Fig. Photoexcited carrier dynamics in colloidal quantum dot solar cells

Photoexcited carrier dynamics in colloidal quantum dot solar cells: insights into individual quantum dots, quantum dot solid films and devices The certified power conversion efficiency (PCE) record of colloidal quantum dot solar cells (QDSCs) has considerably improved from below 4% to 16.6% in the last few years. However, the record PCE value of QDSCs is still substantially lower than the theoretical efficiency. So far, there have been several reviews on recent and significant achievements in QDSCs, but reviews on photoexcited carrier dynamics in QDSCs are scarce. The photovoltaic performances of QDSCs are still limited by the photovoltage, photocurrent and fill factor that are mainly determined by the photoexcited carrier dynamics, including carrier (or exciton) generation, carrier extraction or transfer, and the carrier recombination process, in the devices. In this review, the photoexcited carrier dynamics in the whole QDSCs, originating from individual quantum dots (QDs) to the entire device as well as the characterization methods used for analyzing the photoexcited carrier dynamics are summarized and discussed. The recent research including photoexcited multiple exciton generation (MEG), hot electron extraction, and carrier transfer between adjacent QDs, as well as carrier injection and recombination at each interface of QDSCs are discussed in detail herein. The influence of photoexcited carrier dynamics on the physiochemical properties of QDs and photovoltaic performances of QDSC devices is also discussed. 　　 [See more] Yaohong Zhang, Guohua Wu, Feng Liu, Chao Ding, Zhigang Zou and Qing Shen.

Fig. Schematic illustration of hot and cold electron and hole relaxation and transfer at the FAPbI3 QDs/TiO2 and FAPbI3 QDs/NiOx heterojunctions.

Photoexcited hot and cold electron and hole dynamics at FAPbI3 perovskite quantum dots/metal oxide heterojunctions used for stable perovskite quantum dot solar cells Highly luminescent formamidinium lead iodide (FAPbI3) quantum dots (QDs) exhibit high stability and narrowest bandgap energy among lead halide perovskites, thus they have become one of the most promising materials for the development of perovskite QD-based light-harvesting and near infrared-emitting devices. However, little is known thus far about photoexcited carrier dynamics at the interface between FAPbI3 QDs and charge transport layers, which is very important for both fundamental studies and applications of the QD/charge transport layer heterojunctions. Here, we systematically investigate both hot and cold photoexcited carrier (electron and hole) dynamics including relaxation and transfer at the heterojunction interfaces between FAPbI3 QDs and two kinds of well used charge acceptors, i.e., TiO2 and NiOx. We find that (i) the hot carriers in the FAPbI3 QDs are cooled to cold carriers with a cooling rate in the order of 1011 s−1, and (ii) the cold-electron and -hole injection rates are size dependent and are 2.01–2.29 × 109 s−1 and 1.55–1.96 × 109 s−1 at the two types of FAPbI3 QD/MO (metal oxide) heterojunctions, respectively, which are in good agreements with Marcus theory of charge transfer. In addition, the photoexcited carrier injection efficiency at the two heterojunctions is found to be as high as over 99%, which is the most important key for achieving high photovoltaic performance of the FAPbI3 QD solar cells (QDSCs). Prototypes of the two types of heterojunction-based QDSCs, i.e., normal-structure solar cells based on FAPbI3 QD/TiO2 and inverted-structure solar cells based on FAPbI3 QD/NiOx, were developed and the power conversion efficiencies of more than 9% and 5% were obtained, respectively. Moreover, the photovoltaic performance showed a higher storage stability over 100 days. The photovoltaic performance would be improved largely by optimization of each parts in the QDSCs. Our results shed light on perovskite QD-based optoelectronic devices. 　　 [See more] Naoki Nakazawa, Yaohong Zhang, Feng Liu, Chao Ding, Kanae Hori, Taro Toyoda, Yingfang Yao, Yong Zhou, Shuzi Hayase, Ruixiang Wang, Zhigang Zoub and Qing Shen

Fig. a)The device structure adopted in this study. PEAI was used as the passivation layer on the perovskite surface. b)Cross-sectional SEM images of PEAI-treated perovskite devices.

Growth of Amorphous Passivation Layer Using Phenethylammonium Iodide for High‐Performance Inverted Perovskite Solar Cells Organic–inorganic lead halide perovskite solar cells have realized a rapid increase of power conversion efficiency (PCE) in the past few years. However, their performance still suffers trap‐assisted decline due to defects at the surface and grain boundaries of the perovskite film. Herein, a phenethylammonium iodide‐lead iodide (PEAI‐PbI2) passivation layer is formed on the CH3NH3PbI3 perovskite film. The characterization results indicate that the PEAI covering layer leads to the reduction of surface defects and suppression of nonradiative recombination. By manipulating this surface passivation method, a remarkably improved VOC of 1.16 V and an enhanced PCE of 20.8% are achieved. 　　 [See more] Fan Zhang, Qinxun Huang, Jun Song, Yaohong Zhang, Chao Ding, Feng Liu, Dong Liu, Xiaobin Li, Hironobu Yasuda, Koji Yoshida, Junle Qu, Shuzi Hayase, Taro Toyoda, Takashi Minemoto, and Qing Shen.

Fig. (A)Schematic diagram of the architecture of ZnO@SnO2NW-based CQDSCs. (B)The J-V curves of ZnO NW CQDSCs with or without SnO2 passivation.

Improving Photovoltaic Performance of ZnO Nanowires Based Colloidal Quantum Dot Solar Cells via SnO2 Passivation Strategy Colloidal quantum dot solar cells (CQDSCs) based on one-dimensional metal oxide nanowires (NWs) as the electron transport layer (ETL) have attracted much attention due to their larger ETL/colloidal quantum dots (CQDs) contact area and longer electron transport length than other structure CQDSCs, such as planar CQDSCs. However, it is known that defect states in NWs would increase the recombination rate because of the high surface area of NWs. Here, the defect species on the ZnO NWs' surface which resulted in the surface recombination and SnO2 passivation effects were investigated. Comparing with the solar cells using pristine ZnO NWs, the CQDSCs based on SnO2 passivated ZnO NW electrodes exhibited a beneficial band alignment to charge separation, and the interfacial recombination at the ZnO/CQD interface was reduced, eventually resulting in a 40% improvement of power conversion efficiency (PCE). Overall, these findings indicate that surface passivation and the reduction of deep level defects in ETLs could contribute to improving the PCE of CQDSCs. [See more] Shuhei Ozu, Yaohong Zhang, Hironobu Yasuda, Yukiko Kitabatake, Taro Toyoda, Masayuki Hirata, Kenji Yoshino, Kenji Katayama, Shuzi Hayase, Ruixiang Wang and Qing Shen.

Fig. high-efficiency PbSe colloidal quantum dot solar cells (CQDSCs)

Lead Selenide Colloidal Quantum Dot Solar Cells Achieving High Open-Circuit Voltage with One-Step Deposition Strategy Lead selenide (PbSe) colloidal quantum dots (CQDs) are considered to be a strong candidate for high-efficiency colloidal quantum dot solar cells (CQDSCs) due to its efficient multiple exciton generation. However, currently, even the best PbSe CQDSCs can only display open-circuit voltage (Voc) about 0.530 V. Here, we introduce a solution-phase ligand exchange method to prepare PbI2-capped PbSe (PbSe-PbI2) CQD inks, and for the first time, the absorber layer of PbSe CQDSCs was deposited in one step by using this PbSe-PbI2 CQD inks. One-step-deposited PbSe CQDs absorber layer exhibits fast charge transfer rate, reduced energy funneling, and low trap assisted recombination. The champion large-area (active area is 0.35 cm2) PbSe CQDSCs fabricated with one-step PbSe CQDs achieve a power conversion efficiency (PCE) of 6.0% and a Voc of 0.616 V, which is the highest Voc among PbSe CQDSCs reported to date. 　　 [See more] Yaohong Zhang, Guohua Wu, Chao Ding, Feng Liu, Yingfang Yao, Yong Zhou, Congping Wu, Naoki Nakazawa, Qingxun Huang, Taro Toyoda, Ruixiang Wang, Shuzi Hayase, Zhigang Zou and Qing Shen

Fig. Cross-sectional SEM image of a typical Zn1-xMgxO/PbS solar cell

Understanding charge transfer and recombination by interface engineering for improving the efficiency of PbS quantum dot solar cells In quantum dot heterojunction solar cells (QDHSCs), the QD active layer absorbs sunlight and then transfers the photogenerated electrons to an electron-transport layer (ETL). It is generally believed that the conduction band minimum (CBM) of the ETL should be lower than that of the QDs to enable efficient charge transfer from the QDs to the collection electrode (here, FTO) through the ETL. However, by employing Mg-doped ZnO (Zn1-xMgxO) as a model ETL in PbS QDHSCs, we found that an ETL with a lower CBM is not necessary to realize efficient charge transfer in QDHSCs. The existence of shallow defect states in the Zn1-xMgxO ETL can serve as additional charge-transfer pathways. In addition, the conduction band offset (CBO) between the ETL and the QD absorber has been, for the first time, revealed to significantly affect interfacial recombination in QDHSCs. We demonstrate that a spike in the band structure at the ETL/QD interface is useful for suppressing interfacial recombination and improving the open-circuit voltage. By varying the Mg doping level in ZnO, we were able to tune the CBM, defect distribution and carrier concentration in the ETL, which play key roles in charge transfer and recombination and therefore the device performance. PbS QDHSCs based on the optimized Zn1-xMgxO ETL exhibited a high power conversion efficiency of 10.6%. Our findings provide important guidance for enhancing the photovoltaic performance of QD-based solar cells. 　　 [See more] Chao Ding, Yaohong Zhang, Feng Liu, Yukiko Kitabatake, Shuzi Hayase, Taro Toyoda, Ruixiang Wang, Kenji Yoshino, Takashi Minemoto and Qing Shen

Fig. Air-Stable Alloyed CsSn1-xPbxI3 Perovskite Nanocrystals

Colloidal Synthesis of Air-Stable Alloyed CsSn1-xPbxI3 Perovskite Nanocrystals for Use in Solar Cells Organic-inorganic hybrid perovskite solar cells have demonstrated unprecedented high power conversion efficiencies in the past few years. Now, the universal instability of the perovskites has become the main barrier for this kind of solar cells to realize commercialization. This situation can be even worse for those tin-based perovskites, especially for CsSnI3, because upon exposure to ambient atmosphere the desired black orthorhombic phase CsSnI3 would promptly lose single crystallinity and degrade to the inactive yellow phase, followed by irreversible oxidation into metallic Cs2SnI6. By alloying CsSnI3 with CsPbI3, we herein report the synthesis of alloyed perovskite quantum dot (QD), CsSn1-xPbxI3, which not only can be phase-stable for months in purified colloidal solution but also remains intact even directly exposed to ambient air, far superior to both of its parent CsSnI and CsPbI3 QDs. Ultrafast transient absorption spectroscopy studies reveal that the photoexcited electrons in the alloyed QDs can be injected into TiO2 nanocrystals at a fast rate of 1.12 × 1011 s-1, which enables a high photocurrent generation in solar cells. 　　 [See more] Feng Liu, Chao Ding, Yaohong Zhang, Taichi Kamisaka, Taro Toyoda, Teresa Ripolles-Sanchis, Shuzi Hayase, Takashi Minemoto, Kenji Yoshino, Songyuan Dai, Masatoshi Yanagida, Hidenori Noguchi, and Qing Shen

Fig. the change in TC against time for different photoexcited carrier densities

Slow hot carrier cooling in cesium lead iodide perovskites Lead halide perovskites are attracting a great deal of interest for optoelectronic applications such as solar cells, LEDs, and lasers because of their unique properties. In solar cells, heat dissipation by hot carriers results in a major energy loss channel responsible for the Shockley–Queisser efficiency limit. Hot carrier solar cells offer the possibility to overcome this limit and achieve energy conversion efficiency as high as 66% by extracting hot carriers. Therefore, fundamental studies on hot carrier relaxation dynamics in lead halide perovskites are important. Here, we elucidated the hot carrier cooling dynamics in all-inorganic cesium lead iodide (CsPbI3) perovskite using transient absorption spectroscopy. We observe that the hot carrier cooling rate in CsPbI3 decreases as the fluence of the pump light increases and the cooling is as slow as a few 10 ps when the photoexcited carrier density is 7 × 1018 cm−3, which is attributed to phonon bottleneck for high photoexcited carrier densities. Our findings suggest that CsPbI3 has a potential for hot carrier solar cell applications. 　　 [See more] Qing Shen, Teresa S. Ripolles, Jacky Even, Yuhei Ogomi, Koji Nishinaka, Takuya Izuishi, Naoki Nakazawa, Yaohong Zhang, Chao Ding, Feng Liu, Taro Toyoda, Kenji Yoshino, Takashi Minemoto, Kenji Katayama, and Shuzi Hayase

KEYWORDS cesium lead halide perovskite nanocrystals; colloidal nanoparticles; perovskite quantum dot; photoluminescence quantum yield; stable perovskite

Highly Luminescent Phase-Stable CsPbI3 Perovskite Quantum Dots Achieving Near 100% Absolute Photoluminescence Quantum Yield Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability. For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems such as CdSe and PbS, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus quenching luminescence. In this paper, we show that a high room-temperature photoluminescence quantum yield of up to 100% can be obtained in CsPbI3 perovskite QDs, signifying the achievement of almost complete elimination of the trapping defects. This is realized with our improved synthetic protocol that involves introducing organolead compound trioctylphosphine-PbI2 (TOP-PbI2) as the reactive precursor, which also leads to a significantly improved stability for the resulting CsPbI3 QD solutions. Ultrafast kinetic analysis with time-resolved transient absorption spectroscopy evidences the negligible electron or hole trapping pathways in our QDs, which explains such a high quantum efficiency. We expect the successful synthesis of the “ideal” perovskite QDs will exert profound influence on their applications to both QD-based light-harvesting and -emitting devices. 　　 [See more] Feng Liu, Yaohong Zhang, Chao Ding, Syuusuke Kobayashi, Takuya Izuishi, Naoki Nakazawa, Taro Toyoda, Tsuyoshi Ohta, Shuzi Hayase, Takashi Minemoto, Kenji Yoshino, Songyuan Dai and Qing Shen

Fig. Spatial band diagrams of ungraded and graded QD solar cells

Recombination Suppression in PbS Quantum Dot Heterojunction Solar Cells by Energy-Level Alignment in the Quantum Dot Active Layers Using spatial energy-level gradient engineering with quantum dots (QDs) of different sizes to increase the generated carrier collection at the junction of a QD heterojunction solar cell (QDHSC) is a hopeful route for improving the energy-conversion efficiency. However, the results of current related research have shown that a variable band-gap structure in a QDHSC will create an appreciable increase, not in the illumination current density, but rather in the fill factor. In addition, there are a lack of studies on the mechanism of the effect of these graded structures on the photovoltaic performance of QDHSCs. This study presents the development of air atmosphere solution-processed TiO2/PbS QDs/Au QDHSCs by engineering the energy-level alignment (ELA) of the active layer via the use of a sorted order of differently sized QD layers (four QD sizes). In comparison to the ungraded device (without the ELA), the optimized graded architecture (containing the ELA) solar cells exhibited a great increase (21.4%) in short-circuit current density (Jsc). As a result, a Jsc value greater than 30 mA/cm2 has been realized in planar, thinner absorption layer (∼300 nm) PbS QDHSCs, and the open-circuit voltage (Voc) and power-conversion efficiency (PCE) were also improved. Through characterization by the light intensity dependences of the Jsc and Voc and transient photovoltage decay, we find that (i) the ELA structure, serving as an electron-blocking layer, reduces the interfacial recombination at the PbS/anode interface, and (ii) the ELA structure can drive more carriers toward the desirable collection electrode, and the additional carriers can fill the trap states, reducing the trap-assisted recombination in the PbS QDHSCs. This work has clearly elucidated the mechanism of the recombination suppression in the graded QDHSCs and demonstrated the effects of ELA structure on the improvement of Jsc. The charge recombination mechanisms characterized in this work would be able to shed light on further improvements of QDHSCs, which could even benefit other types of solar cells. 　　 [See more] Chao Ding, Yaohong Zhang, Feng Liu, Naoki Nakazawa, Qingxun Huang, Shuzi Hayase, Yuhei Ogomi, Taro Toyoda, Ruixiang Wang, and Qing Shen

Fig. (a, b) A schematic illustration of the ZnO@TiO2/PbS solar cells; (c, d) photographs of the PbS CQDSCs before and after the deposition of the Au contacts;

High reduction of interfacial charge recombination in colloidal quantum dot solar cells by metal oxide surface passivation Bulk heterojunction (BHJ) solar cells based on colloidal QDs and metal oxide nanowires (NWs) possess unique and outstanding advantages in enhancing light harvesting and charge collection in comparison to planar architectures. 　　However, the high surface area of the NW structure often brings about a large amount of recombination (especially interfacial recombination) and limits the open-circuit voltage in BHJ solar cells. This problem is solved here by passivating the surface of the metal oxide component in PbS colloidal quantum dot solar cells (CQDSCs). 　　 [See more] J. Chang, Y. Kuga, I. Mora-Seró, T. Toyoda, Y. Ogomi, S. Hayase, J. Bisquertand, Q. Shen

Fig. Schematic diagram of the PbS/ZnS sensitized solar cells. The solid arrows indicate the excitation of PbS QDs and the injection/transfer of photoexcited charges. The dotted arrows indicate the charge recombination at the TiO2/QD/electrolyte interfaces.

Uncovering the charge transfer and recombination mechanism in ZnS-coated PbS quantum dot sensitized solar cells In this work, the charge transfer and recombination mechanism is uncovered for the PbS/ZnS quantum dot sensitized solar cells (QDSSCs) based on nanoporous TiO2 electrodes. PbS quantum dots (QDs) were in-situ grown on TiO2 nanoparticles through the successive ionic absorption and reaction (SILAR) method, followed by the surface passivation of ZnS for the sensitized electrodes. It was observed that the ZnS coating cycles play a significant role in determining the photovoltaic parameters. [See more] J. Chang, T. Oshima, S. Hachiy, K. Sato, T. Toyoda, K. Katayama, S. Hayase, Q. Shen

Fig. Dependence of normalized TA responses of CH3NH3PbClI2/ TiO2 on pump light intensity. The pump light wavelength is 470 nm and the probe light wavelength is 775 nm.

Charge transfer and recombination at the metaloxide/CH3NH 3PbClI2/spiro-OMeTAD interfaces: Uncovering the detailed mechanism behind high efficiency solar cells In recent years, organometal halide perovskite-based solid-state hybrid solar cells have attracted unexpected increasing interest because of their high efficiency (the record power conversion efficiency has been reported to be over 15%) and low fabrication cost. It has been accepted that the high efficiency was mainly attributed to the strong optical absorption (absorption coefficient: 15 000 cm1 at 550 nm) over a broader range (up to 800 nm) and the long lifetimes of photoexcited charge carriers (in the order of 10 ns – a few 100 ns) of the perovskite absorbers. [See more] Q. Shen, Y. Ogomi, J. Chang, S. Tsukamoto, K. Kukihara, T. Oshima, N. Osada, K. Yoshino, K. Katayama, T. Toyoda S. Hayase

Fig. AFM images of CdSe QDs adsorbed on (001)

Effect of TiO2 Crystal Orientation on the Adsorption of CdSe Quantum Dots for Photosensitization Studied by the Photoacoustic and Photoelectron Yield Methods We describe the adsorption and growth of CdSe quantum dots (QDs) on single crystals of rutile TiO2 with different crystal orientations. We used atomic force microscopy (AFM) to characterize the morphology of the QDs and photoacoustic (PA) spectroscopy to measure the optical absorption. Photoelectron yield (PY) spectroscopy was applied to characterize the valence band maximum (VBM) of the single crystal TiO2. [See more] T. Toyoda, W. Yindeesuk, K. Kamiyama, S. Hayase, Q. Shen

Fig. SEM images of TiO2 electrodes: high magnification of NT

Photoacoustic spectroscopy of TiO2 nanotube electrode adsorbed with CdSe quantum dots and its photovoltaic properties We report on the optical absorption properties and photovoltaic characteristics of nanotube (NT) TiO2 electrodes adsorbed with CdSe quantum dots (QDs), and compared them with those of nanoparticle (NP) TiO2 electrodes adsorbed with CdSe QDs. The CdSe QDs were grown directly on the TiO2 electrodes by the successive ionic layer adsorption and reaction (SILAR) method. [See more] M. Akimoto, Q. Shen, S. Hayase, T. Toyoda

Fig. Scanning electron microscopy (SEM) image of IO-TiO2

Optical absorption of CdSe quantum dots on electrodes with different morphology We have studied the optical absorption of CdSe quantum dots (QDs) adsorbed on inverse opal TiO2 (IO-TiO2) and nanoparticulate TiO2 (NP-TiO2) electrodes using photoacoustic (PA) measurements. The CdSe QDs were grown directly on IO-TiO2 and NP-TiO2 electrodes by a successive ionic layer adsorption and reaction (SILAR) method with different numbers of cycles. The average diameter of the QDs was estimated by applying an effective mass approximation to the PA spectra. The increasing size of the QDs with increasing number of cycles was confirmed by a redshift in the optical absorption spectrum. [See more] W. Yindeesuk, Q. Shen, S. Hayase, T. Toyoda

　　

Fig. NT-TiO2 prepared at a deposition temperature of 10°C adsorbed with CdSe (50,000)

Effect of defects in TiO2 nanotube thin filmon the photovoltaic properties of quantum dot-sensitized solar cells In the liquid-phase-deposition (LPD) method, the deposition temperature is considered to be one of the most important factors in TiO2 nanotube crystal growth. We investigated the effects of the deposition temperature on the surface morphology and defects in TiO2 nanotube (NT–TiO2) thin film electrodes utilizing scanningelectron- microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL), together with the effects of these on the photovoltaic characteristics of CdSe quantum dot (QD)-sensitized NT–TiO2 solar cells. In addition, we studied the effect of these defects on the physical properties, such as the carrier recombination and electron transport at the TiO2 and TiO2/QD interface. [See more] M. Akimoto, T. Toyoda, T. Okuno, S. Hayase, Q. Shen