Fabrication and properties of a high-performance chlorine doped graphene quantum dot based photovoltaic detector

Abstract

Functionalized graphene quantum dot (GQD) based materials play an important role in the development of high-performance, low-cost, large-area optoelectronic devices. The progress, however, is impeded by the poor understanding of the physical mechanism for GQDs in these devices. In this paper, chlorine doped GQD (Cl-GQD) based photovoltaic photodetectors have been fabricated using a solution process, and it was found that the presence of Cl-GQDs can significantly enhance the performance of the device. The improved performance of Cl-GQD based devices has been investigated by systematically studying the structural, morphological, optical, electrical, electrochemical and photoelectrical properties. The important photovoltaic detectors parameters such as the saturation current densities (J₀), barrier heights (Φ_b), built-in potentials (V_bi), carrier concentrations (N) and depletion layer widths (W_d) have been calculated and discussed by studying the I–V and C–V characteristics under different illuminations. The frequency dependent capacitance and conductance have also been discussed. The results provide guidance for developing high-performance graphene based optoelectronic devices.

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1 Introduction

Graphene quantum dots (GQDs) have been regarded as a kind of new multifunctional material due to their unique optical, electrical and photoelectric properties. GQDs show obvious advantages over graphene as far as the modulation of energy-level related properties is concerned. Recently GQDs have been investigated in photovoltaic devices.^1,2 It has been widely reported that graphene plays the carrier transport layer role in the devices because of the outstanding conductivity and shows ultrafast photoresponses and broadband absorption.^3–6 For example, Zhang et al. fabricated a graphene–polymer nanocomposite membrane, they firstly used it to fabricate a mode lock fiber laser. A 700 fs pulse width at a wavelength of 1590 nm has been directly generated from the laser.⁷ Zheng et al. reported the saturable absorption of graphene at the microwave frequency band.⁸ Compared with graphene, GQDs have more attractive features such as photoluminescence and non-zero bandgaps,^9–11 which are tunable by controlling the size and surface modification. Their facile solution processing, quantum confinement effect, bandgap tunability, and multiexciton generation make them particularly attractive as the charge generation layer in photodetectors.^12–14

GQD preparation methods include chemical synthesis,^10,15 graphene oxide (GO) reduction,^9,16 carbon nanotube (CNT) disintegration transformations,¹⁷ etc. The properties of GQDs have been modified by doping with different elements. Li et al. prepared sulphur doped GQDs (S-GQDs) and Cl-GQDs through a facile hydrothermal method, and the electronic structures of the GQDs have been tuned by introducing S-related (or Cl-related) energy levels between the π and π* energy levels of C, leading to efficient and multiple emissions.^18,19 Qu et al., using urea, prepared nitrogen doped GQDs (N-GQDs), resulting in a great improvement in the photoluminescence (PL) quantum yield (QY) of GQDs.²⁰ Vertical heterostructures made with multilayer GQDs sandwiched between graphene sheets by Kim et al., achieved high detectivity (>10¹¹ cm Hz^1/2 W⁻¹) and high responsivity (0.2–0.5 A W⁻¹) in a broad spectral range from ultraviolet (UV) to near infrared (NIR).¹² Tang et al. reported that layered structure N-GQDs possess broadband emission ranging from 300 to >1000 nm, and the responsivity of the photodetector is as high as 325 V W⁻¹ under 405 nm laser irradiation.¹⁴

Compared with graphene photodetectors, the response wavelength of a GQD based photodetector is adjustable due to its size dependent energy bandgap. Nowadays, many researchers focus their attention on the preparation of GQD based heterojunction photovoltaic detectors. However, the charge transport properties of GQD-based photoelectric devices have been rarely reported. The carrier transport mechanism and photoconductive behaviour of organic heterojunctions are different from that of conventional inorganic devices,^21,22 under illumination the excitons are generated in the photoactive layers (e.g. P3HT (poly(3-hexylthiophene))/Cl-GQDs) and dissociated into electrons and holes at the donor/acceptor interfaces. The interface states play an important role in the determination of barrier height and other electronic parameters, affecting the photovoltaic device performance.²³ Therefore, it is important to make clear the charge transport properties of GQD based photoelectric devices.

Both quantum confinement (size, edge effect etc.) and doping may affect the bandgap of GQDs. In this work, we mainly study the influence of doped chlorine on the properties of GQDs. The bandgap of GQDs measured by cyclic voltammetry in our work is 1.36 eV, similar to the value (0.8–2.508 eV) reported in the literature.^24–26 The electronic and photoelectric properties of GQD based photovoltaic detectors have been investigated by systematically studying the current density–voltage (J–V) and the C–V characteristics of ITO/PEDOT:PSS/Cl-GQD:P3HT/Al devices under different illuminations. The important electronic and photoelectric parameters such as barrier height (Φ_b), built-in potential (V_bi), carrier concentration (N) and depletion layer width (W_d) have been calculated and discussed accordingly.

2 Experimental section

2.1 Materials

All the chemical reagents used in the experiments were used without further purification. High-purity reagents (e.g. NMP (N-Methylpyrrolidone)) (99.9%) were purchased from Tianjin FengChuan Chemical Reagent Technology Co. Ltd., PEDOT:PSS and P3HT were purchased from Sigma-Aldrich.

2.2 Preparation of Cl-GQDs

Cl-GQDs were prepared using a liquid-phase exfoliation method. Degreasing cotton was fully carbonized into carbon fibers (CFs) after heating in an air atmosphere at 1000 °C for 30 min. The CFs were ground adequately. 1.0 g of CFs was chlorinated with hydrochloric acid (HCl 100 mL, 6 mol L⁻¹), halogen and oxygen functional groups were introduced on layered structure CFs. Here, HCl plays two roles: one is to provide a Cl doping source; the other is to exfoliate the CFs into small pieces. After that, the turbid liquid was washed with distilled water until the pH reached neutral. Then, the chlorinated CFs (0.5 g) were added into 80 mL of NMP as solution A. Solution A was treated using an ultrasonic dispersion machine for 10 h. The resultant dispersion was centrifuged for 45 minutes at 4000 rpm. After centrifugation, the top clear solution was collected.

2.3 Fabrication of photovoltaic detectors

The P3HT/Cl-GQDs devices were prepared as follows: P3HT and Cl-GQDs were dissolved in chlorobenzene at a concentration of 15 mg mL⁻¹. The solutions were mixed for 4 h in an ultrasonic bath. The mixed solution was filtered with a 0.22 μm polytetrafluoroethylene (PTFE) syringe filter. ITO substrates were cleaned consecutively with acetone, ethanol and distilled water in an ultrasonic bath. After blow-drying, the substrates were cleaned with Ar plasma. PEDOT:PSS solution was first spin-coated on the ITO substrates with a speed of 2000 rpm. After being dried at 80 °C for 10 min, the active layers of the photovoltaic devices were deposited with P3HT/Cl-GQDs solutions at a spinning speed of 2000 rpm. The Al electrode was thermally deposited onto the surface of the active layers in a vacuum chamber (∼10⁻⁴ Pa). The P3HT devices (contrast devices) were prepared using a similar fabrication process but in the absence of Cl-GQDs in the active layers.

2.4 Characterization

HRTEM (high-resolution transmission electron microscopy) was performed on a JEM-2100 electron microscope operating at 200 kV. The Raman spectrum was recorded at ambient temperature on a Renishaw inVia Raman microscope with an argon-ion laser at an excitation wavelength of 514.5 nm. The FTIR spectra were measured by a Thermo Nicolet Avatar 360 spectrometer using the KBr pellet technique. Optical properties were characterized by UV-Vis, UV-Vis-NIR (U-4100) and fluorescence (Hitachi F-7000) spectrometers. Functional groups on the surface of the Cl-GQDs were verified by XPS (X-ray photoelectron spectroscopy) (PHI VersaProbe II) using 50 W AlKα radiation. The surface morphology and phase image of photovoltaic devices were determined by SEM (scanning electron microscope) (FEI Quanta 200) and AFM (atomic force microscope) (SPA-400), respectively.

3 Results and discussion

3.1 Characterization of Cl-GQDs

The crystalline structures of graphite and the Cl-GQDs²⁷ are shown in Fig. 1(a) and (b), respectively. The lattice parameter of 0.213 nm (d₁) corresponds to the (11̄0) and (1̄10) planes of graphite, and 0.246 nm (d₂) corresponds to the hexagonal lattice.²⁸ Moreover, the basal plane spacing is 0.335 nm (d₃).^14,29

Fig. 1 (a) The basal plane spacing (d₃ = 0.335 nm) of graphite. (b) A schematic diagram of a Cl-GQD. (c) The HRTEM image of a typical single Cl-GQD; the spacing between the lattice fringe is 0.213 nm. (d) The HRTEM image of the Cl-GQDs; the size is about 5 nm. (e) The HRTEM image of the Cl-GQDs with lattice fringe spacings of 0.246 nm and 0.345 nm, inset: the FFT pattern of a selected area (red square) of a Cl-GQD. (f–h) The line profile analyses (blue line) of the Cl-GQDs.

Fig. 1(c) is the high-resolution transmission electron microscopy (HRTEM) image of a typical single Cl-GQD with a lattice spacing of 0.213 nm, whose line profile analysis is shown in Fig. 1(f). Fig. 1(d) is the HRTEM image of the Cl-GQDs; the size is about 5 nm. Fig. 1(e) shows other Cl-GQDs, similarly, the lattice fringes can be clearly observed;, the size is also about 5 nm. In this image, two sets of lattice fringes are observed: 0.246 nm is the in-plane graphene lattice constant, 0.345 nm (d) corresponds to the basal plane spacing of the Cl-GQDs. Their line profile analyses are shown in Fig. 1(g) and (h), respectively. The plane spacing is slightly larger than that of bulk graphite because the introduced functional group increases the interlayer spacing of the Cl-GQDs. The fast Fourier transform (FFT) pattern of the selected area (red square) is shown in the inset of Fig. 1(e), two bright points result from the diffraction of (11̄0) and (1̄10) planes.

Fig. 2(a) shows Raman and Fourier transform infrared (FTIR) spectra of the Cl-GQDs. It can be seen from the Raman spectrum that there are three dominating vibrational peaks centered at 1340 cm⁻¹ (D peak), 1598 cm⁻¹ (G peak) and 2692 cm⁻¹ (2D peak), respectively. The FTIR spectrum shows that there are four peaks located at 603 cm⁻¹, 804 cm⁻¹, 1050 cm⁻¹ and 1089 cm⁻¹, respectively. According to the Sadtler Handbook of Infrared Spectra,³⁰ 1050 cm⁻¹ and 1089 cm⁻¹ may be the C–Cl vibrational peaks of chlorinated aromatic hydrocarbons, while 603 cm⁻¹ and 804 cm⁻¹ are the C–Cl vibrational peaks of chlorinated aliphatic hydrocarbons. This indicates the effectiveness of the Cl doping in the Cl-GQDs. As for the dopant concentration, it can be adjusted by tuning the HCl concentration within limits. The band at 1619 cm⁻¹ is attributed to the stretching vibration of C=C, which is the dominating feature of the Cl-GQDs. Another peak at 1724 cm⁻¹ may be attributed to C=O stretching. The bending vibration of C–H (1388 cm⁻¹, 881 cm⁻¹), the stretching vibration of C–H (2921 cm⁻¹) and the stretching vibration of O–H (3438 cm⁻¹)^31,32 are also observed in the FTIR spectrum.

Fig. 2 (a) The Raman and FTIR spectra of the Cl-GQDs. (b) The UV-Vis absorption and PL spectra of a Cl-GQD aqueous solution. (c) The C 1s XPS spectrum of the Cl-GQDs. (d) The Cl 2p XPS spectrum of the Cl-GQDs. (e) The UV-Vis-NIR absorption spectra of the P3HT and Cl-GQDs films. (f) The cyclic voltammetry curve of the Cl-GQDs deposited on an ITO electrode in 0.1 M Na₂SO₄ with a scan rate of 10 mV s⁻¹.

X-ray photoelectron spectroscopy (XPS) measurements were carried out to investigate the chemical bonding of the Cl-GQDs. It is worth noting that the pristine degreasing cotton contains no Cl, because the XPS measurements have been carried out in our prior works for Cl-free GQDs which were obtained from the degreasing cotton derived carbon fiber without HCl modification. Fig. 2(c) shows the high-resolution C 1s XPS spectrum of the Cl-GQDs; the measured C 1s XPS spectrum is deconvoluted into 4 peaks, which exhibits a different type of bonding to C. Fig. 2(d) is the Cl 2p XPS spectrum; it can be deconvoluted into 4 peaks, i.e., –OCCl (198.2 eV, Cl 2p_3/2), –OC₆Cl₄O (199.3 eV, Cl 2p_3/2), –C₆H₄Cl (200.5 eV, Cl 2p_3/2) and –CH₂Cl (201.9 eV, Cl 2p_1/2), respectively.^19,33,34 The difference in binding energies for Cl 2p_1/2 and Cl 2p_3/2 is due to the spin–orbit splitting of the Cl 2p core level.

In order to investigate the optical properties of the Cl-GQDs, the UV-Vis absorption and photoluminescence (PL) spectra of Cl-GQD aqueous solutions were measured, and the results are shown in Fig. 2(b). Similar to other doped GQDs,^18,35 the PL spectra indicate that the emission peak and the intensity depend on the excitation wavelength (λ_Ex). The excitation wavelength ranges from 320 nm to 500 nm with an increment of 30 nm. The strongest PL emission peak appeared at around 458 nm when the sample was excited with λ_Ex = 350 nm. The origin of the luminescence could be attributed to zigzag sites with the carbene-like triplet ground state.³⁶ For the UV-Vis absorption spectra, multiple absorption peaks located at 197, 238, and 282 nm were observed. The multiple absorptions could be explained as follows: when the Cl-GQDs are irradiated by a certain energy light, the absorbed photons may cause electron transitions between the C π and C π* energy levels. The effective doping of Cl may introduce additional energy levels, which diversifies the electron transitions.¹⁹ Three types of absorption at 197 nm (6.29 eV), 238 nm (5.20 eV) and 282 nm (4.39 eV) correspond to three electron transition pathways. This shows the potential applications in light-conversion or optoelectronic devices. The UV-Vis-NIR absorption spectra of the P3HT and Cl-GQDs films coated on quartz plates are shown in Fig. 2(e), which were utilized to determine the optical bandgap of the as-prepared materials. For the Cl-GQD solid thin film, three absorption peaks located at 299, 342, and 399 nm were observed, which is similar to that of the Cl-GQD aqueous solutions, but with a redshift of about 100 nm. This may be induced by photon reabsorption (the Cl-GQDs could be aggregated during film-forming). The redshift has also been reported before and is attributed to the increase in sp² content.^13,37 Additionally, the visible and broad NIR absorption bands are highlighted in cyan and pink, respectively, in Fig. 2(e). The NIR absorption could be attributed to the large conjugated system containing extensive delocalized π electrons in the layered Cl-GQDs.¹⁴ Furthermore, the UV-Vis-NIR absorption spectra of the P3HT films were measured; the noticeable peak located at 520 nm indicates that P3HT mainly absorbs visible light. In Fig. 2(e), the noise near to 900 nm is caused by the change of light source in the spectrometer rather than the signals of samples.

3.2 Electrochemical measurements

The energy levels of the as-prepared Cl-GQDs were evaluated by an electrochemical method.^32,38 The oxidation potential and the reduction potential correspond to the valence band edge and the conduction band edge, respectively. The electrochemical measurements were carried out using a CHI627D electrochemical analyser. The cyclic voltammetry (CV) curves were measured in a 0.1 M Na₂SO₄ solution in a standard three-electrode cell with a Ag/AgCl (3 M KCl) and platinum mesh (PM) as the reference and counter electrodes, respectively. The working electrode was prepared by drop-casting of a Cl-GQD solution onto the indium tin oxide (ITO) electrode. The CV curves were recorded from −1.5 V to +2.0 V at a scan rate of 10 mV s⁻¹. The control sample (blank ITO) was measured under the same conditions. The obtained CV curves are shown in Fig. 2(f). Compared with the control sample, we note that the −1.36 V cathode peak (the reduction of oxygen functionalities) shifts towards more negative potentials, while the −0.75 V cathode peak and the 0.61 V anode peak shift towards more positive potentials. This hysteresis effect may be due to different surface functional groups of the Cl-GQDs and ITO. These CV characteristics are similar to those previously reported for graphene oxide (GO).^39,40 Thus, from the reduction and oxidation peaks and the proposed equations,⁹ the HOMO and LUMO of the Cl-GQDs are calculated to be −5.01 eV and −3.65 eV, respectively, and the corresponding electrochemical energy gap E_g is 1.36 eV. In addition, an extra oxidation peak at 1.27 V is observed, which may be related to chlorine-containing functional groups.

Furthermore, the oxidation and reduction reactions of the Cl-GQDs were directly investigated on a Au electrode and PM counter electrodes in a N-methylpyrrolidone (NMP)-based Cl-GQD solution using ∼0.05 M Na₂SO₄ as a supporting electrolyte. The measured cyclic voltammetry curve is shown in Fig. S1 (ESI†). It can be seen that the CV curve exhibits two oxidation peaks located at 0.61 V and 1.36 V, respectively. However, the reduction peak is not obvious, probably due to the difference in solvents.

3.3 Characterization of devices

To explore the applications of Cl-GQDs in optoelectronic fields, P3HT/Cl-GQDs organic heterojunction photovoltaic detectors were fabricated. To improve the charge extraction and transport properties, an interfacial buffer layer (poly(3,4-ethylenedioxythiophene)–poly(styrene sulfonate), POEDOT:PSS) was spin-coated onto ITO. The active layers were prepared by spin-coating a solution of 15 mg mL⁻¹ P3HT in chlorobenzene with Cl-GQD contents of 0 and 10 wt%. Al was finally vacuum deposited as the top electrode. The details of the fabrication of the photovoltaic detectors are described in the Experimental section. In our organic heterojunction photovoltaic detector, the donor and acceptor are mixed and deposited as one active layer; the performance of the device strongly depends on the morphology of active layer. Atomic Force Microscopy (AFM) topographic images are widely used to measure the morphology of devices.^41,42 Herein, the use of AFM phase images to differentiate the component phase of heterogeneous composites was proposed. Fig. 3(a) and (c) are the AFM phase and topographic images of the device using pure P3HT as the active layer, respectively. The film is uniform with a root-mean-square (RMS) roughness of 1.09 nm. In the inset of Fig. 3(a), a statistical distribution of the equivalent disc radius (r_eq) is obtained, and the average particle size is about 4 nm. The RMS roughness of the film was increased to 1.24 nm when the film was introduced with 10 wt% Cl-GQDs (Fig. 3(d)). The phase image (Fig. 3(b)) exhibits relatively uniform morphology, which indicates that the Cl-GQDs dispersed well in the P3HT matrix. It should be noted that the statistical distribution of the equivalent disc radius (r_eq) (Fig. 3(b)) shows two sizes: the one with an average particle size of 4 nm is due to P3HT, the other with an average particle size of 15 nm corresponds to the Cl-GQDs. The average particle size increases because the Cl-GQDs aggregated during film-forming. Comparing the two distributions, an approximation of the content of Cl-GQDs in the blended film is estimated to be 10%. In addition, the AFM image of the Al film was also tested, and the obtained RMS roughness is 2.84 nm as shown in Fig. S2 (ESI†).

Fig. 3 (a) AFM phase image of the P3HT film, inset: size distribution of particles in the P3HT film. (b) AFM phase image of the P3HT/Cl-GQDs film, inset: size distribution of particles in the P3HT/Cl-GQDs film. (c and d) The AFM topographic images of the P3HT and P3HT/Cl-GQDs films with an RMS roughness of 1.09 nm and 1.24 nm, respectively. (e) SEM image of the P3HT/Cl-GQDs film. (f) The enlarged SEM image of a section of (e).

To further investigate the distribution of Cl-GQDs in the active layer, scanning electron microscopy (SEM) characterization was conducted, and the obtained image is shown in Fig. 3(e). It can be seen from Fig. 3(e) that the Cl-GQDs are uniformly distributed in the P3HT polymer matrix. Fig. 3(f) is the enlarged image of a section of Fig. 3(e). It can be clearly seen that the size of the Cl-GQDs is about 10–20 nm; the Cl-GQD content is estimated to be about 10%. The SEM results accord well with those of AFM.

3.4 Current–voltage measurements

Schematic diagrams of the Cl-GQD-based photovoltaic detector and its photocurrent generation mechanism are shown in Fig. 4(a) and (b), respectively. The photo-generated carriers are produced in the device under illumination through the process [hν → e⁻ + h˙], and under the built-in potential, photo-generated electron and hole pairs separate and move towards the Al and ITO electrodes. Fig. 5(a) and (b) show the log J vis-à-vis V curves of the pure P3HT and P3HT/Cl-GQDs devices, respectively (light source: fluorescent lamp, 1.7 mW cm⁻², the same below). The as-fabricated devices showed good rectification behavior, suggesting that the charge transport may be described by the thermionic emission theory. The total current density for typical current–voltage characteristics is given by:^43,44where V is the applied voltage, T is the absolute temperature, q is unit charge, k_B is the Boltzmann constant and η is the ideality factor related to the slope. J₀ is the saturation current density obtained by extrapolating the current density from the log-linear plot to V = 0 and is given by:andwhere A* is the effective Richardson constant, Φ_b is the barrier height, h is the Planck constant, and m* is the effective electron mass. For P3HT, m*(1) equals 1.48.⁴⁵

Fig. 4 (a) A schematic diagram of the Cl-GQD-based photovoltaic detector. (b) A schematic illustration for photo-generated carriers. The photo-generated electron (solid circle) and hole (open circle) pairs are separated by the built-in field.

Fig. 5 (a and b) Logarithmic plots of J vs. V for the P3HT and P3HT/Cl-GQDs photovoltaic detectors, respectively, under dark and illumination conditions. Variation of the capacitance with the applied voltage and plots of 1/C² vs. V for the P3HT and P3HT/Cl-GQDs devices under illumination at various frequencies: (c) 200 Hz, (d) 1 kHz, (e) 5 kHz, respectively. (f) A schematic illustration of the working mechanism of the Cl-GQD-based photovoltaic detector.

For P3HT/Cl-GQDs, the effective electron mass is achieved according to the equation:

m*(2) = m*(P3HT)ω(P3HT) + m*(Cl-GQDs)ω(Cl-GQDs) (4)

where ω(P3HT) and ω(Cl-GQDs) are mass fractions for P3HT and the Cl-GQDs, respectively, ω(Cl-GQDs) = 10% (known from preparation). m*(Cl-GQDs) ≈ 0.05 (referred from graphite⁴⁶ and carbon nanotubes⁴⁷). m*(2) has been calculated to be 1.34.

When electron transport across the barrier is dominated by thermionic emission as described by eqn (1), the logarithmic plots of the J–V curves display a linear region under forward bias.⁴⁴ As seen in Fig. 5(a) and (b), the overlying dashed straight-line segments of our fitting allow us to extract J₀ and η for each device. The values of Φ_b under different conditions can be calculated according to eqn (2), they are listed in Table 1. Fig. 5(f) gives the working mechanism of the Cl-GQD-based photovoltaic detector.

Table 1 The ideality factors, saturation current densities and barrier heights of the P3HT and P3HT/Cl-GQDs bulk heterojunctions

Device structure	Working conditions	η	J0 (mA cm^−2)	Φb (eV)
P3HT	Dark	1.14	2.12	0.54
	Illumination	1.08	4.97	0.38
P3HT/Cl-GQDs	Dark	1.76	1.34	0.41
	Illumination	1.24	13.48	0.35

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3.5 Capacitance–voltage and capacitance–frequency measurements

Generally, C–V measurements can provide important information on the nature of the semiconductor interface and charge transport. Fig. 5(c)–(e) represent the C–V plots and the variation of 1/C² with applied voltage for two devices at room temperature under illumination at various frequencies (200 Hz, 1 kHz and 5 kHz, respectively). It can be clearly seen that the capacitance of the P3HT/Cl-GQDs device increases more evidently than that of the P3HT device under the same conditions. This is because the Cl-GQDs themselves also behave as a light absorber, which has been proved by the UV-Vis-NIR absorption characterization. The introduction of Cl-GQDs in the P3HT matrix may induce more charge. However, all capacitances decrease with increasing frequency. This can be explained as follows: at lower frequencies the interface states can follow the alternating current signal. At high frequencies the interface states remain constant because they cannot follow the alternating signal.⁴⁸ In addition, in order to explore the frequency (f) dependent capacitance and conductance (G), the C–f and G–f characteristics for the P3HT and P3HT/Cl-GQDs devices were measured at a series of voltages ranging from −0.5 to 0.5 V at room temperature, and the results are shown in Fig. S4 (ESI†).

The C–V relationship under a bias can be expressed as:⁴⁹

where V_bi is the built-in potential at zero bias, ε₀ is the permittivity of a vacuum, ε_r is the relative permittivity of the material, N is the carrier concentration in the depletion layer and A is the photosensitive area (20 mm²). The x-intercept is V_bi, and the carrier concentration N can be calculated from the slope of the linear section of the 1/C² versus V plot:

For P3HT, ε_r(1) = 3.0.⁵⁰ For the Cl-GQDs, ε_r(Cl-GQDs) ≈ 1.88.⁵¹ Similar to the handling of the effective electron mass, the P3HT/Cl-GQDs composite dielectric constant has been calculated as follows,

ε_r(2) = ε_r(P3HT)ω(P3HT) + ε_r(Cl-GQDs)ω(Cl-GQDs) (7)

The calculated ε_r(2) is 2.89.

The 1/C²–V characteristics of the two devices under illumination at 200 Hz, 1 kHz, and 5 kHz are shown in Fig. 5(c)–(e), respectively. The calculated built-in potentials and carrier concentrations are listed in Table 2.

Table 2 The built-in potential, carrier concentration and depletion layer width of the P3HT and P3HT/Cl-GQDs bulk heterojunctions at various frequencies

Device structure	Vbi (V)			N (10^18 cm^−3)			Wd (nm) (V = 0)
	200 Hz	1 kHz	5 kHz	200 Hz	1 kHz	5 kHz	200 Hz	1 kHz	5 kHz
P3HT	0.78	0.73	0.70	1.50	1.34	1.15	13.13	13.44	14.21
P3HT/Cl-GQDs	0.64	0.60	0.86	2.01	1.75	1.44	10.09	10.47	13.81

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The depletion layer width (W_d) is expressed as:⁴⁹

The depletion layer widths at zero bias voltage were also calculated and are listed in Table 2. As the frequency increases, the built-in potential and charge carrier concentration decrease while the depletion layer width increases. Under the same conditions, the charge carrier concentration of the P3HT/Cl-GQDs device is 30% higher than that of the P3HT device; as far as the depletion layer width is concerned, however, the W_d of the P3HT/Cl-GQDs device is smaller than that of the P3HT device.

4 Conclusions

Cl-GQD based photovoltaic photodetectors were fabricated, and the presence of Cl-GQDs in the photoactive layer may significantly enhance the photoresponse of the device. The device was stable, the J–V curve remained unchanged after several measurements, and the device remained undamaged under a light power density of up to 165 mW cm⁻². The distribution and morphology of the Cl-GQDs in a P3HT matrix have been investigated by AFM and SEM. The physical mechanism for the function of the Cl-GQDs has been disclosed. The improved performance of the Cl-GQDs has been uncovered after systematically studying the optical, electrical, electrochemical, photoelectrical properties of the devices. The important device parameters such as the saturation current densities, barrier heights, built-in potentials, carrier concentrations and depletion layer widths have been discussed after studying the I–V, C–V and C–f characteristics in detail. The obtained results showed that the carrier concentrations increased by 30% in the presence of Cl-GQDs (∼10% weight ratio), and the depletion layer width decreased compared to Cl-GQD free devices. The present study may help us to design and fabricate high-performance graphene based optoelectronic devices.

Acknowledgements

This work was supported by National Natural Science Foundation of China (no. 61106098, 51201150), and the Key Project of Applied Basic Research of Yunnan Province, China (no. 2012FA003).

Notes and references

1. F. J. Lim, K. Ananthanarayanan, J. Luther and G. W. Ho, J. Mater. Chem., 2012, 22, 25057–25064.
2. J. H. Lee, J. H. Shin, J. Y. Song, W. F. Wang, R. Schlaf, K. J. Kim and Y. Yi, J. Phys. Chem. C, 2012, 116, 26342–26348.
3. F. Xia, T. Mueller, Y. M. Lin, A. Valdes-Garcia and P. Avouris, Nat. Nanotechnol., 2009, 4, 839–843.
4. V. Patil, A. Capone, S. Strauf and E. H. Yang, Sci. Rep., 2013, 3, 2791.
5. B. Y. Zhang, T. Liu, B. Meng, X. Li, G. Liang, X. Hu and Q. J. Wang, Nat. Commun., 2013, 4, 1811.
6. C.-H. Liu, Y.-C. Chang, T. B. Norris and Z. Zhong, Nat. Nanotechnol., 2014, 9, 273–278.
7. H. Zhang, Q. Bao, D. Tang, L. Zhao and K. Loh, Appl. Phys. Lett., 2009, 95, 141103.
8. Z. W. Zheng, C. J. Zhao, S. B. Lu, Y. Chen, Y. Li, H. Zhang and S. C. Wen, Opt. Express, 2012, 20, 23201–23214.
9. V. Gupta, N. Chaudhary, R. Srivastava, G. D. Sharma, R. Bhardwaj and S. Chand, J. Am. Chem. Soc., 2011, 133, 9960–9963.
10. L. B. Tang, R. B. Ji, X. M. Li, K. S. Teng and S. P. Lau, Part. Part. Syst. Charact., 2013, 30, 523–531.
11. R. Sekiya, Y. Uemura, H. Murakami and T. Haino, Angew. Chem., 2014, 53, 5619–5623.
12. C. O. Kim, S. W. Hwang, S. Kim, D. H. Shin, S. S. Kang, J. M. Kim, C. W. Jang, J. H. Kim, K. W. Lee, S. H. Choi and E. Hwang, Sci. Rep., 2014, 4, 5603.
13. S. K. Lai, L. Tang, Y. Y. Hui, C. M. Luk and S. P. Lau, J. Mater. Chem. C, 2014, 2, 6971–6977.
14. L. B. Tang, R. B. Ji, X. M. Li, G. X. Bai, C. P. Liu, J. H. Hao, J. Y. Lin, H. X. Jiang, K. S. Teng, Z. B. Yang and S. P. Lau, ACS Nano, 2014, 8, 6312–6320.
15. M. L. Mueller, X. Yan, J. A. McGuire and L.-S. Li, Nano Lett., 2010, 10, 2679–2682.
16. D. Pan, J. Zhang, Z. Li and M. Wu, Adv. Mater., 2010, 22, 734–738.
17. D. B. Shinde and V. K. Pillai, Chem.–Eur. J., 2012, 18, 12522–12528.
18. X. M. Li, S. P. Lau, L. B. Tang, R. B. Ji and P. Z. Yang, Nanoscale, 2014, 6, 5323–5328.
19. X. Li, S. P. Lau, L. Tang, R. Ji and P. Yang, J. Mater. Chem. C, 2013, 1, 7308.
20. D. Qu, M. Zheng, L. Zhang, H. Zhao, Z. Xie, X. Jing, R. E. Haddad, H. Fan and Z. Sun, Sci. Rep., 2014, 4, 5294.
21. Y.-F. Lao, A. G. U. Perera, L. H. Li, S. P. Khanna, E. H. Linfield and H. C. Liu, Nat. Photonics, 2014, 8, 412–418.
22. S. Hussain, C. Cao, W. S. Khan, G. Nabi, Z. Usman, A. Majid, T. Alharbi, Z. Ali, F. K. Butt, M. Tahir, M. Tanveer and F. Idress, Mater. Sci. Semicond. Process., 2014, 25, 181–185.
23. S. Zeyrek, E. Acaroğlu, Ş. Altındal, S. Birdoğan and M. M. Bülbül, Curr. Appl. Phys., 2013, 13, 1225–1230.
24. J. Lu, P. S. Yeo, C. K. Gan, P. Wu and K. P. Loh, Nat. Nanotechnol., 2011, 6, 247–252.
25. S. H. Jin, D. H. Kim, G. H. Jun, S. H. Hong and S. Jeon, ACS Nano, 2012, 7, 1239–1245.
26. G. Eda, Y. Y. Lin, C. Mattevi, H. Yamaguchi, H. A. Chen, I. S. Chen, C. W. Chen and M. Chhowalla, Adv. Mater., 2010, 22, 505–509.
27. J. Zhao, L. Tang, J. Xiang, R. Ji, J. Yuan, J. Zhao, R. Yu, Y. Tai and L. Song, Appl. Phys. Lett., 2014, 105, 111116.
28. L. B. Biedermann, M. L. Bolen, M. A. Capano, D. Zemlyanov and R. G. Reifenberger, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 79, 125411.
29. J. Slonczewski and P. Weiss, Phys. Rev., 1958, 109, 272.
30. W. H. S. Sadtler, in The Sadtler Handbook of Infrared Spectra, Bio-Rad Laboratories, Inc., Informatics Division, 1978.
31. W. Zhang, D. Dai, X. Chen, X. Guo and J. Fan, Appl. Phys. Lett., 2014, 104, 091902.
32. D. Yu, Y. Yang, M. Durstock, J.-B. Baek and L. Dai, ACS Nano, 2010, 4, 5633–5640.
33. J. Y. Kim, W. H. Lee, J. W. Suk, J. R. Potts, H. Chou, I. N. Kholmanov, R. D. Piner, J. Lee, D. Akinwande and R. S. Ruoff, Adv. Mater., 2013, 25, 2308–2313.
34. The NIST XPS Online, http://srdata.nist.gov/xps/EngElmSrchQuery.aspx?EType=PE%26CSOpt=Retri_ex_dat%26Elm=Cl, accessed 15 September 2012.
35. L. B. Tang, R. B. Ji, X. M. Li, K. S. Teng and S. P. Lau, J. Mater. Chem. C, 2013, 1, 4908–4915.
36. L. R. Radovic and B. Bockrath, J. Am. Chem. Soc., 2005, 127, 5917–5927.
37. Q. Shou, J. Cheng, L. Zhang, B. J. Nelson and X. Zhang, J. Solid State Chem., 2012, 185, 191–197.
38. J. Tang, K. W. Kemp, S. Hoogland, K. S. Jeong, H. Liu, L. Levina, M. Furukawa, X. Wang, R. Debnath, D. Cha, K. W. Chou, A. Fischer, A. Amassian, J. B. Asbury and E. H. Sargent, Nat. Mater., 2011, 10, 765–771.
39. Y. Shao, J. Wang, M. Engelhard, C. Wang and Y. Lin, J. Mater. Chem., 2010, 20, 743.
40. A. Y. S. Eng and M. Pumera, Electrochem. Commun., 2014, 43, 87–90.
41. A. Tamanai, S. Beck and A. Pucci, Displays, 2013, 34, 399–405.
42. L. Scudiero, H. Y. Wei and H. Eilers, ACS Appl. Mater. Interfaces, 2009, 1, 2721–2728.
43. S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, John Wiley & Sons, Inc., Hoboken, New Jersey, Canada, Third edn, 2007.
44. S. Tongay, M. Lemaitre, X. Miao, B. Gila, B. R. Appleton and A. F. Hebard, Phys. Rev. X, 2012, 2, 011002.
45. X.-H. Xie, W. Shen, R.-X. He and M. Li, Bull. Korean Chem. Soc., 2013, 34, 2995–3004.
46. P. Wallace, Phys. Rev., 1947, 71, 622–634.
47. D. Shah, N. A. Bruque, K. Alam, R. K. Lake and R. R. Pandey, J. Comput. Electron., 2007, 6, 395–400.
48. V. Rajagopal Reddy, V. Janardhanam, J.-W. Ju, H.-J. Yun and C.-J. Choi, Solid State Commun., 2014, 179, 34–38.
49. M. Ramar, C. K. Suman, R. Manimozhi, R. Ahamad and R. Srivastava, RSC Adv., 2014, 4, 32651–32657.
50. G. Garcia-Belmonte, A. Munar, E. M. Barea, J. Bisquert, I. Ugarte and R. Pacios, Org. Electron., 2008, 9, 847–851.
51. K. Uchida and A. Oshiyama, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 79, 235444.

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Footnote

† Electronic supplementary information (ESI) available: Electrochemistry measurements for CV of Cl-GQDs, AFM images, C–V curves under high frequencies, C–f and G–f characteristics. See DOI: 10.1039/c5ra02358k

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