Hole-extraction and photostability enhancement in highly efficient inverted perovskite solar cells through carbon dot-based hybrid material

Highlights

• Carbon Dots hybrid material is used in place of PEDOT:PSS as hole transporting material.

• UV-Absorbing Carbon Dots is used as downshifting layer to improve the efficiency and the long term stability of the device.

• The efficiency improved significantly from 14.7% to 16.8% of the best device with optimized Cdots content.

• The presence of Cdots on graphene oxide increases the rate of hole injection and decreases charge recombination.

Abstract

We report the effect of the integration of carbon dots (Cdots) in high-performance inverted planar-heterojunction (PHJ) perovskite solar cells (PSCs). We used Cdots to modify the hole-transport layer in planar PSC devices. By introducing Cdots on graphene oxide (GO) as hole-transporting layer, the efficiency of the PSC improved significantly from 14.7% in the case of bare GO to 16.2% of the best device with optimized Cdots content. When applying Cdots with an engineered absorption in the UV range as downshifting layer, the device performance was further improved, attaining a maximum PCE of 16.8% (+14%); the stability of the device was also enhanced of more than 20%. Kelvin probe force microscopy (KPFM) and cyclic voltammetry (CV) were employed to analyze the electronic band alignment at the interface between GO/Cdots and the perovskite film. Holes were extracted and transferred to the conductive substrate more efficiently in the presence of Cdots, thus delaying charge recombination. Photoluminescence (PL), transient PL decays and transient photovoltage (TPV) decays investigated the charge-transfer kinetics and proved the retardation of charge recombination. This work reveals an effective enhancement of the performance of planar PSCs by using Cdots/GO as hole transport material.

Introduction

The generation of electric power directly from solar radiation represents a promising opportunity to address the increasing demand for clean energy, while decreasing the environmental impact caused by excessive CO₂ emissions from fossil fuels. Solar energy is the most abundant renewable energy source and solar cells provide a convenient means to harvest it. Because of the costly fabrication and maintenance of conventional solar cells, such as those based on Si, a new generation of more cost-effective solar cells, the so-called third generation, is being investigated. Various materials systems and architectures have been proposed and realized, such as dye-sensitized solar cells (DSSC) [[1], [2], [3]], organic-polymer solar cells (OPV) [4,5], quantum-dot solar cells (QDSC) [6,7] and perovskite solar cells (PSC) [[8], [9], [10]]. To effectively compete with commercial silicon solar cells, a significantly increased efficiency, enduring stability and decreased price per watt of generated power are required.

Among various methods that have been developed, an efficient approach to enhance the performance of third-generation solar cells is to improve the charge extraction using a suitable charge-transport layer (CTL). There are mainly two kind of CTL: hole transport layer (HTL) and electron transport layer (ETL). An effective CTL increases the extraction of the photo-generated charge carriers from the absorbing layer while avoiding their recombination. Strategies to increase charge collection include the use of various metal oxides, organic polymers and small organic molecules [[11], [12], [13], [14], [15]]. Among them, carbon nanomaterials, such as carbon nanotubes (CNT) [16], fullerene [17] and graphene nanosheets [18], are interesting alternative CTL materials that have been successfully employed in various solar cells, such as DSSC [19,20], OPV [21,22] and PSC [[23], [24], [25], [26], [27]]. In general, carbon nanoallotropes possess intriguing opto-electronic properties that can improve the efficiency of solar cells.

The latest addition to the carbon nanomaterials family are carbon dots (Cdots) [28]. They are composed of discrete, quasi-spherical nanoparticles of size less than 10 nm. They have emerged as prospective competitors to conventional semiconductor quantum dots (QDs) and recently they have been proposed as possible CTL for solar cells [29,30]. Cdots are exclusively composed of non-toxic, earth abundant elements (C, N, H and O) and can be synthesized in large quantities via a simple hydrothermal or microwave approach [31]. Relative to conventional semiconducting QDs, their advantages include being non-toxic, cheap, chemically stable and simply prepared from abundant carbon-based feedstock [29].

Within third-generation solar cells, organic or inorganic halides having a perovskite (PSK) crystal structure represent an opportunity to develop high efficiency solar cells with solution processes at modest production cost. In less than a decade, their power conversion efficiency (PCE) increased from 10% to over 23% [32,33]. Their excellent optical properties and their facile fabrication resulted in a rapid growth in research and publications on this topic [8,34,35].

The planar inverted configuration is one of the most widely used configurations of PSC. In this architecture, the cell is commonly illuminated from the p-side, resulting in a structure glass/ITO/HTL/perovskite/ETL/metal which functions in a superstrate configuration [10]. In this layout, the HTL is critical for achieving high V_oc and overall high performance [21,32].

So far, most inverted solar cells have used (3,4-ethenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as HTL. Although PEDOT:PSS-based PSK solar cells have been reported with high efficiency, severe charge injection losses at the interface can limit the V_oc [33]. Furthermore, due to its acidity, hygroscopic properties and inhomogeneous electricity, devices based on PEDOT:PSS showed poor long-term stability. Various promising HTLs, such as NiO_x [34], MoO₃ [35], CuSCN [36], and others, have also been explored to fabricate efficient inverted PSK solar cells in the superstrate configuration. In general, all these HTLs showed good performance at the cost of long and complicated fabrication processes.

Graphene structures have been shown to be effective CTLs and transparent conductive electrodes in PSCs [36]. For example, graphene oxide (GO), a well-known derivative in the graphene family, was applied as a hole-transport layer in inverted planar-heterojunction (PHJ) PSC and yielded a PCE exceeding 12% [24,36]. Although GO is an efficient hole-extracting material, hole propagation on the oxygen groups limits the transfer of extracted holes and causes charge recombination. Even if reduced graphene oxide (rGO) can solve this problem, there remain issues that should be addressed: rGO requires protracted and complicated preparation and yields poor surface coverage; in GO-based devices the hole propagation from GO to the indium tin oxide (ITO) substrate is slow [25].

Cdots have been already employed as electron transport layer, or as additive in the perovskite layer itself showing promising results [[37], [38], [39]]. Recently it has been also studied theoretically their use as hole transport layer [40]. In this work, to overcome the limitations of the GO layer, we introduce fluorescent Cdots to form a homogeneous GO/Cdots HTL to develop high efficiency inverted PHJ PSCs. Based on our knowledge, this is the first report in which is highlighted experimentally the hole transport ability of the Cdots. Exploiting the excellent opto-electronic properties of Cdots [29], this work demonstrates that an optimal amount of Cdots in optimized proportion in a composite with GO nanosheets can significantly improve hole extraction from the perovskite film at the interface with GO/ITO. The addition of Cdots creates an intermediate energy level that shifts the GO work function, enabling an improved alignment of energy levels with the perovskite film. In this way, the holes can be extracted more efficiently than from GO alone. The increased hole-transport property at the interface of ITO/GO/PSK increases the short-circuit current density (J_SC) and open-circuit voltages (V_OC), yielding a maximum efficiency of 16.2%, as opposed to a PCE of 14.7% obtained from the GO-based perovskite device.

Long term stability is one of the most critical parameters for solar cells in general (also because it affects overall cost), and is a major challenge for PSCs specifically. In particular, the stability of PSC under ultraviolet light remains a major problem [41,42]. To address this issue, we prepared UV-absorbing Cdots and embedded them in a polymer matrix to serve as the downshifting layer over the perovskite cell. The incorporation of Cdots as a protective layer increases the light-soaking stability by more than 20% and further increases the PCE of the solar cell to 16.8%.