The very fast evolution in certified efficiency of lead-halide organic-inorganic perovskite solar cells to 24.2%, on par and even surpassing the record for polycrystalline silicon solar cells (22.3%), bears the promise of a new era in photovoltaics and revitalisation of thin film solar cell technologies. However, the presence of toxic lead and particularly toxic solvents during the fabrication process makes large-scale manufacturing of perovskite solar cells challenging due to legislation and environment issues. For lead-free alternatives, non-toxic tin, antimony and bismuth based solar cells still rely on up-scalable fabrication processes that employ toxic solvents. Here we employ non-toxic methyl-acetate solution processed (CH₃NH₃)₃Bi₂I₉ films to fabricate lead-free, bismuth based (CH₃NH₃)₃Bi₂I₉ perovskites on mesoporous TiO₂ architecture using a sustainable route. Optoelectronic characterization, X-ray diffraction and electron microscopy show that the route can provide homogeneous and good quality (CH₃NH₃)₃Bi₂I₉ films. Fine-tuning the perovskite/hole transport layer interface by the use of conventional 2,2′,7,7′-tetrakis (N,N′-di-p-methoxyphenylamino)−9,9′-spirbiuorene, known as Spiro-OMeTAD, and poly(3-hexylthiophene-2,5-diyl - P3HT as hole transporting materials, yields power conversion efficiencies of 1.12% and 1.62% under 1 sun illumination. Devices prepared using poly(3-hexylthiophene-2,5-diyl hole transport layer shown 300 h of stability under continuous 1 sun illumination, without the use of an ultra violet-filter.

A convenient 1-step spincoating method for the hybrid perovskite MAPb(I_1−xBr_x)₃(Cl)_y, with PbCl₂ as lead source, enables tuning of the bandgap.

A novel biphenyl fumaronitrile as an acceptor and triphenylamine as donor conjugated building blocks are used for the first time to successfully synthesize donor–acceptor–donor molecule (D‐A‐D) 2,3‐bis(4′‐(bis(4‐methoxyphenyl)amino)‐[1,1′‐biphenyl]‐4‐yl)fumaronitrile (TPA‐BPFN‐TPA). The new TPA‐BPFN‐TPA with low‐lying HOMO is used as a dopant‐free hole‐transporting material (HTM) in mesoporous perovskite solar cells. The performance of the solar cells using this new HTM is compared with the traditional 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamino)‐ 9,9′‐spirobifluorene (Spiro‐OMeTAD) HTM based devices for outdoor and indoor performance evaluation. Under 1 sun illumination, dopant‐free TPA‐BPFN‐TPA HTM based devices exhibit a power conversion efficiency (PCE) of 18.4%, which is the record efficiency to date among D‐A‐D molecular design based dopant‐free HTMs. Moreover, the stability of unencapsulated TPA‐BPFN‐TPA‐based devices shows improvement over Spiro‐OMeTAD‐based devices in harsh relative humidity condition of 70%. Another exciting feature of the newly developed HTM is that the TPA‐BPFN‐TPA‐based devices exhibit improved PCE of 30% and 20.1% at 1000 lux and 200 lux illuminations, respectively. This new finding provides a solution to fabricate low indoor (low light) and outdoor (1 sun) perovskite solar cell devices with high efficiency for cutting‐edge energy harvesting technology.

This study reports three newly developed dopant-free hole-transporting materials (HTMs) for perovskite solar cells. The design is based on a quinacridone (QA) dye as the core with three different extended end-capping moieties, namely, acenaphthylene (ACE), triphenylamine (TPA) and diphenylamine (DPA), attached to the QA core. These HTMs were synthesized and used to successfully fabricate in mesoscopic TiO₂/CH₃NH₃PbI₃/HTM perovskite devices. Under AM 1.5G illumination at 100 mW cm⁻², the devices achieved a maximum efficiency of 18.2% for ACE-QA-ACE, 16.6% for TPA-QA-TPA and 15.5% for DPA-QA-DPA without any additives, whereas reference devices with doped spiro-OMeTAD as the HTM achieved a PCE of 15.2%. Notably, the unencapsulated devices based on the novel dopant-free HTMs exhibited impressive stability in comparison with the devices based on doped spiro-OMeTAD under a relative humidity of 75% for 30 days. These linear symmetrical HTMs pave the way to a new class of organic hole-transporting materials for cost-efficient and large-area applications of printed perovskite solar cells.

A new dopant free stable HTM using anthanthrone dye has been reported with 11.5% efficiency.