Understanding the Beneficial Role of Grain Boundaries in Polycrystalline Solar Cells from Single-Grain-Boundary Scanning Probe Microscopy

Abstract

The superior performance of certain polycrystalline (PX) solar cells compared to that of corresponding single‐crystal ones has been an enigma until recently. Conventional knowledge predicted that grain boundaries serve as traps and recombination centers for the photogenerated carriers, which should decrease cell performance. To understand if cell performance is limited by grain bulk, grain surface, and/or grain boundaries (GBs), we performed high‐resolution mapping of electronic properties of single GBs and grain surfaces in PX p‐CdTe/n‐CdS solar cells. Combining results from scanning electron and scanning probe microscopies, viz., capacitance, Kelvin probe, and conductive probe atomic force microscopies, and comparing images taken under varying conditions, allowed elimination of topography‐related artifacts and verification of the measured properties. Our experimental results led to several interesting conclusions: 1) current is depleted near GBs, while photocurrents are enhanced along the GB cores; 2) GB cores are inverted, which explains GB core conduction. Conclusions (1) and (2) imply that the regions around the GBs function as an extension of the carrier‐collection volume, i.e., they participate actively in the photovoltaic conversion process, while conclusion (2) implies minimal recombination at the GB cores; 3) the surface potential is diminished near the GBs; and 4) the photovoltaic and metallurgical junction in the n‐CdS/p‐CdTe devices coincide. These conclusions, taken together with gettering of defects and impurities from the bulk into the GBs, explain the good photovoltaic performance of these PX cells (at the expense of some voltage loss, as is indeed observed). We show that these CdTe GB features are induced by the CdCl2 heat treatment used to optimize these cells in the production process.

Numerical simulation of carrier collection and recombination at grain boundaries in Cu(In,Ga)Se2 solar cells

“…From the experimental evidence it appears that type inversion applies rather to CdTe solar cells 4,5 whereas in CIGS ͑Refs. 7-9, 12, 16, and 17͒ the band bending around the GB is relatively small.…”

Nanoimaging of Open‐Circuit Voltage in Photovoltaic Devices

“…[10][11][12][13][14][15][16][17][18][19][20][21] In particular, Kelvin probe force microscopy (KPFM) has been implemented to probe the electrical characteristics of a variety of PV materials and devices, ranging from organic materials [ 9,[22][23][24] and oxides [ 25 ] to III-V semiconductors for multijunction designs [26][27][28] and polycrystalline thin fi lms. [ 18,[29][30][31][32][33][34][35] The local optoelectronic properties and changes in material composition have also been mapped using near-fi eld scanning optical microscopy (NSOM) probes as local sources of excitation.…”

Local Electrochemical Functionality in Energy Storage Materials and Devices by Scanning Probe Microscopies: Status and Perspectives

“…Overall, while EFM and KPFM were successfully applied for studies of solar materials, [98][99][100] and semiconductor and electroceramic devices, [ 88 ] only few applications to energy materials are available. Future potential-once the systems with controlled environment become accessible, and when these measurements can be implemented in liquids, either with different signal generation mechanisms or with the insulated probes.…”