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Solution-Processed Short-wave Infrared PbS Colloidal Quantum Dot / ZnO Nanowire Solar Cells Giving High Open Circuit Voltage Haibin Wang, Takaya Kubo, Jotaro Nakazaki, and Hiroshi Segawa ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00505 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 20, 2017
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ACS Energy Letters
Solution-Processed Short-wave Infrared PbS Colloidal Quantum Dot / ZnO Nanowire Solar Cells Giving High Open Circuit Voltage Haibin Wang1, Takaya Kubo1*, Jotaro Nakazaki1 and Hiroshi Segawa1,2*
1
Research Center for Advanced Science and Technology, The University of Tokyo,
153-8904 Tokyo, Japan. Graduate School of Arts and Sciences, The University of Tokyo, 153-8902 Tokyo, Japan. 2
AUTHOR INFORMATION *E-mail:
[email protected];
[email protected] 1
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ABSTRACT: A systematic investigation into the performance of PbS quantum dot (QD)/ZnO nanowire (NW) solar cells in the near-infrared (NIR) and short-wave infrared (SWIR) regions was carried out. The solar cells were confirmed to convert a wide range of solar energy (3.54–0.62 eV, corresponding to 0.35–2.0 µm). We found that the solar cells working in the SWIR region had a high open-circuit voltage (Voc). A relatively high Voc of 0.25 V was achieved even in solar cells whose photocurrent onsets were at approximately 0.64 eV (1.9 µm); this Voc is as high as that of Ge solar cells, which have been used for III-V compound semiconductor triple-junction solar cells. Although short-circuit current density and fill factor have to be further increased, these results indicate that solution-processed colloidal QD solar cells with ZnO NWs are promising candidates for the middle and/or bottom subcells of multijunction solar cells.
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There is a theoretical upper limit of the power conversion efficiency of single junction solar cells. The Shockley–Queisser limit under one-sun illumination1 is approximately 31%; this theoretical limit can be estimated through the use of an absorber with an absorption edge of 970 nm (1.28 eV). In practice, a GaAs single junction solar cell (λg=873 nm, Eg =1.42eV) has achieved 28.8%, which is close to the efficiency limit, and the power conversion efficiency of a c-Si solar cell (λg=1100 nm, Eg =1.13eV) was found to be as high as 26.6%.2 Although recent advancements in solar cell performances have narrowed the gap between the power conversion efficiency obtained in experiments and the theoretical limit. There is still the issue that more than 70% of the solar energy that reaches single junction solar cells is not converted. For ultrahigh efficiency solar cells to be developed, one would need to maximize the use of the infrared portion of the solar spectrum. Various designs for photovoltaic structures that could be used in ultrahigh efficiency solar cells have been proposed; one such example is that of multi-junction structures.3,4 Four-junction solar cells based on III-V compound semiconductors have yielded power conversion efficiencies of 37.9 % under one-sun illumination (AM1.5 100 mA cm-2) and 46 % under 508-sun illumination. However, multi-junction solar cells have only been used in a limited number of application areas, such as aerospace because conventional multijunction solar cells rely heavily on expensive technologies to be produced. If multijunction solar cells could be constructed with cheaper technologies, then a drastic reduction in the cost of power generation and an expansion of the areas to which it could be applied could be achieved. Polymer solar cells are made up of conjugated polymers5,6 and perovskite solar cells7 are composed of organometal halide perovskite compounds (e.g. CH3NH3PbI3); of 3
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the solar cells that are low-temperature, solution-processable, these two are particular promising. These types of solar cells capture visible photon energy efficiently and are suitable for the top and/or middle cells of multijunction solar cells.8-11 This means that there are a wide range of options for the top and/or middle subcells. However, there are few materials to choose from for the bottom cells, as few materials harvest solar energy below energies of about 1.0 eV (1.24 µm). The development of low cost and efficient short-wave infrared solar cells is therefore essential for the development of ultrahigh efficiency solar cells. Lead chalcogenide colloidal quantum dots (CQDs), such as PbS and PbSe, are materials that seem to be promising for use as the middle and/or bottom cells of multi-junction solar cells. This is because the absorption bandgap of bulk PbS is located in the infrared region (0.4 eV or 3.10 µm) and can be readily tuned by controlling quantum dot synthesis conditions.12 Furthermore the assembly of colloidal QDs gives rise to semiconducting behavior, which could result in relatively high carrier mobility.13 Most importantly, however, all of these properties can be obtained with low-temperature solution-based technologies.14 The most widely studied PbS QD solar cell structures are based on depleted heterojunction that have formed by the deposition of a QD layer on top of an electron-accepting layer such as ZnO or TiO2 (referred to as planar-type solar cell). The power conversion efficiency of planar-type solar cells has been increasing rapidly, and one was recently reported to be over 11%.15,16 Improving the light harvesting efficiency of these cells is crucial to increasing their power conversion efficiencies. Making the active layer thicker is not always the most effective way to increase this efficiency. This is because the typical minority carrier diffusion length of PbS QD layers is about 200-300 nm, which gives an indication of the maximum possible thickness of an active 4
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ACS Energy Letters
layer. To enhance carrier transportation in the layer of the solar cells, various types of solar cell structures have been proposed as methods that would enable the carrier transportation in the active layer of solar cells to be enhanced.17,18 One example is that of a quantum funnel structure, which is a structure composed of different-sized QD layers that have been stacked in such a way that they form a cascade-like energy level alignment.19 The efficient ambipolar carrier transport nature of organometal halide perovskite compounds such as CH3NH3PbI320 have been successfully incorporated into this structure to fill the inter dot gaps, thereby facilitating carrier transport between the QDs.21 Mixtures of PbS QDs and ZnO nanowires or TiO2 nanorods are proposed as methods for developing carrier transport pathways.22-29 We have also previously fabricated PbS QD/ZnO nanowire (NW) solar cells (referred to herein as NW-type solar cells), and were able to successful increase the thickness of the active layer by up to about 1.8 µm by controlling the morphology of ZnO nanowires.24 This enabled us to simultaneously obtain a high near-infrared light harvesting efficiency and high carrier transportation (Figure 1a).30-32 These PbS QD-based solar cells were also found to be highly stable.32 Most of the solar cells that have been reported on so far have used PbS quantum dots giving the first exciton absorption band around 950 nm,16, 18, 33-35 which could produce the single junction limit. But there is a limited amount of literatures reporting on CQD-based solar cells working in the low photon energy region (0.8 eV), Voc of the NW-type solar cells was lower than those of the planar type solar cells, whereas for lower energies (
