 1. 2420

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ACS Energy Letters Table 1. Photovoltaic Parameters of the OSCs Based on Different Active Layers active layer

Jsc (mA/cm2)

CS01:PC71BM (1:2) CS01:PC71BM (1:2) (SVA) CS01:MPU3 (1:2) CS01:MPU3 (1:2) (SVA) CS01:PC71BM:MPU3 (1:0.5:1.5) (as cast) CS01:PC71BM:MPU3 (1:0.5:1.5) (SVA)

7.13 (±0.08) 10.48 (±0.09) 9.48 (±0.07) 13.04 (±0.05) 12.18 (±0.07) 16.27 (±0.06)

Voc (V) 0.84 0.79 1.10 1.07 1.02 0.97

(±0.03) (±0.02) (±0.04) (±0.05) (±0.02) (±0.03)

FF 0.34 0.58 0.32 0.56 0.52 0.63

(±0.02) (±0.03) (±.02) (±0.03) (±0.02) (±0.02)

PCE (%) 2.03 4.80 3.34 7.81 6.46 9.94

(±0.08) (±0.07) (±0.08) (±0.06) (±0.04) (±0.08)

(1.95)a (4.73)a (3.26)a (7.74)a (6.41)a (9.86)a

a

Average of eight devices.

nonradiative recombination loss.51,52 The Eloss values for OSCs based on SVA-treated CS01:MPU3 and CS01:PC71BM are 0.45 and 0.94 eV, respectively. As the ΔE LUMO for CS01:MPU3 is very low (0.14 eV) compared to that for CS01:PC71BM (0.73 eV), the radiative recombination loss may be attributed to the low Eloss.53,54 The reduced Eloss is the reason for the higher Voc for the CS01:MPU3-based OSCs. The active CS01:MPU3 layer showed poor absorption below 500 nm, and it was therefore reasonable to incorporate PC71BM, which exhibits strong absorption in the range of 300−500 nm, as a third component to improve the lightharvesting efficiency of the active layer and thus increase the PCE of the corresponding device. Furthermore, the HOMO and LUMO energy levels of PC71BM are lower than those of MPU3 (Figure 3). This type of cascade energy level alignment improves the charge transfer. Initially, the weight ratio MPU3 (acceptor 1):PC71BM (acceptor 2) was carefully varied while the weight ratio of CS01:(MPU3+PC71BM) was kept at 1:2 based on the binary OSCs. The photovoltaic data are summarized in Table S3. The increase in the amount of PC71BM in the CS01:MPU3 active layer leads to higher Jsc and FF values, while the Voc value was reduced slightly. The best photovoltaic result was observed for the active layer CS01:PC71BM:MPU3 with a weight ratio of 1:0.5:1.5 in chloroform (CF) (PCE of 6.46 ± 0.04% with Jsc = 12.18 mA/ cm2, Voc = 1.02 V, and FF = 0.52). A further increase in PC71BM beyond 0.5 led to decrease in the Jsc value due to the insufficient absorption in the longer-wavelength region. In order to improve PCE, the ternary active layer was treated by SVA using tetrahydrofuran (THF) for 2 min. The J−V characteristics of the OSC based on the optimized ternary active layer are shown in Figure 4, and the photovoltaic parameters are provided in Table 1. The OSCs based on the optimized ternary active layer showed an overall PCE of 9.94 ± 0.08% with Jsc = 16.27 mA/cm2, Voc = 0.97 V, and FF = 0.63. The increase in Jsc for the ternary active layer is due to the complementary absorption spectra. The FF value of the ternary device also improved significantly to 0.63 and this is related to the morphology modulation and better charge balanced mobility due to the incorporation of PC71BM, which plays a key role in the enhancement of the PCE. The IPCE spectra of the optimized ternary OSC are shown in Figure 6. These closely resemble the absorption spectra of the ternary active layer (Figure 5), and this indicates that all components used in the ternary active layer contribute to the photocurrent generation. The Jsc value estimated from the IPCE spectra is 16.22 mA/cm2, and this is very close to the value obtained from the J−V characteristics under illumination. The slight decrease in the Voc after the incorporation of PC71BM is due to the deeper LUMO energy level of PC71BM when compared to MPU3, a difference that gives a lower energy offset with respect to the HOMO energy level.55 The

Figure 5. Optical absorption spectra of binary [CS01:PC71BM (black) and CS01:MPU3 (red)] and ternary active layers [CS01:PC71BM:MPU3 as cast (blue) and SVA (green)].

Figure 6. IPCE spectra of the OSCs based on optimized CS01:PC71BM (black), CS01:MPU3 (red), and CS01:PC71BM:MPU3 (blue).

OSCs are 10.39 and 12.95 mA/cm2 for CS01:PC71BM and CS01:MPU3 active layers, respectively, and these match well with the values obtained from the J−V measurements. One of the critical issues for OSCs is their relatively large energy loss (Eloss), defined as Eloss = Eg/q − Voc, where Eg is the optical band gap of the donor or acceptor, which is in the range of 0.7−1.0 eV. The energy loss of OSCs has three causes, including the unavoidably large driving force for exciton dissociation (≈ 0.2−0.3 eV) and additional radiative and 2421

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ACS Energy Letters values of ΔELUMO and ΔEHOMO for the CS01:MPU3 binary BHJ active layer are less than 0.3 eV, and this may hinder the photoinduced charge transfer and also limit the PCE of the corresponding OSC. Compared to MPU3, PC71BM has lower LUMO and HOMO energy levels, and this difference acts as an energy driver to enhance the driving force for charge transfer.56 As a result, when compared to the OSC based on CS01:MPU3, the optimized CS01:PC71BM:MPU3-based counterpart showed an enhanced PCE of 9.84% with a small energy loss of 0.56 eV and a high Voc of 0.96 V. As discussed above, the PL intensity of the CS01:MPU3 blend is quenched by 84% and 79% relative to pristine CS01 and MPU3, respectively. The insufficient PL quenching of the CS01:MPU3 may be attributed to the unsatisfactory driving force for charge transfer ΔGCT in the BHJ active layer. However, it was observed that in the optimized CS01:PC71BM:MPU3, the PL intensity decreased by 94% and 90% relative to the pristine CS01 and MPU3, respectively (as shown in Figure 4). The strong PL quenching indicates more efficient charge transfer from donor to acceptor and acceptor to donor. The enhanced IPCE response of CS01 and MPU3 in the CS01:PC71BM:MPU3 blend also suggests more efficient charge transfer from the donor to acceptor and acceptor to donor. The more efficient charge transfer in the ternary blend is attributed to the lower LUMO and HOMO energy levels of PC71BM, and this acts as an energy driver and leads to the enhancement in the Jsc and FF. Discussion of the charge recombination is included in the Supporting Information. In summary, small-molecule OSCs have been fabricated with a low band gap nonfullerene acceptor, MPU3, as an acceptor and a benzothiadiazole-substituted small-molecule donor. After optimization of the CS01:MPU3 active layer, the OSC showed an overall PCE of 7.81%, which is higher than that of the CS01:PC71BM counterpart processed under identical conditions. The higher LUMO energy MPU3 is beneficial for high Voc and low energy loss (0. 48 eV), while the low band gap and absorption profile, which extended up to 900 nm, are beneficial for higher Jsc. Finally, the OSC based on an optimized ternary CS01:PC71BM:MPU3 gave a PCE as high as 9.94% with a Jsc value of 16.27 mA/cm2 and a low energy loss of 0.56 eV. The increase in the PCE for ternary OSCs is attributed to the fact that PC71BM acts as the energy driving force for the exciton dissociation because of the lower values of the HOMO and LUMO energy levels when compared to those of MPU3.

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Emilio Palomares: 0000-0002-5092-9227 Author Contributions #

M.P. and C.R.S. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.P. thanks ICIQ-BIST and ICREA for their support. The Spanish Ministerio de Ciencia, Innovación y Universidades and The Agencia Estatal de Investigación (AEI) are also acknowledged for the CTQ2016800042-R/AIE project and the FPI grant to C.R.S. (BES-2014-068795). F.L. thanks MINECO (Spain) for financial support (CTQ2016-79189-R) and the FPU grant to M.P.U. (FPU16/01687)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b01400. Optical absorption spectra, tables with complementary photovoltaic parameters, and additional electronic studies about the incorporation of PC71BM (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fernando Langa: 0000-0002-7694-7722 2422

DOI: 10.1021/acsenergylett.8b01400 ACS Energy Lett. 2018, 3, 2418−2424

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DOI: 10.1021/acsenergylett.8b01400 ACS Energy Lett. 2018, 3, 2418−2424