Negative Correlation between Intermolecular vs Intramolecular

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Negative Correlation between Intermolecular vs Intramolecular Disorder in Bulk Heterojunction Organic Solar Cells Nakul Jain, Urvashi Bothra, Dhanashree Moghe, Aditya Sadhanala, Richard Friend, Christopher R. McNeill, and Dinesh Kabra ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14628 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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ACS Applied Materials & Interfaces

Negative Correlation between Intermolecular vs Intramolecular Disorder in Bulk Heterojunction Organic Solar Cells Nakul Jaina, Urvashi Bothraabc, Dhanashree Moghea, Aditya Sadhanalad, Richard H Friendd, Christopher R. McNeillc and Dinesh Kabraa* a

Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India. [email protected] b

IITB-Monash Research Academy, IIT Bombay, Mumbai-400076, India.

c

Department of Materials Science and Engineering, Monash University, Wellington Road, Clayton, VIC 3800 Australia. d Optoelectronics Group, Cavendish Laboratory, University of Cambridge, UK (CB3 0HE) Keywords- Urbach energy, Electroluminescence (EL), intermolecular, intramolecular, transport length (Ld), Bulkheterojunction Solar Cells Abstract: By varying the concentration of a solvent additive, we demonstrate the modulation of intermolecular (donor:acceptor (D:A) interface) and intramolecular (bulk) disorder in blends of the low band gap polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopental[2,1-b;3,4-b’]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) blended with [6,6]-Phenyl-C71butyric acid methyl ester (PC71BM). Using the solvent additive concentration of 1,8-diiodooctane (DIO) in the host processing solvent, the disorder in the bulk and at the interface is studied in terms of Urbach energy, electroluminescence (EL) broadening and EL quantum efficiency (ELQE). The Urbach energy varies from 80 meV to 39 meV for bulk and 39 meV to 51 meV for D:A interface. An interesting feature is that changes in the Urbach energy of the D:A are opposite to those of the Urbach energy of bulk: i.e. the disorder at the D:A interface increases as the disorder in the bulk decreases with increase in DIO concentration. Our study evidently suggested a negative correlation between intermolecular and intramolecular property in bulk heterojunction solar cell. 1 ACS Paragon Plus Environment

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Furthermore, scanning photocurrent microscopy measurements show that the effective hole transport length is double in magnitude for cells processed from 3 vol.% DIO in comparison to cells processed from 0 vol.%. This increase in effective transport length is explained by an increase in the delocalization of the electronic states involved in charge transport as confirmed by dark J-V knee-voltage, JSC and EU-bulk measurements. Henceforth, we provide a functional relationship between the additive induced bulk heterojunction morphology and the optoelectronic properties of PCPDTBT-based solar cells. Introduction Substantial research has been carried out to improve the efficiency of organic solar cells by understanding their device physics1-3. Organic solar cells offer the potential of light-weight and flexible devices with a practical power conversion efficiency, a low-cost of manufacture combined with large-area production. A promising approach to increase the efficiency of organic solar cells (OSCs) is to use low band-gap materials as absorbers. These materials increase the overlap of the photoactive layer’s absorption with the solar spectrum in the low-energy NIR wavelength region and thereby increase the photon collection of the device. BHJ solar cells based on conjugated polymer blends have emerged as a promising technology in the development of low-cost photovoltaic power generation with single cell power conversion efficiency no exceeding 14%4-6. In bulk heterojunction blends, the donor and acceptor phases are dispersed to form an interpenetrating nanoscale network that provides percolation pathways along with large interfacial area, which is essential for efficient transport and charge separation. Organic solar cells based on low band gap polymer also typically require the use of solvent additives to optimize performance78

, which has been associated with improved crystallinity and purer domains of acceptor and donor

molecules9-10. The role of the solvent additive on the resulting morphology8, 10-12, charge carrier

2 ACS Paragon Plus Environment

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dynamics13 and device performance7, 9, 14 has been examined in detail. Recently, the localized density of states (DOS) within blend with and without additives has been probed by conductance measurement using Scanning Tunnelling Microscopy/Spectroscopy.15 A qualitative estimate shows that the DOS broadens as a result of DIO addition due to an increase in the domain size of donor and acceptor phases as well as the presence of mixed phase in between the large domains. In OPVs, the donor:acceptor interface is studied using Marcus electron transfer theory introduced by Vandewal et al.16-18. Moreover, connecting the Urbach tail with charge transport properties of various molecular semiconductor films are studied in great detail by the Sirringhaus Group19. In this manuscript we use these two important concepts to correlate change in molecular packing in BHJ via solvent additive to solar cell parameters.

In this report, we evaluate the role of solvent additive on the disorder in the bulk and at the donor:acceptor interface of PCPDTBT:PC71BM blends. Here, we consider intermolecular disorder to be between donor- acceptor molecules, whereas intramolecular disorder is between donor-donor molecules. We use reduced external quantum efficiency (EQE) measurements, EL studies and spatial photocurrent microscopy to get an insight into the bulk vs. interfacial properties of photo induced charge carriers. We determine the energetic disorder (Urbach energy) from the tail region of the EQE spectrum in blends processed for different concentrations of the solvent additive. Photocurrent microscopy measurements on the device provide an effective transport length for holes, which correlate with the bulk ordering or delocalization of electronic states of the material. Interestingly, we note that increasing in bulk ordering are coupled with increases in D:A interface disorder, i.e., there is an inverse monotonic relationship between these two parameters as a function of solvent additive concentration. We note that our approach is not limited to this BHJ



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system and can be further extended to non-fullerene based BHJ systems to provide important insight into material/device parameters. 2 Results: 2.1 Device Structure and Characteristics Figure 1a represents the chemical structure of the donor polymer PCPDTBT and that of the acceptor molecule PC71BM. Devices were fabricated with an inverted structure of ITO/ZnO/PCPDTBT:PC71BM/MoO3/Ag. The work-functions of the different interlayers as well as the HOMO-LUMO levels of the organic molecules are shown in figure1b with energy level values taken from literature20-21. The active bulk heterojunction layer was prepared using different DIO concentration in the host solvent cholorobenzene (CB). Five different DIO concentration of 0%, 1%, 3%, 5%, and 10% by volume in the host CB were chosen for this study. Detailed information regarding the device fabrication procedure is described in experimental section. Solar cell parameters (namely VOC, FF and JSC) were extracted from the illuminated J-V curves (figure S1) and presented in figure 1c, 1d and 1e respectively. The solar cell parameters are also tabulated in Table 1. VOC is found to vary from 0.68 V to 0.58 V when the DIO concentration is increased from 0 vol.% to 10 vol.%. JSC increases from 10.7 mA/cm2 (0 vol.% DIO) to 14.2 mA/cm2 (3% DIO) and then slightly decreases to 12.8 mA/cm2 (5 vol.% DIO) and further to 11.9 mA/cm2 (10% DIO). Fill Factor (FF) increases from 0.42 (0 vol.% DIO) to 0.50 (3% DIO) and then decreases to 0.48 (5 vol.% DIO) and to 0.47 (10 vol.% DIO). The resulting power conversion efficiency (PCE) is found to be optimized for a DIO concentration of 3 vol.%, increasing from 3.1 % (0 vol.% DIO) to 4.3% for 3 vol.% DIO. With further increase of DIO concentration, the PCE decreases to 3.6 % (5 vol.% DIO) and further to 3.3 % (10 % DIO). The JSC calculated from the measured EQE spectra


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(as shown in supporting information Fig. S11) matches with JSC obtained from illuminated J-V and are tabulated in table1. 2.2 Urbach Energy To understand the nature of the bulk and CT states of the blend, high sensitivity EQE measurements were performed with the results shown in figure 2a and 2b. The photocurrent response (absorption region) of PC71BM is up to 1.7 eV16, 22 and PCPDTBT is up to 1.4 eV23. Thus the photocurrent response from 1.4 eV and above mainly results from excitation of polymer chains in the bulk of the blend, that is, polymer chains away from a donor:acceptor interface. The photocurrent response below 1.4 eV corresponds to excitation of CT states, reflecting the electronic nature of the donor:acceptor interface.24 To understand the nature of the bulk and interfacial CT states in terms of disorder, we have determined the Urbach energy. The absorption tail that extends beyond the absorption band edge in conjugated polymers is termed the Urbach tail25 and the energetic value of the Urbach tail gives an estimate of the disorder. The Urbach energy is calculated by26 *

𝐸𝑄𝐸 𝐸 ∝ 𝛼 𝐸 ∝ exp ( )…………………………(1) *+

Where α is the absorption coefficient, E is the incident photon energy and EU is the Urbach energy26-31. In figure 2a and 2b, the EQE is fitted with equation 1 and the extracted EU values are tabulated in Table 2. The fit indicated in figure 2a corresponds to the bulk part of the donor material and the fit of figure 2b corresponds to the D:A interface. The fitted data shown here are for 0 vol.%, 1 vol.% and 3vol.% DIO devices which show pretty distinct Urbach tails for each concentration of DIO. Data for 5 vol.% and 10 vol.% DIO can be found in the supporting information (figure S2). For bulk PCPDTBT chains located within donor-rich domains, the Urbach energy varies from ~ 80 meV (0 vol.%) to 39 meV (10 vol.%). The Urbach energy of the D:A interface in contrast 5 ACS Paragon Plus Environment


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increases from ~ 39 meV (0 vol.%) to 51 meV (10 vol.%) indicating that the addition of DIO in the host solvent increases the energetic disorder at the D:A interface. The charge transfer region can also be fitted with the Gaussian equation and reorganization energy (λ) and ECT can be extracted. To understand the difference between these two functions, CT region is fitted by adding the two equations: Gaussian + exponential. The fitted curve is shown in the figure S12 and the extracted parameters are presented in Table S1. The extracted EU-CT values are the same as extracted when using only the exponential equation 1 as expected due to the nature of the Urbach tail, which is a signature of the availability of exponential DOS beyond allowed states due to disorder. 2.3 Electroluminescence (EL) from BHJ OSCs To gain more insight regarding the nature of the donor:acceptor interface, EL measurement was performed on the same set of blend devices. As the EL emission in BHJ solar cell is dominated by CT emission and arises from the recombination of charge carriers at the D:A interface, it gives direct information about the EL broadening and disorder of the CT state26. Figure 2c presents the EL spectra of PCPDTBT:PC71BM cells for 0 vol.%, 1 vol.% DIO (spectra for 5 vol.% and 10 vol.% are shown in figure S3) with the calculated Full Width at Half Maximum (FWHM) values tabulated in table 2. The width of the EL emission (parameterised by the FWHM) provides another measure of interfacial disorder, with a larger FWHM reflecting higher interfacial disorder. The FWHM of EL is found to increase from 251 meV to 333 meV with increasing in from 0 vol.% to 10 vol.%. It is observed that for the additive DIO content of more than 3%, the broadening of the EL is almost the same and follows the same trend as EU-CT. The EL quantum efficiency with respect to injected current density is plotted in figure 2d. We note that the device without additive saturates after J ~ 0.2 A/cm2(which is observed earlier as


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well32), whereas for DIO 3% ELQE does not saturate until up to J ~ 0.7 A/cm2. It is also clearly observable that the ELQE vs. J curve of the 1 vol.% DIO device is very different compared to the 3 vol.% device with a steeper rise at low values of J and a slower rise at higher values of J. The ELQE of the 5 vol.% and 10 vol.% DIO with comparison to 3% DIO is shown in supporting information Figure S4 and does not saturate for the injection current density up to 0.6 A/cm2. 2.4 Transport Length Scale of Holes It is known that the crystalline nature of PCPDTBT improves with the addition of DIO10 and thus is expected to improve hole transport. To get a quantitative estimate of the effect of DIO on hole mobility, we have calculate the effective transport lengths of holes by using scanning photo-current microscopy33. The experimental setup, which is used to calculate the transport lengths, is shown in figure S8. The details about the full setup and parameters are described in the experimental section. A 638 nm laser with a spot size of 460 nm was focused onto the device and photogenerated holes & electrons are simultaneously collected by the electrodes. When the laser spot moves off the device active area and away from the hole collecting electrode (x direction as shown in figure 3a), the photo generated electron can be collected through the ITO easily but the hole has to travel in the plane of the device to reach the hole collecting electrode (silver electrode). So as the laser spot moves away from the silver (Ag) electrode the yield of holes reaching the Ag electrode reduces, with the decay of photocurrent as a function of d (distance from silver electrode) providing a measure of the transport length of the holes. The raw photocurrent scanning data for all cells and identification of d = 0 is shown in figure S8. Figure 3a shows plots of photocurrent vs. d for cells fabricated with 0 vol.%, 1 vol.% and 3 vol.% DIO, with the corresponding exponential fits to calculate the decay length of the holes33. The corresponding data and fits for cells fabricated with 5 vol.% and 10 vol.% are shown in Figure S9. Table 3 summarised the calculated effective hole



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transport lengths (LDh). From these results LDh is found to increase from 84 µm for 0 vol.% DIO, to 105 µm for 1 vol.% DIO and further to 166µm for 3 vol.%. Further increases in DIO content results in similar LDh values to the 3 vol.% DIO case, with values for LDh of 158 µm (5 vol.%) and 160 µm (10 vol.%). The dark J-V curves for all devices are performed shown in figure 3c. It is clearly notable from the dark J-V curves that knee voltage continuously decreases with increasing DIO concentration; such crystallinity-induced changes in dark J-V knee voltage has also been observed for the poly(3-hexylthiophene) based system31. 3. Discussion In general, the VOC is strongly affected by the change in D:A interface morphology, domain size, domain purity and the interconnecting network present within the blend34. Moving from 0 vol.% DIO to 10 vol.% DIO leads to an increase in EU-CT, suggesting an increase in the disorder at the D:A interface. This increased EU-CT is attributed to the presence of more defect states at the D:A interface, which leads to a decrease in the VOC. The EU-CT increases significantly from 0 vol.% DIO to 3vol.% DIO and almost saturates for 5 vol.% DIO and 10 vol.% DIO. This saturation may be because the morphology of the blend aggregated state of the polymer remains essentially for 5 vol.% DIO and 10 vol.% DIO10. Evidently, VOC also almost saturates with DIO content increasing from 5 vol.% to 10% vol.%. Increased disorder at the D:A interface with increasing DIO content is also supported by the observed increase EL FWHM (Fig. 2c). As the EL originates from the CT state, this broadening will provides direct information about the disorder at the interface. Disorder or defects at the donor:acceptor interface is further confirmed by ELQE, as a function of injected carrier density, which is a direct measure of traps present at the CT interface32, 35. For 0 vol.% DIO, ELQE is voltage independent and suggests that all the recombination happens through bimolecular recombination.


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As the concentration of DIO increases from 1 vol.% to 10 vol.%, trap-assisted recombination reflected by voltage dependent ELQE characteristics32. For devices fabricated with 5 vol.% and 10 vol.% DIO, the ELQE starts to saturate at the same current density, consistent with the similar VOC values measured for these two cells. To demonstrate the negative correlation between EL FWHM and VOC, these parameters are plotted side-by side as a function of DIO content in Fig. 4a, clearly showing that as the EL FHWM increases with DIO content the VOC systematically decreases. Similarly, to demonstrate how the intra-molecular and inter-molecular properties covary with respect to addition of DIO, EUbulk CT

and EL FWHM as a function of DIO are plotted side-by side in figure 4b, while EU-bulk and EU-

as a function of DIO are plotted side-by-side in figure 4c. Similar to the case for EL FWHM

and VOC, EU-bulk and EL FWHM are found to be strongly negatively correlated along with EU-bulk and EU-CT. Thus we conclude that in the PCDPTBT:PC71BM system the use of DIO can be used to improve the bulk properties such as crystallinity and domains purity but at the expense interface properties resulting in a decrease in VOC. To understand the effect of DIO concentration on bulk properties, the effect of hole transport length was estimated experimentally, with transport length directly correlated to mobility of the charge carriers33. The device fabricated with 0 vol.% DIO possesses the low hole transport length (84 μm) and contributes to the low JSC and FF (Fig.1). As the DIO concentration increases from 0 vol.% DIO to 3 vol.% DIO, the effective transport length increases which is correlated with an increase in both JSC and FF. For 3 vol.% DIO, the effective hole transport length increases to 166 μm suggesting that the holes are sufficiently mobile in the blend as compared to the blend processed with 0 vol.% DIO. The effective hole transport lengths calculated for devices fabricated with 5 vol.% and 10 vol.% DIO are similar to that of the 3 vol.% DIO device, albeit with a slight



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decrease that is relatively insignificant or within error. The increase in effective hole transport length for DIO based devices suggests long range ordering of polymer chains facilitating charge transport. This long range polymer chain ordering (or fibril like structure)10 facilitates delocalization of the electronic states, resulting in a shallower HOMO energy of the donor15, 31 and improved hole mobility. This will reduce the diagonal band gap i.e. difference between the HOMO(donor)-LUMO(acceptor), thereby reducing the knee, i.e., built-in voltage (Fig S14). This enhances JSC (Fig. 1, Table 1) and fill-factor of solar cells under illumination condition (Fig. 1).. The bulk Urbach energy also suggests that polymer chains are becoming more ordered with the addition of DIO in the host solvent. Examining plots of internal quantum efficiency (IQE) with respect to DIO concentration shown in figure S11 further suggests that there may be an increase in absorption due to increased roughness (see AFM images in Fig. S10). However, for the 3 vol.% DIO case where there is optimal packing which not-only facilitates higher IQE (Fig. S11) but also improved the charge transport (improved fill factor, Fig. S1, Table 1) to extract photo-induced charges to respective electrodes. Thus this study shows how improving bulk properties can negatively impact interfacial properties leading to a negative correlation between the intermolecular vs. intra-molecular ordering. 4. Conclusion: This study has systematically assessed the impact of the solvent additive DIO on the bulk and interfacial properties of PCPDTBT:PC71BM blends and its connection to device properties. The addition of solvent additive was found to improve the bulk ordering of PCPDTBT chains evidenced by a decrease in the bulk Urbach energy with a resulting in improvement in the transport length of holes within the PCPDTBT domains. These results suggest that improvement in bulk ordering with DIO treatment leads to delocalization of the HOMO state of polymer, which in turn


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enhances JSC and causes a reduction in the knee-voltage in the dark J-V curves. With regard to interfacial properties, from the EL studies and measurement of EU-CT, we conclude that DIO treatment leads to an increase in disorder at the D:A interface causing a reduction in VOC. We have further demonstrated that solar cell parameters can be modulated by reducing the intramolecular disorder (increased JSC and FF) at the cost of increased intermolecular disorder (decreased VOC). Our approach of combining measurement of interfacial and bulk Urbach energies, EL emission and effective transport lengths can be readily applied to other systems such as high efficiency nonfullerene solar cells which are characterized by high JSC (around 27mA/cm2) but are limited by open circuit voltage36. 5 Methodology PCPDTBT was purchased from 1-material and used as received. The molecular weight (MW) was more than 35 kDa with dispersity less than 3. The acceptor molecule, PC71BM was purchased from Solenne BV with purity level greater than 99%. Solvents were purchased from Sigma-Aldrich. Indium Tin Oxide (ITO) coated glass substrate were purchased from Lumtec and were cleaned by using ultrasonicating in soap solution, DI water, acetone and IPA for 10 minutes each. After drying with nitrogen, the substrates were then transferred to plasma asher and plasma cleaned for 10 minutes in oxygen plasma at 0.5 mbar pressure. The Zinc Oxide (ZnO) solution, which was prepared from 0.1 M Zinc acetate dihydrate in 2-methoxyethenol, was spin coated on ITO substrates at 2000 rpm for 50 seconds and annealed at 200 °C in ambient air for 10 mins. The substrates were transferred to the glove box for active layer deposition. The blend solution was prepared from PCPDTBT:PC71BM (1:2) in chlorobenzene (CB) and stirred overnight at 70 °C. DIO was added to the host solvent in 1%, 3%, 5% and 10% by volume before spin coating. The PCPDTBT:PC71BM solution was spin-coated at 2000 rpm for 50 seconds. The samples without



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DIO were directly transferred to the evaporation chamber for metal deposition. The sample with DIO were kept in a Petri-dish for 2-hours and then transferred to a vacuum chamber for overnight for solvent evaporation. MoO3 and Ag were deposited on PCPDTBT:PC71BM film at 10-6 torr vacuum. The thickness of MoO3 and Ag deposited were 15 nm and 100 nm respectively. Microscope scanning- A 638 nm TOPTICA class 3B laser diode was mechanically chopped (80 Hz) and collimated on the sample through 60X (0.7 N.A.) objective in an inverted Olympus IX73 microscope. The photocurrent from the device is measured using SR830 lock-in amplifier coupled with the same mechanical chopper mentioned previously. A Thorlabs DET10A/M Si based detector was used to collect the reflection of the laser from device. The output of the detector was connected to another SR830 lock-in amplifier. Both the Lock-in amplifiers and scanning stage are interfaced via LabView VI. LabView software was customized to collect the photocurrent and reflection simultaneously with respect to (x or y) position of the device. Acknowledgement: We acknowledge Department of Science &Technology DST-AISRF and Global Challenges Research Fund (GCRF), SUNRISE for funding. We also acknowledge the support from the NCPRE IITB for device fabrication & characterizations facility. NJ and UB acknowledge the Council of Scientific and Industrial Research (CSIR), India, and IITB-Monash Research Academy, respectively for the research fellowship. DM acknowledges IIT Bombay for the fellowship. Supporting Information: Illuminated J-V, EQE, IQE, Atomic Force Microscopy, EL for different additive concentration; scanning photo current microscopy setup; Photocurrent verses reflection current from device; Gaussian plus exponential fitting with reduced EQE; EL fitting with Marcus equation; knee voltage from dark J-V.


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17. Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca, J. V., Relating the Open-Circuit Voltage to Interface Molecular Properties of Donor:Acceptor Bulk Heterojunction Solar Cells. Physical Review B 2010, 81 (12), 125204. 18. Elumalai, N. K.; Uddin, A., Open Circuit Voltage of Organic Solar Cells: An in-Depth Review. Energy & Environmental Science 2016, 9 (2), 391-410. 19. Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H., Approaching Disorder-Free Transport in High-Mobility Conjugated Polymers. Nature 2014, 515 (7527), 384-388. 20. Kröger, M.; Hamwi, S.; Meyer, J.; Riedl, T.; Kowalsky, W.; Kahn, A., Role of the Deep-Lying Electronic States of Moo3 in the Enhancement of Hole-Injection in Organic Thin Films. Appl. Phys. Lett. 2009, 95 (12), 123301. 21. Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll, M.; Dindar, A.; Haske, W.; Najafabadi, E.; Khan, T. M.; Sojoudi, H.; Barlow, S.; Graham, S.; Brédas, J.-L.; Marder, S. R.; Kahn, A.; Kippelen, B., A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics. Science 2012, 336 (6079), 327-332. 22. Chow, P. C. Y.; Albert-Seifried, S.; Gélinas, S.; Friend, R. H., Nanosecond Intersystem Crossing Times in Fullerene Acceptors: Implications for Organic Photovoltaic Diodes. Adv. Mater. 2014, 26 (28), 4851-4854. 23. Scharber, M. C.; Lungenschmied, C.; Egelhaaf, H.-J.; Matt, G.; Bednorz, M.; Fromherz, T.; Gao, J.; Jarzab, D.; Loi, M. A., Charge Transfer Excitons in Low Band Gap Polymer Based Solar Cells and the Role of Processing Additives. Energy & Environmental Science 2011, 4 (12), 5077-5083. 24. Jain, N.; Chandrasekaran, N.; Sadhanala, A.; Friend, R. H.; McNeill, C. R.; Kabra, D., Interfacial Disorder in Efficient Polymer Solar Cells: The Impact of Donor Molecular Structure and Solvent Additives. Journal of Materials Chemistry A 2017, 5, 24749-24757 25. Urbach, F., The Long-Wavelength Edge of Photographic Sensitivity and of the Electronic Absorption of Solids. Phys. Rev. 1953, 92 (5), 1324-1324. 26. Gong, W.; Faist, M. A.; Ekins-Daukes, N. J.; Xu, Z.; Bradley, D. D. C.; Nelson, J.; Kirchartz, T., Influence of Energetic Disorder on Electroluminescence Emission in Polymer:Fullerene Solar Cells. Physical Review B 2012, 86 (2), 024201. 27. Kurik, M. V., Urbach Rule. physica status solidi (a) 1971, 8 (1), 9-45. 28. Zhao, B.; Abdi-Jalebi, M.; Tabachnyk, M.; Glass, H.; Kamboj, V. S.; Nie, W.; Pearson, A. J.; Puttisong, Y.; Gödel, K. C.; Beere, H. E.; Ritchie, D. A.; Mohite, A. D.; Dutton, S. E.; Friend, R. H.; Sadhanala, A., High Open-Circuit Voltages in Tin-Rich Low-Bandgap Perovskite-Based Planar Heterojunction Photovoltaics. Advanced Materials 2017, 29 (2), 1604744. 29. Street, R. A.; Song, K. W.; Northrup, J. E.; Cowan, S., Photoconductivity Measurements of the Electronic Structure of Organic Solar Cells. Physical Review B 2011, 83 (16), 165207. 30. Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo, M. A.; Manca, J. V.; Voorhis, T. V., Charge Transfer State Versus Hot Exciton Dissociation in Polymer−Fullerene Blended Solar Cells. J. Am. Chem. Soc. 2010, 132 (34), 11878-11880. 31. Chandrasekaran, N.; Gann, E.; Jain, N.; Kumar, A.; Gopinathan, S.; Sadhanala, A.; Friend, R. H.; Kumar, A.; McNeill, C. R.; Kabra, D., Correlation between Photovoltaic Performance and Interchain Ordering Induced Delocalization of Electronics States in Conjugated Polymer Blends. ACS Applied Materials & Interfaces 2016, 8 (31), 20243-20250. 32. Wetzelaer, G.-J. A. H.; Kuik, M.; Blom, P. W. M., Identifying the Nature of Charge Recombination in Organic Solar Cells from Charge-Transfer State Electroluminescence. Advanced Energy Materials 2012, 2 (10), 1232-1237.


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33. Kabra, D.; Narayan, K. S., Direct Estimate of Transport Length Scales in Semiconducting Polymers. Advanced Materials 2007, 19 (11), 1465-1470. 34. Credgington, D.; Durrant, J. R., Insights from Transient Optoelectronic Analyses on the OpenCircuit Voltage of Organic Solar Cells. The Journal of Physical Chemistry Letters 2012, 3 (11), 1465-1478. 35. Kuik, M.; Nicolai, H. T.; Lenes, M.; Wetzelaer, G.-J. A. H.; Lu, M.; Blom, P. W. M., Determination of the Trap-Assisted Recombination Strength in Polymer Light Emitting Diodes. Appl. Phys. Lett. 2011, 98 (9), 093301. 36. Song, X.; Gasparini, N.; Ye, L.; Yao, H.; Hou, J.; Ade, H.; Baran, D., Controlling Blend Morphology for Ultrahigh Current Density in Nonfullerene Acceptor-Based Organic Solar Cells. ACS Energy Letters 2018, 3 (3), 669-676.



-2

-4 -5

-3.9

-4.2 ITO/ZnO

PC71BM

-6

PC71BM

PCPDTBT c) 0.70

-6.0

JSC(mA/cm2)

FF (%)

FF

50

40

0.55 0%

1% 3% 5% DIO (Vol%)

10%

-4.7 Ag

-5.3

e)

d) 60

0.60

PCPDTBT

MoO3 -6.8

-7

VOC

0.65

-3.55

-3

0%

1% 3% 5% DIO (Vol%)

10%

6

14

5

12

4

10

3 JSC

8 6

2

PCE

0%

1% 3% 5% DIO(Vol%)

PCE (%)

b) E(eV)

a)

VOC (volt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 22

10%

1

Figure 1: a) Molecular structure of PCPDTBT and PC71BM, b) Device structure with their respective work function and HOMO-LUMO energy levels, c), d) and e) VOC, FF, and JSC with PCE respectively with Respect to different concentration of DIO solvent additive in the host solvent.


Page 17 of 22

a)

b)

100 PCPDTBT PDS 0% 1% 3%

10

EQE (%)

EQE (%)

10

1

0% 1% 3%

1 0.1

0.1

0.01

c)

1.4 Energy (eV)

1.6

d) 0%

1.0

1%

1.0

1.2 1.4 1.6 Energy (eV)

1.8 8.0x10-8

4x10-7

3%

6.0x10-8

ELQE (a.u)

3x10-7

0.5

4.0x10-8

0% 1% 3%

2x10-7

2.0x10-8

1x10-7

0.0

0.8

1.0 1.2 1.4 Energy (eV)

1.6

ELQE (a.u)

1.2

Nor. EL (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

0.2

0.4 J(A/cm2)

0.6

0.8

Figure 2: Normalized EQE spectrum of PCPDTBT:PC71BM at a) bulk and b) D:A CT interface. Dashed black line is a linear fit to estimate the Urbach energy and solid line is absorption of pure PCPDTBT film to differentiate between bulk vs CT interfacec) Electroluminescence spectrum and d) ELQE vs injected carrier density for device prepared with different concentration of DIO solvent additive in the host solvent (The ELQE of device fabricate with 3 vol.% DIO is multiplied by 3 to show that in same graph).



a)

Ag

ITO

PCPDTBT:PCBM

x

y

Ag h

d (µm)

ITO Laser

Objective

c)

b) 1.0

160

0.9

0.6 0.5

120 80

0.8 0.7

40

0% 1% 3% 5% 10% DIO conc. (vol%)

0% 1% 3%

J (mA/cm2)

Ldh (nm)

Nor. Photo-current

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

30

0% 1% 3%

20 10 0

0 20 40 60 80 100 120 140 d (µm)

-0.5

0.0 0.5 applied bias (V)

1.0

Figure 3: a) Block diagram of scanning photocurrent microscopy to measure the transport lengths b) Transport length of the holes for devices fabricate with different concentration of DIO solvent additive. Dashed line is fitted with mono exponential to calculate the effective hole transport length (inset there is LDh is plotted with respect to different DIO concentration based devices) c) Dark current vs applied voltage of devices with different solvent additive concentration.


Page 19 of 22

a) 0.80

FWHM VOC

0.65

200

0.60

150 120 EU-bulk FWHM

80

250 200

60 40

FWHM (meV)

300

100

c)

0.70

VOC (V)

250

EU-bulk (meV)

b)

0.75

150 50

120 EU-bulk

100

EU-CT

40

80 60

30

EU-CT (meV)

FWHM (meV)

300

EU-bulk (meV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 0%

1% 3% 5% 10% DIO conc. (Vol %)

Figure 4: Negative correlation between a) FWHM and VOC, b) EU-bulk and EL peak FWHM and c) EU-bulk and EU-CT, with addition of solvent additive for BHJ PCPDTBT:PC71BM organic solar cells derived from IPCE spectrum, EL spectroscopy and light J-V characteristics. 19 ACS Paragon Plus Environment


Page 20 of 22

Table 1: Device parameters of PCPDTBT:PC71BM prepared using different concentration of DIO solvent additive in the host solvent.

a

DIO conc.

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

0 vol.%

0.68

10.7 (10.1)a

41.7

3.1

1 vol.%

0.63

12.0 (11.3) a

44.2

3.4

3 vol.%

0.60

14.2 (13.9) a

50.2

4.3

5 vol.%

0.58

12.8 (12.5) a

48.2

3.6

10 vol.%

0.58

11.9 (11.7) a

47.2

3.3

JSC calculated from EQE

Table 2: EU-bulkand EU-CT energy as calculated by fitting with Equation 1 to EQE of the devices. The FWHM is calculated by the EL measured for the same devices. DIO conc.

EU-Bulk (meV)

EU-CT (meV)

FWHM (meV)

0%

80.5 ± 0.4

38.6 ± 0.3

251

1%

58.6 ± 0.6

41.7 ± 0.2

268

3%

42.7 ± 1

50.5 ± 0.3

309

5%

39.6 ± 1

50.4 ± 0.2

328

10%

38.9 ± 1

50.7 ± 0.2

333


Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3: Hole effective transport length (LDh) for device prepared using different concentration of DIO additive in the host solvent. DIO Conc.

LDh (µm)

0%

84 ± 7

1%

105 ± 4

3%

166 ± 8

5%

158 ± 4

10%

160 ± 11



0% 1% 3%

Ldh

J (mA/cm2)

EU-Bulk

TOC

bias (V)

0% 1% 3%

VOC

EQE (%)

DIO %

EU-CT

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 22

Energy (eV)


Negative Correlation between Intermolecular vs Intramolecular

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