*Article* **Mg Doped CuCrO2 as E**ffi**cient Hole Transport Layers for Organic and Perovskite Solar Cells**

**Boya Zhang 1, Sampreetha Thampy <sup>1</sup> , Wiley A. Dunlap-Shohl 2, Weijie Xu <sup>1</sup> , Yangzi Zheng 3, Fong-Yi Cao 4, Yen-Ju Cheng 4, Anton V. Malko 3, David B. Mitzi <sup>2</sup> and Julia W. P. Hsu 1,\***


Received: 19 July 2019; Accepted: 11 September 2019; Published: 13 September 2019

**Abstract:** The electrical and optical properties of the hole transport layer (HTL) are critical for organic and halide perovskite solar cell (OSC and PSC, respectively) performance. In this work, we studied the effect of Mg doping on CuCrO2 (CCO) nanoparticles and their performance as HTLs in OSCs and PSCs. CCO and Mg doped CCO (Mg:CCO) nanoparticles were hydrothermally synthesized. The nanoparticles were characterized by various experimental techniques to study the effect of Mg doping on structural, chemical, morphological, optical, and electronic properties of CCO. We found that Mg doping increases work function and decreases particle size. We demonstrate CCO and Mg:CCO as efficient HTLs in a variety of OSCs, including the first demonstration of a non-fullerene acceptor bulk heterojunction, and CH3NH3PbI3 PSCs. A small improvement of average short-circuit current density with Mg doping was found in all systems.

**Keywords:** Mg doped CuCrO2; hole transport layer; organic solar cells; perovskite solar cells

#### **1. Introduction**

With continued increase in power conversion efficiency (PCE), organic and perovskite solar cells (OSCs and PSCs, respectively) are promising for low cost clean electricity generation [1–3]. Further enhancing the PCE of OSCs and PSCs requires not only the development of better absorber materials, but also suitable transport layer materials. In OSCs and PSCs, the absorber is sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL), whose primary functions are to set up the built-in field across the absorber and selectively extract their respective carriers, while blocking the other type of carriers. In bulk heterojunction (BHJ) OSCs, the photogenerated excitons dissociate at the donor/acceptor interface to charged carriers. In PSCs, photoabsorption directly generates electrons and holes. These carriers then drift in opposite directions due to the built-in electric field, and travel through the transport layers to the electrodes [4]. Thus, both ETL and HTL play an important role in carrier extraction and device performance. For effective hole extraction from the absorber, the material used as HTL should possess good optical and electrical properties in addition to good physical and chemical stability. Extensive studies have been devoted to organic materials, such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate and 2,2 ,7,7 -Tetrakis[*N*,*N*-di(4-methoxyphenyl)amino]-9,9 -spirobifluorene, as HTLs [5–7]. However, these materials are expensive and degrade under air exposure [8,9]. Alternatively, metal oxides are shown to be promising candidates for HTL due to their low cost and improved stability [10]. Commonly used metal oxides are MoO*<sup>x</sup>* and WO3 [11,12], but these *n*-type semiconductors do not block electrons [13,14]. Among *p*-type HTLs, NiO*x* has been shown to have promising performance [15,16]. However, it suffers from low conductivity and high visible light absorption [17,18]. Therefore, developing new inorganic *p*-type HTLs is crucial to achieve highly efficient and stable devices.

Delafossite (AMO2; A = Cu1<sup>+</sup> or Ag1<sup>+</sup> and M is a trivalent metal) compounds are *p*-type oxides and have drawn significant interest since Kawazoe et al. reported CuAlO2 as a transparent oxide with room temperature conductivity up to 1 S cm−<sup>1</sup> [19]. Since then, many Cu-based delafossites have been synthesized with M = Al, Sc, Cr, Mn, Fe, Co, Ga, and Rh [20]. CuCrO2 (CCO) is particularly attractive due to its high conductivity [21]. Theoretical calculations and X-ray photoelectron spectroscopy (XPS) studies showed that Cu *d* states are dominant at the valence band maximum (VBM) and the intrinsic CCO conduction is through a Cu<sup>I</sup> /CuII mixed valence hole mechanism [22,23]. The size of CCO nanoparticles can be very small, ~10 nm [24,25]. In solar energy harvesting, it was first used in *p*-type dye sensitized solar cells (DSSCs) [25]. Moreover, CCO has been shown as a promising HTL in OSCs and PSCs [24,26–29]. To further increase the hole concentration, efforts have been made to replace the trivalent Cr3<sup>+</sup> cation with a divalent dopant such as Ni2<sup>+</sup>, Mg2+, or Zn2<sup>+</sup> [30–32]. In particular, Mg has been shown to be an excellent dopant to increase CCO conductivity [22,31,33]. Theoretical calculations showed that Mg doping induces low-formation energy defects just above the VBM in CCO and introduces new Cu *d* states in the bandgap, thus leading to a Cu<sup>I</sup> /CuII mixed valence and higher conductivity [22,33]. Compared with Be and Ca, defect states introduced by Mg are closest to the VBM, making it a more effective dopant [34]. Several experimental studies also confirm that Mg doping increases electrical conductivity [31,35–37]. In *p*-type DSSCs, Mg doped CCO (Mg:CCO) has performed superior to undoped CCO [38,39].

Based on our results of using undoped CCO as HTL in OSCs [24] and PSCs [27], we hypothesized that Mg:CCO could further improve the solar cell performance. Mg:CCO has not been applied as HTL in OSCs and, to our knowledge, there is only one report of using Mg:CCO as HTL in PSCs [40]. In this work, we examine the effect of Mg doping on CCO nanoparticles and their performance as HTLs in OSCs and PSCs. The CCO and Mg:CCO nanoparticles are synthesized by a hydrothermal method. The influence of Mg doping on structural, chemical, morphological, optical, and electronic properties of CCO films are carefully characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), ultraviolet–visible (UV-vis) absorption spectrometry, photo-electron spectroscopy in air (PESA), and Kelvin probe (KP) techniques. Finally, spin-coated CCO and Mg:CCO nanoparticle films are used as HTLs in three different BHJ OSCs and methylammonium lead iodide (MAPbI3) PSCs. Time-resolved photoluminescence (TRPL) is applied to probe charge transfer between MAPbI3 and HTL, and XPS is used to examine elemental diffusion. This is the first work to apply Mg:CCO nanoparticle films as HTLs in OSCs and the first demonstration in a non-fullerene acceptor BHJ system.
