Combination of Optical and Electrical Loss Analyses for a Si

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Combination of Optical and Electrical Loss Analyses for a Si-Phthalocyanine Dye-Sensitized Solar Cell Keng-Chu Lin, Lili Wang, Tennyson L. Doane, Anton Kovalsky, Sandra Pejic, and Clemens Burda J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2014 Downloaded from http://pubs.acs.org on June 21, 2014

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The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Combination of Optical and Electrical Loss Analyses for a Si-Phthalocyanine Dye-Sensitized Solar Cell

Keng-Chu Lina,b, Lili Wanga, Tennyson Doanea, Anton Kovalskya, Sandra Pejica, and Clemens Burdaa,b,* a

Department of Chemistry, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH 44106, USA Fax: 216-368-3006; Tel: 216-368-5918; E-mail: [email protected] b

Department of Materials Science and Engineering, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH 44106, USA Fax: 216-368-3006; Tel: 216-368-5918; E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract

In order to promote the development of solar cells with varying types of sensitizers including dyes and quantum dots, it is crucial to establish a general experimental analysis that accounts for all important optical and electrical losses resulting from interfacial phenomena. All of these varying types of solar cells share common features where a meso-porous scaffold is used as a sensitizer loading support as well as electron transport material, which may result in light scattering. The loss of efficiency at interfaces of the sensitizer, the mesoporous TiO2 nanoparticle films, the FTO conductive layer, and the supportive glass substrate should be considered in addition to the photoinduced electron transport properties within a cell. Based on optical parameters, one can obtain the internal quantum efficiency (IQE) of a solar cell, an important parameter that cannot be directly measured but must be derived from several key experiments. By integrating an optical loss model with an electrical loss model, many solar cell parameters could be characterized from electro-optical observables including reflectance, transmittance, and absorptance of the dye sensitizer, the electron injection efficiency and the charge collection efficiency. In this work, an integrated electro-optical approach has been applied to SiPc (Pc 61) dye-sensitized solar cells for evaluating the parameters affecting the overall power conversion efficiency. The absorptance results of the Pc 61 dye sensitized solar cell provide evidence that the adsorbed Pc 61 forms non-injection layers on TiO2 surfaces when the dye immersion time exceeds 120 min, resulting in shading light from the active layer rather than an increase in photoelectric current efficiency.


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Keywords: Dye sensitization; Grätzel cell; phthalocyanine; electron injection; absorptance; energy loss analysis 1. Introduction The photoelectrochemical strategy of sensitizing a wide band gap semiconducting metal oxide with a molecular dye was first demonstrated in the 1960’s1 as an effective means of free carrier generation. This sensitization modification on the surface of the metal oxide extended the absorbance spectrum towards the visible spectral region which comprises a significant fraction of the solar spectrum.2 The dye, which must have a higher reduction potential of the excited state relative to the conduction band edge of semiconductor, can absorb visible light photons to inject the photo-generated excited electrons into the semiconductor. This is an efficient strategy for populating the conduction band of wide band gap semiconductors with mobile excess electrons under visible light irradiation. However, the development of efficient dye-sensitized solar cells using single-crystal semiconductor electrodes was limited by the low loading of dye on the electrode surface, resulting in poor light harvesting efficiencies and therefore low photocurrent densities.3 Significant improvement of conversion efficiency for dye-sensitized solar cells (DSSCs) was achieved by replacing the planar semiconductor electrode by a mesoporous film of a nanocrystalline semiconductor which possessed a high surface area.4 The geometric surface area of this kind of mesoporous electrode was three orders of magnitudes higher than that of a planar electrode for a 10 μm thick film, allowing for large amounts of sensitizer bound to the electrode’s surface and enhancing the light harvesting efficiency.5 This turned out to dramatically improve the performance of DSSCs, which typically consist of dye-sensitized mesoporous TiO2


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electrodes in conjunction with an electrolyte containing an iodide-based redox system for dye regeneration, increasing the overall power conversion efficiency to 10 %.6 The efficiency has been optimized to exceed 12 % by applying the co-sensitization (define) technique and using a higher redox potential, cobalt-based electrolyte.7,8 However, the progress for improving light conversion efficiency has slowed in the past decade due to a few inherent challenges in DSSCs. The first of these challenges is related to the light harvesting efficiency of cells. When the light falls incident on a DSSC, the undesired processes of reflection and non-exciton-generating absorption occur both within the TiO2-coated FTO glass substrate and on its surface. These optical losses should be minimized so that the light can be efficiently absorbed by the adsorbed dye sensitizers on the TiO2 surface. Therefore, in order to optimize the light harvesting capability, it is necessary to understand the optical properties of the TiO2-coated FTO glass substrate. By applying different optical measurement techniques, one can obtain all optical properties of dye-sensitized TiO2 electrodes, including transmittance, reflectance and absorptance. According to comprehensive studies conducted by Grätzel et al.,9,10 it is important to understand the optical and electrical loss channels in order to optimize the overall power conversion efficiency of a dye-sensitized solar cell. In general, light harvesting efficiency dominates the performance of a DSSC relative to electrical losses within the cell. Following the absorption of a photon and simultaneous electronic excitation of the adsorbed dye sensitizer, the photoexcited electron can undergo a series of transport processes including the desired route for photocurrent generation: charge separation, electron injection, diffusion into TiO2 networks and eventual arrival at the FTO contact. The overall efficiency of these desired processes can be accounted for by monitoring electrical losses. Combining both the measured optical and


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electrical characteristics, one can generate a breakdown of energy loss channels in a DSSC under irradiation. As illustrated in Fig. 1, the blue arrows indicate the optical loss channels when a DSSC is under irradiation. After the capture of a fraction of the incident light by the dye sensitizer molecules (green arrow, dye absorptance) – which leads to the formation of photoexcited electrons at the dye/TiO2 –the electrical loss processes–, including injection loss and collection loss (brown arrows), take place during the photoexcited electron transportation within an irradiated cell. Ultimately, the electrical output (red arrow in Fig. 1) denotes the residual converted energy from the sunlight one can utilize in an external circuit.

Figure 1. The optical and electrical loss channels in a DSSC under solar irradiation. (figure adapted and modified from reference 9). A significant portion of incident light is lost owing to reflectance, transmittance and to absorptance that is not ascribed to the dye. The photoinduced electrons can either inject into the conduction band of TiO2 from the excited state or relax to the molecular ground state (injection loss). The injected electrons undergo a series of energy loss pathways, such as recombination with the surrounding oxidized dye sensitizer or capture at TiO2 surface trap states (collection losses).


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Previously, we prepared a series of DSSCs with different Pc 61 dye immersion times and performed electrical characterization, including I-V properties and IPCE, and excited-state sensitizer dye lifetime measurements.11

The results revealed that the cell with 120 min

immersion time had the shortest fluorescence lifetime and the highest power conversion efficiency. Based on the experimental result, we hypothesized that if the immersion time exceeded 120 min, multilayers of the dye formed on the TiO2 surfaces and non-injection layers would affect the photoinduced electron transportation and further decrease the power conversion efficiency. While measuring the I-V curves of a DSSC provides the most straightforward insight into the cell performance, optical analyses offer the other opportunity to identify and further optimize individual components within photovoltaic devices.12 In this work, we investigate the Pc 61 dye loading effects in terms of optical properties of Pc 61-TiO2 films with different immersion times, hypothesizing that this process would provide insights into the arrangement of the adsorbed Pc 61 dye molecules on the TiO2 surface. In addition, we determine the injection efficiency and collection efficiency of the Pc 61-based sensitized solar cell. Most importantly, by systematically combining the analysis of optical and electrical quantum efficiencies, the loss channels in a PV device can be identified as well as quantified. Specifically, we take special care to measure diffusely scattered photons – diffuse transmittance and diffuse reflectance – in order to fully account for all important optical loss, as was done by Korgel et al. in studying optical properties of Si and Ge nanowire fabric.13 Here, we have utilized the obtained information about the involved losses and begun to optimize the Pc 61 loaded DSSC.


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2. Experimental Section 2.1. Preparation of Pc 61-sensitized Photoelectrodes The structure of Pc 61 reported in our previous work11 is shown in Figure 2. The substrates used in this study consist of a mesoporous anatase TiO2 film, which is deposited onto microscopy glass slides and transparent conducting oxide-coated glass (fluorine-doped-tin oxide (FTO) glass, TEC 8, Hartford Glass) by a doctor-blade technique. The commercially available TiO2 paste DSL 18NR-T, purchased from Dyesol, containing 20 nm diameter particles, was used as obtained. The films were then sintered at 450 °C for 30 minutes in air with a heating rate of 2 °C /min. The resulting films had a controlled thickness of 7 μm. Prior to dye sensitization, surface protonation of TiO2 was necessary to accelerate dye adsorption and improve the overall cell performance.14-16 A typical TiO2-coated substrate was immersed in an HCl solution (pH=2) for 15 min with a substrate temperature of ~ 120 °C followed by washing with ethanol. In order to investigate the effect of the dye loading on photo-induced electron injection into DSSCs, a series of HCl treated photoanodes were immersed in a 60 μM Pc 61 dye solution for different time periods, varying from 5 min to 6 hrs. After the dipping process was complete, the dye loaded TiO2 substrates were rinsed with ethanol thoroughly and dried with compressed air. Alumina nanoparticles used as a control in this work were γ-alumina, 50 nm in diameter, from CH Instruments. The alumina paste, consisting of 33 wt% of alumina nanoparticles in ethanol/water (1:1 v/v) solution, was deposited using a doctor-blade technique on microscopy glass slides and sintered according to the same procedure as described above. The resulting opaque alumina layer was also ~ 7 μm thick and dye sensitization was carried out as for the TiO2 films.


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Si

OH

O N N

N N

Si

N N

O N

N

O

O OH

Si

Figure 2. The structure of Pc 61.11

2.2. Optical Measurements Internal quantum efficiency (IQE), also known as absorbed photons to current conversion efficiency (APCE), is an important characteristic for evaluating the performance of solar cells, indicating how efficiently absorbed photons are converted to current. Assessment of the IQE takes into account the optical losses within the cells such as transmittance and reflectance in addition to charge injection. According to the relation between incident photon to current efficiency (IPCE) and APCE, as shown in equation (1), the APCE is calculated by correcting the IPCE for the light harvesting efficiency of the dye-sensitized TiO2 electrodes.17

IPCE ( )  LHE ( )  APCE ( )  LHE ( )  inj ( ) cc ( )

(1)

Here, LHE(λ), is the light harvesting efficiency of the dye-sensitized TiO2 electrode – which depends on the extinction coefficient of the dye and the amount of adsorbed dye –ϕinj(λ) is the quantum yield for electron injection from the excited dye to the metal oxide semiconductor, and ηcc(λ) is the efficiency of collecting the injected electrons into the external circuit.18 If one assumes that light scattering is minimal, the LHE(λ) can be expressed according to the BeerLambert Law as:


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LH E     1  10

   

(2)

where A(λ) is the optical absorbtance of the dye-sensitized TiO2 films. However, the BeerLambert Law does not apply if the films scatter light and the contribution of the reflectance of real DSSCs should be considered.9 According to energy conservation, the sum of the absorptance, transmittance and reflectance of incident irradiation flux is equal to unity. Hence, the LHE(λ) can be calculated from the following equation19: LHE     A'     1  Ttotal     Rtotal   

(3)

where A′(λ) is the absorptance of the dye-sensitized TiO2 film, Ttotal(λ) is the total spectral transmittance (the fraction of incident light which passes through the sample) and Rtotal(λ) is the total reflectance (the fraction of incident light reflected and scattered off the sample). The total transmittance, Ttotal(λ), which includes both direct and diffuse transmission (Ttotal = Tdir + Tdiff), of dye-sensitized TiO2 electrodes were measured using an Ocean Optics USB2000 fiber optic UV-vis spectrometer coupled to a FOIS-1 Fiber Optic integrating sphere consisting of a 1.5” Spectralon sphere with a 9.5 mm aperture diameter, and the spectral transmittance curves were recorded by the Ocean Optics SpectraSuite software. The integrating sphere can be used to collect the transmitted light from all angles. A 150 W Xenon arc lamp with a UV filter was used as the light source. In order to conform to the real operational condition of DSSCs, the cell was oriented so that the light fell incident on the counterelectrode glass side rather than transmitting through the dye coated TiO2 layer first. According to the ASTM Standard Test Method E 903-96 (Standard Test Method for Solar Absorptance, Reflectance, and Transmittance of Materials Using Integrating Spheres), the Ttotal(λ) can be obtained by the equation (4):


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Ttotal      S  Z  100  Z 

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(4)

where Sλ is the spectrum recorded with the sample at the entrance aperture of the integrating sphere, Zλ is the zero line reading when the sample beam is blocked, and 100λ is the 100% line reading with no sample. The direct transmittance, Tdir, (the fraction of incident light passing directly through the cell surface) of dye-sensitized TiO2 electrodes was determined with a Varian Cary Bio50 UV-vis spectrometer. Subsequently, the diffuse transmittance, Tdiff, can be calculated by the difference between the total and direct transmittance. The diffuse reflectance, Rdiff(λ), of the dye-sensitized TiO2 electrodes were measured by a Varian Cary Bio50 UV-vis spectrometer equipped with an external remote diffuse reflectance accessory (Barrelino, Harrick Scientific) which allows sampling of spot sizes of 1.5 mm in diameter. The diffuse reflectance spectra in the range of 400 nm to 800 nm were corrected by using an appropriate reference baseline (MgO powder) set to 100 % total reflectance. The specular reflectance, Rspe(λ), is defined as the ratio of the reflected incident light measured at an angle of 45° from the sample relative to the same experiment using a broad band mirror as a function of wavelength. A monochromatic incident light was generated from a 150 W Xenon lamp after focusing through a monochromator (Chromex 250). The intensity of the reflected light from a mirror and the dye coated TiO2 samples were measured at 45° using a laser power meter (Coherent Fieldmate). Hence, the total reflectance, Rtotal(λ) (which includes both diffuse and specular reflectance, Rtotal = Rdiff + Rspe), of dye-sensitized TiO2 electrodes were obtained. Finally, the absorptance, A′(λ), of dye-sensitized TiO2 electrodes can be calculated from the equation (3). Taking the absorptance of pure TiO2 on FTO glass substrate as a background, the absorptance of the dye alone can be calculated. Thus, any scattering or transmission from the TiO2 film can be


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taken into account. 2.3. Injection Quantum Yield The electron injection quantum yield, ϕinj, can be expressed by

inj  kinj / (kinj  ko )

(5)

where kinj and ko are the excited state relaxation rate constants for electron injection and noninjection processes of the excited dye, respectively.20 The kinj can be obtained by calculating the difference between ktitania, the rate constant for electron injection for the dye adsorbed on titania, and the kalumina, the rate constant for the dye adsorbed on a large band gap metal oxide (alumina), which has a higher conduction band edge potential relative to the excited state of dye, thus ko = kalumina. Using the large band gap material as reference substrate, no charge injection processes take place and only self-quenching or non-radiative decay processes occur to depopulate the dye excited state. In general, these rate constants can be expressed by the inverse of the monitored emission lifetimes, τ, which were determined by time-resolved photoluminescence lifetime measurement (TRPL). Using streak camera measurements, the emission decay traces for the Pc 61 dye adsorbed on titania and alumina samples were monitored at a center wavelength of 695 ± 1 nm after 600 nm excitation and the resulting traces could be normalized and fit with a monoexponential decay function to obtain the excited state lifetimes.

3. Results and Discussions 3.1. Optical Loss Analysis 3.1.1. TiO2-coated FTO glass substrate Since the photoanode of dye-sensitized solar cell is a laminated structure, composed of a


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soda-lime glass, transparent conducting oxide such as fluorine-doped tin oxide (FTO), and a dyeimpregnated mesoporous TiO2 layer, the incident photons undergo a series of interface transitions where reflectance, transmittance and absorptance processes occur, resulting in undesired optical losses. In order to systematically understand the intrinsic optical properties of the dye-sensitized TiO2 films, it is necessary to start with the bare FTO glass substrate. Fig. 3a shows the optical properties of the FTO glass substrate, consisting of a 2.2 mm soda-lime glass with a 600 nm FTO layer coated on one side, as a function of wavelength. The total transmittance for the FTO glass is 85 % over the visible region, with the anomalous small spike shown at 760 nm in the total transmittance spectrum being an artifact from the Xenon arc lamp light source. The total reflectance of the FTO glass is around 10 % in the visible region. Hence, the calculated absorptance spectrum in Fig. 3a indicates that about 5 % of the incident light is captured by the FTO glass substrate throughout the visible spectral range. After depositing a 7 μm thick mesoporous TiO2 layer on top of the FTO glass, the total transmittance of the composite sample lowers from 85 % to 73 % and the total reflectance increases to 16 % at 400 nm, as shown in Fig. 3b. The decreasing transmittance and the increasing total reflectance are mainly due to the absorption features of anatase TiO2, which possess an energy band gap of 3.2 eV (390 nm) and the light scattering effect of the TiO2 nanoparticle interfaces, respectively.9 After immersing the TiO2-coated FTO glass in Pc 61 dye solution for 5 min, the Pc 61-TiO2 film exhibits a significant absorptance in the red region with the peak at 670 nm, corresponding to the absorption peak of the Pc 61 dye sensitizer, shown in Fig. 3c. The adsorbed Pc 61 dye sensitizers on the TiO2 surface, however, did not alter the overall reflectance spectrum of the Pc 61-TiO2 film (yellow area), which may due to the low dye loading on the TiO2 surface (5 min immersion time).


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Figure 3. The optical properties consisting of total transmittance (blue), total reflectance (yellow) and calculated absorptance (black) as a function of wavelength of different samples are presented in a stack plot. (a) a FTO glass substrate, (b) a mesoporous TiO2-coated FTO glass substrate and (c) a Pc 61-TiO2 film after 5 min dye immersion time.

3.1.2. Effect of dye loading on transmittance Based on the results shown in Fig. 3c, it is clear that the main contribution of Pc 61 to the optical properties of the composite cell is absorptance of incident light. The effect of loading times on the optical properties of the Pc 61-TiO2 films was further explored to see whether increasing dye concentration led only to increased absorptance or also to increased scattering and reflectance, potentially affecting the overall performance of the DSSSC. A series of photoanodes with different dye immersion times, varying from 5 min to 6 hrs, were prepared and


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characterized.

Figure 4. (a) Direct transmittance spectra and (b) total transmittance spectra of the samples with different dye immersion times as a function of wavelength. The gray curves indicate the spectra of TiO2/FTO glass before dye immersion which were used as control base lines.

Fig. 4a shows the direct transmittance spectra of the samples with different immersion times. The direct transmittance spectrum of the FTO glass substrate in the visible region shows oscillation phenomena originating from the interference reflections between the FTO layer and the glass substrate.21,22 At immersion times over 90 min, the direct transmittance of the Pc 61TiO2 films at around 670 nm approached zero due to the high dye loading on the TiO2 surface. Utilizing the Ocean Optics spectrometer with an integrating sphere, the total transmittance spectra of the samples with different immersion time were obtained, as shown in Fig. 4b. It presents similar features and trends as seen in the direct transmittance spectra. The percentage differences between them refer to the diffuse transmittance component, which is ascribed to the light scattering effect of the mesoporous TiO2 films. In order to further understand the contribution of the light scattering effect from Pc 61 coated TiO2 films, the diffuse transmittance spectra with different dye immersion times as a function of wavelength, shown in Fig. 5, were obtained as the difference between the total transmittance spectra (Fig. 4b) and the direct transmittance spectra (Fig. 4a). The results reveal


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that the diffuse transmittance amplitudes remained constant with increasing immersion time (after 5 min), and suggest that the increased dye loading does not contribute to more diffuse transmittance.

Figure 5. Diffuse transmittance spectra with different dye immersion times as a function of wavelength obtained as the difference of spectra in Fig 4b and spectra in Fig 4a (Tdiff = Ttotal - Tdir). The gray curve is the TiO2/FTO glass spectrum before dye immersion, which was used as a control.

3.1.3. Effect of dye loading on reflectance The diffuse reflectance spectra of the Pc 61-TiO2 films with different immersion times (Fig. 6a) were collected on the Cary Bio50 UV-vis spectrometer equipped with an external diffuse reflectance accessory. The results illustrate that for Pc 61-TiO2 films with immersion times shorter than 120 min, < 10 % of the incident light was diffusely reflected in the visible region. According to Rayleigh scattering theory, light scattering is inversely proportional to the fourth power of the wavelength, which results in more efficient scattering of blue light relative to red light as shown in Fig. 6a.23 However, when the immersion time is longer than 120 min, the Pc 61-TiO2 films show a decreasing diffuse reflectance in the blue region, which may be due to the increase in short-wavelength absorption of Pc 61. Resulting from the measurement, the


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average specular reflectance of the Pc 61-TiO2 films with different immersion times are around 5 % from 400 nm to 800 nm. The total reflectance spectra of the samples with different dye immersion times can be determined by combining the diffuse and specular reflectance together, shown in Fig. 6b. In comparing the spectra of the total transmittance (Fig. 4b) and total reflectance (Fig. 6b), the highly loaded spectra showed decreased reflectance in the blue region, which is due to more light absorption of Pc 61 on the TiO2 films.

Figure 6. (a) Diffuse reflectance spectra and (b) total reflectance spectra of the photoanode samples with different dye immersion times as a function of wavelength. The gray curves show the TiO2/FTO glass spectra before dye immersion.

3.1.4. Effect of dye loading on absorptance Using the total transmittance and total reflectance properties of the Pc 61-TiO2 films measured at different immersion times, we used the equation (3) to calculate the absorptance of these samples. Fig. 7a shows that the calculated absorptance of the Pc 61-TiO2 films with immersion times longer than 90 min at 670 nm are all ~ 80 %. The 20 % optical loss of the incident light is simply due to intrinsic losses by transmittance and reflectance of the photoanode.


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Figure 7. (a) Calculated absorptance spectra and (b) UV-Vis absorptance spectra of Pc 61-TiO2 films with different dye immersion times. The gray curve in each panel indicates the spectrum of as-prepared TiO2 coated FTO glass before dye immersion.

In order to further investigate whether aggregates or multi-layers contribute to optical losses for the Pc 61 dye sensitizer on the TiO2 surfaces, it is important to obtain the absorptance spectra of the dye alone. The absorptance spectra of the Pc 61 alone were corrected by subtraction of the absorptance of the TiO2-coated FTO glass, shown in Fig. 8. A growing shoulder at 560 nm was observed when the immersion time increased and was consistent with the absorbance spectra of Pc 61-TiO2 films in Fig. 7b. Based on previous work24 as well as structure considerations of Pc 61, aggregates of Pc 61 are not thermodynamically favored to form on the TiO2 surface due to the bulky axial ligands on each side of the phthalocyanine ring of Pc 61. However, the observed growing shoulder at 750 nm in the absorptance spectra could be due to the formation of interacting Pc 61 dye layers on the nanocystalline TiO2 surface at long deposition times. Finally, the absorption was higher at longer immersion time with a corresponding decrease in the reflectance and transmittance, indicating an increase in absorptance but with no correlation to better overall DSSC efficiency or shorter dye excited state lifetime. This is strong evidence that Pc 61 forms non-injection layers on the TiO2 surface if the immersion time exceeds 120 min, which primarily contributes to optical loss by preventing photons from reaching the active dye layer.


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Figure 8. The absorptance spectra of Pc 61 dye alone at different dye immersion time. The spectra were corrected by subtracting the absorptance of the TiO2-coated FTO glass.

3.1.5. Incident Photon-to-current Efficiency (IPCE) vs. Absorbed Photo-to-current Efficiency (APCE) In general, APCE cannot be obtained directly via a single measurement because this characteristic not only correlates to the incident photon to current conversion efficiency but also includes the inherent optical loss properties of the solar cell. The APCE value can be determined by using the absorptance of the dye-sensitized TiO2 film and IPCE value in conjunction with the equation (1). In Fig. 9, the dash-dotted spectrum exhibits the intense light harvesting efficiency in the red region, where the Pc 61 sensitizer has strong absorption (over 80 %). However, the APCE result also shows that only ~ 5 % of the absorbed photons in this spectral region are converted into electrons for the Pc 61-sensitized TiO2 photoelectrode with 120 min immersion time. This low APCE may result from the undesired electron transfer pathways in a completed DSSC, such as9: 1. Dye multilayers were formed on the TiO2 surface and light was absorbed but the photoexcited electrons did not inject into the conduction band of TiO2.


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2. The injected electrons in the TiO2 conduction band undergo back electron transfer to the ground state of the dye or recombine with the oxidized species (I3-) at the interface of TiO2/electrolyte. Hence, it is necessary to further investigate the electron injection and collection efficiency of the Pc 61-based sensitized solar cells.

Figure 9. Incident photon-to-current efficiency (IPCE; blue curve), absorbed photo-to-current efficiency (APCE; red curve), and absorptance (LHE; dash-dotted curve) of the Pc 61-TiO2 film with 120 min immersion time. The IPCE and APCE are plotted as a function of wavelength.

3.2. Electrical Loss Analysis 3.2.1. Electron injection yield In general, the time scale for photogenerated electrons to inject from the dye’s photoexcited state to the conduction band of TiO2 is in the ps to ns time range.20,25,26 Utilizing the picosecond-time-resolved photoluminescence lifetime measurements, the excited state lifetime of the dye-sensitizer on the mesoporous TiO2 films can be obtained, which provides critical insight into the photoinduced electron injection to the conduction band of the metal oxide in


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competition to the radiative relaxation back to the molecular ground state. The electron injection yield was determined based on the equation (5) by comparing the fluorescence decay rate constant (kalumina = 1/τalumina) for the dye adsorbed on alumina with that of the dye adsorbed on titania (ktitania = 1/τtitania)27:

k titania  k inj  k alumina

(6)

kinj  1  inj

(7)

where kinj is the rate constant for the electron injection from the photoexcited dye into titania. Fig. 10a shows the energy level diagram of Pc 61, TiO2 and Al2O3 and Fig. 10b illustrates the binding of the dye in a Pc 61-TiO2 film and Pc 61-Al2O3 film. The time-resolved PL decay of a Pc 61-TiO2 film and Pc 61-Al2O3 film with same immersion times (120 min) were measured in previous work.11 The PL decay could be fitted with a monoexponential decay and the obtained excited state lifetime of the Pc 61-Al2O3 film is τ*(Alumina) = 3 ns, which is three times longer than that of the Pc 61-TiO2 film τ*(Titania) = 1 ns.

Figure 10. (a) The energy level diagram of TiO2, Al2O3 and Pc 61. (b) Schematic representation of a Pc


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61-TiO2 film and Pc 61-Al2O3 film. The green arrows indicate the electron injection process. The noninjection processes (radiative (fluorescence) and non-radiative) of the excited dye are denoted by red and blue arrows, respectively. The electron lifetime of Pc 61-TiO2 and Pc 61-Al2O3 film with 120 min immersion time are 1 ns (τ*(Titania)) and 3 ns (τ*(Alumina)), respectively.11

Hence, with the equations (6) and (7), one can obtain the electron injection yield of 67 % for the Pc 61-TiO2 film with 120 min immersion time. Although the two axial ligands consisting of long carbon chains on the Pc 61 molecule can prevent Pc aggregation on the TiO2 surface, the effective distance between Pc 61 and TiO2 may result in the deserved lower overall charge injection yield. Indeed, comparison to the other literature reports for Pc derivatives adsorbed on TiO2 surfaces revealed an excess of 80 % electron injection yields,28,29 further supporting that the low injection yield of Pc 61 can be ascribed to the relatively inefficient electronic coupling between the excited states of the Pc 61 dye and the conduction band (CB) of TiO2. The electron injection process is sufficiently slow to compete kinetically with the decay of the excited state of the dye sensitizer to the ground state via fluorescence. For the Pc 61-TiO2 films, a residual fluorescence emission can be detected using a streak camera after excitation, indicating that the radiative decay process competes with the electron injection and results in the overall loss of photoinduced electron injection to the CB of TiO2. By switching from TiO2 to Al2O3 as substrate one can observe the photo-physical changes that occur on one versus the other metal oxide surface. One such change is the much wider band gap of alumina, which makes electron injection into alumina impossible. While the observed photo-physical changes from one to another surface have to be interpret with the necessary caution, one finds a striking change in photophysics when switching from TiO2 to Al2O3. The photoluminescence lifetime becomes longer by a factor three. Combined with the much wider band gap of Al2O3 it seems plausible that the extended lifetime of the sensitizer dye


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is due to the lack of electron injection into the alumina. We would like to point out that other photo physical processes, such as carrier trapping, and intersystem crossing, always can complicate a system and its analysis. Here, we assume that the major effect is due to the expected difference in electron injection into the two metal oxides. We note that an overestimation of the injection rate would also overestimate the collection losses. 3.3 Charge Collection Efficiency Using the required optical and electrical parameters for the Pc 61-sensitized solar cells including IPCE, light harvesting efficiency (LHE) and electron injection yield (ŋinj), the charge collection efficiency (ŋcc) can be further determined with equation (1).20 Using the sample with 120 min immersion time, shown in Fig. 9, the values of IPCE and LHE at 670 nm are 4.51 % and 80 % for the Pc 61-TiO2 film, respectively. Consequently, the charge collection efficiency of the sample with 120 min immersion time is ~ 8 %. The low electron collection efficiency could be due to the fact that an extremely slow electron transport takes place in mesoporous interconnected TiO2 networks, where diffusion processes tend to dominate.30 The transport of the injected electrons within TiO2 networks toward the FTO back contact can last from microseconds to milliseconds. During the diffusion process, the injected electrons are exposed and subjected to different deactivation pathways, such as recombination with the surrounding oxidized dye sensitizer or the reducible electrolyte species (I3-) on the TiO2 surfaces, as well as being captured at TiO2 surface trap states.3,31 It has been proposed that the slower diffusion rate for injected electrons transported in nanoscale TiO2 networks results in higher probability of recombination and consequently, the lower charge collection efficiency and overall power conversion efficiency of DSSCs,32,33 especially in mesoporous TiO2 nanoparticle electrodes, which possess abundant interfacial boundaries between nanoparticles which generate scattering


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centers and slow the overall electron diffusion.34 Thus, while the work to this point has demonstrated that optical losses are significant obstacles to developing efficient DSSCs, it becomes clear that improving charge collection efficiency through the preparation of novel photoanode architectures is essential for optimizing DSSC applications. This work demonstrates the facility of measuring separately the transmittance, absorbtance, and reflectance of the components of a photovoltaic device, rather than obtaining a single catchall measurement of efficiency. Separating transmittance, absorbtance, and reflectance gives insight into the type of losses being observed, thus providing evidence for deducing the causes of loss. In particular, although scattering – measured as diffuse transmittance and diffuse reflectance – is less significant in DSSCs because most light is absorbed by the uniformly distributed sensitizer dye inside the active layer,9 its effect should be incorporated into the performance evaluation of hetero-structured solar cells in which the sensitizer is not distributed as uniformly as in DSSCs, such as quantum dot sensitized solar cells.35 In the present work, it has been demonstrated that this analysis leads to concrete conclusions regarding optimal loading of organic dyes in DSSCs – a central question in DSSC research.36,37 The analysis of optical and electronic efficiencies is important for any hetero-structured solar cell which contains different functional components. The thickness of the sensitizer layer is a crucial aspect of its function, as it leads to the balance between optimal absorption of photons vs. optimal carrier diffusion. Although optical losses in DSSCs and other types of hetero-structured solar cells may result from different mechanisms, the method described herein constitutes a useful piece of the puzzle in allowing researchers to identify aspects of optical and electrical losses and by providing direct hints towards which components and step can be further optimized.


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4. Conclusions By integrating the spectral analysis demonstrated by Korgel et al.13 into the optoelectronic loss model published by Grätzel9, we show that solar cells can be studied in greater detail. We integrated a series of optical and electric test measurements, and demonstrate a systematic optical and electrical loss analysis for Pc 61-dye-sensitized solar cells. The optical analyses provide a quantification of the real absorptance of the Pc 61 dye sensitizer on the TiO2 surfaces by considering the light loss from the substrate layer, including total transmittance and reflectance. Consequently, there are two conclusive results one can extract from the obtained optical information. First, it provides evidence that Pc 61 forms non-injection layers on the mesoporous TiO2 surface if the immersion time exceeds 120 min, which mainly contributes to optical loss by preventing photons from reaching the active dye layer and results in low power conversion efficiency of the device rather than direct influencing reflectance or scattering. Second, the internal quantum efficiency, or absorbed photon to current conversion efficiency (APCE) has been determined indirectly through systematic optical and electric measurements. Besides the optical loss analyses, the electrical losses during the injection and collection processes can be determined from conducting time-resolved photoluminescence measurements and the APCE calculation. Finally, we were able to perform a quantitative and comprehensive optical and electrical loss analysis, which is significant for the development and optimization of future solar cells. Based on the results in Pc 61-based DSSCs, the low charge collection efficiency in the presented solar cell is due both to losses during photon injection as well as to the grain boundaries between TiO2 nanoparticles in the nanostructured network. Hence, it becomes clear that developing better photoanode architectures will further enhance electron transport properties and will greatly improve the overall power efficiency of future solar cells.


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Acknowledgement The authors thank Prof. Malcolm Kenney for donation of Pc 61.

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Combination of Optical and Electrical Loss Analyses for a Si

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