2, Universidade do Minho, Braga, , Portugal
Inorganic semiconductor based pn junctions constitute an integral part of most optoelectronic devices, such as photovoltaic (PV) solar cells and light emitting diodes (LEDs). Although the inorganic materials offer higher stability and superior electro-optic properties relative to organics, they present exorbitant cost and scale-up challenges. This paper presents a radically different approach to circumvent such challenges. It takes advantage of naturally formed nanocrystalline pn junctions, resulting from judicious coupling of semiconductor materials and electrochemical processes. Certain combinations can lead to highly practical inorganic material systems, comprising process-induced bulk pn junction nanostructures. The paper will illustrate an exemplary material system, based on single-step electrodeposited copper-indium-selenide (CISe) ordered defect chalcopyrite compounds. Thermodynamically driven reactions enable the electrodeposition of highly-ordered, interlinked, space-filling CISe films, in a single step. This approach creates a low-cost processing platform to produce nanocrystalline films, with all the attributes necessary for efficient bulk homojunction (BHJ) operation. Surface analytical microscopies and spectroscopies reveal unusual phenomena and extraordinary electro-optical properties that could potentially maximize spectral absorption and reduce recombination loss. They support the fortuitous formation of surprisingly ordered, sharp, abrupt 3-dimensional, nanoscale CISe pn BHJs. The specific BHJ structure enables efficient separation and transport of free carriers and essentially performs the same functions as planar pn junctions. The CISe nanocrystals are very different from colloidal nanocrystals used in state-of-the-art BHJs; they exhibit mixed conductivity and high doping densities. These distinctive innate attributes of CISe films naturally create ordered nanoscale morphology and facilitate interconnections between the nanocrystals to form the BHJ structure. This totality manifests a highly significant advance in semiconductor processing as it creates an accessible, low cost solution-based method to fabricate high quality pn BHJ nanocrystalline material systems that can be directly used in PV or LED devices. Furthermore, with the incorporation of finely band-aligned contact electrode materials, the CISe BHJ film can transition into high performance flexible devices and roll-to-roll processing in simple thin-film form factor for easy scale-up.