Perovskite solar cells (PSCs) have made incredibly fast progress in past years, pushing the efficiency approaching 26%, which is comparable to the best silicon solar cells. One of the features...

Having demonstrated incredibly fast progress in power conversion efficiency, rising to a level comparable with that of crystalline silicon cells, lead‐based organic–inorganic hybrid perovskite solar cells are now facing the stability tests needed for industrialization. Poor thermal stability (<150 °C) owing to organic constituents and interlayer diffusion of materials (dopants), and environmental incompatibility due to Pb has surged the development of organic‐free, Pb‐free perovskites and dopant‐free hole transport materials (HTMs). The recent rapid increase in efficiency of cells based on inorganic perovskites, crossing 18%, demonstrates the great potential of inorganic perovskites as thermally stable and high‐efficiency cells. Although all kinds of Pb‐free perovskites lag in efficiency in comparison to the hybrid and inorganic perovskites, they also demonstrate better structural and environmental stability. The performance of dopant‐free HTMs matching/surpassing dopant‐containing HTMs makes the former a better choice for stability. Even though the efforts to enhance the stability of Pb‐based hybrid perovskites should continue by different techniques, organic‐free and lead‐free perovskites, and dopant‐free HTMs must be pursued with greater interest for the future. This review describes the present issues and possible strategies to address them, and thus will help to improve the overall performance of robust organic‐free, Pb‐free, and dopant‐free perovskite solar cells.

CsPbI2Br perovskite solar cells have attracted much attention because of the rapid development in their efficiency and their great potential as a top cell of tandem solar cells. However, the V OC outputs observed so far in most cases are far from that desired for a top cell. Up to now, with various kinds of treatments, the reported champion V OC is only 1.32 V, with a V OC deficit of 0.60 V. In this work, we found that aging of the SnCl2 precursor solution for the electron-transporting layer can promote the V OC of CsPbI2Br solar cells by employing a dopant-free-polymer hole transport material (HTM) over 1.40 V and efficiency over 15.5% with high reproducibility. With the champion V OC of 1.43 V, the V OC deficit was reduced to <0.50 V, which is achieved for the first time. This simple technique of SnCl2 solution aging forms a uniform and smooth amorphous SnO x film with pure Sn4+, elevates the conduction band of SnO x , and reduces the interfacial gaps and the trap state density of the device, resulting in enhancement in average V OC from ∼1.2 V in the nonaged case to ∼1.4 V in the aged case. Furthermore, the device using an aged SnCl2 solution also exhibits a much better long-term stability than that made of the fresh solution. These achievements in dopant/additive-free CsPbI2Br solar cells can be useful for future research on CsPbI2Br and tandem solar cells.

Lead halide perovskite single layers with three grain sizes are subjected to proton-beam irradiation in order to assess the durability and radiation tolerance of perovskite solar cells (PSCs) against space radiation. Proton-beam irradiation is chosen because proton beams significantly affect solar cell performance in the space environment. We evaluate the effects of proton beams by focusing on the grain structure, crystal structure, and carrier lifetime of a perovskite single layer by using scanning electron microscopy, X-ray diffraction, photoluminescence (PL) spectra, and time-resolved PL (TRPL). The results show that proton irradiation does not significantly affect the grain structure and crystal structure of perovskite layer; the TRPL results show that the carrier lifetime inside the grain is constant up to a fluence of 1 × 10 14 p + /cm 2 and decreases significantly at a fluence of 1 × 10 15 p + /cm 2 . Proton-beam radiation tolerance of the grain inside the perovskite layer is dominant in the radiation tolerance of PSCs.

Interfacial engineering, grain boundary, and surface passivation in organic–inorganic hybrid perovskite solar cells (HyPSCs) are effective in achieving high performance and enhanced durability. Organic additives and inorganic doping are generally used to chemically modify the surface contacting charge transport layers, and/or grain boundaries so as to reduce the defect density. Here, a simple but tricky one‐step method to dope organic–inorganic hybrid perovskite with Ge for the first time is reported. Unlike Ge doping to all‐inorganic perovskites, application of GeI2 in organic–inorganic perovskite precursors is challenging due to the extremely poor solubility of GeI2 in hybrid perovskite ink, leading to failure in the formation of uniform films. However, it is found that addition of methylammonium chloride (MACl) into the precursor remarkably increases the solubility of GeI2. This MACl‐assisted Ge doping of hybrid perovskites produces high‐quality crystalline film with its surface passivated with nonvolatile GeI2 (GeO2) and the volatile MACl additive also improves the uniformity of GeO2 distribution in the perovskite films. The resulting Ge‐doped mixed cation and mixed halide perovskite films with composition FA0.83MA0.17Ge0.03Pb0.97(I0.9Br0.1)3 show superior photoluminescence lifetime, power conversion efficiency above 22%, and greater stability toward illumination and humidity, outperforming photovoltaic properties of HyPSCs prepared without the Ge doping.

Semi-transparent solar cells draw a great deal of attention because their applications include, for instance, photovoltaic windows. General approach to semi-transparent cells is using thin active layers or island-type structures. Here we take human luminosity function into account, and develop solar cells that harvest photons in the wavelength regions in which human eyes are less sensitive to light. We used an organic-inorganic hybrid perovskite, which is sensitive to light particularly in the blue and deep-blue regions, and plasmonic silver nanocubes that enhance light harvesting in the red and deep-red ranges. In order to tune the plasmonic wavelength to that range, we took advantage of electrode-coupled plasmons (ECPs). We prepared non-plasmonic semi-transparent solar cells, and reduced the active layer thickness and introduced ECPs, so that the visual transparency index and power conversion efficiency of the cell were improved by 28% and 6%, respectively, of the initial values.

were the main concern, and a variety of fabrication processes were suggested, such as anti-solvent and two-step methods. [1][2][3][4] Researchers shifted focus from synthetic methods to additional treatments on the existing perovskite layers that provide high PCEs and performance since the discovery of multiple processes to obtain flawless and homogeneous perovskite films. [5][6][7][8][9][10][11] 2D perovskite materials, such as phenethylamine halides (PEAX), have been favored by researchers lately for surface treatments. [8,[12][13][14] The surface treatment using the 2D materials significantly improves general photovoltaic properties of 3D perovskites. However, the working mechanism of surface treatments using 2D materials remains unclear. The surface passivation effect best explains the positive effects of surface treatments on 3D perovskites. [13][14][15][16] 2D perovskites existing either in the 3D perovskite grain boundaries or on the top of perovskite surfaces reduce the defects in 3D perovskites because the defect sites mostly exist near the grain boundaries. Multiple research groups reported improved stability against air and moisture because the rigid and robust low-dimensional materials protected perovskite layers. [13,[15][16][17] A recent report by You et al. suggested that unannealed phenethylamine iodide (PEAI) was more effective for surface passivation than annealed PEAI. [8] You et al. claimed that the 2D perovskite materials fabricated using heat were less effective compared to other 2D perovskites. However, many other reports state that the 2D perovskite materials were effective in surface treatments irrespective of their fabrication process. [15,16,[18][19][20] As the clear reasons behind this have not manifested, the right way of using these materials in PSCs has not still been established.The term passivation used in many reports regarding PSCs is also ambiguous and blended with the term interlayers. [21,22] Especially, 2D perovskite materials coated on the perovskite layers such as PEAX are often referred to passivation layers because they are highly reactive with perovskite layers to form protective layers. [8,23,24] Herein, we propose a mechanism for the application and analysis of 2D perovskite materials in surface treatments. We developed new hole transport layers (HTLs) consisting of a mixture of two small molecules having donor-acceptor (D-A) structures instead of using spiro-OMeTAD, a typical HTL for PSCs. This is because the electronic properties of spiro-OMeTAD dynamically change over time due to its self-oxidization making the precise Surface modification of 3D hybrid perovskites using 2D perovskites, such as phenethylamine halides (PEAX), increases the overall power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). The effect is based on a surface passivation phenomenon where PEAX is in direct contact with the perovskite and hole transport layer (HTL). However, it is herein observed that the PCE of PSCs containing PEAX increases significantly when they a...

Semitransparent solar cells can be applied to photovoltaic windows, which harvest light from both sides. Here we report that the visual transparency of perovskite solar cells can be improved by suppressing the optical scattering at the perovskite layer, through making the surface ultrasmooth, without undue reduction of the perovskite thickness and efficiency. Ultrasmooth perovskite films prepared by a short spinning with vacuum drying method, mean roughness of which is 6 nm or lower, exhibit suppressed scattering (<2%) and high visual transparency. The power conversion efficiency (PCE) of 14.26% is achieved with a thick Ag top electrode (100 nm). A semitransparent cell with a thin Ag top electrode (10 nm) coated with MoO3 (20 nm) exhibits scattering of <1% and PCE of 11.0 and 8.7% for the front and back incidence, respectively. The ultrasmooth and compact preovskite layer leads also to fast photocurrent responses with negligible or sufficiently small hysteresis.