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Sb2Se3 is a promising candidate for low toxicity, low-cost and high efficiency solar cells. Thin film solar cells employing Sb2Se3 as the absorber have achieved power conversion efficiencies of up to 6.5%. However, these solar cells are typically fabricated using rapid thermal evaporation, a fast deposition technique but suffer from entangled substrate and source temperatures. Here, we report on the fabrication of stable and efficient CdS/Sb2Se3 thin film solar cells using close-space sublimation process, which allows separate control of the source and substrate temperatures. This enables better control of Sb2Se3 film deposition and reduction of interfacial diffusion, leading to higher quality absorber films and improved device heterojunctions. Additionally, we introduced a novel low-cost material 4,4',4'',4'''-(9-octylcarbazole-1,3,6,8-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)aniline) (CZ-TA) as the hole transport layer (HTL) to create n-i-p structured devices. This CZ-TA HTL reduced the back contact resistance and promoted photogenerated carrier collection, boosting device photo conversion efficiency to 6.84%.

We report on compositional and morphological engineering of mixed methylammonium (MA) and formamidinium (FA) lead triiodide (MA1−xFAxPbI3) perovskite absorber layers to produce highly efficient planar and flexible perovskite solar cells (PVSCs) with reduced hysteresis. Incorporation of FA into the MAPbI3 extends the absorption edge of the perovskite to longer wavelengths, leading to enhanced photocurrent of the resultant PVSCs. Moreover, adding a small amount of lead thiocyanate (Pb(SCN)2) additive into mixed perovskite precursor solutions significantly enlarges the grain size and prolongs the carrier lifetime, leading to improved device performance. With optimal compositional and morphological engineering, the average power conversion efficiency (PCE) improves from 15.74±0.74% for pure MAPbI3 PVSCs to 19.40±0.32% for MA0.7FA0.3PbI3 PVSCs with 3% Pb(SCN)2 additive, exhibiting a high reproducibility and small hysteretic behavior. The best PVSC achieves a PCE of 20.10 (19.85)% measured under reverse (forward) voltage scan. Furthermore, the compositional and morphological engineering allowed the fabrication of efficient flexible PVSCs on indium-doped SnO2 (ITO)/polyethylene terephthalate (PET) substrates, with the best PCE of 17.96 (16.10)% with a VOC of 1.076 (1.020) V, a JSC of 22.23 (22.23) mA/cm2 and a FF of 75.10 (71.02)% when measured under reverse (forward) voltage scan. Our approach provides an effective pathway to fabricate highly efficient and reproducible planar PVSCs.

We show that the cooperation of lead thiocyanate additive and a solvent annealing process can effectively increase the grain size of mixed-cation lead mixed-halide perovskite thin films while avoiding excess lead iodide formation. As a result, the average grain size of the wide-bandgap mixed-cation lead perovskite thin films increases from 66 ± 24 to 1036 ± 111 nm, and the mean carrier lifetime shows a more than 3-fold increase, from 330 ns to over 1000 ns. Consequently, the average open-circuit voltage of wide-bandgap perovskite solar cells increases by 80 (70) mV, and the average power conversion efficiency (PCE) increases from 13.44 ± 0.48 (11.75 ± 0.34) to 17.68 ± 0.36 (15.58 ± 0.55)% when measured under reverse (forward) voltage scans. The best-performing wide-bandgap perovskite solar cell, with a bandgap of 1.75 eV, achieves a stabilized PCE of 17.18%.

Among the variety of approaches, such as current density–voltage (J–V) measurements with different voltage scan directions and rates, steady-state efficiency measurements, and maximum power point tracking (MPPT), MPPT is the most reliable method for characterizing the efficiency of organic–inorganic lead halide perovskite solar cells with strong hysteretic behavior. However, MPPT based on the commonly used simple perturb-and-observe (P&O) algorithm can still significantly underestimate the true maximum power point if the hysteresis is severe and a relatively small dwell time at each bias step is used. Here, we present a predictive P&O algorithm to model the current data which allows the prediction of the steady-state current density for each bias set point. As a result, our predictive MPPT speeds up the tracking process and measures the true maximum power point, regardless of the degree of hysteresis, suggesting a useful approach for characterizing the performance of hysteretic solar cells.

Perovskite photovoltaic cells (PVs) have attracted significant worldwide attention in the past few years. Although the stability of the power conversion is a concern, there is great potential for perovskites to enter the global PV market. To determine the future potential of perovskites, we performed a cradle-to-gate environmental life cycle (LCA) for two different perovskite device structures suitable for low cost manufacturing. Rather than examining current laboratory deposition processes like dipping and spinning, we considered spray and co-evaporation methods that are more amenable to manufacturing. A structure with an inorganic hole transport layer (HTL) was developed for both solution and vacuum based processes, and an HTL-free structure with printed with back contact was modeled for solution based deposition. The environmental impact of conventional Si PV technology was used as a reference point. The environmental impacts from manufacturing of perovskite solar cells were lower than that of mono-Si. However, environmental impacts from unit electricity generated were higher than all commercial PV technology mainly because of the shorter lifetime of perovskite solar cell. The HTL-free perovskite generally had the lowest environmental impacts among the three structures studied. Solution based methods used in perovskite deposition were observed to decrease the overall electricity consumption. Organic materials used for preparing the precursors for perovskite deposition were found to cause a high marine eutrophication impact. Surprisingly, the toxicity impacts of the lead used in the formation of the absorber layer were found to be negligible. Energy payback times were estimated as 1.0–1.5 years.

Efficient lead (Pb)-free inverted planar formamidinium tin triiodide (FASnI3 ) perovskite solar cells (PVSCs) are demonstrated. Our FASnI3 PVSCs achieved average power conversion efficiencies (PCEs) of 5.41% ± 0.46% and a maximum PCE of 6.22% under forward voltage scan. The PVSCs exhibit small photocurrent-voltage hysteresis and high reproducibility. The champion cell shows a steady-state efficiency of ≈6.00% for over 100 s.

Lead thiocyanate in the perovskite precursor can increase the grain size of a perovskite thin film and reduce the conductivity of the grain boundaries, leading to perovskite solar cells with reduced hysteresis and enhanced fill factor. A planar perovskite solar cell with grain boundary and interface passivation achieves a steady-state efficiency of 18.42%.

Both tin oxide (SnO2) and fullerenes have been reported as electron selective layers (ESLs) for producing efficient lead halide perovskite solar cells. Here, we report that SnO2 and fullerenes can work cooperatively to further boost the performance of perovskite solar cells. We find that fullerenes can be redissolved during perovskite deposition, allowing ultra-thin fullerenes to be retained at the interface and some dissolved fullerenes infiltrate into perovskite grain boundaries. The SnO2 layer blocks holes effectively; whereas, the fullerenes promote electron transfer and passivate both the SnO2/perovskite interface and perovskite grain boundaries. With careful device optimization, the best-performing planar perovskite solar cell using a fullerene passivated SnO2 ESL has achieved a steady-state efficiency of 17.75% and a power conversion efficiency of 19.12% with an open circuit voltage of 1.12 V, a short-circuit current density of 22.61 mA cm−2, and a fill factor of 75.8% when measured under reverse voltage scanning. We find that the partial dissolving of fullerenes during perovskite deposition is the key for fabricating high-performance perovskite solar cells based on metal oxide/fullerene ESLs.

We report on the synthesis of methylammonium tin triiodide (MASnI3) thin films at room temperature by a hybrid thermal evaporation method and their application in fabricating lead (Pb)-free perovskite solar cells. The as-deposited MASnI3 thin films exhibit smooth surfaces, uniform coverage across the entire substrate, and strong crystallographic preferred orientation along the 〈100〉 direction. By incorporating this film with an inverted planar device architecture, our Pb-free perovskite solar cells are able to achieve an open-circuit voltage (Voc) up to 494 mV. The relatively high Voc is mainly ascribed to the excellent surface coverage, the compact morphology, the good stoichiometry control of the MASnI3 thin films, and the effective passivation of the electron-blocking and hole-blocking layers. Our results demonstrate the potential capability of the hybrid evaporation method to prepare high-quality Pb-free MASnI3 perovskite thin films which can be used to fabricate efficient Pb-free perovskite solar cells.

Recent progress has shown that low-temperature processed tin oxide (SnO2) is an excellent electron selective layer (ESL) material for fabricating highly efficient organic–inorganic metal-halide perovskite solar cells with a planar cell structure. Low-temperature processing and a planar cell structure are desirable characteristics for large-scale device manufacturing due to their associated low costs and processing simplicity. Here, we report that plasma-enhanced atomic layer deposition (PEALD) is able to lower the deposition temperature of SnO2 ESLs to below 100 °C and still achieve high device performance. With C60-self-assembled monolayer passivation, our PEALD SnO2 ESLs deposited at ∼100 °C led to average power conversion efficiencies higher than 18% (maximum of 19.03%) and 15% (maximum of 16.80%) under reverse voltage scan for solar cells fabricated on glass and flexible polymer substrates, respectively. Our results thus demonstrate the potential of the low-temperature PEALD process of SnO2 ESLs for large-scale manufacturing of efficient perovskite solar cells.

We present efficient metal oxide-free and annealing-free planar perovskite solar cells with the regular cell structure using vacuum-deposited fullerenes C 60 and C 70 as the electron-selective layers and vacuum-processed perovskites as the light absorbers. The devices with an ultrathin C 60 layer (5.5 nm) yielded an average power conversion efficiency of 14.3% and a maximum efficiency of 15.7%. The best-performing cell produced a steady-state efficiency of 14.6%. The high performance is attributed to the efficient blocking of holes and extraction of electrons by C 60 due to a favorable energy level alignment between the C 60 and the fluorine-doped tin oxide electrodes. With the realization of efficient cells, the annealing-free vacuum deposition of perovskite absorbers and C 60 or C 70 electron-selective layers and intermediate layers demonstrates its power for fabricating all-perovskite tandem solar cells.

Efficient lead halide perovskite solar cells have been realized using SnO2 as electron selective layers (ESLs). Here, we report on the effects of the annealing temperature of solution-processed SnO2 ESLs on the performance of perovskite solar cells. We find that the cells using low-temperature annealed SnO2 (LT-SnO2) ESLs outperform the cells using high-temperature annealed SnO2 (HT-SnO2) ESLs, exhibiting higher open circuit voltages and fill factors. Structural, electrical, optical, and electrochemical characterizations reveal the origin of the performance differences: LT-SnO2 produces better film coverage, wider band gap, and lower electron density than that of HT-SnO2. The confluence of these properties results in more effective transportation of electrons and blocking of holes, leading to lower interface recombination. Therefore, LT-SnO2 ESLs are preferred for manufacturing perovskite solar cells on flexible substrates.

Efficient organic–inorganic lead halide perovskite solar cells with regular structure typically use solution-processed electron selective layers. Here, we demonstrated efficient planar perovskite solar cells using vacuum-processed C60 compact electron selective layers at room temperature. The best-performing planar perovskite solar cell has shown a power conversion efficiency of 15.14% with an open circuit voltage of 1.08 V and a fill factor of 74.51% measured under reverse voltage scanning. Our results suggest that vacuum-processed fullerene electron selective layer is a good candidate for fabricating all perovskite tandem solar cells.

Tandem solar cells using only metal-halide perovskite sub-cells are an attractive choice for next-generation solar cells. However, the progress in developing efficient all-perovskite tandem solar cells has been hindered by the lack of high-performance low-bandgap perovskite solar cells. Here, we report efficient mixed tin–lead iodide low-bandgap (∼1.25 eV) perovskite solar cells with open-circuit voltages up to 0.85 V and over 70% external quantum efficiencies in the infrared wavelength range of 700–900 nm, delivering a short-circuit current density of over 29 mA cm−2 and demonstrating suitability for bottom-cell applications in all-perovskite tandem solar cells. Our low-bandgap perovskite solar cells achieve a maximum power conversion efficiency of 17.6% and a certified efficiency of 17.01% with a negligible current–voltage hysteresis. When mechanically stacked with a ∼1.58 eV bandgap perovskite top cell, our best all-perovskite 4-terminal tandem solar cell shows a steady-state efficiency of 21.0%. All-perovskite tandem solar cells hold the promise of high efficiencies whilst safeguarding the ease of fabrication intrinsic to perovskites. Here, Zhao et al. present a certified 17% efficient tin and lead perovskite solar cell, which is integrated as the low-bandgap component of a tandem device with 21% efficiency.

Mixed tin (Sn)-lead (Pb) perovskites with high Sn content exhibit low bandgaps suitable for fabricating the bottom cell of perovskite-based tandem solar cells. In this work, we report on the fabrication of efficient mixed Sn-Pb perovskite solar cells using precursors combining formamidinium tin iodide (FASnI3) and methylammonium lead iodide (MAPbI3). The best-performing cell fabricated using a (FASnI3)0.6(MAPbI3)0.4 absorber with an absorption edge of ∼1.2 eV achieved a power conversion efficiency (PCE) of 15.08 (15.00)% with an open-circuit voltage of 0.795 (0.799) V, a short-circuit current density of 26.86(26.82) mA/cm(2), and a fill factor of 70.6(70.0)% when measured under forward (reverse) voltage scan. The average PCE of 50 cells we have fabricated is 14.39 ± 0.33%, indicating good reproducibility.

Formamidinium lead triiodide (FAPbI3) is considered as an alternative to methylammonium lead triiodide (MAPbI3) because of its lower band gap and better thermal stability. However, owing to the large size of FA cations, it is difficult to synthesize high-quality FAPbI3 thin films without the formation of an undesirable yellow phase. Smaller sized cations, such as MA and Cs, have been successfully used to suppress the formation of the yellow phase. Whereas FA and MA lead triiodide perovskite solar cells (PVSCs) have achieved power conversion efficiencies (PCEs) higher than 20 %, the PCEs of formamidinium and cesium lead triiodide (FA1−xCsxPbI3) PVSCs have been only approximately 16.5 %. Herein, we report our examination of the main factors limiting the PCEs of (FA1−xCsxPbI3) PVSCs. We find that one of the main limiting factors could be the small grain sizes (≈120 nm), which leads to relatively short carrier lifetimes. We further find that adding a small amount of lead thiocyanate [Pb(SCN)2] to the precursors can enlarge the grain size of (FA1−xCsxPbI3) perovskite thin films and significantly increase carrier lifetimes. As a result, we are able to fabricate (FA1−xCsxPbI3) PVSCs with significantly improved open-circuit voltages and fill factors and, therefore, enhanced PCEs. With an optimal 0.5 mol % Pb(SCN)2 additive, the average PCE is increased from 16.18±0.50 (13.45±0.78) % to 18.16±0.54 (16.86±0.63) % for planar FA0.8Cs0.2PbI3 PVSCs if measured under reverse (forward) voltage scans. The champion cell registers a PCE of 19.57 (18.12) % if measured under a reverse (forward) voltage scan, which is comparable to that of the best-performing MA-containing planar FA-based lead halide PVSCs.

Close-space sulfurization (CSS) of sputtered metal precursors is developed for the first time to fabricate Cu2ZnSnS4 (CZTS) thin-film solar cells. To optimize the CSS parameters, a series of CZTS thin films are prepared by sulfurizing stacked metal precursors with various sulfurization temperatures and durations. These films are studied by different characterization techniques and then fabricated into solar cell devices. According to the XRD, Raman and SEM, single-phase CZTS thin films with densely packed large grains are formed after CSS. The best cell performance with an efficiency of 5.2% is achieved for the CSS at 590°C for 30 min.

Through detailed device characterization using cross-sectional Kelvin probe force microscopy (KPFM) and trap density of states measurements, we identify that the J–V hysteresis seen in planar organic–inorganic hybrid perovskite solar cells (PVSCs) using SnO2 electron selective layers (ESLs) synthesized by low-temperature plasma-enhanced atomic-layer deposition (PEALD) method is mainly caused by the imbalanced charge transportation between the ESL/perovskite and the hole selective layer/perovskite interfaces. We find that this charge transportation imbalance is originated from the poor electrical conductivity of the low-temperature PEALD SnO2 ESL. We further discover that a facile low-temperature thermal annealing of SnO2 ESLs can effectively improve the electrical mobility of low-temperature PEALD SnO2 ESLs and consequently significantly reduce or even eliminate the J–V hysteresis. With the reduction of J–V hysteresis and optimization of deposition process, planar PVSCs with stabilized output powers up to 20.3% are achieved. The results of this study provide insights for further enhancing the efficiency of planar PVSCs.