InGaAsP/InGaAs double-junction solar cells(DJSCs)with approximate bandgap combination of 1.0/0.75 eV are used in four-junction configuration to harvest 900~1 700 nm sunlight,and are crucially important for device performances [
Journal of Infrared and Millimeter Waves, Volume. 40, Issue 1, 7(2021)
Reducing Voc loss in InGaAsP/InGaAs dual-junction solar cells
Smaller Voc of 1.0 eV/0.75 eV InGaAsP/InGaAs double-junction solar cell(DJSC) than the Voc sum of individual subcells has been observed, and there is little information of the origin of such Voc loss and how to minimize it. In this paper, it is disclosed that the dominant mechanism of minority-carrier transport at back-surface-field(BSF)/base interface of the bottom subcell is thermionic emission, instead of defect-induced recombination, which is in contrast to previous reports. It also shows that both InP and InAlAs cannot prevent the zinc diffusion effectively. In addition, intermixing of major III-V element occurs as a result of increasing thermal treatment. To suppress the above negative effects, an initial novel InP/InAlAs superlattice(SL) BSF layer is then proposed and employed in bottom InGaAs subcell. The Voc of fabricated cells reach 997.5 mV, and a reduction of 30 mV in Voc loss without lost of Jsc, compared with the results of conventional InP BSF configuration, is achieved. It would benefit the overall Voc for further four-junction solar cells.
Introduction
InGaAsP/InGaAs double-junction solar cells(DJSCs)with approximate bandgap combination of 1.0/0.75 eV are used in four-junction configuration to harvest 900~1 700 nm sunlight,and are crucially important for device performances [
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where
Experience on III-V semiconductor devices reveals that the diffusion and intermixing at heterojunction interface of InP system always leads to device performance degradation[
In this paper,the evolution of dopant diffusion and recombination at BSF/base interface with increasing thermal treatment is studied. Based on experimental results,we propose a novel InP/InAlAs superlattice(SL)BSF layer for bottom InGaAs subcell. A reduction of 30 mV in Voc loss is achieved,compared with the results of conventional InP BSF configuration. It shows that such SL BSF would benefit the Voc enhancement for four-junction solar cells.
1 Experiments
Growth are done on n-type <100> InP substrates using MOVPE technique. The primary group III and group V precursors used are trimethylgallium(TMGa),trimethylindium(TMIn),trimethylgallium(TMAl),arsine(AsH3),and phosphine(PH3). The dopant precursors used are silane(SiH4)and diethylzinc(DEZn). V/III ratio of 200~300 and growth temperature of 650 °C are employed,as described previously [
Three periods of isotype p+-barrier(100nm)/p--InGaAsP(500nm)/p+-barrier(100nm)/p++-In(Al0.1Ga0.9)As(100nm)double heterojunctions(DHs),separated by InGaAs spacer layers,are grown in the same stack of MOVPE layers,as illustrated in
Figure 1.Cross-section of MOVPE stack containing three BSF/InGaAsP/BSF DHs.
SIMS measurements are performed using Cs+ primary beam with a fixed 5 kV acceleration. The positive ions of the quasi-molecular cluster are collected and detected. TRPL measurements with a temporal resolution of ~200 ps are performed at room temperature. An H-10330-75 PMT is used to collect PL signals.
The schematic cross-section of InGaAsP/InGaAs(1.0/0.75 eV)DJSC structure is shown in
Figure 2.Cross-section of the InGaAsP/InGaAs double-junction solar cell structure.
In-house photovoltaic current density-voltage (J-V)measurements are performed under AM0 solar simulator,without GaAs filter. External quantum efficiency(EQE)measurements are performed to give qualitative insight into the spectral response. Cells are placed on 25 °C cooled stages during measurements.
2 Results and discussions
Figure 3.Element profiles (a) and PL decay curves (b) for InP-barrier and InAlAs-barrier DH1s.
It evidences in
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where d is the thickness of confined layer in the DH. For high-quality materials,the
The value of B could be calculated according to Ref.[
The primary mechanism of carrier transport at heterojunction interface includes thermionic emission and defect-induced recombination. When thermionic emission dominates,the recombination current writes [
where m1 and m2 are effective mass of confined material and barrier,and ΔE is band offset. It is quite obvious that S would exponentially decreases as the band offset increasing. Notice that electron is the minority-carrier in p-InGaAsP DHs. The values of S for DHs,in the scenario of thermionic emission dominating,are estimated using ΔEc and m1,2 from Ref.[
For InGaAs,the conduction band offset ΔEc for InP barrier and InAlAs barrier are 0.25 eV and 0.52 eV,respectively. The larger offset in conduction band means smaller thermionic emission velocity. Meanwhile,the valence band offset ΔEv for InP barrier and InAlAs barrier are 0.35 eV and 0.17 eV[
Figure 4.Overall SIMS results of (a) InP-barrier DH stack and (b) InAlAs-barrier DH stack.
Figure 5.PL decay curves of (a) InP-barrier DHs and (b) InAlAs-barrier DHs.
As shown in
The steady-state PL of DHs confirms the above hypothesis. As shown in
Figure 6.Steady-state PL of (a) InP-barrier DHs and (b) InAlAs-barrier DHs. Weak peaks marked by asteroids in DH1 and DH2 are related to the spacers.
The results of InGaAsP/InGaAs DJSCs using both InP and InAlAs BSF layers confirm the advantages and effectiveness of InAlAs BSF layer in practical device.
Figure 7.(a) Light J-V and (b) spectra response curves for InGaAsP/InGaAs solar cells using InP and InAlAs BSF layers
It is well established that SL serve as effective barrier for element diffusion or intermixing,and dislocation threading,and it has been widely used in semiconductor devices such as high electron mobility transistors,laser diodes,electro absorption modulators [
Figure 8.Light J-V for InGaAsP/InGaAs DJSC using 5-period InP/InAlAs SL BSF layer.
3 Conclusions
In general,the use of novel SL BSF layer in the bottom subcell reduces the Voc loss in InGaAsP/InGaAs DJSC.
Experiments show that,the mechanism of minority-carrier transport at BSF/base interface of the bottom subcell of InGaAsP/InGaAs DJSCs is dominated by thermionic emission,instead of defect-induced recombination,which is in contrast to previous reports. It also shows that both InP and InAlAs cannot prevent the zinc diffusion effectively. In addition,intermixing of major III-V element occurs as a result of increasing thermal treatment.
Based on the above results,an initial 5-period InP/InAlAs SL BSF layer is designed and employed in bottom InGaAs subcell of DJSC. A Voc of 997.5 mV,a Jsc of 15.8 mA/cm2 and an FF of 0.824 are obtained. The Vocapproaches 1.0 V,resulting in a Wocof 752.5 mV. A reduction of 30 mV in Voc loss for DJSC is achieved,compared with the results of conventional InP BSF configuration. It suggests that such SL BSF would benefit the Voc enhancement for four-junction solar cells.
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Hong-Bo LU, Xin-Yi LI, Ge LI, Wei ZHANG, Shu-Hong HU, Ning DAI, Gui-Ting YANG. Reducing Voc loss in InGaAsP/InGaAs dual-junction solar cells[J]. Journal of Infrared and Millimeter Waves, 2021, 40(1): 7
Category: Research Articles
Received: Apr. 16, 2020
Accepted: --
Published Online: Aug. 30, 2021
The Author Email: LI Xin-Yi (lixy_sisp@163.com), DAI Ning (lixy_sisp@163.com)