PVSC 2013-CdTe thin films progress

I had the opportunity to go to PVSC 39 in Tampa, Florida with fellow Highly Qualified Personnel (HQP). There were a lot of interesting speeches but I will only focus on a couple of them here – particularly those focusing on CdTe thin films progress. CdTe is one of the most attractive materials for production of low cost thin film solar modules [1]. The record efficiency for CdTe solar cells has been established to be 16.7% for 10 years. In the past 2 years, the CdTe record was broken several times and increased from 16.7% to 18.7%. However, there has been no significant change in the open-circuit voltage which was in the range of 840-860 mV for over 20 years. Many arguments have been made to justify the apparent V_oc limitation, most frequently: poor hetero-interface with CdS, the difficulty in doping polycrystalline CdTe, midgap defect levels, or non-uniformities at the nano- or micro-scale. Paths for open-circuit voltage above 900 mV are:

• Doping: Increasing doping level of CdTe is believed to increase the built-in potential and reduce recombination at the back-surface. Present doping levels are of the order 10¹⁴ /cm³ and different ways are proposed to increase them.
• Lifetime: Higher lifetime(s) are expected to be a sign of less recombination in the junction and quasi-neutral region, and, hence, improved V_oc and carrier collection. With higher lifetime, it is expected that a greater fraction of the recombination may occur at the back-contact due to increased electron diffusion through the absorber.

Gloeckler from First Solar announced a new record efficiency of 19.1% for CdTe, although not yet certified by NREL.

Despite these promising results, the gap between solar cell and module efficiencies is still wide (3-5%) [2]. This so-called “solar gap” constitutes a major challenge for commercial viability of photovoltaics. One explanation, proposed by M. Alam and his group at the University of Purdue, says that the “solar gap” is due to the monolithic series connection of thin films that causes shunt leakage current. Analysis of the shunt leakage current show that an I-V curve can be modeled by the diode equation and the shunt current, which has a non-linear relation with the voltage, as shown in Fig. 1. It was shown by Alam et al, that as a consequence of the series connection of cells, large shunts have a twofold impact on module performance. First, they modify the operating point of their neighboring good sub-cells, thereby lowering their output power. Second, a large fraction of this (already reduced) power, generated by the neighbours, is consumed by the shunted sub-cell. Interestingly this phenomenon is not unique to CdTe photovoltaics but more of a universal phenomenon and studies on CdTe, CIGS, OPV and amorphous silicon thin films show the same behaviour. At PVSC they have described a post-deposition scribing technique for electrically isolating these distributed shunts in monolithic thin film PV modules. The localized scribes minimize the losses due to defective shunts by restricting lateral current drain from its (otherwise defect-free) neighbors.

Fig. 1 Measured I_Dark (squares) can be represented by a parallel combination of diode with series resistance I (green), and a parasitic shunt component (red), with a symmetric (around V = 0) non-Ohmic voltage dependence. Reproduced from [3] with permission of The Royal Society of Chemistry.

References:

[1] W. N. Shafarman, “What’s next for Cu(InGa)Se2 Thin Film PV: Opportunities and Challenges”, 39th IEEE Photovoltaic Specialists Conference, 2013

[2] S. Dongaonkar, M. A. Alam, “Reducing the Cell to Module Efficiency Gap in Thin Film PV using In-line Post-Process Scribing Isolation”, 39th IEEE Photovoltaic Specialists Conference, 2013

[3] S. Dongaonkar, S. Loser, E. J. Sheets, K. Zaunbrecher, R. Agrawal, T. J. Marks, and M. A. Alam, “Universal statistics of parasitic shunt formation in solar cells, and its implications for cell to module efficiency gap,” Energy & Environmental Science, vol. 6, pp. 782–787, 2013

-Ahmed Gabr

Ph.D Candidate, Year 3

SUNLAB, University of Ottawa

PVSC 2013 in Tampa, Florida

This year, the Photovoltaics Specialists Conference was held in Tampa.  In the middle of June in Florida, you could really feel the sun.  It was hot.  The temperature accounting for humidity was easily into the 40’s each day.   And this was awesome for me, since I almost feel perpetually cold in Ottawa.  An ultra-hot day feels like a blessing so I didn’t mind it at all. We conveniently stayed right across the street from the Tampa Convention Centre, which was also conveniently connected to our hotel with a bridge.  All it took was a quick 30 seconds in the heat and it was back to the frigidity of an air conditioned building.

 

The actual conference was much more along the interests of the students in the network.  And you could tell.  Students were picking out their sessions as soon as they got their hands on a physical copy of the conference schedule.  I was amazed by the sheer volume of talks and posters at the conference.  It seems as though each of these presenters had a unique research topic as well.  I was completely unaware of the breadth of the research activity in the field of solar energy.  Practically any strategy you could think of had a researcher working on exactly that.

The organization of the conference was also equally impressive.  Talks were kept on time.  The seating was great.  Even the coffee was in adequate supply!  The conference centre itself was a great venue too.  There was enough space for everyone to roam about freely (and then some!).

The PVIN booth was manned as usual by quite a few students at a time.  It seems being at the booth allowed you to interact with more people (it also helped being close to the coffee). While at the booth, it was nice to meet other researchers, industry professionals and manufacturers of solar products and characterization tools as well.  Being a research-oriented conference, it really felt like the place to be as a student studying photovoltaics and I couldn’t have asked for a better conference experience.

 

 

-Ross Cheriton

Ph.D Candidate, Year 1

SUNLAB, University of Ottawa