HTL-free Perovskite Solar Cells Research Articles

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Summary Capacitance-based techniques have been used to measure the electrical properties of halide perovskite solar cells (PSCs) such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. Here, we show that capacitance-based techniques cannot be used to reliably analyze the properties of defects in the perovskite layer or at its interface, because the high-frequency capacitance signature is due to the response of charge carriers in the hole-transport layer (HTL). For HTL-free PSCs, high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer because there is no trapping and de-trapping of charge carriers in the perovskite layer. We further find that the low-frequency capacitance signature can be used to calculate the activation energy of the ionic conductivity of the perovskite layer, but the overlapping effects with charge transport materials must be avoided.

Performance of low-cost mixed cationic carbon-based solar cells prepared through compositional engineering under ambient conditions

Perovskite solar cells (PSC) have reached a record efficiency of up to 25.2 % in a matter of few years. This has spawned a lot of interest in investigating the commercial scalability of the perovskites by reducing the cost. Carbon can act as a cheaper alternative to gold and aluminium to act as counter electrode. In the present study, we have tuned the perovskite layer by the addition of FA (Formamidinium) and Cs (Cesium) through compositional engineering. The optimized configuration of perovskite was then utilized in carbon monolithic electrode stack to form a carbon-based perovskite solar cell. Emphasis is more on the performance of these carbon-based PSC. Three batches of carbon-based PSC were prepared with two-hole transport layers (P3HT, Spiro-OMeTAD) and one with no hole transport layer. All the carbon-based PSC were then subjected to EIS studies. The carbon-based HTL-Free PSC had a power conversion efficiency (PCE) of up to 9.5 % while cells with HTLs as P3HT and Spiro-OMeTAD had 8.5 % and 13.4 %, respectively. Stability wise, HTL-free devices performed exceedingly well (up to 76 days). Under endurance testing against light and heat, P3HT- based devices managed to retain 75 % of PCE after 25 h constant illumination and performed better than HTL-free and Spiro-OMeTAD based cell under thermal stress of up to 240 ℃.

Effects of thiourea on the perovskite crystallization for fully printable solar cells

Fully printable HTL free perovskite solar cells (PSCs) are the most attractive candidates for large scale production, due to their low cost and high stability. In this kind of PSCs, perovskite precursor solutions are dropped on the three layer mesoscopic scaffolds, so it is important to improve the crystallinity as well as the penetrability into the mesoscopic scaffolds. Thiourea that has two NH2 groups and one S-donor can form hydrogen bond with the perovskite crystals or lewis acid-base adducts to retard the growth of perovskite crystals. In this work, it is found that thiourea facilitate the penetrability of perovskite precursor solution into the mesoscopic scaffolds and the crystallinity of perovskite crystals. It is suggested that such effects are associated with the cross-liker function and with the retarding function of thiourea. With adding thiourea into the perovskite precursor solution, electron transfer from perovskite crystals to FTO and the recombination resistance are enhanced, so that the performance of thiourea based device was improved from 10.20% (the pristine device) to 13.03%. Along with this, the thiourea can reduce iodine molecule to I− ion, so that perovskite films with thiourea have a higher stability than the pristine film.

Graded Heterojunction Engineering for Hole-Conductor-Free Perovskite Solar Cells with High Hole Extraction Efficiency and Conductivity.

Despite great progress in the photovoltaic conversion efficiency (PCE) of inorganic-organic hybrid perovskite solar cells (PSCs), the large-scale application of PSCs still faces serious challenges due to the poor-stability and high-cost of the spiro-OMeTAD hole transport layer (HTL). It is of great fundamental importance to rationally address the issues of hole extraction and transfer arising from HTL-free PSCs. Herein, a brand-new PSC architecture is designed by introducing multigraded-heterojunction (GHJ) inorganic perovskite CsPbBrx I3-x layers as an efficient HTL. The grade adjustment can be achieved by precisely tuning the halide proportion and distribution in the CsPbBrx I3-x film to reach an optimal energy alignment of the valance and conduction band between MAPbI3 and CsPbBrx I3-x . The CsPbBrx I3-x GHJ as an efficient HTL can induce an electric field where a valance/conduction band edge is leveraged to bend at the heterojunction interface, boosting the interfacial electron-hole splitting and photoelectron extraction. The GHJ architecture enhances the hole extraction and conduction efficiency from the MAPbI3 to the counter electrode, decreases the recombination loss during the hole transfer, and benefits in increasing the open-circuit voltage. The optimized HTL-free PCS based on the GHJ architecture demonstrates an outstanding thermal stability and a significantly improved PCE of 11.33%, nearly 40% increase compared with 8.16% for pure HTL-free devices.

Optimal design of efficient hole transporting layer free planar perovskite solar cell

Hole transporting layer (HTL) free organo-metal halide perovskite solar cells have shown great promise in simplifying device architecture, fabrication process and enhancing stability. However, the simple elimination of the HTL from the standard sandwiched configuration suffers from relatively poor device performance; additionally, the mechanism of the HTL-free perovskite solar cell is still unclear. Herein, we applied a one-dimensional modeling program wxAMPS to investigate the planar HTL-free perovskite solar cells by adjusting the absorber thickness, doping and the absorber/back contact band alignment. The simulation results reveal the importance of the moderate absorber thickness as well as the p-doping perovskite rather than intrinsic as in sandwich structure to the overall device efficiency. In the meanwhile, reducing the mismatching of the absorber/back contact by using higher work function back contact material in replacement of commonly utilized Au electrode is more favorable to improve the device performance. Through optimizing, high performance HTL-free perovskite solar cell with efficiency approaching 17% could be achieved. This study is helpful in providing theoretical guidance for the design of HTL-free perovskite solar cells.

Cu2O particles mediated growth of perovskite for high efficient hole-transporting-layer free solar cells in ambient conditions

Inverted hole-transporting-layer (HTL) free planar perovskite solar cells are fabricated in ambient conditions. The perovskite layers are prepared via a two-step process on electrodeposited Cu2O particles, and a compact, homogenous, and pin-hole free morphology with extremely large grains are obtained. The high quality of the perovskite films is attributed to that the Cu2O particles can act as the growing sites of crystal nucleus of perovskite films during annealing. The optimized device shows a power conversion efficiency of 9.64%. Such a PCE is one of the highest values among the reported inverted HTL-free perovskite solar cells even it is fabricated in a high humidity condition.

Flexible, hole transporting layer-free and stable CH3NH3PbI3/PC61BM planar heterojunction perovskite solar cells

Hole transporting layer (HTL)-free CH3NH3PbI3/PC61BM planar heterojunction perovskite solar cells were fabricated with the configuration of ITO/CH3NH3PbI3/PC61BM/Al. The devices present a remarkable power conversion efficiency (PCE) of 11.7% (12.5% best) under AM 1.5G 100 mW cm−2 illumination. Moreover, the HTL-free perovskite solar cells on flexible PET substrates are first demonstrated, achieving a power conversion efficiency of 9.7%. The element distribution in the HTL-free perovskite solar cell was further investigated. The results indicated that the PbI2 enriched near the PC61BM side for chlorobenzene treatment via the fast deposition crystallization method. Without using HTL on the ITO, the device is stable with comparison to that with poly(3,4-ethylenedioxylenethiophene): poly(styrene sulfonate) (PEDOT:PSS) as HTL. In addition, the fabricating time of the whole procedure from ITO substrate cleaning to device finishing fabrication only cost about 3 h for our mentioned devices, which is much more rapid than other structure devices containing other transporting layer. The high efficient and stable HTL-free CH3NH3PbI3/PC61BM planar heterojunction perovskite solar cells with the advantage of saving time and cost provide the potential for commercialization printing electronic devices.