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*,†,‡ †
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
‡
S Supporting Information *
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 triplejunction 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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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 polymers,5,6 and perovskite solar cells7 are composed of organometal halide perovskite compounds (e.g., CH3NH3PbI3); of the solar cells that are low-temperature and solution-processable, these two are particularly 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 lowcost 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 multijunction solar cells. This is because the absorption band gap 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
here 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 = 1.42 eV) 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 = 1.13 eV) 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 multijunction 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, multijunction solar cells have been used in a only limited number of application areas, such as aerospace, because conventional multijunction solar cells rely heavily on expensive production technologies. If multijunction solar cells could be constructed with cheaper technologies, then a drastic reduction © 2017 American Chemical Society
Received: June 12, 2017 Accepted: August 18, 2017 Published: August 18, 2017 2110
DOI: 10.1021/acsenergylett.7b00505 ACS Energy Lett. 2017, 2, 2110−2117
Letter
http://pubs.acs.org/journal/aelccp
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ACS Energy Letters
Figure 1. (a) Schematic of the structure of the NW-type solar cells (FTO/ZnO-NW-PbS/Au). (b) Absorbance spectra of the PbS QD octane solutions with different first exciton absorption peaks. (c) Energy diagrams of the PbS QD/ZnO NW solar cells. The valence band edges (VB, red lines) values of PbS QD films were estimated by the onset of photoelectron spectrum (AC-3 Riken Keiki, Figure S2). The conduction band edges (CB1, blue lines; CB2, green lines) were calculated by adding the optical band gap energy (Egex), as determined from the first exciton absorption peak (Table 1) of the QD films, and band gap energy, EgEQE, was estimated from the photon energy giving an EQE of 1% (the inset of Figure 2a), respectively. (d) Absorbance spectra of the NW-type solar cells measured before Au back contact deposition. The spectrum of ZnO-NW formed on the FTO substrate is plotted in gray.
carrier mobility.13 Most importantly, however, all of these properties can be obtained with low-temperature solutionbased technologies.14 The most widely studied PbS QD solar cell structures are based on depleted heterojunctions that form 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 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 interdot 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 to date have used PbS quantum dots giving the first exciton absorption band around 950 nm,16,18,33−35 which could produce the singlejunction limit. But there is a limited amount of literature 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 (
