On the Design and Development of Hole Selective Contacts for Organic and Silicon Solar Cells /

Abstract

This work deals with the design and development of hole selective contacts (HSCs) for solar cell technologies such as the bulk heterojunction polymer, and silicon (Si) technologies. HSCs are charge extraction layers (CELs) that facilitate only hole collection at the anode terminal. Design of CELs is driven by the fact that not only charge carriers photogeneration but also their efficient collection with minimal recombination on their way to the device electrodes is important. CELs not only act as sinks for the respective carriers and reduce recombination, but may also serve as optical spacers and buffers, besides preventing ion diffusion from the respective electrodes to the absorber layer. In general, wide band gap, high work function and sufficient (P-type) conductivity are key criteria for efficient HSCs and the reasons that make HSCs much scarcer among the CELs, and hence the motivation for this work. In this regard, sulfonated, reduced graphene oxide (S-RGO) and substoichiometric molybdenum oxide (MoOx, x < 3) systems were developed as HSC materials for P3HT:PC61BM based bulk heterojunction, and metal oxide based Si heterojunction solar cells, respectively. S-RGO is a partially reduced graphene oxide with –SO3H attached to its graphene structure. It is a solution processable, P-type material with tunable electro-optical properties. To develop S-RGO and study the effects of processing conditions on the properties of S-RGO, we first prepared its precursor, graphene oxide (GO). High quality few-layer GO was prepared from graphite powder through a modified Hummers method. S-RGO was then developed by fuming/concentrated sulfuric acid treatment of the as synthesized GO in ambient. Further, three variants of S-RGO varying in C/O and C/S were developed, and it was demonstrated that the optical band gap (Eg) and electrical conductivity () of S-RGO are tunable, and depend on the level of reduction and sulfonation of GO. Whereas, reduction and sulfonation were found dependent on SO3 content, acid strength and gas tightness of the reaction mixture. The easily tunable/adjustable Eg and  make S-RGO a potential hole extraction layer (HEL) material for several donor-acceptor systems. For P3HT:PC61BM based PSCs, it was observed that the shape of the J–V performance curve is tunable with the choice of HEL, and that performance superior to that of PEDOT:PSS is achievable with a properly designed S- II RGO. Our results imply that properly designed S-RGO can be used to replace some of the state of the art HEL materials for a host of device applications. MoOx, which is a wide band gap, high work P-type transparent semiconductor, was prepared through reactive-RF sputtering from molybdenum trioxide (MoO3) target. The effect of sputtering atmosphere on the stoichiometry, transparency, and thickness of MoOx films was analyzed. Moreover, MoOx optical behavior as a function of annealing temperature (≤ 250 °C) and annealing atmosphere (air/inert N2) was explored. Further, we investigated the effects caused by substrate upon annealing in inert, on the optical behavior of MoOx/substrate system. Our results suggest that annealing in inert makes MoOx/a-Si:H/glass more absorbing than MoOx/glass, however a-Si:H/glass remains almost unaffected by annealing. Thus, leading us to conceive that hydrogen (H) diffusion from a-Si:H to MoOx combined with a reduction in Mo oxidation states upon annealing in inert, are the basic reasons for the more pronounced optical absorption of MoOx/a-Si:H/glass system. By adopting appropriate processing measures, H diffusion into the MoOx layer can be mitigated, and thus MoOx HSC based Si heterojunction devices (SHJ-SCs) with better light management can be realized.