Improving the charge carrier transport of organic solar cells by incorporating a deep energy level molecule

Abstract

Tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), a strong molecular acceptor, has been proved to be an excellent candidate to achieve the p-type doping effect. When F4-TCNQ is incorporated into a poly(3-hexylthiophene) (P3HT): indene-C₆₀ bisadduct (ICBA) active layer, superior behavior upon inducing polymer donor excited electron transport is demonstrated due to the addition of a deep-lying lowest unoccupied molecular orbital (LUMO) from F4-TCNQ, leading to the realization of organic solar cells (OSCs) with an improved power conversion efficiency (PCE) of 5.83%, accounting for 29.6% enhancement. In the system of active layer, the low LUMO of F4-TCNQ can easily accept electrons, remarkably reducing electron/hole recombination, which contributes to the enhancement of the photoconductivity and charge carrier mobility, resulting in higher short-circuit current density (J_sc), and achieving a more balanced charge carrier transport, as well as an ideal fill factor (FF).

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1. Introduction

Currently, environmental pollution and energy shortage are the most severe challenges for human existence. The study of solar cells could solve these two problems effectively, and in the past few years, impressive progress has been made in solar energy, both in experimental research and market activity.^1–4 Among the solar cells, organic solar cells (OSCs) have received much attention,^5–7 although their power conversion energy (PCE) is still low for large-scale application. In the research of OSCs, poly(3-hexylthiophene) (P3HT) and indene-C₆₀ bisadduct (ICBA) are excellent photovoltaic materials with the potential to obtain high open-circuit voltage (V_oc) and fill factor (FF) when they are used to fabricate devices.^8–11 ICBA is selected as the acceptor material by matching up P3HT electron donor instead of the typical fullerene acceptor (6,6)-phenyl C₆₁ butyric acid methyl ester (PCBM); the main reason for this is that the deep-lying lowest unoccupied molecular orbital (LUMO) of ICBA (−3.7 eV) is much higher than that of PCBM (−4.3 eV), which will faithfully result in a higher V_oc (exceeding 0.80 V).^12–14 However, the devices with P3HT:ICBA generally demonstrate lower short-circuit current density (J_sc). Some methods should be put forward and applied to solve this problem from the aspects of optical properties and electronic characteristics.

Intentional doping of the materials is necessary for improving performance of the organic photovoltaic devices,^15–17e.g., for tunnelling contacts with efficient carrier injection and for the generation of space charge layers in p–n junctions.^18,19 Studies on ternary solar cells have made a great deal of achievements with improved efficiency and they have been repeatedly reported on.^20–23 Doping could also increase the quantity of charge carriers (electron or hole or both), leading to higher conductivity, which is important in many devices to lower the contact losses.^24–28 Organic molecular materials (n-type and p-type) are promising for cell devices due to their adjustable and high absorption as well as electrical behaviour comparable to crystalline semiconductors in many respects. For instance, to achieve some improvement of holes, the p-type doping in organic layers with strong molecular acceptors like tetracyanoquinodimethane (TCNQ) or dicyano-dichloroquinone (DDQ) is necessary and has been proved to be a good idea to improve the performance of OSCs previously.²⁹ In this work, we doped the tetrafluoro-tetracyanoquinodimethane (F4-TCNQ, C₁₂N₄F₄, fluorinated form of TCNQ) into active layer fabricating devices with the structure of the ITO/polyethylenimine (PEI)/P3HT:ICBA:F4-TCNQ/WO₃/Ag. F4-TCNQ is a strong molecular acceptor that possesses high electron affinity. Meanwhile, there are no molecular dopant clustering because charged F4-TCNQ anions will generate repulsion between each other at high coverage. Thus, efficient and stable p-doping will be achieved. Previously, F4-TCNQ has been also doped into carbon nanotube field-effect transistors leading to decrease in both the channel resistance and contact resistance by layer deposition of F4-TCNQ molecules.³⁰ Meanwhile, Wee et al. has proved that F4-TCNQ is an excellent candidate for functionalizing graphene.³¹ In our study, the main purpose is to improve the lower J_sc of pristine P3HT:ICBA device, matching with its high V_oc and FF to obtain the desired performance. We show that the incorporation of F4-TCNQ exhibits the following advantages: (1) increased photoconductivity and charge carrier mobilities due to the electrons from P3HT trapped by deep-lying LUMO of F4-TCNQ, reducing the electron/hole recombination, which contributes to the boost of J_sc, and (2) a more balanced charge carrier transport because of the apparent improvement of hole mobility, which benefits the enhanced FF. As a result, higher J_sc of 9.85 mA cm⁻² and FF of 68.0% were achieved, resulting in an optimal PCE of 5.83% for the doped device with 0.10 wt% F4-TCNQ.

2. Experimental section

2.1. Device fabrication

F4-TCNQ was doped into an active layer and three different doping concentrations of 0.07 wt%, 0.10 wt% and 0.13 wt% (the weight ratio of F4-TCNQ to the total weights of P3HT and ICBA) were selected to improve the performance of the OSCs. The control device without F4-TCNQ was also fabricated using the same experimental parameters. Firstly, PEI solution with the concentration of 2 mg mL⁻¹ was spin-coated onto the pre-cleaned ITO glasses, postannealing at 100 °C for 10 min. Then active layer solution including 15 mg P3HT and 15 mg ICBA in 1 mL 1,2-dichlorobenzene (DCB) was prepared. Different weights of F4-TCNQ were added to obtain doped active layer solutions. Well-blended active layer solutions were spin-coated onto the PEI layer at 1500 rpm for 30 s, annealing at 150 °C for 25 min in a glove box. Finally, WO₃ (10 nm) and Ag (100 nm) were thermally evaporated as a hole transport layer and anode, respectively. Every active area of the OSC devices was about 0.064 cm².

2.2. Measurements and characterization

The film morphology was analyzed using a Bruker Dimension Icon Atomic Force Microscope (AFM). Current density–voltage (J–V) characteristics were measured using a Keithley 2601 source meter under 1 sun with an Oriel 300 W solar simulator intensity of ∼100 mW cm⁻². The measurement of the incident photon-to-current efficiency (IPCE) was performed using a Crowntech QTest Station 1000 AD. The light absorption spectra were recorded on a UV 1700, Shimadzu. The impedance, Mott–Schottky curves and dark capacitance versus voltage (C–V) characteristics were analyzed by a Precision Impedance Analyzer 6500B Series of Wayne Kerr Electronics.

3. Results and discussion

The chemical structure of F4-TCNQ and the energy diagram of the active layer materials are shown in Fig. 1(a) and (b), respectively. The LUMO of F4-TCNQ is ∼5.2 eV, deeper than that of the P3HT and ICBA, indicating that F4-TCNQ is a strong electron acceptor. In dark, when doping F4-TCNQ into the active layer, electrons from the highest occupied molecular orbital (HOMO) of the P3HT will preferentially occupy the deep-lying LUMO of the F4-TCNQ,^32–34 which leads to an increase of the background hole concentration in the P3HT, contributing to the enhancement of the conductivity for doped devices.

Fig. 1 (a) Chemical structure of F4-TCNQ and (b) energy diagram of active layer materials, AFM morphology images of (c) pristine P3HT:ICBA film and (d) P3HT:ICBA:0.13 wt% F4-TCNQ hybrid film.

The morphology of organic donor:acceptor thin films significantly influences exciton migration and dissociation,^35–37 charge carrier transport,^3,38 and charge carrier recombination,^39–41 which is the major bottleneck that hampers the progress in PCEs of OSCs. Hence, the surface morphology of active layer films using AFM were measured firstly, shown in Fig. 1(c) and (d), including height images of P3HT:ICBA films without and with 0.13 wt% F4-TCNQ. There was an ignorable difference in average roughness of the surface between them (from 0.547 to 0.567 nm). Uniform surface morphology indicates that F4-TCNQ was evenly distributed in the active layer and there was no molecular clustering existence even for the highest doping concentration.

The comparison of J–V curves of devices without and with different concentration F4-TCNQ in Fig. 2(a) apparently indicates that J_sc was markedly improved by the incorporation of F4-TCNQ, leading to increased PCE for doped devices. The average photovoltaic parameters from thirty identical devices for each type are listed in Table 1. For the optimal doped device, the device with 0.10 wt% F4-TCNQ demonstrated higher PCE of 5.83% with increased J_sc (from 8.19 to 9.85 mA cm⁻²) and FF (from 63.2% to 68.0%), accounting for 29.6% enhancement in the PCE compared with the control device. When deviating from the optimal doping concentration, J_sc, FF and PCE began to decrease, while V_oc remained at the same level. The performance of the device with higher doping concentration of 0.13 wt% began to decrease, which could have arisen from the decrease of the charge carrier mobilities. The effect of molecular doping on mobility rationally conforms to the analytic model of charge carrier mobility in doped organic semiconductors by D. Neher et al.,⁴² and the detailed charge carrier mobility would be calculated and compared in the subsequent works. Seen from Table 1, the V_oc of all devices are about 0.87 V, which is the common performance for the solar cells based on P3HT:ICBA. The enhanced PCE of doped devices are mainly attributed to the increase of J_sc and FF, owing to the improved optical and electronic properties. Under illumination, the LUMO of F4-TCNQ could trap the electrons from P3HT, reducing the electron and hole recombination, which contributes to the increase of the photoconductivity and the charge carrier mobilities. Thus J_sc would be significantly improved and the more balanced charge carrier transport benefits the increased FF. In order to identify enhanced optical properties of doped devices arising from the existence of the F4-TCNQ, the IPCE curves for devices without and with F4-TCNQ were obtained and shown in Fig. 2(b). In the IPCE spectra, the 0.10 wt% F4-TCNQ device demonstrated a maximum IPCE of 60% at 530 nm, while the control device showed a maximum IPCE of 51% at 520 nm. Doped devices exhibited improved photon utilization in a wide wavelength range from 350 nm to 650 nm, being consistent with the enhanced light absorption range of active layers shown in the inset of Fig. 2(b).

Fig. 2 (a) J–V characteristics of control device and doped devices with different concentrations of F4-TCNQ, (b) IPCE curves of the inverted OSCs without/with the F4-TCNQ and inset is the light absorption of the active layers corresponding to all fabricated devices.

Table 1 Photovoltaic properties of the devices doped without and with different concentrations of F4-TCNQ, including V_oc, J_sc, FF, PCE, μ_e, μ_h and μ_e/μ_h

Doping concentration (wt%)	V oc (V)	J sc (mA cm^−2)	FF (%)	PCE (%)	μ e (cm^2 V^−1 s^−1)	μ h (cm^2 V^−1 s^−1)	μ e/μh
0	0.87 ± 0.01	8.19 ± 0.06	63.2 ± 0.2	4.50 ± 0.10	4.92 × 10^−4	9.06 × 10^−5	5.43
0.07	0.87 ± 0.01	9.36 ± 0.07	66.3 ± 0.1	5.40 ± 0.11	6.47 × 10^−4	1.94 × 10^−4	3.34
0.10	0.87 ± 0.01	9.85 ± 0.07	68.0 ± 0.1	5.83 ± 0.12	6.91 × 10^−4	2.45 × 10^−4	2.82
0.13	0.87 ± 0.01	8.84 ± 0.06	65.6 ± 0.2	5.05 ± 0.11	5.68 × 10^−4	1.62 × 10^−4	3.51

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Under illumination, excited electrons in P3HT have a chance of jumping into the F4-TCNQ, trapped by the LUMO of the F4-TCNQ instead of being combined, which decreases the electron and hole recombination. In order to verify the existence of excited electron transport from P3HT to F4-TCNQ in the active layer, photoluminescence (PL) and time-resolved PL decay measurements were carried out. Fig. 3(a) is the PL spectra of neat P3HT:ICBA and P3HT:ICBA:0.10 wt% F4-TCNQ films. The PL intensity of the doped film is relatively lower than that of the neat P3HT:ICBA film, indicating that electron/hole recombination was apparently suppressed. Thus, part of the photo-generated electrons of P3HT were transferred to F4-TCNQ instead of being combined,^43,44 which was further testified by the time-resolved PL decay measurements subsequently, shown in Fig. 3(b). It can be noted that the response of P3HT in the mixed film is shorter than that of the undoped film, suggesting the presence of an energy transfer process (i.e., electron transfer) from P3HT to F4-TCNQ.^32,45–47 On this occasion, the charge carrier recombination in P3HT was reduced; therefore the hole concentration would be significantly increased, contributing to the improvement of electrical conductivity.

Fig. 3 (a) PL spectra and (b) time-resolved PL decay curves of P3HT:ICBA films doped without and with 0.10 wt% F4-TCNQ.

In order to calculate the quantity of charge carrier in devices, Mott–Schottky curves and dark C–V characteristics of the control device and optimal device were measured in a bias range of 0–1 V at 5 KHz. From Fig. 4(a) and (b), the built-in voltage (V_bi) and charge carrier density (n) of the P3HT:ICBA devices can be attained. According to the value of V_bi, n can be calculated based on

where A is the device area, e is the elementary charge, d is the thickness of the active layer, and C is the chemical capacitance.^48,49 The capacitance increased at first, reaching a peak, and then decreased. Roughly, for the C–V curve, when V < V_bi, C is determined by the depletion layer modulation, when V >V_bi, C is determined by the storage of minority carrier. For the control device, V_bi is about 0.86 ± 0.01 V (from the blue line in Fig. 4(a)) and n is 2.23 × 10¹⁶ cm⁻³ (from the black line in Fig. 4(a)) while from Fig. 4(b), V_bi for the optimal device is about 0.86 ± 0.01 V and n is 3.27 × 10¹⁶ cm⁻³. It is clear that the incorporation of F4-TCNQ leads to a noticeable enhancement of the charge carrier density, which could effectively improve the photoconductivity of devices, contributing to the increase of J_sc.

Fig. 4 Mott–Schottky (blue line) and C–V (black line) curves of (a) control device and (b) doped device with 0.10 wt% F4-TCNQ.

To investigate charge carrier transport balance, hole-only and electron-only devices were fabricated and the corresponding charge carrier mobilities were estimated. Firstly, hole-only devices with the structure of ITO/WO₃/P3HT:ICBA:F4-TCNQ/WO₃/Ag were fabricated, where WO₃ is an electron blocking layer. Hole mobilities of all devices without and with F4-TCNQ were calculated by the charge transfer model of space-charge-limited-current (SCLC) and the Mott–Gurney law that includes field-dependent mobility,^50,51 given by

where ε₀ is the permittivity of free-space, ε_r is the relative dielectric constant of the active layer (about 3 for organic materials), L is the thickness of the active layer (about 120 nm) and μ is the mobility. From the dark J–V characteristic shown in Fig. 5(a), hole mobilities were compared between undoped and doped devices, which revealed the increase of hole mobility from 9.06 × 10⁻⁵ to 2.45 × 10⁻⁴ cm² V⁻¹ s⁻¹, indicating that the doping of the F4-TCNQ favorably enhanced hole transport attributed to the increase of the hole concentration and conductivity. Electron-only devices were also fabricated with the structure of ITO/PEI/P3HT:ICBA:F4-TCNQ/BCP/Ag, where BCP is the hole blocking layer. J–V characteristics were measured under the same conditions with those of hole-only devices, which is shown in Fig. 5(b). It can be found that electron mobilities of doped devices are slightly higher than that of the control device, increasing from 4.92 × 10⁻⁴ to 6.91 × 10⁻⁴ cm² V⁻¹ s⁻¹. The enhancement of the electron mobility should arise from the electron trapping of the LUMO of the F4-TCNQ, reducing the electron/hole recombination. Electron and hole mobilities of all devices are shown in Fig. 5(c), and the corresponding values are summarized in Table 1. It is worth noting that the value of μ_e/μ_h for doped devices achieved a more balanced charge carrier transport, resulting in the improvement of the FF. The mobility of the 0.13 wt% doped device is lower than that of the 0.10 wt% doped device, which could well explain the decrease of the performance of the 0.13 wt% doped device. Subsequently, J–V characteristics measured for ITO/P3HT:ICBA/Ag and ITO/P3HT:ICBA:F4-TCNQ/Ag are shown in Fig. 5(d). The device with this simple sandwich structure could verify the charge carrier transport capacity (considering the electron and hole) without the influence of the buffer layer and the results have indicated that F4-TCNQ could enhance the charge carrier transport capacity.

Fig. 5 Dark current density versus bias characteristics of (a) hole-only devices and (b) electron-only devices, (c) charge carrier mobilities of all hole-only and electron-only devices, and (d) current density versus bias with the device structure of ITO/P3HT:ICBA/Ag without and with 0.10 wt% F4-TCNQ.

When F4-TCNQ was added into the active layer, the background hole concentration in polymer donor was increased due to the ground-state electron transport from P3HT to F4-TCNQ, which contributes to the improvement of the conductivity of doped devices. To examine the effect of this on resistance, the impedance spectroscopy was examined in the dark with an alternating current signal of 1 V in a frequency range of 20 Hz to 1 MHz, shown in Fig. 6. As we can see, the shapes of impedance spectra are desirable semicircles that are beneficial to investigate the resistance in OSCs. In the impedance spectroscopy, the diameters of the semicircles are connected with the series resistance (R_s). It is noteworthy that the diameters of the semicircles for these doped devices are smaller than those of the control device, which suggests decreased R_s, thus reducing the current losses across the contacting materials.⁵²

Fig. 6 Impedance spectra of the devices without and with different concentrations of F4-TCNQ measured with a frequency range from 20 Hz to 1 MHz in dark.

4. Conclusions

In summary, we deliberately doped F4-TCNQ with deep-lying LUMO into the active layer to induce excited electron transport, achieving p-type doping of P3HT:ICBA for optimized device performance resulting in a high PCE of 5.83%. The incorporation of F4-TCNQ could effectively decrease the electron/hole recombination and increase the charge carrier density. On the one hand, J_sc was effectively improved due to the increased photoconductivity. Meanwhile, impedance and R_s are significantly decreased, suppressing current loss, which also facilitates to increase J_sc. On the other hand, electron and hole transport became more balanced due to enhanced charge carrier mobilities, which is beneficial for improving FF.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (61275035, 61370046, and 11574110), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2013KF10), the Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No. 2014A030306005), and the Foundation for High-level Talents in Higher Education of Guangdong Province, China (Yue Cai-Jiao [2013]246, Jiang Cai-Jiao[2014]10) for the support of the work.

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