CsPbI3-perovskite quantum dot solar cells: unlocking their potential through improved absorber layer characteristics and reduced defects

Perovskite quantum dots (CsPbI3-PQDs), a translucent material, have gained great interest in the PV industries owing to their unified virtues of perovskites and quantum dots. However, researchers have found that perovskite solar cells (PSCs) suffer from issues like low stability at high relative humidity, energy states imbalance, severe hysteresis, and an easy decomposition under ultraviolet (UV) radiation that severely restrict their industrialization. Quantum dots (QDs) are excellent materials with numerous admirable traits that have been extensively employed in PSCs to overcome the aforementioned problems. To achieve high performance of the examined device, the CsPbI3-PQDs has been stacked between two charge transport layers, i.e., Cl@SnO2 (to facilitate electrons towards cathode) and P3HT (to facilitate holes towards anode). In this context, study of variations in different parameters such as thickness and acceptor density of the CsPbI3-PQDs absorber layer has been done. After varying the thickness and acceptor density of the CsPbI3-PQDs layer, the cell’s performance is optimized at thickness of 400 nm and acceptor density of 1 × 1017/cm3 delivering higher PV parameters power conversion efficiency (PCE):16.17%, open circuit voltage (VOC):1.02 V, short circuit density (JSC):18.06 mA cm−2 and fill factor (FF): 87.06% respectively. Thereafter, the effects of bulk defects in CsPbI3-PQDs and the interface between CsPbI3-PQDs and Cl@SnO2 have been explored in this work. For the cell to work at its best, the bulk defect density and interface defect density, respectively, should not be more than 1 × 1014 /cm3 and 1 × 1013 /cm2. Afterwards, a comprehensive study has been done by varying the front electrode transparency (from 40% to 95%) to improve the device performance. With 95% of front electrode transparency, the performance of device is improved due to increase in the photon coupling.


Introduction
All organic-inorganic hybrid perovskites have lately attracted a lot of attention in the photovoltaic (PV) area due to their remarkable photoelectric characteristics [1][2][3][4]. Photovoltaic, light-emitting diodes (LEDs), and photo detectors are examples of optoelectronic devices that use lead halide perovskites (LHPs) as semiconducting materials with a bandgap in the visible region of the spectrum [5][6][7][8][9][10][11]. Their surprising optoelectronic characteristics, such as high absorption coefficient, high charge carrier mobility, and extended carrier diffusion length, are to account for the use of LHP's [12][13][14][15][16][17][18]. The photostability and thermal stability of perovskite solar cells have consistently remained a central area of interest and concern [19]. In this context, Ji et al [19] introduced a novel pre-protected strategy that combines the benefits of sodium ion doping to enhance the stability of metal tri-halide perovskite quantum dots (PQDs) while preserving their exceptional optoelectronic properties. By Any further distribution of this work must maintain attribution to the author-(s) and the title of the work, journal citation and DOI. employing this coating approach, the researchers achieved improved stability of PQDs, thereby addressing a critical challenge in the field of perovskite quantum dot research [19]. However, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has rapidly grown during the last ten years, rising from 3.8% to 25.2% [20][21][22]. It is notable that such high PCEs may be achieved using thin lead (Pb) halide perovskite active layers that have undergone low-temperature solution processing [23,24]. These active layers are typically less than 500 nm, which is far smaller than the active layers employed in popular inorganic thin-film solar cell technologies with equivalent PCEs. It reveals that using organic-inorganic Pb halide perovskite as active layers will be a promising method for developing low-cost, high-efficiency thin-film solar cells. Additionally, Pb halide perovskite solar cells have appreciably high open circuit voltages (V OC ). Also compared to the best single-crystal Si solar cell, the best perovskite solar cell is even smaller in size and more efficient in terms of their PV performances [25][26][27].
A single junction perovskite-based solar cell with a PCE of 12% was proposed in 2013 by Hyuck et al [28]. The PCE of the perovskite-based solar cell was discovered to be 15% in the very next year by Zhou Yang et al [29]. Perovskite material received a lot of interest in the years 2017 to 2020 because of its exceptional optical characteristics and affordable cost [22,30,31]. The research community discovered that the perovskite-based single junction solar cell had an efficiency of above 25% [32][33][34][35][36]. One challenge that needs to be solved is the destabilization of perovskite at high relative humidity. To overcome this, researchers created a significant mixed halide perovskite. To increase the stability of the entire device, J.H. Noh et al blended CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3 to form CH 3 NH 3 PbI 3-X Br X (X = 0-3) [37]. Researchers have also discovered the possibility of merging perovskite with quantum dots to increase stability and size compatibility [38][39][40]. Due to perovskite quantum dots (PQDs) comparatively simple synthesis, tunable bandgap and high optical absorption, chances for developing next-generation of optoelectronic devices as well as opportunities for investigating fundamental aspects at the nanoscale unit are created [41][42][43][44]. The best contender among all semiconductors for the creation of a highly efficient solar cell is PQD with a favorable nano-size effect, distinctive multi-exciton generation (MEG), increased phase stability, and exceptional cell hysteresis suppression [45].
Importantly, the ability to absorb both visible and infrared (IR) light as well as the enhancement of charge transportation brought on by the nano-size effect and quantum confinement make PQD a more competitive alternative to other metal halide perovskites [46,47]. In this context, Erin M. et al reported in 2017 the development of colloidal PQD solar cells for PVs with a PCE more than 13%. Other PQDs have also been suggested with significant PCE, such as those by Wenzhen et al and Yuan et al, who revealed PQD-based solar cells in 2019 with respective PCEs of 12% and 14% [41,48]. Further, Wen et al have investigated the application of carbon quantum dots (CQDs) as an additive in the methyl ammonium iodide solution to enhance the production of high-quality CH 3 NH 3 PbI 3 (MAPbI3) perovskite films [49] . The incorporation of CQDs reduces the unsolicited trap-state density that consequently reduces the carrier recombination, and hence improves the PV performance. The utilization of CQDs in the modified MAPbI 3 cell led to a significant increase in the average PCE up to 19.17% [49]. Yang et al introduced the titanium carbide (Ti 3 C 2 Tx)-MXene quantum dot-modified tin oxide (MQDs-SnO 2 ) as ETL in perovskite solar cells (PSCs) [50]. They highlighted the potential of nanocrystalline SnO 2 as ETL in achieving highly efficient and stable PSCs, particularly for low-temperatureprocessed flexible PSCs. Additionally, the MQDs-SnO 2 layer exhibits excellent charge extraction properties, leading to a steady-state power conversion efficiency of up to 23.3% and exceptional stability against humidity and light soaking for the corresponding PSCs [50].
In this work, CsPbI 3 QD layer is taken as active layer of the device. This layer is examined under the various parameter variations such as thickness, acceptor density, bulk defect density, and interface defect density and front electrode optical transparency in order to increase the performance of the solar cell. There are three sections to this study: 2, 3, and 4. Section 2 provides an illustration of the device structure and the simulation approach, and section 3 provides an interpretation of the simulation results. The section 4 of this work has been presented with the conclusion and the study's future scope.  [51]. The simulations in this work have been conducted using a one-dimensional solar cell simulation tool called SCAPS 1d, developed by the Department of Electronics and Information Systems (ELIS) at the University of Gent in Belgium. In the proposed device, the cesium lead iodide (perovskite) quantum dots (CsPbI 3 -QD) layer acts as the absorber layer. To facilitate the movement of electrons at the interface of the second and third layers, a chlorine passivated tin oxide (Cl@SnO 2 ) layer has been used as the electron transport layer (ETL). Additionally, a hole transport layer (HTL) is also included in the device, with Poly 3-Hexylthiophene (P 3 HT). Figure 1

Simulation based results and discussion
In the simulator, the thickness, acceptor density, and optical transparency of the active layer (CsPbI 3 QD) have been optimized. Additionally, the bulk defect density and interface defect density of (CsPbI 3 QD) have also been studied using the results obtained from the simulator. The software SCAPS 1D has been used to perform all the parameter variations and optimizations in the CUC. The entire process flow for the device simulation is shown in figure 2.

Thickness optimization
Optimizing the thickness of the absorber layer in a solar cell is essential as it affect the efficiency of the cell. A thicker absorber layer can increase the amount of Sunlight absorbed by the cell, which can lead to a higher power output. However, there is a trade-off between the thickness of the absorber layer and other factors such as the resistance and cost of the cell. Therefore, it is important to carefully optimize the thickness of the absorber layer to find the right balance between efficiency and cost-effectiveness. In other words, the optimum thickness tradeoff between maximum absorption and minimum recombinations. Thus, to obtain best PV performance, the thickness of the CsPbI 3 QD layer in CUC is varied from 50 to 400 nm. Figure 3(a) shows the external quantum efficiency (EQE) for photon wavelength range of 300 to 750 nm. The graphs shows that the quantum efficiency for the high energy photons is appreciably higher than that of low energy photons. Therefore, it has been observed that significant improvement in photon absorption, generation and thereby collection with increase in  It clearly depicts that the current density delivered by the cell increases with thickness, owing to enhanced effective absorption of low energy photons deeper inside the CsPbI 3 QD layer. The deeper absorption subsequently enhances the generation of electron and hole pairs followed by the collection and eventually the current density. Figure 3(c) summaries the PV parameters of the CUC for CsPbI 3 QD layer thickness. It is worth noting that with increase in CsPbI 3 QD layer thickness from 50 nm to 400 nm, short circuit current density (J SC ) increases substantially from 9.27 to 18.06 mA cm −2 i.e., approximately 3 times. Further, it has been obtained that with increase in CsPbI 3 QD layer thickness from 50 to 400 nm, the open circuit voltage (V OC ) delivered by the CUC increases from 0.95V to 1.02V i.e., 13%. While the FF of the cell also increases from 84.19 to 87.06% with increase in CsPbI 3 QD layer thickness from 50 to 400 nm. The cumulative impact of J SC , V OC and FF decide the conversion efficiency of the cell. Results depicts that the PCE of the CUC increases from 7.45% to 16.17% with an increase in CsPbI 3 QD layer thickness from 50 to 400 nm. In conclusion, the optimized CsPbI 3 QD layer thickness i.e., 400 nm, the PV parameters of the CUC are recorded as PCE-16.17%, V OC −1.02V, J SC −18.06 mA cm −2 , and FF-87.06%.

Acceptor density optimization
Apart from thickness, the acceptor density (N A ) of the CsPbI 3 QD layer also plays a critical role in affecting the photovoltaic (PV) performance of the CUC. To investigate the impact of N A of the CsPbI 3 QD layer on PV parameter of the CUC, N A of CsPbI 3 QD has been varied from 1 × 10 17 to 1 × 10 19 cm −3 . As seen in figure 4(a), the EQE curve illustrates that the overall performance of the solar cell improves for higher N A . However, as seen in figure 4(b), there is a decrease in the short-circuit current (J SC ) as the N A increases. Specifically, the J SC drops from 19.76 to 18.06 mA cm −2 as the N A increases from 1 × 10 17 to 1 × 10 19 cm −3 , as shown in figure 4(c), due to an increase in the built-in potential of the junction, which leads to a reduction in the area of the space charge region and subsequently, the collection of charge carriers at the load.
Additionally, figure 4(c) also demonstrates improved PV parameters as the N A increases. The device's power conversion efficiency (PCE) increases from 13.64% to 16.17% with an increase in N A from 1 × 10 17 to 1 ×  1 × 10 19 cm −3 indicating that the voltage is inversely proportional to the doping. Finally, at N A of 1 × 10 19 cm −3 , the CUC recorded a maximum PCE of 16.17%, a V OC of 1.02 V and a fill factor (FF) of 87.60%. The reduction in the resistivity of the CsPbI 3 QD layer with increasing N A lowers the series resistance, which plays a key role in increasing the FF of the CUC.

Impact of bulk defect density on PV performance
Another important factor that can significantly affect the performance of the CUC is the total defect density of the CsPbI 3 QD layer. As the defect density increases, the charge carrier diffusion length decreases, leading to a higher recombination of charge carriers. This results in a degradation in solar cell performance. The J-V and EQE curves of the CUC, shown in figures 5(a)-(b), demonstrate the impact of bulk defect density (from 1 × 10 14 −1 × 10 17 /cm 3 ) affect the device. It can be seen that as the defect density of the CsPbI 3 QD layer increases, the device's J SC and V OC decreases. Figure 5(c) shows the PV parameters obtained from the variation in defect density of CsPbI 3 QD layer in the simulator. As can be seen in the figure 5(c), the PCE decreases from 16.17% to 16.12%, 15.79%, 14.57%, 12.08, and 8.57%. Additionally, the increment in defect density also affects the FF of the CUC, dropping from 87.60% → 87.36% → 85.83% → 80.54% → 73.61% → 65.54%. The J SC of the CUC is also affected by the increment of the defect density, reducing from 18.06 mA cm −2 to 13.38 mA cm −2 . Therefore, for better stability and performance of the CUC, it is crucial to have as low a total defect density of the active layer as possible. According to the results of the simulation, the optimal defect density of the CsPbI 3 QD layer is 1 × 10 14 /cm 3 , where a PCE of 16.17%, V OC of 1.02 (V), J SC of 18.06 mA cm −2 and FF of 87.60% have been recorded. The optimized BDD is also in accordance with previous publications [58,59].

Cumulative impact of thickness, BDD and N A on PV performance
To summaries, the combined impact of CsPbI 3 QD thickness, N A , and BDD on PV conversion efficiency of CsPbI 3 QD solar cell have been shown by varying all the parameters all together in the simulator. Figure 6 shows that thickness has a greater impact on the conversion efficiency of the CUC's w.r.t N A and BDD. The best performance of the CUC has been obtained at 400 nm thick CsPbI 3 QD. While variations in N A and BDD affect the PCE of the CUC marginally. 3.5. Impact of interface defect density (IDD) on PV performance Different fabrication issues such as poor quality of wafers, incomplete removal of contaminants, improper doping, poor quality of passivation layer and or metal contacts lead to generation of interface defects. Thus, in this subsection, impact of the IDD present at Cl@SnO 2 and CsPbI 3 interface, has been analyzed. The range of IDD considered is from 1 × 10 13 to 1 × 10 18 /cm 2 . As the IDD increases, the diffusion length reduces and charge carrier recombination becomes more pronounced, resulting in a decrease in overall PCE. Additionally, the lifetime of the CUC is also reduced due to the higher recombination of charge carriers at high IDD. As can be seen in figures 7(a)-(b), the PCE decreases from 16.17% → 16.15% → 15.90% → 14.70% → 12.87% → 11.09% with an increase in IDD from 1 × 10 13 to 1 × 10 18 /cm 2 , while the J SC remains almost constant at 18.06 mA cm −2 . The other PV parameters, V OC and FF, also decrease with the increment of IDD (V OC : 1.02 to 0.84 V and FF: 87.60% to 72.08%, respectively) due to the high recombination of charge carriers in the device. Therefore, the best performance of the CUC is achieved at an IDD of 1 × 10 13 /cm 2 according to the results obtained from the variation of IDD in the simulator. The optimized IDD is also in accordance with previous publications [60,61].

Optical transparency optimization
After studying the influence of thickness, acceptor density, defect density, and interface defect density, the impact of transparency of the front electrode of the CUC has also been inspected using the SCAPS-1D software. In this context optical transparency of the front electrode has been varied from 50% to 95% to inspect the best performance of CUC. Consider the fact that complete non-transparent contact covering of the  front surface prevents photon interaction within the active layer. As a result, during the simulation, the front electrode's transparency is changed in order to simulate the front electrode with various levels of transparency. Investigation of the cell PV performance with various front electrode transparency levels has been done with the help of EQE and J-V curve as depicted in figures 8(a)-(b). After variation in front electrode transparency from 50% to 95% it is found that PCE and J SC of the CUC getting improved. Figure 8(c) depicts that PCE improves from 8.51% → 10.21% → 11.92% → 13.62% → 15.34% → 16.17% and J SC also increases like 9.50 → 11.4 0→ 13.30 → 15.21 → 17.11 → 18.06 mA cm −2 . The V OC and FF saturated during this variation through simulator. Therefore, it can be concluded that the increment in the transparency enhances the optical coupling of the photons inside the layer which improved the light generated charge current density and performance of the CUC.

Conclusion
In this work, optimization of thickness and acceptor density of the CsPbI 3 QD layer of the CUC have been done. SCAPS 1D software based numerical simulation has been utilized in order to optimize above parameters. Initially, thickness of the CsPbI 3 QD layer has been optimized at 400 nm, thereafter acceptor density of CsPbI 3 QD layer is analyzed and optimized at 1 × 10 19 /cm 3 . The V OC is found to be directly proportional to N A , while J SC decreases with increasing N A . Additionally, the bulk defect density of the CsPbI 3 QD layer is also taken into consideration to minimize the recombination of charge carriers and improve the overall performance of the cell. It is examined that the maximum bulk defect density should not exceed 1 × 10 14 /cm 3 in CsPbI 3 QD layer for optimal PV performance. Furthermore, the impact of interface defect density at CsPbI 3 QD and Cl@SnO 2 layers is also examined, and it is found that it should not be higher than 1 × 10 13 /cm 2 for the better PV characteristics. The results show that the highest PCE of the CsPbI 3 QD based solar cell is achieved at 16.17% with a V OC of 1.02V.
Research is also conducted to examine the influence of the transparency of the front electrode to further improve the PV characteristics of the CUC. Results shows that the PCE of the CUC is 16.17% at 95% front electrode transparency, this further increment in the PCE of the CUC is due to increase in photons coupling. These findings provide valuable information for optimizing the performance of CsPbI 3 QD based solar cells.