Exploiting the potential of 2-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile as an efficient donor molecule in vacuum-processed bulk-heterojunction organic solar cells

Abstract

A comprehensive experimental study is reported on the optical and electrical characteristics of 2-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile (DPTMM) when used as molecular donor in an organic solar cell (OSC) device structure. A major property of this new donor-type material is an unusually deep highest-occupied molecular orbital (HOMO) level that leads to a high open-circuit voltage (V_oc). A reasonably high hole-mobility was also observed in a hole-injection diode configuration. These are both promising factors for high-performance OSCs. In order to fully explore the potential of DPTMM in bulk-heterojunction-based OSCs, a step-wise experimental strategy was applied to optimize film composition and cell architecture. By co-evaporating the DPTMM with C₆₀ to promote exciton dissociation by maximizing the heterojunction area power conversion efficiency (PCE) of 3.0% was achieved. Finally, inserting a buffer layer and a spatial gradient of the donor/acceptor ratio was found to provide better conduction paths for charge carriers. The maximum obtained PCE was 4.0%, which compares favorably with the state-of-the-art of high-performance OSCs. All optimized devices show quite unusual high V_oc values up to 1 V.

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1. Introduction

Organic solar cells (OSCs) are considered to be a promising future energy technology – one which benefits from the possibility of tailoring materials and enabling cost-effective production of mechanically flexible solar modules. Over the last two decades, the synthesis of new functional materials^1–5 and the design of novel device architectures have been the foremost factors that have led to dramatic improvements in the power conversion efficiency (PCE) of OSCs.^6–9 However, their absolute level of performance is still too low to respond to existing commercial demand. It is well-established that three key parameters define the power conversion efficiency of OSCs: the open-circuit voltage (V_oc), the short circuit current (J_sc) and the fill factor (FF).^10–12 Therefore, it is crucial to understand the critical factors that correlate best with changes in these parameters and concomitant strategic improvement based on relevant device physics is of central importance.

Employing bulk-heterojunction (BHJ) architecture in an OSC is greatly advantageous in comparison to planar bilayer heterojunction devices. A BHJ organic layer is formed by deposition of mixed solution or co-evaporation of donor and acceptor materials, whereas a planar device is based on two independently and sequentially deposited organic films. It has been generally proven that the overall characteristics of BHJ cells showed better device performance than planar ones, especially in terms of J_sc and FF. These improvements are mainly explained by the increased interfacial area at which a donor (p-type) and an acceptor (n-type) materials form an intimate contact and where the charge transfer between two materials takes place.^13–16 Accordingly, such a structure is thought to enhance the dissociation of excitons at interfaces. However, random structural phases in a BHJ layer may disrupt the transport of generated charge carriers, because there is possibility of clusters that are not well electrically connected to the other clusters. Therefore, the control of the morphology of BHJ layer represents a key issue whereas high-mobility organic semiconductors which can overcome deficiencies in carrier transport in BHJ are desirable.

There are intensive research efforts to find organic semiconductors that can meet the material requirements for BHJ OSCs. Although low band gap processable conjugated polymers remain a major class of donor materials for organic photovoltaics,^2,3 the past few years have witnessed the growing importance of small conjugated molecules.¹ Due to their perfectly defined chemical structures, the reproducibility of their synthesis and their easier purification, molecular donors allow a more straightforward analysis of structure–property relationships than polydisperse polymers. In addition small conjugated molecules allow the preparation of BHJ solar cells either by solution⁵ or vacuum process.⁴ Small π-conjugated molecules have represented a major class of active materials in OSCs during the early developments of the field.^17,18 For example, phthalocyanine derivatives have been considered as promising materials due to their high molar absorptivity, high hole mobility, and long exciton lifetime.^19,20 However, photovoltaic devices with phthalocyanine derivatives generally exhibited relatively low V_oc. Beyond any consideration of the potential impact of the film microstructure on V_oc,²¹ the most critical factor determining V_oc is the highest-occupied molecular orbital (HOMO) of the donor material itself relative to the lowest-unoccupied molecular orbital (LUMO) of the paired acceptor material.²² In this context, the search for molecular donor materials showing an increased ionization potential is a key strategy for finding alternative phthalocyanine-based materials that might present a higher V_oc in an OSC device.

An interesting approach consists of the design of molecular donors in which an internal charge transfer (ICT) is created by the introduction of electron acceptor (A) groups in the structure of the donor.²³ This ICT concomitantly leads to the extension of the absorption spectrum towards longer wavelengths and particularly to the decrease of the HOMO level which results in an increase of V_oc of the corresponding OSC and also in a better stability of the molecule against oxidation. Thus conjugated molecules such as D–A, D–A–D or A–D–A systems, combining electron-donor (D) and electron-acceptor (A) groups, have led to molecular donors exhibiting interesting photovoltaic performance.^24,25 In addition, the use of a buffer layer with the photoconversion layer has recently gained increasing attention in OSC research.^26,27 An organic buffer layer is usually inserted between the charge-injection electrode and the active layer for the purpose of enhanced device operation. The buffer layer is introduced to improve charge injection/collection by providing an intermediate transport level. Morphological tuning of the active layer can be also expected in the case where the active layer is deposited on top of a buffer layer for which the surface energy is different from that of the bare electrode. The overall improvement in PCEs observed in this type of system has been found to be linked to more ideal diode current–voltage curves, lacking the so-called s-shape related to a leakage current contribution.²⁸

We herein report on the demonstration of high-performance, evaporation-based BHJ OSCs using a small donor molecule based on a D–A structure, namely 2-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile (DPTMM) which has been recently used in planar bilayer heterojunction OSCs.^29,30 Through multiple series of experiments, and by employing a co-evaporated BHJ system design and the tailored film composition with a proper buffer layer at the hole-collection side, the PCE of a DPTMM/C₆₀ cell was raised to 4.0%. In particular, an outstanding value of V_oc near 1.0 V suggests that DPTMM can be considered as a promising candidate for an efficient molecular donor in high-performance OSCs. Also, the simplicity of the molecular structure of DPTMM associated to its ease of preparation represents important parameters which must be taken into account for the future development of OSCs.

2. Experimental

2.1. Materials

Fig. 1 shows the chemical structures of all materials used in this study. DPTMM (Fig. 1a) was used as the donor material in all experiments. The synthesis of this new donor material has been reported in a recent paper.²⁹ The HOMO and LUMO levels of DPTMM were found to be 5.96 eV and 3.79 eV, respectively.

Fig. 1 Chemical structures of the investigated compounds. (a) DPTMM, (b) C₆₀, (c) α-NPB and (d) PEDOT:PSS.

2.2. Solar cell fabrication and measurements

All solar cell samples were fabricated on a pre-cleaned, patterned indium-tin-oxide (ITO) coated glass substrate purchased from Xinyan Technologies and with a sheet resistance of 20 Ω □⁻¹. Before depositing organic layers, the patterned ITO glasses were cleaned by ultrasonification in isopropyl alcohol (IPA) and treated by UV–ozone with a NOVASCAN UV-exposure system. Subsequently, the substrates were spin-coated with poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) (Baytron 4083). The thickness of this layer was approximately 40 nm after drying at 108 °C for 2 minutes. The samples were then transferred into the thermal evaporator. Small molecules were thermally evaporated at a pressure below <10⁻⁷ Torr. For the BHJ active layer, the material composition ratio (DPTMM and C₆₀) was obtained by two quartz sensors. One quartz sensor measured C₆₀ evaporation rate while another sensor detected the deposition rate of both DPTMM and C₆₀, from both of which could be estimated the film composition (see ESI†). The devices were finished by depositing 1.2 nm of lithium fluoride (LiF) and a 100 nm thick aluminum layer as a top electrode through a shadow mask, defining a device area of 0.28 cm². The structures of the devices used in this study are summarized in Table 1. The current–voltage (I–V) measurements of the fabricated OSCs were performed under the illumination of a simulated AM 1.5G solar light (100 mW cm⁻²) connected to a computer-controlled Keithley 2635 source measurement unit (SMU) inside a nitrogen-filled glove box.

Table 1 Summary of the structures of the solar cells used is this study

Device X	Structure (nm) ITO/PEDOT:PSS (40 nm)/X/C60 (30 nm)/LiF (1.2 nm)/Al (100 nm)
A	DPTMM (10 nm)/DPTMM:C60 60 : 40 (30 nm)
B	DPTMM (15 nm)/DPTMM:C60 60 : 40 (30 nm)
C	DPTMM (20 nm)/DPTMM:C60 60 : 40 (30 nm)
D	α-NPB (5 nm)/DPTMM (15 nm)/DPTMM:C60 60 : 40 (30 nm)
E	α-NPB (5 nm)/DPTMM (15 nm)/DPTMM:C60 55 : 45 (30 nm)
F	α-NPB (5 nm)/DPTMM (15 nm)/DPTMM:C60 50 : 50 (30 nm)
G	α-NPB (5)/DPTMM (15 nm)/DPTMM:C60 70 : 30…DPTMM:C60 50 : 50 (30 nm)

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2.3. Hole only device fabrication and measurements

Hole only devices were fabricated using the same indium-tin-oxide (ITO) coated glass substrates following the same cleaning procedure as for solar cell fabrication. Firstly, PEDOT:PSS (Baytron AI4083) was spin-coated on the ITO anode, followed by thermal annealing at 108 °C for two minutes. The thickness of the PEDOT:PSS layer was approximately 40 nm. Subsequently, 120 nm of DPTMM was deposited by thermal evaporation at a pressure of <10⁻⁷ Torr. Twenty nanometers of Au was then evaporated on top of the DPTMM layer, with a final capping layer of 30 nm of Al to avoid mechanical deterioration of the Au contact during the electrical characterization. The active layer of 2.9 × 10⁻² cm² was defined by shadow-masking of the top contact. The current–voltage (I–V) characteristics were acquired with a Keithley 4200 semiconductor characterization system.

2.4. Optical characterization

Organic layers for absorption spectra measurement were deposited on glass substrates and the films for ellipsometric measurements were deposited on Si wafers. The absorption spectra were measured using a Jenway 6800 spectrophotometer. The refractive and extinction indexes n and k were obtained from measurement performed by MM16 Muller-ellipsometer from Horriba Jobin-Yvon. Spectroscopic ellipsometry is a useful method to study optical characteristics in OSCs.^31–34 This technique is based on the relative change of polarization in incident and reflected light. The refractive and extinction indexes n, k of C₆₀, DPTMM, and DPTMM:C₆₀ (60 : 40) blend layers were obtained by fitting the results from Muller ellipsometric measurements. In order to fit the optical data, a parametric model based on four oscillators was used. This model was derived from the simple Forouhi-model, and was used in order to give a Lorentzian-shape to the expression of n and k.^35–37

3. Results and discussion

3.1. Optical properties of DPTMM

Fig. 2 shows the absorption spectra of the DPTMM and C₆₀ compounds, as well as a thin film of co-deposited DPTMM:C₆₀ (1 : 1). The DPTMM compound has a broad absorption spectrum from 300 nm to 700 nm, with an absorption peak centered at 520 nm. The extracted values for n and k of the C₆₀ layer (Table 2) are close to those found in the literature.³³ Ellipsometric measurements of the DPTMM:C₆₀ blend are well fitted by using a combination of both ellipsometric models obtained for C₆₀ and DPTMM separately. Note that the measured k spectra are in good agreement with the absorption spectra of Fig. 2. Equally important as film absorption, the propagation of light in a multilayer structure is influenced by interference effects. More specifically, the electromagnetic field distribution strongly depends on the thickness and optical constants of each layer.^38,39 The spatial distribution of absorption and exciton generation characteristics were modeled by using a constant value of n and k at 520 nm (where DPTMM shows the highest absorption). The selected values of n and k are shown in Table 2.

Fig. 2 Absorption spectra of compounds DPTMM, C₆₀, and DPTMM:C₆₀ in solid film.

Table 2 Selected values of n and k at 520 nm used for optical simulation

	Material
	Glass	ITO	PEDOT:PSS	DPTMM	DPTMM:C60 (60 : 40)	C60	LiF	Al
n	1.51	2.00	1.52	1.81	1.92	2.26	1.41	0.71
k	0	0.01	0.03	0.83	0.39	0.16	0	5.37

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Fig. 3a displays the two corresponding intensity profiles (the squared moduli of the electromagnetic field amplitude |E(x)|²) as a function of position in the device for both a bi-layer device and BHJ structure. It is observed that the BHJ structure has its maximum light intensity in the co-deposited layer, whereas a bilayer structure has its maximum near the PEDOT:PSS region. Fig. 3b shows the calculated exciton generation rate G(x), taking into account the extinction coefficient of each layer. Total generated exciton was calculated by integration of G(x) throughout the entire active zone of 190 nm < x < 260 nm in both structures. It is found that the total density of generated excitons is ∼20% higher in the bilayer structure than that in the BHJ structure. However, it should be noted that from the total ‘generated’ excitons, only ‘dissociated’ excitons create electron–hole pairs that finally contribute to the energy conversion. We can compare the numbers of effective dissociable excitons by assuming an average exciton diffusion length of 15 nm.^40–42 Integration zone was therefore confined to the 15 nm distance zone from the DPTMM/C₆₀ interface for the bilayer structure and to the mixed layer plus 15 nm margins for the BHJ structure (Fig. 3b). The exciton generation by PEDOT:PSS layer was neglected because generated excitons dissociate ineffectively at a PEDOT:PSS/DPTMM interface. By using this method it is found that the maximum dissociable exciton in the BHJ structure is ∼65% higher than in the bilayer structure.

Fig. 3 (a) Distribution of electromagnetic field intensity for bi-layer device (ITO/PEDOT:PSS (40 nm)/DPTMM (30 nm)/C₆₀ (40 nm)/LiF (1.2 nm)/Al (100 nm)) and BHJ device (ITO/PEDOT:PSS (40 nm)/DPTMM (10 nm)/DPTMM:C₆₀ (60–40%) (30 nm)/C₆₀ (30 nm)/LiF (1.2 nm)/Al (100 nm)) at 520 nm incident wavelength. (b) Profiles of the exciton generation rate versus the x-position in the solar cell structures.

3.2. Mobility measurement on DPTMM

To estimate the hole-transport properties of the DPTMM molecular films, we fabricated and analyzed a hole-injection diode. Fig. 4a shows the current density–voltage (J–V) characteristics of such a device, containing a 120 nm DPTMM layer. It is observed that the current is higher when the PEDOT:PSS side is more positively biased than the Au electrode, which is mainly attributed to the difference in the injection barriers.^43,44 In such a situation, an energetic asymmetry leads to a formation of non-zero internal field, so that the measured J–V characteristics should be corrected by the corresponding built-in potential (V_bi).⁴⁵ Fig. 4b shows that, by assuming V_bi of 0.3 V, the current injected from the PEDOT:PSS follows a quadratic dependence on V–V_bi at high voltage, indicating space-charge-limited conduction (SCLC).^46,47 We can fit the experimental data in this regime to the Mott–Gurney law including the voltage correction term aswhere μ is the charge-carrier mobility, ε_s is the semiconductor permittivity, and L is the semiconductor thickness. By assuming a dielectric constant of 3, the extracted hole mobility μ of DPTMM in Fig. 4b is 1.0 × 10⁻⁵ cm² V⁻¹ s⁻¹, which favorably compares to the reported SCLC mobility values of various organic semiconductors in a diode configuration.^48–50

Fig. 4 (a) Current–voltage characteristics of a DPTMM hole-only diode. The inset is the device structure. (b) The current–voltage curve for the injection from PEDOT:PSS corrected by the built-in potential (V_bi). The mobility extraction is done by fitting the measured data to the Mott–Gurney law in the high-voltage region.

3.3. Optimization and analysis of DPTMM-based solar cells

The performance characteristics of all the photovoltaic devices in this study are summarized in Table 3. The device performance of these OSCs exhibited good reproducibility, for every set of experiments several cells were tested (see ESI†).

Table 3 Summary of the device parameters of the fabricated solar cells

Device	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)
A	0.94	5.58	55	3.0
B	0.95	6.29	49	2.9
C	0.88	4.00	56	2.0
D	1.02	5.64	55	3.2
E	1.00	5.46	60	3.3
F	1.00	5.85	65	3.8
G	0.99	6.29	64	4.0

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Fig. 5 shows the illuminated J–V characteristics of the BHJ solar cells made with DPTMM and C₆₀. Devices A, B and C differ in the thickness of the pure DPTMM layer (10, 15 and 20 nm respectively) in contact with the BHJ layer. Note that a pure DPTMM layer was also inserted, and this provides an energy level gradient favoring hole extraction from the mixed layer.¹⁶ It has, however, been reported that a pure hole transporting layer can hinder electron transport toward the PEDOT:PSS side.⁵¹ A comparison of devices A, B and C provides an insight into the effect of DPTMM thickness on photovoltaic characteristics. When increasing the DPTMM thickness from 10 to 20 nm, a reasonable outcome would be an increase in the series resistance. In general, the series resistance (R_s) is estimated by the slope of a J–V curve at the V_oc point. However, all three devices in Fig. 5 have a serious kink effect⁵² which does not allow the determination of reliable R_s value at the V_oc point. Therefore, we alternatively defined a quasi-series resistance R_s at 0.7 V for all three curves, where a reliable linear regression toward the V_oc point is possible. The extracted values are 134, 147, and 248 Ω cm² for devices A, B and C, respectively. It is expected that an increasing R_s would be linked to a deterioration of other cell parameters. However, the observed monotonous increase in R_s does not lead to any simple tendency in J_sc and FF as shown in Table 1. The performance enhancements are observed in an increase in J_sc from device A to B and in an increase in FF from device B to C. It is inferred that the increased DPTMM thickness from device A to B provides greater light absorption volume, which leads to a higher J_sc, compensating the negative effect of R_s. The increase in FF from device B to C can be explained by improved charge balance property which is further investigated hereinafter.

Fig. 5 Measured current–voltage characteristics of bulk hetero junction solar cells with different thicknesses of DPTMM layer under 1sun AM 1.5, (ITO/PEDOT:PSS (40 nm)/DPTMM (10–20 nm)/DPTMM:C₆₀ (60–40%) (30 nm)/C₆₀ (30 nm)/LiF (1.2 nm)/Al (100 nm)).

It is thought that the serious kink shape observed in devices A to C (Fig. 5) is due to the energy level mismatch at the hole extraction/collection side, mainly due to the deep HOMO level of DPTMM (−5.96 eV) comparatively to the work function of the PEDOT:PSS material (−5.2 eV). Another set of experiments was carried out by adding a buffer layer of (4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl) (α-NPB) to provide an intermediate HOMO level (−5.4 eV) between PEDOT:PSS and DPTMM.⁵³ A thickness of five nanometers was selected from various reports concerning hole extraction layers.^54–56 The effect of inserting a 5 nm α-NPB layer is best seen by comparing device D with device B (Fig. 5 and 6), both of which have the same DPTMM thickness and the same BHJ composition. By comparing J–V curves and the extracted parameters in Table 2, it is observed that the buffer layer effectively removed the kink and a concomitant overall performance improvement was achieved. In spite of a slight reduction in J_sc due to an added bulk resistive component, a promising PCE value of 3.2% was obtained in device D.

Fig. 6 J–V characteristics of device with α-NPB hole extraction layer (circle) ITO/PEDOT:PSS (40 nm)/α-NPB (5 nm)/DPTMM (10 nm)/DPTMM:C₆₀ (60 : 40) (30 nm)/C₆₀ (30 nm)/LiF (1.2 nm)/Al (100 nm) and gradient doped BHJ device (triangle) ITO/PEDOT:PSS/α-NPB (5 nm)/DPTMM (15 nm)/DPTMM (70%):C₆₀ (30%)…DPTMM (50%):C₆₀ (50%) (30 nm)/C₆₀ (30 nm)/LiF (1.2 nm)/Al (100 nm).

It is well established that the donor/acceptor volume ratio in a BHJ layer has a significant influence on photovoltaic characteristics, as this ratio determines hole/electron charge balance properties.^10–12 Keeping the α-NPB buffer layer geometry, we changed the bulk composition of the DPTMM:C₆₀ layer by controlling the deposition rate of each material in device D, E, and F. As shown in Table 1, it is found that reducing the relative amount of the donor DPTMM leads to a further performance improvement, mainly through improved J_sc and FF, which gives a significant PCE of 3.8%. When decreasing the DPTMM ratio from 60 to 50%, J_sc may be decreasing due to poorer exciton generation. However, a better hole–electron balance in the devices leads to a lesser amount of accumulated space charge in the electrode. This counter effect provides better charge transport and carrier collection at each electrode, finally leading to a higher J_sc.^57–59

As a last development strategy, a BHJ layer with a spatial gradient in donor/acceptor ratio (detailed in ESI†) was fabricated. This strategy was based on previous reports on a similar technique, which is known to promote charge generation and improve transport properties while providing a gradually varying electronic structure from p- to n-side.⁶⁰ In addition, a relatively low J_sc in device F compared to that in device B implies that there is a need for additional energetic adjustment to achieve both the optimum exciton harvest and charge balance properties. Therefore, an average donor : acceptor ratio of 60 : 40 was employed, with a gradient in composition from DPTMM (70%):C₆₀ (30%) at the anode side to DPTMM (50%):C₆₀ (50%) at the cathode side. Fig. 7 shows the J–V characteristics of the gradient BHJ device G in the dark and under illumination. This device shows an impressive PCE of 4.0% and a J_sc of 6.29 mA cm⁻². This high performance is a result of the extensive investigation of geometrical and material factors in order to improve exciton dissociation, charge balance and collection properties. The PCE of 4.0% proves that the DPTMM:C₆₀ system has a high potential for evaporation-based BJH solar cells when an optimized device structure is employed. It is worthy to note that in all optimized devices V_oc up to 1 V have been achieved. Such high V_oc value is quite unusual for OSCs devices.

Fig. 7 Current density–voltage response plots for gradiently doped BHJ device. ITO/PEDOT:PSS (40 nm)/α-NPB (5 nm)/DPTMM (15 nm)/DPTMM (70 nm):C₆₀ (30%)…DPTMM (50%):C₆₀ (50%) (30 nm)/C₆₀ (30 nm)/LiF (1.2 nm)/Al (100 nm). Total amount of DPTMM and C₆₀ maintain the ratio of 60:40. V_oc = 0.99 V, J_sc = 6.29 mA cm⁻², FF = 64%, and PCE = 4.0%.

4. Conclusion

In summary, a novel material (DPTMM) was studied in order to investigate its potential as a donor material in organic solar cells (OSCs). In this study bulk-heterojunction (BHJ) devices were fabricated by vacuum co-evaporation process. This study put emphasis on combining thin-film characterization and photovoltaic device optimization schemes. By means of optical modeling, the BHJ architecture of DPTMM and C₆₀ was shown to promote exciton dissociation, which in turn led to a significant improvement in device performance. The use of an α-NPB buffer layer eliminated the contact-related kink shape near V_oc point by providing an energetic bridge between PEDOT:PSS and DPTMM. The charge-balance property was controlled through various donor/acceptor compositions and a fully optimized device was fabricated with a composition gradient that favors both the exciton separation and charge transport balance in the complex BHJ cell geometry. The maximum obtained PCE was 4.0%, which compares favorably to the state-of-the-art of high-performance OSCs. For all devices, unusual high values of V_oc up to 1 V have been achieved. The overall PCE improvement to the fully optimized BHJ device clearly shows that there is significant room for device design and process optimization to fully exploring the potential of a given material. This result demonstrates that simple and easily to produce small conjugated molecules such as PTMM is a promising strategy for high-performance small molecule-based organic solar cells.

Acknowledgements

The work of C. H. Kim was supported by the Vice Presidency for External Relations (DRE) in Ecole Polytechnique through a PhD fellowship. The authors would like to thank Dr H. Derbal-Habak for ellipsometric measurements. The authors gratefully thank the Renewable Energy Science & Technology (REST) Master program from ParisTech for enabling supervised student access to the research lab. The Ministère de la Recherche is acknowledged for the PhD grant to A. Leliège.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47059h

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