N-Type Surface Design for p-Type CZTSSe Thin Film to Attain High Efficiency

As a low-cost substitute that uses no expensive rare-earth elements for the high-efficiency Cu(In,Ga)(S,Se)₂ solar cell, the Cu₂ ZnSn(S,Se)₄ (CZTSSe) solar cell has borrowed optimization strategies used for its predecessor to improve its device performance, including a profiled band gap and surface inversion. Indeed, there have been few reports of constructing CZTSSe absorber layers with surface inversion to improve efficiency. Here, a strategy that designs the CZTSSe absorber to attain surface modification by using n-type Ag₂ ZnSnS₄ is demonstrated. It has been discovered that Ag plays two major roles in the kesterite thin film devices: surface inversion and front gradient distribution. It has not only an excellent carrier transport effect and reduced probability of electron-hole recombination but also results in increased carrier separation by increasing the width of the depletion region, leading to much improved V_OC and J_SC . Finally, a champion CZTSSe solar cell renders efficiency as high as 12.55%, one of the highest for its type, with the open-circuit voltage deficit reduced to as low as 0.306 V (63.2% Shockley-Queisser limit). The band engineering for surface modification of the absorber and high efficiency achieved here shine a new light on the future of the CZTSSe solar cell.

A 14.7% Organic/Silicon Nanoholes Hybrid Solar Cell via Interfacial Engineering by Solution-Processed Inorganic Conformal Layer

We demonstrated a high-performance Si-organic hybrid heterojunction solar cell utilizing low-temperature and liquid-phase-processed TiO₂ as an interlayer between poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Si nanoholes to produce a conformal contact on the surface of the Si nanostructure. The hydrophilic TiO₂/Si-nanohole surface enabled the PEDOT:PSS to flow into the spacing of the close-packed nanoholes. Scanning electron microscopy images were used to confirm the PEDOT:PSS nanohole filling induced by the TiO₂. With forming gas annealing of the double-sided TiO₂, high V_oc (0.63 V) and J_sc (35.7 mA/cm²) values were obtained, yielding a high power conversion efficiency of 14.7%. The high V_oc was attributed to the surface passivation of Si by annealed TiO₂. The X-ray photoelectron spectroscopy investigation at the TiO₂/Si interface indicates the TiO_x signal decreased and the TiO₂ and SiO_x signals increased after annealing. The Si-O bonding found in the O 1s study appeared in the form of Si-O-Si bonding to serve surface passivation. The band alignment of the PEDOT:PSS/TiO₂/n-Si hetero-interfaces was postulated and plotted. The V_bi in the system after annealing was assumed to be higher because of the reduction of bulk and surface states that yield high V_oc. After annealing, the V_bi increased from 0.805 to 0.905 V. The reduction of surface recombination velocity proved the passivation ability of TiO₂ after annealing. With proven surface passivation and conformal PEDOT:PSS/Si nanohole interfaces for enhanced contact, this Si-organic hybrid heterojunction solar cell with solution-processed TiO₂ interlayers has excellent potential for application as a high-efficiency and low-cost Si solar cell.

Gated Hall effect of nanoplate devices reveals surface-state-induced surface inversion in iron pyrite semiconductor

Understanding semiconductor surface states is critical for their applications, but fully characterizing surface electrical properties is challenging. Such a challenge is especially crippling for semiconducting iron pyrite (FeS2), whose potential for solar energy conversion has been suggested to be held back by rich surface states. Here, by taking advantage of the high surface-to-bulk ratio in nanostructures and effective electrolyte gating, we develop a general method to fully characterize both the surface inversion and bulk electrical transport properties for the first time through electrolyte-gated Hall measurements of pyrite nanoplate devices. Our study shows that pyrite is n-type in the bulk and p-type near the surface due to strong inversion and yields the concentrations and mobilities of both bulk electrons and surface holes. Further, solutions of the Poisson equation reveal a high-density of surface holes accumulated in a 1.3 nm thick strong inversion layer and an upward band bending of 0.9-1.0 eV. This work presents a general methodology for using transport measurements of nanostructures to study both bulk and surface transport properties of semiconductors. It also suggests that high-density of surface states are present on surface of pyrite, which partially explains the universal p-type conductivity and lack of photovoltage in polycrystalline pyrite.