Adding Copper Bromide as a Hole Transport Layer in Solar Cells

By Samudrapom DamReviewed by Susha Cheriyedath, M.Sc.Jul 18 2022

A paper recently published in the journal ACS Applied Energy Materials demonstrated the feasibility of using copper bromide (CuBr) as a hole transport layer (HTL) to realize a better and more stable device performance in perovskite solar cells (PSCs).

Study: Copper Bromide Hole Transport Layer for Stable and Efficient Perovskite Solar Cells. Image Credit: Diyana Dimitrova/Shutterstock.com

Background

The development of metal halide PSCs has invigorated the field of thin-film photovoltaics (PVs), as the power conversion efficiencies (PCEs) have increased from 3.8% to over 25% within a decade, moving closer to the maximum theoretical efficiency of 33%.

However, metal halide PSCs' long-term stability and efficiency are limited by charge accumulation at the interfacial defects and significant non-radiative losses. Previous studies have demonstrated that p-type contact is a major source of defects in PSCs, and passivation using HTLs can considerably increase their photoluminescence quantum efficiency (PLQE).

Poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) is typically used as an HTL in p-i-n devices owing to its high optical transmittance and electrical conductivity. However, the mismatch between the valence band (VB) edge of the perovskite and the work function (WF) of the PEDOT:PSS can limit the open circuit voltage (V_OC) of the devices.

Additionally, PEDOT:PSS can react with the active layer of the perovskite and corrode the electrodes. The reactivity of organic HTLs with the oxidized products of iodides can also lead to degradation of the device. These factors necessitated the identification of new HTLs as substitutes for organic HTLs in PSCs.

In recent years, inorganic p-type semiconductor HTLs have gained considerable attention as they are stable, can be prepared using low-cost and simple processes, and have high charge collection and hole mobility.

Specifically, cuprous salts, such as copper iodide (CuI) and copper thiocyanate (CuSCN), have demonstrated significant potential as an alternative to organic HTLs. However, the solution processing of CuSCN is complicated, while aggregates formed in the CuI films upon annealing lead to undesired effects.

CuBr can be used to overcome the limitations of CuSCN and CuI as it is hydrophobic in nature and easily solution-processable. Moreover, CuBr possesses the advantages of both CuSCN and CuI. Although CuBr is inexpensive, efficient, and has a 5.34 eV VB edge that is close to that of a perovskite, its feasibility as an HTL in PSCs has not been explored until now.

The Study

In this study, researchers fabricated inverted PSCs based on a p−i−n device architecture containing indium tin oxide (ITO)/HTL/ methylammonium lead iodide (MAPbI₃)/[6,6]-phenyl C₆₁ butyric acid methyl ester (PC₆₁BM)/C₆₀−N/ silver (Ag), with CuBr and PEDOT:PSS as HTLs, and investigated the feasibility of using CuBr as an HTL for stable and efficient p-i-n PSCs.

Researchers performed photocurrent density-voltage (J−V) curve measurements under 100 mW cm⁻² irradiation, impedance spectroscopy using an impedance analyzer, capacitance-voltage (C−V) measurements, and external quantum efficiency (EQE) measurement to characterize the fabricated devices. All C−V and J−V measurements were performed inside a nitrogen-filled glovebox.

Researchers also performed ultraviolet photoelectron spectroscopy (UPS) measurements, electric force and topographical scanning probe measurements, and contact angle measurements to characterize the CuBr and PEDOT:PSS films on the ITO substrates.

Observations

Researchers successfully fabricated inverted MAPbI₃-based PSCs. The CuBr HTL-based devices outperformed PEDOT:PSS-based devices in terms of stability and efficiency.

The average PCEs of CuBr-based devices and PEDOT:PSS-based devices were 15.76 ± 0.82% and 10.94 ± 0.65%, respectively, which indicated that CuBr-based devices possessed a higher PCE compared to PEDOT:PSS-based devices. A maximum PCE of 17.65% was observed in a CuBr-based device with higher V_OC and short circuit density (J_SC).

CuBr-based devices demonstrated a higher current density of 21.2 mA cm⁻² from an EQE compared to 16.2 mA cm⁻² in PEDOT:PSS devices, indicating that the interfacial resistance for holes was lower in CuBr-based devices as photocarriers were generated near the perovskite/HTL interface at shorter wavelengths.

Perovskite grains with an average size of 530 nm were observed in the perovskite films on the CuBr HTL compared to the grains with an average size of 260 nm in films on the PEDOT:PSS HTL. Large grain sizes led to lower grain boundary-related defects, which enhanced the performance of CuBr-based devices.

Moreover, the perovskite films on the CuBr HTL displayed a higher crystallinity compared to the films on PEDOT:PSS HTL. The larger perovskite grain size and higher crystallinity of the perovskite films in CuBr-based devices were attributed to the CuBr templating effect that facilitated the growth of perovskite film. The contact angle measurements showed the hydrophobic nature of CuBr layers that led to reduced reactivity at interfaces and larger grain sizes and crystallinity.

The CuBr-based devices showed a larger built-in potential (V_bi) of 1.03 V compared to 0.77 V observed in PEDOT:PSS-based devices. The large V_bi suppressed the recombination loss and enhanced the V_OC. The higher V_OC and V_bi in CuBr-based devices were attributed to their lower trap densities.

The CuBr-based devices demonstrated a higher electrical conductivity and recombination resistance compared to PEDOT:PSS-based devices. Additionally, a substantially reduced PL decay time was observed in perovskite films on CuBr HTL compared to films on PEDOT:PSS HTL, indicating that the photogenerated charges/holes in the perovskite film were extracted efficiently at the CuBr−perovskite interface. The high electrical conductivity indicated efficient transportation of the charges to the HTL.

Furthermore, the CuBr-based PSCs displayed superior stability when stored within an inert nitrogen-filled glovebox under white fluorescent light. The CuBr-based devices maintained 95% of their initial PCE compared to 53% retained by the PEDOT:PSS-based devices after 476 h. The pH neutrality of the CuBr did not degrade the underlying ITO anode, resulting in long-term stability.

The unencapsulated CuBr-based PSCs also displayed higher stability under 30% relative humidity by maintaining 96% of their initial PCE after 244 h compared to 61% maintained by the PEDOT:PSS-based PSCs.

Taken together, the findings of this study demonstrated the feasibility of using CuBr as an HTL for fabricating efficient and stable PSCs.

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