1.

Introduction

Organic thin-film photovoltaic (OPV) devices have been receiving a great deal of attention as candidates for next-generation solar cells or photodetectors because of their potentially low cost, light weight, and capability of large-area, flexible device fabrication.^{1, 2, 3, 4, 5}

The development of new materials, including both small molecules and polymers, and the implementation of new device structures have led to significant improvement in power conversion efficiency (PCE).^{6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18} At present, PCEs of 8% under simulated sunlight illumination have been attained.^{19, 20, 21} With regard to small molecular materials for OPV devices, polycrystalline materials have usually been used because of their relatively high charge-carrier mobilities. Phthalocyanines, and perylene pigments and fullerenes are typical examples of electron donors and acceptors that give a high PCE. Recently, growing attention has also been paid to amorphous molecular materials for use in OPV devices. Following the report that a planar pn-heterojunction OPV device using N,N^′-bis(α-naphthyl)-N,N^′-diphenyl-[1,1^′-biphenyl]-4,4^′-diamine as an electron donor and C₆₀ as an electron acceptor exhibits 1% PCE,²² there have been extensive studies on OPV devices using a variety of amorphous molecular materials as electron donors and fullerenes as electron acceptors, and a PCE up to over 2% has been attained.^{23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36} With regard to device structures, p-i-n,^{7, 12, 14} bulk-heterojunction,^{6, 8} and tandem structures,^{11, 13, 17} as well as a planar pn-heterojunction structure, together with the incorporation of an exciton-blocking layer^{9, 10, 16} or optical spacers,^{15, 16} have been employed to improve the PCE.

Unlike inorganic semiconductors, organic semiconductors that are used for OPV devices are essentially insulators. The observed current density that flows in the external circuit of OPV devices at given cell voltages under simulated sunlight illumination significantly decreases as the series resistance in the equivalent circuit increases; this results in a low fill factor (FF) and PCE. Making organic layers as thin as possible and reducing the contact resistance at the interface between organic layers and electrodes are required to improve the PCE of OPV devices. Charge-transfer doping of organic layers is an effective method for reducing the series resistance in the equivalent circuit. It is well known that charge-transfer doping, i.e., p- or n-type doping, of both π-conjugated polymers,^{37, 38} and polymers containing pendant π-electron systems³⁹ produces electrically conducting polymers. Electrochemically p-doped pendant polymers^{40, 41} and chemically doped polymers⁴² have been used as materials for OPV devices and organic light-emitting diodes (OLEDs). Recently, charge-transfer doping has been extended to amorphous molecular materials,⁴³ and p- or n-doped crystalline and amorphous molecular materials have been applied for OPV devices and OLEDs.^{12, 44, 45, 46, 47, 48} It has been shown that p-doping of zinc phthalocyanine and amorphous molecular materials, e.g., 4,4^′,4^″-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) and 4,4^′,4^″-tris(diphenylamino)triphenylamine (TDATA), with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) increases the electrical conductivities of these organic materials;^{12, 44, 45, 46, 47, 48} as a result, hole injection from the electrode is enhanced in OLEDs, and the operating voltage is significantly reduced.^{45, 46, 47, 48} With regard to OPV devices, it has not been fully clarified how charge-transfer doping affects device performance parameters such as open-circuit voltage (V_OC), short-circuit current density (J_SC), and FF.

In the present study, we have investigated the effect of p-type doping of the donor layer with F₄TCNQ on the performance of planar pn-heterojunction OPV devices. Amorphous molecular materials with low solid state ionization potentials (Ips), m-MTDATA (Ip 5.1 eV,⁴⁹) and 4,4^′,4^″-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA) (Ip 5.15 eV⁵⁰), and C₆₀ were used as electron donors and an electron acceptor, respectively. These electron donors with the low solid-state ionization potentials are thought to be suitable for p-type doping, enabling electron transfer to an electron acceptor, F₄TCNQ, in the ground state. In addition, grain-boundary-free amorphous molecular materials that form smooth, uniform amorphous thin films are expected to allow uniform doping.^{51, 52}

2.

Experimental

3.

Results and Discussion

m-MTDATA and TDATA doped with F₄TCNQ have been used as a hole-transporting layer in OPV devices;^{13, 45, 46} however, the performance data of planar pn-heterojunction OPV devices using m-MTDATA as an electron donor and C₆₀ as an electron acceptor is not yet available in the literature. In the present study, planar pn-heterojunction OPV devices consisting of m-MTDATA or 2-TNATA as an electron donor and C₆₀ as an electron acceptor, ITO/PEDOT:PSS (ca. 30 nm)/m-MTDATA (50 nm)/C₆₀ (50 nm)/LiF (0.1 nm)/Al (80 to 150 nm) (device A) and ITO/PEDOT:PSS (ca. 30 nm)/2-TNATA (50 nm)/C₆₀ (50 nm)/LiF (0.1 nm)/Al (80 to 150 nm) (device B), were fabricated, and their performance was examined. Then, the effect of p-type doping of the donor layer with F₄TCNQ on the cell performance was investigated. For this purpose, the following devices were fabricated: ITO/PEDOT:PSS (ca. 30 nm)/m-TDATA: F₄TCNQ (10:1.0) (50 nm)/C₆₀ (50 nm)/LiF (0.1 nm)/Al (80 to 150 nm) (device D) and ITO/PEDOT:PSS (ca. 30 nm)/2-TNATA :F₄TCNQ (10:1.0) (50 nm)/C₆₀ (50 nm)/LiF (0.1 nm)/Al (80 to 150 nm) (device E). Figure 1 shows the structures of the fabricated OPV devices.

Fig. 1

Structures of fabricated devices.

Figures 2 and 2 show the current density (J) − voltage (V) characteristics of device A and device D, and device B and device E, respectively, in the dark and under AM1.5G illumination at an incident light intensity of 100 mW cm⁻². Table 1 summarizes the performance of the fabricated OPV devices, V_OC, J_SC, FF, and PCE, under AM1.5G illumination at an incident light intensity of 100 mW cm⁻². Device A using m-MTDATA as an electron donor exhibited a V_OC of 0.40 V, a J_SC of 0.7 mA cm⁻², a FF of 0.24, and a PCE of 0.07%. Device B using 2-TNATA as an electron donor showed similar performance to that of device A, exhibiting a V_OC of 0.45 V, a J_SC of 0.5 mA cm⁻², a FF of 0.25, and a PCE of 0.05%. For comparison, a corresponding pn-heterojunction device using TPD as an electron donor and C₆₀ as an electron acceptor, ITO/PEDOT:PSS (ca. 30 nm)/TPD (50 nm)/C₆₀ (50 nm)/LiF (0.1 nm)/Al (80 to 150 nm) (device C), was also fabricated and its performance was examined. Device C exhibited a V_OC of 0.67, a J_SC of 1.4 mA cm⁻², a FF of 0.42, and a PCE of 0.38% (Table 1).

Table 1

Device performance under AM1.5G illumination at an intensity of 100 mW cm−2.

Fig. 2

J–V characteristics of (a) device A and device D, and (b) device B and device E in the dark and under AM1.5G illumination at an incident light intensity of 100 mW cm⁻².

It has generally been understood that V_OC corresponds to the difference between the highest occupied molecular orbital (HOMO) level of electron donor and the lowest unoccupied molecular orbital (LUMO) level of electron acceptor.⁴ The V_OC values from 0.40 to 0.45 V observed for device A and device B roughly correspond to the difference between the HOMO level of m-MTDATA or 2-TNATA (5.1, 5.15 eV)^{5, 49, 50} and the LUMO level of C₆₀ (4.5 eV).² Likewise, the V_OC value of 0.67 V observed for device C roughly corresponds to the difference between the HOMO level of TPD (5.45 eV)⁵⁰ and the LUMO level of C₆₀. Since m-MTDATA and 2-TNATA have similar solid-state ionization potentials and hole drift mobilities of ∼3 × 10⁻⁵ cm² V⁻¹s⁻¹ at 1.0 × 10⁵ V m⁻¹ at room temperature,^{54, 55, 56, 57} the similar performance observed for device A and device B seems to be reasonable. As light is mainly absorbed by C₆₀, slight differences in the ultraviolet absorption spectra between m-MTDATA and 2-TNATA did not appreciably affect the performance under simulated AM 1.5G sunlight illumination.

Very low PCEs of these devices stem from relatively low V_OC and small J_SC and FF. The relatively low V_OC is attributed to the low HOMO levels of these electron donors, and the small J_SC value is attributed to the absence of visible light absorption by m-MTDATA and 2-TNATA (Fig. 3) and to their very low hole drift mobilities. It has been reported that the dissociation process of photogenerated hole-electron pairs at the donor/acceptor interface to generate charge carriers in competition with the charge recombination process is greatly influenced by charge-carrier mobilities of organic materials used and that a tenfold increase in mobility dramatically improves J_SC and FF, doubling the maximum power output.⁵⁸ The limited FF for these devices results from the large series resistance (R_s) of these devices. In fact, the R_s values calculated from the J−V curves under illumination were ca. 210 and 60 Ωcm² for device A and device B, respectively. The relatively small shunt resistance (R_sh) values under illumination (ca. 650 and 1200 Ωcm² for device A and B) were also thought to be responsible for the low FF. Higher performance of device C than that of device A and device B is attributable to the higher HOMO level and much higher hole drift mobility of TPD (1.0 × 10⁻³ cm² V⁻¹ s⁻¹ at 1.0 × 10⁵ V cm⁻¹)⁵⁷ relative to those of m-MTDATA and 2-TNATA.

Fig. 3

Electronic absorption spectra of vapor-deposited films of m-MTDATA (30 nm) and 2-TNATA (30 nm).

The p-doping of the donor layer with F₄TCNQ resulted in the increase of both J_SC and FF and a significant decrease of V_OC. That is, device D and device E exhibited 1.7 to 2.0 times higher J_SC and 1.5 to 1.6 times larger FF, respectively, but significantly lower V_OC than device A and device B. As a result, device D and device E using F₄TCNQ-doped materials gave 1.6 to 1.7 times larger PCEs than device A and device B, as shown in Fig. 2 and Table 1. The increase of both J_SC and FF caused by the p-doping of the donor layer with F₄TCNQ can be explained in terms of the increased hole mobility of F₄TCNQ-doped m-MTDATA and 2-TNATA and the decreased series resistance of the devices using these doped donor materials. The increase of charge-carrier mobility by p-type doping has been reported with regard to 1,3,5-tris[N,N-bis(4,5-dimethoxyphenyl)aminophenyl]benzene⁵⁹ and zinc phthalocyanine.⁶⁰ The decrease in the resistance of donor materials by p-type doping was shown by the analysis of the J−V curves of device D and device E under illumination. The R_s values calculated from the J−V curves under illumination for device D and device E were ca. 8 and 7 Ω cm², respectively, which are much smaller than those calculated from the J−V curves of device A and device B using undoped electron donors. The increase in electrical conductivities of m-MTDATA and TDATA by the p-doping with F₄TCNQ has been reported.^{45, 46}

The observed current density (J_obs) that flows in the external circuit of OPV devices is given by the subtraction of the photocurrent density (J_ph) from the dark current density (J_d) [Eq. 1]. Since J_SC is the current density observed at the zero cell voltage where J_d is zero, J_SC is equal to J_ph. If it is simply assumed that the photocurrent observed for the OPV device corresponds to that observed for photoconductors, J_ph is expressed as Eq. 2, where L is the thickness of the sample, I₀ is the total number of photons arriving at the unit surface area of the cell per second, α is the absorption coefficient, η is the photogeneration efficiency of charge carriers, τ is the carrier lifetime, e is the elementary electric charge, μ is the charge carrier mobility, and E is the electric field. The parameters involved in J_ph that are affected by the p-doping of the electron donor are suggested to be μ and η. That is, the increase in J_ph is attributable to the increase of μ for the p-doped donor materials and to the increase of η owing to the increased μ.

The dark current density is the sum of the current density that flows through the cell and the leakage current density that flows through the shunt resistance [Eq. 3] in the equivalent circuit of OPV devices (Fig. 4). J_obs is expressed as Eq. 4, where J₀ is the reverse saturated dark current density, R_s and R_sh represents series and shunt resistances, respectively, V is the cell voltage, n is the diode ideal factor, k is the Boltzmann constant, and T is the absolute temperature. Numerical calculation for Eq. 4 clearly shows that J_obs at given cell voltages under V_OC significantly decreases and hence, FF also significantly decreases as the series resistance R_s increases. The decrease in the R_s of the m-MTDATA and 2-TNATA layer caused by the p-doping with F₄TCNQ is responsible for the enhancement of both J_obs and FF. There was no shunt resistance decrease by the doping (ca. 500 and 1200 Ωcm² for device D and device E). Considering the low HOMO level of m-MTDATA and 2-TNATA (5.1, 5.15 eV)^{5, 49, 50} and the LUMO level of F₄TCNQ (5.24 eV),⁴⁴ electron transfer from the electron donor to the electron acceptor F₄TCNQ is suggested to take place in the dark, resulting in the increase in the electrical conductivity of the F₄TCNQ-doped m-MTDATA and TDATA

Fig. 4

Equivalent circuit of OPV devices.

V_OC is the voltage where J_obs is zero. Although Eq. 4, which is applied for inorganic semiconductor photovoltaic devices, predicts that the V_OC value is not affected by R_s, the decrease of V_OC caused by the p-doping is due to the hole injection from the ITO electrode into the p-doped donor layer at a lower applied voltage because of the decreased resistance of the p-doped donor layer. The V_OC value decreases as such injection dark current starts to flow at a lower cell voltage. Comparison of the J–V characteristics in the dark between device A and device D [Fig. 2] and between device B and device E [Fig. 2] clearly shows that the dark current density for device D and device E abruptly starts to increase at a lower cell voltage of ca. 0.2 V as compared with ca. 0.4 V for device A and device B. It is understood that charge-carrier injection from the electrode into the organic layer takes place for OPV devices and that hole injection from the anode into the p-doped donor layer is facilitated because of the increase in the electrical conductivity of the p-doped material.

It was expected that the reduction of V_OC can be compensated by incorporation of a thin layer of undoped m-MTDATA on the ITO electrode. From this viewpoint, the following device was fabricated: ITO/PEDOT:PSS (ca. 30 nm)/m-MTDATA (10 nm)/m-MTDATA: F₄TCNQ (10:1.0) (40 nm)/C₆₀ (50 nm)/LiF (0.1 nm)/Al (80 to 150 nm) (device F). Device F exhibited higher V_OC than that for device D, maintaining almost the same J_SC and FF as those obtained for device D, and hence, led to further enhancement of PCE (Fig. 5 and Table 1).

Fig. 5

J–V characteristics of device D and device F in the dark and under AM1.5G illumination at an incident light intensity of 100 mW cm⁻².

4.

Summary

In the present study, we have investigated how the p-type doping of electron donors with low solid-state ionization potentials, m-MTDATA and 2-TNATA, with a strong electron acceptor F₄TCNQ, affects J_SC, V_OC, FF, and PCE of OPV devices, where C₆₀ is used as an electron acceptor. The results demonstrated that the p-doping of the donor layer with F₄TCNQ causes the increase in both J_SC and FF accompanied by the decrease in V_OC, leading to higher PCE. These characteristic features are explained in terms of the decreased bulk resistance of the donor layer caused by the p-doping. The reduction of V_OC caused by the p-doping was partly compensated by the incorporation of a thin layer of undoped m-MTDATA on the ITO electrode, and hence, PCE was further enhanced. The present study shows that charge-transfer doping of organic materials to reduce their bulk resistance is an effective approach for improving the PCE of OPV devices.