Lithium Polystyrene Sulfonate as a Hole Transport Material in Inverted Perovskite Solar Cells

Abstract

Despite the exceptional efficiency of perovskite solar cells (PSCs), further improvements can be made to bring their power conversion efficiencies (PCE) closer to the Shockley‐Queisser limit, while the development of cost‐effective strategies to produce high‐performance devices are needed for them to reach their potential as a widespread energy source. In this context, there is a need to improve existing charge transport layers (CTLs) or introduce new CTLs. In this contribution, we introduced a new polyelectrolyte (lithium poly(styrene sulfonate (PSS))) (Li:PSS) polyelectrolyte as an HTL in inverted PSCs, where Li+ can act as a counter ion for the PSS backbone. The negative charge on the PSS backbone can stabilize the presence of p‐type carriers and p‐doping at the anode. Simple Li:PSS performed poorly due to poor surface coverage and voids existence in perovskite film as well as low conductivity. PEDOT:PSS was added to increase the conductivity to the simple Li:PSS solution before its use which also resulted in lower performance. Furthermore, a bilayer of PEDOT:PSS and Li:PSS was employed, which outperformed simple PEDOT:PSS due to high quality of perovskite film with large grain size also the large electron injection barrier (ϕe) impeded back diffusion of electrons towards anode. As a consequence, devices employing PEDOT:PSS / Li:PSS bilayers gave the highest PCE of 18.64%.

Solution‐Processed Metal Ion Polyelectrolytes as Hole Transport Materials for Efficient Inverted Perovskite Solar Cells

“…Because very thin films are used, electrons can back‐diffuse and tunnel through the metal:PSS interlayers, which is one reason that the FF values of the materials are somewhat low compared to PEDOT:PSS. [ 41,43 ]…”

“…Because very thin films are used, electrons can back-diffuse and tunnel through the metal:PSS interlayers, which is one reason that the FF values of the materials are somewhat low compared to PEDOT:PSS. [41,43] Figure 3b summarizes the normalized PCE data for all of the metal: PSS-based devices. Here, the benchmark device (PE-DOT:PSS) showed the highest performance compared to other HTLs.…”

Recent progress on the use of graphene-based nanomaterials in perovskite solar cells

“…[33][34][35][36][37][38][39][40] Perovskite materials have several advantages including strong absorption in a widespread window up to 800 nm, long carrier diffusion lengths (>175 mm), high ambipolar charge transporting ability, low recombination loss, low exciton binding energy (<25 meV), high carrier mobility, low cost, simple processability, remarkably high PCE, and composition and bandgap tunability. [41][42][43][44][45][46][47][48][49][50] Typically, the PSCs are divided into 3 kinds of mesoporous ni-p, planar n-i-p, and planar p-i-n (Fig. 2).…”

Improved Hole Injection in Hybrid Light‐Emitting Transistors Incorporating Lithium and Copper(II) Poly(Styrene Sulfonate)

“…By incorporating metal-based PSS polyelectrolytes (metal:PSS) admixed with PEDOT:PSS into HILs, significantly improved performance in solar cells has been demonstrated due to reduced hole-injection barriers (Φ h ) at the HIL/active layer interface and via a p-doping effect. [25][26][27][28][29] Metal:PSS polyelectrolytes feature outstanding solubility in polar solvents such as water or methanol and additionally offer pH neutrality relative to PEDOT:PSS. These attributes lead to easy processing on organic and hybrid devices.…”

“…Metal:PSS additive (A) compositions contain a relatively small volume of PEDOT:PSS (10 μL) added to 1 mL of metal:PSS solution while metal:PSS mixtures' (M) compositions comprise a 3:7 ratio of metal:PSS to PEDOT:PSS solutions; these compositions were used as they were previously shown to work well as HILs in solar cell devices. [25][26][27][28] We investigated the influence of these different interlayers on the band structure and performance of HLET devices and found that employing Cu:PSS M resulted in significantly enhanced brightness of up to 4.89 × 10 3 cd m −2 , with an EQE of up to 0.45%, which was a dramatic improvement compared to HLETs without no HIL (no emission was observed) and pristine PEDOT:PSS (2.17 × 10 2 cd m −2 with an EQE of 0.01%). To understand how the hybrid polyelectrolyte HILs influenced the charge injection and recombination in ntype HLET devices, we carried out ultraviolet photoelectron spectroscopy (UPS) analysis and photoluminescence (PL) quenching studies.…”