Perovskite Self-Passivation with PCBM for Small Open-Circuit Voltage Loss

It is well known that [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) is a common n-type passivation material in PSCs, usually used as an interface modification layer. However, PCBM is extremely expensive and is not suita-ble for future industrialization. Herein, the various concentrations of PCBM as an additive are adopted for PSCs. It not only avoids the routine process of spin coating the multi-layer films, but also reduces the PCBM material and cost. Meanwhile, PCBM can passivate the grain surface and modulate morphology of perovskite films. Furthermore, the most important optical parameters of solar cells, the current density (J sc ), fill factor (FF), open-circuit voltage (V oc ) and power conversion efficiencies (PCE) were improved. Espe-cially, when the PCBM doping ratio in CH 3 NH 3 PbI 3 (MAPbI 3 ) precursor solution was 1 wt%, the device obtained the smallest V oc decay (less than 1%) in the p-i-n type PSCs with poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) as hole transport layer (HTL) and fullerene (C 60 ) as electron transport layer (ETL). The PSCs V oc stability improvement is attributed to enhanced crystallinity of photoactive layer and decreased non-radiative recombination by PCBM doping in the perovskites.

Energy and Power Engineering transport capacity, efficient conduction of electrons and holes, rapid increase in power conversion efficiencies (PCE) [1]- [8]. In 2009, Miyasaka and coworkers introduced a seminal work involving organic perovskite solar cells demonstrated with a low PCE of 3.8% [9]. After a decade of development, the highest recorded and certified PCE has been reached 25.2%, meeting the needs of commercial solar cells [10]. However, photovoltaic cells are more expensive than conventional energy sources, such as fossil fuels, wind, hydro, and nuclear, due to the manufacturing materials involved. Reducing the amount of materials used or synthesizing cheaper alternatives with similar functions to facilitate the commercialization of photovoltaic cells was necessary [11]. The PSCs device structure can be divided into two categories, a normal n-i-p architecture and an inverted p-i-n architecture [12] [13] [14] [15]. The normal n-i-p structure often needs mesoporous TiO 2 as the ETL, and the production of this material requires high temperature environment (>400˚C) [16] [17]. For PSCs with an inverted p-i-n structure, the conventional organic p-type material is usually PEDOT:PSS [14] [18], poly (triarylamine) (PTAA) [19] [20]. In contrast, inorganic p-type materials have nickel (II) oxide (NiO x ) [21] [22], copper thiocyanate (CuSCN) [23] [ 24], and copper iodide (CuI) [24] [25], while the n-type material is mainly fullerenes and its derivatives. Therefore, researchers prefer inverted p-i-n structures involving a low temperature preparation process and low hysteresis effect [26].
In the production of PSCs, the commonly used electronic transport materials and electronic trap states passivator of p-i-n structure include PCBM, C 60 and other fullerene derivatives [27] [28] [29]. Many studies have proved that in high-efficiency PSCs, C 60 and its derivative PCBM have excellent electron acceptability due to the spherical structural strain, hence making this kind of material the most effective passivation agent. However, since the PCBM has a higher electronic acceptance and excellent passivation effect on the active layer, it is widely used as a material of ETL. However, as an ETL passivation layer, C 60 is capable of passivation depth defects, but its passivation effect is worse than that of PCBM. Due to its much cheaper price, the use of C 60 as a passivation layer device promotes the commercialization process. In this work, considering the high price of PCBM, the method of micro-doping not only avoids the tedious process of spin coating multi-layer film, but also produces high-quality perovskite-PCBM heterojunction photoactive film [29]. In the whole device, the quality of active layer is the most critical factor affecting the PCE of photovoltaic cells. To obtain dense perovskite film, the purity of precursor solution should be high, and the solute should be completely dissolved. The solubility of lead iodide (PbI 2 ), Methyl ammonium iodide (MAI), and PCBM in organic solvents is limited, especially after the incorporation of PCBM. Therefore, in the preparation of doping solution, PCBM is stirred first and dissolved with N,N-Dimethyl formamide (DMF) and Dimethyl sulfoxide (DMSO) mixed solvent, and then perovskite precursors are added to form a slightly doped perovskite precursor solution. The addition of PCBM affected the morphology of perovskite, and an appropriate amount of PCBM can increase the grain size of perovskite film. A simple and convenient one-step method is used to spin coat the perovskite films. Figure  1(a) shows the structure of the devices in this work. Although the PSCs have higher carrier mobility and longer carrier life-times, the whole device will still have non-radiative recombination, which limits their V oc value below the Shockley-Queisser theory [30] [31]. In this experiment, it was found that doped PCBM could not only passivate perovskite, increase grain size, but also improve the cells' V oc . When the PCBM doping concentration in MAPbI 3 precursor solution was from 0 -1 wt%, the J sc , FF and V oc of the devices increased, the V oc of the planar ITO/PEDOT:PSS/MAPbI 3 with PCBM/C 60 /BCP/Ag (p-i-n) device structure reached more than 0.99 V, and the V oc loss was less than 1% compared with the theoretical value.

Device Fabrication
The patterned ITO glass substrates were subsequently cleaned in an ultrasonicator with ITO detergent, acetone, isopropyl alcohol and deionized (DI) water for 15 minutes each. After that, the ITO substrates were treated in a UV ozone oven for 15 min. First, PEDOT:PSS was spin-coated at 4000 rpm for 30 s onto the ITO glass substrate, followed by annealing at 150˚C for 10 min. Pristine perovskite precursor solution was prepared by dissolving PbI 2 and MAI (1:1 molar ratio) in the solvent mixture of DMF and DMSO (9:1 v/v) for a total concentration of 1.5 m in the glovebox. For doped perovskite precursor solution, perovskite concentration remained unchanged, and PCBM concentration was calculated according to the amount of PbI 2 . The solution was stirred at room temperature overnight. Then, the perovskite layer was spun onto the hole transport layer (HTL) at 3000 rpm for 10 s and 5000 rpm for 50 s. Next the substrate was annealed on a hot plate at 100˚C for 20 min. Finally, the devices were finished by thermally evaporating C 60 (50 nm), BCP (8 nm), and Ag (80 nm) in the order.

Optoelectronic Characterization
The perovskite thin film surface morphology images were characterized by atomic force microscopy (Hitachi AFM 5100N). The top-view scanning electron microscopy (SEM, JEOL JEM5610) of the perovskite, which were spin coated on ITO/PEDOT:PSS substrates. Steady state photoluminescence (PL) spectra and PL mapping were acquired by Raman spectrometer (LabRAM HR Evolution) with 532 nm laser unit, which was manufactured by HORIBA FRANCE SAS.
The time-resolved photoluminescence spectra (FLS1000, TRPL) were obtained with the range of wavelength from 600 nm to 800 nm by exciting at 532 nm laser. The absorption spectra of the perovskite films were determined by UV-visible spectrophotometer. The crystallinity of the perovskite thin film was monitored by X-ray diffraction (XRD, Rigaku, SmartLab3kW

Results and Discussion
PL spectra are an effective and non-destructive method to characterize the optical properties of semiconductor materials. By analyzing a PL spectrum, the bandgap, impurity type and activation energy of semiconductor material can be obtained. The TRPL indicates that electron relaxation and non-radiative recombination physical mechanisms can be obtained by detecting the change of photoluminescence intensity at a certain wavelength with time by pulsed excitation.
PL and TRPL were used to investigate the effect of the PCBM additive on recombination and carrier lifetime of the perovskite film. It can be seen from the pristine and PCBM-perovskite thin films PL spectrum shown in Figure 1  It was found from a series of doping concentration J-V curves (Figure 4(a)) that the concentration of PCBM in the perovskite precursor would affect the photovoltaic parameters of the whole device. The corresponding photovoltaic parameters are listed in Table S1 (Support information). The J sc of the device is related to the band gap, thickness, surface morphology, and carrier transport properties of the perovskite layer [33]. The addition of a certain concentration of PCBM can improve carrier mobility and thus obtain a higher J sc , as illustrated in Figure 4(b). It was found that the FF of the device increased when the additive was very small (Figure 4(c)). Since the FF of the device is affected by series and parallel resistance, it reflects the ideal degree of the diode of the device. Especially for the V oc of the device, When PCBM was added to the photoactive film as an additive, the V oc of the device changed from 0.82 to 0.99 V (Figure 4(d)). Generally, the inverted p-i-n PSCs with PEDOT:PSS as HTL and C 60 as ETL exhibit a V oc less than 1.0 V.  [39]. Combining with the above equation, it can be seen from the energy level diagram (Figure 4(e)) that when the concentration of photoactive layer doped PCBM was 1 wt%, the V oc loss of the device was 1%. In theory, for PSCs, the V oc loss is related to the perovskite bandgap (E g ) and the elementary charge (e), determined by the equation of ( loss g oc V E e V = − ) [40] [41] [42]. In general, the high V oc loss can be attributed to the high trap density of the perovskite surface causing severe interface defects non-radiative recombination, and an undesired energy-level mismatching between the n-type layer and p-type in the device [40] [43]. The loss mechanism of V oc is an important and necessary research topic for    Figure S1 (Support information). It can be seen from Figure 4(f) that a noticeable EQE enhancement was characterized for PCBM incorporated PSCs. Furthermore, the EQE spectrum was also measured for devices constructed from different additive concentrations of PCBM in order to evaluate the efficient photocurrent at various wavelengths and to provide an additional measure of material band gap from the beginning to the end of photocurrent generation.

Conclusion
In summary, through a series of optical characterization such as PL, SEM and UV-vis, it was shown that the addition of small amount of PCBM to the precursor can form a self-passivating, high-quality perovskite thin film. As a result, the solar cells obtain a higher V oc , J sc , and FF. From the theoretical formula, the attained of 0.99 V is among the smallest V oc loss for p-i-n type PSCs with PEDOT:PSS as HTL and C 60 as ETL in the obtained perovskite band gap of 1.61 eV. Systematic perovskite thin film optical characterization of the PSCs V oc enhancement indicated that the improvement is mainly attributed to the enhanced crystallinity of photoactive layer and interface efficient charge carrier dissociation with decreased non-radiative recombination. Therefore, PCBM as micro-additive may be applicable to other organic-inorganic hybrid photoelectronic devices.  Figure S1. The PCE of pristine and PCBM as additive in the absorber layer perovskite solar cells.