Grain Boundaries Contribute to the Performance of Perovskite Solar Cells by Promoting Charge Separations

Abstract

Historically seen as a limitation, grain boundaries (GBs) within polycrystalline metal halide perovskite (MHP) films are thought to impede charge transport, adversely impacting the efficiency of perovskite solar cells (PSCs). In this study, we employ home-built confocal photoluminescence microscopy, combined with photocurrent detection modules, to directly visualize the carrier dynamics in the MHP film of PSCs under real operating conditions. Our findings suggest that GBs in high-efficiency PSCs function as carrier transport channels, where a notable enhancement in photocurrent is observed. Femtosecond transient absorption and Kelvin probe force microscopy measurements further validate the existence of a built-in electric field in the vicinity of GBs, offering additional driving force for charge separation and establishing channels for swift carrier transport along the GBs, thereby expediting subsequent charge collection processes. This study elucidates the pivotal role of GBs in operational PSCs and provides valuable insights for the fabrication of high-efficiency PSCs.

1. Introduction

Polycrystalline semiconductor films are widely utilized in a variety of devices, including solar cells [1–6], thin-film transistors (TFT) [7–10], thin-film thermoelectric generators [11, 12], and microelectromechanical systems (MEMS) [13]. The performance of these devices hinges on the polycrystalline film’s microstructures, particularly grain boundaries (GBs) [14]—the interfaces that separate individual grains oriented in distinct crystallographic directions [15]. Metal halide perovskite solar cells (PSCs) have achieved fast progress in power conversion efficiency (PCE), which is largely determined by the quality of the metal halide perovskite (MHP) polycrystalline film that contains a large quantity of GBs [16, 17]. Comprehending the role of GBs in polycrystalline MHP film is imperative for the rational design of the active layer and, ultimately, for enhancing device performance [18–20].

In the realm of PSCs, the GBs in MHP films have predominantly been labeled as detrimental factors that influence device stability [21, 22] and efficiency [17, 23–25]. This perspective is primarily attributed to the markedly greater defect density at GBs in comparison with the bulk phase [23]. It is widely accepted that GBs inherently hinder charge transport through their scattering effects or trapping mechanisms, consequently exacerbating nonradiative recombination and leading to energy loss [25, 26]. However, PSCs using single-crystal MHP films or those with ultra-large grains did not achieve higher PCE as anticipated, suggesting that the impact of GBs may not be entirely negative [27, 28]. Actually, in other solar cells such as polycrystalline CdTe [6] and copper indium gallium selenide (CIGS) [29], certain specific GBs have been found to paradoxically enhance the efficiency of charge transport. Recent studies have proposed that GBs in MHP films may enhance the efficiency of PSCs by facilitating the separation of electrons and holes [30, 31] However, this conclusion was drawn from the investigation on isolated MHP films, rather than on fully assembled or operational PSCs. Therefore, the role of GBs (such as whether they facilitate carrier transport as in CdTe or CIGS solar cells) is indeed unclear in an operational PSC.

In this work, we employ home-built confocal photoluminescence (PL) microscopy, combined with a photocurrent detection module, to directly map the local photocurrent and PL in operational PSCs with different PCE values with a sub-micrometer spatial resolution.