Carrier‐selective contacts using metal compounds for crystalline silicon solar cells

Abstract

Solar cells rely on the efficient generation of electrons and holes and the subsequent collection of these photoexcited charge carriers at spatially separated electrodes.High wafer quality is now commonplace for crystalline silicon (c-Si) based solar cells, meaning that the cell's efficiency potential is largely dictated by the effectiveness of its carrier-selective contacts. The majority of contacts currently employed in industrial production are based on highly doped-silicon, which can introduce negative side-effects including Auger recombination or parasitic absorption depending on whether the dopants are diffused into the absorber or whether they are incorporated into silicon layers deposited outside the absorber. Given the terawatt scale of deployment of c-Si solar cells, the search for alternative contacting schemes that can offer potential benefits in terms of performance, cost, ease of processing or stability is highly relevant. One such category of contacting schemes, with the potential to avoid the above mentioned issues, is that which employs metal compounds as the 'carrierselective' layer. The last 7 years has seen a surge in interest on this topic and a few promising families of materials have emerged, most prominently the alkali/alkalineearth metal compounds and the transition-metal oxides. The number of successful selective-contact demonstrations of materials within these families is fast increasing with the best solar cell demonstrations now exceeding 23%. However, in addition to improving their efficiency performance, several challenges remain if such contacts are to be considered for industrial adoption. These are mainly associated with poor stability, lack of compatibility with transparent electrodes and inability to be deposited using standard industrial techniques. This review covers the historical developments, current status and future prospects of metal-compound based selective contacts in the context of c-Si photovoltaics.

Dopant‐free passivating contacts for crystalline silicon solar cells: Progress and prospects

“…Other than dielectrics, this interlayer can also be some semiconductors with appropriate energy band structure and work function (W F ) to selectively transport the desired type of charge carrier (i.e., electron or hole). In this case, the contact is termed as “carrier‐selective contact” or “passivating contact” 8,19,20 . As the name suggests, the electron‐selective contact should collect electrons while block holes, vice versa for the hole‐selective contact.…”

“…In this case, the contact is termed as "carrier-selective contact" or "passivating contact". 8,19,20 As the name suggests, the electronselective contact should collect electrons while block holes, vice versa for the hole-selective contact. Such an asymmetric collection of charge carriers by the passivating contact is defined as the carrier selectivity.…”

Growth Mechanism and Film Properties of Atomic-Layer-Deposited Titanium Oxysulfide

“…In many electronic and optoelectronic devices including displays, solar cells, and batteries, tailoring of thin film properties such as conductivity, transparency, and band positions is paramount for maximizing device performance. − Such tailoring of the material properties often entails controlling the film composition and introduction of foreign atoms in the films. For metal oxides, for example, the functionality is often enhanced by the introduction of dopants, for example, Al doping in ZnO or Sn doping in In 2 O 3 , or by moving to ternary or even quaternary oxides such as InGaZnO in the field of thin film transistors .…”

“…Another common approach to tailor the properties of metal oxides is the introduction of sulfur. Prime examples are zinc oxysulfide (ZnO x S y ) and indium oxysulfide (InO x S y ) layers, which find their application as cadmium-free buffer layers in Cu­(In,Ga)­Se 2 (CIGS) solar cells. − In this case, the introduction of sulfur enables improved charge extraction by minimizing the conduction band offset with the CIGS layer . On the other hand, tin oxysulfide (SnO x S y ) compounds have been studied for application as a channel material in transparent electronics .…”

“…In this work, TiO x S y films with controllable S content are prepared by atomic layer deposition (ALD). ALD is a well-studied and commonly employed deposition technique for the synthesis of metal oxysulfides. ,,,− In general, the popularity of ALD can most likely be ascribed to its submonolayer-level control over film thickness, high uniformity, and its ability to conformally deposit films over demanding surface topologies. Moreover, ALD can yield a very high level of control over the film composition, which is most commonly achieved by combining multiple ALD processes in a supercycle fashion .…”