High-Efficiency Colloidal Quantum Dot Photovoltaic Devices Using Chemically Modified Heterojunctions

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electron-accepting TiO 2 scaffolds require a high-temperature annealing process (≈500 °C), which limits their use with flexible substrates. Planar heterojunctionstructured PSCs have also been widely investigated because they enable adoption of low-temperature electron accepting layers (EALs). Zinc oxide, ZnO, is a strong EAL candidate because it offers good electrical properties even when prepared at low temperatures. [12][13][14][15][16] However, ZnO is used less often than mesoporous TiO 2 because it has several drawbacks. The surface properties of the ZnO layers are not favorable for growth of uniform perovskite layers with large crystal grains. This affects device performance significantly. [2,3,[7][8][9][10]17] Thus, efforts have been undertaken to improve perovskite layer quality by enhancing ZnO surface hydrophobicity. [18][19][20][21] Another shortcoming is the reverse reaction from perovskite to PbI 2 that can occur at ZnO/perovskite interfaces during perovskite layer formation. [14,[22][23][24] Two-step sequential deposition method has been employed to fabricate perovskite active layers on ZnO-EALs. This results in more reproducible synthesis of continuous, pinholefree perovskite layers than conventional single deposition methods. [15,21,[25][26][27][28][29] The perovskite layers in these reports were formed in two steps: (i) PbI 2 layer formation and (ii) perovskite layer formation, which occurred when the PbI 2 layer was immersed into an alkyl-ammonium iodide solution and annealed (80-100 °C). The sequential deposition method partially alleviated the burden of the reverse reaction that occurs at ZnO/perovskite interfaces, while helping to achieve a PCE of ≈16%. [15,30] Thus, the development of these strategies may offer a chance to further enhance low-temperature ZnO based PSC performance.Herein, we present high-efficiency, low-temperature PSCs using a strategy that combines self-assembled monolayer (SAM) modification of ZnO-EALs with sequential preparation of perovskite active layers. SAMs of the newly synthesized, highly polar molecules were constructed on ZnO-EALs and the perovskite layers were formed via sequential deposition. The SAMs acted as ZnO wetting control layers and as electric dipole layers. [1,13,19,20,[31][32][33] Modifying our SAMs enhanced the hydrophobicity of ZnO, which improved perovskite formation quality. Simultaneously, the electric dipole effect induced via Herein, this study reports high-efficiency, low-temperature ZnO based planar perovskite solar cells (PSCs) with state-of-the-art performance. They are achieved via a strategy that combines dual-functional self-assembled monolayer (SAM) modification of ZnO electron accepting layers (EALs) with sequential deposition of perovskite active layers. The SAMs, constructed from newly synthesized molecules with high dipole moments, act both as excellent surface wetting control layers and as electric dipole layers for ZnO-EALs. The insertion of SAMs improves the quality of PbI 2 layers and final perovskite layers during sequential dep...

Section: Resultsmentioning

confidence: 99%

High‐Efficiency Low‐Temperature ZnO Based Perovskite Solar Cells Based on Highly Polar, Nonwetting Self‐Assembled Molecular Layers

Azmi

Hadmojo

Sinaga

et al. 2017

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158

164

“…[15] The EQE spectra were obtained using monochromated light with 100 Hz chopper frequency under a dark condition (Newport, IQE-300). A solar simulator with a 150 W Xenon lamp (Newport) was calibrated using a monosilicon photodetector certified at the National Renewable Energy Laboratory (NREL).…”

Section: Methodsmentioning

confidence: 99%

Improved Processability and Efficiency of Colloidal Quantum Dot Solar Cells Based on Organic Hole Transport Layers

Aqoma

Mubarok

Lee

et al. 2018

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IntroductionColloidal quantum dot (CQD) based photovoltaic devices (CQDPVs) have emerged as promising next-generation solar cells owing to low-cost solution processibility at low temperature, easy bandgap tunability into the near infrared (NIR, λ > 800 nm) regime, and multiple exciton generation. [1,2] The power conversion efficiency (PCE) of CQDPVs has improved High-efficiency solid-state-ligand-exchange (SSE) step-free colloidal quantum dot photovoltaic (CQDPV) devices are developed by employing CQD ink based active layers and organic (Polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) and poly(3-hexylthiophene) (P3HT)) based hole transport layers (HTLs). The device using PTB7 as an HTL exhibits superior performance to that using the current leading organic HTL, P3HT, because of favorable energy levels, higher hole mobility, and facilitated interfacial charge transfer. The PTB7 based device achieves power conversion efficiency (PCE) of 9.60%, which is the highest among reported CQDPVs using organic HTLs. This result is also comparable to the PCE of an optimized device based on a thiol-exchanged p-type CQD, the current-state-of-the-art HTL. From the viewpoint of device processing, the fabrication of CQDPVs is achieved by direct single-coating of CQD active layers and organic HTLs at low temperature without SSE steps. The experimental results and device simulation results in this work suggest that further engineering of organic HTL materials can open new doors to improve the performance and processing of CQDPVs.

“…[11] As a result of these properties, PbS QD solar cells (QDSCs) have demonstrated a remarkable improvement in the efficiency when used in conventional, rigid solar cell architectures. [17] To reduce the charge recombination at the interfaces, various strategies have been considered, such as engineering the energy band levels as well as the employment of a buffer layer. For example, by addressing factors such as the relatively low built-in potential, the excessive surface trap sites and interfaces, which cause an open circuit voltage (V oc ) deficit, and the low charge carrier dissociation and collection rate, it has been possible to enhance the performance of the solar cells.…”

mentioning

confidence: 99%

Charge Transport Modulation of a Flexible Quantum Dot Solar Cell Using a Piezoelectric Effect

Cho

Giraud

Hou

et al. 2017

that maintain the device performance on the application of strain/stress. [3][4][5][6][7][8] To further advance current flexible solar cell technologies, however, a larger absorption efficiency is required so as to have a high solar cell efficiency with a photoactive layer that is as thin as possible in order to attain a high degree of flexibility. In this regard, colloidal quantum dots (QDs) are one of the most promising materials as they have been shown to exhibit a high light absorption coefficient, a tunable band gap across the solar spectrum, and are solution processible. [9,10] Among the various types of QD materials, lead sulfide (PbS) QDs are considered to be one of the most attractive materials for solar cell applications because of their large Bohr radius, wide tuning range of the band gap, low material cost, and air stability. [11] As a result of these properties, PbS QD solar cells (QDSCs) have demonstrated a remarkable improvement in the efficiency when used in conventional, rigid solar cell architectures. [12] However, there is still room for further improvement in the QDSC performance. For example, by addressing factors such as the relatively low built-in potential, the excessive surface trap sites and interfaces, which cause an open circuit voltage (V oc ) deficit, and the low charge carrier dissociation and collection rate, it has been possible to enhance the performance of the solar cells. [13][14][15][16] In particular, the heterojunction between the electron transport layer (ETL) and the PbS QD layer plays a key role in governing the overall performance of the PbS QDSCs because charge trapping and recombination occurs much faster at this heterojunction than at other locations between and within the PbS QD layers. [17] To reduce the charge recombination at the interfaces, various strategies have been considered, such as engineering the energy band levels as well as the employment of a buffer layer. [18][19][20] However, flexible QDSCs are more susceptible to the loss of charge carriers at the junction due to recombination pathways that arise from the straining of the semiconducting layers during the fabrication process, which is a major limitation that has to be dealt with for an improvement in the performance of flexible QDSCs to be realized. [9,10] Therefore, fundamental strategies for structurally and actively controlling the junction properties are required to boost extraction and reduce the recombination of Colloidal quantum dots are promising materials for flexible solar cells, as they have a large absorption coefficient at visible and infrared wavelengths, a band gap that can be tuned across the solar spectrum, and compatibility with solution processing. However, the performance of flexible solar cells can be degraded by the loss of charge carriers due to recombination pathways that exist at a junction interface as well as the strained interface of the semiconducting layers. The modulation of the charge carrier transport by the piezoelectric effect is an effective way of resolving and im...