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Polymer Photovoltaic Cells with Rhenium Oxide as Anode Interlayer.

Wei J, Bai D, Yang L - PLoS ONE (2015)

Bottom Line: It is found that insertion of ReO3 interfacial layer results in the decreased performance for P3HT: PCBM based solar cells.The results indicated the fact that a portion of ReO3 decomposed during thermal evaporation process, resulting in the formation of a buffer layer with a lower work function.As a consequence, a higher energy barrier was generated between the ITO and the active layer.

View Article: PubMed Central - PubMed

Affiliation: School of Management, Tianjin University of Technology, Tianjin, China.

ABSTRACT
The effect of a new transition metal oxide, rhenium oxide (ReO3), on the performance of polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend as buffer layer was investigated. The effect of the thickness of ReO3 layer on electrical characteristics of the polymer solar cells was studied. It is found that insertion of ReO3 interfacial layer results in the decreased performance for P3HT: PCBM based solar cells. In order to further explore the mechanism of the decreasing of the open-circuit voltage (Voc), the X-ray photoelectron spectroscopy (XPS) is used to investigate the ReO3 oxidation states. Kelvin Probe method showed that the work function of the ReO3 is estimated to be 5.13eV after thermal evaporation. The results indicated the fact that a portion of ReO3 decomposed during thermal evaporation process, resulting in the formation of a buffer layer with a lower work function. As a consequence, a higher energy barrier was generated between the ITO and the active layer.

No MeSH data available.


Optical transmittance of ReO3 film with different thickness on quartz substrate.
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pone.0133725.g003: Optical transmittance of ReO3 film with different thickness on quartz substrate.

Mentions: The J-V characteristics curve for the devices under an intensity of 100 mW/cm2 white light illumination with different thickness of ReO3 layer is shown in Fig 2. The detailed results are also given in Table 1. The J-V curve for device with ITO/PEDOT: PSS as buffer layer is also shown for comparison. The device with PEDOT: PSS buffer layer provides a significant improvement in the device performance. The PCE is 4.15%, with Jsc = 13.54 mA/cm2, Voc = 0.60 V, and FF = 51.1%. Furthermore, the insertion a 0.2 nm ReO3 layer between PEDOT: PSS and the active layer results in the decrease in Jsc of 12.64 mA/cm2. The Voc decreases to 0.58V, and FF of 46.4%. The overall PCE is therefore 3.40%. The PCE of the device with 1 nm ReO3 is 2.9%, with Jsc = 12.04 mA/cm2, Voc = 0.55V, and FF = 43.8%. Consequently, the PCE decreases significantly, rising from 4.15% to 2.9%, a 30% decrease compared with the control devices. Fig 2b shows the incident photon-to-current conversion efficiency (IPCE) curves (or the external quantum efficiency (EQE) spectra) of devices with different thickness of ReO3 interfacial layer. The IPCE spectra of photovoltaic cells compare very well with those previously reported for P3HT: PCBM blend films. The IPCE results are in agreement with the measured performance. Interestingly, the reported work function of the ReO3 and PEDOT: PSS is about 6.0 eV and 5.2 eV, respectively. Theoretically, the performance of the device using ReO3 should be better than that of the control device. The observed result is very different from what we have expected. Therefore, we also fabricated devices consisting only ReO3 (without PEDOT: PSS) as a buffer layer. The PCE of the optimum device with 10 nm ReO3 layer is 2.8%, with Jsc = 13.6 mA/cm2, Voc = 0.45V, and FF = 53.6%. For organic BHJ solar cells, their maximum value of Voc max depends on the characteristics of the organic/metal contacts. The maximum Voc for the devices with ohmic contacts is governed by the energy difference between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor. On the other hand, in the case of non-ohmic contacts, Voc max is given by the difference in work functions between the anode and the cathode, which follow the metal-insulator-metal model [12]. In our devices, the donor and acceptor materials are the same for all devices. The difference in work functions between the anode (ΦITO = 4.7eV) and the cathode (ΦAl = 4.28 eV) is 0.42 eV, which is much closer to the measured Voc = 0.45V. To clarify the origin of the Voc decrease in the devices, the optical absorption for ReO3 film with different thickness on quartz substrate was measured by an ultraviolet-visible spectrometer. The results are shown in Fig 3. The absorption spectrum is very similar. The average optical transmittance was higher than 98% in the visible range (380-780nm). This small difference could not attribute to more than 30% Voc decrease in our experiment. Therefore, the only possible explanation for the observed phenomena may be caused by reduction in the efficiency of the carrier collection at electrodes. We suspected that a decomposition reaction of ReO3 was happened during thermally evaporation, resulting in the formation of a buffer layer with a lower work function. As a consequence, a higher energy barrier was generated between the ITO and the active layer. Kelvin Probe method was used to measure the work function of the ITO substrate coated with PEDOT and ReO3, it is revealed that the WF is decrease from 5.20 eV for ITO/PEDOT: PSS to 5.13 eV after thermal evaporation of ReO3. Ultraviolet photoelectron spectroscopy (UPS) was also performed to verify the accuracy of the WF of ReO3. UPS spectra of ReO3 in S1 Fig shows that the WF of ReO3 is about 4.94 eV, which is very closely with the results obtained by Kelvin Probe method.


Polymer Photovoltaic Cells with Rhenium Oxide as Anode Interlayer.

Wei J, Bai D, Yang L - PLoS ONE (2015)

Optical transmittance of ReO3 film with different thickness on quartz substrate.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4520519&req=5

pone.0133725.g003: Optical transmittance of ReO3 film with different thickness on quartz substrate.
Mentions: The J-V characteristics curve for the devices under an intensity of 100 mW/cm2 white light illumination with different thickness of ReO3 layer is shown in Fig 2. The detailed results are also given in Table 1. The J-V curve for device with ITO/PEDOT: PSS as buffer layer is also shown for comparison. The device with PEDOT: PSS buffer layer provides a significant improvement in the device performance. The PCE is 4.15%, with Jsc = 13.54 mA/cm2, Voc = 0.60 V, and FF = 51.1%. Furthermore, the insertion a 0.2 nm ReO3 layer between PEDOT: PSS and the active layer results in the decrease in Jsc of 12.64 mA/cm2. The Voc decreases to 0.58V, and FF of 46.4%. The overall PCE is therefore 3.40%. The PCE of the device with 1 nm ReO3 is 2.9%, with Jsc = 12.04 mA/cm2, Voc = 0.55V, and FF = 43.8%. Consequently, the PCE decreases significantly, rising from 4.15% to 2.9%, a 30% decrease compared with the control devices. Fig 2b shows the incident photon-to-current conversion efficiency (IPCE) curves (or the external quantum efficiency (EQE) spectra) of devices with different thickness of ReO3 interfacial layer. The IPCE spectra of photovoltaic cells compare very well with those previously reported for P3HT: PCBM blend films. The IPCE results are in agreement with the measured performance. Interestingly, the reported work function of the ReO3 and PEDOT: PSS is about 6.0 eV and 5.2 eV, respectively. Theoretically, the performance of the device using ReO3 should be better than that of the control device. The observed result is very different from what we have expected. Therefore, we also fabricated devices consisting only ReO3 (without PEDOT: PSS) as a buffer layer. The PCE of the optimum device with 10 nm ReO3 layer is 2.8%, with Jsc = 13.6 mA/cm2, Voc = 0.45V, and FF = 53.6%. For organic BHJ solar cells, their maximum value of Voc max depends on the characteristics of the organic/metal contacts. The maximum Voc for the devices with ohmic contacts is governed by the energy difference between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor. On the other hand, in the case of non-ohmic contacts, Voc max is given by the difference in work functions between the anode and the cathode, which follow the metal-insulator-metal model [12]. In our devices, the donor and acceptor materials are the same for all devices. The difference in work functions between the anode (ΦITO = 4.7eV) and the cathode (ΦAl = 4.28 eV) is 0.42 eV, which is much closer to the measured Voc = 0.45V. To clarify the origin of the Voc decrease in the devices, the optical absorption for ReO3 film with different thickness on quartz substrate was measured by an ultraviolet-visible spectrometer. The results are shown in Fig 3. The absorption spectrum is very similar. The average optical transmittance was higher than 98% in the visible range (380-780nm). This small difference could not attribute to more than 30% Voc decrease in our experiment. Therefore, the only possible explanation for the observed phenomena may be caused by reduction in the efficiency of the carrier collection at electrodes. We suspected that a decomposition reaction of ReO3 was happened during thermally evaporation, resulting in the formation of a buffer layer with a lower work function. As a consequence, a higher energy barrier was generated between the ITO and the active layer. Kelvin Probe method was used to measure the work function of the ITO substrate coated with PEDOT and ReO3, it is revealed that the WF is decrease from 5.20 eV for ITO/PEDOT: PSS to 5.13 eV after thermal evaporation of ReO3. Ultraviolet photoelectron spectroscopy (UPS) was also performed to verify the accuracy of the WF of ReO3. UPS spectra of ReO3 in S1 Fig shows that the WF of ReO3 is about 4.94 eV, which is very closely with the results obtained by Kelvin Probe method.

Bottom Line: It is found that insertion of ReO3 interfacial layer results in the decreased performance for P3HT: PCBM based solar cells.The results indicated the fact that a portion of ReO3 decomposed during thermal evaporation process, resulting in the formation of a buffer layer with a lower work function.As a consequence, a higher energy barrier was generated between the ITO and the active layer.

View Article: PubMed Central - PubMed

Affiliation: School of Management, Tianjin University of Technology, Tianjin, China.

ABSTRACT
The effect of a new transition metal oxide, rhenium oxide (ReO3), on the performance of polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend as buffer layer was investigated. The effect of the thickness of ReO3 layer on electrical characteristics of the polymer solar cells was studied. It is found that insertion of ReO3 interfacial layer results in the decreased performance for P3HT: PCBM based solar cells. In order to further explore the mechanism of the decreasing of the open-circuit voltage (Voc), the X-ray photoelectron spectroscopy (XPS) is used to investigate the ReO3 oxidation states. Kelvin Probe method showed that the work function of the ReO3 is estimated to be 5.13eV after thermal evaporation. The results indicated the fact that a portion of ReO3 decomposed during thermal evaporation process, resulting in the formation of a buffer layer with a lower work function. As a consequence, a higher energy barrier was generated between the ITO and the active layer.

No MeSH data available.