Integrating a Triplet-triplet Annihilation Up-conversion System to Enhance Dye-sensitized Solar Cell Response to Sub-bandgap Light

Summary

An integrated device, incorporating a dye-sensitized solar cell and triplet-triplet annihilation up-conversion unit was produced, affording enhanced light harvesting, from a wider section of the solar spectrum. Under modest irradiation levels a significantly enhanced response to low energy photons was demonstrated, yielding a record figure of merit for dye-sensitized solar cells.

Abstract

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents and hence higher efficiencies. Photon up-conversion by way of triplet-triplet annihilation (TTA-UC) is an attractive technique for using these otherwise wasted low energy photons to produce photocurrent, while not interfering with the photoanodic performance in a deleterious manner. Further to this, TTA-UC has a number of features, distinct from other reported photon up-conversion technologies, which renders it particularly suitable for coupling with DSC technology. In this work, a proven high performance TTA-UC system, comprising a palladium porphyrin sensitizer and rubrene emitter, is combined with a high performance DSC (utilizing the organic dye D149) in an integrated device. The device shows an enhanced response to sub-bandgap light over the absorption range of the TTA-UC sub-unit resulting in the highest figure of merit for up-conversion assisted DSC performance to date.

Introduction

Dye-sensitized solar cells (DSCs) have been proclaimed as a promising concept in affordable solar energy collection^1-3. In spite of this enthusiasm, widespread commercialization has yet to occur. A number of reasons have been put forward for this, with one pressing issue being the relatively high energy of the absorption onset, limiting the achievable light harvesting efficiency of these devices⁴. Although this can be overcome, lowering the absorption onset is typically accompanied by a drop in open circuit voltage, which disproportionately erodes any gains in current density^{5, 6}.

The general operation of DSCs involves electron transfer from a photoexcited dye to a semiconductor (typically TiO₂), followed by the regeneration of the oxidized dye by a redox mediator. Both these processes appear to require substantial driving forces (potential) in order to proceed with high efficiency⁷. With such significant inherent losses, it becomes obvious that the optimal absorption onset for these devices is reasonably high in energy. Similar problems exist for organic photovoltaics (OPV), due once again to the large chemical driving forces required for effective charge separation. Accordingly, predictions of upper solar-to-electric conversion efficiency limits to single junction devices based on both of these technologies involve absorbers with wide (effective) band gaps⁴.

In order to overcome the light harvesting issue raised above, a number of approaches have been taken. This includes the ‘third generation’⁸ approaches of tandem structures^{9, 10} and photon upconversion^11-14.

Recently¹¹ we reported an integrated device composed of a DSC working and counter electrode, with a triplet-triplet annihilation based up-conversion (TTA-UC) system incorporated into the structure. This TTA-UC element was able to harvest red light transmitted through the active layer and chemically convert it (as described in detail below) to higher energy photons which could be absorbed by the active layer of the DSC and generate photocurrent. There are two important points to note about this system. Firstly, TTA-UC has many prospective advantages over other photon upconversion systems¹¹; secondly it demonstrates a feasible architecture (proof-of-principle) for the incorporation of TTA-UC, which had been lacking from the TTA-UC literature up to that point.

The process of TTA-UC^15-24 involves the excitation of ‘sensitizer’ molecules, in this case Pd porphyrins, by light with energy below the device onset energy. The singlet-excited sensitizers undergo rapid intersystem crossing to the lowest-energy triplet state. From there, they can transfer energy to a ground-state triplet-accepting ‘emitter’ species such as rubrene, as long as the transfer is allowed by free energy²⁵. The first triplet state of rubrene (T₁) is greater than half the energy of its first excited singlet state (S₁) but less than half the energy of T₂, meaning that an encounter complex of two triplet-excited rubrenes can annihilate to give one singlet excited emitter molecule (and the other in the ground state) with a fairly high probability. Other states, statistically predicted, are most likely energetically inaccessible for rubrene²⁶. The singlet excited rubrene molecule can then emit a photon (as per fluorescence) with energy sufficient to excite the dye on the working electrode of the DSC. This process is shown in Animation 1.

TTA-UC offers a number of advantages compared to other UC systems, such as a broad absorption range and incoherent nature^{27, 28}, making it an attractive option for coupling with DSC (as well as OPV). TTA-UC has been demonstrated operating at relatively low light intensities and in diffuse lighting conditions. Both DSC and OPV are most efficient in the low light intensity regime. Solar concentration is expensive and only justifiable for high efficiency, high cost devices. The relatively high performance of TTA-UC systems in low intensity lighting conditions is attributable to the process involving sensitizer chromophores with strong, broad absorption bands in concert with long-lived triplet states which are capable of diffusing in order to come into contact with interacting species. In addition, TTA-UC has been found to have high intrinsic efficiency from a kinetic study²⁶.

Although TTA-UC operates at low light intensity, there is still a quadratic relationship between incident light intensity and emitted light (at least at low light intensities). This is due to the bimolecular nature of the process. To account for this and the varied experimental conditions (particularly light intensity) reported by different groups, a figure of merit (FoM) system should be employed to meter the performance enhancement offered by upconversion. This FoM has been defined as ΔJ_SC/ʘ, where ΔJ_SC is the increase in short circuit current (usually determined by integration of the Incident Photon to Charge Carrier Efficiency, IPCE, with and without the upconversion effect) and ʘ is the effective solar concentration (based on the photon flux in the relevant region, that is the Q-band absorption of the sensitizer)²²⁹.

Herein, a protocol for producing and correctly characterizing an integrated DSC-TTA-UC device is reported, paying special attention to potential pitfalls in device testing. It is hoped that this will serve as a basis for further work in this field.

Protocol

1. DSC Fabrication

1.1. Working Electrode Preparation

1. Clean one whole sheet of F:SnO₂ coated glass (110 mm × 110 mm × 2.3 mm, <8 Ω/□) by sonication sequentially in soapy water, then acetone and finally ethanol (10 min each).
2. Deposit a dense layer of TiO₂ following the steps below:
   1. Dry glass using compressed air and heat glass to 450 °C on hotplate (conductive side up).
   2. Dilute Titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol) with ethanol in a 1:9 ratio.
   3. Spray the dilute solution onto heated glass from a distance of ~100 mm, with five sprays across the glass sheet.
   4. Spray one round per 10 sec for 12 rounds.
   5. Keep glass at 450 °C for a further 5 min, before switching off the hotplate. Leave the glass on hotplate and allow it to slowly cool to RT.
3. Place the glass onto the screen printer table (once again conductive side up). Insert screen and align pattern to glass. Add TiO₂ paste to screen and print one or two layers. If depositing two layers, remove glass plate from printer between prints, cover and allow to settle for ~5 min, then heat to 125 °C for 10 min before returning to the printer to print a subsequent layer.
4. Once the final print is made run a full sinter program. Heat the electrodes to 150 °C at 12.5°/min, hold 10 min, then to 325 °C at 11.7 °/min, hold 5 min, then to 375 °C at 10 °/min, hold 5 min, then to 450 °C at 10.7 °/min, hold 30 min and finally to 500 °C at 10 °/min, hold 15 min. Slowly cool to RT after this.
5. Cut the master plate into individual electrodes ensuring there is sufficient room around the printed film for the gasket to be applied after. Remove any glass shards using compressed air.
6. Immerse electrodes in a 20 mM TiCl₄ solution (aq.), cover loosely and place container in a preheated oven (70 °C for 30 min). Subsquently,  wash the electrodes thoroughly and sinter once more at 500 °C for 30 min.
7. Once cooled to below 100 °C, immerse the electrodes in a 0.5 mM dye solution. In this case use, D149 in acetonitrile:tertbutanol (1:1).
8. After dyeing O/N remove the electrodes and rinse vigorously in acetonitrile for ~30 sec then allow to sit for a further 30 sec. Withdraw electrodes from the rinsing bath and dry with compressed air.

1.2. Counter Electrode Preparation

1. Cut another sheet of 2.3 mm F:SnO₂ glass into 18.3 mm × 27.5 mm pieces.
2. Immerse counter electrode in water and drill a small hole in the corner (φ = 1 mm, 2.5 mm from each corner) to use as a filling port, using a diamond tipped dental burr mounted in a small bench drill.
3. Clean counter electrode as per section 1.1.1
4. Dry counter electrode and place on a tile with conductive side up. Apply one drop of platinic acid solution (H₂PtCl₆, 10 mM in ethanol) and spread with the end of a pipette. Place tile onto preheated (400 °C) hotplate for 15 min. After this, remove glass and tile and allow to cool on a bench.

1.3. Reflector

1. Cut a piece of nonconductive 2 mm glass to 18.3 mm × 27.5 mm and drill two holes in adjacent corners along the long edge, using the same technique as for the counter electrode (section 1.2.2).
2. Clean glass, once using the same protocol as above (1.1.1)
3. Tape the clean, dry glass to the bench on three sides, using low residue tape. Apply a drop of Al₂O₃ paste (2.0 g of 0.3 µm Al₂O₃ particles, 2 ml colloidal Al₂O₃ + 1 ml ethanol) and draw down with a glass rod.
4. Allow film to dry, remove tape and sinter glass at 500 °C for 30 min.

1.4. Device Assembly

1. Cut two batches of hot melt adhesive gaskets.
   NOTE: The first, for the DSC, is 25 µm thick and has internal dimensions of 17 mm × 8 mm and external dimensions of 21 mm × 12 mm. The sec, for the upconversion chamber, uses 60 µm gasket material doubled over to give 120 µm thickness. When folded, this gasket has internal dimensions of 17 mm × 21 mm and external dimensions of 21 mm × 25 mm.
2. Place the first gasket in the corner of the counter electrode, ensuring the filling port is accessible. Place the working electrode over this, such that the printed area is entirely inside the gasket, and obtain a good seal.
3. Move this assembly to a hotplate (120 °C) and apply pressure until gasket softens and melts, which can be observed visually as the gasket wets the glass surfaces. Remove assembly and allow to cool.
4. Place second gasket on reflector, once again ensuring filling ports are not covered. Place DSC on top such that the printed area is directly in front of the printed alumina reflector. Once again heat device while applying pressure, until gasket softens and adheres, as in section 1.4.3. This assembly is shown in Figure 1.

1.5. Filling Cavities

1. Prepare an electrolyte solution of 0.1 M LiI, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide and 0.05 M iodine in methoxypropionitrile.
2. Place the device in a small plastic container with vacuum tube attached, with the counter electrode facing upwards.
3. Put a drop of the electrolyte solution over the hole and a piece of glass on top. Apply vacuum for a few sec to extract air from the DSC cavity, before releasing, which will draw electrolyte into the cavity.
4. Prepare the seals by laminating hot melt gasket material onto aluminum foil. Leave these on a hotplate, gasket material side up. Clean the back of the counter electrode thoroughly, then seal by pressing device against the gasket material for ~5 sec.
5. Prepare TTA-UC solution by dissolving 0.6 mM of Pd dye (tetrakis(3,5-di-tert-butylphenyl)-6’-amino-7’-nitro-tetrakisquinoxalino[2,3-b'7,8-b''12,13-b'''17,18-b''''-porphyrinato) palladium(II)) and 22 mM of rubrene in benzene. Deaerate this solution thoroughly using three liquid nitrogen freeze-pump-thaw cycles.
6. Inside a glovebox, introduce the TTA-UC solution into the back cavity, allowing capillary forces to draw it through. Once full, once again clean the surface thoroughly and seal using another piece of aluminum backed gasket material.

2. Measurement

2.1. Electrical Contacts

1. Apply solder to exposed F:SnO₂ of working and counter electrodes using sonic soldering iron and appropriate solder.
2. Attach wires to anode and cathode using normal solder.
3. Apply UV curable epoxy to open edges.
   NOTE: This is done to serve as a secondary encapsulation of the device against oxygen ingress and solvent evaporation, as well as increasing the robustness of the device, particularly the wire attachment.
4. Attach the anode and cathode wire to an open-ended BNC cable through a terminal block.

2.2. IPCE Measurement Setup

1. Using the setup shown schematically in Figure 2, mount the integrated device onto a cell holder.
2. Illuminate a section of the integrated device (~2 mm × 1 mm) with a 670 nm continuous wave laser beam (the ‘pump beam’) via a mirror on an adjustable mount.
3. Illuminate the integrated TTA-UC DSC with incoherent quasi-monochromatic light (the ‘probe beam’) generated using an Xe lamp, passed first through a 405 nm longpass filter, then a chopper wheel operating at 29 Hz, a monochromator, an angled glass slide (used here as a ~4% beam splitter) and a parabolic mirror. Generate a background triplet population in the TTA-UC layer by exciting the UC layer with the pump beam, which is incident at such an angle that it does not illuminate the probed DSC active layer but the UC layer only.
4. Align the pump and probe beam on the TTA-UC layer using the adjustable mirror mount. Measure the short circuit current generated by the probe as it is scanned across the visible spectrum in 5 nm increments using a dynamic signal acquisition device, current amplifier and in-house control software.
5. Simultaneously record the power variation of the probe beam reflected from the glass slide with a power meter and a photodiode with analog output fed to the signal acquisition device. Correct the J_SC from the device by the probe variation in the software.
6. Displace the pump beam slightly using the adjustable mirror mount, such that it hits the active layer of the device adjacent to the probe beam. Repeat the measurement with the pump and probe beam misaligned.
7. Record six sets of measurements with alignment and misalignment at the same positions for better signal to noise ratio.
8. Reduce the pump beam intensity by placing on the pump beam different neutral density filters with known transmissions at 670 nm, and repeat steps 2.2.4 to 2.2.7 for a range of intensities.
9. Measure the integrated device J_SC without the pump beam source active.
10. Measure the probe power incident on the DSC in terms of current generated by the photodiode by placing the photodiode at the sample position.
11. Measure the transmission of the studied device with the UC chamber removed using a UV Visible Spectrophotometer to obtain the transmission spectrum, T_DSC.
    NOTE: This may be alternately done in between steps 1.4 and 1.5.

2.3. Pump Source Characterization

1. Measure the pump beam power at the DSC position for each filtering condition used, using the photodiode and power meter (as described in section 2.2.10).
2. Take a photograph of the pump beam projecting onto a piece of grid paper at a position equivalent to where the TTA layer was during the experiment. Heavily attenuate the beam if necessary to prevent saturation of the camera detector. Use this image and image analysis software to determine the pump spot size.

3. Data processing

3.1. Interpolate All Data to 1 nm Increments.

3.2. IPCE Determination

1. Calculate the photon flux (φ) reaching the integrated device from the probe power measured as current generated by the photodiode (I) and electric charge (q):
2. Calculate the cell incident photon to converted electron efficiency (IPCE₀) of the device from the J_SC measurement without pump illumination and the probe flux.
3. Take the ratios between measurements with pump and probe aligned and misaligned to obtain the relative enhancements from activating the up-convertor.
4. Solar concentration factor determination
5. Convert the extinction coefficient of the sensitizer into absorption cross section, σ.
6. Obtain the excitation rate of the sensitizer under the standard AM1.5G solar spectrum, (k_φ) by taking the products of photon flux density from the solar spectrum, transmittance of the DSC and the sensitizer (σ) at each wavelength and then summing the products across the sensitizer Q-band absorption, typically 600 nm to 750 nm.
7. Calculate, from the powers and spot size of the pump source, the photon flux densities of the pump with different neutral density filters. Then take the products of the flux densities, transmittance of the DSC and the sensitizer at 670 nm to obtain the pump excitation rates.
8. Calculate the solar concentration factor (ʘ) from the ratio of pump excitation rate to the excitation rate under AM1.5G conditions.

3.3. Model Fitting and Figures of Merit Determination

1. Fit a model of the relative enhancement = 1 + constant × (T_DSC/IPCE₀) × [(σ_pump× σ_probe ) / (σ_pump + σ_probe)], onto the experimental enhancement results, where σ_pump and_σprobe are cross-sections with respect to the pump and probe wavelengths; σ_pump is fixed for each pump intensity and σ_probe varies with wavelength.
2. Estimate the enhancement in J_SC obtained from the upconversion effect (ΔJ_SC) from the differences between IPCE_UC and IPCE₀ and the solar flux density.
3. Calculate the FoM by normalizing ΔJ_SC by the square of solar concentration factor, since TTA-UC has a quadratic dependence on power input at low excitation intensity.

Results

Figures 3A - D display enhancement responses measured under different measurement conditions, with the effects discussed in more detail below. From the raw current density enhancements it should be clear that the results in Figure 4A and 4B are attributable to upconversion, with the peak current enhancement and IPCE enhancement matching well with the absorption spectrum of the sensitizer, attenuated by transmission through the active layer of the DSC.

Discussion

This protocol provides a means to achieve photon up-conversion enhanced DSC and detail on how to correctly measure such a device. The FoM allows for the simple calculation of anticipated ΔJ_SC improvements to be expected at different light intensities, including at 1 sun. The values shown here are invariant with light intensity (inset of Figure 4), as per expectation when the system is below its saturation threshold³³. With the FoM, we can standardize the enhancement effect of T...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
(tetrakis(3,5-di-tert-butylphenyl)-6’-amino-7’-nitro-tetrakisquinoxalino[2,3-b'7,8-b''12,13-b'''17,18-b''''-porphyrinato) palladium(II))	in house	in house	Chem. Commun., 4851–4853 (2007)
1,2-dimethyl-3-propylimidazolium iodide	Solaronix	33150	Material warning: Irritant
405 nm longpass filter	Semrock	BLP01-405R-25	-
670 nm laser	Thorlabs	LDS5 + CPS198	-
Acetone	Chemsupply	AA008-20L-P	Material warning: Flammable
Acetonitrile	Sigma	271004	Material warning: Flammable
Alumina	Alfa Aesar	12733
Alumina	Leeco	810-782
Back filling chamber	Sistema	1303	Klip it round, modified
Benzene	Scharlau	BE0033	Material warning: Toxic
BNC cable	Jaycar	RG- 59U
Cerasolzer	MBR	CS186
Chopper wheel	Thorlabs	MC1000A
Control software	in house	in house	Written in LabVIEW
Current Amplifier	Standford Research	SR 570
D149 dye	1m	OSO149
Dental burr	Priority dental supplies	835.104.008
Detergent	Palmolive	Original
Diamond wheel	Frameco	14220
Drill	Dremmel	220
Dynamic dignal acquisition device	National Instruments	USB-4431	Analog to Digital
Ethanol	Univar	214	Material warning: Flammable
Glovebox	IT systems
H2PtCl6	Sigma	334472	Material warning: corrosive
Hot melt adhesive gasket	Solaronix	Meltronic 1170-25	Surlyn
Hot melt adhesive gasket	Solaronix	Meltronix 1170-60	Surlyn
Hotplate	Harry Gestigkeit	PR 5 3T / PZ28-3T
Hotplate	IKA	RCT basic
Image analysis software	National Institutes for Health	Image-J
Iodine	Sigma	326143	Material warning: corrosive
Laser engraver	Universal Laser Systems	PLS6WM
Liquid Nitrogen	Air Liquide
Lithium Iodide	Aldrich	518018	Material warning: toxic
Methoxypropionitrile	Sigma	65290	Material warning: Flammable
Mirror	Thorlabs	PF10-03-P01
Mirror mount	Thorlabs	KM100
Monochromator	Spectral Products	CM110
Neutral density filters	Edmund Industrial Optics	64-352
Parabolic mirror	Newport	50329AL, 50338AL
Photodiode	Newport	918D-UV-OD3
Power meter	Newport	1936-C
Rubrene	Sigma	551112
Semi-automatic screen printer	Keywell	KY-500FH
Spray pyrolyser	Glaskeller
Tape	3M	Magic Tape
Terminal block	Jaycar	HM3194
tert-Butanol	Sigma	360538	Material warning: Flammable
TiCl4	Sigma	89545	Material warning: corrosive
Tile	Johnson tiles
Tile cutter	DTA	DTA-310
TiO2 paste	Dyesol	NR18-T	-
Titanium diisopropoxide bis(acetylacetonate) (75% in isopropanol)	Aldrich	325252	Material warning: Flammable
Ultrasonic soldering iron	MBR	USS-9200
UV cure epoxy	Dymax	425	Material warning: Irritant
UV cure system	Dymax	BlueWave 50
UV Visible Spectrophotometer	Varian Cary	1E
Vacuum cuvette	Custom made	Custom made
Vacuum pump		Rotary backed diffusion pump
Wipes	Kimtech	34120KC	Kimwipes
Xe lamp	Energetiq	LDLSTM EQ-1500	White light source