Abstract
1. Introduction
Methylammonium mixed lead halides (MAPbXs), which are known for their excellent optical and electrical properties, have drawn a great deal of attention for photovoltaic applications in recent years[
Tin halide MASnI3 (MA = CH3NH3) perovskite solar cells have been intensively investigated for lead-free alternatives[
In order to further improve the conversion efficiencies of MASnI3 PSC, it is required to deeply understand the internal electron dynamics and corresponding interfacial engineering, and to clarify the limiting physical mechanism of the conversion efficiency. It is well known that an excellent interface quality is the key factor for a high efficiency PSC due to a strong interface recombination of defect states.
Numerical simulation is now becoming increasingly important for the understanding, design, and optimization of high-efficient solar cells. The simulation method allows an intuitive examination of each parameter in the solar cell and thus identifies the optimal conditions for operating. Up to now, there has been no report concerning the numerical simulation of MASnI3 PSC.
In this paper, influences of defect states in the absorption layer and on the interfaces of absorber/electron transport layer (ETL) or absorber/hole transport layer (HTL) on MASnI3 PSC efficiency are investigated by a one-dimensional device simulation with SCAPS-1D (version 3.3.02) [
2. Device simulation parameters
The simulated PSC structure is the transparent conductive oxide (TCO) glass/TiO2/MASnI3/Spiro-OMeTAD/metal back contact, and the sunlight is illuminated from TCO glass. TiO2 acts as ETL, Spiro-OMeTAD as HTL, and MASnI3 as the absorption layer. The absorber is assumed to be fully depleted and the electric field is formed by high densities of electron and hole of TCO and HTL, respectively. In order to simulate a more realistic PSC, we intentionally insert a very thin interface layer in the absorber region with a large number of defect states. The interface defect layer (IDL) is assumed to take into account the interface recombination, and IDL-ETL and IDL-HTL are inserted between ETL/absorber and absorber/HTL layers, respectively. Material parameters were selected from experimental data and theoretical results. The main simulated parameters are listed in Table 1[
3. Results and discussion
The influence of the thickness and doping concentration of each layer of PSC on the device performance were extensively investigated[
Because the self-doping process of Sn2+ is easily oxidized to Sn4+, the tin-based perovskite exhibits a p-type conductive behavior[
The properties and the amount of defects in the perovskite absorber play a key role in determining device performance, because photogenerated carriers are mainly generated in this layer and the carrier recombination is dominant in determining the VOC of PSC. Therefore, for simplicity, we did not consider the effects of the defects in HTL or ETL. The device performances are improved significantly with the reduction of the defect density Nt in MASnI3 thin film, as shown in Fig. 1. When the Nt in MASnI3 is lower than 1 × 1012 cm−3, the PSC efficiency (Eff) is 36% without considering the influence of interface defects between the interfaces of the ETL/absorber and absorber/HTL. Further analysis shows that Nt in MASnI3 is lower than 1 × 1010 cm−3,Eff is no longer increased and reaches its maximum value (40.5%, no display in Fig. 1). For the solar cell of a single homogenous junction with Eg = 1.34 eV, the theoretical Eff limit by the Shockley–Queisser theory is 33.7%[
Figure 1.(Color online) Solar cell parameters of PSC with (or without) IDL as a function of defect density
Without considering the influence of the interface defect, the VOC is almost linearly reduced with the exponential increase of Nt. VOC is given by
where q is the electronic charge, k is the Boltzmann constant, T is the temperature, IL is the light-generated current, and I0 is the saturation current. While IL typically has a small variation, the key factor affecting VOC is I0, since this may vary by orders of magnitude. The saturation current (I0) depends on recombination in the solar cell, and can be written by
where A is the quality factor, ni is the intrinsic carrier concentration, and Ln (Lp) is the electron (hole) diffusion length. The most important factor affecting I0 is diffusion length, which is given by
According to Eqs. (1)–(3), the VOC is almost linearly decreased with the exponential increase of Nt, which is consistent with the simulation results. The JSC has almost no change in the case of Nt lower than 1 × 1016 cm−3, however, JSC decreases significantly with the further increase of Nt, as shown in Fig. 1. For a low Nt, Ln (or Lp) is larger than the absorber thickness, thus the impact of Nt to JSC is rather small. However, for a large Nt (for example, 1 × 1016 cm−3), Ln (or Lp) is smaller than the absorber thickness, so JSC decreases rapidly with the increase of Nt. In fact, a low trap-assisted recombination rate in perovskite thin film has been reported by Herz et al.[
In case of considering the influence of an interface defect, the interface defect density (Dit-ETL or Dit-HTL) was set to a very low value ~1 × 109 cm−2. When Nt is larger than 1 × 1014 cm−3, the insertion of the interface defect layer with a low Dit has almost no influence on the PSC performance, as shown in Fig. 1. As Nt is lower than 1 × 1014 cm−3, the presence of interface defects deteriorates the PSC performance, which is caused by the interface recombination loss. Of course, if Dit was set to a larger value (for example, 1 × 1014 cm−2), the interface defect has an important influence on PSC performance in case of any value of Nt.
The interface quality has a decisive influence on the efficiency of thin film solar cell because the presence of interface defects can lead to high levels of recombination. In real devices, it is difficult to characterize the interface defect density and recombination rate. In the simulation, the influence of an interface state on the device performance can be revealed by drastically changing the interface defect density. Here, two interface defects are considered, and they are the interface defect Dit-ETL located between the ETL and the absorber (TiO2/MASnI3) and the interface defect Dit-HTL between the absorber and HTL (MASnI3/HTL). Two Dit values change from 1010 to 1014 cm−3. In the perovskite absorption layer, the defect type (neutral, donor, or acceptor) has almost no influence on PSC performances, and the simulated results were not shown here. However, different defect types of Dit-ETL and Dit-HTL were considered, and their influences on PSC were investigated in depth.
For the simplicity, the neutral defect type was analyzed at first. The efficiencies of PSC with different defect densities at ETL/absorber and absorber/HTM interfaces were demonstrated, as shown in Fig. 2. As Dit-ETL or Dit-HTL was changed from 1010 to 1014 cm−2, the other parameter Dit-HTL or Dit-ETL was fixed at 1010 cm−2. The Eff decreases from 29.02% to 27.44% when Dit-HTL increases from 1010 to 1014 cm−2. However, the Eff decreases from 29.02% to 22.57% with the increase of Dit-ETL from 1010 to 1014 cm−2. Therefore, the interface quality at the absorber/HTL interface has a greater impact on PSC efficiencies than that at the ETL/absorber interface. When the sunlight changes its incident direction, namely, from the HTL to the ETL, the simulated results are just the opposite, as illustrated in Fig. 2. These results indicate that the interface quality of the sunlight incident side of PSC is far more important than that of the back side.
Figure 2.(Color online) Efficiency of PSC with neutral interface defects as a function of interface defect density.
The difference is caused by the collection probability of photogenerated carriers, which is that a light generated carrier absorbed in a certain region of the device will be collected by the p-n junction and therefore contribute to the light-generated current. Similarly, if the carrier is generated more than a diffusion length away from the junction, then the collection probability of this carrier is quite low. If the carrier is generated closer to a region such as a surface with high recombination, then the carrier will be recombined. For high absorption coefficient of the pervoskite absorber, the number of photogenerated electron-hole pairs at the light irradiation side is high at the surface and significantly becomes small toward the back side. The presence of the localized recombination site, such as a surface or interface, has a stronger influence on the collection probability than a less severe localized recombination site. Therefore, as the sunlight irradiates from the side of ETL/absorber, the higher excess carrier density will be generated, which leads to a larger recombination rate at ETL/absorber than that at absorber/HTL. Thus, the interface defect at ETL/absorber has stronger influence on device performance than absorber/HTL. On the contrary, if the device is irradiated from the HTM side, the defect density of the absorber/HTM interface has a strong impact on the device performance, as shown in Fig. 2. In order to enhance the efficiency of the perovskite solar cell, improving the interface quality of the front side is much more important than that of the back side.
We have discussed the behavior of the interface with neutral defects, next we will analyze the impacts of the interface with donor or acceptor defects on PSC performance. If Dit-HTL was fixed at 1010 cm−2 with neutral defects, the dependence of device efficiency on Dit-ETL with different defect type was given in Fig. 3. When the defect type of Dit-ETL is neutral or donor, the influence of interface defects on device efficiency is the same, and the Eff decreases from 29.02% to 22.50% with the increase of Dit-ETL from 1010 to 1014 cm−2. However, in the case of acceptor defects, the Eff drops sharply from 29.02% to 0.96% when Dit-ETL increases from 1010 to 1014 cm−2, which indicates that Dit-ETL with acceptor defects has a far more important influence on device efficiency than Dit-ETL with neutral or donor defects. Similarly, the impact of Dit-HTL with donor defects on device efficiency is much stronger than that of Dit-HTL with neutral or acceptor defects (not shown in this paper).
Figure 3.(Color online) Efficiency of PSC with donor (or acceptor) interface defects as a function of interface defect density.
The further investigations on the band bending and carrier recombination were shown in Fig. 4. In order to analyze only the band bending and recombination at the interfaces of ETL/absorber and absorber/HTL, the horizontal axis of Fig. 4 starts from 0.2 μm (the thickness of ETL) and ends at 0.82 μm (IDL-ETL (0.01 μm) + absorber (0.6 μm) + IDL-HTL (0.01 μm)). As Dit-ETL with acceptor defects increases, the reduction of the band bending is obviously enhanced, as shown in Fig. 4(a). However, in case of Dit-ETL with donor (or neutral) defects, the interface defect density has almost no influence on band bending, as shown in Fig. 4(b). The band bending affects the photo-generated carrier flow, which leads to the decrease in VOC and deterioration of device performance. The dependences of the interface defect type and defect density on carrier recombination rate are also shown in Figs. 4(c) and 4(d). Similar to the influence on band bending, Dit-ETL with acceptor defects has a much greater impact on recombination rate than Dit-ETL with donor (or neutral) defects.
Figure 4.Distributions of total recombination and energy band in the cell thickness direction for varying interface defect density.
When interface defect states of the n-type-ETL/p-type-absorber or p-type-absorber/n-type-HTL heterojunction are occupied, charges are trapped at the interface in the space charge region of the heterojunction. As the defect type of Dit-ETL is donor or neutral, the space charge region is not significantly affected, and the electric field extends up to the neutral region of the perovskite absorber with the increase of Dit-ETL . However, for Dit-ETL with acceptor defects, the trapped charges prevent the extension of the space charge region into the absorber since the polarity of the interface charge is the same as the polarity of the majority carrier of the perovskite absorber. Therefore, the band bending in the absorber is reduced, which results in a lowering of the effective barrier and consequently a decrease in VOC. In addition, a reduction of the band bending causes an enhancement of the majority carrier concentration at the interface, which leads to an increased recombination probability. Similarly, Dit-HTL with donor defects has a much greater impact on the band bending and recombination rate than Dit-HTL with acceptor (or neutral) defects.
In the actual preparation process of Sn-based perovskite solar cells, some divalent Sn compounds, for example, SnF2, as an additive, can be used as Sn vacancy suppressors to reduce the defect density in the perovskite absorber and improve device performance[
4. Conclusion
The planar heterojunction CH3NH3SnI3 solar cell was simulated with one-dimensional device simulator SCAPS, which is widely used in inorganic thin film solar cells. The defects in the perovskite absorber and at the ETL/absorber or absorber/HTL interface were deeply investigated. In the case of defect density in a perovskite absorber lower than 1 × 1016 cm−3, JSC has almost no change and high efficiency (> 19.0%) can be obtained. The absorber interface density at the sunlight incident side is a decisive factor for efficiency due to the high absorption coefficient of the pervoskite absorber. The defect density at the ETL/absorber interface with acceptor defects has a far more important influence on device efficiency than that with neutral or donor defects. Similarly, the impact of the defect density at the absorber/HTL interface with donor defects on device efficiency is much stronger than that with neutral or acceptor defects. The polarity of the interface charge has a different impact on the band bending and recombination rate. Considering the effects of defect states, a simulated cell efficiency of more than 29% is achieved by optimization of the various parameters, which highlights the great potential of perovskite in achieving high efficiency. The simulation based on this study will be useful for further understanding of device operation and efficiency improvement.
Acknowledgements
The author would like to thank Professor Marc Burgelman, Department of Electronics and Information Systems, University of Gent for the development of the SCAPS software package and allowing its use.
This work was supported by the Zhejiang Provincial Natural Science Foundation of China (No. LY17F040001), the Technology Development Project Program of Hengdian Group DMEGC Magnetics Co., Ltd (No. 2016330001002138), the Open Project Program of Surface Physics Laboratory (National Key Laboratory) of Fudan University (No. KF2015_02), and the Open Project Program of National Laboratory for Infrared Physics, Chinese Academy of Sciences (No. M201503).
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