Two chemically tailored new conjugated copolymers, HSL1 and HSL2, were developed and applied as hole selective layers to improve the anode interface of fullerene/perovskite planar heterojunction solar cells. The introduction of polar functional groups on the polymer side chains increases the surface energy of the hole selective layers (HSLs), which promote better wetting with the perovskite films and lead to better films with full coverage and high crystallinity. The deep highest occupied molecular orbital levels of the HSLs align well with the valence band of the perovskite semiconductors, resulted in increase photovoltage. The high lying lowest unoccupied molecule orbital level provides sufficient electron blocking ability to prevent electrons from reaching the anode and reduces the interfacial trap-assisted recombination at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/perovskite interface, resulting in a longer charge-recombination lifetime and shorter charge-extraction time. In the presence of the HSLs, high-performance CH₃NH₃PbI _x Cl_{3− x} perovskite solar cells with a power conversion efficiency (PCE) of 16.6% (V _oc: 1.07 V) and CH₃NH₃Pb(I_0.3Br_0.7) _x Cl_{3− x} cells with a PCE of 10.3% (V _oc: 1.34 V) can be realized.

An amino-functionalized copolymer with a conjugated backbone composed of fluorene, naphthalene diimide, and thiophene spacers (PFN-2TNDI) is introduced as an alternative electron transport layer (ETL) to replace the commonly used [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) in the p–i–n planar-heterojunction organometal trihalide perovskite solar cells. A combination of characterizations including photoluminescence (PL), time-resolved PL decay, Kelvin probe measurement, and impedance spectroscopy is used to study the interfacial effects induced by the new ETL. It is found that the amines on the polymer side chains not only can passivate the surface traps of perovskite to improve the electron extraction properties, they also can reduce the work function of the metal cathode by forming desired interfacial dipoles. With these dual functionalities, the resulted solar cells outperform those based on PCBM with power conversion efficiency (PCE) increased from 12.9% to 16.7% based on PFN-2TNDI. In addition to the performance enhancement, it is also found that a wide range of thicknesses of the new ETL can be applied to produce high PCE devices owing to the good electron transport property of the polymer, which offers a better processing window for potential fabrication of perovskite solar cells using large-area coating method.

NiO is a promising hole transporting material for perovskite solar cells due to its high hole mobility, good stability, easy processibility, and suitable Fermi level for hole extraction. However, the efficiency of NiO-based cells is still limited by the slow hole extraction due to the poor perovskite/NiO interface and the inadequate quality of the two solution-processed material phases. Here, large influences of a monolayer surface modification of NiO nanocrystal layers with ethanolamine molecules are demonstrated on the enhancement of hole extraction/transport and thus the photovoltaic performance. The underlying causes have been revealed by a series of studies, pointing to a favorable dipole layer formed by the molecular adsorption along with the enhanced perovskite crystallization and the improved interface contact. Comparatively, the solar cells based on a diethanolamine-modified NiO layer have achieved a rather high fill factor, indeed one of the highest among NiO-based perovskite solar cells, and high short-circuit photocurrent density (J_sc), resulting in a power conversion efficiency of ≈16%, most importantly, without hysteresis.