Colloidal quantum dot solar cells on curved and flexible substrates

Colloidal quantum dots (CQDs) are semiconductor nanocrystals synthesized with, processed in, and deposited from the solution phase, potentially enabling low-cost, facile manufacture of solar cells. Unfortunately, CQD solar cell reports, until now, have only explored batch-processing methods—such as spin-coating—that offer limited capacity for scaling. Spray-coating could offer a means of producing uniform colloidal quantum dot films that yield high-quality devices. Here, we explore the versatility of the spray-coating method by producing CQD solar cells in a variety of previously unexplored substrate arrangements. The potential transferability of the spray-coating method to a roll-to-roll manufacturing process was tested by spray-coating the CQD active layer onto six substrates mounted on a rapidly rotating drum, yielding devices with an average power conversion efficiency of 6.7%. We further tested the manufacturability of the process by endeavoring to spray onto flexible substrates, only to find that spraying while the substrate was flexed was crucial to achieving champion performance of 7.2% without compromise to open-circuit voltage. Having deposited onto a substrate with one axis of curvature, we then built our CQD solar cells onto a spherical lens substrate having two axes of curvature resulting in a 5% efficient device. These results show that CQDs deposited using our spraying method can be integrated to large-area manufacturing processes and can be used to make solar cells on unconventional shapes

Systematic optimization of quantum junction colloidal quantum dot solar cells

The recently reported quantum junction architecture represents a promising approach to building a rectifying photovoltaic device that employs colloidalquantum dot layers on each side of the p-n junction. Here, we report an optimized quantum junctionsolar cell that leverages an improved aluminum zinc oxide electrode for a stable contact to the n-side of the quantum junction and silverdoping of the p-layer that greatly enhances the photocurrent by expanding the depletion region in the n-side of the device. These improvements result in greater stability and a power conversion efficiency of 6.1% under AM1.5 simulated solar illumination

Folded-Light-Path Colloidal Quantum Dot Solar Cells SUBJECT AREAS: NANOPARTICLES APPLIED OPTICS ELECTRICAL AND ELECTRONIC ENGINEERING SOLAR CELLS

Colloidal quantum dot photovoltaics combine low-cost solution processing with quantum size-effect tuning to match absorption to the solar spectrum. Rapid advances have led to certified solar power conversion efficiencies of over 7%. Nevertheless, these devices remain held back by a compromise in the choice of quantum dot film thickness, balancing on the one hand the need to maximize photon absorption, mandating a thicker film, and, on the other, the need for efficient carrier extraction, a consideration that limits film thickness. Here we report an architecture that breaks this compromise by folding the path of light propagating in the colloidal quantum dot solid. Using this method, we achieve a substantial increase in short-circuit current, ultimately leading to improved power conversion efficiency. S olution-processed solar cells offer the promise of low cost, large-area processing, and, prospectively, high solar power conversion efficiencies Results In the present work, we sought to increase the interaction of light with the absorbing CQD medium by passing light through the CQD layer multiple times. Multi-pass optical management shemes have been shown to be effective in crystalline silicon We sought to assess quantitatively the extent to which the FLP strategy could enhance absorption, hence current. We began by exploring the spectral behaviour in the case of a 45u tilt angle. We show in The contribution of the additional passes through the CQD layer translates into added photon current through spectral multiplication with the AM 1.5 solar photon fluence spectrum 2

Photojunction Field-Effect Transistor Based on a Colloidal Quantum Dot Absorber Channel Layer

The performance of photodetectors is judged via high responsivity, fast speed of response, and low background current. Many previously reported photodetectors based on size-tuned colloidal quantum dots (CQDs) have relied either on photodiodes, which, since they are primary photocarrier devices, lack gain; or photoconductors, which provide gain but at the expense of slow response (due to delayed charge carrier escape from sensitizing centers) and an inherent dark current vs responsivity trade-off. Here we report a photojunction field-effect transistor (photoJFET), which provides gain while breaking prior photoconductors’ response/speed/dark current trade-off. This is achieved by ensuring that, in the dark, the channel is fully depleted due to a rectifying junction between a deep-work-function transparent conductive top contact (MoO₃) and a moderately n-type CQD film (iodine treated PbS CQDs). We characterize the rectifying behavior of the junction and the linearity of the channel characteristics under illumination, and we observe a 10 μs rise time, a record for a gain-providing, low-dark-current CQD photodetector. We prove, using an analytical model validated using experimental measurements, that for a given response time the device provides a two-orders-of-magnitude improvement in photocurrent-to-dark-current ratio compared to photoconductors. The photoJFET, which relies on a junction gate-effect, enriches the growing family of CQD photosensitive transistors

Depleted Bulk Heterojunction Colloidal Quantum Dot Photovoltaics

The first solution-processed depleted bulk heterojunction colloidal quantum dot solar cells are presented. The architecture allows high absorption with full depletion, thereby breaking the photon absorption/carrier extraction compromise inherent in planar devices. A record power conversion of 5.5% under simulated AM 1.5 illumination conditions is reported

Depleted-heterojunction colloidal quantum dot photovoltaics employing low-cost electrical contacts

With an aim to reduce the cost of depleted-heterojunction colloidalquantum dotsolar cells, we describe herein a strategy that replaces costly Au with a low-cost Ni-based Ohmic contact. The resultant devices achieve 3.5% Air Mass 1.5 power conversion efficiency. Only by incorporating a 1.2-nm-thick LiF layer between the PbSquantum dot film and Ni, we were able to prevent undesired reactions and degradation at the metal-semiconductor interface

Advances in colloidal quantum dot solar cells: The depleted-heterojunction device

Colloidal quantum dot (CQD) photovoltaics combine low-cost solution processibility with quantum size-effect tunability to match absorption with the solar spectrum. Recent advances in CQD photovoltaics have led to 3.6% AM1.5 solar power conversion efficiencies. Here we report CQD photovoltaic devices on transparent conductive oxides and show that our devices rely on the establishment of a depletion region for field-driven charge transport and separation. The resultant depleted-heterojunction solar cells provide a 5.1% AM1.5 power conversion efficiency. The devices employ infrared-bandgap size-effect-tuned PbS colloidal quantum dots, enabling broadband harvesting of the solar spectrum. (C) 2010 Elsevier B.V. All rights reserved

Depleted-Heterojunction Colloidal Quantum Dot Solar Cells

Colloidal quantum dot (CQD) photovoltaics combine low-cost solution processability with quantum size-effect tunability to match absorption with the solar spectrum. Rapid recent advances in CQD photovoltaics have led to impressive 3.6% AM1.5 solar power conversion efficiencies. Two distinct device architectures and operating mechanisms have been advanced, The first the Schottky device was optimized and explained in terms of a depletion region driving electron-hole pair separation on the semiconductor side of a junction between an opaque low-work-function metal and a p-type COD film. The second the excitonic device employed a CQD layer atop a transparent conductive oxide (TCO) and was explained in terms of diffusive exciton transport via energy transfer followed by exciton separation at the type-II heterointerface between the CQD film and the TCO. Here we fabricate COD photovoltaic devices on TCOs and show that our devices rely on the establishment of a depletion region for field-driven charge transport and separation, and that they also exploit the large bandgap of the TCO to improve rectification and block undesired hole extraction. The resultant depletedheterojunction solar cells provide a 5.1% AM1.5 power conversion efficiency. The devices employ infrared-bandgap size-effect-tuned PbS CQDs, enabling broadband harvesting of the solar spectrum. We report the highest open-circuit voltages observed in solid-state CQD solar cells to date, as well as fill factors approaching 60%, through the combination of efficient hole blocking (heterojunction) and very small minority carrier density (depletion) in the large-bandgap moiety