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Article Cite This: ACS Omega 2017, 2, 7777-7783

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Unraveling the Characteristic Shape for Magnetic Field Effects in Polymer−Fullerene Solar Cells Marco Antonio Cabero Zabalaga,†,‡ Jiaqi Wei,†,‡ Huaiwen Yang,†,‡ Bing Bing Fan,§ Yanming Sun,*,§ and Weisheng Zhao*,†,‡ †

Fert Beijing Institute, BDBC, ‡School of Electronic and Information Engineering, and §Heeger Beijing Research and Development Center, School of Chemistry and Environment, Beihang University, Beijing 100191, China S Supporting Information *

ABSTRACT: Spin-dependent effects in organic solar cells (OSCs) are responsible for tuning the electric current when an external magnetic field is applied. Here, we report the magnetic field effect (MFE) on wide-bandgap (WBG) solar cells based on the polymers PBDT(O)-T1 and PBDT(Se)-T1 blended with PC70BM. Furthermore, we propose an experimental method based on the electrical transport (i−V) measurements to unveil the negative magneto conductance (MC) at small bias. The observed curves in a double-logarithmic scale display a particular S-like shape, independent of the OSC power conversion efficiency (PCE) or MC amplitudes. Additionally, from the slope of the S-like shape curve, it is possible to identify the fullerene concentrations that would result in the minimum MC and the maximum PCE. Our work opens up a door to find more patterns to describe MFE and PCE in polymer−fullerene solar cells, without the application of external magnetic or luminous sources.

1. INTRODUCTION Organic devices have drawn great interest because of its promising capacities compared to inorganic semiconductors in photovoltaic applications, regardless of their limitations in power conversion efficiency (PCE) and stability. Their long spin diffusion length makes them attractive and promising in the field of organic spintronics to transfer and process information. The magneto conductance (MC) describes the spindependent effects (SDEs) or magnetic field effects (MFEs) in organic solar cells (OSCs).1−21 It has been characterized by Lorentzian line shape curves.22 The MC curve can be divided into three regions that correspond to an ultra-small field effect (USFE) upto ∼10 mT, low field effect (LFE) upto ∼300 mT, and high field effect (HFE) from ∼300 mT onwards.11,19 Exploring the MC in organic devices has been a motivation for experimental and theoretical studies.5−22 SDEs have been found in OSCs composed of poly(9,9dioctylfluorenyl-2,7-diyl) (PFO),23 P3HT:PCBM,24,53 and MDMO-PPV:PCBM.25 Recently, wide-bandgap (WBG) polymers have attracted much attention due to their important applications in multiple junction OSCs.37,38 In Alq3 lightemitting diodes,39−45 the electrically injected current was increased by up to 5% after the application of an external magnetic field.5 Different mechanisms have been proposed relying on the spin-dependent interactions.31−33 The spin-dependent interactions between charged particles will result in different effects on the electric current that flows through the organic device. First, within the bipolaron mechanism,31,51,54−69 the reaction between particles of the same electric charge takes effect between one quasistationary © 2017 American Chemical Society

and another nearby free carrier. The respective alignment of the spins is important during the interaction. According to the Pauli exclusion principle, this event is more likely to occur for a pair of charges in a singlet configuration than in a triplet state. At low magnetic fields, the random hyperfine field46 mixes the spin states of the charged particles lifting the spin blocking, whereas at high magnetic fields, the hyperfine field interaction is overruled and the spin blocking is regained. As a consequence, the magnetic field dependence of the bipolaron mechanism leads to a negative effect on the MC.25,70 Second, the e−h mechanism proposed by Prigodin et al.32 indicates that electron−hole precursor pairs can dissociate to form free polarons or recombine after the formation of the exciton. If these reactions are spin selective,48 it is possible to consider different recombination rates for singlet and triplet states. Because there is an internal competition between dissociation and recombination, the application of an external magnetic field overrules the hyperfine interactions resulting in less mixing and giving rise to a positive MC.32 The performance of the polymer−fullerene solar cells is closely dependent on the amount of PCBM (fullerene) blended in the organic layer.52 The electric current in the polymer−fullerene solar cells is increased, indicating the efficient dissociation of electron−hole (e−h) precursor pairs and leading to the highest PCE.34−36 Under this scenario, the most likely mechanism that governs the SDE is the e−h mechanism. It is important to mention that at HFE, the polarons will have a slightly different g-factor. As a Received: October 1, 2017 Accepted: October 31, 2017 Published: November 10, 2017 7777

DOI: 10.1021/acsomega.7b01470 ACS Omega 2017, 2, 7777−7783

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result, additional spin mixing occurs at large fields, referred to as the Δg-mechanism.25 Last, we will discuss the triplet−polaron mechanism. Because triplets have longer lifetime compared to singlets, an increased population of triplets, as a result of the magnetic field enhanced intersystem crossing (ISC) process, can boost the free charge generation, resulting in a positive MC. In other words, the average dissociation rate decreases (negative MC) with the increase of triplets and the diffusion rate increases (positive MC). Therefore, the negative and positive values of the MC comprise both components due to dissociation and diffusion. In the triplet−polaron mechanism, the hyperfine field is responsible for spin mixing,19 which plays an important role in the LFE (between singlet and triplets) and the HFE (between doublets and quadruplets) regions of the MC curve.25 The random recombination of electron and hole spin polarization of the injected current statistically yields a ratio of 1:3 in organic materials.26,27 After the application of an external magnetic field, the singlet and triplet recombination ratio can change, affecting the electric current through the device28,29 and resulting in a positive or negative MC.6,30 The separation distance between electron and holes in the polymer−fullerene blend is critical to detect the presence of SDE. Several studies have been performed to measure the MFE in OSCs with the application of external magnetic fields. In this manuscript, we propose a novel method to identify, the negative MC at small bias, and obtain the fullerene concentrations for the minimum MC and maximum PCE. This proposal is based on the electrical transport (i−V) curves obtained at different fullerene concentrations. We conjecture that the curve patterns, such as the shape and slope, offer information about the fullerene concentration for the smallest MC and the maximum PCE without the application of external magnetic or luminous sources. Here, we fabricate the polymer−fullerene WBG solar cells PBDT(O)-T1 and PBDT(Se)-T1, blended with the fullerene PC70BM to investigate the MC in the dark and the PCE amplitudes under illumination at different fullerene concentrations and applied voltages. Then, we modify the electron transport layer (ETL) and the hole transport layer (HTL) for both solar cells and kept the active layer at 50% of fullerene concentration to measure the MC and the PCE. Additionally, electrical transport (i−V) measurements are obtained without the application of external magnetic fields or illumination in different configurations and fullerene concentrations. There have been similar studies for tuning the MC in OSCs,25 however in this manuscript, we focus on two new polymers blended with the fullerene PC70BM and propose a novel method to investigate the MC and the PCE without the use of external magnetic or luminous sources.

Figure 1. WBG polymer−fullerene PBDT(O)-T1:PC70BM solar cell. (a) Molecular structure and (b) device structure under investigation.

concentration. Notably, for all the measurements, it is observed that the USFE and LFE remain positive for all concentrations when the applied voltage is higher than the built-in voltage. Figure 3a (on the left side of the y-axis) reproduces the MC at 1T measured at different fullerene concentrations. We can observe that the smallest value of MC corresponds to the fullerene concentration of 65% and reaches an oscillatory state beyond 75%. We also measured the MC when the applied voltage changes from a higher to a lower value compared to the built-in voltage. In Figure 3b, “Yes” denotes the change from positive to negative MC only at particular fullerene concentrations. 2.2. MC in PBDT(O)-T1:PC70BM Solar Cells. To examine the electrical transport (i−V curve) under illumination at different fullerene concentrations, PCE measurements were performed. We return to Figure 3a (right side of the y-axis), where we observe different rates of change in the PCE curve that correlates with different morphology regions in the polymer−fullerene solar cell.15,25,47,50 The PCE curve can be divided into two intervals, the first from 0 to 50% and the second from 50 to 95%. The maximum PCE was obtained at 50% fullerene concentration, and it is considered as the inflection point. In the first interval, after starting with a negligible value at 0% fullerene concentration due to quenching sites of excitons,25 the rate of change in the PCE as a function of fullerene concentration is positive, reaching the maximum value at 50% of fullerene concentration. The maximum value of the PCE is ascribed to the balance in the generation and the transfer of excitons and extraction of polarons. As the fullerene concentration increases beyond 50%, the rate of change turns to negative. In the interval of 50−95%, the PCEs decay owing to the imbalance of holes with respect to the electrons in the active layer and the fullerene aggregation dominated by single-carrier electron and hole currents.15,47 2.3. Analysis of the MC and PCE in PBDT(O)T1:PC70BM Solar Cells. As reported in Section 1, the MC curve displays three regions. In this section, we will focus only on the HFE region obtained at 1 T, which is plotted in Figure 3a. According to the horizontal axis, the MC curve can be divided into three regions with respect to the fullerene concentrations, the first one located between 0 and 50%, the second between 50 and 65%, and the third in the interval of 75−95%. On the other hand, the PCE curve in Figure 3a is divided into two regions with respect to the fullerene concentration, the first one from 0 to 50% and the second from 50 to 95%.

2. RESULTS AND DISCUSSION 2.1. MC in PBDT(O)-T1:PC70BM Solar Cell. We measured the MC as a function of external magnetic field, fullerene concentration, and applied bias voltage in the polymer− fullerene solar cell PBDT(O)-T1:PC70BM shown in Figure 1. The current density was set at ∼6 mA mm−2. In Figure 2, we observe the MC at three fullerene concentrations: (a) 25, (b) 50, and (c) 75%. In Figure 2a, we note a positive USFE and LFE that reaches a plateau after ∼300 mT. Moreover, in Figure 2b, we observe both USFE and LFE positives, whereas in the HFE region the MC becomes negative. Finally, in Figure 2c, the MC follows the same trend as the OSC at 25% of fullerene 7778


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Figure 3. Global trends for PBDT(O)-T11−x:PC70BMx measured at different fullerene concentrations. (a) The blue circles represent the PCE measured under Air Mass 1.5 Global (AM 1.5 G). The black squares represent the maximum value of the MCat 1T. (b) Sign change in the MC, when the applied voltage is switched from a higher to a lower value than the built-in voltage.

polymer PBDT(O)-T1,15 providing an ideal scenario for an electron−hole pair formation as well as charge dissociation. Indeed, the high PCE corroborates this statement. We notice that the PCE reaches the maximum value at 50% of fullerene concentration. However, at smaller or higher fullerene concentrations, the PCE decreases because of the imbalance of the secondary generated charge carriers and the morphological changes due to different fullerene concentrations.38 Based on this observation, we suggest the electron−hole mechanism32 to be responsible for the positive USFE and LFE in the region. It is remarkable to mention that passing through 50 and 65% of fullerene concentration, the HFE becomes negative. We assume that this change is caused by the Δgmechanism as a fingerprint of an appropriate generation and dissociation of charged carriers and the new spin mixing that takes place. Finally, in the region beyond 65%, the imbalance of charge carriers due to the increasing concentration and aggregation of fullerene, separates the solar cell into two phases, performing as a single charge carrier device.15,47,50 In this region, the PCE decreases as the concentration of fullerene increases due to intermolecular interactions and diminished dissociation of charged particles. The charge transport of the single charge carrier device dominates this process. Once again, triplet exciton densities can be very high because of the long triplet lifetime of the electrically injected current that competes with the limited dissociation of charge carriers. We suggest the

Figure 2. The MC in PBDT(O)-T11−x:PC70BMx at three different concentrations (a) x = 25%, (b) x = 50%, and (c) at x = 75%. The electric current was set at similar values for all the concentrations. The applied voltage was set to be higher than the built-in voltage. (See Figure S1 in the Supporting Information for more details about the MC at other fullerene concentrations).

First, at the beginning of the first region (0% of fullerene concentration), the imbalance of charge carriers allows the solar cell to act as a single-carrier device. Under this scenario, dissociation of charge carriers becomes an unlikely event for PCE. The electrically injected current spin polarizations statistically yields the singlet and triplet ratios of 1:3,26,27 where triplet excitonic states densities are higher for their long triplet lifetime.25 Therefore, the triplet-polaron33,75 mechanism becomes the most likely candidate to explain the positive USFE, LFE, and HFE in the first region. Second, when the content of fullerene is increased up to 50%, the fullerene is homogeneously distributed throughout the 7779


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triplet−polaron mechanism as a likely candidate for the positive USFE, LFE, and HFE at concentrations beyond 75%. After suggesting the mechanisms responsible for line shapes in Figure 3a, we analyzed the origin of the signs in the three regions of the MC curves. For that, we recall Figure 2. In the USFE region, a competition between the low external magnetic field and the hyperfine field interaction that degenerate the triplet states20,71,73,74 leads to the suppression of intersystem crossing (ISC) between singlet and triplet polaron pairs. As a consequence, the sign of the MC in this region will be determined by the spin-dependent interaction of the elements in the active layer of the OSCs, where the spin−orbit coupling (SOC) plays an important role, especially when heavy organic elements such as selenium are present. In our experiment, the weak nature of the SOC of oxygen in the PBDT(O)-T1 polymer is overruled by the external magnetic field, leading to a positive MC at all concentrations. For the LFE and HFE regions, the sign of the MC is ascribed to the intermolecular secondary charges collected by the OSC electrodes, where processes such as ISC between singlet and triplet states will affect the charge carrier population owing to different recombination and dissociation rates leading to a positive MC in the LFE region when the applied voltage is higher than the built-in voltage and a negative MC when the applied voltage is smaller than the built-in voltage. Moreover, in the HFE region, different Lande factors for e−h pairs lead to the enhancement of ISC at 50 and 65% of fullerene concentration.25,71,72 In the following section, we report the results of the MC and PCE measurements in a second solar cell based on the polymer PBDT(Se)-T1 at 50% of fullerene concentration. Its molecular structure differs only in one atom with respect to PBDT(O)T1, as can be seen in Supporting Information Figure S2. Furthermore, we measure the electrical transport (i−V) curves without external sources of illumination or magnetic fields for both OSCs.

Figure 4. Double-logarithmic current−voltage (i−V) curves in PBDT(O)T11−x:PC70BMx at different fullerene concentrations. Note the characteristic plateau in the first region of the curves at fullerene concentrations in the range of 50−85% as a fingerprint of negative MFE at small bias. The curve with the most pronounced slope (mmMFE) indicates the OSC with the minimum MFE. On the other hand, the curve with the slope (mMPCE) indicates the OSC with the maximum PCE. By inspection of the family of curves obtained at different fullerene concentrations, it is possible to identify the fullerene concentration for the minimum MC, the maximum PCE, and negative MC at small bias.

It is appropriate to mention that the curves have been obtained only from the electrical transport (i−V) measurements without the application of any external magnetic field or illumination. To verify whether the characteristic S-like shape and the plateau provide the same information in a solar cell where oxygen (O) is replaced by (Se), we replaced the polymer PBDT(O)-T1 by PBDT(Se)-T1 and kept the fullerene content at 50%.38 Proceeding in the same way as PBDT(O)-T1, we performed the MC, PCE, and electrical transport (i−V) measurements. The reported PCE in PBDT(Se)-T1 was 8.4% compared to 4% in PBDT(O)-T1, whereas the MCat 1T in PBDT(Se)-T1 reported 0.02% compared to 0.07% in PBDT(O)-T1. Strikingly, regardless of these differences in MC and PCE, the double-logarithmic current−voltage (i−V) curve of the electrical transport (i−V) measurements revealed the S-like shape in both devices. (See Figure S3 in the Supporting Information to compare the double-logarithmic current− voltage (i−V) for both OSCs). When measuring the MC at small bias in PBDT(Se)-T1, we also found it to be negative, as reported in PBDT(O)-T1. We must point out that the SOC due to the element selenium plays an important role in the USFE in the PBDT(Se)-T1 polymer. The strong SOC of selenium affects the hyperfine coupling interaction, leading to a negative MC. Under these experimental findings, we conjecture that the plateau in the first region of the S-like shape curve is a signature of negative MC in OSCs at small bias and the most pronounced slope unravels the OSCs with the smallest MC when the family of curves is investigated at different fullerene concentrations. To verify the role of the electron transport layer (ETL) and the hole transport layer (HTL) in the characteristic S-like shape, we replaced the ETL and HTL to obtain the electrical transport (i−V) curves in several configurations at 50% of fullerene concentration. (See Figures S4 and S5 in the

3. DOUBLE-LOGARITHMIC CURRENT−VOLTAGE (I−V) CURVES IN PBDT(O)-T1:PC70BM AND PBDT(SE)-T1:PC70BM Different fullerene concentrations cause different amplitudes for the MC and PCE in the OSCs. Moreover, we investigated the possibility to describe the MC and PCE prior to the use of external sources such as magnetic fields or illumination. For that, we examined in detail the electrical transport (i−V) curves in a double-logarithmic scale. In Figure 4, we observed the complete set of (i−V) measurements at all concentrations in PBDT(O)T1:PC70BM. It can be seen that samples at 50, 65, 75 and 85% fullerene concentration exhibit an S-like shape with a typical plateau in the first region of the curve.23,49 Strikingly, in Figure 3b, the sign of the MC is negative only at these concentrations when the applied voltage was smaller than the built-in voltage (small bias). We consider that the plateau in the first region of the S-like shape curve is a signature of negative MC. Besides, the most pronounced slope of the S-like shape curve in the exponential region coincides with the device at minimum MC magnitude that was found to be at 65% of fullerene concentration. Additionally, from Figure 4, it is possible to identify the curve that corresponds to the device with the maximum PCE that corresponds to the sample at 50% of fullerene concentration. 7780


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Supporting Information). Surprisingly, we confirm that the plateau in the first region of the S-like shape is present only in devices that display a negative MC at small bias. Under this evidence, we conjecture that the S-like shape curve is independent of the ETL or HTL of the OSCs and the chemical composition of the polymer. Once more, we confirmed that the plateau in the first region of the S-like shape curve is a fingerprint of negative MC.

Notes

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ACKNOWLEDGMENTS

4. CONCLUSION We presented the experimental results of the magnetic field effects (MFEs) in the OSCs based on the polymers PBDT(O)T1 and PBDT(Se)-T1 blended with the fullerene PC70BM. From the electrical transport (i−V) curves in a doublelogarithmic scale, the characteristic S-like shape and the plateau in the first region of the curve unveil a negative MC at small bias. Furthermore, the slopes of the curves provide information about the OSCs with the maximum PCE and the minimum MC independent of the MC and PCE amplitudes and active layer and injection layer structures. Our work opens up a door to find more patterns to describe the MFE and PCE in polymer−fullerene solar cells, without the use of external magnetic fields or illumination.

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REFERENCES

The authors declare no competing financial interest.

This work is partially funded by Beijing Advanced Innovation Center for Big Data and Brain Computing, Beihang University. The authors would like to thank also the support by the International Collaboration projects (No. 2015DFE12880 and B16001).

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5. EXPERIMENTAL SECTION To explore the electrical transport (i−V) curves, we analyze the MC in the OSC benzo[1,2-b:4,5-b′]dithiophene (BDT)-based copolymer (PBDT(O)-T1) blended with the fullerene PC70BM.38 The molecular and device structures are shown in Figure 1. The PC70BM fullerene has been introduced to the polymer PBDT(O)-T1 in various concentrations. The PCE and MC were measured at room temperature, different fullerene concentrations and applied bias voltages. Furthermore, the electrical transport (i−V) measurements were performed and analyzed in the form of double-logarithmic current−voltage (i− V) curves. Finally, a second solar cell benzo[1,2-b:4,5b′]dithiophene (BDT)-based copolymer PBDT(Se)-T1 blended with the fullerene PC70BM was electrically characterized, and the values of PCE, MC, and double-logarithmic current−voltage (i−V) at 50% of fullerene concentration were compared to the values of PBDT(O)-T1 (see supplementary note 1 in the Supporting Information for more details).

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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/acsomega.7b01470. See Supporting Information for a complete data of the Experimental Section and SDEs at other fullerene concentrations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.S.). *E-mail : [email protected] (W.Z.) ORCID

Yanming Sun: 0000-0001-7839-3199 Author Contributions

M. Antonio Cabero Zabalaga and J. Wei contributed equally to this paper. 7781


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