成果報告書詳細
管理番号20130000000765
タイトル*平成24年度中間年報 新エネルギー技術研究開発 革新的太陽光発電技術研究開発(革新型太陽電池国際研究拠点整備事業)ポストシリコン超高効率太陽電池の研究開発
公開日2014/5/9
報告書年度2012 - 2012
委託先名国立大学法人東京大学先端科学技術研究センター
プロジェクト番号P07015
部署名新エネルギー部
和文要約
英文要約Title: R & D on Innovative Solar Cells (International Research Center for Innovative Solar Cell Program) "Research and Development of Post-silicon solar cells for ultra-high efficiencies" (FY2008-FY2014) FY2012 Annual Report

Structure of InGaAs/GaAsP strain-balanced quantum wells was optimized so that the absorption edge is 1.2 eV and the current gain by the absorption of quantum wells is 5.9 mA/cm2. The degradation in fill factor was minimized by tunneling-assisted carrier transport through 3 nm barriers. The growth of such a structure was also attempted on Ge substrate for implementing a 3-junction cell with superlattice in its middle cell. Voc of such a superlattice cell showed almost no drop with respect to the reference cell under the concentration ratio of x 100 while the gain in current was still preserved.
To realize III-V-based 4-junction tandem solar cells, we have improved the re-growth conditions of GaInNAsSb layers on the Ge bottom cell by MBE. Growth of mirror-like surfaces were obtained, and this led to the improvement of photovoltaic properties of GaInNAsSb/Ge 2-junction tandem cells; Jsc = 19.1mA/cm2, Voc = 0.65V, and effi. = 8.6% were obtained for non-AR coated sample. Improvement of GaInNAs quality on vicinal Ge substrates was further investigated by MOVPE. For single junction GaInNAsSb cell, Voc was found to increase more rapidly than the reference GaAs with increasing sunlight concentration.

InGaN cells were developed aiming at the mechanically-stacked top cell. We have succeeded in growth of high quality nitrogen-polar GaN and InGaN films by pulsed sputtering deposition (PSD) on sapphire substrates. Both n- and p-type nitrogen-polar GaN films can be obtained by Si and Mg doping, respectively. These achievements will contribute to better carrier transport and higher efficiency.

On the other hand, a mechanically-stacked bottom cell, consisting of ZnO nanowires and PbS quantum dots, was fabricated and the highest quantum efficiency of 45% at 1.05 μm was obtained.
The development of intermediate-band cells with quantum dots was continued. In order to realize intermediate-band photovoltaic operation using quantum-dot superlattice, we have fabricated and characterized InGaAs/GaAsSb quantum dots solar cells (QDSCs) with a type-II band alignment heterostructure. As a result, the radiative recombination rate in QDs was suppressed and the photocurrent production due to 2-step photon absorption was increased. The cell conversion efficiency reached 17.3% at 1 sun. Further, we have fabricated InGaAs/AlGaAs QDSCs. AlGaAs serves as a high band-offset barrier layer which suppresses thermal escape of electrons out of InGaAs QD layers. The quantum efficiency by QDs of > 4% was achieved. The light trapping structure was developed in which the substrate was removed for eliminating parasitic light absorption. The 3-μm-thick GaAs cell with quantum wells, the backside of which was periodically grooved, exhibited 4.3 times larger light absorption by quantum wells than the case of a single light pass.
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