© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Inverted Colloidal Quantum Dot Solar Cells Gi-Hwan Kim, Bright Walker, Hak-Beom Kim, Jin Young Kim, Edward H. Sargent,* Jongnam Park,* and Jin Young Kim* for charge carrier generation in the CQD layer drops to near- zero proximate to the metal interface. [16,17] This further reduces the amount of light that can be absorbed by the PbS layer, and limits degrees of freedom in engineering maximal absorption within a given thickness of CQD material. To address these issues, it is attractive to architect the CQD solar cell so that more light is absorbed by an active layer of limited thickness, enabling avenues to enhanced PCE. In this work, we build a solar cell that is inverted in architecture com- pared to the standard, previously-reported, CQD photovoltaic device. This enables us to explore the introduction and engi- neering of an optical cavity for light enhancement. Specifically, we insert an optical spacer in a CQD photovoltaic device in order to enhance PCE by tuning the spatial distribution of the optical field. [18,19] A materials advance is pursued, achieved, and explained in the present work that is crucial to the realization of the inverted, optically-tuned, architecture. We employ a substan- tially transparent ZnO optical spacer between the photoactive CQD layer and the reflective metal electrode in an inverted device configuration. To do so, we develop the means to inte- grate a low-temperature, solution-processed, n-type ZnO layer as an optical spacer atop the CQD absorber. We show that the spacer offers significant prospects for enhancing light absorp- tion in the CQD layer since careful choice of ZnO thickness enables judicious placement of the optical field maxima within the absorber by optical simulations. Moreover, we demonstrate by experiments that the light harvesting and the photovoltaic efficiency can be significantly improved in CQD solar cells by tuning the thickness of the active layer and inserting an optical spacer between the active layer and the reflective electrode. As discussed herein, the new architecture offers an improve- ment in performance relative to the reference device that goes beyond optical cavity enhancement alone. We investigate using ultraviolet photoemission spectroscopy the detailed electronic structure of each of the materials incorporated into the mate- rials stack in each case. In this way, we offer experimentally- substantiated spatial band diagrams of the device that detail the origins of the enhanced voltage in the device. Figure 1 shows the device structures studied in this work. The devices were fabricated on indium tin oxide (ITO) coated substrates and all processing steps, except a brief immersion in a 3-mercaptopropionic acid (MPA) solution and the final metal electrode deposition, were performed by spin-coating in air. The ZnO optical spacer was prepared using a diethyl zinc precursor and deposited on the CQD layer. The reactive diethyl zinc precursor decomposes rapidly to form ZnO in the presence of air and only mild thermal annealing (110 °C) is required to prepare the film via this route. The low processing temperature of the ZnO layer is critical to the success of the inverted architecture; other methods of ZnO preparation which DOI: 10.1002/adma.201305583 G.-H. Kim, Dr. B. Walker, H.-B. Kim, Prof. J. Park, Prof. J. Y. Kim Interdisciplinary School of Green Energy Ulsan National Institute of Science and Technology (UNIST) Ulsan 689–798, South Korea E-mail: jnpark@unist.ac.kr; jykim@unist.ac.kr Dr. J. Y. Kim, Prof. E. H. Sargent Electrical and Computer Engineering University of Toronto 10 King’s College Rd, Toronto ON, Canada M5S 3G4 E-mail: ted.sargent@utoronto.ca Dr. J. Y. Kim Fuel Cell Research Center Korea Institute of Science and Technology Seoul 136-791, South Korea Colloidal quantum dot (CQD) solar cells have emerged as a promising class of solar cell with the potential to be manufac- tured at low cost. [1] PbS CQDs in particular are readily synthe- sized from earth-abundant elements, [2,3] and their bandgap can be conveniently and widely tuned via control over nanoparticle size. [4] Efficient, air-stable solar cells can be fabricated under ambient conditions using solution processing techniques. [5] PbS CQDs can behave either as a p-type or n-type semicon- ductors, [6] allowing their use in a wide variety of architectures including hybrid organic/inorganic devices. [7] These materials absorb light at wavelengths of 1100 nm and beyond, offering the potential for large photocurrents compared to many widely- employed organic and inorganic materials. Additionally, CQD solar cells offer routes to efficiencies exceeding the Shockley- Queisser limit for solar cells [8,9] via their ability to exploit mul- tiple excitons, [10,11] their promise in hot carrier extraction, [12] and their enablement of size-tuned tandem and multijunc- tion cells. Recent advances in the understanding of CQD solar cells and device fabrication have led to the demonstration of quantum efficiencies over 100%, [10,11] and increases in certified power conversion efficiency (PCE) to 7%. [13–15] The PCE of a CQD solar cell is directly proportional to the generated photocurrent, which is determined primarily by the fraction of incident photons absorbed in the CQD layer and by the number of extracted carriers per absorbed photon. A CQD film thickness of∼1 μm is typically required to absorb all inci- dent photons, but such thick films are often accompanied by insufficient charge transport for complete carrier extraction, limiting the optimal layer thickness to∼300 nm or less and thus reducing the amount of light absorbed by the CQD layer. Also, in the device structure widely-employed in prior CQD solar reports (here referred to as “standard”), the PbS CQD layer is located immediately adjacent the reflective metal elec- trode. As a result, the intensity of the optical field responsible Adv. Mater. 2014, DOI: 10.1002/adma.201305583