Mg Doped CuCrO2 as Efficient Hole Transport Layers for Organic and Perovskite Solar Cells

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.

Figure 1

(a) X-ray diffraction (XRD) patterns of CuCrO₂ (CCO) (black curve), 5% Mg:CCO (red curve), and 10% Mg:CCO (blue curve) powders. Two CCO polytypes, 3R-CCO (Powder diffraction files (PDF) #39-0247, pink sticks) and 2H-CCO (PDF#89-6743, purple sticks) are detected in all three XRD patterns. Prominent reflections are indexed between 30° to 40°, and ~62.0°. (b) Rietveld refinement of XRD patterns. The experimental (blue solid circle), calculated (red curve) and, difference (grey curve) patterns are shown. Structural files for the refinement are 3R-CCO (Crystallography Open Database (COD) No. 8104066) and 2H-CCO (COD No. 8104065).

Figure 2

Energy-dispersive X-ray spectroscopy (EDX) spectra for (a) 5% Mg:CCO and (b) 10% Mg:CCO. Mappings of Mg element (inset) show uniform distribution.

Figure 3

TEM images of (a) CCO, (b) 5% Mg:CCO, and (c) 10% Mg:CCO nanoparticles. Individual and double nanoparticles, as shown inside white circles in (a)–(c) are used to calculate size distributions shown in Table 2 and Figure 4a.

Figure 4

Box plots of (a) TEM-determined particle sizes from Figure 3 and (b) dynamic light scattering (DLS)-determined sizes for CCO (black color), 5% Mg:CCO (red color), and 10% Mg:CCO (blue color) nanoparticles. The percentiles are set to 90% whisker top, 75% box top, 25% box bottom, and 10% whisker bottom for each data set. In (a), ~50 individual nanoparticles are used in each data set. In (b), ~12 batches of DLS measurements are used in each data set.

Figure 5

High resolution TEM (HR TEM) image of 5% Mg:CCO nanoparticle. Lattice spacings corresponding to the (012) reflection in 3R-CCO and the (102) reflection in 2H-CCO polytypes are indicated.

Figure 6

X-ray photoelectron spectroscopy (XPS) spectra of (a) Cu 2p_3/2, (b) Cr 2p_3/2, (c) Mg 1s, and (d) O 1s orbitals for 5% Mg:CCO (red) and 10% Mg:CCO (blue) nanoparticles. The measured XPS spectra are represented by cross symbols. The fitted XPS spectra are represented by solid curves. The black lines show the binding energies for Cu(OH)₂, Cu¹⁺, Cr³⁺_I, Cr³⁺_II, Mg²⁺, O_I, O_II, and O_III. Cr³⁺_I represents Cr³⁺ as oxide, Cr³⁺_II is Cr³⁺ as hydroxide; O_I represents the lattice oxygen, O_II is hydroxyl species, and O_III is adsorbed water.

Figure 7

SEM images of (a) CCO, (b) 5% Mg:CCO, and (c) 10% Mg:CCO films on ITO substrates.

Figure 8

(a) Ultraviolet–visible (UV-vis) absorbance and transmission (inset) spectra and (b) Tauc plots and linear fits of the band edge (straight lines) for CCO (black curve), 5% Mg:CCO (red curve), and 10% Mg:CCO (blue curve) films.

Figure 9

Box plots of (a) work function and (b) ionization energy for CCO (black color), 5% Mg:CCO (red color), and 10% Mg:CCO (blue color) films. The percentiles are set to 90% whisker top, 75% box top, 25% box bottom, and 10% whisker bottom for each data set. In (a), 20 batches of WF measurements are used in each data set. In (b), 13 batches of IE measurements are used in each data set.

Figure 10

Average J-V curves (number of devices for each system is given in the footer of Table 4) of (a) P3HT:PC₆₁BM OSCs, (b) PFBT2Se2Th:PC₇₁BM OSCs, (c) PTB7-Th:ITIC OSCs, and (d) MAPbI₃ PSCs measured in AM 1.5G 100 mW cm⁻² illumination with CCO (black curve), 5% Mg:CCO (red curve), and 10% Mg:CCO (blue curve) hole transport layers (HTLs). In (d), solid J-V curves are measured under forward scan, dashed J-V curves are measured under reverse scan, and the inset is the external quantum efficiency (EQE) measurements of representative MAPbI₃ cells with CCO (black curve), 5% Mg:CCO (red curve), and 10% Mg:CCO (blue curve) HTLs.

Figure 11

Normalized XPS (a) survey, (b) Cu 2p, (c) Cr 2p, and (d) Mg 2p spectra at the surfaces of MAPbI₃ films on top of ITO/HTL. HTLs are CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue). In (a), all peaks are indexed by the component elements (C, N, Pb, and I) of MAPbI₃. In (b–d), the dotted lines show binding energies for the Cu 2p, Cr 2p, and Mg 2p core levels of CCO and Mg:CCO. The positions of Cu 2p_1/2, Cu 2p_3/2, Cr 2p_1/2, and Cr 2p_3/2 peaks are indexed according to our previous CCO reports [24,27]. The position of Mg 2p peak is indexed according to the report from Hoogewijs et al. [65]. In (d), the peaks correspond to the I 4d orbitals; peaks due to the Mg 2p orbitals are not observed.

Figure 12

(a) Photoluminescence (PL) emission spectra and (b) time-resolved photoluminescence (TRPL) decay for ITO/MAPbI₃ (green), ITO/CCO/MAPbI₃ (black), ITO/5% Mg:CCO/MAPbI₃ (red), and ITO/10% Mg:CCO/MAPbI₃ (blue). In (b), the lines are fits to three exponential decays: dotted green line for ITO/MAPbI₃, solid grey line for ITO/CCO/MAPbI₃, dashed brown line for ITO/5% Mg:CCO/MAPbI₃, and dotted-dashed blue line for ITO/10% Mg:CCO/MAPbI₃. The inset table in (b) shows the fitted PL lifetimes for all samples.

Figure 13

The stabilized photocurrents and efficiencies for the representative MAPbI₃ cells with (a) CCO, (b) 5% Mg:CCO, and (c) 10% Mg:CCO HTLs. In (a,b), the applied bias is at 0.8 V. In (c), the applied bias is initially at 0.85 V. After 50 s, it switches to 0.8 V.

Figure 14

The average J_sc barcharts with error bars for P3HT:PC₆₁BM OSCs, PFBT2Se2Th:PC₇₁BM OSCs, PTB7-Th:ITIC OSCs, and MAPbI₃ PSCs under forward and reverse scans for CCO (black color), 5% Mg:CCO (red color), and 10% Mg:CCO (blue color) HTLs.