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Japanese Journal of Applied Physics 55, 056601 (2016) http://doi.org/10.7567/JJAP.55.056601

Interface electric properties of Si/organic hybrid solar cells using impedance spectroscopy analysis Dan Wang1, Juye Zhu2, Li Ding2, Pingqi Gao2, Xiaoyin Pan1, Jiang Sheng2*, and Jichun Ye2* 1

Department of Physics, Ningbo University, Ningbo 315211, People’s Republic of China Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, People’s Republic of China

2

*E-mail: [email protected]; [email protected] Received December 14, 2015; accepted February 4, 2016; published online March 31, 2016 The internal resistance and capacitance of Si/organic hybrid solar cells (Si-HSC) based on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) are investigated by electrochemical impedance spectroscopy (EIS). Three types of Nyquist plots in Si-HSC are observed firstly at different bias voltages, while suitable equivalent circuit models are established to evaluate the details of interface carrier transfer and recombination. In particular, the carrier transport property of the PEDOT:PSS film responds at a high frequency (6 ' 104–1 ' 106 Hz) in three-arc spectra. Therefore, EIS could help us deeply understand the electronic properties of Si-HSC for developing high performance devices. © 2016 The Japan Society of Applied Physics

1.

Introduction

Considerable attention has been focused on the development of Si=organic hybrid solar cells (Si-HSC) based on the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) polymer, because of the low-cost fabrication and high power conversion efficiency (17.4%) of such device.1) Generally, the fundamental components of Si-HSC consist of an n-type crystalline silicon wafer, a p-type highly conductive polymer (PEDOT:PSS), and the metal electrodes. During the light illumination, a photon excites an electron from the valence band into the conduction band of silicon, creating photo-generated electron–hole pairs. Then, the electron–hole pairs are separated by the built-in electric field into the electrons and holes; the electrons travel through the silicon and the holes transit through the PEDOT:PSS film to the external circuit. In this basic process of charge collection, the doping concentration of silicon, PEDOT:PSS film quality, their interfaces of Si=PEDOT:PSS, and electrodes are recognized to determine the charge collection and power conversion efficiencies.2,3) Therefore, a deeper comprehension of hybrid solar cells is warranted for high photovoltaic performance, based on the charge collection processes, energy band structure, and stability. In a single heterojunction (or homojunction), the capacitance–voltage (C–V ) measurement is performed to analyze the surface electric information at a fixed frequency, which determines the doping depth profiles of semiconductor materials, the built-in voltage, and the oxide properties of a metal–oxide–semiconductor structure.4–6) According to the Mott–Schottky model, the equivalent circuit can describe the behavior of a capacitance and a resistor in parallel, which represents the electric information in the space charge region. Unfortunately, other parameters such as contact and bulk resistances in solar cells (or the parameters of a more complicated system) cannot be obtained from this equivalent circuit. Furthermore, the current density–voltage (J–V ) measurement is also commonly performed to provide physical insight into the detailed electric parameters of solar cells, based on the diffusion and recombination currents in the dark or under light illumination.7,8) This is a powerful method of evaluating the charge loss, resistance, and junction quality of

solar cells. However, it cannot extract the electric information regarding each interface and each active layer of solar cells. Fortunately, for a complicated electrochemical system, electrochemical impedance spectroscopy (EIS) possibly enables the analysis of the detailed information of each part based on a suitable frequency region. EIS is a steady-state method that considers the current response to the application of an AC voltage as a function of frequency; it is used to investigate internal electrical processes, and analyze the associated parameters quantitatively and qualitatively in light-emitting diodes,9) Li-ion batteries,10) dye-sensitized solar cells,11) and perovskite solar cells.12) EIS has been introduced to analyze the internal resistance and capacitance of Si-HSC;13) however, it is not clear how to establish the equivalent circuit model to evaluate the electrical quality of each part in the Si-HSC. In this paper, we propose an appropriate equivalent circuit of Si-HSC to evaluate the interface qualities including internal resistance and capacitance by EIS. Furthermore, the dependence of each internal resistance (capacitance) element on the various applied voltages is investigated to examine the carrier transfer and recombination processes. 2.

Experimental methods

The Si-HSC device is fabricated using a previously described procedure.2) The n-type float-zone (FZ) crystalline silicon wafers [1–5 Ω·cm, (100), one-side-polished] are cleaned by the RCA cleaning process, with H-terminated surfaces. After exposing to ambient atmosphere for 60 min, the highly conductive PEDOT:PSS (Clevios PH1000) solution mixed with dimethyl sulfoxide (DMSO, 5%) and zonyl fluorosurfactant (0.1%) is spin-coated on the Si substrate at a spin speed of 1200 rpm, forming the p-type film after thermal treatment at 140 °C for 10 min. The front Ag pattern and Al rear electrode are both prepared by thermal evaporation for a layer thickness of 150 nm. The J–V characteristics of the Si-HSC devices are analyzed using a digital source meter (Keithley 2400) under AM 1.5 simulated solar illumination at 100 mW=cm2. The C–V curve of Si-HSC is measured using a Keithley 4200-SCS parameter analyzer at a signal frequency of 1 KHz. EIS is measured using an impedance analyzer (Solartron Analytical 1260A), connected to a potentiostat

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Fig. 1. (Color online) Full equivalent circuit model of Si-HSC device (Rseries: series resistance; RSi=Al=CSi=Al: charge transfer resistance and capacitance at Si=Al; RSi: transport resistance in the wafer; Rpn=Cpn: charge transfer resistance and capacitance at Si=PEDOT:PSS; Rtrans: transport resistance in the PEDOT:PSS nanoparticles; RPE=CPE: charge transfer resistance and capacitance between PEDOT:PSS particles; RPE=Ag=CPE=Ag: charge transfer resistance and capacitance at PEDOT:PSS=Ag; Cg: capacitance from the dielectric contribution of the heterojunction).

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Results and discussion

Resistance generally describes electron transfer or transport events, whereas capacitances are related to electronic carrier accumulation and distribution at junctions and defect traps in the device. The full equivalent circuit model of Si-HSC is illustrated in Fig. 1. The resistance and capacitance at each point from EIS can be extracted to provide detailed information about the electronic process, using a suitable RC element. In the Si-HSC structure, the PEDOT:PSS is covered on the silicon surface to form the heterojunction, leaving some micro-voids at the interface to be a mass of recombination centers.14) At the rear electrode, the Schottky barrier of the Al=Si interface retards the charge collection based on the low doping concentration of silicon, owing to the fact that the Al work function is smaller than that of the silicon wafer.15) According to the Schottky model, the barrier height (ϕb) can be predicted from the work function (ϕm, 4.31 eV16)) of the Al film and the electron affinity ( χ, 4.05 eV) of silicon expressed as b ¼ m ;

ð1Þ

thus, the ϕb value is 0.26 eV. At the front electrode, an ohmic contact of the PEDOT:PSS=Ag interface allows the carriers to transfer unimpeded. In total, there are two major interfaces that affect the charge transfer and accumulation at Al=Si and Si=PEDOT:PSS. Furthermore, the PEDOT:PSS film is composed of PEDOT:PSS nanoparticles, in which the PEDOT molecules coil together to form the core, shell-coated with a PSS insulating layer.17,18) This barrier can retard the charge hopping such that the charge carriers accumulate around the necks between nanoparticles. Firstly, the photovoltaic performance of the Si-HSC device is characterized by the J–V measurement under AM 1.5 simulated solar illumination, as shown in Fig. 2(a). The Si-HSC device with a photocurrent density (Jsc) of 28.16

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Fig. 2. (Color online) (a) J–V characteristics of Si-HSC under AM 1.5 simulated solar illumination at 100 mW=cm2; the inset shows the J–V curve of Si-HSC in the dark. (b) C −2–V plot of Si-HSC; experimental data are represented by dots and the fit linear data are represented by a line.

mA=cm2, an open-circuit voltage (Voc) of 0.50 V, and a fill factor (FF ) of 71.37% yields a power conversion efficiency of 10.07%. The series resistance and shunt resistance of the device are 10.19 and 19782.77 Ω·cm2, respectively. In addition, the inset of Fig. 2(a) shows the current–voltage characteristics in the dark. According to the Shockley diode equation eV J ¼ Js exp 1 ; ð2Þ nT where T is the absolute temperature (298 K), κ is the Boltzmann constant, n is the diode ideality factor, and Js is the reverse saturation current density. The n of Si-HSC is obtained from the slope of the linear region in the forward region (0.3–0.6 V) and found to be 2.63. Js is also extracted from the dark J–V curve to be 3.32 × 10−7 A·cm−2. Js can be expressed as bi 2 Js ¼ A AT exp ; ð3Þ T where A is the contact area, A+ is the effective Richardson constant (about 252 A cm−2 K−2 for n-type silicon), and Φbi is the barrier height of Schottky diodes. From the obtained Js value, the Schottky barrier height of Si-HSC is calculated to be 0.78 eV. From the relationship19)

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ð4Þ


Jpn. J. Appl. Phys. 55, 056601 (2016)

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Fig. 3. (Color online) Experimental impedance spectra (Nyquist plots) of Si-HSC at the different bias voltages of (a) 0.2, (b) 0.6, and (c) 1.0 V in the dark; (d) violet circle of Nyquist plot in (c); in (a) to (c), the simplified equivalent circuit model for the spectra fitting is represented, experimental data are represented by dots, and fitting data according to the relevant models are represented by lines, respectively.

where Vbi is the built-in voltage, Vapp is the applied voltage, εr is the relative dielectric constant, ε0 is the vacuum permittivity, and NA is the acceptor impurity concentration, Vbi can be extracted to be 0.57 V from the fitting line of the C −2–V curve in Fig. 2(b). Furthermore, the diode resistance R is then described as20) 1 eV / exp ; ð5Þ R nT where R is inversely proportional to the exponential function of V in the dark. During EIS measurement, a harmonically modulated small AC amplitude voltage (UAC: ΔUeiwt ) is applied to allow the flow of a small AC current (IAC: ΔIei(wt−φ)). The phase shift φ is observed between the current and the applied voltage, and the impedance is received from the EIS signal21) U i’ e : Zð!Þ ¼ ð6Þ I The physical meaning of the network in Fig. 1 corresponds to the impedance of charge diffusion and drift in the heterojunction solar cell. The equivalent circuit displays the internal distribution of electrochemical potential in response to the modulated small perturbation of the external electrical potential (UAC) at a steady state.22) The simplest equivalent circuit for a solar cell as an ideal diode consist of a parallel resistor and a capacitor in series with another resistor, as shown in Fig. 3(a). As the diffusion-reaction model, the impedance results are expressed as

Zð!Þ ¼ Z 0 jðwÞZ 00 " 1=2 # Rt Rr !k 1=2 i! 1=2 ¼ coth 1þ ; ð7Þ !k 1 þ i!=!k !d where ZA and ZAA are the real and imaginary parts of the impedance, respectively. The relationship between ZA and ZAA is displayed in the Nyquist plot; Rt is the charge transport resistance, Rr is the charge transfer resistance related to the recombination of minority carriers at the interface, ωd = 1=RtCt is the characteristic frequency of diffusion in a finite layer, ωk = 1=RrCr is the rate constant for recombination, ω is the angular coefficient. It is worth noting that ωd and ωk are related, with the inverse values of the commonly used as parameters of transport time and lifetime.22) Figure 3 shows typical examples of the three characteristic patterns of impedance spectra (Nyquist plots) that are found in the Si-HSC at the different bias voltages and the relevant equivalent circuits that are required to fit them. Semicircles are observed in the measured frequency range of 1–106 Hz. In all cases, the experimental data are plotted as dots, while the fitting values of the models are plotted as lines. At a low voltage (0.2 V), the EIS spectrum only presents a semicircle, as plotted in Fig. 3(a), indicating only a RC element at the interface of the Si=PEDOT:PSS heterojunction. The Si-HSC device embodies a simple diode model, with a resistance element RPN and a capacitance element CPN. These elements are estimated by fitting the experimental data based on the equivalent circuit model in the inset to be

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1.13 × 106 Ω and 3.36 × 10−10 F, respectively. In this solidstate device, the capacitance is attributed to the spatial charge distribution and diffusion capacitance in the depletion region of the Si=PEDOT:PSS interface. However, this capacitance is governed by the junction capacitance Cg at a low applied voltage (Vapp < Vbi). The charges originating from negatively charged defects is distributed along the depletion region, the width of which varies with the applied voltage. Furthermore, the minority carrier lifetime (τn) can be obtained from the relationship n ¼ RPN CPN ;

ð8Þ

indicating 379.68 µs at the Si=PEDOT:PSS interface at a low bias voltage. The series resistance element is found in the high frequency range (>106 Hz) to be 8488 Ω, possibly including the series resistance of the external circuit, the resistance of the Al=Si interface, and the charge diffusion resistance in the silicon layer. Rseries is normally determined by the resistance of the external circuit, but in the EIS measured data, its value is clearly affected by unknown RC elements; this does not enable us to precisely explain the series resistance from the EIS spectra at a low applied voltage. The total resistance of a solar cell from the EIS measurements (Rtotal = RPN + Rseries) is 1.21 × 106 Ω, much larger than the series resistance from light illumination, because minority carriers drift through the Si=PEDOT:PSS heterojunction with a small resistance, driven by the built-in electric field under light illumination. However, during EIS measurement, majority carriers should overcome the barrier of the built-in electric field to be able to cross the Si=PEDOT:PSS heterojunction with large resistance. At a moderate bias voltage (0.6 V, around Voc), Fig. 3(b) shows a Nyquist plot of Si-HSC with two semicircles. These semicircles in the frequency ranges of 5 × 103–1 × 106 and 1–5 × 103 Hz are attributed to the RC elements related to the charge transfer=recombination processes at the Si=PEDOT:PSS and Al=Si interfaces, respectively. These RC elements are in tandem and analyzed using the equivalent circuit model in the inset. The resistances RPN and RAl=Si are estimated to be 6.99 × 104 and 4.78 × 104 Ω, respectively. Additionally, the capacitances CPN and CAl=Si are 3.63 × 10−10 and 1.05 × 10−8 F, respectively. When the bias voltage is in excess of the built-in voltage (Vapp > Vbi), the depletion zone of the Si=PEDOT:PSS heterojunction may collapse such that CPN is governed by the diffusion capacitance Cpn from excess carriers, displaying the minority carrier recombination process. However, the total applied voltage is used by the two RC elements at the Si=PEDOT:PSS and Al=Si interfaces. As a result, the bias voltage cannot ensure enough energy to overcome the barrier of the Si=PEDOT:PSS heterojunction. The CAl=Si of the Al=Si interface is still dominated by the junction capacitance. The minority carrier lifetimes of the two interfaces are deduced to be 25.37 and 501.90 µs for the Si=PEDOT:PSS and Al=Si interfaces, respectively. Expectedly, the Rseries decreases to some extent at 4208 Ω, owing to the absence of the Al=Si junction, but still is affected by the magnitudes of major RC elements. At a large bias voltage (1.0 V), the three-arc impedance spectrum of Si-HSC is shown in Fig. 3(c), and Fig. 3(d) shows the low range (violet circle) of the ZA–ZAA plot from

Fig. 3(c). The three arcs in the frequency ranges 6 × 104–1 × 106, 1 × 104–6 × 104, and 1–1 × 104 Hz are attributed to the RC elements related to the charge transfer processes in the PEDOT:PSS film, at the Si=PEDOT:PSS interface, and at the Al=Si interface, respectively. Owing to the fact that the magnitude of the RC element at the Si=PEDOT:PSS interface is much larger than those at the other interfaces, the data of the ZA–ZAA plot suddenly change around the measured frequency affected by the changes in RC elements. The RC element in the high frequency range describes the charge transit in the PEDOT:PSS film, with a RPE of 610 Ω and a CPE of 1.03 × 10−8 F. The transit time (τd ) of carriers across the PEDOT:PSS film is obtained from d ¼ RPE CPE :

ð9Þ

Thus, the τd of the PEDOT:PSS film is 6.28 µs. The minority carrier diffusion coefficient (D) can be calculated from the PEDOT:PSS thickness using the relation23) D ¼ L2 =d :

ð10Þ

On the basis of the ca. 100 nm PEDOT:PSS film, the value of D is 1.59 × 10−8 m2=s. If the thickness of the PEDOT:PSS film increases, the value of τd will also increase. In addition, τd directly depends on the PEDOT:PSS nanoparticle connection and PSS thickness, which retard the charge transport. Nowadays, many studies are carried out to increase conductivity of the PEDOT:PSS film, by changing the PEDOT molecular structure from the coiling conformation to the linear or expanded coil conformation, decreasing or removing the PSS insulating layer.24) τd decreases with increasing the conductivity of the PEDOT:PSS film, improving the charge collection efficiency in order to promote the Jsc of solar cells. When the bias voltage increases, the depletion zone of the Si=PEDOT:PSS heterojunction collapses and more minority carriers are trapped in the interface defects to be saturated so that the capacitance saturates. In this situation, RPN is related to the recombination of minority carriers at the Si=PEDOT:PSS interface, indicating that the status of current leakage is the same as that of the shunt resistance of the device. Meanwhile, the RPN value becomes larger, and the recombination of minority carriers is retarded more significantly. The RC elements of the Si=PEDOT:PSS interface are a RPN of 1.58 × 104 Ω and a CPN of 4.22 × 10−10 F, with a τn of 6.67 µs, while RPN is in agreement with the shunt resistance of the device. Additionally, RAl=Si is only 1381 Ω and CAl=Si is 1.19 × 10−8 F at the Al=Si interface, resulting in a τn of 16.43 µs. Rseries decreases markedly to 37.92 Ω at a large voltage, precisely indicating the resistance of the external circuit. The total resistance of the solar cell (Rtotal = RAl=Si + RPN + RPE + Rseries) is about 1.78 × 104 Ω. Therefore, the RC element information of Si-HSC can be characterized by a major semicircle plus two additional minor features. The total resistance of four resistance elements observed by EIS measurement decreases with increasing bias voltage, in accordance with the R of the dark J–V curve. In addition, the capacitance of the Si=PEDOT:PSS interface increases with increasing bias voltage, consistent with the results of C–V measurement. We can obtain the information of carrier transfer and recombination from the EIS spectra to design high-photovoltaic-performance Si-HSC. Jsc can be improved by enhancing the charge collection efficiency as

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follows: (1) RAl=Si is reduced by forming the ohmic contact through the reduction in the Al work function25) or the heavy doping concentration of n-type silicon; (2) RPE is reduced by improving the conductivity of the PEDOT:PSS film by the cosolvent addition, such as ethylene glycol or dimethyl sulfoxide, removing the PSS layer and changing the molecular structure to a linear formation; (3) the RPN of the major semicircle is increased by passivating the silicon surface of the Si=PEDOT:PSS interface to retard the carrier recombination.2) As discussed above, methods 1 and 2 are used to improve the charge collection efficiency of Si-HSC. Voc can be improved by modification procedures as follows: (1) RPN is increased to reduce the carrier loss for decreasing the Js; (2) CPN is increased to obtain a larger barrier height of Schottky heterojunction.26) The RC elements of these suggestions can be proved and analyzed using the EIS spectra such that the development of Si-HSC will be promoted by the EIS measurement. 4.

Conclusions

The internal resistance and capacitance elements of Si-HSC are characterized by EIS based on a suitable equivalent circuit model, indicating the details of minority carrier transfer and recombination at internal interfaces. At different bias voltages, we observe three types of EIS spectra (Nyquist plots) belonging to the different RC elements in the Si-HSC, especially the carrier transport property in the PEDOT:PSS film. Internal resistance elements depend on the bias voltages, and the capacitance elements are determined by the junction and diffusion capacitances. Note that EIS measurements would help us understand the carrier transfer and recombination processes of Si-HSC deeply, and facilitate the realization of high-performance devices. Acknowledgements

This work was financially supported by the Thousand Talent Program for Young Outstanding Scientists of People’s Republic China, the Instrument Developing Project of the Chinese Academy of Sciences (Grant No. yz201328), the National Natural Science Foundation of China (Grant No. 21403262), the Zhejiang Provincial Natural Science Foundation (Grant Nos. LY14F040005 and LR16F040002),

the International S&T Cooperation Program of Ningbo (Grant No. 2015D10021), and the Natural Science Foundation of Ningbo (Grant No. 2014A610041).

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