Enhanced photovoltaic response of organic solar cell by singlet-to-triplet exciton conversion

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Enhanced photovoltaic response of organic solar cell by singlet-to-triplet exciton

conversion

Chia-Ming Yang, Chi-Hui Wu, Hua-Hsin Liao, Kuei-Yuan Lai, Hong-Ping Cheng, Sheng-Fu Horng, Hsin-Fei Meng, and Jow-Tsong Shy

Citation: Applied Physics Letters 90, 133509 (2007); doi: 10.1063/1.2716209

View online: http://dx.doi.org/10.1063/1.2716209

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/13?ver=pdfcov Published by the AIP Publishing

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Enhanced photovoltaic response of organic solar cell by singlet-to-triplet

exciton conversion

Chia-Ming Yang, Chi-Hui Wu, Hua-Hsin Liao, Kuei-Yuan Lai, Hong-Ping Cheng, and Sheng-Fu Hornga兲

Department of Electric Engineering, National Tsing Hua University, Hsinchu, 300 Taiwan, Republic of China

Hsin-Fei Meng

Institute of Physics, National Chiao Tung University, Hsinchu, 300 Taiwan, Republic of China

Jow-Tsong Shy

Department of Physics, National Tsing Hua University, Hsinchu, 300 Taiwan, Republic of China

共Received 15 November 2006; accepted 18 February 2007; published online 29 March 2007兲 Ir complex was doped to conjugated polymers, and the photoinduced absorption of triplet excitons in host materials was examined. A greatly enhanced intersystem crossing rate was observed, despite the decrease in triplet exciton lifetime. The authors find that the steady-state triplet exciton population in host polymer would increase by an order of magnitude. Conjugated polymer/colloidal CdSe nanocrystal hybrid solar cells were fabricated and the effect of Ir-complex doping on photovoltaic response was studied. It was found that due to the enhanced singlet-to-triplet conversion, greatly enhanced photovoltaic response of these hybrid organic solar cells was observed. The results suggest that triplet solar cells may be achieved by doping conventional photovoltaic materials with transition-metal complexes. © 2007 American Institute of Physics. 关DOI:10.1063/1.2716209兴

Organic solar cells have been under intensive investiga-tion because of their promising cost effectiveness and envi-ronmental benignity. Due to strong Coulomb interaction in organic materials, photoexcited charged carriers quickly form singlet excitons.1–3These photogenerated excitons have to be dissociated before the charged carriers can be eventu-ally collected by the electrodes. Donor-acceptor-type hetero-junctions were demonstrated to be effective for exciton dissociation.4–6To achieve high power conversion efficiency, long exciton diffusion length is required for donor-acceptor heterojunction to affect efficient charge separation. The pho-togenerated singlet excitons are, however, prone to decay radiatively because of their dipole-allowed spin state, leading to a very short exciton lifetime and thus short diffusion length. The diffusion length of singlet excitons in most pho-tovoltaic materials is usually 5 – 10 nm,7,8much shorter than the absorption depth required for efficient light absorption. Although special solar cell structures, such as bulk hetero-junction, can be used to achieve both efficient light absorp-tion and charge separaabsorp-tion, they impede severely charge transport and are difficult to use in multilayered structures. Compared with singlet excitons, the radiative decay of triplet excitons is dipole forbidden. The lifetime of triplet excitons is therefore much longer than singlet excitons.9,10 Conse-quently it is advantageous to convert the photogenerated sin-glet excitons to triplet excitons for efficient solar energy har-vesting. Solar cells based on triplet material have been proposed;11 however, the triplet organic material employed has low charged carrier mobilities, and there is no direct measurement of the triplet exciton population. Low carrier mobilities will decrease charge transport and carrier collec-tion efficiency, leading to a compromised photovoltaic

per-formance. In this letter, instead of using triplet exciton ma-terials, the singlet-to-triplet exciton conversion is achieved by doping conventional conjugated polymers with transition-metal complex and the triplet exciton population is charac-terized spectroscopically. We found that upon blending Ir complex, the intersystem crossing共ISC兲 rate in polyfluorene can be greatly enhanced and the steady-state triplet exciton population would increase by an order of magnitude, leading to a greatly improved photovoltaic response.

Although most conjugated polymer related solar cells employ regio regular poly共3-hexylthiophene兲共P3HT兲 be-cause of its higher carrier mobilities, the photoinduced ab-sorption共PA兲 signal of triplet excitons overlaps with polaron peak in regioregular P3HT, and precise determination of the triplet PA is difficult. We therefore chose poly共9,9-dioctylfluorenyl-2,7-diyl兲 end capped with dimethylphenyl 共PFO兲 for PA measurements. Iridium 共III兲 tris关2-共4-totyl兲pyridinato-N,C2兴关Ir共mppy兲3兴 is added to enhance the

singlet-to-triplet exciton conversion. The Ir-complex phos-phor is blended into the polymer with various wt % in tou-lene. Both the PFO and Ir共mppy兲3 were purchased from

American Dye Source. The film thickness for PA experi-ments is 750± 50 Å. For comparison with photovoltaic re-sponse, organic/inorganic hybrid solar cells12,13 made of Ir-complex doped PFO/CdSe and Ir-Ir-complex doped P3HT/ CdSe are used. With these organic/inorganic hybrid structures, the Ir complex is expected to be confined in the conjugated polymers, and the effect of Ir-complex doping on conjugated polymers alone can be studied. The device struc-ture is conventional and consists of indium tin oxide/ poly共3,4-ethylenedioxythiophene兲poly共styrenesulfonate兲共50 nm兲/blended layer 共90 nm兲/Al. P3HT was purchased from Aldrich Inc. Colloidal CdSe nanodots are synthesized from CdO and and Se precursors with trioctylphosphine

ox-a兲_{Electronic mail: [email protected]}

APPLIED PHYSICS LETTERS 90, 133509共2007兲

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ide共TOPO兲 surfactant. The synthesized CdSe nanocrystal is purified in methane and ethane, and the TOPO surfactant is removed later by pyridine. Chloroform is used as solvent for the blended layers in the hybrid solar cells, which consists of 0.2 wt % 共1.8 wt %兲 of conjugated polymer 共CdSe兲 when there is no Ir-complex doping. The concentration dependence of singlet-to-triplet conversion on Ir complexes has been in-vestigated, and the ISC was found to be greatly enhanced for doping more than 5%.14In this experiment, an additional 0.02 wt % Ir共mppy兲3 is added to the solvent, which

consis-tutes 10 wt % as compared to the polymers.

The PA signal at 1.45 eV for triplet excitons in PFO is measured.15A 405 nm diode laser is used as the excitation source. At this wavelength, the absorption of PFO is high, whereas the absorption of Ir共mppy兲3 is weak. Although not

shown here, the PA of triplet exciton in PFO was found be weak at room temperature. The PA measurement were there-fore performed at 100 K to achieve higher signal-to-noise ratio. The room-temperature photovoltaic response is also measured with the diode laser of 405 nm wavelength at a power of 2.3 mW/ cm2_{. From the rate equation, the PA}

mag-nitude is proportional to␶T/

冑

␶T

2_␻2_{+ 1, where}_␶

Tis the triplet

exciton lifetime and ␻= 2␲f is the angular modulation

fre-quency of optical pumping.15,16 The triplet exciton lifetime

␶Tcan be determined by the crossover frequency 1 / 2␲␶Tof the lower frequency limit, for which␻␶TⰆ1, and higher fre-quency limit for which␻␶TⰇ1. Moreover, the dc component of PA is directly correlated to the steady-state population of triplet excitons.

Figure 1共a兲 shows the PA spectrum taken at 100 Hz modulation. It can be seen that the PA of triplet exciton at 1.45 eV is not affected by doping with either Ir共mppy兲3 or

organic fluorescent dye 4-共dicyanomethylene兲-2-t-butyl-6-共1,1,7,7-tetramethyljulolidyl-9-enyl兲-4H-pyran 共DCJTB兲, the latter of which does not have strong spin-orbital coupling, and was added as comparison. From Fig. 1共b兲, we observe that while the triplet exciton lifetime ␶T and steady-state population are not affected by the doping of DCJTB,␶T de-creases from 0.4 ms for pure PFO to 0.14 ms, and the steady-state triplet exciton population is increased by an or-der of magnitude upon doping of 10 wt % Ir共mppy兲3. This

implies that the large effects of Ir共mppy兲3 result from the

heavy-metal spin-orbital coupling. The decrease in triplet ex-citon lifetime is attributed to the enhanced radiative decay and the energy transfer to Ir共mppy兲₃by Dexter energy trans-fer. Since the steady-state population of triplet excitons is related to the product of their lifetime by their generation rate from ISC, the increase in the PFO triplet exciton popu-lation must be due to an even greater enhancement of ISC rate in PFO upon doping with Ir共mppy兲₃. It is remarkable that the Ir complex has a tremendous effect on the ISC of the host polymer, besides their possible roles as carrier traps. Therefore, it may be possible to dope conjugated polymers of high carrier mobilities for enhanced singlet-to-triplet ex-citon conversion while maintaining reasonably high charge carrier mobilities.

The dark and photovoltaic responses of hybrid solar cells made from blended PFO/CdSe with and without Ir共mppy兲3

doping are shown in Fig.2. It is observed that as the dark current decreases, the short-circuit current increases by 200% with the addition of 10 wt % Ir共mppy兲₃. The decrease in dark current is attributed to the presence of Ir共mppy兲₃ trapping sites for charge transport and the increase in short-circuit current, the greatly enhanced long-lived triplet excitons. It is

FIG. 1.共a兲 PA spectra of PFO without and with doping of Ir共mppy兲3and

DCJTB. The PA spectrum is taken at 100 K with modulation frequency of 100 Hz.共b兲 Frequency-dependent PA signal of pure PFO and PFO doped with Ir共mppy兲3and DCJTB. While the doping with DCJTB shows no

ef-fects, the doping with Ir共mppy兲₃leads to decrease in triplet exciton lifetime from 0.4 to 0.14 ms and ten times increase in the steady-state triplet exciton population. The inset in共a兲 are the chemical structures of PFO, Ir共mppy兲3,

and DCJTB.

FIG. 2. Dark and photovoltaic responses of hybrid solar cells made from PFO/CdSe with and without Ir共mppy兲3, showing 200% increase in

short-circuit current and 50% increase in open-short-circuit voltage upon doping of Ir共mppy兲3.

133509-2 Yang et al. Appl. Phys. Lett. 90, 133509共2007兲

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also notable that the open-circuit voltage is increased by 50%. The increase in both short-circuit current and open-circuit voltages leads to much enhanced photovoltaic re-sponse for the PFO/CdSe hybrid solar cells with addition of Ir共mppy兲3.

Figure 3 shows the dark and photovoltaic responses of hybrid solar cells made from blended P3HT/CdSe with and without Ir共mppy兲3doping. Again, the short-circuit current is

found to increase by 100% upon the addition of 10 wt % Ir共mppy兲3, revealing a much increased population of triplet

excitons. The dark current is found to increase with Ir-complex doping. It is to note that, in contrast to the case of PFO, Ir共mppy兲3, with electron affinity 共EA兲 and ionization

potential 共IP兲 being 2.4 eV and 5.4 eV respectively, would not affect charge trapping in P3HT 共EA=3.0 eV, IP = 5.1 eV兲. By reducing the formation of short-lived singlet excitons from injected charge carriers, the enhanced ISC with Ir共mppy兲3 doping may thus lead to increase in dark

current. It also suggests that the carrier mobilities in P3HT may not suffer much with Ir-complex doping. Although we are not able to measure the triplet PA signal from P3HT blended with Ir共mppy兲3directly, we expect similar

enhance-ment of ISC and thus singlet-to-triplet exciton conversion. The enhancement of photovoltaic response of P3HT/CdSe hybrid solar cells is therefore expected. It is also noteworthy that Ir共mppy兲₃is highly phosphorescent and may not be the optimal choice for solar energy harvesting. Nevertheless, our results clearly suggest the possibility of enhanced

singlet-to-triplet exciton conversion by doping conventional photovol-taic materials with transition-metal complexes. A large vari-ety of possible combinations is therefore available to optimize the solar cell design. Moreover, with this approach, multilayered organic solar cells that make use of long-lived triplet excitons may be feasible.

To sum up, we analyzed the PA signal of triplet excitons in PFO and found an order of magnitude increase in the steady-state triplet exciton population upon the addition of Ir共mppy兲3. This increase in triplet exciton population is

at-tributed to the increase in ISC rate by Ir共mppy兲3. Hybrid

solar cells made of PFO/CdSe and P3HT/CdSe with the ad-dition of Ir共mppy兲₃ were fabricated, and great enhancement in their photovoltaic response was observed.

This work is supported by the National Science Council with project “Study on conjugated polymer related nano-structured solar cells.” The authors would like to thank H. S. Chen from Material and Chemical Research Laboratories at Industrial Technology Research Institute for the help of syn-thesis with CdSe nanodots.

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133509-3 Yang et al. Appl. Phys. Lett. 90, 133509共2007兲