Point contacts at the copper-indium-gallium-selenide interface—A theoretical outlook

Abstract

For a long time, it has been assumed that recombination in the space-charge region of CIGS is dominant, at least in high efficiency solar cells with low band gap. The recent developments like KF post deposition treatment and point-contact junction may call this into question. In this work a theoretical outlook is made using three-dimensional simulations to investigate the effect of pointcontact openings through a passivation layer on CIGS solar cell performance. A large set of solar cells is modeled under different scenarios for the charged defect levels and density, radius of the openings, interface quality and conduction band offset. The positive surface charge created by the passivation layer induces band bending and this influences the contact (CdS) properties, making it beneficial for the open circuit voltage and efficiency, and the effect is even more pronounced when coverage area is more than 95 %, and also makes a positive impact on the device performance, even in the presence of a spike at CIGS/CdS heterojunction.

Surface Passivation of CIGS Solar Cells Using Gallium Oxide

“…Since the doping levels in CIGSe material of these experiments are comparatively low (10 15 cm −3 ), an oxide with positive charge could form a depletion layer and improve charge collection at the front CIGS‐CdS interface. This effect has been theoretically studied by Sozzi et al and Bercegol et al In contrast, AlO x shows high negative charge densities between −2 × 10 11 to −1 × 10 12 which is consistent with previous observations …”

“…The electrical characterization of these cells did indeed show an absolute increase of 2.6% in efficiency originating from a substantial increase of 56.3 mV in V OC , 1.04 mA cm −2 in J SC , and 8.22% in FF with a gallium oxide layer as compared to the reference. Therefore as an outlook, future experiments can focus on alternative buffer layers or deposition techniques, point contact openings through thick gallium oxide layers (such as those simulated by Sozzi et al and Bercegol et al,) and also low‐temperature conductance measurements on MIS structures to quantify D it .…”

“…ф fp was calculated for an acceptor concentration of N a = 5 × 10 15 cm −3 and intrinsic carrier concentration ( n i ) of 7 × 10 8 cm −3 . n i was calculated from conduction‐band density of states (N C = 7 × 10 17 cm −3 ) and valence‐band density of states (N V = 1.5 × 10 19 cm −3 ) and k being the Boltzmann constant …”

“…This can be due to a “chemical passivation” which reduces dangling bonds and active interface defects. It may also be due to a “field‐effect passivation” which can, based on the polarity of charges in the region, either repel minority charge carriers and reduce recombination or repel majority carriers sufficiently to cause an inversion at the surface …”

Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer

“…Simply, the chemical passivation allows for the decrease of the total number of electrically active defects. [38][39][40][41] Ultrathin devices have recently been studied in detail by numerous groups [42][43][44][45][46][47] as they have the potential to reduce the material costs and manufacturing times. Such field is beneficial for the electrical performance of the solar cell, since it drives minority carriers away from the highly recombinative rear contact into the space charge region.…”