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Molecular packing and electronic processes in amorphous-like polymer bulk heterojunction solar cells with fullerene intercalation.

Xiao T, Xu H, Grancini G, Mai J, Petrozza A, Jeng US, Wang Y, Xin X, Lu Y, Choon NS, Xiao H, Ong BS, Lu X, Zhao N - Sci Rep (2014)

Bottom Line: In this work we carried out a systematic investigation on a high PV efficiency (>6%) BHJ system consisting of a newly developed 5,6-difluorobenzo[c] thiadiazole-based copolymer, PFBT-T20TT, and a fullerene derivative.Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and found that more efficient hole transport is achieved in the PFBT-T20TT:fullerene BHJ as compared to pure PFBT-T20TT, showing a different trend as compared to the previously reported highly crystalline polymer:fullerene blend with a similar intercalation manner.Our study correlates the fullerene intercalated polymer lamella morphology with device performance and provides a coherent model to interpret the high photovoltaic performance of some of the recently developed weakly-ordered BHJ systems based on conjugated polymers with branched side-chain.

View Article: PubMed Central - PubMed

Affiliation: Department of Electronic Engineering, Chinese University of Hong Kong, New Territories, Hong Kong.

ABSTRACT
The interpenetrating morphology formed by the electron donor and acceptor materials is critical for the performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaic (PV) cells. In this work we carried out a systematic investigation on a high PV efficiency (>6%) BHJ system consisting of a newly developed 5,6-difluorobenzo[c] thiadiazole-based copolymer, PFBT-T20TT, and a fullerene derivative. Grazing incidence X-ray scattering measurements reveal the lower-ordered nature of the BHJ system as well as an intermixing morphology with intercalation of fullerene molecules between the PFBT-T20TT lamella. Steady-state and transient photo-induced absorption spectroscopy reveal ultrafast charge transfer (CT) at the PFBT-T20TT/fullerene interface, indicating that the CT process is no longer limited by exciton diffusion. Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and found that more efficient hole transport is achieved in the PFBT-T20TT:fullerene BHJ as compared to pure PFBT-T20TT, showing a different trend as compared to the previously reported highly crystalline polymer:fullerene blend with a similar intercalation manner. Our study correlates the fullerene intercalated polymer lamella morphology with device performance and provides a coherent model to interpret the high photovoltaic performance of some of the recently developed weakly-ordered BHJ systems based on conjugated polymers with branched side-chain.

No MeSH data available.


Related in: MedlinePlus

Room temperature current vs. voltage on a log-log plot for pure PFBT-T20TT and PFBT-T20TT: PC71BM blends.
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f5: Room temperature current vs. voltage on a log-log plot for pure PFBT-T20TT and PFBT-T20TT: PC71BM blends.

Mentions: Thin-film transistor (TFT) and space charge-limited current (SCLC) measurements were used to investigate the in-plane and out-of-plane film mobilities respectively. Pure PFBT-T20TT exhibits an optimized hole mobility (μ) of 1.02 × 10−2 cm2·(V·s)−1 in field-effect transistor (FET) configurations (Figure S1, Supplementary Information). Intermixing fullerene molecules with polymer can either increase or decrease the hole mobility of the polymer by several orders of magnitude1744. It has been observed in P3HT: PCBM blends that intimate mixing of P3HT and PCBM disrupts P3HT crystallization4546, resulting in a decrease of hole mobility17. In contrast, hole mobility of PCDTBT and poly(2-methoxy-5-{3′,7′-dimethyloctyloxy}-p-phenylene vinylene) (MDMO-PPV) films are increased when fullerene derivatives are added47. To gain insights into how PC71BM intercalation affect the hole transport in PFBT-T20TT, we fabricated hole-only diodes based on both pure PFBT-T20TT and PFBT-T20TT: PC71BM BHJ. In our hole-only diodes we used MoO3/Pd (~5.3 eV) as the electron blocking contact and PEDOT: PSS (~5.0 eV) as the hole injection contact. A thin layer of MoO3 can prevent Pd diffusion into the organic layer during evaporation. Both contacts can provide large barriers for electron injection and ensure efficient hole injection. Figure 5 shows the JV characteristics of the devices, together with the fitting curves generated from the space charge limited model174447. The fitting results shown in Table 1 clearly suggest that the hole mobility increases in the blend. This can be understood by the cartoon presented in Figure 3. For the diode configuration the direction of hole transport is perpendicular to the substrates. In the pure PFBT-T20TT device the polymer backbones mostly take the edge-on orientation, and the hole transport is limited by hopping between the largely separated lamella of backbones. On the other hand, when blended with PC71BM, the orientation of PFBT-T20TT becomes randomly oriented. This morphology helps formation of more efficient percolation pathways for holes, thus resulting in enhanced hole mobility. Note here the hole mobility of 1:4 blend is improved from that of the pure polymer film, but not as high as the 1:2 blend. This is reasonable since more PC71BM loading leads to improved ordering in the amorphous PC71BM domains, which will impede the hold transport to some extent. To obtain a complete picture of the charge transport process, we also fabricated and characterized electron-only devices for pure PC71BM and PFBT-T20TT:PC71BM blends (Figure S6, Supplementary Information). The results show that although the electron mobility is reduced from 2.34 × 10−3 cm2·(V·s)−1 for pure to 1.05 × 10−3 cm2 (V·s)−1 for the 1:2 blend, the electron and hole mobility is on the same order and therefore the transport can be well balanced in the 1:2 blended PV cells.


Molecular packing and electronic processes in amorphous-like polymer bulk heterojunction solar cells with fullerene intercalation.

Xiao T, Xu H, Grancini G, Mai J, Petrozza A, Jeng US, Wang Y, Xin X, Lu Y, Choon NS, Xiao H, Ong BS, Lu X, Zhao N - Sci Rep (2014)

Room temperature current vs. voltage on a log-log plot for pure PFBT-T20TT and PFBT-T20TT: PC71BM blends.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4048884&req=5

f5: Room temperature current vs. voltage on a log-log plot for pure PFBT-T20TT and PFBT-T20TT: PC71BM blends.
Mentions: Thin-film transistor (TFT) and space charge-limited current (SCLC) measurements were used to investigate the in-plane and out-of-plane film mobilities respectively. Pure PFBT-T20TT exhibits an optimized hole mobility (μ) of 1.02 × 10−2 cm2·(V·s)−1 in field-effect transistor (FET) configurations (Figure S1, Supplementary Information). Intermixing fullerene molecules with polymer can either increase or decrease the hole mobility of the polymer by several orders of magnitude1744. It has been observed in P3HT: PCBM blends that intimate mixing of P3HT and PCBM disrupts P3HT crystallization4546, resulting in a decrease of hole mobility17. In contrast, hole mobility of PCDTBT and poly(2-methoxy-5-{3′,7′-dimethyloctyloxy}-p-phenylene vinylene) (MDMO-PPV) films are increased when fullerene derivatives are added47. To gain insights into how PC71BM intercalation affect the hole transport in PFBT-T20TT, we fabricated hole-only diodes based on both pure PFBT-T20TT and PFBT-T20TT: PC71BM BHJ. In our hole-only diodes we used MoO3/Pd (~5.3 eV) as the electron blocking contact and PEDOT: PSS (~5.0 eV) as the hole injection contact. A thin layer of MoO3 can prevent Pd diffusion into the organic layer during evaporation. Both contacts can provide large barriers for electron injection and ensure efficient hole injection. Figure 5 shows the JV characteristics of the devices, together with the fitting curves generated from the space charge limited model174447. The fitting results shown in Table 1 clearly suggest that the hole mobility increases in the blend. This can be understood by the cartoon presented in Figure 3. For the diode configuration the direction of hole transport is perpendicular to the substrates. In the pure PFBT-T20TT device the polymer backbones mostly take the edge-on orientation, and the hole transport is limited by hopping between the largely separated lamella of backbones. On the other hand, when blended with PC71BM, the orientation of PFBT-T20TT becomes randomly oriented. This morphology helps formation of more efficient percolation pathways for holes, thus resulting in enhanced hole mobility. Note here the hole mobility of 1:4 blend is improved from that of the pure polymer film, but not as high as the 1:2 blend. This is reasonable since more PC71BM loading leads to improved ordering in the amorphous PC71BM domains, which will impede the hold transport to some extent. To obtain a complete picture of the charge transport process, we also fabricated and characterized electron-only devices for pure PC71BM and PFBT-T20TT:PC71BM blends (Figure S6, Supplementary Information). The results show that although the electron mobility is reduced from 2.34 × 10−3 cm2·(V·s)−1 for pure to 1.05 × 10−3 cm2 (V·s)−1 for the 1:2 blend, the electron and hole mobility is on the same order and therefore the transport can be well balanced in the 1:2 blended PV cells.

Bottom Line: In this work we carried out a systematic investigation on a high PV efficiency (>6%) BHJ system consisting of a newly developed 5,6-difluorobenzo[c] thiadiazole-based copolymer, PFBT-T20TT, and a fullerene derivative.Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and found that more efficient hole transport is achieved in the PFBT-T20TT:fullerene BHJ as compared to pure PFBT-T20TT, showing a different trend as compared to the previously reported highly crystalline polymer:fullerene blend with a similar intercalation manner.Our study correlates the fullerene intercalated polymer lamella morphology with device performance and provides a coherent model to interpret the high photovoltaic performance of some of the recently developed weakly-ordered BHJ systems based on conjugated polymers with branched side-chain.

View Article: PubMed Central - PubMed

Affiliation: Department of Electronic Engineering, Chinese University of Hong Kong, New Territories, Hong Kong.

ABSTRACT
The interpenetrating morphology formed by the electron donor and acceptor materials is critical for the performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaic (PV) cells. In this work we carried out a systematic investigation on a high PV efficiency (>6%) BHJ system consisting of a newly developed 5,6-difluorobenzo[c] thiadiazole-based copolymer, PFBT-T20TT, and a fullerene derivative. Grazing incidence X-ray scattering measurements reveal the lower-ordered nature of the BHJ system as well as an intermixing morphology with intercalation of fullerene molecules between the PFBT-T20TT lamella. Steady-state and transient photo-induced absorption spectroscopy reveal ultrafast charge transfer (CT) at the PFBT-T20TT/fullerene interface, indicating that the CT process is no longer limited by exciton diffusion. Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and found that more efficient hole transport is achieved in the PFBT-T20TT:fullerene BHJ as compared to pure PFBT-T20TT, showing a different trend as compared to the previously reported highly crystalline polymer:fullerene blend with a similar intercalation manner. Our study correlates the fullerene intercalated polymer lamella morphology with device performance and provides a coherent model to interpret the high photovoltaic performance of some of the recently developed weakly-ordered BHJ systems based on conjugated polymers with branched side-chain.

No MeSH data available.


Related in: MedlinePlus