spectral range for singe junction solar cells, and according to the Shockley-Queisser model the photocurrent is expected to be below 9.7 mA cm −2 . [2] However, a relatively high V oc of 1.4-1.5 V was reported, which makes this material system a great candidate for applications such as tandem configuration or other systems requiring spectral splitting. [2][3][4][5][6][7] Significant efforts were undertaken to investigate and optimize the V oc of CH 3 NH 3 PbBr 3 solar cells. [2][3][4][5][6][7][8][9] Engineering advanced hole transport layers (HTL) such as carbon nanotubes, [2] 2,2′,7,7′-tetrakis(N,Ndi-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD), [5,7] poly (indenofluoren-8-triarylamine) (PIF8-TAA), [3,4] and 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP) [6] resulted in V oc within a range of 1.4-1.5 V for solution-processed perovskite solar cells. Sheng et al. demonstrated a high V oc of 1.45 V using the architecture TiO 2 / CH 3 NH 3 PbBr 3 /spiro-OMeTAD via a vaporassisted depositon. [7] Kim et al. modified the TiO 2 surfaces with carboxyl groups and employed no HTL for CH 3 NH 3 PbBr 3 solar cells, producing a V oc of 1.37 V. [8] Dymshits et al. reported a V oc of 1.35 V for Al 2 O 3 /CH 3 NH 3 PbBr 3 perovskite solar cells without HTL. [9] These studies focused more on the V oc improvement via interface engineering and via optimizing the CH 3 NH 3 PbBr 3 film quality. Despite significant progress toward unraveling the V oc limitation of CH 3 NH 3 PbBr 3 solar cells, the limiting V oc (V oc,rad ) is still unknown to the best of our knowledge. Besides, Perovskite solar cells based on CH

Effective engineering of surface ligands in semiconductor nanocrystals can facilitate the electronic interaction between the individual nanocrystals, making them promising for low-cost optoelectronic applications. Here, the use of high purity Cu 2 ZnSnS 4 (CZTS) nanocrystals as the photoactive layer and hole-transporting material is reported in low-temperature solution-processed solar cells. The high purity CZTS nanocrystals are prepared by engineering the surface ligands of CZTS nanocrystals, capped originally with the long-chain organic ligand oleylamine. After ligand removal, CZTS nanocrystals show substantial improvement in photoconductivity and mobility, displaying also an appreciable photoresponse in a simple heterojunction solar cell architecture. More notably, CZTS nanocrystals exhibit excellent holetransporting properties as interface layer in perovskite solar cells, yielding power conversion efficiency (PCE) of 15.4% with excellent fill factor (FF) of 81%. These findings underscore the importance of removing undesired surface ligands in nanocrystalline optoelectronic devices, and demonstrate the great potential of CZTS nanocrystals as both active and passive material for the realization of low-cost efficient solar cells.

In colloidal nanoparticle (NPs) devices, trap state densities at their surface exert a profound impact on the rate of charge carrier recombination and, consequently, on the deterioration of the device performance. Here, we report on the successful application of a ligand exchange strategy to effectively passivate the surface of cuprite (Cu2O) NPs. Cu2O NPs were prepared by means of a novel synthetic route based on flame spray pyrolysis. FTIR, XRD, XPS, and HRTEM measurements corroborate the formation of cubic cuprite Cu2O nanocrystals, excluding the possible presence of undesired CuO or Cu phases. Most importantly, steady-state emission and transient absorption assays document that surface passivation results in substantial changes in the intensity of emissive excitonic states--centered at copper and oxygen vacancies--and in the lifetime of excitons near the band edge. To shed light onto ultrafast processes in Cu2O nanocrystals additional pump probe experiments on the femtosecond and nanosecond time scales were carried out. Two discernible species were observed: on one hand, an ultrafast component (~ps) that relates to the excitons; on the other hand, a long-lived component (~μs) that originates from the defects/trap states.

The production of high-performance, solution-processed kesterite Cu2ZnSn­(S x ,Se1–x )4 (CZTSSe) solar cells typically relies on high-temperature crystallization processes in chalcogen-containing atmosphere and often on the use of environmentally harmful solvents, which could hinder the widespread adoption of this technology. We report a method for processing selenium free Cu2ZnSnS4 (CZTS) solar cells based on a short annealing step at temperatures as low as 350 °C using a molecular based precursor, fully avoiding highly toxic solvents and high-temperature sulfurization. We show that a simple device structure consisting of ITO/CZTS/CdS/Al and comprising an extremely thin absorber layer (∼110 nm) achieves a current density of 8.6 mA/cm2. Over the course of 400 days under ambient conditions encapsulated devices retain close to 100% of their original efficiency. Using impedance spectroscopy and photoinduced charge carrier extraction by linearly increasing voltage (photo-CELIV), we demonstrate that reduced charge carrier mobility is one limiting parameter of low-temperature CZTS photovoltaics. These results may inform less energy demanding strategies for the production of CZTS optoelectronic layers compatible with large-scale processing techniques.