Efficiency Enhancement of Organic and Thin-Film Silicon Solar Cells

Article pubs.acs.org/JPCC

Efficiency Enhancement of Organic and Thin-Film Silicon Solar Cells with Photochemical Upconversion Tim F. Schulze,† Jens Czolk,‡ Yuen-Yap Cheng,† Burkhard Fückel,† Rowan W. MacQueen,† Tony Khoury,† Maxwell J. Crossley,† Bernd Stannowski,§ Klaus Lips,⊥ Uli Lemmer,‡ Alexander Colsmann,‡ and Timothy W. Schmidt*,† †

School of Chemistry, The University of Sydney, NSW 2006, Australia Light Technology Institute, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany § Competence Centre Thin-Film- and Nanotechnology for Photovoltaics Berlin (PVcomB), Helmholtz-Zentrum Berlin für Materialien und Energie, 12489 Berlin, Germany ⊥ Institute for Silicon Photovoltaics, Helmholtz-Zentrum Berlin für Materialien und Energie, 12489 Berlin, Germany ‡

ABSTRACT: The efficiency of thin-film solar cells with large optical band gaps, such as organic bulk heterojunction or amorphous silicon solar cells, is limited by their inability to harvest the (infra)red part of the solar spectrum. Photochemical upconversion based on triplet−triplet annihilation (TTA-UC) can potentially boost those solar cells by absorbing sub-bandgap photons and coupling the upconverted light back into the solar cell in a spectral region that the cell can efficiently convert into electrical current. In the present study we augment two types of organic solar cells and one amorphous silicon (a-Si:H) solar cell with a TTA-upconverter, demonstrating a solar cell photocurrent increase of up to 0.2% under a moderate concentration (19 suns). The behavior of the organic solar cells, whose augmentation with an upconverting device is so-far unreported, is discussed in comparison to aSi:H solar cells. Furthermore, on the basis of the TTA rate equations and optical simulations, we assess the potential of TTA-UC augmented solar cells and highlight a strategy for the realization of a device-relevant current increase by TTA-upconversion. organic bulk heterojunction (OPV)13,14 devices, potentially offer cheaper energy than commercially available crystalline silicon solar cells.15 They require less material to manufacture, and offer the possibility of reel-to-reel manufacturing.12,16 However, many single-junction thin-film cells make inefficient use of the red part of the solar spectrum due to their comparably large band gaps. These cells could potentially benefit from photonic upconversion, i.e., the physical process whereby low energy photons are converted to higher energy photons. There are coherent phenomena known to achieve this, such as second harmonic generation,17 but there are also processes which do not rely on coherent light sources and are thus potentially operative under the solar spectrum.18,19 Of these, photochemical upconversion based on triplet−triplet annihilation in organic molecules20,21 offers the spectral versatility22,23 required for application. Here, a sensitizer species absorbs light and crosses to a triplet state, the energy of which is rapidly and efficiently transferred to a second species, the emitter. The emitter triplet states are roughly half the energy of its first

1. INTRODUCTION Solar energy offers the opportunity for our society to make the inevitable transition toward environmental sustainability.1 It is abundant, with several kilowatt-hours delivered per square meter per day, on average. This energy can be converted directly to electricity by photovoltaics cells,2 or stored in solar fuels such as hydrogen.3 In both of these cases, however, an energy threshold in the light-harvesting material which serves to create the photovoltage needed to drive a current or chemical reaction, in turn leads to inefficient use of the solar spectrum. Indeed, of the energy loss mechanisms suffered by photovoltaic cells with a single threshold in the visible part of the spectrum, the most prominent is the transmission of subthreshold photons.4 This effect, and the thermalization of energy in excess of the band gap lead to single threshold photovoltaic devices being limited to an energy conversion efficiency of 33.7% under the standard AM1.5G solar spectrum.5,6 By introducing multiple energy thresholds, the limiting energy conversion efficiency of both organic7−9 and inorganic solar cells can be increased, the latter type reaching efficiencies exceeding 40%.10,11 However, these multijunction devices are inherently more complex and expensive to manufacture. Thin film solar cells, such as amorphous silicon12 and in particular © 2012 American Chemical Society

Received: September 28, 2012 Published: October 3, 2012 22794

dx.doi.org/10.1021/jp309636m | J. Phys. Chem. C 2012, 116, 22794−22801

The Journal of Physical Chemistry C

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excited singlet state, so when two such triplets, |T1⟩|T1⟩, combine to create a supramolecular singlet, internal conversion to the |S1⟩|S0⟩ state occurs to yield subsequent upconverted fluorescence.24 This process is illustrated in Figure 1. Though a

Figure 2. Molecular structures of the sensitizer (PQ4PdNA) and emitter (rubrene) species. Figure 1. Schematic of the energy levels and processes involved in triplet−triplet annihilation upconversion: After absorption of a low energy photon with energy hν1, the sensitizer molecule undergoes immediate intersystem crossing (ISC) to its first excited triplet state T1. Ensuing fast triplet electronic energy transfer (TET) stores this energy in the first triplet state of the emitter molecule, sacrificing a small amount of energy to drive the transfer process. Two emitters coming together then facilitate triplet−triplet annihilation (TTA), bringing about one emitter molecule in its first excited singlet state S1, which leads to fluorescence at energy hν2.

cPQ4PdNA = 0.9−1.5 × 10−3 M and crubrene = 0.5−1.1 × 10−2 M. The preparation of the upconverter solution consisted of degassing in a custom-made glass cuvette by at least three freeze−pump−thaw cycles (∼10−6 mbar) to prevent quenching of the triplet states by molecular oxygen. In all measurements presented here we partly filled the cuvette containing the upconverter solution with 100 μm-diameter Ag-coated glass spheres. As a consequence, the UC solution is restrained to reside in the roughly 100 μm sized cavities between the reflective beads, which helps to multipass the incoming light to increase absorption, while also improving the out-coupling of upconverted light. The front of the thus-prepared UC cuvette was optically coupled to the solar cell with a thin film of immersion oil (Sigma-Aldrich, n20 D = 1.516). 2.2. Solar Cells. The successful application of an upconverting device to a solar cell requires a device architecture with transparent contacts. In the past, various concepts have been studied for the fabrication of semitransparent organic solar cells. While transparent bottom electrodes can easily be realized from indium tin oxide (ITO) or conductive polymers, the deposition of transparent top-electrodes appears much more challenging. One approach for the deposition of a transparent top-electrode is sputtering of transparent conductive oxides such as ITO or aluminum doped zinc oxide (ZnO:Al) in conjunction with an ultrathin alkali salt interlayer.32−34 Previously, transparent top electrodes from the highly conductive polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) have been utilized35,36 as well as semitransparent top electrodes from ultrathin metal layers.37 In this work, due to the special demands of augmentation with a TTA-upconverter, we utilize a semitransparent organic solar cell device architecture comprising an ITO bottom cathode and a metal oxide/metal/metal oxide top electrode. As illustrated in Figure 3, semitransparent polymer solar cells were fabricated at KIT from ITO-coated glass samples (sheet resistance R□ = 13 Ω/□), structured with hydrochloric acid and subsequently cleaned with acetone and isopropanol. The samples were exposed to an oxygen plasma (60 W, 2 min), before the zinc oxide buffer layer was spin coated from a filtered zinc acetylacetonate hydrate/ethanol solution (50 °C, 20 mg/ mL, Sigma-Aldrich) according to the process description in ref 38. The photoactive blend comprising poly[[9-(1-octylnonyl)9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] and [6,6]-phenyl C71-butyric acid methyl ester (PCDTBT:PC71BM, 1:4 blend ratio, 20 mg/ mL solid content, PCDTBT supplied by Konarka, PC71BM from Solenne BV) was spin coated from dichlorobenzene solution resulting in a 70 nm thick active layer. Alternatively, a

quadratic process at low efficiency, once a certain triplet concentration is reached, the triplet decay is dominated by bimolecular reactions and the response on illumination density changes to linear.25−28 We recently demonstrated that the external quantum efficiency (EQE) of a hydrogenated amorphous silicon (aSi:H) solar cell can be enhanced by several percent in the spectral region where the porphyrin sensitizer material absorbs.29,30 However, the employed solar cells were not of state-of-the-art quality with conversion efficiencies of about 2%, and had low transmittance in the absorption range of the sensitizer. Therefore, the upconversion figure of merit (FOM), i.e., the current increase by upconversion at the illumination density equal to 1 sun, can be improved by employing a cell with a higher peak EQE, and a higher transmission in the red part of the spectrum. In this article, we describe the augmentation of two types of semitransparent organic solar cells, having high transmission of red light and yet comparably high conversion efficiency, with a TTA-upconverter consisting of a palladium porphyrin sensitizer and rubrene as emitter. These results are compared to a semitransparent p-i-n a-Si:H cell with significantly improved conversion efficiency as compared to the previous studies. We show that the upconversion FOM can indeed be improved with a judicious choice of cell. Furthermore, we demonstrate that our model for the upconversion-assisted EQE30 is robust and predicts the spectral responses of these very different cell/UC combinations equally well. Finally we assess the potential of TTA-UC assisted thin-film cells and highlight a strategy for reaching device-relevant current enhancements.

2. EXPERIMENTAL SECTION 2.1. Upconversion Materials. As in our previous study,30 we used nitroaminopalladiumtetrakis porphyrin (PQ4PdNA) as sensitizer and rubrene as the emitter (structures shown in Figure 2), dissolved in toluene. Rubrene was used as purchased from Sigma-Aldrich, while PQ4PdNA was synthesized in house.31 The concentrations of the molecules in toluene were 22795



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case, the higher overall transmission was achieved by using a smooth 800 nm thick ZnO:Al front TCO and employing a high-bandgap μc-SiOx:H p-layer. These measures also allowed to increase the i-layer thickness to 135 nm, while maintaining the thicknesses for the highly doped p- and n-layers (10 and 20 nm, respectively), and the transparent ZnO:Al back contact (300 nm). As a result, these new a-Si:H solar cells had both increased transmission in the (infra)red part of the solar spectrum, and a higher peak EQE and overall conversion efficiency. With illumination through the glass and employing white paint as a simple Lambertian back reflector, respectable conversion efficiencies of 7.5% were reached (3.9% for illumination through the ZnO/(n)a-Si:H side without back reflector). 2.3. Measurement and Analysis Scheme. As the upconversion process is nonlinear for low illumination densities,25−28 a simple EQE measurement is insufficient to characterize a solar cell/UC assembly. Indeed, even employing a high-power white light source, the monochromated EQE probe beam typically has photon fluxes corresponding to ≪1 sun, and therefore the linear response of the intrinsically quadratic UC process is negligible. To remedy this issue, the upconverter is illuminated by a continuous laser beam at the peak absorption wavelength of the sensitizer (670 nm) during the lock-in detection of the EQE, in order to create a background triplet concentration and thus increase the yield of upconverted photons. Note that due to the weak absorption of the solar cells in that spectral range, the photocurrent itself is not significantly biased by the red laser. However, to eliminate any effect of the 670 nm bias on the solar cell response, we did not switch off the laser to measure the EQE without UC contribution, but laterally displaced the 670 nm pump beam on the active area, thus misaligning it with the EQE probe beam. Thereby, the solar cell still sees the same illumination conditions, but within the area probed by the EQE measurement the UC signal is negligible. A sketch of the EQE setup is shown in Figure 4, showing the aligned laser and probe beam.

Figure 3. Sketch of the solar cell architectures employed in the present study. Left: Organic bulk heterojunction solar cells. Right: Hydrogenated amorphous silicon solar cells. Note that the glass encapsulation of the semitransparent OPV cells allowed attachment of the upconverter either to the ITO cathode side (a), or the Ag anode side (b) of the solar cells. The 40 nm MoO3 layer is only present in the P3HT:ICBA cells. The upconverter unit is optically connected to the solar cell glass substrate/encapsulant by means of immersion oil.

blend from poly(3-hexylthiophene-2,5-diyl) and 1′,1″,4′,4″tetrahydro-di[1,4]methanonaphthaleno[5,6]fullerene-C 60 (P3HT:ICBA, 1:1 blend ratio, 40 mg/mL solid content, P3HT from Rieke Metals, ICBA from Solenne BV) was spin coated from dichlorobenzene solution, solvent annealed and subsequently thermally treated (150 °C) resulting in a 180 nm thick active layer. Then a 10 nm molybdenum trioxide (MoO3) interlayer and a semitransparent 13 nm silver anode were thermally evaporated in vacuum at a base pressure below 3 × 10−6 mbar. For the P3HT:ICBA solar cells a light coupling layer of 40 nm MoO3 was evaporated on top of the silver anode under the same conditions. The encapsulated PCDTBT:PC71BM solar cells were measured under AM1.5G irradiation from a Newport 300 W solar simulator. Very good power conversion efficiencies of up to 3.8% under illumination through the ITO cathode and 2.4% for illumination through the silver anode were achieved. P3HT:ICBA solar cells exhibited power conversion efficiencies of up to 3.1% and 1.7%, respectively. For comparison to the OPV cells, we prepared inorganic thin-film solar cells made of hydrogenated amorphous silicon. In our previous studies29,30 we used semitransparent a-Si:H p-in solar cells with i-layer thicknesses of 100 nm. This number was limited by the required transmission in the absorption region of the sensitizer, due to (i) high intrinsic absorption in the low-bandgap p-layer, and (ii) a smooth onset of the subbandgap transmission caused by the scattering of light from the textured front TCO, leading to longer effective pathlengths in the solar cell. The resulting thin i-layer led to rather poor conversion efficiencies of the cells of around 2 ± 0.2%. Here we aimed for a higher transmission in the 670 nm region while maintaining or increasing the peak EQE values. Again, semitransparent a-Si:H p-i-n solar cells were deposited at PVComB, employing the same equipment as before.29 In this

Figure 4. EQE setup as used to measure the upconversion-augmented solar cells. Light from a white-light source (WL) is long-pass filtered (F), monochromated (M), chopped at 118 Hz (C) and focused by lenses (L) onto the solar cell/upconverter unit (OPV/UC or a-Si:H/ UC). A glass slide (G) is used to couple part of the light onto a photodiode power meter (PM) to monitor its intensity. 670 nm light from a laser diode (LD) is focused onto the same spot on the solar cell by means of lenses (L) and a beam splitter (BS). A λ/2 plate is used to control the polarization and thus the reflectance of the BS. The contribution of the 670 nm light bias and chopped EQE probe beam to the solar cell current signal are deconvoluted by means of lock-in detection. 22796



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To calculate the current enhancement that would be brought about at normal solar cell oparation − i.e. at 1 sun −, we need to take into account the effective number of suns imposed by this light bias. This is done by matching the rate of sensitizer excitation by the 670 nm laser to that which would be brought about by a certain concentration of the AM1.5G spectrum, as filtered by the solar cell. The rate of excitation of the porphyrins, kϕ(⊙), by 1 sun (⊙) is calculated by multiplication of the AM1.5G solar spectrum, ΦAM1.5G, in photons cm−2 s−1 nm−1 by the transmission of the solar cell, TSC, and integrating the product of this with the sensitizer absorption cross section, σ in cm2 kϕ⊙ =

∫ ΦAM1.5G(λ)TSC(λ)σ(λ) dλ

(1)

We calculated values of kϕ⊙ = 2.0−2.7 s−1, depending on the choice of solar cells. The irradiation Ib of the bias in photons per area per time is used to calculate the experimental pump rate, i.e., kϕb = σ(670 nm)TSC(670 nm)Ib. The ratio C = kϕb/ kϕ⊙ then gives the effective solar concentration C, which ranged between 17 and 30 suns in the present case. We compare the solar cell performance with and without upconverter by taking the ratio between the EQE taken with the 670 nm beam aligned with the EQE probe beam (EQEUC), and the EQE with misaligned UC pump beam (EQE0). In analyzing the organic solar cells, it has to be taken into account that these cells show a faint dependence of the photocurrent on the spatial positions of the EQE probe beam and the 670 nm bias laser on the active area. Resulting from that, there is a slight global scaling of the EQE0 curve with respect to EQEUC, brought about by misaligning the bias laser for taking the EQE without upconversion contribution. To correct for that, we scaled EQE0 such that the two curves match at 550 nm, well outside the wavelength range of the upconversion effect. The scaling factor ranged between 0.9937 and 1.0091 and corresponds to a baseline correction of less than 1% in the EQEUC/EQE0 ratio. In any case, the magnitude of the upconversion signal with respect to the spectral regions unaffected by upconversion is not changed by this procedure.

Figure 5. Spectral alignment of sensitizer absorption and emitter emission spectra (a) with transmission and EQE curves of employed solar cells (b, P3HT:ICBA solar cell; c, PCDTBT:PC71BM solar cell; d, a-Si:H solar cell).

upconverter in both cases. The respective upconversion signals in the EQE ratio EQEUC/EQE0 are shown in Figure 6. All solar cell/UC combinations show very pronounced EQE enhancements in the absorption range of the sensitizer molecules, with peak enhancements surpassing 12% for the P3HT:ICBA solar cell illuminated through the Ag anode. Care should be taken when assessing the amplitude of the EQEUC/EQE0 signals, as the height of the peak in the EQE ratio strongly depends on the value of EQE0 at the sensitizer absorption peak. In case of a sharp band edge of the photovoltaic absorber material, EQE0 would be zero and the EQEUC/EQE0 ratio would diverge even for a poor upconverter. This sensitivity toward the wavelength dependence of the cell performance can be readily observed in Figure 6, as a nominal identical upconverter yields very different EQEUC/EQE0 peak values. We therefore refrain from a quantitative discussion of these peak enhancements, and instead calculate a figure-of-merit for upconversion which reflects the current enhancement that would be brought about at 1 sun illumination, to be discussed in the next section. Despite this, there is a certain sense to analyzing EQEUC/EQE0, as the difference of EQEUC and EQE0being the logical alternative data setis affected by a very uneven distribution of noise: To the blue of the sensitizer absorption peak where the solar cell quantum efficiency sets in and the lamp delivers more photons, significantly larger current values have to be subtracted than to the red of the peak, leading to a larger scatter of the data. Therefore, EQEUC/EQE0 is a much cleaner set of data and provides a sound foundation for comparison to the modeled UC enhancement signal. Consistent with expectation, the form of the enhancement curve is different for each solar cell/UC combination, while the illumination

3. RESULTS AND DISCUSSION 3.1. Spectral Alignment. The goal of the solar cell design and the choice of sensitizer (PQ4PdNA) and emitter (rubrene) species was to ensure a favorable spectral alignment of solar cell transmission and spectral response with sensitizer absorption and emitter emission peaks. The result can be observed in Figure 5, where the absorption cross section of the sensitizer molecules and the fluorescence spectrum of the rubrene emitter are plotted along with the transmission and EQE curves of the employed OPV and a-Si:H solar cells. In the region of the porphyrin absorption peak, the transmission of all cells surpasses 40%, while the EQEs of the cells are at (or close to) their maximum of 50−80% where rubrene emits the upconverted photons. Note that the OPV cells exhibit an EQE which strongly depends on the illumination direction, which will be discussed below. 3.2. EQE Measurements. Using the measurement scheme described above, we measured EQEs with and without upconverting effect for the three different types of solar cells. The glass encapsulation of the OPV cells allowed them to be combined with the UC cuvette in both orientations, i.e., illuminating either through the ITO cathode or through the Ag anode, while ensuring the same optical coupling to the 22797



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imprinted on the EQE curve. Instead, the long-wavelength part of the absorption peak obtains more weight, depending on the exact form of the TSC and EQE0 curves, which explains the differing form of the curves. 3.3. Upconversion Figure-of-Merit. Next, we calculated the current enhancement that results from the augmentation of the different solar cells with the upconverter at the effective number of suns imposed by the 670 nm light bias, by the equation 780nm

ΔjSC = Ce

where e is the elementary charge and C the effective solar concentration. From ΔjSC, the upconversion figure of merit ζ is calculated as ζ = C−2ΔjSC, to take into account the effective solar concentration C and the quadratic nature of the UC process at the illumination densities employed here. The results are compiled in Table 1, stating also the cell type, the molecular concentrations, and the effective illumination density of the 670 nm bias laser. Interestingly, the upconversion FOMs differ within one order of magnitude among the different cell types and orientations. It appears that the OPV cells perform less efficiently with the upconverter as compared to the a-Si:H cells. The reason for this is the pronounced dependence of these particular devices’ EQEs on orientation which mainly stems from optical losses in the Ag electrode. In the usual operation scheme with illumination through the substrate glass and ITO cathode, the upconverter is penalized by the absorptive Ag layer on the rear which is then positioned in between the active blend and the upconverting material. This leads to a much lower FOM for this orientation, although at first glance the EQE and T spectra look almost as promising as for the a-Si:H cell. For illumination through the Ag anode on the other hand, the upconverted photons can be harvested by the cell with fewer optical losses, but with a significantly lower quantum efficiency as compared to the other case (cf. Figure 5), leading to an improved FOM as compared to the other orientation, but still somewhat lower than for the a-Si:H cell. Although an a-Si:H p-i-n cell generally benefits from illumination through the highly defective p-layer, whose inherent recombination is lowered by increased generation when being at the front of the cell, the orientation dependence is less pronounced than for the OPV cells. This explains the comparably good EQE curve and performance of the a-Si:H cell in conjunction with the upconverter, despite the setup requiring illumination of the a-Si:H cells through the n-layer as the optical coupling to the upconverter can only be done via the substrate glass in the present setup. In the future, we will investigate whether inverted a-Si:H cells and OPV cells comprising anodes with higher transparency can further

Figure 6. Relative EQE enhancement of different solar cells by TTAupconversion, and fitted model of the upconversion effect. Note that the OPV cells can be combined with the upconverter in two orientations, leading to different UC signals, as discussed in the text. Effective solar concentrations employed here were 30 suns (a), 29 suns (b), and 19 suns (c).

direction of the organic solar cells causes a global scaling. In the case of the PCDTBT:PC71PM cells, the UC signal upon illumination through the ITO is barely visible. Previously30 we have shown that the expected upconversion signal in the ratio EQEUC/EQE0 can be modeled from the known spectral response of the solar cell EQE0, its transmission TSC and the absorption cross section σ of the sensitizer molecules: EQE UC(λ) EQE0(λ)

=1+χ×

TSC(λ) σ(λ)σb EQE0(λ) σ(λ) + σb

∫λ=600nm ΦAM1.5G(λ) × (EQEUC(λ) − EQE0(λ)) dλ

(2)

Here, σb is the porphyrin absorption cross section at the wavelength of the light bias (670 nm). As a first step of analysis we applied the model of eq 2, which contains only one free parameter, χ, that incorporates the efficiency of the upconverter and its optical coupling to the solar cell. Fitting this single parameter we obtain excellent fits to all the enhancement peaks, as can be observed in Figure 6. Note that due to the transmission increasing and EQE0 decreasing with wavelength, the peak structure of the sensitizer absorption is not directly

Table 1. Current Enhancement and Upconversion Figure-of-Merit for the Different Solar Cells Employed in the Study cell type a

P3HT:ICBA P3HT:ICBAb P3HT:ICBAa PCDTBT:PC71PM PCDTBT:PC71PM (n)a-Si:H/(i)a-Si:H/(p)μc-SiOx:H a

illum. through

cPQ4PdNA, mmol

crubrene, mmol

C, ⊙

Δjsc, mA/cm2

Ag Ag ITO Ag ITO ZnO

1.3 1.4 1.3 1.3 1.3 0.9

11 35 11 11 11 13

29.5 17.3 29.5 28.9 28.9 19

0.126 0.048 0.066 0.129 0.020 0.275

FOM ζ, mA/cm2/⊙2 1.44 1.60 0.75 1.54 0.24 7.63

× × × × × ×

10−4 10−4 10−4 10−4 10−4 10−4

Cell 1. bCell 2, nominally identical to cell 1. 22798



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increase the upconversion yields, and explore more sophisticated integration schemes. The figures of merit for upconversion-assisted thin-film solar cells presented here are the highest reported. With the present result we improve our latest FOM for upconverter-enhanced aSi:H cells30 by a factor of 2.2, while as compared to our very first report29 a factor of >25 was realized by improving the sensitizer molecule, the optical coupling and the solar cells. The present results outperform a-Si:H solar cells augmented with rare-earth upconverting materials39 by a factor of >1200, as can be seen by the comparison of FOMs provided in Figure 7. The

f=

k TTANT k1 + k TTANT

describing the competing roles of the first-order decay by k1 and the second-order TTA process. While k1 and kTTA are fundamental properties of the molecules and their environment and therefore not trivially engineered, we are left to increase the emitter triplet concentration NT in order to maximize the UC yield. Assuming the regime of inefficient TTA, thus k1 ≫ kTTANT, solving eq 3 yields the following equation for the steady-state triplet concentration: NT =

application of photochemical upconversion to organic bulk heterojunction solar cells is unreported, and the comparably high FOMs for these cells reached in the present study underline the general significance of TTA-UC as a candidate mechanism for augmentation of thin-film solar cells. 3.4. Prospects for TTA-UC Applied to Solar Cells. The highest photocurrent enhancement obtained in this study corresponds to a 0.2% relative current increase at 19 suns, which still is a fairly moderate enhancement. Therefore, toward the envisioned device application, an important question to answer is whether the concept of TTA-upconversion is in principle able to yield relevant solar cell enhancementsi.e., on the order of 0.1−1 mA/cm2, with the materials at hand. To provide a comprehensive answer, we need a model which couples the TTA-upconversion dynamics as described by rate equations25,26,28,40,41 with the optics of the upconverter coupled to the solar cell. The rate equation describing the density of emitters being in their triplet states under steady state conditions is dNT = kϕNS − k1NT − k TTANT 2 dt

kϕNS k1

(5)

Again, we have limited access to the first order rate constant, but we can engineer the two quantities in the numerator. It is clear that the triplet concentration and therefore the upconversion efficiencygoverned by f from eq 4is enhanced when we increase the sensitizer excitation rate kϕ and/or the sensitizer concentration NS. We explored the significance of these quantities in a simple 1D optical model of an upconverter consisting of PQ4PdNA and rubrene in a cuvette with an ideal specular back reflector, coupled to a solar cell. We found that such arrangement exhibits an ideal thickness of the upconverting layer, which coincides with the inverse absorption coefficient 1/α = 1/(σmaxNS) of the solution at the absorption peak of the sensitizer molecule, σmax. In this configuration, the upconversion yield is increased by a factor of 2.6 (3.6 in case of a Lambertian reflector) as compared to an infinitely long cuvette.29 Further details on these simulations will be reported elsewhere. With the presently employed back reflector consisting of Ag-coated silica beads we are reasonably close to the optically optimized state.30 While these numbers indeed illustrate the need to optimize the optics of the upconverter/solar cell assembly to realize the highest kϕ with the given solar spectrum, the choice of a suited thickness and back reflector alone does not suffice to reach a relevant current increase. Taking into account the concentrations used in the measurements, rate constants as previously measured26,40 as well as the a-Si:H solar cell transmission and EQE curves, we find a theoretical maximum current increase of ζth = 1.24 × 10−3 mA/cm2 that could be expected at 1 sun for the present situation. That number compared with the FOMs reported in Table 1 shows that we have already reached about 62% of the current enhancement by UC theoretically possible with the present setup. Given the fact that the simulation does not take into account any optical losses, e.g., stemming from a lossy back reflector or imperfect coupling between UC cuvette and solar cell, this is a satisfying result. On the other hand, we found the TTA quantum efficiency averaged over the cuvette length, to be only 0.13% in this situation, which illustrates the drastic need for improvement of this number. The key quantity toward this end is the sensitizer concentration, which directly impacts the TTA efficiency through eq 4. Increasing this number by a factor of 100 (while applying the same scaling also to the emitter concentration) yields a maximum current enhancement potential of about ζth = 0.1 mA/cm2, which would immediately propel TTA-UC into the range of relevant solar cell current increase. The TTA quantum efficiency averaged over the cuvette is then already 7.3%, to be compared with the highest experimentally reported number of 30%40 and the theoretical

Figure 7. Evolution of the current improvement in thin-film solar cells by upconversion facilitated by TTA and, for comparison, by rare-earth materials39 (RE). Since our very first report, the FOM for UC applied to a-Si:H solar cells was increased by more than a factor of 25.

0=

(4)

(3)

with kϕ being the excitation rate of the sensitizer as used above, while k1 is the first-order decay constant of emitter triplets, kTTA is the TTA rate constant, NS is the sensitizer concentration and NT is the emitter triplet concentration.40 The proportion f of emitter triplets decaying via TTA is given by 22799



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HZB was funded by the BMBF and the state government of Berlin (SENBWF) in the framework of the program “Spitzenforschung und Innovation in den Neuen Ländern” (03IS2151).

maximum of 50% (as two photons are merged in the TTA process). Assuming a TTA process at 30% efficiency, the maximum current enhancement with the presently employed molecules would be ζth = 0.4 mA/cm2. One should note that unlike for rare-earth elements, where the absorption range is defined by atomic levels, molecular engineering23,31 or the simultaneous use of two sensitizer species22 could, in the present case, allow broadening of the spectral range of absorption of the sensitizer molecules, bringing about a further increase of the current enhancement.

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4. CONCLUSION In conclusion, we have augmented two semitransparent organic bulk heterojunction solar cells and one amorphous silicon solar cell with a photochemical upconverter based on triplet−tripletannihilation, demonstrating current enhancements of up to 0.2% at 19 suns. While this number is still 2−3 orders of magnitude below a device-relevant current enhancement, we have shown TTA-upconversion to be in principle able to reach that desired regime. On the basis of a rate equation modeling approach similar to refs 25 and 28, we have highlighted TTAupconversion to be highly promising for the application in thinfilm solar cells, with the sensitizer concentration being the key quantity to be optimized. The limited solubility of the molecular species in organic solvents will not allow concentrating the UC solution by a further factor of 100, but incorporating them into solid-state compounds such as nanoparticles42,43 or polymer films44−46 might allow the concentration to be increased by the required 2 orders of magnitude while maintaining the efficient TET and TTA rates.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +61 2 93 51 27 81. Fax: +61 2 93 51 33 29. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank X. Yang, M. Wright, and A. Uddin from the University of New South Wales for providing OPV test samples and for fruitful discussions. T.F.S. and B.F. acknowledge the Alexander von Humboldt-Foundation for respective Feodor Lynen fellowships. Y.C. acknowledges The University of Sydney for a Henry, Bertie and Florence Mabel Gritton Scholarship. K.L. is indebted to the Deutsche Forschungsgemeinschaft (DFG) for Grant 583727, which initiated this German−Australian bilateral cooperation. J.C., A.C., and U.L. would like to thank the DFG for funding of the project TRAPOS within the SPP 1355. A.C. acknowledges support by the Federal Ministry of Education and Research (BMBF) under contract 03EK3504 (Project TAURUS). We thank A. Stanco and Lastek for the gift of the 670 nm diode laser, M. Kaegi for making the solar cell holder, and T. Hänel (HZB) as well as S. Kirner (PVComB) for EQE measurements. This research project is funded by the Australian Solar Institute (A-023), with contributions from The New South Wales Government and The University of Sydney. Aspects of this research was supported under Australian Research Council’s Discovery Projects funding scheme (DP110103300). Equipment was purchased with support from the Australian Research Council (LE0668257). The work related to the solar cell preparation at 22800



Article

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Efficiency Enhancement of Organic and Thin-Film Silicon Solar Cells

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