Study on negative incident photon-to-electron conversion efficiency of quantum dotsensitized solar cells Chunhui Li, Huijue Wu, Lifeng Zhu, Junyan Xiao, Yanhong Luo, Dongmei Li, and Qingbo Meng Citation: Review of Scientific Instruments 85, 023103 (2014); doi: 10.1063/1.4865115 View online: http://dx.doi.org/10.1063/1.4865115 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigation of the influence of coadsorbent dye upon the interfacial structure of dye-sensitized solar cells J. Chem. Phys. 141, 174709 (2014); 10.1063/1.4900640 Charge transport in zirconium doped anatase nanowires dye-sensitized solar cells: Trade-off between lattice strain and photovoltaic parameters Appl. Phys. Lett. 105, 153901 (2014); 10.1063/1.4898091 TiO2 nanospheres and spiny nanospheres for high conversion efficiency in dye-sensitized solar cells with gel electrolyte J. Appl. Phys. 115, 134504 (2014); 10.1063/1.4870473 Study on the effect of measuring methods on incident photon-to-electron conversion efficiency of dye-sensitized solar cells by home-made setup Rev. Sci. Instrum. 81, 103106 (2010); 10.1063/1.3488456 Negative electrochemical capacitance for a double-quantum-dot device J. Appl. Phys. 98, 086103 (2005); 10.1063/1.2099535
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 023103 (2014)
Study on negative incident photon-to-electron conversion efficiency of quantum dot-sensitized solar cells Chunhui Li, Huijue Wu, Lifeng Zhu, Junyan Xiao, Yanhong Luo, Dongmei Li, and Qingbo Menga) Key Laboratory for Renewable Energy, Chinese Academy of Sciences, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
(Received 31 October 2013; accepted 25 January 2014; published online 14 February 2014) Recently, negative signals are frequently observed during the measuring process of monochromatic incident photon-to-electron conversion efficiency (IPCE) for sensitized solar cells by DC method. This phenomenon is confusing and hindering the reasonable evaluation of solar cells. Here, cause of negative IPCE values is studied by taking quantum dot-sensitized solar cell (QDSC) as an example, and the accurate measurement method to avoid the negative value is suggested. The negative background signals of QDSC without illumination are found the direct cause of the negative IPCE values by DC method. Ambient noise, significant capacitance characteristics, and uncontrolled electrochemical reaction all can lead to the negative background signals. When the photocurrent response of device under monochromatic light illumination is relatively weak, the actual photocurrent signals will be covered by the negative background signals and the resulting IPCE values will appear negative. To improve the signal-to-noise ratio, quasi-AC method is proposed for IPCE measurement of solar cells with weak photocurrent response based on the idea of replacing the absolute values by the relative values. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865115] IPCE can be calculated by the following formula:
I. INTRODUCTION
As one of the environmentally friendly and cost-effective third-generation photovoltaic solar cells, sensitized solar cell attracts much attention of researchers, such as dye-sensitized solar cells (DSCs) and quantum dot-sensitized solar cells (QDSCs). Though much progress has been achieved, the efficiency of most sensitized solar cells is still far below that of monocrystalline silicon solar cell.1, 2 In fact, recombination is serious in the sensitized solar cell due to the large specific surface area and the complex photoanode/sensitizer/electrolyte interface structure. For QDSCs, usually a ZnS passivation layer covering the QDs sensitized photoanodes is needed to reduce the photogenerated electrons recombining with the redox couple in the electrolyte.3 It was reported that the recombination happening at the CdSe QDs/electrolyte interface rather than at the TiO2 /electrolyte interface was the most active recombination pathways.4 Modeling high-efficiency QDSCs indicated that existing technology was deficient and recombination had to be overcome for further improvement of QDSC.5 Therefore, there still need more deep works to explore the interior mechanism of sensitized solar cell for its development. Monochromatic incident photon-to-electron conversion efficiency (IPCE), which is also called external quantum efficiency and defined as number of photogenerated electrons per number of incident photons, is an important indicator of working mechanism of solar cells. According to its definition,
a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. 0034-6748/2014/85(2)/023103/6/$30.00
I P CE(λ) =
124 × Jsc (λ) , λ × P in(λ)
(1)
where λ is the wavelength of incident monochromatic light in unit of nanometer, Jsc (λ) is the short-circuit photocurrent density of solar cell under monochromatic illumination in unit of μA/cm2 , and Pin(λ) is the monochromatic light intensity in unit of mW/cm2 . IPCE is composed of three parts: light harvesting efficiency, charge separating efficiency, and charge collecting efficiency.6 The case of charge carrier generation, diffusion, extraction, and recombination inside the solar cells can be reflected by analyzing the IPCE results. For example, IPCE measurements have been used to determine the electron injection efficiency and charge separation efficiency at the TiO2 /dye/electrolyte interface, and electron diffusion length reflecting the recombination situation of the electrons photoinjected into TiO2 in DSCs.7, 8 Thus, the accurate measurement of IPCE is an important issue for right evaluation of solar cells. The traditional AC method with a chopping frequency of dozens of Hz, usually used for silicon-based solar cells, has been found not suitable due to the relatively slow response rate of DSCs.9 It is reported that the response time of DSCs is related to electron trapping in the nanocrystalline TiO2 and depends on illumination conditions10 and also on chemical composition11 of the cell. According to our previous work, the response time of N719 DSCs can reach as long as 1.5 s at the wavelength of 750 nm.12 Then DC method is proposed and widely applied for the IPCE measurement of sensitized solar cells, where the incident monochromatic light continuously
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irradiates on the solar cells and long enough waiting time is set before data acquisition. Recently, negative IPCE values of sensitized solar cells are frequently encountered by DC method, especially when the photocurrent response of cell is relatively small under the monochromatic light illumination of low intensity. This negative IPCE is confusing and severely hindered the reasonable evaluation of solar cells. In this paper, we studied the phenomenon of negative IPCE values measured by DC method in detail via taking QDSC as an example. It is the negative background signal that directly leads to the negative IPCE values. At last, to avoid the negative signal and more accurately reflect the spectral response efficiencies of devices, quasi-AC method is proposed for IPCE measurement of solar cells with weak photocurrent response. II. EXPERIMENT A. QDSC fabrication
Several kinds of QDSCs were fabricated with different components. F-doped SnO2 (FTO) glasses (Nippon Sheet Glass; sheet resistance: 15 /square) were used for preparing both photoanodes and counter electrodes. First, TiO2 and SnO2 mesoporous films13, 14 were prepared for loading QDs, respectively, followed by TiCl4 treatment. Subsequently, two kinds of QDs, CdS,15 and PbS,14, 16 were assembled on the photoanode film by the successive ionic layer adsorption and reaction (SILAR) technique. Screen-printed Cu2 S counter electrodes17 and PbS counter electrodes by SILAR method were applied, respectively. Finally, the sensitized photoanode and counter electrode were sealed together with ethylenevinyl acetate (EVA) copolymer thin film under hot press. The standard polysulfide electrolyte used here was 1 M Na2 S and 1 M S in aqueous solution. More details about QDSC fabrication are illuminated in the supplementary material.18 The fabricated QDSCs are labeled for distinguishment. For example, TiO2 /10CdS–Cu2 S represents a QDSC whose photoanode is TiO2 film sensitized with 10 CdS SILAR cycles and counter electrode is Cu2 S. B. IPCE measurement
IPCE spectra were measured using a home-made setup which could also record the short-circuit photocurrent (Isc ) of solar cell.12, 19 Xenon lamp in combination with a monochromator was used to provide the monochromatic light illumination whose intensity was limited to 1.5 mW/cm2 . Based on the optical beam-splitting technique, the incident light irradiated on the QDSC sample and on a calibrated silicon diode as a reference at the same time. Thus, the influence of possible fluctuations of the monochromatic light source could be suppressed. A shutter controlled by the computer, which could be quickly turned on and turned off, was inserted at the exit of the monochromator. The Isc of QDSC was converted to voltage signal and recorded by a data acquisition card. Two methods were used to measure the IPCE spectra of QDSCs: (1) DC method: The monochromatic light irradiated on the sample continuously. The waiting time before data ac-
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quisition was long enough to ensure that Isc had reached to a steady-state value; and (2) quasi-AC method: The monochromatic light was chopped by the shutter. At each wavelength, the Isc steady-state values were recorded before and after the shutter was switched on, and the difference was used to calculate the IPCE value. Quasi-AC method can effectively suppress common-mode noise and improve the signal-to-noise ratio of the measurement results. Measurement processes by both the two methods are automatically controlled by a computer. III. RESULTS AND DISCUSSION A. Phenomenon of negative IPCE values by DC method
DC method was first used to measure IPCE spectra of the fabricated QDSCs. Two representative curves of QDSCs are shown in Fig. 1(a). It is obvious that negative IPCE values appear in the short wavelength region. The extent of negative values for PbS sensitized SnO2 QDSC is severer than that for CdS sensitized TiO2 QDSC. IPCE value of near negative 200 is recorded for PbS sensitized SnO2 QDSC around 300 nm, which badly depresses the reliability of the test results. As a control, the IPCE values of a silicon diode are
(a)
(b)
FIG. 1. Phenomena of negative IPCE values: (a) IPCE and (b) Isc of CdS sensitized TiO2 QDSC and PbS sensitized SnO2 QDSC. Inset of (b) is the enlarged curves in the wavelength range of 300–360 nm. The corresponding values of a silicon diode are also shown for comparison.
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(a)
(b)
(c)
(d)
FIG. 2. Isc background signals in the dark of QDSCs with (a) different CdS SILAR cycles and different kinds of QDs, (b) different concentrations of electrolyte: the standard 1 M polysulfide electrolyte (1 M Na2 S and 1 M S in aqueous solution), the dilute 0.1 M polysulfide electrolyte (0.1 M Na2 S and 0.1 M S in aqueous solution), the dilute 0.01 M polysulfide electrolyte (0.01 M Na2 S and 0.01 M S in aqueous solution), and the control group of Milli-Q ultrapure water (denoted 0 M), (c) different counter electrodes: Cu2 S counter electrode and PbS counter electrode, (d) different photoanode film materials: TiO2 mesoporous film and SnO2 mesoporous film. Insets of (a), (b), and (c) are the enlarged curves.
all above zero in the whole measuring wavelength range. The corresponding Isc spectra of solar cells in Fig. 1(a) are recorded at the same time and shown in Fig. 1(b). The enlarged curves in inset of Fig. 1(b) indicate that the negative photocurrent values in the short wavelength region are the immediate cause leading to the phenomenon of negative IPCE values by DC method. In fact, the phenomenon of negative IPCE values also appears in the long wavelength region (Fig. S1 in the supplementary material).18 As the denominators in calculating IPCE values, the intensities of incident monochromatic light in the long wavelength region are stronger than those in the short wavelength region (Fig. S2 in the supplementary material).18 Thereby, negative IPCE values in the long wavelength region are not so obvious as those in the short wavelength region. B. Isc background signals in the dark
It is confusing that a negative photocurrent response appears for solar cells. If there is no reverse photovoltaic process happened in solar cells, what contributes to the apparently reverse photocurrent? It is noticed that the Isc signal collected by
DC method is an absolute value which may be influenced by the testing environment and apparatus. To evaluate the effect of environmental factors and find out the deeper reason of the negative Isc arising, the response signals of QDSCs without illumination of any monochromatic light, called Isc background signals here, were recorded and shown in Fig. 2. Several kinds of QDSCs were fabricated as illuminated in the experiment part. The Isc background signals of QDSCs with different QDs, different concentrations of electrolyte, different counter electrodes, and different mesoporous photoanode films are investigated, respectively. Fig. 2(a) shows the Isc background signals in the dark of QDSCs with different CdS SILAR cycles and different kinds of QDs. Fig. 2(b) shows the Isc background signals of TiO2 /PbS–Cu2 S QDSCs with different concentrations of electrolyte: the standard 1 M polysulfide electrolyte (1 M Na2 S and 1 M S in aqueous solution), the dilute 0.1 M polysulfide electrolyte (0.1 M Na2 S and 0.1 M S in aqueous solution), the dilute 0.01 M polysulfide electrolyte (0.01 M Na2 S and 0.01 M S in aqueous solution), and the control group of Milli-Q ultrapure water (denoted 0 M). Fig. 2(c) shows the Isc background signals of QDSCs with different counter electrodes: Cu2 S counter
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electrode and PbS counter electrode. Fig. 2(d) shows the Isc background signals of QDSCs with different photoanode film materials: TiO2 mesoporous film and SnO2 mesoporous film. As shown in Fig. 2, the Isc background signals of QDSCs without irradiation are negative in many cases. Although there are more of less differences for all the Isc background signals of QDSCs with different fabrication components, the differences among Isc background signals of QDSCs with different amount of QD deposition, different kinds of QDs, different concentrations of electrolyte, and different counter electrodes are much smaller than the difference between Isc background signals of QDSCs with different photoanode film materials. The difference of Isc background signals reaches 1 μA between QDSC with TiO2 mesoporous film and QDSC with SnO2 mesoporous film. It is noticed that the highest Isc is only about 5 μA over the whole measuring wavelength range for SnO2 QDSC as shown in Fig. 1(b). Thus, the Isc background signal of negative 1 μA is large enough to influence the resulting photocurrent response characterization, and the overall IPCE values of SnO2 QDSC in Fig. 1(a) are on the low side. C. Origins of negative Isc background signals
Multiple factors may cause this negative Isc background signals for sensitized solar cells. Fig. 3 shows the continuous four measurement results of Isc background signals for the same QDSC sample with 20 CdS SILAR cycles sensitized TiO2 photoanode and PbS counter electrode. The four results are not identical, which accords with the feature of noise. Thus, it is believed that the fluctuant Isc background signals are related to noise. The slight differences of Isc background signals may be caused by the ambient noise coming from the testing environment and apparatus, such as the stray light in the environment, the fluctuation of the power supply system, the interference in the circuit system, and so on. Because all these influencing factors are uncertain, the Isc background signals of QDSCs are unstable, can be positive or negative, and change over time. As a result, the calculated IPCE values also
FIG. 3. Continuous four measurements of Isc background signals in the dark for the same QDSC sample with 20CdS sensitized TiO2 photoanode and PbS counter electrode. Continuous two measurements of Isc background signals for the same silicon diode are also shown for comparison.
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FIG. 4. Schematic diagram of the photocurrent-collecting circuit. I represents as a constant-current source. CIN is the total input capacitance of the transimpedance amplifier. R is the transimpedance. The output voltage signal Vout is equal to the product of the input current signal I and R.
fluctuate wildly and sometimes even show themselves negative. For comparison, the Isc background signals of the silicon diode are relatively stable, while a slight fluctuation still exists and sometimes Isc background signals of Si also drop below zero (Fig. 3). As for the large Isc background signal of SnO2 QDSC, there must be other factors at play besides the ambient noise. Then the difference of electrical properties between SnO2 QDSC and other QDSCs is considered. In our apparatus of IPCE measurement, the data are collected after being conversed from current signal to voltage signal by a transimpedance amplifier. The schematic diagram of the datacollecting circuit is shown in Fig. 4 and the noise current of this circuit increases as the total input capacitance (CIN ) of transimpedance amplifier increases.20 Fig. 5 shows the equivalent circuit model of sensitized solar cells.21, 22 The capacitance element C1 at the electrolyte/counter electrode interface and C2 at the photoanode/electrolyte interface contribute to the input capacitance of transimpedance amplifier. SnO2 /PbS–Cu2 S cell has the same C1 as TiO2 /PbS–Cu2 S cell in view of the same standard electrolyte and the same counter electrode used. According to Wang group’s work,23 C2 of SnO2 QDSC is obviously higher than that of TiO2 QDSC. Thus, it is believed that the higher capacitance of SnO2 QDSC may lead to an increase of the input capacitance and then an increase of the noise current. The relative instability of Isc background signals for sensitized solar cells compared to the silicon diode (Fig. 3) may be also caused by the distinct capacitance characteristics of sensitized solar cells. In addition, it is observed from the enlarged curves in Fig. 2(b) that the more dilute electrolyte gives the less negative Isc background signal. To confirm this trend, a batch
FIG. 5. Equivalent circuit model of sensitized solar cells. D represents as a diode. A constant-current source Iph and shunt resistance Rsh are in parallel with D. The sum of R1 , R2 , and Rh corresponds to the series resistance. C1 is the capacitance element at the electrolyte/counter electrode interface. C2 is the capacitance element at the photoanode/electrolyte interface.
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FIG. 6. Isc background signals in the dark of SnO2 /PbS–Cu2 S QDSCs with different electrolytes: the standard 1 M polysulfide electrolyte (1 M Na2 S and 1 M S in aqueous solution), Milli-Q ultrapure water, and no electrolyte. Two measurement results for every cell are exhibited here.
of SnO2 /PbS–Cu2 S QDSCs was newly fabricated with different electrolytes. Two measurement results of Isc background signal for every cell are exhibited in Fig. 6. As shown in Fig. 2(d), the Isc background signal of SnO2 /PbS–Cu2 S QDSC with standard 1 M electrolyte reaches negative 1 μA, which proves the repeatability of experiments. Same as above, the fluctuation of Isc background signals is observed for the same QDSC, and the Isc background signals of cells with different electrolytes sometimes overlap. However, the fluctuation range is large for QDSC with high concentration of electrolyte. For QDSC without electrolyte, the fluctuation of Isc background signals is extremely small. Therefore, it is indicated that the negative Isc background signal also has a relationship with electrolyte and uncontrolled electrochemical reaction.
D. Weak photocurrent response of QDSCs
According to above discussions, the ambient noise, significant capacitance characteristics, and uncontrolled electrochemical reaction inside sensitized solar cells may result in an Isc background signal below zero. The intensity of monochromatic light used in the IPCE measuring process is quite low. In this case, the actual photocurrent response of solar cell is weak. This small response signal is probable to be covered by the background signals, which leads to a low signal-to-noise ratio. Fig. 7 shows the time response of Isc for SnO2 /PbS– Cu2 S QDSC. The monochromatic light illumination of different wavelengths is switched on at t = 0. As shown in Fig. 7, the Isc background signal of SnO2 /PbS–Cu2 S QDSC is negative in the dark. After the monochromatic light illumination of 300 nm is switched on, there is no obvious photocurrent signal observed. Although obvious photocurrent signal appears at 350 nm, the photocurrent signal adding Isc background signal is still below zero and the apparent IPCE value will be calculated negative. As the illumination wavelengths become longer, the photocurrent response of SnO2 /PbS–Cu2 S QDSC becomes stronger, and Isc rises near zero at 380 nm and above
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FIG. 7. Time response of Isc for SnO2 /PbS–Cu2 S QDSC. The monochromatic light illumination of different wavelengths is switched on at t = 0.
zero at 400 nm. Positive IPCE value at 400 nm can be expected. Fig. S3 in the supplementary material18 shows a similar situation in the long wavelength region. At present, the efficiencies of most QDSCs are below 6%, so their photocurrent responses are relatively weak especially in the short wavelength region. Thus, the influence of the background signals on the IPCE results measured by DC method cannot be ignored, and the phenomenon of negative values is common.
E. IPCE measurement by quasi-AC method
Noise cannot be eliminated quietly though some ambient noise may be suppressed by a certain way. On the other hand, as the background signals are not identical for different QDSCs and change over time, artificial deduction of the background signals of QDSCs without monochromatic light illumination will not work. Then, in this case, how to decrease the influence of the unstable background signals and how to eliminate negative IPCE values? Quasi-AC method is suggested here for the IPCE measurement of QDSCs giving weak photocurrent response. In quasi-AC method, the signals are recorded before and after the incident light is switched on, and the difference between Isc with illumination and Isc in the dark is used to calculate the IPCE value. That is, the absolute values in DC methods are replaced by the relative values in quasi-AC method. Thus, the influence of the background signals can be effectively avoided and the signal-to-noise ratio of the measurement results is improved. Fig. 8(a) shows the IPCE spectra of TiO2 /5CdS–Cu2 S QDSC and SnO2 /PbS–Cu2 S QDSC measured by DC method and quasi-AC method, respectively. For TiO2 /5CdS–Cu2 S QDSC, the two spectra measured by two methods are almost overlapped at the wavelength range from 350 nm to 700 nm. Below 350 nm negative IPCE values are effectively eliminated in quasi-AC method compared to DC method. For SnO2 /PbS–Cu2 S QDSC, besides the badly negative values below 350 nm disappear benefiting from applying quasi-AC method, IPCE values measured by quasi-AC method are always higher than that measured by DC method above 350 nm. This again indicates that the negative Isc background signals
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(a)
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pacitance characteristics of sensitized solar cells. The higher capacitance results in more negative Isc background signals. By variation of electrolyte, it is found that the negative Isc background signal also has a relationship with uncontrolled electrochemical reaction. When the photocurrent response of device is relatively weak under the illumination of monochromatic light at certain wavelengths, the actual photocurrent signals will be covered by the negative background signals and the resulting IPCE values will appear negative. At last, to improve the signal-to-noise ratio and avoid negative values, quasi-AC method is proposed for IPCE measurement of solar cells with weak response. ACKNOWLEDGMENTS
(b)
The authors appreciate the financial supports of National Key Basic Research Program (973 Project No. 2012CB932903), National Natural Science Foundation of China (Nos. 20725311, 51072221, and 21173260), Beijing Science and Technology Committee (Z131100006013003), and the Knowledge Innovation Program of the Chinese Academy of Sciences. 1 A.
FIG. 8. (a) IPCE spectra of TiO2 /5CdS–Cu2 S QDSC and SnO2 /PbS–Cu2 S QDSC measured by DC method and quasi-AC method, respectively. (b) The corresponding Isc . Inset of (b) is the enlarged curves in the wavelength range of 300–360 nm.
depress the whole IPCE spectrum by DC method because of the weaker photocurrent response of SnO2 /PbS–Cu2 S QDSC. Fig. 8(b) shows the corresponding Isc of IPCE spectra in Fig. 8(a), all Isc by quasi-AC method are above zero. It is noticed that the test time of DC method is relatively short. Thus, DC method is applied in most cases especially when the photocurrent response of solar cells is strong enough. However, if the response of solar cells is weak and comparable to the background signal, quasi-AC is more suitable to achieve more accurate measurement results.
IV. CONCLUSION
To find out the cause of negative IPCE values for sensitized solar cells measured by DC method, QDSCs with different components were fabricated and their IPCE characteristics were studied in details. It is the negative Isc background signal of QDSC without illumination that directly leads to the negative IPCE values in DC method. The background signals coming from noise are different for different solar cells, change over time, and can be positive or negative. The relatively larger Isc background signal of SnO2 QDSC compared to QDSCs with other different components indicates that the instability of background signals is probably related to the ca-
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 023103 (2014)
Study on negative incident photon-to-electron conversion efficiency of quantum dot-sensitized solar cells Chunhui Li, Huijue Wu, Lifeng Zhu, Junyan Xiao, Yanhong Luo, Dongmei Li, and Qingbo Menga) Key Laboratory for Renewable Energy, Chinese Academy of Sciences, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China
(Received 31 October 2013; accepted 25 January 2014; published online 14 February 2014) Recently, negative signals are frequently observed during the measuring process of monochromatic incident photon-to-electron conversion efficiency (IPCE) for sensitized solar cells by DC method. This phenomenon is confusing and hindering the reasonable evaluation of solar cells. Here, cause of negative IPCE values is studied by taking quantum dot-sensitized solar cell (QDSC) as an example, and the accurate measurement method to avoid the negative value is suggested. The negative background signals of QDSC without illumination are found the direct cause of the negative IPCE values by DC method. Ambient noise, significant capacitance characteristics, and uncontrolled electrochemical reaction all can lead to the negative background signals. When the photocurrent response of device under monochromatic light illumination is relatively weak, the actual photocurrent signals will be covered by the negative background signals and the resulting IPCE values will appear negative. To improve the signal-to-noise ratio, quasi-AC method is proposed for IPCE measurement of solar cells with weak photocurrent response based on the idea of replacing the absolute values by the relative values. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865115] IPCE can be calculated by the following formula:
I. INTRODUCTION
As one of the environmentally friendly and cost-effective third-generation photovoltaic solar cells, sensitized solar cell attracts much attention of researchers, such as dye-sensitized solar cells (DSCs) and quantum dot-sensitized solar cells (QDSCs). Though much progress has been achieved, the efficiency of most sensitized solar cells is still far below that of monocrystalline silicon solar cell.1, 2 In fact, recombination is serious in the sensitized solar cell due to the large specific surface area and the complex photoanode/sensitizer/electrolyte interface structure. For QDSCs, usually a ZnS passivation layer covering the QDs sensitized photoanodes is needed to reduce the photogenerated electrons recombining with the redox couple in the electrolyte.3 It was reported that the recombination happening at the CdSe QDs/electrolyte interface rather than at the TiO2 /electrolyte interface was the most active recombination pathways.4 Modeling high-efficiency QDSCs indicated that existing technology was deficient and recombination had to be overcome for further improvement of QDSC.5 Therefore, there still need more deep works to explore the interior mechanism of sensitized solar cell for its development. Monochromatic incident photon-to-electron conversion efficiency (IPCE), which is also called external quantum efficiency and defined as number of photogenerated electrons per number of incident photons, is an important indicator of working mechanism of solar cells. According to its definition,
a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. 0034-6748/2014/85(2)/023103/6/$30.00
I P CE(λ) =
124 × Jsc (λ) , λ × P in(λ)
(1)
where λ is the wavelength of incident monochromatic light in unit of nanometer, Jsc (λ) is the short-circuit photocurrent density of solar cell under monochromatic illumination in unit of μA/cm2 , and Pin(λ) is the monochromatic light intensity in unit of mW/cm2 . IPCE is composed of three parts: light harvesting efficiency, charge separating efficiency, and charge collecting efficiency.6 The case of charge carrier generation, diffusion, extraction, and recombination inside the solar cells can be reflected by analyzing the IPCE results. For example, IPCE measurements have been used to determine the electron injection efficiency and charge separation efficiency at the TiO2 /dye/electrolyte interface, and electron diffusion length reflecting the recombination situation of the electrons photoinjected into TiO2 in DSCs.7, 8 Thus, the accurate measurement of IPCE is an important issue for right evaluation of solar cells. The traditional AC method with a chopping frequency of dozens of Hz, usually used for silicon-based solar cells, has been found not suitable due to the relatively slow response rate of DSCs.9 It is reported that the response time of DSCs is related to electron trapping in the nanocrystalline TiO2 and depends on illumination conditions10 and also on chemical composition11 of the cell. According to our previous work, the response time of N719 DSCs can reach as long as 1.5 s at the wavelength of 750 nm.12 Then DC method is proposed and widely applied for the IPCE measurement of sensitized solar cells, where the incident monochromatic light continuously
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irradiates on the solar cells and long enough waiting time is set before data acquisition. Recently, negative IPCE values of sensitized solar cells are frequently encountered by DC method, especially when the photocurrent response of cell is relatively small under the monochromatic light illumination of low intensity. This negative IPCE is confusing and severely hindered the reasonable evaluation of solar cells. In this paper, we studied the phenomenon of negative IPCE values measured by DC method in detail via taking QDSC as an example. It is the negative background signal that directly leads to the negative IPCE values. At last, to avoid the negative signal and more accurately reflect the spectral response efficiencies of devices, quasi-AC method is proposed for IPCE measurement of solar cells with weak photocurrent response. II. EXPERIMENT A. QDSC fabrication
Several kinds of QDSCs were fabricated with different components. F-doped SnO2 (FTO) glasses (Nippon Sheet Glass; sheet resistance: 15 /square) were used for preparing both photoanodes and counter electrodes. First, TiO2 and SnO2 mesoporous films13, 14 were prepared for loading QDs, respectively, followed by TiCl4 treatment. Subsequently, two kinds of QDs, CdS,15 and PbS,14, 16 were assembled on the photoanode film by the successive ionic layer adsorption and reaction (SILAR) technique. Screen-printed Cu2 S counter electrodes17 and PbS counter electrodes by SILAR method were applied, respectively. Finally, the sensitized photoanode and counter electrode were sealed together with ethylenevinyl acetate (EVA) copolymer thin film under hot press. The standard polysulfide electrolyte used here was 1 M Na2 S and 1 M S in aqueous solution. More details about QDSC fabrication are illuminated in the supplementary material.18 The fabricated QDSCs are labeled for distinguishment. For example, TiO2 /10CdS–Cu2 S represents a QDSC whose photoanode is TiO2 film sensitized with 10 CdS SILAR cycles and counter electrode is Cu2 S. B. IPCE measurement
IPCE spectra were measured using a home-made setup which could also record the short-circuit photocurrent (Isc ) of solar cell.12, 19 Xenon lamp in combination with a monochromator was used to provide the monochromatic light illumination whose intensity was limited to 1.5 mW/cm2 . Based on the optical beam-splitting technique, the incident light irradiated on the QDSC sample and on a calibrated silicon diode as a reference at the same time. Thus, the influence of possible fluctuations of the monochromatic light source could be suppressed. A shutter controlled by the computer, which could be quickly turned on and turned off, was inserted at the exit of the monochromator. The Isc of QDSC was converted to voltage signal and recorded by a data acquisition card. Two methods were used to measure the IPCE spectra of QDSCs: (1) DC method: The monochromatic light irradiated on the sample continuously. The waiting time before data ac-
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quisition was long enough to ensure that Isc had reached to a steady-state value; and (2) quasi-AC method: The monochromatic light was chopped by the shutter. At each wavelength, the Isc steady-state values were recorded before and after the shutter was switched on, and the difference was used to calculate the IPCE value. Quasi-AC method can effectively suppress common-mode noise and improve the signal-to-noise ratio of the measurement results. Measurement processes by both the two methods are automatically controlled by a computer. III. RESULTS AND DISCUSSION A. Phenomenon of negative IPCE values by DC method
DC method was first used to measure IPCE spectra of the fabricated QDSCs. Two representative curves of QDSCs are shown in Fig. 1(a). It is obvious that negative IPCE values appear in the short wavelength region. The extent of negative values for PbS sensitized SnO2 QDSC is severer than that for CdS sensitized TiO2 QDSC. IPCE value of near negative 200 is recorded for PbS sensitized SnO2 QDSC around 300 nm, which badly depresses the reliability of the test results. As a control, the IPCE values of a silicon diode are
(a)
(b)
FIG. 1. Phenomena of negative IPCE values: (a) IPCE and (b) Isc of CdS sensitized TiO2 QDSC and PbS sensitized SnO2 QDSC. Inset of (b) is the enlarged curves in the wavelength range of 300–360 nm. The corresponding values of a silicon diode are also shown for comparison.
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(a)
(b)
(c)
(d)
FIG. 2. Isc background signals in the dark of QDSCs with (a) different CdS SILAR cycles and different kinds of QDs, (b) different concentrations of electrolyte: the standard 1 M polysulfide electrolyte (1 M Na2 S and 1 M S in aqueous solution), the dilute 0.1 M polysulfide electrolyte (0.1 M Na2 S and 0.1 M S in aqueous solution), the dilute 0.01 M polysulfide electrolyte (0.01 M Na2 S and 0.01 M S in aqueous solution), and the control group of Milli-Q ultrapure water (denoted 0 M), (c) different counter electrodes: Cu2 S counter electrode and PbS counter electrode, (d) different photoanode film materials: TiO2 mesoporous film and SnO2 mesoporous film. Insets of (a), (b), and (c) are the enlarged curves.
all above zero in the whole measuring wavelength range. The corresponding Isc spectra of solar cells in Fig. 1(a) are recorded at the same time and shown in Fig. 1(b). The enlarged curves in inset of Fig. 1(b) indicate that the negative photocurrent values in the short wavelength region are the immediate cause leading to the phenomenon of negative IPCE values by DC method. In fact, the phenomenon of negative IPCE values also appears in the long wavelength region (Fig. S1 in the supplementary material).18 As the denominators in calculating IPCE values, the intensities of incident monochromatic light in the long wavelength region are stronger than those in the short wavelength region (Fig. S2 in the supplementary material).18 Thereby, negative IPCE values in the long wavelength region are not so obvious as those in the short wavelength region. B. Isc background signals in the dark
It is confusing that a negative photocurrent response appears for solar cells. If there is no reverse photovoltaic process happened in solar cells, what contributes to the apparently reverse photocurrent? It is noticed that the Isc signal collected by
DC method is an absolute value which may be influenced by the testing environment and apparatus. To evaluate the effect of environmental factors and find out the deeper reason of the negative Isc arising, the response signals of QDSCs without illumination of any monochromatic light, called Isc background signals here, were recorded and shown in Fig. 2. Several kinds of QDSCs were fabricated as illuminated in the experiment part. The Isc background signals of QDSCs with different QDs, different concentrations of electrolyte, different counter electrodes, and different mesoporous photoanode films are investigated, respectively. Fig. 2(a) shows the Isc background signals in the dark of QDSCs with different CdS SILAR cycles and different kinds of QDs. Fig. 2(b) shows the Isc background signals of TiO2 /PbS–Cu2 S QDSCs with different concentrations of electrolyte: the standard 1 M polysulfide electrolyte (1 M Na2 S and 1 M S in aqueous solution), the dilute 0.1 M polysulfide electrolyte (0.1 M Na2 S and 0.1 M S in aqueous solution), the dilute 0.01 M polysulfide electrolyte (0.01 M Na2 S and 0.01 M S in aqueous solution), and the control group of Milli-Q ultrapure water (denoted 0 M). Fig. 2(c) shows the Isc background signals of QDSCs with different counter electrodes: Cu2 S counter
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electrode and PbS counter electrode. Fig. 2(d) shows the Isc background signals of QDSCs with different photoanode film materials: TiO2 mesoporous film and SnO2 mesoporous film. As shown in Fig. 2, the Isc background signals of QDSCs without irradiation are negative in many cases. Although there are more of less differences for all the Isc background signals of QDSCs with different fabrication components, the differences among Isc background signals of QDSCs with different amount of QD deposition, different kinds of QDs, different concentrations of electrolyte, and different counter electrodes are much smaller than the difference between Isc background signals of QDSCs with different photoanode film materials. The difference of Isc background signals reaches 1 μA between QDSC with TiO2 mesoporous film and QDSC with SnO2 mesoporous film. It is noticed that the highest Isc is only about 5 μA over the whole measuring wavelength range for SnO2 QDSC as shown in Fig. 1(b). Thus, the Isc background signal of negative 1 μA is large enough to influence the resulting photocurrent response characterization, and the overall IPCE values of SnO2 QDSC in Fig. 1(a) are on the low side. C. Origins of negative Isc background signals
Multiple factors may cause this negative Isc background signals for sensitized solar cells. Fig. 3 shows the continuous four measurement results of Isc background signals for the same QDSC sample with 20 CdS SILAR cycles sensitized TiO2 photoanode and PbS counter electrode. The four results are not identical, which accords with the feature of noise. Thus, it is believed that the fluctuant Isc background signals are related to noise. The slight differences of Isc background signals may be caused by the ambient noise coming from the testing environment and apparatus, such as the stray light in the environment, the fluctuation of the power supply system, the interference in the circuit system, and so on. Because all these influencing factors are uncertain, the Isc background signals of QDSCs are unstable, can be positive or negative, and change over time. As a result, the calculated IPCE values also
FIG. 3. Continuous four measurements of Isc background signals in the dark for the same QDSC sample with 20CdS sensitized TiO2 photoanode and PbS counter electrode. Continuous two measurements of Isc background signals for the same silicon diode are also shown for comparison.
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FIG. 4. Schematic diagram of the photocurrent-collecting circuit. I represents as a constant-current source. CIN is the total input capacitance of the transimpedance amplifier. R is the transimpedance. The output voltage signal Vout is equal to the product of the input current signal I and R.
fluctuate wildly and sometimes even show themselves negative. For comparison, the Isc background signals of the silicon diode are relatively stable, while a slight fluctuation still exists and sometimes Isc background signals of Si also drop below zero (Fig. 3). As for the large Isc background signal of SnO2 QDSC, there must be other factors at play besides the ambient noise. Then the difference of electrical properties between SnO2 QDSC and other QDSCs is considered. In our apparatus of IPCE measurement, the data are collected after being conversed from current signal to voltage signal by a transimpedance amplifier. The schematic diagram of the datacollecting circuit is shown in Fig. 4 and the noise current of this circuit increases as the total input capacitance (CIN ) of transimpedance amplifier increases.20 Fig. 5 shows the equivalent circuit model of sensitized solar cells.21, 22 The capacitance element C1 at the electrolyte/counter electrode interface and C2 at the photoanode/electrolyte interface contribute to the input capacitance of transimpedance amplifier. SnO2 /PbS–Cu2 S cell has the same C1 as TiO2 /PbS–Cu2 S cell in view of the same standard electrolyte and the same counter electrode used. According to Wang group’s work,23 C2 of SnO2 QDSC is obviously higher than that of TiO2 QDSC. Thus, it is believed that the higher capacitance of SnO2 QDSC may lead to an increase of the input capacitance and then an increase of the noise current. The relative instability of Isc background signals for sensitized solar cells compared to the silicon diode (Fig. 3) may be also caused by the distinct capacitance characteristics of sensitized solar cells. In addition, it is observed from the enlarged curves in Fig. 2(b) that the more dilute electrolyte gives the less negative Isc background signal. To confirm this trend, a batch
FIG. 5. Equivalent circuit model of sensitized solar cells. D represents as a diode. A constant-current source Iph and shunt resistance Rsh are in parallel with D. The sum of R1 , R2 , and Rh corresponds to the series resistance. C1 is the capacitance element at the electrolyte/counter electrode interface. C2 is the capacitance element at the photoanode/electrolyte interface.
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FIG. 6. Isc background signals in the dark of SnO2 /PbS–Cu2 S QDSCs with different electrolytes: the standard 1 M polysulfide electrolyte (1 M Na2 S and 1 M S in aqueous solution), Milli-Q ultrapure water, and no electrolyte. Two measurement results for every cell are exhibited here.
of SnO2 /PbS–Cu2 S QDSCs was newly fabricated with different electrolytes. Two measurement results of Isc background signal for every cell are exhibited in Fig. 6. As shown in Fig. 2(d), the Isc background signal of SnO2 /PbS–Cu2 S QDSC with standard 1 M electrolyte reaches negative 1 μA, which proves the repeatability of experiments. Same as above, the fluctuation of Isc background signals is observed for the same QDSC, and the Isc background signals of cells with different electrolytes sometimes overlap. However, the fluctuation range is large for QDSC with high concentration of electrolyte. For QDSC without electrolyte, the fluctuation of Isc background signals is extremely small. Therefore, it is indicated that the negative Isc background signal also has a relationship with electrolyte and uncontrolled electrochemical reaction.
D. Weak photocurrent response of QDSCs
According to above discussions, the ambient noise, significant capacitance characteristics, and uncontrolled electrochemical reaction inside sensitized solar cells may result in an Isc background signal below zero. The intensity of monochromatic light used in the IPCE measuring process is quite low. In this case, the actual photocurrent response of solar cell is weak. This small response signal is probable to be covered by the background signals, which leads to a low signal-to-noise ratio. Fig. 7 shows the time response of Isc for SnO2 /PbS– Cu2 S QDSC. The monochromatic light illumination of different wavelengths is switched on at t = 0. As shown in Fig. 7, the Isc background signal of SnO2 /PbS–Cu2 S QDSC is negative in the dark. After the monochromatic light illumination of 300 nm is switched on, there is no obvious photocurrent signal observed. Although obvious photocurrent signal appears at 350 nm, the photocurrent signal adding Isc background signal is still below zero and the apparent IPCE value will be calculated negative. As the illumination wavelengths become longer, the photocurrent response of SnO2 /PbS–Cu2 S QDSC becomes stronger, and Isc rises near zero at 380 nm and above
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FIG. 7. Time response of Isc for SnO2 /PbS–Cu2 S QDSC. The monochromatic light illumination of different wavelengths is switched on at t = 0.
zero at 400 nm. Positive IPCE value at 400 nm can be expected. Fig. S3 in the supplementary material18 shows a similar situation in the long wavelength region. At present, the efficiencies of most QDSCs are below 6%, so their photocurrent responses are relatively weak especially in the short wavelength region. Thus, the influence of the background signals on the IPCE results measured by DC method cannot be ignored, and the phenomenon of negative values is common.
E. IPCE measurement by quasi-AC method
Noise cannot be eliminated quietly though some ambient noise may be suppressed by a certain way. On the other hand, as the background signals are not identical for different QDSCs and change over time, artificial deduction of the background signals of QDSCs without monochromatic light illumination will not work. Then, in this case, how to decrease the influence of the unstable background signals and how to eliminate negative IPCE values? Quasi-AC method is suggested here for the IPCE measurement of QDSCs giving weak photocurrent response. In quasi-AC method, the signals are recorded before and after the incident light is switched on, and the difference between Isc with illumination and Isc in the dark is used to calculate the IPCE value. That is, the absolute values in DC methods are replaced by the relative values in quasi-AC method. Thus, the influence of the background signals can be effectively avoided and the signal-to-noise ratio of the measurement results is improved. Fig. 8(a) shows the IPCE spectra of TiO2 /5CdS–Cu2 S QDSC and SnO2 /PbS–Cu2 S QDSC measured by DC method and quasi-AC method, respectively. For TiO2 /5CdS–Cu2 S QDSC, the two spectra measured by two methods are almost overlapped at the wavelength range from 350 nm to 700 nm. Below 350 nm negative IPCE values are effectively eliminated in quasi-AC method compared to DC method. For SnO2 /PbS–Cu2 S QDSC, besides the badly negative values below 350 nm disappear benefiting from applying quasi-AC method, IPCE values measured by quasi-AC method are always higher than that measured by DC method above 350 nm. This again indicates that the negative Isc background signals
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(a)
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pacitance characteristics of sensitized solar cells. The higher capacitance results in more negative Isc background signals. By variation of electrolyte, it is found that the negative Isc background signal also has a relationship with uncontrolled electrochemical reaction. When the photocurrent response of device is relatively weak under the illumination of monochromatic light at certain wavelengths, the actual photocurrent signals will be covered by the negative background signals and the resulting IPCE values will appear negative. At last, to improve the signal-to-noise ratio and avoid negative values, quasi-AC method is proposed for IPCE measurement of solar cells with weak response. ACKNOWLEDGMENTS
(b)
The authors appreciate the financial supports of National Key Basic Research Program (973 Project No. 2012CB932903), National Natural Science Foundation of China (Nos. 20725311, 51072221, and 21173260), Beijing Science and Technology Committee (Z131100006013003), and the Knowledge Innovation Program of the Chinese Academy of Sciences. 1 A.
FIG. 8. (a) IPCE spectra of TiO2 /5CdS–Cu2 S QDSC and SnO2 /PbS–Cu2 S QDSC measured by DC method and quasi-AC method, respectively. (b) The corresponding Isc . Inset of (b) is the enlarged curves in the wavelength range of 300–360 nm.
depress the whole IPCE spectrum by DC method because of the weaker photocurrent response of SnO2 /PbS–Cu2 S QDSC. Fig. 8(b) shows the corresponding Isc of IPCE spectra in Fig. 8(a), all Isc by quasi-AC method are above zero. It is noticed that the test time of DC method is relatively short. Thus, DC method is applied in most cases especially when the photocurrent response of solar cells is strong enough. However, if the response of solar cells is weak and comparable to the background signal, quasi-AC is more suitable to achieve more accurate measurement results.
IV. CONCLUSION
To find out the cause of negative IPCE values for sensitized solar cells measured by DC method, QDSCs with different components were fabricated and their IPCE characteristics were studied in details. It is the negative Isc background signal of QDSC without illumination that directly leads to the negative IPCE values in DC method. The background signals coming from noise are different for different solar cells, change over time, and can be positive or negative. The relatively larger Isc background signal of SnO2 QDSC compared to QDSCs with other different components indicates that the instability of background signals is probably related to the ca-
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