(327g) The Profound Impact of Transient Heat Transfer on the Photovoltaic Properties of Solution-Processed Cu(In,Ga)Se2

Photovoltaics (PV) offer a path towards energy independence and a renewable energy future. However, current commercial PV production is limited by the need for batch processing and high initial investment. Decreased material usage and manufacturing costs associated with the solution-processing of thin film PV materials such as Cu(In,Ga)Se₂ (CIGS) offer a route towards larger scale PV production and deployment. Unfortunately, recent progression in the power conversion efficiency (PCE) of solution-processed CIGS has been slow, and the buildup of understanding through worldwide collaborative research has been hampered by batch to batch and lab to lab irreproducibility.

This work presents a potential solution to these issues by identifying the previously unaccounted for variable of transient heat transfer during the high temperature growth step of CIGS fabrication. Previous studies have explored selenium condensation during the high temperature growth step as a function of differing zone temperatures¹ and total pressure within the system², with changes in selenium condensation having a significant impact on the resulting PV properties. However, until now, heat transfer resistances and their impact on selenium condensation and device performance have not been thoroughly studied.

In this work, subtle changes in transient heat transfer are shown to have a large impact on final device performance, with identical films processed under nominally the same conditions controllably displaying final device PCE’s from less than 5% to greater than 14.5% through variation of only thermal resistances within the furnace system. A model demonstrating how transient heat transfer determines the interface quality and current collection is proposed, and use of these ideas is shown to reliably manipulate all relevant PV parameters

In our selenization process, precursor Cu(In,Ga)S₂ films doctor-bladed onto a glass substrate are placed into a graphite box and surrounded by solid selenium, with both the bottom of the substrate and the selenium resting on the bottom of the box. This box is then inserted into a pre-heated refractory furnace at the desired temperature of selenization. Modeling of the fundamental 1-D time-dependent heat transfer equation during heat-up identifies L²/α as the determining parameter of the temperature of the substrate surface relative to the graphite box in contact with the selenium, where L is the thickness of the substrate and α is its thermal diffusivity. A higher L²/α value of the substrate leads to an increased and prolonged temperature difference between the CIGS top surface and the provided selenium resting directly on the graphite. Taking care to reduce any mass transfer effects, this change in temperature difference then corresponds to a change in the driving force for condensation of selenium from the box atmosphere onto the CIGS film. Thus, it was chosen to utilize a set of identical precursor films and vary only substrate thickness/position in the box in order to probe the impacts of different L²/α values.

I-V measurements coupled with Voltage-dependent EQE and other optoelectronic measurements show that an increased L²/α corresponding to an increased driving force for selenium condensation leads to increased current collection at longer wavelengths, which suggests the promotion of larger active grain depth and is consistent with other studies on liquid fluxes driving grain growth. Interestingly, the V_OC and Fill Factor, which are often associated with the quality of the CIGS/buffer interface, first increase and then decrease with increasing driving force for selenium condensation. In order to explain this trend, it is hypothesized that the top-down growth mechanism that occurs with sulfur to selenium conversion^1,3 may lead to the formation of a favorable Copper-deficient layer (CDL) if the quantity of condensed Selenium increases to an appropriate level for the amount of Copper provided in the precursor film⁴. Past this desired ratio, the formation of detrimental CuSe_x leads to rapid deterioration of device quality with excess selenium condensation. This theory then leads to the existence of optimum substrate thermal properties for interface quality based on the total copper and selenium content of the precursor film and the desired temperature of the selenization. V_OC’s matching that of the best solution-processed device have already been realized through this novel form of property control, and further optimization should allow for PCE’s to surpass current records in the very near future.

This novel understanding of the high temperature growth step then eliminates a previously unaccounted for variable that hampered globally cohesive property improvement and disguised the true impact of other parameters studied in previous works. The thickness and thermal properties of substrates along with the detailed setup within the furnace are often not reported in publications. With the significant impact of these properties on device performance shown here, replicating every key condition described in an external study is then not possible. Further, unintentional changes to thermal resistance can easily occur if tape residue or other foreign objects create gaps and/or increased resistances between the bottom of the substrate and its source of thermal energy. Thus, batch to batch and lab to lab reproducibility requires the understanding and consideration of transient heat transfer described here.

Integrating this idea with the conclusions from other studies, selenium condensation must be understood and controlled as a function of transient heat transfer, maximum temperature, film porosity, and system pressure. This must then be targeted with respect to the copper content of the initial film in order to truly optimize solution-processed CIGS devices. As alternative/flexible substrates with vastly different thermal properties are explored for high throughput roll-to-roll applications, this understanding is even more critical in order to translate the knowledge and processes from traditional substrates to those desired in commercial applications. This new understanding of PV property control is not only limited to CIGS and should also translate to many other solution processed material systems such as Cu(Zn,Sn)Se₂ where similar high temperature growth steps involving a liquid flux agent are used.

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2. Jiang, J. et al. Highly efficient copper-rich chalcopyrite solar cells from DMF molecular solution. Nano Energy 69, 104438 (2020).

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4. Nishimura, T., Sugiura, H., Nakada, K. & Yamada, A. Accurate control and characterization of Cu depletion layer for highly efficient Cu(In,Ga)Se2 solar cells. Prog. Photovoltaics Res. Appl. 27, 171–178 (2019).