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Carrier density effect on recombination in PTB7-based solar cell.

Moritomo Y, Yonezawa K, Yasuda T - Sci Rep (2015)

Bottom Line: By means of the time-resolved spectroscopy, we confirmed that the exciton-to-carrier conversion process takes place within ~1 ps at the D/A interface of the PTB7/C70 HJ device.We further determined the absolute magnitude of n by combination of the time-resolved and electrochemical spectroscopies.We confirmed that a similar behaviors is observed in the PTB7/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) bulk heterojunction (BHJ) device.

View Article: PubMed Central - PubMed

Affiliation: Graduate School of Pure and Applied Science, Univ. of Tsukuba, Tsukuba 305-8571, Japan.

ABSTRACT
Organic solar cells (OSCs) are promising alternatives to the conventional inorganic solar cells due to their low-cost processing and compatibility with flexible substrates. The development of low band-gap polymer, e.g., poly-[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3]thiophenediyl]] (PTB7), increases the power conversion efficiency (PCE) in the last decade. Here, we investigated the interrelation between the instantaneous carrier density (n) per donor (D)/acceptor (A) interface area and the carrier density (ncollected) collected as photocurrent in PTB7/C70 heterojunction (HJ) device. By means of the time-resolved spectroscopy, we confirmed that the exciton-to-carrier conversion process takes place within ~1 ps at the D/A interface of the PTB7/C70 HJ device. We further determined the absolute magnitude of n by combination of the time-resolved and electrochemical spectroscopies. We found that the carrier recombination becomes dominant if n exceeds a critical concentration (nc = 0.003 carriers/nm(-2)). We confirmed that a similar behaviors is observed in the PTB7/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) bulk heterojunction (BHJ) device. Our quantitative investigation based on the HJ device demonstrates that the fast carrier escape from the D/A interface region is indispensable for high PCE, because the carrier accumulation nonlinearly accelerates the carrier recombination process.

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(a) ΔOD spectra (open circles) of PTB7/C70 bilayer film at 0.9 ps and at 400 nm together with the spectral decomposition into acceptor exciton (A*: blue curve), donor exciton (D*: green curve), and donor carrier (D+ : red curve) components. The ΔOD spectra of the PC71BM neat, PTB7 neat, and PTB7/C70 bilayer films at 1, 1, and 10 ps are regarded as the A*, D*, and D+ components, respectively. (b) Absolute number of acceptor exciton (nA*), donor exciton (nD*) and donor carrier (nD+) per an absorbed photon against the delay time. The magnitudes of nA*, nD* and nD+ were evaluated by the spectral decomposition of the ΔOD spectra of the PTB7/C70 bilayer film. The solid curves are results of the least-squares fittings with an exponential function.
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f2: (a) ΔOD spectra (open circles) of PTB7/C70 bilayer film at 0.9 ps and at 400 nm together with the spectral decomposition into acceptor exciton (A*: blue curve), donor exciton (D*: green curve), and donor carrier (D+ : red curve) components. The ΔOD spectra of the PC71BM neat, PTB7 neat, and PTB7/C70 bilayer films at 1, 1, and 10 ps are regarded as the A*, D*, and D+ components, respectively. (b) Absolute number of acceptor exciton (nA*), donor exciton (nD*) and donor carrier (nD+) per an absorbed photon against the delay time. The magnitudes of nA*, nD* and nD+ were evaluated by the spectral decomposition of the ΔOD spectra of the PTB7/C70 bilayer film. The solid curves are results of the least-squares fittings with an exponential function.

Mentions: We investigated ΔOD spectra of PTB7/C70 bilayer, PC71BM neat, and PTB7 neat films at 400 nm (see Supplementary Fig. S3 online). The ΔOD spectra of the PC71BM (PTB7) neat film show a characteristic photoinduced absorption (PIA), which should be ascribed to the acceptor exciton (A*) [donor exciton (D*)] . The ΔOD spectra of the PTB7/C70 bilayer film show a broad PIA centered at 1150 nm. In the late stage (>10 ps), the profile of the PIA is essentially unchanged. In addition, the spectral profile is similar to that of the doping-induced spectrum of the PTB7 neat film (see Supplementary Fig. S4 online). Therefore, we ascribed the PIA to the photo-created donor carriers (D+). In the early state (<10 ps), however, an additional absorption component is observed around 1500 nm. The additional component should be ascribed to the PIAs due to A* and D*. We decomposed the ΔOD spectra (ϕexp) of the PTB7/C70 bilayer film into the components of A* (ϕA*), D* (ϕD*), and D+ (ϕD+). The ΔOD spectra of the PC71BM (PTB7) neat film at 1 ps was regarded as ϕA* (ϕD*) while the ΔOD spectra of the PTB7/C70 bilayer film at 10 ps was regarded as ϕD+. The spectral weights of the respective components were evaluated by least-squares fitting of the observed spectra (ϕexp) with the linear combination of ϕA*, ϕD*, and ϕD+ : ϕcal = CA*ϕA* + CD*ϕD* + CD+ϕD+. The coefficients, CA*, CD* and CD+, were determined so that the evaluation function, , becomes the minimum. Figure 2a show an example of the spectral decomposition at 0.9 ps. We clearly observed that the photo-excitation at 400 nm excites D* in addition to A*.


Carrier density effect on recombination in PTB7-based solar cell.

Moritomo Y, Yonezawa K, Yasuda T - Sci Rep (2015)

(a) ΔOD spectra (open circles) of PTB7/C70 bilayer film at 0.9 ps and at 400 nm together with the spectral decomposition into acceptor exciton (A*: blue curve), donor exciton (D*: green curve), and donor carrier (D+ : red curve) components. The ΔOD spectra of the PC71BM neat, PTB7 neat, and PTB7/C70 bilayer films at 1, 1, and 10 ps are regarded as the A*, D*, and D+ components, respectively. (b) Absolute number of acceptor exciton (nA*), donor exciton (nD*) and donor carrier (nD+) per an absorbed photon against the delay time. The magnitudes of nA*, nD* and nD+ were evaluated by the spectral decomposition of the ΔOD spectra of the PTB7/C70 bilayer film. The solid curves are results of the least-squares fittings with an exponential function.
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Related In: Results  -  Collection

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f2: (a) ΔOD spectra (open circles) of PTB7/C70 bilayer film at 0.9 ps and at 400 nm together with the spectral decomposition into acceptor exciton (A*: blue curve), donor exciton (D*: green curve), and donor carrier (D+ : red curve) components. The ΔOD spectra of the PC71BM neat, PTB7 neat, and PTB7/C70 bilayer films at 1, 1, and 10 ps are regarded as the A*, D*, and D+ components, respectively. (b) Absolute number of acceptor exciton (nA*), donor exciton (nD*) and donor carrier (nD+) per an absorbed photon against the delay time. The magnitudes of nA*, nD* and nD+ were evaluated by the spectral decomposition of the ΔOD spectra of the PTB7/C70 bilayer film. The solid curves are results of the least-squares fittings with an exponential function.
Mentions: We investigated ΔOD spectra of PTB7/C70 bilayer, PC71BM neat, and PTB7 neat films at 400 nm (see Supplementary Fig. S3 online). The ΔOD spectra of the PC71BM (PTB7) neat film show a characteristic photoinduced absorption (PIA), which should be ascribed to the acceptor exciton (A*) [donor exciton (D*)] . The ΔOD spectra of the PTB7/C70 bilayer film show a broad PIA centered at 1150 nm. In the late stage (>10 ps), the profile of the PIA is essentially unchanged. In addition, the spectral profile is similar to that of the doping-induced spectrum of the PTB7 neat film (see Supplementary Fig. S4 online). Therefore, we ascribed the PIA to the photo-created donor carriers (D+). In the early state (<10 ps), however, an additional absorption component is observed around 1500 nm. The additional component should be ascribed to the PIAs due to A* and D*. We decomposed the ΔOD spectra (ϕexp) of the PTB7/C70 bilayer film into the components of A* (ϕA*), D* (ϕD*), and D+ (ϕD+). The ΔOD spectra of the PC71BM (PTB7) neat film at 1 ps was regarded as ϕA* (ϕD*) while the ΔOD spectra of the PTB7/C70 bilayer film at 10 ps was regarded as ϕD+. The spectral weights of the respective components were evaluated by least-squares fitting of the observed spectra (ϕexp) with the linear combination of ϕA*, ϕD*, and ϕD+ : ϕcal = CA*ϕA* + CD*ϕD* + CD+ϕD+. The coefficients, CA*, CD* and CD+, were determined so that the evaluation function, , becomes the minimum. Figure 2a show an example of the spectral decomposition at 0.9 ps. We clearly observed that the photo-excitation at 400 nm excites D* in addition to A*.

Bottom Line: By means of the time-resolved spectroscopy, we confirmed that the exciton-to-carrier conversion process takes place within ~1 ps at the D/A interface of the PTB7/C70 HJ device.We further determined the absolute magnitude of n by combination of the time-resolved and electrochemical spectroscopies.We confirmed that a similar behaviors is observed in the PTB7/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) bulk heterojunction (BHJ) device.

View Article: PubMed Central - PubMed

Affiliation: Graduate School of Pure and Applied Science, Univ. of Tsukuba, Tsukuba 305-8571, Japan.

ABSTRACT
Organic solar cells (OSCs) are promising alternatives to the conventional inorganic solar cells due to their low-cost processing and compatibility with flexible substrates. The development of low band-gap polymer, e.g., poly-[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3]thiophenediyl]] (PTB7), increases the power conversion efficiency (PCE) in the last decade. Here, we investigated the interrelation between the instantaneous carrier density (n) per donor (D)/acceptor (A) interface area and the carrier density (ncollected) collected as photocurrent in PTB7/C70 heterojunction (HJ) device. By means of the time-resolved spectroscopy, we confirmed that the exciton-to-carrier conversion process takes place within ~1 ps at the D/A interface of the PTB7/C70 HJ device. We further determined the absolute magnitude of n by combination of the time-resolved and electrochemical spectroscopies. We found that the carrier recombination becomes dominant if n exceeds a critical concentration (nc = 0.003 carriers/nm(-2)). We confirmed that a similar behaviors is observed in the PTB7/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) bulk heterojunction (BHJ) device. Our quantitative investigation based on the HJ device demonstrates that the fast carrier escape from the D/A interface region is indispensable for high PCE, because the carrier accumulation nonlinearly accelerates the carrier recombination process.

No MeSH data available.


Related in: MedlinePlus