Thermal evaporated CuI film thickness-dependent performance of perovskite solar cells

Highlights

• CuI hole transport films are first fabricated by vacuum thermal evaporation procedure for n-i-p perovskite solar cell.

• 200 nm thick CuI film exhibits the best carrier extraction ability.

• The device with optimized CuI film achieves an efficiency of 8.07%.

• With the dual modification of reduced graphene oxides, the efficiency can be improved to 8.69%.

Abstract

CuI, as a promising hole transport material, possesses high hole mobility and is compatible with perovskite materials about band structure. Herein, CuI hole transport films are first fabricated by vacuum thermal evaporation procedure for n-i-p perovskite solar cell. The results show that vacuum thermal evaporated CuI films have no obvious influence on perovskite underlayer and 200 nm thick CuI film has the best carrier extraction ability. To further improve the stability of the device, reduced graphene oxides are introduced to avoid the accelerated attenuation caused by the reaction of CuI with Au. Finally, the power conversion efficiency of the dual-modified device improves to 8.69%, with the short circuit current density of 18.86 mA cm⁻², the open circuit voltage of 0.832 V and the filling factor of 0.550. The work demonstrates a potential application of vacuum thermal evaporated CuI films in organic-inorganic hybrid perovskite solar cells.

Introduction

Over the past few years, organic-inorganic hybrid perovskite solar cell (PCS) has become one of the most promising candidates for the next generation of solar cells because of its excellent photoelectric performance and low production cost [[1], [2], [3]]. Generally, PSCs can be classified into two structures: normal structure (n-i-p) and inverted (p-i-n) structure. Lately, the power conversion efficiency (PCE) record of n-i-p structured PSCs has increased to 25.2% [4]. For a typical n-i-p structure PSC, the widely used hole transport materials (HTM) are organic polymers [5]. For the time being, the most commonly used organic hole conductor is spiro-OMeTAD (2,2′,7,7′-tetrakis(N,N-di-p-me-thoxyphenylamine) 9,9′-spirobifluorene). However, the synthesis method of spiro-OMeTAD is complex and expensive. What's more, spiro-OMeTAD has low hole mobility and conductivity. It only performs efficiently in the case of doping lithium salt and/or Co(III) complex [6,7]. While this kind of dopants are sensitive to water and oxygen, thus restricting the long-term stability of PSC device. Recently, a variety of inorganic p-type materials have been successfully employed as HTMs in PSC devices, such as CuSCN [8], CuI [9], Cu₂O [10], CuS [11], CuGaO₂ [12], CuAlO₂ [13], NiO_x [14], MoS₂ [15]. In contrast with organic hole conductors, inorganic hole transport conductors are low cost and have high mobility and stability, making them good alternatives.

Among the new inorganic HTMs, CuI, a cheap and stable p-type semiconductor, shows higher hole mobility and ideal band alignment, laying a theoretical foundation for obtaining high efficiency PSCs [16]. Besides, the report show that the hysteresis of CuI-based PSCs is much lower than that of traditional spiro-OMeTAD-based PSCs [17]. To date, several reports display the successful application of CuI as HTM in PSC devices. In 2014, Prashant V. Kamat et al. employed CuI as HTM in PSCs for the first time. They prepared CuI HTL on CH₃NH₃PbI₃ perovskite film by drop-casting method. The as-resulted n-i-p structured PSC device achieved a PCE of 6.0% [18]. In 2015, Yue Shen et al. prepared CuI HTL in n-i-p structured PSC by spraying CuI solution of propyl sulfide with spray gun, receiving a PCE of 5.8% [19]. In 2016, Tao Yu et al. found that the adverse corrosion reaction between CH₃NH₃PbI₃ and Cu cathode could form p-type CuI layer in situ. The effect promoted an amazing evolution which boosted the efficiency of PSC dozens of times as from 0.1% to over 3.0% [20]. In the same year, Ahmad Moshaii et al. prepared CuI HTL in n-i-p structured PSC by gas-solid phase transformation from Cu to CuI, achieving a PCE of 7.4% [16]. In 2017, Kenta Takahashi and Yoshikazu Suzuki prepared CuI HTL by spin coating CuI solution. After aging 20 days, the device achieved an optimum photovoltaic performance with the PCE value of 6.5% [21]. In the same year, Bahram Abdollahi Nejand et al. deposited Cu layer on top of the perovskite layer which contained excess amount of CH₃NH₃I. The extra CH₃NH₃I provided I to react with Cu for ease of producing CuI HTL at the interface of Cu metal contact and perovskite layers. The prepared device obtained a PCE of 9.24% [22]. In 2019, Punniamoorthy Ravirajan et al. incorporated CuI powder layer between perovskite layer and Pt top-contact using solvent-free powder pressing method. The champion PSC yielded a PCE of 8.1% [23]. The p-i-n structured PSC with CuI HTL also exhibited superior photovoltaic performance. Zuqiang Bian et al. assembled inverted PSCs with solution-processed CuI film as HTL. The corresponding device acquired an impressive PCE value of 16.8% [24]. Although CuI has been successfully used in PSCs, disadvantages exist in some preparation methods. For solution processed CuI films, the existence of organic solvents may corrode perovskite underlayers. For chemical methods, CuI film prepared by iodization is uneven and the reaction is difficult to control. Therefore, it remains a challenge to fabricate CuI film by appropriate methods.

In this work, CuI films prepared by vacuum thermal evaporation process are used as HTLs in n-i-p structured PSCs for the first time. According to the literature, the thickness of CuI film has a great influence on optical bandgap and film resistance, manifesting that appropriate film thickness is conducive to the optoelectrical properties of CuI based device [25]. Therefore, in this work, the film thickness of CuI is controlled by evaporation time. After optimization, the PSC device with proper thickness acquires a PCE of 8.07%. On this basis, reduced graphene oxides (rGO) are employed to modify the dual side of CuI films. As reported, rGO has excellent conductivity and thermal conductivity. On one hand, the rGO interlayer between perovskite light absorption layer and CuI HTL helps reduce interfacial recombination and enhance hole extraction [26]. On the other hand, the rGO interlayer between CuI HTL and Au top electrode can act as a barrier for I⁻ ion and Au diffusion, decelerating the undesired reaction between I⁻ ion and Au [27,28]. Through dual interfacial engineering, the PCE of the CuI based PSC improves to 8.69%.