Bishop, Matthew T.
(2019)
Single-source precursor deposition of low-toxicity sulphides for thin film photovoltaic devices.
PhD thesis, University of Nottingham.
Abstract
Solar power has long been considered as an environmentally friendly method of energy generation, enjoying continued development and implementation. However, silicon solar cells (the most commercially successful type of solar cell) require high energy input for the extraction of silicon, high material quality, and is an economically expensive technology. This thesis investigates the use of alternative solar cells, namely: hybrid solar cells, using both organic and inorganic semiconductors, with an additional study focusing on utilising non-toxic materials for fabricating thin film inorganic solar cells. Overcoming obstacles associated with the future commercialisation of these techniques (i.e. associated toxicity and fabrication challenges) is the main purpose of this work. A key focus will be placed on metal xanthates, utilised as the inorganic precursor for each of these devices, as they have been proven to be effective, low temperature precursors for metal sulphide deposition.
Initially, it was important to determine that the synthesised metal xanthates (cadmium ethyl xanthate, copper ethyl xanthate, tin ethyl xanthate and zinc ethyl xanthate) were effective precursors for deposition (decomposing to the appropriate metal sulphide) and to check the thermal stability of the purchased polymer (Poly(3-hexylthiophene-2,5-diyl)(P3HT)). It was demonstrated that all metal xanthate precursors had the expected mass loss after decomposition, elemental ratios and crystal structures, proving the feasibility of these precursor materials. It was also shown that P3HT had no significant mass loss up until 300 \textsuperscript{o}C, allowing for metal xanthate precursors to be deposited in conjunction with P3HT. Overall, the feasibility of each precursor, to be utilised within this work, was demonstrated .
The main focus of this work was on overcoming the obstacles for the commercialisation of hybrid Bulk Heterojunctions (BHJs) and Cu\textsubscript{2}ZnSnS\textsubscript{4} (CZTS) based photovoltaic devices, respectively. Three keys areas were addressed as obstacles to be investigated, these were: \textbf{(i)} the use of CdS (a highly toxic material) as an acceptor material within photovoltaic devices, \textbf{(ii)} investigating the use of spray coating for creating these devices (a technique that allows for easy up-scale to commercialisation), and \textbf{(iii)} investigating potential routes to improve the efficiencies of these devices.
To address the toxicity associated with the use of CdS, the chemical composition of the acceptor material was altered by incorporating ZnS into the layer, to form functional photovoltaic devices with reduced toxicity. This was conducted systematically for hybrid BHJs, investigating how the efficiency of a device alters while reducing the toxicity. It was shown that a toxicity reduction of up to 25 \% for human toxicity (HT) and 19 \% for the terrestrial ecotoxicity (TET) could be achieved while actually enhancing the performance of the device. Although, higher ZnS loaded devices performed worse compared to their more toxic counterparts. This CdS:ZnS mix (the highest performing ratio from work conducted into hybrid BHJs) was used an alternative acceptor material for CZTS based photovoltic devices, showing a minor improvement in the efficiency of the device (with a Power Conversion Efficiency (PCE) of 0.16 and 0.15 \% respectively) while reducing the toxicity of the device.
To investigate the use of spray coating P3HT:ZnS and CZTS based photovoltaic devices were fabricated and tested. The PCE of P3HT:ZnS bulk heterojunctions significantly outperformed the P3HT:ZnS devices previously presented using spin coating and matched the previously optimised P3HT:CdS devices. This shows the potential of completely replacing CdS with ZnS, reducing the toxicity of up to 99 \% and 73 \% for HT and TET respectively, while maintaining the same performance. For CZTS devices, an effective fabrication route for spray coating metal xanthate precursors is unknown. To decipher this, different precursor ratios were deposited. It was shown that the tin rich sample (CZT2) displayed the most accurate overlap with CZTS. This observation was explained via a kinetic investigation into the decomposition of the metal xanthate precursors, showing that the decomposition of copper and zinc precursors aligned very well, at 1.26 and 1.24 s\textsuperscript{-1} respectively, while the rate constant for the tin precursor was calculated to be 1.09 s\textsuperscript{-1}. Therefore, in order to form a material with the correct ratio, a tin rich precursor mixture was required.
The versatile nature of spray coating was further exploited, utilising the flexibility of this method to fabricate graded CZTS:CdS devices, forming areas with an overlapping architecture similar to that of a bulk heterojunction. However, it was noted that these graded interfaces resulted in chemical changes in the photoactive materials. This in turn resulted in devices with a graded interface of more than 20 \% of the total deposition time to show no photoactivity and display no characteristic features of a CZTS device. However, it was shown that photoactive devices with a graded overlap performed three times better than the ungraded or spin coated samples, demonstrating the feasibility of this technique.
To further show the utility of P3HT:ZnS devices, carbon nanotubes (CNTs) were incorporated into the active layer for spin-coated devices. It was shown that PCEs of the devices significantly improved after just a 5 wt\% addition of any CNT. It was also shown that incorporating CNTs into the devices allowed for similar PCEs to be achieved compared to the P3HT:CdS device, demonstrating how a dopant can allow for ZnS to completely replace CdS, while maintaining a similar PCE.
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