Song and Jeong Nano Convergence (2017) 4:21 DOI 10.1186/s40580-017-0115-0 REVIEW Open Access Colloidal quantum dot based solar cells: from materials to devices Jung Hoon Song1 and Sohee Jeong1,2* Abstract Colloidal quantum dots (CQDs) have attracted attention as a next-generation of photovoltaics (PVs) capable of a tun- able band gap and low-cost solution process. Understanding and controlling the surface of CQDs lead to the sig- nifcant development in the performance of CQD PVs. Here we review recent progress in the realization of low-cost, efcient lead chalcogenide CQD PVs based on the surface investigation of CQDs. We focus on improving the electrical properties and air stability of the CQD achieved by material approaches and growing the power conversion efciency (PCE) of the CQD PV obtained by structural approaches. Finally, we summarize the manners to improve the PCE of CQD PVs through optical design. The various issues mentioned in this review may provide insight into the commer- cialization of CQD PVs in the near future. Keywords: Colloidal quantum dots, Nanocrystals, Solar cells, Photovoltaics, Lead chalcogenides 1 Introduction efcient collection of high-energy charges, which are dif- Colloidal quantum dots (CQDs) are chemically-prepared fcult to achieve in conventional PVs [5]. In this article, semiconductor nanocrystals, which have diameter is less we discuss the current state of high-efciency CQD PV than twice the Bohr radius describing the spatial exten- development. sion of exciton (electron–hole pair) in semiconductors. Te CQDs have attracted attention over the past dec- 2 Size‑dependent physical properties of CQDs ade due to a solution based synthetic methods [1], easily When the size of a bulk semiconductor is reduced, dis- tunable optoelectronic properties [2], and superior pro- crete energy levels appear in the energy band owing to cessing capabilities for optoelectronic applications [3]. the quantum confnement efect, as shown in Fig. 1a. Specifcally, lead chalcogenide CQDs are considered as a A CQD consists of a semiconductor core and surface prominent material for a next-generation photovoltaics ligands. In the case of a core, synthesis methods capable (PVs) [4] owing to their wide tunable bandgaps covering of controlling the size of binary or ternary compound from visible to near-infrared wavelength regime arising semiconductors, such as II–VI (CdSe, CdS) compound from a large Bohr exciton radius and narrow bulk band- semiconductors, III–V (InP, InAs), IV–VI (PbS, PbSe), gap. Also, the excitons in lead chalcogenide CQDs can be and III–V ­(CuInS2, ­CuInSe2), have been developed and easily separated into electrons and holes because of their reported. As shown in Fig. 1a, the bandgap of IV–VI high dielectric constant and extinction coefcient. More (PbS, PbSe) CQDs can be controlled to absorb light in importantly, because their material properties can be the range of 600–3000 nm, which is suitable for solar cell easily controlled using electron density functional design, materials. Additionally, most of the reported high-ef- lead chalcogenide CQDs are expected to exhibit efcient ciency CQD PVs have been fabricated using IV–VI (PbS, utilization of low-energy and low-intensity photons and PbSe) CQDs. Terefore, in this article, we mainly focus on the IV–VI (PbS, PbSe) CQDs. Typically, when the energy bandgap of a semiconductor decreases in a single- *Correspondence: [email protected] junction PV, the open-circuit voltage ­(V ) decreases 1 Nano‑Convergence Systems Research Division, Korea Institute OC of Machinery and Materials (KIMM), Daejeon 34113, Republic of Korea and the short-circuit current ­(JSC) increases by absorb- Full list of author information is available at the end of the article ing more light. Figure 1b shows that single-junction CQD © Korea Nano Technology Research Society 2017. This article is distributed under the terms of the Creative Commons Attribu- tion 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Song and Jeong Nano Convergence (2017) 4:21 Page 2 of 8 Fig. 1 a AM 1.5G solar spectrum (from ASTM G173-03 reference spectra), band diagram, and frst exciton energy of PbS CQDs with various diam- eters. b Characteristics of Schottky junction PbS CQD PVs as a function of the frst exciton energy (reprinted with permission from ref. 6, Copyright 2013 American Physical Society) PVs also exhibit the relationship between energy band- semiconductors exhibit p-type doping polarity under gaps and solar cell characteristics as described above [6]. anion-rich conditions and n-type doping polarity under In CQD solids, the hole mobility increases depending on cation-rich conditions. Tus, doping can be controlled the size of the CQDs [7], explained by the decrease in the by using these ligands. When a CQD flm is fabricated, total number of interparticle hops and the reduction of the mobility of the charge carriers is determined by the the coulombic charging energy of an individual particle. length of ligands that passivate the surface of the CQDs. As shown in Fig. 2b, the mobility and coupling of the 3 Surface modifcation and characteristics control CQDs increase with the use of shorter-length ligands Nanomaterials such as CQDs generally have a high sur- [9]. In addition, when surface ligands bind to the surface face-area-to-volume ratio and the surfaces are prone to of CQDs, the electron distribution of the CQD surface form dangling bonds, which causes defects. Especially, and ligand changes, resulting in the formation of dipole reducing surface defects is very important for obtain- moment on the CQD surface. Te strength and direction ing high-efciency CQD PVs. A CQD fabricated using of the surface dipole moment are determined by the sur- wet-chemical synthesis consists of a semiconductor core face ligands. Terefore, the position of the energy band and surface ligands, as shown in Fig. 2a [8]. Te physi- is shifted by the surface dipole moment, as shown in cal characteristics of the CQD can be controlled by the Fig. 2c–d [10]. CQDs have many surface defects because surface ligands owing to its high surface-area-to-volume of their high surface-area-to-volume ratio. Terefore, the ratio. Surface ligands generally have an amphiphilic surface ligands play an important role in reducing these structure consisting of a polar head group and a non- surface defects. Figure 2e–f show that additional passi- polar aliphatic group. Te rear portion of the ligand, vation using halide atoms reduces the surface defects of which is composed of aliphatic groups, provides steric CQDs and increases photoluminescence (PL) [11, 12]. stabilization and dispersibility, which is the ability to Furthermore, as passivation prevents the oxidation of dissolve in organic solvents. Te head group typically CQDs, their air stability can be improved [11, 12]. Con- contains amines, carboxylate, thiolate and phospho- sequently, the surface ligands of CQDs play an impor- nate. Tese functional groups bind to the cationic met- tant role in controlling the dispersibility, air stability, and als on the CQD surfaces and produce nonstoichiometric electrical properties (such as doping, mobility, electronic CQDs, resulting in a doping efect. Typically, compound structure, and surface defects). Terefore, understanding Song and Jeong Nano Convergence (2017) 4:21 Page 3 of 8 Fig. 2 a Schematic diagram of a PbS CQD consisting of a semiconductor core and surface ligands (reprinted with permission from ref. 8, Copy- right 2014 American Association for the Advancement of Science). b Charge carrier mobility of PbS CQD flms with various surface ligand length (reprinted with permission from ref. 9, Copyright 2013 American Chemical Society). c Energy band position of PbS CQD flms for d diferent surface ligands (reprinted with permission from ref. 10, Copyright 2013 American Chemical Society). Air stability analysis using e the absorbance spectrum and f quantum yield of PL as a function of storage time and its dependence on NH 4Cl treatment on the surface of PbSe CQDs (reprinted with permission from ref. 11, Copyright 2014 American Chemical Society) the dependence of CQD characteristics on surface early stages of the development of CQD PVs, the PCE ligands is essential for obtaining high-efciency CQD was increased in accordance with the structural changes PVs. of the devices. However, since 2012, the development of technologies that control the surface of CQDs has 4 Enhancement of power conversion efciency resulted in dramatic improvements of the performance through solar cell structure design of CQD PVs. In Sect. 4, we will describe the structural Te power conversion efciency (PCE) of CQD PVs has development of CQD PVs. Initially, CQDs with high increased very rapidly since national renewable energy extinction coefcients were used instead of dyes in dye- laboratory (NREL) certifcation began in 2010 and the sensitized solar cells. In this case, the CQD absorbs light highest PCE in the NREL chart is currently 13.4%. In the to form excitons, and electrons and holes separated Song and Jeong Nano Convergence (2017) 4:21 Page 4 of 8 from the excitons are generally transferred through TiO 2 the excitons generated by light are easily separated by and the electrolyte, respectively (Fig. 3a) [13]. Conse- the internal feld of the diode due to their high dielectric quently, the electric characteristics of the CQDs have constant, and the separated electrons and holes move in relatively low infuence on the device in the dye-sensi- the CQD thin flm. Terefore, their electronic properties tized solar cell. However, the CQDs with uncontrolled itself largely infuence on the CQD solar cells. Such elec- electrical properties can be formed as a monolayer on tronic properties can be controlled by chemically surface TiO2 to absorb only a small amount of light.