成果報告書詳細
管理番号20160000000188
タイトル*平成27年度中間年報 「高性能・高信頼性太陽光発電の発電コスト低減技術開発/革新的新構造太陽電池の研究開発/超高効率・低コストIIIーV化合物太陽電池モジュールの研究開発(量子ドットセル評価)」
公開日2016/12/14
報告書年度2015 - 2015
委託先名国立大学法人神戸大学
プロジェクト番号P15003
部署名新エネルギー部
和文要約
英文要約Title:Development of high performance and reliable PV modules to reduce levelized cost of energy, Research and Development of innovative new structure solar cells, Research and Development of ultra-high efficiency and low-cost III-V compound semiconductor solar cell modules (Characterization of quantum dot solar cells) (FY2015-FY2017)FY2015 Annual Report

We studied the two-step photon absorption (TSPA) process via quantized states formed in intermediate-band solar cells (IBSCs), including a superlattice (SL) structure of InAs/GaAs QDs. An IBSC structure including undoped 9-layer stacked InAs/GaAs QDs was fabricated using solid-source molecular beam epitaxy. Nominal GaAs spacer layer thickness was 4 nm, which is thin enough to couple the electronic states along the stacking direction and forms a miniband. The built-in electric field applied to the QDSLs was expected to be 7 kV/cm. We carried out photoluminescence (PL) and its excitation (PLE) measurements, as well as the external quantum efficiency (EQE) measurements under two-color photoexcitation for VB IB and IB CB. The photoluminescence (PL)-excitation spectrum demonstrates a weak absorption edge at approximately 950 nm. To clarify the origin of this, we examined the PL peak wavelength as a function of the excitation wavelength. We found that the PL peak starts shifting with the excitation wavelength in the region longer than 950 nm when the inhomogeneously distributed fundamental QDSL states were directly excited. The critical wavelength showing the peak shift produced the edge structure at 950 nm in the PLE spectrum. Therefore, this absorption edge located above the inhomogeneously distributed fundamental states can be attributed to the higher excited states of the QDSLs. TSPA of sub-bandgap photons efficiently occurs when electrons are pumped from the valence band to the states above the inhomogeneously distributed fundamental states of QDSLs.
In addition, we conducted the excitation power dependence of the PL intensity to study separate electron and hole energy relaxation. The excitation power dependence when selectively exciting the absorption edge at 950 nm demonstrates a superlinear relation, though the slopes for the excitations at the WL and GaAs states were linear. The superlinear dependence indicates that the excited electron and hole are separately relaxing into the QDSL states. Since the energy distribution of the first excited states overlaps with that of the fundamental states, which reduces the excitation cross-section for the excited states, the higher excited states appeared above the inhomogeneously distributed fundamental states play an important role in the efficient spatial carrier separation in the miniband. To confirm the separate electron-hole energy relaxation, we studied the radiative recombination lifetime by the time-resolved PL measurements. The 800-nm excitation which is above the GaAs band gap dominantly exhibited a single exponential decay with the decay time of 1.1 ns. The decay time slightly increases with the delay. On the other hand, when excited at 900 nm, the slow decay component turns significant. This slow decay component yields to a stretched exponential profile given by a fractional power law representing a continuous distribution of lifetimes, which can be attributed to recombination of spatially separated electron and hole in the miniband of the excited states. Furthermore, this slow decay time becomes longer. with increasing the internal electric field by applying DC bias voltage. Since the strong electric field causes significant carrier separation, the slower decay at the higher electric field convincingly support our discussion. These results demonstrate an important role of IB electron lifetime in improving the conversion efficiency of IBSCs.
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