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Two-step photon up-conversion solar cells

View Article: PubMed Central - PubMed

ABSTRACT

Reducing the transmission loss for below-gap photons is a straightforward way to break the limit of the energy-conversion efficiency of solar cells (SCs). The up-conversion of below-gap photons is very promising for generating additional photocurrent. Here we propose a two-step photon up-conversion SC with a hetero-interface comprising different bandgaps of Al0.3Ga0.7As and GaAs. The below-gap photons for Al0.3Ga0.7As excite GaAs and generate electrons at the hetero-interface. The accumulated electrons at the hetero-interface are pumped upwards into the Al0.3Ga0.7As barrier by below-gap photons for GaAs. Efficient two-step photon up-conversion is achieved by introducing InAs quantum dots at the hetero-interface. We observe not only a dramatic increase in the additional photocurrent, which exceeds the reported values by approximately two orders of magnitude, but also an increase in the photovoltage. These results suggest that the two-step photon up-conversion SC has a high potential for implementation in the next-generation high-efficiency SCs.

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Two-step photon up-conversion current at biased conditions.(a) Current-voltage curve obtained with light illumination. The black and blue lines correspond to irradiation by a 780 nm LD and a 1,300 nm LD, respectively. The magenta line indicates the result obtained with irradiation by both the 780 nm and 1,300 nm LDs. The inset shows the magnification of the open-circuit voltage. All light sources operated in the continuous-wave mode without any optical choppers. Dependence of up-converted characteristics of (b) the short-circuit current (ΔJsc) and (c) open-circuit voltage (ΔVoc) on the 1,300 nm LD excitation power. The solid lines indicate fitted curves determined by a model proposed in the Methods section. The error bars represent the standard error in the measurements. (d) Dependence of evaluated n value on the reverse-bias voltage according to the relationship, ΔJsc ∝ Pexn, where Pex is the 1,300 nm excitation power density. (e) Comparison of the ΔJsc-ΔVoc relation between increasing the 1,300 nm LD power density and increasing the temperature. ΔJsc and ΔVoc in Fig. 7e indicate the difference in Jsc and Voc against the values (black circle) measured by a 780 nm LD with the intensity of 47 mW cm−2 at 290 K.
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f7: Two-step photon up-conversion current at biased conditions.(a) Current-voltage curve obtained with light illumination. The black and blue lines correspond to irradiation by a 780 nm LD and a 1,300 nm LD, respectively. The magenta line indicates the result obtained with irradiation by both the 780 nm and 1,300 nm LDs. The inset shows the magnification of the open-circuit voltage. All light sources operated in the continuous-wave mode without any optical choppers. Dependence of up-converted characteristics of (b) the short-circuit current (ΔJsc) and (c) open-circuit voltage (ΔVoc) on the 1,300 nm LD excitation power. The solid lines indicate fitted curves determined by a model proposed in the Methods section. The error bars represent the standard error in the measurements. (d) Dependence of evaluated n value on the reverse-bias voltage according to the relationship, ΔJsc ∝ Pexn, where Pex is the 1,300 nm excitation power density. (e) Comparison of the ΔJsc-ΔVoc relation between increasing the 1,300 nm LD power density and increasing the temperature. ΔJsc and ΔVoc in Fig. 7e indicate the difference in Jsc and Voc against the values (black circle) measured by a 780 nm LD with the intensity of 47 mW cm−2 at 290 K.

Mentions: Next, we studied TPU phenomena at biased conditions using two-colour photo-excitations. Figure 7a shows typical current–voltage curves obtained for the TPU-SC with InAs QDs with illumination from the 780 nm LD and the additional 1,300 nm LD. The excitation power density of the 780 nm LD was 110 mW cm–2. When only the 1,300 nm LD was used for the excitation, no changes were observed in the photocurrent and photovoltage, indicating that the below-gap photons for GaAs do not cause non-linear two-photon absorption in GaAs. When irradiated by the 780 nm LD, the TPU-SC produces both photocurrent and photovoltage and the 780 nm photons traverse Al0.3Ga0.7As and excite GaAs. The excited electrons drift towards n-Al0.3Ga0.7As and are obstructed at the hetero-interface. The accumulated electrons at the hetero-interface are partially extracted by thermal and tunnelling processes and thus generate electric power. By adding the 1,300 nm LD illumination, we observed an obvious enhancement in the photocurrent; for a density of 320 mW cm–2, the photocurrent increased by 0.6 mA cm–2. This value is rather high and approximately two orders of magnitude greater than previously reported values, as described in the Discussion section. Generally, the intraband excitation strength is proportional to the electron density in the initial state. Because of the carrier separation in the internal electric field, extremely long-lived electrons are densely accumulated at the hetero-interface and fill all the confinement states of the InAs QDs and the wetting layer. Here, it must be noted that we confirmed an increase in the photovoltage by adding the 1,300 nm LD illumination. This demonstrates that TPU enhances quasi-Fermi level splitting, which is a key feature that characterizes the operation of the TPU-SC. When irradiated by the 780 nm LD, the SC operates only in the GaAs region, and the open-circuit voltage is predominantly limited by GaAs. TPU populates electrons in Al0.3Ga0.7As and consequently, the quasi-Fermi levels split further.


Two-step photon up-conversion solar cells
Two-step photon up-conversion current at biased conditions.(a) Current-voltage curve obtained with light illumination. The black and blue lines correspond to irradiation by a 780 nm LD and a 1,300 nm LD, respectively. The magenta line indicates the result obtained with irradiation by both the 780 nm and 1,300 nm LDs. The inset shows the magnification of the open-circuit voltage. All light sources operated in the continuous-wave mode without any optical choppers. Dependence of up-converted characteristics of (b) the short-circuit current (ΔJsc) and (c) open-circuit voltage (ΔVoc) on the 1,300 nm LD excitation power. The solid lines indicate fitted curves determined by a model proposed in the Methods section. The error bars represent the standard error in the measurements. (d) Dependence of evaluated n value on the reverse-bias voltage according to the relationship, ΔJsc ∝ Pexn, where Pex is the 1,300 nm excitation power density. (e) Comparison of the ΔJsc-ΔVoc relation between increasing the 1,300 nm LD power density and increasing the temperature. ΔJsc and ΔVoc in Fig. 7e indicate the difference in Jsc and Voc against the values (black circle) measured by a 780 nm LD with the intensity of 47 mW cm−2 at 290 K.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
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f7: Two-step photon up-conversion current at biased conditions.(a) Current-voltage curve obtained with light illumination. The black and blue lines correspond to irradiation by a 780 nm LD and a 1,300 nm LD, respectively. The magenta line indicates the result obtained with irradiation by both the 780 nm and 1,300 nm LDs. The inset shows the magnification of the open-circuit voltage. All light sources operated in the continuous-wave mode without any optical choppers. Dependence of up-converted characteristics of (b) the short-circuit current (ΔJsc) and (c) open-circuit voltage (ΔVoc) on the 1,300 nm LD excitation power. The solid lines indicate fitted curves determined by a model proposed in the Methods section. The error bars represent the standard error in the measurements. (d) Dependence of evaluated n value on the reverse-bias voltage according to the relationship, ΔJsc ∝ Pexn, where Pex is the 1,300 nm excitation power density. (e) Comparison of the ΔJsc-ΔVoc relation between increasing the 1,300 nm LD power density and increasing the temperature. ΔJsc and ΔVoc in Fig. 7e indicate the difference in Jsc and Voc against the values (black circle) measured by a 780 nm LD with the intensity of 47 mW cm−2 at 290 K.
Mentions: Next, we studied TPU phenomena at biased conditions using two-colour photo-excitations. Figure 7a shows typical current–voltage curves obtained for the TPU-SC with InAs QDs with illumination from the 780 nm LD and the additional 1,300 nm LD. The excitation power density of the 780 nm LD was 110 mW cm–2. When only the 1,300 nm LD was used for the excitation, no changes were observed in the photocurrent and photovoltage, indicating that the below-gap photons for GaAs do not cause non-linear two-photon absorption in GaAs. When irradiated by the 780 nm LD, the TPU-SC produces both photocurrent and photovoltage and the 780 nm photons traverse Al0.3Ga0.7As and excite GaAs. The excited electrons drift towards n-Al0.3Ga0.7As and are obstructed at the hetero-interface. The accumulated electrons at the hetero-interface are partially extracted by thermal and tunnelling processes and thus generate electric power. By adding the 1,300 nm LD illumination, we observed an obvious enhancement in the photocurrent; for a density of 320 mW cm–2, the photocurrent increased by 0.6 mA cm–2. This value is rather high and approximately two orders of magnitude greater than previously reported values, as described in the Discussion section. Generally, the intraband excitation strength is proportional to the electron density in the initial state. Because of the carrier separation in the internal electric field, extremely long-lived electrons are densely accumulated at the hetero-interface and fill all the confinement states of the InAs QDs and the wetting layer. Here, it must be noted that we confirmed an increase in the photovoltage by adding the 1,300 nm LD illumination. This demonstrates that TPU enhances quasi-Fermi level splitting, which is a key feature that characterizes the operation of the TPU-SC. When irradiated by the 780 nm LD, the SC operates only in the GaAs region, and the open-circuit voltage is predominantly limited by GaAs. TPU populates electrons in Al0.3Ga0.7As and consequently, the quasi-Fermi levels split further.

View Article: PubMed Central - PubMed

ABSTRACT

Reducing the transmission loss for below-gap photons is a straightforward way to break the limit of the energy-conversion efficiency of solar cells (SCs). The up-conversion of below-gap photons is very promising for generating additional photocurrent. Here we propose a two-step photon up-conversion SC with a hetero-interface comprising different bandgaps of Al0.3Ga0.7As and GaAs. The below-gap photons for Al0.3Ga0.7As excite GaAs and generate electrons at the hetero-interface. The accumulated electrons at the hetero-interface are pumped upwards into the Al0.3Ga0.7As barrier by below-gap photons for GaAs. Efficient two-step photon up-conversion is achieved by introducing InAs quantum dots at the hetero-interface. We observe not only a dramatic increase in the additional photocurrent, which exceeds the reported values by approximately two orders of magnitude, but also an increase in the photovoltage. These results suggest that the two-step photon up-conversion SC has a high potential for implementation in the next-generation high-efficiency SCs.

No MeSH data available.


Related in: MedlinePlus